II SEMESTER FACTS CONTROLLERS Sub Code: 18EPE23 CIE:50 Hrs/Week: 04 SEE:50 Total Hrs: 50 Exam Hours: 03 COURSE OBJECTIVES: 1. The objective of this course is to introduce variety of controllers with new technology based on power electronics. 2. The students have an opportunity to learn principle of operation, modeling and applications of different FACTS controllers in AC transmission systems. COURSE OUTCOMES: The Student will be able to 1. Acquire the basic concept of general power system considerations and knowledge of different FACTS controllers. . 2. Describe the principle of operation, modeling, steady state and dynamic characteristics of TCSC,SVC,STATCOM,SPST and UPFC controllers. 3. Analyze, determine and investigate the effect of individual FACTS controllers on power flow and voltage profile in line. Course Content: Module-1: FACTS: Concept and General System Considerations: Basics of Power Transmission Networks and Interconnection, Flow of power in AC system, - Control of Power Flow in AC - Transmission Line, Limits of loading capability, Power flow and dynamic stability consideration of a Transmission Interconnection, Relative importance of controllable parameters, and Basic types of FACTS controllers, Brief description and Definitions of FACTS controllers, Application of FACTS Controllers in Distribution Systems. 10 Hours Module-2: Shunt Compensators: Analysis of Uncompensated AC Line - Passive Reactive Power Compensation - - Shunt Compensation Connected at the Midpoint of the Line. Basic principle of operation of StaticVAr Compensator (SVC)- Voltage control by SVC – advantages of slope in dynamic characteristics – influence of SVC on system voltage. Modeling of SVC. Applications – enhancement of transient stability – steady state power transfer – prevention of voltage instability. 10 Hours Module-3: Series Compensators: Compensation by a Series Capacitor Connected at the Midpoint of the Line, Basic model concept of Thyristor Controlled Series Capacitor (TCSC), Operation of the TCSC – different modes of operation – modeling of TCSC – variable reactance model – modeling for stability studies. Applications – improvement of the system stability limit – enhancement of system damping – voltage collapse prevention. 10 Hours 1|Page Module-4: Static Phase Shifting Transformer (SPST): General - basic principle of a PST configurations of SPST, Improvement of transient stability using SPST - damping of low frequency power oscillations -applications of SPST- Some Representative Examples. Comparison between Series and Shunt Capacitor. 10 Hours Module-5: Emerging FACTS Controllers: Static Synchronous Compensator (STATCOM) – operating principle – V-I characteristics , applications: Steady state power transferenhancement of transient stability – prevention of voltage instability. SSSC-operation of SSSC and the control of power flow –modeling of SSSC in load flow and transient stability.Unified Power Flow Controller (UPFC) – Principle of operation – modes of operation – applications – modeling of UPFC for power flow studies.Special Purpose FACTS Controllers - Interline Power Flow Controller - operation and control. 10 Hours TEXT BOOKS: 1. NarainG.Hingorani, Laszio. Gyugyl, “Understanding FACTS Concepts and Technology of Flexible AC Transmission System”, IEEE Press, Standard Publishers, Delhi 2001. 2. K.R.Padiyar,” FACTS Controllers in Power Transmission and Distribution”, New Age International(P) Limited, Publishers, New Delhi, 2008. REFERENCE BOOKS: 1. Mohan Mathur, R., Rajiv. K. Varma, “Thyristor – Based Facts Controllers for Electrical Transmission Systems”, IEEE press and John Wiley & Sons, Inc. 2|Page Module-1: FACTS: Concept and General System Considerations: Basics of Power Transmission Networks and Interconnection, Flow of power in AC system, Control of Power Flow in AC - Transmission Line, Limits of loading capability, Power flow and dynamic stability consideration of a Transmission Interconnection, Relative importance of controllable parameters, and Basic types of FACTS controllers, Brief description and Definitions of FACTS controllers, Application of FACTS Controllers in Distribution Systems. Power Transmission Networks and Interconnections • Purpose of the transmission network is to pool power plants and load centers in order to minimize the total power generation capacity and fuel cost. • Transmission interconnections enable taking advantage of diversity of loads, availability of sources, and fuel price to supply electricity to the loads at minimum cost with a required reliability. • With that perspective, transmission is often an alternative to a new generation resource. • Less transmission capability means that more generation resources would be required regardless of whether the system is made up of large or small powerplants. • small distributed generation becomes more economically viable if there is a backbone of a transmission grid. • One cannot be really sure about what the optimum balance is between generation and transmission unless the system planners use advanced methods of analysis which integrate transmission planning into an integrated valuetransmission/generation planning scenario. • Cost of transmission lines and losses, as well as difficulties encountered in building new transmission lines, would often limit the available transmission capacity. • It seems that there are many cases where economic energy or reserve sharing is constrained by transmission capacity, and the situation is not getting any better. • On the other hand, as power transfers grow, the power system becomes increasingly more complex to operate and the system can become less secure for riding through the major outages. 3|Page • It may lead to large power flows with inadequate control, excessive reactive power in various parts of the system, large dynamic swings between different parts of the system and bottlenecks, and thus the full potential of transmission interconnections cannot be utilized. • Power systems of today, by and large, are mechanically controlled.Problem with mechanical devices is that control cannot be initiated frequently, because these mechanical devices tend to wear out very quickly compared to static devices. • There is a widespread use of microelectronics, computers and high-speed communications for control and protection of present transmission systems; • Power system planners, operators, and engineers have learned to live with this limitation by using a variety of ingenious techniques to make the system work effectively, but at a price of providing greater operating margins and redundancies. • In recent years, greater demands have been placed on the transmission network, and these demands will continue to increase because of the increasing number of nonutility generators and heightened competition among utilities themselves. • Increased demands on transmission, absence of long-term planning, and the need to provide open access to generating companies and customers, all together have created tendencies toward less security and reduced quality of supply. • The FACTS technology is essential to alleviate some but not all of these difficulties by enabling utilities to get the most service from their transmission facilities and enhance grid reliability. • It must be stressed, that for many of the capacity expansion needs, building of new lines or upgrading current and voltage capability of existing lines and corridors will be necessary. FLOW OF POWER IN AN AC SYSTEM In ac power systems, given the insignificant electrical storage, the electrical generation and load must balance at all times. If generation is less than load, the voltage and frequency drop, and thereby the load, goes down to equal the generation minus the transmission losses. However, there is only a few percent margin for such a self-regulation. If voltage is propped up with reactive power support, then the load will go up, and consequently frequency will keep dropping, and the system will collapse. 4|Page Alternately, if there is inadequate reactive power, the system can have voltage collapse. When adequate generation is available, active power flows from the surplus generation areas to the deficit areas, and it flows through all parallel paths available which frequently involves extra high- voltage and medium-voltage lines. Often, long distances are involved with loads and generators along the way. There are in fact some major and a large number of minor loop flows and uneven power flows in any power transmission system. Power Flow in Parallel Paths • Consider a simple case of power flow [Figure 1.1(a)], through two parallel paths (possibly corridors of several lines) from a surplus generation area, shown as an equivalent generator on the left, to a deficit generation area on the right. • Without any control, power flow is based on the inverse of the various transmission line impedances. • Apart from ownership and contractual issues over which lines carry how much power, it is likely that the lower impedance line may become overloaded and thereby limit the loading on both paths even though the higher impedance path is not fully loaded. • There would not be an incentive to upgrade current capacity of the overloaded path, because this would further decrease the impedance 5|Page Figure 1.1 Power flow in parallel paths: (a) ac power flow with parallel paths; (b) power flow control with HVDC; 6|Page (c) power flow control with variable impedance; (d) power flow control with variable phase angle. • Figure 1.1 (b) shows the same two paths, but one of these has HVDC transmission. With HVDC, power flows as ordered by the operator, because with HVDC power electronics converters power is electronically controlled. Also, because power is electronically controlled, the HVDC line can be used to its full thermal capacity if adequate converter capacity is provided. Furthermore, an HVDC line, because of its high-speed control, can also help the parallel ac transmission line to maintain stability. However, HVDC is expensive for general use, and is usually considered when long distances are involved, such as the Pacific DC Intertie on which power flows as ordered by the operator. • As alternative FACTS Controllers, Figures 1.1(c) and 1.1(d) show one of the transmission lines with different types of series type FACTS Controllers. By means of controlling impedance [Figure 1.1(c)] or phase angle [Figure 1.1(d)], or series injection of appropriate voltage (not shown) a FACTS Controller can control the power flow as required. Maximum power flow can in fact be limited to its rated limit under contingency conditions when this line is expected to carry more power due to the loss of a parallel line. 1.2.2 Power Flow in a Meshed System • To understand the free flow of power, consider a simplified case in which generators at two different sites are sending power to a load centre through a network consisting of three lines in a meshed connection (Fig. 1.2). • Suppose the lines AB, BC, and AC have continuous ratings of 1000 MW, 1250 MW, and 2000 MW, respectively, and have emergency ratings of twice those numbers for a sufficient length of time to allow rescheduling of power in case of loss of one of these lines. 7|Page Figure 1.2 Power flow in a mesh network: (a) system diagram; (b) system diagram with Thyristor-Controlled Series Capacitor in line AC; (c) system diagram with Thyristor-Controlled Series Reactor in line BC; (d) system diagram with ThyristorControlled Phase Angle Regulator in line AC. • If one of the generators is generating 2000 MW and the other 1000 MW, a total of 3000 MW would be delivered to the load center. For the impedances shown, the three lines would carry 600, 1600, and 1400 MW, respectively, as shown in Figure 1.2(a). Such a situation would overload line BC (loaded at 1600 MW for its continuous rating of 1250 MW), and therefore generation would have to be decreased at B, and increased at A, in order to meet the load without overloading line BC. • Power flows in accordance with transmission line series impedances (which are 90% inductive) that bear no direct relationship to transmission ownership, contracts, thermal limits, or transmission losses. If, a capacitor whose reactance is —5 ohms at the synchronous frequency is inserted in one line [Figure 1.2(b)], it reduces the line's impedance from 10 ohm to 5 ohm, so that power flow through the lines AB, BC, and AC will be 250,1250, and 1750 MW, respectively. • It is clear that if the series capacitor is adjustable, then other power-flow levels may be realized in accordance with the ownership, contract, thermal limitations, transmission losses, and a wide range of load and generation schedules. Although this capacitor could be modular and mechanically switched, the number of operations would be severely limited by wear on the mechanical components because the line loads vary continuously with load conditions, generation schedules, and line outages. • Other complications may arise if the series capacitor is mechanically controlled. A series capacitor in a line may lead to subsynchronous resonance (typically at 10-50Hz for a 60 Hz system). This resonance occurs when one of the mechanical resonance frequencies of the shaft of a multiple-turbine generator unit coincides 8|Page with 60 Hz. minus the electrical resonance frequency of the capacitor with the inductive impedance of the line. If such resonance persists, it will soon damage the shaft. • Also while the outage of one line forces other lines to operate at their emergency ratings and carry higher loads, power flow oscillations at low frequency (typically 0.3-3 Hz) may cause generators to lose synchronism, perhaps prompting the system's collapse. • If series capacitor is thyristor-controlled, o can be varied as often as required. o can rapidly damp any SSR conditions, a o can damp low frequency oscillations in the power flow. o allow the transmission system to go from one steady-state condition o Avoid risk of damage to generator shaft and system collapse . • In other words, a thyristor-controlled series capacitor can greatly enhance the stability of the network. • it is practical for part of the series compensation to be mechanically controlled and part thyristor controlled. • Similar results may be obtained by increasing the impedance of one of the lines in the same meshed configuration by inserting a 7 ohm reactor (inductor) in series with line AB [Figure 1.2(c)]. Again, a series inductor that is partly mechanically and partly thyristor-controlled, it could serve to adjust the steadystate power flows as well as damp unwanted oscillations. • Another option, a thyristor-controlled phase-angle regulator could be installed instead of a series capacitor or a series reactor in any of the three lines to serve the same purpose. • In Figure 1.2(d), the regulator is installed in the third line to reduce the total phase-angle difference along the line from 8.5 degrees to 4.26 degrees. • The same results could also be achieved by injecting a variable voltage in one of the lines. Note that balancing of power flow in the above case did not require more than one FACTS Controller, and indeed there are options of different controllers and in different lines. LIMITS OF THE LOADING CAPABILITY? To make the best use of the transmission asset, and to maximize the loading capability (taking into account contingency conditions), it is important to know the limitation of transmission loading capability There are three kinds of limitations for loading capability: • Thermal • Dielectric • Stability 9|Page Thermal • • • • • • • • • • • • A large current flow increases the losses in the form of heat. This results in increased conductor temperatures. Excessive temperature may result in expansion and resultant sag of conductors causing decreased clearance to ground. Temperature extremes have an "annealing effect" causing reduced mechanical strength of aluminium. Thermal capability of an overhead line is a function of the a)ambient temperature, b) wind conditions, c) condition of the conductor, and d) ground clearance. It varies due to the variable environment and the loading history. The nominal rating of a line is generally decided on a worst ambient environment case scenario. There are also off-line computer programs that can calculate a line's loading capability based on available ambient environment and recent loading history. There are the on-line monitoring devices that provide a basis for on-line real-time loading capability. These methods have evolved over a period of many years, and, given the age of automation (typified by GPS systems and low-cost sophisticated communication services), it surely makes sense to consider reasonable, day to day, hour to hour, or even real-time capability information. Normal loading of the lines is decided on a loss evaluation basis under assumptions which may have changed for a variety of reasons;. Increasing the rating of a transmission circuit involves consideration of the real-time ratings of the transformers and other equipment. Realtime loading capability of transformers is also a function of ambient temperature, aging of the transformer and recent loading history. Off-line and on-line loading capability monitors can also be used to obtain real time loading capability of transformers. There is the possibility of upgrading a line by changing the conductor to that of a higher current rating, which may in turn require structural upgrading. Finally, there is the possibility of converting a single-circuit to a double-circuit line. Once the higher current capability is available, then the question arises of how it should be used. Will the extra power actually flow and be controllable? Will the voltage conditions be acceptable with sudden load dropping, etc.? The FACTS technology can help in making an effective use of this newfound capacity. Dielectric • • • • • • The dielectric limit refers to the insulation capability of the transmission line From an insulation point of view, many lines are designed very conservatively conservatively based on worst ambient environment case scenario. Exceeding dielectric limits (maximum electric field strength) results in failure of insulation, causing faults For a given nominal voltage rating, it is often possible to increase normal operation by +10% voltage (i.e., 500 kV-550 kV) or may be even higher. Care is then needed to ensure that dynamic and transient overvoltages are within limits. Modern gapless arresters, or line insulators with internal gapless arresters, or powerful thyristor-controlled overvoltage suppressors at the substations can enable significant increase in the line and substation voltage capability. 10 | P a g e • The FACTS technology could be used to ensure acceptable over-voltage and power flow conditions. Stability There are a number of stability issues that limit the transmission capability. These include: • Transient stability - corresponds to the stability attained after a fault occurs & suddenly a large part of load is bypassed. Then there is a large unbalance in the system. Then also gradually the system attains the stability. • Dynamic stability- like transient stability but here help of an external device is taken to regain the stability whereas in transient stability the stability was attained within the power system itself without the help of any external device. • Steady-state stability - is the stability the power system attains after slight unbalance. suppose a small amount of load is disconnected, but after a very short duration again the power system will regain its steady state. • Frequency collapse- If the frequency deviates from desired value , then generators start to go out of synchronism which triggers events in the power system causing voltage, frequency, power imbalance due to which a power system will collapse. • Voltage collapse- is typically associated with reactive power demand of load not being met due to shortage in reactive power production and transmission. • Subsynchronous resonance- Sub-Synchronous Resonance is an electrical power system condition where, electrical network exchanges energy with turbine generator at one or more natural frequency of combined system, below the synchronous frequency of the system Power flow and dynamic stability consideration of a Transmission • Figure 1.3(a) shows a simplified case of power flow on a transmission line. Locations 1 and 2 could be any transmission substations connected by a transmission line. • Substations may have loads, generation, or may be interconnecting points on the system and for simplicity they are assumed to be stiff busses. • Ex and E2 are the magnitudes of the bus voltages with an angle δ between the two. The line is assumed to have inductive impedance X, and the line resistance and capacitance are ignored. • As shown in the phasor diagram [Figure 1.3(b)] the driving voltage drop in the line is the phasor difference EL between the two line voltage phasors, E1 and E2. The line current magnitude is given by I = EL/X, 11 | P a g e / = E/X, and lags EL by 90° • For a typical line, angle δ and voltage drop along the line, is small compared to the line voltages. • Given that a transmission line may have a voltage drop at full load 1% per10 km, and assuming that a line between two stiff busbars (substations) is 200 km long, the voltage drop along this line would be 20% at full load, and the angle δ would be small. • If we were to assume, for example, that with equal magnitudes of Ex and E2, and X of 0.2 per unit magnitude, the angle 8 would be only 0.2 radians or 11.5 degrees. • The current flow on the line can be controlled by controlling EL or X or δ. • Figure 1.3(b) shows that the current flow phasor is perpendicular to the driving voltage (90° phase lag). If the angle between the two bus voltages is small, the current flow largely represents the active power. Increasing or decreasing the inductive impedance of a line will greatly affect the active power flow. Thus impedance control, which in reality provides current control, can be the most cost-effective means of controlling the power flow. 12 | P a g e • Figure 1.3(c), corresponding to Figure 1.3(b), shows a phasor diagram of the relationship between the active and reactive currents with reference to the voltages at the two ends. because it is assumed that there are no active power losses in the line. • Thus, varying the value of X will vary P, Q1 and Q2 in accordance with Eq.(1.1), Eq.(1.2), and Eq.(1.3), respectively. Assuming that E1 and E2 are the magnitudes 13 | P a g e of the internal voltages of the two equivalent machines representing the two systems, and the impedance X includes the internal impedance of the two equivalent machines, Figure 1.3(d) shows the half sinewave curve of active power increasing to a peak with an increase in δ to 90 degrees. • Power then falls with further increase in angle, and finally to zero at δ = 180°. It is easy to appreciate that without high-speed control of any of the parameters E1, E2, Ei — E2, X and δ, the transmission line can be utilized only to a level well below that corresponding to 90 degrees. This is necessary, in order to maintain an adequate margin needed for transient and dynamic stability and to ensure that the system does not collapse following the outage of the largest generator and/or a line. • Increase and decrease of the value of X will increase and decrease the height of the curves, respectively, as shown in Figure 1.3(d). For a given power flow, varying of X will correspondingly vary the angle between the two ends. • Power/current flow can also be controlled by regulating the magnitude of voltage phasor E1 or E2. However, it is seen from Figure 1.3(e) that with change in the magnitude of E1 the magnitude of the driving voltage phasor E1 — E2 does not change by much, but its phase angle does. This also means that regulation of the magnitude of voltage phasor E1 and/or E2 has much more influence over the reactive power flow than the active power flow, as seen from the two current phasors corresponding to the two driving voltage phasors E1 — E2 shown in Figure 1.3(e). • Current flow and hence power flow can also be changed by injecting voltage in series with the line. It is seen from Figure 1.3(f) that when the injected voltage is in phase quadrature with the current (which is approximately in phase with the driving voltage, it directly influences the magnitude of the current flow, and with small angle influences substantially the active power flow. • Alternatively, the voltage injected in series can be a phasor with variable magnitude and phase relationship with the line voltage [Figure 1.3(g)]. It is seen that varying the amplitude and phase angle of the voltage injected in series, both the active and reactive current flow can be influenced. Relative importance of controllable parameters It is worth noting a few basic points regarding the possibilities of power flow control: _ Control of the line impedance X (e.g., with a thyristor-controlled series capacitor) can provide a powerful means of current control. _ When the angle is not large, which is often the case, control of X or the angle substantially provides the control of active power. _ Control of angle (with a Phase Angle Regulator, for example), which in turn controls the driving voltage, provides a powerful means of controlling the current flow and hence active power flow when the angle is not large. _ Injecting a voltage in series with the line, and perpendicular to the current flow, can increase or decrease the magnitude of current flow. Since the current flow lags the driving voltage by 90 degrees, this means injection of reactive power in series, (e.g., 14 | P a g e with static synchronous series compensation) can provide a powerful means of controlling the line current, and hence the active power when the angle is not large. _ Injecting voltage in series with the line and with any phase angle with respect to the driving voltage can control the magnitude and the phase of the line current. This means that injecting a voltage phasor with variable phase angle can provide a powerful means of precisely controlling the active and reactive power flow. This requires injection of both active and reactive power in series. _ Because the per unit line impedance is usually a small fraction of the line voltage, the MVA rating of a series Controller will often be a small fraction of the throughput line MVA. _ When the angle is not large, controlling the magnitude of one or the other line voltages (e.g., with a thyristor-controlled voltage regulator) can be a very cost-effective means for the control of reactive power flow through the interconnection. _ Combination of the line impedance control with a series Controller and voltage regulation with a shunt Controller can also provide a cost-effective means to control both the active and reactive power flow between the two systems Basic types of FACTS controllers In general, FACTS Controllers can be divided into four categories: _ Series Controllers _ Shunt Controllers _ Combined series-series Controllers _ Combined series-shunt Controllers Figure 1.4(a) shows the general symbol for a FACTS Controller: a thyristor arrow inside a box. Series Controllers: [Figure 1.4(b)] The series Controller could be a variable impedance, such as capacitor, reactor, etc., or a power electronics based variable source of main frequency, subsynchronous and harmonic frequencies (or a combination) to serve the desired need. In principle, all series Controllers inject voltage in series with the line. Even a variable impedance multiplied by the current flow through it, represents an injected series voltage in the line. As long as the voltage is in phase quadrature with the line current, the series Controller only supplies or consumes variable reactive power. Any other phase relationship will involve handling of real power as well. Shunt Controllers: [Figure 1.4(c)] As in the case of series Controllers, the shunt Controllers may be variable impedance, variable source, or a combination of these. In principle, all shunt Controllers inject current into the system at the point of connection. Even a variable shunt impedance connected to the line voltage causes a variable current flow and hence represents injection of current into the line. As long as the injected current is in phase quadrature with the line voltage, the shunt Controller only supplies or consumes variable reactive power. Any other phase relationship will involve handling of real power as well. Combined series-series Controllers: [Figure 1.4(d)] This could be a combination of separate series controllers, which are controlled in a coordinated manner, in a multiline transmission system. Or it could be a unified Controller, Figure 1.4(d), in which series Controllers provide 15 | P a g e independent series reactive compensation for each line but also transfer real power among the lines via the power link. The real power transfer capability of the unified series-series Controller, referred to as Interline PowerFlow Controller, makes it possible to balance both the real and reactive power flow in the lines and thereby maximize the utilization of the transmission system. Note that the term "unified" here means that the dc terminals of all Controller converters are all connected together for real power transfer. Combined series-shunt Controllers: [Figures 1.4(e) and 1.4(f)] This could be a combination of separate shunt and series Controllers, which are controlled in a coordinated manner [Figure 1.4(e)], or a Unified Power Flow Controller with series and shunt elements [Figure 1.4(f)]. In principle, combined shunt and series Controllers inject current into the system with the shunt part of the Controller and voltage in series in the line with the series part of the Controller. However, when the shunt and series Controllers are unified, there can be a real power exchange between the series and shunt Controllers via the power link. It is important to appreciate that the series-connected Controller impacts the driving voltage and hence the current and power flow directly. Therefore, if the purpose of the application is to control the current/power flow and damp oscillations, the series Controller for a given MVA size is several times more powerful than the shunt Controller. As mentioned, the shunt Controller, on the other hand, is like a current source, which draws from or injects current into the line. The shunt Controller is therefore a good way to control voltage at and around the point of connection through injection of reactive current (leading or lagging), alone or a combination of active and reactive current for a more effective voltage control and damping of voltage oscillations. This is not to say that the series Controller cannot be used to keep the line voltage within the specified range. After all, the voltage fluctuations are largely a consequence of the voltage drop in series impedances of lines, transformers, and generators. Therefore, adding or subtracting the FACTS Controller voltage in series (main frequency, subsynchronous or harmonic voltage and combination thereof) can be the most cost-effective way of improving the voltage profile. Nevertheless, a shunt controller is much more effective in maintaining a required voltage profile at a substation bus. One important advantage of the shunt Controller is that it serves the bus node independently of the individual lines connected to the bus. Series Controller solution may require, but not necessarily, a separate series Controller for several lines connected to the substation, particularly if the application calls for contingency outage of any one line. However, this should not be a decisive reason for choosing a shuntconnected Controller, because the required MVA size of the series Controller is small compared to the shunt Controller, and, in any case, the shunt Controller does not provide control over the power flow in the lines. On the other hand, series-connected Controllers have to be designed to ride through contingency and dynamic overloads, and ride through or bypass short circuit currents. They can be protected by metal-oxide arresters or temporarily bypassed by solid-state devices when the fault current is too high, but they have to be rated to handle dynamic and contingency overload. The above arguments suggest that a combination of the series and shunt Controllers [Fig(e) and (f)] can provide the best of both, i.e., an effective power/ current flow and line voltage control. For the combination of series and shunt Controllers, the shunt Controller can be a single unit serving in coordination with individual line Controllers [Figure 1.4(g)]. This arrangement can provide additional benefits (reactive power flow control) with unified Controllers. 16 | P a g e FACTS Controllers may be based on thyristor devices with no gate turn-off (only with gate turn-on), or with power devices with gate turn-off capability. Also, in general, as will be discussed in other chapters, the principal Controllers with gate turn-off devices are based on the dc to ac converters, which can exchange active and/ or reactive power with the ac system. When the exchange involves reactive power only, they are provided with a minimal storage on the dc side. However, if the generated ac voltage or current is required to deviate from 90 degrees with respect to the line current or voltage, respectively, the converter dc storage can be augmented beyond the minimum required for the converter operation as a source of reactive power only. This can be done at the converter level to cater to short-term (a few tens of main frequency cycles) storage needs. In addition, another storage source such as a battery, superconducting magnet, or any other source of energy can be added in parallel through an electronic interface to replenish the converter's dc storage. Any of the converter-based, series, shunt, or combined shunt-series Controllers can generally accommodate storage, such as capacitors, batteries, and superconducting magnets, which bring an added dimension to FACTS technology [Fig(h), (i), and (j)]· 17 | P a g e Figure 1.4 Basic types of FACTS Controllers: (a) general symbol for FACTS Controller; (b) series Controller; (c) shunt Controller; (d) unified seriesseries ontroller; (e) coordinated series and shunt Controller; (f) unified series-shunt Controller; (g) unified Controller for multiple lines; (h) series Controller with storage; (i) shunt Controller with storage; (j) unified series-shunt Controller with storage. The benefit of an added storage system (such as large dc capacitors, storage batteries, or superconducting magnets) to the Controller is significant. A Controller with storage is much more effective for controlling the system dynamics than the corresponding Controller without the storage. This has to do with dynamic pumping of real power in or out of the system as against only influencing the transfer of real power within the system as in the case with Controllers lacking storage. Here also, engineers have to rethink the role of storage, particularly the one that can deliver or absorb large amounts of real power in short bursts. A converter-based Controller can also be designed with so-called high pulse order or with pulse width modulation to reduce the low order harmonic generation to a very low level. A converter can in fact be designed to generate the correct waveform in order to act as an active filter. It can also be controlled and operated in a way that it balances the unbalance voltages, involving transfer of energy between phases. It can do all of these beneficial things simultaneously if the converter is so designed. DESCRIPTION AND DEFINITIONS OF FACTS CONTROLLERS This section briefly describes and defines various shunt, series, and combined Controllers • For the converter-based Controllers there are two principal types of converters with gate turn-off devices. • These are the so-called voltage-sourced converters and the current-sourced converters. As shown in the left hand side of Figure 1.5(a), the voltage-sourced converter is represented in symbolic form by a box with a gate turn-off device paralleled by a reverse diode, and a dc capacitor as its voltage source. As shown in the right-hand side of Figure 1.5(a), the current-sourced converter is represented by a box with a gate turn-off device with a diode in series, and a dc reactor as its current source. • For the voltage-sourced converter, unidirectional dc voltage of a dc capacitor is presented to the ac side as ac voltage through sequential switching of devices. 18 | P a g e • Through appropriate converter topology, it is possible to vary the ac output voltage in magnitude and also in any phase relationship to the ac system voltage. The power reversal involves reversal of current, not the voltage. • When the storage capacity of the dc capacitor is small, and there is no other power source connected to it, the converter cannot supply or absorb real power for much more than a cycle. The ac output voltage is maintained at 90 degrees with reference to the ac current, leading or lagging, and the converter is used to absorb or supply reactive power only. • For the current-sourced converter, the dc current is presented to the ac side through the sequential switching of devices, as ac current, variable in amplitude and also in any phase relationship to the ac system voltage. The power reversal involves reversal of voltage and not current. The current-sourced converter is represented symbolically by a box with a power device, and a dc inductor as its current source • From overall cost point of view, the voltage-sourced converters seem to be preferred, and will be the basis for presentations of most converter-based FACTS Controllers. 19 | P a g e Figure 1.5 Shunt-connected Controllers: (a) Static Synchronous Compensator (STATCOM) based on voltage-sourced and current-sourced converters; (b) STATCOM with storage, i.e., Battery Energy Storage System (BESS) Superconducting Magnet Energy Storage and large dc capacitor; (c) Static VAR Compensator(SVC), Static VAR Generator (SVG), Static VAR System (SVS), ThyristorControlled Reactor (TCR), Thyristor- Switched Capacitor (TSC), and Thyristor-Switched Reactor (TSR); (d) Thyristor-Controlled Braking Resistor. on new Controllers or variations of known Controllers. The IEEE PES Task Force of the FACTS Working Group defined Terms and Definitions for FACTS and FACTS Controllers. Along with a brief description of FACTS Controllers, appropriate IEEE Terms and Definitions are also presented in this section in italic for reference. IEEE terms and definitions. Flexibility of Electric Power Transmission. The ability to accommodate changes in the electric transmission system or operating conditions while maintaining sufficient steadystate and transient margins. Flexible AC Transmission System (FACTS). Alternating current transmission systems incorporating power electronic-based and other static controllers to enhance controllability and increase power transfer capability. It is worthwhile to note the words "other static Controllers" in this definition of FACTS implying that there can be other static Controllers which are not based on power electronics. FACTS Controller. A power electronic-based system and other static equipment that provide control of one or more AC transmission system parameters Application of FACTS Controllers in Distribution Systems 20 | P a g e FACTS was developed originally for transmission network; has been extended for improvement of Power Quality (PQ) in distribution systems operating at low or medium voltages. The power quality referred primarily to the continuity of power supply at acceptable voltage and frequency. However, the prolific increase in the use of computers, microprocessors and power electronic systems has resulted in power quality issues involving transient disturbances in voltage magnitude, waveform and frequency. The nonlinear loads not only cause PQ problems but are also very sensitive to the voltage deviations. In the modern context, PQ problem is defined as Any problem manifested in voltage, current or frequency deviations that result in failure or misoperation of customer equipment" The PQ problems are categorized as follows 1. Transients (a) Impulsive (b) Oscillatory 2. Short-duration and Long-duration variations (a) Interruptions (b) Sag (dip) (c) Swell 3. Voltage unbalance 4. Waveform distortion (a) DC offset (b) Harmonics (c) Interharmonics (d) Notching (e) Noise 5. Voltage Flicker 6. Power frequency variations FACTS controllers for improving PQ termed them as Custom Power Devices (CPD). These are based on VSC and are of 3 types given below. 1. Shunt connected Distribution STATCOM (DSTATCOM) 2. Series connected Dynamic Voltage Restorer (DVR) 3. Combined shunt and series, Unified Power Quality Conditioner (UPQC). • • • • • • DVR is similar to SSSC while UPQC is similar to UPFC. In spite of the similarities, the control strategies are quite different for improving PQ. A major difference involves the injection of harmonic currents and voltages to isolate the source from the load. For example, a DVR can work as a harmonic isolator to prevent the harmonics in the source voltage reaching the load in addition to balancing the voltages and providing voltage regulation. A UPQC can be considered as the combination of DSTATCOM and DVR. A DSTATCOM is utilized to eliminate the harmonics from the source currents and also balance them in addition to providing reactive power compensation (to improve power factor or regulate the load bus voltage). The terminology is yet to be standardized. The term `active ¯filters' or `power conditioners' 21 | P a g e is also employed to describe the custom power devices. ABB terms DSTATCOM as `SVC light'. Irrespective of the name, the trend is to increasingly apply VSC based compensators for power quality improvement. Application of FACTS 1. Submarine cables. Cables have a large capacitance, and hence ac cables require a large charging current (reactive power) an order of magnitude larger than that of overhead lines. As a result, for over a 30 km or so stretch of ac submarine cable, the charging current supplied from the shore will fully load the cable and leave no room for transmitting real power. The charging current flowing in the cables can only be reduced by connecting shunt inductors to the cable at intervals of 15-20 km, thus requiring appropriate land location. With HVDC cable on the other hand, distance is not a technical barrier. Also, the cost of dc cable transmission is much lower than that of ac which works to HVDC's advantage to cover new markets for long distance submarine transmission. In this area, FACTS technology (e.g., the UPFC) can provide an improvement by controlling the magnitude of one of the end (e.g., the receiving-end) voltages so as to keep it identical to that of the other one. In this way, the effective length of the cable from the standpoint of the charging current can be halved. This approach may provide an economical solution for moderate submarine distances, up to about 100 km, but for long distance transmission HVDC will remain unchallenged. 2. Long distance overhead transmission. If the overhead transmission is long enough, say 1000 km, the saving in capital costs and losses with a dc transmission line may be enough to pay for two converters (note that HVDC represents total power electronics rating of 200% of the rated transmission capacity). This distance is known as the break-even distance. This break-even distance is very subject to many factors including the cost of the line, right-of-way, any need to tap the line along the way, and often most important, the politics of obtaining permission to build the line. Nevertheless, it is important to recognize that while FACTS can play an important role in an effective use of ac transmission, it probably does not have too much influence on the breakeven distance. Thus, the principal role of FACTS is in the vast ac transmission market where HVDC is generally not economically viable. 3. Underground transmission. Because of the high cost of underground cables, the break even distance for HVDC is more like 100 km as against 1000 km for overhead lines. 22 | P a g e Again, FACTS technology probably does not have much influence in this break-even distance. In any case, to date there have been no long distance underground projects, either ac or dc, because, in an open landscape, overhead transmission costs so much less than underground transmission (about 25% of the costs of underground transmission). Cable transmission, on the other hand, has a significant potential of cost reduction, both in the cost of cables and construction cost. 4. Connecting ac systems of different or incompatible frequencies. For historical reasons, the oceans in effect separate the globe's electric systems into 50 Hz and 60 Hz groups. The 60 Hz normal frequency pervades all the countries of the Americas, excepting Argentina and Paraguay. Those two countries and all the rest of the world have a 50 Hz frequency except Japan, which is partly 50 Hz and partly 60 Hz. In general, the oceans are too huge and deep to justify interconnections of 50 and 60 Hz systems. Thus there is a limited market for HVDC for connecting 50 and 60 Hz systems. Questions Module 1 1. Explain power flow in ac power system for cases a). Parallel network b) Mesh network 20 M 2. Elaborate the applications of FACTS 6M 3. Describe in detail FACTS and FACTS controller 8M 4. Elaborate the basic types of FACTS controller 16 M 5. Elaborate Power flow and dynamic stability consideration of Transmission 14M 6. Elaborate the series and shunt types of FACTS controller 8M 7. Elaboarte types of limitations for loading capability of transmission 12M 8. Explain importance of controllable parameters 8M 9. Explain Power Transmission Networks and Interconnections 8M 10. Elaborate the applications of FACTS in distribution Systems 8M One mark question 1. What are the factors that affects Thermal capability of an overhead line. 2. What limits the loading capability of transmission lines? 3. What is open access? 4. State three kinds of limitations for loading capability 5. What are stability issues that limit the transmission capability. 6. Define Flexible AC Transmission System (FACTS) as per IEEE definition. 7. State and draw types of voltage source converter-based Controllers 8. What is frequency collapse in power system? 23 | P a g e 9. What is voltage collapse in power system? 10. What is subsynchronous resonance? 11. What is steady state stability? 12. What is transient stability? 13. State the classification of the power quality problem Module-2: Shunt Compensators: Analysis of Uncompensated AC Line - Passive Reactive Power Compensation - - Shunt Compensation Connected at the Midpoint of the Line. Basic principle of operation of StaticVAr Compensator (SVC)- Voltage control by SVC – advantages of slope in dynamic characteristics – influence of SVC on system voltage. Modeling of SVC. Applications – enhancement of transient stability – steady state power transfer – prevention of voltage instability. Analysis of Uncompensated AC Line Lossless Distributed Parameter Lines • Most power-transmission lines are characterized by distributed parameters: 24 | P a g e series resistance, r; series inductance l; shunt conductance, g; and shunt capacitance, c—all per-unit (pu) length. • These parameters all depend on the conductors’ size, spacing, clearance above the ground, and frequency and temperature of operation. • Characteristic behavior of a transmission line is dominated by its l and c parameters. Parameters r and g account for the transmission losses. • Consider a small element of the line of length (dx) at a distance x from the receiving end, I(x + dx) = I(x) + (ydx)V (x + dx) V (x + dx) = V (x) + (zdx)I(x) where y = g + jb; z = r + jx; • we get the following differential equations for V and I. (2.1) (2.2) (2.3) The fundamental equations governing the propagation of energy along a line are the following wave equations: 25 | P a g e These equations are used to calculate voltage and current anywhere on line, at a distance x from the sending end, in terms of the sending-end voltage and current and the line parameters Where 26 | P a g e Likewise, power at the receiving end is given as It is concluded that for a lossless line, Ps = − Pr , However, Qs ≠ Qr because of the reactive-power absorption/ generation in the line. Midpoint Conditions of a Symmetrical Line The magnitude of the midpoint voltage depends on the power transfer. This voltage influences the line insulation and therefore needs to be well understood. For a symmetrical line where the end voltages are held at nominal values, the midpoint voltage shows the highest magnitude variation. In terms of the midpoint voltage Vm, the receiving-end voltage of a symmetrical line, from Eq., is given as 27 | P a g e Figure shows the load angle δ and the midpoint-normalized per-unit voltage π£Μπ as functions of P/ P0 per-unit power transfer on the line. For this line, it is observed that for light loading below the surge-impedance load, the midpoint voltage exceeds Vnom and reaches its highest value at no load. Furthermore, for stability considerations should an operating load angle of, say, δ =30 be chosen to define the full-load rating of the line (0.588P0), the midpoint voltage of the line will be 1.1058 pu. These expected overvoltages in the range 0.1–0.2 pu from full load to no load are not within acceptable limits. Therefore, special techniques must be used to control these overvoltages It is possible to control the overall voltage profile of such a line connecting a controllable reactive-power source, called a var compensator, to it so that below the surge-impedance loading P0, the var compensator absorbs reactive power, and above P0, it supplies reactive power 28 | P a g e the var requirement from the midpoint var controller is Qv = 2Qm The conclusions are as follows: 1. As the length of the line increases on account of the line-charging capacitances, the line experiences significant overvoltages at light-load conditions. 2. Overvoltages can be limited by using fixed- or switched-shunt reactors at the line ends as well as at intermediate buses where needed. 3. The application of midpoint or intermediate bus-voltage controllers (var compensators) enhances the power-transmission capacity of a long line. 4. In practical cases, var controllers are sized by carefully selecting their continuous-operating range to hold the connecting-bus voltage within an acceptable range of values in the normal line-loading range. PASSIVE COMPENSATION • Reactive-power control for a line is often called reactive-power compensation. • External devices or subsystems that control reactive power on transmission lines are known as compensators. 29 | P a g e • • • • • A compensator mitigates the undesirable effects of the circuit parameters of a given line. The objectives of line compensation are invariably 1. to increase the power-transmission capacity of the line, 2. to keep the voltage profile of the line along its length within acceptable bounds to ensure the quality of supply to the connected customers as well as to minimize the line-insulation costs. Because reactive-power compensation influences the power-transmission capacity of the connected line, controlled compensation can be used to improve the system stability Reactive-power compensators are dimensioned, and their types are selected on the basis of both their technical and cost effectiveness. Passive reactive-power compensators include series capacitors and shuntconnected inductors and capacitors. Shunt Compensation • Shunt devices may be connected permanently or through a switch. Shunt reactors compensate for the line capacitance, and because they control overvoltages at no loads and light loads, they are often connected permanently to the line, not to the bus. • Figure shows the arrangements of shunt reactors on a long-distance, highvoltage ac line. 30 | P a g e • Many power utilities connect shunt reactors via breakers, thereby acquiring the flexibility to turn them off under heavier load conditions. • Shunt reactors are generally gapped-core reactors and, sometimes, aircored. • Shunt capacitors are used to increase the power-transfer capacity and to compensate for the reactive-voltage drop in the line. • The application of shunt capacitors requires careful system design. • The circuit breakers connecting shunt capacitors should withstand highcharging in-rush currents and also, upon disconnection, should withstand more than 2-pu voltages, because the capacitors are then left charged for a significant period until they are discharged through a large timeconstant discharge circuit. • Also, the addition of shunt capacitors creates higher-frequency–resonant circuits and can therefore lead to harmonic overvoltages on some system buses. Series Compensation • Series capacitors are used to partially offset the effects of the series inductances of lines. • Series compensation results in the improvement of the maximum powertransmission capacity of the line. • The net effect is a lower load angle for a given power-transmission level and, therefore, a higher-stability margin. • The reactive-power absorption of a line depends on the transmission current, so when series capacitors are employed, automatically the resulting reactive-power compensation is adjusted proportionately. • Also, because the series compensation effectively reduces the overall line reactance, it is expected that the net line-voltage drop would become less susceptible to the loading conditions. • In an interconnected network of power lines that provides several parallel paths, for power flow between two locations, it is the series compensation of a selected line that makes it the principal power carrier. • Series compensation is defined by the degree of compensation; for example, a 1-pu compensation means that the effective series reactance of a line will be zero. A practical upper limit of series compensation, on the other hand, may be as high as 0.75 pu. • One impact of the passive compensation of lines is that whereas the shuntinductive compensation makes the line electrically resonant at a supersynchronous frequency, the series compensation makes the line resonant at a subsynchronous frequency. 31 | P a g e • The subsynchronous resonance (SSR) can lead to problematic situations for steam turbine–driven generators connected to a series-compensated transmission line. • These generators employ multiple turbines connected on a common shaft with the generator. This arrangement constitutes an elastically coupled multimass mechanical system that exhibits several modes of lowfrequency torsional resonances, none of which should be excited as a result of the subsynchronous-resonant electrical transmission system. • The application of series compensation requires several other careful considerations. • The application of series capacitors in a long line constitutes placing a lumped impedance at a point. Therefore, the following factors need careful evaluation: 1. The voltage magnitude across the capacitor banks (insulation); 2. The fault currents at the terminals of a capacitor bank; 3. The placement of shunt reactors in relation to the series capacitors (resonant overvoltages); and 4. The number of capacitor banks and their location on a long line (voltage profile). Effect on Power-Transfer Capacity • The consideration of series compensation invariably raises the issue of its comparison with shunt compensation. • A simple system analysis to develop a basic understanding of the effect of β shunt compensation on power-transmission capacity. β series compensation on power-transmission capacity. Series compensation on power-transmission capacity Consider a short, symmetrical electrical line as shown in Fig.below. For an uncompensated line, and assuming Vs = Vr = V, the power equation becomes 32 | P a g e Figure The series compensation of a short, symmetrical transmission line CASE1 :Series Compensation If the effective reactance of a line is controlled by inserting a series capacitor, and if the line terminal voltages are held unchanged, then a ΔXl change in the line reactance will result in a ΔIl change in the current, where 33 | P a g e Therefore, from above Eq. , the corresponding change in the power transfer will be Using above Eqs.. we can write Shunt Compensation Reconsider the short, symmetrical line described in Fig. (a). Apply a shunt capacitor at the midpoint of the line so that a shunt susceptance is incrementally added (DBc), as shown in Fig. b. For the system in this figure, the power transfer in terms of the midpoint voltage on the line is Fig The midpoint-capacitor compensation of a short, symmetrical line 34 | P a g e Assuming an operating load angle δ=30, we get the ratio of the ratings of series (ΔQse) to shunt (ΔQsh) compensators to be 0.072, or 7.2%. 35 | P a g e From the discussion, it is clear that the var net rating of the series compensator is only 7.2% of that required of a shunt compensator for the same change in power transfer. Therefore, concludes that the series-capacitive compensation is not only achieved with a smaller MVAR rating, but also that it is automatically adjusted for the entire range of the line loading. However, the cost of the compensator is not directly related only to the MVARrating. series capacitor costs increase because they carry full line current and also both their ends must be insulated for the line voltage. Practical application of series capacitors requires isolation and bypass arrangements as well as protection and monitoring arrangements. Basic principle of operation of StaticVAr Compensator (SVC)- Voltage control by SVC – advantages of slope in dynamic characteristics – influence of SVC on system voltage. Modeling of SVC. Applications – enhancement of transient stability – steady state power transfer – prevention of voltage instability. An SVG (static var generators) is a static electrical device, system that is capable of drawing a controlled capacitive or inductive current from an electrical power system, thereby generating or absorbing reactive power. An SVC is a shunt-connected static generator andο― or absorber of reactive power in which the output is varied to maintain or control specific parameters of an electrical power system. SVS is a combination of different static and mechanically switched var compensators (capacitors andο― or reactors) in which the outputs are coordinated. A var compensating system (VCS) is a combination of SVSs and rotating var compensators in which the outputs are coordinated. The general characteristics of SVCs are given in the list that follows. 1. The lowering of maintenance requirements from the absence of rotating parts. 2. The very fast control-response time. 3. The feasibility of individual phase control. 4. The diminished losses. 5. The high reliability. 6. The lack of contribution to system short-circuit capacity. 36 | P a g e 7. The generation of harmonics by SVCs except thyristor-switched capacitors (TSCs). 8. The variation of SVC reactive-power generation as the square of terminal voltage when it is operating outside the linear controllable range, leading to a substantial reduction in reactive-power support at lower voltages. THYRISTOR-CONTROLLED REACTOR (TCR) A TCR is one of the most important building blocks of thyristor-based SVCs. Although it can be used alone, it is more often employed in conjunction with fixed or thyristor-switched capacitors to provide rapid, continuous control of reactive power over the entire selected lagging-to-leading range. • Single-Phase TCR A single-phase TCR comprises an anti-parallel–connected pair of thyristor valves, T1 and T2, in series with a linear air-core reactor, as illustrated in Fig. • • • • • The anti-parallel–connected thyristor pair acts like a bidirectional switch, with thyristor valve T1 conducting in positive half-cycles and thyristor valve T2 conducting in negative half-cycles of the supply voltage. The firing angle of the thyristors is measured from the zero crossing of the voltage appearing across its terminals. The controllable range of the TCR firing angle, a, extends from 90 0 to 180 0 . A firing angle of 90 0 results in full thyristor conduction with a continuous sinusoidal current flow in the TCR. As the firing angle is varied from 90 0 to close to 180 0, the current flows in the form of discontinuous pulses symmetrically located in the positive and negative half-cycles, 37 | P a g e • • Once the thyristor valves are fired, the cessation of current occurs at its natural zero crossing, a process known as the line commutation. The current reduces to zero for a firing angle of 180 0 . Thyristor firing at angles below 90 0 introduces dc components in the current, disturbing the symmetrical operation of the two antiparallel valve branches. (1) 38 | P a g e 39 | P a g e • • The TCR thus acts like a variable • However, as the firing angle is increased beyond 90 0 , the current becomes nonsinusoidal, and harmonics are generated. susceptance. (admittance,) Variation of the firing angle changes the susceptance and, consequently, the fundamental-current component, which leads to a variation of reactive power absorbed by the reactor because the applied ac voltage is constant. • If the two thyristors are fired symmetrically in the positive and negative half-cycles, then only odd-order harmonics are produced. Three phase TCR The Thyristor-Switched Reactor - (TSR) • TSR is a special case of a TCR in which the variable firing-angle control option is not exercised. • Instead, the device is operated in two states only: either fully on or fully off. • If the thyristor valves are fired exactly at the voltage peaks corresponding to a c 90 0 for the forward-thyristor valve T1 and 2700 (90 0 + 180 0 ) for the reverse-thyristor valve T2, full conduction 40 | P a g e • The maximum inductive current flows in the TCR as if the thyristor switches were replaced by short circuits. • However, if no firing pulses are issued to the thyristors, the TSR will remain in a blocked-off state, and no current can flow. • TSR ensures a very rapid availability of rated inductive reactive power to the system. • When a large magnitude of controlled reactive power, Q, is required, a part of Q is usually assigned to a small TSR of rating, say, Qο― 2; the rest is realized by means of a TCR also of a reduced rating Q 2. • This arrangement results in substantially decreased losses and harmonic content as compared to a single TCR of rating Q. THE FIXED-CAPACITOR–THYRISTOR-CONTROLLED REACTOR (FC–TCR) -- SVC Configuration • TCR provides continuously controllable reactive power only in the lagging power-factor range. • To extend the dynamic controllable range to the leading power-factor domain,(SUPPLYING REACTIVE POWER) a fixed-capacitor bank is connected in shunt with the TCR. 41 | P a g e • TCR MVA is rated larger than the fixed capacitor to compensate (cancel) the capacitive MVA and provide net inductive-reactive power should a lagging power-factor operation be desired. • The fixed-capacitor banks, usually connected in a star configuration, are split into more than one 3-phase group. • Each capacitor contains a small tuning inductor that is connected in series and tunes the branch to act as a filter for a specific harmonic order. • For instance, one capacitor group is tuned to the 5th harmonic and another to the 7th, • At fundamental frequency, the tuning reactors slightly reduce the net MVA rating of the fixed capacitors. 42 | P a g e Operating Characteristic 1 Without the Step-Down Transformer (Directly connected to supply line) • The operating V-I characteristic of an FC–TCR compensator is illustrated in Fig. below. • The fixed capacitor extends the operating-control range of the SVC to the leading side. • The SVC current, ISVC, can be expressed as a function of system voltage, V, and compensator susceptance, BSVC, as follows, 43 | P a g e • Figures (b) and (c) show the operating characteristic and the susceptance of this type of compensator, respectively, and both also show that VAR(Reactive power ) production as well as absorption is possible. • By dimensioning the ratings of the TCR and the capacitor, respectively, the production and absorption ranges can be selected according to the system requirements. 2 With the Step-Down Transformer – ( High power transmission line) An FC–TCR SVC is usually connected to the high-voltage power system( 110kv , 220kv ,400kv ) by means of a step-down coupling transformer, as shown in Fig. 44 | P a g e From Eq. (a), • BL is a negative quantity. • An analysis of Eq. (a) shows that the total susceptance BSVC of the static var compensator does not change linearly with BTCR. • However, if (BC / Bo ) << 1 and (BL/ Bo ) << 1, which is usually the case, the nonlinearity is relatively small. • This assumption implies that the reactance of the coupling transformer is greatly smaller than the reactance of either the fixed capacitor or TCR • Equation (a) can then be approximated by a linear relation, as follows The Transformer-Secondary Voltage The voltage at the secondary of the transformer is 45 | P a g e MECHANICALLY SWITCHED CAPACITOR–THYRISTOR CONTROLLED REACTOR (MSC–TCR) • In certain applications, especially those involving few capacitor switchings, an MSC–TCR has been shown to offer acceptable performance at much lower compensating system costs than a TSC–TCR. • The different MSC–TCR circuit configurations are shown in Fig. • Advantages of the MSC–TCR scheme1. lower capital cost from the elimination of the thyristor switches in the capacitor branches; 2. reduced operating costs in terms of losses. • Disadvantage - slower speed of response. • Mechanical switches can close in two cycles and open in about eight, compared to one-half to one cycle with thyristor switches. • To compensate for the slower speed and to achieve a level of transient stability similar to a thyristor switched capacitor–thyristor-controlled reactor, 25% higher–rated MSC–TCR SVC may be needed. 46 | P a g e • Mechanical switches also possess a finite life, typically 2000–5000 operations, compared to the infinite switching life of thyristors. • Problem with the MSC–TCR relates to the trapped charge that is invariably left on the capacitor after de-energization. • The residual charge on the capacitors is usually dissipated in about five minutes through discharge resistors (built into the capacitor units). • If the capacitor is switched on within five minutes after de-energization, the trapped charge may lead to increased switching transients. • MSCs can be switched in only when the capacitors are discharged. • TCR in an MSC–TCR is designed to have a lower inductance .( compared to a TCR in a TSC–TCR SVC of similar rating). • MSCs are usually switched two to four times a day; they are connected during heavy-load conditions and removed under light-load conditions. • An MSC–TCR may not be very suitable for voltage-control applications in a system experiencing frequent disturbances. 47 | P a g e THE THYRISTOR-SWITCHED CAPACITOR (TSC) The circuit shown in Fig. consists of a capacitor in series with a bidirectional thyristor switch, is supplied from an ideal ac voltage source (without resistance nor reactance present in the circuit). Figure Switching of a capacitor at a voltage source: (a) a circuit diagram and (b) the current and voltage waveforms Analysis of the current transients after closing the switch brings out two cases: 1. The capacitor voltage is not equal to the supply voltage when the thyristors are fired. Immediately after closing the switch, a current of infinite magnitude flows and charges the capacitor to the supply voltage in an infinitely short time. The switch realized by thyristors cannot withstand this stress and would fail. 2. The capacitor voltage is equal to the supply voltage when the thyristors are fired, as illustrated in Fig. (b). The analysis shows that the current will jump immediately to the value of the steady-state current. The steady state condition is reached in an infinitely short time. Although the magnitude of the current 48 | P a g e does not exceed the steady-state values, thyristors have an upper limit of di/ dt values that they can withstand during the firing process. Here, di/ dt is infinite, and the thyristor switch will again fail. It can therefore be concluded that this simple circuit of a TSC branch is not suitable. Switching a Series Connection of a Capacitor and Reactor To overcome the problems discussed above , a small damping reactor is added in series with the capacitor, as depicted in Fig.. Let the source voltage be v(t) = V sin ω0t where ω0 is the system nominal frequency. (a) The switching strategies to limit the transients to acceptable limits ,are based on very processes to decide when the thyristors should be fired. The following two simple firing schemes are the basis for the switching strategies: .1. If VC0 < V at the time of demand for the capacitor, it is switched on, as soon as the voltage across the valve reaches zero and the capacitor voltage is equal to the supply voltage 2.If the capacitor is overcharged (VC0 > V) at the time of demand, it is switched on, when the supply voltage reaches the crest and the voltage across the valve is minimal. This scheme is also called a forced switch-on. 49 | P a g e The TSC Configuration A basic single-phase TSC consists of an anti-parallel–connected thyristor-valve pair that acts as a bidirectional switch in series with a capacitor and a current limiting small reactor, as shown in Fig • The thyristor valves are turned on at an instant when minimum voltage is sensed across the valves to minimize the switching transients. • Besides the initial transients, the TSC current is sinusoidal and free from harmonics, thus no need for any filters. • The small-series inductor is installed to limit current transients during overvoltage conditions and planned switching operations, as well as when switching at incorrect instants or at the inappropriate voltage polarity. • The inductor magnitude is chosen to give a natural resonant frequency of four to five times the system nominal frequency, which ensures that the inductance neither creates a harmonic-resonant circuit with the network. • A 3-phase TSC unit comprises three single-phase TSCs connected in a delta, which are usually supplied by the delta secondary winding of a stepdown transformer, as depicted in Fig. (b). An alternative 3-phase, 4-wire star-connected TSC configuration is shown in Fig. (c). 50 | P a g e Figure Different TSC configurations: (a) a single-phase TSC branch; (b) a 3phase delta-connected TSC; and (c) a 3-phase Y-connected transformer secondary with neutrals connected. Operating Characteristic The TSC has a discrete voltage–current operating characteristic as shown in Fig.. The shape of this characteristic is a function of the number of TSC units, their individual ratings, and a hysteresis voltage ΔV, which is built in to avoid undesirable frequent switchings of capacitors. In a closed-loop voltage control operation, the TSC regulates the bus voltage within the range Vref ± ΔV/ 2. 51 | P a g e 52 | P a g e THYRISTOR-SWITCHED CAPACITOR– THYRISTOR CONTROLLED REACTOR (TSC–TCR) 1 Configuration • TSC–TCR compensator usually comprises n TSC banks and TCR that are connected in parallel. • The number of branches n, is determined by practical considerations that include the operating voltage level, maximum var output, current rating of the thyristor valves, bus work and installation cost, etc • TSC bank // TCR • The rating of the TCR is chosen to be 1/ n of the total SVC rating. • The TSC is switched in using two thyristor switches (connected back to back) at the instant in a cycle when the voltage across valve is minimum and positive. This results in minimum switching transients. • In steady state, TSC does not generate any harmonics. • To switch off a TSC, the gate pulses are blocked and the thyristors turns off when the current through them fall below the holding currents • More flexible than FC-TCR - The feature of disconnecting the capacitor in is not available with FC–TCRs. 53 | P a g e V-I characteristic of the operation of a TSC-TCR • Fig. shows the V-I characteristic of the operation of a TSC-TCR. Fig – V-I characteristics of TSC-TCR 54 | P a g e • The capacitors of the TSC can be switched in discrete steps, whereas continuous control within the reactive power span of each step is controlled by the TCR. • The total capacitive output range is divided into n intervals. In the first interval, the output of the var generator is controllable in the zero to QCmax/n range, where QCmax is the total rating provided by all TSC branches • In this interval, one capacitor bank is switched in (by firing, , thyristor of TSC1, ) and, simultaneously, the current in the TCR is set by the appropriate firing delay angle so that the sum of the var output of the TSC (negative) and that of the TCR (positive) equals the capacitive output required. • In the second, third, ..., and nth intervals, the output is controllable in the QCmax/n to 2QCmax/n, 2Qcmax/n to 3QCmax/n, ..., and (n 1)Qcmax/n to QCmax range by switching in the second, third, ..., and nth capacitor bank and using the TCR to absorb the surplus capacitive vars. • TSC-TCR can quickly operate to disconnect all the capacitors from the compensator, preventing the resonant oscillations. • The main function of the TCR is for generating inductive VAR to increase the flexibility of VAR compensation. • VAR rating of the TCR has to be larger than that of one TSC to provide enough overlap for TSC switching in and out action. • The VAR rating of the TCR in this type of SVC is lower than that of the TCR in FC-TCR • The maximum inductive range of the SVC corresponds to the rating of the relatively small TCR. • Four Types of Power Losses – 1. Reactor resistive loss , 2. Capacitor losses 3. Thyristor losses 4. Controller and filter losses - (In electronic control part of TCR and TSC bank) • The advantages of TSC-TCR type SVC over FC-TCR type are (i) the reduction in the reactor size and consequently the harmonics generated (ii) greater flexibility in control 55 | P a g e (iii) better performance under system fault conditions. Calculation of the Operating-Range Limits • BC3 as the susceptance of all three TSC branches in parallel and • BC3 is considered zero for the TSC scheme at the absorption limit, for all capacitors are switched off. • Susceptance at production limit is • SVC susceptance in the TSC–TCR scheme as follows: Current Characteristic TSCs and the TCR share the current and contribute to the total SVC current.. Generally, the TSC and TCR currents ISVC = IC + IL where IC = V2nBCn, IL = V2nBTCR n - number of TSC branches turned on , 56 | P a g e where nc = 1, 2, . . . is the number of TSC circuits in operation and BCn is the total susceptance of n TSC branches. Substituting BTCR = 0 and BTCR = BL in the above equations, respectively, results in the currents at the absorption limit and at the production limit for different numbers of TSCs. 57 | P a g e Figure-a gives the total susceptance BSVC as a function of the susceptance of the controlled reactor BTCR for the example data. These characteristics are of importance for control design, for the controls vary BTCR and the effect on the system is caused by BSVC. 58 | P a g e 59 | P a g e VOLTAGE CONTROL BY SVC β’ The voltage-control action can be explained through a simplified block representation of the SVC and power system shown below. The power system is modeled as an equivalent voltage source Vs behind equivalent system impedance Xs as viewed from the SVC terminals. Figure 5.2 (a) A simplified block diagram of the power system and SVC control system; (b) a phasor diagram of the ac system for the inductive SVC current β’ system impedance Xs corresponds to the short circuit MVA at the SVC bus ,is obtained as Xs = (Vb / Sc). MVAb in p.u. Where, Sc = the 3 phase short circuit MVA at the SVC bus Vb = the base line-line voltage 60 | P a g e MVAb = base MVA SVC bus voltage is given by , Vs = VSVC + ISVC Xs (1) β’ The SVC current results in a voltage drop of ISVCXs in phase with the system voltage Vs. β’ The SVC bus voltage decreases with the inductive SVC current and increases with the capacitive current. β’ Equation (1) represents the power-system characteristic or the system load line β’ The intersection of the SVC dynamic characteristic and the system load line provides the quiescent operating of the SVC as illustrated in the below figure. Characteristics of the simplified power system and the SVC β’ The voltage control action in the linear range is described as 61 | P a g e Where ISVC is positive if inductive and ISVC is negative if capacitive. β’ It is emphasized that the V-I characteristics described here relate SVC current or reactive power to the voltage on the high-voltage side of the coupling transformer. 62 | P a g e SVC Applications ο • Static var compensators (SVCs) constitute a mature technology that is finding widespread usage in modern power systems for load compensation as well as transmission-line applications. • In high-power networks, SVCs are used for voltage control and for attaining several other objectives such as damping and stability control. 1. Enhancement of Transient Stability - Power-angle curves β’ An enhancement in transient stability is achieved primarily through voltage control exercised by the SVC at the interconnected bus. β’ enhancement in transient stability can be obtained from the power-angle curves,of uncompensated and midpoint SVC–compensated SMIB system as shown in Fig. β’ Assume that both systems are transmitting the same level of power and are subject to an identical fault at the generator terminals for an equal length of time. β’ The power-angle curves for both systems are depicted in Fig. 63 | P a g e β’ Iinitial operating point in the uncompensated and compensated systems are indicated by rotor angles d1 and dc1. These points correspond to the intersection between the respective power-angle curves with the mechanical input line PM, which is same for both the cases. β’ In the event of a 3-phase-to-ground fault at the generator terminals, even though the short-circuit current increases enormously, active-power output from the generator reduces to zero. β’ Because the mechanical input remains unchanged, generator accelerates until fault clearing, by which time the rotor angle has reached values d2 and dC2 and the accelerating energy, A1 andAC1, has been accumulated in the uncompensated and compensated system, respectively. β’ When the fault is isolated, the electrical power exceeds the mechanical input power, and the generator starts decelerating. β’ The rotor angle, however, continues to increase until δ3 and δc3 from the stored kinetic energy in the rotor. 64 | P a g e β’ The decline in the rotor angle commences only when the decelerating energies represented by A2 and AC2 in the two cases, respectively, become equal to the accelerating energies A1 and AC1. β’ The power system in each case returns to stable operation if the post-fault angular swing, denoted by d3 and dC3, does not exceed the maximum limit of dmax and dc max, respectively. β’ The far the angular overswing from its maximum limit, the more transient stability in the system. β’ An index of the transient stability is the available decelerating energy, termed the transient-stability margin, and is denoted by areas Amargin and Ac margin in the two cases, respectively. β’ Clearly, as Ac margin significantly exceeds Amargin, the system-transient stability is greatly enhanced by the installation of an SVC. The increase in transient stability is thus obtained by the enhancement of the steady-state power-transfer limit provided by the voltage-control operation of the midline SVC. 2. Steady State Power Transfer Capacity β’ An SVC can be used to enhance the power-transfer capacity of a transmission line, which is also characterized as the steady-state power limit. β’ Consider a single-machine infinite-bus (SMIB) system with an interconnecting lossless tie line having reactance X shown in Fig. The SMIB system: (a) an uncompensated system (b) an SVC-compensated system 65 | P a g e β’ Let the voltages of the synchronous generator and infinite bus be V1∠ δ and V2 ∠ 0, respectively. The power transferred from the synchronous machine to the infinite bus is expressed as β’ Power varies as a sinusoidal function of the angular difference of the voltages at the synchronous machine and infinite bus, β’ The maximum steady-state power that can be transferred across the uncompensated line without SVC corresponds to δ = 90 0; it is given by Fig - variation of line real-power flow and SVC reactive-power flow in a SMIB system. (double Pmax , reactive-power rating of the SVC is four times the maximum power transfer in an uncompensated case 66 | P a g e β’ The variation of linear real-power flow and SVC reactive-power flow in a SMIB system β’ Let the transmission line be compensated at its midpoint by an ideal SVC. β’ The term ideal corresponds to an SVC with an unlimited reactive-power rating that can maintain the magnitude of the midpoint voltage constant for all real power flows across the transmission line. β’ SVC bus voltage is then given by Vm ∠–δ/2.The electrical power flow across the half-line section connecting the generator and the SVC is expressed as ,(Assuming Vm= V1 =V2 =V) β’ The power transfer in the other half-line section interconnecting the SVC, and the infinite bus is also described by a similar equation Hence , maximum transmittable power across the line (occurs at δ/ 2 =900 ) is given by which is twice the maximum power transmitted in the uncompensated case and. β’ Midpoint-located ideal SVC doubles the steady-state power limit and increases the stable angular difference between the synchronous machine and the infinite bus from 900 to 1800 . β’ If the transmission line is divided into n equal sections, with an ideal SVC at each junction of these sections, maintaining a constant-voltage magnitude (V), then the power transfer (P′c) of this line can be expressed theoretically by 67 | P a g e β’ Maximum power, P′c max = nV2/ X. Hence , with n sections the power transfer can be increased n times that of the uncompensated line. β’ It can be shown that the reactive-power requirement, QSVC, of the midpoint SVC for the voltage stabilization is given by 3. Enhancement of Power System Damping β’ The power-transfer capacity along a transmission line is limited by several factors; such as , the thermal limit, the steady-state stability limit, the transientstability limit, and system damping. β’ In certain situations, a power system may have —even negative— damping; therefore, a strong need arises to enhance the electrical damping of power systems to ensure stable, oscillation-free power transfer. β’ A typical scenario of the magnitude of various limits, especially where damping plays a determining role , is depicted graphically in Fig. β’ The behavior of generator oscillations is determined by the two torque components: the synchronizing torque and damping torque. β’ The synchronizing torque ensures that the rotor angles of different generators do not drift away following a large disturbance. 68 | P a g e β’ Magnitude of the synchronizing torque determines the frequency of oscillation. Meanwhile, damping torque influences the decay time of oscillations. β’ Even if a power system is stable, the oscillations may be sustained for a long period without adequate damping torque. β’ SVCs are employed primarily for voltage control; they do not contribute to system damping. However by incorporating auxiliary control, an SVC can significantly improve the electrical damping of power systems 69 | P a g e 4. Prevention of Voltage Stability β’ Voltage instability is caused by the inadequacy of the power system to supply the reactive-power demand of certain loads, such as induction motors. β’ Drop in the load voltage leads to an increased demand for reactive power, if not met by the power system, leads to a further decline in the bus voltage. This decline eventually leads to a rapid decline of voltage which may have a cascading effect on neighboring regions that causes a system voltage collapse. β’ Consider an SVC connected to a load bus, (Fig. a). The load has varying power factor and is fed by a lossless radial transmission line. β’ The voltage profile at the load bus, which is situated at the receiver end of the transmission line, is shown in Fig. (b). β’ For a given load-power factor, as the transmitted power is gradually increased, a maximum power limit is reached beyond which the voltage collapse takes place. β’In this system, if the combined power factor of the load and SVC is appropriately controlled through the reactive-power support from the SVC, a 70 | P a g e constant voltage of the receiving-end bus can be maintained with increasing magnitude of transmitted power, and voltage instability can be avoided. Modelling of SVC For a detailed study of SVC control interactions, it is necessary to perform transient simulation for which SVC is modelled in detail including the switching of the thyristor valves in TCR and TSC. The transient network is modelled by differential equations rather than algebraic equations . However for stability study it is not necessary to consider the switching of valves and assume that SVC generates only fundamental current. In addition, the network transients are neglected and represented by algebraic equations of the type: [Y ]V = I ( 1) Module-3: Series Compensators: Compensation by a Series Capacitor Connected at the Midpoint of the Line, Basic model concept of Thyristor Controlled Series Capacitor (TCSC), Operation of the TCSC – different modes of 71 | P a g e operation – modeling of TCSC – variable reactance model – modeling for stability studies. Applications – improvement of the system stability limit – enhancement of system damping – voltage collapse prevention Compensation by a Series Capacitor Connected at the Midpoint of the Line SERIES COMPENSATION – 1 Fixed-Series Compensation • Series capacitors offer certain major advantages over their shunt counterparts. • With series capacitors, the reactive power increases as the square of line current, • whereas with shunt capacitors, the reactive power is generated proportional to the square of bus voltage. • For achieving the same system benefits as those of series capacitors, shunt capacitors that are three to six times more reactivepower–rated than series capacitors need to be employed . • Furthermore, shunt capacitors typically must be connected at the line midpoint, whereas no such requirement exists for series capacitors. • Let Qse and Qsh be the ratings of a series and shunt capacitor, respectively, • To achieve the same level of power transfer through a line that has a maximum angular difference of δmax across its two ends. • Then • Specifically, for δmax of 350, Qse will be approximately 10% of Qsh. • Even though series capacitors are almost twice as costly as shunt capacitors (per-unit var) because of their higher operating voltages, the overall cost of series compensation is lower than shunt compensation 2. The Need for Variable-Series Compensation Compensation of transmission lines by series capacitors is likely to result in the following : 72 | P a g e 1. enhanced power flow and loadability of the series-compensated line 2. additional losses in the compensated line from the enhanced power flow 3. increased sensitivity of power flow in the series-compensated line from the outage of other lines in the system. (Also, the increased sensitivity of the compensated line to other network outages may cause a line loading that exceeds the enhanced loadability level of the line itself. ) These undesirable effects can be avoided by employing variable levels of series compensation instead of fixed compensation. Series compensation can be varied, depending on the enhancement of power transfer desired at that time, without affecting other system-performance criteria TCSC CONTROLLER • Basic conceptual TCSC module comprises a series capacitor, C, in parallel with a thyristor-controlled reactor, LS, [Fig.(a)]. • However, a practical TCSC module also includes protective equipment normally installed with series capacitors,[Fig. (b)]. • Metal-oxide varistor (MOV), - nonlinear resistor, is connected across the series capacitor to prevent the occurrence of high-capacitor over- voltages. MOV allows the capacitor to remain in circuit even during fault conditions and helps improve the transient stability 73 | P a g e • Also installed across the capacitor is a circuit breaker, CB, for controlling its insertion in the line. Also CB bypasses the capacitor if severe fault or equipment-malfunction events occur. • A current-limiting inductor, Ld, is incorporated in the circuit to restrict both the magnitude and the frequency of the capacitor current during the capacitor-bypass operation. • If the TCSC valves are required to operate in the fully “on” mode for prolonged durations, the conduction losses are minimized by installing an ultra–high-speed contact (UHSC) across the valve. • UHSC metallic contact offers a lossless feature similar to circuit breakers. The metallic contact is closed shortly after the thyristor valve is turned on, and it is opened shortly before the valve is turned off. • During a sudden overload of the valve, and also during fault conditions, the metallic contact is closed to relieve the stress on the valve. 74 | P a g e • An actual TCSC system usually comprises a cascaded combination of many such TCSC modules, together with a fixed-series capacitor, CF. This fixed series capacitor is provided primarily to minimize costs. Fig C – Typical TCSC system • A conceptual TCSC system with basic TCSC modules [see Fig. C]. The capacitors—C1, C2, . . . , Cn—in the different TCSC modules may have different values to provide a wider range of reactance control. • The inductor in series with the antiparallel thyristors is split into two halves to protect the thyristor valves in case of inductor short circuits. OPERATION OF THE TCSC Basic Principle • A TCSC is a series-controlled capacitive reactance that can provide continuous control of power on the ac line over a wide range. • From the system viewpoint, the principle of series compensation is to increase the fundamental-frequency voltage across an fixed capacitor (FC) in a series compensated line through appropriate variation of the firing angle, α . • This enhanced voltage changes the effective value of the series-capacitive reactance. 75 | P a g e • A simple understanding of TCSC functioning can be obtained by analyzing the behavior of a variable inductor connected in parallel with an FC, [see Fig. d.] Figure d- A variable inductor connected in shunt with an FC. • The equivalent impedance, Zeq, of this LC combination is expressed as • The impedance of the FC is given by -j(1/ ωC). • If ωC − (1/ ωL) > 0 or, in other words, ωL > (1/ ωC), the reactance of the (XL>Xc ) FC is less than that of the parallel-connected variable reactor . • Inductor increases the equivalent reactance of the LC combination above that of the FC. • If ωC− (1/ ωL)=0, a resonance develops that results in an infinitecapacitive impedance—an unacceptable condition. • If, ωC − (1/ ωL) < 0, the XL< Xc • In the variable-capacitance mode of the TCSC, as the inductive reactance of the variable inductor is increased, the equivalent-capacitive reactance is gradually decreased. • The minimum equivalent-capacitive reactance is obtained for extremely large inductive reactance or when the variable inductor is open-circuited, in which the value is equal to the reactance of the FC itself. • The behavior of the TCSC is similar to that of the parallel LC combination. • The difference is that the LC-combination analysis is based on the presence of pure sinusoidal voltage and current in the circuit, whereas in the TCSC, because of the voltage and current in the FC and thyristor-controlled reactor (TCR) are not sinusoidal because of thyristor switchings. 76 | P a g e 3 Advantages of the TCSC (Thyristor Controlled Series Capacitor) Use of thyristor control in series capacitors potentially offers the following advantages: 1. Dynamic control of power flow in selected transmission lines within network to enable optimal power-flow conditions and prevent the loop flow of power. 2. Damping of the power swings from local and inter-area oscillations. 3. Suppression of subsynchronous oscillations. At subsynchronous frequencies, TCSC presents an inherently resistive–inductive reactance. subsynchronous oscillations cannot be sustained in this situation and consequently get damped. 4. Decreasing dc-offset voltages. The dc-offset voltages, invariably resulting from the insertion of series capacitors, can be made to decay very quickly (within a few cycles). 5. Enhanced level of protection for series capacitors. A fast bypass of series capacitors can be achieved through thyristor control when large over-voltages develop across capacitors following faults. Likewise, capacitors can be quickly reinserted by thyristor action after fault clearing to aid in system stabilization. 6. Voltage support. TCSC can generate reactive power that increases with line loading, thereby aiding regulation of local network voltages and improve of any voltage instability. 7. Reduction of the short-circuit current. During events of high short-circuit current, TCSC can switch from the controllable-capacitance to controllable-inductance mode, thereby restricting the short-circuit currents. 77 | P a g e Modes of TCSC Operation • There are essentially three modes of TCSC operation (a) the bypassed-thyristor mode; (b) the blocked-thyristor mode; (c) the partially conducting thyristor (capacitive and inductive) mode; • Modes of TCSC operation are illustrated in Fig. and described in the following section . 78 | P a g e Figure 1 Different operating modes of a TCSC: (a) the bypassed-thyristor mode; (b) the blocked-thyristor mode; (c) the partially conducting thyristor (capacitive) mode; and (d) the partially conducting thyristor (inductive) mode 1 Bypassed-Thyristor Mode (α=900) • In this bypassed mode, both thyristors conduct 180 degree consecutive when they have right condition of conducting. • Gate pulses are applied as soon as the voltage across the thyristors reaches zero and becomes positive, resulting in a continuous sinusoidal of flow current through the thyristor valves. • The TCSC module behaves like a parallel capacitor–inductor combination. • The TCR branch is in circuit completely in this mode. The TCSC impedance is: • • • • In above Equation , negative X TCSC means the overall impedance is capacitive and positive X TCSC means the overall inductive impedance. As XL is smaller than XC, the TCSC impedance is inductive in bypass mode. Hence , the net current through the module is inductive. Also known as the thyristor-switched-reactor (TSR) mode, the bypassed thyristor mode is distinct from the bypassed-breaker mode, in which the circuit breaker provided across the series capacitor is closed to remove the capacitor or the TCSC module in the event of TCSC faults or transient overvoltages across the TCSC. This mode is employed for control purposes and also for initiating certain protective functions. Whenever a TCSC module is bypassed from the violation of the current limit, a finite-time delay, Tdelay, must elapse before the module can be reinserted after the line current falls below the specified limit. 79 | P a g e 2 Blocked-Thyristor Mode (α=1800 ) • This mode, also known as the waiting mode, the firing pulses to the thyristor valves are blocked. • If the thyristors are conducting and a blocking command is given, the thyristors turn off as soon as the current through them reaches a zero crossing. • The TCSC module is thus reduced to a fixed-series capacitor, and the net TCSC reactance is capacitive. • In this mode, the dc-offset voltages of the capacitors are monitored and quickly discharged using a dc-offset control without causing any harm to the transmission-system transformers. 3 Partially Conducting -Thyristor, or Vernier, Mode • This mode allows the TCSC to behave either as a continuously controllable capacitive reactance or as a continuously controllable inductive reactance. • It is achieved by varying the thyristor-pair firing angle in an appropriate range. • However, a smooth transition from the capacitive to inductive mode is not permitted because of the resonant region between the two modes. • • A variant of this mode is the capacitive-vernier-control mode, in which the forward voltage thyristor valve is triggered slightly before capacitor voltage crosses zero (i.e thyristors are fired when the capacitor voltage and capacitor current have opposite polarity) to allow current to flow through inductive branch, adding to capacitive current. • If firing angle is between zero and 90 degree, the impedance of TCR branch will be Where α is delay angle of thyristors turning on from zero crossing of line current and σ in conducting angle σ= 2π-α ; • The firing angle of TCR is assigned as XL(α)>XC for TCSC operation in capacitive mode , This situation will happen in angles greater than α min_Cap and smaller than 90 degree. • There is a resonance angle that XL(αresonance) will be equal to XC • • This condition causes a TCR current that has a direction opposite that of the capacitor current, (This effectively increases the observed capacitance of the TCSC without requiring a larger capacitor within the TCS ), thereby resulting in a loop-current flow in the TCSC controller. 80 | P a g e Fig b • According to Fig.b , in delay angle α , after zero crossing of the line current, the proper thyristor (Th+) turns on in t1 when the line current and capacitor voltage are in opposite sign. Afterwards a resonance circuit forms from L and C that resonates half cycle. The ON thyristor turns off in t2. capacitor voltage inversing from -VC0 to +VC0 is result of half cycle resonance , The capacitor voltage without switching is shown also in dash line in Fig.b.. The capacitor voltage increasing due to resonance circuit creates larger capacitive impedance for TCSC. • In this operating condition, the current through the TCR is opposite to the current through the capacitor. This results in current loop inside the TCSC module [see below fig] • The loop current increases the voltage across the capacitor, effectively enhancing the equivalent-capacitive reactance and the series-compensation level for the same value of line current 81 | P a g e . Fig Parallel resonant circuit will resonate with 1/ (2π√ LC ) frequency. Parallel resonance corresponds to high impedance ,voltage and internal currents. LC resonance will work for half cycle because of conduction thyristors in one direction • The loop current increases as α is decreased from 1800 to αmin. The maximum TCSC reactance permissible with α = αmin is typically two-anda-half to three times the capacitor reactance at fundamental frequency. Inductive-vernier mode - Another variant is the inductive-vernier mode, • For TCSC operation in inductive mode, the firing angle of TCR branch must be so that XL(α)<XC. This case happens when α be greater than zero and smaller than α max_ Ind • In this mode, the direction of the circulating current is reversed and the controller presents a net inductive impedance. 82 | P a g e Fig c- Switching in inductive mode and half cycle resonance of LC circuit • after delay angle α from zero crossing of line current, when the polarity of the capacitor voltage and line current are same, the switching is done in t1. Again a resonance circuit consists of L and C. This circuit resonates half cycle and the ON thyristor will turn off • Angle switching thyristors can change inductive reactance controlled choke from a minimum value (α = 0, XTCR = XL) theoretically to infinity (α = π/2, XTCR = κ_). • For sufficiently small inductive reactance of reactor towards capcitive reactance of capacitor (XL < XC), the operating diagram of TCSC contains inductive and capacitive mode operation of TCSC and the transition between areas is the resonance region • TCSC will be in capacitive mode for the firing angle greater than αresonance and will be in inductive mode for the firing angle smaller than this angle. • Because of resonance in αresonance, impedance changing of TCSC is high around this angle and it is very sensitive to firing angle deviations. Therefore an inhibited area always defines between capacitive and inductive region as shown in Fig.a that it is defined by α min_Cap and α max_ Ind boundary 83 | P a g e 84 | P a g e MODELING OF TCSC A TCSC involves continuous-time dynamics, relating to voltages and currents in the capacitor and reactor, and nonlinear, discrete switching behavior of thyristors. Deriving an appropriate model for such a controller is an intricate task. 1. Variable-Reactance Model β’ A TCSC model for transient- and oscillatory-stability studies, used widely for its simplicity, is the variable-reactance model depicted in Fig. β’ In this quasi-static approximation model, the TCSC dynamics during power- swing frequencies are modeled by a variable reactance at fundamental frequency. β’ The other dynamics of the TCSC model—the variation of the TCSC response with different firing angles. β’ It is assumed that the transmission system operates in a sinusoidal steady state, with the only dynamics associated with generators and PSS. β’ This assumption is valid, because the line dynamics are much faster than the generator dynamics in the frequency range of 0.1–2 Hz that are associated with angular stability studies. β’ The reactance-capability curve of a single-module TCSC,. exhibits a discontinuity between the inductive and capacitive regions. 85 | P a g e β’ However, this gap is lessened by using a multimode TCSC. The variablereactance TCSC model assumes the availability of a continuous-reactance range. β’ This model is generally used for inter-area mode analysis, and it provides high accuracy when the reactance-boost factor (=XTCSC/ XC) is less than 1.5. 86 | P a g e 2. Transient – Stability Model β’ In the variable-reactance model for stability studies, a reference value of TCSC reactance, Xref, is generated from a power-scheduling controller based on the power-flow specification in the transmission line. β’ The reference Xref value may also be set directly by manual control in response to an order from an energy-control center, and it essentially represents the initial operating point of the TCSC; it does not include the reactance of FCs (if any). β’ The reference value is modified by an additional input, Xmod, from a modulation controller for such purposes as damping enhancement. β’ Another input signal, this applied at the summing junction, is the open-loop auxiliary signal, Xaux, which can be obtained from an external power-flow controller. β’ A desired magnitude of TCSC reactance, Xdes, is obtained that is implemented after a finite delay caused by the firing controls and the natural response of the TCSC. This delay is modeled by a lag circuit having a time constant,TTCSC, of typically 15–20 ms . β’ The output of the lag block is subject to variable limits based on the TCSC reactance-capability curve shown in Fig. β’ The resulting XTCSC is added to the Xfixed, which is the reactance of the TCSC installation’s FC component. β’ To obtain per-unit values, the TCSC reactance is divided by the TCSC base reactance, Zbase, given as 87 | P a g e where , kVTCSC = the rms line–line voltage of the TCSC in kilovolts (kV) MVAsys = the 3-phase MVA base of the power system β’ The TCSC model assigns a positive value to the capacitive reactance, so Xtotal is multiplied by a negative sign to ensure consistency with the convention used in load-flow and stability studies. β’ The TCSC initial operating point, Xref, for the stability studies is chosen as β’ The reactance capability curve of the multimodal TCSC shown in Fig. can be approximated by the capability curve shown in Fig.b 88 | P a g e Figure b A simplified reactance-capability curve of a multimodule TCSC β’ This figure can be conveniently used for the variable-reactance model of TCSC, and the capability curve that the figure depicts includes the effect of TCSC transient overload levels. β’ It should be noted that the reactance limit for high currents is depicted in Figb. as a group of discrete points for the different modules. β’ During periods of over current, only some TCSC modules move into the bypassed mode, for the bypassing of a module causes the line current to decrease and thus reduces the need for the remaining TCSC modules to go into the bypass mode. β’ However, for the case of modeling, only one continuous-reactance limit— denoted by a vertical line in Figb is considered for all TCSC modules. 89 | P a g e β’ All reactance are expressed in per units on XC; all voltages, in per units on ILrated. XC and all currents, in amps. In the capacitive region, the different TCSC reactance constraints are caused by the following: 1. The limit on the TCSC firing angle, represented by constant reactance limit Xmax 0. 2. The limit on the TCSC voltage VCtran. The corresponding reactance constraint is give by 3.The limit on the line current (ILtran) beyond which the TCSC transpires into the protective-bypass mode: β’ The effective capacitive-reactance limit is finally obtained as a minimum of the following limits: β’ In the inductive region, the TCSC operation is restricted by the following limits: o The limit on the firing angle, represented by a constant-reactance limit Xmin 0. 90 | P a g e o The harmonics-imposed limit, represented by a constant-TCSCvoltage limit VLtran. The equivalent-reactance constraint is given by 3. Long - Term – Stability Model β’ The capability curves of the TCSC depend on the duration for which the voltage- and current-operating conditions persist on the TCSC. β’ In general, two time-limited regions of TCSC operation exist: the transientoverload region, lasting 3–10 s, and the temporary-overload region, lasting 30 min; both are followed by the continuous region. For long-term dynamic simulations, an overload-management function needs to be incorporated in the control system. β’ The overall reactance-versus-line-current (X-I) capability curve of the TCSC is depicted in Fig. c with the relevant data presented in Table 1 91 | P a g e β’ This function keeps track of the TCSC variables and their duration of application, and it also determines the appropriate TCSC overload range, for which it modifies the Xmax limit and Xmin limit. It then applies the same modifications to the controller. β’ The variable-reactance model does not account for the inherent dependence of TCSC response time on the operating conduction angle. β’ Therefore, entirely incorrect results may be obtained for the highconduction-angle operation of the TCSC or for whenever the power-swing frequency is high (>2 Hz) . 92 | P a g e β’ However, the model is used widely in commercial stability programs because of its simplicity, and it is also used for system-planning studies as well as for initial investigations of the effects of the TCSC in dampingpower oscillations. β’ A reason for the model’s widespread use lies in the assumption that controls designed to compensate the TCSC response delay are always embedded in the control system by the manufacturer and are therefore ideal. β’ Hence the response predicted by the model is a true replica of actual performance. β’ In situations where this assumption is not satisfied, a more detailed stability model is required that accurately represents the inherent slow response of the TCSC. Applications of TCSC(Thyristor Controlled Series Capacitor) One mark question - CAPACITOR 1. What is meant by TCSC? 93 | P a g e THYRISTOR CONTROLLED SERIES TCSC is a thyristor controlled series capacitor. It has one parallel connected thyristor controlled inductor and a series capacitor connected with the transmission line. It provides continuous variable capacitive reactance and variable inductive reactance to control the transmission line parameters. 2.Write down the expression for equivalent impedance, capacitive and inductive reactance of a TCSC. Equivalent impedance of a TCSC, Zeq = -j ((1) / (ωc-1/ ωL)) If (ωc - (1/ ωL)) < 0 - the TCSC provides variable capacitive reactance mode. If (ωc - (1/ ωL)) > 0 - the TCSC provides variable inductive reactance mode. 3. What are the different modes of operation of TCSC? Bypassed- thyristor mode Blocked - thyristor mode Partially conducting thyristor or Vernier mode. 4. What are the modeling techniques involved in TCSC? Variable reactance model (1. Transient stability model 2. Long term stability model) 5. What is the need for modeling of a TCSC? A TCSC involves continuous - time dynamics, relating to voltages and currents in The capacitor and reactor, and nonlinear discrete switching behavior of thyristors. So it is Very important to derive a model for a TCSC controller to maintain the stability of a power system. 94 | P a g e 95 | P a g e