EE-4063 Power Electronic and Applications II ASSIGNMENT 1 Name :C.A.N. Yapa Index No :110651F Power Conditioners 1. Power line disturbances are defined as any changes in power supply characteristics (voltage, frequency, current) that would cause interferences to the normal operation of electrical equipment. Types of power line disturbances i. ii. iii. iv. v. vi. Transients Interruptions Undervoltages or sag Overvoltages or swell Waveform distortions Voltage variations i. Transients This is the most damaging type of power line disturbance which can be further categorized as impulsive transients and oscillatory transients. Impulsive transients sudden high peak events that raise the voltage and/or current levels in either positive or negative direction. Oscillatory transients are sudden changes in the steady state condition of a signal’s voltage, current or both at both the positive and negative limits; oscillating at the natural system frequency. These oscillations usually decay to zero within a cycle. ii. Interruptions This could be defined as the complete loss of supply voltage or load current and depending on the duration it persists an interruption can be further categorized as given below. Instantaneous – 0.5 to 30 cycles Momentary – 30 cycles to 2 sec Temporary – 2 sec to 2 min Sustained – greater than 2 min iii. Undervoltages or sags Sag is defined as a reduction of AC voltage at a given frequency for the duration of 0.1 cycles to 1minute’s time whereas undervoltages are the result of long – term problems that create sags. iv. Overvoltages or swell Swell is having an increase in AC voltages for a duration of 0.5 cycles to 1 minute’s time and overvoltages are the result of long – term problems that create swells. v. Waveform distortions Waveform distortions could be further categorized as; DC offset – DC currents that are induced into the AC distribution system which could transverse the AC power system and add an unwanted current component. Harmonics – Corruption of the fundamental sine wave at frequencies that are multiples of the fundamental. Notching – Periodic voltage disturbances caused by electronic devices. Noise – Unwanted voltage or current superimposed on the power system voltage or current waveform. vi. Voltage variation A systematic variation of the voltage waveform or a series of random voltage changes of small dimensions. 2. Lightning Switching of inductive or capacitive loads Utility fault clearing Damages to the supply grid system due to destructive weather Equipment failure Starting of large loads Sudden load reductions Disturbances due to power electronic devices Poor earthing systems Electrostatic discharge Lightning – This is the most damaging cause which basically causes impulsive transients on the system both from direct lightning strikes as well as due to the induced current effect of the electromagnetic field created by strikes to nearby conductive structures. Switching of inductive or capacitive loads – Transients appear on an energized circuit when capacitor banks are automatically switched into the system. Starting of large loads – An induction motor draws approximately six times the nominal current while starting up hence result in a significant voltage drop to the rest of the circuit it resides. Sudden load reductions – This would cause an overvoltage condition in the system and typically occurs during midnight where less loads are connected to the grid. Disturbances due to power electronic devices – DC offsets are caused due to failure of the rectifiers that are connected to the system and as well as notches in the voltage waveform are due to electromagnetic interference of the power electronic drives which are extensively used in industries. Further the advanced electronic drives inject harmonics to the grid supply at the Point of Common Coupling (PCC) and cause noise in the voltage waveform. Effects of the above mentioned disturbances on the sensitive equipment Transients cause errors in results and damage the equipment such as burning of the entire electronic control system. Undervoltage or overvoltage situations result in errors in the equipment, shut down or damage to sensitive equipment. Sags can slow down the data processing systems hence resulting loss of data. Voltage variations might result in permanent loss of data. Harmonics affect the stability of the power supply hence cause malfunctioning of sensitive equipment. Further harmonics alter the waveform parameters such as zero crossing and voltage peak which are the typical quantities on which most of the modern power electronic controls depend on thus affect the operation of the equipment adversely. Interruptions shut off or damage the sensitive equipment. 3. Power conditioners have the ability to regulate and improve the quality of the power supplied by delivering dynamic adjustments and removing spikes, surges, noise, sag and frequency irregularities which affect adversely on the system performance. A list of commonly used power conditioning techniques is as described below. Surge protective devices Voltage regulators Uninterruptible Power Supplies Isolation Power Factor correction i. Surge protective devices These devices are usually incorporated to the system with active line clamping thus connected directly across the incoming power supply. They consist of Metal Oxide Varistors (MOV), Transorbs, gas discharge or spark gap devices which will protect the equipment from transients. ii. Voltage regulators Voltage regulation is accomplished through several techniques including tap changing, feroresonant transformer and double conversion methodology. In tap changing mechanism transformer taps are switched automatically by a line voltage sensing circuit hence to maintain the output voltage as closer as possible to the nominal voltage. A feroresonant transformer is a non – linear transformer that is designed to provide passive voltage regulation using magnetics. In double conversion, the incoming AC is rectified, filtered and regulated and this regulated DC power is fed to an inverter to obtain the regulated AC power. iii. Uninterruptible Power Supplies iv. Isolation The power line input/output isolation is achieved through a transformer in various configurations including simple standard core transformers and power toroidal type. Isolation transformers provide broad band noise filtering hence reducing power line disturbances. v. Power Factor correction This is implemented either with active or passive circuits and is usually incorporated with another power conditioning technique. Active power factor correction restores the power waveform peak (undervoltages) which is reduced due to heavy loading of the system whereas passive power factor correction is typically carried out by the utility with the connection of large capacitor banks to the grid. UPS 1. An Uninterruptible Power Supply provides protection against power outages and handles the voltage regulation aspect as well during power line overvoltage and undervoltage situations. Further these are used to suppress incoming line transients and harmonic disturbances. 2. 3. i. Rectifier The main functions of the rectifier of an UPS are to supply power to the inverter and to maintain the battery bank charged. Different circuit arrangements can be incorporated in order to achieve the required operation of the rectifier. Phase controlled rectifier circuit The control is obtained through the controlling of the firing angle of the thyristors. Diode rectifier bridge in cascade with a step down dc – dc converter In order to obtain a regulated dc voltage supply, the above described circuit arrangement can be utilized. Controlling is achieved through PWM schemes that will handle the switching of the buck converter. DC – DC converter with electrical isolation Output voltage is controlled using PWM schemes similar to the non – isolated arrangement of the dc – dc converter. ii. Battery There are many battery systems that could be incorporated with the UPS but the most commonly used is the lead – acid battery. Rectifier and the separate battery charging circuit Under normal operation with the line voltage present, a trickle charge voltage is applied across the battery hence it draws a small amount of current and maintains itself in a fully charged state. During a power outage the battery will supply the load. After the power has been restored the UPS is brought back to the fully charged state which is achieved initially through a constant charging current state until the voltage reaches the trickle voltage level and then charging in the constant voltage mode (at the trickle voltage) with the current decreasing to the trickle charging current. iii. Inverter The UPS utilizes PWM dc – dc inverters giving either a single phase or three phase ac output. For large UPS systems, several inverter units are connected in parallel and connected through transformers with phase shift so that the inverter could operate at a lower switching frequency. This could be achieved through either low – frequency PWM, selective cancellation or a square – wave switching scheme. The control of the above mentioned inverters are obtained through PWM switching schemes. Further it is also possible to use an inverter arrangement which incorporates resonant dc – dc converters connect to an integral – half – cycle frequency converter through an isolation transformer. 4. A static transfer switch is used to transfer the load from the UPS to the grid supply and vice versa and a schematic is as given below. During the line is energized the load will be supplied through the grid supply whereas during a line outage the load will be transferred to the UPS system. In using a static transfer switch, the output of the inverter should be synchronized with the grid supply therefore during the transferring process; the load will see a minimal amount of disturbance. VAR Compensation 1. VAR compensation adjusts the reactive power that is supplied to the system hence maintain the system voltage within a specified range around the nominal value (in Sri Lankan context +/- 5%) and further provide dynamic voltage regulation which enhances the stability of the system. This can be achieved through either power factor correction capacitor banks which compensates for slow changes in reactive power whereas the Static Var Compensators provide quick control over the compensation. Var compensation in turn reduces the current drawn by the equipment that is connected to the AC system hence reduces the I2R losses. 2. i. Thyristor – controlled inductors A thyristor – controlled inductor acts as a variable inductor which is capable of providing inductive vars to the system rapidly. Two back – to back thyristors which conduct in alternate half cycles of the supply frequency are controlled by varying the position of the voltage waveform at which they are gated into conduction. Both the thyristors are fired at equal delay angles. Full conduction is obtained at a delay angle of 900 where as partial conduction is obtained between delay angle of 900 and 1800. Increasing delay angle will reduce the fundamental component of the current through the inductor which is equivalent to increasing of the effective inductance that is connected to the system. Equivalent per phase circuit is as shown below. Variation of the inductor current with the delay angle is as shown below. ii. Thyristor – switched capacitors Thyristor switched capacitor is defined as a shunt – connected, thyristor switched capacitor whose effective reactance is varied in a stepwise manner by full or zero conduction operation of the thyristor valve. The susceptance is adjusted by controlling the number of parallel capacitors in conduction. Each capacitor always conducts for an integral number of half – cycles. This control mechanism is in contrast to the phase control method of thyristor – controlled inductors. The capacitor bank will be brought to zero conduction by blocking the gate pulse to both the thyristors. When turning on, the thyristors should be gated at the proper instant of maximum ac voltage to limit overcurrents. The inductors are used to limit these switching overcurrents. Incorporating a large number of thyristor – switched small capacitor banks it is possible to vary the reactive power in small but discrete steps. iii. Switching converter with minimum energy storage elements The thyristor – controlled inductors and thyristor – switched capacitors incorporate large energy storage devices in compensating vars of the system and are associated with an inherent time lag thus unable to provide instantaneous control. Thus in order to overcome these issues, switch mode converters that are operated in current control mode will be utilized thus the ac current could be controlled quickly in the magnitude as well as the relationship of it to the voltage. Since the average power drawn by these switch mode converters is zero, a DC source at the input of the converter is not required instead a capacitor with a minimum energy storage capacity is sufficient whose voltage will be maintained by the converter. 3. i. Photovoltaic array interconnection The solar cell characteristics for a given solar insolation level and temperature comprises of two components, i.e. constant current component and the constant voltage component. The maximum power is delivered by the PV array at the knee point of the characteristics which is the point at which the above mentioned two components meet. In order to ensure that the array always operates at the maximum power point, a tracking system is incorporated where the current drawn and the resulting power output is observed and adjusted at regular intervals. In an instance where an increase in current results in an increase in power, the current will be further increased until the power output starts to decline and if an increase in current results in a reduction in the power output, the current will be reduced until the power output reaches a maximum value. The below described two interconnection methods are thus possible. Single phase interconnection Two alternative interconnection methods are possible in the single phase arrangement. a. Line frequency, phase controlled converter that always operates in the inverter mode and electrical isolation provided through a 60Hz transformer. AC side filter and var compensators will be required since the output current has a lagging power factor, containing harmonics. b. Switch mode, pulse – width- modulated converter with electrical isolation provided using a 60Hz transformer. High frequency ac voltage is produced at the primary of the isolation transformer which is rectified at the secondary end and interfaced with the line voltage through line – frequency, line – voltage commutated thyristor inverter. Three phase interconnection As the power output exceeds several kilowatts it is required to incorporate a three phase interconnection. This could be achieved through switching mode dc-ac inverter operating in the current control mode where electrical isolation is provided through a 60Hz three phase transformer. ii. Wind and small hydro interconnection In wind power, the power output is proportional to the cube of the wind speed and in small hydro power generation, this is proportional to the head and the flow rate. Thus in order to harness maximum power output from these technologies it is desirable to allow the turbine speed to vary in a wide range which is not possible with a synchronous generator which dictates a constant speed. Thus an induction generator connected to the grid is incorporated and the efficiency of power generation is optimized by rectifying the generator output and then interfaced with the grid supply with the means of a switching dc – ac inverter with proper electrical isolation is provided through a 60Hz transformer. iii. Interconnection of energy storage systems to the utility grid This technique could be incorporated to connect energy storage systems which could store energy generated from efficient generating plant during low load conditions, to the utility grid during the peak hours hence minimize the generation cost which arises due to utilization of fossil fuel or gas fired power plants to cater the peak demand. These energy storage devices can be batteries, fuel cells or superconductive inductors in which energy is stored in the form of a magnetic field. Batteries and fuel cells produce a dc output voltage thus the single phase or three phase interconnection methods which are incorporated with the photovoltaic arrays could be utilized. For superconductive inductors, 12 pulse line commutated converters in which the delay angle is controlled to obtain continuously varying converter operation from full rectifier mode to full inverter mode could be incorporated in connecting them to the utility grid. Residential and Industrial Applications 2. Artificial lighting High frequency fluorescent lighting is the best example for power electronic applications in artificial lighting. The block diagram of this system is as shown below. The high frequency electronic ballast converts the low frequency input (typically 50Hz) to a high frequency output in the range of 25 – 40 kHz. The schematic of the high frequency electronic ballast is given below. High frequency ac is obtained by a dc – high frequency ac inverter which could be either a resonant converter or a switch mode converter such as the half bridge topology. Further a dimming circuit could be incorporated with these gas discharge lamps which also involve power electronic controls. By varying the firing angle of the triac which is fired by the means of a diac and a RC triggering circuit, the intensity of the light could be varied. Less firing angle will result in more voltage across the lamp hence more intensity of illumination and vice versa. During the operation in each half cycle, depending on the value of VR1 the voltage across the capacitor C1 will vary hence if it reaches a value greater than the breakover voltage of the diac the gate of the triac will be triggered. Utility Interface 1. Harmonics are of two variations, voltage harmonics and current harmonics. The main causes for these harmonic types are the static power converters that are used for various purposes in the industry such as adjustable speed drives, Switch – Mode Power Supplies (SMPS) and Uninterruptible Power Supplies (UPS). These converters utilize semiconductor devices for power conversion from ac to dc, dc to dc, dc to ac and ac to ac and draw nonlinear currents hence distort the supply voltage waveform at the Point of Common Coupling (PCC). The nonlinear currents contain high amplitude short pulses which are rich in harmonics and produce voltage drop across system impedance. Below described are certain sources of harmonics. Phase controlled rectifiers (thyristor rectifiers) – These converters inject voltage notches and draw power at a very low displacement power factor. This is mainly due to the fact that the power through the converter is reversible although the current is unidirectional which is not the case with uncontrolled rectifiers (diode rectifiers) where both the current and power flow are unidirectional Welding equipment Variable speed drives Periodic switching of voltages and currents AC generators due to non-sinusoidal air gap and flux distribution Switching devices like SMPS, UPS and CFL Effects of harmonics on the utility Increase the generator heating due to increased iron losses and copper losses which both are proportional to the frequency hence increase due to increase of harmonics in the voltage and current waveforms Due to the effect of the harmonics, the transformers are affected by the increase of the core losses due to eddy current and hysteresis and copper losses and stray flux losses which contributes to the overheating of the windings and insulation. Further resonance between the winding inductance and supply capacitance might occur, causing additional losses in the transformer Nonlinear load being connected to the distribution transformers, due to the triplen harmonics in the phase currents not getting cancelled off results in a high current in the neutral conductor overheating it and burning off Cable losses, dissipated as heat, are substantially increased when carrying harmonic currents due to elevated I2R losses Circuit breakers of the system might trip prematurely and the operating time of the fuses will be reduced due to the increased heating effect of the harmonic currents Power cables carrying harmonic loads act to introduce EMI (electromagnetic interference) in adjacent signal or control cables via conducted and radiated emissions Any telemetry, protection or other equipment which relies on conventional measurement techniques or the heating effect of current will not operate correctly in the presence of nonlinear loads Conventional meters are normally designed to read sinusoidal-based quantities. Nonlinear voltages and currents impressed on these types of meters introduce errors into the measurement circuits which result in false readings 2. With the intention improving the power quality, standards and guidelines have been imposed which specify the limits on the magnitude of the harmonic currents that could be inject to the system by the consumers. EN 50 006, Limitation of Disturbances in Electricity Supply Networks caused by Domestic Appliances Equipped with Electronic Devices”, European Standard prepared by Comitè Europèen de Normalisation Electro technique, CENELEC IEC Norm 555 – 3, prepared by International Electrical Commission West German Standards VDE 0838 for household appliances, VDE 0160 for converters and VDE 0712 for fluorescent lamp ballasts IEEE Guide for Harmonic Control and Reactive Compensation of Static Power Converters, ANSI/IEEE Std. 519-1981, revised in 1992 to 512-1992 The revised IEEE – 519 specifies requirements on the user as well as on the utility The table below defines the levels of harmonic currents that an industrial user can inject onto the utility distribution system (120V through 69kV). The below given table specifies the voltage distortion limits that can be reflected back onto the utility distribution system. 3. i. Passive Harmonic Filters (Line Harmonic Filters) Passive or Line Harmonic Filters (LHF) are also referred to as harmonic trap filters and are used to eliminate or control lower order harmonics which are more dominant specifically 5th, 7th, 11th and 13th. It is used either as a standalone part integral to a large nonlinear load such as 6 – pulse drive or it can be used for a multiple small single – phase nonlinear load by connecting it to a switchboard. LHF comprises of a passive L – C circuit which is tuned to a specific harmonic frequency which is needed to be eliminated. The operation of this filter relies on the resonance principle that occurs due to the variation in frequency in the inductor and the capacitor. These filters are susceptible to the changes in source and load impedances and attract harmonics from other sources thus proper design consideration should be given taking these into account. ii. Active filters Active filters are commonly used in industrial applications for both harmonic mitigation and reactive power compensation. Unlike passive L–C filters, active filters do not present potential resonance to the network and does not depend on the source impedance. The commonest configuration is the shunt – connected active filters consisting of an IGBT bridge and DC bus architecture. When rated correctly in terms of harmonic compensation current, the active filters provide the nonlinear load current with the harmonic component that is required for its proper functioning while the source will have to provide only the fundamental component thus minimizing waveform distortions. 4. Electro Magnetic Interference (EMI) is generated in switching converters and power electronic devices due to rapid changes in voltage and currents, in their operation with other equipment as well as with its own proper operation. EMI is transmitted in two forms, radiated and conducted. Conducted interferences are further divided into two forms namely, differential mode and common mode. EMI standards Various CISPR, IEC, VDE, FCC standards specify the maximum limit on the conducted EMI. The conducted noise will be measured by mean of the Line Impedance Stabilization Network (LISN) which is specified impedance network and compared against the set standards. EMI filters An EMI filter is an electronic passive device which is used in order to suppress conducted interference that is present on the power line. EMI filters can be used to suppress interference that is generated by the device or by other equipment in order make a device more immune to electromagnetic interference signals present in the environment. Most EMI filters consist of components that suppress differential and common mode interference. In either power supplies or electronic equipment, it is the function of the EMI filter to keep any internally generated noise contained within the device and to prevent any external ac line noise from entering the device. The inductive part of the EMI filter is designed to act as a low-frequency pass device for the AC line frequencies and a high-frequency blocking device. Other parts of the EMI filter use capacitors to bypass or shunt unwanted high-frequency noise away from the sensitive circuits. The net result is that the EMI filter significantly reduces or attenuates any unwanted noise signals from entering or leaving the protected electronic device.
