Industrial Electronics Society , IECON 2017 - 43rd Annual Conference of the IEEE Design Interruption Process for HVDC System using Vacuum circuit Breaker Based on Artificial Current Zero 1 2 3 Ali Raza , Waheed Aftab Khan , Manzoor Ellahi , Bilal Masood 4 1. Electrical Engineering Department, Superior Group of Colleges, [email protected] 2. Faculty of Engineering & Technology, Superior Group of Colleges, [email protected] 3. Faculty of Engineering & Technology, Superior Group of Colleges, [email protected] 4. Faculty of Engineering & Technology, Superior Group of Colleges, [email protected] Abstract: This paper deals with the interruption scheme designed using Vacuum Circuit Breaker (VCB) based on zero crossing characteristics, implemented electromechanically for High Voltage Direct Current(HVDC) system. The operation is carried out using Mechanical scheme as it has minimum energy loss in comparison to Solid-State and Hybrid schemes.TheMechanical scheme consists of series connection of multiple modules, variable resistors, VCB, and RC snubber circuit. The commutation is performed through a commutation branch that consists of LC circuitry and a switch. An inverse current is injected in the system through commutation branch at the instant of fault occurrence to interrupt the arcing process. MATLAB is used for designing the simulation of the proposed scheme. The results of the simulation are analyzed and improvements from previous schemes are discussed. Keywords:HVDC,VCB, branch, Interruption 1. Hybrid, Commutation INTRODUCTION The classical HVDC system relied on the point to point distribution technology but, with the increased focus on enhancing DC transmission, there is a need of interconnection of multiple transmission lines. This will cause to increase the transmission and operating cost. In Current Source Converters (CSC)based systems, complex filters and capacitors were required to remove the harmonics of alternating current (AC) . Moreover the flow of power was unidirectional. The reversal of polarity in this technology was difficult while bidirectional flow of power is the essential part of modern distribution system [1, 2]. The solution of this problem is to use multiple terminal high voltage DC (MTHVDC) technology. Voltage source converter (VSC) technology can be utilized in MTHVDC transmission system. The design of VSC is based on the Insulated gate bipolar transistors (IGBT). Voltage and power in the VSC technology are controllable. The added advantage in using VSC is the use of small size filters than the CSC because only high frequency harmonics rise in the VSC based systems. In spite of many advantages, the rate of change of short circuit current is very high because of low inductance in the DC system [3, 4]. Therefore, there is a need of reliable circuit breaker to interrupt the fault current in a certain time otherwise whole system will have to shut down . The main issue of the DC system is to design a reliable interruption scheme. Nowadays, three interruption schemes are mostly used such as, solid state (SS) DC interruption scheme; electromechanical schemeand lastly hybrid technique for interruption process. The Mechanical scheme had a main constraint of very poor interruption time. The proposed scheme for DC interruption which is based Artificial Zero-Crossing (AZC) is shown in fig 1. There are four branches in this scheme for interruption of fault, vacuum circuit breaker (VCB), metal oxide varistors and commutation branch consists of pre charge capacitor C in series with inductance L. Multiple VCBs are connected to increase the voltage level. In the occurrence of fault, VCB is opened first. When impulse of current reaches a certain safe value then contacts of VCB gets open and switch S of commutation branch is closed. High frequency arc is generated due to discharge of commutation capacitor through the inductance and branch switch S. These high frequency oscillations are overlapped on the VCB current. Thus, artificial zero crossing scheme forces the arc which is produced due to VCB to quench. There is a high arc extinguish ability of vacuum due to perfect insulation. Back-up Circuit Breaker VCB MOV+Inductor VCB MOV+Inductor Communication Branch Fig. 1: Proposed Interruption Scheme Back-up Circuit Breaker Industrial Electronics Society , IECON 2017 - 43rd Annual Conference of the IEEE Solid state circuit breaker based on semiconductors and this circuit breaker performs very fast switching. This technology can also be said ultra-fast switching technology. IGBTs or different semiconductor based switches are mostly used in this type of circuit breaker. These switches are used in series and parallel combination to control the circuit. Further research is carried on to make better the circuit breaker [6, 7]. Fig.2: SS CB with surge arrestor  2. Literature Review A significant research has been performed on HVDC circuit breakers. An important development is attained by ABB. Modularized scheme is used by ABB for designing of HVDC circuit breaker. Each module has 80KV capability. In this scheme mechanical switch and ultra-fast switch technique of power electronics is used. An experimental work is carried out for verification of this scheme on 320KV with 2.6KA rated current in . Y. Niwa and fellows have designed a dc circuit breaker using vacuum for arc quenching purpose. Commutation is created by thyristor using as a switch. The rating of this breaker is 1500KV with 100KA. It is tested on traffic power system railway track . 3KV high speed DC vacuum circuit breaker is designed by M. Batosik and fellows . Shi et is performed an experiment using triggering gap on voltage. In this scheme current was stressed to attain zero limits at 30KA while the voltage level was about 3KV . 2.1 SS circuit breaker parallel with a surge arrestor In this type of circuit breaker solid state semiconductor switch T plays a main circuit breaker parallel with an arrestor as shown in fig. 2 . In normal operating condition switch T is in on state and current flows through the switch. When fault occurs in a system switch T will detect the fault and gets open and load current will flow from surge arrestor. It increases the voltage across surge arrestor. If we know the fault current, arrestor voltage and dc voltage then we can find the opening time of circuit breaker and we can also find the absorbed energy by the arrestor. 2.2 SS circuit breaker with freewheeling diode In this type topology a freewheeling diode and surge arrestor are connected in a branch while this branch is parallel across voltage dc source as shown in fig. 3. In normal operating condition switch is closed and current flows through switch T. In the occurrence of fault switch gets open and current flows through the diode. The inductance starts to demagnetize through the surge arrestor and fault will go to decrease. The surge voltage can be expressed as a sum of arrestor voltage and dc voltage source. If we know the inductance and fault current then we can find absorbed energy during interruption process. W=(1/2)*Li^2. T Ldc DC Surge Arrestor This paper has been divided into six sections,starting from Literature Review and Electromagnetic Repulsion (EMR) in VCB is given in section 2 and 3 respectively. Sections 4 and 5 give details of Research Methodology and Interruption Process. Simulation results are presented and discussed in section 6, followed by conclusion. + - IL D Short Circuit Facult Fig.3: SS CB with freewheeling diode  2.3 Passive Resonance Circuit Breaker Passive resonance circuit breaker shown in fig. 4was an important part for arc quenching at lower current values in CSCHVDC systems as discussed in . Surge Arrestor Ia Cc Lc L Io Is CB Fig.4: Pure mechanical HVDC circuit breaker  3. Electro Magnetic Repulsion (EMR) in VCB The fig. 5shows the structural diagram of Vacuum Circuit Breaker (VCB) . A pulse of current in the Industrial Electronics Society , IECON 2017 - 43rd Annual Conference of the IEEE coil is generated when a fault occurs in the system. The alternating current creates magnetic field around the coil. Thisinduced magnetic field in the metal plate produces eddy current causing a repulsive force is between the coil and metal plate.Since the current flows due to changing magnetic field, it always flows in the opposite direction toits cause. Therefore moving plate moves rapidly. (2) Where, S shows the displacement of metal plates. The movement EMR can also be calculated using equation 3, (3) Fdamp represents the resistance between the movements of EMR, Fhold is the permanent magnet force and m is the mass of moving part. 4. Fig.5: Structure diagram of VCB  VCB circuit topology works on the principle of RLC oscillatory discharging mode. Current is flowing in positive direction to avoid reverse charging of capacitor. This works in RL discharging mode. The fig. 6 shows the equivalent circuit diagram for the metal plate that is composed of series inductance and resistance. M C L1 D + R1 I1 L2 I2 The block diagram of proposed scheme is show in fig. 1. This scheme is divided into four pieces, such as main VCB used as central circuit breaker (CCB), secondly the varistors branch, thirdly commutation branch to create zero crossing in the scheme for interruption purpose and lastly the backup circuit breaker to eliminate the residual current. The monitoring and control section is not given in fig.2. The proposed scheme is in shown in Fig. 7. The scheme is based on the modular principle. Therefore CCB contains multiple modules in series up to “n” number of modules, which are based on the design of the system voltage. Each module comprises of two parallel branches a low voltage designed VCB and varistor. The working principle of VCB is based on repulsion by electromagnetism using permanent magnet. If fault occurs in a system then electromagnetic repulsion system respond quickly. R2 SCR Fig.6: The equivalent circuit of EMR  The pulse of current during the process can be written such as in equation 1, (1) While α=R/2L and Research Methodology = Where,Uo represents the pre-charged voltage and Leq shows the equivalent inductance of metal plate. According to law of conservation of energy , the power supply is the sum of heat loss, magnetic energy and working process. The electromagnetic force is calculated by using following equation 2, The usage of modularization of VCB has two reasons over a single high voltage VCB. This technique has two advantages over previous schemes. Firstly, if contact stroke is increased from 2mm than dielectric strength between vacuum gaps increases nonlinearly . The design of high voltage VCB is difficult to achieve due to this behavior of vacuum gap. To eliminate this problem, multiple VCBs are connected in series [16, 17]. The vacuum gap of multiple small VCBs can be equal or greater than main single circuit breaker with some factor. The enhancement factor of breakdown voltage can be derived as follows. The breakdown voltage of vacuum gap can be calculated as , (4) Where Ub is break down voltage while the range of “α” is 0.4-0.7 and k is constant. Then n numbers of vacuum gaps are connected in series to calculate breakdown voltage described in following equation . Industrial Electronics Society , IECON 2017 - 43rd Annual Conference of the IEEE (5) The breakdown voltage Ubn for single vacuum gap and enhancement factor are given in below equations. (6) Secondly, in this scheme a commutation capacitor is pre-charged by this back up circuit breaker. (7) In another respect, modularize scheme is more power efficient then single VCB gap due to small axial dimensions of vacuum circuit breaker at low voltage level. Therefore the opening speed of circuit breaker will be increased during fault interruption. Moreover the bearing transient recovery voltage is also on small scale as compared to single long vacuum gap. It indicates that commutation current is injected at small value after separation of contacts. It reflects that fault current can interrupt rapidly due to high speed of contacts opening and small commutation stroke. VCB1 Ro L BCB MCB L MCB 5. Interruption Process of the Proposed Scheme To explore the behavior of residual current due to weak arc quenching capability of triggered sphere gap through switch S, simulation is performed. Therefore arc will be interrupted at forced zero current by MCB (using VCB). During the interruption process back up switches are used to interrupt the residual current. The interruption process is as follows. BCB Uo RL S Lastly, the backup circuit breaker acts as an isolator. It isolates the electrical supply from the DC circuit breaker. T0: The system is operating in normal state while 650A current is flowing and commutation capacitor is pre-charged up to 80KV. VCB2 L Ic L might be some residual current flowing due to long burning of arc in triggered sphere gap after quenching of arc from main circuit breaker forcedly to zero. This residual current C T1: A short circuit fault takes place. The monitoring and control system detects the fault and gives the opening signal to EMR system of every module of CCB. It starts operating to open the contacts of breaker quickly due to the arc is formation. Fig.7: Schematic diagram of proposed scheme The high increasing rate of fault current can be controlled by series RL snubber circuit before the modules of VCB. The metal oxide varistor is designed to control the peak value of transient recovery voltage. It is parallel connected with VCB because it also absorbs the energy during the interruption process. Commutation branch contains inductor L and capacitor C and their values can be designed on the basis of desired frequency and magnitude of countercurrent, this single branch is paralleled to multiple modules. To attain bi-directional interruption, a trigger sphere gap is adopted as a switch S.A back up circuit breaker is used on the left side of the main circuit breaker. As described above, the arc quenching capability of triggered sphere gap (DCCB) is very weak in bidirectional arc extinguish technique. DCCB cannot interrupt bidirectional current completely. Therefore T2: After reaching the short circuit current up to 4.6KA. Commutation switch is triggered as it injects high frequency oscillating commutated current when CCB approaches a certain safe limit to withstand the transient recovery voltage T2-T3: During the first commutation process currents starts to shift from CCB to commutation branch. In this process commutation capacitor starts to discharge and its voltage continuously decreases. T3: At the end of first commutation process current is completely transferred to commutation branch. The arc in CCB quenches due to forciblytaking the current to zero, as a result high energy is generated. To absorb this energy a series inductor with metal oxide varistor is used. T3-T4: The capacitor starts charging after discharging the capacitor during first commutation process with positive voltage in reverse direction. Due to increase of voltage of commutation capacitor, Industrial Electronics Society , IECON 2017 - 43rd Annual Conference of the IEEE the CCB voltage also increases. Therefore current starts to move from commutation branch to varistor branch. T4: This is the second commutation process in which total current is shifted to arrestor branch. T5: The current through switch S crosses zero crossing after flowing through varistor. Arc can be quenched in commutation switch at zero crossing if an auto switch is adopted as experimental and simulation results shown in [8, 9]. In spite of zero crossing there is no guarantee that arc can be fully quenched. It is totally dependent on the history of arc and geometry of sphere gap. It concludes that only commutation switch S is incapable to cut off current by itself in the simulation. As a result, residual current is still flowing in the commutation branch. T5-T6: The current frequency of commutation and metal oxide varistor is up to 8 kHz. This current is decreasing gradually. In this process, the total current which is flowing is the sum of commutation current, varistor branch current and charging capacitor. T6: The total flowing current will be zero after cut off through the varistor branch. In spite of the fact that the commutation switch can’t quench the arc itself. Due to long time arc burning an oscillating current up-to 430Acan flow in the DC system. The oscillating frequency can be calculated such as follows, The arc in the triggered sphere gap can be quenched at a certain current level. It depends on many factors as discussed above. Therefore, triggered sphere gap is not a dependable arc quenching switch. To eliminate the residual current, backup circuit breaker is used as shown in simulation results fig.11. If contacts of backup circuit breaker BCB1 are opened at T6 then arc will be quenched and current flow in triggered sphere gap will also be zero. At the end of process, the commutation capacitor starts to discharge through the resistor, after that the second backup circuit breaker BCB2 can also be opened to isolate the whole circuit. By this way reclosing is done. System Parameters Voltage 80 Source kV (U) Ro ShortCircuit Inducta nce (Lo) Load Resista nce (RL) MCB Module Protectio n Voltage .79 Oh m 23m H 150 Oh m 80 kV Commutation Branch Charging -40 voltage of kV Commutat ion capacitor Commutat 7.77k ion Hz frequency Peak 4.1kA Current (Ip) Capacitan ce (C) 12µF Inductanc e (L) 35µH Table 1: Scheme Parameters of the proposed Technique Parameters have been adopted according to commutation frequency. Commutation frequency is the point where artificial zero crossing starts. In the view of simulation results, current commutation technique has a favorable interruption. Commutation process depends on commutation branch and mainly on frequency of commutation current. If the frequency of commutation increases then the value of capacitor and inductor will decrease for the same value of voltage and current. This indicates smaller volume and lower cost. It is also to be noted that voltage across commutation capacitor and peak of desired countercurrent should be greater than the short circuit current. As it is discussed above, triggered sphere gap has weak arc quenching capability to achieve bi-direction interruption. In this method, commutation current will overlap to increase short circuit current of the same direction in the first half cycle. The current is interrupted in the next half cycle. More energy is decomposed when superimposition of currents occurs. This is a big obstruction in the way of successful interruption. This drawback can be overcome by injecting high frequency current. For example 50 KHz frequency is used . 6. Simulation Results The results of proposed scheme are shown in following figures. The following results describe the effect of residual current and the significance of Industrial Electronics Society , IECON 2017 - 43rd Annual Conference of the IEEE backup circuit breaker. Firstly, the results are shown without using the backup circuit breaker. 6.1 Interruption results circuit breaker without Backup Fig. 10: Commutation Voltage Fig. 8: Source Current A normal current of 650A is flowing while a fault occur at 2ms,and then current starts to rise as shown in Fig. 8. As it approaches the safe limit 4.2KA then VCB contacts on and switch S is closed. Arc is quenching at zero crossing at 6.8ms but residual current continues to flow until backup circuit breaker works. Fig. 10 depicts that as commutation capacitor is inversely pre-charged when switch S is on then commutation capacitor starts to discharge to create commutation. When all the current is transferred from CCB to Metal Oxide Varistor then it gets charged again in reverse direction. As residual current continues to flow, therefore its voltage drops again. 6.2 To interrupt residual current Secondly, to interrupt the residual current backup circuit breaker is used, Fig. 11: Source Current with backup circuit breaker Fig.9: Commutation Current When the contacts of VCB open then the transient recovery voltage approaches to certain safe limit as depicted in Fig. 9. As a result high current flows and at the same time commutation switch is closed. Due to which oscillating current is injected in the system. In Fig.11 shows that the residual current continues to flows after breaking the circuit breaker, therefore it is a need of backup circuit breaker to interrupt the current zero. As shown in Fig. 11, at 7.8ms backup circuit breaker interrupts the source current. Industrial Electronics Society , IECON 2017 - 43rd Annual Conference of the IEEE 6.3 Commutation current with backup circuit breaker Fig 12: Commutation Current with backup circuit breaker The Fig. 12 gives the Commutation current with backup circuit breaker. High transient current flows in the interruption process due to high transient recovery voltage. While after switching backup circuit breaker operatesresulting in no current flow through the system due to commutation current. the commutation process. The residual current can be controlled by using backup circuit breaker. According to simulation results the residual current can be damped more rapidly by using high frequency. Moreover, the thermal joule losses i.e. integral I2*t created due to residual current can be eliminated using high frequency. The value of commutation inductor and commutation capacitor can be different from each other. According to simulation small inductance and large capacitance with lower charging voltage can be more effectivein increasing the rate of change of transient recovery voltage and interruption capability will become better. REFERENCES D. Jovcic, D. van Hertem, K. Linden, J. P. Taisne and W. Grieshaber, "Feasibility of DC transmission networks," 2011 2nd IEEE PES International Conference and Exhibition on Innovative Smart Grid Technologies, Manchester, 2011, pp. 1-8.doi: 10.1109/ISGTEurope.2011.6162829  M. Callavik, M. Bahrman and P. Sandeberg, "Technology developments and plans to solve operational challenges facilitating the HVDC offshore grid," 2012 IEEE Power and Energy Society General Meeting, San Diego, CA, 2012, pp. 1-6. doi: 10.1109/PESGM.2012.6344742 Fig. 13: Commutation Voltage with backup circuit breaker  M. K. Bucher and C. M. Franck, "Contribution of Fault Current Sources in Multiterminal HVDC Cable Networks," in IEEE Transactions on Power Delivery, vol. 28, no. 3, pp. 1796-1803, July 2013. doi: 10.1109/TPWRD.2013.2260359 When fault occurs, the commutation capacitor starts to discharge and charge again in the reverse direction, as shown in Fig. 13. But the charge on the capacitor starts to decrease after openingof the backup circuit breaker.  K. Tahata et al., "HVDC circuit breakers for HVDC grid applications," 11th IET International Conference on AC and DC Power Transmission, Birmingham, 2015, pp. 1-9.doi: 10.1049/cp.2015.0018 7. CONCLUSION This scheme works perfectly and interruption time is 4.8ms. As a comparison from ultra-fast switching the interruption time is slightly greater but scheme works much faster than previous mechanical circuit breakers. Also,as compared to previous scheme it is cost effective because there is no use of RC snubber circuit which require high value capacitor bank. 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