Final Year Project: Simulation of virtual-current chopping...
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Final Year Project: Simulation of virtual-current chopping in circuit breakers in electrical supply systems Lui Chak Hing (14073317D) Supervisor: Dr C.W. Yu Abstract Methodology Discussion Introduction Results Conclusion Objectives • Vacuum circuit breaker (VCB) provides highly enhanced current interruption and dielectric recovery features. It has the ability to interrupt the high frequency (HF) current effectively. In some cases, HF frequency current may superimpose on the power- frequency current. This can cause multiple reignitions and serious voltage escalations under some conditions. • In this project, computer simulation will be performed in order to study and analyse the properties of such process. This model replicates the original circuit breaker properties and simulate the phenomenon of the transients. • The construction of the VCB model is conducted in the simulation software EMTP-RV. The developed model is implemented in a simple-phase and a three-phase testing circuit. Simulation is then performed to obtain the transient behaviour of VCB and analyse the effects on multiple reignitions phenomena under different parameter values. • Afterwards, the developed VCB model is implemented in a typical electrical supply circuit. The circuit involves the switching of an arc furnace transformer. The behaviour of virtual current chopping and the effects on overvoltage phenomena of VCB will be analysed. • Circuit breakers are widely used and have become the important equipment for the protection of transmission and distributions systems. They play a vital role in clearing faults and isolating defective parts of network clearly and rapidly. • VCB are commonly adopted in power systems with medium voltage range due to many advantages, such as small size, less maintenance, excellent performance on the interruption and recovery of dielectric strength. VCB can interrupt current with a very high value of di/dt , ranging from 150A/μs to 1000A/μs typically. • However, the switching transient overvoltage phenomenon due to repeated reignitions, current chopping and virtual current chopping are commonly found in VCB. • When VCB opens, electric arc arises and the arc will extinguish at a natural current zero ideally. When the arc conducts a small current, the arc becomes unstable and may disappear before natural current zero. This is called current chopping. • When current chopping happens, the transient recovery voltage (TRV) arises between the gap of VCB. When the TRV is higher than the level of dielectric strength of vacuum gap, reignition will happen and consequently arc will appear again. The process will cause a flow of HF current due to the stored charges in the stray capacitance on either side of the gap . The power-frequency current can be superimposed by the HF current. • VCB can interrupt the HF current at one of the current zeros. When the interruption occurs, multiple reignitions may happen and it may produce undesirable voltage surges. Voltage escalation will continue until the contact separation of VCB is enough to withstand the TRV or the magnitude of power- frequency current is high enough to prevent zero crossing of the sum of HF current and power frequency current. In this situation, there is no interruption and the current will continue for one more half cycle. If the HF current of one phase is flowing into other two phases of the system through the electrical couplings of the load, the HF current can superimpose on the power- frequency current of these phases and force the power- frequency current to zero. Virtual current chopping happens. • Modelling of VCB is performed. Then simulation of virtual current chopping will be done and the effects of overvoltage transients in electrical supply system will be analysed. • The project will be mainly conducted in the simulation software EMTP-RV. • A mathematical VCB wit the characteristics of dielectric strength and HF current quenching capability is constructed in the software environment. • In this project, computer simulation is performed on different network configurations by using the simulation software EMTP- RV. These configurations include the testing circuits of single phase and three phases. An application circuit which is about the switching of an arc furnace transformer is also simulated. • Developing a VCB model incorporating different parameters of the circuit breaker in the simulation software EMTP-RV • Performing the simulation in the single-phase and three-phase testing circuits • Analysing the simulation results and investigating the transient behaviour of VCB with different parameters • Applying the VCB model to a practical circuit which is related to the switching of an arc furnace transformer • Simulating the virtual current chopping process and analysing some protective measures which can reduce the overvoltage and reignitions behaviour of VCB • For single-phase simulation, different cases are involved in order to investigate the VCB behaviour during the opening process. Different parameters (dielectric strength of VCB and HF current quenching capability) are set to analyse to effects on reignitions phenomena and HF current property of VCB. The results show that overvoltage and multiple reignitions are more likely to occur when the dielectric strength is smaller or the HF current quenching capability is higher. Also, there are HF oscillations during reignitions. • For the three-phase simulation, similar simulation tasks are performed. The results show that VCB experiences reignitions at different phases under various time intervals. The HF current produced at one phase will through other phases and force the power-frequency current to zero. The forced current zero in that phase will cause TRV in the breaker. As a result, further reignitions will produce and this will lead failure of the VCB. • The VCB model developed in this project is put into a practical circuit for the purpose of simulation. It involves the switching of an arc furnace transformer. A lot of switching operations are needed during the operation of the transformer. In the circuit, the phenomena of virtual current chopping can cause the failure of the VCB and other devices. Therefore, adding the RC surge suppressor or surge arrestor is necessary. The process of virtual current chopping can be simulated by using the developed VCB model. • After adding the surge arrestor to the arrestor, the overvoltage behaviour was limited and hence the phenomena of virtual current chopping were improved greatly. As a result, the addition of surge arrestor can help to protect the system. • Sensitivity Analysis: Single-phase simulation • The characteristics of VCB and phenomena of transient overvoltage are discussed. A VCB model is built and implemented in the simulation software. A single-phase and three-phase testing circuit is developed to perform the simulation with different parameters and observe the phenomena of multiple reignitions and voltage escalations of VCB. • The results of sensitivity analysis shows that the parameters of arcing time and the recovery of dielectric strength play a significant role for the occurrence of multiple reignitions and overvoltage. The quenching capability of HF current has less effect. Consequently, when estimating the overvoltage phenomena of VCB, it is suitable to consider the recovery of dielectric strength in the middle range with smaller arcing time to look for the worst situation so that the appropriate protection scheme can be provided. • The developed VCB model is implemented in an application network which involves switching of a transformer. The phenomena of virtual current chopping is observed in the process of simulation. One possible way of reducing the problem of virtual current chopping is the addition of the RC surge suppressor or surge arrestor. This can reduce the chance of equipment failure and hence protect the system. Practical circuit simulation Three-phase simulation Case 1 Case 2 Case 1 Case 2
Transcript of Final Year Project: Simulation of virtual-current chopping...
Final Year Project:
Simulation of virtual-current chopping in circuit breakers in electrical supply systems
Lui Chak Hing (14073317D)
Supervisor: Dr C.W. Yu
Abstract Methodology Discussion
Introduction
Results
Conclusion
Objectives
• Vacuum circuit breaker (VCB) provides highly enhanced current interruption and dielectric recovery features. It has the ability to interrupt the high frequency (HF) current effectively. In some cases, HF frequency current may superimpose on the power-frequency current. This can cause multiple reignitions and serious voltage escalations under some conditions.
• In this project, computer simulation will be performed in order to study and analyse the properties of such process. This model replicates the original circuit breaker properties and simulate the phenomenon of the transients.
• The construction of the VCB model is conducted in the simulation software EMTP-RV. The developed model is implemented in a simple-phase and a three-phase testing circuit. Simulation is then performed to obtain the transient behaviour of VCB and analysethe effects on multiple reignitions phenomena under different parameter values.
• Afterwards, the developed VCB model is implemented in a typical electrical supply circuit. The circuit involves the switching of an arc furnace transformer. The behaviour of virtual current chopping and the effects on overvoltage phenomena of VCB will be analysed.
• Circuit breakers are widely used and have become the important equipment for the protection of transmission and distributions systems. They play a vital role in clearing faults and isolating defective parts of network clearly and rapidly.
• VCB are commonly adopted in power systems with medium voltage range due to many advantages, such as small size, less maintenance, excellent performance on the interruption and recovery of dielectric strength. VCB can interrupt current with a very high value of di/dt , ranging from 150A/µs to 1000A/µs typically.
• However, the switching transient overvoltage phenomenon due to repeated reignitions, current chopping and virtual current chopping are commonly found in VCB.
• When VCB opens, electric arc arises and the arc will extinguish at a natural current zero ideally. When the arc conducts a small current, the arc becomes unstable and may disappear before natural current zero. This is called current chopping.
• When current chopping happens, the transient recovery voltage (TRV) arises between the gap of VCB. When the TRV is higher than the level of dielectric strength of vacuum gap, reignition will happen and consequently arc will appear again. The process will cause a flow of HF current due to the stored charges in the stray capacitance on either side of the gap . The power-frequency current can be superimposed by the HF current.
• VCB can interrupt the HF current at one of the current zeros. When the interruption occurs, multiple reignitions may happen and it may produce undesirable voltage surges. Voltage escalation will continue until the contact separation of VCB is enough to withstand the TRV or the magnitude of power-frequency current is high enough to prevent zero crossing of the sum of HF current and power frequency current. In this situation, there is no interruption and the current will continue for one more half cycle. If the HF current of one phase is flowing into other two phases of the system through the electrical couplings of the load, the HF current can superimpose on the power-frequency current of these phases and force the power-frequency current to zero. Virtual current chopping happens.
• Modelling of VCB is performed. Then simulation of virtual current chopping will be done and the effects of overvoltage transients in electrical supply system will be analysed.
• The project will be mainly conducted in the simulation software EMTP-RV.
• A mathematical VCB wit the characteristics of dielectric strength and HF current quenching capability is constructed in the software environment.
• In this project, computer simulation is performed on different network configurations by using the simulation software EMTP-RV. These configurations include the testing circuits of single phase and three phases. An application circuit which is about the switching of an arc furnace transformer is also simulated.
• Developing a VCB model incorporating different parameters of the circuit breaker in the simulation software EMTP-RV
• Performing the simulation in the single-phase and three-phase testing circuits
• Analysing the simulation results and investigating the transient behaviour of VCB with different parameters
• Applying the VCB model to a practical circuit which is related to the switching of an arc furnace transformer
• Simulating the virtual current chopping process and analysingsome protective measures which can reduce the overvoltage and reignitions behaviour of VCB
• For single-phase simulation, different cases are involved in order to investigate the VCB behaviour during the opening process. Different parameters (dielectric strength of VCB and HF current quenching capability) are set to analyse to effects on reignitionsphenomena and HF current property of VCB. The results show that overvoltage and multiple reignitions are more likely to occur when the dielectric strength is smaller or the HF current quenching capability is higher. Also, there are HF oscillations during reignitions.
• For the three-phase simulation, similar simulation tasks are performed. The results show that VCB experiences reignitions at different phases under various time intervals. The HF current produced at one phase will through other phases and force the power-frequency current to zero. The forced current zero in that phase will cause TRV in the breaker. As a result, further reignitions will produce and this will lead failure of the VCB.
• The VCB model developed in this project is put into a practical circuit for the purpose of simulation. It involves the switching of an arc furnace transformer. A lot of switching operations are needed during the operation of the transformer. In the circuit, the phenomena of virtual current chopping can cause the failure of the VCB and other devices. Therefore, adding the RC surge suppressor or surge arrestor is necessary. The process of virtual current chopping can be simulated by using the developed VCB model.
• After adding the surge arrestor to the arrestor, the overvoltage behaviour was limited and hence the phenomena of virtual current chopping were improved greatly. As a result, the addition of surge arrestor can help to protect the system.
• Sensitivity Analysis:Single-phase simulation
• The characteristics of VCB and phenomena of transient overvoltage are discussed. A VCB model is built and implemented in the simulation software. A single-phase and three-phase testing circuit is developed to perform the simulation with different parameters and observe the phenomena of multiple reignitions and voltage escalations of VCB.
• The results of sensitivity analysis shows that the parameters of arcing time and the recovery of dielectric strength play a significant role for the occurrence of multiple reignitions and overvoltage. The quenching capability of HF current has less effect. Consequently, when estimating the overvoltage phenomena of VCB, it is suitable to consider the recovery of dielectric strength in the middle range with smaller arcing time to look for the worst situation so that the appropriate protection scheme can be provided.
• The developed VCB model is implemented in an application network which involves switching of a transformer. The phenomena of virtual current chopping is observed in the process of simulation. One possible way of reducing the problem of virtual current chopping is the addition of the RC surge suppressor or surge arrestor. This can reduce the chance of equipment failure and hence protect the system.
Practical circuit simulation
Three-phase simulation
Case 1
Case 2
Case 1
Case 2
