The Analysis and Discussion in the Effective Application of the Dispatcher Training Based on Case Teaching Method with the Cause from the Action of the Gap Protection of Main Transformer

The combination of theory and practice is a difficult problem on dispatcher training. Through a typical example of case, this paper provides an effective case teaching method for dispatcher training, and combines the theoretical discussion of the rule of experience with cases and achieves vividness. It helps students to understand and catch the key points of the theory, and improve their practical skills.


Introduction
During the dispatcher teaching process of State Grid Corporation, the practical skills is one of the most important parts. However, the improvement of practical skills is based on the understanding of theorical knowledge, how to combine the theory and practice is a difficult problem on dispatcher training. Through a typical example of case, this paper makes a deep mining and analysis of the key points of dispatcher training, correlates these points closely, puts them into application, and improves the training effect.

Operation Mode of QingYuan Substation before Accident
No.1 & No.2transformers of QingYuan Substation are operating in parallel. For No.1 transformer, its 110kV and 220kV neutral points are both direct grounding, and zero sequence protection is operating. For No.2 transformer, its 110kV and 220kV neutral points are both ungrounding and its gap protection is operating.

Formatting author names Accident Phenomenon
According to Figure 1 & 2, the key alarm information of QingYuan Substation is as follows.
The protection of No.1156 circuit breaker on QingYun2 Line 1 acts, but the circuit breaker does not open.
The protection of 110kV bus connection circuit breaker in QingYuan Substation acts, the circuit breaker opens correctly, and its 110kV Bus Line 2 loses voltage. The protection of all the three sides of the No.2 transformer acts, and the circuit breakers all open correctly.
The 10kV automatic connection device of QingYuan Substation acts correctly, and there is no load loss.

3.1Difference Between Zero Sequence Protection and Gap Protection
Both of the zero sequence protection and gap protection are the backup protection of transformer. As for a system with neutral point direct grounding, while there are zero sequence current on the primary side, there would be fault current induced on the secondary side. If both the peak value and duration of the fault current reach the setting value, the zero sequence protection would act. For gap protection, it acts while the discharging gap of neutral point is breakdown and there is fault current on the primary side of the current transformer on the gap.
The logic of the zero sequence protection is as follows. The bus connected breaker opens while the protection section 1 acts, the circuit breaker of this side of the transformer opens after a delay time while the protection section 2 acts, the circuit breakers of all the three sides open while the protection section 3 acts. For gap protection, the circuit breakers of all the three sides while it acts.

3.2Whether the Zero Sequence Protection and Gap Protection Could Be Operating at the Same Time?
What Is the Harm if Both of Them Use the Same Current Transformer? Answer for Q1.These two protections could not be operating at the same time. Hence they could not use the same current transformer. If their current transformers are separated, they could be set independently, and there is no need of manual operation. While the neutral point is grounding, the zero sequence protection is operating and the gap protection is out of service, while the neutral point is ungrounding, on the opposite, the gap protection is operating and the zero sequence protection is out of service, automatically and safely.
Answer for Q2. There are some disadvantage if they use the same current transformer. While the neutral point is grounding, once the gap protection is not out of service, and unfortunately there is a grounding fault of system, the gap protection would misoperate and request the the current breakers of the transformer to open. Even through the setting value of the gap protection is quite small, the overcurrent of grounding fault is always too big to damage the devices.

3.3The Instruction of the Protection of Transformer
Through the instruction of protection of transformer, students could have a better understanding of the case. The setting values of the protection is as shown in Fig.4. The direction from HV-side backup protection to the transformer is the positive direction, the direction from MV-side and LV-side backup protection to the bus line is the positive direction.

Why the Hybrid Voltage & Current Blocking Protection Does not Act?
Consider the direction of impedance of the MV-side current transformer. If the direction to 110kV is positive, and if the backup protection of MV-side of the transformer is voltage blocking, and the setting value is appropriated, the hybrid voltage & current blocking protection would act. In another word, the action condition of the protection is positive direction, setting value of current, and operating time. Similarly, the tripping operation matrix could be set. According to the analysis of the setting value of protection, the operating time of the hybrid voltage current blocking protection is 2.2 seconds, and the setting value of zero sequence protection is 1 second. Hence this protection does not act, and the zero sequence protection acts.

3.5The consideration of factors of grounding natural point of transformer
Ensure the sensibility and selectivity of zero sequence protection is enough. And ensure the stability of grounding current.
In order to prevent the overvoltage to damage the devices, make sure while splitting the system, which is caused by operation or tripping, the neutral point of the system keeps grounding.
The insulation ability and instruction of the transformer determine the operating mode of the neutral point. For example, the neutral point of the self-coupling transformer.  grounding with small resistance, after the section 1 of the zero sequence protection starts, the gap protection would be delayed. In actual operation, if they do not use the same current transformer, the arrester may be breakdown before the discharging gap, and the system turns to a grounding one, and the 110kV zero sequence protection operates, and the current breakers of all the three sides of No.2 transformer open after the secondary time limit.

Conclusion
Most of the students feel confused about the case of the gap protection of main transformer, and may even feel difficult to analyze the action logic and steps. The reason of this is as follows.
1. The abstract theories are too difficult to understand, and students feel unclear about the setting values of the protection, the difference between the protections, the preference of the protections.
2. The knowledges are difficult to be connected efficiently. In the process of the analysis of the case, it is difficult to combine the theories and the phenomenon together and make any analysis.
The case teaching method is a great method to integrate the theories with practice. In the process of dispatcher training, teachers should make full use of the typical cases and combine the key theorical knowledge and practice. it would improve the ability of analyzing and solving problem of the students.
In addition, if teachers could make the knowledge visualized by preparing some demonstration, it would help students to learn better and enhance the teaching effects.