1. What are the current methods in differential protection to prevent the effects of excitation inrush current?

Answer: The method to prevent inrush current effects is to use a differential relay with a quickly saturating iron core.

2. What protections should be installed for 500kV shunt reactors and their functions?

Answer: High-voltage shunt reactors should be equipped with the following protection devices:

(1) High-impedance differential protection. Protects against phase-to-phase and ground faults in reactor windings and bushings.

(2) Turn-to-turn protection. Protects against inter-turn short-circuit faults in the reactor.

(3) Gas protection and temperature protection. Protects against various internal faults in the reactor, oil level reduction, and temperature rise.

(4) Overcurrent protection. Overcurrent caused by phase-to-phase or ground faults in the reactor and leads.

(5) Overload protection. Protects the reactor windings from overload.

(6) Neutral point overcurrent protection. Protects against overcurrent in the neutral point small reactor caused by external ground faults of the reactor.

(7) Neutral point small reactor gas protection and temperature protection. Protects against various internal faults, oil level reduction, and temperature rise in the small reactor.

3. Discuss the characteristics of medium-impedance fast busbar protection.

Answer: Fast busbar protection is a medium-impedance busbar differential protection with restraining characteristics. Its selection element is a medium-impedance current differential relay with ratio restraint characteristics, which solves the problem of maloperation caused by current transformer saturation during external faults. The protection device is based on instantaneous current measurement and comparison. During internal busbar faults, the starting element and selection element of the protection device can operate before the current transformer saturates, thus achieving very fast operation speed. The characteristics of medium-impedance fast busbar protection are:

(1) For parallel operation of double busbars, when a fault occurs on one busbar, the protection device maintains high selectivity under all circumstances.

(2) For parallel operation of double busbars, if faults occur successively on both busbars, the protection device can sequentially trip all connected elements on both busbars.

(3) For internal bus faults, the complete operation time of the protection device shall not exceed 10ms.

(4) During normal switching operations of double buses, the protection device operates reliably.

(5) In the event of an internal bus fault during double bus switching operations: If two switches of one circuit simultaneously bridge both buses and a bus fault occurs, the protection device can rapidly disconnect all connected elements on both buses. If two switches of one circuit do not simultaneously bridge both buses and a bus fault occurs, the protection device still maintains high selectivity.

(6) For external bus faults, regardless of whether the current transformers on the line are saturated or not, the protection device reliably avoids incorrect operation.

(7) During normal operation or switching operations, if a break occurs in the AC current circuit of the bus protection, the protection device will block the entire protection system after a set delay and issue an alarm signal for the AC current circuit break.

(8) In substations where similar circuit breakers are used or the tripping time difference of circuit breakers is minimal, the protection device ensures that the bus coupler circuit breaker trips first in the event of a bus fault.

(9) For faults occurring between the current transformer of the bus coupler switch and the bus coupler switch itself, the bus protection and switch failure protection will sequentially trip all connected elements on both busbars.

(10) On 500kV busbars, when transient-type current transformers are used and the busbar configuration involves double bus coupler switches, the initiating elements may operate without restraint characteristics. On 220kV busbars, to prevent maloperation of protection during double bus coupler switch configurations, both initiating elements and selection elements are equipped with ratio restraint characteristics.

4. Why is a voltage blocking element used in busbar current differential protection?

Answer: To prevent maloperation of the differential relay or accidental triggering of the output intermediate relay from causing incorrect operation of the busbar protection, a voltage blocking element is employed.

5. How is the voltage blocking element implemented?

Answer: The voltage blocking element is implemented using low-voltage relays and zero-sequence overvoltage relays connected to the secondary side of voltage transformers on each bus. Three low-voltage relays respond to various phase-to-phase short-circuit faults, while the zero-sequence overvoltage relay detects various ground faults.

6. Why is bus charging protection set up?

Answer: The bus differential protection should ensure rapid and selective disconnection of the faulty bus when a bus or a bus section is energized. To more reliably clear faults on the energized bus, current or zero-sequence current protection is installed on the bus coupler or bus section switches as bus charging protection.

The wiring for bus charging protection is simple, and its settings can ensure high sensitivity. Where conditions permit, this protection can serve as a temporary protection specifically for a new line being charged by a dedicated bus.

Bus charging protection is only activated during bus charging and should be promptly deactivated once the charging is confirmed to be successful.

7. What are the "Two Tickets and Three Systems"?

Answer: The two tickets refer to the work ticket and the operation ticket;

The three systems refer to the shift handover system, patrol inspection system, and equipment periodic testing and rotation system.

8. How many states do the equipment in the power system have?

Answer: The equipment states in the power system are generally categorized into four types: operation, hot standby, cold standby, and maintenance.

9. What are the operational regulations for the power system?

Answer: Power system duty dispatchers should comply with the following regulations during operations:

Operation order ticket system, command repetition system, supervision system, and voice recording system.

10. What are the forms of dispatching operation instructions?

Answer: The forms of dispatching operation instructions include: single instruction, step-by-step instruction, and comprehensive instruction.

11. What conditions should be met for grid closed-loop operation?

Answer: (1) The phases must be consistent. For the first time closing the loop or when maintenance may cause phase changes, it must be verified through measurement that the phases on both sides of the loop-closing point are consistent.

(2) If it is an electromagnetic loop network, the difference in transformer connection groups within the loop should be zero. Under special circumstances, after verification through calculations ensures that relay protection will not malfunction and that related loop equipment will not be overloaded, a transformer connection difference of 30 degrees is permitted for loop closing operations.

(3) No component within the looped network should be overloaded after closing the loop.

(4) The voltage at each busbar must not exceed the specified limit.

(5) Protection relays and safety automation devices must be compatible with the looped operation mode.

(6) Stability must meet the prescribed requirements.

12. Under what circumstances can the accident-involved unit take immediate action without awaiting dispatch instructions before reporting the incident?

Answer: In the event of the following accident situations, the accident unit may take immediate action without waiting for dispatch instructions and report afterwards:

(1) When there is a threat to personnel or equipment, take measures according to on-site procedures.

(2) When the auxiliary power of a power plant or substation is partially or completely interrupted, restore it using other power sources.

(3) When a system accident causes frequency reduction, each power plant should increase unit output and bring standby generating units online to connect to the system.

(4) If the system frequency drops to the action value set for under-frequency load shedding or low-frequency disconnection devices, and these devices fail to operate, manually trip the switches they should have opened after confirming no error.

(5) Incidents specified in dispatching regulations and on-site procedures that can be handled without awaiting instructions from the duty dispatcher.