1. What is a microcomputer protection device?

Answer: A microcomputer protection device is an automatic device that can reflect the faults or abnormal working status of various electrical equipment in the power system, and act on circuit breakers to trip or send signals.

2. What are the basic tasks of microcomputer protection?

Answer: (1) It can automatically, quickly, and selectively use circuit breakers to disconnect faulty equipment from the system, ensuring that no faulty equipment quickly resumes normal operation and preventing further damage to the faulty equipment;

(2) It can reflect the abnormal working status of electrical equipment and act on signals or cut off equipment that will cause damage or develop into faults if it continues to operate according to the conditions of operation and maintenance. Relay protection that responds to abnormal states usually does not require immediate action and can be delayed to a certain extent.

3. What are the basic requirements for microcomputer protection?

Answer: Microcomputer protection plays an extremely important role in the safe, stable, and reliable operation of power systems, and timely cutting off faults. Therefore, relay protection should meet the following requirements:

(1) Selectivity: When a system failure occurs, the protection device is required to only disconnect the faulty equipment to ensure that the equipment without faults continues to operate, thereby minimizing the scope of power outages and achieving the goal of selective action.

(2) Swiftness: In the event of a system failure, if the fault cannot be quickly removed, it may escalate. For example, during a short circuit, the voltage drops significantly, causing the user's motor near the short circuit point to slow down or stop due to braking, and disrupting the user's normal production; On the other hand, when a short circuit occurs, the generator cannot deliver power, which can cause stability damage to the power system. Furthermore, during a short circuit, the faulty equipment itself will experience a large short-circuit current, and due to the effects of electric force and thermal effects, the equipment will also be severely damaged. The longer the short-circuit current passes through, the more severe the equipment damage. So after a power system failure, the relay protection device should act as quickly as possible and cut off the fault.

(3) Sensitivity: The protective device should be able to accurately respond to faults and abnormal working conditions that occur within its protection range. That is to say, the protective device can not only operate sensitively in the maximum operating mode when there is a three-phase metallic short circuit, but also have sufficient sensitivity and reliable operation in the minimum operating mode and when there is a two-phase short circuit with a large transition resistance.

(4) Reliability: The reliability of a protective device is very important, which means that when a fault occurs within its protection range, it should not refuse to operate due to its own defects, and should not malfunction in any situation that does not belong to its operation. Otherwise, the use of unreliable protective devices can become the root cause of expanding accidents and directly causing accidents.

4. What are the microelectromechanical protections for transformers? What are the functions of various microcomputer protections?

Answer: Transformers are one of the important equipment in the power system, and their faults will have a serious impact on the reliability of power supply and the normal operation of the system. At the same time, large capacity transformers are also very valuable equipment. Therefore, it is necessary to install high-performance and reliable protective devices according to the capacity and importance of transformers.

Transformer faults can be divided into internal faults in the oil tank and external faults.

The main internal faults of the fuel tank are:

Phase to phase short circuit: single-phase turn to turn short circuit; Short circuit faults such as single-phase grounding short circuits will generate arcs, which can burn out coils, their insulation, and iron cores, and even cause strong vaporization of transformer oil insulation materials, leading to serious consequences such as oil tank explosions.

External faults of the fuel tank include:

Phase to phase short circuit between insulation sleeve and lead wire; Single phase grounding short circuit, etc.

The abnormal operation modes of transformers include:

Overcurrent caused by external short circuit; Overload caused by various reasons; The oil level inside the fuel tank has decreased.

According to the type of fault and abnormal operation mode of the transformer, the following protective devices should be installed:

(1) Gas protection for internal short circuits and oil level reduction in transformer oil tanks.

(2) Longitudinal differential protection or current quick break protection for multi-phase short circuits in transformer windings and lead out lines, grounding short circuits in high grounding current grid side windings and lead out lines, and inter turn short circuits in windings.

(3) Provide backup overcurrent protection (or composite voltage starting overcurrent protection or negative sequence current protection) for external phase to phase short circuits, including gas protection and longitudinal differential protection (or current quick break protection).

(4) Zero sequence current protection for external grounding short circuits in high grounding current power grids.

(5) Overload protection for symmetrical overload, etc.

5. What are the protections for assembling a 600MW generator transformer?

Answer: (1) Differential protection for generator transformer units; (2) Generator longitudinal differential protection; (3) Main transformer differential protection; (4) Generator demagnetization protection; (5) Generator out of step protection; (6) Generator reverse power protection; (7) Generator low-frequency protection; (8) Overexcitation protection; (9) Generator stator grounding protection; (10) Generator overcurrent protection; (11) Generator inverse time negative sequence overcurrent protection; (12) Generator stator overload protection; (13) Generator water cut-off protection; (14) Main transformer neutral point zero sequence current protection; (15) Main transformer gas protection; (16) Main transformer pressure release protection.

6. What are the differences in the functions of main transformer differential and gas protection? Can both types of protection be reflected when there is an internal fault in the transformer?

Answer: Differential protection is the main protection for transformers; Gas protection is the main protection for internal faults in transformers.

The protection range of differential protection is the primary electrical part between the differential transformers on both sides of the main transformer, including:

(1) Multiphase short circuit occurs in the main transformer lead and transformer coil;

(2) Severe single-phase turn to turn short circuit;

(3) Grounding fault on coil lead wire in high current grounding system.

The gas protection range is:

(1) Multiphase short circuit inside the transformer;

(2) Interturn short circuit, short circuit between turns and iron core or outer skin;

(3) Iron core failure (heating and burning);

(4) Oil level drops or leaks;

(5) Poor contact of tap changer or poor welding of wires.

Differential protection is installed on transformers, generators, segmented busbars, and power lines, while gas protection is a unique protection for transformers.

When there is an internal fault in the transformer (except for minor inter turn short circuits), both differential and gas are reflected, because when there is an internal fault in the transformer, the speed of the oil and the increase in the reflected primary current may activate both types of protection. As for which protection is activated first, it also depends on the nature of the fault.

7. What types of faults are protected against neutral point zero sequence overcurrent, gap overcurrent, and zero sequence overvoltage in the main transformer? What is the principle of protection setting?

Answer: The neutral point zero sequence overcurrent, gap overcurrent, and zero sequence overvoltage of the main transformer are used to protect the grounding short circuit fault on the lead out line of the equipment itself. They are generally used as backup protection for grounding faults in the 110-220 kV system on the high-voltage side of the transformer. Zero sequence current protection is the zero sequence protection during neutral grounding operation of the transformer; And zero sequence voltage protection is the zero sequence protection when the neutral point of the transformer is not grounded during operation; Gap overcurrent is used in the operation mode where the neutral point of the transformer is grounded through the discharge gap.

Zero sequence overcurrent protection has a small starting current, usually around 100 amperes, for about 0.2 seconds. Zero sequence overvoltage protection is set to twice the rated phase voltage based on experience. To avoid transient overvoltage caused by single-phase grounding, the time is usually set to 0.1-0.2 seconds. The length of the discharge gap of the neutral point on the 220KV side of the transformer is generally 325 millimeters, and the effective value of the breakdown voltage is 127.3 kilovolts. When the voltage at the neutral point exceeds the breakdown voltage, the gap is broken down and the zero sequence current passes through the neutral point. The protection time is set to 0.2 seconds.

8. What are primary protection and backup protection?

Answer: Main protection refers to the ability to meet system stability, equipment safety, and basic requirements in the event of a short circuit fault. It first acts on tripping and selectively cuts off the protection of the protected equipment and the entire line fault.

Backup protection refers to the protection used to cut off faults when the main protection or circuit breaker refuses to operate.

9. What is the function of forced excitation of the generator?

Answer: (1) Increase the stability of the power system.

(2) After the short circuit is cut off, it can quickly restore the voltage.

(3) Improve the reliability of overcurrent protection actions with time limits.

(4) Improve the self starting conditions of the electric motor during system accidents.

XZTJB-1200 6 Phase Protection Relay Tester