September 25, 2020

Relay Protection [Technology Interview] Frequently Asked Questions

1. What is the task of relay protection in the power system?

When a protected power system component fails, the relay protection device of the component should promptly and accurately issue a trip command to the circuit breaker that has been removed from the faulty component, so that the fault component can be disconnected from the power system in time to maximize the Reduce the damage to the power system components themselves, reduce the impact on the safe power supply to the power system, and meet certain specific requirements of the power system (such as maintaining the transient stability of the power system, etc.).

Responsive to the abnormal working condition of the electrical equipment, and send out signals according to the abnormal working conditions and equipment operation and maintenance conditions (for example, whether there are frequent duty personnel), so that the personnel on duty can carry out the treatment, or the equipment can automatically adjust, or those Continue to operate the electrical equipment that caused the accident to be removed. Relay protection devices that respond abnormally to the operating conditions are allowed to carry a certain time delay action.

2. What are the basic requirements for relay protection devices? What are the contents of each request?

Relay protection devices must meet four basic requirements: selectivity, quickness, agility and reliability.

Selectivity is that when the system fails, the relay protection device should have the option to cut off the fault to ensure that the non-faulty part continues to operate and the power outage range is reduced as much as possible.

Snap action means that the relay protection should act on the tripping of the circuit breaker at the fastest possible speed to break the fault or halt the development of abnormal conditions.

Sensitivity refers to the relay protection device's ability to reflect faults within its scope of protection, that is, the relay protection device should be able to sensitively experience and reflect the possible faults and abnormal operation modes of the protected equipment.

Reliability means that when a fault that belongs to it should occur, it can operate reliably, that is, no rejection action will occur; and in any other case that does not belong to it, reliability does not act, ie no misoperation occurs.

3. What is primary protection and what is backup protection?

Main protection: To meet system stability and equipment safety requirements, the protection of the protected equipment and line faults can be selectively cut off at the fastest speed.

Back-up protection: When the main protection or circuit breaker refuses to move, the protection to cut off the fault, including near backup and far backup.

Far backup protection: backup protection that is achieved by the protection of adjacent power equipment or lines when the main protection or circuit breaker is refused to move.

Near backup protection: when the main protection is refused to move, backup protection is realized by another protection of the power equipment or the line; when the circuit breaker refuses to move, backup protection is implemented by breaker failure protection.

4. What is auxiliary protection and what is abnormal protection?

Supplementary protection To compensate for the lack of primary protection and backup protection performance, or for simple protection that increases when the primary and backup protections are removed from service, this protection is a secondary protective relay.

Abnormal operation protection: Responsive protection against abnormal operating conditions of protected power equipment or lines.

5. What is the breaker failure protection?

It means that when the circuit breaker refuses to act after the protection line of the faulty line has issued a tripping pulse, it can cut off the relevant power plant or other related circuit breakers within the substation with a short time limit to minimize the outage protection range.

6. Why is 220KV and above systems equipped with breaker failure protection and what is its role?

220KV and above transmission lines generally have large transmission power and long transmission distance. In order to improve the transmission capacity of the line and the stability of the system, phase-breaking circuit breakers and rapid protection are often used.

Because the circuit breaker has the possibility of operation failure, when the circuit fails and the circuit breaker refuses to move, it will pose a great threat to the power grid. Therefore, the circuit breaker failure protection device should be installed, and the circuit breaker that will selectively reject the failure will be opened. The circuit breaker of the busbar connected to the device is disconnected to reduce equipment damage, reduce the power outage range, and improve safety and stability.

7. What is the oscillation of the power system? What causes the oscillation?

The phenomenon of loss of synchronization between two systems or power plants operating in parallel is called oscillation.

There are many causes of oscillation, most of which are due to the long time of the cut-off failure and cause the system to be destroyed. In weakly connected systems, it may also be caused by misoperation, loss of magnetization of the generator or trip of the fault, disconnection of a line or equipment. Causes oscillations.

8. What is the difference between power system oscillation and short circuit?

When the system is oscillating, both the voltage and current values ​​of the system are oscillating back and forth, and the short-circuit current and voltage are abrupt. During the oscillation, the current and voltage values ​​change slowly, while the short-circuit current and voltage values ​​suddenly change.

The phase angle between voltage and current at any point in the system changes with the change of the power angle; when the short circuit occurs, the phase angle between the current and voltage is basically constant.

9. What is the impact on relay protection devices when the power system oscillates? Which protective devices are not affected?

Mainly affect current relays and impedance relays. The protection that is not affected by the oscillation in principle is differential protection.

10. What is the maximum and minimum operating mode of the system?

In the setting calculation of relay protection, the maximum and minimum operating modes of the power system are generally considered.

The maximum operating mode means that the system's equivalent impedance is the shortest at the end of the protected object, and the short-circuit current through the protection device is the maximum operating mode.

The minimum operating mode refers to the operating mode in which the equivalent impedance of the system is the largest under the same short-circuit condition as described above, and the short-circuit current through the protection device is the minimum.

11. What are the basic requirements for distance protection?

When a direct short circuit occurs on the protected circuit, the measured impedance of the relay should be proportional to the distance between the bus and the short circuit point;

In the short circuit outside the positive direction should not exceed the action;

Should have clear directionality;

It should still be able to operate in the region due to a short-circuit resistance with a large transition;

Does not operate at the minimum load impedance;

It can prevent malfunction when the system oscillates.

12. What is the polarization of the distance relay?

In addition to the distance measurement voltage, when constructing the distance relay's amount of action, another specific AC quantity must also be passed as a reference quantity to detect Uph phase switching when a fault occurs outside the protection zone. This reference amount of communication is generally referred to as the polarization of the distance relay.

13, what is the impedance direction relay?

The impedance direction relay means that it not only measures the size of the impedance, but also can judge the direction of the fault.

14. Why are some distance protected sections I and II required to be subjected to oscillatory locking devices, while paragraph III is not subjected to oscillatory blocking devices?

The oscillation period of the system is generally 0.15 to 1.5 seconds. The contact point of a certain impedance relay affected by the system oscillation will close once and return once in one cycle. If the closed time is longer than the action time of the distance protection device in the segment, the segment will Due to system oscillation and misoperation, the operation time of the distance protection I and II is generally short and does not exceed the oscillation period. Therefore, the oscillation blocking device is required, and the operation time of the distance III is generally greater than the oscillation period, so no Oscillation blocking device.

15. How many ways to start reclosing?

Two types: The circuit breaker control switch and the circuit breaker position do not correspond to the start mode and the protection start mode.

16, what is the acceleration after reclosing?

When the line fails, the protection selectively operates to cut off the fault and the recloser conducts a coincidence to restore the power supply. If the fault coincides with a permanent fault, the protection device opens the circuit breaker without time delay and no selective action. The method is called post-acceleration acceleration.

17. Why does the overhead line install an automatic reclosing device and the cable line does not have a reclosing device?

Automatic reclosing is designed to avoid transient failures causing line blackouts. For overhead lines, there are many faults that are momentary faults (such as bird damage, lightning strikes, and pollution). These faults, when they occur in the worst case scenario, trip open circuit. After the device disappears, the effect of reclosing is very significant.

As the cable lines are buried underground, the faults are mostly permanent and the effect of reclosing is not significant, so automatic reclosing devices are not installed.

18. What is the overcurrent protection of composite voltage starting?

The over-current protection of the composite voltage starter is based on the over-current protection. The protection consists of a composite voltage starting component consisting of a negative sequence voltage relay and a low-voltage relay connected to the phase-to-phase voltage. Only when the current measuring element and the voltage starting element are in motion can the protective device act on the tripping.

19. What are the advantages and disadvantages of the gas transformer protection?

Advantages: simple structure, rapid operation, high sensitivity, can reflect the short circuit and interturn short circuit in the transformer oil tank when the number of turns is small, the fault circuit current is very large, may cause severe overheating, but the external phase of the lead line current The change may be very small, and the protection of various reactive currents is difficult to operate. Gas protection has special advantages for removing such faults.

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