Phase angle difference between two electromotive forces

1. What are the main differences between the changes of electrical quantities during system oscillation and short circuit?

1) In the process of oscillation, the electrical quantity determined by the phase angle difference between the electromotive

forces of generators in parallel operation is balanced, while the electrical quantity in short circuit is abrupt.

2) In the process of oscillation, the angle between voltages at any point on the power grid changes with the difference of

phase angle between system electromotive forces, while the angle between current and voltage is basically unchanged

during short circuit.

3) In the process of oscillation, the system is symmetrical, so there are only positive sequence components in electrical

quantities,and negative sequence or zero sequence components will inevitably appear in electrical quantities during

short circuit.

2. What is the principle of the oscillation blocking device widely used in the distance protection device at present?

What kinds are there?

It is formed according to the speed of current change during system oscillation and fault and the difference of each

sequence component. Commonly used are oscillation blocking devices composed of negative sequence components

or fractional sequence increments.

3. What is the distribution of zero sequence current related to when a short circuit occurs in a neutral directly grounded system?

The distribution of zero sequence current is only related to the zero sequence reactance of the system. The size of zero

reactance depends on the capacity of grounding transformer in the system, the number and position of neutral point

grounding. When the number of transformer neutral point grounding is increased or decreased, the zero sequence

reactance network of the system will change, thus changing the distribution of zero sequence current.

4. What are the components of HF channel?

It is composed of high frequency transceiver, high frequency cable, high frequency wave trap, combined filter, coupling

capacitor,transmission line and earth.

5. What is the working principle of phase difference high-frequency protection?

Directly compare the current phase on both sides of the protected line. If the positive direction of current on each side

is specified to flow from the bus to the line, the phase difference of current on both sides is 180 degrees under normal

and external short circuit faults.In case of internal short circuit fault, if the phase difference between the electromotive

force vectors at both ends suddenly occurs, the phase difference of the current at both ends is zero. Therefore, the phase

relationship of the power frequency current is transmitted to the opposite side by using high-frequency signals. The

protection devices installed on both sides of the line act according to the received high-frequency signals representing

the current phase of both sides when the phase angle is zero, so that the circuit breakers on both sides trip at the same

time, So as to achieve the purpose of fast fault removal.

6. What is gas protection?

When the transformer fails, due to heating or arc burning at the short-circuit point, the transformer oil volume expands,

pressure is generated, and gas is generated or decomposed, resulting in the oil flow rushing to the conservator, the oil level

drops, and the gas relay contacts are connected, which acts on the circuit breaker tripping. This protection is called gas protection.

7. What are the scope of gas protection?

1) Polyphase short circuit fault in transformer

2) Turn to turn short circuit, turn to turn short circuit with iron core or external short circuit

3) . Core failure

4) Oil level drops or leaks

5) Poor contact of tap switch or poor wire welding

8. What is the difference between transformer differential protection and gas protection?

The transformer differential protection is designed according to the principle of circulating current method, while the

gas protection is set according to the characteristics of oil and gas flow caused by internal faults of the transformer.

Their principles are different,and the scope of protection is also different. Differential protection is the main protection

of transformer and its system, and the outgoing line is also the scope of differential protection. Gas protection is the main

protection in case of internal fault of transformer.

9. What is the function of reclosing?

1) In case of temporary failure of the line, the power supply shall be recovered quickly to improve the reliability of power supply.

2) For high-voltage transmission lines with bilateral power supply, the stability of parallel operation of the system can

be improved,thus improving the transmission capacity of the line.

3) It can correct the false tripping caused by poor circuit breaker mechanism or relay misoperation.

10. What requirements should reclosing devices meet?

1) Fast action and automatic phase selection

2) Any multiple coincidence is not allowed

3) Automatic reset after action

4) . Manual tripping or manual closing shall not reclose in case of fault line

11. How does the integrated reclosing operate?

Single phase fault, single-phase reclosing, three-phase tripping after reclosing permanent fault; Phase to phase fault

trips three phases,and three phases overlap.

12. How does three-phase reclosing operate?

Any type of fault trips three phases, three-phase reclosing, and permanent fault trips three phases.

 
13. How does single-phase reclosing operate?

Single phase fault, single phase coincidence; Phase to phase fault, non coincidence after three-phase tripping.

 
14. What inspection work should be carried out for the voltage transformer newly put into operation or overhauled

when it is connected to the system voltage?

Measure phase to phase voltage, zero sequence voltage, voltage of each secondary winding, check phase sequence

and phase determination

15. What circuits should the protective device withstand the power frequency test voltage of 1500V?

110V or 220V DC circuit to ground.

16. What circuits should the protective device withstand the power frequency test voltage of 2000V?

1) . Primary to ground circuit of AC voltage transformer of the device;

2) . Primary to ground circuit of AC current transformer of the device;

3) Backplane line to ground circuit of device (or screen);

17. What circuits should the protective device withstand the power frequency test voltage of 1000V?

Each pair of contact to ground circuit working in 110V or 220V DC circuit; Between each pair of contacts, and

between the dynamic and static ends of contacts.

18. What circuits should the protection device withstand power frequency test voltage of 500V?

1) DC logic circuit to ground circuit;

2) DC logic circuit to high-voltage circuit;

3) 18~24V circuit to ground with rated voltage;

19. Briefly describe the structure of electromagnetic intermediate relay?

It is composed of electromagnet, coil, armature, contact, spring, etc.

20. Briefly describe the structure of DX signal relay?

It is composed of electromagnet, coil, armature, dynamic and static contact, signal board, etc.

21. What are the basic tasks of relay protection devices?

When the power system fails, some electrical automatic devices are used to quickly remove the fault part from

the power system.When abnormal conditions occur, signals are sent in time to narrow the fault range, reduce

the fault loss and ensure the safe operation of the system.

22. What is distance protection?

It is a protection device that reflects the electrical distance from the installation of protection to the fault point

and determines the action time according to the distance.

23. What is high-frequency protection?

One phase transmission line is used as the high-frequency channel to transmit high-frequency current, and two

half sets of protection of power frequency electrical quantities (such as current phase, power direction) or other

quantities reflected at both ends of the line are connected as the main protection of the line without reflecting the

external fault of the line.

24. What are the advantages and disadvantages of distance protection?

The advantage is high sensitivity, which can ensure that the fault line can selectively remove the fault in a relatively

short time, and is not affected by the system operation mode and fault form. Its disadvantage is that when the

protection suddenly loses the AC voltage,it will cause the protection to malfunction. Because impedance protection

acts when the measured impedance value is equal to or less than the set impedance value. If the voltage suddenly

disappears, the protection will act wrongly. Therefore, corresponding measures should be taken.

25. What is high-frequency locking directional protection?

The basic principle of high-frequency blocking directional protection is based on comparing the power directions on

both sides of the protected line. When the short circuit power on both sides flows from the bus to the line, the protection

will act to trip. Since the high-frequency channel has no current normally, and when an external fault occurs, the side

with negative power direction sends high-frequency blocking signals to block the protection on both sides, it is called

high-frequency blocking directional protection.

26. What is high-frequency blocking distance protection?

High frequency protection is the protection to realize the quick action of the whole line, but it cannot be used as the

backup protection of bus and adjacent lines. Although distance protection can play a role of backup protection for bus

and adjacent lines, it can only be quickly removed when faults occur within about 80% of the lines. High frequency

blocking distance protection combines high frequency protection with impedance protection. In case of internal fault,

the whole line can be quickly cut off, and the backup protection function can be played in case of bus and adjacent line fault.

27. What are the protective pressing plates that should be removed during the regular inspection of relay protection

devices in our factory?

(1) Failure startup pressing plate;

(2) Low impedance protection of generator transformer unit;

(3) Zero sequence current protection strap at high voltage side of main transformer;

28. When PT breaks, which corresponding protective devices should be exited?

(1) AVR device;

(2) Standby power automatic switching device;

(3) Loss of excitation protection;

(4) Stator interturn protection;

(5) Low impedance protection;

(6) Low voltage lockout overcurrent;

(7) Low voltage of bus;

(8) Distance protection;

29. Which protection actions of SWTA will trip the 41MK switch?

(1) OXP overexcitation protection three section action;

(2) 1.2 times V/HZ delay for 6 seconds;

(3) 1.1 times of V/HZ delay for 55 seconds;

(4) ICL instantaneous current limiter operates in three sections;

30. What is the function of the inrush current blocking element of the differential protection of the main transformer?

In addition to the function of preventing maloperation of transformer under inrush current, it can also prevent maloperation

caused by current transformer saturation in case of faults outside the protection area.