Title of electric power paper

Power system relay protection

Principle of Power System Relay Protection is an important professional course of this major. This course describes the basic principles, concepts and methods of power system relay protection. Now the main content of relay protection course is reviewed by asking questions.

I. Basic concepts

1, why is it generally required to calculate the short-circuit current of power system with unit value system?

Answer: The average rated voltage of all levels is taken as the base value of all levels. The nominal voltage of all levels is 1, and the nominal value transformation ratio of transformer is 1. The nominal values of impedance, voltage and current of all voltage levels do not need to be reduced.

2. What is the principle, function and composition of power system relay protection?

Answer: Principle: It reflects the sudden change of some physical quantities before and after the failure of the protected line or equipment.

Function: automatically, quickly and selectively control the circuit breaker to eliminate faults, and send corresponding signals to abnormal operation.

Composition: It consists of measuring element, logic element and executing element.

3. What are the basic requirements of power system relay protection?

A: Four characteristics-selectivity, rapidity, sensitivity and reliability.

4. What is the zero sequence voltage of single-phase grounding in small grounding current system?

A: It is approximately equal to the phase voltage.

5. What is the starting current, return current and return coefficient of the relay?

Answer: Relay starting current: the minimum current that makes the relay just start.

Relay return current: the maximum current that can be returned after the relay is started.

Relay return coefficient: the ratio of relay return current to action current.

6. What is the principle, function and composition of current transformer?

A: Principle: electromagnetic induction principle.

Function: Change the rated high current to the low current of 5A or1a..

Composition: It consists of iron core, primary winding and secondary winding.

7. From the use point of view, what are the measures to reduce the error of current transformer?

Answer: The current error of the current transformer is caused by the excitation current of the current transformer.

Measures to reduce the error: (1) increase the cross section of the secondary conductor of the current transformer and reduce the resistance of the secondary load;

(2) Select a current transformer with large transformation ratio;

(3) Two current transformers of the same model are connected in series.

8. Briefly describe the protection characteristics and time limit characteristics of three-stage protection.

Answer: Section I (quick break)-generally protects a part of the protected line (generally 25% ~ 85% of the total length of the protected line), with zero second action.

The second section (speed limit)-protect the whole length of the line, generally 0.5s as the main protection of the line.

Section ⅲ-As a backup protection, its action time limit should be one time limit stage longer than the backup protection of the next line or equipment.

9. Why do I and II sections of current three-section protection adopt two-phase two-relay wiring, while section III (overcurrent protection) adopts two-phase three-relay or three-phase three-relay wiring?

Answer: Three-stage current protection is generally used in small grounding current systems. Its Class I and Class II are connected by two-phase double relays, so there is a 2/3 chance that only one loop will be cut off when two points of different outgoing lines and different phases are grounded. In the third section, two-phase three-relay or three-phase three-relay wiring is adopted to improve the sensitivity of remote backup Y δ- 1 1 wiring transformer to two-phase short circuit on the opposite side.

10, why does the power directional relay with directional current protection that reacts to phase-to-phase short circuit adopt 900 wiring?

Answer: There is no dead zone for two-phase short circuit.

1 1. Briefly describe the advantages of three-stage protection for zero-sequence directional current.

Answer: Reflect the sensitivity of grounding fault; The protection range is stable and is not affected by the system operation mode; The zero sequence ⅲ segment has less limited time series and short action delay; System oscillation will not fail; Zero sequence power direction relay has no dead zone; The wiring is simple and reliable.

12. What is the principle of distance protection? What are the advantages of distance three-section protection?

A: Principle: It reflects the sudden change of measuring impedance.

Advantages: the measured impedance changes greatly before and after the fault, which is nearly one order of magnitude larger than the current or voltage reflecting a single physical quantity; It is the comparison between the measured impedance and the set impedance, and the protection range is stable, which is generally not affected by the system operation mode; It is directional.

13, write the action impedance equation of directional impedance relay.

Answer:

14. Write the expression of moving edge condition of directional impedance relay.

A: Amplitude comparison:

Phase comparison:.

15. What wiring should be used for interphase short-circuit impedance relay?

Answer: Generally, wiring is adopted, and directional impedance relay can be connected.

16, write the expression of measuring impedance when the system oscillates.

Answer: To measure the oscillation impedance:

17。 Why should the third phase voltage be added to the rectifier relay that reacts to phase-to-phase short circuit?

Answer: In order to avoid two-phase short circuit in opposite directions, the memory voltage loses its directionality after it disappears.

18. What is the function of rectifying impedance relay plus memory voltage?

Answer: It is used to eliminate the dead zone of directional impedance relay, so as to protect the export short circuit and reverse short circuit from losing directionality.

19. Briefly describe the principle of phase difference high frequency protection.

Answer: The phase difference high frequency protection is the differential protection of the line. The phase of the current on both sides of the line is transmitted to the opposite side by high frequency carrier, and the power frequency current phase of the current on both sides is compared. In-phase break protection sends out tripping pulse, out-of-area fault, power frequency current inversion, locking protection.

20. What are the advantages and disadvantages of lateral differential current protection for parallel transmission lines?

Answer: Advantages: selectively and quickly control the circuit breaker to cut off the faulty line.

Disadvantages: there are continuous action zones and dead zones.

2 1. What protection should be installed for transmission lines with various voltage levels?

Answer: 220kV: Install two sets of high-frequency protection with different principles, three-section distance, three-section or four-section zero sequence direction and comprehensive reclosing;

1 10kV: install three sections of distance, three sections of zero sequence direction and three-phase primary reclosing;

35kV and 60kV: install three-section current, insulation monitoring and three-phase primary reclosing;

10kV: install two-stage current, insulation monitoring and three-phase primary reclosing.

22, what are the requirements for three-phase automatic reclosing at a time?

Answer: (1) The protection outlet action trips the circuit breaker, and the circuit breaker can be reclosed after a delay of 0.5 ~ 1s; The amount of overlap should be determined.

(2) Manual jump or remote jump cannot overlap.

(3) The closing condition of the circuit breaker is not met.

(4) Bilateral power supply shall meet the conditions of coincidence in the same period.

(5) reclosing should be able to cooperate with protection.

23, what is the transformer inrush current? What are its characteristics?

Answer: Excitation inrush current: the excitation current when the voltage is restored after the transformer is closed with no load and the external fault is removed.

Features: the fundamental component of the maximum inrush current is 6 ~ 8 times of the rated current of the transformer, and decays to 0.5 ~ 1 times of the rated current after 0.5 ~/s, which contains higher harmonics and DC components, and the discontinuity angle is 600 ~ 840.

24. What are the conditions for installing special differential protection on the bus?

Answer: Considering the stability of the power system, the bus fault of the hub substation may cause system oscillation; Double buses or single buses run in parallel, and selective bus differential protection is needed when the bus fault is removed.

25, 10MVA capacity transformer generally should be equipped with what protection.

Answer: main protection: transformer differential protection and gas protection;

Backup protection: phase-to-phase fault overcurrent protection (low-voltage lockout overcurrent protection and compound voltage lockout overcurrent protection), zero-sequence overcurrent protection for ground fault and transformer overload protection.

Second, drawing questions

1. Draw the schematic diagram of current secondary protection for 10kV overhead transmission line.

1LHA 1LJ 3LJ +bh -bh

1LJ 1XJ BCJ

1LHC 2LJ 4LJ 2LJ

3LJ 1SJ

4LJ

1SJ 2XJ

BCJ DL TQ

2 draw a wiring diagram for insulation monitoring of small current grounding system? (and write down the set voltage).

3. Draw the schematic diagram of zero sequence current filter.

4. Draw the wiring diagram of inverse time limit current two-stage protection principle.

5. Draw the wiring diagram of transformer Yd- 1 1 wiring differential protection principle.

6. Draw the principle wiring diagram of acceleration protection after automatic reclosing.

7. Draw the diode ring comparison circuit diagram with amplitude and phase comparison.

Amplitude comparison actuator

8, draw the starting motor rotor two-point grounding protection wiring diagram.

9. Draw the wiring diagram of transformer overload protection.

10. Draw an action diagram of an impedance relay with the offset characteristics in all directions and the offset is about-0.1Zzd on the complex plane with the set impedance of zzd.

Zzd

rare

-0. 1Zzd

Third, the judgment question

1, the return coefficient of excess relay is greater than 1, and the return coefficient of insufficient relay is less than 1.

2. The contacts opened when the relay is not charged are called normally open contacts, and the contacts closed when it is not charged are called normally closed contacts.

3. The connection coefficient of relay protection is the ratio of the current flowing into relay to the secondary current of current transformer.

4. There are basically two neutral point operation modes in power system: large grounding current system and small grounding current system.

5. The zero sequence voltage of single-phase grounding in small grounding current system is phase voltage, and that of single-phase grounding in large grounding current system is about 1/3 phase voltage.

×6. Parallel line lateral differential protection has no dead zone.

* 7, generator longitudinal differential protection generally no dead zone.

×8, the three-stage protection of zero-sequence direction current has a dead zone.

3. Distance protection is the comparison between measured impedance and set impedance.

The action characteristic of impedance relay refers to the action range or figure of impedance relay on complex impedance plane.

× 10, and the protection range of distance protection section II is not affected by increasing current.

1 1, phase difference high frequency protection When the length of the protected line is greater than 180km, the three-phase short circuit will produce successive actions.

× 12, the phase comparison of current transformer in and out of bus differential protection must be consistent.

The relay protection principle of 13 power system is generally three-stage protection reflecting single-ended physical quantities and differential protection reflecting multi-ended physical quantities.

The impedance relay14 "00" wiring which reflects the phase-to-phase short circuit can also correctly reflect the ground short circuit fault.

The connection mode of 15 and impedance relay phase voltage and phase current plus 3KI0 cannot correctly reflect the two-phase short circuit.

Fourth, the calculation problem

1. The wiring and parameters of the power grid are shown in the figure.

D 1 D2 D3 D4

1 10kV 1DL 50k m 2DL 10kV 3DL 10k m 0.4kv

S=20MVA 2MVA

Ud%= 10

The reactance of transmission lines at all levels is 0.4Ω/km; ; ; ; ,

Try to calculate the setting of 3DL 10kV line current two-level protection or1dl1kloc-0/0kv transmission line distance three-level protection (without checking the precision current) or 20MVA transformer differential protection (BCH-2 or BCH- 1 differential relay can be used).

Scheme 1, 3DL 10kV line current secondary protection setting calculation:

Ⅰ phase (current quick break) active current:

Sensitivity check:

=3.7 qualified

Secondary action current

= 。

Ⅲ (overcurrent protection) primary action current.

1 15A=243A

Sensitivity check

= =

Secondary action current

Action time of current protection