Reprint--What are the main relay protections of generators?

Source: Power Jianghu

1. Generator loss of magnetism protection

Loss of magnetism protection is used as a loss of magnetism fault protection when the excitation current of the generator decreases abnormally or disappears completely. It is composed of excitation low voltage Ufd(P), system low voltage, static impedance, TV disconnection and other criteria that automatically change with the active power in the setting value, and acts in signal generation and demagnetization respectively.

The excitation low voltage Ufd(P) criterion and the static impedance criterion are both related to the static and stable boundary, which can detect whether the generator loses static stability due to magnetization. The statically stable impedance criterion operates at the statically stable boundary after demagnetization.

The TV disconnection criterion is judged to be the TV secondary circuit disconnection when any of the following two conditions are met: │Ua+Ub+Uc-3U0│≥Uset (voltage threshold) or three-phase voltage is lower than 8V, and 0.1A<IA<ISET (current threshold) is judged to be the TV secondary circuit disconnection, and the magnetism loss protection is locked. │UA+UB+UC-3U0│≥USSET is used to distinguish TV single-phase or two-phase disconnection, and low-voltage criterion to judge three-phase voltage loss.

When the system oscillates due to non-demagnetization factors such as short circuit or short circuit removal in the power system, protection measures are taken to lock Ufd(P) to prevent the false exit of the protection.

After the excitation low voltage Ufd(P) criterion is activated, a demagnetization signal is sent by t1(2s). When the excitation low voltage Ufd(P) criterion and the static impedance criterion are satisfied, and there is no TV secondary circuit disconnection, the trip command is issued through t2 (6s). When the excitation low-voltage Ufd(P) criterion, the static impedance, and the system low-voltage criterion are all satisfied, and there is no TV secondary circuit disconnection, a trip instruction is issued through t3(1s).

2. Generator over-excitation protection

Over-excitation protection is the protection of the reaction generator from excessive magnetic density due to the decrease in frequency or too high voltage. The over-excitation protection is divided into two fixed values, high and low, the low fixed value sends a signal and reduces the excitation voltage after a fixed delay of 5s (the function of reducing the excitation voltage and excitation current is not used for the time being), and the high fixed value is operated in the demagnetization after the reverse time limit. The upper limit of the reverse time delay is 5 seconds, and the lower limit is 200 seconds.

3. Generator stator grounding protection

As the generator stator single-phase grounding fault protection, the generator stator grounding protection is composed of two parts: the fundamental zero sequence voltage part and the third harmonic voltage, and the fundamental zero sequence voltage protects the single-phase grounding fault of the stator winding from the end of the machine to the tail of the machine, which is composed of the principle of reflecting the zero sequence voltage at the end of the generator, and is operated in the de-demagnetization after the time limit T1 (3S); The third harmonic voltage protects the single-phase grounding fault of the stator winding from the tail to 30% of the machine end, which is composed of the neutral point of the generator and the third harmonic principle of the machine end, and acts on the signal after the time limit T2 (5S). The two constitute 100% stator grounding protection. The protection is equipped with PT disconnection locking.

4. Generator stator inter-turn protection

The protection is composed of the longitudinal zero sequence voltage and the negative sequence direction criterion of the fault division, and the PT disconnection and locking measures are set as the main protection for the internal turn and phase short circuit of the generator and the open welding of the stator winding. The negative sequence direction criterion of the fault component is realized by detecting the negative sequence power flowing out of the generator, and the longitudinal zero-sequence voltage criterion is realized by detecting the longitudinal 3U0 output by the 3PT open triangle winding that is directly connected to the neutral point of the generator but not grounded. The protection action is at a full stop.

5. Out-of-step protection

The protection uses a three-impedance element that detects the number of pole slips and determines the position of the oscillation center by the trajectory change of the impedance. In the case of short-circuit fault, system oscillation, voltage loop disconnection, etc., the protection does not malfunction. Protects the general action from the signal; When the oscillation center is inside the generator-transformer group, the protection I section starts and sends a trip command through t1 (0.5s), and the action is demagnetized in the detrain; When the center of oscillation is outside the generator-transformer set, the protection II stage starts to signal via t2(2s). The protection is equipped with a current locking device to ensure that the current does not exceed the rated out-of-step breaking current of the circuit breaker when the circuit breaker is disconnected.

6. Low-frequency cumulative protection

The cumulative effect of the impact of the frequency reduction of the low-frequency cumulative protection system on the steam turbine, the protection is composed of a sensitive frequency relay and a counter, and the auxiliary contact of the outlet circuit breaker is locked (that is, the low-frequency cumulative protection is also withdrawn when the generator exits operation), the cumulative system frequency is lower than the time when the frequency is set at 47.5Hz, and when the cumulative time reaches the setting value of 3000 seconds, it will be activated by a delay of 30 seconds. The device can be monitored in real time during operation: display of fixed value, frequency f and cumulative time. The differential protection of the transformer group, the differential protection of the transformer and the differential protection of the high variable are the main protection of the internal phase short-circuit fault of the protected element, and the principle of ratio braking is adopted. In the event of an out-of-zone fault, the unbalanced current generated by the inconsistent CT characteristics of each side is reliably avoided, and the fault protection in the zone operates sensitively. In order to avoid the protection misoperation under the action of transformer excitation inrush current, the protection adopts second harmonic locking. The protection is equipped with the function of differential flow speed interruption without second harmonic locking, and the fault is cut off instantly when the differential current reaches the setting value. The protection has a CT disconnect latching function (actually unused). CT wire break discrimination is the same as generator differential protection.

7. Overload protection of excitation circuit

The excitation circuit overload protection is used as the overcurrent or overload protection of the rotor excitation circuit, which is connected into a three-phase type, which is composed of two parts: fixed time limit and reverse time limit.

The fixed time limit part of the action current is set according to the condition that it can be reliably returned under the maximum rated current of normal operation, and the time limit T1 (5S) is used to act on the signal and reduce the excitation current (the function of reducing the excitation current is not used); The action characteristics of the anti-time part are determined according to the overload capacity of the generator excitation winding, and the protection action is demagnetized in the de-column, and the upper limit of the inverse time limit is 10 seconds.

8. Generator rotor is grounded at one point

The generator rotor one-point grounding protection is used to reflect the generator rotor circuit one-point grounding fault, the protection adopts the principle of ping-pong switching, takes turns to sample the positive and negative pole voltage of the rotor circuit to the ground, and calculates the rotor grounding resistance and grounding position in real time by solving two different grounding loop equations. The protection is delayed by 2 seconds to act on the signal.

9. Generator symmetrical overload protection

The protection device is composed of two parts: fixed time limit and anti-time limit, and the fixed time limit part is activated in the signal after a time limit of 5 seconds. The inverse time limit action characteristics are determined according to the ability of the generator to withstand the overload current, and the action is delisted. The protective device reacts to the heat accumulation process of the generator stator.

10. Generator negative sequence overload protection

The protection device is composed of two parts: fixed time limit and anti-time limit, and the fixed time limit action current is set according to the negative sequence current value that avoids the generator for a long time and the current value that avoids the unbalance of the negative sequence current filter under the maximum load, and the time limit is 3 seconds to act on the signal. The action characteristics of the reverse time limit are determined according to the ability of the generator to withstand the negative sequence current, and the action is demagnetized. The protective device reacts to the heat accumulation process of the generator rotor.

11. Generator overvoltage protection

Before the generator is connected to the grid, if the terminal voltage reaches 1.3 times the rated value, the generator overvoltage will be demagnetized after a delay of 0.5 seconds. When the generator is connected to the grid, it automatically exits this protection. Whether the generator-transformer unit is connected to the grid is determined by the auxiliary contact status of the switch on the 220KV side of the main transformer.