How to better protect the generator set? Please ask the experts of Dongguan Generator Mo Lifeng Electromechanical Equipment Co., Ltd. to answer?

Mr. Li Li, Mo Lifeng electromechanical equipment expert of Dongguan generator, replied:

What protection does the generator set have?

1, generator loss-of-excitation protection

Loss-of-excitation protection is used as loss-of-excitation fault protection for generator excitation current to drop abnormally or disappear completely. It is composed of excitation low voltage Ufd(P), system low voltage, static impedance, TV disconnection and other criteria. Used for signaling and demagnetization respectively.

Both excitation low voltage Ufd(P) criterion and static impedance criterion are related to the static stability boundary, which can detect whether the generator loses static stability due to loss of excitation. When the boundary is static after the excitation is lost, the static impedance standard comes into play.

The TV disconnection criterion meets any of the following two conditions │Ua+Ub+Uc-3u 0│Uset (voltage threshold) or the three-phase voltage is lower than 8V and 0.1a.

When the power system oscillates due to non-loss-of-excitation factors such as short circuit or cut-off short circuit, the protection takes measures to lock Ufd(P), which can prevent the protection from being exported by mistake. T 1(2s) sends out a loss-of-excitation signal after the excitation low voltage Ufd(P) criterion acts. When the excitation low voltage Ufd(P) standard and static impedance standard are met, and the TV secondary circuit is not disconnected, t2(6s) issues a trip command. When the excitation low voltage Ufd(P) criterion, static impedance and system low voltage criterion are satisfied and the TV secondary circuit is not disconnected, t3( 1s) issues a trip command.

2, generator excitation protection

Overexcitation protection is a kind of protection that reflects the high working magnetic density of iron core caused by the decrease of generator frequency or the increase of voltage. Overexcitation protection is divided into high and low fixed values. The low fixed value signals and reduces the excitation voltage after a fixed delay of 5s (the function of reducing the excitation voltage and current is not used for the time being), and the high fixed value is used for de-excitation after the inverse time limit action. The upper limit of inverse time delay is 5 seconds and the lower limit is 200 seconds.

3, generator stator grounding protection

Generator stator grounding protection, as a single-phase grounding fault protection of generator stator, consists of fundamental zero-sequence voltage and third harmonic voltage. The fundamental zero-sequence voltage protects the single-phase grounding fault of the stator winding in the 95% area from the machine end to the tail, which is composed of the principle of reflecting the zero-sequence voltage at the machine end of the generator. After the time limit of t 1(3s), the train loses power. The third harmonic voltage protects the single-phase grounding fault of the stator winding in the tail-to-tail area of the generator, which is composed of the generator neutral point and the third harmonic principle of the tail, and acts on the signal after the time limit t2(5s). They constitute 100% stator grounding protection. This protection has the function of PT disconnection locking.

4, generator stator turn-to-turn protection

The protection consists of longitudinal zero sequence voltage and negative sequence direction criterion of fault point, and PT disconnection locking measures are set for interturn and interphase short circuit of generator and stator winding as main protection. The fault component negative sequence direction criterion is realized by detecting the negative sequence power flowing from the generator, and the longitudinal zero sequence voltage criterion is realized by detecting the longitudinal 3U0 output from the 3PT open delta winding whose neutral point is directly connected to the generator neutral point without grounding. All protective actions are stopped. 0w & amp； n+ H2 N: C2

5, out-of-step protection

The protection adopts three impedance elements, and the number of sliding poles is detected by the trajectory change of impedance, and the position of oscillation center is determined. In the case of short circuit fault, system oscillation, voltage loop disconnection, etc. , to protect against misoperation. Protection generally acts on signals; When the oscillation center is in the generator-transformer unit, the protection section I is started, and the trip command is issued through t 1(0.5s), so that the train loses power; When the oscillation center is outside the generator-transformer group, the protection section II starts to send signals through t2(2s). The protection is equipped with a current locking device to ensure that the current when the circuit breaker is disconnected does not exceed the rated out-of-step breaking current of the circuit breaker.

6, low frequency accumulation protection

Low-frequency cumulative protection reflects the cumulative impact of system frequency reduction on steam turbines. The protection consists of sensitive frequency relay and counter, and is locked by the auxiliary contact of the outlet circuit breaker (that is, the low-frequency cumulative protection also exits when the generator exits the operation). The cumulative system frequency is lower than the frequency setting value of 47.5Hz, and when the cumulative time reaches the setting value of 3000 seconds, it will delay for 30 seconds to send a signal. The device can monitor in real time during operation: display fixed value, frequency f and accumulated time. Generator-transformer differential protection, transformer differential protection and high voltage differential protection are the main protections for internal phase-to-phase short circuit faults of protected components, and the principle of ratio braking is adopted. When a fault occurs outside the area, it can reliably avoid the unbalanced current caused by the inconsistency of CT characteristics on each side, and the fault protection in the area can act sensitively. In order to avoid misoperation of the protection under the action of transformer inrush current, the protection adopts second harmonic locking. The protection is equipped with differential current breaking function, and there is no second harmonic locking. When the differential current reaches the set value, the fault is instantly removed. The protection has the function of CT disconnection locking (actually not used). CT disconnection discrimination is the same as generator differential protection.

7, excitation circuit overload protection

Excitation circuit overload protection is used to protect the rotor excitation circuit from overcurrent or overload, which is connected to three-phase mode and consists of fixed time limit and inverse time limit.

The action current of the time-limited part is set according to the condition that it can return reliably under the maximum rated current of normal operation. After the time limit t 1(5s), it acts on the signal and reduces the excitation current (the function of reducing the excitation current is useless); The action characteristics of the inverse time limit part are determined according to the overload capacity of the generator excitation winding, and the protection action is column disconnection. The upper limit of the inverse time limit is 10 second.

8, generator rotor one-point grounding protection

One-point grounding protection of generator rotor is used to reflect the one-point grounding fault of generator rotor circuit. The protection adopts the ping-pong switching principle, samples the positive and negative voltages of the rotor circuit to the ground in turn, and calculates the rotor grounding resistance and grounding position in real time by solving two different grounding circuit equations. This protection works on the signal after a delay of 2 seconds.

9. Symmetric overload protection of second-hand generators

The protection device consists of fixed time limit and inverse time limit, and the fixed time limit acts on the signal after 5 seconds. The characteristics of inverse time limit action are determined according to the generator's ability to withstand overload current, and the action is column disconnection. The protection device can reflect the heat accumulation process of generator stator.

10, generator negative sequence overload protection

The protection device consists of definite time limit and inverse time limit. The time-limited action current is set according to the long-term allowable negative sequence current value of the second-hand generator and the current value of the negative sequence current filter to avoid imbalance under the maximum load, and acts on the signal after a time limit of 3 seconds. The characteristics of inverse time limit action are determined according to the generator's ability to withstand negative sequence current, and the action is column disconnection. The protection device can reflect the heat accumulation process of generator rotor.

1 1, generator overvoltage protection

Before the generator is connected to the grid, if the generator terminal voltage reaches 1.3 times the rated value, the generator overvoltage will lose power after a delay of 0.5 seconds. When the generator is connected to the grid, it will automatically exit the protection. Whether the generator-transformer unit is connected to the grid is judged by the auxiliary contact state of the 220KV side switch of the main transformer.

12, generator longitudinal differential protection

According to the different input currents of differential components, it can be divided into complete longitudinal differential protection and incomplete longitudinal differential protection.

Alternating current access circuit diagram of generator longitudinal differential protection

In the above figure: Ja, Jb and Jc are differential components of the three phases of the generator A, B and C respectively;

A, b and c three-phase input terminals of generator.

1) complete longitudinal differential protection

The complete longitudinal differential protection of generator is the main protection of generator phase-to-phase fault. Because the model and transformation ratio of TA on both sides of the differential element are exactly the same, it is less affected by its transient characteristics. Its action sensitivity is also high, but it can't reflect the turn-to-turn short circuit of stator winding and the open welding of bar.

2) The longitudinal differential protection is imperfect.

Incomplete longitudinal differential protection can not only protect stator windings from phase-to-phase short circuit, but also reflect stator bar open welding and partial turn-to-turn short circuit. However, because the neutral point side only introduces the current of one branch, the differential current caused by the unequal currents of each branch should be considered in the setting calculation. In addition, when the TA models on both sides of the differential element are different and the transformation ratio is different, it is greatly influenced by the transient process of the system.

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