Fault analysis of low excitation loss and tripping

The two 200 MW units of Wuhan Steel and Power Co., Ltd. are QFSN type water hydrogen turbine generators produced by Dongfang Electric Machinery Co., Ltd. the seven circuit 110 kV tie lines are respectively connected to the three transformer power stations of Wuhan Iron and Steel Co., Ltd., metallurgical substation and rolling substation, and are closely connected to the 220 kV system, with sufficient reactive power reserve in the system. In order to assess the leading phase operation ability of the unit and the regulation ability of the system voltage, the leading phase operation test is carried out on the generator unit, and according to the test results, the voltage regulation rate of 110 kV bus voltage is 0.21 ~ 0.5 kv/10 Mvar, and the voltage regulation rate of 220 kV system is 0.1 ~ 0.254 kv/10 Mvar

1 fault profile

, Wuhan Steel and Electric Co., Ltd. No. 1 a) oil pipe is the standby machine connecting oil pump and jaw. No. 2 machine has loss of excitation fault under manual excitation operation mode, loss of excitation protection action switches auxiliary power, boiler MFT action enters the experimental procedure and trips

before the fault occurred, unit 2 was in full load operation, with generator active power of 200 MW, reactive power of 20 Mvar and terminal voltage of 15.29 kV. At 05:02:00, the No. 2 central control room sent out the "loss of excitation" signal, and the 6 kV fast switching device "021 switching" and "022 switching", then the boiler MFT acted and the unit tripped

2 fault analysis

2.1 analysis of low excitation loss of excitation fault during generator operation

the wiring of generator excitation system is shown in Figure 1. Because the auxiliary exciter burns, the excitation system temporarily operates in manual mode, which requires the operator to adjust reactive power in time according to the change of active power, so that the generator operates in a stable state. See Table 1 for the changes of parameters during the loss of excitation of generator under low excitation

see table

it can be seen from table 1 that the active power of the generator before loss of excitation is 198.65 MW, while the reactive power is only 16.29 Mvar. At this time, the unit operates on the upper part of the static stability curve, the static stability margin is small, and the generator terminal voltage and 6 kV auxiliary voltage are low. When the unit is subject to small disturbance from the outside, and the load gradually increases from 198.65 MW to 205.04 MW, due to the operator's failure to increase the excitation in time, the power angle of the generator will gradually increase, so that the reactive power of the generator will gradually decrease, and the voltage of the generator terminal and the voltage of the auxiliary power supply will also decrease. While the manual excitation power supply is from 380 V auxiliary power, without adjusting the manual excitation, the voltage and current of the generator rotor will decrease accordingly, Thus, the reactive power and voltage of the generator will be further reduced, and then the rotor voltage and current will be reduced. Such repetition will eventually lead to the loss of excitation of the generator. Since the generator is loaded with full load before loss of excitation, after entering asynchronous operation, its equivalent reactance is reduced more, and the generator absorbs more reactive power from the system, and the maximum reactive power leading phase is -150 Mvar

Figure 1 excitation system wiring

2.2 analysis of generator loss of excitation protection action

the generator loss of excitation protection of Wuhan Steel and Power Co., Ltd. is composed of three parts: static limit machine end impedance ZK-1, rotor low voltage ZY-1, and system bus low voltage DY-1. The impedance is set to 32 Sword, and "mandrill must be more accurate"? 5。， As the locking element in case of system fault or system oscillation, the low voltage of rotor is set to 100 V, and the low voltage of system bus is set to 80% UN. The protection logic block diagram is shown in Figure 2

Figure 2 generator loss of excitation protection logic

disconnection action is to trip the generator outlet switch; The program trip action will trip the main switch, de excitation switch and 6kV branch switches of the generator 9 s after the main throttle valve is closed

according to the data in Table 1, the generator gradually enters the out of step operation stage from low excitation loss of excitation operation, and the reactive power leading phase is up to 150 Mvar. At this time, the measured impedance at the machine end transitions from the normal position in the first quadrant to the fourth quadrant, and enters the asynchronous operation impedance park. When the rotor low voltage is lower than 100 V, the loss of excitation protection T1 delays 0.5 s to act on the outlet, sends the "loss of excitation" signal and switches the auxiliary power. During the loss of excitation of the whole generator, the reactive power loss of the system is nearly 170 Mvar. According to the voltage regulation rate provided by the unit's phase advance test, 0.21 ~ 0.5 kv/10 Mvar (that is, for every 10 Mvar less reactive power of the generator, the 110 kV bus voltage decreases by 0.21 ~ 0.5 kV), then the 110 kV bus voltage decreases by about 3.57 ~ 8, even after cleaning and separation of waste plastic 5 kV, and considering the role of other units in the system, the voltage of the 110 kV system will not drop to 80% of its rated value. The controlled data recording also shows that when the generator is out of step, the voltage of the 110 kV system is still higher than 80% UN, so the system low voltage condition of the loss of excitation protection is not met, and the loss of excitation protection T2 does not trip

2.3 analysis of boiler MFT action

when the furnace pressure fluctuates sharply (± 1500 Pa) or the main fuel is lost (the pulverized coal feeder is fully tripped), the boiler MFT action will be caused. After MFT action, the generator outlet switch and de excitation switch will be tripped together with a delay of 9 s

Wuhan Gangdian Co., Ltd. reformed the frequency converter of the powder feeder in 1999, and its frequency converter is FR-E500 frequency converter produced by Mitsubishi Electric Co., Ltd. The original control circuit of the powder feeder remains, that is, the signal is still sent by the same operator to the controller, and then the controller outputs a 4 ~ 20 mA DC signal to the frequency converter to control the speed of the powder feeder. The working voltage of fr-e 500 frequency converter is 325 ~ 528 V, that is, the frequency converter works normally when the working voltage is 0.8 ~ 1.3 UN. If it exceeds this range, the frequency converter will stop working, so that the pulverizer trips, causing loss of main fuel and MFT action. When the auxiliary voltage is too low or the power is cut off instantaneously, the self holding time of the frequency converter is set to 1 s. when the power supply voltage is lower than the minimum working voltage of the frequency converter or disappears for more than 1 s, the frequency converter has no output and the powder feeder trips

it can be seen from table 1 that the whole process of generator from starting phase leading to loss of excitation protection action is 33 S. during this process, the voltage of 6 kV Bus decreases significantly until it is lower than 80% (5.04 kV) of the rated value. If the voltage drop of transformer is not considered, the voltage of 380 V bus also decreases to below 80% (320 V) of the rated value, and the instantaneous loss of voltage occurs when the loss of excitation protection action switches the auxiliary power (the switching time of auxiliary power is about 120 ms), Finally, the frequency converter stopped working, so that the powder feeder tripped completely, and the boiler MFT tripped

it can be seen from the above analysis that after the loss of excitation of the generator at low excitation, the loss of excitation protection only acts on the "loss of excitation" signal and switches the auxiliary power. Because the auxiliary power drops too low and lasts for a long time, the output of the frequency converter of the pulverized coal machine stops, the pulverized coal machine trips, and the boiler MFT acts and trips

3 suggestions and measures

3.1 strengthen the technical and working heart training of operators

the manual excitation mode of generator only temporarily meets the needs of power generation when the automatic excitation regulator fails or the auxiliary exciter does not have the ability to be put into operation. During the manual excitation operation of the unit, the operator should have a high degree of dedication, concentrate on monitoring the panel, and be able to adjust the excitation in time according to the change of load, so that the ratio of active and reactive power of the generator is less than 3. At the same time, the operator should pay close attention to the emergency repair and restore the automatic excitation operation as soon as possible

3.2 reset of loss of excitation protection

when the generator loss of excitation causes static stability damage and enters asynchronous operation, a large amount of reactive power is absorbed from the system. At this time, the system voltage has not yet fallen to the low voltage setting value, and the loss of excitation protection cannot act to disconnect the unit from the system, which will overload the adjacent units and threaten the system stability. At the same time, the voltage of 6 kV Bus drops sharply, seriously threatening the safe operation of auxiliary plant equipment; It will also cause stator overcurrent, stator armature winding temperature rise, stator end heating and rotor heating, which pose a great threat to the safety of the unit itself. Therefore, according to the actual situation of ABS/pbt alloy on site, reasonable calculation and appropriate setting should be carried out, so that when the generator loses excitation, the protection can disconnect the unit from the system in time, so as to ensure the stability of the system and the safety of the unit itself

3.3 change the working power supply of the frequency converter

since the working power supply of the frequency converter of the powder feeder is connected to the 380 V auxiliary power, when the auxiliary power voltage is abnormally reduced or disappears (such as the switching is slow), the normal operation of the frequency converter will be affected, resulting in the tripping of the powder feeder and the action of the MFT. Therefore, the working power supply of the frequency converter can be connected to the auxiliary UPS, so that the operation of the frequency converter will not be affected by the auxiliary voltage

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