Analysis of the abnormality of DC voltage divider isolation amplifier power failure lockout

作者： GOZ Electric 时间：2024-07-11 09:33:08 阅读：20

1.1 DC voltage divider isolation amplifier power failure lockout abnormality During the commissioning of a certain station, it was found during the test that after the pole bus DC voltage divider isolation amplifier lost power, it would output a reverse voltage with a peak value of about -3500kV, which is equivalent to the actual output value of about -21V on the isolation amplifier side, and the decay time is about 3s.

1.2 Simulation waveform analysis of power failure of the isolation amplifier on the rectifier and inverter sides The power failure of the pole bus isolation amplifier on the rectifier and inverter sides was simulated using RTDS (real time digital simulation, RTDS). It was found that if the pole bus isolation amplifier of the main system on the rectifier side lost power, the control system measured a reverse high voltage, and the control mode was switched from current control to arc extinction angle voltage control. The trigger angle increased to about 160°, the DC voltage decreased, and the DC current dropped to 0A.

The control system detects the calculated value and measured value of the pole bus DC voltage in real time. If the difference is large, a fault alarm signal will be issued and the system will be switched. However, after the DC current drops to 0A, the control system will exit the DC voltage detection function, resulting in the DC voltage being unable to recover for a long time. If it is lower than the DC line low voltage protection setting of 0.3p.u. and exceeds 4s, the pole will be locked after the restart fails. During normal operation, the DC system controller realizes the conversion of the control mode by mutual limiting. The arc extinction angle controllers on both the rectifier and inverter sides limit the maximum value of the voltage controller output, the rectifier side voltage controller limits the minimum value of the current controller output, and the inverter side voltage controller limits the maximum value of the current controller. After the DC voltage divider isolation amplifier of the main system on the inverter side loses power, the voltage controller acts to reduce the trigger angle to reduce the DC voltage. If the control system detects that the DC voltage is lower than 0.6p.u., the shutdown angle γ is adjusted to the output value of the arc extinction angle control to avoid continuous reduction of the DC voltage. At the same time, because the DC current is not 0A, the control system detects that the calculated value and measured value of the DC voltage of the pole bus are quite different, and the system is switched after a delay of 1s. The DC system is restored without causing lockout. After the neutral bus DC voltage divider isolation amplifier on the rectifier side and the inverter side loses power, if the control system detects that the measured value of the neutral bus voltage is significantly different from the calculated value, the system will be switched, which will not cause DC lockout.

2. Solutions

Currently, there are two types of DC voltage divider isolation amplifiers available in the station, with rated voltages of ±10V and ±25V respectively. Among them, the isolation amplifier with a rated voltage of ±25V has the problem of outputting high voltage after power failure, while the isolation amplifier with a rated voltage of ±10V does not have this problem. After the isolation amplifier with a rated voltage of ±10V loses power, its output voltage drops directly to 0V, and no reverse high voltage appears. The working principles of the isolation amplifier with a rated voltage of ±10V and the isolation amplifier with a rated voltage of ±25V are basically the same. Both convert the DC voltage signal into a square wave signal, and realize the isolation of the electrical circuit through a small transformer, and the output circuit is equipped with a resistor-capacitor filter circuit. The filter circuit of the isolation amplifier with a rated voltage of ±10V stores less energy, which can be absorbed by the circuit and decay rapidly when released after power failure. However, the isolation amplifier with a rated voltage of ±25V has a relatively high voltage, and the filter circuit stores a lot of energy. The energy released after power failure cannot decay quickly, resulting in a reverse overvoltage phenomenon at the output. The technical specification requires that the rated voltage of the secondary circuit of the DC voltage divider is 5V, and the DC voltage range of the pole bus and the DC voltage of the neutral bus are 1.5 times and 4 times the rated DC voltage respectively. To meet the range requirements, the pole bus DC voltage divider should use an isolation amplifier with a range of ±10V (2 times the rated voltage), and the neutral bus should use an isolation amplifier with a range of ±25V (5 times the rated voltage). In order to unify the equipment model, the equipment manufacturer selected the isolation amplifier with a range of ±25V for the pole bus, valve group and neutral bus DC voltage dividers in the post-Zhaqing DC project, which may cause DC lockout. Replacing the pole bus DC voltage divider isolation amplifier with a rated voltage of ±25V with an isolation amplifier with a rated voltage of ±10V can solve this problem.

3. Conclusion

During the commissioning of a station, it was found that the DC voltage divider isolation amplifier had a hidden danger of DC lockout caused by a single component failure. The problem was analyzed and a solution was proposed, which has reference significance for other projects. For projects that have been put into operation, the isolation amplifier can be replaced in conjunction with power outage maintenance. New projects should avoid using isolation amplifiers that may bring the above-mentioned hidden dangers. For new projects, it is recommended to specify in the technical specifications that the DC voltage divider adopts optical signal transmission to avoid the problems of difficult shielding and post-fault analysis caused by the use of electrical signal transmission.