The fault of the inverter tapping out of step in the Gaozhao DC Zhaoqing converter station

introduction

The converter transformer plays an important role in the HVDC transmission system, such as participating in AC/DC conversion, electrical isolation of AC/DC system, suppressing DC fault current, and weakening the overvoltage of AC system invading DC system. The reliable operation of the converter transformer is of great significance to the DC transmission system.

In the HVDC transmission system, the purpose of controlling the valve side voltage is achieved by controlling the position of the tap. According to the different controlled quantities, tap control can be divided into fixed angle control and fixed voltage control. The former requires keeping the angle a or y within a certain range, and the latter requires keeping Udi0 (no-load DC voltage) within a certain range. Since the fixed angle control mode has a fast response speed and can reduce the frequent adjustment of the tap, the converter tap in the HVDC transmission system usually adopts the fixed angle control mode.

1 Basic principles of commutation transformer tap changer

The oil-immersed resistor on-load tap-changer consists of a voltage regulating circuit, a selection circuit, a transition resistor, a drive and control circuit, and various protection devices, as shown in Figure 1.

1.1 Voltage regulation circuit

The positive and reverse excitation voltage regulation circuits have a larger adjustment range (more than 15%) and are generally used for transformers with higher voltage levels.

The voltage regulating winding is connected to the basic winding in a positive or negative way, so that the magnetic fluxes B1 and B2 generated in the two winding cores are added or subtracted, thereby changing the turns ratio of the primary and secondary windings to achieve voltage regulation.

The regulation range on the regulating winding is doubled.

1.2 Selecting the circuit

The selection circuit is part of the voltage regulation circuit, and its task is to select the position of the winding tap. The device that selects the tap position is called a tap selector. In addition, there is a polarity selector in the positive and negative excitation voltage regulation circuits. The selection circuit requires the tap to be selected without load. Therefore, the contacts of the tap selector are divided into two groups, single and double numbers, corresponding to the tap numbers. When the double group of moving contacts is running with load, the single group of moving contacts can select the adjacent tap without load. Because it will not cause arcing, the contacts of the selection circuit do not need to be placed in a special oil tank.

1.3 Transition resistor and switch

In order to ensure that the current in the load is uninterrupted during the switching process, the partial bridging phenomenon of the voltage regulating winding will inevitably occur during the switching process. In order to limit the circulating current of the bridged winding from being too large, a resistor (transition resistor) must be connected in series. The device that selects the tap in the selector and finally completes the rapid switching between adjacent taps is called a transfer switch. It consists of moving contacts, static contacts, transition resistors, and rapid action mechanisms. Since arcs are generated during the switching process, these components are installed in a well-sealed independent oil tank. The most important thing in the tap switch is the transition resistor and the transfer switch. If the tap is not adjusted in place, it may cause the tap switch to continuously burn arcs in the oil tank, causing the pressure of the oil filter to continue to increase, and finally damaging the tap switch.

2-tap gear transmission principle

The transmission principle of the tap position is shown in Figure 2. After the tap position is adjusted, the tap position transmission board S61P forms a corresponding positive potential, so that the diode matrix module turns on the corresponding diode, and then forms a BCD code (000001 represents -6 gear, 000010 represents -5 gear, and so on). The diode matrix module transmits the corresponding BCD code information to the converter interface panel SU200 device through the shielded control cable. The BCD code of the corresponding gear of the tap switch is displayed on the SU200 interface, and the gear information is transmitted to the pole control system through the field bus.

Analysis of the Causes of 3-Tap Out-of-Step

The transformer tap loss signal is generated by the 00SREL module (EC3.341P1641) and must meet the following conditions at the same time, as shown in Figure 3.

(1) The tap positions of the six converter transformers on the same pole are not completely consistent (OUTST module):

(2) Avoid the 15S delay of the converter tap changer adjustment (NMonTimR module):

(3) Avoid the 15S switching delay of the pole control system (N0TEDG module).

When the gear position of any of the 6 converter tap changers is inconsistent with the others, the OUTST output is 1.

Based on the analysis of the entire signal loop and the fault condition of the converter transformer, we believe that the fault point can be basically located at the following locations:

(1) SU200 device failure)

(2) Fault in BCD output module (position transmission board S61P and diode matrix module S61M)

(3) Internal fault of the tap changer body

(4) The tap changer motor loses power.

4 Analysis of the real cause of frequent commutation transformer out of step

According to the on-site conditions when the tap switch lost step many times, the on-site tap switch motor power supply was normal, and the gear position display was normal before and after the tap switch was adjusted. It can be judged that the power supply of the SU200 device and the tap switch was normal. In addition, after the tap switch was adjusted, the on-site tap switch gear display had been adjusted in place. Until the signal was restored, the on-site tap switch did not move again. We believe that the reason for the loss of step of the commutation frequency converter was due to the failure of the BCD output module of the commutation transformer tap switch. After each tap adjustment, the position transmission board S61P needs to change its position and re-trigger the diode of S61M. If S61P is slightly stuck during adjustment, resulting in poor contact, it may cause the S61M diode to fail to conduct normally, so that the SU200 device does not receive any BCD code, causing all gear indicators of SU200 to go out.

5 Conclusion

When the commutator transformer tap failure occurs frequently, although the gear positions of the 6 commutator transformers on site are consistent, the tap adjustment will not be inadequate and the tap changer will not continue to arc in the oil tank. However, when the automatic power adjustment needs to adjust the tap, the pole control system fails to operate the tap due to the maintenance of the out-of-step signal, which may cause the voltage on the valve side of the corresponding pole to be high or low, causing the voltage on the commutator valve to be high or low, and even develop into continuous commutation failure, thus affecting the safe and stable operation of the corresponding pole. The author believes that in the future, when the commutator transformer tap failure signal appears and the commutator transformer gear position is consistent, the position transmission board of the commutator transformer tap should be repaired in time.