Researchers Zhang Xiongqing and Wan Fangpei of the State Grid Zhoushan Power Supply Company wrote an article in the 3rd issue of "Electrical Technology" in 2019 (the paper is titled "Analysis of a 35kV transformer neutral point arrester failure"), introducing various overvoltages that may occur at the neutral point of the transformer in an ungrounded system, and protecting the equipment from damage by installing a neutral point arrester. The correct selection of high-voltage arrester is directly related to the normal operation of the system.
Through the analysis of the reasons for the breakdown of the high-voltage arrester at the neutral point of the main transformer caused by a line ground fault, the fault location of the arrester was found, and the selection principles of the neutral point arrester of the transformer and the precautions during the maintenance process were proposed.
For transformers in substations, due to the intrusion of multi-phase or single-phase lightning shock waves, the operation of the power system and the overvoltage generated by asymmetric faults, the neutral point of the transformer is subjected to excessive voltage. Therefore, the neutral point of the ungrounded transformer needs to be equipped with a protective device to protect against lightning and operating overvoltage. When a single-phase ground fault occurs in the power grid or it operates in non-all phases, the transient and steady-state voltages of the neutral point power frequency increase, and protection devices are also required for protection.
At present, the neutral point protection device of the transformer is gradually improved, and the probability of neutral point failure is also greatly reduced. However, neutral point overvoltage protection is still an indispensable and important part of the power grid operation. If not paid enough attention, it may cause serious accidents to the entire power grid.
1. Transformer neutral point overvoltage (omitted)
In power systems, overvoltages are generally divided into two categories, namely external overvoltage and internal overvoltage.
2. Selection of neutral point arrester for fully insulated transformer
In the power system, the insulation level of the neutral point of transformers 35kV and below is the same as the insulation level of the line end, that is, a fully insulated transformer. The neutral point of a fully insulated transformer can be protected by a protection level no greater than the usual phase-to-ground arrester.
When the line is abnormal, such as grounding, operating overvoltage and lightning strike, the overvoltage of the transformer neutral point to ground is smaller than the line overvoltage. Therefore, the rated voltage of the neutral point metal oxide arrester can be selected to a relatively small value. It is recommended to use the following three types of arresters as neutral point overvoltage protection.
1) Select typical phase-to-ground arresters of the same voltage level.
2) The design is the same as that of a typical phase-to-ground arrester. The rated voltage can be reduced, but not less than 60% of the rated voltage of a typical phase-to-ground arrester.
3) Special arresters that reduce the level of protection.
3.Case analysis
At 05:48:29 on February 23, 2018, the Xiatao 3551 submarine cable between the Xiashi substation and the Taohua substation experienced a B-phase grounding, and the neutral point arrester of the No. 1 main transformer of the Taohua substation broke down. The main transformer neutral point metal oxide arrester model is Y1.5W-30/80. The manufacturer is Ningbo Guochuang. It was produced in 2009. U1mA=45kV, rated voltage 30kV, system voltage 35kV. See Table 2 for historical data. The main transformer is a fully insulated transformer, the model is SZ10-20000/35, the manufacturer is Jiangsu Huapeng, and the wiring group is YNd11.
3.1 What happened after the accident
Before the fault occurred, the 35kV Taohua Transformer Xiatao 3551 line was running, but there was basically no current. 35kV bus-switch operation, Taodeng 3553 line hot standby, 1# main transformer operation, 2# main transformer hot standby, 35kV Section I busbar, Section II busbar operation, 10kV Section I busbar, Section II busbar operation, 10kV busbar switch operation, 35kV standby automatic switching operation, 35kV system adopts ungrounded operation mode.
At 5:48:29 on February 23, Xiatao 3551 line protection started. At 5:48:29:695, overcurrent stage II action occurred, and the Xiatao 3551 line switch jumped. The fault waveform diagram shows that phase B of Xiatao 3551 line is grounded. Xiatao 3551 line has no positive sequence current or negative sequence current, only zero sequence current, and the three-phase currents are equal in size and phase, and the effective value reaches 4.25A.
3.2 Cause analysis
After investigation, it was found that the Xiatao 3551 submarine cable between the Xiashi substation and Taohua substation had phase B grounding, and the neutral point arrester of the 1# main transformer broke down, as shown in Figure 1 and Figure 2 respectively.
After the B-phase grounding of the Xiatao 3551 submarine cable occurred, zero sequence current immediately appeared in the Xiatao 3551 line, which was much higher than the current during normal operation. However, the voltage of the two AC phases of the line was still phase voltage, not line voltage. Therefore, it can be concluded that after the single-phase grounding of phase B in the Xiatao 3551 line, the neutral point arrester of the 1# main transformer immediately broke down.
The Taohua substation has only one Xiatao 3551 line incoming line and no other incoming power supply. After the Xiatao 3551 line B phase is grounded and the neutral point of the 1# main transformer is grounded, the AC two-phase high-voltage winding of the transformer receives power from the system and is the primary side, and its 10kV low-voltage winding is the secondary side. When phase B of the line is grounded, the positive and negative sequence impedances on the high-voltage side are approximately infinite, which is equivalent to an open circuit on the low-voltage side of the transformer.
The voltages of the three-phase low-voltage windings are Ua, Ub, and Uc phase low-voltage windings connected in series. The voltage Ua+Uc is applied to the B-phase low-voltage winding. Therefore, the low-voltage winding of the B-phase transformer is equivalent to the primary side, and its high-voltage winding is equivalent to the secondary side. The output voltage UbN of the B-phase high-voltage winding supplies a short-circuit current to the line. Since the low-voltage side is equivalent to a series circuit, the currents are equal in magnitude and direction. The high-voltage side winding also has currents of equal magnitude and direction. This is consistent with the fault wave recording data. It further proves that after phase B of the line is grounded, the 1# variable neutral point arrester also breaks down immediately.
The 35kV system is an ungrounded system. Under normal circumstances, after a single-phase grounding occurs, the system is allowed to operate for a short time. At this time, the ungrounded phase voltage rises to the line voltage, the grounded phase voltage is 0, and the neutral point voltage rises to the phase voltage. After a single-phase grounding occurs in the Taohua transformer, calculated according to equation (2), the voltage at the neutral point of the 1# main transformer may be 23.1kV, while the rated voltage on the nameplate of the arrester manufacturer is 30kV; if the arrester is well insulated, immediate breakdown is unlikely to occur.
According to the selection principles of the neutral point arrester manufacturer of fully insulated transformers, 60% of the rated voltage of the typical phase-to-ground arrester is selected according to the second article. Therefore, it can be ruled out that the equipment selection does not meet the requirements and overvoltage causes arrester breakdown.
Under normal operation conditions of the 1# main transformer neutral point arrester, there is basically no voltage at the neutral point. It can be ruled out that the ZnO arrester valve plate is aging, causing insulation degradation and causing arrester breakdown. Therefore, the breakdown of the neutral point arrester of the 1# main transformer is due to moisture, which causes the insulation to decrease, resulting in breakdown. The historical data of the 1# main transformer neutral point arrester is qualified, which can rule out that the valve plate and internal parts were not completely dried during the production process, and some moisture was retained, causing the arrester to become damp.
It can be concluded from this that the neutral point arrester of the 1# main transformer is affected by moisture during operation. The upper and lower sealing surfaces of the 1# main transformer neutral point arrester are fixed by bolts, and the sealing is good. The pressure release nozzle is shown in Figure 2. The pressure release device has been activated. It can be seen from the figure that the sealing of the nozzle mainly relies on surface glue. There is no sealing ring and no fixation. As the operating years increase, the glue will age, resulting in poor sealing and moisture entry. Therefore, the dampness of the neutral point arrester of the 1# main transformer is caused by the poor sealing of the cover plate of the pressure release device.
To sum up, the pressure release cover of the neutral point arrester of the 1# main transformer was poorly sealed and penetrated due to moisture, causing the neutral point of the 1# main transformer to be grounded, causing the Xiatao 3551 protection action. In order to effectively prevent the single-phase grounding of the line from causing breakdown of the main transformer neutral point arrester, it is recommended to choose a manufacturer with mature production technology and a good reputation. Select a typical design phase-to-ground arrester for the rated voltage, ensure the entry of the equipment, carry out C-level maintenance strictly according to the cycle, and promptly replace the lightning arrester with hidden dangers, eliminate the hidden dangers from the source, and ensure the safe and stable operation of the system.
