Analysis and treatment of frequent tripping of air switch of transformer cooler

introduction

Large-capacity transformers must use appropriate cooling devices to dissipate enough heat to maintain safe and stable operation of the transformer. When the transformer cooler is abnormal, it will have a huge impact on the transformer. The heat generated cannot be effectively dissipated and accumulates, which will seriously affect the internal insulation and life of the transformer. Therefore, whether it is a partial failure of the cooler or a serious failure of the cooler to stop, it should be taken seriously, discovered and handled in time to prevent it from happening.

1Pre-operation status of transformer cooler in a 500kV substation

The 500kV substation transformer was produced by Guangzhou Siemens Transformer Co., Ltd., model 0DFs10-250000/500, cooling mode 0NAN/0NAF (natural oil circulation air cooling), and the start and stop of the cooler is controlled by the transformer oil temperature or the high-voltage side load current. The transformer was put into operation in December 2013. Since 2017, the cooler air switch has frequently tripped, causing some fans of the cooler to fail to operate normally, affecting the heat dissipation effect of the transformer, especially during the peak summer period, increasing the instability of the transformer operation and leaving a major safety hazard.

2 Analysis of the cause of the cooler air switch tripping

2.1 Theoretical Analysis

The control circuit of the transformer cooling system is shown in Figure 1. There are two possibilities for the cooler air switch to trip: (1) When the fan starts, the starting current is too large, and the cooler air switch trips after being impacted by the large current; (2) After the fan starts and runs for a period of time, the current passing through the cooler air switch reaches the trip value and trips. In order to determine the cause of the trip, the monitoring background s0E was checked and it was found that the transformer cooler fault fan was started and tripped 4 hours later. Therefore, the first reason can be ruled out, that is, the transformer cooler air switch tripped due to the long-term operation of the cooler fan.

2.2 Equipment Analysis

The air switch model of the transformer cooler is Schneider GV2-ME05C, with a rated voltage of 690V, a rated current range of 0v6.~1v0A, a uniform setting of the on-site setting Ik of 0v6.A, and a rated impulse withstand voltage of 6kV: the cooler fan power is 0v2kw, the rated voltage is 400V, and the rated current If=0v56A. The line voltage of the power system of the substation is about 400V. By comparison, it can be seen that Ik>If. In theory, the air switch setting meets the use requirements of the cooler.

2.3 On-site equipment inspection

Check each cooler fan. The rotating components of the transformer cooler fan are all rotating normally. If the fan rotates abnormally, lacks lubrication, and rotates difficultly, the current will increase when the fan is running. Check the air cooling device control box. The temperature measurement found that the temperature inside the box is generally around 50℃. It can be seen that the operating environment of the box where the cooler air switch is located is relatively harsh.

2.4 Determine the cause

After analysis, it was found that since Schneider's GV2-ME05C ​​air switch uses the thermal trip principle, it will not trip when the rated current passes through it. However, after inspection, it was found that the operating environment of the box where the cooler air switch is located is generally around 50°C. The higher ambient temperature is superimposed on the thermal trip element, causing the cooler fan to operate under normal load current due to the trip element temperature reaching the trip value, eventually causing the air switch to trip.

3. Risk analysis of cooler air switch tripping

The transformer cooler is a heat dissipation method that starts when the main transformer is at high temperature and high load, which is of great significance to the normal operation of the transformer. The heat dissipation process of the transformer is firstly to radiate heat naturally through the oil tank wall and the cooler, and then to exchange heat with the ambient air by convection. To stabilize the temperature of the transformer, it is necessary to reach a thermal equilibrium state. Newton's cooling law is as follows:

Where α is the heat transfer coefficient, A is the heat transfer area, and AT is the difference between the fluid body temperature (the average temperature of the fluid on the cross section) and the wall temperature.

It can be seen from formula (1) that accelerating the air flow on the radiator surface (increasing the temperature difference) accelerates the convective heat transfer and improves the efficiency of convective heat transfer. That is, using a circulating air cooling system to accelerate the air flow and reduce the temperature difference between the radiator and the air, thereby improving the cooling efficiency of the transformer. This is the heat dissipation principle of the transformer cooler.

The control method of the transformer cooler is two independent power supplies, the base group is the first group of fans, the even group is the second group of fans, and each fan is equipped with an independent air switch. When any air switch in the group trips, a corresponding power failure signal of the fan group is issued, and the fan corresponding to this air switch stops operating and loses the heat dissipation effect. If the transformer stops operating with fewer fans, the impact will be relatively weak. However, if the problem is not handled in time, the transformer fans will stop operating one after another, and the overall heat dissipation effect of the transformer will be seriously affected.

The transformer cooler fault signal is a frequent signal. It is issued after the fan starts normally for a certain period of time. There have also been many cooler failure events, which affect the normal operation of the transformer. If the abnormal temperature is caused by the abnormal operation of the cooler and it cannot be repaired during operation, the dispatcher should apply to the on-duty dispatcher to shut down the transformer. If it cannot be shut down immediately, the load of the transformer should be adjusted to the capacity within the allowable operating temperature (60% sn) [2]. For the complete shutdown of the cooling device, first check whether the power supply voltage is normal. If the automatic switch circuit cannot be automatically switched due to a working power failure, immediately go to the main transformer cooler control box to switch the working power (in the process, to prevent the starting current from being too large and the contactor from burning, several groups of coolers can be stopped first) and the cooling device is restored. If it cannot be handled in a short time, it can continue to operate when the load and temperature allow. During the handling process, the main transformer load and temperature should be closely monitored. Even if there is no overload and the oil temperature is <75℃, the maximum operating time of the transformer when the cooler is completely stopped should be <1h. It can be seen that the abnormal situation of the cooling system brings great risks and hidden dangers to the normal operation of the transformer.

4. Cooler air switch tripping treatment method

From the above cause analysis and risk analysis, it can be seen that the main problem is the matching of the transformer cooler air switch setting value and the fan working current, that is, whether the transformer cooler air switch setting value can avoid the addition of the normal working current of the fan and the ambient temperature in the air-cooled control box, but the transformer cooler air switch setting value cannot be too large. If it is set too large, when the fan is really abnormal, the air switch cannot trip correctly to disconnect the abnormal fan, and an over-tripping occurs, which may eventually cause the cooling device to stop completely. In view of the above situation, the transformer cooler air switch on-site setting value Ik is uniformly set to 0v8A, which can not only avoid the normal working current of the fan, but also correctly trip the faulty fan when the fan is abnormal.

5 Conclusion

In summary, this article focuses on analyzing the causes and treatment methods of frequent tripping of transformer cooler air switches. Through effective analysis and research, reasonable improvements have been made to solve the difficult problems that repeatedly occur in substation operation. Although the matter is small, it has benefited a lot. Combined with the importance of the cooling system to the transformer, it can be seen that ensuring the normal operation of the cooling system is the prerequisite for ensuring the normal operation of the transformer. The operating personnel should strengthen the study of the transformer cooling system and regularly track and analyze the operation of the cooling system. This can not only continuously accumulate experience, but also effectively discover and deal with problems and protect the equipment.