In order to effectively analyze the fault situation of the box-type transformer, the double-secondary winding box-type transformer (ZGS11-Z.T-1000/38.5) is selected in this paper, which can be equipped with two centralized inverters. The structure of its power generation unit is shown in Figure 1. The transformer adopts three-phase three-column structure design, low pressure side has 2 windings, the overall structure is divided into high, low pressure chamber, tank and so on three parts. In actual operation, common faults of box-type transformer include low-voltage winding grounding fault, high-voltage side line breaking fault and high-low-voltage side short circuit fault, etc., which will be analyzed in detail below.
1 Typical faults of box transformers of photovoltaic power stations
1.1 Analysis of low-voltage winding grounding fault
Because neutral wiring is not designed in the variable structure design of photovoltaic power station box, if single-phase grounding fault occurs on the low voltage side, it will cause great harm to insulation. Different running state of centralized inverter, the performance of the fault will be different. First of all, when the illumination is insufficient, the power generation unit will no longer work, the centralized inverter will be out of the grid-connected state, so that the standby state of the inverter will no longer as a generation load, but by means of box transformer from the grid to absorb electricity. At this time, the box transformer is responsible for the task of distribution. If single-phase grounding occurs in the box transformer low-voltage position, the line voltage accepted by the inverter will not change, and the inverter will work normally, but the phase voltage will rise. Under long-term operation, it will cause insulation damage on the low-voltage side of the box transformer, and even multi-point grounding. Secondly, if there is light, the centralized inverter will change into a grid-connected state, through the analysis of its wiring mode, box change ungrounded mode of operation, in the one-way grounding state, it is difficult to form an effective circuit with the earth, that is, there will be no grounding current, inverter outlet line voltage will not change, but the control system at the beginning of the design, the main monitoring is still line voltage, No grounding abnormality will be found, and the inverter will still work normally. However, affected by grounding, the overall efficiency of the inverter will be reduced, thus affecting the benefit of photovoltaic power generation.
1.2 Analysis of high-voltage side line break fault
According to the different position of broken line, box transformer broken line fault is divided into high voltage lead and winding broken line. When the high voltage lead break fault occurs on the high voltage side of the box, it will directly cause the inverter trip and the generator set will stop due to the fault. It can be found through the test that there will be sound and special smell inside the box transformer, and the resistance of the winding of the fault phase will be infinite. By measuring the DC resistance, it can be found that the resistance between the normal phase and the fault phase winding will also be infinite, so that the occurrence of line breaking fault can be judged basically. However, when the high-voltage winding line break fault occurs, the fault symbol will be different, mainly manifested in that the DC resistance will not be infinite, but only twice the normal DC resistance of the two phases. By measuring the voltage after the fault, it can be found that on the high voltage side, the line voltage of the fault phase and the adjacent phase will decrease, generally to 50% of the rated line voltage, while the line voltage of the normal phase will not change. At the low voltage side, the line voltage of the phase at the low voltage side corresponding to the broken line phase decreases obviously, but will not become zero, which is mainly caused by the induced voltage.
1.3 Short-circuit fault analysis on high and low voltage side
Usually in the operation of the box-type transformer, inter-phase short-circuit fault occurs. When the high/low voltage side occurs inter-phase short-circuit, it will lead to the circuit breaker trip on the corresponding side of the box-type transformer. Moreover, under the impact of the fault, the box-type transformer is often accompanied by internal abnormal sound, oil injection, odor and so on. Generally speaking, the analysis of the short circuit fault of the box transformer, to start with the protection of the action situation, general understanding of the box transformer fault situation, and then the box transformer to the maintenance of the state, do a good job of safety measures, and the photovoltaic power generation unit uncolumn, a detailed view of the fault. In addition, sometimes at the initial stage of failure, it may only be the inter-phase fault of the box-type transformer. If the fault develops further, the internal winding of the transformer will be burned down, the iron core will collapse and other greater losses. In this case, the box-type transformer can only be scrapped. In this paper, the development process of an actual fault of the tank transformer is analyzed. The initial fault occurred in the short-circuit between the low-voltage side phases. Under the action of short-circuit impact discharge, the breakdown and short-circuit between the high and low-voltage windings would be caused by more serious short-circuit discharge, which would further damage the iron core. It is found that the direct cause of short circuit between high and low voltage windings is the insulation weakness itself.
2 Preventive measures for photovoltaic power station box transformer failure
2.1 Add insulation monitoring devices
For the box transformer selected in this paper, it belongs to the three-phase three-wire star connection. If single-phase grounding occurs, there will be no significant change in line voltage due to the absence of neutral point, which increases the difficulty of grounding fault troubleshooting. Continuous work of the grounding inverter may cause the deterioration of grounding fault, or even bring greater risks. Therefore, the use of insulation monitoring device can add a guarantee for the grid-connection of the inverter unit. If there is a ground fault, the insulation fault alarm will be sent directly and the fault unit will be disassembled. In addition, in order to better deal with the ground fault of photovoltaic box transformer, inverter with neutral connection should be used, and its connection group should be yn11 type.
2.2 Strengthen daily insulation monitoring
In order to ensure the operation safety of photovoltaic power stations, regular monitoring should be carried out in strict accordance with regulations, especially in insulation monitoring, so as to discover insulation defects of box-type transformers in a more timely manner and reduce the probability of equipment failure in the station. In the actual operation and maintenance of photovoltaic power station, it is necessary to increase the frequency of insulation monitoring.
2.3 Pay attention to oil sample testing
According to the analysis of the photovoltaic power station’s transformer failure, the internal insulation defect is the key cause of the high voltage failure. In order to avoid similar failures, it is necessary to do a good job of the oil sample inside the transformer. From the internal insulation defects to cause failure, are gradually accumulated in a long time, its deterioration in the process of heat release or discharge problems, and then make the oil composition of the New Year greetings. Through regular testing, insulation can be more effectively detected, and if necessary, intervention measures should be taken to avoid insulation deterioration. In the case of transformer operation, it is necessary to strengthen the monitoring of oil temperature, and avoid the transformer failure due to high oil temperature by increasing the frequency of inspection and testing.
2.4 Do a good job of technology selection in the construction phase
In order to ensure the long-term operation safety of photovoltaic power station and reduce the failure probability of box-type transformer, it is necessary to start from the construction stage of photovoltaic power station, do a good job in site selection, electrical design, equipment selection and other work, to ensure that the box product itself is excellent quality, but also meet the overall design requirements of the power station.
3 Closing remarks
To sum up, this paper takes a typical box transformer as an example, combined with the characteristics of photovoltaic power generation, analyzes its common transformer faults such as grounding, line breaking and short circuit. In order to effectively avoid the box change fault, the photovoltaic power station operation and maintenance side should strengthen the daily insulation monitoring, pay attention to the oil tank test work, under the conditions allowed, but also add insulation monitoring device, effectively ensure the safe operation of the box transformer.
