Analysis and Rectification of Transformer Overvoltage Short Circuit

The No. 1 main transformer SFSZ8-25000/110 of a 110kV substation was damaged due to an accident. The company returned the transformer to the manufacturer and formed a joint investigation team to analyze the cause of the accident and formulate a repair plan. The weather was clear on the day of the accident, with a temperature of up to 39°C. At 13:26, the operator suddenly heard an explosion and abnormal sounds inside the No. 1 main transformer. Then the heavy gas and differential protection of the No. 1 main transformer were activated, disconnecting the three-side circuit breakers of the main transformer, and the transformer was out of operation.

Transformer damage

(1) Winding. The upper 3-4 turns of the low-voltage winding a and b phases were short-circuited and burned, and arched upward, with the insulation at the upper end damaged.

(2) Iron core. The edges of the core columns of the corresponding low-voltage winding phases a and b were burned. Two pressure nail limbs of the upper clamp phase a fell off from the weld, and one pressure nail limb of phase c also fell off.

(3) Insulation of the device body. The 50mm thick laminated wood board at the end of phase a was broken, and two Φ42 pressure nails together with the Φ55 pressure nail bowl penetrated. Phase b also had a pressure nail that penetrated the pressure nail limb board. The paper tubes inside the low-voltage windings of phases a and b were also partially damaged.

Accident Analysis

Based on the damage of the transformer, after intuitive analysis, it is believed that the transformer

The accident caused overvoltage and short circuit shock.

1 Overvoltage shock

The low-voltage winding of this transformer is continuous, as shown in Figure 1. When the continuous winding is subjected to the wave, the potential difference between the first few segments is high, especially the axial and radial synthetic electric fields at K are very strong, which is very easy to be broken down when overvoltage occurs. From the short-circuit burning of 3-4 turns of the low-voltage winding a phase and b phase and the burning of the core column edge, it can be explained that the low-voltage winding a phase and b phase are subjected to overvoltage impact, which breaks down the core and forms a turn-to-turn breakdown short circuit.

2 Short circuit shock

Figure 1 Schematic diagram of continuous winding line segment

The transformer will generate large axial and radial forces when short-circuited. The radial force Fd causes the outer winding to be subjected to outward tension and the inner winding to be subjected to inward pressure (as shown in Figure 2a). The axial force can be divided into axial internal force and axial external force. The axial internal force Fq1 compresses the winding (as shown in Figure 2b), and the axial external force Fq2 is caused by the imbalance of the ampere-turns of the inner and outer windings. It causes the inner and outer windings to produce relative displacement and increase the asymmetry (as shown in Figure 2c). The pressure nail limbs on the transformer clamp fall off, the laminated wood board breaks, and the pressure nails penetrate the pressure plate, indicating that a large electric force is generated in the axial direction; and the low-voltage inner paper tubes of phases a and b break, which is caused by the low-voltage windings being subjected to inward pressure. These phenomena indicate that the transformer is impacted by the short-circuit current. The transformer is short-circuited because the low-voltage windings of phases a and b break through the core under overvoltage.

With the above two basic analyses, the remaining question is what causes the overvoltage of phases a and b. On the day of the fault, the weather was clear, there was no atmospheric overvoltage caused by lightning, and there was no temporary overvoltage. It might be an operating overvoltage, and there are only several types of operating overvoltage: ① arc grounding overvoltage of neutral point ungrounded system; ② overvoltage of no-load line closing; ③ overvoltage of no-load line cutting off; ④ overvoltage of no-load transformer cutting off, etc. According to the operation records of the day, it can be seen that ②③④ are impossible to occur, leaving only ①. The 10kV side of the transformer adopts D-type connection, without neutral point, and belongs to the neutral point ungrounded system. When single-phase intermittent arc grounding occurs, arc overvoltage will be formed on the non-fault phase, and the overvoltage on the non-fault phase can reach up to 3.5 times the rated voltage. The transformer a and b phases are subjected to overvoltage, and it may be single-phase intermittent arc grounding of phase c. Therefore, we searched the transformer's low-voltage phase C line and found that the wall bushing from the 10kV line to the switch room had exploded and the metal conductor was partially grounded.

So far, the cause of the accident has been found: first, there is a defect in the 10kV c-phase wall bushing, which exploded under high temperature and heavy load, forming a 10kVc-phase single-phase intermittent arc grounding, thereby forming arc overvoltage on phase a and phase b, causing the end of the low-voltage winding of phase a and phase b to break through the iron core, causing the transformer to short-circuit. Phase a and phase b then form a turn-to-turn short circuit, the main transformer differential protection and heavy-watt period protection are activated, disconnecting the three-side circuit breakers of the transformer, and the transformer is out of operation.

Figure 2 Winding stress

5 Emergency repair plan and corrective measures

In view of the damage of the transformer, a detailed emergency repair plan was formulated, including rewinding the winding, reprocessing and replacing the core silicon steel sheets, and remaking and processing the insulation parts of the transformer body. At the same time, the following rectification measures were formulated.

(1) From this accident, it can be seen that the transformer has the weakness of weak resistance to overvoltage and short-circuit impact. When the manufacturer repaired the transformer, the following suggestions were put forward. ① Add a capacitor ring at the head end of the low-voltage winding, increase the insulation of the end and tail turns, and increase the oil channel to ensure that it will not break down during overvoltage impact and improve the impact electrical strength. ② Achieve "axial compression and radial support". For axial compression requirements: a. Replace the laminated wood board with epoxy laminated glass cloth board with higher hardness and better insulation strength; b. Perform a close pressure treatment on the winding pads and end ring pads; c. Dry the winding at constant pressure; d. The welding control of the nail limb plate must be strengthened, and a support plate must be added. For radial support requirements: the soft paper tube inside the low-voltage winding is replaced with a hard paper tube as the skeleton to prevent radial electric force from compressing the low-voltage winding and causing the core to break down and ground.

(2) Transformer operators of power supply companies should strengthen equipment inspection and equipment defect management during operation to prevent accidents.