Transformer is the most common equipment in the power system. It plays a very important role. However, we often see some faults in transformers. There are many types of faults and the causes are also different. The industry often divides transformer faults into two categories, namely faults within the tank and faults outside the tank.

 

The causes of faults in the box include: phase-to-phase short circuit, inter-turn short circuit of the winding, and single-phase grounding short circuit. The occurrence of this kind of failure will bring some impacts and hazards. For example, after high temperature is generated in the oil tank, it is easy to cause the transformer to explode.

 

The causes of external faults include: various phase-to-phase short circuits and grounding (to the transformer shell) short circuits may occur at the leads and insulating sleeves. This can cause the coil to deform and then cause the insulation of the winding to decrease.

 

We can avoid the occurrence of transformer faults through some preventive tests. Next, we will learn about various protection tests and protection scopes in detail!

 

Preventive measure: gas protection

 

Gas protection is a relay protection test used to prevent internal faults of the transformer. If there is a fault inside the equipment, such as a short circuit between turns, and gas is generated due to heat inside the oil tank, the gas protection should operate. It can reflect very well. There is a fault within the transformer.

 

Generally speaking, if the capacity of oil-immersed transformer is higher than 800 kVA, a gas protection device must be installed. As shown in the picture below, the installation position of the gas device is also particular. For example, it is usually installed on the connecting pipe between the oil tank and the oil conservator. When installing the transformer, attention should also be paid to keeping it at a certain inclination. The inclination is between 1% and 1.5%, so that the gas generated when the transformer tank is heated can be smoothly discharged from the gas device to the storage tank. Oil tank.

1. Under normal circumstances, the fuel tank of the gas device is filled with oil, which will cause the upper and lower contacts to be disconnected. If the transformer fails, the oil level in the tank will drop, which will cause the upper contact to connect, thus sending out an alarm signal.

2. Heavy gas action means that when the transformer oil tank generates a lot of gas, it will impact the baffle of the tank, which will cause the lower oil cup of the gas device to fall and connect with the lower contact point, causing a direct trip. This is a heavy gas action. .

3. When the transformer leaks oil, it will cause the upper and lower oil cups of the gas device to fall one after another and connect to each contact point respectively, causing the circuit breaker to trip.

 

Preventive measure: longitudinal differential protection

 

The longitudinal differential protection is used to protect the internal faults of the transformer and avoid phase short circuits in its bushings and lead wires. Its protection range is between the current transformers on one and two sides of the transformer. When a short circuit occurs in this range, it will A protection that operates due to unbalanced current in the differential circuit.

 

Protection measure: current quick-break protection

 

Current quick-break protection, like longitudinal differential protection, is used to prevent internal faults of the transformer and failures of the outlet bushings. Since the two methods achieve the same effect, under what circumstances must current quick-break protection rather than longitudinal differential protection be used?

The answer is that when the operating capacity of a single transformer is less than 10,000 kVA, and when the capacity of the transformers working in parallel is not higher than 6,300 kVA, in this case, there is no need to install longitudinal differential protection, but to install current quick-break protection.

Overcurrent protection devices are formulated based on the principle of the maximum load current of the line, so the selectivity of the action of this protection device is guaranteed. The time limit for the protection device to operate is directly proportional to the degree of harm. That is to say, the longer the time limit, the more serious the harm will be. In order to solve this problem, we usually adopt the method of increasing the current setting value to limit the protection action range, which greatly reduces the time limit of the protection action. The whole process is current quick-break protection.

 

Protection measure: overcurrent protection

 

Overcurrent protection is a little different from the previous three measures. This method is used to prevent internal and external faults of the transformer. It is a backup protection for the longitudinal differential protection and the current quick-break protection. It has a time-limited action to trip the circuit breakers on each power supply side. .

 

Protection measure: overload protection

 

This method is used to ensure that overload conditions occur during normal operation of the transformer. It can only be installed when the transformer determines that there is an overcurrent that may be caused by overload. This only requires installing a current relay on one phase.

When the transformer is overloaded, the current relay operates, and then the time relay gives a certain delay, and finally the signal relay is turned on to send out an alarm signal.

 

Protection measure: zero sequence current protection

 

The zero-sequence current protection consists of two parts, namely the current component and the voltage component, which are mainly installed on the current transformer of the neutral point grounding lead of the transformer. After the protection action, the circuit breakers on the primary and secondary sides of the transformer are cut off. For systems with a neutral point directly grounded on the low-voltage side (three-phase four-wire system), when the protection sensitivity of the high-voltage side cannot meet the requirements, special zero-sequence current protection should be installed.

 

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