Transformers are commonly used in power supply systems to increase or decrease the operating voltage. Transformers are the key gas machinery equipment that cannot be lacked in the power supply system. Its failure will cause serious harm to the reliability of the power supply system and the optimal operation of the system. In addition, the large-space power engineering transformer is also a very precious machine and equipment. Therefore, relay protection devices with excellent characteristics and reliable posture should be installed according to the transformer volume level and key level.

The fault of the transformer can be divided into two kinds of faults outside the automobile fuel tank and inside the automobile fuel tank.

The faults outside the fuel tank of the automobile are mainly two-color short circuits on the waterproof bushing and the grounding wire of the transformer and the short circuit of the grounding device. The faults in the automobile fuel tank include two-color short circuit of the winding, short circuit of the grounding device, short circuit between turns and the ablation of the transformer core. The electric isolation caused by the fault in the fuel tank of the automobile will not only destroy the insulating layer of the winding, damage the core of the transformer, but also cause a lot of vapors due to the decomposition reaction of the insulating layer material and the transformer oil, which may cause damage to the automobile fuel tank of the transformer. explosion occurs. For various faults caused by the transformer, the protector should be able to remove the transformer as soon as possible.

The practical activities show that the two-color short circuit on the transformer waterproof bushing and the transformer grounding wire, the short circuit of the grounding device, and the inter-turn short circuit of the winding are the more common fault methods; while the two-color short circuit in the transformer tank is relatively rare.

The key to the abnormal operation of the transformer is: the overcurrent caused by the external short circuit of the transformer, the overload caused by the load exceeding the short circuit capacity for a long time, the cooling capacity reduction caused by the failure of the cooling fan or oil leakage, etc. This abnormal operation can overheat the windings and transformer core.

In addition, for a star-shaped wiring transformer with an ungrounded neutral line, when the external grounding device is short-circuited, it may cause overvoltage of the neutral line of the transformer, which may harm the insulation layer of the transformer; Under abnormal operating conditions, the transformer can be over-excited and the regulator can be overheated, resulting in over-temperature of the transformer core and other metal structures. When the transformer is in abnormal operation, the relay protection device should send out an alarm signal according to its serious level, so that the operating staff can deal with it in time and take reasonable countermeasures to ensure the safety of the transformer.

1. Maintenance of transformer placement

(1) CBM maintenance: Defensive vehicle fuel tank failure and reduction of oil level.

(2) Differential protection or current quick-break maintenance: The protection against faults on the transformer winding waterproof bushing and the transformer grounding line (current quick-break maintenance is generally used for transformers below 10005kVA).

(3) Overcurrent protection: two-color short circuit outside the defense force (as reserve maintenance for CBM maintenance and differential protection).

(4) Zero-sequence current maintenance defense force Overcurrent caused by short circuit of external grounding device (in the power network immediately to the neutral line).

Second, the basic principle of transformer differential protection

In order to grasp the basic principle of transformer differential protection with a better brand image, it is like the figure below. When left F left ≠ F right, the sum of the forces received is not zero, and the block is unbalanced. When F left = right block receives The coordination of the forces is zero, and the block is stationary and in equilibrium.

Differential protection is based on the basic principle of Kirchhoff's basic law of current that "for any connection point in the power circuit at any time, the algebraic sum of the current flowing through the connection point is always zero". The differential protection regards the transformer being maintained as a contact, and the current and voltage transformers are placed on each side of the transformer, and the secondary sides of the current and voltage transformers on each side of the transformer are wired according to the differential wiring method, that is, the The houses with the same optical activity are facing the side of the busbar, and the same optical activity terminals will be connected, and the differential protection automotive relays will be connected in series.

The current passing through the electromagnetic coil of the automotive relay is the difference between the secondary currents of the current and voltage transformers on each side. In other words, the differential protection automotive relay is connected to the differential protection control loop; in theory, everything is normal. When the fault occurs outside the maintenance area, the current injected into the transformer is the same as the discharged current (the converted current), and the current in the differential protection control loop is zero. (To make the current value of the secondary side of the main transformer high voltage and low voltage measuring current and voltage transformers the same, be sure to select current transformers with different transformation ratios.) However, there are unbalanced currents in the differential protection control loop through various elements, so the difference The starting current value of the dynamic protection posture needs to be measured and set. When the differential current exceeds the whole time constant of the differential protection device, the maintenance posture will trip the isolation switches on each side of the maintenance transformer, so that the faulty transformer will cut off the switching power supply.

There are several key reasons for the unbalanced current of the transformer:

1. Unbalanced current caused by transformer magnetizing inrush current Ily;

2. Unbalanced current caused by different current positions on both sides of the transformer;

3. Unbalanced current caused by the difference between the measured ratio and the specific ratio;

4. Unbalanced current caused by different models of current transformers on both sides;

5. The unbalanced current caused by the transformer with load regulating the sub-connector.