1. protection differential.

In relay protection, current differential protection is a kind of protection. Differential protection is based on the principle that the sum of the currents flowing into the nodes in the circuit is equal to zero.

1.1 the principle of differential protection.

The differential protection is carried out using Kirchhoff's current theorem. When the transformer is working normally or external fault, it is regarded as an ideal transformer, and then the current of the transformer and the outflow current (converted current) are equal, and the differential relay does not work. When there is a fault inside the transformer, both sides (or three sides) should provide short-circuit current to the fault point. The secondary current felt by the differential protection is proportional to the fault point current and the action of the differential relay.

1.2 differential protection function.

It is mainly used to protect various interphase short-circuit faults in double-winding or three-winding transformer windings, and can also be used to protect short-circuit faults between single-phase transformers. The current transformer is installed on both sides of the winding transformer, and the secondary side is wired according to the circulating current method, that is, if the same terminal of the current transformer on both sides faces the bus, connect the same terminal and connect the current relay between the two connections. The current flowing in the relay coil is the difference between the secondary currents of the current transformers on both sides, that is, the differential relay is connected to the differential circuit. Theoretically, the current of the differential circuit is zero during normal operation and external faults. In fact, because the characteristics of the current transformers on both sides cannot be completely consistent, there is still an unbalanced current in the differential circuit during normal operation and external short circuit.

The scope of transformer differential protection is the electrical equipment between the current transformers, which constitutes the transformer differential protection, and the wires connecting these equipment. Since the differential protection will not fail outside the protection zone, the differential protection does not need to cooperate with each other in the action value and action time limit of the adjacent component protection outside the protection zone, so when a failure occurs in the zone, it can be Instantly move.

Differential protection is a protection device that reflects the current difference on both sides of the protection element (or area). Differential protection is to protect the transformer from internal short-circuit faults. Current transformers are installed on both sides of the transformer. When the normal load or external short circuit, the current of the bad relay is unbalanced current. Under the proper selection of the transformation ratio and wiring mode of the current transformers on both sides, the unbalanced current value is very small, which is lower than the operating current of the bad protection, so the protection does not operate. When the transformer is short-circuited, the current of the bad relay is greater than the action current of the bad protection, and the bad protection action trips. Due to the primary and secondary current of the transformer, the voltage is different, the phase is different, the characteristics of the current transformer is different, and the power supply side has different excitation current, which will lead to unbalanced current through the relay, so corresponding measures must be taken to eliminate the influence of unbalanced current.

1.3 main measures to protect the differential.

(1) Reduce unbalanced current under steady-state conditions.

The current transformer on each side of transformer differential protection shall be Class D current transformer dedicated for transformer differential protection; the secondary load of differential protection circuit shall meet the requirement of 10% error when passing through large external steady-state short-circuit current.

(2) Reduce the secondary load of the current transformer.

This effectively amounts to a reduction in the terminal voltage on the secondary side and a corresponding reduction in the excitation current of the current transformer. The commonly used method to reduce the secondary load is to reduce the resistance of the control cable (appropriately increase the electrical cross-section and shorten the length of the control cable as much as possible); Use a weak current control current transformer (secondary rated current is 1A), etc.

(3) Use a current transformer with a small air gap.

The core magnetism of the current transformer is small, and the current transformer is not easy to be saturated when the side current is large. Therefore, the excitation current is small, which is advantageous in reducing the unbalanced current. It also improves the transient characteristics of the current transformer. Ratio differential protection is a type of differential protection. Differential protection should adopt the principle of ratio differential protection: to prevent faults outside the transformer area (through faults), the CT transmission characteristics of the high and low voltage sides are inconsistent, resulting in differential flow, exceeding a fixed value and action. When proportional braking with differential protection is adopted, as the cross current increases, the threshold of differential activation will be increased to ensure that the cross fault will not be misled.

The second is gas protection.

Gas protection is the main protection component of transformer internal fault. It can be sensitive to transformer inter-turn and inter-layer short circuit, iron core fault, casing internal fault, winding internal fracture, insulation fault and oil surface drop. Whenoil-immersed transformerIn the event of an internal failure, the insulating material is broken down and produced by the arc.

A large amount of gas is generated, which flows from the oil tank to the oil conservator, and its strength varies with the degree of failure,

The protection of gas flow and oil flow is called gas protection, also known as gas protection.

2.1 the scope of protection of the gas.

Gas protection is the main protection of the transformer, which can reflect all the faults in the oil tank. Including: multi-phase short circuit in the oil tank, short circuit between winding turns, short circuit between winding and iron core or shell, iron core failure, oil surface drop or oil leakage, poor contact of connecting switch or poor welding of wires, etc. The gas protection action is fast, sensitive, reliable and simple in structure. However, it cannot reflect the failure of the external circuit of the fuel tank (such as the outlet line), so it cannot be used as the only protection device to protect the internal failure of the transformer. In addition, under the interference of some external factors (such as earthquakes), gas protection can easily go astray.

2.2 gas relay action principle.

When the transformer internal fault fault, the gas generated will gather in the upper part of the gas relay to reduce the oil level. When the oil level drops to a certain level, the buoy sinks, connects the mercury connector and sends a signal. If the fault is serious, the oil flow will impact the baffle to deflect, and drive the connecting rod behind the baffle to rotate upward, provoking the connecting ring connected to the mercury connector snap ring, so that the mercury connector rotates to both sides perpendicular to the oil flow. The two mercury connectors are connected at the same time to trip or signal the switch.

2.3 how to protect and install gas.

Install the gas relay on the connecting pipe from the transformer to the oil conservator. Pay attention:

(1) First, close the disc valve on the gas relay pipe. If the disc valve is not closed tightly or there are other conditions, the oil in the oil conservator can be discharged if necessary to prevent a large amount of oil from overflowing during work.

(2) Before installing a new gas relay, check whether there is an inspection certificate, correct caliber, flow rate, and whether internal and external components are damaged. If there is a temporary lashing inside, it should be removed. Later, check the reliability of the float, damper, signal and trip contacts, and close the exhaust valve.

(3) The gas relay should be installed horizontally, and the direction of the arrow marked on the top cover should point to the oil conservator. In the project, the pipe axis of the relay should be slightly higher than one end of the oil conservator, but the inclination from the horizontal should not exceed 4%.

(4) Open the disc valve to fill the gas relay with oil, and exhaust from the exhaust valve after filling the oil. If the oil pillow has capsules, attention should be paid to the method of oil filling and exhaust to minimize and avoid gas entering the oil pillow.

(5) When protecting the wiring, it is necessary to prevent wrong connection and short circuit, avoid live operation, and prevent the rotation of the guide rod and the oil leakage of the small flow head.

(6) Insulation shaking test and transmission test shall be carried out before putting into operation.

2.4 operation and maintenance of gas protection devices.

(1) General precautions:

A. When the transformer is in normal operation, the light gas protection bidding signal position, heavy gas protection bidding trip position;

B. the differential protection of transformer must be put into operation normally during the period when the heavy gas protection is changed to the signal position,

c. When working on the transformer, it must be noted that the withdrawal of heavy gas protection may lead to gas protection misoperation.

(2) When the following work occurs, the heavy gas protection must be put into the signal position; After the work is finished, the heavy gas protection is put into the trip position:

(3) When there are the following tasks, heavy gas protection must be thrown to the signal position; after the work is over, the transformer load runs for 24 hours, and the heavy gas protection is thrown to trip after professional inspection.

As the main protection of the transformer, during the operation of the transformer, at least one gas protection and differential protection must be put into operation, and both must not be withdrawn at the same time. Gas protection and differential protection as the main protection of oil-immersed transformer.