Design of differential protection device for microcomputer transformer

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The differential protection of the transformer is the main protection of the transformer, which is installed according to the principle of circulating current. It is mainly used to protect various phase-to-phase short-circuit faults that occur inside the winding of double-winding or three-winding transformers and their lead-out lines, and can also be used to protect transformers from single-phase inter-turn short-circuit faults.

Current transformers are installed on both sides of the winding transformer, and the secondary side is wired according to the circulating current method, that is, if the sibling end of the current transformer on both sides is facing the bus side, the homogeneous terminal is connected, and the current relay is connected in series between the two wirings.

The main protection of the transformer responds to the internal and external faults of the transformer, and the protection action is on the switch, which separates the transformer from the system. However, the response to a few turns of the short circuit in the winding is not as good as that of gas protection.

The transformer differential protection with reed protection depends on the transformer wiring group (e.g., y δ-11; y/y、δy-1，δy0-11）。The microcomputer differential protects the general current y y wiring to compensate for the current. The phase difference is 180 degrees.

Other phase splitting zone protection exits differential protection.

Differential or differential can be directly added to the current, the differential action of the small point of the current, the differential action of the large point, sometimes the differential speed will not have a line to guess the message, then double the differential action current can be.

If it is a 3-side differential, it is generally OK to do the star angle (high and low side, medium and low side), and the 2-side differential is the same as above. Harmonic braking, adding fundamental current to a certain phase, and adding harmonic current at the same time, can be locked.

Compare the magnitude of the phase and value of the current on each side of the protected device. Due to the fact that the rated currents on the high-voltage side and the low-voltage side of the transformer are not equal, and the phase of the currents on each side of the transformer is often different. Therefore, in order to ensure the correct operation of longitudinal differential protection, it is necessary to appropriately select the transformation ratio of the current transformer on each side, and the compensation of the current phase on each side so that the secondary current on both sides is equal during normal operation and short-circuit fault outside the area.

The longitudinal differential protection principle of the DC line is similar to the differential protection principle of the AC system, and it is also by comparing the difference between the current of the local station and the opposite station, and when the difference is greater than the fixed value, the outlet is protected by a certain delay.

However, in addition to the short-term inconsistency of the current on both sides during the power adjustment, when the direct call is in normal operation, the current on both sides is a very stable direct flow, and there is no synchronization point that can be used as a reference.

As a backup protection, the DC line longitudinal differential protection has reduced the requirements for synchronization, and because the high-voltage DC transmission is mostly used for long-distance transmission, the DC line is relatively long, even if the channel transmission time is longer, and even the carrier channel is used, the action characteristics of the DC line longitudinal differential protection have little impact.

1. Principle of differential protection.

Differential protection works using Kirchhoff's current theorem, when the transformer is working normally or when it is faulty outside the zone, it is regarded as an ideal transformer, then the current flowing into the transformer and the outflow current (converted current) are equal, and the differential relay does not operate.

When the transformer is faulty internally, both sides (or three sides) provide short-circuit current to the fault point, and the secondary current felt by the differential protection is proportional to the fault point current, and the differential relay acts.

Second, the role of differential protection.

The transformer differential protection is the main protection of the transformer, which is mainly used to protect various interphase short-circuit faults that occur in the winding of the double-winding or three-winding transformer and its lead line, and can also be used to protect the transformer from single-phase inter-turn short-circuit faults.

(1) The composition of longitudinal differential protection.

The longitudinal differential protection works according to the principle of comparing the magnitude and phase of the current at the beginning and end of the protected element (No. 1 main transformer). In order to achieve this comparison, a group of current transformers TA1 and TA2 are arranged on both sides of the protected element, and their secondary side is wired according to the circulation method, that is, if the positive polarity terminals of the current transformer at both ends are placed on the side close to the busbar, then their secondary copolarity terminals are connected, and then the coil of the differential relay is incorporated to constitute differential protection. Among them, the differential relay coil circuit is called the differential circuit, and the circuit on both sides is called the two arms of differential protection.

2) The working principle of longitudinal differential protection According to Kirchhoff's first law, the vector sum of the currents on each side of the transformer is represented in the formula, and its physical meaning is: when the transformer is operating normally or the external fault, if the excitation current loss and other losses are ignored, the current flowing into the transformer is equal to the current flowing out of the transformer. Therefore, longitudinal protection should not be moved.

When the internal fault of the transformer, if the load current is ignored, only the current flowing into the transformer and not the current flowing out of the transformer, the longitudinal protection action will cut off the transformer. See the principle of transformer longitudinal protection wiring.

Reference:

The reason is very simple, it is the difference between the outflow and inflow current, and this amount is used to protect.

The differential protection of the transformer is installed according to the principle of circulating current. Two current transformers of the same model are installed on both sides of the transformer, and their secondary use of circulation method wiring, in the case of normal and external faults, the current on both sides of the transformer is equal in size and in opposite directions, and no current flows in the differential relay; In the event of a phase-to-phase short circuit within the differential protection range, a large amount of current flows through the differential relay.

Transformer differential protection range: all equipment, leads, aluminum wires, etc. between the circuit breakers on the high voltage side and the low voltage side of the transformer.

Differential protection of transformers: primary protection of transformers.

Ask the differential protection whole test (six groups of current), the high-voltage side A is O degree, B is 24O degree, C is 120 degrees, what is the phase of ABC on the low-voltage side, Ya 11, Ya Ya 12