What is a transformer differential protection device? And how to explain it?

A common function of transformer differential protection devices.

1) Ratio differential quick break protection.

When any phase difference current is greater than the differential speed breaking setting fixed value, the instantaneous action outlet relay.

2) Ratio differential protection.

Ratio differential protection is a relatively complex protection function, involving some fixed parameters, such as ratio braking coefficient, second harmonic braking coefficient, inflection point fixed value, etc. In the calculation of fixed values, there is also exquisiteness. For the main transformer, the common wiring method is y.

However, there are other ways that are not common, so you need to be careful when setting the fixed value.

2. The installation method of the transformer protection device.

Normally, the device is often combined with backup protection to form a complete transformer protection, for this reason, in practical applications, it is mostly installed in the form of a group screen.

3. What technical parameters need to be referred to when selecting transformer differential protection?

1) Timing mode, the current commonly used timing method is B code time alignment, and sometimes network timing is also used.

2) Communication mode, at present, for the protection device, the communication mode has dual network port, single network port, one network port + 485 communication mode and so on. Communication protocols include 103 protocol, modbus and 61850, and so on.

3) For the user station, the conventional protection device is generally used, and for the system project, that is, the power grid project, the smart grid will be involved, and the protection device of the two is completely different.

4) For the quantity requirements of opening in and out, there is also the distinction between two circles and three circles, which shall be subject to the actual situation.

Transformer differential protection.

Rationale: A current transformer that constitutes the differential protection of a transformer.

electrical equipment, and the wires connecting them. Since the differential protection does not act on the fault outside the protected area, the differential protection does not need to cooperate with the protection of adjacent components outside the protected area in the action value and action time limit, so it can act instantaneously when the fault in the area is located.

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 cycle current method, that is, if the sibling end of the current transformer on both sides is facing the bus side, the homogeneous terminal will be used.

and connect the current relay in series between the two wirings. The current flowing in the relay coil is the secondary current difference between the two current transformers, which means that the differential relay is connected to the differential circuit. Theoretically, the differential loop current is zero during normal operation and external faults.

Cause of malfunction. 1. Secondary side load.

A short-circuit current is flowing.

, it cannot meet the requirements of the CT10% error curve. When the capacity of the current transformer access system changes or the new protection is put into operation, it is not possible to ignore whether the 10% error curve of the CT for nuclear protection meets the requirements according to the maximum short-circuit current of the transformer and the measured secondary load of the differential circuit when the short-circuit fault in the differential protection zone is passed.

2. The grounding method of the secondary current circuit of differential protection is improper.

When the differential protection secondary current loop is grounded, the secondary current loop of the TA on each side must be connected to the ground grid through a point, because the grounding network of the substation is not absolutely equipotential with each other.

There is a certain potential difference between the different points.

When a short-circuit fault occurs, there is a large current flowing into the ground grid, and the potential difference between the points is large.

If the secondary current loops of differential protection are connected to different points of the ground grid, the current generated by the potential difference between them will flow into the protection device, affecting the accuracy of the operation of the differential protection device or even making it misoperate. Therefore, the secondary current circuits of the CT on each side should be connected in parallel to the differential current circuit of the protection device, and all the current circuits must be grounded at the common point of parallel connection.

The principle is the same as that of line longitudinal protection, which compares the phase and value of the current on each side of the protected equipment. ­

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. ­

Peculiarity. The excitation current (excitation current) of the transformer flows through only one side of the transformer, so an unbalanced current will be formed when it is reacted to the differential circuit through the current transformer.

In steady-state operation, the excitation current of the transformer is not large, only 2-5% of the rated current. When a fault occurs outside the differential range, the excitation current decreases due to the voltage drop. Therefore, the unbalanced current formed in both cases is very small and has little effect on the differential protection of the transformer.

However, when the transformer is put into no-load and the voltage is restored after the external fault is removed, a large excitation current, i.e., excitation inrush current, may occur.

Differential protection is made according to the principle that the sum of the currents flowing into the nodes in the circuit is equal to zero. Differential protection treats the protected electrical equipment as a contact, so that the current flowing into the protected equipment is equal to the current flowing out, and the differential current is equal to zero.

When the equipment fails, the current flowing into the protected equipment and the current flowing out are not equal, and the differential current is greater than zero. When the differential current is greater than the setting value of the differential protection device, the protection action will jump off the circuit breakers on each side of the protected equipment, so that the faulty equipment will be disconnected from the power supply.

The following items should be checked after transformer differential protection:

1. Check whether the transformer body is abnormal, check whether the insulator within the differential protection range has flashover, damage, and whether the lead wire is short-circuited;

2. If there is no obvious fault in the equipment within the differential protection of the transformer, whether there is a fault in the relay protection and the secondary circuit, and whether the DC circuit is grounded at two points;

3. When there is no abnormality after the above inspection, it should be tried immediately after the load is removed, and then tripped, and it should not be sent again;

4. If it is caused by the failure of the relay or the secondary circuit and the DC two-point grounding, the differential protection should be withdrawn, and the transformer should be powered, and then the secondary circuit barrier and DC grounding should be dealt with;

5. When the differential protection and heavy gas protection trip the transformer at the same time, the transformer shall not be put into operation without internal inspection and test.

1. Principle of differential protection.

Differential protection is the use of Kirchhoff.

When the current theorem works, when the transformer is working normally or when it is faulty outside the area, 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 there is an internal fault in the transformer, two (or three) sides provide short-circuit current to the fault point.

The difference protection is proportional to the sum of the secondary currents perceived.

At the point of fault, the current is 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.

The principle of differential protection works using Kirchhoff's current theorem, when the transformer is working normally or when it is faulty outside the area, 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 is proportional to the fault point current, and the differential relay acts.

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

Advantages and disadvantages of differential protection.

The advantage of differential protection is that it can quickly and selectively remove faults within the protection range, the wiring is correct, the adjustment is appropriate, and there will be no misoperation.

The disadvantage is that it can not reflect the internal transformer internal faults such as overheating burns of the iron core, the reduction of the oil level, etc., such as the inter-turn short circuit of a few turns in the transformer winding, although the short-circuit turn short-circuit current is very large, which will cause serious overheating of the local winding and produce a strong oil flow to the direction of the oil pillow, but the phase current is not large, and the differential protection will not react. Therefore, differential protection is not a substitute for gas protection.

Differential protection is the main protection to prevent the internal fault of the transformer, which is widely used in substations of 35 thousand volts and above, mainly used to protect various phase short circuit faults that occur inside the winding of the double or triple winding transformer and its lead line, and can also be used to protect the transformer from single-phase inter-turn short circuit faults.

The scope of differential protection is the electrical equipment between the current transformers that make up the differential protection of the transformer and the wires connecting these devices. Since the differential protection will not act on the fault outside the protected area, the differential protection does not need to cooperate with the protection of adjacent components outside the protected area in the action value and action time limit, and can be set to instantaneous action when the obstacle occurs in the area; The principle of the number of differential protection feet is simple, the amount of electrical used is simple, the protection range is clear, and the action does not need to be extended, so it is used for transformer protection.

The differential protection of the transformer is the main protection of the transformer, which is installed according to the principle of circulating current.

The so-called differential protection of transformers refers to the protection composed of the comparison of the values and phases of the primary and secondary currents of the transformer. The longitudinal differential protection device is generally used to protect the interphase short circuit that occurs on the transformer coil and the lead line and the single-phase ground short circuit in the high-current grounding system. For internal faults such as turn-to-turn short circuits in transformer coils, only backup protection is usually used.

The transformer differential protection is composed according to the principle of circulating current, and the principle of transformer longitudinal protection requires that when the transformer is in normal operation and the longitudinal protection zone (the longitudinal protection zone is the range between the current transformer TA1 and TA2) is faulty, the current flowing into the differential relay is zero, so as to ensure that the longitudinal protection does not operate.

However, due to the different rated currents on the high-voltage side and the low-voltage side of the transformer, in order to ensure the correct operation of longitudinal protection, it is necessary to appropriately select the conversion ratio of the current transformer on both sides, so that the two currents are equal during normal operation and external fault.