Introduction to transformer protection
Transformers generally use protection methods. The abnormal working conditions of transformers mainly include overload, overcurrent caused by external short-circuit, neutral overvoltage caused by external grounding short-circuit, oil level reduction caused by oil tank leakage, or temperature caused by cooling system failure Elevate etc. In addition, large-capacity transformers, due to their high rated working magnetic flux density, are proportional to the voltage-frequency ratio. When operating under overvoltage or low frequency, it may cause overexcitation faults of the transformer. In view of the above situation, large transformers generally adopt the following protection methods:
1. Gas protection: It protects the internal short circuit of the transformer and the failure of the oil level reduction.
2. Differential protection, current quick-break protection: protect transformer windings or phase-to-phase short-circuits of lead wires, ground short-circuits of large ground current systems, and short-circuits between winding turns.
3. Overcurrent protection: protect the external phase-to-phase short circuit, and serve as a backup protection for gas protection and differential protection (or current quick-break protection).
4. Zero-sequence current protection: protect the external single-phase grounding short circuit of the large ground current system.
5. Overload protection: protects symmetrical overload, only acting on signals.
6. Over-excitation protection: protect the transformer's over-excitation from exceeding the allowable limit.
Transformer gas protection reacts to various faults and oil level drops inside the transformer tank. Oil-immersed transformers of 0.8MVA and above and oil-immersed transformers in workshops of 0.4MVA and above shall be equipped with gas protection. When a fault in the oil tank produces a slight gas or the oil level drops, it should act on the signal instantaneously; when a large amount of gas is produced, it should act to disconnect the circuit breakers on each side of the transformer. The pressure regulating device of the oil-immersed transformer with load regulation should also be equipped with gas protection.
The second protection method generally adopted for transformers: longitudinal differential protection or current quick-break protection
The longitudinal differential protection or current quick-break protection of the transformer lead wire, bushing and internal short circuit fault. The protection momentarily acts to open the circuit breakers on each side of the transformer.
1. For factory transformers below 6.3MVA and transformers operating in parallel, as well as factory backup transformers below 10MVA and transformers operating separately, when the backup protection time is greater than 0.5s, current quick-break protection should be installed.
2. For 6.3MVA and above factory working transformers and parallel operation transformers, 10MVA and above factory backup transformers and stand-alone operation transformers, and 2MVA and above transformers whose current quick-break protection sensitivity does not meet the requirements, they should be installed Longitudinal differential protection.
3. For transformers with high voltage side voltage of 330kV and above, dual longitudinal differential protection can be installed.
4. For generator-transformer group, when there is a circuit breaker between the generator and the transformer, the generator is equipped with a separate longitudinal differential protection. When there is no circuit breaker between the generator and the transformer, the generator and transformer group of 100MVA and below share the longitudinal differential protection; the generator of 100MVA and above. In addition to the generator and transformer sharing the longitudinal differential protection, the generator shall also be equipped with a longitudinal differential protection separately. For generators and transformers of 200~300MVA, a separate longitudinal differential protection can also be added to the transformer, that is, double fast protection is adopted.
It reacts to short circuit between external phases of the transformer and performs gas protection and longitudinal differential protection (or current quick-break protection) backup overcurrent protection, overcurrent protection for low voltage starting, overcurrent protection for composite voltage starting, negative sequence current protection and impedance protection , After the protection is activated, it should be activated with a time limit for tripping.
1. Overcurrent protection is suitable for step-down transformers.
2. The overcurrent protection of compound voltage starting is suitable for step-up transformers, system contact transformers and step-down transformers whose overcurrent protection does not meet the sensitivity requirements.
3. Negative sequence current and single-phase low-voltage starting over-current protection can be used for 63MVA and step-up transformers.
4. When the above 2 and 3 protections cannot meet the sensitivity and selectivity requirements, impedance protection can be used.
Four common protection methods for transformers: zero sequence current protection
The zero-sequence current protection that responds to the external grounding short circuit of the transformer in the large ground current system. In the 110kV and above large ground current system, if the neutral point of the transformer may be grounded, zero-sequence current protection should be installed for the step-up transformer or step-down transformer of the two-sided or three-sided power supply as the backup protection of the transformer main protection, and As a backup protection for adjacent components.
What is zero sequence current protection
The device that uses the zero sequence current generated when grounding to make the protection operate is called zero sequence current protection. Special zero-sequence current transformers are used on cable lines to achieve grounding protection. Put the zero-sequence current transformer on the ground three-core cable, and connect the current relay to the secondary coil of the transformer. In normal operation or without ground fault, since the vector sum of the three-phase current of the cable is equal to zero, the zero-sequence transformer is two The current of the secondary coil is also zero (only a small unbalanced current), so the current relay does not operate. When a ground fault occurs, a larger current will appear in the secondary coil of the zero-sequence transformer, causing the current relay to act in order to send a signal or remove the fault.
Five common protection methods for transformers: overload protection
Overload protection that responds to symmetrical overload of the transformer.
For transformers of 400kVA and above, when the number of transformers is operated in parallel or separately and used as a backup power source for other loads, overload protection should be installed according to the possible overload conditions. For autotransformers and multi-winding transformers, the protection device should be able to reflect the overload conditions of the common winding and each side. In most cases, the overload current of the transformer is three-phase symmetrical. Therefore, the overload protection only needs to be connected to one-phase current, and the current relay is implemented, and a certain delay is applied to the signal. When choosing which side of the protection to install, it should be considered that it can reflect the overload situation of the coils on all sides of the transformer. In substations where there are no personnel on duty, the overload protection can act to trip or disconnect part of the load when necessary.
Transformer protection mode six: over-excitation protection
Over-excitation protection in response to over-excitation of the transformer.
In the current design of large transformers, in order to save materials, reduce cost, and reduce transportation weight, the rated working magnetic flux density of the iron core is designed to be higher, about 1.7~1.8 T, which is close to the saturation magnetic density (1.9~2 T). In the case of overvoltage, it is easy to produce overexcitation. In addition, because the magnetization curve is relatively "hard", during overexcitation, the excitation impedance decreases due to the saturation of the core, and the excitation current increases quickly. When the working magnetic density reaches 1.3 to 1.4 times the normal magnetic density, the excitation current can reach the rated current Level. Secondly, because the excitation current is a non-sine wave and contains many high-order harmonic components, and the eddy current loss of the iron core and other metal components is proportional to the square of the frequency, it can cause serious overheating of the iron core, metal components, and insulating materials. High and long duration may damage the transformer. Therefore, 500kV transformers on the high-voltage side should be equipped with over-excitation protection.
The purpose of installing the transformer overexcitation protection is to detect the overexcitation of the transformer, send a signal or act on a trip in time, so that the overexcitation of the transformer does not exceed the allowable limit, and to prevent the transformer from being damaged due to overexcitation.