Analysis and countermeasures of transformer differential protection malfunction
The Power Transformer is one of the key main equipment in the power system. It undertakes voltage conversion, power distribution and transmission, and provides power services. Therefore, the normal operation of the transformer is an important guarantee for the safe, reliable, high-quality and economical operation of the power system. As the main protection of the main equipment, the microcomputer-based longitudinal differential (abbreviated as longitudinal differential or differential) protection has been continuously improved, but there are still some false operations, which will cause abnormal shutdown of the transformer, affect the power generation and supply of the power system, and even cause system oscillation, which is very unfavorable to the stable operation of the power generation and supply of the power system. Therefore, the causes of false operation of the transformer differential protection of newly built or equipment-updated power plants and substations are analyzed, and countermeasures to prevent transformer differential false operation are proposed.
1 Transformer differential protection
Transformer differential protection generally includes: differential quick-break protection, ratio differential protection, and ratio differential protection of secondary (fifth) harmonic braking. Regardless of the differential protection function, the differential current is obtained by the vector sum of the currents on each side of the transformer. When the transformer is operating normally or there is a fault outside the protection zone, the differential current is approximately zero. When there is a fault in the protection zone, the differential current increases. Now take the double-winding transformer as an example for explanation.
1.1 Action characteristics of ratio differential protection The action characteristics of ratio differential protection are shown in Figure 1. When the transformer has a minor fault, such as when the number of turns short-circuited is small, there is no braking amount, which makes the protection have a higher sensitivity when the transformer has a minor fault. In the case of a more serious out-of-zone fault, there is a larger braking amount, which improves the reliability of protection.
The main difference between the second harmonic braking is the fault current or the excitation inrush current, because when the transformer is put into operation without load, a relatively large excitation inrush current will be generated, accompanied by a second harmonic component. In order to prevent the transformer from malfunctioning, the harmonic braking principle is adopted. By judging whether the second harmonic component reaches the set value, it is determined whether the transformer is faulty or the transformer is put into operation without load, thereby determining whether the ratio differential protection is activated. The second harmonic braking ratio is generally 0.12 to 0.18. For some large transformers, in order to increase the reliability of protection, the braking principle of the fifth harmonic is also adopted.
1.2 The role of differential quick-break protection Differential quick-break protection is to quickly trip the circuit breakers on both sides of the transformer and cut off the fault point in the case of a more serious in-zone fault. The setting value of differential quick-break is based on the larger of the excitation inrush current of the transformer and the unbalanced current caused by the through fault under the operation mode. The setting value is generally (4~14)Ie.
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2 Analysis of the causes of transformer differential protection misoperation
According to the possibility of transformer differential protection misoperation, it is roughly divided into three aspects: newly built power plants and substations, operating power plants and substations, and power plants and substations with equipment renewal and transformation. This classification method is not mutually exclusive, but is only for the convenience of giving priority to practical problems when analyzing problems.
2.1 Analysis of the causes of transformer differential protection misoperation in newly built power plants and substations The misoperation of transformer differential protection in newly built substations accounts for a large proportion of the transformer differential protection misoperation, but the misoperation in this case is generally discovered within 72 hours of the transformer being put into operation and tested with load. Based on field experience, the following aspects can be summarized:
2.1.1 Misoperation of transformer differential protection caused by unreasonable setting value The ratio of differential quick-break setting value and secondary harmonic braking The incorrect selection of differential setting value causes misoperation. Differential quick-break is to quickly trip the circuit breakers on both sides of the transformer and cut off the fault point in the case of a more serious fault in the area. The setting value of the differential quick-break is based on the larger of the excitation surge current of the transformer and the unbalanced current caused by the through fault under the operating mode. The setting value is generally (4~14)Ie. For the calculation department of the protection setting value, especially the setting calculation department of the non-power system, the differential quick-break setting value is often taken as (5~6)Ie based on operating experience. In this way, the circuit breaker will trip falsely when the transformer is closed under no-load. In particular, the influence of the excitation surge current on the protection. After the transformer protection equipment of a power plant in Guangdong was updated and renovated, the above false tripping phenomenon occurred due to the excitation surge current generated by the no-load transformer. Ratio differential means that when a minor fault occurs inside the transformer, the protection will trip the circuit breakers on both sides without braking, so that the protection has a higher sensitivity when the transformer has a minor fault; in the case of an out-of-zone fault, braking is performed through a certain ratio to improve the reliability of the protection; at the same time, the secondary harmonic generated when the transformer is closed without load is used to distinguish whether it is a fault current or an excitation inrush current, so as to achieve protection braking. Generally, the differential current and braking current are calculated under rated conditions, but the transformer on site is in the general operating mode. Due to the influence of the current transformer ratio, simultaneous coefficient, and calculation error, a certain differential current will be formed during the actual operation of the transformer, resulting in the misoperation of the ratio differential protection. The incorrect selection of the setting value of the secondary current transformer (TA) wiring method causes misoperation. For microcomputer protection, the transfer of the current phase angle of the high and low voltage sides is completed by software. Regardless of whether the high voltage side adopts Y-type wiring or △-type wiring, the correct differential current can be obtained. Compared with traditional conventional relay protection, the actual application is more convenient and flexible, but it is also due to this flexibility and convenience that the differential protection on site often misoperates. For transformer differential protection, if the setting value of the secondary current transformer (TA) connection method is not selected correctly, the phase angle transfer on the high-voltage side cannot be achieved, and the differential current between the high-voltage and low-voltage sides cannot be balanced under normal operation, resulting in false operation of the differential protection.
