The main transformer differential protection is the main protection method of the transformer. The basic principle is to react the difference between the inflow and outflow currents of the protected transformer at each end. The fault in the protection zone is greater than the setting value in the protection circuit. The differential protection instantaneous action However, in the case of a fault outside the protected area, the main differential protection should not be operated. Influenced by factors such as transformer excitation current, wiring mode, current transformer error, etc., the unbalanced current is generated in the differential circuit, and the existence of the magnetizing inrush current in the unbalanced current often causes the transformer differential protection to malfunction, giving the transformer a difference. The realization of dynamic protection brings difficulties, so the use of measures to reduce the unbalanced current and its impact on protection is the main contradiction that needs to be solved to achieve the main transformer differential protection.
2 Analysis of main transformer differential protectionIn the selection of the current transformer used for the main transformer differential protection, in addition to the D-class iron core transformer with air gap, the current transformer ratio should be increased appropriately to reduce the short-circuit current multiplier. The excitation inrush current is weakened, the unbalanced current generated in the differential circuit is reduced, and the sensitivity of the differential protection is improved. This is especially effective for avoiding malfunctions outside the protection zone, especially the most severe three-phase metallic short circuit. The following will be analyzed by example:
Example: A three-phase three-winding step-down transformer with a capacity of S e = 40.5 MVA, voltage 110 ± 2 × 2.5% kV / 385 ± 2 × 2.5% kV / 11kV, wiring: Ydd11-11, transformer rated current: 213A /608A/2130A. The main transformer differential protection uses the BCH-2 differential relay. The basic calculation is shown in Table 1. It has been determined that the 110kV side is the base side.
The setting value of the main transformer differential protection part is as follows (calculation process is omitted):
The number of calculations of the differential coil: W cd.js = 6.3 åŒ , the actual number of turns is rounded down, taking W cd.js = 6 åŒ ;
The actual operating current of the relay: I dz = 10A;
Sensitivity K 1m = 2.1.
The substation had a three-phase metallic short circuit due to external force damage at the 10kV line outlet, and the 10kV line current was quickly disconnected, causing the main transformer differential protection to malfunction. We initially analyzed that the short-circuit point is too close to the protection, and it is the most serious three-phase metallic short circuit. The short-circuit current is extremely large. When the external fault is removed and the voltage is restored, a large amount of magnetizing inrush current occurs, so that the differential circuit is generated. The unbalanced current is greater than the setting current value, causing the main transformer differential protection to malfunction. However, if the protection setting is increased, if the protection setting is increased to 11A, the sensitivity becomes smaller K1m=1.91<2, which does not meet the sensitivity requirement.
Table 1
Project high voltage side medium voltage side low voltage side rated voltage (kV) 110 35 11 rated current (A) 213 608 2130 current transformer wiring DYY current transformer calculation ratio 608/5 2130/5 Current transformer selection ratio 400/5 750/5 3000/5 Current flowing into the differential circuit (A) 213/(400/5)×3=4.6 608/(750/5)= 4.05 2130/(3000/5)=3.55 Unbalanced current (A) 0 4.6-4.05=0.55 4.6-3.55=1.05
After comprehensive analysis, it is considered that the BCH-2 type has a fast saturation variable current differential relay to avoid certain defects in the magnetizing inrush current. From the characteristics of the magnetizing inrush current, for the three-phase transformer, when there is voltage recovery, there are at least two phases of magnetizing inrush current, that is, there may be one phase in the three-phase magnetizing inrush current without aperiodic components, and the time-saturated converter will lose its effect. . Analyze the protection setting, the differential protection current transformer ratio is selected to be somewhat low, and tends to be saturated. In this way, when the most serious three-phase metallic short circuit occurs, the current transformer increases its error due to saturation, which not only increases the unbalance current, but also seriously overloads the current transformer. Increasing the current transformer ratio can reduce the short circuit current multiple and reduce the unbalanced current in the differential loop, thus effectively reducing the unbalance current generated by the magnetizing inrush current and the out-of-zone fault.
According to the above analysis, a method of increasing the ratio of the current transformer is used, and the data is as shown in Table 2 (the same parts as in Table 1 are omitted).
Table 2
Project high-voltage side medium-voltage side low-voltage side current transformer selection ratio 500/5 900/5 4000/5 current flowing into the difference circuit (A) 608(900/5)=3.38 2130/(4000/5)=2.66 Unbalanced current (A) 0 3.69-3.38=0.31 3.69-2.66=1.03
The setting value of the main transformer differential protection part is as follows (calculation process is omitted):
The number of calculations of the differential coil: W cd.js = 8.1 åŒ , the actual number of turns is rounded down, taking W cd.js = 8 åŒ ;
The actual operating current of the relay: I dz = 9A;
Sensitivity K 1m = 2.44.
It can be seen from Table 1, 2 and part of the setting data that after the current transformer ratio increases, the secondary current 110kV side is reduced from 4.6A to 3.69A, the 35kV side is reduced from 4.05A to 3.38A, and the 10kV side is reduced from 3.55A to 2.66A; Unbalanced current in the differential circuit: the 35kV side is reduced from 0.55A to 0.31A, the 10kV side is reduced from 1.05A to 1.03A; the sensitivity is increased from 2.1 to 2.44. That is to reduce the current of the secondary circuit of the current transformer and the unbalanced current in the differential circuit, and improve the sensitivity of the protection device, so that the current transformer does not cause the most serious three-phase metallic short circuit outside the main differential protection zone. The error is increased due to severe overload, which effectively weakens the unbalance current generated by the magnetizing inrush current and the out-of-zone fault. After the self-increase current transformer ratio, when the 10kV line has a similar short-circuit fault, the main transformer differential protection has not been mis-operated.
3 ConclusionIn order to prevent the protection malfunction caused by the unbalanced current of the main transformer differential protection, when selecting the protection current transformer, in addition to the D-class iron core transformer with air gap, increase the current transformer appropriately. The ratio is reduced, which can reduce the short circuit current multiple, reduce the unbalance current generated in the differential circuit, effectively weaken the magnetizing inrush current, and improve the sensitivity of the differential protection. This is one of the effective methods to avoid malfunctions of the main transformer differential protection caused by faults outside the protection zone, especially the most serious three-phase metallic short circuit.
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