Main Transformer Differential Protection Principle

1 Introduction The main transformer differential protection is the main protection method of the transformer. The basic principle is to reflect the difference between the inflow and outflow currents at each end of the protected transformer. In the protection zone, the current value in the differential circuit is greater than the setting value, differential protection. Instantaneous action, but outside the protection zone, the main transformer differential protection should not act. Transformer excitation current, wiring mode, current transformer error and other factors, so that the differential circuit produces unbalanced current, unbalanced current in the presence of magnetizing inrush current, often can lead to transformer differential protection misoperation, to the transformer poor The realization of dynamic protection brings difficulties, so adopting measures to reduce unbalanced current and its impact on protection is the main contradiction that needs to be solved to achieve differential protection of main transformers.

2 main transformer differential protection analysis In the main transformer differential protection used in the choice of current transformer, in addition to should be selected with the air gap of the D-class core transformer, but also should be appropriately increased current transformer ratio to reduce Short circuit current multiples, which can effectively weaken the magnetizing inrush current, reducing the imbalance current generated in the differential circuit, improve the sensitivity of differential protection. This is especially effective in avoiding faults outside the protected area, especially the most serious three-phase metal short circuit caused by the main transformer differential protection malfunction. The following will be analyzed by examples:

Example: A three-phase three-winding step-down transformer with capacity Se=40.5MVA, voltage 110±2×2.5%kV/385±2×2.5%kV/11kV, connection mode: Ydd11-11, transformer rated current: 213A/ 608A/2130A. The main transformer differential protection adopts BCH-2 type differential relay. The basic calculations are shown in Table 1. The 110-kV side has been determined as the basic side.

The setting values ​​of the main transformer differential protection are as follows (the calculation process is omitted):

The differential coil's calculation parameter: Wcd. Js = 6.3 匝, the actual number of rounding down, taking Wcd. Js=6匝;

Actual operating current of relay: Idz=10A;

Sensitivity K1m=2.1.

The substation has experienced a three-phase metallic short circuit caused by external force damage at the outlet of the 10 kV line, and the 10 kV line has a short circuit current, which has caused the main transformer differential protection to malfunction. Our preliminary analysis is that the short-circuit point is too close to the protection and is the most serious three-phase metal short circuit. The short-circuit current is extremely large. When the external fault is removed and the voltage recovers, a large value of magnetizing inrush current appears and the differential circuit generates. The unbalanced current is greater than the setting current value and the main transformer differential protection malfunctions. However, if the protection setting is increased, if the protection setting is increased to 11A, the sensitivity becomes smaller, K1m=1.91<2, and the sensitivity requirement cannot be satisfied.

After a comprehensive analysis, it was considered that using the BCH-2 type with a speed-saturated current-differential differential relay to avoid the existence of certain defects in the excitation inrush current. From the characteristics of magnetizing inrush current, for three-phase transformers, when the voltage recovers, there are at least two phases with varying degrees of inrush current, that is, there may be one phase without aperiodic components in the three-phase excitation inrush current, and the speed saturation converter will lose its function. . Analysis of the protection setting, differential protection current transformer ratio selected somewhat lower, and tends to saturation. In this way, when the most serious three-phase metal short circuit occurs, the error of the current transformer due to saturation increases, which not only increases the unbalanced current, but also causes the current transformer to be heavily overloaded. Increasing the transformation ratio of the current transformer can reduce the multiple of the short circuit current and reduce the unbalance current in the differential circuit, which can effectively reduce the unbalance current caused by the excitation inrush current and the fault outside the zone.

The setting values ​​of the main transformer differential protection are as follows (the calculation process is omitted):

The differential coil's calculation parameter: Wcd. Js = 8.1 匝, the actual number of rounding down, taking Wcd. Js=8匝;

Actual operating current of relay: Idz=9A;

Sensitivity K1m=2.44.

According to Table 1, 2 and some setting data, the current transformer's transformation ratio is increased, 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 loop: 35kV side decreased from 0.55A to 0.31A, 10kV side decreased from 1.05A to 1.03A; sensitivity increased from 2.1 to 2.44. In other words, the secondary loop current of the current transformer and the unbalanced current in the differential circuit are reduced, the sensitivity of the protection device is improved, and the most serious three-phase metallic short circuit outside the main transformer differential protection zone occurs, the current transformer is not caused. The error is increased due to severe overload, which effectively weakens the unbalanced current caused by magnetizing inrush current and out-of-zone faults. After the increase of the current transformer ratio, the 10kV line recurred similar short circuit fault, the main transformer differential protection has not been misacted.

3 conclusions In order to prevent the main transformer from differential protection due to the influence of unbalanced current protection caused by misoperation, in the choice of protection with current transformers, in addition to should be selected with air gap D-class core transformer, the appropriate increase in current Transformer ratio, this can reduce the short circuit current multiple, reducing the imbalance current generated in the differential circuit, effectively weakening the magnetizing inrush current, improve the sensitivity of differential protection. This is one of the effective ways to avoid faults outside the protected area, especially the most serious three-phase metal short circuit caused by the main transformer differential protection malfunction.