Understand the characteristics of transformer installation protection device
Ideally, when the transformer differential protection is in normal operation and external short circuit, the current flowing into the differential relay is zero and the protection device does not operate. However, in fact, the working behavior of the transformer differential protection is very different from that of the generator differential protection in normal operation and external short circuit. Because when the transformer differential relay is short-circuited outside the protection zone, it is affected by some factors that may cause false operation, and these factors generally do not appear in the generator differential protection. The specific manifestations are as follows:
(1) The voltage levels of the current transformers on each side used in the transformer differential protection are different, and the transformation ratio, capacity and core saturation characteristics are inconsistent. Even if the balanced coil or autoconverter method is used for compensation, the transformation ratio between the current transformers on each side may still not match. In normal operation and external short circuit, the unbalanced current flowing in the differential circuit may still be relatively large.
(2) The transformer adjustment tap changes the transformer transformation ratio, causing mismatch between the selected current transformers and increasing the unbalanced current.
(3) Since the transformer winding is connected in Y∕△, the current phases on each side are inconsistent. Therefore, the connection group of the current transformer should be adapted to it. However, the selection of the current transformer ratio may still be mismatched and produce unbalanced current due to the selection of the standard ratio.
(4) The transformer generates an excitation surge during the voltage recovery process when closing the switch at no load or removing the external fault.
Due to the influence of these special factors, the unbalanced current of the transformer differential protection is much larger than that of the generator differential protection. The unbalanced current or excitation surge flows into the differential circuit as in the case of internal fault, which may cause the differential protection to malfunction. In order to ensure the selectivity of the differential protection, the operating current of the relay must avoid the maximum unbalanced current during the external fault. Therefore, the larger the unbalanced current, the larger the relay setting operating current, which will reduce the sensitivity of the protection during the internal fault. In order to improve the sensitivity of the differential protection, it is necessary to try to reduce the unbalanced current. Therefore, reducing the unbalanced current and its influence is the key issue in realizing differential protection.
