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Analysis of Transformer Longitudinal Differential Protection Faults and Solutions

First, the role of longitudinal differential transformer protection:

          Transformer longitudinal differential protection is mainly used to react the transformer winding and its casing, leading to the short-circuit between phases, but also can react transformer winding inter-turn short circuit and neutral point grounding system side winding, casing, pinout single Short to ground.

Second, the transformer longitudinal differential protection of unbalanced current causes:

          (1). Inrush current transformer. When the transformer no-load input or external fault excision voltage recovery, the value may appear a large amount of inrush current. Magnetizing inrush current up to 6-8 times the rated current, the waveform side of the timeline bias, it contains a large non-periodic components and higher harmonic components (mainly the second harmonic components). Inrush current only appears in the power supply side of the transformer, so it will flow into the differential circuit completely through the current transformer into the secondary current. 

          (2) The wiring of the winding on the different sides of the transformer is different. When the winding on both sides of the transformer by Y, d11 way wiring, the current transformer on both sides of the phase difference of 30 degrees. Therefore, to make the current transformer secondary current on both sides of the transformer in the same value, there is still a large unbalanced current loop, as shown on the right. 

          (3) The calculated ratio of each side transformer varies with the selected standard ratio. As the transformer high-voltage side and low-voltage side of the rated current is different, so in the realization of the vertical differential protection of transformers must choose different than the current transformer. In the selection of current transformers, the current transformers on both sides of the calculated ratio and the standard ratio does not exactly match, will also cause unbalanced current. 

          (4). Each side of the current transformer model. As the transformer rated voltage and rated voltage on each side are different, which uses the current transformer models vary, their characteristics are inconsistent, will cause unbalanced current. 

          (5). Change the transformer tap changer in operation. Change the transformer tap changer will change the transformer ratio, the current transformer secondary current balance relationship is destroyed, there will be unbalanced current.

Third, reduce and escape differential protection of unbalanced current differential measures:

          (1). Phase compensation. In order to eliminate unbalanced current caused by phase difference of Y.d11 connecting transformer due to current on both sides, the differential protection of such transformers should adopt phase compensation wiring. The method is to connect the current transformer on the Y side of the transformer into a delta shape, and connect the current transformer on the side of the transformer to a Y shape to compensate for the phase difference of 30 degrees. 

          (2) Adopt BCH-2 type differential relay. Transformer magnetizing inrush current contains a large non-periodic components, BCH-2-type differential relay with a short-circuit coil with a speed-saturated converter, it escaped the current containing non-periodic components with good performance. Therefore, the use of BCH-2-type differential relay transformer differential protection can effectively avoid the impact of magnetizing inrush.

Fourth, the transformer differential protection and gas protection have what fault?

          Transformer differential protection is to defend the transformer winding and the phase lead short-circuit, and the transformer grounding current system side winding and pin-out of the ground fault protection. Gas protection is to protect the transformer tank from various failures and reduce the oil surface protection, in particular, it is to reflect the transformer winding turn short circuit has significant advantages.

Fifth, the newly installed power transformer longitudinal differential protection should be put into operation before the check?

          (1) Check whether the differential protection is put into use (should be put into use) when the transformer is charged. 

          (2) before the load differential protection will be disabled, with the load measurement of each side of the RMS current and phase. 

          (3) with the load side of the phase difference voltage (or difference current), to check the correctness of the current loop wiring. 

          (4). Transformer charging switch 5 times to check the performance of differential protection excitation magnetic inrush current.

Sixth, the current longitudinal differential protection of the tuning

          The setting of the current longitudinal differential protection is divided into the setting of the operating current and the setting of the sensitivity coefficient. 

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