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A three-phase, 132 kV/33 kV star-delta connected power transformer is protected by a differential protection scheme. Determine the ratio of the CTs on the HV side of the transformer if the ratio on the LV side is 400/5. How are the CT secondaries connected?

Answer :

To determine the ratio of the current transformers (CTs) on the high-voltage (HV) side of a three-phase, star-delta connected transformer, we must ensure that the differential protection scheme operates correctly by matching the currents flowing through the HV and LV sides.

Step-by-step Solution:

  1. Transformer Voltage Ratio:
    The transformer has a primary voltage (HV side) of 132 kV and a secondary voltage (LV side) of 33 kV. The turns ratio or voltage ratio of the transformer is:

    [tex]\text{Voltage Ratio} = \frac{132 \text{ kV}}{33 \text{ kV}} = 4[/tex]

    This means the transformer steps down the voltage by a factor of 4.

  2. CT Ratio on the LV Side:
    The given CT ratio on the LV side is 400/5.

  3. Equivalent CT Ratio on the HV Side:
    To match the primary and secondary sides for differential protection, the HV side CT should reflect the same number of secondary turns as the LV side CT when referred to the same base. We achieve this by using the transformation ratio of the transformer. Therefore, the HV CT ratio should be adjusted by the same factor as the increase in voltage across the transformer:

    [tex]\text{Equivalent CT Ratio on HV Side} = \left(\frac{Voltage Ratio}{LV CT Ratio} \right) \times \text{HV Rated Current}[/tex]

    The LV side is rated 400/5, which means the HV side CT should be set to reflect this ratio multiplied by the voltage ratio:

    Thus, the HV CT ratio should be 1600/5 (because the voltage ratio is 132/33 = 4, hence 400 x 4 = 1600).

  4. Connection of CT Secondaries:

    Since the transformer is star-delta connected, the CT secondaries should also be connected in star on the HV side. This method helps cancel out any zero-sequence currents that may appear during an internal winding fault, effectively stabilizing the differential relay operation.

Conclusion:

  • The ratio of CTs on the HV side should be 1600/5.
  • The CT secondaries on the HV side should be connected in a star configuration.

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Rewritten by : Barada