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CT and PT Turns Ratio

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Calculation of CT and PT Turns Ratio

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The first point to consider when testing the transformation ratio of CT (Current Transformer) and PT (Potential Transformer)

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is that the accuracy class is defined for the working conditions of nominal burden and nominal current for these transformers

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For example, for a CT with a 0.5 accuracy class and a 5 VA burden

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the transformation ratio error will be a maximum of 0.5% at 1 A current

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assuming a 5 VA load is connected to the CT terminal.

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A common method used for testing in substations is to short-circuit the CT secondary and inject current (typically 20%) into the primary

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measure the current flowing through the secondary, and record the transformation ratio

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For PT, the usual method involves applying voltage (2-12 kV) to the PT primary and measuring the open-circuit voltage at the PT secondary

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As evident from the common testing methods, these transformers are not tested under their actual working conditions

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and the tests merely determine the "turns ratio" between the primary and secondary

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which may be outside the designed accuracy class. Therefore, to determine the transformation ratio error, the test must be conducted under different conditions

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For testing the transformation ratio and accuracy class of the CT under different loads and currents, voltage is applied to the secondary, and the primary voltage is measured

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In the first stage of this test, the "turns ratio" between the primary and secondary is obtained

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The voltage for this stage of the test is calculated by dividing the nominal burden by the nominal secondary current

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In addition to this voltage, half and double of it are also applied to achieve better measurement accuracy

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 By applying higher voltages, the CT turns ratio is obtained

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At this stage, the transformation ratio error must be calculated at various burdens and currents (based on nominal current)

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For example, transformation ratio error calculations are provided for three different scenarios

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1) 100% current and nominal burden.

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2) 100% current and 50% nominal burden.

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3) 50% current and 50% nominal burden.

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Using this calculation method, the CT transformation ratio error can be obtained under various current and burden conditions

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PT Testing

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For PT testing, the turns ratio between the PT primary and secondary must first be determined

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To do this, apply 450V to the PT primary, and measure the open-circuit voltage at the PT secondary

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. In this way, the "turns ratio" between the primary and secondary is obtained

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In the next step, the transformation ratio error at different burdens must be determined

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. For example, transformation ratio error calculations are provided for two different scenarios, assuming a secondary resistance of 0.1 ohms

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1) 100% nominal burden.

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2) 50% nominal burden.