IEEE C57.13.1-2017 pdf free download – IEEE Guide for Field Testing of Relaying Current Transformers.
12. Excitation test
12.1 Safety Ensure that the CT is de-energized and that the CT has been de-magnetized. Applying voltage to a portion of the secondary winding will cause the proportional induced voltage across full winding due to autotransformer action. CT’s should not remain energized at voltages higher than the knee-point value for times longer than that required for the readings.
12.2 Type of test This subclause describes the various test methods used when performing the excitation test. This test verifies the operating characteristics of the current transformer when compared to manufacturer’s data. The two common methods are the secondary voltage method and the primary current injection method.
Whether performing the voltage or current method, the selection of the instruments used for the measurements is especially important for this test. The ammeter should be an rms instrument and the voltmeter should be an average-reading voltmeter. This average-reading voltmeter will make the voltage less dependent on the harmonics caused by the non-linear winding impedance being connected to a source of finite impedance. It can be either an analog type consisting of a d’Arsonval instrument connected across a full-wave rectifier, or a digital one. The bandwidth of the instrument should extend at least to the third harmonic. It should be calibrated to give the same numerical indication as an rms voltmeter on sine-wave voltage (this is the case for all general-purpose meters). SeeAnnex B for illustrations of the effect of different measuring instruments.
12.2.1 Voltage method This method is used most often as it does not require high-current test equipment.An ac test voltage is applied to the secondary winding of the CT while the primary winding is left open-circuited as shown in Figure 10. It is prudent to demagnetize the CT before performing this test (see 5.1).
Avariable ac voltage is applied to the secondary winding of the CT, and the current flowing into the winding at each selected value of voltage is recorded. Readings near the knee of the excitation curve are especially important in plotting a comparison curve. For CTs with taps, the secondary tap should be selected to assure that the CT can be saturated with the test equipment available. The highest tap that can accommodate the requirement shall be used. Compare the readings obtained with the manufacturer’s published data.IEEE C57.13.1 pdf download