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How to Test Instrument Transformers Using CVA500

Introduction

CVA500 is DV Power’s device specially made for testing instrument transformers. Besides current transformers (CT), it can also be used for condition assessment of inductive voltage transformers (VT) and capacitive voltage transformers (CVT).

This application note will provide overview of tests that can be performed with current and voltage transformer analyzer CVA500.

CT saturation test (knee point detection)

Knee point of a CT is a very important parameter. It is represented as a voltage at CT secondary terminals above which the CT becomes saturated. When the CT is saturated, it doesn’t convert primary current to secondary current according to its rated turns ratio. CTs in such state can create problems when they are used for supplying protective relays. They may operate normally at low primary current levels but not operate at all during fault currents. To avoid saturation in normal conditions, knee point must be lower than the rated voltage of CT burden.

Some of the following conditions can cause CT saturation:

  • CT burden is higher than rated
  • Very high current flowing through the CT primary (i.e. fault current)
  • Low current flowing through CT primary with open‐circuit secondaries
  • DC current flowing through either winding (e.g. after winding resistance test)

CT knee point test is performed by applying variable AC voltage to a CT secondary. Voltage and current are recorded as voltage is changed over time, and plotted on a square log-log graph (Figure 1).

Knee point is determined based on one of the following 3 criteria:

  1. ASA 10/50. The point in the square log-log graph where 10% increase of the voltage causes 50% increase of the current. This criteria is defined by the ANSI C57.13 and IEC 61869-2 standards.
  2. IEEE 30°. The point in the square log-log graph in which tangent makes an angle of 30° to the horizontal I-axis. This criteria is defined by the ANSI C57.13 standard and is applied to CTs with a gapped core.
  3. IEEE 45°. The point in the square log-log graph in which tangent makes an angle of 45° to the horizontal I-axis. This criteria is defined by the ANSI C57.13 standard and is applied to CTs with a non-gapped core.
Figure 1. Example of a square log-log graph

Before saturation test, CVA500 performs demagnetization of CT core.

Turns ratio test and polarity check

CVA500 performs a turns ratio test using a voltage method. AC voltage is applied to a transformer side with higher number of turns, and induced AC voltage at the transformer side with lower number of turns is measured. The ratio of these two voltages represents turns ratio of the transformer. The test is performed on a no-loaded instrument transformer, with burden disconnected from the transformer secondary side.

Considering that a CT has higher number of turns on its secondary side, CVA500 applies voltage at X1-X5 terminals which are connected to the CT secondary side. CVA500 H terminals are connected to the CT primary side (Figure 2). Accordingly, when testing turns ratio of a VT or a CVT, X terminals of CVA500 should be connected to the voltage transformer primary side, and H terminals of CVA500 should be connected to the voltage transformer secondary side.

Turns ratio can be determined simultaneously during CT saturation test. In that case, CVA500 measures ratio in the area below knee point.

Figure 2. Connecting CVA500 to a multi-tap CT

The polarity check is performed during turns ratio test by measuring the phase shift between the applied and measured voltages. Polarity is considered correct if the phase angle is around 0°; otherwise, if the phase angle is around 180°, the polarity is considered reversed.

Winding resistance test

Winding resistance may change over a period of time depending on the specimen age, use, external conditions, and loading effect. Therefore, measuring the DC resistance of secondary winding is important for accessing the true condition, state, and accuracy of an instrument transformer.

This test is performed by injecting DC current to a secondary winding, measuring voltage drop, and calculating resistance by dividing voltage with current. The entire process is completely automated by CVA500.

It is a standard practice to measure winding resistance of all taps, and all cores. The resistance should be compensated to a referent temperature of 75 °C.

Insulation resistance test

This test is performed by applying DC voltage between two isolated parts, and by measuring leakage current that flows between those parts. Insulation resistance is calculated as the ratio of the applied voltage (DC) to the total leakage current (R=V/I). Leakage current should be as small as possible, therby IR value should be as high as possible (R=V/I).

The purpose of the insulation resistance test on a transformer is to check the integrity of the winding insulation. Primary and secondary windings are isolated from each other and isolated from earth. This insulation can degrade over time due to excessive temperature or moisture or exposure to a voltage higher than rated. In this case, the voltage level that insulation was previously able to withstand may drop, which will increase the leakage current between two isolated parts. Increased leakage current will be reflected as decreased insulation resistance.

Prior to testing, the neutral ground should be removed from secondary terminals, and transformer should be isolated from any associated burden. CVA500 shorts the primary winding under test by internally connecting H1(P1) and H2(P2) terminals, and the secondary winding under test by internally connecting X1(S1) and X2(S2)-X5(S5) terminals. This way, the transformer under test becomes a three-terminal specimen – primary, secondary, and ground. The insulation resistance is normally measured between all three specimens:

  • Primary to secondary
  • Primary to ground
  • Secondary to ground

In case of a CT with multiple cores, it is also useful to check the insulation resistance between core under test and all other cores. In that case, all other cores should be externally shorted. Primary H1(P1) and H2(P2) terminals of the CVA500 instrument should be connected to other cores in that case.

Insulation resistance readings should remain constant over a time. Therefore, attention should be paid to the trending of insulation resistances. A sudden decrease of insulation resistance values may indicate possible insulation degradation and further investigation is required to diagnose the problem.

Burden test

The instrument transformer is loaded with a relay or a meter. The total load on the instrument transformer secondary side is called burden. It consists of connecting wires (lead resistance) and the resistance of the relay/meter. The instrument transformer rated accuracy is valid if the transformer is loaded with its rated burden. Otherwise, accuracy may be significantly affected. Therefore, burden test is performed to verify that the specifications of burden connected to the instrument are appropriate and that transformer accuracy is not affected.

A burden test is typically performed by injecting full rated secondary current value (5 A or 1 A) in case of a CT, or by applying rated secondary voltage (110 V or 100 V) in case of a voltage transformer. CVA500 measures burden voltage, current, power, impedance, φ (phase angle between voltage and current), and cosφ.

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February 8, 2023

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