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Transformer Testing Using Handheld Turns Ratio and Winding Resistance Tester

Introduction

TWR-H is a handheld, battery-powered device intended for turns ratio and winding resistance measurement of single-phase and three-phase transformers. In addition, it can also demagnetize transformers.

The connection to the transformer is single-phase, i.e. the instrument can be connected to one phase at a time. Figure 1 shows the example of connecting to a high voltage (HV) and a low voltage (LV) winding of a three-phase transformer.

Transformer testing with TWR-H
Figure 1 – Connecting TWR-H to one phase of a three-phase transformer

The fact that the unit is handheld and battery-powered, and that it can be connected to one phase at a time, makes this device ideal for testing distribution transformers, where cables can be easily moved from one phase to another.

Winding Resistance Test

A common issue when testing winding resistance of distribution transformers is the selection of test current. Distribution transformers have a high turns ratio, and therefore a high difference between rated HV and LV currents. Testing HV and LV winding simultaneously using the same test current can be challenging. Test current must be less than or equal to 10% of the HV rated current in order to avoid significant heating of HV winding. However, such a test current is very often too low for LV winding and can cause incorrect LV winding resistance measurements.

For this reason, it is recommended to test HV and LV winding with two different test currents, ideally with 10% of HV and LV rated currents, respectively. This is possible with TWR-H as it has two output DC sources. First DC source, intended for testing transformer HV side, can provide up to 2 A DC. Second DC source, intended for testing transformer LV side, can provide up to 10 A DC. These currents are within 10% of rated currents of most distribution transformers. This ensures quick and accurate winding resistance measurement.

Demagnetization

Injecting a DC current through a transformer winding will magnetize the transformer core. For that reason, the transformer core will definitely be magnetized after winding resistance tests. The residual magnetism can cause various problems such as erroneous diagnostic electrical measurements on a transformer (especially excitation current and FRA), an inrush and asymmetrical currents at start-up of power transformer, or incorrect operation of protective relays due to magnetized core. It is recommended to perform transformer demagnetization after winding resistance tests, and prior to turns ratio measurements.

TWR-H performs transformer core demagnetization by applying DC current, and changing its polarity and decreasing its magnitude down to zero, following a proprietary developed program. By reducing the magnitude of the applied current to the zero value, the total magnetic flux, or residual magnetism, is also annulled. Demagnetization is performed on the transformer HV side, using the first DC current source. The starting demagnetization current should be the same as the test current used for HV winding resistance test.

Turns Ratio Test

TWR-H is equipped with an AC source that can provide up to 40 V AC. Our researches showed that this voltage is high enough to obtain accurate turns ratio results on distribution transformers. Together with turns ratio, TWR-H also calculates ratio deviation, and measures excitation current and phase angle.

A low test voltage of 1 V AC can also be selected, which is necessary for verification of a current transformer (CT) turns ratio. By measuring the phase angle, the polarity of the CT can be checked.

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February 4, 2020

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