Energy Metering – Routine Testing of Voltage Transformers

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Energy Metering

In the present day, when energy drives our lives, we need to keep a check on our consumption and control the same. Our customers at Powerpoint Engineering regularly pose questions relating to Energy Metering. In this article, I have put together the following information that I hope will give you a better understanding of Energy Metering and the process of monitoring and tracking energy consumption.

Instrument Transformers (ITs)

Instrument transformers play a crucial role in metering and monitoring. ITs are electrical devices that operate in very high voltages or currents and transform it to lower values so that the magnitude can be measured by normal range instruments. They are mainly used for energy billing, system protection and protective relaying purposes, and load survey.

There are two types of ITs: current and voltage transformers. Current transformers are used for measuring currents of high magnitude, whereas voltage transformers (potential transformers) step down voltages of high magnitudes to a lower voltage. To ensure that a transformer works efficiently, it has to be put through a number of testing procedures. A type of testing called routing testing and its sub-types are discussed further in detail.

The Road to Testing

Before the tests are conducted on the transformer, we need to consider a lot of other factors – one of them being performance. This can be determined from a transformer’s respective equivalent circuit. Neither transformer can replace each other as their functionalities differ. This is because voltage transformers are designed in such a way that the through impedance is very low. But in current transformers, the excitation impedance is made very high.

When it comes to the protection aspect of instrument transformers, ratio error and phase angle are of concern. This needs an introduction to turns ratio and voltage ratio. Turns ratio is the number of turns in the high voltage (primary) winding to that in the low voltage (secondary) winding. Voltage ratio is the ratio of primary voltage to the secondary voltage.

When testing a single unit, there are many facets that need to be addressed such as ratio error and phase angle. In ITs with high transformation ratios, the voltage ratio is different from the turns ratio. This is called the ratio error. IEEE standard C57.13 defines this amount as the ratio correction factor (RCF). RCF is the ratio of true ratio to marked ratio.

Another feature is the phase angle. When the secondary output is slightly out of phase with the primary input, it is called phase angle. It is measured in minutes and is positive if the secondary signal leads to the primary.

Transformer Testing

Generally, many tests are conducted on transformers that are newly built and also serviced ones. There are mainly two types of tests – routine and type. Both these tests are done by the manufacturer. Routine testing is done to ensure good performance of individual units. In addition to these two tests, others are conducted at the site premises to ensure the appliance meets the customer’s specifications and design.

There are many routine tests performed on a transformer. One of them is the voltage ratio test. This is with respect to the turns ratio of an IT. The working of a transformer largely depends on this turns ratio. Hence, this type of test is very essential. This testing can be done by a transformer turn ratio (TTR) meter. To reduce any physical hazards while conducting the test, low voltages are applied through the high voltage winding so that the induced voltage is lower. The ratio that is obtained after the test is compared with the ratio given on the nameplate. It should agree with the factory-specified value of 0.5% or within that.

There is also another vital test, called the winding resistance test. It is conducted as both type test and routine test. It helps in determining the calculation of I²R losses and winding temperature. It is also a field test that helps in identifying loose connections, broken strands of conductor and likewise.

Voltage Ratings and Power Quality

Voltage Ratings are the voltages at which the electrical device is designed to work on. Power quality is one of the most important aspects of any system and the manufacturer must keep in mind the power quality the device is going to deliver at the end of the day as it matters the most to any consumer. There are many quality problems, which include voltage sags and dips, momentary interruptions and harmonics. Sometimes, one disturbance might in turn cause another. An instrument transformer that surpasses all these tests and is assured to perform well can be called a near-to-ideal transformer.

For more information please contact Garrett Kelly BSc MIET at, Senior Project Engineer at Powerpoint Engineering or call us on 057 866 2162

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