Specification of Voltage Transformer

The specifications of a voltage transformer mean the parameters by which the equipment is specified. The specifications of a voltage transformer are as follows.

  1. Rated Secondary Voltage
  2. Rated Outputs OR VA Ratings
  3. Class of Accuracy
  4. Rated Voltage Factors
  5. Insulation Levels

Rated Secondary Voltage

As per standards the rated voltage of a potential transformer is 110V.  Although we specify this voltage as line voltage but in actual case the secondary voltage is 110/√3 volts. This is because it is normal practice to earth one terminal of primary of the transformer.

Rated Output

Theoretically we can design a voltage transformer with any VA output. But as per standard only some specific VA ratings are available in the market. These ratings are 10, 25, 50, 75, 100, 150, 200 and 500 VA for each single phase unit. Sometimes a voltage transformer may have two or more than two secondary cores. Then these values are the sum of the output of all the secondary winding of a single voltage transformer.

Class of Accuracy

This is a one of the important specifications of voltage transformers. As we know that there are always some errors in the performances of a voltage transformer. This is because no one can make an ideal one. There are some ratio errors as well as phase angle errors in these transformers. Although we can minimize these errors to a good extent. But for that the costing of the equipment rises. Practically for different purposes we can allow the errors up to a limited extend. So, intentionally we compromise with the errors to optimize the cost of the potential transformers.

This is because voltage transformers are used for different purposes. In some cases erroneous performance of a voltage transformer does not affect the actual requirement. Again in some cases the errors must be minimum for obtaining desired results.

The class of accuracy or simply accuracy classes represent the error limits of a potential transformer.

In the specification of the voltage transformers there are some accuracy classes defined. Each of the accuracy classes corresponds to permissible voltage error and phase angle error at rated frequency. The specified error limits are valid for any voltage between 80 to 120% with burdens from 25 to 100% of rated burdens at 0.8% lagging power factor. This is the general rule of representing error limits of a potential transformer.

Again the accuracy classes specification of voltage transformers is different for measuring core and protection core.

Class of Accuracy for Measuring Voltage Transformer

The standard accuracy class of voltage transformer are 0.1, 0.2, 0.5, 1.0 and 3.0. Each of these values indicates a certain voltage ration error limit in percentage and a certain phase displacement error limit in minutes.

Class Voltage (Ratio) Error Phase Displacement
0.1 ±0.1% ±5′
0.2 ±0.2% ±10′
0.5 ±0.5% ±20′
1.0 ±1.0% ±40′
3.0 ±3.0%

Class of Accuracy for Protection Voltage Transformer

It is needless to say an electrical protection scheme does not require very fine accuracy of instrument transformers. Hence a protection voltage transformer or protection core of a voltage transformer does not require high level of accuracy. So the accuracy limit in these cases may be anything from 3.0 to 10.0.

As per definition the protection voltage transformers maintains these limit of errors between any voltage from 5% to 110% of rated voltage within 25 to 100% burden of rated burden of a power factor 0.8 lagging. The limits of ratio errors and phase displacement error for protection voltage transformers of different accuracy classes are given below.

Accuracy Class Ratio Error Phase Angle Error
3.0 ±3% ±120′
5.0 ±5% ±300′
10.0 ±10% ±600′

Rated Voltage Factors

It is a determined by the maximum operating voltage and the primary winding earthing conditions. The recommended voltage factors different earthing conditions are given in table below.

Rated Voltage Factor Rated Time Method of Connecting Primary Winding and System Earthing Conditions.
1.1 Continuous Between Lines and between transformer star point and earth.
1.1 Continuous Between line and earth in an effectively earthed neutral system.
1.5 30 seconds Between line and earth in an effectively earthed neutral system.
1.1 Continuous Between line and earth in a non-effectively earthed neutral system(with automatic earth fault tripping).
1.9 30 seconds Between line and earth in a non-effectively earthed neutral system(with automatic earth fault tripping).
1.1 Continuous Between line and earth in an isolated neutral system or in resonant earthed system.
1.9 8 hours Between line and earth in an isolated neutral system or in resonant earthed system.

Rated Insulation Level

Rated insulation levels mean the rated voltage with stand capacities potential transformers. The rated insulation levels include nominal system voltage rating, highest system voltage rating, highest system voltage rating and power frequency voltage.

Nominal Voltage

The nominal voltage is such voltage at which a potential transformer operates. That is the normal system voltage. Such as 11KV for 11KV system, 33KV for 33KV system, 132KV for 132KV system etc.

Highest System Voltage

Highest system voltage is the maximum possible system voltage which may continue for any period of time. This is the maximum system voltage which can occur during no load or low load conditions. So, the PT must be capable of withstanding this higher voltage for any duration. For example, the highest system voltage of 11KV system is 12KV. Similarly, the highest system voltage of 33KV system is 36KV. The same for a 132KV system is 145 KV etc.

Power Frequency Withstand Voltage

There may be some situations in the power system when due to sudden withdrawal of huge load; there will be exercise hike in the system voltage. That high voltage may be of power frequency voltage but it does stay for every nominal period. So, the insulation level of a voltage transformer must be such that it can withstand these high voltages at least for a certain time. Normally we consider with the stand capacity of a PT for such power frequency high voltages for 30 sec period. For example power frequency withstand voltage of an 11KV PT is 28KV.

Impulse Withstand Test Voltage

There is another voltage rating or insulation level of a PT. We call this as the impulse withstand test voltage with standard full wave positive and negative polarity KV peak. Due to lightning and switching impulses there may be very high voltage imposed on the insulation’s of the PT. But we are quite fortunate that, these impulse high voltages stay for very short periods. The duration of these impulse high voltages range from microseconds to few milliseconds. So, we can design an insulation of a potential transformer in such a way that it can withstand the standard applied impulse voltages.

The disturbances of these impulse voltages are very uneven with different wave forms and frequencies. So, it is impossible to judge which type of impulses strikes on the insulation of a PT during its lifespan. So, we have standardized some Impulse voltages for different nominal system voltage. We design the insulation for the standard impulse voltages. That means during testing a potential transformer should withstand these standard impulse voltages. Such as 75KV is the impulse voltage level for an 11KV potential transformer.

Table of Insulation Levels for Different Voltage Levels

A table of insulation levels for different voltage levels as given below.

Nominal Voltage Highest System Voltage Power Frequency Withstand Voltage Impulse Withstand Test Voltage
11KV(r.m.s) 12KV(r.m.s) 28KV(r.m.s) 75KV(peak)
33KV(r.m.s) 36KV(r.m.s) 70KV(r.m.s) 170KV(peak)
132KV(r.m.s) 145KV(r.m.s) 275KV(r.m.s) 650KV(peak)
220KV(r.m.s) 245KV(r.m.s) 460KV(r.m.s) 1050KV(peak)
400KV(r.m.s) 420KV(r.m.s) 680KV(r.m.s) 1500KV(peak)


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