Before going into the impact
of Burden on the performance of a Potential transformer, we will discuss about
Burden. The Burden of an Instrument Transformer is the rated Volt-Ampere
loading which is permissible without errors exceeding the limits for a
particular class of Instrument Transformer.

Basically, there are two
Classes of an Instrument Transformer, one is Protection Class and another one
is Metering Class. Protection Class (PS) is one which is used for the protection
scheme where as Metering Class is used for the purpose of Metering.

Now, we will discuss about
errors in a Potential Transformer. There are two types of error in a Potential
Transformer, Ration Error and Phase Angle Error.

**Ratio Error:**

Transformation Ratio of a
Potential Transformer varies with the operating condition and therefore the
voltage induced in the secondary of a Potential Transformer will also vary. The
error in secondary voltage of a Potential Transformer is defined as

% Ration Error = (K

_{n}-R)×100/R
Where K

_{n}= Rated primary voltage / Rated secondary voltage
R = Primary Voltage /
Secondary Voltage

**Phase Angle Error:**

In an ideal Potential
Transformer, there should not be any phase difference between the primary
voltage and the secondary voltage reversed. But in a practical Potential
Transformer there exists a phase difference between them.

The important thing to note
that, Ratio error is of importance when we use PT for Protection purpose but
when we use PT for Metering then both Ration and Phase Angle Error are
important to consider.

Now we are on the stage to
discuss the impact of Burden on the performance of a Potential Transformer.

**Impact of Burden on the Performance of PT:**

The effects of Burden on the
performance of a Potential Transformer are as follows:

If we increase the Burden,
the secondary as well as primary current will increase but as the primary
voltage will remain constant (because primary of a PT is connected to the line),
the secondary voltage will decrease. Therefore, the voltage ratio will
increase. Thus we see that increasing the Burden, increases the Error in the
Ratio. Also, as the power factor of the Burden reduces the transformation ratio
of Potential Transformer increases.

PT accuracy performance
changes linearly with burden and can be plotted as

Here RCF is Ratio Correction
Factor, which is defined as defined as the factor that when multiplied by the potential
transformer output will yield the correct result.

Mathematically,

RCF = Transformation
Ratio/Nominal Ratio =R / K

_{n}_{}

The transformer nameplates
show a “marked ratio” usually an even number, such as 20 to 1. The actual ratio
of primary to secondary quantity may be slightly higher or lower than the
marked value by an amount1 called ratio error. For example, if the actual ratio is 20.2 to 1,
then the RCF is 1.01 and the ratio error is 1%.

RCF = 20.2/20 = 1.01

So, % Ratio Error = (RCF-1)×100
= 1%

**Thank you!**

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