What is a potential transformer

Potential transformer (PT)

definition - The potential transformer can be defined as an instrument transformer for converting voltage from a higher to a lower value. This transformer lowers the voltage to a safe limit that can easily be measured with a normal low voltage instrument such as voltmeter, wattmeter, and watt-hour meter, etc.

Construction of a potential transformer

The potential transformer is made with a high quality core that works at a low flux density so that the magnetizing current is small. The connection of the transformer should be designed so that the variation of the voltage ratio with the load is minimal and the phase shift between input and output voltage is also minimal.

The primary winding has a large number of turns and the secondary winding has a very small number of turns. The coaxial winding in the potential transformer is used to reduce the creep reactance. Isolation costs are also reduced by dividing the primary winding into the sections, thereby reducing the isolation between the layers.

Connection of a potential transformer

The potential transformer is connected in parallel with the circuit. The primary windings of the potential transformer are directly connected to the power circuit whose voltage is to be measured. The secondary connections of the potential transformer are connected to the measuring device, for example voltmeter, wattmeter, etc. The secondary windings of the potential transformer are magnetically coupled through the magnetic circuit of the primary windings.

The primary terminal of the transformer is rated for 400 volts to several thousand volts, and the secondary terminal is always rated for 400 volts. The ratio of the primary voltage to the secondary voltage is called the transformation ratio or turns ratio.

Types of potential transformers

The potential transformer is mainly classified into two types; H. The conventional winding types (electromagnetic types) and the capacitor voltage potential transformers.

Conventional winding transformers are very expensive because of the insulation requirements. Capacitor potential transformer is a combination of capacitor potential divider and magnetic potential transformer with a relatively small ratio.

The circuit diagram of the capacitor potential transformer is shown in the figure below. With the stack of the high voltage capacitor from the potential divider, the capacitors of two sections become C.1 and C2and the Z is the burden.

The voltage applied to the primary side of the intermediate transformer is normally on the order of 10 kV. Both the potential divider and the intermediate transformer have the ratio and isolation requirement suitable for economical construction.

The intermediate transformer must be very good, a small ratio error and a phase angle give the satisfactory performance of the whole unit. The secondary terminal voltage is given by the formula below.

Ratio and phase angle errors of a potential transformer

In an ideal potential transformer, the primary and the secondary voltage is exactly proportional to the primary voltage and exactly in phase opposition. However, this is practically impossible to achieve due to the primary and secondary voltage drops. Thus, both the primary and the secondary voltage are introduced into the system.

Voltage ratio error - The stress ratio error is expressed in terms of the measured stress and given by the formula below.

Where Kn is the nominal ratio, i.e. H. the ratio of the nominal primary voltage and the secondary nominal voltage.

Phase angle error - The phase angle error is the error between the secondary terminal voltage, which is exactly the opposite of the primary terminal voltage.

The increase in the number of instruments in the relay connected to the secondary side of the potential transformer increases the errors in the potential transformers.

Load on a potential transformer

The load is the total external volt-ampere load, the secondary at rated secondary voltage. The nominal load of a PT is a VA load that must not be exceeded if the transformer is to work with its nominal accuracy. The nominal load is given on the nameplate.

The limiting or maximum load is the greatest VALast at which the potential transformer works continuously without heating its windings beyond the permissible limits. This load is a multiple of the nominal load.

Vector diagram of a potential transformer

The vector diagram of the potential transformer is shown in the following figure.

Where Is - secondary current
E.s - secondary induced emf
V.s - secondary terminal voltage
R.s - secondary winding resistance
Xs - secondary winding reactance
Ip - primary current
E.p - mainly induced emf
V.p - primary terminal voltage
R.p - primary winding resistance
Xp - Primary winding reactance
Kt - Turn ratio
IO - excitation current
Im - magnetizing component of IO
Iw - core loss component of IO
Φm - main river
Phase phase angle error

The main flow is taken as a reference. In the measuring transducer, the primary current is the vector sum of the excitation current I.O and the current is equal to the reverse secondary current I.s multiplied by the ratio of 1 / kt. The Vp is the voltage that is applied to the primary connection of the potential transformer.

The voltage drops due to the resistance and reactance of the primary winding due to the primary current are given by I.pXp and mepR.p. When the voltage drop is subtracted from the primary voltage of the potential transformer, the primary induced emf appears at the connections.

This primary EMF of the transformer is converted into secondary winding and secondary EMF E by mutual inductions. This emf drops through the resistance and reactance of the secondary winding, and the resulting voltage will appear across the secondary terminal voltage and is denoted by V.s.

Applications of potential transformers

  1. It is used for measurement purposes.
  2. To protect investors.
  3. To protect the impedance of the generators.
  4. To synchronize the generators and feeders.

The potential transformers are used in the protection relay scheme, since the potential coils of the protection device are not directly connected to the system in the case of high voltage. It is therefore necessary to reduce the voltage and isolate the protective equipment from the primary circuit.