Potential Transformers: Definition, Working, Errors, Applications

A potential transformer (P.T.) is an instrument transformer that is employed in power systems for measurement and protection purposes. The main function of a potential transformer in a power system is to measure high alternating voltage.

What are Potential Transformers (PTs)?

Potential transformers or voltage transformers are used in electrical power systems for stepping down the system voltage to a safe value which can be fed to low-rating meters and relays. Commercially available relays and meters used for protection and metering, are designed for low voltage. This type of transformer is generally employed for voltages above 380 V which feeds the potential coils of indicating and metering instruments and relays.

Operation

A voltage transformer theory or potential transformer theory is just like a theory of general-purpose step-down transformer. The primary windings of the potential transformer are connected directly to the power circuits either between two phases or between a phase and ground depending on the rating of the transformer and its application. Various indicating and metering instruments and relays are connected to the secondary windings.PT has lower turns winding at its secondary. The system voltage is applied across the terminals of the primary winding of that transformer, and then a proportional secondary voltage appears across the secondary terminals of the PT.

Standard ratios

The primaries of PT are rated from 400V to several thousand volts. The secondary voltage of the PT is generally 110 V. In an ideal potential transformer, when the rated burden gets connected across the secondary; the ratio of primary and secondary voltages of the transformer is equal to the turns ratio. The two terminal voltages are in a precise phase opposite to each other. But in the actual transformer, there must be an error in the voltage ratio as well as in the phase angle between primary and secondary voltages.

Errors in Potential Transformer

Total primary current Ip is the vector sum of excitation current Io and the current equal to the reversal of secondary current Is multiplied by the ratio 1/KT

Ratio Error

Ideally, a PT should give a secondary voltage exactly proportional to the primary voltage. Due to the voltage drop, this cannot be achieved. Hence, the ratio and phase angle errors are introduced by a PT. If the system voltage is Vp, ideally Vp/KT should be the secondary voltage of PT, but in reality; the actual secondary voltage of PT is Vs. The difference between the ideal value Vp/KT and actual value Vs is the voltage error or ratio error in a potential transformer, it can be expressed as,

Phase Angle Error

The angle ′β′ between the primary system voltage Vp and the reversed secondary voltage vectors KT.Vs is the phase error. The phase angle error ′β′  is usually given in minutes. It indicates by what small angle ′β′  the secondary voltage Vs departs from in exact opposition to the applied primary voltage Vp.

Potential transformer Accuracy classes

Cause of Error in Potential Transformers

The voltage applied to the primary of the potential transformer first drops due to the internal impedance of the primary.  Then it appears across the primary winding and then transformed proportionally to its turns ratio, to the secondary winding. This transformed voltage across the secondary winding will again drop due to the internal impedance of the secondary, before appearing across burden terminals. This is the reason for errors in the potential transformer.

Capacitive voltage transformer

There are generally two types of potential transformers:

1. Conventional wound type and
2. capacitor voltage transformer.

For voltages exceeding 100 KV (phase) the conventional type of transformers becomes expensive as more insulation is required. For reducing the cost, the capacitive potential transformer is used in the system. The CVT is cheap, and its performance is not much inferior to the highly insulated transformer. The capacitive voltage transformer step-down the high voltage input signals and provide the low voltage signals which can easily measure through the measuring instrument. The Capacitive voltage transformer (CVT) is also called a capacitive potential transformer. The capacitive potential divider, the inductive element, and the auxiliary transformer are the three main parts of the capacitive potential transformer.

Working of CVT

The capacitive potential divider is used in combination with the auxiliary transformer and the inductive element. The capacitive potential divider step-down the extra high voltage signals into a low voltage signal. The output voltage of the capacitive potential transformer is a further step down with the help of the auxiliary transformer.

The capacitor or potential divider is placed across the line whose voltage is used to be measured or controlled. Let the C1 and C2 be the capacitor placed across the transmission lines. The output of the potential divider acts as an input to the auxiliary transformer. The capacitors placed near the ground have high capacitances as compared to that placed near the transmission line. The high value of capacitances means the impedance of that part of the potential divider becomes low. Thus, low voltages pass to the auxiliary transformer. The auxiliary transformer is further step-down the voltages.

The N1 and the N2 are the numbers of turns on the primary and the secondary winding of the transformer. The meter used for measuring the low value of voltage is resistive, and the potential divider is capacitive. Thereby, the phase shift occurs, and the output will be affected. To overcome this problem, the inductance is placed in series with the auxiliary transformer. This inductance L consists of the leakage flux of the auxiliary winding of the auxiliary transformer. The value of inductance is given as