Potential Transformers

Potential Transformers(PTs)

Potential transformer or voltage transformer are used in electrical power system for stepping down the system voltage to a safe value which can be fed to low ratings 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 the 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 is connecetd 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 connecetd to the secondary windings.PT has lower turns winding at its secondary. The system voltage is applied across the terminals of primary winding of that transformer, and thenproportionate 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 rated burden gets connected across the secondary; the ratio of primary and
secondary voltages of transformer is equal to the turns ratio. The two terminal voltages are in precise phase opposite to each other.But in actual transformer, there must be an error in the voltage ratio as well as in the phase angle between primary and secondary voltages.
The errors in potential transformer or voltage transformer can be best explained by phasor diagram below:

Errors in Potential Transformer

Is- Secondary current.
Es-Secondary induced emf.
Vs-Secondary terminal voltage.
Rs-Secondary winding resistance.
Xs-Secondary winding reactance.
Ip-Primary current.
Ep- Primary induced emf.
Vp- Primary terminal voltage.
Rp- Primary winding resistance.
Xp- Primary winding reactance.
KT- Turns ratio =Numbers of primary turns/number of secondary turns.
I0- Excitation current.
Im – Magnetizing component of I0
Iw- Core loss component of I0
Φm – Main flux.
β- Phase angle error.
Total primary current Ip is the vector sum of excitation current Io and the current equal to reversal of secondary current Is multiplied by the ratio 1/KT

 If Vp is the system voltage applied to the primary of the PT, then voltage drops due to resistance and reactance of primary winding due to primary current Ip will come into picture. After subtracting this voltage drop from Vp , Ep will appear across the primary terminals.This Ep is equal to primary induced emf. This primary emf will transform to the secondary winding by mutual induction and transformed emf is Es. Again this Es will be dropped by secondary winding resistance and reactance, and resultant will actually appear across the burden terminals and it is denoted as Vs.

Ratio Error

Ideally a PT should give 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 system voltage is Vp, ideally Vp/KTshould be the secondary voltage of PT, but in reality;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 exact opposition to the applied primary voltage Vp.

Potential transformer Accuracy classes

Cause of Error in Potential Transformer

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 of errors in 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 their 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 capacitive potential transformer. The capacitive potential divider, inductive element and the auxiliary transformer are the three main parts of the capacitive potential transformer.

Working of CVT

     fig:capacitor Potential transformer circuit Diagram

 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 further step- down by 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 capacitor places near to 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 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 the leakage flux of the auxiliary winding of the auxiliary transformer. The value of inductance is given as
If the current flowing in the output or secondary circuit is negligible the

Application

The potential transformers are used to:
(i) feed instruments to measure the voltage level
(ii) feed relay to protect the power system components from undesirable voltage fluctuations
(iii) safely isolating secondary control circuitry from the high voltages.

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