Potential Transformers in detail
Read more on transformers
Potential transformers are instrument transformers used to feed the potential coils of indicating and metering relays. These transformers make the ordinary low voltage instruments suitable for the measurement of high voltages and isolate them for high voltage.
The primary winding of the transformer is directly connected to the high voltage power circuits between two phases or between a phase and ground depending on the transformer rating and its application. The secondary of the potential transformer is connected various measuring devices and relays. The primary winding has a large number of turns and the secondary winding has lesser number of turns than the primary winding. These two windings are magnetically coupled. The number of secondary turns depends upon the purpose for which the potential transformer is used.
The primary of the potential transformers are rated from 400 V to several thousand volts. Most of the potential transformers have the secondary rating of 110 V. The ratio of the rated primary voltage to the rated secondary voltage is known as turn or transformation ratio.
Are potential transformer and power transformer the same?
The theory of operation of a potential transformer is essentially same as that of the power transformer. The main difference between a potential transformer and a power transformer is that the load current of the potential transformer depends purely on the exciting current and its secondary impedance. The secondary impedance of the potential transformer will be resistive in nature. The potential transformers are rated in terms of their maximum burden it delivers without exceeding specified limits of error, whereas the power transformer is rated secondary output it delivers without exceeding a specified temperature rise. The output of PTs is limited is usually limited to few hundred volt amperes while the output of a power transformer varies from several KVA to several MVA.
Errors occurring in potential transformers
An ideal Potential transformer should give secondary voltage exactly in phase with the primary voltage and exactly in phase opposition. But practically due to some primary and secondary voltage drops the exact proportional voltage cannot be obtained at the secondary. Also exact phase opposition is not obtained. Thus both the ratio and phase angle errors are introduced by a PT.
The voltage ratio error is generally expressed in terms of the measured voltage and is given as
Percentage ratio error = (KnVs - Vp)/ Vp x 100%
Kn = number of secondary winding turns / number of primary winding turns,
= turns ratio
Vp, Vs = primary and secondary voltages respectively,
Phase angle error
The phase angle error δ is usually given in minutes and indicates by what angle the secondary voltage departs from the applied primary voltage. The increase in the number of measuring instruments and relays connected to the secondary of the PT increases the burden thereby increasing the ratio and phase angle error.
Characteristics of Potential Transformer According to their Accuracy class
Positive or negative permissible voltage ratio error in percentage
Positive or negative permissible voltage ratio error in minutes
The “burden” is the impedance presented by the measuring instrument on the secondary terminals of the transformer. In other words, total external volt ampere load connected on the secondary at the secondary rated voltage. It is mainly resistive in nature and has a large value. The ratio and phase angle errors are also influenced by the primary voltage. Operation in excess of 10% of over voltage may cause increased errors and excessive heating. The errors must be kept within the limits. In order to achieve this PTs are designed to have low reactance, low loss and high magnetizing reactance.
Low reactance is achieved by interlacing both primary and secondary on core limb. For high magnetizing reactance minimum iron path and high permeability steel are required. Low loss can be obtained by using low loss steel and thin lamination. The most important thing for low PT errors is to make the burden Zb as high as possible.
The ratio and phase angle errors of any standard ASA PT are so small that they may be neglected for the protective relaying purpose if the burden is within the thermal volt- ampere rating of the PT. This thermal volt- ampere rating corresponds to the fill load rating of the power transformer.
Model circuit of Potential transformer
Rated burden is the VA burden at the secondary of the PT at which the PT operated within its rated accuracy.
The rated VA rated within which the PT operates without violating the permissible temperature limits of its windings. This burden is several times greater than its rated burden.
Types of Potential Transformer
There are two types of potential transformers. They are
1. Electro magnetic (Conventional) type potential transformers.
2. Capacitive type potential transformers.
The conventional type potential transformers are used for voltages below 100 kV. For the voltages above 100 kV the conventional potential transformer becomes more costly due to its insulation requirements. So, for voltages above 100 kV capacitive type potential transformers are used. Capacitive PT consists of a capacitive potential divider and an intermediate transformer of relatively small ratio. The intermediate transformer must have small ratio error and phase angle error to give satisfactory performance of the complete unit.