Potential Transformers

Potential Transformers
Potential Transformers

Instrument transformers are commonly employed to reduce the extremely high voltages and currents seen in power transmission and distribution systems to a more manageable level for measurement purposes. This is due to the fact that safety relays and measuring meters or instruments do not operate on high voltage and cannot be directly attached to such a circuit in order to safeguard the system or take measurements.These transformers not only lower the voltage and current levels, but they also separate the high-powered main circuit from the measurement or safety circuit.While potential transformers convert high voltage to low voltage, current transformers lower the current to an operating range suitable for instruments or relays. We will go over the potential transformers in great depth in this article.

What is Potential Transformer

An example of a voltage step-down transformer is the potential transformer, which is used to assess voltages by reducing them from high levels to lower ones. They run in parallel or across the line that needs monitoring.This transformer is built and operates on the same basic idea as a regular power transformer. The typical abbreviation for potential transformers is PT.A primary winding with many turns is connected across the high voltage side, or the line that needs to be protected or measured. The voltmeters, energy meters’ potential coils, relays, and other control devices are linked to the secondary winding, which has a lower number of turns. These potential transformers can be either single-phase or three-phase. The 110 V secondary output voltage is a design feature that is independent of the main voltage rating.

A modest current passes through PT’s secondary due to the high impedance of other meters’ voltmeters and potential coils. Thus, when there is no load, PT operates similarly to a standard two-winding transformer. The PT’s low load means that its VA ratings are low as well, typically ranging from 50 to 200 VA. As illustrated in the image, one end of the secondary side is fastened to the ground for safety reasons.You can provide the transformation ratio in the same way as with a standard transformer:

N divided by N2 equals V divided by V2

If you have the voltmeter reading and the transformation ratio, you may use the above equation to find the high voltage side voltage.


Potential transformers, or PTs, use bigger cores and conductors than regular transformers. PTs are created with the goal of achieving greater precision, hence material economy is not prioritized during the design process.PTs are designed with a high-quality core that operates at lower flux densities. This allows for a minimal magnetizing current, which in turn minimizes load losses. For PTs, the best designs include both a core and a shell. While shell-type PTs are more commonly used at low voltages, core-type PTs are utilized at high voltages.

The leakage reactance can be decreased by utilizing co-axial windings for both the primary and secondary circuits. Placing a low-voltage secondary winding adjacent to the core helps bring down the insulating cost. In addition, the insulation between coil layers is reduced for high voltage PTs by dividing the high voltage primary into parts of coils. Cotton tape and disappeared cambric are employed as laminations for these windings. The spaces between the coils are filled with stiff fiber separators.

Potential Transformers
Potential Transformers

These are meticulously engineered to keep the voltage ratio constant regardless of changes in load and to minimize phase shift between the input and output voltages. To handle voltages above 7 kilovolts, oil-filled PTs are utilized. The primary lines in these PTs are connected using oil-filled bushings.

Many Forms of Potential or Voltage Transformers

Outdoor potential transformers and interior potential transformers are the two main categories.

1. Potential Transformers for Outdoor Use

Outdoor relaying and metering applications make use of these voltage transformers, which can be either single or three phase and have a variety of working voltages. The voltage ratings of these single-and three-phase electromagnetic transformers reach up to 33 kilovolts. One kind of outdoor potential transformer is the electromagnetic kind, while the other is the capacitive voltage transformer (CVT), and both can handle voltages above 33KV.

Conventional Potential Transformer, Electromagnetic or Wound Type

Typical oil-filled wire-wound transformers are comparable to this. The electromagnetic PT type, depicted in the figure below, involves connecting the tap tank to the line terminal. The oil tank is placed on an insulator support and has a filling plug.

There is an oil drain plug and ground terminal at the bottom. In this case, the main connection is between the ground and either the second or third phase. Thus, the primary is terminated at the bottom and grounded with other terminals, while one end is linked to the main line at the top.

Subsequently, the metering and relay circuits are linked to the secondary terminals, which include an earth terminal, which are situated in the terminal box at the base. Insulation considerations limit their application to voltages below 132 KV.

Transformers that store voltage in capacitors

The capacitive potential divider is linked to the ground and main line phases. These CVTs can be either bushing type or coupling-capacitor-type. The development of capacitance further determines their rated burden (or load), yet electrically, these two varieties are less or more similar.Using aluminum foil and oil-impregnated paper, a series-connected stack of capacitors forms a coupling capacitor type. By connecting the main and secondary terminals across the capacitors, one can achieve the desired primary and secondary voltages.

Condenser type bushings that are tapped are used by the CVT of the bushing type. Because of their reduced cost, CVTs find more use in power line carrier communication.

Indoor Transformers with Potential

These are also offered as molded, magnetic PTs in single or three phase varieties. You have the option of a fixed or drawout mounting method. All primary winding components in these PTs have earth insulation equal to or more than their rated insulation capability. In an interior setting, these are engineered to precisely run measuring instruments, relays, and other control equipment.

Voltage transformers, often known as PTs, can be categorized into two main types: metering transformers and protection transformers.

Voltage Transformer Mistakes

In a perfect voltage transformer, the secondary winding voltage is directly proportional to the primary voltage and is in perfect phase opposition. However, this is not the case in real PTs as a result of the power factor of the secondary load and voltage reductions in the primary and secondary resistances. As a result, voltage transformers experience faults related to phase angle and ratio. Give us the specifics.

Think about the phasor diagram of the potential transformer up there:

  • in which
  • No load current is represented by Io.
  • A current without a load has a magnetizing component [Im].
  • Unit of watts of no-load current
  • Induced voltages in the primary winding (Ep) and the secondary winding (Es)
  • Np is the number of turns in the main winding and Ns is the number of turns in the secondary winding.
  • Ip and Is stand for main and secondary current, respectively.
  • The primary winding resistance (Rp) and the secondary winding resistance (Rs) are the same thing.
  • Xp is the reactance of the main winding and Xs is the reactance of the secondary winding.

the phase angle inaccuracy

From the primary voltage Vp, subtract the primary resistive (IpRp) and reactive drop (IpXp) to produce the primary induced voltage, or EMF Ep. The secondary terminal voltage Vs is calculated by vectorially subtracting the secondary winding resistance drop (IsRs) and reactance drop (IsXs) from the secondary generated EMF Es. These drops generate a ratio error because the potential transformer’s nominal ratio differs from the PT’s actual ratio.

Ratio Mistake

The potential transformer’s ratio error is the difference between the nominal ratio and the actual ratio of transformation.

Error in Percentage Ratio = (Kn – R) / R × 100

In what cases

The transformation ratio, denoted as Kn, is either rated or nominal.

You may calculate Kn by dividing the rated primary voltage by the rated secondary voltage.

Phasor Angle Mistake

The ideal PT would have a main voltage and a reversed secondary voltage with no phase angle at all. However, in reality, there is a phase mismatch between the reversal of Vp and Vs (as seen in the picture above), which causes phase angle error. It is the difference in phase between the main voltage and the secondary voltage, inverted.

Transformers should have windings with appropriate reactances and internal resistance to reduce these errors. Furthermore, the exciting current’s magnetizing and core loss components shouldn’t be too high for the core.

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