What is Potential Transformer (PT)?

What is Potential Transformer (PT)? . A Potential Transformer (PT), also known as a voltage transformer, is a type of instrument transformer designed to scale down high voltages in electrical systems to safer, measurable levels. These transformers enable precise voltage measurements and protect equipment and personnel from hazardous high-voltage conditions. PTs are vital in power generation, distribution, and industrial systems, ensuring accurate voltage monitoring and efficient operation.

Read More : Maintenance of Transformer

What is a Transformer?

Transformer is a device that transfers electrical energy from one circuit to another through mutual induction. It has two coils i.e. primary & secondary that are magnetically coupled & electrical isolated. They are used for increasing or decreasing the voltage & current levels without changing their frequency. There are different types of transformers used for specific applications such as power transformers, autotransformers, instrument transformers etc.

The instrument transformer can be classified into a current transformer (CT) and potential transformer (PT). Just like a current transformer is used to reduce the current levels for measurement, PT is used for reducing the voltage level.

What is Potential Transformer (PT)?

The potential transformer may be defined as an instrument transformer used for the transformation of voltage from a higher value to the lower value. This transformer step down the voltage to a safe limit value which can be easily measured by the ordinary low voltage instrument like a voltmeter, wattmeter and watt-hour meters, etc.

Potential Transformers (PT) in Power Systems

A Potential Transformer is an instrument transformer within power systems. It facilitates accurate voltage transformation from higher to lower values for measurement and protection purposes. Commonly referred to as a PT, a Potential Transformer maintains precise voltage ratios and phase relationships while presenting minimal load to the circuit, thereby ensuring reliable secondary connected metering.

Stepping down voltages to safer levels enables easy measurement using standard low-voltage instruments such as voltmeters, wattmeters, and watt-hour metres. What’s more, commercial voltmeters are typically compatible with PT outputs and are designed to handle negligible loads and maintain accuracy in voltage measurement.

Construction of Potential Transformer

The potential transformer is made with high-quality core operating at low flux density so that the magnetising current is small. The terminal of the transformer should be designed so that the variation of the voltage ratio with load is minimum and the phase shift between the input and output voltage is also minimum.

The primary winding has a large number of turns, and the secondary winding has a much small number of turns. For reducing the leakage reactance, the co-axial winding is used in the potential transformer. The insulation cost is also reduced by dividing the primary winding into the sections which reduced the insulation between the layers.

Working of Potential Transformer

The working of PT is similar to any conventional transformer. The electrical energy is transferred between the primary & secondary winding through magnetic induction.

The alternating voltage at the primary generates alternating magnetic flux in the transformer core. Since both windings use the same core, this alternating flux induces a voltage in the secondary winding. Thus current starts to flow in the secondary winding.

Since the primary has a greater number of turns compared to fewer secondary turns, the voltage induced in the secondary is very low. The secondary voltage is measured by using a standard low voltage voltmeter. Using the turn ratio equation of the transformer, we can calculate the primary voltage.

V_P/V_S = N_P/N_S

Where

V_P = Primary Voltage
V_S = Secondary Voltage
N_P = No. of Turns in Primary
N_S = No. of Turns in Secondary

Since the voltmeter has very high impedance, very low current flow through the secondary windings of the PT. for the same reason, the PT has very low VA ratings around 200VA.

Connection of 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 terminals of the potential transformer are connected to the measuring instrument like the 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 400V to several thousand volts, and the secondary terminal is always rated for 400V. The ratio of the primary voltage to the secondary voltage is termed as transformation ratio or turn ratio.

Types of Potential Transformers

There are several types of Potential Transformers:

Electromagnetic Potential Transformer

This is the most common type of Potential Transformer, which operates on the principle of electromagnetic induction. It consists of a primary winding connected across the high-voltage circuit and a secondary winding connected to the measuring instruments. Insulation at high voltages presents challenges, especially for voltages higher than 10 kV.

Capacitive Potential Transformer (CPT)

In high-voltage power systems, a CPT is used to detect voltage. It operates on the principle of the capacitive voltage divider, using it to reduce the voltage level of the high-voltage circuit to a measurable level. CPTs are frequently used to measure the voltage of transmission lines due to their precise voltage measurement, high insulation resistance, and minimal signal distortion.

Inductive Potential Transformer

This type operates on the principle of the inductive voltage divider, using it to reduce the voltage level of the high-voltage circuit to a measurable level.

Resistor-Capacitor Potential Transformer

This type uses a combination of resistor and capacitor to reduce the voltage level of the high-voltage circuit to a measurable level.

Optical Potential Transformer

This type uses an optical sensor to measure the voltage level of the high-voltage circuit and is commonly used in high-voltage systems where electromagnetic interference can cause inaccuracies in measurement.

Errors in Potential Transformer

In an ideal transformer, the primary & secondary voltage are in exact proportion as its turns ratio & they are both in-phase. But practically, there is a voltage drop at primary due to its reactance which creates voltage ratio error & phase-shift error. Here are some of the errors that may occur in PT.

Ratio Error

The ratio error is the change in the voltage ratio due to the variation in load. Varying load changes the magnetizing current & the core losses that affect the secondary voltage of the PT.

In simple words, its nominal ratio differs from its actual ratio. Ratio error is given by

Ratio Error = (Nominal Ratio – Actual Ratio) / Actual Ratio

Ratio Error = (K_n – R)/R

% Ratio Error = {(K_n – R)/R} x 100

Where

K_n = Nominal Ratio (Rated Ratio)
R = Actual primary to secondary voltage Ratio

The nominal ratio is the ratio of rated primary voltage to rated secondary voltage.

Read More : What Is an Ideal Transformer?

Voltage Ratio Error

The voltage ratio error is the difference between the ideal voltage & the practical or actual voltage. Here is the formula to find the voltage ratio error

Voltage Ratio Error = (V_P – K_nV_S)/ V_P

% Voltage Ratio Error = {(V_P – K_nV_S)/ V_P} x 100

Where

K_n = Nominal Ratio (Rated Ratio)
V_P = Actual Primary Voltage
V_S= Actual Secondary Voltage

Phase Angle Error

The phase angle error is the difference between the phase of primary voltage & the reversed secondary voltage. Ideally, the primary voltage is in phase with the secondary voltage in reverse. But practically, there is the reactance of the windings that shifts the phase of the secondary voltage creating phase angle error.

Where

V_P= primary voltage
E_P = Primary Induced EMF
R_P = Primary Winding Resistance
X_P = Primary Winding Reactance
β = Phase angle Error.
I_P = primary current
I_o = Excitation current
I_m = Magnetizing current (part of I_o)
I_w = core loss current (part of I_o)
K_n = Turn Ratio of Transformer
Φ_m = Main Flux
V_S= Secondary voltage
E_S = Secondary Induced EMF
R_S = Secondary Winding Resistance
X_S = Secondary Winding Reactance
I_S = Secondary Current

The reference of the given phasor diagram is the main flux Φ_m. The primary induced voltage is achieved by the subtraction of losses due to the primary winding resistance R_P, & reactance X_P. The voltage drop due to primary windings is I_PR_P, & the reactance of the windings is I_PX_P.

The excitation current I_o is the vector sum of magnetizing current I_m& core loss current I_W. The vector sum of excitation current I_o & the reversal secondary current I_S multiplied by turn ratio 1/K_nresults in the primary current I_P.

Due to mutual induction, the primary emf will transform into the secondary emf E_S in the secondary windings. The secondary voltage V_S that appears at the output of the secondary windings is derived by subtracting the voltage drops due to the secondary windings resistance R_S & reactance X_S.

Applications of Potential Transformers

Potential Transformers are an essential component of power systems, providing accurate and reliable measurements of high voltages for various applications, as listed below.

They are used with ammeters and wattmeters to measure voltage, current, and power in high-voltage transmission lines.
They signal protective relays when a fault occurs on a high-voltage line.
They provide voltage signals for control circuits, such as automatic voltage regulators, reactive power compensators, and other control systems.
They are used in testing high-voltage equipment, such as transformers and circuit breakers.
They measure voltage and current during testing.
They isolate communication systems from high-voltage power systems, preventing interference with communication signals.
They are used in research to measure high voltages and currents in laboratory experiments and to calibrate other electrical equipment.

Advantages of Potential Transformers

Some advantages of Potential Transformers include:

They provide accurate and reliable measurements of high voltages.
They are essential for the safe and efficient operation of power systems.
They isolate high-voltage power systems from lower-voltage control and measurement systems, ensuring the safety of personnel and equipment.
They are typically less expensive than other high-voltage measurement options.
They are highly reliable and require minimal maintenance, reducing downtime and increasing operational efficiency.
They have low power consumption, helping to reduce energy costs and environmental impact.
They are easy to install and can be mounted on poles or structures, reducing installation time and cost.

Disadvantages of Potential Transformers

Some disadvantages of Potential Transformers include:

They are designed to operate within a specific frequency range and may not provide accurate measurements outside this range.
They are designed to measure high voltages within a specific range and may not be suitable for extremely high-voltage applications.
They can be sensitive to external factors such as temperature, humidity, and electromagnetic interference, which may affect accuracy.
They require complex wiring to ensure they are properly connected to other components in the power system, which can be time-consuming and challenging.
They are not designed to provide accurate voltage measurements at low voltage levels, which may limit their use in specific applications.

Frequently Asked Questions (FAQs)

1. What is the primary purpose of a potential transformer?

A potential transformer is used to step down high voltages in electrical systems for safe measurement and monitoring.

2. What are the types of potential transformers?

The two main types are electromagnetic potential transformers and capacitive voltage transformers (CVTs).

3. How is a potential transformer different from a current transformer?

A potential transformer measures voltage, while a current transformer measures current.

4. What factors affect the accuracy of a potential ?

Factors include load, frequency, temperature, and the quality of the core material and windings.

What is Potential Transformer (PT)?

What is a Transformer?

What is Potential Transformer (PT)?

Potential Transformers (PT) in Power Systems

Construction of Potential Transformer

Working of Potential Transformer

Connection of Potential Transformer

Types of Potential Transformers

Electromagnetic Potential Transformer

Capacitive Potential Transformer (CPT)

Inductive Potential Transformer

Resistor-Capacitor Potential Transformer

Optical Potential Transformer

Errors in Potential Transformer

Ratio Error

Voltage Ratio Error

Phase Angle Error

Applications of Potential Transformers

Advantages of Potential Transformers

Disadvantages of Potential Transformers

Frequently Asked Questions (FAQs)

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