Comparison Between Current Transformer And Potential Transformer

Current Transformer Vs Potential Transformer

What is a Current Transformer(C.T)?

A Current Transformer (CT) is a type of instrument transformer.it is used to measure alternating current (AC) in electrical systems. It consists of a primary winding and a secondary winding.

The primary winding carries the current to be measured and the secondary winding generates a proportionally reduced current.

Current Transformer Vs Potential Transformer

Ratings of a current transformer(CT)

Primary Current Rating
Secondary Current Rating
Turns Ratio
Burden
Accuracy class
Rated frequency
Insulation Level
Short-time current rating
Thermal Rating

Primary Current Rating

This is the maximum current that the CT can measure on the primary side. It is specified in amperes (A). For example, a CT might be rated 100A, 200A, 500A, etc.

Secondary Current Rating

The standard secondary current ratings are 5A or 1A. This is the output current of the CT when the primary current is at its rated value.

Turns Ratio

The turns ratio is the ratio of primary current to secondary current. For instance, a CT with a 1000:5 ratio means that 1000A on the primary side will produce 5A on the secondary side.

Burden

The burden is the load connected to the secondary of the CT.it is expressed in ohms (Ω) or volt-amperes (VA). The burden rating ensures the CT can operate accurately within the specified load range, such as 5VA, 10VA, 15VA, etc.

Accuracy Class

The accuracy class indicates the error limits of the CT. The Common accuracy classes are 0.1, 0.2, 0.5, 1, and 3. For example, a 0.5-class CT has a maximum error of 0.5%.

Rated Frequency

The frequency at which the CT is designed to operate is typically 50 Hz or 60 Hz.

Insulation Level

This rating specifies the CT’s ability to withstand high voltage without breaking down. It includes the maximum voltage the CT can handle continuously (e.g., 600V, 11kV) and the insulation class (e.g., Class B, Class F).

Short-Time Current Rating

The maximum current the CT can withstand for a short duration (e.g., 1 second, 3 seconds) without damage, is often expressed in kA.

Thermal Rating

The thermal rating indicates its rated current without exceeding a specified temperature rise.

Current Transformers (CTs)

CTs are connected in series with the circuit being measured.
CTs have a toroidal core with primary and secondary windings.
CTs are used to measure and monitor electric current in power systems.
CTs are used to measure and transform high currents into proportional lower currents.
They consist of a primary winding connected in series with the current-carrying conductor.
The secondary winding of the CT is connected to measuring instruments. protective relays, meters, or other devices.
CTs have a high class of accuracy.
They provide electrical isolation between the high-current power system and the secondary circuit.
CTs are commonly used for metering, protective relaying, fault detection, current monitoring, and load analysis.
CTs have a secondary current rating in milliamperes (mA).
CTs have a burden in the form of resistance or impedance that affects the accuracy of current measurements.
CTs have different accuracy classes based on their intended precision and error tolerance.
CTs are typically larger and bulkier.
CTs are more susceptible to core saturation.
CTs have a voltage rating on the primary side, which should not be exceeded to prevent insulation breakdown.
CTs are designed to withstand high short-circuit currents.
CTs are safer to handle compared to PTs.
CTs have a negligible phase angle error.
CTs are typically connected to the main current-carrying conductor.
CTs are usually designed for higher insulation levels.
CTs are designed for specific frequency ranges of 50 or 60 Hz.

What Is Potential Transformer(P.T)?

A Potential Transformer (PT), is also known as a Voltage Transformer (VT).it is an instrument transformer that transforms voltages from high to lower levels.

Current Transformer Vs Potential Transformer

Ratings of Potential Transformer(P.T)

Primary Voltage Rating
Secondary Voltage Rating
Voltage Ratio
Burden
Accuracy Class
Rated Frequency
Insulation Level
Thermal Rating
Voltage factor
Short-Circuit Withstand Capability

Primary Voltage Rating

This is the high voltage of PT measured on the primary side. It is expressed in kilovolts (kV). For example, a PT might be rated for 11kV, 33kV, 66kV, etc.

Secondary Voltage Rating

The standard secondary voltage ratings are 110V or 120V. This is the output voltage of the PT when the primary voltage is at its rated value.

Voltage Ratio

The voltage ratio is the ratio of primary voltage to secondary voltage. For instance, a PT with a 66kV/110V ratio steps down.

Burden

The burden is the load connected to the secondary of the PT.it is expressed in volt-amperes (VA) such as 50VA, 100VA, etc.

Accuracy Class

The accuracy class indicates the error limits of the PT. Common accuracy classes are 0.1, 0.2, 0.5, 1, and 3. For example, a 1-class PT has a maximum error of 1%.

Rated Frequency

The frequency at which the PT is operated is typically 50 Hz or 60 Hz.

Insulation Level

This rating specifies the ability to withstand high voltage without breaking down. It includes the maximum voltage the PT can handle continuously (e.g., 11kV, 33kV).

Thermal Rating

The thermal rating indicates the PT’s ability to carry its rated voltage without exceeding a specified temperature rise.

Voltage Factor

The voltage factor is the PT's to operate correctly at a voltage higher than the rated voltage for a specified time.

Short-Circuit Withstand Capability

This rating indicates the PT's ability to withstand short-circuit conditions on the secondary side without damage.

Potential Transformers (PTs)

PTs are connected in parallel to the circuit being measured.
PTs have a laminated core with primary and secondary windings.
PTs are used to measure and transform high voltages into proportional lower voltages.
PTs, also known as voltage transformers.it is used to measure and monitor voltage levels in power systems.
They consist of a primary winding connected in parallel with the power system voltage.
The secondary winding of the PT is connected to measuring instruments, protective relays, meters, or other devices.
PTs have a voltage rating on the secondary side.
PTs may not be able to handle such high currents.
PTs are generally not safer to handle compared to CTs.
PTs have built-in voltage transient protection mechanisms.
PTs can introduce phase angle errors.
PTs are relatively more frequency-independent.
They provide electrical isolation between the high-voltage power system and the secondary circuit.
PTs have a secondary voltage rating, usually in volts (V).
PTs have a burden that affects the accuracy of voltage measurement.
PTs have different accuracy classes.
PTs are typically smaller in size.
PTs are less affected by core saturation.

Conclusion

Current Transformers (CTs) and Potential Transformers (PTs) are both essential in measuring. CTs are used to step down high currents while PTs reduce high voltage values for measurements.

The choice between using a CT or a PT depends on the needs of the power system. Both play a vital role in ensuring the safety and accuracy of electrical power distribution.