Current vs Potential Transformers: Differences You Need to Know

Current transformers and potential transformers are two important types of transformers used in electrical power systems. Both types of transformers serve different purposes and have distinct characteristics that make them suitable for specific applications. Current transformers are used to measure the current flowing in a circuit while potential transformers are used to measure the voltage in a circuit.

In this article, we will take a closer look at the differences between current transformers and potential transformers including their construction, operation and applications.

Differences Between Current and Potential Transformer

The main differences between a current transformer and a potential transformer are:

1. Definition

The major difference between a current transformer and a potential transformer is the definition. A current transformer steps down the current of the primary circuit to conventional standard levels for measuring and controlling purposes. The potential transformer is used for stepping down voltages to nonhazardous and conventional voltage levels for measuring and controlling purposes.

2. Connection

The primary winding of the potential transformer is connected in parallel with the monitored circuit, while the primary winding of the current transformer is connected in series.

3. Stepping down levels

The secondary windings of potential and current transformers proportionally transform the primary levels to typical values of 120 V and 5 A. Monitoring devices such as wattmeters, power-factor meters, voltmeters, ammeters and relays are often connected to secondary circuits.

4. Standards

It is important to know their international standards in use; some of the more common are listed below. Most major countries originally developed their own standards.

5. Primary winding stress

The primary winding is subjected to the same dynamic and thermal stresses as the rest of the primary system when large short-circuit currents and voltage transients are present. For current transformers, it is the maximum asymmetrical peak current that can be detrimental and for potential transformers, it is the magnitude of overvoltage, both of which are dependent on the duration of the event causing the disturbance. It must also be sized to safely carry the maximum continuous current without exceeding the insulation system’s temperature class.

6. Damage conditions

When a potential transformer’s secondary winding is shorted, the resulting high current will produce high winding losses. Consequently, if the transformer is not properly protected, it will be damaged and may become a fire hazard.

A current transformer hooked up to a live feeder should never have its secondary open-circuited. This would result in a higher level of magnetization which in turn would induce a high potential across the secondary terminals. This will endanger working personnel and will destroy the current transformer (owing to excessive core loss).

7. Level of magnetization

The magnetization level of a potential transformer depends on supply line voltage but the magnetization level of a current transformer depends on the current through its secondary.

8. Applications

A current transformer is used in conjunction with current measuring devices (like an ammeter). On the other hand, a potential transformer is used in conjunction with voltage-measuring devices (like a voltmeter)

9. Variants

The current transformer is divided into four categories, including bushing, bar, split-core and wound-type. The potential transformer is also divided into three categories, including electromagnetic, capacitive-coupled and optical type.

10. Effect of change in frequency

The reduction of frequency results in an increase in the core flux of a potential transformer, with a corresponding increase in the exciting current Io and a reduction in reactance volt drops. The effect of frequency variation is less important than that of variation of load and load power factor in a current transformer. The variation of magnetizing and iron-less magnetizing force, with the variation of frequency, is comparatively small if the frequency variation does not exceed 10-20 kHz, so an approximation, may be considered independent of frequency.

11. Transformation ratio

The transformation ratio in a current transformer is high, while in the potential transformer, the ratio is low.

12. Core

The current transformer is designed with silicon steel lamination but a potential transformer is designed with top-quality steel operating at low flux densities.

13. Primary current

In a current transformer, the primary current doesn’t depend upon secondary side circuit conditions. On the other hand, in the potential transformer, the primary current relies on secondary side circuit conditions.