Difference between CVT and PT

Quiz

In electrical engineering, there are several different types of transformers for different applications. Two such transformers are CVT and PT. CVT stands for Capacitive Voltage Transformer and PT stands for Potential Transformer.

Both CVT and PT are the types of instrument transformers and thus are used in protection and metering applications. However, they are different from each other in many aspects. The primary difference between CVT and PT is that a CVT (Capacitive Voltage Transformer) utilizes a capacitor voltage divider and a step-down transformer to convert a high voltage into a low voltage, while a PT (Potential Transformer) utilizes only a step-down transformer to reduce the voltage level.

There are numerous other differences between CVT and PT which we will discuss in this article. But before that lets have an overview of CVT and PT individually.

What is a CVT?

A CVT (Capacitive Voltage Transformer) is a device used in electrical systems to reduce the high voltages of power lines into low voltages suitable for measurement and protection.

A CVT is a specially designed step-down transformer in which a capacitor voltage divider is employed along with a step-down transformer for reduction of the voltage level.

The circuit diagram of a typical capacitive voltage transformer or CVT is depicted in the following figure

We can see that it consists of the following two major components

Capacitor Voltage Divider This circuit consists of two series connected capacitors. This capacitor voltage divider is connected across the transmission line whose voltage is to be reduced. This voltage divider reduces the voltage of the power line significantly by using the principle of voltage division between capacitors. This reduced voltage is then applied to the electromagnetic unit of the CVT.

Step - Down Transformer It is another essential component of a CVT which is tagged as the electromagnetic unit in the CVT. The function this transformer is to further reduce the voltage from the capacitor voltage divider to a value that can be safely applied to a protecting or metering device.

Applications of CVT

It is clear that a CVT consists of a voltage transformer and a capacitor element. For this reason, it is referred to as a capacitive voltage transformer.

CVTs are used in electrical power systems for the following purposes: Measurement of high-voltages of transmission lines, Protection purposes, For monitoring insulation levels in system, Filtering harmonics from the system, etc.

What is a PT?

PT (Potential Transformer) is another type of voltage step-down transformer used in power systems for reducing the high voltages of transmission lines to a low value suitable for metering and protection purposes.

Sometimes, a PT is also referred to as a voltage transformer or VT. Like CVT, PT is also an instrument transformer because it is employed for metering and protection of the system.

The circuit diagram of a PT or potential transformer is depicted in the following figure

It can be seen that a PT consists of three main components namely,

Primary Winding A coil to which high input voltage is applied for stepping it down.

Secondary Winding A coil from which reduced voltage is taken as output.

Magnetic Core To carry the magnetic flux from primary winding to the secondary winding for mutual induction.

It is clear that the construction of a PT or potential transformer is similar to an ordinary voltage step-down transformer used in power system. Also, the working principle of a PT is based on electromagnetic induction only.

Applications of Potential Transformer

PTs are also used in metering, protection, and control applications in electrical power systems. Some of the key applications of PTs are as follows

Measurement of voltage in transmission and distribution lines,
Protection of power system and operate the protective relays,
Monitoring the power system for voltage fluctuations to reduce power quality problems,
To provide control signals to control and communication devices, etc.

After getting a brief overview of CVT and PT, let us now highlight the important differences between them.

Difference between CVT and PT

The following table highlights all the significant differences between CVT (Capacitive Voltage Transformer) and PT (Potential Transformer)

Parameter	CVT	PT
Full form	CVT stands for Capacitive Voltage Transformer or Capacitor Voltage Transformer.	PT stands for Potential Transformer.
Components	CVT has a capacitor voltage divider and a step-down transformer to reduce the high voltages.	PT has a step-down transformer only for reducing the high voltages.
Working principle	The working of a CVT is based on the principle of voltage division and electromagnetic induction.	The working of a PT is based on the principle of electromagnetic induction.
Voltage step-down process	A CVT first reduces the voltage using a capacitor voltage divider and then this reduced voltage is further stepped down using a step-down transformer.	A PT reduces the voltage using a step-down transformer only.
Voltage rating	CVTs are rated for extra high voltage levels, typically greater than 220 kV.	PTs are generally rated for low, medium, and high voltage levels up to 220 kV.
Core losses	Due to small sized magnetic core, CVTs has low iron or core losses.	PTs have a large magnetic core and thus these have very high core or iron losses.
Frequency response	CVTs have a wider frequency response.	The frequency response of PTs is narrower.
Coupling of PLCC	CVTs can be used for coupling of PLCC (Power Line Carrier Communication) due to their high frequency response.	PTs are not suitable for PLCC coupling.
Weight	CVT are lighter in weight due to their small core size.	PTs are relatively heavier than CVTs.
Cost	CVTs are less expensive for extra-high voltage applications.	PTs are relatively expensive than CVTs for extra high voltage applications.
Design complexity	CVTs have a relatively complex design due to the presence of additional capacitor voltage divider.	PTs have a simple design as they use only an electromagnetic unit.
Reliability	CVTs are more reliable at extra-high voltages.	PTs are not much reliable as the CVTs. This is due to the problems associated with their magnetic core and windings.
Insulation level	CVTs have a very high insulation level, thus they can be used in extra high voltage applications.	PTs do not have insulation level as high in CVTs. Thus, they are generally used in medium and high voltage applications only.
Effect of frequency variations	CVTs use capacitor component. Hence, their performance is affected by the variations in frequency.	The performance of PTs is less affected by the variations in frequency.
Applications	CVTs are mainly used in extra-high voltage power systems for voltage measurement, protection, and power line carrier communication.	PTs are used in low to high voltage power systems for protection and voltage measurement.

Conclusion

From the above discussion, we can conclude that the primary function of a CVT and a PT is that same, i.e., reducing high-voltage to a lower value for measurement and protection purposes. But they are completely different from each other in construction, working principle, applications, and many other aspects.

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