Electrical Machines - CVT Transformer

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In electrical engineering, CVT stands for Capacitive Voltage Transformer. It is a specially designed electrical transformer used for stepping down the high voltage of incoming supply to a low voltage for metering, protection and automation. It is an important component of an electrical substation. Sometimes, CVT is also referred to as CPT (Capacitive Potential Transformer).

In this article, we will learn about CVT, its function, advantages, and applications in electrical power systems. So, lets start with the basic definition of CVT.

What is a CVT Transformer?

CVT or Capacitive Voltage Transformer is an instrument transformer used in electrical systems, mostly in substations, for reducing the voltage to a level suitable for measurement and protection purposes.

Therefore, a CVT is a voltage stepdown transformer that reduces the high voltage to a low voltage. CVT is basically a highinsulated transformer that mainly used for measurement of highvoltage of greater than 100 kV.

When we compare a highinsulated transformer with a standard transformer, the highinsulated transformers are very expensive. Therefore, to reduce the cost of these highinsulated transformers, a capacitive voltage transformer technology is used. In simple words, we can say that a CVT provides same level of insulation and have a lower price as compare to another type of highinsulated transformer.

Working Principle of CVT Transformer

Let us now understand the basic construction and working of a capacitive voltage transformer.

A typical capacitive voltage transformer consists of the following three main components

• Capacitive Voltage Divider
• Auxiliar Transformer
• Inductive Element

The circuit diagram of the CVT is shown in the following figure.

The main function of the capacitive voltage divider and the auxiliary transformer is to reduce the high voltage to a low value.

The capacitive voltage divider is connected across the transmission line and in series with the primary winding of the auxiliary transformer. Therefore, the auxiliary transformer gets the input supply from the output of the capacitive voltage divider.

The capacitive voltage divider consists of two capacitors namely, C₁ and C₂ Among these capacitors, the capacitor C₂ which is placed closer to the ground terminal have a higher capacitance value as compared to the capacitor C₁. Due to this high capacitance of C₂ , the impedance of this part of the divider circuit becomes low. Thus, it produces a reduced voltage at its output.

This low voltage is then transmitted to the primary winding of the auxiliary transformer which is further stepped down by the transformer and supplied to a metering unit for measurement. It is also important to note that the metering unit behaves a resistive load.

So, we can observe that the voltage divider on the input side is of capacitive nature while the metering unit on the output side is of resistive nature. This creates a phase shift between the input and output voltages and affects the measurement. To overcome this issue, a compensating inductor is connected in series with the primary winding of the auxiliary transformer as shown in the circuit diagram.

The inductance value L of this compensating inductor is given by,

$$\mathrm{L \: = \: \frac{1}{\omega^2(C_{1} \:+\: C_{2})}}$$

This inductance value L consists of the leakage flux of the auxiliary transformer. Therefore, it also compensates the voltage drops that occur in the auxiliary transformer due to reduction in the current from the capacitive voltage divider. But in practice, this compensation is not possible due to inductive losses.

Voltage Transformation Ratio of CVT Transformer

The voltage transformation ratio of a CVT transformer is given by,

$$\mathrm{\frac{V_{\circ}}{V_1}\:=\: \left[\frac{C_1}{C_1 \:+\: C_2}\right]\:\times \: \frac{N_2}{N_1}}$$

Here,

• V₁ is the input voltage
• V_° is the output voltage
• N₂ is the secondary winding turns of the auxiliary transformer
• N₁ is the primary winding turns of the auxiliary transformer

Since, for a CVT transformer, C₂ > C₁, then the value of [C₁ / (C₁ + C₂)] will be low. Hence, this decreases the value of voltage.

Name Plate Details of CVT Transformer

The nameplate of the capacitive voltage transformer (CVT) has the following parameters marked on it

• Manufacturers Information
• Insulation Level
• Standard
• Capacitance
• Primary Capacitance
• Secondary Capacitance
• Highest System Voltage (HSV)
• Voltage Factor
• Frequency
• Simultaneous Burden
• Thermal Burden
• Total Weight
• Total oil Weight
• Manufacturing Year
• Primary and Secondary Voltages
• VA Burden
• Accuracy Class
• Fuse rating

Let us discuss about each of these parameters in detail.

Manufacturers Information

The nameplate of CVT contains details about the manufacturer. This information mainly includes the name of the manufacturer, logo, and contact details.

Insulation Level

This information has two values where one is the RMS value and the other is the peak value. The RMS value mentioned is known as power frequency withstand voltage and is given in kV_rms.

The peak value mentioned is known as the lighting impulse voltage and is given in kV_peak. These voltage values are used for determining the insulation characteristics of the CVT and are known as its insulation levels.

Standard

This parameter provides information about industry standard according which the CVT is manufactured.

Capacitance

A CVT has a combination of two capacitors C₁ and C₂ connected in series. The capacitance is the equivalent capacitance of these two capacitors which is provided on the nameplate.

Primary Capacitance

In the CVT, the capacitance of the capacitor C₁ is called the primary capacitance. Its value is provided on the nameplate of the CVT transformer.

Secondary Capacitance

The capacitance of the capacitor C₂ is known as secondary capacitance. Its value is also given on the name plate of the CVT transformer.

Highest System Voltage (HSV)

The highest system voltage is the voltage for the CVT transformer is designed to operate. This voltage is also known as rated voltage of the CVT and its value is always provided on the nameplate.

Voltage Factor

Voltage factor is nothing but the value of highest voltage that the CVT can handle for a specific time. It is mentioned on the name plate of the CVT in Per Unit System.

For example, if on the nameplate, it is given that 1.15 continuous and 1.3 for 25 seconds. Then, it indicates that the CVT can handle 115% of the rated voltage continuously and it can handle 130% of the rated voltage for 25 seconds.

Frequency

It is the rated frequency of the supply voltage at which the power is generated, transmitted, and distributed. It can be 50 Hz or 60 Hz depending on the supply system used in the country.

Simultaneous Burden

This parameter of the CVT nameplate provides information about the VA that can be taken from all the secondary windings at the same time without affecting the accuracy of any of the cores.

It is an important parameter because the burden connected to one of the secondary windings can affect the accuracy of the other secondary winding. This is mainly because the secondary windings of the CVT are wound on the same core.

Thermal Burden

It is the value of total amount of VA that can be taken from the CVT transformer at a time without increasing its temperature above a particular limit defined by the industry standard.

For example, if the thermal burden of 250 VA is mentioned on the nameplate of a CVT, then it shows that 250 VA can be taken from the CVT without exceeding the specified temperature limit.

Total Weight

This parameter provides the total weight of the CVT transformer in kilograms.

Total Oil Weight

This parameter gives the weight of the oil used in the CVT transformer for insulation and cooling purpose.

Manufacturing Year

This parameter provides the detail of year in which the CVT transformer is manufactured.

Primary and Secondary Voltages

This parameter shows the per phase voltage of the CVTs primary and secondary windings.

VA Burden

Burden is the total VA connected to the secondary winding of the CVT transformer. On the nameplate, the burden is given that we can connect to each winding of the CVT.

Accuracy Class

CVT transformer is used in power systems for measurement and protection purposes. And, different windings are used for metering and protection. Therefore, different accuracy classes are specified for metering and protection purposes. The main purpose of accuracy class is to specify the tolerable error in the output.

For example, if a metering class of CVT has an accuracy class of 0.5, then it shows that the tolerable error in voltage can be equal to ± 0.5%.

Similarly, if we have a protection class of CVT and it has an accuracy class of 3P and the applied voltage is 2% of the rated voltage. Then, this shows that the error in the voltage of the CVT must be within ± 6%.

Fuse Rating

In a CVT, fuses are provided on the secondary side for protection purpose.

This is all about name plate details of a CVT transformer. Let us now discuss the major advantages and applications of CVT transformers.

Functions of CVT Transformer

A CVT transformer is used in power systems for the following purposes

• Accurate measurement of high voltages in power system.
• To provide voltage signals for monitoring and control applications.
• To detect abnormal conditions in the system and actuate the protection relays.
• For metering purposes.
• To filter out the harmonic components, etc.

Advantages of CVT Transformer

The main advantages of a capacitive voltage transformer are highlighted below

• CVT has a high accuracy in voltage measurement.
• CVTs can operate in a wide range of frequencies and hence these can be used in both AC and DC systems.
• CVTs have high insulation levels and thus can withstand very high voltages.
• CVTs require less maintenance. Hence, their operation cost is low.
• CVTs can be used with a wide range metering and protection devices.
• CVTs are less expensive than other highly insulated transformers.
• Due to compact size, CVTs require a less space for installation.
• CVTs are simple to design and manufacture.

Applications of CVT Transformer

CVT is primarily used for metering and protection purposes in power systems. The following are some major applications of CVT transformers

• CVT transformers are used for measuring high voltage in power systems for calculating energy consumption.
• CVTs are also used for protection purposes to detect abnormal conditions in the power system and operate the protection relays.
• In instrumentation and control, CVTs are used for monitoring and controlling the system voltage to improve the power system stability.
• In substation automation systems, CVTs are used for providing voltage signals for monitoring and controlling different automation components.
• CVTs are also employed in testing of highvoltage electrical systems.

This is all about CVT or Capacitive Voltage Transformer.

Conclusion

From the above discussion, we can conclude that a CVT is a high insulation instrument transformer used in power systems for metering and protection purposes. It is used because of its high accuracy and low cost as compared to other highly insulated transformers. In this article, I have explained major concepts related to CVT transformer.

FAQs Related to CVT Transformer

This section answers some most frequently asked questions related to capacitive voltage transformer (CVT)

1. What is the full form of CVT?

The full form of CVT is Capacitive Voltage Transformer. Sometimes, it is also expended as Capacitor Voltage Transformer.

2. What is the purpose of a CVT?

A CVT is used for the following two purposes

• Measurement of high voltages
• Protection of power system

3. Is the difference between VT and CVT?

VT stands for voltage transformer. A VT is a normal electromagnetic device that can stepup or stepdown the voltage. On the other hand, CVT is a voltage stepdown transformer which consists of a capacitive voltage divider along with a transformer for voltage stepdown purpose.

4. Why is CVT better?

The following are some key factors that make a CVT better

• Versatility, as it can perform measurement, protection, monitoring, control, and many other important functions.
• Less expensive as compared to other types of highly insulated transformers.
• Compact design, etc.

5. What are 3 benefits of CVT?

The following are the 3 main benefits of CVT

• Small size
• Low cost
• High accuracy