Voltage Transformer for Metering and Protection

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A voltage transformer is nothing but a type of instrument transformer used in electrical substations for metering and protection purposes. Measurement of voltage levels in an electrical substation is important for accurate measurement and reliable protection, as both are crucial for safe operation of the substation.

In power distribution and transmission systems, voltages are usually high, typically 11 kV up to 765 kV, and the measuring instruments and relays cannot be directly connected to such high voltages. To overcome this issue, voltage transformers (VT) or Potential Transformers (PT) are used.

The primary function of a voltage or potential transformer is to reduce the system voltage to a standard low value such as 110 V or 100 V, so that it can be safely measured or applied to control equipment like relays. Let’s discuss in detail about the voltage transformer used in electrical substations for metering and protection applications.

What is a Voltage Transformer?

A voltage transformer is a specially design electrical transformer that reduces high system voltage to a safe low value for metering and protection applications. Voltage transformer is also referred to as potential transformer, and it is categorized as a type of instrument transformer, because it is only used for instrumentation purposes like measuring system voltage and providing protection against faults.

In Indian electrical power systems, the use case of voltage transformers is regulated by following standards and rules −

• IS 3156 (Parts I to IV) for instrument transformers
• IEC 6189-3 for voltage transformers
• CEA regulations
• CBIP manuals

Functions of Voltage Transformers in Substations

Voltage transformers are mainly used to perform two key functions in an electrical substation, and they are −

• Metering or measurement of system voltage
• Protection of power system

In terms of operation, the function of a voltage transformer is to step down the high system voltage to a standard low voltage for metering and protection.

Voltage Transformer for Metering

In electrical substation, it is very important to accurately measure the system voltage for both technical and commercial reasons. Although the system voltage is very high and thus it cannot be directly connected to the measuring instrument or voltmeter. Therefore, a voltage transformer is employed to step down the voltage to a standard low value suitable for metering.

Voltage Transformer for Protection

Voltage transformers are also used in protection applications. When a voltage transformer is used in protection, then it focuses on reliability during fault conditions in the system. In protection system, voltage transformer provides a reference voltage required by the protection relays to operate.

Accuracy Class of Voltage Transformer

For a voltage transformer, accuracy class can be defined as the maximum permissible error under normal operating conditions. The accuracy class of voltage transformer has impact on meter readings when VT is used for metering and relay operation when VT is used for protection.

When voltage transformer is used for metering purposes, then high accuracy is required. The standard accuracy classes for metering voltage transformer are 0.1, 0.2, 0.5, and 1.0. Here the number represents the percentage ratio error limit at the rated voltage and burden. The following table gives the maximum ratio error and maximum phase displacement for these classes −

In Indian electrical substations, the following are some recommendations for metering voltage transformers −

• Accuracy class 0.2 is used as standard for ABT (Availability-Based Tariff) and SEM (Special Energy Meters) at inter-utility scale.
• Accuracy class 0.5 can be used for load survey meters and PQ meters inside the utilities.

On the other hand, the accuracy during fault condition is more important for protection voltage transformers. Two standard accuracy classes are used for protection VTs which are 3P and 6P, where P stands for Protection.

The following table provides protection accuracy classes their ratio error and phase displacement −

In Indian substations, these accuracy classes for VTs are recommended as follows −

• The accuracy class 3P is used in critical applications such as distance protection, directional protection, etc.
• The accuracy class 6P can be used for less sensitive protection schemes such as over- or under-voltage protection.

Types of Voltage Transformers

The following two types of voltage transformers are mainly used based on the system voltage class and application requirements −

• EMVT (Electromagnetic Voltage Transformer) − This type of voltage transformer is primarily used in low-voltage or medium voltage substations.
• CVT (Capacitive Voltage Transformer) − This type of voltage transformer is used in high-voltage or extra-high voltage electrical substations.

The following table provides some key details of both types of voltage transformers −

Characteristics of Voltage Transformer

While selecting a voltage transformer during designing a substation, it is important to consider the following characteristics −

• Rated Primary Voltage − As the primary winding of the voltage transformer is connected to the system voltage, hence it must be selected according to the nominal system voltage of the substation. In India, it cloud be 230 V to 765 kV.
• Rated Secondary Voltage − This voltage must be compatible with meters and relays. In India, it is generally 110 V or 100 V.
• Accuracy Class − It defines the error limit in ratio and phase angle. For metering VTs, it could be 0.1, 0.2, 0.5, or 1.0, and for protection VTs, it can be 3P or 6P.
• Burden − It is also known as VA rating and can be defined as sum of power consumed by all connected devices like meters, relays to the secondary winding. In India, it generally lies in the range of 25 VA to 100 VA. It is a good practice to select a voltage transformer having rated burden greater than or equal to actual connected burden.
• BIS (Basic Insulation Level) − It is defined as the ability of the voltage transformer to withstand surge voltages due to lightning and switching. In India, it is taken approximately 5 times of the rated system voltage, for example, in a 132 kV substation, BIS is taken approx. 650 kVp.
• Thermal and Short Time Ratings − The voltage transformer must be cable in handling continuous rated burden under rated conditions and it must also cable in withstanding shortovercurrent or overvoltage during the fault conditions.

How to Select a Voltage Transformer in Substation Design?

The step-by-step process for selecting a right voltage transformer in substation design is given below −

Step 1 − First of all, define the application for which the voltage transformer will be used. For example, metering, protection, or both. For example, in India, CVTs of rating 132 kV or above have two cores namely, one for metering with accuracy class 0.2 and other one is for protection with accuracy class 3P.

Step 2 − Select the type of voltage transformer. In India, for voltage levels less than 66 kV, electromagnetic voltage transformers are used and for voltage levels 132 kV and above, capacitive voltage transformers are used.

Step 3 − Select the rated primary voltage i.e., the primary voltage of voltage transformer must match the system voltage.

Step 4 − Select the rated secondary voltage i.e., the secondary winding voltage of the voltage transformer must match the voltage rating of meters and relays. In India, most meters and relays have a rated voltage of 110 V.

Step 5 − Determine the accuracy class of the voltage transformer. For metering VTs, it is 0.1, 0.2, 0.5, or 1.0, and CEA mandates class 0.2 VT for metering applications. For protection VT, accuracy class could be 3P or 6P, and as per CEA, class 3P VT is used for distance or directional relays and class 6P VT is used for simple voltage protections.

Step 6 − Calculate the burden or VA rating by summing up the power requirement of all connected meters and relays to the secondary winding of voltage transformer. For example, if we connect an ABT meter of 5 VA, PQ meter of 10 VA, and a relay of 10 VA, then total VA required will be 25 VA and thus we will select a voltage transformer of 50 VA burden rating for safety margin.

Step 7 − Verify the basic insulation level of the voltage transformer, so it can withstand lighting and switching surges. This can be done simply by multiplying the system voltage rating by 5 (in India it is taken approx. 5 times of the rated system voltage).

Step 8 − Check the thermal and short-time ratings of the voltage transformer so it can handle thermal load during continuous operation and short-duration overvoltage during faults. In India, it is generally specified as withstand capability for 1 second or 3 seconds.

Conclusion

Voltage transformers or potential transformers are important components in an electrical substation, as they play a vital role in enabling accurate, reliable, and safe operation of measuring and protection systems of the substation. In this chapter, we presented a comprehensive view of voltage transformers for substation design and explained how to select a right voltage transformer for a particular substation design.