CT and VT Selection for MV Switchgear: A Specification Guide for Protection and Metering

Selecting medium-voltage switchgear involves more than determining rated voltage, continuous current and short-circuit capacity. Current transformers (CTs) and voltage transformers (VTs) are equally important because they provide the electrical measurements required for protection, metering and control.

Incorrect CT VT selection can result in inaccurate metering, protection relay maloperation, CT saturation or costly modifications during commissioning.

Two switchgear panels with identical primary electrical ratings may have significantly different performance because of their instrument transformer configurations.

For EPC contractors, utilities, industrial facilities and renewable energy developers, CT and VT requirements should therefore be established before issuing the switchgear Request for Quotation (RFQ).

This guide explains the essential parameters engineers should include in an instrument transformer specification.

Why CT and VT Selection Matters in MV Switchgear

Current transformers and voltage transformers convert primary electrical quantities into proportional secondary signals suitable for measuring instruments and protection relays.

A CT reduces primary current to a standardised secondary current, commonly 1 A or 5 A.

A VT provides a proportional lower-voltage signal for voltage measurement, directional protection, synchronisation and energy metering.

Although both devices support measurement and protection, their operating characteristics and specification requirements differ.

The medium voltage switchgear specification checklist highlights CT ratios, accuracy classes, burdens and protection relay interfaces as essential equipment parameters.

These details must be coordinated with the electrical studies, protection philosophy and metering requirements before equipment procurement.

Understanding Metering CTs and Protection CTs

One of the most important aspects of current transformer selection is distinguishing between measurement and protection applications.

Metering CTs Prioritise Accuracy

Metering CTs are designed to provide accurate current measurements within their specified operating range.

They support energy meters, multifunction meters and power monitoring equipment.

Common IEC metering accuracy classes include 0.2, 0.2S, 0.5 and 0.5S, depending on the required measurement performance.

For revenue metering, the applicable network or metering authority may impose additional requirements.

Metering CT performance must also be evaluated against the actual burden and expected operating current range.

Protection CTs Must Perform During Faults

Protection CTs have a different responsibility.

They must reproduce current signals with sufficient accuracy during abnormal conditions so that protective relays can detect faults correctly.

Protection classes such as 5P and 10P are commonly used for applicable overcurrent protection arrangements.

For example, a 5P20 CT has a specified composite error limit at an accuracy limit current corresponding to 20 times rated primary current under its rated burden conditions.

However, the appropriate class depends on fault levels, relay requirements and the protection scheme.

A CT suitable for normal metering may therefore be unsuitable for high-current fault protection.

How to Select the Correct CT Ratio

An MV switchgear CT ratio should accommodate maximum operating current while maintaining adequate protection sensitivity.

For example, an 800/1 A CT provides a nominal secondary current of 1 A when the primary current is 800 A.

Selecting an unnecessarily high ratio can reduce secondary current during low-level faults, potentially affecting relay sensitivity.

Conversely, an undersized ratio may create problems during normal loading or overload conditions.

Engineers should consider maximum demand, transformer rating, future expansion, minimum fault current and maximum short-circuit current.

CT Core Allocation and Secondary Circuits

A single CT assembly may contain multiple secondary cores dedicated to different functions.

Typical arrangements include separate cores for:

• Main protection

• Backup protection

• Operational metering

• Revenue metering

• Differential protection

  
  

Core allocation should be defined in the protection and metering schedules.

Sharing a core between multiple devices requires verification of total burden, accuracy and the secondary circuit arrangement.

Engineers must also specify terminal polarity, earthing and suitable test or shorting facilities.

CT secondary circuits must never be left open while primary current is flowing, because dangerous voltages can develop.

Accuracy Class, Burden and CT Saturation

CT performance cannot be evaluated from ratio and accuracy class alone.

The connected burden is equally important.

Burden represents the load imposed by connected relays, meters, wiring and associated secondary circuit components.

It is commonly expressed in volt-amperes (VA) at the rated secondary current.

If the actual burden exceeds the CT's specified capability, its accuracy may deteriorate.

For protection applications, excessive burden can contribute to premature saturation during faults.

When Does Knee-Point Voltage Matter?

Some protection applications require more detailed CT performance information.

Differential protection, restricted earth fault and certain high-impedance protection schemes may require CT excitation characteristics and knee-point voltage data.

A knee-point requirement is not automatically necessary for every CT.

It should be determined by the protection relay manufacturer and the engineering calculations for the specific application.

Where required, the specification should identify excitation characteristics, winding resistance and other relevant performance parameters rather than relying only on a generic accuracy class.

Voltage Transformer Selection for Protection and Metering

While CTs measure current, VTs provide the voltage signals required by protection relays, meters and control systems.

Correct voltage transformer selection begins with the nominal system voltage and intended measurement arrangement.

Engineers should establish whether the VT measures phase-to-phase or phase-to-earth voltage.

The primary ratio must match the network and insulation arrangement, while the secondary voltage must be compatible with the connected equipment.

Common secondary voltage arrangements may include 110 V or 110/√3 V, depending on the system and connection configuration.

Accuracy and Secondary Burden

VT accuracy requirements depend on the intended function.

Revenue metering may require a different accuracy class from operational voltage indication or protection applications.

The total secondary burden should include connected relays, meters and wiring.

Where multiple secondary windings are provided, their respective functions and ratings should be clearly identified.

This helps avoid ambiguity between metering, protection and residual voltage applications.

VT Protection, Isolation and Earthing Requirements

VT installations require appropriate protection and isolation arrangements.

Depending on the switchgear design, primary-side protection may use fuses or another approved arrangement. Secondary circuits may include protective devices and isolation facilities.

Engineers should confirm:

• Primary and secondary protection

• Secondary earthing

• Fuse failure indication

• Isolation and testing access

• Residual voltage requirements

• VT compartment arrangement

Loss of VT voltage can affect directional, under-voltage and other voltage-dependent protection functions.

Protection logic should therefore consider VT circuit failures and appropriate supervision.

Where applicable, VT design must also account for ferroresonance risk and the system earthing arrangement.

Revenue Metering Requirements Need Early Coordination

Revenue metering is often subject to more demanding requirements than general operational measurement.

For Australian utility and renewable energy projects, CT and VT requirements should be coordinated with the applicable metering arrangements, network connection requirements and metering service provider.

These may specify accuracy classes, dedicated cores or windings, secondary wiring, test facilities and sealing requirements.

Metering accuracy should be evaluated as a complete measurement chain rather than by considering the CT or VT individually.

A technically suitable instrument transformer may still be unacceptable if it does not meet the applicable revenue metering specification.

Integrating CTs and VTs With Protection Relays

Instrument transformers must be coordinated with the complete protection architecture.

A CT for protection relay application should consider the required relay functions, fault levels, accuracy performance and secondary circuit configuration.

Functions such as overcurrent, earth fault, differential and directional protection can impose different CT or VT requirements.

For example, directional protection requires suitable voltage and current references, while transformer differential protection may impose specific CT performance requirements.

Modern IEC 61850 substation automation can improve communication between intelligent devices, but it does not eliminate the need for accurate instrument transformer signals.

The CT and VT specification should therefore be reviewed together with relay selection, protection studies and SCADA requirements.

Physical Integration Inside MV Switchgear

CT and VT requirements also affect the mechanical design of the switchgear.

CT dimensions, installation positions and cable routing must fit within the available compartment space.

VTs may be fixed, withdrawable or installed in dedicated compartments, depending on the equipment architecture.

Leistung Energie's medium voltage switchgear components guide identifies instrument transformers as important elements of measurement, monitoring and protection systems.

Physical interfaces should be confirmed before manufacturing drawings are approved, particularly for compact switchgear and installations requiring multiple CT cores or specialised metering arrangements.

CT and VT Specification Checklist Before RFQ

A complete instrument transformer specification should contain enough information for suppliers to offer technically comparable equipment.

Specification Area	What Engineers Should Define
System voltage	Nominal voltage and insulation requirements
CT ratio	Primary and secondary rated current
CT cores	Number and function of cores
CT accuracy	Metering and protection classes
CT burden	Rated VA and connected burden
CT saturation	Accuracy limit and special requirements
Knee-point	Required excitation data where applicable
VT ratio	Primary and secondary voltage
VT accuracy	Metering and protection requirements
VT burden	Rated VA and connected burden
VT protection	Fuse, isolation and earthing arrangement
Revenue metering	Dedicated cores, accuracy and test facilities
Relay interface	Protection functions and input compatibility
Testing	Ratio, polarity, insulation and accuracy checks
Documentation	CT/VT datasheets, drawings and test reports

IEC 61869-1 provides general instrument transformer requirements, while IEC 61869-2 and IEC 61869-3 address current transformers and inductive voltage transformers respectively.

The applicable editions and project-specific requirements should be identified during procurement.

Review CT and VT Requirements Before Factory Acceptance Testing

Factory Acceptance Testing should verify that the supplied instrument transformers match the approved specification.

Relevant checks may include ratio, polarity, secondary wiring, insulation, nameplate information, terminal identification and protection relay integration.

Accuracy and excitation test requirements should be established according to the applicable standards and project scope.

The EPC contractor should also confirm that the final CT and VT schedules match the protection study and approved electrical drawings.

Discovering a missing protection core or incorrect VT ratio during commissioning can create substantial delays.

Conclusion

Successful CT VT selection requires more than choosing standard transformer ratios.

Metering accuracy, protection performance, burden, saturation, core allocation, voltage ratios, revenue metering and relay integration must all be considered together.

For EPC contractors and electrical engineers, these requirements should be confirmed before releasing the MV switchgear RFQ.

A complete CT and VT specification improves supplier comparison, reduces engineering changes and supports reliable protection and measurement throughout the installation's operating life.

Planning an MV Switchgear Project?

Leistung Energie Australia provides medium voltage switchgear solutions for utility, renewable energy, mining and industrial applications.

If your project requires specific CT ratios, metering accuracy classes, protection cores or VT configurations, these requirements should be established early during switchgear selection.

Contact Leistung Energie Australia to discuss your MV switchgear, instrument transformer and protection system requirements for your upcoming project.