Selection of Current Transformers for Protection Relays or Tripping Relays

Current Transformers perform two critical roles in the power supply eco-system. They are used as current measurement devices and as protection relays. However, there are different types of Current Transformers and several factors govern the appropriate choice of the Current Transformer.

The objective of this study is to understand the key considerations before choosing the right Current Transformer for a particular application.

If you are looking to read about the basic function of Current Transformers, compliance standards, and applications, read our earlier blog here.

These Current Transformers are employed to detect a fault, not for measuring the current of a power system or the connected equipment. The fundamental difference in the requirement of a measuring and a protective transformer is in terms of accuracy, saturation level and VA burden.

How to select the right Current Transformer?

Unlike a measuring CT, a protection CT has a high saturation level to allow high primary current to transform substantially to the secondary as required, depending on the current setting of the protective or tripping relays. Therefore for protection CTs, accuracy class is of little relevance up to the primary rated current, but a true reflection in the secondary is more important of a fault condition in the primary.

Generally, it is not feasible to meet both requirements of metering and protection by one CT. Two sets of transformers are usually employed for a power circuit, one for measurement and the other for protection.

Protection CTs are generally available in wound or ring type. They find wide application in Control Panels, Monitoring Systems and Protective Systems.

KWP Series – Wound Primary Protective Current Transformers from KSI

KWP Series are a range of Protective Current Transformers. These CTs are used to activate the protective relay in case of a fault current and isolate a part of or the full system from the main supply.

Features:

• Designed as per IS-16227, IEC-61869, C-57 or Customer-specific requirement
• Approved and widely used by various State Power Supply companies
• Requires little or no maintenance
• Tested and certified at CPRI Bangalore, India (NABL)
• Secondary Current 5A or 1A
• Primary Current up to 5000A
• Secondary Burden 1VA to 30VA
• Dual Ratios can be offered
• High accuracy on request
• Mounting arrangement on request
• Construction Styles Offered :
  • Varnished Fibre Glass or PVC Tape Insulated
  • Resin cast
  • ABS or Glass Filled Nylon Moulded

Learn more: KWP Series – Wound Primary Protective Current Transformers from KSI

KRP SERIES – Ring Type Protective Current Transformers from KSI

KRP series of Ring type also called as window type, allow bus bars or cables to be passed through the CT and act as a Primary for the CT. These CTs are used to activate the protective relay in case of a fault current and isolate a part of or the full system from the main supply.

Features:

• Designed as per IS-16227, IEC-61869, C-57 or Customer-specific requirement
• Approved and widely used by various State Power Supply companies
• Requires little or no maintenance
• Tested and certified at CPRI Bangalore, India (NABL)
• Secondary Current 5A or 1A
• Primary Current up to 5000A
• Secondary Burden 1VA to 30VA
• Dual Ratios can be offered
• High accuracy on request
• Mounting arrangement on request
• Construction Styles Offered :
  • Varnished Fibre Glass or PVC Tape Insulated
  • Resin cast
  • ABS or Glass Filled Nylon Moulded

Learn more: KRP SERIES – Ring Type Protective Current Transformers from KSI

Read related blogs on

Summation Measuring Current Transformers for Power Systems

Selection of Current Transformers for Metering and Measurement Applications

Customized Current Transformers for Industry Applications

Author: Anuradha C

An integral part of the content creation team at KS Instruments, Anuradha is a corporate trainer in the IT/telecom domain with over 18 years of experience. She served in senior technical and management positions in Huawei and TCS for 10+ years