CT Selection for Medium Voltage Switchgear

CT Selection for Medium Voltage Switchgear

Current Transformers (CTs) are critical components in Medium Voltage, Metal Clad Switchgear.  Selecting CTs for Switchgear relay and metering applications requires several important considerations. You need to select the correct ratio, burden, and accuracy class as well as ensuring the CT can withstand fault currents.

CT Ratio

CTs have a rated secondary output at 110-125% of expected load and no more than 100A secondary current at maximum primary fault current. CTs can be supplied with either a single ratio or a multi-ratio design. If you are unsure of your primary load profile, multi-ratio designs are a good option.

CT Burden

CT burden is the maximum secondary load permitted, expressed in volt-amperes (VA) or ohms impedance, to ensure the CT can meet its accuracy rating.  ANSI/IEEE standards list burdens of 2.5 – 45VA at 90% power factor (PF) for Metering CTs and 25 – 200VA at 50% PF for Relaying CTs.

CTs burden has become less of an issue with the emergence of microprocessor relays. Microprocessor and solid-state relays impose much lower burdens than the earlier electromechanical relays.

CT Accuracy Class

There are two sources of error in CTs, namely ratio and phase angle error. These occur due to heating losses and magnetizing core losses.  These affect the accuracy of the CT.  The ANSI/IEEE accuracy class standards are +/- 0.3, 0.6, or 1.2%. Higher accuracies are used for metering applications and lower accuracies are using in relaying.  Typically, 0.3% accuracy is used for watthour metering where 0.6% to 1.2% accuracy are used for relaying and indicating instruments.

CT Polarity

CTs are marked with a dot or other polarity identification so that the primary and secondary terminals have the same polarity. Polarity is not required for overcurrent sensing but is important for differential relaying and other relaying functions.

CT Cross-Reference Lists

Suppliers like Peak Demand provide cross-reference lists here and here to allow you to select the correct CT to replace popular models from other CT suppliers.  CTs identified in these cross-reference lists will match the mechanical and electrical performance of the CTs from other vendors.

You can find Peak Demand’s Switchgear CT page here.

Article written by Jon Rennie  

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