Burden of a Current Transformer
A current transformer converts a high primary current with a secondary winding around a magnetizable core to a lower secondary current. A current transformer is passive, therefore no external power supply is connected, and what is secondary connected forms a load for the current transformer. In short, the connecting wires and the connected meter form the load of the current transformer. In technical terms this is called load in VA (volt amps). This load is of influence on the accuracy of the current transformer.
In the design of the current transformer, internal losses and the external load of the current transformer are taken into account. The current transformer acts as a current source. Therefore, the Midwest Current Transformer designers take the burden of the current transformer into account.
The IEC 61869-2 standard for current transformers specifies the following about the load of the current transformer:
- The load of a measuring transformer is between 25% and 100% of its nominal load. The idea behind this is that when the load of the current transformer is too low, it may no longer comply with the class and can return a value that is too positive.
Knowing your burden and calculating your VA is the key to finding the proper current transformer for your application.
The VA rating of the current transformer is determined through two straightforward equations.
Power = Current x Voltage e.g. VA = Is • V
Voltage = Current x Resistance e.g. V= Is • R
VA = Volt-Amp Rating (VA)
Is = Secondary Current (Amps)
R = Burden (ohms)
Combining these equations results in:
VA = (Is)2 • R
In some cases, required VA ratings are listed on the device the current transformer will connect with, ie. a meter, relay, etc. Always contact your Midwest Current Transformer representative for assistance finding the proper current transformer for your application.