Why place more efforts on magnetizing characteristics for CT than VT?

Technical Notes of Instrument Transformers
The Role that Magnetizing Impedance Plays

Instrument Transformer is special transformer, due to its operation condition, the equivalent circuit differes with respect to tranfer parameters.

Difference in equivalent circuit between CT and VT
1.VT. due to its operation limit, the secondary voltage runs rarely out of the limit value (80%~120), even in fault conditions, protection equipment will prevent them long keep in the state of out-limit. so it is reasonable to assume that the magnetizing impedance during operation is constant with respect to protection, and linear with respect to metering.Also, as VT is connected in paralle with power lines, only voltage is considered. the magnetizing impedance has very little influence on the burden voltage.

2.CT, secondary current is considered, its operation range is (1%~120%,to say,S class), magnetizing impedance varies significantly thus bypass secondary current from the burden, and affect the state value from primary being transfered to secondary. especially in fault conditions, such as primary line short-circuit, magnetizing impedance is a very important parameters to be considered.

In consideration of metering accuracy:

Both CT and VT characterize their error graph by nonlinearity of magnetizing impedance (MI). In principle, MI bypasses burden current, which influence the voltage drop on secondary lines and thus affecting the signal supplied to the burden.

MI is an interesting parameter that still attracts big conern, both for metering and protections.

J.ZH
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