CURRENT TRANSFORMER PERFORMANCE USED IN PROTECTIVE RELAYING

Current transformer steady-state performance can be calculated from ANSI accuracy classification for C type or excitation data supplied by the manufacturer for C and T types. The secondary voltage as given by the maximum fault current reflected on the secondary side multiplied by the connected burden should not exceed the assigned C accuracy class.

Conversely, the permissible current transformer burden for a given accuracy class can be calculated. Performance using excitation data calculates the excitation current at the secondary voltage, which should be a small percentage of the operating current.

For T-type transformers, the manufacturer should supply overcurrent ratio curves from 1 to 22 times normal current and for all standard burdens up to the one that causes a ratio error of 50%. The transient performance should consider the dc component of the fault current because it has far more effect in producing severe saturation of the current transformer than the ac component.

Before a current transformer reaches its saturation flux, it may still accurately reproduce the offset fault current for a short duration. The time to saturate is a function of magnitude of the short-circuit current and its offset, secondary burden, system time constant, and current transformer time constant and its characteristics.

The current transformers may saturate within the first half cycle at large fault currents. The remanence in the core is the result of a previous current interruption, at other than current zero. This can force the current transformer into saturation sooner than expected.

Time to desaturate is dependent upon the same parameters as the time to saturate and is on the order of five cycles, largely because of the power system time constant. If a current transformer is chosen based upon steady-state performance, current transformer saturation due to dc offset will not cause problems on the operation of time overcurrent relays, as long as the dc component decays to zero in the time the relay is expected to operate.

For high-speed relays, such as instantaneous relays, differential and distance relays, and other relay types operating in less than two cycles, the current transformers should be selected so as not to saturate in less than 2 cycles. The rms value of the distorted output current of a current transformer can be calculated.

Relays that respond to only the fundamental may be more seriously affected than what the rms value indicates. Selecting as high a ratio as an application will permit and series and parallel connections of current transformers are some of the means to reduce saturation.

Current transformers with an air gap have a fairly high exciting current and low residual flux and can be used in some differential schemes; however, their use is limited in the modern protective schemes.

Current transformer saturation, waveform of secondary current output for various degrees of saturation