AUXILIARY CURRENT TRANSFORMERS USED FOR CIRCUIT PROTECTION

Auxiliary cts are used for the following reasons:

a) Circuit isolation to permit independent grounding
b) Change in ratio to match current requirements
c) To produce a phase shift in a three phase circuit
d) To reverse polarity
e) To limit main ct fault burden by saturating during faults
f) To reduce the burden on the main ct by reducing the apparent impedance of a portion of the burden by the square of the auxiliary ct ratio
g) Zero sequence shunt or trap

The auxiliary ct should be selected with an adequate continuous current rating and voltage capability for the requirements of its connected burden. The addition of an auxiliary ct adds burden to the main ct, but the net effect on the main ct may be either a decrease or an increase in burden, depending on whether the current is stepped down or up.

The apparent impedance to the main ct of the portion of the burden in the secondary of the auxiliary ct is that portion multiplied by the square of the auxiliary ct ratio. For example, a 1.0 W burden in the secondary of a 2:1 auxiliary ct would appear as 0.25 W to the main ct, but would appear as 4.0 W for a 1:2 auxiliary ct.

For this reason, current ste pup applications should be avoided when practical.


To ensure good performance under fault conditions, the knee-point voltage of the auxiliary ct should be considered in relation to its connected burden, without regard to the knee-point voltage of the main ct.

However, an auxiliary ct with an unnecessarily high knee-point voltage may have an undesirably high internal burden, which is seen directly by the main ct.

If circuit isolation is not required, it is advantageous to use auxiliary cts in the autotransformer connection for maximum capability or minimum burden. The use of the autotransformer connection usually results in better transient response. Ratios that are not available with a two winding arrangement can be obtained using the autotransformer arrangement.

Figure below shows how additional ratios can be obtained with 5:5 A, 10:5 A, and 15:5 A two-winding auxiliary cts connected as autotransformers. Only the step down ratios are shown. Stepping up current with auxiliary cts is not usually good practice as the connected burden will be increased as the square of the turns ratio of the auxiliary ct.

Two-winding auxiliary cts connected as autotransformers