Current Transformer Tests:
Current Transformer Tests are generally mounted in the switchgear when it is delivered to site and may not be easily accessible. The normal tests which are carried out can easily be performed with the CTs mounted in the switchgear. These tests are:
Ratio Check and
This check is necessary to see the relative polarity of the primary and secondary terminals when terminals are not marked or to establish the correctness of the marking if already marked. A simple test circuit for this purpose is shown in Fig. (8.7).
The primary is connected across a low voltage battery through a push button switch. The secondary of the CT is short circuited through a moving coil permanent magnet centre zero type ammeter. On closing the push-button the d.c. ammeter A should indicate a positive flick and on opening a negative flick.
This type of Current Transformer Tests is usually carried out during the primary injection tests described in 8.5.1. The ratio of the readings of the ammeters in the primary and secondary circuit of the CT under test is taken which should approximate to the ratio marked on the CT.
CTs intended for different purposes will have different magnetization characteristics. On a single bushing there may be several CTs of the same ratio but having different characteristics to perform different duties such as protection, metering or operation of ammeters only.
A magnetization test is a convenient method for the identification of the different CTs which will have differing knee points. The knee point as already defined is the secondary voltage at which an increase of 10% in voltage will result in an increase of 50% in the magnetizing current.
A simple method of checking the magnetization characteristics of a CT is to apply a gradually increasing sinusoidal voltage of standard frequency to the secondary winding leaving the primary open circuited.
The Magnetization current and the voltage applied are measured with the help of an ammeter and voltmeter. The shape of this curve indicates as to when the saturation occurs in the CT core.