Page 860 - Basic Electrical Engineering

It is seen from Fig. 11.31 that the there exists an angle of phase difference
of θ between I and nI . Idealy nI and I should not have any phase angle
1
2
2
1
between them.
We have seen that there exists a difference between the actual
transformation ratio and the turns ratio of a CT. This largely depends upon
the loss component of the no-load current, i.e., I . The transformer phase
c
angle depends upon the magnetizing component of the no-load current, i.e.,
I . By proper design, the magnitude of I and I are kept low to maintain the
m
m
c
ratio error and phase angle error to the minimum.

Figure 11.32 Principle of operation of an insulation tester illustrated

11.12.2 Potential Transformers

Potential transformers are similar to two winding transformers except that the
secondary volt-ampere loading is very low. The primary winding is the high

voltage winding which is connected across the lines whose voltage is to be
measured. The secondary winding is normally rated at 110 V and is

connected across a voltmeter or the pressure coil of a wattmeter. For details
on potential transformer the students may refer to any advance book on

measurement and instrumentation.

11.13 MEGGER AND MEASUREMENT OF INSULATION RESISTANCE
The insulation resistance of any cable or electrical system is very high. To

measure such high resistance we use an instrument called megger. The