Page 522 - Industrial Power Engineering and Applications Handbook

15/496 Industrial Power Engineering and Applications Handbook
voltage developed in the secondary circuit in the for motors and discussed in Section 9.6.1 is advisable as a process
event of an accidental secondary open circuit on test to monitor the quality of an HV insulation system, during the
load. The inter-turn insulation of the windings course of manufacture and using the Fame value for future reference
should be capable of withstanding an inter-turn when checking the quality of insulation when energizing or during
overvoltage of 4.5 kV peak across the complete a field test.
secondary winding. The test may be conducted
by keeping the secondary winding open circuited
and applying a primary current less than or equal List of formulae used
to the rated primary current for 1 minute, sufficient
to produce a voltage at the secondary terminals Differential ground fault protection
equal to 4.5 kV (peak).
(VI Power frequency withstand test on primary Current setting of the relay,
windings
This test is same as for item (iv) (a), under type Zpf = n (N x Z, + Is,) (15.1)
tests. Ipf = minimum fault current through the primary rcquired
to trip the relay
Note n = turn ratio of the CTs
A repeat power frequency test, if considered necessary, must be Z,,, = magnetizing current of the CTs corresponding to
performed at 80% of the prescribed test voltage. See also Section the V,
14.5. Z,, = relay current setting
N = no. of CTs falling in parallel
(vi) Partial discharge measurement
(vii) Verification of accuracy To limit the peak voltage
This test is the same as under type test item (vi)
3 Special tests v, = 2Jz ,'V,(V, - v,, (15.2)
The following additional tests may be conducted when
considered necessary: Vp = peak voltage across the relay
(i) Chopped lightning impulse test. Refer to IEC V, = theoretical maximum CT secondary voltage across
60044- 1 the relay circuit at the maximum internal fault
(ii) Measurement of the dielectric dissipation factor current
(tan a, applicable to only liquid immersed primary V, = knee point voltage
windings, rated for 110 kV and above.
Further reading
Note
For lower voltage systems, say, 2.5 to 10 kV, measurement of Pmtecrive Relays and Application Guide, GEC Measurement, General
dielectric loss factor tan 6, along similar lines, to those recommended Electric Co. Ltd, Stafford, Uk.