Rockwell Automation 865 Differential Protection Relay - Manual (Page 147)

Applications 8-5
865-UM001A-EN-P – July 2009
Approximation of peak voltage during inside fault using a non-linear
model for a saturating CT (Equation 8.4):
kV 2 . 3 ) 100 12600 ( 100 2 2 = − ⋅ =
sp
V
This is a too high value and a VDR must be used to reduce the
voltage below 3 kV.
A zinc oxide varistor (i.e. VDR, METROSIL) of 1 kV will limit the
voltage. Using a 400 J model allows two 20 VA CTs feeding ten
times their nominal power during one second before the energy
capacity of the varistor is exceeded.
Iron core current transformers (CT) are accurate in amplitude and
phase when used near their nominal values. At very low and at very
high currents they are far from ideal. For over-current and differential
protection, the actual performance of CTs at high currents must be
checked to ensure correct function of the protection relay.
CT classification according IEC 60044-1, 1996
CT model
Figure 8.3 – A CT equivalent circuit
Note: L
m
is the saturable magnetisation inductance, L is secondary
of an ideal current transformer, R
CT
is resistance of the CT secondary
winding, R
W
is resistance of wiring and R
L
is the burden i.e. the
protection relay.
Composite error ε C
Composite error is the difference between the ideal secondary
current and the actual secondary current under steady-state
conditions. It includes amplitude and phase errors and also the
effects of any possible harmonics in the exciting current.
Current Transformer
Selection