Page 639 - Industrial Power Engineering and Applications Handbook

18/604 Industrial Power Engineering and Applications Handbook

Considerations Parameters
-
p = 1 .I5 for a lightning surge in a KID transformer, :. V,, = 0.4 x 386 x 1.15
= 177.6 kV peak
which is too high compared to LV side LlWL of the
transformer of 75 kVPeak (protective level not to be more CP 62.5
than 75/1.2 = 62.5 kV peak, Table 18.2), and calls for <
either an arrester on the LV side too or provision of a c, + c, + c 177.6
few surge capacitors across the secondary windings
such that,
The value of C' can be calculated if values of C, and C,
are known, which can be obtained from the transformer
manufacturer.
Note Even then a surge protection is essential for the
tertiary if the tertiary is provided.
v,
(ii) Inductive transference V,, = p.9.r. -
n
Assuming, p= 1 for a switching surge (in inductive
transference we have to consider long-
duration surges only)
9= 1.8 for a switching surge
7
43
r= - from Figure 18.14 for a 'f&
fi
transformer with surges of opposite
polarity appearing on two phases.
n = 145/12 = 12.1 :_ & 294
v,, = 1 x 1.8 x - -
x
V, = V,,, (switching) 2 12.1
= 294 kV peak = 37.9 kVpeak
and the power frequency withstand capacity of the LV = 28 kV, s.
windings or 28 fi
kV,,,,
:. Protective margin - 28fi - 1.04
37.9
which is too low. It is, however, possible to make it up by
selecting the arrester on the primary side with a lower
switching V,,,. Consult the arrester manufacturer for it
or provide an arrester on the secondary side also.
Moreover, the response factor, q is considered very high,
which may not be true in actual service and an arrester
at the secondary may not be necessary in all probability.
The BIL of the interconnecting cables and the terminal
equipment on the secondary must be at least equal to
the capacitive and inductive transferences of the primary
surges as determined above. If it is not so, the V,,, of
the primary arrester must be re-chosen or an arrester
also provided on the secondary side.

18.5.3 Effect of resonance frequencies that may cause such a phenomenon to help
to take corrective steps or modify the parameters C and
This applies to systems up to 245 kV, where inductive L of the transformer at the design stage.
impedance is significant. An arrester basically is for equipment protection and
It is possible that at certain frequencies the capacitive must be installed at all main equipment heads that are
and inductive couplings of the transformer may resonate exposed to internal or external surges and whenever the
(X, = X,) (Section 24.4.1 and 2) during the course of a amplitude of such surges, V,, is expected to exceed the
long-duration surge and give rise to yet higher voltage BIL of the equipment.
transferences. For critical installations it is advisable to Figure 18.15 shows a power network with generation,
identify (e.g. by TNA) the likely surges and their transmission and distribution of power, illustrating the