Page 638 - Industrial Power Engineering and Applications Handbook

Surge arresters: application and selection 18/603

Considerations Parameters
Transformer voltage ratio 132/11 kV
Connections %ID
Rating 60 MVA
Approx. surge impedance from graphs of Figure 17.7(b) 50 0
by extrapolation (obtain accurate value from the
manufacturer)
Surge travels from higher voltage side of the transformer
to the lower voltage side. Consider a surge protection
on the primary, with details as follows:
BIL of transformer from Table 13.2.
Power frequency LlWL
(Choosing a higher level, as the system being exposed withstand voltage
to the atmosphere). ~~~
,
HV side 275 kVr , 650 kV peak
LV side 28 kvrms 75 kV peak
~~~ ~~
Primary side vm 145 kV,,,
1 p.u. 145 x lr2 = 118 kV
lr3
Secondary side V, 12kVrrms

\' 2
1 p.u 12 x - = 9.8 kV
1' 3
Max. continuous operating voltage, MCOV (Vm/% 3);
HV side 145 = 83.7 kV
v3
LV side 12 = 6.9 kV
-\I 3
Characteristics of the arrester chosen from Table 18.9, 120 kV
class 111; Standard rating, V,
(for detailed working and exact design parameters for
selecting the arrester, refer to Example 18.3)
Max residual voltage (lightning) at 10 kA, V,,, (8/20 ps) 355 kV peak
Max. residual voltage (switching) at 1 kA, V,,, (30/60 ps) 294 kV peak
Max. residual voltage (FOW) at 10 kA, Vr,, (1/20 ps)
TOV capability for 10 s 386 kV peak
203 kV peak (140% of V, which is OK)
Reflection of surges:
Z, of jumpers through which will travel the surge to the
transformer 200 R
As the transformer HV side is already protected by an 2XV1X- 50
arrester, it is not necessary to consider the influence of 50 + 200
refraction of surges at point A, which is quite meagre in
this case, = 0.4 V,
The reflected wave will dampen the incidence surge by
50 - 200
Surge transferences through the HV side of the V, . ~ 50 + 200 = - 0.6 V,
transformer
(i) Capacitive transference (initial voltage spike)
C
Assuming = 0.4
c, + cs
V, = 2.5 P.U.
Since an arrester is provided at location A, it is appropriate
to substitute V, by V,,, (FOW) = 386 kV peak