Page 658 - Industrial Power Engineering and Applications Handbook

Surge arresters: application and selection 181623

Segment A Segment B
Basic parameters Z,, = 300 R Zsl = 300
Zsz = 300 R Z,, = 40 R
(a) Reflection of surges: Low Low
As the arresters are being provided at the
primary of each transformer, this aspect need
not be considered
(b) Surge transferences to the LV side
(i) Capacitive transference
(initial voltage spike)

CP
Assuming 0.4
cp + cs
The actual value may be much less than this when
obtained from the transformer manufacturer
p for a YlA transformer 1.15
Vt(F0W) 945 kV peak
:. v,, = 0.4 x 945 x 1.15
= 434.7 kV peak
BIL (lightning) of the transformers’ LV sides from for 15.75 kV LV for 132 kV LV
Table 13.2 (V, = 17.5 kV) (V, = 145 kV)
= 95 kV peak = 650 kV peak
Minimum protective level required 9511.05 = 90.5 kV peak 650/1.05 = 619 kV peak
which is too low compared to VI, and requires an arrester on the LV side Protective margin available
or provision of surge capacitors across the secondary windings, such that
-
cp <- 90.5 _-2 650 1.49
C‘
c, + c, i 434.7 474.7
whichis adequate and no more
protection is necessary
The value of C’ can be calculated if values of C, and C, are known.
Note Even then a surge protection will be essential for the tertiary,
if a tertiary is provided.
Note The above analysis also corroborates that surge transferences are more severe in high ratio transformers than low ratio ones (Section
18.5.2)
(ii) Inductive transference Vti = p.9.r. VJn
Assuming p = 1 for a long-duration switching surge for inductive transference
q = 1.8 for a long-duration switching surge
Jr
r = - from Figure 18.14 for a YIA transformer with
2
surges of opposite polarity appearing on two phases
420
420
n= - -
17.5 145
= 24 = 2.9
VI = V,, (switching) 676 kV peak
& 676 =lX1.8X-X- & 676
x
:. vti = 1 x 1.8 x - -
2 24 2 2.9
= 43.9 kV peak = 363 kV peak
Power frequency withstand capacity of the LV 38 kV r.m.s. 275 kV r.m.s
windings from Table 13.2
38 fi --
275 -,h
-
:. Protective margin -- -
43.9 363
= 1.22 = 1.07
This is adequate and no additional This is too low. But it is possible to make it up
surge protection is necessary by selecting the arrester at the primary with a
lower switching V,,,, if possible, or provide an
arrester at the secondary. 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 design engineer can use a more
realistic factor based on his past experience and
the data available from similar installations.