NAME : René SMEETS
COUNTRY : the Netherlands
REGISTRATION NUMBER : 14127
GROUP REF. : A3
PREF. SUBJECT : 1
QUESTION N° : 1-5
The generation of very fast transient overvoltages is a severe concern in switching of
unloaded GIS sections with disconnectors. In the figure, a measured VFTO (internally in the
GIS is shown)
As the authors of paper A3.110 rightfully point out, the role of the trapped charge, remaining
on the disconnected busbar section, is of eminent importance. After all, the next closing
operating can lead to a 2 pu jump in voltage when load is left after at 1 pu and supply is at -1
pu. This leads to the most onerous very fast transient overvoltages.
In three-phase metal enclosed switchgear, such as very common at 145 kV level (see figure),
is was found from tests that the level of trapped voltage can exceed 1 pu. This is due to the
capacitive coupling between the phases. This interaction allows an increase of trapped voltage
even after the switching process in this phase has completed. The ongoing switching
transients in the neighbouring phase can charge the already stabilized phase beyond 1 pu.
This is illustrated in the measured case shown here. The dashed voltages are measured at
supply side, the drawn ones are at load side. A special three-phase measurement system is
used. The result shows the disconnecting operation of a three-phase disconnector in a 145 kV
three-phase enclosed GIS, submitted to standard IEC tests. As usual, the disconnecting
operation is associated with a number of breakdown and recovery events, to be identified by
the vertical steps, each of which excite the very fast transient overvoltages.
As can be seen, due to the charge reversals in phase 1 and 3, the already stabilized phase 2
receives part of these charges though the phase-to-phase stray capacitances.
As a result, the charge, finally trapped at phase 2 leads to a 1.2 pu DC voltage. A subsequent
closing of the disconnector may show very fast transient overvoltages beyond the expected
magnitude.
In the design phase of thee-phase enclosed GIS, the phase-to-phase stray capacitance and
more specifically the ratio of phase-to-phase and phase-to-ground capacitance should be kept
as small as possible to avoid surprises.
200
measured voltage-to-ground (kV)
3
150
100
2
50
0
-50
1
-100
time (us)
97300512.09
-150
4
5
6
7
8
9
10
4
72
36
0
R
280
328
376
424
472
520
328
376
424
472
520
328
376
424
472
520
72
36
0
S
280
72
36
0
T
280
time (ms)