NAME : René SMEETS
COUNTRY : the Netherlands
REGISTRATION NUMBER : 14127
GROUP REF. : A3
PREF. SUBJECT : 2
QUESTION N° : 2-19
Back-to-back capacitor bank switching is the energization of a capacitor bank while adjacent
banks in the station are already energized, see the figure. Back-to-back capacitor bank
switching is usually associated with a large inrush current that flows during the pre-arcing
phase in the closing operation of a circuit breaker. The very high transient current may
damage contacts and/or other parts such as the nozzle of SF6 breakers.
In the figure, a complete back-to-back switching test sequence is shown. For vacuum circuit
breakers, in which the switching and the insulation function is shared by the same set of
contacts, damage to contacts may imply a reduced dielectric withstand in the open position.
This seems to be confirmed by test statistics, as outlined in paper A3-201. In this paper results
are presented of 433 capacitor bank switching test series in the voltage range 12 – 550 kV
during the past 11 years. It has been found that with increasing rated voltage, the probability
of late breakdown after capacitive current interruption increases rapidly. In more than 80% of
the test series, very short lasting late breakdown events (NSDD) were observed in the voltage
class 30 – 40.5 kV after back-to-back capacitor bank switching.
For the application of vacuum switchgear at high-voltage, as studied in CIGRE WG A3.27,
this might imply that mitigation of inrush current is essential, once high-voltage vacuum
switchgear is applied for back-to-back capacitor switching. By the way, also for high-voltage
SF6 breakers passing the back-to-back capacitive test is a major challenge.
The standardized IEC value of 20 kApeak is often been criticized as being too high, and
especially, in high-voltage applications, inrush current limiting reactors are routinely applied
in capacitor banks, as the work of CIGRE A3.26 shows.
For single capacitor switching, inrush current is rather limited and vacuum switchgear, up to
84 kV is successfully certified and applied, exploiting the advantage of a long electrical
lifetime.
Physically, damage to the vacuum breaker's contact surface can be measured by looking at the
field emission current that is directly drawn out of the metal by applying very high voltage
across the vacuum gap. The field intensification due to the metallic microstructure at the
damaged site leads to several tens to hundreds of micro-amperes that we can measure now
during standard IEC tests.
The observed field emission current at limited values of inrush current was in all cases very
low, as can be seen in the graph. Nevertheless, low field emission activity does not directly
imply low risk of breakdown.
pre-strike
contact touch
current interruption
1
40
re-strike
40
load current
20
0.5
20
0
0
0
-0.5
-20
-1
100
-40
100
50
50
50
0
0
-20
-40
100
inrush
current
re-strike
current
0
recovery voltage
-50
-50
198
200
202
-100
100
150
200
250
300
350
400
450
500
550
600
-100
650
572
574
100
percentage of test-series
with NSDD(s)
-100
196
-50
single bank
back‐to‐back
80
60
40
20
0
12‐15.5 kV
17.5‐29 kV
30‐40.5 kV
rated voltage
20
inrush current
3.3 kApk
10
800
600
0
400
-10
200
-20
FE current 90 uA
0
0
20
2
4
inrush current
21 kApk
10
6
0
200
300
400
500
800
600
0
400
-10
200
-20
100
FE current 760 uA
0
0
2
4
6
0
100
200
300
400
500
576
578