Download HV Vacuum circuit-breakers: Challenges of capacitive load

NAME :
Richter, Frank
COUNTRY :
Germany
REGISTRATION NUMBER : CE113578
GROUP REF. : A3
PREF. SUBJECT :PS1
QUESTION N° : Q1-9
HV Vacuum circuit-breakers: Challenges of capacitive load switching
In general, capacitive current switching is a crucial requirement for circuit breaker applications.
Switching currents with capacitive attribute constitutes a major challenge for high-voltage vacuum circuit
breakers with single-break vacuum interrupters. This is especially the case concerning very low restrike
behavior, which is mandatory for class C2 classification of IEC-requirements [1]. The difficulty of fulfilling
requirements increases from switching unloaded lines and cables to the switching of single capacitor bank to
back-to-back capacitor bank applications (see table 1).
Load
Inrush current
Load current
Cap. voltage factor
Unloaded lines and cables
negligible
Small
up to 1.7 p.u.
Single capacitor bank
small
Small
up to 1.4 p.u.
Back-to-back capacitor bank
high
Small
up to 1.4 p.u.
Table 1 : Overview on different capacitive loads
The capacitive current switching of unloaded lines and cables is an application using requiring comparably low
currents up to a few hundred amps. The dielectric stress after current zero represents the main challenge for the
circuit breaker. This dielectric stress results from the nearly fully charged capacitance on the load side at the time
of current zero and the oscillating voltage of the supply side. The circuit breaker must withstand the differential
voltage at the terminals. This results in an oscillating direct voltage stress in (1-cos(ωt)) wave shape. The
dielectric stress is not a transient load, but starts with zero steepness and reaches the maximal value after 10 ms
for a 50 Hz and 8.3 ms for a 60 Hz configuration.
I
UCB
LN
~
UN
CN
UA
UL
CL
Neglecting :
1
ωL N <<
⇒ U A ≈ U N ; C N << C L
ωC N
Fig. 1.
Switching of capacitive loads [2]
Single capacitor bank and back-to-back applications present the same dielectric loads with slightly higher
breaking currents. For back-to-back applications, the making operations may cause different issues because the
circuit breaker contacts close under significant influence of high-frequency inrush currents. This could result in
welds between the contact surfaces. These welds break in the ensuing opening operation and can influence the
capacitive switching behavior [3]. The welding behavior is a contact material property.
The cause for voltage restrikes in vacuum can be manifold and initiated by various circumstances. One reason
could be the exceedance of the maximum permissible field strength between the arcing contacts of the vacuum
interrupter. This is not only a phenomenon in the phase of contact separation, but also possible with an
insufficient final contact stroke (dielectric design of vacuum interrupters).
Results indicate delayed voltage breakdowns in vacuum interrupters despite sufficient opening velocity and
contact distance. This phenomenon is termed late breakdown and can be observed even after several ten
milliseconds periods. The explanation for this behavior could vary.
Zadeh [4] measured field emission currents in vacuum interrupters, which occurred during the period, when the
(1-cos(ωt)) voltage was applied. Her research revealed high field emission currents shortly just prior to the
restrike event. This would seem to imply that field emission currents initiate the breakdown (dielectric design of
vacuum interrupters).
There is another theory which states that particles, activated by mechanical shocks, are responsible for initiating
the restrike phenomenon. Gebel and Hartmann [5] proved a close connection between mechanical shocks in the
kinematic chain and late breakdowns. Even if these investigations were carried out with high current arc
interruption, it is conceivable that this might also appear during capacitive current switching.
Giere et al. [2] investigated the influence of different closing velocities and the capacitive voltage factors on the
restrike performance. The summarized result is shown in Figure 2 (kinematic chain).
25
20
15
Relative restrike
probability [%]
10
5
2
1,7
0
0
100
Closing velocity [%]
1,4
140
Capacitive voltage
factor [p.u.]
200
Figure 2: Results of investigation of the influence of closing velocity and cap. voltage factor on the restrike performance [2]
References:
[1]
[2]
[3]
[4]
[5]
IEC 62271-100, 04-2008, "High Voltage switchgear and controlgear – Part 100: Alternating-current circuit-breakers," Edition 2.0.
Giere et. al: „Capacitive Current Switching Capability of 72.5 kV High-Voltage Vacuum Interrupters”, ISDEIV 2012, Tomsk
T. Delachaux, F. Rager, D. Gentsch, "Study of vacuum circuit breaker performance and weld formation for different drive closing
speeds for switching capacitive current," XXIVth International Symposium on Discharges and Electrical Insulation in Vacuum,
September 2010.
M. Koochack Zadeh, V. Hinrichsen, R. Smeets, A. Lawall, "The Impact of Capacitor Bank Inrush Current on Field Emission
Current in Vacuum," XXIVth International Symposium on Discharges and Electrical Insulation in Vacuum, September 2010.
R. Gebel, W. Hartmann, "Mechanical shocks as cause of late discharges in vacuum circuit breakers," IEEE Transactions on
Insulation, Vol. 28, No. 4, 1993, pp.468.