Analyses of vacuum circuit breaker switching
transients in medium voltage networks with respect to
LC filters of solar converters
T. Kuczek, M. Florkowski, W. Piasecki ABB Corporate Research Center
13A Starowislna St., Krakow, Poland
[email protected], [email protected], [email protected]
Abstract: This paper presents possible transient states related
to vacuum circuit breaker operation in PV power plant. Case study of transformer de-energization under no-load conditions was analyzed. The variation with respect to up-to-date studies is related to determination of solar converters LC filters influence on multiple arc re-ignitions occurrence during vacuum circuit breaker operation. Research covers both laboratory measurement as well as PSCAD simulations.
key words: vacuum circuit breaker, transients, switching, LC filters, solar converters, simulations verification
I. INTRODUCTION
The motivation for this study was driven by significant
development and expansion of photovoltaic (PV) power plants
in modern electrical power systems. Photovoltaic market has
grown by a factor of hundreds of percent [1] during last 13
years. It covers both rise of small household installations as
well as large photovoltaic plants with the peak power
capability of 500 kWp and more [2]. Photovoltaic cells
generate power that is dependent on solar irradiation and
ambient temperature [3, 4]. Voltage and current at DC are
converted by means of power electronic inverter to the AC side
and then transferred to the medium voltage level by LV/MV
transformer [6]. Such circuit may be subjected to de-
energization operation that is usually performed by vacuum
circuit breakers (VCB). At certain network conditions such
operation may result in high peak of the transient recovery
voltage, which can lead to significant overvoltages generation
at the transformer terminals. Such effects were analyzed for
example in [7] and [8], but for other applications like no-load
transformer or arc furnace transformer, not for PV related. This
paper is focused on determination of influence of solar
converter’s LC filters on transformer de-energization during
no-load conditions. Research covered both laboratory
experiments as well as PSCAD numerical simulations.
II. PV PLANT LAYOUT OF CONCERN
The layout of concern in this study is presented in Figure 1.
PV panels generate DC voltage and current, which are
dependent on solar irradiance and ambient temperature.
Manufacturers of PV panels provide nonlinear characteristics
of generation with respect to above mentioned factors [3], [4].
The DC power has to be converted into AC by means of
DC/DC and DC/AC power electronic converters. For DC/DC
boost converters are most commonly utilized since DC voltage
has to be appropriately adjusted in order to transfer the power
by the inverter from PV panels into the external grid. The LC
filters are required in order to limit the ripple and the THD in
voltage and current [5]. They are designed according to desired
output current, voltage, switching frequency and allowable
limits of peak-to-peak ripple and THD. For example, 5% value
can be found in the literature [6].
PV
LV
VCB
external
grid
DC
DC
DC
AC
MV
MV cableLC
Figure 1. Equivalent circuit of grid connected PV plant
As mentioned earlier, LV/MV transformer may be subjected
to switching off operations by means of vacuum circuit breaker.
This phenomena is well described in literature [7, 8, 9].
However, this paper focuses on influence of converter’s LC
filters on switching conditions during vacuum circuit breaker
operation. This is due to the fact that those filters may change
the total impedance as seen from operated vacuum circuit
breaker’s terminals, thus influencing the natural frequency of
the circuit being switched.
Based on above clarification, several laboratory tests of
switching off operations were conducted in network prepared
on AGH University of Science and Technology in Kraków,
Poland. Measurement results allowed to develop PSCAD
model that was utilized for experimental results verification as
well as further analyses.
III. VACUUM CIRCUIT BREAKER OPERATION
Vacuum circuit breakers use vacuum as a quenching
medium for electrical arc suppression, which appears across
breaker’s contacts during any switching operations. Thanks to
this method, the dielectric withstand between circuit breaker
contacts is approximately 10 times larger than in air at
atmospherical pressure. As a result, it is possible to decrease
the gap between contacts inside the vacuum chamber of the
circuit breaker. Several conditions have to be fulfilled in order
to successfully interrupt the current, which happens when the
instantaneous current value drops below a threshold level
referred to as chopping current. Due to this feature VCB
switching may result in significant overvoltage hazards for
switched devices, such as electric machines, transformers or
shunt reactors. Several factors have influence on overvoltages
generation. The simplified circuit that represents external
network, vacuum circuit breaker and switched off transformer
is illustrated in Figure 2. It can be easily adopted to switching
off operations of transformers, shunt reactors or motors.
Figure 2. Transformer switching off – simplified single line diagram;
U – network source voltage, LZ, CZ – network inductance and capacitance,
LP1, LP2 – inductance of connections at both sides of vacuum circuit breaker W,
C0, R0, L0 – equivalent capacitance, inductance and resistance of transformer
Opening operation results in Transient Recovery Voltage
(TRV) that appears between the VCB’s contacts. The natural
frequency fn of its oscillations is determined by the inductance
and capacitance of L0-C0 circuit according to formula:
002
1
CLfn
(1)
During the contacts movement, TRV arises parallel to the
dielectric withstand UR of the vacuum gap. Most common
approach is to represent it as linear function in time domain:
UR = A(t – topen)+B (2)
where:
A – Rate of Rise of Dielectric Strength (RRDS),
B – initial dielectric withstand,
topen – opening time instant.
The RRDS depends on velocity of contacts movement as
well as the condition of vacuum inside breaking chamber.
Typically it is reported that RRDS value accounts between
2 kV/ms to 50 kV/ms [10]. Understanding of two above
mentioned curves (TRV and dielectric withstand) is essential
for description of entire process of current breaking. When
contacts start to open an electric arc is initiated. Once it is close
to its natural zero crossing, it is suddenly chopped – in modern
VCBs at approximately 3-5 Amps [11]. It results in TRV rising
– every time, when the TRV exceeds the dielectric withstand,
an arc is re-ignited and then chopped again. The process is
repetitive and it lasts until the dielectric withstand exceeds the
TRV. This effect may be hazardous to transformer’s insulation,
since at certain condition it may lead to significant overvoltage
escalation with high peak values as well as steepness, which
may exceed maximum permissible values.
IV. LABORATORY MEASUREMENTS
A. Test Stand Description
The laboratory tests covered measurement of overvoltages
arising during de-energization of 20 kVA distribution
transformer (Fig. 3). It was supplied from low voltage supply
network through step-down autotransformer connected in
series with 250 kVA distribution transformer. The
autotransformer was operating at 0.1 kV in order to set the
voltage at the 20 kVA transformer to 6 kV. Utilized vacuum
circuit breaker was rated at 12 kV and 1250 A of current. It
was motor spring operated. Electrical parameters of all
components utilized in the system are listed in Table I.
0.1 kVVCB
supply
network
6 kV
85 m MV
cable
voltage
6 kV 0.4 kV
1 m wire
connection
L
C
20 kVA250 kVA
Figure 3. Laboratory measurement test stand equivalent circuit
TABLE I COMPONENTS OF LABORATORY TEST STAND
Component Parameters Value
250 kVA
transformer
UP/US 15.75 kV / 0.4 kV
I0 0.25 %
uk% 4.5 %
20 kVA
transformer
UP/US 6 kV / 0.4 kV
I0 4.23 %
uk% 4.3 %
winding
capacitances
Cp_g = 3 nF
Cp_ph_ph = 0.2 nF
Cp_s = 1 nF
MV cable length 85 m
impedance Z 50 Ω
Cp_g – capacitance between primary winding and ground
Cp_ph_ph – capacitance between phases of primary winding
Cp_s – capacitance between primary and secondary winding
B. Measurement Results Three separate configurations of switching off operations
were performed, namely:
configuration 1: LC filters not connected, configuration 2: L = 200 µH, C = 1 µF (wye ungrounded),
configuration 3: L = 200 µH, C = 25 µF (wye ungrounded).
Voltage was measured for each scenario at the 20 kVA
distribution transformer’s primary terminals. Several switching
off operations were performed in each scenario in order to
eliminate statistical disorders. Representative waveforms are
illustrated in Figure 4 to Figure 6.
At first Figure 4 was analysed, since this gives direct input to
PSCAD simulations (section V). First configuration presents a
typical scenario, when transformer is subjected to switching off
operation under no-load conditions. It results in generation of
multiple arc re-ignitions. Based on measured waveforms, the Rate of Rise of Dielectric Strength can be easily determined.
As marked in Fig. 4, during the tests it was equal to 4.5 kV/ms.
When analysing further figures, one may see several effects of
LC filters presence.
0 5 10 15 20-10
-5
0
5
10
t [ms]
Up
[kV
]
Figure 4. Laboratory test, configuration 1, LC filters not connected
0 5 10 15 20-10
-5
0
5
10
t [ms]
Up
[kV
]
Figure 5. Laboratory test, configuration 2, L = 200 µH, C = 1 µF
0 10 20 30 40 50-10
-5
0
5
10
t [ms]
Up
[kV
]
Figure 6. Laboratory test, configuration 3, L = 200 µH, C = 25 µF
It is visible that LC filter decreases the overall natural
frequency of the disconnected circuit (including transformer
and MV cables). For configuration with 1 µF capacitance the
overall effect is less visible than for 25 µF. It comes out of the
fact that the higher the capacitance of the filter, the lower is the frequency of oscillations, according to formula (1). In each
case, after successful current breaking, voltage oscillations are
damped within dozens of miliseconds, depending on the
capacitance of the filter. Damping comes out of resistive losses
of transformers and cables. For configuration 3 multiple arc re-
ignitions are almost totally eliminated – some are visible, but
those are negligible. Summarized measurement results were
presented in Table II. It compares maximum overvoltage peak
values, du/dt of a single spark and number of multiple arc re-
ignitions.
TABLE II LABORATORY TESTS SUMMARY
Configuration
Up du/dt fn no of arc
re-ignitions
[kVp] [kV/µs] [Hz] [maximum
per phase]
1 without LC filters 8.8 27.1 108 9
2 L = 200 µH, C = 1 µF 7.8 15.9 76 5
3 L = 200 µH, C = 25 µF 5.9 4.8 40 1
V. PSCAD SIMULATIONS
A. Model Description The model was based on the network diagram presented in
Figure 3. Parameters of the circuit were based on nominal
ratings as well as achieved measurement results, as mentioned
earlier (Fig. 4). PSCAD diagram is presented in Figure 7.
R=0
3 [n
F]
cable_85...#1#2 #1 #2
0.2
[nF
]
0.2
[nF
]
0.2
[nF
]
1 [nF]
200 [uH]
25
[uF
]
25
[uF
]
25
[uF
]
0.1
[oh
m]
VCB
0.1
[oh
m]
0.2 [nF]
0.2 [nF]
0.2 [nF]
A B C
LV_5_m_ca... 3 [n
F]
1.8 [nF]
4.8
[nF
]
LV_10_m_c...#1#2
Figure 7. PSCAD diagram
Distribution transformer models were represented by means
of “3-phase 2-winding transformer” component from PSCAD
masters library. The no-load current was measured during
laboratory experiments and it was equal to 4.23% (81 mARMS).
It determines the equivalent inductance of the transformer at
no-load state. Phase-to-phase, winding-to-ground and winding-to-winding capacitances were added, too. Magnetization of the
core was also included with knee point at 1.2 p.u. More
attention was paid to the 85 m MV cable model. During the
numerous sets of preliminary simulations it was determined
that the best convergence in terms of overvoltage waveforms is
achieved when cable was modelled as surge impedance. Wave
propagation speed as well as high frequency resistance for
damping were also established in the same way. The goal was
to achieve the same overvoltage peak values, TRV’s steepness
and frequency of oscillations after successful current breaking.
The following final data was used for cable modelling:
surge impedance Zc = 50 Ω,
wave propagation speed v = 200 m/μs,
per unit resistance R0 = 0.05 mΩ/m.
Finally, the vacuum circuit breaker model was prepared
according to principles described in section III. The controlling
algorithm of vacuum circuit breaker was based on PSCAD
CSMF: Continuous System Model Functions. The “A”
parameter from formula (2) was set to 4.5 kV/ms (measured).
Entire process of model verification is explained in Figure 8.
experimental results:
Up_L , fn_L , duL/dt
switching off
no
switching off
yes
no
yes
model verified VCB
switching off
fn_L = fn_S
Up_L = Up_S
fitting of:
cables: v, R0
transformers: capacitances, saturation
VCB: opening time instant
fitting of:
VCB: RRDS
Figure 8. PSCAD model fitting process
1/fn
RRDS = 4.5 kV/ms
B. Simulation Results Based on clarification presented above, numerous sets of
simulations were conducted. All system aspects were taken
into consideration. Source voltage phase shift with respect to
time zero was also adjusted in order to achieve the best
possible convergence with measurement results. Calculated
simulation results are presented in Figure 9, Figure 10 and Figure 11.
0.0500 0.0550 0.0600 0.0650 -10.0
-5.0
0.0
5.0
10.0
sec
U [ k
Vp]
Voltage at 20 kVa transformer primary terminals
Figure 9. PSCAD simulation, configuration 1, LC filters not connected
0.0500 0.0550 0.0600 0.0650 -10.0
-5.0
0.0
5.0
10.0
sec
U [
kV
p]
Voltage at 20 kVa transformer primary terminals
Figure 10. PSCAD simulation, configuration 2, L = 200 µH, C = 1 µF
0.040 0.050 0.060 0.070 0.080 0.090 -10.0
-5.0
0.0
5.0
10.0
sec
U [ k
Vp]
Voltage at 20 kVa transformer primary terminals
Figure 11. PSCAD simulation, configuration 3, L = 200 µH, C = 25 µF
VI. SUMMARY AND CONCLUSIONS
The research conducted herein covered both laboratory
measurement as well as numerical simulations in PSCAD
software package. The following most important conclusions
and observations can be pointed out:
1) during de-energization of unloaded transformer without any
additional LC filters on the low voltage side, multiple arc re-
ignitions are well visible,
2) connection of LC filters has significant influence on
switching off conditions, since capacitance of the filter affects
the overall natural frequency of the transformer, 3) increase of LC filter’s capacitance results in decrease of
maximum overvoltage, steepness, number of strikes and
natural frequency of oscillations,
4) with 25 µF capacitance, multiple arc re-ignitions are almost
eliminated (only one singular arc re-strike per phase is visible)
5) in this particular case inductance of the filter is negligible
since it is several orders less than the inductance of the
transformer itself,
6) regarding the real application with photovoltaic installation
(Fig. 1) it can be concluded that from the point of view of
switching off operations’ strategies, de-energization of the transformers under no-load conditions is less hazardous thanks
to presence of capacitance of the filter,
7) de-energization with 25 µF LC filters reveals saturation
effect of the transformer’s magnetic core at lower frequencies,
8) based on conducted laboratory tests, PSCAD numerical
model was developed, achieved simulation results are
satisfactory in terms of overvoltage waveforms parameters,
namely maximum overvoltage peak value, steepness and
number of multiple arc re-ignitions, however, 100% perfect
convergence is hard to obtain due to lack of more precise data
regarding transformer capacitances and magnetization
characteristic.
This paper presents only a part of research that is under
consideration. Another issue is measurement of switching on
operations, which is out of scope of this article. Next directions
of research will cover analyses of other vacuum circuit breaker
related transient states in PV power plants. Main interest will
be put on investigation on de-energization under load
conditions during feedback generation as well as during short
circuit. Thanks to fitted models after laboratory experiments,
such analyses can be conducted in PSCAD software.
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