Int. Journal of Applied Sciences and Engineering Research, Vol. 4, Issue 5, 2015
© 2015 by the authors – Licensee IJASER- Under Creative Commons License 3.0
Research article
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ISSN 2277 – 9442
High frequency spectrum analysis of transient overvoltage across
VCB switched large induction motors
Ebrahim M.A1, Elyan T, Wadie F2, Abd-Allah M.A1
1- Faculty of Engineering at Shoubra, Benha University, Shoubra, Cairo, Egypt
2- Faculty of Engineering, Egyptian Russian University, Badr City, Egypt
DOI: 10.6088/ijaser.04086
Abstract: Transient overvoltage Due to Switching Operation of large induction motors by vacuum circuit
breaker can result in severe stator failures. The protection of the motor with MOA surge arresters only, is not
guaranteed. A full motor protection will be provided by using C-R surge suppressors to eliminate multiple
re-ignitions. The optimum selection of the C and R values depend on the frequency and values of the
vervoltages. In this paper, Fast Fourier Transform (FFT) was used to analyze the transient over voltages
generated during switching of vacuum circuit breaker (VCB). A waste water treatment plant pumping station
driven by induction motors was used as a case study. The case study was simulated by using Alternative
Transients Program (ATP) to mimic a typical system behavior in actual operation. The results of the analysis
showed the frequencies of the transient components of the switching over voltage. Such information could
be used for selection of proper filters that could be used to eliminate such components and provide additional
protection required.
Key words: Transient overvoltage; vacuum circuit breaker; Induction motor; Fast Fourier Transform; ATP.
1. Introduction
The transient overvoltages generated across the contacts of vacuum circuit breaker (VCB) during its
switching, have been widely studied (Lerche M, 2009). The importance of such studies is due to the
probability for the arc to reignite which might cause serious damage to connected loads. Additional
protection measures were proposed by researchers (Xue, 2013). In order to define the suitable protection,
simulation of electrical network understudy should be done. Modeling of VCB, as a part of the simulation,
would require an understanding of the events related to switching transient overvoltages including current
chopping, multiple reignitions and virtual current chopping (Helmer, 1996). The current chopping refers to
the interruption of the arc before reaching its zero crossing ((Xue, 2013).). Once the current is chopped, the
transient recovery voltage (TRV) appears across the contacts of VCB. If the TRV exceeds the dielectric
strength of vacuum gap, a reignition of the vacuum arc occurs. A high frequency (HF) current is
superimposed on the power frequency current. If the HF current due of one phase forces the power
frequency currents of the other two phases to reach zero, multiple reignitions will occur in the other phases
also. This is known as virtual current chopping (Helmer, 1996).
An electrical network of a waste water treatment plant pumping station was used to mimic a case study.
The case study and the analysis of its results will be introduced after the modeling of different electrical
circuit elements. The paper explicitly describes the system components modeling using ATP. A simulation
of case study and its results is then presented. FFT analysis of the results was performed. Finally,
—————————————
*Corresponding author (e-mail: fady-wadie@eru.edu.eg)
Received on June 2015; Published on October, 2015
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High frequency spectrum analysis of transient overvoltage across VCB switched large induction motors
conclusion was drawn.
2. Modeling of electrical network elements
The main elements required to be modeled will be the vacuum circuit breaker (VCB), induction motor at
different operating conditions (starting or full load), transformer, cables and busbar. The VCB as the main
element in this simulation would require the most attention to the accuracy of its model.
2.1 Modeling of VCB
Although vacuum circuit breaker has been under investigation by many researchers, there is still no
universal model. This can attributed to the limitation in the information received from manufacturers due
to its confidentiality. Therefore, the stochastic model of VCB has been adopted by many researchers
( Helmer J, 1996). The model of VCB incorporates different stochastic properties inherited to the breaker
operation in order to control the actual state of the breaker during the computer simulation. They are
chopping current, rate of recovery of the dielectric strength, high frequency current quenching capability
and arcing time.
The point at which the current begins to decline is the chopping level and the value of the current at this
point is called the chopping current (
). The actual chopping current is non-deterministic, however earlier
research established different mean chopping levels for different load currents and contact material (Wong
SM, 2013). The mean chopping current is estimated according to equation 1.
(1)
The withstand voltage is modeled as linearly dependent for the first millimeter after contact opening with
the voltage stress taken as a uniform field. Thus a linear relationship between contact distance and time is
normally assumed. A typical relation is as shown in equation 2 (Kondala Rao B, 2006).
(2)
Where U
: The withstand voltage
: The moment of contact separation
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High frequency spectrum analysis of transient overvoltage across VCB switched large induction motors
A
B
: The rate of rise of dielectric strength
: Breaker’s TRV just before current zero.
The high frequency current quenching capability (HFQC) of VCB is represented by using equation 3. The
rate-of-change of the current at a current zero determines whether or not there is a successful extinction. If
the HFQC is higher than the rate-of-change of the current at a current zero, the VCB will be able to
interrupt the current (Borghetti A, 2011).
(3)
Where C : The rate of rise of HFQC of VCB
D : The quenching capability of VCB Just before its contact separation.
The arcing time of the breakers is the time between the contact separation and the following current zero
and it is random in nature. The higher the arcing time, the sufficient is the time provided for the breaker to
develop its dielectric strength. Vacuum circuit breaker model on ATP will be selected as an ideal switch.
This assumption can be justified as the voltage drop across the arc is small compared to the voltage
transient (Popo M, 1999). Its state is only characterized by the two possibilities open or closed. The state of
the switch is controlled by MODELS tool available on ATP as shown in figure 1.
Figure 1: VCB model on ATP/EMTP using MODELS tool
The controlling algorithm implemented in MODELS-tool of the ATP is as follows:
1. The breaker is considered to remain closed after mechanical opening (as the arc hasn’t
extinguished yet) till the arcing current becomes lower than the chopping current.
2. After changing the breaker status to being opened, the TRV across the VCB contacts is
compared to the withstand voltage of the gap of the VCB. If the TRV exceeds the withstand
voltage, a reignition of the arc occurs and the VCB becomes closed again.
3. The HFQC of VCB is compared to the rate-of-change of the current at a current zero. If the
HFQC exceeds the rate-of-change of the current, the VCBwill be able to interrupt the current.
The last two steps are repeated till the breaker is fully opened. The model was validated using the results
from HaoyanXue.
2.2 Modeling of induction motor
Modeling of induction motor depends on the loading of the motor. During the starting of the induction
motor, the rotor speed is very low; therefore the generated back electromotive force (EMF) is also very low
as compared to the source voltage. The low back EMF cannot keep the TRV at a low level after opening
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High frequency spectrum analysis of transient overvoltage across VCB switched large induction motors
the contacts of VCB. Therefore, the modeling of induction motor under starting condition can be regarded
as doing switching operation under the situation where the rotor is locked. The T-equivalent circuit of
induction motor is proposed to represent the electrical characteristics of motor under starting condition
(Xue, 2013). Although theT-equivalent is simple and useful, it does not consider the grounding capacitance
for each phase and the natural oscillation of motor windings. In order to fix this problem, the modified
equivalent network is proposed and shown in figure 2. For full load operation of the motor, ATP’s
Universal Machine Type 3(UM3) is used to model the induction motor.
Figure 2 Modified T-equivalent circuit of induction motor under starting condition
2.3. Modeling of other circuit elements
The transformer was modeled by ATP’s saturable transformer model with capacitance elements to
represent capacitive coupling and stray capacitances (Wong SM 2003). The JMARTI frequency dependent
line/cable model was used for modeling cables dealing with wide frequency range . The parameters of the
cables were obtained by employing the subroutine CABLE CONSTANTS in ATP using the dimensions,
geometrical and physical data of the cables. The busbar reactance is the dominant element and the length
of the busbar influences the number of reignitions in the vacuum circuit breaker. The ohmic loss of
shortbusbar is so small that it can nearly be overlooked, and the busbar can be simplified as an inductance
with a proper length, as setting in ATP/EMTP (Cai S 2013).
3. Case study
An electrical network of a waste water treatment plant pumping station was used as a case study. The
network operates at 11kV, and uses two 5MVA 11KV/3.3 KV delta/star transformers to operate ten 3.3KV,
350 kW induction motors. The motors are switched by vacuum circuit breakers. A single line diagram of
the network is shown in figure 3.
Figure 3Single line diagram of the pumping station under study
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High frequency spectrum analysis of transient overvoltage across VCB switched large induction motors
The results obtained from the simulation could be divided into four main cases as follows:
1. Case 1: simulation results from VCB 4 with motors operating at full load.
2. Case 2: simulation results from VCB 4 with one of the motors at starting condition.
3. Case 3: simulation results from VCB 1 with motors operating at full load.
4. Case 4: simulation results from VCB 1 with one of the motors at starting condition.
In each case, the effect of changing the A, B, C and D parameters were changed according to tables 1 and 2
(Wong SM, 2003). The effect of arcing time was also taken into account.
Table 1 Rate of rise of dielectric
Table 2 High frequency quenching
strength of VCB
capability of VCB
A (V/μs)
B (V)
C (A/μs²)
D (A/μs)
2
20
30
50
0
0
1000
0
-0.034
0
0
0.31
255
100
600
155
The results of case1 showed that reignition occurs only for VCB with low RRDS of 2 V/μs. This can be
explained as the generated transient overvoltage across the VCB is much faster in its recovery than the
RRDS of the VCB leading to such reignition to occur. It’s also noticed the occurrence of virtual current
chopping phenomenon as shown in figure 4.
(a)
(b)
Figure 4 Case 1: (a) Voltage across VCB (b) Voltage at motor terminalat RRDS = 2 V/μs, HFQC = 600
A/μs and arcing time = 75 μs
The results of case 2 showed that at RRDS of 2 V/μs and 20 V/μs, the circuit breaker failed to open. When
the RRDS was increased to 30 V/μs, the circuit breaker succeeded in opening after suffering multiple
reignitions with high severity due to the occurrence of virtual current chopping. Finally at RRDS of 50
V/μs, the circuit breaker succeeded in opening without suffering any reignition. Figure 5 shows the voltage
at the motor terminal at case 2. The results of case 3 showed great similartity in conclusion with that of
case 1. While those of case 4 showed that at RRDS of 2 V/μs the breaker suffered a single reignition and
failed to open.
Ebrahim M. A et al.,
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High frequency spectrum analysis of transient overvoltage across VCB switched large induction motors
(a)
(b)
(c)
(d)
Figure 5: Case 2: Voltage at motor terminal at HFQC = 600 A/μs and arcing time = 50 μs
(a) RRDS = 2 V/μs, (b) RRDS = 20 V/μs,(c) RRDS = (30 V/μs + 1000V) and (d) RRDS = 50 V/μs
4. Fast Fourier Transform analysis of simulation results
Fast Fourier Transform was used to analyze the results obtained from the simulations. MATLAB software
package provides a special function for FFT which can be used during the analysis. Therefore, the results
were transferred from ATP to MATLAB in order to use the FFT function.
The frequency spectrum of that case 1 showed two additional voltage components beside the power
frequency component. The additional components vary in amplitude from one phase to another. However,
the components always occur at approximately 4.6 kHz and 36 kHz. Changing the arcing time or HFQC
resulted in changing the amplitudes of both components but didn’t affect their frequencies. Also, the
voltage at the terminal of the motor had the same frequency spectrum with lower amplitudes at high
frequency range and higher amplitudes at lower frequency range as shown in figure 6.
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High frequency spectrum analysis of transient overvoltage across VCB switched large induction motors
(a)
(b)
(c)
(d)
Figure 6: Case1: Frequency spectrum of phase voltage A at RRDS = 2 V/μs, HFQC = 600 A/μs, arcing time
= 75 μs (a and b) Across contacts of VCB, (c and d) At the terminal of the motor
(a)
(b)
Figure 7: Case 2 : Frequency spectrum of phase voltage B at RRDS = 20 V/μs, HFQC = 600 A/μs, arcing
time = 50 μs(a) Across contacts of VCB (b) At the terminal of the motor
The frequency spectra of the voltage across the VCB contacts in case 2 showed that several voltage
components appear at frequency range up to 300 kHz. The major components mostly lie in two frequency
ranges. The first range lies between 4 kHz and 7 kHz, while the second range lies between 60 and 80 kHz.
Also, it should be noted that within case, some phases had failed to open. Therefore, no frequency analysis
was required for the phases that failed to open. The frequency spectrum for the voltage at motor terminal at
RRDS of 2 V/μs showed two main voltage components appear at approximately 18 and 23 kHz. At
RRDS= 20 V/μs, an additional component at 3.5 kHz appeared as shown in figure 7.
Ebrahim M. A et al.,
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High frequency spectrum analysis of transient overvoltage across VCB switched large induction motors
The Frequency spectra of the results of case 3 showed three high frequency voltage components are noted
at 7.4 kHz, 18 kHz and 21 kHz at RRDS of 2 V/μs. The frequency spectra of the results of case 4 showed a
high frequency voltage component at range of 7.5 kHz.
5. Conclusions
The overvoltages generated during the switching of VCB in a pumping station of a waste water treatment
plant were simulated. Modeling of the VCB, the induction motor and other electrical elements within the
plant was performed using ATP. The simulation was conducted for two locations of VCB, over the full
range of operating conditions and severe disturbances.
The simulation results showed that the most severe case was when one of the motors at starting condition
and the VCB closer to the motor was under higher stress. Effect of changing the arcing time and HFQC of
VCB were taken into consideration.
The FFT analysis of the transient overvoltages had provided valuable information regarding the frequency
of the transient overvoltage components. The frequencies defined in this research could be used as guide
for designing a suitable filter to be used for their elimination and provide the additional protection
required.
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