Post on 21-Jul-2020
THE HONG KONG POLYTECHNIC UNIVERSITY
DEPARTMENT OF ELECTRICAL ENGINEERING
1
Project ID: FYP_71
Simulation of virtual-current chopping in circuit breakers in
electrical supply systems
by
Lui Chak Hing
14073317D
Final Report
Bachelor of Engineering (Honours)
in
Electrical Engineering*
Of
The Hong Kong Polytechnic University
Supervisor: Dr C.W. Yu Date: 31/3/2018
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DEPARTMENT OF ELECTRICAL ENGINEERING
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Contents
Abstract................................................................................... 3
Acknowledgement................................................................... 4
Chapter 1:
Introduction........................................................................ 5 - 9
Chapter 2:
Objectives.............................................................................. 10
Chapter 3:
Background.................................................................... 11 - 13
Chapter 4:
Methodology.................................................................. 14 - 24
Chapter 5:
Results............................................................................ 25 - 39
Chapter 6:
Discussion...................................................................... 40 - 42
Chapter 7:
Conclusion............................................................................. 43
References...................................................................... 44 - 46
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Abstract
Vacuum circuit breaker (VCB) provides highly enhanced current interruption
and dielectric recovery features. It has the ability to interrupt the high frequency
(HF) current effectively. In some cases, HF frequency current may superimpose
on the power-frequency current. This can cause multiple reignitions and serious
voltage escalations under some conditions. In this project, computer simulation
will be performed in order to study and analyse the properties of such process.
A mathematical VCB model which incorporates different phenomena during the
operation is constructed. These include the HF current quenching capability and
the recovery of dielectric strength of VCB. This model replicates the original
circuit breaker properties and simulate the phenomenon of the transients. The
construction of the VCB model is conducted in the simulation software EMTP-
RV. The developed model is implemented in a simple-phase and a three-phase
testing circuits. Simulation is then performed to obtain the transient behaviour
of VCB and analyse the effects on multiple reignitions phenomena under
different parameter values. The simulation work reveals that there may be a
number of multiple reignitions when the dielectric strength is low or arcing time
is short. Afterwards, the developed VCB model is implemented in a typical
electrical supply circuit. The circuit involves the switching of an arc furnace
transformer. The behaviour of virtual current chopping and the effects on
overvoltage phenomena of VCB will be analysed. Finally, suitable protective
measures such as surge protection are necessary in order to eliminate the
problem of virtual current chopping and hence the connected system can be
protected.
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Acknowledgement
In the project, I would like to take this chance to express my thanks to my
supervisor Dr C.W. Yu.
I am very grateful to Dr Yu who offers me this challenging project. The
valuable advice given by Dr Yu has been a great help in doing the Project.
In addition, I wish to thank all of the people for their undivided support and
encouragement. They have given me lots of help and contributed greatly to the
Project.
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Chapter 1: Introduction
Circuit breakers are widely used and have become the important equipment
for the protection of transmission and distributions systems for more than a
hundred years. They play a vital role in clearing faults and isolating
defective parts of network clearly and rapidly. They also take the role of
normal load and fault switching [1]. In the early days, the design of circuit
breaker is very simple and air is used as an arc interruption medium.
Starting from the 20th century, there was a rapid development of oil-filled
and air-blast forced cooling circuit breaker. Later, the development of SF6
and vacuum circuit breaker (VCB) started and has been in use. When
compared to early designs, the improved design of circuit breaker can
provide the advantage of reduction of chamber volume and improvement of
the dielectric features. Nowadays, oil-insulated and gas-insulated circuit
breakers are still irreplaceable for high voltage levels. For medium voltage
levels, VCB have dominated the switching functions in power systems.
A circuit breaker can be viewed as one moving and one fixed contact. These
contacts are either placed in a special container which contains the particular
extinguishing medium (e.g. gas and oil) or a vacuum bottle. The contacts are in
closed state under normal operation and current will flow through the device
without major losses. If a signal of opening is sent to the circuit breaker, the
contacts are separated by an external mechanism. When the separation of
contacts starts, current still continues to flow between the contacts as an electric
arc. The presence of electric arc is due to the supplied energy and the arc will be
present until the energy is eliminated by some ways. In an AC system, there are
two zero-crossing points for current in every cycle. Circuit breaker can have the
chance to extinguish the electric arc in these current zeros because of the zero
energy input in these points. The design of a circuit breaker should fulfil certain
criteria such as interrupting at natural current zeros, meeting the thermal
interruption requirements and withstanding dielectric stresses caused during the
interruption process.
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Figure 1.1 - A typical VCB
Out of many types of circuit breakers, VCB (shown in Figure 1.1) are
commonly adopted in power systems with medium voltage range due to many
advantages, such as small size, less maintenance, excellent performance on the
interruption and recovery of dielectric strength [2]. VCB can interrupt current
with a very high value of 𝑑𝑖
𝑑𝑡 , ranging from 150A/µs to 1000A/µs typically [3].
However, everything has two sides. The switching transient overvoltage
phenomenon due to repeated reignitions, current chopping and virtual current
chopping are commonly found in VCB [2].
Figure 1.2 - Current chopping
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When VCB is opening, electric arc arises and the arc will extinguish at a natural
current zero ideally. When the arc conducts a small current, the arc becomes
unstable and may disappear before natural current zero [2]. The above process
is described as current chopping as shown in Figure 1.2 and is commonly found
in VCB during the interruption of inductive and capacitive current [1].
When current chopping happens, the transient recovery voltage (TRV) arises
between the gap of VCB. Once the level of TRV is higher than the level of
dielectric strength of vacuum gap, reignition will happen and consequently arc
will appear again [3]. The process will cause a flow of HF current due to the
stored charges in the stray capacitance on either side of the gap of the breaker.
The power-frequency current can be superimposed by the HF current, as shown
in Figure 1.3 [4].
Figure 1.3
VCB can interrupt the HF current at one of the current zeros. When the
interruption occurs, the process of multiple reignitions may happen and it may
produce undesirable voltage surges [3]. Voltage escalation will continue until
the contact separation of VCB is enough to withstand the TRV or the magnitude
of power-frequency current is high enough to prevent zero crossing of the sum
of HF current and power frequency current. In this situation, there is no
interruption and the current will continue for one more half cycle [1]. This
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process can repeat several times. If the HF current of one phase is flowing into
other two phases of the system through the electrical couplings of the load, the
HF current can superimpose on the power-frequency current of these phases and
force the power-frequency current to zero. Virtual current chopping happens
and this will lead to severe overvoltage in three phases [2].
It is reported that a number of transformer insulation failures have occurred in
the systems with the installation of VCBs. This is possibly due to the switching
actions of VCBs, although these transformers have complied with associated
requirements and passed all standard tests previously [5]. A study on the
investigation of transformer failures revealed that the HF transients are the
major cause of the insulation failures (around 34%) [6].
The switching transients modelling in VCB is important in order to analyse the
overvoltage behaviour which may happen at different stages. However, the
accurate properties and performance of VCB are difficult to simulate because of
the confidential information of suppliers and the limitation of experiments [2].
A number of VCB models exist and the factor of arc thermal instabilities is
considered. However, there is currently no universal precise arc model due to
the complexity of the arc physics [3].
The simplest circuit breaker model is assumed to have zero impedance in closed
state and infinite impedance in open state. The breaker will open at the first
current zero after giving the corresponding signal and TRV of the breaker can
be obtained by using this model. The consideration of arc (time-varying) is
needed in more complicated models. This requires the parameters of arc which
are sometimes difficult to obtain. For advanced models, the circuit breaker can
be represented as a dynamically varying conductance or resistance which is
determined by the past values of voltage and current in the arc.
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In this project, modelling of VCB will be performed. Then simulation of virtual
current chopping will be done and the effects of overvoltage transients in
electrical supply system will be analysed.
This project is organised by the following sequence.
Chapter 1 is the introduction of the project, including the history of circuit
breaker and the transient behaviour of VCB.
Chapter 2 is the objective of the project, which will introduce what outcomes
can be introduced after finishing the project.
Chapter 3 will mainly provide different research ideas proposed by different
researcher in the area of circuit breaker.
Chapter 4 introduces the methodology that is mainly about how the VCB model
is constructed and what simulation tasks are required in the project.
Chapter 5 mainly presents the results of the simulation in different circuit
configurations.
Chapter 6 will mainly provide explanations of the simulation results and some
extensive ideas
Chapter 7 concludes the whole project.
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Chapter 2: Objectives
In this project, several objectives would like to be achieved, including
Developing a VCB model incorporating different parameters of the circuit
breaker in the simulation software EMTP-RV
Performing the simulation in the single-phase and three-phase testing
circuits
Analysing the simulation results and investigating the transient behaviour
of VCB with different parameters
Applying the VCB model to a practical circuit which is related to the
switching of an arc furnace transformer
Simulating the virtual current chopping process and analysing some
protective measures which can reduce the overvoltage and reignitions
behaviour of VCB
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Chapter 3: Background
The problems that arise from the switching of VCB have been a major research
subject for many years. A lot of work has been conducted on the analysis of
physical behaviour regarding the arc circuit interaction and current interruption.
The effect that VCB has when switching different networks boosted the
research after the commercial application of VCB in the past decades. In the
early days, the research was focussed mainly on the high level of chopping
current of VCB. Greenwood [7] performed the research work on explaining
how chopping current reacts with motor and transformer circuits. Later, the
behaviour of current chopping was further analysed. For example, Czarnecki
and Lindmayer [8, 9, 10] conducted a research on the effects of contacting
materials on VCB interrupting performance around current zero. From the result
of the research, an expression for the chopping current value was obtained. This
was a difficult task due to the statistical nature of current chopping phenomena.
Damstm and van den Heuvel [11] and Gibbis et al. [12, 13] later performed the
work on comparing the chopping current levels of various SF6 breakers with
those of VCBs. The results showed that the levels of VCB chopping current is
distributed randomly with a value from a range that is dependent on the types of
contacting materials. Smeets [14] conducted a further research on the behaviour
of low current arc. A more generally applicable expression for evaluating the
level of chopping current was obtained.
Researchers have done quite a lot of studies on VCB modelling and overvoltage
transient behaviour. A wide range of work are studied and analysed in this
research area. In recent years, some researchers have developed a mathematical
VCB model that comprises the random characteristics of various phenomena
that take place in the VCB operation, including the arcing time, recovery of
dielectric strength, quenching capability of HF current and current chopping
level of the VCB [2]. Different combinations of these properties were chosen
and the performance of a number of breaker models were analysed. From this, a
brief idea can be obtained on how to model VCB properly by considering these
characteristics.
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A model of single-phase VCB was presented in an academic paper [15],
including the factors which have the effects on overvoltage produced as
reignitions happen during the opening of VCB. These included some circuit
parameters and breaker characteristics (e.g. interrupting capability and rate of
change of dielectric strength, etc.). Some random events e.g. arcing time were
also included. It had the assumption that the dielectric strength of VCB
increases with time linearly. The VCB model was constructed to analyse these
factors. The model was verified by measuring current and voltage values across
different VCBs in a testing circuit.
A model of three-phase VCB was developed in the simulation software in
another paper [1]. The model incorporated some characteristics such as arcing
time, chopping current level, quenching capability of current and dielectric
strength between the contacts. Some were evaluated based on statistical
approach such as using a normal distribution with a certain standard deviation.
A research [16] was conducted on the simulation of voltage escalation and the
behaviour of reignition in VCBs when they are generator circuit breakers. Nine
particular models of VCB were investigated. The model include an RC branch
parallel to the breaker to represent the gap stray capacitance. Similar to other
research papers, the VCB features were modelled by linear expressions for
dielectric strength of the breaker and HF current quenching capability. The
research showed that multiple restrikes in the process may generate a voltage
escalation when the arcing time is short (e.g. 0μs – 100μs).
Other scholars have done similar research work and studied the transient
behaviour of VCB in different practical situations. Some examples include the
shunt reactor and arc furnace transformer switching [1] [4]. These studies can
provide some directions and ideas on this project.
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Some researchers [2] conducted the study on transient behaviour which happen
at different parts within an offshore wind farm. The models for transformers,
cables, wind turbines and circuit breakers are reviewed.
The problem of motor switching have also attracted great interest because
vacuum contactors are widely applied in motor circuits. The overvoltage
phenomena, which result from multiple reignitions, current chopping and virtual
current chopping, can vary greatly depending on different motor conditions (e.g.
unloaded, normal load or starting up). By considering different motor circuits,
analysis of motor switching can be performed. These studies were performed by
a number of researchers such as Smeets et al. [17] and Smeets [18, 19].
Besides the energisation and de-energisation of motors, capacitive loads can
cause overvoltage phenomena as well. Pu and Damstm [20] conducted a study
that combined the computer simulation and experiments on capacitive load. The
results showed that the actual phenomena are affected by both the breaker
parameters and the network
Overvoltage transient of VCB can lead to transformer failures [5]. Some studies
about this issue have been analysed and presented [5] [21]. This research area is
important in studying transformer protection. Various simulation work and
testing have been performed by researchers [22]. The background information is
useful when doing the related work in this project.
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Chapter 4: Methodology
The project will be mainly conducted in the simulation software EMTP-RV.
With the help of the software, complex systems can be simulated and analysed.
The detailed properties and phenomena of VCB are discussed in this section.
Afterwards, a VCB model that includes the characteristics of current chopping,
voltage escalations and multiple reignitions will be introduced [3]. A
mathematical VCB is then constructed in the software environment. The
parameters of the breaker are obtained from different research papers.
When switching a VCB, transient phenomena may happen in different
situations. Even for the small differences in the circuit parameters values can
lead to large differences in the final results. The overvoltage behaviour in VCB
is random in nature and it is dependent on the actual configuration of the
system, breaker conditions and associated parameters. Various ways can be
employed for analysing the overvoltage behaviour and voltage oscillation
generated during the switching of VCB. These include theoretical quantitative
analysis, laboratory/field testing and computer simulation.
For laboratory testing, one major problem is the difficulty of getting a realistic
load model. Also, it has the difficulty in constructing the testing circuits that
match the practical situations. For field test, the drawbacks are difficult to
perform and expensive. There is a possible risk that the circuit breakers and
circuit components may damage. For theoretical analysis, it can be a useful tool
to obtain insight in the problems related to VCB switching. However, it cannot
model the circuit breaker and system in detail. Some models (e.g. transformers
and cables, etc.) are frequency dependent. A lot of time may be consumed to
analyse the complicated network. Nowadays, computer simulations are widely
used for the modelling of different complicated systems.
An appropriate VCB model is essential in order to simulate the transient
behaviour in electrical system. However, the accurate breaker model is very
difficult to obtain because of the limited information and the complicated
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breaker behaviour during operation [2]. Therefore, some researchers have
proposed the stochastic model [3].
When the VCB is opening, the dielectric strength of the contacts increases with
time as well. When the TRV is higher than the dielectric strength of VCB
contacts, reignition will happen and the model will generate a closed signal.
When the rate of change of HF current at a zero crossing is lower than the
quenching capability of HF current of VCB, the model will send an opening
signal to the switch [2]. The HF current will be interrupted. If multiple
reignitions happen, the above-mentioned process will repeat until the dielectric
strength of VCB is able to withstand the TRV.
The general VCB model includes some characteristics inherent to the VCB
operation to control the actual behaviour of the breaker during the process of
simulation [1]. These characteristics include the current chopping level, arcing
time, recovery of dielectric strength and HF current quenching capability [3].
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Arcing time
Arcing time is the time interval between the opening of VCB contact and the
subsequent current zero.
Chopping current level
Current chopping is the phenomenon that the power-frequency current is
suppressed before its natural zero crossing in the breaker. If the capacitive or
inductive current is interrupted, the arc appears and power-frequency current
conducts through the arc [3]. When the power frequency current of first pole to
clear reaches to the low level, the arc becomes highly unstable or even
disappear. The power frequency current will be chopped before its natural zero
crossing. Current chopping is a major disadvantage of VCB because it will be
accompanied with the transient overvoltage which will affect the load side due
to oscillations [2]. The chopping current is non-deterministic. Some researchers
have proposed different mean chopping levels for various situations [3]. In the
simulation, the chopping current is calculated by [3]:
𝑖𝑐ℎ = (𝜔 ∙ 𝑖 ∙ 𝛼 ∙ 𝛽)𝑞
where:
ω = 2 ∙ π ∙ 50Hz
i = amplitude of the 50Hz current
α = 6.2 ∙ 𝑒−16𝑠
β = 14.3
q = (1 − 𝑞)−1
The chopping currents calculated from the formula are similar to those of
present VCB which use Cu/Cr as the contact materials [3]. The current
chopping level is dependent on the moment of separation of the VCB contacts
(the closer the contact opens to current zero, the higher the chopping level).
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In the simulation, current chopping is not included in the VCB model because it
is assumed that multiple pre-strikes and re-strikes will lead to serious
overvoltage. Also, the current chopping level becomes relatively low after the
development of modern VCB.
Recovery of dielectric strength
In general, two breakdown mechanisms exist in VCB, namely the cold gap
breakdown and the hot gap breakdown. In this project, the cold gap breakdown
is considered. Researchers have shown that there is a linear relationship
between the dielectric strength value and the contact distance. The equation is
given as follow [3]:
U = A(t − 𝑡𝑜𝑝𝑒𝑛) + 𝐵
where:
t: software internal time
𝑡𝑜𝑝𝑒𝑛: VCB opening time
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Quenching capability of HF current
The power-frequency current is superimposed by the HF current if reignition
happens. The HF current has some zero crossings and the breaker has the
capability to extinguish the HF current in one of the zero crossings [3]. The rate
of change of HF current at zero crossing determines whether the current can be
interrupted by the VCB or not.
In earlier study, the quenching capability of HF current is modelled by [3]:
𝑑𝑖
𝑑𝑡= 𝐶(𝑡 − 𝑡𝑜𝑝𝑒𝑛) + 𝐷
where:
𝑑𝑖
𝑑𝑡: rate of change of the current
C: slope of the equation
t: software internal time
𝑡𝑜𝑝𝑒𝑛: VCB opening time
D: intercrpt of the equation
When the rate of change of HF current in one zero crossing is lower than the
calculated di/dt value, the HF current will be extinguished.
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Figure 4.1a and 4.1b show the flow charts of opening and closing operation of
the VCB model.
Figure 4.1a - Flow chart for VCB opening operation
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Figure 4.1b - Flow chart for VCB closing operation
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Figure 4.2 - A controlled switch
The VCB model is characterised by a controlled switch (shown in Figure 4.2)
and the switch will be open or closed based on a specific mechanism. The state
of the VCB is determined by voltage, current and the previous state of the
breaker. The flow charts for VCB opening and closing operation are shown in
Figure X
For the closing operation, the dielectric strength of the VCB will start to
decrease. Once the VCB voltage exceeds the dielectric strength of the breaker at
a particular time, a closing is generated. If the slope of the current at zero-
crossing point is lower than the HF current quenching capability, an opening
signal is generated and hence the pre-strike is interrupted. The process will
continue until the dielectric strength of the breaker reduces to zero.
When the VCB starts to open, the dielectric strength of the breaker will begin to
increase with time. If the slope of the current at the zero-crossing point is lower
than the HF current quenching capability of the breaker, the switch will be
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open. TRV will appear across the contacts of the VCB. If the TRV exceeds the
dielectric strength of the breaker, the switch will be closed and hence a
reignition is simulated. The process will continue until one of the following
conditions happen:
The VCB can interrupt the current successfully when the TRV is smaller
than the dielectric strength of the breaker.
The VCB cannot interrupt the HF current after the last reignition. The
current interruption is accomplished at the next power-frequency current
zero when the dielectric strength is higher.
The VCB cannot achieve the interruption in any period.
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Figure 4.3 - EMTP-RV software
Figure 4.4 - VCB model
Figure 4.4 shows the developed VCB model that is implemented in the EMTP-
RV environment.
In this project, computer simulation is performed on different network
configurations by using the simulation software EMTP-RV. These
configurations include the testing circuits of single phase and three phases. An
application circuit which is about the switching of an arc furnace transformer is
also simulated. By using the simulation results, transient behaviour of VCB can
be analysed under different situations. Figure 4.5, 4.6 and 4.7 show the different
network configurations.
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Figure 4.5 - Single-phase testing circuit
Figure 4.6 - single-phase testing circuit
Figure 4.7 - Switching of a transformer
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Chapter 5: Results When a load is disconnected by the circuit breaker, one of the situations may
occur in general:
The load current is interrupted successfully by the VCB and reignition
will not occur.
Multiple reignitions occur and VCB interrupts the HF current. After the
process is repeated for several times, the VCB withstands the transient
recovery voltage and interrupt the current successfully.
The VCB cannot interrupt the HF current and power-frequency current
conducts through the arc. Successful interruption is extended in the next
zero point of power frequency current.
The VCB cannot interrupt the current in any position of the period.
Serious damage to equipment may occur.
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Simulation of single-phase cases
Testing circuit
Figure 5.1 - Single-phase testing circuit
Circuit parameters
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Results
Case 1
Data:
Recovery of dielectric strength A = (50V/µs)
Quenching capability (D = 100A/µs)
Waveforms:
Figure S1 C1-1 - Voltage across the contacts of VCB
Figure S1 C1-2 - VCB current
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Figure S1 C1-3 - Voltage on the load side
The results are shown in the above figures. The increase of TRV is always slower
than the increase of dielectric strength of VCB. As a result, the VCB can
withstand the TRV and interrupts the load current successfully. When the current
chopping occurs in the VCB, a transient overvoltage with several kHz oscillations
appears in the load because the energy stored in LL is transferred into CL.
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Case 2
Data:
Recovery of dielectric strength (A = 20V/µs)
Quenching capability (D = 100A/µs)
Waveforms:
Figure S1 C2-1 - Voltage across the contacts of VCB
The TRV exceeds the dielectric strength of VCB at a particular time. The first
reignition occurs. The voltage across the VCB starts to have a high frequency
oscillation after the zero power-frequency current. The high frequency
oscillation is induced from the interaction of CL, CS and LK. The expression is
given by:
𝑓1 =1
2π√𝐿𝑘𝐶𝑠𝐶𝐿
𝐶𝑠 + 𝐶𝐿
= 1.8𝑀𝐻𝑧
The above HF voltage oscillation will be damped after very short time. The
frequency of next voltage oscillation is much lower and can be determined by:
𝑓2 =1
2π√𝐿𝐿𝐶𝐿
= 4.6𝑘𝐻𝑧
It represents the natural frequency of the load.
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Figure S1 C2-2 - VCB current
Figure S1 C2-3 - VCB current of the first HF oscillation
Figure S1 C2-4 - VCB current of the second HF oscillation
As a direct result of reignition, the HF current is injected into network and two
HF oscillations can be observed. The first HF oscillation is due to the
interaction of CS and LS. The frequency can be determined by:
𝑓3 =1
2π√𝐿𝑠𝐶𝑠
= 50𝑀𝐻𝑧
This HF current is damped quickly and it is not interrupted in its zero point by
the VCB. The second HF current is caused by the interaction of LK and CL. The
frequency can be determined by:
𝑓4 =1
2π√𝐿𝑘𝐶𝐿
= 0.25𝑀𝐻𝑧
Figure S1 C2-5 - Voltage on the load side
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Case 3
Data:
Recovery of dielectric strength (A = 20V/µs)
Quenching capability (D = 200A/µs)
Waveforms:
Figure S1 C3-1 - Voltage across the contacts of VCB
Figure S1 C3-2 - VCB current
Figure S1 C3-3 - VCB current of the first HF oscillation
Figure S1 C3-4 - VCB current of the second HF oscillation
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The quenching capability of HF current of VCB plays a vital role in interrupting
the second HF current. When the slope of the HF current at current zero is smaller
than the HF current quenching capability of the VCB, this HF current will be
interrupted. In this case, the HF quenching capability is higher (D = 200), the
VCB can interrupt the HF current earlier when compared to D = 100. As a result,
TRV will occur earlier so the chance of another reignition will increase because
the dielectric strength of the VCB is lower in the early time. The arc will then be
extinguished earlier and hence the chance of having HF current zeros will
increase. The process will continue until the power frequency current increases
to a level where there are no current zeros with the slope of 200 or less. The arc
will continue to exit until the time of second current zero crossing, when the
dielectric strength of the VCB can withstand the TRV.
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Simulation of three-phase cases
Testing circuit
Figure 5.2 - Three-phase testing circuit
Circuit parameters
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Results
Case1
Waveforms:
Figure S2 C1-1 - Voltage across the VCB contacts at phase A
Figure S2 C1-2 - Voltage across the VCB contacts at phase B
Figure S2 C1-3 - Voltage across the VCB contacts at phase C
Figure S2 C1-4 - VCB voltages of all phases
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Figure S2 C1-5 - VCB current at phase A
Figure S2 C1-6 - VCB current at phase B
Figure S2 C1-7 - VCB current at phase C
Figure S2 C1-8 - VCB currents of all phases
Figure S2 C1-9 - Load voltages of all phases
The results of the simulation show that the reignitions phenomena is not serious
and the VCB can withstand the TRV after few cycles.
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Case 2
Waveforms:
Figure S2 C2-1 - VCB voltages of all phases
Figure S2 C2-2 - VCB current of all phases
Figure S2 C2-3 - Load voltages of all phases
The results of the simulation show that there is a serious and multiple
reignitions in this VCB when compared to the previous case. High frequency
oscillations appear in the current waveforms of the VCB. The circuit breaker
can only withstand reignitions after a number of cycles.
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Simulation of the circuit involving switching of a transformer
Testing circuit
Figure 5.3 Switching of an arc furnace transformer
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Results
Case 1
Description:
The circuit has no surge protection.
Waveforms:
Figure S3 C1-1 - VCB voltages of all phases
Figure S3 C1-2 - VCB current of all phases
Figure S3 C1-3 - Load voltages of all phases
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Case 2
Description:
The circuit has a surge arrestor with the rating of 35.35kV.
Waveforms:
Figure S3 C2-1 - VCB voltages of all phases
Figure S3 C2-2 - VCB current of all phases
Figure S3 C2-2 - Load voltages of all phases
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Chapter 6: Discussion
The simulation works are mainly based on three circuits, including a single-phase
and a three-phase testing circuit. In addition, an application circuit involving the
switching action of an arc furnace transformer is simulated as well.
For the simulation of single-phase testing circuit, different cases are involved in
order to investigate the VCB behaviour during the opening process. Different
parameters (dielectric strength of VCB and HF current quenching capability) are
set to analyse to effects on reignitions phenomena and HF current property of
VCB. The results show that overvoltage and multiple reignitions are more likely
to occur when the dielectric strength is smaller or the HF current quenching
capability is higher. Different frequency values are evaluated and this shows that
there are high frequency oscillations during the process of reignitions in VCB.
For the three-phase testing circuit, similar simulation tasks are performed. The
results show that VCB experiences reignitions at different phases under various
time intervals. The HF current produced at one phase will through other phases
and force the power-frequency current to zero. The forced current zero in that
phase will cause TRV in the breaker. As a result, further reignitions will produce
and this will lead failure of the VCB.
Sensitivity analysis of parameters is important to find the trend which would
cause serious multiple reignitions and escalation of voltage in the circuit breaker.
According to a research paper [3], 48 combinations of VCB parameters are
analysed.
The paper gives a summary on the effect of the quenching capability of HF
current and the recovery of dielectric strength with different ranges of arcing time
on voltage escalation. When the recovery of dielectric strength is in middle range
(between 20V/µs and 30V/µs), voltage escalation is usually higher than other
cases. For arcing time, when it is small (0µs - 100µs), voltage escalation is more
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serious than those arcing time ranging from 100µs to 300µs. High voltage
escalation also usually occurs when the slope of HF current quenching capability
is negative. From the result, some combinations of the parameters may cause
reignitions and voltage escalation. This may happen when the recovery of
dielectric strength is in middle range, the arcing time is short or the quenching
capability of HF current is decreasing with time.
The results show that the arcing time and the recovery of dielectric strength are
the most important parameters for the estimation of TRV. The effect of the
quenching capability of HF current on voltage escalation is less noticeable. The
illustration figures are shown in Figure 6.1.
Figure 6.1
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The VCB model developed in this project is put into a practical circuit for the
purpose of simulation. It involves the switching of an arc furnace transformer. A
lot of switching operations are needed during the operation of the transformer.
VCB is a suitable type of circuit breaker for this application due to the cheap
maintenance cost and the capability of switching high current at lots of switching
cycles. However, switching of transformer may lead to high stress on the network.
In the circuit, the phenomena of virtual current chopping can cause the failure of
the VCB and other devices. Therefore, adding the RC surge suppressor or surge
arrestor is necessary. The process of virtual current chopping can be simulated by
using the developed VCB model. Suitable values of the surge suppressor and
surge arrestor can be found to minimise the overvoltage effect.
In the simulation of the practical circuit, Case 1 is the normal loading without
adding surge arrestor. The results of the simulation show that reignitions
happened during the operation of the circuit breaker. Hence, this caused serious
virtual current chopping. Case 2 is the circuit with the addition of surge arrestor
which has the rating of 35.35kV. After adding the surge arrestor to the circuit, the
overvoltage behaviour was limited and hence the phenomena of virtual current
chopping were improved greatly. As a result, the addition of surge arrestor can
help to protect the system.
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Chapter 7: Conclusion
In conclusion, the characteristics of VCB and phenomena of transient overvoltage
are discussed. A VCB model is built and implemented in the simulation software.
A single-phase and three-phase testing circuit is developed to perform the
simulation with different parameters and observe the phenomena of multiple
reignitions and voltage escalations of VCB.
Moreover, the results of sensitivity analysis show that the parameters of arcing
time and the recovery of dielectric strength play a significant role for the
occurrence of multiple reignitions and overvoltage. The quenching capability of
HF current has less effect. Consequently, when estimating the overvoltage
phenomena of VCB, it is suitable to consider the recovery of dielectric strength
in the middle range with smaller arcing time to look for the worst situation so that
the appropriate protection scheme can be provided.
The developed VCB model is implemented in an application network which
involves switching of a transformer. The phenomena of virtual current chopping
is observed in the process of simulation. One possible way of reducing the
problem of virtual current chopping is the addition of the RC surge suppressor or
surge arrestor. This can reduce the chance of equipment failure and hence protect
the system.
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