Post on 10-Dec-2016
Modelling and Control of a Hybrid Circuit breaker with Fault Current Limiting
Ability
Rudraksh Kapoor*, Student IEEE Member and Anshuman Shukla
+, IEEE Member
Department of Electrical Engineering,
Indian Institute of Technology, Bombay
*rudrakshpeps@iitb.ac.in, +ashukla@ee.iitb.ac.in
Abstract- A new fault current limiter and circuit interrupter is
proposed using a hybrid circuit breaker. The hybrid circuit
breaker consists of a mechanical switch in parallel with
bidirectional conducting IGBTs with anti-parallel diode
modules. The varistor used in this HCB circuit to control over
voltage during switching is modelled and its characteristics are
presented. Modelling of the mechanical switch using Mayr arc
model is presented. The fault current limitation is achieved by
switching the power semiconductor devices using hysteresis
control. The performance of the proposed circuit is confirmed
through simulation investigations.
I. INTRODUCTION
Fault in an electrical system cannot be avoided.
Therefore, protection device in a network is essential.
Conventional circuit breaker in the electrical network, protect
the network against fault by dissipating the circuit inductive
energy in form of an arc. The arching in the conventional
circuit breaker during fault clearing is essential to follow the
conservation of energy law [1]. The dissipation of energy can
be done either in a medium such as air, oxygen, etc. or in a
medium of electronegative gas such as sulphur hexafluoride.
Hence, depending upon the medium conventional circuit
breakers are classified [2]. But, due to arching these devices
are subjected to regular maintenance and if these devices are
used in the electrical network, network element has to be over
rated due to moving mechanical parts which has large time
constant.
To overcome the shortcoming of a conventional circuit
breaker alternate means has to be adopted. With the
advancement in power semiconductor devices, current and
voltage rating, using these devices in place of a mechanical
switch is possible [3]. Due to PN junction in a device, there
will be no arcing while breaking and unlike conventional
circuit breaker there switching is fast. Therefore the block
diagram of a breaker in which power semiconductor devices
are used in place of a conventional circuit breaker is shown in
Fig. 1 and these breakers are termed as solid-state circuit
breakers (SCB). The power electronic module consists of a
main path and an auxiliary path. Controlled switch in the main
path carries the load current during normal operation and
when fault occurs in the network, auxiliary path assist in
breaking the main path either using current commutation or
voltage commutation technique [4]. When current breaks in
the electrical network, very high voltage builds across the
main path, which can damage the power semiconductor
devices in the main path. Hence an energy absorbing device is
also required which will limit the voltage across the devices
and absorbs the circuit energy. Even with an advantage of no
arching and fast switching, these devices during normal
operation due to high on-state resistance, limit the application
of an SCB up to low voltage level only.
Power ElectronicModule
Source
Energy Absorbing Device
Source/ Load
Fig 1: Block diagram of an SCB
With the advancement of an electrical network in terms of
connectivity, interconnection, high losses across an SCB
during normal operation and slow speed of transition of a
conventional circuit breaker make them unsuitable for to
match the need of the future grid. Therefore, a block diagram
of a circuit breaker which has an advantage of both
conventional circuit breaker as well as an SCB is shown in
Fig. 2 and these breakers are termed as hybrid circuit breaker
(HCB).
Power ElectronicModule
Source
Energy Absorbing Device
Source/ Load
MechanicalSwitch
Fig 2: Block diagram of an HCB
The mechanical switch an HCB is the conventional circuit
breaker which carries the load current during normal operation
and therefore, limits the on-state conduction losses. Power
electronic module, which consists of power semiconductor
device(s) and passive element, is responsible for fast and
efficient transition of the breaker. The power electronics
module either injects impedance across the main path or
injects current or voltage across the mechanical switch to force
commutates the current form the mechanical switch [4]. For
modernized and complex electrical network, an HCB is a
suitable option because of it has many advantages over
conventional circuit breaker and an SCB. But, unlike
conventional circuit breaker an SCB or an HCB cannot be
customised because these devices forcefully created current
zero across the main path. So, these devises has to be designed
for the worst case. But, due to large interconnection and
modernization short circuit current level is even increasing the
forecasted value. Hence, an HCB should also have a current
2012 IEEE International Conference on Power Electronics, Drives and Energy Systems December16-19, 2012, Bengaluru, India
978-1-4673-4508-8/12/$31.00 ©2012 IEEE
limiting ability so that they can match the future requirement
of the electrical network.
This paper presents the modelling and analysis of an HCB
which has a current limiting ability. The next section discusses
the need of a fault current limiter in an HCB when used in an
electrical network and the topology studied in this research. In
section III, the mathematical modelling of various elements
used in an HCB are discussed. The mechanical switch is
modelled using mayr arc model and its simulation results are
presented. Mathematically modelled energy absorbing device
which is used in an HCB is also presented. Section IV
describes the controller for the proposed topology and the
simulation results. In the end a conclusion is presented in
section V.
II. NEED OF FAULT CURRENT LIMITER IN AN
HCB
The peak short-circuit current in the interconnected
electrical network varies depending upon the location of the
fault and if a single source-load system is considered as shown
in Fig. 3(a), peak short circuit current will vary depending
upon the impedance provided at the fault location. The peak of
a short-circuit current with varying short-circuit impedance is
shown in Fig. 3(b). Therefore even designing an HCB for the
worst case may not be sufficient because the peak short circuit
current will rise beyond the forecasted value, in the future.
Therefore, a current limiter in the electrical network in must.
ACSourc
e
Zs
Mayr Arc Modeled
Circuit Breaker
Load
Fault
(a)
(b)
Fig 3: (a) A source load system in which short circuit fault occurs a different
fault location. (b) Peak current with respect to the impedance provided at the
fault location.
Future grid requires the breaker which can break the
electrical network as well as can limit the short circuit current.
With the advancement in power electronics, the concept of an
SCB and an HCB was introduced. As these devices are costly,
if they also have current limiting ability in them, utilities can
easily start adopting these devices as an alternate solution for
the protection of an electrical network.
Hence, M.M.R. Ahmed et. al. [5] proposed as an SCB
with the current limiting ability. But, due to high on-state
losses during normal operation, use of such devices will be
limited to low voltage level only. However, an HCB with
current limiting ability which is inspired by the SCB topology
described in [5] is shown in Fig. 4(a). The topology can be
again modified to a less number of controlled switches, as
shown in Fig. 4(b). However, for analysis Fig. 4(a) is only
considered. The proposed topology will reduce the on-state
losses considerably during normal operation and will limit the
short circuit current into the hysteresis band before breaking.
ACSo
urc
e
Zs
Mayr Arc Modeled
Circuit Breaker
Lo
ad
Varistor
R_snub C_snub
(a)
ACSou
rce
Zs
Mayr Arc Modeled
Circuit Breaker
Loa
d
Varistor
R_snub
C_snub
(b)
Fig 4: (a) An HCB with current limiting ability. (b) An HCB current limiting
ability and less controlled switches.
III. MATHEMATICAL MODELLING OF AN HCB
ELEMENTS
In this section elements used in the topology shown in
Fig. 4 are mathematically modelled. Each modelled element
explains its characteristics and helps in understanding the
element performance in the topology. The elements used are
the circuit breaker which is modelled using the mayr arc
model, the non-linear resistor (the varistor) which is modelled
using basic equation and the controlled switched (an IGBT)
which are connected in series for bidirectional current flow
with an anti-parallel diodes. The circuit breaker and the
varistor are modelled in this section and the analysis has been
conducted in MATLAB and the simulation results have been
discussed.
A. Non Linear Resistor
A non-linear resistor is a device in which current do not
varies linearly with voltage. This happens because when
current flows through non-linear resistor it produces heat. The
production of heat in a system can make them behave as a
conductor or a resistor. The change in nature of a device due
to the production of heat leads to a non-linear nature of a
device. A mathematical model of non-linear resistor (the
varistor) [6] used in topology as an energy absorber and
suppress the over voltage, which is being discussed further.
The following Fig. 5 shows an application of such a
nonlinear resistance to simulate the varistor used in the
network. Which is enabled us to study the V-I characteristics
of the device. The parameters used for the varistor modelling
in simulation are not the same as what used in a topology
described in next section but are mentioned in Table I. The
study of the varistor is being made to understand the
characteristics of a device. The mathematical equation
governing the varistor is given by [6]:
(
)
(1)
Where,
i, v = Instantaneous current and voltage across the varistor
Vo = Protection voltage
Io = Maximum current
α = Exponent defining the nonlinearity
The i(v) characteristics of the varistor is simulated in
MATLAB/SIMULINK, using user defined function library,
where instantaneous current is a function of voltage. The Fig.
6(a)-(c), shows the current, the voltage and the V-I
characteristics of the varistor. The results shown are simulated
using eulers numerical integration technique and may differ
with different integration technique due to high frequency
transient operation. However, the resultant V-I characteristics
derived from (1) makes it suitable for the use in the topology
used in this analysis.
Table I. Varistor model parameters for simulation
Vo(KV) 1
Io(KA) 6
Α 15
ACSour
ce
Zs
Loa
d
Varistor
Fig. 5: The electrical network with modelled varistor.
(a)
(b)
(c)
Fig 6: (a) Current (b) Voltage and (c) V-I Characteristics of the varistor
B. Circuit Breaker
As stated before, an arc appears across the breaker
contacts when the conventional circuit breaker operates to
interrupt the circuit. There are many methods available in the
literature on arc modelling and the arc model defined by the
mayr arc equation has been adopted from [7] in this paper.
Arc model is simulated to evaluate the interruption
performance of the circuit breaker which is shown in Fig. 3(a).
The simulation model block-set is taken from [8]. Many
aspects can be determined using arc model such as arc voltage,
interrupting success or failure of circuit breaker. The mayr arc
equation describing the arc model is:
(
) (2)
Where,
G = Arc conductance
τ = Arc time constant
E = Arc Voltage
I = Arc current
Po = Arc constant power loss
When a fault is detected, breaking signal is given to the
circuit breaker and breaking commence. Parameters chosen
for this study are same as what chosen for the simulation study
of an HCB, listed in Table II, which break the circuit in one
cycle of natural frequency effectively. Current through
mechanical switch and arc voltage across circuit breaker
contacts are shown in Fig. 7.
Table II. Mayr acr model parameter for simulation
Gi(S) 3.265
Po(W) 645
τ(s) 12e-6
Fig 7: Arc voltage and current of a mayr arc modelled circuit breaker
From the simulation of circuit breaker arc it is clear that
the arc behaves as a non-linear resistance; therefore both arc
current and voltage pass the zero value at the same time. As
the power input into the arc channel is zero at that time, the
current zero crossing is the place where the interruption takes
place.
In the arc model it is clear that the transient recovery
voltage (TRV) builds up over the breaker. This voltage
consists of the 2-parameter TRV and the high frequency
voltage oscillation of the line side, which you can be seen in
the Fig. 7.
IV. SIMULATION STUDY OF AN HCB WITH CURRENT
LIMITING ABILITY
In [5], a fault current limiting and interrupting device was
proposed based on an SCB circuit. The hysteresis current
control method was used to control the power semiconductor
devices for limiting the fault current within a hysteresis band.
In the present paper, an HCB circuit of Fig. 4 (a) is considered
and the fault current is controlled within a limit using
hysteresis current control. This circuit performs as a fault
current limiter as well as a circuit interrupter. The normal
current is carried by the conventional circuit breaker, which is
modelled using mayr arc equation described above. As there is
no current flowing through the power semiconductors devices
in normal operation, the on-state loss in this HCB circuit is
significantly less than that in an equivalent SCB circuit
discussed in [5]. In the event of a fault, a controller shown in
Fig. 8 commutates the current to the parallel branch which
controls the current in a hysteresis band. This parallel branch
comprises of bidirectional IGBT switches with anti-parallel
diodes. These switches and the varistor limit the short-circuit
current within desired limit using hysteresis current control.
When short-circuit current after commutation form the
conventional circuit breaker hits the lower bound of the
hysteresis band, IGBT switches are triggered to provide the
low impedance path and short-circuit current rises. While
short-circuit current hits the upper bound, the gate signal from
the IGBT switches is removed and the varistor insert high
impedance to limit the short-circuit current. As IGBT switches
are turned-off at non-zero current a snubber circuit [9] in
parallel with power semiconductor device is also required to
suppress the over voltage during switching to keep the current
in a desired limit. The varistor, as discussed above in the
previous section, also limit the voltage across the power
semiconductor devices and absorb the circuit inductive energy
when low impedance fault occurs in a network. This proposed
model follows all the desired requirements which are
favourable for the network elements.
A. Working principle of the hybrid fault current limiting and
interrupting device
In Fig. 8, a controller used to control the fault current in
the HCB circuit of Fig. 4 (a) is shown. In the event of low
impedance fault the current starts rising. If the supply current
rises beyond the reference value of the fault current i*, the
comparator output sets high. A memory block is also required
for an AC circuit to perform the fault controlling action
because there will be a natural zero crossing of the AC current
and as the fault current value becomes less then i* the
controller stops the corrective action. The fault signal is sent to
the HCB which triggers the conventional circuit breaker
contacts to start opening. This results in striking of an arc
which causes a high impedance path to start building across
the contacts of mechanical contact. The high impedance starts
building across the conventional circuit breaker and as a result
current commutates to the bidirectional IGBT switches with
anti-parallel diode at current zero. The technique of current
commutation to the power electronics module (Fig. 2) in an
HCB due to arc impedance is termed as impedance injection
technique in an HCB [4]. The fault current continues to build
up through the semiconductor path. At a pre-fixed value Imax
the power semiconductor devices are turned-off which forces
the current to flow through the parallel-connected varistor as
the mechanical switch is open. While the fault current is
flowing through the varistor, the non-linear nature of the
varistor increases the impedance offered by it, as shown in the
simulation study conducted in the previous section, which
helps in reducing the fault current magnitude. It is to be noted
that the parallel connected RC snubber circuit is used to limit
the high voltage spikes across the power semiconductor
devices after they are turned off. Due to the high impedance
offered by varistor, the current reduces and at the magnitude
of Imin the IGBTs are turned on again to allow the fault current
to flow through the semiconductor path again. When the fault
current magnitude again reached Imax, the IGBTs are turned off
again and this entire process is repeated. By following this
sequential process the fault current can be limited to the
maximum value of Imax irrespective of fault location and its
impedance. As shown in Fig. 4 (a), two IGBTs are used in the
HCB circuit. One of the IGBTs is turned-on to conduct
depending on the current direction in the ac circuit. The
current then flows through the antiparallel diode of the other
IGBT which is not turned-on.
i*
Isu
pp
ly
Comparator
Com
para
tor
Imax
Isu
pp
ly
Imin
IGBT
Switches
Gate Signal
Trip signal to
mayr acr modeled
circuit breaker
1Memory
Fig 8: Controller diagram of the fault current limiting HCB
B. Simulation Result
The afore-presented HCB circuit is simulated with the
parameters listed in Table III and control function is
performed by the controller of Fig. 8.
Table III. HCB circuit parameter for simulation
Gi(S) 3.265
Po(W) 645
τ(s) 12e-6
R_snub(Ω) 15
C_snub(f) 0.06e-6
Vo(V) 650
Io(A) 150
α 25
Figs. 9(a) and (b) show the supply current and current
through the power semiconductor devices path. The varistor is
connected across the power semiconductor devices which
serve two purposes; first it maintains the voltage across the
power semiconductor devices under limit which is shown in
Fig. 9(c) and second it provides high impedance path to
maintain hysteresis operation. Base values of current and
voltage are i* and maximum permissible voltage across power
semiconductor switches, respectively. The simulation results
confirm the operation of the proposed hysteresis based current
controller for an HCB. It is important to note that the
parameters used in TABLE III for the varistor model is
different from TABLE I parameters but the V-I characteristics
obtained with the TABLE III model is similar to the
previously simulated varistor model characteristics as shown
in Fig. 9(d). Another important observation is that, even
though with the same parameters for mayr arc model current
in an HCB commutates in half cycle (Fig. 9(a)) while current
in Fig. 7 clearly indicates breaking in one complete cycle of
the natural frequency. Therefore the impedance injection
breaking technique also proved in this simulation study in an
HCB.
(a)
(b)
(c)
(d)
Fig 9: (a) Supply current in the electrical network. (b) Current through bidirectional IGBT switches with antiparallel diode. (c) Voltage across an
HCB. (d) V-I characteristics of the modelled varistor in an HCB
V. CONCLUSIN
Hybrid circuit breaker can be a feasible option for many
applications as it offers least losses in normal conducting
mode and also performs breaking action very fast with no or
very less arcing across the mechanical contacts. An HCB is
presented in this paper which can also be controlled to limit
the fault current within a desired hysteresis limit. It uses of the
varistor and the snubber circuit with controlled switching of
the power semiconductor devices to limit the current within
the hysteresis band. The modelling of the varistor and the
conventional circuit breaker used in the simulation study is
presented. The Modelling of the varistor explained its non-
linear nature. The modelled circuit breaker using mayr arc
model verified the successfulness of breaking as well as
proved the non-linear resistive nature of an arc while breaking.
The working principle of controller used to perform the
desired operation is also presented. Simulation results
obtained have verified the viability and effectiveness of the
proposed controller for the HCB circuit. This device is
expected to be loss efficient, more reliable and able to perform
both the fault current limiting and breaking action. Therefore
capable to meet the future needs from the circuit breaker of the
complex and interconnected power system.
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