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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