direct current hybrid breakers

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DIRECT CURRENT HYBRID BREAKERS:

A DESIGN AND ITS REALIZATION


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Cover: The Hindu temple in Lake Bratan, Bali, Indonesia.


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DIRECT CURRENT HYBRID BREAKERS:

A DESIGN AND ITS REALIZATION

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de

Technische Universiteit Eindhoven, op gezag van de

Rector Magnificus, prof.dr. M. Rem, voor een

commisie aangewezen door het College voor

Promoties in het openbaar te verdedigen

op donderdag 4 Mei 2000 om 16.00 uur

door

Ali Mahfudz Surya Atmadji

geboren te Semarang, Indonesi


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iv

Dit proefschrift is goedgekeurd door de promotoren:

prof. ir G.C. Damstra

en

prof. dr.-ing. H. Rijanto

CIP-DATA LIBRARY TECHNISCHE UNIVERSITEIT EINDHOVEN

Atmadji, Ali M.S.

Direct current hybrid breakers : A design and its realization / by Ali

M.S. Atmadji. - Eindhoven : Technische Universiteit Eindhoven, 2000.Proefschrift. - ISBN 90-386-1740-2

NUGI 832

Trefw.: kortsluitingsbeveiliging / kortsluitstromen /

vacuumschakelaars / elektrische schakelaars.

Subject headings: current limiters / short-circuit currents /

vacuum circuit breakers / switchgear testing.

Copyright 2000 by A.M.S. Atmadji, Departement of Electrical Engineering, EindhovenUniversity of Technology, Eindhoven, The Netherlands.


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v

Do not believe in anything simply because you have heard it. Do not

believe in anything simply because it is spoken and rumored by many. Do

not believe in anything simply because it is found written in your religious

books. Do not believe in anything merely on the authority of your teachers

and elders. Do not believe in traditions because they have been handed

down for many generations. But after observation and analysis, when you

ind that anything agrees with reason and is conducive to the good and

benefit of one and all, then accept it and live up to it.

Buddha

To my parents

Samoeri and Koestiwati


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vi

Summary

The use of semiconductors for electric power circuit breakers instead of conventional breakers

remains a utopia when designing fault current interrupters for high power networks. The major

problems concerning power semiconductor circuit breakers are the excessive heat losses and their

sensitivity to transients. However, conventional breakers are capable of dealing with such matters. Acombination of the two methods, or so-called hybrid breakers, would appear to be a solution;

however, hybrid breakers use separate parallel branches for conducting the main current and

interrupting the short-circuit current. Such breakers are intended for protecting direct current (DC)

traction systems. In this thesis hybrid switching techniques for current limitation and purely solid-

state current interruption are investigated for DC breakers.

This work analyzes the transient behavior of hybrid breakers and compares their operations with

conventional breakers and similar solid-state devices in DC systems. Therefore a hybrid breaker was

constructed and tested in a specially designed high power test circuit. A vacuum breaker was chosen

as the main breaker in the main conducting path; then a commutation path was connected across the

vacuum breaker where it provided current limitation and interruption. The commutation path

operated only during any current interruption and the process required additional circuits. These

included a certain energy storage, overvoltage suppressor and commutation switch. So that when

discharging this energy, a controlled counter-current injection could be produced. That counter-

current opposed the main current in the breaker by superposition in order to create a forced current-

zero. One-stage and two-stage commutation circuits have been treated extensively.

This study project contains both theoretical and experimental investigations. A direct current short-

circuit source was constructed capable of delivering power equivalent to a fault. It supplied a direct

voltage of 1kVDCwhich was rectified having been obtained from a 3-phase 10kV/380V supply. The

source was successfully tested to deliver a fault current of 7kA with a time constant of 5ms. The

hybrid breaker that was developed could provide protection for 750VDC traction systems. The

breaker was equipped with a fault-recognizing circuit based on a current level triggering. An

electronic circuit was built for this need and was included in the system. It monitored the system

continuously and took action by generating trip signals when a fault was recognized. Interruption

was followed by a suitable timing of the fast contact separation in the main breaker and the current-zero creation. An electrodynamically driven mechanism was successfully tested having a dead-time

of 300s to separate the main breaker contacts. Furthermore, a maximum peak current injection of

3kA at a frequency of 500Hz could be obtained in order to produce an artificial current-zero in the

vacuum breaker. A successful current interruption with a prospective value of 5kA was achieved by

the hybrid switching technique. In addition, measures were taken to prevent overvoltages.

Experimentally, the concept of a hybrid breaker was compared with the functioning of all

mechanical (air breaker) and all electronical (IGCT breaker) versions. Although a single stage

interrupting method was verified experimentally, two two-stage interrupting methods were analyzed

theoretically.


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vii

Samenvatting

Het gebruik van halfgeleider schakelaars om conventionele schakelaars te vervangen blijft een

utopia voor de foutstroom onderbreking in elektrische netten. Voor de halfgeleider

stroomonderbrekers zijn er beperkingen zoals het grote warmte verlies en de gevoeligheid voor

transienten waar conventionele schakelaars juist heel goed tegen bestand zijn. Samenstellingen van

beide soorten schakelaars noemt men hybride schakelaars. Hybride schakelaars maken gebruik van

twee afzonderlijke paden; voor de doorgaande nominale stroom en voor de foutstroom

onderbreking. Zulke schakelaars zijn grotendeels bedoeld voor de beveiliging van tractiesystemen.

In dit proefschrift zijn hybride technieken voor de stroombegrenzing en volledige halfgeleider

stroomonderbreking behandeld voornamelijk in gelijkstroom circuits.

In dit werk wordt een analyse gepresenteerd van het transient gedrag van hybride schakelaars en

worden hun functies vergeleken met conventionele en halfgeleider schakelaars. Een ontwerp voor

een hybride schakelaar is gerealiseerd en beproefd in een hiertoe opgebouwd gelijkstroom test

circuit. Een vacuum schakelaar is gekozen als de hoofdschakelaar in het hoofdpad. Hieraan parallel

is een commutatie pad aangebracht dat voorziet in stroombegrenzing en stroomonderbreking. Het

commutatie pad wordt alleen gedurende een stroomonderbreking bedreven om de commutatie van

de hoofdstroom mogelijk te maken. Het commutatie proces vereist componenten voor het opslaan

van energie en het onderdrukken van overspanningen. Door het vrijgeven van opgeslagen energie

kan een gecontroleerde tegenstroom injectie worden bewerkstelligd. Deze tegenstroom forceert een

stroom nuldoorgang in het hoofdpad. Een en twee-trap commutatie circuits zijn vergeleken.

Het onderzoek bevat zowel theoretisch als experimenteel werk. Een gelijkstroom circuit is gebouwd

om de kortsluitstroom te leveren van 7kA met een tijdkonstant 5ms. De bron heeft een nominale

spanning van 1kVDC door gelijkrichting van twee distributie transformatoren (10kV/380V). Het

ontwerp van de hybride schakelaar is gericht op toepassing voor het beveiligen van 750VDCtractie

systemen. De schakelaar is uitgerust met een foutdetectiesysteem gebaseerd op een stroomlevel trip.

De stroom in het circuit wordt bewaakt waarbij uitschakel commando gegenereerd wordt zodra de

stroom in het circuit de ingestelde waarde overschrijdt. Het feitelijke onderbrekingsproces wordt

bepaald door de snelheid van contactscheiding in de vacuum schakelaar en het creren van de

benodigde nuldoorgang. Een snelle contactscheiding na ongeveer 300s is gerealiseerd met een

elektrodynamische aandrijving. Een injectie stroom met een frequentie van 500Hz en amplitude

3kA is gebruikt voor het creren van de nuldoorgang in de vacuum schakelaar. Een successvolle

stroomonderbreking van een prospective gelijkstroom van 5kA is met de hybride techniek

gerealiseerd. Bovendien is een geschikte overspanning onderdrukking bereikt. Het hybride concept

is experimenteel vergeleken met volledig mechanische en volledig electronische (IGCT)

schakelaars. Terwijl alleen de een-trap commutatie circuit ook experimenteel is uitgevoerd, zijn 2

twee-trap commutatie circuits alleen theoretisch geanalyseerd.


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viii

CONTENTS

Summary vi

Samenvatting vii

1 Concepts of direct current limitation and interruption ........................ 11.1 Introduction ............................................................................................................. 1

1.2 Current limiting and interrupting techniques ........................................................... 3

1.2.1 Conventional direct current air breakers ....................................................... 3

1.2.2 Current Limiting Fuses ................................................................................. 6

1.2.3 Pyrotechnique ................................................................................................ 6

1.2.4 Positive Temperature Coefficient Resistors (PTCR) ..................................... 7

1.2.5 Superconducting Current Limiter (SCCL) ..................................................... 8

1.2.6 Solid-state breakers (SSBs) ......................................................................... 9

1.3 Hybrid switching techniques ................................................................................... 10

1.4 Outline of thesis ....................................................................................................... 14

1.5 References and reading lists .................................................................................... 14

2 Analysis of commutating circuits for hybrid breakers .......................... 192.1 Introduction ............................................................................................................... 19

2.2 Analysis of the active commutation circuit ............................................................... 22

2.3 Dimensions for the components of the parallel circuit ............................................. 32

2.4 Simulating one-stage interruptions using MATLAB ................................................ 35

2.4.1 Successful interruption using a bi-directional switch .................................. 38

2.4.2 Successful interruption at the first current-zero using a uni-directional

switch ........................................................................................................... 392.4.3 Successful interruption at the second current-zero using a uni-directional

switch ........................................................................................................... 40

2.4.4 Unsuccessful interruption ............................................................................. 40

2.5 Protection against excessive overvoltages ................................................................ 41

2.5.1 Linear energy absorbing devices as the primary protection ......................... 44

2.5.2 Non-linear energy absorbing elements as the secondary protection ............ 45

2.5.3 Snubber circuits as the tertiary protection .................................................... 46

2.5.4 Applications of the freewheeling diode ....................................................... 48

2.5.5 Combining all the components ..................................................................... 48

2.6 Circuit simulation using PSPICE .............................................................................. 492.6.1 Device modelling ......................................................................................... 49

2.6.2 Simulation diagram ...................................................................................... 51

2.6.3 Simulation results using PSPICE ................................................................. 52

2.7 Conclusions ............................................................................................................... 57

2.8 References and reading lists ...................................................................................... 57

3 Two-stage commutation circuits for direct current interrupters ......... 593.1 Introduction .............................................................................................................. 59

3.2 Basic principles of the first variant .......................................................................... 61

3.3 Basic principles of the second variant ...................................................................... 67

3.4 Computer simulation using PSPICE ......................................................................... 72

3.4.1 The short-circuit simulation of a DC source with a prospective current of

10kA .............................................................................................................

73


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3.4.2 The one-stage DC interruption of 10kA with Itrip=5kA ................................ 74

3.4.3 The first variant of two-stage DC interruption with Itrip=5kA ...................... 75

3.4.4 The second variant of two-stage DC interruption with Itrip=5kA ................. 76

3.5 Conclusions .............................................................................................................. 77

3.6 References and reading lists .................................................................................... 77

4 Fault identification and direct current measurement ........................... 794.1 Introduction .............................................................................................................. 79

4.2 Realization of a detection circuit .............................................................................. 80

4.3 Direct current transducers ......................................................................................... 81

4.4 Rogowski-coils as current transducers ..................................................................... 84

4.5 Conclusions .............................................................................................................. 88

4.6 References and reading lists ..................................................................................... 88

5 Fast electrodynamic drives for the hybrid breaker ............................... 91

5.1 Introduction .............................................................................................................. 915.2 Description of the electrodynamic drive system ....................................................... 92

5.3 Mathematical analysis of the electrodynamic drive system ...................................... 94

5.3.1 Analysis of the electrodynamic drive using the coupled coils theory .......... 95

5.3.2 Analysis of the electrodynamic drive using equivalent lumped parameters 103

5.4 Comparison between simulation and measurement results ...................................... 107

5.5 Conclusions .............................................................................................................. 116

5.6 References and reading lists ..................................................................................... 116

6 Test circuit for DC breakers ..................................................................... 1196.1 Introduction ............................................................................................................... 119

6.2 Analysis of rectifier circuits for a direct current short-circuit source ....................... 120

6.2.1 One 3-phase rectifier .................................................................................... 121

6.2.2 Two 3-phase rectifiers in series .................................................................... 123

6.3 Realization of the direct current short-circuit source (DCSCS) ................................ 129

6.3.1 Sequential timing operation ......................................................................... 130

6.3.2 Overvoltage suppression .............................................................................. 131

6.3.3 Surge phase-currents in the transformer secondary when switching-on ...... 134

6.3.4 Overcurrent protection by I2t fusing ............................................................. 138

6.3.5 Protection from overheating.......................................................................... 139

6.4 Simulation results ...................................................................................................... 140

6.4.1 Simulation of a 10kA prospective short-circuit current ............................... 1416.4.2 A short-circuit current directly after the bridge ............................................ 142

6.5 Measured and simulated results ................................................................................ 143

6.5.1 An open circuit test ...................................................................................... 144

6.5.2 Short-circuit test ........................................................................................... 144

6.6 Conclusions ............................................................................................................... 145


7 Experimental and modelling results ........................................................ 1497.1 The air breaker experiment ....................................................................................... 149

7.2 The hybrid breaker experiment ................................................................................. 151

7.2.1 Hybrid breaker test without anti-parallel diode across the vacuum breaker 151

7.2.2 Hybrid breaker test with anti-parallel diode across the vacuum breaker ..... 156

7.3 The solid-state breaker experiment ........................................................................... 159


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7.2.1 A brief description of the Integrated Gate Commutated Thyristor (IGCT) .. 160

7.2.2 Experimental and simulated results using IGCT .......................................... 161

7.4 Conclusions ............................................................................................................... 164


8 General conclusions and future developments ....................................... 1678.1 General conclusions .................................................................................................. 1678.2 Future developments ................................................................................................. 169

Appendix A ....................................................................................................... 171

List of symbols ................................................................................................. 173

Acknowledgements .......................................................................................... 177

Biography ......................................................................................................... 179


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Chapter 1

Concepts of direct current limitation and interruption

Abstract

This chapter presents an overview of available electric current limitation and interruption

techniques for protecting direct current systems. Some of them were installed in networks for long

periods while others are still in the development stage. Attention was focused on hybrid switching

techniques which were the subject of this study. Finally, the form of this thesis is discussed.

1.1 Introduction

Faults in electric currents impose severe thermal and mechanical stresses on electrical systems and

their related apparatus and the severity depends on the peak current value and the time of the

interruption. Thermal overloading can result in the burning of lines or cables, while electrodynamic

forces can deform bus bars or the coils of reactors and transformers. Moreover, arcing resulting

from a fault can initiate explosions. Protection against such events is usually provided by installing

circuit breakers or current limiters in the line to be protected. A conventional AC circuit breaker is

capable of conducting high continuous currents and has a substantial short-circuit interrupting

capacity; but it is not able to perform current limitation at nominal high current ratings. On the otherhand, fuses which are the best known current limiting devices, have a relatively low continuous

current rating. Due to this contradictory situation, an ideal circuit breaker should have the following

features which are difficult to combine into one concept:

fast breaking action (at earliest current-zero);

minimal arcing after contact separation (to reduce contact erosion);

minimal conduction losses (a small voltage drop across the contacts);

reliable and efficient protection against all types of faults;

repetition of switching operation (allowing contacts to reclose after a fault clearance);

prevention of excessive overvoltage (during operation).While these features are applicable for all circuit breakers, the task of direct current breakers is even

heavier because current limitation is required in the absence of current-zeros.

Direct current (DC) can be used for a large voltage range. According to the provisions of standards,

DC voltages are classified as low voltages (LV) up to 1200V (for instance, urban vehicles use

750V), systems for 1500V and 3000V are generally referred to as medium voltages (MV) and high

voltage (HV) is up to 1500kV. High voltage direct current (HVDC) technology applies especially to

high power transmission lines and for the back-to-back stations of AC systems. In the medium

voltage range, direct current is used principally in electric traction, electric heating devices andsome drives. In the low voltage range, direct current is used for most kinds of urban and mine

electric traction, in various drives and converter systems. Short-circuit parameters for specific


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2 Chapter 1

circuits are very different. Time constant values in HV circuits generally are rather high. In LV and

MV circuits, time constants are in the range of 5 to 30ms, prospective short-circuit currents are in

the range of 10 to 150kA, initial rates of current rise in the range of 0.5 to 15A/s and magnetic

energy of the short-circuits in the range of 5 to 30kJ [1.1].

Current interruption in DC systems is more problematic than in AC systems since there is no natural

current-zero available and the magnetic energy stored in the circuit inductance has to be dissipated.

Breakers must not only be able to interrupt but also to reduce the current to zero within a certain

time [1.2,3,4,5]. During the interruption process, an excessive high voltage should not be created in

the system.

A current-zero can be created in two ways. The first one is the traditional method used in DC

circuits: a switching device develops arc voltages significantly in excess of the system voltage. The

second method creates a virtual current-zero by producing a counter-current from auxiliary

commutation circuits. This counter-current is usually provided by a capacitor bank. The diagram in

Figure 1.1shows the classification of fault clearances in DC systems [1.4].

Inverse voltage method

DC Interruption

Current Limiting

Current commutation method

Current Oscillation

Self oscillation

Arc Switches & LC orRLC (active)

LC+Arc(passive)

Forced oscillation

Impulse circuitLC & Switches

R+Arc+Switches

FusesExplosive chargefuses

Non-linear materialor devices

ConventionalDC and HVDCbreakers

PTC-resistorSuperconductor

Hybrid breakersPure solid-statebreakers

Unknown

Figure 1. 1 Classification of DC interrupting methods; where PTC: Positive Temperature Coefficient,

R: resistor, RLC and LC: oscillating loops with and without damping.


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Concepts of direct current limitation and interruption 3

1.2 Current limiting and interrupting techniques

A current limiting device can be seen as a series of elements in the line; they offer low impedance to

the load current and high impedance to the fault current. In principle, it is not necessary for the

current limiting device itself to create the final current-zero. An auxiliary interrupter can be

connected in series in order to interrupt the limited current. In the following sub-sections, a number

of current limiting techniques are summarized.

1.2.1 Conventional direct current air breakers

Classical direct current interruption utilizes arc plasma in order to build up the inverse voltage

opposing the supply voltage for the current-zero creation. In the closed position, conventional

mechanical breakers are able to conduct high continuous currents with low power dissipation. In the

open position, these breakers provide excellent isolation. During the switching process, the arc

plasma causes contacts to erode and it generates noises and hot gasses. Moreover, these switches

generally react slowly. Hence, they hardly limit the maximal fault current, due to their slow opening

and long arcing times which together take longer than 20ms, which is usually above the time

constant of a circuit.

Interrupting DC is accompanied by different phenomena depending on the systems parameters and

the exact location of the breaker. For example, see Figure 1.2, for a given simple 1kV DC system

containing the total lumped resistance RS=100m and inductance LS=400H with a breaker and

load. In the closed position, the breaker has a low resistance. During a fault, the current has to be

interrupted. The prospective short-circuit current is 10kA. The fault is distinguished from a normal

current load by the setting of a trip current value. As soon as the current exceeds that trip value, the

electromagnetic device in the circuit breaker (CB) separates the contacts creating an arc between the

electrodes.

LoadV

CBi(t)

LS R

S

ES

Figure 1. 2 A typical DC system with a conventional breaker.

The current can be reduced to zero only if the breaker can generate and maintain a switching arc

voltage of VCB that ishigher than the systems voltage ESfor long enough. While this occurs, the


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4 Chapter 1

breaker dissipates the inductive energy and any excess energy delivered by the source during the

interruption process. Obviously, this method is suitable for conventional air breakers. Figure 1.2

shows the application of a conventional breaker in a simple circuit.

Much depends on the way that the switching arc voltage VCB is generated, this may be represented

by a function of several different quantities, such as: current, time derivative, stored magnetic

energy, time, etc. The equation for voltages in the circuit (Figure 1.2) is given by the expression:

E V V VS R L CB= + + (1.1)

where: V R i t R S= , V Ldi

dtL S= and V f i

di

dti dt t CB=

, , , .

During the switching process, the energy stored in the system must be dissipated by the circuit

resistance and the breaker. The energy dissipated in the resistance is calculated by:W R i dt R S= 2 (1.2)

And the arcing energy is given by the relationship:

W V i dt CB CB= (1.3)The let-through energy integral for the breaker can be computed using the expression:

i dt i t 2 2 .

To demonstrate the interruption process, a switching arc voltage VCB across the breaker was

represented empirically by some idealized algebraic functions, in order to simulate the relationship

between the voltage across the breaker and the current through it. The trip current for opening the

breaker was set to 2kA. After a successful interruption, a transient recovery voltage appears across

the breaker. Now two cases: A and B, for empirical switching arc voltage traces will be presented

(Table 1.1).

Table 1. 1Switching arc voltage patterns.

Case A Case B

V t

S t t

t t

t t tCB

=

0 0

1

1

1 2

V t

S t t

t t

t t tCB

=

0

3

0

1

1

1 2

where: t1 is tripping time, t2 is current-zero time and Sis the slope of the switching arc voltage. The

interruption time is defined as the time difference between t2 and t1 . In case A, the rate of change of

the switching arc voltage Swas about 250V/ms which is a typical value for conventional breakers.

The switching arc voltage increased and suppressed the current within 6.8ms, see the left hand

column of Figure 1.3. In case B, the switching arc voltage grew three times faster (750V/ms). The

interrupting time then became 2.95ms, see the right hand column of Figure 1.3. The energy balance

for both cases can be calculated too as shown below the current and voltage graphs.


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Case A Case B

ICBVCB

0 2 4 6 8 100

500

1000

1500

2000

2500

3000

3500

4000

time [ms]

Current[A],Voltage[V]

Current and voltage

ICBVCB

0 2 4 6 8 10-500

0

500

1000

1500

2000

2500

3000

time [ms]

Current[A],Voltage[V]

Current and voltage

WTotWRWCB

0 2 4 6 8 100

5

10

15

20

25

time [ms]

Energy[kJ]

Energy balance

WTotWRWCB

0 2 4 6 8 100

1

2

3

4

5

6

7

8

time [ms]

Energy[kJ]

Energy balance

Figure 1. 3 DC interruption for different patterns of the arc voltage with a trip current I trip=2kA;

where: ICBand VCBare the current in and voltage across the breaker and W Tot, WRand WCBrepresent the

energy dissipated during the interruption; due to the total, line resistance and in the breaker, respectively.

From these results, it can be seen why the interruption must not be too fast because it caused high

surge voltages and not too slow because it caused long energy dissipation times that might damage

the contacts. The simulated results are summarized in Table 1.2.

Table 1. 2 The energy balance of the interruption;

WRand WCBfor the dissipated energy in the line resistance and the breaker respectively.

WR

[Joule]

WCB

[Joule]

tint

[ms]

Imax

[A]

Case Awith S=250V/ms 6322 13909 6.8 3929

Case Bwith S=750V/ms 1586 5576 2.95 2807

Adequate current limiting capacity could be achieved by minimizing the arcing time and generating

switching voltages 1.1 to 1.5 times higher than the supply voltage. This was possible by using a

special cooling mechanism to destabilize the arc plasma. Clearly, the appearance of switchingvoltages across the breaker could cause energy dissipation through the arcing process. It was


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6 Chapter 1

released mainly as heat to the surroundings. At the same time, this energy could damage and

corrode the contacts, thus shortening the breaker life and reducing its interrupting capacity.

In this thesis, a conventional air breaker was investigated and it is described in Chapter 7.

1.2.2 Current limiting fuses

The simplest current limiting device is the fuse [1.6], which is able to conduct a continuous current,

sense a fault automatically, limit the current, dissipate the energy and interrupt the fault. Current

limiting fuses unlike circuit breakers become operational before the short-circuit current reaches a

prospective peak value and thereby they effectively limit the let-through current to lower values.

Due to this current limiting action and the subsequent rapid interruption and isolation of the circuit,

thermal and electrodynamic effects on components of the circuit are reduced to a minimum. The

most important characteristics of fuses are low continuous current ratings, small size, cheapness and

suitability for both low and medium voltage AC and DC systems.

Self-recovery fuses based on sodium (Na) have been developed in the past [1.7], but so far further

development is uncertain; however, it should be noted that the fuse arc voltage will be

superimposed on the system and must, therefore, be limited to avoid excessive overvoltages. On the

other hand, energy considerations and fuse size limitations require that the fuse arc voltage is

typically up to twice the line to neutral voltage. Disadvantages of a fuse are its limited continuous

current rating and the need for replacement after each fault. These shortcomings can be partly

overcome by using triggerable current limiting devices with a parallel path for the continuous

current.

1.2.3 Pyrotechnique

By separating the continuous current conduction and interruption duties of triggerable current

limiting devices, the fusible element is shunted by a link which can be removed as required. Theinterrupting duty is provided by a fuse once the current has been commutated from the shunting

device. In principle, such breakers consist of two main components, firstly a special copper

conductor which can carry large continuous currents during normal operation, but it can be sheared

at high speed by a pyrobreaking technique when overloaded, and, secondly, current limiting fuses

which are mounted in parallel with the large continuous current conductor [1.8]. A diagram of this

device is depicted in Figure 1.4.


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Main conductor

Control Circuit

I

I dI/dt

Fuse

Figure 1. 4 Schematic of the Pyrotechnique

The main current conductor is broken by the pyrotechnique mechanism which is triggered by a

control circuit fed by a sensor system. The pyrotechnique mechanism contains an explosivechemical charge. After that charge explodes, the current commutates to the fuse for controlling

current limitation and interruption. System parameters, such as the current and current slope , are

monitored using appropriate sensors. The signal generated by the sensors is compared with a preset

reference value in the control circuit and this can trigger the chemical charge. The pyrotechnique

circuit interrupter is very useful for protecting electrical systems with high continuous currents

(>5kA) when rapid interrupting is required. Clearly, this device can not be reset. The recovery time

is long and the cost of the replacement is high.

Several manufacturers deliver pyrotechnique products [1.8,9,10]. Their main uses are in medium

voltage AC networks although they can be used in DC systems too (but not for traction, due to fast

reclosing requirements).

1.2.4 Positive Temperature Coefficient Resistors (PTCR)

A composite of polymer and metal forms the main part of this device which like fuses, is inserted in

the line where it carries a rated current continuously. In such situations, the ohmic losses are

sufficiently low in order to prevent any real resistance increase. When the current suddenly

increases, the internal heating will exceed the natural cooling capability. If the temperature of the

polymer resistor increases above a critical limit, its resistance changes stepwise up to ten orders of

the normal magnitude (Figure 1.5). Consequently, the current is limited. A load breaker then

interrupts the final current. When the internal temperature returns to the ambient temperature, the

PTCR resumes normal service after the short-circuit has been removed; therefore, this device can be

used repetitively.


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8 Chapter 1

20 40 60 80 100 120 140

10-2

100

102

104

106

108

Temperature [C]

Spec.

Resistivity[cm]

Figure 1. 5 Specific resistivity as a function of temperature TiB2.

These devices have been tested in 220V AC networks [1.11] when a prospective current of 16kA

can be reduced to just 3kA. Recently, a device for 12kV networks was announced [1,12] for

repetitive current limitation of prospective currents of 4 to 14kA within 1ms. Their reliability and

economy are not yet generally accepted and suitability for DC networks is also unknown as yet.

1.2.5 Superconducting Current Limiters (SCCL)

Because superconducting materials operate below the ambient temperature they require cooling in

order to maintain their superconducting properties. A fault current brings the superconducting

material to its normal resistive state which limits the fault current to an acceptable level. Basically,

two methods are employed; firstly, the so-called resistive method which uses a superconducting

element for transferring the fault current to a shunt resistor thereby limiting it, see Figure 1.6(a).

Secondly, a system of coupled coils in which the secondary winding is connected to the

superconducting material, see Figure 1.6 (b). This is also known as the inductive method

[1.13,14,15].

Rshunt

iline

Cryogenic shield

Load breaker

(a)iline

Cryogenic shield

Load breaker

Lprim

Lsec

(b)

Figure 1. 6Schematic of the superconducting current limitation types; (a) resistive (b) inductive.

Resistive type

Commutating the current is accomplished by switching the superconducting element from a state of

zero resistance (this occurs below the critical temperature TC) to its resistive state, by increasing thetemperature above TC. The critical temperature TCdepends on the superconducting material. Under

normal conditions, the load current flows through the superconductor but after a fault, the resistance


21/195


of the superconductor becomes much greater than the shunt resistance. So, after commutation only a

small current flows in the superconducting element. A load breaker for fully rated continuous

current can finally interrupt the limited current.

Inductive type:Under normal conditions, coupled coils consisting of a normal conducting primary coil and a

superconducting secondary coil act as a short-circuited transformer, so that a low impedance is

introduced into the primary circuit. But when the current exceeds a certain value, the current

induced in the secondary coil becomes too high, resulting in a change in the state of the

superconducting material. A high impedance value will then appear on the primary side and it limits

the fault current. Finally, a load breaker can disconnect this current. The secondary side can also

include a stack of short-circuit rings composed of superconducting material [1.16].

These interrupting devices have the following advantages : there are no moving parts; and there are

low losses, but the main drawback is their need for permanent cooling. Apparently, the

superconducting current limiter may become economically attractive for medium voltage AC

networks; however, the inductive type is unsuitable for DC systems.

1.2.6 Solid-State Breakers (SSB)

Since the invention of power semiconductors (power diode, thyristor, GTO-thyristor, power

transistor, IGBT, power MOSFET, and recently the IGCT), these components have been considered

for load switching in power networks [1.17,18,27,30,32,37]. Power semiconductor switches provide

a fast acting arcless mechanism with great reliability and reduced maintenance. There are some

disadvantages, however, such as their sensitivity to transient overvoltage and overcurrent. Such

transients can break down the junctions of power semiconductors. Also the power losses in them

can be relatively high which will limit their current ratings. Effective cooling is required too. Figure

1.7 shows an overview of the voltage-current-range capacities of solid-state devices [1.28]. The

IGCT has a rating comparable with the GTO.

101

102

103

104

101

102

103

104

SCR

GTO-thyristor

HPBT

IGBTSIT

MOSFET

Current maximum [A]

Figure 1. 7Application ranges of power semiconductor devices

where SCR: Silicon Controlled Rectifier or Thyristor; GTO-thyristor: Gate Turn-Off thyristor;

HPBT: High Power Bipolar Junction Transistor; IGBT: Insulated Gate Bipolar Transistor;

SIT: Static Induction Transistor; MOSFET: Metal Oxide Semiconductor Field Effect Transistor;


22/195

10 Chapter 1

As a controllable solid-state switch with its highest rating for forward currents and blocking

voltages, the thyristor is still invincible, followed by the GTO-thyristor and the IGCT. Since such

devices are controlled by currents, they can be unsuitable for some applications. Transistor-based

devices which are controlled by voltage are faster, but generally, they have much lower current

ratings and blocking voltages. Figure 1.8shows a general application of solid-state breakers with

auxiliary protective devices.

Commutation Circuit

Voltage Limiting Element

Control Circuit

I

SSB dI/dt

Snubber Circuit

I

Figure 1. 8 Schematic of the Solid-state breaker (SSB).

Research and testing of breakers based on pure solid-state switches have been reported in many

papers both for AC and DC systems. Basically, two methods are known; one and two-stage

interruptions. One-stage interruption is the commonest type where the interruption process can be

difficult, because the device must reduce the overcurrent to zero [1.19,20,21,22,33,43,49,56].

During this process, the solid-state switches may undergo stresses and not be able to interrupt the

current, particularly in high voltage or high current systems. A combination of both series and

parallel arrangements of the solid-state switches may help solve the problem. However, a new

problem arises, that is, the sharing of voltages and currents among those switches. On the other

hand, two-stage interruption facilitates the interruption process by firstly reducing the fault current

to a much lower value after which the current is interrupted in the second stage [1.23].

In this thesis, a new solid-state device (IGCT) was investigated and the results are presented in

Chapter 7.

1.3 Hybrid switching techniques

Purely mechanical and solid-state breakers have both positive and negative points. Table 1.3

summarizes and compares a number of breaker features.


23/195


Table 1. 3 Comparison of mechanical and semiconductor breakers.

Feature mechanical breaker semiconductor breaker

Switching mechanism metallic contact and arc PN-junction

Contact resistance - m few m

Power loss very small relative high

Voltage drop at rated current less than 10mV 1-2V

Galvanic isolation Yes No

Isolation capability very high limited (sensitive for overvoltage)

Overload capability very high limited by I2t value

Delay/response time few ms-20ms few s

Life expectancy limited by contact erosion theoretically unlimited

Contact reliability high very high

Frequent switching ability high very high

Surge capabilities high limited (device dependence)

Overvoltage protection not necessary snubber circuit/varistor

Size & volume compact and small relatively big due to cooling being

necessary

Maintenance necessary not necessary

Cost relatively low relatively high

Integrating solid-state devices with a mechanical breaker in a combined configuration is called the

Hybrid Switching Technique(HST) [1.24,25,26,31,35,36,40,42,44,48,49]. Intentionally, the positive

points from each method are retained and the negative points are eliminated. As a result of the fast

actions of semiconductors, the moving mechanism of the main contact is critical. The hybrid

switching technique is very suitable for limiting currents especially for repetitive use.

Generally, within a hybrid switching system, two different mechanical switches are incorporated; a

main breaker and an isolation switch; the main breaker is accompanied by a solid-state switch inparallel. The main breaker provides a path for the continuous current, while the isolation switch

allows dielectric separation of the load after a current interruption. The solid-state switch will

operate only when the main current has to be interrupted. Figure 1.9shows the basic components of

hybrid switching. A commutation path is connected in parallel with the main breaker, it includes a

snubber circuit as a transient suppressor and a voltage limiting element as an energy absorber.

During normal operation, the snubber circuit and voltage limiting element provide high impedance

paths. The commutation path is introduced by solid-state switches and only operates during the

interruption process. All the switches are controlled by electronic circuits.


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12 Chapter 1

Snubber Circuit

Isolation Switch

Voltage Limiting Element

I

CommutationCircuit

Main Breaker

Solid-stateSwitch

Figure 1. 9 Basic components of hybrid switching techniques.

The fact that the reaction times of solid-state switches are much quicker than those of the

mechanical ones, means that the mechanical drive of hybrid breakers must be as fast as possible

[1.53]. The higher the rated current, the greater the mass of the mechanism that is needed. Also, the

main breaker MB must be able to maintain insulation at the time of the first current-zero event;

consequently, a vacuum breaker is most suitable because of its excellent insulating properties after

the current-zero. For the development of a high-speed current limiting circuit breaker based on

hybrid switching techniques, the features needed are listed in Table 1.4[1.29,50,52].

Table 1. 4 Design requirements for hybrid breakers.Subject Purpose Methods

High-speed operation fast fault detecting time suitable criteria for faults in a certain

network based on parameters i , di/dtfast main breaker MB opening

time adoption of a fast electrodynamic drive

system

decrease the entire mass of the movingpart of the MB

High-current interruption fast current commutation from

main breaker MB to

commutating path

reduction of circuit inductance on thecommutation path

increase the arc voltage in the MB

adaptation of main breakerMB and commutating devices

application of fast switches forinitiating the counter-current, (high

di/dt and dv/dt capabilities)

limitation of the overvoltage

during the interruption using proper overvoltage protection

devices (snubber and non-linear

resistance)

free-wheeling diodes to absorb the load-stored inductive energy

increase the capacitance value anddecrease its initial voltages


25/195


Economic considerations will follow these engineering design aspects of hybrid breakers in the

field. Investigations of contact erosion with HST are reported in [1.34,38], whilst the role of ZnO as

a voltage clipper during operation is discussed in [1.45,46].

An interest in developing HST breakers has been shown by a few electric power companies andtheir breakers are detailed in Table 1.5.

Table 1. 5 Commercial types of HCB for fault current limitation.

ACEC (1992) Meiden (1995) Fuji

(1994)

Zwar (1996)

rated voltage 750V,1.5kV,3kV

(DC)

1.5kV(DC) 400V(AC) 3kV(DC)

rated current 6kA 4kA 2kA 250, 400A

interruption

time


26/195

14 Chapter 1

capacitance of 1.44F with initial voltage of 400V [1.58]. Nevertheless, this solution could not

penetrate into existing DC applications because of their triggering criteria and economics.

1.4 Outline of thesis

The outline of this thesis is given below.

Chapter 2 characterizes one-stage hybrid interruption techniques using analytic and numeric

solutions.

Chapter 3 presents two-stage interruption methods that alleviate the component problems, with the

aid of analysis and simulations.

Chapter 4 describes DC measurement and fault detection methods.

Chapter 5 describes and models the fast opening mode of the prototype breaker developed using a

specially designed electrodynamic drive.

Chapter 6 gives an explanation of the direct current short-circuit source with the models required for

the experiments.

Chapter 7 covers the experimental and simulation results including those for an air breaker, a hybrid

breaker and a solid-state breaker.

Chapter 8 presents and discusses the conclusions that can be drawn from the work described in this

thesis giving recommendations for future work.

1.5 References and reading lists

[1.1] Bartosik, M., Progress in D.C. breaking, Proc. 8th Int. Conf. Switching Arc Phenomena,

Summary of discussed items on fuses, Lodz, Poland 3-6 Sept. 1997, Vol. 2, p. 24-41.

(Published in 1998)

[1.2] Kenn Lian, DC Breaker Applications, HVDC Circuit Breaker Symposium 1972, IEEE

Summer Power Conference, p. 9-10.

[1.3] Schaufelberger, F.G., HVDC Circuit Breakers- Application, HVDC Circuit Breaker

Symposium 1972 IEEE Summer Power Conference, p. 13-4.

[1.4] Pucher, W., Fundamentals of HVDC Interruption,Electra, No. 5, 1968, p. 24-38.

[1.5] Lee, A., et. al., The development of a HVDC SF6 breaker, IEEE Trans. on Power

Apparatus and Systems, Vol. PAS-104, No. 10, October 1985, p. 2721-9.

[1.6] Newbery, P. and Wright, A., Electric fuses, Proc. IEE, Vol. 124, No. 11R, November

1977, p. 909-24.

[1.7] Nakayama, H. et.al., Development oh high voltage, self-healing current limiting element

and verification of its operating parameters as a CLD for distribution substations, IEEE

Trans. on Power Delivery, Vol. 4, No. 1, January 1989, p. 342-8.

[1.8] Benouar, M., Pyrotechnique circuit interrupter for the protection of electrical systems,

IEEE Trans. on Power Apparatus and Systems, Vol. PAS-103, No. 8, August 1984, p.

2006-10.[1.9] -, Is-limiter, ABB Calor Emag Schaltanlagen AG, 1996.


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[1.10] Das, J.C., Limitations of fault current limiters for expansion of electrical distribution

systems, IEEE Trans. on Industry Applications, Vol. 33, No. 4, July/August 1997, p.

1073-82.

[1.11] Skindhj, J., et.al., Repetitive current limiter based on polymer PTC resistor, IEEE

Trans. on Power Delivery, Vol. 13, No. 2, April 1998, p. 489-94.

[1.12] Strumpler, R., et.al., Novel medium voltage fault current limiter based on polymer PTCresistors,IEEE Trans. on Power Delivery, Vol. 14, No. 2, April 1999, p. 425-30.

[1.13] Tixador, P., et.al., Hybrid superconducting a.c. fault current limiter principle and previous

studies,IEEE Trans. on Magnetics, Vol. 28, No. 1, January 1992, p. 446-9.

[1.14] Gray, K.E., and Fowler, D.E., A superconducting fault-current limiter, Journal of

Applied Physics, 49(4) April 1978, p. 2546-50.

[1.15] Noe, M., Supraleitende Strombegrenzer als neuartige Betriebmittel in

Elektroenergiesystemen, PhD Dissertation 1998, Hannover University. (In German)

[1.16] Tanaka, T, et.al, Electrical insulation in HTS power cables, fault-current limiters and

transformers,Electra, No. 186, October 1999, p. 11-29.

[1.17] Smith, R.K., et. al., Solid state distribution current limiter and circuit breaker: application

requirements and control strategies, IEEE Trans. on Power Delivery, Vol. 8, No. 3, July

1993, p. 1155-64.

[1.18] Ueda, T., et. al., Solid-state current limiter for power distribution system,IEEE Trans.

On Power Delivery, Vol. 8, No. 4, October 1993, p. 1796-1801.

[1.19] Jinzenji, T., and Kudor, T., GTO DC circuit breaker based on a single-chip

microcomputer,IEEE Trans. on Industrial Electronics, Vol. IE-33, No. 2, May 1986, p.

138-43.

[1.20] Salama, M.M.A., et. al., Fault-current limiter with thyristor-controlled impedance (FCL-

TCI),IEEE Trans. on Power Delivery, Vol. 8, No. 3, July 1993, p. 1518-27.

[1.21] Chokhawala, R., and G. Castino, IGBT Fault current limiting circuit, IEEE Industry

Applications Magazine, September/October 1995, p. 30-5.[1.22] Zyborski, J., J. Czucha and M. Sajnacki, Thyristor circuit breaker for overcurrent

protection of industrial d.c. power installations, Proc. IEE, Vol. 123, No. 7, July 1976, p.

685-8.

[1.23] McEwan, P.M., and Tennakoon, S.B., A two stage DC thyristor circuit breaker, IEEE

Trans. on Power Electronics, Vol. 12, No. 4, July 1997, p. 597-607.

[1.24] Atmadji, A.M.S., Hybrid switching: a review of current literature,Int. Conf. on Energy

Management and Power Delivery 1998, Mar. 1998 Singapore, p. 631-8.

[1.25] Amft, D., and Drummer, G., Hohere Schaltstuecklebensdauer durch

Hybridschutztechnik,Elektrie24, 1970, H.5, p. 165-7. (In German)

[1.26] Humann, K., and Koppelmann, F., Lichtbogenfreies von Wechselstrom mit mechanischen

Schaltern in Verbindung mit Paralleldioden im Niederspannungsbereich,

Elektrotechnische Zeitschrift ETZ-A, Bd. 86, 1965, H. 15, p. 496-500. (In German)

[1.27] Baliga, J.,Modern power devices, Wiley-Interscience, 1987.

[1.28] Chen, D.Y., Power Semiconductors: fast, though and compact, IEEE Spectrum

Magazine, September 1987, p. 30-5.

[1.29] Genji, T., et. al., 400V class high-speed current limiting circuit breaker for electric power

system,IEEE Trans. on Power Delivery, Vol. 9, No. 3, July 1994, p. 1428-35.

[1.30] Bonhomme, H., and Legros, W., Use of Power semiconductors in circuit breakers,

Proceedings of the fifth International PCI Conf., September 28-30 1982, Geneva

Switzerland, p. 319-25.

[1.31] Hartig, G., and Wedell, H., Betrachtungen uber Ausgleichvorgange bei derParallelschaltung von mechanischen Schaltstrecken und Halbleiterleistungsventilen,

Elektrie27, 1973, H. 6, p. 309-10. (In German)


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16 Chapter 1

[1.32] Humann, K., and Koppelmann, F., Kontaktloses Schalten mit steuerbaren

Halbleiterelementen im Niederspannungsbereich, Elektrotechnische Zeitschrift ETZ-A,

Bd. 86, 1965, H. 17, p. 552-7. (In German)

[1.33] Bonhomme, H., et.al., A 6kV/500A Switching device with thyristors : dream or reality,

Proceedings of the sixth International PCI Conf., April 1983, Orlando, USA, p. 1-5.

[1.34] Greitzke, S., Untersuchungen an Hybridschaltern, Dissertation TU Braunschweig, 1988.

(In German)

[1.35] Bonhomme, H., et. al., A semistatic switching device,Int. Conf. on Power Electronics

and Variable-Speed Drives, PEVSD '84, London, May 1984, p. 27-9.

[1.36] Krstic, S., and P.J. Theisen, Push-Button Hybrid Switch,IEEE Trans. on Components,

Hybrids and Manufacturing Technology, Vol. CHMT-9, No. 1, March 1986, p. 101-105.

[1.37] Holroyd, F.W., and Temple, V.A.K., Power Semiconductor devices for hybrid breakers,

IEEE Trans. on Power Apparatus and Systems, Vol. PAS-101, No.7, July 1982, p. 2103-8.

[1.38] Greitzke, S., and Lindmayer, M., Commutation and erosion in hybrid contactor systems,

IEEE Trans. on Components, Hybrids and Manufacturing Technology, Vol. CHMT-8, No.

1, March 1985, p. 34-9.[1.39] Collart, P., and Pellichero, S., A new high speed DC circuit breaker: the DHR, IEE

Colloquium on Electronic-aided current limiting circuit breaker developments and

applications, No. 1989/137, p. 7/1-3.

[1.40] Chaly, M., et. al., Switching arc in combined switching systems, 3th Int. Symp. on

Switching Arc Phenomena(SAP), Lodz, Poland, September 1977, Part 1, p. 153-6.

[1.41] Collart, P., and Pellichero, S., A super high speed intelligent circuit breaker, GEC

Alsthom Technical Review, No. 9, 1992, p. 35-42.

[1.42] Theisen, J., et. al., 270-V DC Hybrid Switch,IEEE Trans. on Components, Hybrids and

Manufacturing Technology, Vol. CHMT-9, No. 1, March 1986, p. 97-100.

[1.43] Lasota, R., Reduction of switching arc energy in direct current hybrid switches with GTO

thyristors, 7th International Conference Switching Arc Phenomena(SAP), 27 September -

1 October 1993, Lodz, Poland, p. 264-7.

[1.44] Shammas, N.Y.A., Combined conventional and solid-state device breakers, IEE

Colloquium on Power semiconductor devices, No. 1994/247, p. 5/1-5.

[1.45] Hasan, S., et.al., The critical switching parameters of a new hybrid AC low voltage circuit

breaker without and with ZnO varistor, 6th Int. Symp. On Short-Circuit Currents in

Power System, September 1994, Liege Belgium, p. 3.11.1-8.

[1.46] Czucha, J., et.al., AC low-voltage arcing fault protection by hybrid current limiting

interrupting device, 7th Int. Symp.on Short-Circuit Currents in Power Systems, September

1996, Warsaw Poland, p. 3.8.1-5.

[1.47] Brice, C.W., et.al., Review of Technologies for Current-Limiting Low-Voltage CircuitBreakers,IEEE Trans. on Industry Applications, Vol. 32, No. 5, Sept./Oct. 1996, p. 1005-

10.

[1.48] Lasota, R., The work of hybrid switches in low voltage direct current circuits, Fifth

International Symposium on Switching Arc Phenomena (SAP), September 1985, Lodz,

Poland, p. 152-5.

[1.49] Lasota, R., Some problems of arc energy limitation in the DC hybrid switches with power

MOSFET, Sixth International Conference on Switching Arc Phenomena (SAP),

September 1989, Lodz, Poland, p. 40-3.

[1.50] Bartosik, W., Theoretical and practical aspects of fault direct current switching-off by

counter-current, Proc. of the Int. Conf. on Electrical Contacts, Arcs, Apparatus and Their

Applications, May 3-7 1989, Xi'an China, p. 5-12.


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[1.51] Takehara, K. and Yamada, Y., High-speed circuit-breaker incorporating a digital current

detector to support safe operation of electric trains, Meiden review [International

Edition],1995, No. 3, p. 29-32.

[1.52] Shammas, N.Y.A., and Naumovski, N., Combined conventional and solid-state device

breakers 'Hybrid circuit breakers' , Proc. 29th Univ. Power Eng. Conf, UPEC 1994,

Galway (Ireland), p. 716-9.[1.53] Czucha, J., and J. Zyborski, Ultra fast hybrid circuit breaker for AC network theoretical

analysis, 29th University Power Engineering Conf, UPEC 1994, Galway Ireland, Vol. 1,

p. 173-6.

[1.54] -, Ultra high-speed direct current-limiting vacuum circuit breakers, Manufacture Catalogue

Sheet, 1996, Zwar, Poland.

[1.55] Bartosik, W., et. al., Arcless DC Hybrid circuit breaker,Eight International Conference

on Switching Arc Phenomena(SAP), September 1997, Lodz Poland, p. 115-9.

[1.56] Dawson, F.P., et.al., A fast DC current breaker,IEEE Trans. on Industry Applications,

Vol. IA-21, No. 5, Sept./Oct. 1985, p. 1176-81.

[1.57] Dokopoulos, P. and Kriechbaum, K., Gleichstromschalter fuer 73kA und 24kV in der

Plasmaphysik, Elektrotechnische Zeitschrift ETZ-A, Bd. 97, 1976, H.8, S. 499-503. (In

German)

[1.58] Dijk, E. van., Experimental results obtained with the 1 MA resonant series counterpulse

opening switch system, developed at TNO, 11th IEEE International Pulsed Power Conf.,

June 29 - July 2 1997, Baltimore, Maryland, USA, p. 287-92.


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18 Chapter 1


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Chapter 2

Analysis of commutating circuits for hybrid breakers

Abstract

For DC networks, current limiting devices are necessary for disconnecting faulty circuits rapidly.

This work presents an analysis of the hybrid techniques which apply to current commutation.

Firstly the basic commutation circuit known as one-stage interruption is described. Results from

simulations of the complete system are presented giving an estimation of the possible transient

behavior during the interruption processes. Analytical and numerical solutions have been obtained

for the relevant differential equations.

2.1. Introduction

The fact that DC systems have no natural current-zero, becomes a problem when currents have to be

interrupted. Principally, breakers may use two ways of producing current-zeros. According to one

method, an arc voltage is created between the electrodes of the breaker which opposes the supply

voltage. The breaker has to be able to produce arc voltages greater than the systems voltage in order

to produce the current-zero. The success of arc plasma quenching depends on the ability of the

surrounding medium to absorb all the inductive energy stored in the system. Unfortunately, thismethod eventually results in long arcing times causing considerable erosion of the contacts of the

breaker. The greater the inductive energy content of the system, the longer the arcing times

necessary. An effective current limitation may be hampered by the chance of the contacts opening

and a fast voltage building up in the early stages of the interruption process.

Another way of interrupting a current is known as current commutation. The commutation process

requires additional circuits to be connected in parallel across the main breaker. Generally, such

circuits are able to store a certain amount of energy and by discharging this energy, a controlled

counter-current injection can be made. This counter-current injection opposes the main current in

the breaker (by superposition) in order to produce a forced current-zero. Indeed, current-zero can

only be produced if the counter-current injected is greater than the instantaneous fault current;

consequently, it is very important to identify the fault current level in which the counter-current

injection will be able to force the current to zero. This method reduces the arcing time effectively

thereby reducing contacts erosion [2.1]. The basic DC commutation system is shown in Figure 2.1

[2.2,3].


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20 Chapter 2

MOV

RLoad

ES

iS iB S1

DFWCC LC

iMOV

S3

S2

-

+

iCRT

LT LLoadvC

Figure 2. 1Basic DC systems with a commutation circuit; S1: main breaker, S2: auxiliary switch,

S3: load breaker, CCand LC: commutation capacitor and coil, and MOV: metal oxide varistor.

A DC source ESwith circuit resistance RT and inductance LT is connected in series with a main

breaker S1 and a load breaker S3 followed by a load. The circuit resistance and inductance may

comprise the value of the DC source and linking lines or tracks. The current normally passes

through the main breaker S1. A commutation circuit is connected in parallel across the main breaker

S1; it consists of capacitor CC, coil LCand auxiliary switch S2. The metal oxide varistor (MOV)

connected across S1 have a clamp voltage protecting devices in the system. The capacitor CCcan be

initially pre-charged, as is required of the active commutation mode, otherwise it is called the

passive commutation mode. Because the load is inductive, the system may require a freewheelingdiode DFWin parallel with the load side. The freewheeling diode DFWwill bypass the circuit current

when the current slope changes to negative. Intentionally, this is very useful for avoiding any energy

being transferred from the downstream lines (transmission lines and inductive loads) to the

commutation capacitor CC during the interruption. In contrary, the source side inductive energy

cannot be bypassed using the freewheeling diode.

In the active mode, a current oscillation provided by the precharged commutation capacitor CCwill

arise instantly and it will grow to oppose the current in the main breaker S1 when the auxiliary

switch S2 is closed. A trip command provided by a fault sensor controls closing of the auxiliary

switch S2 and opening the main breaker S1. A proper combination of LC and CC will create an

oscillation that generates at least one current-zero crossing in the main breaker S1. After an

interruption at current-zero in the main breaker S1, the main current i S will commutate to the

parallel path thereby changing the polarity of the capacitor CC. Oscillation of the commutated

current will create another current-zero crossing in the switch S2 that will be determined by the

upstream line and the commutation parameters. Therefore, the capacitor will be charged up to a

value depending on the initial voltage, the system voltage and a voltage related to the stored

inductive energy in the upstream line. In short, the residual capacitor voltage will depend on the

network parameters to a great extent. When the main breaker S1 is not separated at the first current-

zero, the current interruption can be produced at the second current-zero crossing. If the switch S2 is


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Analysis of commutating circuits for hybrid breakers 21

bi-directional, the damped current will oscillate in the circuit until it becomes zero and the capacitor

voltage becomes equal to the supply voltage. As a matter of fact, this oscillation enables the stored

inductive fault energy to be dissipated in the circuit resistance. However, the switch S2 and the

rectifier station are generally uni-directional. As a consequence, after the first current-zero occurs in

the switch S2, it opens and the capacitor CCwill have to withstand high voltages. At this instant,current interruption is achieved. Finally, the load breaker S3 can be opened without any arcing. For

a successful commutation, the main breaker S1 must be able to maintain the isolation between its

electrodes at and after the current -zero creation. This active commutation circuit is known as the

one-stage interruption method. Disadvantages of this method include:

the need of a continuous external voltage for charging the capacitor CC;

high overvoltages across the breaker when a current interruption occurs and this requires

voltage limiting devices, such as arresters, MOVs, etc.;

CCmust have a large capacitance value, consequently, it must have a large size and a high

price;

the commutation circuit may be unable to fulfill its function after an interruption failure.

Apart from the active mode described above, a passive mode is needed sometimes. In the passive

commutation mode, it can be assumed that a short circuit has been caused on the load-side, resulting

in fault current iS=iBflowing in the circuit. When the fault current iSexceeds the critical limit, the

main breaker S1 will open drawing an arc between its electrodes. The switch S2 subsequently must

be closed in order to initiate a counter-current iCin the branch S1. A proper combination of LCand

CCwill create an oscillation that generates at least one current-zero crossing in the main breaker S1.

For the passive mode, the current commutation needs a longer time due to the interaction between

the arc and the LCCC-loop. An oscillatory current will be created by an uncharged capacitor that is

repeatedly charged and discharged by the arc voltage in the course of current interruption. The

condition for current interruption in the main breaker S1 is created solely by passive elements in

parallel with the breaker and by the properties of the arc itself. When the contacts are separated, arc

plasma is formed. The arc voltage will increase further as a result of arc lengthening and the heat

loss increases. During a short period, the current in the LCCC branch will show a growing

oscillation. At a time when its magnitude is equal to the main current, current-zero in the main

breaker S1 can be produced. The main current iS commutates entirely to the parallel path.Consequently, the source will charge up the capacitor increasing its voltage. At a moment that the

current is zero in the auxiliary switch S2, the capacitor will be fully charged so that its voltage will

reach its highest value. As a result, the interruption succeeds. If the auxiliary switch S2 is bi-

directional, the oscillation can continue until the capacitor voltage is equal to the supply voltage,

otherwise the interruption will occur as soon as the current becomes zero.

Every DC system has a maximum fault current. Obviously, the rate of change of the fault current

depends on the line inductance. Since the energy stored in the commutation capacitor is limited too,

there will be another significant quantity of energy available for creating a successful current-zero.

Therefore, the maximum trip current for recognizing a fault has to be determined carefully for each

DC system. As an illustration of the current interruption procedure, a DC system with a prospective


34/195

22 Chapter 2

fault current of 10kA will now be analyzed. For the fault current, a rate of change between 1 and

10A/s has been assumed. Figure 2.2shows typical DC faults and their current slopes.

=1ms

=3ms

=6ms

=9ms

0 2 4 6 8 100

1

2

3

4

5

6

7

8

9

10

time [ms]

Current[kA]

(a)

=1ms

=3ms=6ms

=9ms

0 2 4 6 8 100

1

2

3

4

5

6

7

8

9

10

time [ms]

di

dt__[

A/s]

(b)

Figure 2. 2Typical DC faults with Ipros=10kA and 4 different time constants;

(a) the currents and (b) the current slopes.

This study will now concentrate on the active counter-current injection method which is controlled

by a uni-directional solid-state switch. That will lead to the realization of a hybrid breaker with a

current limiting ability using the current commutation principle which can limit a 5kA prospective

DC fault to just 3kA in 1kV/1kA DC systems.

2.2. Analysis of the active commutation circuit

A typical sequence for one-stage DC interruption is illustrated in Figure 2.3. For convenience, the

current is represented by a straight line rising from zero.

t

i

i

1st CZ

t3

tiB

Isp

t

t1

iiS

I t1iC

(a) t0tt1

(c) t2tt3

(b) t1tt2

vC

VCO

t

t

VCE

iS

t2

iS=iC

t2 t3

t1

t2VCt2

t1

t0

t0

It1

It2

vC

vC

t

2nd CZ

Figure 2. 3The sequence of one-stage DC fault interruption; CZ current-zero crossing in the main breaker.


35/195


At time t1, the source current iSreached a level It1, see (a), the threshold for starting the interruption

process. A counter-current injection iCwas applied to oppose the source current iS, see (b). As a

result, the current in the breaker iBwould decrease. At instant t2, the current iBbecame zero and the

voltage across the commutation capacitor reached VCt2. After that, current-zero was produced and

the current iSwas completely commutated from the main breaker to the parallel path. During theinterval t1-t2, the source current iSkept increasing. When charging in the interval t2-t3, the source

current was firstly increased due to the stored magnetic energy transfer and that remaining in the

capacitor CC. Subsequently, charging changed to the opposite polarity. At time t3, the source current

iS reached zero when the capacitor CC was fully charged, see (c). The capacitor current was

eventually interrupted and its final voltage (VCE) increased to a higher value but having an opposite

polarity. Hereafter, the load breaker S3 could be opened in order to isolate the fault from the source.

The final voltage across the capacitor CChad to be limited which depended on the magnetic energy

stored in the system, the initial voltage of the capacitor, and the supply voltage. When that voltage

reached the clipping value of the arrester, it limited the overvoltage. This process prevented any

further voltage rise as the arrester partly absorbed the inductive DC-line energy (W Lim= 1 22).

Obviously, a proper choice of arrester voltage for the clip was vital. Energy absorption by the

arrester would lead to a decay of the fault current at a certain time depending on the line inductance

and the last current value before the commutation. However, in a very high inductive system, the

arrester might not be capable of absorbing such amounts of energy repetitively. If this energy was

excessive, it might cause permanent damage or even destruction of associated devices (S2, MOV,

capacitor, etc.). Accordingly, the whole breaker would not interrupt the current and it would lose its

ability to function repetitively.

For the sake of clarity, the following analysis does not include circuit resistance. Furthermore, the

line inductance on the source side LTwas considerably larger than in the commutation coil LC. The

energy required for a counter-current injection depended on the capacitance value and the initial

voltage (W C VCO C CO= 1 22). Such energy had to be prepared and maintained permanently. The larger

currents had to be interrupted so that more energy was needed. Charging energy for the capacitor

could be supplied by the main voltage system itself or by means of an external supply.

In an oscillatory circuit without damping, the maximum counter-current injection could be

determined roughly by the equation (2.1) :

i VC

LC CO

C

C

= . (2.1)

Obviously, a high initial voltage would result in a high initial rate of change of the counter-current.

A rough expression for this slope of the current is di dt V LC CO C max = . The following initial

conditions apply; S1 is closed, S2 is open, v t VC t CO = =0 and the circuit current could be consideredto increase linearly. Assuming, that the current in the source iS reached the trip value, then S2


36/195

24 Chapter 2

closed and initiated a counter-current iC. This counter-current can be represented linearly, see

Figure 2.3(b), so that i t tV LC CO C = and the capacitor voltage can be written as :

v tC

i d Vt

L CV VC

C

C

t

CO

C C

CO CO = + +1 20

2

At time t2 , current in the breaker becomes zero; which can be defined as current-zero time tz

i t i t i t B z S z C z = . This current will be i t i t V LC z z z CO C = = and the capacitor voltagechanges to v t t V L C V C z z CO C C CO = +2 2 . Subsequently, current from the source will follow thecommutation path, see Figure 2.1. This current obeys the following differential equation:

E v t L Ldi

dtv tS C z T C

CC+ = + + (2.2)

with the initial current i t iC z z = .The solution of this differential equation is :

i tE v t

L Lt t i t t C

S C z

o T C

o z z o z =

+

+

+

sin cos (2.3)

where : oT C CL L C

=+

1

.

Introducing a new parameter :

tan

=

+

+=

+

i L L

E v t

i

C E v t

z o T C

S C z

z

o C S C z

(2.4)

and using the trigonometric equivalent, the capacitor current from equation (2.3) can be rewritten

as:

i tE v t

L Lt t

C E v t t t

C

S C z

o T C

o z

o C S C z

o z

= +

+

+

=+

+

cossin

cossin

(2.5)

By integrating this current, the capacitor voltage becomes : v t

C

i d KCC

C

t

t

z

= +

1

and substituting : t tz= , the integration constant can be found : K ES= .

The capacitor voltage is governed by :

v tE v t

t t ECS C z

o z S =

+

+ cos

cos

(2.6)

The maximum current of the capacitor obtained from equation (2.5) at tx occurs when :

sin o x z o

t t + =

1, so that i

C E v t

C

o C S C z

omax

max

cos=

+

.

From equation (2.4), the maximum current iz max can be defined as :


37/195


tan max

max

oz

o C S C z

i

C E v t =

+ (2.7)

so that :

t t

i

C E v t x z o

z

o C S C z= + +

1

2

arctanmax

max (2.8)

Further, from the trigonometry, we can define oz

C

i

i=

arcsin max

max

or sin max

max

oz

C

i

i= .

The time when the capacitor voltage is zero at the instant when v tC y = 0, can be derived from

equation (2.6), namely coscos

o y zS

S C z

t tE

E v t + =

+

so that :

tE

E v tty

o

S

S C z

z=+

+1

arccos cos

(2.9)

The current becomes zero when i tC int = 0. The time tint is called the total interrupting time and it iswritten as :

t

i

C E v t t

z

o C S C z

o

zint

arctan

=

+

+

(2.10)

From equation (2.7) :

oz

o C S C z

i

C E v t =

+max

maxtan .

Substituting this into equation (2.9) and extracting CC, gives :

Ci

E v tE

E v t

t tCz

o S C zS o

S C z

o

y z=

++

max

maxtan arccoscos

(2.11)

By definition : L L L CL T C

o C= + =

12

, therefore : L

C E v t

iL

o C S C z

z=

+tan max

max

2

After substituting (2.11) and performing algebraic manipulation, this becomes :

LE v t t t

iE

E v t

L

o S C z y z

zS o

S C z

o

=+

+

tan

arccoscos

max

max

(2.12)

To make the expressions (2.11) and (2.12) appropriate, it is necessary to define three new terms:

Ci t

v tt tCo

C z

C z

y z= max

max

, Lv t

i tt tLo

C z

C z

y z= max

max

and parameter kv t

E

C z

S

= max

.

A per unit basic expression can be obtained by simplifying the above ratio as :


38/195

26 Chapter 2

C

C

k

E

E v t

C

Co

oS o

S C z

o

=

+

+

1

11

tan arccoscos

(2.13)

and

L

L

k

E

E v t

L

Lo

o

S o

S C z

o

=

+

+

tan

arccoscos

1 1

(2.14)

The graphs in Figure 2.4 represent equations (2.13) and (2.14) for two different k values. The

horizontal axis represents the term sin o from equation (2.7).

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.91

2

3

4

5

6

7

8

9

10

izmax

icmax

____

CC

CCo

____

k=1

k=4

(a)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90

2

4

6

8

10

12

14

16

18

20

izmax

icmax

____

LL

LLo

____

k=1k=4

(b)

Figure 2. 4 (a) The unit base of the capacitor and (b) The unit base of the circuit inductance.

Next, in order to make the circuit analysis more realistic, circuit resistance had to be included in the

analytical solution. Figure 2.5 presents the extended circuit to show the interrupting sequences.

Additionally, discharged energy stored in the commutation capacitor can be taken into account by

extending an energy absorbing circuit across the capacitor. The energy absorbing circuit (LA, and

RA) played a part only after the capacitors polarity changed, so that in the idling (waiting) state, thepre-charged capacitor had to retained its stored energy continuously. Therefore a reverse biased

diode D1was required. The following new symbols are introduced now : the MS-make switch and

the Thy-thyristor as an auxiliary switch. When considering ideal DC systems, the following

assumptions can be made :


39/195


the DC voltage source ESis constant in time and the internal resistance is negligible;

switches are ideal (no voltage drop or heat loss) and they introduce no transients duringswitching;

the lines and devices have linear characteristics without limitation for the rate of change ofcurrent and voltage.

Therefore, the interruption process could be separated into intervals in which each interval

represents a linear differential equation. In this way, each differential equation could be solved

successively to give analytical solutions.

+

MS

S1-

Thy

LA

RA

D1

+

-

MS

AB

+

MS

-

Thy

C

+

MS

-

Thy

D

+

MS

-

Thy

E

+

MS

-

RA

LA

D1

ES

ES

ESES

ESES

RS LS

RS LSRS LS

RS LS

RS LS RS LS

CC

LC,RC

CC

CC CC

LC,RC

LC,RC

vCS1 S1

Figure 2. 5The sequence of the one-stage interruption; MS: make switch, S1: main breaker.

By solving the differential equations that corresponded to each interval, the analytical solution was

obtained, where the end state of the previous interval was introduced as the initial state of the

following interval.

A The first interval0 1 t t

In this interval, when make switch MS was closed, the current can be expressed by the differential

equation :

Ldi t

dt

R i t v tSS

S S S

+ = , (2.15)

Generally, before a fault occurred, the rated current IR flowed, so that the initial condition becomes:

i t IS t R = =0 otherwise i tS t = =0 0.

The source is defined as a constant voltage source: v t ES S = for all values of t . The source currentiScan be solved from the expression:

i t I I e I S R

t

R( )=

+

1 (2.16)

where: is the time constant ( =L

RS

S), LSand RSare the inductance and the resistance at the fault

location relative to the source; I is the steady-state fault current (prospective) determined only


40/195

28 Chapter 2

from the resistance and DC voltage system, (IE

R

S

S

= ). For convenience, it is assumed that IR is

zero. In the commutation path, the capacitor voltage and its current are constants

v t v t V C C CO( ) ( )= =1 and i tC( )= 0 . Initially, the current in the breaker was equal to the current in the

source until the commutation process occurred during the next interval i t i t B S( ) ( )= . At the end ofthe first interval, the source current becomes i t IS t( )1 1

= (where a counter-current injection would

be performed).

B The second intervalt t t1 2

The counter-current iCwas injected during the second interval in which the current in the main

b

direct current hybrid breakers

Documents

socalled hybrid breakers

dc breakers

current limiters

current limitation

thesis hybrid

main current andinterrupting

fault current interrupters

main breaker