vacuum contactor switching phenomena.pdf

8/11/2019 VACUUM CONTACTOR SWITCHING PHENOMENA.pdf

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SWTCHNG PHENOMENA N MEUM VOLTAGE SYSTEMS GOO ENGNEERNG PRACTCE ON THE APPLCATON OF

VACUUM CRCUTBREAKERS AN CONTACTORS

A MAGPO B 3240

91050 G

D AGPO B 3240

91050 G

Abstract - This paper summarises the transient phenomenaand overvoltages when switching with vacuum circuitbreakers and vacuum contacors in medium voltage systems

( kV U 52 kV). t describes the interacion betweenswitching devices and network as well as the most influencing factors for the shape and magnitude of overvoltages.

Based on studies, laboratory and field tests, engineeringand mitigation guidelines are presented intended to cope with

transient overvoltage phenomena. t is the aim of this paperto demonstrate that adequate surge protection ensures safeand reliable system operation with vacuum switching devices.

x vacuum circuit-breaker, vacuum contactor,switching transients, medium voltage, current chopping,multiple reignition, surge arrester, RC circuit, protection.

I. INTRODUCTION

During the recent decades, starting in the 970ies, vacuumreplaced almost all earlier arc-quenching media for circuitbreakers and contactors in medium voltage systems. Today

about 80% of new installations employ vacuum switchingtechnology, the others use SF gas. Old quenching principles, such as e.g. bulk-oil, minimum-oil, air-magnetic or airblast, almost disappeared from the market.

Fg. 1 Vacuum neupe

n the public perception vacuumcircuit-breakers (VCB) and switching overvoltages sometimes areseen as inseparable twins, although the physical eecs alsooccur with all other switchingdevices. any incidents havebeen reported where the breakdown of insulation was combined

with high-frequency switching

transients, said to be a characteristic of vacuum breakers. Thoseevents triggered intensive research all over the world.

Numerous investigations have been carried out to assessthe switching transients associated with vacuum circuitbreakers and to design adequate surge protection. n fact,the VCB was more intensively investigated since its marketentrance than any other breaker type before. The merit ofthis intensive research is deep scientific knowledge about theswitching behaviour, the interaction between circuit-breakerand network. On this basis concepts for surge protectioncould be developed which resulted in higher safety and reliability than with any other switching principle. This is why

most operators use VCB in their network today.

At the PCC Europe Conference 2007 a paper [] waspublished which reported failures in oil & gas installations

where vacuum circuit-breakers were installed, i t analysed thecauses and proposed surge protection measures. The present article is intended as a follow-up. t discusses the basictransient phenomena when switching of inductive or capacitive currents and presents good engineering practice foradequate insulation coordination and surge protecion based

on theoretical investigations, laboratory and field tests.

II. SWITCHING IN MEDIUM VOLTAGE SYSTEMSEvery switching operation causes more or less coined

transients. nducive and capacitive circuits (Table ) producedierent transients in terms of frequency, wave shape andmagnitude. n adjustable circuits both inductive and capacitive conditions may occur depending on the relevant application.

TABE Swchng dues n medum volage ssems

Type of iruit Swithing duties examplesInducve ccus Tansome no-load)

MooShun eacoAc unace

Ac suppesson colTacon powe suppl

Capacve ccus Cable Ovehead lneCapaco bank

Hamonc) Fle ccu

Adjusable ccus GeneaoPowe convee

n detail the switching transients depend on the

interaction between breaker and network,

influence of network parameters,characteristics of the HV equipment connected.

Thorough system engineering must take into account threeconsecutive steps when configuring surge protection for anindividual installation:

basic surge protection depending on the load

extended measures corresponding with the network

precautionary measures related to the industrial(production) process.

The following secions describe the switching transientsand mitigation methods in principle.


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III. INDUCTIVE CIRCUITSWhen inductive circuits are switched, overvoltages can oc

cur with all switching principles especially during switching o[2]. Three possible causes are to be considered.

A. Current chopping

Current chopping can occur, when switching o very smallinductive currents, for example up to approx. 20 A withunloaded transformers. f the current is suddenly interrupted(chopped) before the natural current zero, magnetic energyremains in the iron of the disconnected circuit, proportional tothe chopped current, see Fig. 2. This magnetic energy isdischarged through the capacitance, mainly the cable capacitance, see equation (). Transposing it (2) estimates themaximum overvoltage (first peak). As and C are given bythe system, the only item that can be influenced is the magnitude of the chopped current i So, one objecive is to develop breakers with small chopping currents. However, thechopping eect cannot be reduced at will, because intensive

arc quenching is required for other switching duties, such asswitching of capacitive currents.

t ms

Fig. 2 Current chopping during inductive switching

() = 2 2whee L load nducance

C load sde capacance choppng cuen

UN ssem volageUL volage a he load

(2)

Uax maxmum volage a load

With circuit-breakers, using gas for arc quenching, the design of the arc quenching unit determines the chopping cur

rent. n contrast to this the chopping value of vacuum circuitbreakers only depends on the contact material. oderncontact material ranges at 3 to 4 A for circuit-breakers; andfor some contactors the value can even be below A.

.5 P,

P Pobab ooccuence0 Vaue o

0 choppg cen.

0

0

00

4

A

6

Fig. 3 Chopping current of vacuum circuit-breakers

Thus the overvoltages on switching o unloaded transformers are far below being critical. Switching tests on magnetizing currents from less than A up to 20 A at operating voltages between 6 kV and 30 kV yield overvoltage factors (seeAppendix) of maximum k = 3..B. Multiple reignitions and viual current chopping

When switching small inducive currents (approx. 20 A upto some hundred amps), higher overvoltages can be generated if the arc reignites aer the first current interruption, andif the device is then able to interrupt the high-frequency transient current, which appears aer the reignition. This processalways includes a transient reaction between the capacitanceon the system side and that on the load side. f it occursrepetitively, it is defined as multiple reignition. The voltageincreases with each reignition (voltage escalation), so thathigh overvoltages can result.

U

IIII1

v

I

n

v

]

vi iel eng

Fig. 4 Reignitions during inductive switching (principle)

With an increasing voltage, caused by reignition, the correspondingly high frequency transient current rises with eachreignition. f this transient current (i in Fig. 5 and 6) is coupled inductively / capacitively into the other two phases,

which are still carrying the power-frequency current, highfrequency current zeros (i, i) may also appear there. f thebreaker interrupts in one of these current zeros, this is calledvirtual (induced) current chopping. The high-frequency curent only flows in the immediate vicinity of the switchgea,

whereas the load still carries the 50 Hz current. Current interruption at a HF current zero described above has the sameeect on the 50 Hz current as real current chopping at thistime. So, equation (2) mentioned in subclause A will alsoestimate the height of this possible overvoltage.

Fig. 5 Coupling to thephases S and R due tomultiple reignitions in

phase T


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High-frequency

currnt zo in t last plstla,cud by nduction

Fig. 6 Virtual current chopping in the phases S and R(principle, transients not drawn to scale)

As the virtual current chopping can occur at high instantaneous values of the 50 Hz current, high overvoltages arepossible, which must be controlled by surge arresters.

f (

t

n w tn u.

Breakingcuet

Unloaded tansfomes Ac funace tansfomes Neutal eathng tansfomes

wth petesen col

Motos beeng stated

5 Shunt eactos6 Tansfomes wth

shot-ccuted secondaes

7 System shot-ccuts8 Shot-ccuts wth eactos n sees

Fig. 7 Switching duties with inductive circuits(example of 2 kV systems)

ultiple reignitions and virtual current chopping do not occur on every breaking operation or in every circuit. Figure 7marks the range of higher overvoltage. The current (abscissa) is given by load inductance and system voltage. Thefrequency (ordinate) results from the load inductance and thecapacitance of the connecing cables (ranging from only fewmeters to some hundred meters). Five prerequisites must besimultaneously fulfilled for multiple reignitions plus virtualcurrent chopping:

. Switching o a purely inductive load (e.g. motor during run-up, locked rotor or jogging).

2. Opening of the first pole-to-clear 0.5 ms prior tocurrent zero. This corresponds in three-phase systems to a probability of occurrence of 5 % at 50 Hz(or 8% at 60 Hz)

.

1 A 50 Hz: cuen zeo occus eve 3.3 ms n one o he heephases. Thus he pobabl s 0.5 ms / 3.3 ms 0,15

3

3. Build-up of the dielectric strength in the contact gaplagging rise of transient recovery voltage (TRV).

4. Combination of supply side and load side capacitances and inducances result in a di/dt value thatenables the breaker to quench at high-frequencycurrent zero.

5. The current to be switched o has to be smaller

than the limit current up to which virtual currentchopping may occur. For vacuum circuit-breakerthe value is 600 A.

C. Closing transints

Closing transients can occur with all switching devices, regardless of the method of arc extinction and arc quenchingmedium used. Every closing of an elecrical circuit in highvoltage systems will cause pre-arcing with transient oscillations. The following theoretical treatment describes the closing conditions in the most unfavourable case, in which themaximum transients occur. Damping and other eects are

neglected.

.

. ewok capacacesm o cabes coeced o e bsba)

Fig. 8 Network and load capacitances on closing

The arrangement in Fig. 8 shows an inducive load whichis connected to the switchgear by a cable. The load capacitance C results from the cable, whereas the stray capacitance of the load (e.g. transformer, motor) is some multiples

of ten less, so it can be neglected in this treatment. anycables are connected to the busbars, which results in a very

large network capacitance (C - . Thus full transientvoltage appears at the load side of the breaker.

Fig. 9 Voltages at time t and trace in first pole-to-close

n the time scale of the oscillations described in Fig. 9, thepower frequency voltage remains almost constant At time tthe line to earth voltage UL is at the maximum p.u. (Fig.9 le) and the first closing pole is in line . The pre-arcingover the contact gap causes a travelling wave which runsperiodically along the cable. n the first pole-to-close, , thisbuilds an oscillating voltage with an overvoltage factor k = 2(Fig. 9 right). The travelling wave sees the inductive load as aquasi open cable end, since the wave resistance of, for example, motors and transformers lies in the region of 000Ohms, in contrast to 0 Ohms of cables. At the breaker, thecable connection has a wave resistance approaching zero.Thus the natural cable frequency f = v (41); where v =wave velocity (approx. 50 m/lJs for XPE cables), = lengthof cable.


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L2----------- ------------------------'

,

I

: : -----

/; 7

;

J

Fig. 0 Oscillations in last poles-to-close before contact touch

Simultaneously with the closing of the first pole an equalising oscillation of frequency fL is set up in the two last closing poles 2 and 3. This is represented by the circuit in Fig.0. Through the bridging of the contact gaps in the last polesto close, the oscillation fL moderates into an oscillation withthe natural frequency of the cable f (Fig. ) which thenoscillates into the instantaneous value of the operating frequency voltage (half line-to-earth voltage). Overvoltages upto k = 3 result.

[]f

o -

-

3Fig. Voltage trace in last poles to close

Related to the peak value of the line-to-earth voltage,

overvoltages with factor k = 2 can occur in the conductor ofthe first pole to close and, in the conducors of the last polesto close, with k = 3 resp. k = 5 (bipolar). Theoretically, avalue of k = 6 is possible for the overvoltage between thephases of the last poles to close. n practice, the stresses arefar less than the theoretical maximum values, because of thestatistical distribution of the overvoltages and damping effects.

IV. CAPACITIVE CIRCUITSA distinction has to be made here between switching o

and switching on capacitive loads as dierent stresses mayarise.

A. Switching o

When switching o at a current zero (time t), the capacitorremains charged at the peak value of the source voltage (u).The system voltage (UN) continues changing sinusoidally andreaches its opposite peak value aer 0 ms. The recoveryvoltage (dierence between Uc and UN) now starts risingslowly. n this case, the stress is not caused by the rate ofrise as with inducive switching, but by the absolute value ofthe voltage. f there is a new ignition within 5 ms aer arcquenching, this is called a reignition. This type of new ignitionis not dangerous. f the new ignitions take place aer a deenergized pause of more than 5 ms, they are called restrikes.

However, if they appear aer approx. 0 ms, these restrikescan be the origin of high switching overvoltages for the following reason: The restrike recharges the residual energy of

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the capacitor. Due to this, the voltage theoretically oscillatesto a value corresponding to the capacitor voltage the instantaneous system voltage (time t), but this value is notfully reached due to the existing system damping. ultiplerecharging (more reignitions) can generate very high switching overvoltages, so that the switchgear insulation may beoverstressed and flashover may occur.

N System voltagec Capacitor voltage Votage across beakec Caacto curentLN System ndctace

C Capacitace on oad sde

u

f

.

Fig. 2 Restrikes during capacitive switching

For safe operation of capacitive circuits it is essential thatrestrikes do not occur.

B. Closing onto and paraleing of capacitors(backtoback switching)

10

Makin

oo a soccu o e no

e vortage maxmum w maxmumae o se o he cue

2 Inus cue o backoback swchg

Fig. 3 Current variation when paralleling a capacitor

When the breaker contacs approach each other, prearcing occurs across the open gap before galvanic contact.At this moment, a transient process takes place between thesystem and the capacitor, in which making currents up tosome 0 with frequencies up to several k Hz can appear.As pre-arcing occurs approx. . . . 2 ms prior to galvaniccontact, the full transient current (2 in Fig. 3) flows throughthe arc when switching on capacitors. n contrast to this,

when making onto a short-circuit () the instantaneous current value at this time is much smaller. The dynamic forces ofhigh currents during pre-arcing may slow down contact

CuNuN

us


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movement and hamper latching or promote contact welding.So back-to-back capacitor switching is harder than making 1,0 -

onto short-circuit with the same values of current amplitude.When configuring the application the maximum permissible 09

inrush current of the circuit-breaker must be considered.

V. INFLUENCE OF THE NETWORKSwitching transients do not only depend on the nature of

the load circuit but also on the network on the feeding side.The network configuration influences the shape and themagnitude of transients on the load and busbar side of thebreaker. Thus the network as a whole also determines whichkind of surge protecion must be installed.

A. Cables and traveing waves

Where impulse voltages occur, the voltage along a cable isno longer constant. The cable then acts as a travelling-waveconductor. This eect is obtained when the impulse front timeis shorter than the travelling time in the cable; i.e. in mathe

matical terms when the condition (I v) is met, where =front time of impulse; = cable length; v = propagation velocity of the wave in the cable. Depending on the cable type, v =0.3 to 0.6c (c = velocity of light 300 m/lJs). Cables with alength of less than 30 . . . 40 m ("short cables") do not therefore act as travelling-wave conductors in normal switchgearinstallations as regards the phenomena described here.

B busbar votage oad votage able mpedance Ldmpd

eg motor) reractive index

Z Z + Z +ZFig. 4 Voltage rise at the load due to travelling waves

gnition of the contact gap is a matter of a few nanoseconds and causes a very steep fronted voltage variation totake place on both sides of the contact gap within a fewmicroseconds. Reignition thus gives rise to a travelling wave.

The surge impedance changes at the transition from thecable to the inductive load, e.g. a motor (ZM Z) so thatthe travelling wave is reflected and a voltage rise occurs atthe motor terminals Fig. 4 shows the relationships and thedefinition of the refractive index. Theoretically, the latter can

reach a value of y = 2, but in practice (field and test labmeasurements) only values up to y = .7 have occurred,

where the rise times of the surge are in the order of Js

B. Cables connected to the busbar

Fig. 5 shows the equivalent circuit of an installation inwhich the cables connected to the bus bar are represented bytheir surge impedance Z. On ignition of the contact gap, theimpulse waves occurring on the right and le of the gapdepend on the surge impedance values.

As can be seen from the graph for the load side voltage ULthe impulse voltage stressing of the load side increases withthe number of cables connected to the busbar. Reversely,the voltage stress on the busbar itself rises with only few

cables conneced. There are configurations where surgeprotection is also required for the busbar side (incomingfeeder) of the switchgear installation.

5

08

0,5 I-;:.=

Fig. 5

5 10 5 20

Share of load side voltage as a function of thenumber of cables connected to the busbar

C. Switching status of the busbar

The location of cables connected to the busbar also aecsvirtual current chopping. Chopping is more likely if high

frequency and power-frequency currents share a long common loop along the busbar between incomer and panel to beswitched o (Fig. 6, le). n contrast, cables connectedoutside the power-frequency current flow (Fig. 6 right) reduce the coupling along the bus bar and thus the probabilityof virtual chopping in that the cable capacitances form abypath for the high-frequencies. Altogether, the probability ofoccurrence of overvoltage may widely vary with one and thesame load circuit, depending on the switching status.

z z

Incmer IncmerHg probabit Low probabiit

Fig. 6 Probability of virtual current chopping dependent onthe panel arrangement

VI. INFLUENCE OF THE LOADn view of the impact on the insulation, the magnitude and

wave shape alone are only one part of the decisive factors.The load itself plays the other part of the role. As all induciveloads consist of windings whose design (shape of the conductor, arrangement of the turns, type of main and interturninsulation) determine the stress on the insulation and whichparts of it may be particularly at risk. As motor insulation ismore susceptible to overvoltage (compared to other inductiveloads), it is taken as example in the following.

A. Voltage distribution within windings

Within a winding the distribution of steep surge voltages isnot linear and depends on the rate-of-rise of the overvoltage.Because of the propagation eec the voltage across the coil


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depends mainly on its conductor length. Fig. 7 shows theexample of a 0 kV motor, where the voltages across the lineend coils dropped to noncritical values aer the third coil. Asregards stressing of the motor insulation, a distinction mustbe made between the unipolar component of travelling waves(which stresses the motor winding with respect to earth) andthe bipolar voltage range of the travelling wave (voltage

surge) by which the interturn insulation of the line end coils inparticular is stressed. With rise times in the order of .7 Js,recorded in numerous tests see section V, A the first coilis stressed with 65% of the voltage at the terminals. Thesame may apply in principle to transformer windings.

40

- l

-

--

o ,7 ! 4T1_Fig. 7 Voltage distribution across the coils of a motor wind

ing under synthetic impulse voltage

B. Resonances within windings

Windings have characteristic natural resonances becauseof the inducive and capacitive coupling of the individual

winding turns with one another and to earth. The frequencyresponse of this system is determined by the geometry of thewinding. External pulses may excite resonant oscillations.When the exciting pulse frequency equals a resonance frequency, this may cause impermissible voltage rises at certainpoints inside the winding while the voltage is still in the permissible range at the external terminals.

f the repetition frequency of multiple reignitions is withinthe range of the resonance frequencies, there is the risk ofresonant oscillations in the winding giving rise to high internalovervoltages. This can only be prevented by e.g. means ofan external RC damping circuit, not by surge arresters at theouter terminals.

VII. LAORATORY AND FIELD TESTBesides theoretical investigations numerous practical tests

have been carried out (in addition to the standard type tests)on inductive and capacitive load switching since the earlydevelopment of the vacuum circuit-breaker. n regular intervals some tests are retaken in order to check and ensure

constant characteristics of vacuum circuit-breakers and vacuum contactors.

6

TABE Laboraor and eld ess on nducve load swcng

Site nstallation Test objet Subjet ofinvestigation

Berln Facor Moors Fundamenal re-Tes lab 10 kV searc on overvol-

age penomena, .e.Mannem FGH Tes lab Moors

mulple re-gnonsPEHLA) 6 kV and vrual curren

coppng

Helbronn Termal Researc on epower saon nluence o nework

Isar II Nuclear parameerspower saon Moors6 kV and Invesgaon o e

Plenng Termal 10 kV delecrc sress onpower saon equpmen n real

Termal nsallaonsV klngen power saon Vercaon o var-

Sun reacorous surge proecon

Oenoen Subsaon 30 kVmeods

Transormers Overvolages onFacor 6 kV o 30 kV no-load swcngBerln Tes lab apacor Inrus currens,

banks 20 kV breakng currens

Mengen Seel acor Arc urnace Surge proeconransormer meods

The following focuses on inductive switching and summarises some major issues of overvoltage test results and thecorresponding surge protection when investigating theswitching of motors with vacuum circuit-breakers.

A. Occurrence of multiple reignitions

Not every breaking operation is associated with multiplereignitions and severe overvoltages. Fig. 8 shows the resultof 94 breaking operations with a 0 kV motor under worstcase conditions [3]. 48 of the 94 breaking operations werefollowed by multiple reignitions, this is 6 %. This percentagecorresponds very well with the prerequisite No. 2 (refer tosection . D.) predicting a probability of 5 % (at 50 Hz).

Fig. 8 Test series of 94 breaking operations of a 0 kV 400 kW motor during starting (every vertical linestands for an operation with multiple re-ignitions)


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B. Probabi of occurrence of overvo/tage peaks

When the overvoltage phenomena associated with motorswitching were investigated, in total 2500 switching testswere carried out using dierent system configurations (cabletypes lengths, motor currents, busbar arrangements) andsurge suppression measures in order assess the dielectric

stress and to verif the most eective protection methods.The above mentioned 94 cycles using the worst-case

configuration were evaluated to back up statistically thereliability of the system referred to as "vacuum circuit-breakerfitted with surge arresters. ultiple reignitions occurred in 6% of the startstop cycles and virtual current chopping in %. t might be assumed that each multiple reignition or virtualcurrent chopping went along with unduly high overvoltage.However, this is not true. The magnitude of overvoltage alsois subject to statistical distribution.

i99 %

(

o

I

Maximum vales of unipoar and

biboar overvotages recoded a914 swichg tests usng he

wos -ca se coguao

1

--

-

l

U

I L- t

r

Blar

V

,

\

jRf py

k

\

0 4 6 k DIFig. 9 Cumulative probability of occurrence of

overvoltage peak values (0 kV motor)

Fig. 9 shows the cumulative probability of occurrence ofthe unipolar and bipolar voltages measured at the motorterminals. Out of 94 tests only 2% (reference value for statistical insulation coordination to EC 6007) caused overvoltages higher than 38 kV (unipolar) or 59 kV (bipolar).

VIII. ENGINEERING AND MITIGATION GUIDELINESSurge protection should consider three consecutive steps. Step is the basic protecion required by the nature of

the load circuit. Step 2 considers the network configuration as whole

and special modes of operation. These may requireadditional measures, e.g. protection on the bus bar orfeeding side (incoming feeder).

Step 3 takes into account further criteria related to the

industrial process. Beyond the switching duty and network conditions, other aspects may predominate andadvise protection as a precautionary measure.

7

A. Means of overvo/tage protection

Well proven techniques to cope with surges is to install surge arresters to limit the magnitude of surges, surge capacitors to attenuate their rate-of-rise,

RC circuits to damp high-frequency transients and pre

vent repetitive impacts.n general there is consensus among professional experts

to use these protection elements, and corresponding recommendations were published at earlier conferences, referfor example to [4], [5]. n detail, however, there is a margin ofdiscretion which measures shall be applied so that recommendations may vary. nsulation coordination must weightechnical and economical aspects and conclude in a justifiable balance between eicient protecion for reliable serviceand acceptable costs. A guideline to configure surge protection is given in the following.

B. Basic protection measures - Step

Table lists common switching duties in V systems andthe corresponding recommendations

TABE Sandad suge poecon o MV swchng dues

Swithing dutyDsbuon and/opowe ansome(

Fo nnome eemoo conveeo nce

Moo

Geneao

Conveeansome

Shun eaco

Sho-cculmng eaco

Peesen colFunaeansome

Capacve ccus

Ovehead lneo cable

Capaco bankback-o-back)

Fle ccu

ReommendationNo suge poecon o nomal dsbuonansome.Suge aeses o be ed n case o:- connecon o ovehead lne- nsulaon level s "Ls , IEC 6007-- nsulaon s aged o s level s unknown- swchng ae n nomal sevce s hgh

Suge aeses I

vacuum contactor switching phenomena.pdf

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