SYSTEMS-72.pdf

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A Methodology for the Efficient Application of Controlled Switching to

Current Interruption Cases in High-Voltage Networks

C. D. TSIREKIS

Hellenic TransmissionSystem Operator

Kastoros 72, Piraeus

GREECE

[email protected]

F. D. KANELLOS

Hellenic TransmissionSystem Operator

Kastoros 72, Piraeus

GREECE

[email protected]

G. J. TSEKOURAS

Dept. of Electrical & Computer ScienceHellenic Naval Academy

Terma Hatzikyriakou, Piraeus

GREECE

[email protected]

Abstract: - Transients produced upon the opening operation of a circuit-breaker may be harmful for the

network elements and the switching device. A modern countermeasure for the reduction of such

transients is controlled switching. Fundamental requirement for all controlled switching applications

is the precise definition of the desired switching times. In this paper a new methodology is proposed

for the calculation of the optimum switching instants, taking into account circuit-breakers statisticalscatters in the contact operation time and the slope of the contact gap voltage withstand characteristic.

Key-Words: -Controlled Switching, Switching Transients, ATP-EMTP Simulation, Circuit-Breaker

1 IntroductionControlled switching is a technique that

automatically adjusts the circuit-breaker mechanism

in such a way that switching operation takes place at

a point-on-wave which minimizes switching

transients, such as the phase-to-earth overvoltage,

the inrush current and the transient recovery voltage

(TRV) across the breaker poles [1, 2, 3].

One of the most significant requirements for

proper controlled switching performance is to

reduce the statistical variations of contact operation

times, since they may affect the success of this

method [1, 2, 3, 4]. Circuit-breaker technology has

improved these statistical scatters, allowing thus

utilities and manufacturers to achieve contact

operation times quite close to the preferable ones.

This means that a precise definition of the desired

switching times is required, taking into account the

effects of parameter changes (such as the trappedcharge in a capacitor bank or the impedance of a

load) and the circuit-breaker characteristics (such as

the statistical variations of the contacts operating

times and the contacts gap characteristic of

dielectric strength) on the optimum switching

instant.

In this paper a methodology for the calculation of

the optimum circuit-breaker opening instants is

proposed. Specific restrictions, such as contacts gap

voltage withstand characteristic and variations in

contacts operating times are considered. With the

aid of numerical techniques, the optimum switchinginstants can be easily calculated.

2 Operation Principles2.1 Controlled Switching Arrangement

In general, a typical controlled switching

arrangement consists of the following main parts:

The circuit-breaker, with an independent poleoperation capability.

Devices for the measurement of the instantvalues of the circuit-breaker currents.

A controller, which after receiving the switchingcommand, processes the signals from the

measuring devices, determines the suitable

reference phase angles and sends the breaking

commands to each breaker pole, so that the

breaking occurs at the optimum instant.

According to this, a typical controlled switching

layout is shown in the following figure:

LOAD

SIDE

INTERFACE

SIDE

SOURCE

CONTROLLER

Fig.1: Main parts of a typical controlled switching

arrangement.

Recent Researches in System Science

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2.2 Operation in an Ideal Circuit-BreakerFor the analysis of each part of the operation

sequence during the controlled opening, the

definition of the ideal circuit-breaker is introduced.

Its main characteristics are the following:

No arc is developed during the openingoperation. Therefore, current breaking occurs

simultaneously to the mechanical separation of

the contacts.

The probability of reignition is zero.

The sequence of each partial operation during

controlled opening of an ideal circuit-breaker is

illustrated in the next figure. In this figure and all

the next ones, t and T represent time instants

and time intervals, respectively:

Current acrosscontacts

Breakingcommand (User)

Operatingcommand to poles

Breaking

w delay

breaktt command

T + Toperat ing

startt Fig.2: Operation sequence during controlled

opening of an ideal circuit-breaker

Toperating corresponds to the time intervalbetween the sending of the opening command from

the controller (tstart) and the starting of the contacts

mechanical separation. Since Toperating is very short,

it can be neglected and considered that current

breaking occurs instantly and simultaneously to the

command sending from the controller and the

contacts mechanical separation (tstart = tbreak).

Therefore, the corresponding time delay Tdelay for

each phase is given by the following equation:

Tdelay = tbreak Tw (1)

2.3 Application to a Real Circuit-BreakerIn practical applications, contact gap has a finite

dielectric strength. This affects the controlled

opening operation. In a real HV circuit-breaker,

current breaking (instant tbreak in Fig.2) does not

occur simultaneously to the instant of the contacts

mechanical separation (instant tseparate). The current

flow after the instant of tseparate is kept through the

electric arc developed between the opening contacts

and is broken at one of the next physical current

zeros, since at those instants the energy absorbedby the arc, maintaining it, is zero. However, as

current approaches the physical zero points, the

arc comes to an instable mode. As a result, current

becomes zero slightly before a physical zero

instant (current chopping). Therefore, opening

switching transients are independent of the phase

angle corresponding to the instant of contacts

mechanical separation and controlled opening

cannot reduce this kind of transients directly.

Immediately after the arc extinguishing, a

transient recovery voltage (TRV) is installed across

the contacts gap. The initial rate of rise of recovery

voltage (RRRV) may be quite high, especially in

inductive currents interruption [5]. As a result, it is

strongly probable for TRV to exceed once or

multiple times gap dielectric strength, with the

subsequent occurrence of a number of reignitions.

Each reignition, is equivalent to a temporary

reclosing of the circuit, which generally occursunder more adverse conditions than the closing with

zero initial conditions, due to the accumulated

energy. In any case, reignitions must be avoided, so

that more dangerous transients, such as voltage

escalation, are prevented.

The key for the prevention of restrikes is the

relation between the magnitude of TRV and the gap

dielectric strength. The slope of the latter (Rate of

Rise of Dielectric Strength - RRDS), which depends

on the contacts separation velocity and the gap

dielectric behavior, may be quite smaller than

RRRV. However, it appears earlier (at the instant ofcontacts mechanical separation), while TRV is

established at the instant of electric breaking, as it is

illustrated in the next figure:

Current's "physical zero"

Characteristic of gapdielectric strength

Current Chopping

Breaking current

(TRV)recovery voltage

Transient

Grid phase voltage

separatet breaktarcingT

Fig.3: Successive inductive current interruption. No

reignition due to a long arcing duration (Tarcing)

As mentioned previously, electric breaking

occurs slightly before one of the current zeros after

contacts mechanical separation. Therefore, a

maximum arcing duration (Tarcing) is needed for the

prevention of a reignition. This means that contacts

mechanical separation should occur immediately

after a current zero, so that even when electricbreaking occurs near the first current zero, arcing


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duration Tarcing is quite long (just shorter than a half-

cycle) and the gap dielectric strength is high enough

to avoid reignition. All the above are illustrated in

Fig. 3 and 4, for inductive current interruption

without and with reignition, respectively:

Reignit ion

separatet breaktarcingT

Characteristic of gapdielectric strength

Grid phase voltage

Current's "physical zero"Current Chopping

Breaking current

(TRV)recovery voltage

Transient

Fig.4: The same inductive current interruption, but

with reignition, due to the short arcing duration(Tarcing)

The general result is that the aim of the

controlled switching application in current breaking

cases is the avoidance of reignitions. The sequence

of the various operations is illustrated in Fig.5:

Current acrosscontacts

Breaking commandfrom User

to breaker poles &starting of contacts

breaktt command t separate

arcingTT + w delay

Current breaking

Operating command

separation

Fig.5: Operation sequence during controlled

opening of a real circuit-breaker

2.4 Statistical DeviationsA variety of factors, like environment conditions

(ambient temperature, humidity .), the utilizationfrequency of the circuit-breaker and the whole

device, the age of the arrangement and the

corresponding equipment strain and wear, contribute

to the appearing of statistical deviations in the

operational characteristics of the controlled

switching arrangement, such as the following ones:

The waiting duration w

The time delay delay

The time between the sending of the commandfrom the controller to the circuit-breaker and the

starting of contacts movement The time between the starting of contacts

movement and their mechanical separation

The velocity of contacts separation

The RRDS due to the gaps stochastic nature.

The third and the fourth time intervals are quite

short, so that their statistical deviations can be

neglected. Deviations ofw and delay contribute to a

total deviation to the instant of starting of contactsmovement. Finally, the last two deviations

contribute to the slope of dielectric strength

characteristic. Therefore, the basic statistical

deviations which should be always considered,

despite of their source, can be condensed to the

following two:

Deviation of the starting instant of contactsmovement (), depending only on the

performance of the controller.

Deviation of RRDS, depending only on the

performance of the circuit-breaker.

The maximum deviations can be estimated with

a great probability (up to 99.99%) by the

manufacturers. In particular, a controller with a

maximum of 2 ms is easily constructed. The

same is valid for a maximum of 1 ms, but with

higher cost, while there is no reference for a

reduction of below 0.7 ms. On the other side, a

deviation of RRDS due to contacts velocity does not

exceed 5%, while the deviation due to the gaps

stochastic nature is higher (up to 20%).

The existence of a deviation

causes abilateral parallel shift to the gap dielectric strength

characteristic, creating an area of possible instants

of contacts mechanical separation as wide as 2,

as shown in Fig.6. This results in the reduction of

the arcing duration (Tarcing). The deviation of RRDS

contributes to a further increase of the probability of

a reignition, as shown in the same figure.

Breaking current

(TRV)Recovery Voltage

Transient

TT

Fig.6: Effect of and RRDS deviation to the

instant of contacts mechanical separation

3 Calculation MethodsFrom the preceded analysis, it became obviousthat calculation of the optimum switching instants in


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real networks is quite complicated. In practically all

cases, the current breaking instant (opening instant)

does not coincide with the instant of mechanical

separation of the circuit-breaker contacts (target

instant). The presence of statistical deviations leads

to the claim that instead of a simple optimum

opening instant and the respective target instant, it is

more realistic to talk about an optimum window

of target instants. Furthermore, three-phase

operation of the network leads to the final

conclusion that we should talk about optimum

combination of target instants windows.

From all the above mentioned, it is obvious that a

systematic calculation of the optimum target instants

windows combinations, taking into account all

network parameters, circuit-breaker electrical

characteristics and various statistical deviations. It is

also necessary to assess the controlled switchingperformance for the prevention of reignitions. Both

of these aims are met successfully through the

application of the algorithms described in the next

paragraphs.

3.1 Basic Steps of the AlgorithmThe first step of the algorithm run is the precise

calculation of the waveforms of breaking currents

and TRVs, as they are derived after successful

(without reignition) breaking of the currents in the

preceded phases. It should be noted, that except thewaveform of the breaking current of the first phase,

the waveforms of the rest phases depend in general

on the instants of electric breaking in the preceded

phases. However, if the peak value of the breaking

current is well higher than the chopping level, the

electric breaking in each phase occurs exactly at the

chopping instant and consequently, it is independent

of the instant of contacts mechanical separation.

Therefore, for each combination of values of

chopping level and the electric parameters of the

grid, the waveforms of the current and TRV in each

phase are precisely defined and independent of theinstant of contacts mechanical separation in this

case. This is not valid if chopping level is at least of

the same order of magnitude with the peak value of

the breaking current, since electric breaking may

occur even simultaneously to the instant of contacts

mechanical separation. As a consequence, in this

case the waveforms of the breaking current of the

two last breaking phases, as well as of all TRV, are

functions of the instants of contacts mechanical

separation in the preceded phases.

In the next stage, the user defines the variation

step of the possible instants of contacts mechanical

separation (tstep), which is set equal for the three

phases for minimization of the number of

parameters, as well as the total number of steps. In

addition, the user defines the vectors of the possible

values of RRDS, of the deviation of the velocity of

contacts separation () and of the deviation of the

slope of gap dielectric strength characteristic due to

its stochastic nature (s). Finally, the user defines

the shape of the gap dielectric strength characteristic

through appropriate polynomial factors. Considering

that the left end of the optimum instants window of

contacts mechanical separation is the first discrete

instant after current zero (equal to the sum of

current zero instant and tstep), the calculation of the

optimum instants window is based on the right limit

of the gap dielectric strength characteristic (see Fig.

6). Therefore, for the slope of this characteristic the

deviation given are those which provided for the

right limit of the instants window of the contactsmechanical separation, since this is the most adverse

one. For instance, if the dielectric strength

characteristic is straight, its formula is:

)tt()s1(RRDS)t,t(V separateseparateright = (2)

In the above formula, tseparate is the step-varied

instant of contacts mechanical separation.

Starting from current zero instant in the first

phase (to), the instant of contacts mechanical

separation increase gradually by tstep, until gap

dielectric strength characteristic intersects therectified TRV waveform. Assuming that this takes

place for an instant of contacts mechanical

separation tseparate = tlast. The optimum instants

window for contacts mechanical separation for the

first phase is then [to, tlast - tstep] and the mean value

of this window combined with maximum T is used

for its designation. Considering that deviation is

bilateral, the absolute value of the maximum , so

that the reignition is avoided, is derived from the

following equation:

2tttT osteplast = (3)

In case that chopping level is of the same order

of magnitude with the breaking current peak value,

the process in this stage is modified, so that the left

end of the optimum instants window of contacts

mechanical separation does not necessarily coincide

to current zero. The optimum instants window starts

from the first instant of contacts mechanical

separation which does not lead to reignition.

The same process is repeated for the rest phases.

Finally, the minimum value among the maximumallowed derived for the three phases is chosen as


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the maximum allowed for the whole system. The

combination of the optimum instants windows of

contacts mechanical separation, as well as the

maximum allowed DT, depend on the combination

of the values of the grid parameters, the chopping

level (only when it is well lower than current peak

value), the circuit-breaker dielectric characteristics

and the corresponding deviations.

Any common programming package (Matlab,

Mathcad, C, Fortran etc.) can be used for the

application of this algorithm.

3.2 Calculation MethodsAs mentioned in the previous sections, for the

application of the algorithm of the optimum instants

of contacts mechanical separation, the calculation of

the breaking current in each phase is required. Thiscalculation can be performed via either analytical or

numerical methods. Analytical methods comprise

the solution of systems of differential equations

describing the transient behaviour of the network.

The Current Injection Method is such a method,

which is proposed for the application of the

algorithm. On the other side, the most conventional

method for the numerical solution of the network

during transient conditions is the use of programs

like ATP/EMTP [6, 7]. The application of both

methods is described in the next paragraphs.

3.2.1 Current Injection Method

The widely known Current Injection Method is

used for the network solution in current breaking

cases, assuming a linear network [4, 8]. According

to this technique, the calculation of the transients is

based on the fact that the current elimination at the

instant of its breaking (tbreak) is equivalent to the

injection of the same magnitude and opposite

polarity to the breaking current, as shown in Fig.7.

Therefore, the transient voltages and currents in all

network places are derived from the superimposition

of their instant values which they would havewithout the breaking and the respective values

obtained after the aforementioned current injection

at the instant tbreak. The application of the method

can be summarized in the following steps:

1. Calculation of the steady-state voltages andcurrents in various network places before the

breaking of the first phase.

2. Development of an equivalent network after thereplacement of voltage and current sources with

open- and short-circuits, respectively. The

breaker pole to open is replaced with a current

source, connected at the instant tbreak, with the

same magnitude and opposite polarity to the

breaking current.

3. Calculation of voltages and currents in theequivalent network after the instant tbreak.

4. Results of step 3 are superimposed to those ofstep 1, for the total expressions of the voltages

and currents after the breaking of the first phase.

5. The process is repeated for the breaking of therest phases, using the voltages and currents after

step 4 after the breaking of the preceded phase

instead of the respective steady-state quantities.

Breakingcurrent

Injectedcurrent

Superimposition ofthe two currents

breakt

Result(actual current)

Fig.7: Current injection method

The advantage of Current Injection Method

against the direct network solution is that there is no

need for the (often hard) calculation of the initial

conditions, since their effect is taken into account

through the superimposition. However, its results

are valid only for networks consisting exclusively of

linear elements. Another disadvantage is that the

calculation of the transients are usually quite

complicated, due to a plenty of reasons, like the

unsymmetrical three-phase networks in the

intermediate steps (different breaking instant in each

phase), the inductive and/or capacitive coupling

between the three phases, as well as the existence of

elements with distributed parameters or complicated

models (lines, cables, transformers). For these

reasons, the application of the Current InjectionMethod is just used for the comprehensive approach

of the relative switching phenomena and the

assessment of the performance of controlled

switching application to a specific current breaking

case. In any case, the use of numerical methods is

necessary for a more precise assessment.

3.2.2 Use of ATP/EMTP Program

For the numerical solution of the network in

transient conditions, the widely known ATP/

program is used. The model of Statistical Switch

included in this program is the most suitable circuit-breaker model for controlled switching simulations.


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The use of this model causes automatically the

repeated run of the simulation for different

combinations of instants of contacts mechanical

separation in each phase, following a uniform

probability distribution with user-defined mean time

and standard deviation (see Fig.8), since the interest

is focused on the most adverse transients and not on

the probability of their occurrence.

Fig.8: Probability distribution of opening time Tf(T): Density function, T : Mean time,

: Standard deviation

For instance, in the usual case of the desired

instants window of contacts mechanical separation

corresponds to a 50 Hz half-cycle (10ms), it is

derived that = 2.887 ms.

According to the general principle followed by

ATP/, electric breaking is achieved at the first

instant after contacts mechanical separation at which

current criterion is met. As current criterion is

defined the condition, at which the absolute instantvalue of breaking current is lower than a user-defined

current margin, according to the following figure. It

is obvious, that the above current margin represents

the chopping level.

Fig.9: Illustration of application of current

criterion in an EMTP switch during current

breaking

In the majority of the current breaking cases, a

number of automatic re-runs between 100000 and

150000 is enough for the precise assessment of the

optimum instants windows of contacts mechanical

separation.

The use of any similar numerical software, such

as ATP/EMTP, aims to the precise evaluation of the

performance of controlled switching application, but

it does not ensure the thorough investigation of the

problem. For this reason, it would be better that

such software is used in a second stage, after the

application of an analytical method, such as

Current Injection Method to a simplified network

model, which is more suitable for the identification

of the possible issues.

4 ConclusionsIn this paper, a new methodology has been

proposed, for the calculation of the optimum

switching instants for current interruption cases. The

methodology is based on the Current Injection

Method that eliminates the need for exhaustive

simulations for an initial assessment of the

performance of a possible controlled switching

application. Circuit-breakers characteristics, like

contact operation time scatter and deviation of theslope of the contact gap voltage withstand

characteristic are taken into account in this method.

References:

[1] CIGRE WG13.07, Controlled Switching ofHVAC Circuit-Breakers - Planning,

Specification and Testing of Controlled

Switching Systems,Electra No 197, pp. 23-33,

August 2001.

[2] CIGRE WG13.07, Controlled Switching of

HVAC Circuit-Breakers - Guide forApplication Lines, Reactors, Capacitors,

Transformers, 1st

Part: Electra No 183, April

1999 - 2nd

Part:Electra No 185, August 1999.

[3] CIGRE Task Force 13.00.1, ControlledSwitching: A State-of-the-Art Survey, 1

stPart:

Electra No 163, December 1995 - 2nd

Part:

Electra No 164, February 1996.

[4] C.D. Tsirekis, N.D. Hatziargyriou, B.C.Papadias, Controlled Switching Based on the

Injection Method, International Conference on

Power Systems Transients (IPST 97), Vol. II,

pp. 405-410, Rio de Janeiro, Brazil, June 2001.

[5] CIGRE WG13.02, Interruption of SmallInductive Currents - Chapter 3, Part A, Electra

No 75, March 1981.

[6] Leuven EMTP Center, ATP Rule Book, June1993, Leuven (Belgium).

[7] Bonneville Power Administration, EMTPTheory Book, Oregon (Portland), 1986.

[8] W.M.C. Van Den Heuvel, B.C. Papadias,Interaction Between Phases in Three-Phase

Reactor Switching, 1st

Part:Electra No 91, pp.

11-50, Dec. 1983 - 2

nd

Part: Electra No 112,pp. 57-81, May 1987.

iSWITCH

t

Switch opens

Current forced to zero in next step

Current margin

Current margin

Current

3

f(T)

32

1

+ 3


ISBN: 978-1-61804-023-7 434

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