Digital Testing of High Voltage Circuit Breakers
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INTRODUCTION
Acircuit hreaker is a switching device that the

American National Staiidarcls Institute (ANSI)

defines as: "A mechanical switching device, capable

of making, carrying, ancl breaking currents under normal

circuit conditions and also making, High-voltage circuit breakers play an impartant rolc in

transmission and distribution systems. They must clear

faults and isolate faulted sections rapidly and reliably. In

short, they must possess the followiiig qualities:

In closed position, they are good conductors

In open position, they are excellent insulators

w They can close a shorted circuit quickly and safely

without unacceptable contact erosion

They can interrupt a rated short-circuit current, or

lower current, quickly without generating an abnormal

voltage.

The only physical mechanisiii that can change in a short

period of time from a conducting to an insulating state at

a certain voltage is the arc. It is this principle on which

all circuit breakers are Iiased.

Circuit Breaker Switching and Arc Modeling
The switching action, the basic function oi thc circuit

breaker, refers to the change from conductor to insulator

at a certain voltage. Be€ore interruption, the (shortcircuit)

current flows through the arc channet of the

circuit breaker. Bccause of the nonzero resistance of the

arc channel, this short-circuit current caiises a voltage

across the colitacts of the circuit breaker: the arc voltage.

The arc behaves as a nonlinear resistance. Thus,

both arc voltage and arc current cross the zero-value at

the same time instant. If the arc is coded sufficiently at

the time the current goes through zero, the circuit breaker

interrupts the current, because thc electrical power

input is zero. During current interruption, the arc resistance

increases from practically zero to alinnst infinite in

microseconds. Immediately aftcr current interruption,

the transient recovery voltage builds up across the circuit

breaker. As the gas mixture in the interelectrodc

space does not change to a completely insulating state

instantaneously, the arc resistance is finite at that time,

and a sniall current can flow: thc post-arc current,

Measurements and Data Analysis
I-ligh-resolution ineasurcmcnts of current and voltage in

the critical period around short-circuit current zero

must supply the necessary parameters, characterizing

the breakers' behavior. A tailor-macle high-frequency

measuring system was realized for this purpose. This

system consists of a number of battery-pciwcred, singlechaniicl,

40 MHz, 12 bit AD converters, each storing the

data temporarily in on-board local RAM (2Stik samples

each). The concept of on-site data storage is necessary

for reaching a maximum overall sysLern bandwidth,

Cables to the current and voltage S ~ I I S O ~caSn thus be

kept very short, and thc system can operate 011 floating

potential. The arc voltage is measured with standard

brnad-band RCR-type voltage dividers; current is measurecl

with a spccial Rogowski coil. After the remote

RAM is fillcd, data is transmitted serially through optical

fibers to the proccssirig unit in the command ccntcr. The

greatest challenge with respect to developing the equiprneiit

in this application design lies in the electromagnetic

compatibility, since the microelectronics has to

function in an extremely hostile environment of intense

EM fields of various origin.

Arc-Circuit Interaction Software
At the final stage of the realization of digital testing, measured

arc model parameters will be used as input for the

arc model. Of course, this arc model behaves a a nonlinear

element in the electrical circuit ancl must therefore

be analyzed with a dedicated cnmputer program. The

analysis of arc-circuit interaction iiivolving nonlinear clcments

in relation to stiff differential equations makes it

necessary to perform the calculations with a variable

step size and adjustable accuracy of the computed currents,

voltages, and canductaiices. Because they have

fixed step-size solvers, EMTP ancl comparable programs

are less suitable for this purpose and thercfore a new

approach, the integration of differential algebraic cquations

(DAE) by means of the backward differentiation formulas

@DO method.