RE: Digital Testing of High Voltage Circuit Breaker

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, Highvoltage 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 shortcircuit 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. Beā¬ore interruption, the (shortcircuit)
current flows through the arc channet of the
circuit breaker. Bccause of the nonzero resistance of the
arc channel, this shortcircuit 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 zerovalue 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 postarc current,
Measurements and Data Analysis
Ilighresolution ineasurcmcnts of current and voltage in
the critical period around shortcircuit current zero
must supply the necessary parameters, characterizing
the breakers' behavior. A tailormacle highfrequency
measuring system was realized for this purpose. This
system consists of a number of batterypciwcred, singlechaniicl,
40 MHz, 12 bit AD converters, each storing the
data temporarily in onboard local RAM (2Stik samples
each). The concept of onsite 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
brnadband RCRtype 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.
ArcCircuit 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 arccircuit 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 stepsize 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. 

