Fast Electrical Switch Paper - UTRON, Inc.

8506 Wellington Road, Suite 200


Dennis Massey

Manassas, VA 20109


dennism@utroninc.com

(703) 369
-
5552


Lester Via

www.utroninc.com


lesterv@utroninc.com


UTRON’S FAST ELECTRICAL SWITCH


The speed of
protective device
s used to isolate short circuit faults

is a
n

important

factor in the
d
esign

of
electrical power distribution system
s
.

Higher short circuit currents can be tolerated wh
en

faster protective
devi
ces

are used
.

While
the short circuit
current
can be

limited by
increasing
the
source impedance,
this
results in

greater voltage changes in response
to
t
he normal

variations in

load current

–

an undesirable
effect.

Faster protection devices allow greate
r flexibility in distribution system design, particularly in
systems using locally generated power where
the r
ate of short circuit current rise is not limited
by long transmission lines.



Conventional
60 Hz
AC protective breaker
s are
relatively slow because the

arc formed when
electrical contacts
open allows
the fault
current
to
continue
flow
ing

even though the contacts are
mechanically separated. This arc
only quenches
during a current zero crossing
,

which
o
ccurs
every 8.33 mil
liseconds
,

l
imit
ing

what can be
achieved by mechanical speed

alone
. A fast DC
breaker would be needed to achieve
response
times

shorter tha
n

the half cycle

time of the AC
waveform.


UTRON has developed
a high speed DC breaker
by combining

a
highly effecti
ve

arc
quenching technique
with a high speed
actuation device
. An
Experimental device utilizing this breaker technology successfully interrupted
a 9000 ampere, 4.5 kilovolt
circuit in less than 400 microseconds.

Even higher speeds should be achievable.


The
high
speed of the mechanical
switch
actuator is achieved
by suddenly heating the gas on one side
of a
drive
piston

by discharging
electrical
energy stor
ed

in a
capacitor through the working gas
.

The gas
pressure, initially at 150 psi,
suddenly
increas
es
to

more than 5,000

psi

in less than
50 microseconds
.

This
pressure
increase
provides the force needed to rapidly accelerate the switch
’s moving parts
.

Once
the piston moves a short distance, the working gas is
allowed to pass

around the piston
, pressu
rizing both
sides,
thus
removing the drive force. A hydraulic damper mechanism th
en

brings the rapidly moving
switch parts to a stop in its
fully
actuated position.


The contact arc is quenched
using

a
switch
contact configuration that forces the arc to b
e confined
between t
w
o close fitting
electrically
insulating surfaces
. Th
is

arc must exist
in the form of

a thin sheet
between the close
ly

fitting surfaces
,

increas
ing

its

surface area compared to a free arc which will form a
cylindrical cross section. T
he thin sheet arc, having a much greater surface area, loses
heat
much faster
than a free arc, requiring a much greater voltage to sustain it.
The

arc
, having a negative resistance
characteristic, quickly

quenches when
i
t
s

source voltage is
unable
to sust
ain t
he initial current
.

Metal
oxide varistors (MOV
s
) may be needed to limit the rise in voltage that will occur due to the inductance of
the source and the load. The MOVs are selected to have a breakdown
higher than the highest voltage
normally seen on

the line being protected.