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Control Circuit Transients MULTILIN
GER-3061
Control Circuit
Transients
GE Power Management
1
It has been well established by
field experience that transient
potentials of significant
magnitudes (several kv) can be
induced in the secondary
cables and d-c control wiring in
switching stations. These
overvoltages may be present
during ordinary operating
conditions. Commonly, they
have been referred to as
surges.
With the introduction of high-
speed solid-state relay
equipment and other solid-
state control equipments, the
normal presence of these
surges has Control-Circuit-Transients/' >taken on increased
importance. This is due to
both the fast response of
semiconductor devices and
their susceptibility to damage
by surges. Concurrent with the
advent of solid-state relays
has come the introduction of
higher and higher voltage
systems. This also has affected
the situation adversely,
because the magnitude of
surges in station control
circuits on EHV systems tends
to be more severe than
on lower-voltage systems.
Successful application of solid-
state equipment and controls
to EHV systems requires,
therefore that engineers
understand surges what
causes them, how they get
into control circuits, and what
can be done to prevent false
operation or damage to the
protective relay system.
We will deal here with the
theoretical aspects of one of
the most common causes of
surges. In particular, the
various ways by which surges
from this source can be
coupled into the control circuit
will be examined in detail. For
purposes of discussion,
consider that there are two
classes of electrical conductor
systems in a high-voltage
switching station. These are
described as follows:
1. EHV Power Circuits: All of
the EHV buses and
apparatus, primary circuits
of instrument transformers
and devices and all circuits
operating at high potential;
also, the ground grid and
apparatus grounds.
2. Control Circuits: All
instrument transformer
secondary circuits, all
Control Circuit Transients - Part 1
Application of solid-state controls to EHV systems requires an understanding of surges. Here is an
explanation of how surges are caused and how you can cope with them.
By W. C. KOTHEIMER,
Power Systems Management Dept.,
General Electric Co. 2
battery, d-c control, a-c
auxiliary power, protective
relaying and communication
circuits; normally, these
circuits will operate at
potentials of only a few
hundred volts or less.
Of necessity, these two classes
of conductor systems are
located in near proximity (with
due regard for insulation
distances). They can be quite
close inside of EHV apparatus
such as current and potential
transformers (CTs and PTs).
They can also be quite close in
the ground system. A
consequence of this proximity
is that both intentional and
stray electromagnetic coupling
will exist between these
systems. It follows, therefore,
that an electrical disturbance
in one of the systems will
result in a coupled or induced
effect in the other.
A disturbance in the EHV
system, even though small,
can result in a relatively large
effect in the control-circuit
system. The transient
potentials and currents
resulting can be orders of
magnitude greater than the
normal operating values for
the control circuit. Another
consideration is the effect of
disturbances in one control
circuit on another, or upon
equipment in the same circuit.
This generally causes transient
of lesser magnitude
7, 8, 9, 10
.
Sources of transients
A number of phenomena that
occur on EHV systems give
rise to transient electro-
magnetic field disturbances
and induce transient poten-
tials in secondary or control
circuits. Some of these sources
are: (a) switching of shunt
capacitor banks in parallel, (b)
flashover of protective gaps
due to overvoltages; (c)
restriking of a circuit breaker
and (d) switching a section of
EHV bus by an air break
disconnect switch.
Doubtless, the list should be
much longer. The sources
given merely serve to illustrate
types of phenomena which
can induce control circuit
transients. They are given in
their approximate order of
severity; the most severe, but
not the most prevalent, is
switching of parallel EHV
capacitor banks. This
phenomenon has been known
to induce transient potentials
of about 8 kv in control
circuits.
1
Probably the most prevalent
source of surges is (d) the
switching of a section of EHV bus
by an air break disconnect
switch. Several papers have been
written on this subject.
2, 3, 4, 5.
However, it is still one of
the most elusive of the
phenomena that occur in a
switchyard. Some rather
elaborate explanations have
been proposed relating the
frequency (wavelength) of the
transients generated to the
length of the buses. Actual
frequencies measured in the
field, 300 kHz to 1.5 MHz, do
not, however, lend support to
these explanations, as they 3
would indicate unusually
long bus structures. It is
more probable that other
parameters, in addition to
the length of the buswork,
also affect the character
of these transients, notably
the shunt capacitance to
ground of various apparatus.
For example, the capaci-
tance of coupling capacitor
potential devices (CCPDs) and
bushing capacitances of CTs,
trans-formers, circuit breakers,
etc. In fact, when these
parameters are included in
the analysis, rather good
agreement is obtained with
actual field data.
Role of the shunt capacitance
of EHV apparatus can be
illustrated by an example.
Consider the EHV switching
station shown in one-line
diagram form in Fig. 1.
It consists of a ring bus
structure. Notice the area
of interest encircled on
the diagram. It contains two
current transformers, A and E;
a circuit breaker, B, and two
disconnect switches, C and D.
Consider the case where all of
the breakers are open, all of
the disconnect switches in the
station are open except C and
D, and the transformer bank is
excited from the 161-kv side.
Switch C is then opened.
Figure 2 shows an elevation
view of the area of interest.
The dimensions shown are
important. The two CTs are
120 ft apart and the EHV
conductor connecting the
apparatus is 30 ft above
the ground grid. Only one
phase is shown, but similar
arrangements exist for the
other two phases.
Disconnect switch C is opened
and a restriking arc occurs
between its arms, making and
breaking the charging current
flowing into the open-breaker
voltage-dividing capacitance
and the CT-bushing capaci-
tance in series. This transient
disturbance causes a train of
damped oscillatory currents to
flow around a loop in the
vertical plane consisting of the
two CT bushings, the open-
breaker capacitance, and the
loop inductance in series.
Figure 3 shows the equivalent
circuit. In this circuit the total
self inductance of the loop in
the vertical plane between the
CTs is represented by L1 and
L2 in series, as shown. The
bushing capacitances of the
CTs are CA and CE. The
capacitance across the open
circuit breaker (the built-in
voltage equalizing capacitors)
is CB. The inductance of the
long loop back to the
transformer bank is shown as
L3. The 60-Hz voltage source
equivalent of the transformer
is also shown.
Using the dimensions given in
Fig. 2 and assuming that all of
the conductor diameters are
1 in., the loop inductances L1,
L2, and L3 can be calculated
approximately as 60
µ
h, 60
µ
h,
and 268
µ
h respectively.
13
Now consider the phenomenon
occurring when the disconnect
switch is opening. As in line
dropping by a circuit breaker,
capacitors CA and CB in series
are left with a trapped charge
so that the static potential on
one side of the switch will be V
max. Meanwhile, a half cycle
later the 60-Hz source will have
reversed the potential on the
opposite pole of the open
switch causing the potential
across the switch to be 2 x V
max. Assume that the switch
cannot withstand this difference
of potential and the gap breaks
down. Then an oscillatory
discharge will occur giving rise
to high frequency currents in
the vertical plane loops. For
the case illustrated, the
dominant oscillation occurs in
the loop be