For several years, the increased
application of solid-state devices for protective relaying and
control and for electronic equipment, such as audio tones, carrier
and microwave equipment, event recorders, and supervisory control
equipment, in EHV substations has resulted in many equipment
failures.
Many of these failures have been
attributed to transients or surges in the control circuits connected
to the solid-state devices. Failures due to transients or surges have
been experienced even with conventional electromechanical devices.
The failures being experienced are
attributed to the use of EHV (345 kV and higher voltage levels) as
well as the presence of unusually high short-circuit currents. One of
the major sources of transient voltages is the switching of
capacitances, for example, the operation of a disconnect switch which
generates high-magnitude, high-frequency oscillatory surge currents.
The transient magnetic fields
associated with these high-frequency surge currents are both
electrostatically and magnetically coupled to cables in the area.
Induced voltages have been reported to be as high as 10 kV in cables
without shielding, and the frequencies of these induced voltages have
been reported to be as high as 3 MHz.
In order to avoid insulation breakdown
at 10-kV crest and possible false operation of relays, it is
important that station design includes necessary precautions to limit
the undesirable surges and control circuit transients to an
acceptable minimum.
In any station design there are several
precautions that can be taken. All cable circuits that are used in a
substation should be run radially, with each circuit separated from
any other circuit and with both supply and return conductors
contained within the same cable.
If a conductor is routed from the
control house to a point in the switchyard with the return circuits
following different paths, loops may be formed that are inductive and
are subject to magnetically induced voltages. However, when the two
conductors involved are both affected by the same field, the voltage
appearing between them at the open end should be essentially zero.
Because of ground-mat potential
differences and longitudinally induced voltages in the radial
circuits, proper cable shielding is necessary to maintain lowest
possible voltages on the cable leads. The cables that require
shielding include control, current, and potential transformer
circuits.
The shield should be of as low
resistance as possible, and it should be connected to the ground grid
at least at both ends. To reduce penetration of a magnetic flux
through the nonferrous shield (lead, copper, bronze, etc.), a current
must flow in the shield to produce a counterflux, which opposes the
applied flux.
Ground-grid conductors should be placed
in parallel to and in close proximity to the shield to maintain as
low a resistance between the ends as possible and also to form a
small loop to reduce the reactance between ground and the shield.
Without close coupling of the conductor and ground shield, the
propagation time of the two paths could differ so that a voltage
impulse could arrive at the receiving end with a time difference,
hence causing an unwanted voltage difference.
All control, potential-transformer, and
current-transformer cables should be shielded, with the shield
grounded at the switchyard end and at the control-house end. In
addition, each group or run of conduits and cables should be
installed with a separate No. 4/0 bare stranded copper cable buried
directly in the ground and grounded and bonded to the control-cable
shield at each end of each cable.
The bare copper cable should run as
closely as possible to the cable run. The heavy cable functions to
provide a low-resistance path in an attempt to prevent heavy fault
currents from flowing in the shield and to reduce reactance between
ground and shield.
In order to limit induced voltages, the
control-cable runs should be installed, where possible, at right
angles to high-voltage buses. Where it is necessary to run parallel
to a high-voltage bus for any appreciable distance, the spacing
between cables and high-voltage buses should be made as great as
possible. Distances of at least 50 ft should be maintained.
It is further considered good practice
to have both current-transformer and potential-transformer leads
installed with the ground for the secondary wye neutral made at the
control-house end rather than at the switchyard end. Any rise due to
induced voltages will be concentrated at the switchyard and will
ensure operator safety at the control switchboard in the control
house.
The shield can be grounded by using a
flexible tinned copper braid of from 1/2 to 1 in wide. The
shielded-cable outer insulation is peeled back, exposing the sheath.
The 1-in braid is wrapped around the sheath and soldered carefully to
it.
The other end of the braid is connected
to a lug, and solder should be run over the lug to the braid
connection. The lug is then bolted securely to the ground bus bar.
The flexible copper braid circuits should be kept as short as
possible and should be run directly to the ground bus without any
bends, if possible.
It should be pointed out that the
shields should be grounded at multiple points rather than at a single
point, because of the tendency to lose any advantage from
single-point grounding at 50 kHz and above. As an example, assume
that one input and one output terminal of a system are grounded, each
at different points on a common ground plane.
A small noise voltage will usually
exist across these ground points because of currents flowing in the
finitely conductive ground plane. If either the load or source ground
is lifted, a ground loop is no longer formed, and coupling of
unwanted signals is minimized. This is the advantage of having one
physical ground.
Removal of one of the ground
connections achieves a single-point ground only for dc and low
frequency signals. At higher frequencies, ground loops will be
created by capacitance coupling.
Frequencies below 50 kHz are considered
the arbitrary crossover point for single-point grounding. At EHV, the
transient voltages above 50 kHz represent the more serious problem;
for this reason, all cable shields should be grounded at least at two
points. It should be noted that shielding of control cables is
normally provided for substations operating at voltage levels of 138
kV and above.
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