Página 652 - BURNDY

Versión de HTML Básico

Substation — Welded/EHV
US: 1-800-346-4175
Canada: 1-800-387-6487
Blue highlighted items are industry standard and most frequently ordered.
Connectors for use in EHV Substations must
meet essentially the same electrical and
mechanical requirements as those for other
power connectors. However, operation at
extra high voltages imposes an important
additional requirement. They must not pro-
duce corona discharges that interfere with
radio reception and cause energy loss.
Corona forms when the voltage gradient at
the surface of a conducting material exceeds
a critical value and ionizes the surrounding
air. For conductors, the four basic factors
that determine surface voltage gradient are
distance from ground, conductor diameter,
phase spacing and voltage.
In A.C. circuits, there are two basic kinds of
corona. Negative corona forms during the
negative half cycle, and positive corona dur-
ing the positive half cycle. Negative corona
generally appears as a glow on conventional
conductors at about 20 kV rms/cm. Its ampli-
tude is relatively low and causes no signifi-
cant radio interference. Positive corona
appears as a plume at above 30 kV rms/cm.
Its amplitude is about 50 times higher than
that for negative corona and is the major
cause of radio interference.
EHV Connectors are designed so
that under fair weather operating conditions,
the voltage gradient at the connector surface
will be at a level that will not cause corona
and the resultant radio interference. (RIV)
Cable Connectors
For reasons of economy, EHV systems using
stranded conductor are generally designed
to operate at voltage gradients close to the
negative corona onset level. It is essential,
therefore, that connectors provide corona-
free performance superior to that of the
cable. So our design criterion calls for the
voltage at which corona extinguishes from
the connector to be higher than the voltage
at which it extinguishes from the cable. This
criterion is met by eliminating all projections
and by providing smooth contours on all sur-
faces. On compression elements, the ends
are especially critical. Carefully designed
tapers are provided to keep the voltage gra-
dient at a level lower than that on the con-
ductor. Of course, it is still necessary during
installation to smooth crimped elements.
On accessories, like spacers for bundled
lines, the critical areas are those at the edges
of the bundle. The bundle itself generally
shields those parts that fall within it. Many
projections that would cause corona on a
single conductor line are quiet when they fall
within the shielding influence of a bundle.
However, those parts that fall at the edges
are carefully finished at the factory to insure
corona-free operation.
Tubular Bus Connectors
Station designers choose tubular bus sizes
on the basis of mechanical rather than elec-
trical requirements. For instance, stations
that only need 4" IPS to meet electrical and
corona requirements often have 6" IPS as
main buses. The resultant voltage gradient
on these buses is very low, perhaps only 10
kV rms/cm, well below the corona onset
It is impractical, therefore, to require that
connectors operate quieter than the bus
regardless of voltage. Under some circum-
stances, it might be impossible to meet such
criteria. In most cases, it would be prohibi-
tively expensive to do so.
Of course, theoretically optimum connectors
could be designed for each application,
based on the design voltage gradient for
individual stations. However, in most cases
even differences as great as that between
345 and 500 kV don’t have a meaningful
impact on connector costs. So, from a prac-
tical point of view, it is feasible to design
most connectors for 500 kV operation. This
makes it more convenient for station design-
ers to select and order connectors.
Bus connectors are designed to provide
corona-free performance under conditions of
actual operation. This is done by calculating
the voltage gradient on the surface of the bus
at 500 kV, using the phase spacing and
ground distance typical for this voltage.
Connectors are then designed to operate
corona free when the voltage gradient on the
bus is 10% above this value.
The exceptions to this rule are the flexible
expansion connectors. Those designed for
345 kV are self-shielding. Those for 500 kV
have separate shielding rings. Experimental
work on self-shielding 500 kV expansion
connectors indicates that the margin of safe-
ty is too small to justify recommending them
for this voltage.
Since corona is caused when the voltage
gradient at the surface of a conducting mate-
rial reaches a level that causes the surround-
ing air to break down, then obviously, the
way to prevent corona is to keep the gradient
below this critical level.
From the point of view of the connector
designer, this can be accomplished in
three ways:
1. By providing generous radii on all out-
side surfaces to keep the voltage
stresses to a minimum.
2. By providing shielding rings.
3. By placing the connector within the
shielding influence of some part of the
bus structure.
Since it is impossible for the connector
designer to know the exact configuration of
every bus system where the connectors
might be used, the third approach is not
practical. So, for purposes of developing a
standard line, we concentrate on the first
Whenever possible, connectors are designed
to be self-shielding. This approach leads to
less costly and less obstrusive designs. Only
in the case of complicated connector config-
urations do BURNDY
EHV designs use
corona rings. Examples of such applications
are disconnectable equipment taps, expan-
sion couplers and equipment terminals
which often have configurations that pre-
clude the use of self-shielding designs.