[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

the conductivity of the water-soluble deposits on the insulator surface. It is expressed in terms of the
density of sodium chloride deposited on the insulator surface that will produce the same conductivity.
Following are typical environments for the four contamination levels listed [8]:
Light-contamination areas include areas without industry and with low-density emission-producing
residential heating systems, and areas with some industrial areas or residential density but with frequent
winds and/or precipitation. These areas are not exposed to sea winds or located near the sea.
Medium-contamination areas include areas with industries not producing highly polluted smoke and/
or with average density of emission-producing residential heating systems, areas with high industrial
and/or residential density but subject to frequent winds and/or precipitation, and areas exposed to sea
winds but not located near the sea coast.
Heavy-contamination areas include those areas with high industrial density and large city suburbs
with high-density emission-producing residential heating systems, and areas close to the sea or exposed
to strong sea winds.
Extra-heavy-contamination areas include those areas subject to industrial smoke producing thick,
conductive deposits and small coastal areas exposed to very strong and polluting sea winds.
3.2.6.3 High-Current Bushings within Isolated-Phase Bus Ducts
As already noted, there are applications where temperatures can exceed the thermal capabilities of kraftpaper
insulation used within bushings. One such application is in high-current bushings that connect
between generator step-up transformers (GSUT) and isolated-phase bus duct. Typically, forced air is
used to cool the central conductors in the isolated-phase bus duct, air forced toward the GSUT in the
two outer phases and returned at twice the speed in the center phase. Air temperatures at the outer ends
of the bushings typically range from 80 to 100C, well above the standard limit of 40C. This means that
either a derating factor, sometimes quite severe, must be applied to the bushing’s current rating, or
materials with higher temperature limits must be used.
In older bushings, which were of the solid type, the only materials that were temperature limited
were the gaskets, typically cork neoprene or nitrile. In this case, these gasket materials were changed
to higher-temperature, oil-compatible fluorosilicon or fluorocarbon materials. However, solid-type
bushings do not have low-partial-discharge characteristics. Therefore, as requirements for low-partialdischarge
characteristics arose for GSUTs, a capacitance-grade core was used. As has already been
explained, kraft-paper insulation is limited to 105C, so that higher-temperature materials have been
adapted for this purpose. This material is a synthetic insulation called aramid, i.e., Nomex , and it
has a limiting temperature in the order of 200C. The material with the next-highest limiting temperature
is the mineral oil, and to date, its temperature limits have been adequate for the high-temperature,
high-current bushing application.
3.2.7 Accessories Commonly Used with Bushings
3.2.7.1 Bushing Potential Device
It is often desirable to obtain low-magnitude voltage and moderate wattage at power frequency for
purposes of supplying voltage to synchroscopes, voltmeters, voltage-responsive relays, or other devices.
This can be accomplished by connecting a bushing potential device (BPD) [13] to the voltage tap of a
condenser-type bushing. Output voltages of a BPD are commonly in the 110 to 120-V range, or these
values divided by 1.732, and output power typically ranges from about 25 W for 115-kV bushings to 200
W for 765-kV bushings.
A simple schematic of the BPD and bushing voltage tap is shown in Figure 3.2.8. The BPD typically
consists of several components: a special fitting on the end of a shielded, weatherproof cable that fits
into the voltage tap of the bushing; a padding capacitor that reduces the voltage seen by the BPD; a main
transformer having an adjustable reactance; an adjustable-ratio auxiliary transformer; a tapped capacitor
used to correct the power factor of the burden; a protective spark gap in case a transient voltage appears
on the bushing; and a grounding switch that enables de-energization of the device. All items except the
FIGURE 3.2.8 Bushing potential device.
first are housed in a separate cabinet, typically mounted to the side of the transformer or circuit-breaker
tank. Since the BPD is essentially a series-tuned device, output phase shift is sensitive to output frequency.
The greatest phase shift is experienced when the BPD is loaded to its rating and system voltage is low
relative to the bushing rating.
If the BPD is called upon to carry a burden beyond its capacity, the voltage appearing on the tap rises.
If it rises enough, it will cause the protective gap to operate. This phenomenon is also a consequence of
the series-resonant circuit in the BPD.
3.2.7.2 Upper Test Terminals
In order to perform periodic maintenance tests on bushings, transformers, and other electrical equipment,
it is necessary to disconnect the line leads from the bushing terminals. This often requires the use
of bucket trucks and/or lifting cranes to loosen the connections and lower the leads, particularly on the
higher voltage ratings. This operation therefore requires several people and a substantial amount of time.
A device known as the Lapp test terminal [14] is used to simplify this operation. This device, shown
with the shunting bars opened and closed in Figure 3.2.9, is made of a short length of porcelain, mounting
terminals on both ends, and some shunting bars that connect both terminals during normal operation.
Its bottom terminal connects to the top terminal of the bushing, and the line leads are connected to its
top terminal. When maintenance tests are required, one end of each shunt is loosened, and the shunts
are swung away so that there is no connection from top to bottom. This enables the line to be isolated
from the bushing without actually removing the line, and the testing on the transformer or other
equipment can proceed, saving both time and manpower.
The bushing and outer-terminal design for the bushing must be adequate for the use of the Lapp test
terminal. The outer terminal must be capable of withstanding the moment placed on the top of the test
terminal without permanently bending the bushing’s top terminal or upper part of the central conductor,
and the bushing must be capable of withstanding the extra moment placed on it. The primary location
of concern is at the bottom of the upper insulator, which can be lifted up off the sealing gasket, if one
is used, and allow leakage of the internal insulating oil.
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC