[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)
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1. Design tests<br />
2. Routine tests<br />
3. Special tests<br />
FIGURE 3.2.9 Lapp test terminal. (Photo courtesy of Lapp Insulator Company, Leroy, NY.)<br />
At higher voltages, excessive amounts of corona may occur due to the relatively sharp edges present<br />
on the top or both ends of the Lapp test terminal. Large-diameter corona rings are therefore placed over<br />
one or both ends of the terminal.<br />
3.2.7.3 Draw-Lead Conductors<br />
Normally, bushings have current ratings of 1200 A or higher. Some applications of bushings mounted<br />
in transformers with lower MVA ratings do not require these current-carrying capacities. In these cases,<br />
it is practical to run a smaller-diameter cable inside the hollow central tube in the bushing and connect<br />
it directly to the transformer winding. If the bushing must be removed for some reason, the transformer<br />
oil can be lowered, as necessary, to a level below the top of the transformer tank or turret. Then, the top<br />
of the draw-lead is unfastened from the top terminal of the bushing. The bushing can then be lifted out<br />
of the transformer and replaced with a new one, and the top terminal is reinstalled to the draw-lead.<br />
Finally, the oil can be adjusted to the proper level in the transformer. The use of a draw-lead therefore<br />
enables much faster replacement of bushings and eliminates the need for time-consuming processing of<br />
oil for the transformer.<br />
Current-carrying capabilities of draw-leads are established by transformer manufacturers and are not<br />
standardized at this time. In general, the capability will increase with the cross-sectional area of the cable<br />
and decrease with the length of the cable. For these reasons, larger-sized holes are placed in the central<br />
tubes of bushings with higher voltage ratings. Table 3.2.4 gives the maximum current ratings of drawlead<br />
applications, the current rating of the same bushing for bottom-connected applications, and the<br />
minimum hole size in the central conductor. [6]<br />
3.2.8 Tests on Bushings<br />
3.2.8.1 Categories of Tests<br />
Standards [1] designate three types of tests to be applied to bushings:<br />
TABLE 3.2.4 Current Ratings of Bushings Capable of Being Used with Draw-Leads<br />
Nominal System Voltage, kV<br />
Maximum Draw-Lead<br />
Current Rating, A<br />
Bottom-Connected<br />
Current Rating, A<br />
Minimum Diameter<br />
inside Tube, mm<br />
34.5–69 400 1200 22<br />
138–230 800 1200 41<br />
345–765 800 1200 51<br />
3.2.8.1.1 Design Tests<br />
Design, or type, tests are only made on prototype bushings, i.e., the first of a design. The purpose of<br />
design tests is to ascertain that the bushing design is adequate to meet its assigned ratings, to ensure that<br />
the bushing can operate satisfactorily under usual or special service conditions, and to demonstrate<br />
compliance with industry standards. These tests need not be repeated unless the customer deems it<br />
necessary to have them performed on a routine basis.<br />
Test levels at which bushings are tested during design tests are higher than the levels encountered<br />
during normal service so as to establish margins that take into account dielectric aging of insulation as<br />
well as material and manufacturing variations in successive bushings. Bushings must withstand these<br />
tests without evidence of partial or full failure, and incipient damage that initiates during the dielectric<br />
tests is usually detected by comparing values of power factor, capacitance, and partial discharge before<br />
and after the testing program.<br />
Standards [1] prescribe the following design tests:<br />
Low-frequency wet-withstand voltage on bushings rated 242 kV maximum system voltage and less<br />
Full-wave lightning-impulse-withstand voltage<br />
Chopped-wave lightning-impulse-withstand voltage<br />
Wet-switching-impulse-withstand voltage on bushings rated 345 kV maximum system voltage and<br />
greater<br />
Draw-lead bushing-cap pressure test<br />
Cantilever-withstand test<br />
Temperature test at rated current<br />
3.2.8.1.2 Routine Tests<br />
Routine, or production, tests are made on every bushing produced, and their purpose is to check the<br />
quality of the workmanship and the materials used in the manufacture. Standards [1] prescribe the<br />
following routine tests:<br />
Capacitance and power-factor measurements at 10 kV<br />
Low-frequency dry-withstand test with partial-discharge measurements<br />
Tap-withstand voltage test<br />
Internal hydraulic pressure test<br />
3.2.8.1.3 Special Tests<br />
Special tests are for establishing the characteristics of a design practice and are not part of routine or<br />
design tests. The only special test currently included in standards [1] is the thermal-stability test, only<br />
applicable to extra high voltage (EHV) bushings, but other tests could be added in the future. These<br />
include short-time, short-circuit withstand and seismic capabilities.<br />
3.2.8.2 Dielectric Tests<br />
3.2.8.2.1 Low-Frequency Tests<br />
There are two low-frequency tests:<br />
1. Low-frequency wet-withstand voltage test<br />
2. Low-frequency dry-withstand voltage test<br />
3.2.8.2.1.1 Low-Frequency Wet-Withstand Voltage Test — The low-frequency wet-withstand voltage test<br />
is applied on bushings rated 242 kV and below while a waterfall at a particular precipitation rate and<br />
conductivity is applied. The values of precipitation rate, water resistivity, and the time of application<br />
vary in different countries. American standard practice is a precipitation rate of 5 mm/min, a resistivity<br />
© 2004 by CRC Press LLC<br />
© 2004 by CRC Press LLC