Megger DELTA 4000, Справочное руководство

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24 DELTA 4000 ZM-AH02E
3 TESTING POWER SYSTEM COMPONENTS
are basically a series of concentric capacitors between the
center conductor and ground. This design is employed on
a wide range of voltage levels, up to and including 765kV.
Modern condenser bushings are usually equipped with test
taps. Bushings rated 115 kV and above usually have volt-
age taps, bushings rated below 115kV have test taps. The
availability of either a voltage tap or a test tap allows for the
testing of the main insulation c
1.
The test tap is normally
designed to withstand only about 500 volts while a voltage
tap may have a normal rating of 2.5 to 5 kV. This voltage is
only a concern when performing the c
2
(tap insulation test)
or the inverted ungrounded specimen test (UST), both of
which will be discussed later in this guide. Before applying a
test voltage to the tap, the maximum safe test voltage must
be known and observed. An excessive voltage may punc-
ture the insulation and render the tap useless. If absolutely
no information is available on the tap test voltage, do not
exceed 500 volts.
Bushing troubles
Operating records show that about 90 percent of all pre-
ventable bushing failures are caused by moisture entering
the bushing through leaky gaskets or other openings. Close
periodic inspection to find leaks and make repairs as needed
will prevent most outages due to bushing failures. Such an
external inspection requires little time and expense and will
be well worth the effort. High-voltage bushings, if allowed
to deteriorate, may explode with considerable violence and
cause extensive damages to adjacent equipment.
Flashovers may be caused by deposits of dirt on the bush-
ings, particularly in areas where there are contaminants
such as salts or conducting dusts in the air. These deposits
should be removed by periodic cleaning.
Table 3.9 lists the common causes of bushing troubles and
the inspection methods used to detect them.
Table 3.9
Bushing troubles
Trouble Possible results Methods of detection
Cracked
porcelain
Moisture enters
Oil and/or gas leaks
Filler leaks out Visual inspection
Power factor/Tan delta test
Hot-collar test
Dete-
rioration of
cemented
joints
Moisture enters
Oil and/or gas leaks
Filler leaks out Visual inspection
Power factor/Tan delta test
Hot-collar test
Gasket leaks
Moisture enters
Oil and/or gas leaks
Filler leaks out
Visual inspection
Power factor/Tan delta test
Hot-collar test
Hot-wire test for moisture
Insulation resistance
Moisture in
insulation
Moisture enters
Power factor/Tan delta test
Hot-collar test
Solder seal
leaks Moisture enters
Filler leaks out
Visual inspection
Power factor/Tan delta test
Hot-collar test
Broken
connection
between
ground
sleeve and
flange
Sparking in apparatus
tank or within bushing
Discolored oil
Power factor/Tan delta test
DGA
Voids in
compound
Internal corona
Visual inspection
Power factor/Tan delta test
Hot-collar test
Oil migra-
tion
Filler contamination
Visual inspection
Power factor/Tan delta test
Hot-collar test
No oil
Oil leaks out
Moisture enters
Visual inspection
Power factor/Tan delta test
Hot-collar test
Displaced
grading
shield
Internal sparking
discolors oil Hot-collar test
DGA
Electrical
flashover
Cracked or broken
porcelain
Complete failure
Visual inspection
Hot-collar test
Lightning
Cracked or broken
porcelain
Complete failure
Visual inspection
Test surge arresters
Corona
Internal breakdown
Radio interference
Treeing along surface
of paper or internal
surface Power factor/Tan delta test
Hot-collar test
Hot-wire test
Thermographic scanning
DGA
Short-
circuited
condenser
sections
Increased capacitance
Reduced voltage at
capacitance tap
Adds internal stress to
insulation
Power factor/Tan delta test
Voltage test at capacitance
tap
Capacitance test
Darkened oil
Radio interference
Poor test results Power factor/Tan delta test
Hot-collar test
Bushing tests
Power and dissipation factor & capacitance test
C1 for main insulation
The voltage or test tap allows for testing the main bushing
insulation while it is in place in the apparatus without dis-
connecting any leads from the bushing. The main insulation
is the condenser core between the center conductor and
the tap layer. The test is conducted in the UST test mode
which eliminates the losses going to grounded portions of
the bushing. The UST method measures only the bushing