Note: Descriptions are shown in the official language in which they were submitted.
~12~792
This invention relates to a method and apparatus for
testing insulators and, more particularly, to a method and a
testing device for detecting faulty insulators used on overhead
power transmission lines.
In the transmission of electrical power, transmission
lines are supported or suspended from poles or towers by means
of insulators made of dielectric materials such as porcelain,
glass or other suitable material. These insulators are usually
connected in strings of two or more individual insulators. The
insulators tend to deteriorate over a period of time, particularly
as a result of the combined effects of changes in temperature and
humidity. It is, therefore, necessary to periodically check the
insulators so that defective insulators may be detected and
subsequently replaced.
Many methods and apparatus have been developed in the
past to detect faulty insulators. One such method and apparatus
is disclosed in United States Patent No. 1 923 565. According to
this patent, a forked probe is positioned across an insulator, a
direct current, generated by a D.C. (direct current) generator
(called a Megger), is applied to the insulator and the flow of
~"~ current across a fault in the insulator is detected and shown on
a current indicator. The method and apparatus according to this
patent have several disadvantag~s. The transmission line must
be de-energized during the testing to avoid placing a high voltage
on the test equipment and to ensure the safety of the personnel
that carry out the testing. A separate source of power such as,
for example, a battery must be used, in addition to using a
Megger,to energize the primary induction coil of the transformer
which forms part of the apparatus.
--1--
llZ4792
According to United States Patent No. 1 943 391,
insulation may be tested by impressing a potèntial sufficient
to cause disturbance currents in inhomogeneities in the insula-
tion which are measured. According to United States Patent No.
2 ~81 470, apparatus for measuring high electrical resistance
comprises a source of D.C., a rectifier to pass current flowing
from the D.C. source through the resistance to be measured and
an instrument for measuring the current from the source to the
resistance through the rectifier. The apparatus measures the
resistance of a device and can not be used to detect faulty
insulators on power transmission lines. Accordin~ to
United States Patent No. 2 239 59~, grounded insulation is
tested by applying an A.C. voltage and measuring current or
capacitance. According to U.S. Patent ~lo. 2 q23 879, insulators
are tested by impressing an alternating voltage across an
insulator and measuring the difference between two voltages
which represents the resistive component of current through the
~ insulator. According to U.S. ~atent No. 3 363 172 groun~ed
- insulators are tested by applying a test voltage and measuring
` 20 the current flowing through the insulator; the testing means
- include a transformer and a grounded lead. Most o~ these
disclosures involve grounding the insulator or applying an ~.C.
voltage.
I have now found that insulators on overhead power
transmission lines can be tested quickly, safely and conveni-
ently while the insulators remain in service and the power line
remains fully energized with its normal operating A.C. voltage.
The tester utiliæes a light-weight, high voltage, capacitance
as an energy source in its measuring circuit. Thus, the tester
~2~79Z
comprises a built-in power supply and the ener~y stored in the
high voltage capacitance bleeds through a high blocking resist-
ance so that a large number o~ insulators can be tested in situ
before the device needs to be recharged. The method of testing
comprises applying a high D.C. potential to pre-charge a high
voltage capacitance while preventing the capacitance from
discharging, applying a D.C. potential from the pre-charged
capacitance for a limited period of time to an insulator and
recording any current flow through the insulator on a D.C. meter
while protecting the meter from the high A.C. voltage of the
power line. The testing means comprises a self-contained source
of D.C., means for preventing the discharge of the D.C. source
while charging said source, two probes, a D.C. meter and means
to protect the D.C. meter from high alternating voltage.
More specifically, there is provided a method for
detecting faulty insulators among insulators supporting high
alternating voltage power transmission lines by means of high
voltage direct current and testing means comprising a capacitance,
a blocking resistance, means to avoid discharge of said
capacitance, contact probes and a direct current meter which
comprises the steps of connecting a source of high voltage
direct current to said testing means, charging said capacitance
conæisting of at least one high voltage, oil filled capacitor
from said source, avoiding discharging of the charged capacitor
to said source, disconnecting said source from the testing
means whereby sai~ testing means have a self contained source
of nigh voltage direct current, se~uentially applying the
contact probes of said testing means across insulators and
recording any current flowing across said insulators by means
of said meter, any recorded current being indicative of a
-- 3 --
l~Z479Z
faulty insulator, and carrying out the sequential application
o~ the contact probes across insulators while said insulators
remain in service and the power transmission lines remain fully
energized with normal operating alternating current voltage.
Furthermore, tnere is provided a testing apparatus for detecting
faulty insulators on po~er transmission lines carrying nigh
alternating voltage comprising two contact probes, a capaci-
tance, a blocking resistance and means to measure a direct
current connected in series between said contact probes to
form an open electrical circuit, two charging leads connected
~ across said capacitance for applying a high voltage direct
; current to said charging leads, and means positioned in one ofsaid charging leads to prevent discharging of said capacitance
while a high voltage direct current is applied to said charg-
ing leads.
The invention will now be described with reference to
the accompanying drawing, a portion of which has been cut away
to schematically illustrate the electric circuit means which
form part of the apparatus of the invention.
The tester generally indicated at 1 is contained in
a housing consisting of a cylindrical tube case 2 closed at
both ends by parallel end discs 3 and 3a. A generally V-shaped
base mount 4 is attached to or may form an integral part with
cylindrical tube case 2. Tube case 2, end discs 3 and 3a, as
well as base mount 4 are made of an electrically non-conductive
material. Attached to the bottom of the V of base mount ~ is a
lug 5 and a connector 6 pivotally secured to lug 5 by fastening
means such as a wing nut 6a.
An insulating stick, generally indicated at 7, which
has lug 8 at one end thereof, is pivotally attached to the
79Z
opposite end of connector 6 by fastening means such as wing
nut-bolt 8a. Fastening means 6a and fastening means 8a, when
attached to each other, form a double swivel joint which gives
a tight connection between the stick and the tester at any
desired angle. When the tester is in use, the insulating
stick 7 is rigidly attached to the tester and serves as an
extension to enable positioning of the tester at the insulators
to be tested. The insulating stick, which is standard in the
industry, is usually about 2 to 4 m long and will also permit
the lineman to keep away from sources of high voltage. The
housing of the tester, its base mount and the insulating stick
are made of an insulating material, such as, for example, glass
fiber.
Two metal contact probes g and 10 are positioned in
~; opposite spaced relationship on the outside of tube case 2
next to end disc 3a by means of similarly spaced metal fasten-
ing means 11 and 12, respectively. The contact probes, which
,
are made of a conductive metal such as steel, should be of
sufficient length to reach across the insulator. The contact
~;20 ~ probes may have different shapes and may be made interchage-
able to suit different types or sizes of insulators to be
tested. As shown in the drawing, contact probe 9 is straight
and contact probe 10 has a spiralled portion, which provides a
degree of flexibility when making contact across an insulator,
ensuring good contact. Alternatively~ probe 9 may be of a U-
shaped configuration. The probes may have a bent portion (not
shown) formed in such a manner that fastening of the probes on
the tube case 2 can be made at two points by means of additional
fastening means (not shown) for added rigidity. It is noted
llZ4792
that fastening means 11 containing probe 9 is insulated from
fastening means 12 containing probe 10 by virtue of their
being spaced apart and the insulating properties of the ma-
terial of the tube case 2 and end disc 3a.
A D.C. micro ammeter 13 is mounted centrally in end
disc 3, the dial 14 of meter 13 being approximately flush with
the surface of the disc. The meter has 2 terminals 15 and 16
for connecting of leads. The measuring range of the meter
should be sufficient to indicate the currents that flow through
defective insulators and chosen with regard to the values and
rating of the capacitance 20 and resistance 21. A range of 0
to 200 ~A is genexally considered adequate.
A charging terminal 17, to which a lead from a
; charging device can be attached, is mounted on tube case 2
- opposite base mount 4. Charging terminal 17 serves as the
positive terminal while the negative terminal for the charging
device is combined with fastening means 12. The charging
; device, which is used to provide a D.C. charge for the tester,
is a D.C. power source with a rating in the range of 0.5 to 10 kV.
Turning now to the electrical circuit, which is schema-
tically indicated, a first wire 18 connects negative terminal
and fastening means 12 containing probe 10 with terminal 16 of
meter 13 and a second wire 19 connects fastening means 11
containing probe 9 with terminal 15 of meter 13. Wire 18 contains
a capacitance 20 and a resistance 21, in series, resistance 21
being situated closest to terminal 16. A charging lead 22
connects charging terminal 17 with wire 18 at a point situated
between capacitance 20 and resistance 21.
The capacitance 20 is a single high voltage, oil filled
capacitor of a given value, or a number of high voltage, oil
filled capacitors arranged in parallel which have a total value
~12479Z
equal to the value of the single capacitor. The value of the
capacitance should be sufficiently high so that a large number of
defective insulators can be detected before recharging becomes
necessary. The value of the capacitance should ~e at least
O.02 ~F, and should preferably be in the range of about 0.02 to
0.20 ~F. The voltage rating of the capacitance should be in
the range of about 1 to 10 kV. Similarly, the resistance 21
is a single resistor of a given value, or a number of resistors
arranged in series which have a total value equal to the value
10 of the single resistor. Preferably, a number of resistors,
which are joined end to end in series, are used to create a
string of resistors for better isolation in the high D.C.
voltage circuit. The value of the resistance should be at least
such that a short circuit between probes 9 and 10 results in a
current flow that will cause not more than a full-scale deflec-
tion on scale 14 of meter 13. The value of the resistance must
be high so that a blocking effect exists which causes a low
current of only a few micro amperes to pass through current meter
13 when testing a defective insulator and thus to avoid a rapid
20 discharging of capacitance 20. For example, a resistance value
of at least 2.5 M Q, preferably in the range of about 2.5 to
200 MQ, i9 satisfactory to obtain a defleation on scale 14 of
meter 13 and a slow discharging of the capacitance 20 in dase
of a defective insulator. The rating of the resistance should
be in the range of 0.25 to 2~.
In charging lead 22 is positioned a forwardly biased
diode 23 which allows the charging of capacitance 20 but which
prevents capacitance 20 from discharging. This diode is essential
to allow the tester to be charged only in one direction guarantee-
30 ing correct polarity and to elimînate discharge of the tester
~lZ479Z
(capacitance) due to a short across the charging terminals17 and 12. The presence of the diode also simplifies the
charging procedure, for example by using a Megger, making it
unnecessary to continue the charging while the charging leads
from the charging device are being disconnected.
Micro ammeter 13 is a standard D.C. meter. The
meter should be protected against the high alternating voltage
of the power transmission line by means of a protective circu~-t.
Such protective circuit may include two bypass diodes 24 and 25
and a capacitance 26 which are placed in parallel across terminals
15 and 16 of the meter and between wires 18 and 19. Bypass diode
24 is forwardly biased and bypass diode 25 is reversely biased
which allows A.C. to bypass the D.C. meter, with A.C. flow
through bypass diode 24 for half of the A.C. cycle and through
bypass diode 25 for the other half of the cycle. The capacitance
26, which is preferably a ceramic disc capacitor, allows any
high frequency R.F. signal to bypass the meter. The protective
circuit is a standard circuit, which alternatively, may be
included with meter 1~.
In the use of the tester, the insulating stick 7 is
. .
firmly attached at the appropriate angle to the tester by means
of lug 8 with fastening means 8a of stick 7 and astening means
6a of connector 6 secured to lug 5. A direct current generator
(not shown), such as a Megger, is connected with its leads
(not shown) to charging terminals 17 and 12, observing the
correct polarity. Capacitance 20 is charged by operating
the generator and when a full charge is obtained the generator
is disconnected by removing its leads from charging terminals
17 and 12. The tester is now ready to test insulators. The
contact pro~es 9 and 10 are applied across an insulator for a
short period of time. If an insulator is good, the resistance
1~24792
of the insulator is extremely high, current flow is substan-
tially zero and no reading is observed on scale 14 of meter 13.
If an insulator is defective, the resistance will be lower,
a current will flow through the insulator and a reading is
observed on scale 14, the reading on scale 14 being indica-
tive of the degree of defectiveness of the insulator.
The D.C~ potential applied by the tester has no effect
on the A.C. circuit of the power transmission line. Similarly,
the A.C. voltage of the power line has negligible effects on
the tester. This is the result of à proper choice of the values
and ratings of the components of the tester which gives its
circuit an extremely long time constant. Thus, the capacitor(s)
of capacitance 20 has/have no chance to chargè up in each half of
the cycles of the 60 Hz A.C. voltage, no matter what the value of
this voltage may be. A contact period of the probes across the
insulator of only a few seconds, for example 2 to 5 seconds, is
sufficient to obtain an indication of the condition of the
insulator. A longer contact period results in unnecessary loss
of charge of the capacitance in case of defective insulators.
As the charge of the capacitance can be considerable and the
loss of charge by current flow through defective insulators
can be very low, a fully charged capacitance can be used to
detect many defective insulators. After testing is completed,
any residual charge may be discharged by placing a conductive
path between the probes; the meter 13 will indicate a flow of
current until the tester is discharged.
It is understood that the tester can be used to check
all types of insulators used to suspend or support electrical
_g_
llZ4792
conductors. These include string insulators, suspension
insulators, pin-type insulators, station bus insulators,
switch insulators and transformer bushings which can be
accommodated when different, interchangeable contact probes
are used with the tester.
In the specific and preferred embodiment of the
inuention the following values for the components of the
tester and generator have given excellent results in the
testing of insulators on power transmission lines under full
operating load and have made it possible to detect up to 50
defective insulators before recharging the tester. A 5 kV
Megger was used to charge capacitance 20, which consisted of
7 oil filled capacitors in parallel of 0.02 ~F at 10,000
W.V.D.C. (Working voltage direct current) each, for a total
capacitance of 0.14 ~F with a rating of 10 kV. Resistance 21
consisted of 10 resistors connected end to end in series of
10 MQ each for a total resistance of 100 MQ and a rating of
O.SW over a length of 12 cm. The diode 23, which eliminates
discharge of capacitance 20 due to a short across charging
terminals 17 and 12, i.e. the Megger, had a rating of 6.5 kV.
The micro ammeter was a 0 to 50 ~A D.C. meter with a protective
circuit as described hereinabove wherein bypass diodes 24 and 25
had a rating of 1 kV and lA, and ceramic disc capacitance 26
had a value of 0.02 ~F. All components were either soldered
together or connected by high voltage wire with a rating of 40 kV
working voltage and 110 kV breakdown voltage. All connections
a~ joints were covered by heat shrink material.
In the use of the tester according to the preferred
embodiment, the tester was charged and subsequently applied to
--10--
1~24792
126 insulators, arranged in strings of 6 insulators each, on
an energized, 60 kV, overhead power transmission line for
periods of application to single insulators of about 2 seconds.
46 defective insulators, i.e. a deflection of the needle of the
current meter was observed, were detected. The testing was
carried out by two operators travelling along the power line,
one operator carrying out the testing. Subsequently, the
power line was shut down and all the tested insulators were
replaced. The removed insulators were then tested in a ware-
house and submitted to the conventional test. The results ofthe "in service" test corresponded with those of the conventional
test, i.e. a 100% accuracy. The same reliability of the method
and apparatus according to the invention was obtained when
insulators on a 230 kV power line were tested.
- It will be understood of course that modifications can
be made in the embodiment of the invention illustrated and
described herein without departing fro~ the scope and purview of
the in~ention as defined by the appended claims.
--11--
