Method and apparatus for testing insulators

A method and apparatus are described for detecting faulty insulators used on overhead power transmission lines. The apparatus comprises two contact probes, a capacitance, a blocking resistance and a direct current meter in series between the probes to form an open electrical circuit. Means are provided to prevent discharging of the capacitance while the capacitance is being charged. In the use of the apparatus, the capacitance is charged from a source of high voltage direct current, the discharging of the capacitance to the source is avoided, the contact probes are sequentially applied across insulators and any current flowing across the insulators is recorded by the direct current meter. The sequential application of the contact probes is carried out while the insulators remain in service and the transmission line remains fully energized. The value of the capacitance is such that a large number of insulators can be tested before recharging becomes necessary. The value of the resistance is such that a blocking effect exists which causes a current of only a few micro amperes to pass through the current meter when testing a faulty insulator and thus avoids a rapid discharging of the capacitance.

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 U.S. Pat. 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 
disadvantages. 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. 
According to U.S. Pat. No. 1,943,391, insulation may be tested by 
impressing a potential sufficient to cause disturbance currents in 
inhomogeneities in the insulation which are measured. According to U.S. 
Pat. No. 2,281,470, 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. According 
to U.S. Pat. No. 2,239,598, grounded insulation is tested by applying an 
A.C. voltage and measuring current or capacitance. According to U.S. Pat. 
No. 2,923,879, 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. Pat. No. 3,363,172 grounded insulators are tested by 
applying a test voltage and measuring the current flowing through the 
insulator; the testing means include a transformer and a grounded lead. 
Most of these disclosures involve grounding the insulator or applying an 
A.C. voltage. 
I have now found that insulators on overhead power transmission lines can 
be tested quickly, safely and conveniently while the insulators remain in 
service and the power line remains fully energized with its normal 
operating A.C. voltage. The tester utilizes a light-weight, high voltage, 
capacitance as an energy source in its measuring circuit. Thus, the tester 
comprises a built-in power supply and the energy stored in the high 
voltage capacitance bleeds through a high blocking resistance so that a 
large number of 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 consisting 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 said testing means have a self 
contained source of high voltage direct current, sequentially 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 faulty insulator, and carrying out 
the sequential application of 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, there is provided a testing apparatus for detecting faulty 
insulators on power transmission lines carrying high alternating voltage 
comprising two contact probes, a capacitance, 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 of said charging leads to 
prevent discharging of said capacitance while a high voltage direct 
current is applied to said charging leads.

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 4 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 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 9 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 fastening 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 probes 
may have different shapes and may be made interchangeable 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 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 material 
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 .mu.A is 
generally 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 schematically 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 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 be at 
least 0.02 .mu.F, and should preferably be in the range of about 0.02 to 
0.20 .mu.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 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 deflection 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 
discharging of capacitance 20. For example, a resistance value of at least 
2.5 M.OMEGA., preferably in the range of about 2.5 to 200 M.OMEGA., is 
satisfactory to obtain a deflection on scale 14 of meter 13 and a slow 
discharging of the capacitance 20 in case of a defective insulator. The 
rating of the resistance should be in the range of 0.25 to 2 W. 
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 guaranteeing correct polarity and to eliminate 
discharge of the tester (capacitance) due to a short across the charging 
terminals 17 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 circuit. 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 13. 
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 fastening 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 probes 9 and 10 are applied across an 
insulator for a short period of time. If an insulator is good, the 
resistance of the insulator is extremely high, current flow is 
substantially 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 indicative 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 a 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 charge 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 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 invention 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 .mu.F at 10,000 W.V.D.C. (Working voltage direct current) each, for a 
total capacitance of 0.14 .mu.F with a rating of 10 kV. Resistance 21 
consisted of 10 resistors connected end to end in series of 10 M.OMEGA. 
each for a total resistance of 100 M.OMEGA. and a rating of 0.5 W 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 .mu.A D.C. meter 
with a protective circuit as described hereinabove wherein bypass diodes 
24 and 25 had a rating of 1 kV and 1 A, and ceramic disc capacitance 26 
had a value of 0.02 .mu.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 and 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 126 insulators, arranged in 
strings of 6 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 warehouse and 
submitted to the conventional test. The results of the "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 from the scope and purview of the invention as defined by the 
appended claims.