Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Transformer testing
Reexamination Certificate
2000-09-25
2002-09-03
Le, N. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Transformer testing
C324S546000
Reexamination Certificate
active
06445196
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to apparatus for testing electrical transformers and, more specifically, to testing electrical transformers in a manner and at voltages to enhance the safety of the personnel working with such equipment.
BACKGROUND OF THE INVENTION
Transformers are installed by an electric utility provider at various intervals along electrical distribution power lines and, especially, at points at which electrical voltage is reduced further from transmission line voltage to consumer line voltage. An electrical utility power provider installs transformers. From time to time, these transformers may be damaged by natural forces such as lightning strikes or wind damage, animals or birds, and human factors including vehicles knocking down power lines, any of which can lead to short circuits or overheating.
Once a transformer in an electrical power distribution system is damaged and fails, the customers “downline” therefrom are deprived of electrical power and service.
Any power outage may be caused by one or more of a variety of factors: shorts or downed lines which cause fuses to bum out and interrupt power; a broken line; a transformer which develops an open circuit in a winding; a transformer developing an internal short; and other causes. Note, not all of the above causes of power interruption result in a defective transformer.
Whenever a power outage occurs, at least one electrical lineman must go to the cause to determine how best to make repairs. Before a transformer is returned to service, it is desirable that the electrical properties of the transformer be verified.
The verification of the electrical properties typically is done with a voltmeter and a multimeter. A voltmeter is connected to a pair of high voltage terminals on the transformer, and a multimeter or second voltmeter is connected between low voltage terminals. Next, a voltage is applied to the high voltage terminals. In many instances, voltage so applied is supplied by the electrical power grid. This may mean that a very high voltage may be impressed on the potentially defective transformer. To provide such voltage to the high voltage terminals of the transformer, a fuse is inserted in the fuse holder typically associated with the transformer and the fuse holder is closed to complete the circuit. Re-powering the transformer with a high voltage may cause the transformer to burn and/or explode, potentially endangering the safety or life of the lineman. Thus, safety considerations require that a lineman must move away from the transformer prior to re-powering to protect himself.
For overall safety of the lineman, it is preferable to use a separate lower voltage, alternating current power source which is safer and not likely to cause fire or explosions of transformers. However, a line truck may not have such a lower alternating current voltage source available.
The lineman's safety and ability to perform his job are affected or complicated by adverse weather conditions including high wind and rain during the reconnection to the power grid voltage to the transformer, and reading the output terminal voltages, and the safety measures required when working around such high voltages.
With the connection of the power grid voltage to the transformer H
1
and H
2
voltage terminals, it is also possible that a defective transformer may explode or catch fire, burning or injuring the lineman, or the cooling oil contained within the case of the transformer may spill and may contaminate both the work site and equipment. The typical transformer oil is a hazardous material, requiring an environmental cleanup of polluting cooling oil.
Of course, the transformer may be routinely removed from the pole and replaced with a known non-defective transformer. The expense of such a procedure is quite high and this approach creates a large demand for manpower and an equipment burden, particularly so if the work must be performed under adverse weather conditions. Also, this alternative procedure may result in replacing usable transformers and needlessly spending the cost of an unnecessary replacement.
OBJECTS OF THE INVENTION
It is an object of the invention to provide improved testing equipment for safe and efficient testing of power pole mounted transformers prior to their removal and/or replacement.
It is another object of the invention to reduce the number of pieces of equipment needed for on-site testing of transformers.
It is an additional object of the invention to reduce the exposure to power grid voltages by line personnel.
It is still another object of the invention to simplify the on-site testing of transformers.
The foregoing objects are not intended to limit the scope of the invention in any manner and should not be interpreted as doing so.
With a full and complete understanding of the invention, other objects of the invention will become apparent to one of skill in the art of electrical connections and devices for accomplishing disconnection of electrical connectors.
SUMMARY OF THE INVENTION
A single tester control assembly incorporates into the present invention a plurality of connections and controls to simplify the testing of transformers. This single tester control assembly provides power supply terminals for the attachment of power supply leads to the high voltage terminals of the transformer, conventionally labeled H
1
and H
2
. Bridging the similarly designated and corresponding H
1
and H
2
power supply terminals is a variable resistor, interior to the tester control assembly. A variable resistor may be operator controlled within a finite predetermined range in order to vary the voltage applied to transformer terminals of H
1
and H
2
.
The variable resistor and the H
1
power supply terminal are separated by an operator controllable switch, preferably of the momentary rocker type. This switch will permit the input of alternating current potential to the primary coil of the transformer only after the entire test setup is completed and the operator no longer is touching anything, thus avoiding possible shock to the operator as the test voltage is applied. The conductors to the H
1
and H
2
terminals of the control assembly may be further bridged, downline from the variable resistor bridge, by a indicator visible to the operator showing if any electrical power is being supplied to the test assembly. Alternatively, a visual indicator may be placed in the high voltage line either immediately prior to the H
1
control assembly terminal or between the variable resistor tap and the H
1
control assembly terminal connection. Moreover, a visual indicator is connected inline with a fuse socket or holder that also can be used as a fuse tester. The fuse socket may be in addition to a fuse socket in a high voltage conductor and so is disposed for safety purposes.
Also bridging the leads to the terminals of the test control assembly variable resistor is a voltmeter. The voltmeter measures the output voltage from the variable resistor and the test voltage impressed on the terminals H
1
and H
2
of the transformer. Hence, the test voltage is adjustable and controllable, and the meter provides visual invitation of the test voltage output from the test assembly terminals to the H
1
and H
2
terminals of the transformer.
The input to the electrical network described above is provided through a ground fault circuit interrupter (GFCI). The GFCI is connected to a male three-prong 120 volt AC connector which protrudes from the body of the test control assembly. Input AC voltage may be applied through an extension cord connected to a power source and connected to the test control assembly.
For ease in using a voltmeter in the adverse conditions frequently encountered by a lineman to repair power lines and the like, a pair of terminals is provided to receive the test leads for connection to the output terminals: X
1
and X
2
; X
1
and X
3
; or X
2
and X
3
of the transformer. A multimeter preferably is used to accurately read the output voltages from the secondary coil of the transformer. So that the ha
Le N.
Letson Laurence R.
Nguyen Vincent Q.
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