Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2000-12-21
2002-08-13
Le, N. (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S541000, C324S544000, C324S536000, C324S552000, C324S509000, C324S510000
Reexamination Certificate
active
06433557
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates to sensors used for on-line monitoring of the state (condition) of high voltage insulation in electrical equipment with capacitance (potential) taps and the interconnection therewith to remote measuring devices. The electrical equipment may include bushings of power transformers, shunt reactors or circuit breakers and current transformers.
2. Description of Prior Art
The On-line monitoring of high-voltage insulation of electrical equipment is performed on the equipment under operation, i.e. in the actual operating condition. Equipment de-energization is required only for the initial sensor installation. As increasingly reliable and cost- and labor-effective, this technology is now widespread in numerous applications. Particularly, such monitoring provided concurrently on power frequency and radio frequencies may be an effective tool in prediction and prevention of in-service failures for high-voltage bushings and other equipment with capacitance (potential) taps.
Attention is called to the following Publications:
“Methods and Tools for High-Voltage Equipment Diagnostics”, Energoatomizdat Publishing House, Moscow, by P. Svy 1992.
“Experience in the Application of the On-Line Monitoring System Using Power Frequency and Partial Discharges to High Voltage Transformer and Bushing Insulation”, by Z. Berler, L. Letitskaya and P. Svy, EPRI Substation Equipment Diagnostic Conference VI, Feb. 16-18, 1998, New Orleans, La.
Bushings of power transformers, shunt reactors or circuit breakers and current transformers, with their internal insulation of oil-impregnated paper similar to that used in cables or capacitors, are equipped with so called capacitance or potential taps. A capacitance tap is connected to a metal foil shield inserted inside the insulation. The insulation has certain capacitance and conductance between the high voltage current-carrying conductor and the foil shield. Both the capacitance value and the power factor of the insulation depend upon the insulation condition and could be quantified at the tap output with the equipment on-line. Furthermore, the electrical impulses that accompany partial discharges inside the insulation are also coupled to the output of the capacitance tap and can be detected using circuits of a suitable design.
The capacitance taps were originally designed only for relatively rare off-line insulation tests using a suitable test source at power frequency. During equipment operation they remained grounded. It was recognized that these taps lend themselves as excellent means of on-line monitoring of the insulation. The use of the capacitance tap for an on-line monitor requires a sensing device to be inserted permanently between the live tap contact and the ground. The aforementioned publications teach such an arrangement.
The sensor designed for the power frequency measurement produces a signal proportional to the capacitive current through the bushing insulation. The sensor designed for partial discharges senses the radio frequency impulses and produces a signal of magnitude proportional to the dissipated electrical charges. The repetition rate of such discharges can be determined by a measuring device.
Sensors based on application of current transformers are described in U.S. Pat. No. 5,471,144 “System for Monitoring the Insulation Quality of Step Graded Insulated High Voltage Apparatus” issued Nov. 29, 1995; U.S. Pat. No. 5,574,378 “Insulation Monitoring System for Insulated High Voltage Apparatus” issued Nov. 12, 1996; U.S. Pat. No. 5,640,154 “Insulation Monitoring System for Insulated High Voltage Apparatus” issued Jun. 17, 1997; and U.S. Pat. No. 5,652,521 “Insulation Monitoring System for Insulated High Voltage Apparatus” issued Jul. 29, 1997 and in the Svy reference, P. 107. They consist of a current transformer with a primary winding created by the capacitance tap grounding conductor, and a secondary toroidal winding consisting of several or many turns. This current transformer can be coreless (so-called Rogovsky coil), as suggested in the above mentioned patents for power frequency measurements, or with a ferrite core, as recommended in the Svy Reference for the radio frequency impulse measurements. The advantage of the current transformer-based sensor is its simplicity. A current transformer with its secondary winding loaded with a small resistance has small input impedance, so there is usually no need for a special tap overvoltage protection.
Monitoring of radio frequency (partial discharge) impulses imposes different requirements on sensor design, as opposed to monitoring of signals at power frequency. For partial discharge monitoring it is desirable to detect a frequency band generally between 0.5 and 20 MHz with high sensitivity. Ferrite radio frequency transformers with a small number of turns in the secondary winding are appropriate for this task as they are capable of accurately transmitting short and steep pulses, but they block power frequency signal. A coreless current transformer with a large number of turns in the secondary winding can be employed for power frequency measurement, but it is practically insensitive to weak partial discharge pulses. To meet both requirements, two separate transformers, one of each type, are necessary.
A coreless Rogovsky coil has a low sensitivity even at the power frequency signals. For this reason it was replaced with a resistor shunt connected between the output of the tap and local ground (Russian Patent 292,062, published Feb. 12, 1971). The measured quantity, a power frequency voltage drop across the resistor shunt, is directly proportional to the capacitive current through the bushing insulation. The magnitude of the power frequency signal can be conveniently controlled by the resistance chosen for the shunt. The disadvantage of such an arrangement is that the tap capacitance, between the high voltage line and the output of the capacitance tap, in series with the resistance of the sensor shunt represents a frequency dependent voltage divider. As a result, switching and lightning transients can cause severe overvoltages at the output of the tap due to their very high frequencies. These transients have the potential of destroying not only the measuring circuit, but also the insulation of the tap output or even the bushing. To limit the transients, a surge arrestor is added in parallel to the resistor shunt, as shown in the Svy Reference, on its FIG. 8.2.
A further improvement of the sensor consisted of replacing the resistor shunt with another capacitor, see U.S. Pat. No. 4,757,263 “Insulation Power Factor Alarm Monitor” issued Jul. 12, 1988; U.S. Pat. No. 5,903,158 “Monitoring of Internal Partial Discharges in a Power Transformer” issued May 11, 1999; and U.S. Pat. No. 6,028,430 “Method for Monitoring a Capacitor Bushing, and Monitoring System” issued Feb. 22, 2000. This arrangement features a capacitor divider ratio that is essentially independent of frequency, thus minimizing the exposure of the tap and the low voltage circuits to destructive switching and lightning impulses. A surge arrester is kept in place as a “second line of defense” for rare cases of extremely severe overvoltages.
All of the sensor designs described above are mutually exclusive in that they can satisfy only one application at a time; a power frequency signal detection or a partial discharge detection, but not both. With only one capacitance tap available per bushing, this represented a serious disadvantage as the replacement of a bushing sensor requires outage.
A Publication entitled “On-Line Monitoring of Power Transformer-Trends, New Developments and First Experiences” by T. Leibfried, W. Knorr, K. Viereck, CIGRE, 1998, #12-211, teaches a sensor that can contain both circuits. The sensor relies on the capacitor shunt connected to the tap output and the radio frequency current transformer the primary winding of which is connected in series with the capacitor shunt, either on its grounded side or on its “live” side. Two separate coaxial cables carry pow
Golubev Alexander
Rashkes Viktor S.
Eaton Corporation
Hamdan Wasseem
Moran Martin J.
LandOfFree
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