Patent Description:
This invention relates to methods and apparatus for monitoring the condition and status of high voltage bushings used in conjunction with power transformers and, in particular, to methods and apparatus for doing so safely.

Power transformers are one of the principal elements of the power system and bushings are an important component of transformer equipment. As shown in <FIG>, high voltage bushings <NUM> are coupled and located about the high power transformer terminals. The bushings generally include specially designed electrical terminals for taking out winding ends (leads) through openings provided in the cover or wall of transformer tanks and connecting to incoming and outgoing lines. The bushings act as insulators to prevent a short circuit or "arcing". In large power transformers, the voltages used are so high that the wires cannot be allowed to come too close to each other, or too close to the metal casing of the transformer. If they do get too close, then the voltage can actually jump through the air (electrical breakdown), and create a short circuit. Bushings are therefore an important element in the reliable operation of their associated transformers. On a global scale, defects of bushings reportedly cause from <NUM> % to <NUM> % of the total number of failures of power transformers.

Bushings used in electrical distribution and transmission systems enable high voltages (e.g., from less than 69kVAC to more than 765kVAC) to be connected to devices such as transformers and circuit breakers. High voltage bushings of interest include capacitive layers to provide voltage grading across the bushing. The capacitance may range from less than <NUM> to more than <NUM>,<NUM> Pico farads. As shown in <FIG>, the capacitance of a bushing <NUM> may be represented as being split into two separate capacitors: C1 and C2; with C1 representing the capacitance between the test tap <NUM> and the high voltage point, or power terminal, <NUM> and C2 representing the capacitance between the test tap <NUM> and terminal <NUM> which is grounded. One side of C1 is the draw rod (lead) <NUM> which is the conductive lead to which the high voltage (HV) is applied and which passes through the bushing and the other side of capacitor C1 is connected to test tap <NUM>. One side of the second capacitor C2 is connected to the test tap <NUM> and the other side of C2 is connected to ground <NUM>. These capacitors may be constructed by using a conductive film on an insulating substrate (layer) that in many cases is paper. This insulating layer may be liquid-immersed typically in mineral oil; but, other insulating mediums could be used.

Over time the bushing and its associated capacitor layers may begin to degrade due to a faulty seal which allows ambient air or other contaminants to mix in with the insulating fluid. Also, moisture can accelerate the degradation of the bushing and its capacitive layers. As the capacitive layer begins to fail, there will be tracking on the paper layer due to evolving partial discharge.

Eventually, the C1 capacitor can short from the draw rod <NUM> to the bushing test tap <NUM>. Should this occur, the line voltage which may range from <NUM> kV to <NUM> kV will be seen at the bushing test tap <NUM>. While many existing bushing monitor couplers contain built in surge arrestors, should the surge arrestors fail or become inoperative, the full bushing potential will appear at the measurement hardware. Under this scenario the measurement electronics will be badly damaged possibly causing a localized fire. Even worse, there is the possibility of a serious or even a fatal injury should personnel be near the affected equipment.

A proposed solution to the problem using an isolation transformer to inductively couple the output of the bushing coupler to a monitoring system does not solve the problem. This may be illustrated by reference to a testing scheme disclosed in <CIT> titled Power Factor/Tan(δ) Testing Of High Voltage Bushings on Power Transformers, Current Transformers and Circuit Breakers. <FIG> of the <NUM>,<NUM> patent is redrawn as <FIG> (Prior Art) of the instant application. In the event of the failure of capacitor 18c, the full line voltage will appear at a center tap 22c. If the surge arrestors in the bushing coupler are inoperative, the full line voltage (E1/I1) will appear across the primary of transformer <NUM> as shown in <FIG> which is derived from <FIG>. Transformer <NUM> would then have to sustain the entire voltage seen at the test tap. This could destroy the transformer <NUM> and, even if it does not destroy the transformer <NUM>, an inordinately large and potentially dangerous voltage would be produced at the output side of transformer <NUM> destroying any testing or monitoring equipment (in box <NUM>) coupled thereto and endangering the life of individuals operating the equipment.

<CIT> discloses an abnormality diagnosing system for high voltage power apparatus comprising a detector linked to sensors disposed at various points in the power apparatus. The signals from the sensors are processed and then transmitted to a remotely-located central monitoring panel. There is no disclosure of how the diagnosing system is powered.

In<NPL>, there is disclosed a measurement system which applies a high frequency signal between the bushing tap and earth. The system is protected from high voltages at the tap by means of an arrangement that shorts the bushing tap to ground at mains frequencies. It is thus not clear where the power is derived for the high frequency signal generator and measurement system.

<CIT> discloses a monitoring system for a transformer having a measuring device connected to a tap on the bushing and arranged to transmit signals to a remote monitoring station, for example though fiber. Power to run the measuring device is provided by a battery. <CIT> discloses a mutual inductor-based current and voltage device for outputting an optical signal to a remote ammeter and relay protection device. It is thus not monitoring a bushing and has no connection thereto.

<CIT> discloses a device for monitoring the earthing of a transformer bushing which transmits the measurements to a remote monitoring station, the device being powered by a "solar battery". In <NPL>, a monitoring system is disclosed which monitors the voltage on a test point on the bushing and signals the data to a central control room via a fiber optic link. There is no disclosure of the means by which the system is powered.

<CIT> discloses a method of testing insulation in electrical apparatus by using a test set to apply a test signal to a first lead connected to a first conductor and receiving a test response from a second lead connected to a second conductor or to ground. Analysis of the results yields an insulation power factor indicating the condition of the insulation.

It is an object of the invention to couple the test tap of a bushing to a bushing coupler whose output is transmitted to testing and monitoring equipment such that the testing and monitoring equipment and any operator of the equipment has no physical contact to the bushing coupler and/or the test tap of the bushing.

In accordance with the invention, the voltage at the test tap of a bushing is applied to a bushing coupler which includes electronic circuitry to sense and process the voltages generated at the test tap and convert the voltages into corresponding data signals. The data signals corresponding to the test tap voltages (but not any part of the voltages as in the prior art shown in <FIG>) are then transmitted wirelessly to a bushing monitoring system. The wireless transmission may be, for example, via an optical coupling (e.g., fiber optics) arrangement or via an electromagnetic radiation (e.g., RF transmission) arrangement. Thus, in accordance with the invention, the output of the bushing coupler is wirelessly transmitted to a receiver which is non-conductively connected to and physically isolated from the bushing coupler and the test tap.

Data signals corresponding to, and representative of, the test tap voltage, rather than any portion of the actual test tap voltages, are wirelessly transmitted (i.e., without using an electrically conductive path which could couple potentially lethal voltages) to a bushing monitoring system (i.e., any suitable receiver) and its operator. Consequently, the high voltage signal source (test tap and/or bushing coupler) has no direct (or transformer) contact with the receiver. The bushing test tap voltages can be constantly and accurately sensed while avoiding the possibility of any portion of the actual tap voltage being applied to the testing or monitoring equipment. This makes for an intrinsically safe sensing and monitoring system.

Furthermore, in accordance with the invention, in order to maintain the bushing coupler physically and conductively isolated from the monitoring equipment, the power supply for the electronic circuitry in the bushing coupler is coupled to the test tap and powered by the voltage at said test tap.

A still other aspect of the invention includes a novel method for the accurate calculation of the bushing capacitance. A signal corresponding to, and indicative of, the value of the line voltage (i.e., the line voltage applied to the draw lead of the bushing) is obtained from the electric utility company supplying the line voltage. The significance of getting this utility supplied voltage is that it is highly accurate (e.g., +/- <NUM> VRMS). Every utility is constantly measuring the bus voltage. The utility energy management system (EMS) receives this information and can transmit the voltage with the required accuracy (e.g., via a DNP3, MODBUS, IEC <NUM>, or some other protocol). In accordance with the invention this information and the bushing test tap information are used to calculate the bushing capacitance (i.e., C1 and/or C2).

In the accompanying drawings which are not drawn to scale like reference characters denote like components; and.

As noted above, <FIG> is a schematic diagram of a three phase delta to wye power transformer (with the neutral grounded), encased in a transformer tank, used in a typical power transmission system for a step down application. The voltages applied to the primary of this transformer (via H1, H2, H3), via bushings <NUM>, can vary over a wide range (e. , from less than 69kV to more than <NUM> kV) while voltages on the secondary (at X1, X2, X3) can also vary over a wide range (e.g., from less than <NUM>. 2kV to more than <NUM> kV). As noted above with respect to <FIG> and <FIG>, to prevent the possibility of flashover to the transformer tank or adjacent substation structures, the connections of each input and output line to the transmission or distribution system is made through an insulated bushing <NUM>.

As shown schematically in <FIG>, each bushing includes a test tap13 (meeting standards as set in IEEE Std. <NUM>-<NUM>)with two capacitors (C1, C2), corresponding to the capacitive layers, being connected to the test tap. As shown in <FIG>, capacitor C1 has one side connected to the high voltage draw lead <NUM> and its other side connected to test tap <NUM> and capacitor C2 is connected between test tap <NUM> and ground terminal <NUM>. The bushing test tap <NUM> may be selectively grounded (e.g., via a metal cover plate that bonds the bushing test tap to ground) to reduce the stressing of capacitor C2.

Capacitors C1 and C2 may be formed in any suitable manner. By way of example, capacitors C1 and C2 may be constructed of a metalized or electrically conductive ink layer to form one side of a capacitor on an insulating substrate. The insulating substrate for C1 and C2 is typically manufactured from cellulose but could be made of other high dielectric strength insulating materials. The insulating substrate may be immersed in an insulating liquid to improve the dielectric strength.

In order to determine the condition/status of a bushing it is desirable (if not necessary) to monitor the test tap voltage. As discussed above, known bushing monitoring schemes include circuitry for connecting to the bushing tap <NUM> via metallic conductors or isolation transformers (see prior art <FIG>) to the measurement and monitoring circuitry. A drawback of the known methods is that if the capacitor layer (e.g., corresponding to C1) degrades significantly (e.g., shorts) an inordinately high voltage is applied to the monitoring electronics which would be destructive to the equipment and potentially lethal to any operator of the equipment. In all cases, this poses a great hazard to those that might be nearby or in contact with the equipment when this failure occurs. As already noted, safety or protective measures such as a surge arrester may be included to try to minimize the over voltage hazards. However, these protective and safety devices have been known to fail rendering the protection useless and the danger very real.

Another drawback of the known art is that in order to determine (calculate) the value of C1, the line voltage applied to the draw rod <NUM> has to be known very accurately. There is no simple or easy way to measure the line voltage that accurately.

Applicants' invention solves the problems discussed above. In accordance with the invention, the voltage at the test tap of a bushing is coupled to a bushing coupler which includes electronic circuitry necessary to sense and process the signal voltages generated at the test tap in order to assess the condition of the bushing. The voltage signals sensed and processed in a bushing coupler embodying the invention are converted to corresponding data signals which are wirelessly transmitted to a bushing monitoring system (e.g., a receiver also identified as a bushing diagnostic monitor electronics <NUM> in <FIG> or <NUM> in <FIG>).

Examples of wireless transmission embodying the invention include an optical coupling arrangement as shown in <FIG>, <FIG> and <FIG>. Other examples of wireless transmission include an RF transmission arrangement (as shown in <FIG> and <FIG>) whereby the receiver is physically isolated from the bushing coupler and the test tap. Consequently, the high voltage signal source (test tap and/or bushing coupler) has no conductive (or transformer) contact with the receiver.

<FIG> shows that the voltage at tap <NUM> is applied to a signal conditioning circuit <NUM> which includes circuitry for sensing the tap voltage and for protecting against overvoltage conditions and for producing a conditioned signal at an output. The conditioned output signal of the signal conditioning circuit <NUM> is applied to an analog to digital (A/D) converter circuit <NUM> which functions to convert the conditioned output voltage from the conditioning circuit <NUM> to data signals (e.g., which vary between zero to <NUM> volts in amplitude) which correspond to, and are representative of, the voltage values. Thus, as a first step in the design of a safe circuit, the actual voltages present at the bushing test tap <NUM> are not propagated beyond the A/D converter <NUM>. Rather, digital signals of fixed amplitude (e.g., zero to <NUM> volts) are propagated from the output of A/D converter <NUM> to a microprocessor (also referred to as processor) <NUM>. The fixed amplitude of the signals is determined by the voltage output of the power supply. Processor <NUM> functions to analyze the digital signals and to format the digital signals from the A/D converter <NUM> and to produce corresponding data signals at a transmitting output (Tx) which are applied to an input of a fiber optic (FO) module <NUM>. FO module <NUM> functions to convert the formatted digital data signals into optical (light) signals which are wirelessly transmitted to fiber optics (FO) cable [<NUM>(<NUM>), <NUM>(<NUM>)]. The FO cable [<NUM>(<NUM>), <NUM>(<NUM>)] is connected to a bushing monitoring system <NUM> also identified as bushing diagnostic monitor electronics device <NUM>. Device <NUM> includes circuitry for reconverting the received optic signals to digital signals. Thus, signals corresponding to the test tap voltage are wirelessly transmitted to and received by device <NUM>. Device <NUM> includes electronic circuitry for analyzing (a) the signals corresponding to the test tap voltage and (b) information corresponding to the line voltage (i.e., the high voltage applied to terminal <NUM>). Device <NUM> may also include alarms outputs and DNP <NUM>, MODBUS, or IEC <NUM> communications protocol interface indicating the condition of the bushing and tis capacitors. Device <NUM> may thus be used to track the state of the bushing including selected parameters and to provide appropriate alarms and displays of its condition.

<FIG> also shows that bushing coupler <NUM> includes a power supply <NUM> coupled via a switch S1 to the voltage tap <NUM>. The power supply <NUM> is designed to produce a direct current (DC) voltage which is distributed to the various circuits (e.g., <NUM>, <NUM>, <NUM>, <NUM>) of the bushing coupler via lines <NUM>. As discussed below, to have the power supply <NUM> functional at all times and isolated form the monitoring electronics <NUM>, the power supply <NUM> may also be charged or recharged wirelessly by signals (Rx) produced in FO module <NUM> in response to optic signals generated in device <NUM> and transmitted to FO module <NUM> via cables <NUM>(<NUM>), <NUM>(<NUM>).

In the embodiment shown in <FIG> the signal conditioning circuitry <NUM> of the bushing coupler <NUM> is shown to include a shunt resistor <NUM>, a varistor <NUM>, a low pass filter <NUM> and a high pass filter <NUM>. The outputs of the filters are applied to analog to digital (A/D) converters <NUM> and <NUM> (which correspond to A/D converter <NUM> of <FIG>). The outputs of the A/D converters are applied to a microprocessor <NUM> (which corresponds to processor <NUM> of <FIG>) and whose output is fed to fiber optic (FO) converter <NUM>. In <FIG>, a temperature probe <NUM>, shown connected to tap <NUM>, is used to sense the temperature of the bushing. The probe <NUM> supplies temperature signals to the processor <NUM> since the temperature is a significant factor in determining the value of the bushing capacitors.

The operation of the bushing coupler <NUM> is generally as follows. The voltage present at the tap <NUM> is applied to filters <NUM> and <NUM> whose outputs are respectively fed to A/D converters <NUM> and <NUM>. The filter <NUM> coupled to A/D <NUM> may be used for measurement of partial discharge signals at the bushing tap and filter <NUM> and A/D converter <NUM> may be used to measure C1 capacitance. The A/D converters function to convert the amplitude of the test tap voltages into corresponding digital signals which are then fed to signal processor <NUM>. The signal processor <NUM> is programmed to analyze the data signals and to format the signals to render the data signals suitable for transmission. The formatted transmitted output signals (Tx) of the processor <NUM> are then fed to an electric signal to fiber optic (FO) converter <NUM>, which converts the electric signals to optical signals which are wirelessly transmitted onto fiber optic cables <NUM>(<NUM>), <NUM>(<NUM>).

<FIG> is intended to illustrate that the voltage, sensing and processing circuitry (<NUM>, <NUM>, <NUM>) produces data signals (Tx) which are supplied to and modulate one (or more) light emitting diode(s) (LED), located within an FO module <NUM>. The light signals from the LED are wirelessly transmitted to fiber optic cables <NUM>. The optical signals are transmitted along cable <NUM> to photoreceptors (e.g., photodiode) located in bushing diagnostic monitor <NUM> connected to diagnostic electronics (not shown). Note that there is no conductive connection between the light emitters LED and the fiber optic cable <NUM> and between the cable <NUM> and the photodiode. Accordingly, the light receptors are electrically and physically isolated from the test tap voltage.

In accordance with the invention, the characteristics and representations of the AC voltage signals at the test tap <NUM> are converted into corresponding digital signals within the bushing coupler so that the actual voltages present at the tap <NUM> are not propagated. Then, the data signals corresponding to the voltages are wirelessly transmitted via a module <NUM> to the bushing monitoring system <NUM> without the use of any metallic conductive path. In module <NUM> the electrical signals are dielectrically isolated from the output optic signals. The light output signals from module <NUM> are then coupled via optic fiber cables to a monitoring electronic system <NUM>. Thus, even if capacitor C1 degrades completely (i.e., shorts), the voltages and/or currents produced at test tap <NUM> are not conductively (or physically) coupled to any circuitry beyond the electric to optic interface of FO converter <NUM>. Should a drastic change (e.g., a short circuit) occur in the capacitor layer, the dielectric isolation of the optic fiber will prevent lethal voltages from being propagated and appearing at the monitoring electronic system <NUM>. Thus, it is apparent that the bushing tap voltage has been sensed safely and that possible injury to equipment and operators of the equipment has been virtually eliminated. Note that locating the measurement electronics at the bushing tap makes the measurement of the AC waveform much more accurate and sensitive because the filtering needed is less severe. In addition, should a drastic change in the capacitor layer occur, the dielectric isolation of the optic fiber will prevent lethal voltages from appearing at the monitoring electronic system <NUM>.

The fiber optic module(s) <NUM> converts digitally encoded electrical input signals into one or more FO (fiber optical) output signals. Fiber optics is a technology that uses glass (or plastic) threads (fibers) to transmit data. A fiber optic cable consists of a bundle of glass threads, each of which is capable of transmitting messages modulated onto light waves. The fiber optic cables can be run to the bushing monitor <NUM> to enable the coupling of signals indicative of the voltages at tap <NUM> without coupling any portion of the physical voltage itself.

Another aspect of the invention pertains to the form and operation of the power supply <NUM>. The bushing coupler <NUM> includes a power supply <NUM> which is coupled via switch S1 to tap <NUM>. When the switch S1 is closed, the voltage at tap <NUM> may be fed to a rectifier circuit which, for example, as shown in <FIG> includes a limiting resistor, R5, a rectifying diode D5 and a relatively large capacitor C5 (shown for example to have a value of <NUM> Farads). The output of the rectifier circuit is fed to a voltage regulator <NUM> whose output is applied to line <NUM> to power the bushing circuitry with a regulated direct current (DC) voltage. Thus power supply <NUM> produces a DC voltage to operate the circuits in the bushing coupler <NUM>. The power generated by power supply <NUM> is shown to be distributed via line(s) <NUM>. It should be noted that the electrical power obtainable from the test tap <NUM> may be insufficient to operate the bushing circuitry. Therefore, an aspect of the invention includes circuitry for wirelessly providing power from an external source to the power supply <NUM> in addition to, or as an alternative to, the power derived from the bushing test tap.

To dielectrically isolate the power supply <NUM> from any other equipment, the power supply <NUM> may be charged or recharged by monitor <NUM> supplying optical signals to module <NUM> which includes circuitry for converting the optical signals to electric signals which can then be processed to produce DC voltages coupled via line <NUM> and isolation diode D6 to power supply <NUM> for charging or recharging the power supply <NUM>. The optic to electric signal conversion is done to ensure that the power supply system for the bushing coupler <NUM> and the signal processing are dielectrically isolated from all equipment and that potential hazards are avoided without the need for batteries or an electrical connection to supply power from an external source. Depending on the level of power required it is possible to power the power supply <NUM> solely from the monitor <NUM> via the optic to electric interface of module <NUM>. As shown in <FIG>, selected power signals may be supplied to light emitters LE2 in bushing monitoring system <NUM> which are optically (wirelessly) transmitted via optic cable <NUM> to light receptors LR2 (e.g., a solar cell or any suitable photovoltaic device). The light signals are wirelessly (optically) transmitted to the light receptors, such as solar or photovoltaic cells, which signals are then converted to a direct current (DC) voltage which can be supplied via line <NUM> to power supply <NUM>.

<FIG>, <FIG> and <FIG> also show that either a measured value of line voltage or the highly accurate line voltage information obtained from the utility EMS is applied to the monitoring system <NUM>. The monitoring system <NUM> includes circuitry for comparing the tap voltage obtained from the bushing coupler with the accurate line voltage to calculate the value of C1 which can then be used to determine the status and/or condition of the bushing being monitored.

Since the primary cause of bushing failure is due to the failure of the C1 or C2 capacitor, it is desirable to monitor the value of these capacitors and to do so safely. A method to determine variation in the value of C1 is achieved by reporting tap voltage developed off a low inductance precision resistor (R172 in <FIG>) to the bushing analysis and monitoring device <NUM>. The tap voltage is transmitted wirelessly via the fiber optic connection from the bushing coupler to the bushing analysis device <NUM>. The bushing monitoring device <NUM> is also designed to receive values of the high voltage (HV) applied to the bushing terminal <NUM>. The value of the high voltage can be obtained, for example, by means of measurements or from a potential transformer in the substation or received from the utility's energy management system via either DNP <NUM>, MODBUS, or IEC <NUM>. It is preferable to use the most accurate value available. Values of C1 can be calculated by using the following equation:
<CHM>
Where:.

The calculations for determining C1 is made by appropriately programming the monitoring circuitry <NUM> which can also display the information in various forms. This can be done and displayed automatically and/or with the aid of an operator. Because of the wireless transmission of the signals the measurements can be conducted without concern of injury or damage due to the high line voltages.

Each bushing coupler <NUM> is preferably encased in a metal container which is grounded. The measuring circuitry is typically located in a transformer control cabinet or its own cabinet perhaps with other monitoring equipment.

The bushing coupler of <FIG> is similar to that of <FIG> except that the output signals (Tx) of the processor <NUM> are fed to a radio frequency (RF) transceiver <NUM> which is coupled to an antenna <NUM> which can transmit via RF transmission to antenna <NUM> which is coupled to a bushing monitor electronics <NUM>, which functions in a similar manner to monitor <NUM>. The RF transceiver <NUM> includes a radio frequency (RF) module device used to "wirelessly" transmit and/or receive radio signals between the processor <NUM> and the monitor <NUM>. In <FIG> the wireless communication is accomplished through optical communication. In <FIG> and <FIG> the wireless communication is through Radio Frequency (RF) communication. An advantage of RF transmission over optic transmission is that it does not require line of sight. The bushing couplers of <FIG> and <FIG> like the ones of <FIG>, <FIG> and <FIG> enable the characteristics and representations of the signal at the test tap <NUM> to be transmitted "wirelessly" to their respective monitor <NUM>. The wireless transmission of data to the receiver <NUM> ensures that the potentially dangerous high voltages at the test tap are not conductively or physically coupled to the receiver.

<FIG> is like <FIG> except that it illustrates that the transceiver <NUM> may include modulator and demodulator circuitry for modulating and demodulating the signals being transmitted. The output signals (Tx) may be fed via the modulator and RF circuitry to antenna <NUM> for transmission to antennae <NUM> and decoding within the monitor <NUM> to effectuate wireless, nonconductive, transmission of the signals form the bushing coupler to the bushing monitoring system.

<FIG> and <FIG> show that, in an analogous manner to the showing in <FIG>, <FIG> and <FIG>, the power for the bushing coupler <NUM> can be obtained from RF signals emanating from the monitor electronics <NUM>. That is, selected RF power signals from the monitor <NUM> can be wirelessly transmitted via antenna <NUM> to antenna <NUM> which is coupled to transceiver <NUM> where they are demodulated. The demodulated RF signals then produce a DC voltage which can be coupled via line <NUM> to power supply <NUM> to charge or recharge the power supply <NUM> in an analogous manner to that described above.

The monitor <NUM> (like monitor <NUM>) is programmed to calculate C1. Line voltage information is supplied to monitor <NUM>, where the line voltage can either be a measured value or it can be a highly accurate value supplied by a utility. The information can be continuously monitored and displayed safely without concern of being injured or damaged by a high voltage.

Claim 1:
Apparatus for monitoring the condition and status of a high voltage bushing (<NUM>) used in conjunction with a power transformer, the bushing having a test tap (<NUM>), comprising:
a bushing coupler (<NUM>) having an input coupled to the test tap (<NUM>) of the bushing to generate data signals the coupler including analog to digital converting circuitry (<NUM>, <NUM>) for sensing and converting the voltage present on the test tap into corresponding digital data signals and circuitry (<NUM>) for wirelessly transmitting the data signals to a bushing monitoring system (<NUM>) physically separated from the power transformer, the bushing coupler (<NUM>) including a power supply circuit (<NUM>) for supplying electrical power thereto;
the bushing monitoring system (<NUM>) being configured to receive the digital data signals corresponding to the test tap voltage from the bushing coupler (<NUM>) without being conductively connected to the bushing coupler or the test tap(<NUM>);
characterized in that the power supply circuit (<NUM>) is coupled to the test tap and powered by the voltage at the test tap (<NUM>).