Insulation Monitoring System for Insulated High Voltage Apparatus

Applicant's invention relates generally to a system which monitors and measures the Insulation status with respect to earth ground of a high voltage electrical network and more particularly to a system that quantitatively measures the capacitively coupled charging current in a step-graded paper and foil or capacitive insulation system to produce a signal output proportional to the current for continuous monitoring and alarm level detection to indicate the possibility of failure for the entire insulating system.

Step-graded foil and paper insulation systems and capacitive insulation systems are generally employed on high voltage measurement and control apparatus such as current transformers for the purposes of protecting personnel from shock hazard and electrical instrumentation from equipment damage. An example of a step-graded system comprises multiple alternating conductive and dielectric layers, with the conductive layer of least potential being earth grounded. The alternate layers are used to form an effective series capacitive divider circuit between the high voltage conductors and ground potential. These alternate layers are usually made from foil and paper. The paper dielectric is usually oil impregnated and is generally used in oil-filled instrument transformers, power transformers, condenser bushings and other apparatus for high voltage electrical power systems. Some SF6 gas insulated systems use a metallized film type of capacitive insulation.

Most step graded insulation and capacitive insulation systems are designed such that the capacitance of each pair of alternate layers is equal, thus producing an equal voltage stress on the dielectric between each conductive layer when the apparatus is energized at high voltage. In designs where each layer is of equal capacitance, the total capacitance of the insulation system is equal to the layer capacitance divided by the total number of layers. A charging current through the capacitive circuit exists and is directly proportional to the product of the line voltage, the line frequency, and the total capacitance. With the line voltage and frequency relatively constant, changes in the insulation charging current are due primarily to a degradation in the insulation system. Electrical breakdown between layers results in degradation of the oil purity which leaves carbon deposits, providing a conductive path which effectively constitutes a short circuit between adjacent foil layers. The total capacitance of an insulator exhibiting such degradation increases as the effective number of layers is reduced. This increase in total capacitance will increase the charging current. Furthermore, each of the remaining layers is subjected to an increase in voltage stress. Ultimately, as additional layers break down, the residual voltage stress between the remaining layers may exceed safe operating levels, leading to the eventual, often catastrophic, failure of the entire insulation system.

Conventional high voltage measurement and control equipment which employ foil and paper step-graded or capacitive insulation offer no inherent means for monitoring the insulation charging current. Methods have been developed for monitoring the condition of the insulation apparatus. Most of them employ off-line methods. A power factor test requires that the system be energized with a test voltage and changes in the measured power factor or capacitance over time are recorded to see if there are any significant changes that would indicate a shorted layer. Partial discharge methods are effective in detecting these changes, but must be performed off-line and may not be practical in installations where interruption of service is not economical. Another method, gas-in-oil analysis, requires an oil sample to be drawn and tested to determine the presence of various gas that are generated when the apparatus overheats, usually indicative of a breakdown of the insulation. Some other prior art systems employ a measuring resistor in series with the ground loop and measure the voltage generated by the leakage current. However, direct measurement of this voltage is often misleading due to the lack of compensating networks to overcome the influence of the capacitance of the insulation and effects of electrical interference. Sensing the insulation charging current may not be satisfactorily accomplished by means of a resistive series element in the grounded electrode or by means of a ferrous magnetic core device. In either the resistive or ferrous magnetic sensing method, the capacitive nature of the insulation circuit between the high voltage conductor and ground is disturbed by a resistive or inductive sensor to the point where the magnitude of the insulation current is altered. Other methods inject a current at a lower frequency than the network and detect the resultant current flow in the effective leakage resistance and capacitance. These methods, being applied off-line, are incapable of continuously monitoring for a change in the insulation charging current while the apparatus is in operation. Further, they are often intrusive to the hermetically sealed insulation common to these types of insulation systems.

Commonly assigned U.S. Pat. No. 5,471,144 describes an on-line improvement over these common methods for monitoring the quality of electrical network insulation. In this system, a remote sensing coil produces a voltage output that is linearly proportional to the insulation charging current and a remote, self powered electronic circuit coupled to the sensor modulates a DC current control circuit proportionally to the output voltage of the sensor. An electronic control circuit provides a suitable voltage supply for the modulated current and alarm threshold detection circuits within the control circuit compare the output proportional voltage with predetermined levels. Although this system provides accurate results, installation of the monitor is such that the sensing coil may be mounted under oil in the transformer tank with the transmitter being located in a secondary box of the high voltage apparatus. This further requires an air-oil feed-through to pass wiring between the sensor coil and the transmitter. The insulation monitor becomes dedicated to the transformer and can not be used to monitor another transformer without extensive downtime and effort. Retrofitting an existing installation also requires a shutdown of the high voltage apparatus. Since the coil is mounted within the insulating oil, the unit has to be removed from the installation site, drained and dismantled before the coil can be installed. The unit has to be refilled under vacuum and the insulating oil reprocessed to remove impurities and moisture. This is time consuming and not very cost effective.

A capacitance tap is an existing electrode provided on all condenser bushings used on high voltage power transformers and circuit breakers and is also used on current transformers. The tap provides access to the insulation capacitance for off-line testing purposes and could be used for measuring voltage on-line. It would be an advantage to have an apparatus that couples directly to this tap for monitoring the insulation quality of the high voltage equipment. This will allow onsite retrofitting of existing installations without requiring dismantling and draining of the insulating oil from the equipment.

Accordingly, the principal object of the present invention is to provide an apparatus for the continuous, on-line, conversion of the charging or leakage current of a step-graded or capacitive insulated high voltage apparatus to a signal proportional to the leakage current.

The further objective of the invention is to provide an apparatus for generating the signal proportional to the charging current with a means of installing on the high voltage apparatus without removing the high voltage apparatus from service.

Yet a further objective of the invention is to provide a method and apparatus for providing a remote location for the conversion apparatus with respect to the comparison apparatus.

Another objective of the invention is to provide a method and apparatus for monitoring the leakage current of single phase and the leakage currents of polyphase high voltage systems.

In the preferred embodiment of the invention, the invention is comprised of a system of essential elements including, but not limited to, a remote sensing coil producing a voltage output linearly proportional to the insulation charging current, a remote, self powered electronic circuit coupled to the sensor which modulates a DC current control circuit proportionally to the output voltage of the sensor, and an electronic control circuit providing a suitable voltage supply for the modulated current and alarm threshold detection circuits.

The sensing coil described by the present invention utilizes a low permeability core, which may be in the form of a toroid, wound with a high number of turns to create a low inductance linear coupler from which an output voltage signal is produced which is proportional to the current in the grounded conductor passing through the center of the toroid. The linear coupler also serves to electrically isolate the sensor electronic circuits from the insulation grounding system.

The monitoring system uses a signal current modulation scheme which includes a 4 milliampere (ma) offset zero from which electrical power is derived for the remote electronic circuits and which serves to indicate that the electronics are functioning properly, even in the absence of insulation current. The zero to full scale modulation of 4 to 20 ma is used to conform to existing standards for auxiliary monitoring instruments, indicating meters, annunciators and alarm devices which may be series connected to the control current loop for additional supervisory and reporting capability. The midpoint between 4 ma and 20 ma is selected to represent the insulation current at its expected level for normal power system voltage and total insulation capacitance. That is, when the power system is energized at its nominal operating voltage and the total insulation capacitance has not been degraded from its intended initial value, the insulation charging current, will be equal to a nominal value for which the electronic circuits will cause an additional 8 mA to flow in the control current loop thereby causing a total of 12 ma in the loop. The electronic circuit is adjusted such that this same insulation charging current will produce a full scale control loop current reading of 20 ma when the insulation current reaches a value equal to twice its nominal value. Due to the proportionality of the power system voltage and the insulation capacitance to the insulation charging current, a doubling of the insulation charging current at nominal power system voltage would indicate that the total insulation capacitance has achieved a level of twice its nominal value, indicating that one half of the foil and paper layers in the insulation system have become ineffective. Alarm system thresholds may then be set between the 12 ma and 20 ma control loop current levels to indicate the degree of insulation breakdown that can be tolerated before further measures are taken to investigate the condition of the insulation system.

The system controller, not an object of the present invention, includes a DC voltage source from which the remote sensor electronics power supply and the modulated control loop current are derived. The controller may also include alarm circuits, possibly including time delays, whereby alarm thresholds may be established and alarm contacts made to transfer when the control loop current exceeds predetermined adjustable limits.

In the preferred embodiment of the invention, the remote sensing coil and the monitoring system are housed in an enclosure which readily adapts to a standard capacitance tap usually provided on high voltage equipment. The enclosure assembly replaces the standard capacitance tap (cap-tap) cover used to ground the insulation system at the tap when it is not otherwise being used for test purposes.

Other features and advantages of the invention, which are believed to be novel and nonobvious, will be apparent from the following specification taken in conjunction with the accompanying drawings in which there is shown a preferred embodiment of the invention. Reference is made to the claims for interpreting the full scope of the invention which is not necessarily represented by such embodiment.

FIG. 1 illustrates the construction of a step graded foil and paper insulator showing the alternating layers of foil and paper.

FIG. 2 is an electrical schematic of a capacitive divider circuit formed by the insulation structure of FIG. 1.

FIG. 3 is a block diagram of the essential elements of a monitoring system utilized in the present invention.

FIG. 4 is a typical installation of the monitoring system of FIG. 3 used on an instrument transformer according to prior art.

FIGS. 5a and 5b are a detailed block diagram of the monitoring system of the present invention as shown in FIG. 3.

FIG. 6 is a typical installation of the monitoring system of FIG. 3 used on an instrument transformer according to the present invention.

FIG. 6A is a typical installation of the monitoring system of FIG. 3 used on a high voltage circuit breaker bushing according to the present invention.

FIG. 7 is a cross sectional view of an enclosure for housing the remote sensor and transmitter illustrated in FIG. 6 of the present invention.

FIG. 8 is a detailed schematic diagram of the remote sensor and transmitter illustrated in FIG. 7.

FIG. 9 is a cross sectional view of an alternate enclosure for housing the remote sensor and transmitter illustrated in FIG. 7 of the present invention.

Although this invention is susceptible to embodiments of many different forms, a preferred embodiment will be described and illustrated in detail herein. The present disclosure exemplifies the principles of the invention and is not to be considered a limit to the broader aspects of the invention to the particular embodiment as described.

FIG. 1 shows the typical construction of a step-graded paper and foil insulation system 1 where a high voltage conductor 2 is wrapped with alternating layers of dielectric material such as paper 4 and conductive material such as foil 6. Thus, as the conductive layers are added to the insulation system, the radius, R0-R4, from the high voltage conductor to the conductive layers tends to increase. The capacitance value between any two adjacent conductive layers is directly proportional to the length of the layer and inversely proportional to the LOG ratio of the outside to inside radius of the layer. In order to maintain equal capacitance values from layer to layer, the length of each conductive layers is adjusted to account for the radial buildup. This construction serves to insulate the outermost layer 8, usually at ground potential, from the high voltage conductor 2.

A simplified electrical schematic diagram of the insulation system 1 as it forms a series capacitance circuit from the high voltage conductor 2 to ground 10 is shown in FIG. 2. With the number of capacitors equal to the total number of foil layers, a capacitive voltage divider 12 is formed. The total capacitance of the series capacitors, with equal valued capacitors, is equal to the layer capacitance divided by the number of layers. The actual equivalent circuit will consist of many parallel capacitors created by the capacitances that exists between noncandidate layers. The electrical current 14, also called the charging current, which flows through the capacitor circuit is proportional to the voltage of the high voltage conductor 2 and the total capacitance 12, based on the relationship that

where 1/Ctotal=1/C1+1/C2+1/C3+. . . +1/Cn and Ctotal=Cn/n for Cn=C1+C2+C3+. . . +Cn

On a typical 345 kV high voltage current transformer, the total capacitance may be approximately 705 pf. Operating on a 345 kV line, the line to ground voltage is about 200 kV and the charging current 14 is nominally 53 ma. Under normal operating conditions, the high voltage AC signal 2 has a relatively constant amplitude and frequency. Thus, the charging current 14 remains relatively constant in amplitude and frequency as well. Changes in the capacitive insulation structure, however, will produce changes in the amplitude of the charging current. Using the circuit shown in FIG. 2, which represents a four layer system, as an example, a voltage breakdown between two adjacent foil layers will effectively reduce the total number of layers by 1, which in turn increases the total capacitance 12 by a factor of 1/3. This will cause a proportionate increase in the charging current 14 to over 70 ma. Although FIG. 2 only shows four layers, more typical systems will use 10 to 30 layers, depending on the voltage class of the unit.

FIG. 3 illustrates an entire insulation current monitoring system 18 in a block diagram as disclosed in commonly assigned U.S. application Ser. No. 08/127,207. A step-graded foil and paper insulation system 20 insulates a high voltage conductor 22 from a transformer core and coil assembly 24. The step-graded insulation system includes alternate layers of dielectric material such as paper 23 and conductive material such as foil 25. The lowest potential foil layer 26 is electrically connected to ground 10 with a conductive element 30 which provides a ground path for the insulation charging current 32. A wound, non-ferrous toroidal coil assembly 34 is positioned such that the ground conductor 30 passes directly through the center of the toroidal coil, such that the coil 34 links all of the magnetic flux generated by the charging current 32 passing through the conductor 30. The non-ferrous nature of the coil assembly 34 results in a low inductance coupling back into the series capacitive circuit of the insulation 20 and, therefore, does not alter the magnitude of the charging current 32 being measured. A burden resistor 35 internal to the assembly 34 produces a voltage Vs that is proportional to the charging current 32. The coii assembly 34 also serves to electrically isolate the current monitoring system circuits 18 from the insulation grounding system. Because of the high voltage and currents associated with the system, the coil assembly will have some type of shielding to prevent inductive coupling of adjacent and unwanted magnetic fields.

The coil assembly 34 is electrically connected to a sensor electronic circuit or transmitter 38 by means of a shielded, twisted pair, or other suitable, cable 36. The transmitter 38 performs the functions of amplifying and rectifying the sensor voltage signal Vs. Output driver circuits in the transmitter 38 are used to modulate a DC current I1 in a control loop 40. The modulation results in the current signal I1 containing a proportionate magnitude of the charging current 32. The control loop 40 conforms to a standard 4-20 ma loop and cable 42 which electrically connects the transmitter 38 to a controller 44 is also a shielded, twisted pair or other suitable cable. The controller 44 may be located in a benign control house 43 environment. The control house 43 and the transmitter 38 can be separated by up to 2000 feet.

Controller 44 performs one or more essential functions. The controller 44 includes an isolated DC voltage source, which may provide as much as 30 to 40 volts, and which has the capability of providing 20 ma full scale modulated current in the control loop 40. The current which flows from the voltage source is strictly modulated by the sensor electronics alone, yet the terminal voltage across the controller 44 output is determined by a nominal voltage level required to excite the remote power supply circuits in the transmitter 38, and by the voltage drops associated with the current in the control loop 40. The outputs of the controller 44 are alarm contacts 45 which operate at a predetermined, settable level of leakage or charging current 32 to indicate a change in the insulation charging current 32 of the insulation system 20. If a polyphase system is being monitored, the controller 44 is capable of receiving inputs from transmitters and control loops similar to transmitter 38 and control loop 40.

The insulation current monitoring system 18 is powered by control power 46 which is inputted to the controller 44. The signal current modulation scheme includes a 4 ma offset zero from which electrical power is derived for the remote transmitter 38, eliminating the need for auxiliary power at the high voltage apparatus that is being monitored.

FIG. 4 shows a typical installation of a prior art insulation current monitoring system. An instrument transformer 47, utilizing a step graded foil and paper insulation system, is shown with tank wall 48 at ground potential and the grounded lead 30 from the insulation system brought through the tank wall 48 and the sensing coil 34 before being electrically grounded to the tank wall 48 itself. The sensing coil 34 is shown outside the tank wall for clarity. More typically, the sensing coil 34 is located inside the tank 48, immersed in oil.

The insulation charging current 32 which flows through conductor 30 is sensed by the sensing coil 34, whose output signal Vs is then coupled by cable 36 to transmitter 38 so as to modulate the current I1 in the control loop 40 coupled by cable 42 to the controller 44 in the control house 43. Transmitter 38 is installed internally to the instrument transformer 47 within a secondary box 39. The controller 44 provides remote power to the sensor electronics and monitors the charging current levels in the control current loop 40 as previously described. Only a single phase system is shown and would be typical of a retrofit.

Operation of the insulation current monitoring system 18 can be best understood with reference to FIGS. 5a and 5b, which are detailed block diagrams of the preferred embodiment of an insulation current monitoring system 18 employed in the present invention. The foil layer 26 of the insulation system being monitored is coupled to ground 10 through ground lead 30 which passes through the sensing coil 34. The output voltage Vs of sensor 34 is proportional to the insulation charging current 32 and is coupled to the transmitter 38. For the low leakage currents being measured, it has been found that a proportionality factor or ratio of fifty microvolts per one milliampere of charging current provides an adequate degree of sensitivity. A combination filter, amplifier and rectifier circuit 54 develops a DC voltage signal 56 from voltage Vs that remains proportional to the charging current 32. A voltage controlled current source 58, where the generated current is proportional to the voltage 56, is used to modulate a 4-20 ma current loop 40 for inputting to channel A of controller 44. Controller 44 is shown for a three phase system, but with only phase A sensor 34 and transmitter 38 shown. Additional sensors and transmitters would be required for a three phase system.

The current I1 flowing in the current loop 40 is coupled to the controller 44 via a twisted pair cable 42 as previously detailed. The current controlled current source 62 provides 4 ma of quiescent current flowing in loop 40 with zero charging current 32 when the high voltage apparatus being monitored is not energized. This quiescent current is used to provide power for the sensor 34 and transmitter 38. With the high voltage apparatus energized during initialization, usually during installation, the voltage controlled current source 58 is adjusted to provide 12 ma of loop current. This represents the expected charging current 32 when the high voltage apparatus is operating at its nominal operating voltage. Thus 100% (1.times.) of initial charging current equals 8 ma. Since there is a linear relationship between the charging current 32 and the loop current I1, 200% (2.times.) of charging current will equal 2.times.8 ma or 16 ma which is added to the 4 ma of quiescent current to create 20 ma of loop current I1. A current mirror 64 generates a voltage V1 across resistor 66. V1 is calibrated to be proportional to the charging current 32.

A 2.times. threshold level generator 68 and an adjustable 1.2-2.times. threshold level generator 70 generate voltage levels V2, V3, respectively. V2 is set such that voltage V1, with 20 ma of loop current I1, will equal V2. V3 is adjusted within the range 1.2-2.times. or 13.6-20 ma of loop current I1 for a similar relationship with V1. Schmidt trigger 72 compares V1 with V2 and will enable delay counter 1 when V1 exceeds V2, indicative of an increase in the charging current 32 to a level that is at least twice the original current resulting from a breakdown in the insulating layers of high voltage apparatus. Schmidt trigger 74 compares V1 with V3 in a similar fashion.

Since switching transients may regularly occur on the power system, voltage levels on the power system will momentarily cause the insulation charging current 32, and hence voltage V1, to increase above their steady state levels when such transient conditions occur. Delay counters 1-6 provide a predetermined delay period before initiating the transfer of the alarm contacts 50a,b. This distinguishes between a short term occurrence of a switching voltage transient and a long term steady degradation of the insulation system as evidenced by a steady increase in the insulation charging current. Clock 75 produces a timing pulse 76 for the delay counters 1, 3, and 5, with a divide by ten circuit 77 providing timing pulse 78 having a longer time delay for counters 2, 4, and 6 that monitor the lower levels of charging current 32 in the 1.2-2.times. range. The clock rate for the 2.times. alarm level is adjustable and can provide up to 30 seconds of delay.

If the charging current 32 exceeds the 2.times. or adjustable thresholds 68, 70 for a period longer than the time duration of the delay counters 1-6, outputs 79 or 80, depending on which threshold was exceeded, will energize latching switches 81, 82 respectively. This will in turn energize the 2.times. or 1.2-2.times. alarm contacts 45a or 45b respectively through one of the "OR" functions 84, 86. Alarm indicators 88a or 88b will display the appropriate cause of the trip. The outputs will remain in a tripped state until a master reset 90 is operated. Latching switches 81, 82 are used to provide memory of the condition if control power 46 is interrupted at any time after a trip operation.

Channels B and C will perform in the same manner. OR 84 will operate the 2.times. alarm if any of the three phases exceeds the 2.times. level for the preset time delay period and OR 86 will operate the 1.2-2.times. alarm if any of the three phases exceeds that level for the other preset time delay period. Power for the system is supplied by a switchmode regulator 92, the details of which are well known and are not an object of the present invention.

Most high voltage insulation systems, such as those used in current transformers and on all condenser bushings used on power transformers and circuit breakers, have a capacitance tap provided on all condenser bushings. The capacitance tap is an electrode that provides access to the insulation capacitance for off-line testing and is also occasionally used to measure voltage on-line. A cover protects the capacitance tap and is used to ground the insulation system at the capacitance tap. This cover is more commonly known as a cap-tap cover. The tap connects to the second to last conductive layer of the insulation system and the outermost conductive layer is grounded internally. This creates a capacitive divider between a capacitance C1 from the tap to the high voltage conductor and a capacitance C2 between the tap and ground. Knowing the ratio, C1 to C2, tests can be performed on the high voltage system by applying a known voltage to the tap. During normal operation, the cover shorts out the capacitance C2.

The present invention is an improvement over the prior art in that the sensing coil 34 and transmitter electronics 38 are enclosed in housing 100, which also serves to ground an insulation capacitance tap 102, just as the cap-tap cover previously described, and as shown in FIG. 6. This housing is used in place of the cap-tap cover and is coupled to the capacitance tap 102 of the instrument transformer 47, which utilizes a step graded foil and paper insulation or capacitive insulation system. The tank 48 is at ground potential 10 and the grounded lead 30 from the insulation system is brought through the tank wall by capacitance tap 102 into the housing 100 and through an internal sensing coil 34 before being electrically grounded to the tank 48 itself. The insulation charging current 32 which flows through conductor 30 is sensed by the sensing coil 34, whose output signal Vs is then coupled to transmitter 38 so as to modulate the current I1 in the control loop 40 coupled by cable 42 through pull box 92 to the controller 44 in the control house 43. Pull box 92 is grounded to ground 10 and provides a means for terminating the shield 94 of cable 42. Transmitter 38 and sensor 34 are both installed internally to Cap-Tap cover housing 100. The controller 44 provides remote power to the transmitter electronics 38 and monitors the charging current levels in the control current loop 40 as previously described. Only a single phase system is shown and would be typical of a retrofit. In addition, FIG. 6 shows the capacitance tap 102 as part of an instrument or current transformer. It could also just as easily be part of a circuit breaker 50 having condenser bushings 52 as illustrated in FIG. 6A. The housing 100 containing the sensor 34 and transmitter 38 would be adaptable to connecting to any capacitance tap. The bushing 52 is provided with the capacitance tap 102 and may be connected to a concentric metal electrode or capacitance divider inside the bushing. As an alternative construction, alternating layers of dielectric material, such as paper, and conductive material such as metallized foil are wound around the conductor as shown in FIG. 1. The bushing 52 will have a ground sleeve for grounding to the circuit breaker tank or housing 54.

A cross sectional view of the housing 100 for enclosing the sensing coil 34 and transmitter 38 is illustrated in FIG. 7 of the present invention. A flange 104 is welded to the tank wall 48. The flange 104 contains tapped holes 106 for securing and sealing the capacitance tap 102 to the instrument transformer 47 or other types of high voltage equipment by mounting plate 108 and bolts 110. Capacitance tap 102 consists of an electrode 112 which is coupled to the grounded lead 30 from the insulation system and is isotated from the tank wall by isolation bushing 114 and associated filler material 116. This type of capacitance tap is commonly referred to as a "type A" tap. Coupling a monitoring device to the capacitance tap 102 is normally accomplished by screwing the device into threads 118. Adaptor 120 uses these threads 118 to secure the housing 100 to the instrument transformer 47. Internal to the housing 100, a threaded cylindrical rod 122 is attached to housing 100 and is encircled by the sensing coil 34 and provides the means for securing a circuit board 126 containing the transmitter 38. Also attached to rod 122 are magnetic shields 125, 126 for shielding sensing coil 34 and transmitter 38 from potential electrical and magnetic fields generated within and around the instrument transformer 47. Screw 128 mounts a spring contact 130 to complete the assembly of housing 100. An access hole 126 provides a feedthrough for cable 42 which connects the transmitter 38 to the controller 44 through connector 129.

Previously, when a monitoring device was not attached to the capacitance tap 102, a tap cap cover consisted of a housing with a spring contact attached for mating with the electrode 112. This provided a ground path from the outer conductive layer of the insulation system. With the present invention, rod 122 mates with spring contact 130 when the housing 100 is attached. Thus, the electrical path for the ground current is from the outer conductive layer or foil of the insulation system to the ground lead 30, through the electrode 112 and spring contact 130, through the screw 128 and rod 122, and into the housing 100, which is grounded to the tank wall 48 by the adaptor 120 and the capacitance tap 102. The sensing coil 34 therefore senses all ground current from the insulation system.

FIG. 8 details the sensing coil 34 and transmitter 38 consisting of the rectifier circuit 54 and voltage controlled current source 58. Current loop 40 is connected to terminals J-1 and J-2. The 4 ma quiescent current of the loop current I1 creates a 6 volt rail between 132 and 134 through the action of zener diode VR1 and transistors Q2a, b, and c. Four ma of current flowing in the current loop 40 is sufficient to provide base drive at node 136 for darlington connected transistors Q2b and c. Transistor Q2a will conduct, allowing zener diode VR1 to also conduct, building a voltage at node 136. Regardless of any increase in the loop current, the rail voltage will remain relatively constant and regulated near the 6 volt level.

The output voltage Vs of sensor 34 which is proportional to the insulation charging current 32, is inputted to rectifier circuit 54. Changing the resistance values of resistors RA and RB provide a means for using the insulation current monitor 18 for different adjustment ranges of charging current 32, based on the magnitude of the high voltage line and the total capacitance of the insulated apparatus that the insulation current monitor 18 is monitoring. These resistors, along with potentiometer RV1, scale the incoming voltage Vs to the fixed gain of buffer amplifier 138, which can be an operational amplifier for greater temperature stability. The gain is selected such that AC output voltage V4 does not saturate at the 200% level of allowable insulation charging current 32. Since Vs is scaled to be fifty microvolts per one milliampere of charging current, the gain of amplifier 138 has to be quite high. A two stage cascaded amplifier is employed with the first stage with amplifier 138 providing half of the required gain so as to allow the full peak to peak voltage swing of Vs to be amplified within the voltage rails 132, 134. The higher gain of the first stage will also reduce the effects of the DC offset levels of the operational amplifiers on the following stages, amplifiers 140 and 142 which only will require equal but opposite polarity gains of the amplifiers 140 and 142, and the full wave rectification circuitry. Capacitor C1 couples voltage V4 to amplifiers 140 and 142 which function to generate voltages V5, V6 respectively, that are equal, but phase shifted by 180 degrees. As a result, resistor R16 becomes a load resistor that produces a full wave rectified voltage V7, which is the DC voltage signal 56 as referenced in FIG. 5a, through the alternate conduction of transistor pairs Q1a-Q1c and Q1b-Q1d. Voltage V7 is positive with respect to the voltage rail 134. RC network C4-R17 provides a DC filter for the rectified voltage V7 and the C3-R15 combination provides a balance with the positive DC rail 132. Calibration of the rectifier circuit 54 is accomplished by adjusting potentiometer RV2 for zero voltage across R16 with voltage Vs not present.

The voltage controlled current source 58 portion of transmitter 38 modulates the 4-20 ma current loop 40 through the action of summing amplifier 144 and the voltages at nodes 146 and 148. The voltage at node 146 comprises voltage V7, which is proportional to the charging current 32, a current reference signal V8 derived from voltage divider R22 and R25, and an adjustable voltage offset signal V9 derived from divider R21 and RV3. The voltage at node 148 is a current sample signal V10 derived from current sense resistor R26. Amplifier 144 subtracts the voltage across R26 from the offset voltage to compensate for changes in current loading of the transmitter circuit 38 whereby the output voltage V11=V7+V8+V9-V10. The output voltage V11 provide base drive for transistor Q2d which modulates current I1 in current loop 40. With no signal present at terminals J1, J2, potentiometer RV3 is adjusted to provide 4 ma of current in current loop 40. With 100% of predetermined charging current, as represented by input voltage Vs, potentiometer RV1 is used to adjust the gain of the amplifier stages such that the current I1 in current loop 40 is equal to 12 ma. This will result in I1 equalling 20 ma of current with 200 % of charging current 32 present. 200% of insulation charging current is generally considered a trip level.

During operation of the high voltage system, it can be shown that extremely high voltage levels can be generated when a disconnect switch supply power to the system is operated. Arcing across the switch contacts generate high current pulses or surges which can flow through the transformer insulation and the transformer ground lead 30. The fast rise times of these pulses and inductance of the ground lead are sufficient to raise the potential voltage of the transformer tank 48 to levels well above ground 10. This voltage exceeds the breakdown voltage between the transmitter electronics 38 and housing 100. This could cause a failure of the transmitter 38 and the current monitoring system 18. Accordingly, a bipolar surge protector VS1 connected to terminals J1 and J2 is used to crowbar the high frequency switching surge and pass the current to ground through terminal J3, which is connected to the shield 94 of cable 42. Field installation of the twisted pair, shielded cable 42 can also provide a method of further reducing effects of the high frequency switching surge. In particular, the installation makes use of two cascaded, twisted pair cables for cable 42. The shield 94 of the cable 42 that connects to the transmitter terminal J3 is terminated on the transformer tank 48 and the shield of cable 42 that connects to controller 44 is terminated to the control house 43 ground. The two cables are coupled to each other on a terminal strip inside of the pull box 92 located in the switchyard. The shields of both segments inside the pull box 92 will be terminated to ground. This provides shielding from both electrical and magnetical fields.

A cross sectional view of an alternative assembly 150 for enclosing the sensing coil 34 and transmitter 38 of the present invention is illustrated in FIG. 9. This type of assembly is used with a "type B" capacitance tap 152. A flange 153 is welded to the tank wall 48. The flange 153 contains tapped holes 154 for securing and sealing the capacitance tap 152 to the instrument transformer 47 or other types of high voltage insulation equipment by bolts 155. A spring contact 156 is coupled to the grounded lead 30 from the insulation system by bolt 158 and is isolated from the tank wall by isolation bushing 160 and associated filler material 162. A mounting adaptor 166 provides the means for attaching the assembly 150 to the capacitance tap 152.

Assembly 150 is identical to housing 100 of FIG. 7 with the exception of spring clip 130 and screw 128. In their place an extension probe 164 is screwed into rod 122. Thus, the electrical path for the ground current is from the outer conductive layer or foil of the insulation system to the ground lead 30, through the bolt 158 and spring contact 156, through the probe 164 and rod 122, and into the housing 150, which is grounded to the tank wall 48 by bolts 155 and flange 154. The sensing coil 34 therefore senses all insulation ground current from the instrument transformer 47.

While the specific embodiments have been illustrated and described, numerous modifications are possible without departing from the scope or spirit of the invention. One possible embodiment is to replace the current loop 32 with a modulated fiber optic cable. The transmitter portion 38 of FIG. 6 would include the electronics to convert the charging current 32 to an equivalent fiber optical modulated light signal to the 4-20 ma current loop, the details of which are well known to those skilled in the art. The controller 44 would contain the necessary decoder to demodulate the light signal to input to the Schmidt triggers and other related circuits previously described. An internal power source for the transmitter circuits 38, such as a battery or a means for deriving power from the ground current, would be required however.