Abstract:
A portable partial discharge detecting device for measuring partial discharge in energized electrical systems encloses a relay circuit and a DC power source. A pair of sensor circuits are enclosed within electrically coupling clamping mechanisms and are coupled to the relay circuit at sensor inputs by cabling. The clamping mechanisms engage the ground leads of the electrical system. Trip and alarm networks of the relay circuit continuously compare the picocoulomb values of the partial discharge pulses against user set threshold values. Relays coupled to the trip and alarm networks respond if the threshold values are exceeded. The alarm setting is always set to a pick-up value equal to or less than the trip setting. The trip network relays are coupled to a control circuit of the electrical system and can take the system off-line. The alarm network relays are coupled to an alarm circuit and can warn the user that a certain level of partial discharge has been reached within the electrical system. The preferred system to monitor is a three phase power transformer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to partial discharge detection devices. More particularly, it relates to a device capable of monitoring and detecting partial discharge in an insulation medium of an electrical system and controlling the electrical system coupled thereto in response to such detection. 
     2. Description of Prior Art 
     Partial discharge is an electrical phenomenon that can occur within an insulation medium in any electrical system having electrical conductors. Recently, the term partial discharge has been used to define a specific phenomenon that is different than that known as corona discharge. Partial discharge is a type of localized internal electrical discharge resulting from transient gaseous ionization in an insulation system when the voltage stress exceeds a critical value. Corona, on the other hand, is an external electrical discharge occurring as a result from the ionization of gases of the surrounding air by the high voltage (that which exceeds the critical value). Corona is often heard as acoustical noise about high-voltage transmission lines, representing sustained discharges in gases that have been energized by an intense electric field near the electrical conductors. Corona can often been seen as a bluish purple glow on the surface of and adjacent a conductor. In other words, where partial discharge is an internal discharge, corona is an external discharge. When reviewing prior art which was published before the 1980&#39;s, it is common for authors to refer to corona when really they are addressing partial discharge. For the purposes of this disclosure, a reference using the word corona will be understood to be describing the internal electrical phenomenon occurring within an insulation system known as partial discharge as defined hereinabove. 
     Partial discharge occurring within an insulation medium can be destructive upon the insulator. In particular, the free electrons in the insulator, accelerated by the electric field, which thereby produces the ionization, collide with the atoms of the insulation material resulting in accelerated breakdown of the insulation material. If the insulator is used in a electrical device such as a transformer, breakdown of the insulator could cause failure of the transformer. Failure of a transformer used by an electric generating power company could result in the explosion thereof causing injury to personnel, destruction of valuable property and interruption of electric power service to consumers. For these reasons, devices which can detect and monitor partial discharge in electrical devices are greatly needed. 
     Many attempts have been made at developing a device or system for measuring, monitoring and/or detecting partial discharge. One of the early innovators of improved partial discharge measuring devices was Vogel. U.S. Pat. No. 2,996,664 discloses a device, called a Corona Detector, to which he contributed. The detector seen therein utilizes an oscilloscope to directly display the charge, in coulombs, of a partial discharge pulse emanating from a piece of electrical equipment to be tested. The Vogel device employs a tuned transformer whose secondary winding produces a series of oscillations that directly indicates the charge of the partial discharge pulse in response to the primary winding being excited by the pulse. Unfortunately, the Vogel device does nothing more than detect partial discharge pulses and display a wave form on an oscilloscope. Nothing in Vogel suggests controlling the piece of equipment being tested nor providing a warning signal that the insulation in the electrical device is reaching a critical failure state. Further, the Vogel device requires that the user understand the operation of an oscilloscope, a device which renders readings which are very subjective. Ii is common for the results displayed on a oscilloscope to be interpreted differently by two or more users. 
     Many other attempts have been made to develop devices and methods for detecting partial discharge occurring in electrical systems. Some devices have employed antennas for receiving electromagnetic radiation from power transmissions lines and other devices where partial discharge may occur. Two such devices are shown in U.S. Pat. Nos. 4,775,839 to Kosina et al. and 5,726,576 to Miyata et al. Unfortunately, the use of an antenna for receiving signals relating to partial discharge has many disadvantages. One such disadvantage is the possibility of receiving unrelated electromagnetic radiation signals thereby producing a false reading for the actual device or system to be tested. Elaborate filtering circuits are needed to eliminate these false reads thereby raising the cost and technical sophistication of the partial discharge detecting device. Even with filtration, due to a lack of a controlled test environment (i.e., shielding or other means of containment), random disturbances, known to exist on multiple levels within the electromagnetic spectrum in the ambient air, can contribute to a corrupted test result. Examples of random disturbances include, solar and microwave radiation, beat frequency oscillations, lightening, RF from fluorescent lighting and other naturally and man made occurring phenomenon. Further, if the electrical system to be tested is a shielded power transformer, wherein multiple transformers are located within close proximity of one another (i.e., a power sub-station), it would be difficult to isolate and test a single transformer in the sub-station through the use of a device receiving a signal by means of an antenna. Even presuming proper isolation of a particular signal emanating from a particular piece of equipment, the reception of the signal utilizing an antennae is still extremely “position sensitive.” For instance, since RF and acoustic signals follow the inverse square law, an operator would have great difficultly ascertaining whether the received signal has been attenuated; there is essentially no reference point. Further, the received signal could have been manipulated and/or distorted due to various wave propagation anomalies such as reflection, diffraction and refraction. 
     Yet other attempts at detecting partial discharge have resulted in the development of devices that apply a high frequency AC voltage test signal to the electrical system to be tested in order to determine whether any partial discharge will occur. Such a device can be seen in U.S. Pat. No. 5,365,177 to Hamp, III et al. Inherent disadvantages exist with this type of device, such as, for example, the necessity of providing the AC test voltage. One of the great needs for partial discharge detection devices is that systems in the field, such as power transformers, need to be tested for partial discharge. The operator testing such a transformer is hampered by the need to apply an AC test voltage in the field. Further, in utilizing the Hamp III device, the system to be tested must be removed from operation, thereby preventing a system test under normal operating and load conditions. 
     Yet even further attempts at improving partial discharge detection devices can be seen in U.S. Pat. Nos. 4,897,607 to Grunewald et al., 4,967,158 to Gonzalez, and 5,506,511 to Nilsson et al. These devices employ a method of detecting partial discharge through the measurement and analyzation of high frequency sound waves attributed to partial discharge through the use of transducers, microphones and other sound wave detecting devices. Unfortunately, inherent disadvantages in the use of such devices exist. For example, naturally occurring and man-made acoustic phenomenon exist in all frequencies and incident and co-incident phase modes in ambient air. Such phenomenon is known to be detected by transducers, microphones and the like. It is therefore necessary to employ filtration circuitry in an attempt to remove the undesired random signals from the actual signal to be analyzed. Without filtration, it would be difficult to determine that the reading produced by the detection device is actually that of a partial discharge signal. Further, in the case that the electrical system to be tested is a transformer, the sound wave receiving devices of these prior art references are susceptible to vibrations of the transformer tank walls. In particular, as an acoustic signal propagates from the partial discharge point, it travels through the insulating medium and eventually strikes the tank wall. Accordingly, if a microphone is attached to the tank wall, the signal that the microphone receives may be that of the signal traveling through the steel wall, in that sound waves travel quicker through a solid material than through a liquid or gas. Further, all of these prior art devices require that the system be analyzed in a “pure” test environment. In other words, the system needs to be taken “off-line.” Additionally, pure test environments should include the use of copper shielded rooms or anechoic chambers to ensure that no random disturbances can effect the test results. These type of testing rooms are expensive to build and maintain. In regards to instrument transformers, as used by utility companies, taking them off-line can have detrimental economic consequences, since instrument transformers are used for consumer billing purposes. Still further deficiencies in these prior art devices are that the Nilsson device will not work in a dry-type transformer. And, even though the Gonzalez device incorporates alarm circuitry for alerting that a fault is about to occur, nothing disclosed therein teaches or suggests that the alarm circuitry should work in tandem with switching and/or relaying circuitry which could take the piece of equipment off-line. Further, nothing in Gonzalez suggests or teaches remote monitoring and/or alarming. 
     As discussed above, many disadvantages exist within the prior art. Most prior art devices require that the electrical system to be tested be taken off-line for the purpose of the test. Further, many of the prior art devices lack portability. Still further, most prior art devices employ detection technology that is susceptible to interference from random electromagnetic radiation and corrupted signals. 
     An improved device is needed which overcomes all of the deficiencies seen in the prior art. In particular, the device should be unobtrusive (i.e., passive in nature) such that the electrical system to be tested can remain “on-line” during testing thereof. But, the device should ensure that no feedback is introduced into the system if the system is to remain “on-line” during the test. Further, the improved device should be portable, thereby permitting a technician to take partial discharge readings in the field, regardless of the remoteness of the location. Still further, the device should be designed with detection technology that is more impervious to interference from naturally occurring and man-made electrical phenomenon without the need of sophisticated filtering circuitry or special testing environments (copper shielded room and/or anechoic chamber). Yet still further, the device should incorporate a means for alarming that a fault is possible as well as a means for controlling (i.e., shutting down) the device being tested/monitored in response to the alarm. Yet still even further, the device should be inexpensive and easy to manufacture. 
     SUMMARY OF THE INVENTION 
     The novel partial discharge detection device of the present invention improves upon known prior art detection devices and overcomes all of the deficiencies seen therein. In particular, the device of the present invention is not susceptible to ambient electromagnetic radiation and therefore will not produce a false partial discharge reading based on reception thereof. The use of the device does not require the employment of a special testing room. Further, the detection device of the present invention incorporates alarm and control circuitry. Accordingly, this novel partial discharge detection device can warn a user of an impending fault in the electrical system being tested and thereafter shut down the system. 
     The novel device of the present invention is also passive in nature. In other words, it is not necessary to take the electrical system to be tested “off-line.” This permits the testing of the electrical system under normal operating and load conditions. Further, while the electrical system is being tested “on-line,” no feedback is introduced into the system. The device can be permanently installed, allowing twenty-four hour measuring and monitoring, or be removably attached allowing the detection device to separately test a multitude of electrical systems in a given area (i.e., separately test all of the transformers in a power sub-station). This represents a huge improvement over the prior art which discloses devices at two extremes: (1) permanent devices that can monitor/measure while the system is “on-line,” and (2) removable/portable devices that can monitor/measure the system but only “off-line.” 
     The device of the present invention is also easy and inexpensive to manufacture. The detection device can incorporate all of the its circuitry in a single small box. Accordingly, multiple devices can be purchased by a single company such that all of their field technicians can include the device in their set of field tools. 
     The novel partial discharge detection device employs a DC power source, a sensor circuit and a relay circuit. The power source is common to all circuits. The sensor circuit is a picocoulomb sensor for measuring the electric charge of a pulse of a partial discharge signal occurring in the electrical system to which the detection device is coupled. In a preferred embodiment, the picocoulomb sensor (or sensor circuit) employs either an air or amorphous core coil. Various networks of wide band amplification, high-pass filtration, precision rectification and peak detection pass a voltage to current converted signal to an output drive which is directly coupled to the relay circuitry of the device. 
     The relay circuit receives the signal and applies it through signal conditioning and a time delay circuit. Thereafter, the signal is directed to a pair of comparator networks which provide a reading from which latches, associated with a trip and alarm setting, can act. The trip and alarm settings are adjustable and set through the use of push buttons and dials on the front of the device. A digital panel meter continuously displays the picocoulomb reading with the push-buttons disengaged. Engaging the push-buttons displays the threshold value of the trip setting or alarm setting on the digital panel meter, respectively. The alarm setting threshold value is set to some percentage of the trip setting voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be best understood by those having ordinary skill in the art by reference to the following detailed description when considered in conjunction with the accompanying drawings in which: 
     FIG. 1 is a block diagram illustrating the circuitry employed in a device of the present invention; 
     FIG. 2 is a schematic diagram of the sensing circuitry of the device; 
     FIG. 3 is a schematic diagram of the relay circuitry of the device; 
     FIG. 4 is a perspective view illustrating how the device of the present invention is employed in the preferred embodiment; and 
     FIG. 5 is a front view of the device of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Throughout the following detailed description, the same reference numerals refer to the same elements in all figures. 
     Referring to FIG. 1, a block diagram is shown depicting the circuitry employed in the partial discharge detecting device of the present invention. There are essentially three circuits associated with the partial discharge detecting device and include: a sensor circuit  10 , a relay circuit  12  and a power supply circuit  14 . Relay circuit  12  and power supply circuit  14  are enclosed within a single box, whereas sensor circuit  10  couples to relay circuit  12  and power supply circuit  14  via cabling. In particular, sensor circuit  10  is coupled to relay circuit  12  by a signal pathway  16  and a common pathway  18  (common only to an output drive of sensor circuit  10  and a primary input of relay circuit  12 ). The partial discharge detecting device of the present invention can incorporate a pair of sensor circuits  10  (although not shown in FIG.  1 ). If a pair of sensor circuits  10  are employed, connection to relay circuit  12  is made at primary and secondary inputs, shown as  70  and  72 , respectively, on FIG.  3 . 
     Power supply circuit  14  is a DC power source suppling +(positive) and −(negative) 15 Vdc to sensor and relay circuits,  10  and  12  respectively, and +(positive) and −(negative) 5 Vdc to a digital panel meter (to be discussed in further detail hereinafter). The + and −15 Vdc is common to all circuits of the partial discharge detecting device. Power supply circuit  14  is additionally coupled to a common ground. 
     Further to FIG. 1, power supply circuit  14  includes an input line filter  20 , a high isolation transformer  22  and noise filtration (not shown). Input line filter  20  is a low pass filter for precluding any RF and high frequency noise from passing therethrough. In the preferred embodiment, a high frequency choke and a network of high frequency by-pass capacitors are employed. High isolation transformer  22  converts the line voltage (120 Vac) to 24 Vac for further rectification by a bridge rectifier which outputs the 24 Vdc. In the preferred embodiment, a 120:24 AC step-down transformer is employed. The noise filtration receives the 24 Vdc and ensures that any residual high frequency noise is eliminated from power supply circuit  14 . In the preferred embodiment, the noise filtration is a network of capacitors, resistors, transistors and zener diodes. A first pair of voltage regulators supplies +(positive) and −(negative) 15 Vdc for the electrical components of the partial discharge device. A second pair of voltage regulators supplies +(positive) and −(negative) 5 Vdc for all logic components used in the partial discharge device, such as, for example, the digital panel meter. A pair of capacitors (one for positive and one for negative) are employed at each stage of the power supply circuit for additional filtration and noise elimination in the circuit. Power supply circuit  14  connects to an AC main through input line filter  20  at the “Control Power Input” connection point as shown in FIG.  1 . An alternate or auxiliary power connection point NEUT  90  and PHASE  92  is provided on front panel  78  of partial discharge detecting device  94  as shown in FIG.  5 . In it noted that in the preferred embodiment, connection to an AC power source is employed. Although, in an alternate embodiment, a DC power source could be used. Further, in the preferred embodiment, the “Control Power Input” is connected to a separate power source (the AC main), although in an alternate embodiment, the “Control Power Input” could be coupled to a power source provided from the electrical system to be monitored. 
     With continuing reference to FIG. 1, it is shown that sensor circuit  10  includes the following elements: an input sensing coil  24 , a wide band amplifier  26 , a high-pass filter  28 , a precision rectifier  30 , a peak detector  32  and an output drive  34 . Each of the aforementioned elements of sensor circuit  10  correspond to a network of electrical components shown in FIG.  2 . It is understood that the electrical components and their associated values shown in FIG. 2 are those of the preferred embodiment. Accordingly, it is possible to substitute other electrical components and/or change the values disclosed therein to reach the same result that partial discharge detecting device  94  carries out. Further, if a pair of input sensors are employed with partial discharge device  94 , as done in the preferred embodiment, a pair of FIG. 2 circuits, acting independently of each other, would be employed. When two sensor circuits  10  are used, each have their own dedicated input circuit and output drive. In this disclosure, sensor circuit  10  is also referred to as a picocoulomb sensor and will be discussed in further detail hereinafter. 
     With continuing reference to FIG. 1, input sensing coil  24  couples directly to an electrical system (or the “Monitored Circuit” as shown in FIG. 1) for continuous detection of a partial discharge level present therein. The type of electrical systems that can be tested includes, but is not limited to, transformers, generators, motors, capacitor banks and bushings. As shown in FIG. 4, partial discharge detecting device  94  is connected to a three phase power transformer  96 , the preferred electrical system to monitor. It is noted that the preferred partial discharge detecting device is a portable unit which can be set next to the system to be monitored. FIG. 4 illustrates a mounted device. 
     Input sensing coil  24  is an air core coil (T 1  of FIG. 2) designed to detect the high frequencies inherent in partial discharge for later amplification and signal conditioning by the subsequent circuit. Typically this signal is a high frequency pulse in the area of 200 KHz. Input sensing coil  24  couples directly to wide band amplifier  26  which amplifies all frequencies detected by input sensing coil  24  to a useable level for the high-pass filter as shown in FIGS. 1 and 2. High-pass filter  28  allows all of the high frequency signals of interest to pass therethrough while simultaneously blocking all of the low frequency signals of no interest. The output of high-pass filter  28  is directed to precision rectifier  30  for rectification of the received signal into a DC control signal. The rectified signal is outputted to peak detector  32  which acts to capture the peak of the applied signal from precision rectifier  30 . Although not shown as a separate item in FIG. 1, an additional stage of peak detection and integration is employed (as shown in schematic FIG. 2) to smooth the signal to a relatively steady DC level that is proportional to the picocoulomb input signal. Peak detector  32  directs the DC signal to output drive  34  which acts as a voltage to current conversion circuit (or process control loop). The resulting signal outputted to relay circuit  12  spans 4-20 mA, wherein 4 mA is the offset and represents a 0 pC signal and 20 mA is the full-scale and represents a 2000 pC signal. This portion of sensor circuit  10  acts as a pre-amp providing improved signal-to-noise ratio, thereby ensuring signal integrity for the input of relay circuit  12 . Since the signal passes through some length of cable  98  between sensor circuit  10  and relay circuit  12 , it could be susceptible to extraneous noise. In the preferred embodiment, wide band amplifier  26 , high-pass filter  28 , precision rectifier  30 , peak detector  32  and 4-20 mA output drive  34  are all enclosed within a small sleeve  100  and directly coupled to input sensing coil  24 . Further, input sensing coil  24  is a “clamp-on” style coil for engaging the ground of the electrical system, such as a transformer, as shown in FIG.  4 . In the preferred embodiment, sensing coil  24  is either cast in epoxy or encapsulated in resin. 
     In an alternate embodiment, sensor circuit  10  and relay circuit  12  are enclosed within the same box. In such embodiment, output drive  34  could be removed from sensor circuit  10  due to being in close coupled electrical proximity of one another. One example of the alternate embodiment provides for a single box enclosing sensing circuit  10 , relay circuit  12  and power supply circuit  14 . A large center opening is formed in the box allowing the ground lead of the electrical system to pass therethrough. In such embodiment, input sensing coil  24  would surround the center opening. 
     In yet another alternate embodiment, input sensing coil  24  has an amorphous core. The use of an amorphous core coil has certain advantages over an air core coil, such as, for example, a larger energy transfer capability and a lesser possibility of “burn-out.” Although, saturation of the amorphous core can occur in the presence of high current conditions. Under normal operating conditions, sensor circuit  10  should not see more than 200A of unbalanced or ground currents (which is an acceptable level to receive on a continuous basis). Higher currents, on the other hand, may result in amorphous core coil saturation, which could cause the sensors to be insensitive to picocoulomb signals. If sensor circuit  10  sees more than 200A of sinusoidal current, an alarm output (to be discussed in further detail hereinafter) will trip. This effectively warns an operator that a condition may exist which could cause relays in relay circuit  12  to not work properly in response to receipt of a picocoulomb signal. In other words, saturation of the amorphous core coil could cause a trip, but will not necessarily cause the partial discharge detecting device to fail. It is also noted that the partial discharge detecting device of the present invention is capable of recognizing and reacting to arcing ground faults due to the inherent characteristics of the device (i.e., the device&#39;s ability to recognize a very narrow current pulse). 
     With continuing reference to FIG. 1, it is shown that relay circuit  12  includes the following elements; input signal conditioning  36 , time delay  38 , switch logic  40 , trip setting  42  and an associated comparator  44  and LED status indicator  46 , alarm setting  48  and an associated comparator  50  and LED status indicator  52 , a digital panel meter  54 , a trip setting latched relay  56  (mechanically held), a trip setting reset  58 , an alarm setting relay  60  (electronically held) and an output drive  62 . 
     With reference again to FIG. 1, the 4-20 mA signal is directed from sensor circuit output drive  34  to a pair of input connection points  70  and  72  of relay circuit  12  (see FIG.  3 ). A first input  70  (the primary input), is used to receive the signal from a first picocoulomb sensor (sensor circuit  10 ), while a second input  72  (the secondary input) is used to receive a signal from a second picocoulomb sensor (sensor circuit  10 ). As stated before, a pair of sensor circuits  10  (or picocoulomb sensors) can be employed with the device of the present invention. In such embodiment, each sensor circuit  10  is functionally independent of the other; the only common circuitry being the DC power source. This redundant configuration ensures high reliability such that if one of the two sensor circuits fail, the other will sound an alarm or trip if the signal exceeds approximately twice the preset level. This result is achieved due to the fact that when two sensors are employed, the sum of the partial discharge pulse in picocoulombs is divided between the two sensors. In an alternate embodiment, the 4-20 mA output drive from sensor circuit  10  could be utilized to recognize (and thereby alarm) that one or both of inputs have failed. As shown in FIG. 4 partial discharge detecting device  94  employs a pair of sensor circuits  10 . Referring to both FIGS. 4 and 5, it is shown that device  94  includes a pair of sensor inputs  70  and  72  also known as sensor inputs A and B respectively. 
     Referring again to FIG. 1, the signal, traveling along signal pathway  16  is first directed to input signal conditioning  36  for establishing a full scale 1 volt signal through a current to voltage conversion. Impressing the 20 mA full scale signal across a 50 ohm resistor yields a proportional 0.2-1 volt output signal in relation to the 4-20 mA input signal. Prior to any signal conditioning, the proportional 0.2-1 volt output signal is directed to output drive  62  for connection to an alternate monitoring and/or alarming circuit which can be locally or remotely positioned. The signal outputted to output drive  62  can be directed to the alternate monitoring/alarming circuit in a plurality of different manners, such as, for example, remote telemetry, fiber optics and RF carrier. As shown on FIG. 5, output drive +(positive) and −(negative) connection points,  74  and  76  respectively, positioned on a front panel  78  of the partial discharge detecting device, permit connection to the alternate monitoring/alarming circuit. As shown on FIG. 3, +(positive) connection point  74  corresponds to TB 4  and −(negative) connection point  76  corresponds to TB 3 . 
     Further to FIG. 1, the signal outputted from signal conditioning  36  is directed to time delay  38  to provide a means for removing any transient spikes or switching surges that may cause relay circuit  12  to improperly trip or alarm. Time delay  38  is a continuously adjustable user setting which has an inverse definite minimum time characteristic. Because the input signal passes through the circuitry of time delay  38  before it passes through the circuitry of trip setting  42  and alarm setting  48 , its setting has the same effect on both trip setting  42  and alarm setting  48 . Initially, time delay  38  exhibits an inverse time characteristic. When the input signal reaches a value that is approximately equal to a value that is ten (10) times the setting (of either the trip or alarm setting), time delay  38  reaches its minimum time as determined by the inherent minimum response time of relay circuit  12 . The user adjustable dial  68  on front panel  78  is used to control the response time for signals that are close to either the alarm or trip settings and exhibits an inverse time delay characteristic. That is, the higher a signal above the threshold (in picocoulombs), the faster relay circuit  12  responds until it reaches the minimum response time for relay circuit  12 . After this point, any further increase in the input signal will not cause a corresponding decrease (a faster) response time. 
     Time delay  38  is set by adjusting delay setting dial  68  on front panel  78 , as shown on FIG. 5, and which corresponds to variable resistor R 8  of FIG.  3 . CW (or clockwise) as shown on FIG. 3 corresponds to “MAX” on FIG. 5 of dial  68 , whereas CCW (or counter clockwise), also of FIG. 3, corresponds to “MIN” on FIG. 5 of dial  68 . If dial  68  is set to “MIN”, there is no intentional delay and time delay  38  causes relays  56  and  60  to latch in about 100 milliseconds without consideration of the magnitude of the fault of the signal received from the electrical system coupled to the partial discharge detecting device. 
     After the input signal passes through time delay  38  it is directed to switch logic  40  which provides a means for adjusting/setting trip and alarm setting,  42  and  48  respectively. As seen in FIG. 3, switch M 2  is used to pass a threshold value in picocoulombs of trip setting  42  to meter  54 , whereas M 3  is used to pass a threshold value in picocoulombs of alarm setting  48  to meter  54 . Referring to FIG. 5, M 2  corresponds to trip setting push-button  64  and M 3  corresponds to alarm setting push-button  66 , both located on front panel  78 . Engaging either push-button  64  or  66 , passes the respective value to digital panel meter  54  for displaying a value between 0-2 volts which represents a threshold value in picocoulombs for each setting. With neither push-button engaged, digital panel meter  54  reads the picocoulomb input signal being detected by device  94  at that moment in time. As illustrated in FIG. 5, digital panel meter reads 1999. This value, merely an example of a potential reading, represents 1999 picocoulombs. Accordingly, digital panel meter  54  displays a numeric value between 0-1999 which corresponds to a picocoulomb value between 0-1999 picocoulombs. Since the signal received by relay circuit  12  from sensor circuit  10  (“output” signal) is in the range of 4-20 mA, thereby providing a “span” of 16 mA, the level of partial discharge (that which is detected by device  94  and displayed by meter  54 ) is determined by the following equation (where “PD” equals the level of partial discharge in picocoulombs and the “offset” equals 4 mA):        PD   =           output                 mA     -     offset                 mA         span                 mA       ×   2000                            
     It therefore follows that an output signal having a value of 12 mA (a value used for illustrative purposes only) would equal 1000 picocoulombs:          1000                 picocoulombs     =           12      mA     -     4      mA         16      mA       ×   2000                            
     Such a reading would be displayed on meter  54  as 1000. 
     As shown on FIG. 5, trip setting  42  can be set by adjusting trip setting dial  80 , located on front panel  78  of partial discharge detecting device  94 . Referring to FIG. 3, trip setting dial  80  corresponds to variable resistor R 17 . Further to FIG. 5, alarm setting  48  can be set by adjusting alarm setting dial  82 , also located on front panel  78  of partial discharge detecting device  94 . Referring to FIG. 3, alarm setting dial  82  corresponds to variable resistor R 12 . For both dials,  80  and  82 , CW (or clockwise), as shown on FIG.  3 , corresponds to “MAX” on FIG. 5, and CCW (or counter clockwise), also of FIG. 3, corresponds to “MIN” on FIG.  5 . It is understood that push-buttons  64  and  66 , pass a picocoulomb value to meter  54  for the purpose of setting the respective threshold value of trip and alarm setting  42  and  48  respectively. It is therefore not necessary to engage push-buttons  64  or  66  for setting the values. But without doing so, the user would not know the threshold picocoulomb value to which each setting is set. Of course, the user could adjust either setting and then engage the respective push-button for reading the set value. But in the preferred embodiment, push-buttons  64  and  66  are engaged before setting each respective threshold value. 
     Once the threshold levels are set for both trip setting  42  and alarm setting  48 , each use an associated comparator,  44  and  50  respectively, for detecting any changes (exceeding the threshold value) which gives rise to the latching of the relays associated therewith. Further, trip setting  42  and alarm setting  48  each have an associated bi-color LED, D 4  and D 5  respectively of FIG. 3 (corresponding to LED status indicator  46  and  52  respectively of FIG. 1) for indicating either a “normal” or “tripped” state. As shown on FIG. 5, trip setting LED status indicator  46  is shown as trip output LED  84  and alarm setting LED status indicator  52  is shown as alarm output LED  86 . As further illustrated on FIG. 5, a “normal” state for either output is represented by green illumination of the LED, whereas a “tripped” state for either output is represented by red illumination of the LED. No color in LED  84  and  86  indicates a loss of control power. 
     Referring to FIG. 1, the signal passing through trip setting  42  is directed to relay  56 . In the preferred embodiment, relay  56  is a mechanically latching SPDT set-reset relay. This type of relay has a set and a reset coil. Referring to FIG. 3, K 1 -A is the set coil (although designated “trip” herein) and K 1 -B is the reset coil. The output of relay  56  is coupled to a circuit (“Control Circuit”) which controls the electrical system to which partial discharge detecting device  94  is connected. For example, if the electrical system is a transformer, relay  56  can be coupled to a control circuit for the transformer which takes the transformer off-line before any failure of the transformer. 
     Relay  56  will not respond to a loss of control power. Although, if there is a loss of control power, oversized capacitors in power supply circuit  14  will allow relay  56  to change state, or trip, for as long as one minute after the loss of the control power, so long as the picocoulomb signal exceeds the set threshold value. With partial discharge detecting device  94  energized, relay  56  will remain in a “normal”, or non-tripped state. If the picocoulomb value of the input signal exceeds the threshold (or trip pick-up) set for trip setting  42 , relay  56  will change state or “trip.” 
     Relay  56  can be configured for either automatic or manual reset. Both configurations require the application of control power to affect a reset. Automatic reset is configured by installing a jumper on the terminal strip—TB 11  and TB 12  of FIG.  3 . If automatic reset is used, relay  56  will reset to its “normal” state, after a trip has occurred, when trip setting  42  recognizes that the picocoulomb input signal has fallen below the set threshold value. If manual reset is used, then the user must reset relay  56  by engaging push-button  88  located on front panel  78  as shown in FIG.  5 . Push-button  88  corresponds to switch S 1  of FIG.  3 . Relay  56  can not be reset until the picocoulomb input signal falls below the trip setting threshold value. Accordingly, engaging push-button  88  will not affect the state of relay  56  until the picocoulomb value falls below the threshold. As shown in FIG. 5, the reset can also be remotely controlled by coupling a remote reset switch to connection points  102  and  104  (“Remote Reset”) which correspond to TB 11  and TB 12 , respectively, of FIG.  3 . 
     Referring again to FIG. 1, the signal passing through alarm setting  48  is directed to relay  60 . In the preferred embodiment, relay  60  is an electrically held SPDT relay. The output of relay  60  is coupled to a circuit (“Alarm Circuit”) for signaling (alarming) that a level of partial discharge (in picocoulombs) has been reached which may cause failure to the electrical system to which the partial discharge detecting is coupled. Since relay  60  is electrically held, it additionally provide a means for signaling that there has been a loss of control power. When control power is first applied, relay  60  changes state to its energized position and remains transferred unless there is loss of control power or alarm setting  48  recognizes that the picocoulomb value of the input signal exceeds the set threshold value. Adjusting the alarm setting threshold value will not affect the trip setting threshold value. Although meter  54  reads a picocoulomb value when push-button  66  is engaged, the value is actually a percentage of the trip setting threshold value. The alarm setting threshold value can never be higher than the trip setting threshold value. Since the trip setting threshold value has a range of 0-2000, it follows that the alarm setting threshold value has a range equal to 0-100% of the trip setting. For example, if the trip setting is set at 1000 picocoulombs and the alarm setting is set to 50%, the corresponding alarm threshold value, or the point at which alarm relay  60  would trip, would be 500 picocoulombs. If the trip setting threshold value is changed, the alarm setting threshold value will also change proportionally such that it remains at a percentage value as compared to the trip setting threshold value. It is therefore recommended that the user first set the trip setting threshold value. By way of example, using the figures directly hereinabove, if the trip setting is adjusted to 1500 picocoulombs and the alarm setting is left alone (at 50%), the corresponding alarm threshold value, or the point at which alarm relay  60  would trip, is now 750 picocoulombs. 
     Referring to FIG. 3, it is shown that relay circuit  12  contains a set of auxiliary connection points (or outputs) for the alarm and trip relays  56  and  60  respectively. In particular, alarm relay  60  is associated with connection points TB 5 , TB 6  and TB 7  of K 2 -B Alarm. Whereas, trip relay  56  is associated with connection points TB 8 , TB 9  and TB 10  of K 1 -C Trip. Referring to FIG. 5, alarm output connections  106 ,  108  and  110  are equivalent to TB 5 , TB 6  and TB 7 , respectively, and trip output connections  112 ,  114  and  116  are equivalent to TB 8 , TB 9  and TB 10 , respectively. 
     Referring to FIG. 5, dials  80  and  82  are shown with hash marks. As to dial  80 , “MIN” equals 0 picocoulombs with each subsequent mark representing a 200 picocoulomb increment such that “MAX” equals 2000 picocoulombs. As to dial  82 , “MIN” equals 0% with each subsequent mark representing a 10% increase such that “MAX” equals 100%. Both dials  80  and  82  are sweep style dials allowing for smaller increment settings between each hash mark. 
     It is noted that all electrical systems having conductors exhibit varying partial discharge readings. It is not necessarily the presence of partial discharge that is an indicator of a problem but rather the rise in partial discharge over a period of time. If that period of time is short, a problem could be arising in the system. When partial discharge detecting device  94  is first installed, the user takes a first reading which could be considered the ambient value. If that value, over time, does not change and is one that is acceptable to the user, no action necessarily needs to be taken. But, as that value begins to climb, it is most likely an indicator of some type of problem relating to partial discharge. 
     Equivalent elements and/or components can be substituted for the ones set forth above such that they perform the same function in the same way for achieving the same result.