TECHNICAL FIELD OF THE INVENTION

This invention relates to a partial discharge location device and method for locating partial discharge activity in substation assets. In particular, the present invention relates to a handheld device comprising a pair of probes that can locate the site of partial discharge activity and determine its magnitude and severity, and to a method of detecting and localising a partial discharge event in substation assets.

BACKGROUND

For many years, it has been known to inspect substation assets for partial discharge (PD) activity to provide an early warning of the deterioration of assets. Partial discharge is a well-understood mechanism which begins within voids, cracks or inclusions within solid insulation, or in bubbles with liquid insulation. Partial discharge is a localised dielectric breakdown of a small portion of a solid or liquid electrical insulation system under high voltage stress but which does not completely bridge the space between the conductors. Once begun, partial discharge causes progressive deterioration or degradation of insulation material, and ultimately leads to dielectric breakdown and hence asset failure.

Partial discharge activity can be detected using various laboratory and field-testing methods. One of the most popular and convenient methods of detection is to measure transient earth voltages (TEV) induced in the surrounding metalwork of the substation asset. Transient earth voltages provide a reliable and convenient means for detecting partial discharge.

It is possible to detect partial discharge via periodic inspections using handheld instruments which do not make any electrical connection to the substation asset. A popular handheld instrument for inspection and early detection of PD activity is the UltraTEV™ Plus ² device available from EA Technology Limited. This device includes sensors which can detect transient earth voltage (TEV) and acoustic ultrasonic signals and which enables an operator to distinguish between true PD activity, noise and other interference in a straightforward and quick measurement. Whilst the UltraTEV™ Plus ² is very effective for detecting PD activity it is not always the case that the highest signal magnitude corresponds to the position of the PD source making physical detection difficult. In order to address this problem, the ETltraTEV Locator™ device available from EA Technology Limited is configured having two probes which are connected to a central measurement unit, and by observing which probe encountered the TEV signal first determines the direction of travel and hence the operator can pinpoint the site of the PD activity.

Whilst the market for handheld instruments which can locate, measure and record PD activity in all types of substation assets continues to grow, all current products, including the ETltraTEV Locator™, require a central measurement unit to be situated part way between the probes. This form factor is not always convenient to use, and increases the cost of the device.

It is an object of the present invention to provide a PD location device and method which overcomes or reduces the drawbacks associated with known products of this type. It is an object of the present invention to provide a dual -probe PD location device having no central measurement unit disposed therebetween. It is a further object of the present invention to provide a single-ended timing measurement for determining the difference in pulse time of arrival, allowing PD source representation to be determined as a distance between the two probes which negates the need for matched cables and centralised processing, and which facilitates noise rejection, and allows for multiple PD source location. It is a further object of the present invention to provide a single-ended measurement which enables use when measuring over longer distances, as there is no need for matched length cables connected to a central measurement unit. SUMMARY OF THE INVENTION

The present invention is described herein and in the claims.

According to the present invention there is provided a handheld device for detecting a partial discharge event, the device comprising:

a first probe having a first partial discharge sensor disposed within; and

a second probe having a second partial discharge sensor disposed within, the second probe being electrically connected to the first probe by a connecting cable;

wherein the first and second probes comprise means for measuring a difference in time of arrival of a detected partial discharge event.

An advantage of the present invention is that it negates the need for matched cables and a centralised processing unit, and which facilitates noise rejection, and allows for multiple partial discharge event location.

Preferably, the detected partial discharge event introduces a short duration pulse with a sharp rising edge into the first and second partial discharge sensors.

Further preferably, in use, the first and second probes are held proximate to, or in abutment with, the surrounding metalwork of a substation asset under inspection.

In use, the first and second partial discharge sensors may comprise first and second transient earth voltage capacitive sensors. Preferably, the first and second transient earth voltage capacitive sensors have generally the same capacitance.

Further preferably, the first probe is provided as a standalone partial discharge detection device and the second probe is provided as a plug-in module to the first probe. In use, the measured difference in time of arrival detected at the first and second probes may be graphically represented as a distance between the first and second probes on a display on the first probe. Preferably, the graphical representation is a simple block character, numerical value and/or histogram.

Further preferably, the detected partial discharge event comprises multiple partial discharge events.

In use, the measured difference in time of arrival of a detected partial discharge event may determine the location of the partial discharge event of the substation asset.

Preferably, the handheld device further comprising:

filter means for receiving the output of the first and second partial discharge sensors and rejecting out of band sensed signals;

logarithmic amplifier means for amplifying the filtered signals;

matching amplifier means for transmitting the log proportional signals;

wherein the filter means, logarithmic amplifier and matching amplifier means are disposed in each of the first and second probes; and

comparator means for comparing the transmitted signals from the first and second probes against a partial discharge trigger level;

precedence means for receiving the triggered output from the two input channels and measuring the difference in arrival time of the signals; and

processing means

wherein the comparator means, precedence means and processing means are disposed in the first probe.

Further preferably, the handheld device comprising storage means for storing the pulse amplitudes and arrival time difference. In use, an offset value may be written in the firmware of the processing means which compensates for differing cable lengths between the first and second probes.

Preferably, the partial discharge trigger level is set manually, or set automatically under the control of the processing means.

Further preferably, wherein the device determines the severity and/or location of partial discharge activity of the substation asset. Also according to the present invention there is provided a method for detecting a partial discharge event using a handheld device, the device comprising:

providing a first probe having a first partial discharge sensor disposed within; and providing a second probe having a second partial discharge sensor disposed within, the second probe being electrically connected to the first probe by a connecting cable;

whereby the first and second probes comprise means for measuring a difference in time of arrival of a detected partial discharge event.

Further according to the present invention there is provided a timing circuit for determining the time difference between two pulses, the timing circuit comprising:

a resettable capacitive timing circuit electrically connected to a constant current source across a voltage supply, a first switch and a second switch, and a first logic input and a second logic input;

wherein, upon detection of a rising pulse, the second logic input is asserted to activate the first switch to cause the constant current source to pull current through the resettable capacitive timing circuit providing a voltage proportionate to the charge of the resettable capacitive timing circuit; and,

upon detection of a secondary pulse or a timeout, the first logic input is asserted to activate the second switch to cause the constant current source to pull current directly from the voltage supply, whilst the analogue output voltage is held at the measurement voltage, the analogue output voltage being proportionate to the pulse precedence time. It is believed that a partial discharge location device and method for locating partial discharge activity in substation assets in accordance with the present invention at least addresses the problems outlined above. It will be obvious to those skilled in the art that variations of the present invention are possible and it is intended that the present invention may be used other than as specifically described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 shows a plan view from above of a handheld PD detection device in accordance with the present invention;

Figure 2 illustrates a high-level schematic diagram of the dual-probe PD detection device illustrated in Figure 1;

Figure 3 is a circuit block diagram of the precedence timing circuit illustrated in Figure 2;

Figures 4a to 4c show screenshots from the PD detection device and show how the dual- probe device can be used in a locator mode to show which probe detected a TEV signal first;

Figures 5a to 5c are screenshots from the PD detection device which illustrate how the dual- probe device can be used in a further, advanced locator mode with enhanced location functionality in the form of a histogram between the two probes; and

Figure 6 is a screenshot from the PD detection device which illustrates how multiple PD sources can be detected and localised between the probes. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention has adopted the approach of utilising a PD location device and method for locating PD activity in sub station assets. Advantageously, the present invention provides a dual-probe PD location device having no central measurement unit disposed therebetween. Further advantageously, the present invention utilises a single-ended timing measurement for determining the difference in pulse time of arrival, allowing PD source representation to be determined as a distance between the two probes which negates the need for matched cables and centralised processing, and which facilitates noise rejection, and allows for multiple PD source location. Providing a single-ended measurement enables use when measuring over longer distances, as there is no need for matched length cables connected to a central measurement unit.

Referring now to the drawings, a partial discharge (PD) location device 10 for use in detecting and localising PD activity in substation assets is illustrated in Figure 1. The PD location device 10 comprises a first handheld detection device 12 which in a standalone mode of operation can detect PD activity through the use of an integrated PD sensor 14 located at the head of the device 12. In a preferred embodiment, the PD sensor 14 is a TEV capacitive sensor, and in use the head of the detection device 12 held proximate to, or in abutment with, the surrounding metalwork of the substation asset (not shown in Figure 1) being inspected. The application software running on the device 12 is navigated using buttons 16, and the device 12 includes display 18. Although not shown in Figure 1, various other sensors and accessories can be connected to the device 12 to increase functionality.

In standalone use, an operator can check for PD activity by holding the device 12 close to the surface of the asset (not shown in Figure 1). The very nature of PD activity means that it is not always the case that the highest PD signal magnitude actually corresponds to the position of the PD source so, in use, a second handheld locator probe 20 can be connected to the device 12 when it desired to pinpoint the site of PD activity in substation assets. Electrically connecting the device 12 and the probe 20 is achieved by the use of a length of cable 24 having one end fixed to the locator probe 20, the other end thereof is received in a port 22 on the device 12. The shape of the locator probe 20 is identical to that of the detection device 12, and like the detection device 12 it includes a PD sensor 26 positioned at the head thereof. The probe 20 also includes operating buttons 28 and a power indicator light 30. Both the device 12 and the probe 20 include a series of laterally positioned projections 32 which enhance grip when in use.

The locator probe 20 has a permanent 2m cable 24 fixed thereto, and a 6m extension cable (not shown) can be added to make total location range of 8m. Depending on the range of detection required, the locator probe 20 can be plugged directly into the accessory port 22 enabling high resolution detection with a range of 2m. When a larger area or long cable runs are needed to be investigated, the 6m extension cable can be used to see a larger picture of PD activity, as will be described in further detail below.

After the locator probe 20 is connected to the device 12, the device 12 will automatically program the locator 20 with the correct version of firmware. When the locator probe 20 is ready for use the power indicator LED 30 will light.

Figure 2 shows a high-level schematic diagram of the PD location device 10 shown in Figure 1, and embodied as a locator probe 20 electrically connected to detection device 12. This drawing is a schematic diagram and, in order to aid clarification, many other circuit elements are not shown.

Figure 2 shows that the PD sensors 14, 26 embodied as TEV capacitive sensors are mounted within the head of the detection device 12 and probe 20, respectively. Each sensor 14, 26 is equivalent in its design and sensitivity. The capacitive coupling method employed in each probe 12, 20 allows the TEV signal to be detected without any direct connection to the switchgear or live conductors. Following a PD event, the skilled person will appreciate that a current pulse is injected onto the earth metalwork of the asset inducing a short duration voltage rise of the order of mV and with ns rise times. The coupled signals from the capacitive sensors 24 in response to partial discharge events are then filtered 34 by applying a bandpass filter to reject any out of band signals and reduce susceptibility to noise. Due to the fast rise times, low amplitude and short duration of the coupled signals, it is not possible to directly compare them. A logarithmic amplifier 36 compresses the signal to provide a log proportional output. The output of the logarithmic amplifier 36 is inputted to a matching amplifier 38 which acts as a line driver to transmit the log proportional output to the comparison circuit 40 located in the device 12.

The TEV outputs from both probes 12, 20 are continuously fed into the comparison circuit 40. This circuit compares the amplitude of the incoming signal to a variable threshold controlled by the microprocessor 46. When the threshold is exceeded, an output from the comparator is coupled to the timing circuit input. The comparison circuit 40 consists of two dedicated comparators, one for each channel (the detection device 12 on channel A and the locator probe 20 on channel B in Figure 3).

A microprocessor 46 controls the trigger level for each channel individually allowing separate trigger levels to be applied for each channel. When the trigger levels for either channel is exceeded the output of the respective comparator goes high. Both of these outputs (one for each channel) are fed into the timing circuit 44. The timing circuit 44 is described in further detail in relation to Figure 3, and it works on the principle that the charge state of a capacitor is used to determine the time difference between two pulses (on channels A and B) arriving from the comparison circuit 40.

The microprocessor 46 controls the process and communicates the result back to device 12 for display. The microprocessor 46, amongst other things, sets trigger levels, reads offsets and collates pulses to enable display of histograms.

The interconnecting cable 24 in a preferred embodiment utilises two circuits to allow for 2m and 8m cable lengths. This is in no way intended to be limiting as cable lengths can be varied with calibration in the microprocessor 46 or in the case of larger cable variation a component change in the timing circuit 44. The cable 24 is provided as a coaxial line for the TEV signal and a series of control cables.

A delay line 42 is also added to add a small delay to ensure that precedence events at the extremes of the scale are captured reliably.

Figure 3 shows a circuit block diagram of the timing circuit 44 shown in Figure 2. When the rising edge of a pulse is detected, logic input B is asserted and logic input A is de-asserted. In this logic configuration, a constant current source 48 will pull current through a resettable capacitive timing circuit 50 through switch 52, resulting in a voltage proportionate to the charge of the capacitive circuit to be present at the analogue output.

If a secondary pulse or timeout is detected; logic input A is re-asserted through switch 54 and logic input B de-asserted. The constant current source 48 will now pull current directly from Vcc whilst the analogue output voltage is held at the measurement voltage due to the charge of the capacitive circuit. The output voltage being proportionate to the pulse precedence time t due to the following equation: where V is voltage; C is the capacitance of resettable capacitor 50; and I is the current flowing through the constant current source 48. The analogue output voltage is fed to an analogue-to-digital converter (not shown in Figure 3) on the microprocessor The reset line discharges the capacitor charge once a measurement of the analogue output has been taken.

The locator probe 20 utilises two measurement channels with different charging rates to accommodate a fine and coarse measuring mode required for different cable lengths. Only one channel is enabled at a time. As mentioned above, the calculations inside the microprocessor 46 rely on the cable length 24 and the timing capacitor 50 being known. The cable calibration carried out on each power on of the locator probe 20 which will correct for small differences/tolerances in cable 24 length, by analysing the precedence results of a test pulse.

With the locator probe 20 connected to the detection device 12 the dual-probe device 10 can be used in a basic locator mode to show which probe detected a TEV signal first or in a further, advanced locator mode with enhanced location functionality in the form of a histogram displayed between the two probes.

Figure 4 shows screenshots from the PD detection device 12 with the locator probe 20 connected and when the device 10 is in a basic locator mode of operation. This mode of operation can be used to locate PD activity by showing which probe (either the device 12 or the locator probe 20) detected the PD source 100 first in the substation asset 102.

In use, this is displayed to the operator on the display 18 of the device 12 with a‘First’ indicator 104 panel being indicated on the right-hand side if the device 12 is closest to the source 100, or the left-hand side 106 if the locator probe 20 is closest to the source 100. It is recommended, but not required, that the device 12 is held in the right-hand and the locator probe 20 in the left-hand during testing, so the indicators on the display 18 are on the correct sides. If the PD source 100 is half way between the two probes 12, 20 both‘First’ indicators 104, 106 will be shown, as illustrated in Figure 4c.

Figure 4a shows the PD source 100 being nearer to the locator probe 20. Figure 4b shows the PD source 100 being nearer to the device 12 and Figure 4c shows the PD source 100 being halfway between the two probes 12, 20.

In Figures 4a to 4c, a trigger level 108, 110 is displayed towards the top of the display 18 for each sensor 14, 26 in dB. For accurate determination of precedence it is important that these trigger levels are set to a suitable value. The trigger level sets the threshold of the detection circuits so that no pulses/signals below the trigger level are detected. Therefore it is important to ensure that the trigger levels are set to higher than the noise floor, whilst also comfortably below the maximum level of the PD.

In most situations the trigger levels should be of similar value on each channel and not deviate by more than 5dB. Having the trigger levels more than 5dB apart may effect the accuracy of results.

In use, the dual-probe PD location device 10 can be used in either an auto trigger mode or manual trigger mode.

In auto trigger mode, the locator probe will automatically set the trigger levels to be slightly above the noise floor meaning triggers are always present (unless the noise floor is below OdB). This mode can be used to determine the approximate noise floor of the measurement and to get an approximate overview of the current PD activity.

In manual mode the trigger levels are manually controlled using the either the buttons 28 on the locator probe 20, or buttons 16 on the device 12. Manual mode gives the operator more control meaning the operator can adjust the trigger levels as the situation requires allowing more accurate measurements to be taken.

Figure 5 shows screenshots from the display 18 of device 12 when the PD location device 10 is used in a further, advanced locator mode with enhanced location functionality in the form of a histogram displayed between the two probes 12, 20. This mode is useful when more detail is required than simply knowing which probe 12, 20 a PD source 100 is closest to, or, for example, when there is more than one PD source 100.

The controls, trigger levels and basic operation is the same as for the standard locator mode, however the display 18 instead shows a histogram of all received pulses. Figures 5a to 5c show display 18 images locating a single PD source 100, as follows: When the histogram peak 100' is on the left-hand side of the zero line, the source 100 is determined to be closer to the locator probe 20, as shown in Figure 5a.

When the histogram peak 100' is on the right-hand side of the zero line, the source 100 is determined to be closer to the device 12, as shown in Figure 5c.

When the histogram peak 100' is approximately centered on zero line, the source 100 is determined to be equal distance from both probes 12, 20, as shown in Figure 5b. Finally, as shown in Figure 6, multiple PD sources 112, 114 will show as separate peaks 112', 114' on the histogram. On the example shown in Figure 6, one source 112 is directly between the two probes 12, 20, and one source 114 being closer to the device 12.

The location device 10 further comprise self-test power feeds connected to each of the plurality of PD sensors 14, 26, and which energise a self-test signal to allow the operation and display of the sensors 14, 26 to be verified when the device 12 and probe 20 are placed in an head-to-head arrangement.

Therefore, the device and method according to the present invention quickly and reliably detects and localises PD activity in substation assets.

When used in this specification and claims, the terms“comprises” and“comprising” and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in the terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, separately, or in any combination of such features, can be utilised for realising the invention in diverse forms thereof. The invention is not intended to be limited to the details of the embodiments described herein, which are described by way of example only. It will be understood that features described in relation to any particular embodiment can be featured in combination with other embodiments.

It is contemplated by the inventor that various substitutions, alterations, and modifications may be made to the invention without departing from the spirit and scope of the invention as defined by the claims.