Capacitive probe techniques

When a discharge occurs, an electromagnetic wave is produced which propagates away from the PD site. Where the plant is metal clad, the wave will propagate towards the earthed metal enclosure. Provided there is a gap somewhere in the enclosure, e.g.

a gap in the gasket or gap at the busbar chamber cover in the case of metal clad switchgear, the electromagnetic wave is free to travel to the atmosphere outside the switchboard. The action of the wave connecting with the earthed metalwork produces a transient earth voltage (TEV) which can be detected by a capacitive probe if positioned at the gap. The amplitude of the detected signal is normally in the millivolts to volts range, and this is generally translated into dB for measurement.

The principle is shown in Figure 4.8.

Although this method of detection will give some indication of severity, there is no detail of exact location. When more than one capacitive probe is used, however, it is possible to gain some knowledge of location using the time of flight principle, as

EM wave seal/gasket

probe case

high voltage component

discharge producing electromagnetic emissions

Figure 4.8 Transient earth voltage (courtesy of EA Technology Limited)

the probe nearest the source should detect the discharge first. This may indicate the panel from which the source is emanating.

An advantage of this type of testing is that the non-intrusive nature of the mea-surements allows for no disruption or outage to the plant under test. All items of plant within a substation can be monitored, such as circuit breakers, busbars, current and voltage transformers and cable end joints. Installing the test equipment while the substation is online has obvious benefits, and savings are also made due to the removal of outage costs and the relevant manpower reduction. The advantages and disadvantages of this technique are detailed in the work of Brown [61].

It is possible that external electromagnetic noise will interfere with the readings taken, and it is therefore essential that a background survey be completed prior to commencement. As with all types of PD measurement, if the interference is too severe, it may not be possible to complete the test, as results may not be sufficiently analysed. Problems are encountered when these noise sources produce voltages on other metallic surfaces within the substation using the TEV principle. The background reading should be taken from a metallic surface that is not attached to the switchboard such as a battery charger or doorframe. If a reading above a certain level is detected in this circumstance, the probe is unsuitable for monitoring the plant as it is not possible to differentiate between discharges from the plant or from external sources.

This type of probe can be purchased as a light, portable handheld unit or as a system with a number of probes connected to (typically) an event counter.

Clearly, the handheld unit is convenient, easy to use and relatively cheap.

However, it will only detect discharges at the moment when the test is conducted.

Although this is useful, it does not include any discharges that may present themselves at some other time due to load changes, humidity or temperature changes etc. Partial discharges are often intermittent and therefore a more thorough test regime may be required. In this event, the multiprobe system can be utilised.

Typically, such a system might have eight to ten capacitive probes and these would be connected to the plant using magnetic clamps. The probes are normally threaded and screw into the clamps until flush with the earthed enclosure. In turn, the probes would be connected by separate channels to (typically) an event recorder. Additional channels should be kept free for antennae. The purpose of the antennae is to detect any external electromagnetic noise that may filter into the plant environment, be detected by one or more of the probes and interpreted as partial discharge activity.

By subtracting the events detected by these antennae from those detected by the probes, interference effects are reduced. These antennae should be positioned in the corners of the substation and extended vertically at the same height as the plant under test.

4.3.2 Acoustic detection

Partial discharges produce acoustic noise, as anyone who has listened to the crackling noise in electrical substations will confirm. Although directional microphone systems have been used to detect partial discharges from their airborne acoustic emissions, their application has been largely linked to external busbars, connectors and insulator assemblies. Acoustic detection has found much greater success and wider application through the use of piezoelectric sensors.

Piezoelectric polymers, such as PVDF (polyvinylidene fluoride), when com-pressed, result in the production of an external voltage proportional to the force applied to the polymer. As such, when built into the head of a handheld probe or when fixed, with a suitable paste or clamping mechanism, to an appropriate enclo-sure, they offer a simple means of detecting acoustic signals. Typically, handheld acoustic probes are coupled to an analogue voltmeter. Unfortunately, although it has been shown that for simple geometries the resultant voltage is proportional to the size of the discharge, due to the complex acoustic impedances associated with the propagation of an acoustic pulse to the probe, no effective calibration is possible for high voltage plant. The intensity of the emitted acoustic waves is proportional to the energy released in the discharge. On this basis, the amplitude of the wave is proportional to the square root of the energy of the discharge and, since energy may be taken as proportional to the charge squared, there should be a linear relationship between discharge magnitude and acoustic signal. However, acoustic measurements are more about detecting the presence of discharges, irrespective of magnitude, and locating these within the plant item. Typical applications for a handheld probe would be distribution circuit breaker boxes and small transformers. As the probe is moved around the enclosure, the larger the voltage detected, the closer the probe is to the source of discharge activity. On a much larger scale, and using more sophisticated acquisition instrumentation but the same sensor technology, with a minimum of three probes fixed to the earthed tank of a large transformer, and a reference signal, it is possible to determine both the presence and the accurate location of any discharges present. By measuring the relative times of arrival of the pulse(s) from the discharging site at the three probes, and by assuming a constant velocity of acoustic propagation through the transformer structure, the relative distance from each of the probes to the

discharging site can be computed and triangulated in three dimensions. Commercial software is available to do this and can be readily utilised with a laptop computer for portability.

Knowing the times of arrival of t₁, t₂ and t₃ from the different sensors, and assuming a given velocity of propagation, since distance = velocity/time, three distances can be computed. Knowing these distances, and the location of the probes, a three-dimensional plot can be made; where these spheres intersect is the discharge source. The approach is illustrated in Figures 4.9a and 4.9b.

Lundgaard [62, 63] has produced a useful review of acoustic detection of partial discharges and, as he points out, changes in both signal amplitude and shape occur as the acoustic signal propagates to any sensors. He cites reduced signal amplitude as a result of:

• geometric spreading of the wave

• division of the wave down multiple pathways

• transmission losses in propagating from one medium to another and at disconti-nuities within a given medium

• absorption in materials.

sensor 2 sensor 3 sensor 1 PD t₁

t₂ t₃

discharge emanating acoustic waves in 3D

Sensor 2

sensor 1 sensor 3

tank/enclosure t₁

t₂

t₃

a

b

Figure 4.9

a Partial discharge acoustic emissions arriving at different times at each sensor according to distance from source

b Arrival of pulses at detector at different times according to distance travelled

In relation to changes in signal shape, he cites:

• frequency-dependent velocity effects resulting in different frequency components of a given signal arriving at the transducers at different times

• frequency-dependent propagation paths, again resulting in different wave components arriving at the transducer at different times

• absorption in materials removing high frequency components preferentially.

In addition, there are significant differences in the acoustic velocities in typical types of media encountered in (say) a transformer, i.e.:

• transformer oil at 25^◦C, 1415 ms⁻¹

• core steel, >5000 ms⁻¹

• impregnated pressboard, 1950 ms⁻¹

When one considers the complexity in both structure and materials in (say) a large oil-filled paper insulated transformer, the factors which can affect the signal propagation, and the variation in signal velocities with medium, it is impressive that this technique can be applied to complex structure, large plant.

Typically, in a quiet factory-type environment, discharges in a large power trans-former can be located to within a volume the size of a football within a couple of hours and to within the size of a fist within a working day. That said, there is some variation in the estimated success rate using this technique according to manufacturer. Some claim total success whereas others are more circumspect admitting that they would be unlikely to detect a source, using this technique, embedded deep in a winding.

In the field, utilities also report a useful success rate (typically 50 per cent) using this technique with large power transformers. Indeed, a good example of this is presented in the work by Jones [64]. He reports that faults, using this approach, can be categorised as, for example, coming from:

• bushing connection stress shields

• windings

• winding jacking screws

• core bolts

• winding lead clamps

• tapchanger components.

The key issue here is that, although the absolute magnitude of discharge events cannot be determined from this technique, knowing the location of the source may be just as, if not more, important. For example, a source identified as coming from a winding might give serious cause for concern whereas, if the source is corona from a core bolt, it might not. A source within the winding will erode the paper and could lead to a catastrophic failure in time. A core bolt suffering corona will result in change to the dissolved gas levels but will not age the transformer in any significant way.

The single probe approach can also be used for capacitors and bushings but great care must be taken to ensure that placement of the probe on the capacitor or bushing surface does not distort the electric field resulting in a flashover to the probe.

Finally, it is worth noting that, in the case of distribution circuit breaker boxes, discharges are often intermittent and a single measurement over a short period of time may not be sufficient to ensure an accurate assessment. In these situations, an alternative to the hand held approach is to fix a sensor to the box and leave it on site for a number of days coupled to an event counter (in the same fashion as per capacitive probes). In a substation, a number of probes can be used simultaneously in this way, one to each box, with some form of reference probe also in place, to ensure that any detected events are coming from discharges within one or more of the distribution boxes and not from some external source.

In summary, acoustic techniques are relatively cheap and simple to apply, are utilised online and can detect the presence and location of discharges in the various items of plant discussed. Their disadvantages include their inability to be applied to intrinsically noisy plant, e.g. motors/generators, the need for the sensor to be relatively close acoustically to the discharge source (so the technology cannot, for example, be applied to cables) and their inability to be calibrated in terms of voltage output versus size of discharge.

4.3.3 Thermography and other camera techniques

Given that partial discharges are generally hotter than their surrounding media (see earlier), it might reasonably be thought that the thermal imaging camera could be applied in their detection and measurement. Unfortunately, most partial discharges are enclosed in some way, e.g. within solid insulation or within metal clad enclosures.

Given the relatively low temperatures of partial discharges and the high thermal impedances likely to be present between such discharges and the imaging camera, its use is very limited in this application.

Certainly where the discharges are external to associated plant, e.g. on overhead lines/busbars or post/string-type insulators, such techniques can be used but not in any quantitative way. However, the desire to detect PD from these structures is somewhat limited compared with other items of plant.

Interestingly, in a similar vein, a daylight corona camera has recently been devel-oped [65]. This incorporates independent UV video and visible cameras to capture separate video images of discharges and of associated high voltage plant. The system detects corona in the 240–280 nm region. Corona discharges emit in air mainly in the 230–405 nm range of the UV spectrum. Although the corona emission lines between 240 and 280 nm are not as strong as in the 290–400 nm range, this region is also called the UV solar blind band, i.e. there is no background radiation in this region. Despite the weaker intensity, the UV solar blind imager is able to provide high contrast images due to the complete absence of background radiation.

4.3.4 Chemical detection

Chemical techniques rely on the measurement of byproducts associated with PD activity and thereby from which PD activity can be inferred. This necessarily requires that these byproducts can be detected in some way. Clearly, PD activity associated

Table 4.3 Typical gases absorbed in the oil under the action of PD

H₂ hydrogen (H–H)

CH₄ methane (CH₃–H)

C₂H₆ ethane (CH₃–CH₃) C₂H₄ ethylene (CH₂=CH2)

C₂H₂ acetylene (CH≡CH)

with a closed, unvented void within a section of solid insulation would not lend itself to detection by this method.

Although chemical detection has been applied to various items of plant involving gas circulation over the years, e.g. hydrogen-cooled generators and, with relative success, gas insulated substations, it is primarily in oil-filled equipment that chemical detection has found favour.

Under the action of partial discharges (and, indeed, other fault conditions), oil will degrade through bond scission to form characteristic gases absorbed in the oil.

Typical gases produced are given in Table 4.3.

The quantity and mix of gases produced depends on the nature of the fault, its severity and the associated temperature. The weakest C–H bond can be broken with relatively little energy, i.e. ionisation reactions, with hydrogen being the main recombination gas. As the strength of the molecular bond increases, more energy and/or higher temperature is required to create scission of the C–C bonds and the resulting recombination into gases which have either a single C–C, double C=C or triple C≡C bond.

Being essentially a low energy type fault, partial discharge activity tends to favour the breaking of the weakest C–H bond with the production of hydrogen. Carbon monoxide and carbon dioxide will also be present if the discharge occurs in the presence of cellulose, i.e. paper insulation (as is generally the case in large power transformers).

As little as 50 ml of oil suffices for analyses to be performed. This is important since, although dissolved gas analysis (DGA) has primarily been used for screening of large, oil-filled transformers (due to the capital involved in such assets) and where the loss of sampled oil would be insignificant, the increasing use of the technique with small oil volume plant such as bushings, CVTs etc., has made the volume of oil to remove critical.