Partial discharges and their measurement
4.3 Partial discharge measurement
4.3.4.3 Graphical techniques
As a consequence of IEC Code 599 being inconsistent in producing reliable diagnoses, additional schemes were introduced to complement the Code.
CH4/H2 C2H6/CH4 C2H4/C2H6 C2H2/C2H4
Range Code Range Code Range Code Range Code
≤0.1 5 <1 0 <1 0 <0.5 0
>0.1 < 1 0 ≥1 1 ≥ 1 < 3 1 ≥ 0.5 < 3 1
≥ 1 < 3 1 ≥3 2 ≥3 2
≥3 2
Code Diagnosis
CH4 H2
C2H6 CH4
C2H4 C2H6
C2H2 C2H4
0 0 0 0 normal
5 0 0 0 partial discharge
1, 2 0 0 0 slight overheating <150◦C 1, 2 1 0 0 slight overheating 150–200◦C
0 1 0 0 slight overheating 200–300◦C
0 0 1 0 general conductor overheating
1 0 1 0 winding circulating currents
1 0 2 0 core and tank circulating currents, overheated joints
0 0 0 1 flashover, no power follow through
0 0 1, 2 1, 2 arc, with power follow through
0 0 2 2 continuous sparking to floating potential 5 0 0 1, 2 partial discharge with tracking (note CO) CO2/CO > 11 higher than normal temperature in insulation
Figure 4.11 Rogers fault gas ratios (code based on Joseph B. DiGiorgio’s ‘Dissolved gas analysis of mineral oil insulating fluids’, published by NTT 1996–2002)
Duval [70] developed a triangle, Figure 4.15, based on the relative percentage of methane, ethylene and acetylene gas. The triangle is divided into six regions representing high energy arcing, low energy arcing, corona discharge and hot spots.
The triangle has the advantage that a diagnosis is always given – but, of course, this will always imply a fault! Hence it must only be used in conjunction with individual levels of gases which imply the possibility of a fault.
Other graphical techniques include the Church Nomograph Method, based on data published by Dornenburg and Strittmatter. The data are plotted on sliding loga-rithmic scales, with each scale representing a different gas. Data points are then joined together and the slope of the line between adjacent scales is indicative of the type of fault.
Code Diagnosis
<1.0 < 1.0 <1.0 <0.5 normal
≤0.1 <1.0 <1.0 <0.5 partial discharge – corona
≤0.1 <1.0 <1.0 ≥0.5
<3.0or >3.0 partial discharge – corona with tracking
>0.1
<0.1 <1.0 ≥3.0 ≥3.0 continuous discharge
>1.0
<0.1 <1.0 ≥1.0
<3.0or >3.0 ≥0.5
<3.0or >3.0 arc – with power follow through
>1.0
<0.1 <1.0 <1.0 ≥0.5
<3.0 arc – no power follow through
≥1.0
<3.0or >3.0 <1.0 <1.0 <0.5 slight overheating – to 150◦C
≥1.0
<3.0or >3.0 ≥1.0 <1.0 <0.5 overheating 150–200◦C
>0.1
<1.0 ≥1.0 <1.0 <0.5 overheating 200–300◦C
>0.1
<3.0 <0.5 circulating currents in windings
>1.0
<3.0 <1.0 ≥3.0 <0.5 circulating currents core and tank; overload joints
Note: several simultaneously occurring faults can cause ambiguity in analysis
Figure 4.12 Rogers fault gas ratios (ratio value based) (based on Joseph B.
DiGiorgio’s ‘Dissolved gas analysis of mineral oil insulating fluids’, published by NTT, 1996–2002)
As can be imagined, artificial intelligence in the form of expert systems, artificial neural networks and fuzzy-logic-based systems are finding increasing application in this area.
Before leaving chemical detection of partial discharges, it is appropriate to note that for oil/paper insulated plant, the detection of degradation byproducts of the paper
C2H2/C2H4 CH4/H2 C2H4/C2H6
0 0 0 no fault normal ageing
0 1 0 partial discharges of discharges in gas filled cavities low energy density resulting from incomplete
impregnation, or supersaturation or high humidity
1 1 0 partial discharges of as above, but leading to tracking high energy density or perforation of solid insulation 1→ 2 0 1→ 2 discharges of continuous sparking in oil
low energy between bad connections of different potential or to floating potential; breakdown of oil between solid materials
1 0 2 discharges of discharges with power follow
high energy through; arcing – breakdown of oil between windings or coils or between coils to earth;
selector breaking current
0 0 1 hot spots general insulated
T <150◦C conductor overheating
0 2 0 hot spots
150◦C < T < 300◦C
local overheating of the core due to concentrations of flux;
increasing hot spot temperatures;
varying from small hot spots in core, shorting links in core, overheating of copper due to eddy currents, bad contacts/joints (pyrolitic carbon formation) upto core and tank circulating currents
0 2 1 hot spots
300◦C < T < 700◦C
0 2 2 hot spots
T >700◦C
Figure 4.13 IEC 599 1978 fault gas ratios1
Case Characteristic fault C2H2 C2H4
CH4
H2
C2H4 C2H6 PD partial discharges NS1 <0.1 <0.2
(see notes 3 and 4)
D1 discharges of low energy >1 0.1–0.5 >1 D2 discharges of high energy 0.6–2.5 0.1–1 >2 T1 thermal fault T < 300◦C NS1 >1 but NS1 <1
T2 thermal fault <0.1 >1 1–4
300◦C < T < 700◦C
T3 thermal fault T > 700◦C <0.22 >1 >4
note 1 – in some countries, the ratio C2H2/C2H6is used, rather than the ratio CH4/ H2, also in some countries, slightly different ratio limits are used
note 2 – the above ratios are significant and should be calculated only if at least one of the gases is at a concentration and a rate of gas increase above typical values
note 3 – CH4/H2< 0.2 for partial discharges in instrument transformers; CH4/H2<0.07 for partial discharges in bushings
note 4 – gas decomposition patterns similar to partial discharges have been reported as a result of the decomposition of thin oil film between overheated core laminates at temperatures of 140◦C and above
1NS= non-significant whatever the value
2an increasing value of the amount of C2H2/C2H6may indicate that the hot spot temperature is higher than 1000◦C
Figure 4.14 IEC 60599 1999 edition fault gas ratios interpretation1
in the oil may also imply the presence of partial discharges. Paper degrades to form several furans and these can be detected using various techniques of oil analysis.
Although absolute levels of these byproducts are important, as with DGA, as the ratios of the different byproducts may prove more important in the longer term. It has been argued that, from these ratios, the temperature resulting in the paper degrading can be inferred and, in turn from this, the integrity of the paper [71–74]. Given that the temperature of the fault can be inferred, as with DGA, it should be possible to infer the presence (and, indeed, type) of partial discharges. However, this form of monitoring is not nearly so well established as DGA; the relationship to the detection of partial discharges has yet to be made. However, it is a potential technique for the future and most people would now analyse an oil sample for both gas and furan content.
4.3.5 Comparison among different PD measurement techniques relative to type of plant under investigation
From the foregoing it can be seen that, in general terms, most types of measurement of PD can be made on most types of plant – with some obvious exceptions as mentioned
T1
T2
D2
T3 D1
D + T
% CH4 % C2H4
% C2H2 80
80 60 40 20 60
40
20 80
60 40 PD
20
Key: partial discharges discharges of low energy discharges of high energy thermal fault, T < 300°C thermal fault, 300 °C < T < 700°C thermal fault, T > 700°C PD
D1 D2 T1 T2 T3
Limits of zones PD 98% CH4
D1 23% C2H4 13% C2H2
D2 23% C2H4 13% C2H2 38% C2H4 29% C2H2 T1 4% C2H2 10% C2H4
T2 4% C2H2 10% C2H4 50% C2H4 T3 15% C2H2 50% C2H4
triangle coordinates:
%C2H4= 100x
x+ y + z %C2H4= 100y
x+ y + z %CH4= 100z x+ y + z where x= C2H2 y= C2H4 z= CH4in p.p.m.
Figure 4.15 IEC 60599 1999 edition Duval’s triangle1
PD measurement technique
Type of IEPD tan δ C.T. capacitive antenna acoustic chemical thermography,
plant probes etc.
Generators ∗
Circuit
breaker boxes
Transformers ∗ ∗∗
Motors
Cable end boxes
Bushings
Capacitors ∗∗
CVTs ∗∗
Overhead
busbars, insulators, etc.
∗provided the plant item is air-cooled (i.e. has access vents)
∗∗provided the plant item is oil filled
Figure 4.16 A comparison of different PD measurement techniques in relation to various items of plant
in the text. The decision on which to choose in a given situation becomes a balanc-ing act among the costs of plant failure (includbalanc-ing health and safety considerations, replacement/repair costs and outage losses), the cost in making a given measurement (including the frequency of measurement dictated over given time periods and the cost of potential outages dictated by the need to make solid connections with some measurement systems) and the quality of information provided by the technique. This type of decision is unique in each situation. However, for those embarking on the measurement of PD for their plant, the table shown in Figure 4.16 may prove helpful.
4.3.6 Other items of plant
In addition to the various types of plant highlighted in the foregoing sections, there are two other plant systems which, because of their specialist nature, have had evolved for them special techniques to measure PD. This is not to say that they cannot utilise the techniques described earlier, merely that better techniques are available specifically related to them. The two items of plant are cables and gas-insulated substations (GIS).
4.3.6.1 Cables
Although other techniques can be applied to cables with limited success in PD mea-surements, the technique described as cable mapping [75] has evolved specifically for use with cables where not only is the magnitude and nature of PD required but also their location. Without location, refurbishment of a plant item hidden underground and extending potentially over many kilometres would be extremely daunting.