BUSHING MONITORING

It is very important in bushing monitoring to be aware of all available data. What is this data and how can it help focus your investigation on an individual bushing?

Power factor and capacitance have been used to detect and prevent failure in many bushings, of many different types, using only the individual bushing leakage currents for diagnostic purposes.

The addition of a voltage reference provides far greater capability and will be discussed in a separate article. For now, we have three bushings, with three leakage currents and an interest in diagnostics.

In Figure 2, we show the three phasors associated with the three leakage currents from a set of bushings which, in an ideal balanced system would all be 120°. We have added a very large change in the Phase 3 bushing – a change 100 times greater than expected from bushing deterioration – so we can easily see the effect on the relative phase angles:

• Phase 3-to-1 increases

• Phase 3-to-2 decreases by the same amount that 3-to-1 increased

• Phase 1-to-2 stays the same

Consequently, if we were to plot the data over time, Phase 1-to-2 should stay constant if just bushing 3 is deteriorating. Also note that the sum of the three phase angles is always 360°.

In Figure 3 we can see example results where both traces involving the phase 2 bushing show a sudden change in angle. Note that the remaining trace is relatively constant, indicating that it is just one bushing deteriorating.

Figure 2. Exaggerated phase angle change for Phase 3 bushing showing Phase 1-to-2 staying unchanged

Figure 3. Step change in phase angle for Phase 2 bushing

As expected in Figure 3, the sum of phase angles at any point is 360°, and the traces involving the deteriorating bushings show reflections of the variation, as expected: the increase in one is the same as the decrease in the other.

At the same time as recording the phase angles, we can also look at leakage current magnitude for the three bushings, shown in Figure 4. The natural variation in current magnitude, following a daily cycle shows no particular disturbance at the date/time of the anomaly in Figure 3.

In Figure 5 we can see the relative phase angles for three low voltage bushings recorded over several months. The changes recorded in Figure 3 were small, as expected, but in Figure 4 we are seeing changes of several degrees.

In Figure 5, none of the traces show a constant value, indicating that more than one bushing is deteriorating. The two traces with large changes in phase angle both include the X1 bushing, which is thus the suspect bushing. In addition, the fact that for the X1 bushing the phase angle heads in one direction then reverses implies that the power factor has increased then decreased again.

Figure 5. Relative phase for 3 bushings

Figure 4. Leakage current magnitude for the three bushings

Figure 6. Leakage current magnitude for three bushings

This is an uncommon effect where a bushing shows the effect of internal tracking within the bushing due to contamination or moisture ingress and can eventually lead to a negative power factor being recorded. The effect of tracking can also be seen in the leakage current magnitude over the same time period, as shown in Figure 6.

A rising current is typical of deteriorating bushing, but here the current rises, and then subsequently falls – again, an uncommon event, and one which relates to internal tracking within the bushing.

It is very important in bushing monitoring to be aware of all available data – the raw phase angles and rms leakage currents for each individual bushing are often as useful as the derived power factors and capacitances in terms of deterioration detection, investigation and diagnosis. A voltage reference is recommended when possible, as this helps to remove the effects of system variation and can focus investigation on to an individual bushing.