Elimination of the impact of surface contamination on the results of insulation resistance measurements

SONEL S.A. based in Swidnica (Poland) offers a wide range of instruments for measuring insulation resistance. Sonel MIC-10k1 and Sonel MIC-10s1, with testing voltage up to 10kV and resistance measuring range up to 40 TΩ, are ultra-modern models offered to the most demanding customers. All models produced by Sonel have 3 measuring sockets, marked as R_ISO+, G – Guard and R_ISO-.

When measuring insulation resistance, using only R_ISO+ (positive) and R_ISO- (negative) terminal, the measurement system of the meter receives both the current that flows through the entire cross-section of the tested insulation material (crossover current) and the current that flows on the surface of this material (surface current). It should be noted that the total current consists of the capacity charging current, absorption current and leakage current, which is discussed in this paper. The current flowing inside the insulation depends primarily on the material of the dielectric. The current, which flows through the upper side of the insulating material depends on the level of insulation contamination, humidity and degradation. The following pictures depict this problem. The presented porcelain insulator shown has a layer of brine applied in order to simulate contamination and moisture. After switching on the measuring (test) voltage of 10 kV and setting the "boost" function at Sonel MIC-10s1, we may observe partial discharges (creeping discharges) on the surface of the insulator. Blue-violet and yellow flares accompanied by specific crackling indicate that current flows on the surface of the insulator.

Fig. 1. Current flowing in/on the insulation

When identifying their location, from a considerable distance, e.g. on insulators of LV overhead lines, operators may use Sonel UV-260 (highly sensitive camera for corona discharge), which is capable of recording partial discharge even when there are no light and sound emissions, when the potential gradient does not exceed a value that triggers puncture and arcing or creepage discharges.

Fig. 2. Photo taken with a digital camera

Fig. 3. Photo taken with Sonel UV-260 camera

When using the classic 2-lead measuring method, the total value of leakage current measured by the meter, was I_L = 21.9 μA and the resistance R_ISO = 467 MΩ. After wrapping the insulator with a copper wire in the middle of its length and attaching the third test lead (guard), the surface current was substantially eliminated from the measurement, flowing out of the measuring system of the ammeter in the measuring device.
The measured resistance was R_ISO = 907 GΩ, total leakage of the surface current was limited to the value of I_L= 9.08 nA.

Fig. 4. Two-lead measurement on contaminated insulator

Fig. 5. Three-lead measurement on contaminated insulator

The value of the current that flowed on the surface of the insulator is easily calculated, using the following formula:
I _surf. = I _L(total) - I _int. = 21.900 μA - 0.009 μA = 21.891 μA
This simple experiment shows how the surface current, which depends on the presence of moisture and contamination on the insulation, may falsify the results of the measured resistance, causing unjustified and often costly decisions on the replacement of the insulation components.
After cleaning insulator, the measurements were repeated using 2 and 3 test leads.

Fig. 6. Two-lead measurement after cleaning the insulator

Fig. 7. Three-lead measurement after cleaning the insulator

After cleaning the insulator, the resistance value in 2-lead measurement was R_ISO= 290 GΩ and the leakage current was I_L= 36.5 nA. After connecting the guard lead: R_ISO > 40 TΩ and the leakage current of I_L< 0.01 nA, which proved that the internal resistance of the insulator is larger than the originally measured.
During the first test, the surface current was not entirely eliminated. This may also mean that despite cleaning the surface of the insulator, the surface still conducts current, which may indicate its slight deterioration or its specific properties.
The decision on the efficiency of tested insulator should be based on two measurements. The measurement with guard lead helps to assess the internal condition of the dielectric. In contrast, the 2-lead measurement provides information on the entire condition of the insulation (including the surface). The decision on the replacement should be based on both measurements. Here are some practical tips that may improve the reliability of result in your insulation resistance measurements:

keep negative polarity on the working core in order to reduce the impact of external interference and to take into account the phenomenon of electroendosmosis,
connect ST-1 probe and note the temperature value, as it will be needed to determine the coefficient of temperature compensation for obtained results, i.e. up to 20°C for insulators, cables, transformers and up to 40°C for a rotating machines,
use the guard lead, to exclude the surface leakage current, which depends on surface contamination and moisture, but do not forget about 2-lead measurement, which indicates the overall condition of the insulation,
in environments with high electromagnetic interferences, use filtration offered by the meter,
set the test leads at some distance from each other and without contacting any other objects or the ground to reduce the possibility of leakage currents in leads,
when purchasing an insulation tester, make sure that its test voltage always has at least the value that has been set and that it is lower during measurement. It is also recommended to check whether the test current during charging is constant and consistent with the one set in the tester,
after completing the test, make sure that the circuit is discharged.

The meter will turn off the yellow LED, which indicates the presence of voltage, and it will display a message on discharging the object.
Regular inspection of equipment insulation is a protection measure against electric shock. Remember that human health and life depend on results of such inspections.

Authors:
Grzegorz Chrzanowski
Wojciech Siergiej
SONEL S.A.