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Practical Experience with State-of-the-Art Surge Arrester Monitoring Devices

2017 INMR World Congress, Barcelona-‐Sitges, November 6-‐8, 2017

Philipp Raschke, Research and Development, Tridelta Meidensha GmbH.

Abstract In regards to tests on the new surge arrester monitoring device “smartCOUNT”, Tridelta investigated different effects on surge arrester leakage current, which may lead to misunderstanding the measured data. Temperature and grid related influences like harmonics in the system voltage play a minor role since there are existing methods for rough compensation of them. Weather and pollution still influence the arrester current by adding surface currents and hence errors to the measured values. A humidity ingress or successive degradation of MOV blocks are the most common reasons for surge arrester failure and need to be detected early. Only proper understanding of measured leakage current values, preferably available as periodically logged long term data, guarantees early failure recognition and the making of a correct decision for replacing a surge arrester. Thus, the most frequent cases of leakage current behavior are explained and set in relation to the according practical scenario.

1. Introduction

In the past 30 Years, numerous devices with different technologies for surge arrester monitoring were introduced to the market. Today, leakage current monitoring is a prevalent method for assessing the condition of surge arresters and most importantly for estimating their remaining lifetime. Nevertheless there are effects on the arrester leakage current, which lead to measurement errors, false interpretations and finally to unnecessary replacement or even to unexpected breakdown of an arrester. Utilizing leakage current for surge arrester monitoring often causes confusion since diverse leakage current behavior phenomena must be understood. This paper is based on the first field and test-‐lab experiences with smartCOUNT Arrester Monitoring System which was initially introduced at INMR 2015. The paper shall give an overview of effects on the surge arrester leakage current and may be used as a guideline for interpretation of measurement results and accurate decision making in surge arrester maintenance.

2. Peak current, capacit ive current and resist ive current – the agony of choice

It´s well known, that ZnO Surge Arresters have a complex impedance, which consists of a resistive and a capacitive component due to the molecular structure of Zinc Oxide. Under AC-‐Voltage, this results in two superimposed currents: One sinusoidal capacitive current phase shifted -‐90° to the voltage signal and one resistive current which is in phase with the voltage and is not sinusoidal but rather shaped as a periodic pulse signal (Figure 1).

Figure 1: Capacitive and resistive current in a ZnO surge arrester

Capacitive and resistive current are superimposed to a total leakage current, in which two important values, peak current and 3rd harmonic current (for example at 150Hz), can be determined (Figure 2).

Figure 2: ZnO leakage current (left) and current spectrum (right)

The peak value of the surge arrester current is always orientated on the predominating component (capacitive or resistive) of the current. At low voltage levels (approx. <Uc), the

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peak current orients itself on the peak value of the capacitive component. At higher voltage stresses, primarily in a range above Ur (rated voltage), the peak current orients itself on the peak value of the resistive component. Between those two ranges, the peak current is influenced by harmonic distortion due to the growing resistive component and shows a behavior with low sensitivity to changes in the voltage, or more appropriate to changes in the V-‐I-‐characteristic of the surge arrester (Figure 3). This qualifies the peak current as a good indicator for pure capacitive or resistive currents, but not for mixed current components.

Figure 3: Peak current characteristic of a surge arrester

The capacitive current represents the current flowing through the series capacity of the surge arrester. It behaves proportional to changes in the voltage and consequently doesn´t show a significant sensitivity in the non-‐linear area of the V-‐I-‐Characteristic of the surge arrester.

The resistive current is a good representative value for the surge arrester condition due to its high sensitivity and logarithmic growth over the whole leakage current area of the V-‐I-‐Curve. Metrologically, the resistive current is often based on the third harmonic content of the leakage current, which is extracted from the leakage current spectrum by using a Fourier Transformation algorithm (Figure 4).

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Figure 4: 3rd harmonic current and resistive current

3. Surge arrester monitoring – test lab and f ield experiences

3.1. smartCOUNT f ield test

During the development of the new Tridelta surge arrester monitoring system “smartCOUNT”, an extensive test program was conducted. As integral part of the test program, a field test was carried out to test the behavior of the monitoring system under real conditions. The first stage of the field test was conducted together with TEN (Thüringer Energie Netze GmbH & Co. KG). Three porcelain housed surge arresters, situated at the 110kV substation in Hermsdorf, were equipped with the new monitoring system in April 2017 (Figure 5). Since that time diverse effects on the behavior of surge arrester leakage current in the field were observed and the proper function of the monitoring devices could be proved. The results presented in this paper are based on experiences with the smartCOUNT Monitoring system from this field test, but also from the Tridelta HV test lab.

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Figure 5: smartCOUNT devices in TEN substation Hermsdorf

3.2. Temperature influence and compensation

Temperature is an important factor in leakage current measurements, because ZnO Varistors are semiconductors and their resistance is highly temperature dependent. Consequently the ambient temperature of a surge arrester has an influence on the resistive leakage current of the arrester (Figure 6). As the operators of substations are not interested in external effects on the leakage current, but in effects that come from the surge arrester itself, ambient temperature influences on the resistive leakage current should be compensated.

Figure 6: Temperature dependency of resistive and capacitive leakage current

Figure 6 shows the difference between the resistive and capacitive current characteristics of a ZnO Varistor at 20°C and 40°C. From 20°C to 40°C the resistive current rises significantly by

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factor 2,0 at ratio U/Uc of 0,40 and by only 1,4 at U/Uc of 1,20. The capacitive current only varies slightly. In fact the temperature influence on the resistive current is non-‐linear, depending on the voltage ratio. Of course this is an example taken from one specific varistor type, other varistor brands and diameters will give different values.

Usually temperature impacts on the resistive leakage current from the ambient temperature is compensated by measuring the ambient temperature and by multiplying the measured raw value with a correction factor based on a temperature compensation model. Of course temperature models also differ between varistor types but can be approximated for all varistor types or roughly linearized for one specific ratio U/Uc in order to simplify the compensation procedure.

There is another influence on the temperature compensation algorithm that arises from the temperature measurement itself. Usually the temperature sensor is built inside the monitoring device and the measured temperature differs from the ambient temperature due to the temperature constant of the monitoring device and how long it takes until the ambient temperature has been conducted to the temperature sensor. The temperature constant of the monitoring device and surge arrester should be roughly the same in order to replicate the same temperature impact on both. Furthermore the influence on the temperature sensor and the varistor will be mismatched due to differences in the capability of the arrester housing and shell of the monitoring device to reflect sunlight. Varying sunlight radiation angles and partial shading will influence temperature calculations.

These factors lead to an effect that has been observed on surge arresters in the field, called over-‐compensation, where a compensation value is subtracted, that is larger than the nominal temperature deviation. Finally with rising ambient temperature, the resistive leakage current neither rises, nor stays stable, it sinks. This leads to a ripple in the resistive leakage current, due to night and daytime temperature fluctuations, outlined in Figure 7.

Figure 7: Ripple effect due to temperature over-‐compensation

This problem can be handled by optimizing the temperature compensation model or most effective by measuring at night to mainly eliminate from ambient temperature and sunlight.

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A precise adjustment of the temperature models in the smartCOUNT system based on varistor measurements as well as switching over to measurements at night brought very stable results for the resistive leakage current (Figure 8).

Figure 8: Steady results due to a precise temperature model and nightly measuring

3.3. Grid related side effects and compensation

An essential point that has to be taken into account are influences on the leakage current due to grid related side effects. Due to the strong non-‐linear behavior of the ZnO Material, small changes in the voltage result in large changes in the peak and resistive leakage current, like pointed out in Figure 3 and 4. As an example, if the maximum permissible voltage deviation is ±10% then the resistive current values can vary in a range from less than -‐40% to more than +80% of the nominal current. Generally the error due to positive voltage deviation will be higher, because the non-‐linearity of the current rises with the voltage ratio U/Uc (Figure 9).

Figure 9: Error Range of resistive current due to voltage deviation

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The automatic compensation of voltage deviation is hard to realize because this would require a measurement of the system voltage. To solve this Problem, the smartCOUNT monitoring system has an integrated function, where the actual system voltage is taken into account to compensate voltage fluctuations. This function is called “single-‐shot”, where one particular measurement is carried out and the information about the system voltage is entered by hand. A voltage reading, for example in the substation control-‐room, should be taken immediately before performing a single-‐shot. The compensation of voltage deviation induced measurement errors is achieved by using a compensation curve which exactly fits to the used ZnO Varistor inside the specific surge arrester. Measuring values that have been taken without voltage deviation compensation (like automatically logged leakage currents) may show permanently fluctuating current values (Figure 10).

Figure 10: fluctuating leakage current due to voltage deviation on test lab transformer

Another uncertainty is raised by harmonics in the system voltage (Figure 11). Those harmonics directly influence the resistive leakage current, because it is based on the measurement of 3rd harmonic current in the surge arrester. State-‐of-‐the-‐art monitoring devices contain a field probe, which measures the percentage of the 3rd harmonic in the electrical field. So a correction factor can be calculated for roughly compensating the influence of harmonic voltage to the leakage current. Frequencies above the 3rd harmonic are filtered by the Fourier algorithm.

Figure 11: electrical field plot from smartCOUNT with 10% 3rd harmonic content

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Outages and earth faults will also raise questions, when they are recognized by the monitoring systems. Outages due to failure or maintenance usually manifest leakage current values of 0µA (Figure 12).

Figure 12: Outage related 0µA values

If an earth fault occurs on solidly grounded transmission lines, the affected system is switched off immediately and produces a behavior in the leakage current as graphed in Figure 11. On isolated neutral networks, the affected line will drop to earth potential and both other phases will rise by factor 1.73 (phase-‐to-‐phase voltage). The leakage current on all three phases will behave similarly to the according voltages (Figure 13).

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Figure 13: Leakage current peaks due to earth failure on L1 in a compensated grid

3.4. Fog and rain effects

As high voltage surge arresters are usually installed outside, they are affected by rain, fog and humidity. In the long term field test in Hermsdorf it was investigated, that there are differences in how the moistening of an insulator takes place. Effects from light rain on the leakage current were never recognized. High peak currents on the 2nd and 19th of May correlated directly with very heavy rain and storm, which was figured out by means of meteorological data. Unstable peak values were determined, in the period between the 2nd and 19th of May, to be related to continuous, steady rain, heavy fog and high humidity (Figure 14 and 15).

Figure 14: Leakage current increased by rain, humidity and fog

These high peak current values always fell back to their nominal value under dry conditions. The resistive current was influenced only marginally.

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Figure 15: rain (left) and humidity (right) of May 2017 (data excerpt from weather station)

3.5. Polluted arresters

In heavily polluted areas, like deserts, industrial parks and coastal areas, exposed surge arresters often show deposits of conductive sediments on their housing after a certain time period. A weather test was performed, simulating growing pollution on the arrester housing in order to gain experience on the impact of surface current on the measured leakage current values under these conditions. One porcelain and one silicone housed arrester, both built with the identical electrical design and equal creepage distances were used to research the differences between the two housing technologies (Figure 16 and 17).

SB30 (porcelain housed) SBKC30 (silicone housed) Rated Voltage [kV] 30 30 Test Voltage [kV] 24 24 Creepage distance [mm] 1187 1210 Spec. creepage distance [mm/kV] 51,6 52,6

Figure 16: weather test properties

Figure 17: porcelain and silicone housed surge arrester in weather aging chamber

The test comprises a procedure, similar to ambient conditions in coastal desert climate with salt fog as well as humidity in the morning followed by hot and dry ambient conditions (Figure 18).

Figure 18: weather aging chamber test procedure

The first stage of the test was carried out under voltage with dry air conditions and without salt fog and heat. This was required to analyze the arrester leakage current without influence of surface current to create a benchmark condition for comparison to the results with surface pollution. Under dry conditions the peak current is 1000µA and the resistive current is 130µA for the porcelain as well as the silicone housed surge arrester with this particular electrical setup.

After switching salt fog spray and heating on in the weather chamber, the peak current starts rising. After switching off the salt fog, the peak current falls back to a level that is lower than the maximum value but still higher than the benchmark current. This behavior repeats with every daily cycle with gradual rise in the daily maximum value as well as the subsequent drop-‐off value to which the peak current falls back, when salt fog is switched off and the arrester housing is drying. This behavior is due to the continuous growth of a pollution layer with high salinity and humidity during salt fog exposition. (Figure 19)

Figure 19: high leakage current due to accumulating pollution on a porcelain housed arrester

Especially wet salt layers have a high conductivity, thus the prevailing capacitive current is superimposed by very high sinusoidal currents that are in phase with the voltage. The drop-‐off value rises and the dried off surface pollution layer remains partially conductive. The conductivity of the pollution layer rises with the layer thickness and thereby with the cross

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section of the conductive layer. The resistive current is marginally affected by the pollution of the housing because the total current is approximately sinusoidal and primary contains a predominant 1st harmonic due to surface conductivity, a small 3rd harmonic from the ZnO current and high frequent distortion (9th, 11th and 13th harmonic order) caused by sparking (Figure 20).

Figure 20: replacement circuit (left), superimposed current oscillogram and spectrum (right)

In contrast to these clear effects of the pollution grade to the arrester current of a porcelain housed arrester, the current of the silicone housed arrester doesn´t show any changes in peak current or in the resistive current (Figure 21).

Figure 21: stable leakage current under harsh environment on silicone housed arrester

This significant difference is due to the hydrophobic properties of silicone. Porcelain always shows behavior of Hydrophobicity Class 6 (Wetted areas cover over 90%) and thus carries continuous surface currents. A good quality silicone ranges between Hydrophobicity Class 1 (discrete droplets) and Class 3 (flat discrete droplets). In Class 3, there is still no interconnection between the droplets and no salt layer sticking to the surface. In consequence there is no formation of current across the surge arrester surface (Figure 22).

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Figure 22: polluted surfaces on porcelain housing (left) and silicone housing (right)

Cleaning the polluted surge arresters eliminates surface currents and brings the leakage current back to its normal value (Figure 23).

Figure 23: leakage current with polluted porcelain housing and after cleaning

All so far mentioned effects that have an impact on the arrester leakage current simply falsify measurement results and make it difficult to estimate the real condition of a surge arrester.

3.6. Detecting humidity ingress

Humidity ingress is the main reason for arrester breakdown in field and it´s a fundamental benefit, when a surge arrester monitoring system is capable of detecting it. To investigate the impacts of moisture inside the surge arrester on the arrester leakage current, a laboratory test was carried out. Four centiliters of water were intentionally poured inside a surge arrester. A smartCOUNT surge arrester monitor was mounted on the surge arrester. Then voltage was switched to the arrester and the leakage current was recorded for several

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hours. In the next step, the surge arrester was installed inside an oven and kept under voltage and ambient temperature of 40°C to accelerate the distribution of humidity in the surge arrester core (Figure 24).

Figure 24: arrester in a humidity ingress test (left), moistened varistor shrink stack (right)

During the test at room temperature, the peak current remained at its nominal value of circa 1000µA and the resistive current kept its value of circa 140µA. By heating up the arrester to 40°C, the evaporation of the water inside was accelerated. After 5 hours at 40°C, the arrester was completely up to temperature and the water was distributed all along the varistor stack. The peak current rose to more than 14mA and started fluctuating due to the repeated drying remoistening of the varistor stack (Figure 25). A lesser impact on the resistive leakage was observed due to the low 3rd harmonic content of the surface current, as illustrated in in Figure 26.

Figure 25: peak current with increased moisture inside the arrester

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Figure 26: resistive current with increased moisture inside the arrester

To summarize, humidity ingress can be easily determined because it results in extremely high peak currents but low changes in the resistive current inside the arrester.

3.7. Discovering degraded MOVs

The second most commonly occurring malfunction of surge arresters is varistor damaged or degraded. Defective MOV blocks generally show lower reference voltages, higher reactive-‐power losses and thus higher resistive and capacitive currents. A malfunction of one MOV block inside a high voltage surge arrester affects only small changes in the resistive and capacitive leakage current but increases the voltage stress to all other MOV blocks. Then an accelerated aging of the other blocks takes place and can lead to a growing leakage current. To simulate this behavior and to test how the leakage current behaves in these cases, a test was performed on a surge arrester with defective MOV blocks (previously overloaded in line discharge tests). To realize this the healthy MOV blocks of the surge arrester were exchanged for defective blocks in 4 stages and the leakage current was recorded with a smartCOUNT arrester monitor (Figure 27).

Stage 1 Stage 2 Stage 3 Stage 4 Ur total [kV] 41,70 39,05 33,83 28,49 Uc [kV] 33,36 31,24 27,06 22,79 Utest [kV] 25,50 25,50 25,50 25,50 U/Uc 0,76 0,82 0,94 1,12 Stage 1 Stage 2 Stage 3 Stage 4 Ur MOV 6 [kV] 6,95 4,30 4,30 4,30 Ur MOV 5 [kV] 6,95 6,95 1,73 1,73 Ur MOV 4 [kV] 6,95 6,95 6,95 1,61 Ur MOV 3 [kV] 6,95 6,95 6,95 6,95 Ur MOV 2 [kV] 6,95 6,95 6,95 6,95 Ur MOV 1 [kV] 6,95 6,95 6,95 6,95

Figure 27: test procedure with degraded MOV blocks

Stage 1 represents a healthy arrester with 6 MOV blocks. The peak current remained stable at 1000µA and the resistive current at 30µA. In stage 2 one MOV block was exchanged for a

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slightly damaged block to simulate minor MOV damage. The peak current is still representative of the capacitive current and is proportional to the voltage stress increase. The resistive current changes only slightly. Stage 3 consists of an exchange of another healthy MOV block for a heavily damaged block. This leads to a significant drop of the reference voltage and consequently to a bigger rise in the peak current. In correlation to this the power dissipation rises and the resistive current grows to over 500% of its nominal value because the voltage ratio at this stage corresponds to the operation of a surge arrester near to Uc, where the current-‐voltage characteristic becomes highly non-‐linear. The current values still stood roughly stable at these values. Stage 4 comprises the exchange of another varistor to produce a situation in which the voltage stress of each single varistor is higher than permitted by its continuous operating voltage. The power dissipation in the arrester produces a large amount of heat, which the arrester housing isn´t capable of conducting to the ambient air. Resistive and peak current values continue rising steadily; the arrester is already in thermal runaway mode. The resistive leakage current is now almost 900% of its nominal value, peak current 500% (Figure 28 and 29).

Figure 29: relative change in leakage current in a MOV degrading test

Figure 28: current in a MOV degrading test

This scenario shows the behavior of a slowly degrading ZnO stack, leading finally to thermal runaway, if the damage isn´t recognized in time. The resistve leakage current is very sensitive to damages to the ZnO Varistor and clearly represents power dissipation and thus the arrester health. ZnO degradation is the only condition, that significantly and permanently increases the resistive current.

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4. Recommendations for correct surge arrester analysis

Finally the gained experiences are presented in the following summary that may be used as a guideline for leakage current measurement data interpretation. The given case assumptions will probably look different in the field. The following recommendations and cases may help in decision making for surge arrester maintenance.

è Very important for the correct estimation of the arrester health is the observation of the leakage current trend instead of comparing absolute values with a suspected maximum current threshold.

è Cases might be mistakenly chosen wrong if different symptoms occur at the same time.

è If reoccurring, transient or cyclic leakage current effects (like arrester pollution) should be monitored, choose a low data logging interval (<1 day)

è When measuring with a logging frequency equal to or greater than 1 day, it´s recommended to measure at night to prevent influences of sunlight

è If the situation is not clear, contact the arrester manufacturer and/or inspect the arrester.

Cases Pictograms

Case 1 -‐ Sudden spike in the peak current, later fall back to drop-‐off value, no influence on the resistive current

Arrester is ok! The surge arrester could be affected by rain. Check the past weather review of the area, where the arrester is situated. If no rain was recorded, the arrester may be affected by humidity, fog, dew and/or pollution. Check the arrester housing for surface pollution, if the effect reoccurs.

peak current resiscve current

Case 2 – Periodical spikes and fall backs to drop-‐off value of peak current, continuous rise drop-‐off value, small influence on the resistive current

The surge arrester housing is probably polluted with salt, dirt, chemicals etc. Check the housing for surface pollution and if necessary clean the arrester to prevent tracking erosion or flash over. The leakage current sinks to nominal value after cleaning. Arrester is ok!

Case 3 – Resistive and peak current fall to 0.

The line was switched off. The leakage current has to rise to its nominal value, when voltage is switched to the line. Arrester is ok!

Case 4 – Resistive and peak current fall to a very low value

The voltage across the arrester has dropped. The line is probably affected by earth fault. The current has to rise to its nominal value, when earth fault is fixed. (concerns isolated grid) Arrester is ok!

Case 5 – Resistive and peak current rise for a certain time and fall back to their nominal value.

The voltage across the arrester has risen. Another line of the system is probably affected by earth fault. The current has to fall to its nominal value, when earth fault is fixed. (concerns isolated grid) Arrester is ok!





Case 6 – Daily rise and fall back to drop-‐off value of resistive current, peak current is not affected.

The measurement values are being overcompensated by the temperature compensation function. The arrester or the measurement device is probably partially shaded or the wrong arrester type was assigned in the measurement device setup (wrong temperature model chosen). Arrester is ok!

Case 7 – Very high fluctuating peak currents, low changes in resistive current, no pollution on arrester surface detected

Arrester not ok! The arrester might be compromised by humidity ingress. Check the arrester for surface pollution and clean it, if necessary. If the current doesn´t stop fluctuating, replace the arrester immediately to prevent dangerous arrester breakdown.

Case 8 – Successive rising values of resistive leakage current and capacitive leakage current, no pollution or humidity detected.

Arrester not ok! The arrester may contain degrading MOV blocks. Check the arrester more frequently. Consult the arrester manufacturer. Replace the arrester preemptively before the resistive current reaches the given maximum threshold.

References

[1] IEC 60099-‐5, Surge arresters -‐ Part 5: Selection and application recommendations, 5/2013.




[2] V. Hinrichsen, „Monitoring of High Voltage Metal Oxide Surge Arresters,“ Bilbao, 1997.

[3] STRI, „Guide1, 92/1 Hydrophobicity Classification Guide“.

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