The extreme weather we have been experiencing over the past half-decade seems becoming the new norm. Now without arguing about global warming and who is to blame, the fact of the matter is that most switchgear designers and experts agree that the environment has changed and is rapidly changing. Medium-voltage switchgear installed at industrial or commercial facilities could be exposed to these extreme environmental conditions. Switchgear components are made of varieties of materials and protective coatings according to operating area and service conditions. These switchgear components are subjected to deteriorating corrosion and ageing processes while effects of the high voltage to electrical insulation is mastered by appropriate classification of the switchgear according to the IEC/TS 62271-304 specification. The rate of which strongly depends on the presence of various gases, vapours, and particles and combinations thereof in the surrounding air. This study provides details on how to deal with unusual and extreme environmental conditions by carrying out some special tests that makes them reliable, fail-safe and trouble free operation throughout expected service life in addition to minimum maintenance requirement with less downtime.
1 Introduction
The two important elements of medium-voltage (MV) switchgear, which can affect performance and reliability of equipment are atmospheric corrosion resistance and resistance to ageing both are highly affected by extreme weather conditions. We have examined principal effect of environmental parameters mentioned in IEC 60068 standards to define test protocol to provide reliable solutions to users. To deal with extreme weather conditions and to provide uninterrupted and robust performance special tests protocols are followed for switchgear components. MV switchgear is typically made up of coated and uncoated steel, non-ferrous materials like copper and aluminum and insulating materials like plastics and epoxies. For metals accelerated and cyclic corrosion tests are followed which are simulations of actual conditions of extreme weather and for insulating materials are extended ageing tests are performed by which reliability and performance can be established. Conversely, the effect of extreme environmental conditions is more evident and deteriorating on metals than plastics so more focus is given to metals and treatment involved in this paper.
MV switchgear is an important asset of distribution systems. This switchgear contains variety of materials, e.g. metals, alloys and plastics. These materials undergo many changes during its life cycle. When designed it is for normal service conditions. However, due to the changes in weather conditions designers must consider various aspects of extreme weather.
2 Extreme weather – what is it?
Changes in weather and climate events have significant impacts and are among the most serious challenges to transmission and distribution industries in coping with a changing climate. This is mainly due to long lasting effect on assets and consequences of its deterioration. Extremes are a natural part of even a stable climate system and have associated costs in terms of equipment failure due to variety of reason. Among all, for switchgear we can say that higher corrosion rates and ageing of the equipment is contributing more, if normal service conditions according to the IEC 62271-1 standard are not met. Fig. 1 shows the probability of occurrence of normal and extreme temperature and precipitation. The higher the black line, the more often weather with those characteristics occurs.
Fig. 1 Probability distributions of daily temperature and precipitation [1]
(a) Temperature, (b) Precipitation
The environmental classifications according to Fig. 2 can be used to define climatic conditions from 3K1 to 3K10 for stationary use at weather protected location and 4K1–4K4 for stationary use at non-weather protected locations as shown in Fig. 3 [2]. For the scope of this paper, Figs. 2 and 3 show only temperature, humidity and precipitation.
Fig. 2 Environmental parameter classification for stationary use at weather protected locations [2]
Fig. 3 Environmental parameter classification for stationary use at non-weather protected locations [3]
MV switchgear undergoes to extreme weathering conditions can observe more failure than in the average weathering conditions. Now over a period of time these normal conditions are changing and we have new extremes as shown in Fig. 4. A relatively small shift in the mean produces a larger change in the number of extremes for both temperature and precipitation [1].
Fig. 4 Simplified representation of the changes in temperature and precipitation
(a) Temperature, (b) Precipitation
This means that the probability of failure of equipment is higher. This probability of failure can be depicted in to various levels of equipment designs. MV switchgear is composed components made of metals and electrical insulating material of various types depending on their functional requirement. In weathering conditions like above these materials tends to deteriorates and they failed to perform their intended functions. For metallic structural parts the major reasons for failure is corrosion and for insulating materials it is early ageing. The paper [4] provides comprehensive information on various environmental conditions encountered during installation and operation of switchgear. The actual environmental conditions to which a product is exposed are normally complex and composed of a number of environmental factors and corresponding parameters. The correlation between classified environmental conditions and environmental test is also complex. However, Fig. 5 can be useful for test selection according to objective [5]. Example to choose the test selection is shown by highlighted text, i.e. X.
Fig. 5 Test selection according to objective and application
According to IEC 60068-1 principal effects of single environmental parameters are shown in Fig. 6 [6].
Fig. 6 Principal effects of environmental parameters
Two basic but important parameters were selected, i.e. temperature and humidity. Test was performed to find out its effect under salt mist conditions. To increase robustness during harsh and extreme environmental conditions two approaches were considered.
i.
cyclic corrosion test (CCT)-metallic materials,
ii.
full product test as per IEC/TS 62271-304.
2.1 Cyclic corrosion test (CCT)
Carbon steel is predominantly used to manufacture metallic structure for switchgear. Coated carbon steel is indeed a concern for switchgear manufacturer to maintain the quality of the product for intended lifespan of switchgear. Researchers around the world have experimented with a variety of CCTs that incorporate various combinations of wet/dry cycling, temperature cycling, solution concentration cycling, and in some cases ultraviolet test. However, for switchgear ISO 60068-2-52 standard provides effective validation method to deal with this situation. CCT was performed on various coated steel material typically used for MV switchgear. Severities are defined according to the product usage in extreme environmental conditions and can be described as per Fig. 7 [7].
Fig. 7 Severity according to IEC 60068-2-52
Two equipments have been used to perform CCT according to severity 4. Test specimens are placed in an enclosed chamber (a separate salt mist chamber and a controlled humidity chamber are generally used as shown in Fig. 8), and exposed to a changing climate that comprises of the: spray of neutral (pH 6.5–7.2) salt water solution, which falls-out on to the specimens at a rate of 1.0 to 2.0 ml/80 cm2/h, in a chamber temperature of +15 to +35°C.
Fig. 8 Equipment for cyclic corrosion test
❶ Salt spray test equipment and ❷ Humidity chamber
This is followed by test cycles according to severity 4. Severity 4 is two test cycles according to severity 3. As per severity 3, four test periods, each of 2 h, with a humidity storage period between 20 and 22 h followed. After that, samples were stored in standard atmospheric conditions (23 ± 2° and 45–55% RH) for 3 days [7].
This cyclic corrosion test is done because normal salt mist test does not provide real simulation of practical conditions. The coated steel is checked for corrosion resistance. For severity 1 the number of salt spray hours in 7 days are only 4 h where as for other severity level its 4 h in a day (Fig. 9).
Fig. 9 Accelerated corrosion test cycle
Three sets of samples prepared from carbon steel were tested viz, hot dipped galvanised, aluminium–zinc and zinc–aluminium–magnesium coated steel. It was observed after the test that, special coating with Zn–Al–Mg performed well even in severity level 4 and showing no signs of red rust or white rust, whereas other two sets of samples show signs of white and small patches of red rust.
This test can also be performed on full products but it is better to be done on component or raw material level. It should be noted here that if this test done on product or equipment it should not be energised. Fig. 10 shows the carbon steel plate before and after cyclic corrosion test. It is apparent from the image ❻ that special coating is required to sustain in harsh environmental conditions.
Fig. 10 Carbon steel plates for cyclic corrosion test
There are various tests and standards available according to the requirement to check resistance to environmental conditions. All these standards provide details on specific aspects and test protocol for testing. Fig. 11 provides information on these standards in brief that can be used for various environmental conditions [8].
Fig. 11 Various standards to check resistance to environmental conditions of de-energised switchgear
Results from such tests turn out to correlate reasonably well with in-service performance at normal and special outdoor conditions. Suitability of corrosion test methods for different fields of application for electrical devices from Fig. 11 is defined in Fig. 12 [8, 9].
Fig. 12 Environmental conditions and test standards (Ref: ISA-71.04-1985)
The life expectancy of the MV part and the electromechanical part are now 30 years or more [10]. It is difficult to test metallic components practically for this long period so accelerated corrosion is one of the ways to find the suitability of the material for switchgear. Testing time to reach a metallic mass loss of carbon steel of 670 [8] or 928 g/m2 [11], corresponding to ∼5 years of outdoor exposure in corrosivity category C3 according various standards are too long except the standard ISO 14993 which takes about 7 days of testing which provide extremely good correlation [12]. The method simulates the real conditions by enhancing the influence of environmental on metallic materials.
MV switchgear uses different metallic materials and operate in different pollution conditions. As per ISO 9223, Fig. 13 gives corrosion rate predicted after one year of exposure [13].
Fig. 13 Corrosion rate of metallic materials*
*Corrosion rates are not linear in ISO methodology
Although the ISO methodology represents a rational approach to corrosivity classification, it has several inherent limitations. The atmospheric parameters determining the corrosivity classification do not include the effects of potentially important corrosive pollutants or impurities such as NOx, sulphides, chlorine gas, acid rain and fumes, deicing salts etc. which could be present in the general atmosphere or be associated with microclimates.
2.2 Ageing test
Ageing test as per IEC/TS 62271-304 was performed on the circuit breaker. The equipment was installed in the climatic test room and subjected during three identical test periods of 7 days (Fig. 14), to 2 h damp heat cycles described by zones [14].
Step 1: To rise from 30 to 50°C in 40 min, with the relative humidity above 95%.
Step 2: To maintain 50°C during 20 min with the relative humidity above 95%
Step 3: To decrease from 50 to 30°C in 40 min, with any convenient value of humidity
Step 4: To maintain 30°C during 20 min, with the relative humidity above 80%.
Fig. 14 Ageing test cycle for circuit breaker test
For 5 days, the equipment tested was energised at its rated voltage and subjected to 60 damp heat cycles. After these 5 days the test was stopped. The equipment is de-energised. A visual inspection is done in order to detect the inception of tracking on insulating surfaces. For 2 days, the equipment was maintained in an ambient climate close to the reference atmosphere mentioned in IEC 60060-1.
According to IEC/TS 62271-304 the ageing test is done on circuit breaker to know the level of ageing on the components as well as its effect on performance. Fig. 15 shows the results after ageing test for circuit breaker.
Fig. 15 Circuit breaker – 36 kV, 1250 A and internal components after ageing test
All evaluation tests [14] were completed as per IEC/TS 62271-304 level 1 ageing test. It was observed that the product was meeting requirements as per standard. This test has been stopped at 1176 h, however for extreme environmental conditions special test protocol shall be applied [15].
3 Conclusion
Changing environmental conditions are inevitable and it has certain consequences on MV switchgear. The consequences are in terms of asset deterioration and performance. Accelerated corrosion and ageing remains to be the concern for both newly erected and old equipment. The selection of relevant and economical environmental testing procedure is extremely important. This paper has presented two different concepts to make MV switchgear ready for extreme environmental conditions – accelerated testing for corrosion of metals and alloys and ageing test for whole product. Although the test done for whole product is for severe conditions, but if it is combined with accelerated ageing conditions it provides some assurance for extreme conditions. The details provided in the paper shows that it is possible to do ageing test with shortest possible time if correct procedures are followed. However, severity of test must be considered carefully. In this paper, the scope kept limited to corrosion aspects only, mainly to focus on metallic materials reliability. As we are going towards more and more automated solution to get the details from the equipment these tests are useful to expect a later simulation of product life. This will contribute to the eco-design regulatory frameworks.
4 References
1.
Weather and Climate Extremes in a Changing Climate: ‘US climate change science program synthesis and assessment product 3.3’, June 2008
Cormenier T., Veuillet P., and Ferraro V.: ‘How to control the impact of the severe environments surrounding medium voltage switchgears’. CIRED 2017, 2017, No. 0322
Rossi S. and Ylasaari S.: ‘Correlation between environmental classification and environmental testing methods. Corrosion and Reliability of electronic materials and devices’, The Electrochemical Society, NJ, 1999, vol. 29–94, pp. 102–105
FD ISO/TR 16335: ‘Corrosion of metals and alloys – corrosion tests in artificial atmospheres – guidelines for selection of accelerated corrosion test for product qualification’, 2014
IEC/TS 62271-304: ‘High-voltage switchgear and control-gear – Part 304: design classes for indoor enclosed switchgear and control-gear for rated voltages above 1 kV up to and including 52 kV to be used in severe climatic conditions’, 2008, pp. 6–11
Milan S., Altinay C.K., and Biasse J.M.: ‘Installation conditions and improved MV air insulated switchgear are keys factors for an extended life’. CIRED 2011, 2011, No. 0091
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