ENVIRONMENTAL HUMIDITY CONDITIONS FOR ENCLOSED HIGH-VOLTAGE SWITCHGEAR

NEW TEST PROCEDURES, DEFINITIONS AND SERVICE EXPERIENCE .

D. Konig K.-H. Krefter H. Gremmel H.-J. Straube

High-Voltage Laboratory Vereinigte Elektrizitats- CALOR-EMAG AG CAUIR-EMAG AG Techn. University Darmstadt werke Westfalen, Dortmund Ratingen Ratingen Federal Republic of Germany Federal Republic of Germany Federal Republic of Germany

Sumnary

1. A survey is given about investigations carried out during the last ten years. It was found that condensation of humidity from the surrounding air will always lead to partial discharge currents at the surface of insulating components in enclosed high-voltage switchgear.

2. IEC Publication 721 has presented a new catalogue of classes for the definition of environmental conditions. Definitions used up to now should be replaced by the new classes.

3. IEC Report No. 932 presents different test procedures to simulate the in- fluence of condensation on insulating components of mtal-enclosed high- voltage switchgear. The philosophies behind the different procedures are explained.

4. Test procedures with defined humidity conditions can be carried out only with an especially developed test equipment. The lay-out and the main components are described.

5. Modern types of switchgear are expected to be less sensitive with respect to environmental conditions. For existing switchgear measures are proposed to increase or maintain safety and reliability in service. A new diagnostic system is under test.

Fundamentals

Modern insulation systems of air insulated enclosed switchgear for 'indoor use are characterized by a compact integrated design with the application of solid insulating materials such as epoxy resins, which can withstand the internal dielectric stresses for a long time with high reliability. Compared to outdoor insulation the dielectric surface stresses of this type of insulation system are much higher, but no problems exist in case of dry and clean surface conditions of the solid insulating Components. These conditions are for instance specified for type and routine tests as well as for commissioning tests.

However, simultaneous stresses by high humidity and/or pollution can start micro discharges on the solid insulation sur- faces, which may lead - depending on the type of insulating material under stress - to a more or less rapid ageing of the insulation surface. During the last few years fundamental research work has been done and is being continued, which establishes a very early stage of ageing by the mechanism of the socalled

"electrolytical partial discharge erosion", initiated by micro partial discharges (1) , (2). Figure 1 explains schematically the various stages of this mechanism. Even on technically clean epoxy resin surfaces moisture layers with low layer conductivity under high-voltage stress may give rise to the start of micro partial discharges leading to a stepwise developing erosion process according to Figure 1. Further in the process during the late stage af ageing partial arcs across thermally dried zones will appear and contribute to the erosion phenomena.

The role of conductive pollution for instance by salty deposits at seaside locations, chemical dust in industrial areas et., is still under investigation. Depending on the actual surface conditions of an insulator a strong increase of the contamination level by means of brine polluted layers of a very high volume conductivity surprisingly leads only to a more or less moderate increase of the resulting layer conductivity (2).

Number, design and characteristics of enclosures preventing more or less the ingress of outdoor climate into the switchgear compartments are of great importance with respect to the actual environment, in which the insulation system has to work and which may essentially differ from the outdoor climate conditions. In the general case three different environmental zones exist for an indoor switchgear installation. The insulating components are situated in climate zone I, separated by the switchgear enclosure from the indoor climate inside the building, i.e. from climate zone 11. Outside the building the outdoor climate (zone 111) exists. Special installations are known, which have no separate switch room (zone I11 and zone I1 are identical) or where there is a direct contact between live parts and the indoor climate (zone 11 and zone I are identical).

When discussing the influence of environ- mental conditions on enclosed high-voltage switchgear three main items should be kept in mind:

- a precise definition of the environmental service conditions

- the important role of enclosures as substantial parts of a switchgear,

- a correct simulation of "natural" service conditions in climate zone I during ageing tests.

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New definitions of normal and special service conditions for enclosed high- voltage switchgear

Whenever on high-voltage switchgear technical problems occur due to environ- mental influences a strong demand for more precise and also for more stringent definitions of service conditions arises.

During the last few years a new basis for such definitions was created by Technical Committee No. 7 5 of IEC and published in IEC-Publication 721 . Part 3 of this publication offers a catalogue of a large number of classes of environmental conditions. Each class is characterized by three main parameters:

- type of product application, with identification by first digit 1 for storage, 2 for transport, 3 for use indoors, 4 for use outdoors etc.

- kind of correlating environmental influences with identification by letter B for biological conditions, C for chemically active substances, K for climatic conditions, M for mechanical conditions

- degree of severity of environmental influences, with identification by a further digit.

In the sense of the present topic the classes of climatic conditions - K - for - use at weather protected locations - 3 - are of main interest. Table I shows a part of the assortment of classes available to define the climatic conditions for indoor use.

It is important to mention that the values listed for environmental parameters in table I are not to be regarded as limiting values, that means they may be exceeded for a very short duration or in rare cases. But it also means that none of these values is expected to be maintained as a permanent service condition. The fraction of time when the specified value is exceeded is usually regarded to be less than 0,Ol. The probability of exceeding two parameters at the same time is expected to be only 0.0001 as a consequence.

Further IEC-Publication 7 2 1 gives infor- mation for each climatic condition para- meter which value is to be expected taking into account different types of open-air climates and different types of buildings.

Normal service conditions for metal- enclosed high-voltage switchgear for indoor use were specified since 1 9 8 0 in IEC- Publication 694 in clause 2 .1 .1 .

A comparison of the climatic conditions specified in IEC-Publication 694 to those of class 3K5 of the new catalogue is demonstrated in the climatogram of Fig. 2. The ranges defined by the two sets of parameters are not differing considerably. This is to be considered regarding also the above explanation that the environmental parameters of the new definitions are neither strict limitations

TABLE I - Classification of climatic conditions (as per IEC 721-3 -3 )

Environmental Class par ameter .. 3K4 3K5 3K6 .. Low air temp. "C High air temp. O C Low rel. humidity % High rel. humidity % Low abs. humidity g/m' High abs. humidity g/m' Change of temp. OC/min Low air pressure kPa High air pressure kPa Solar radiation W/m' Heat radiation Movement of surr. air m/s Condensation Wind driven rain, snow Water other than rain Icing

+5 -5 -25 +40 + 4 5 +55

5 5 1 0 95 95 1 0 0 1 1 0 . 5

29 29 29 0 . 5 0 . 5 0 . 5

70 70 70 1 0 6 1 0 6 1 0 6 700 700 1 1 2 0

* ) * ) * )

1.0 1.0 1.0 Yes Yes Yes No No Yes

No Yes Yes * ) * ) * )

* ) Notes are not reproduced

nor permanent service conditions ;*!. Both definitions include the possibility of condensation, but not as a frequent event.

National and international technical committees responsible for high-voltage installations in buildings and for high- voltage switchgear are therefore invited to agree on a new definition for "normal service conditions" selected from IEC- publication 7 2 1 - preferably from class 3K5. Also classes for special service conditions should be selected. 3K5 will cover the situation in the vast majority of switchgear buildings according to the current constructional practice and the natural climatic conditions in central european distribution systems.

Harmonized and accepted definitions of service conditions, however, do not yet establish rules for design or even testing of switchgear under environmental influences. They may be regarded as a first step to establish relevant testing procedures.

New ageing tests on enclosed high-voltage switchgear according to IEC-Report No. 932

WG 4 of IEC SC 1 7 C has worked out test procedures with respect to environmental humidity conditions. This work is going to be published as IEC-Report No. 932 .

The philosophy behind the proposed tests is as follows: Though the existence of long- time ageing phenomena due to severe climate conditions is well known since many years and has been reported in several papers - for instance at the Cired Conferences -, there is still a lack of coordinated work in establishing suitable tests. Based on some available experiences mainly in two countries two different ageing procedures A und B are proposed. The basic idea behind these tests is common and more or less conventional: with the help of climatic

- - -__--__-_--_ * * ) Consequently in the authors' opinion 4 0 C may still remain the basis for the temperatur rise test parameters.

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test chambers such test conditions are to be created that liquid layers on the insulating surfaces of enclosed switchgear appear. Further stress parameters are the test voltage (rated voltage) and the test duration, the latter being different for level 1 and level 2 ageing tests. The creation of liquid layers requires climate cycles with rapid changes of temperature and humidity. In order to create a distinct and long wet period which lasts as long as possible, these changes are choosen as much more rapid than they would occur under "natural" conditions in Europe. The type of liquid layer created by the two different procedures A and B is different, too. In case of procedure A pure condensation will occur, while in case of procedure B additional contamination by continuously atomizing demineralized salt water solution will take place. The application of a diagnostic procedure after the ageing test will clarify, whether the test has been passed successfully or not.

Additionally, however, IEC-Report No. 932 contains as a new idea the introduction of a so called penetration test also combined with a diagnostic procedure, which might enable to decide without expensive and time consuming ageing tests, whether there is a chance to hamper the ingress of the environmental climate by the barrier "enclosure" to such a degree that no relevant influences on the insulation system can be recorded. After the penetration test, performed as a pollution treatment by means of a salty fog atmosphere, partial discharge and leakage current measurements may serve as diagnostic tools. During the diagnostic procedure climatic cycles are applied, which simulate the strongest climate stresses known in Europe, but do not overstress the equipment by too rapid temperature changes. Thus, it can be found out, if an ingress of humidity and/or pollution in electrically stressed areas has taken place that leads to a change in the electrical performance. If no significant influence is recorded the ageing test may be omitted. Although up to now, only few studies performed on enclosed switchgear bays are available, they encourage to proceed in that new way of testing. Further experience is highly appreciated.

A test equipment for ageing and diagnostic test procedures according IEC-Report No. 932

For test procedures as specified in IEC- Report No. 932 a complicated and expensive test equipment is necessary. When the authors started the development, a test bay was available from ageing and humidity tests according Appendix E of IEC-Pub- lication 466(17C(C.O.)33/0ctober 79).

The volume of the testbay is 18 m3. The dual walls and doors are made of glasfiber reinforced polyester plates with a 4 0 mm air gap as a thermal insulation. The roof with an inclination of 30" is from PVC with an additional thermal insulation of poly- styrene. The floor is covered with ceramic tiles on a thermal insulating pavement. A sink to remove condensed water was provided

at the deepest spot of the floor. Obser- vation windows in the door are equipped with heaters to prevent condensation.

The overall arrangement of the test equipment is shown in figure 3.

The three-phase test voltage is applied from a 120 kVA-transformer set outside the test bay via three porcelain bushings in one of the polyester walls. The voltage may be adjusted to every value up to 45 kV (value between phases). The voltage is kept constant to the adjusted value by AVR control.

In the upper part of the test bay four atomizer nozzles are installed. Pressurised air is fed directly or via a humidifier to the nozzles. Deionized water is provided from a constant level container of 60 1. The conductivity of the spray fluid can be kept constant at a value between 1 pS/cm and 20.000 W/cm by automatically adding of a premixed NaCl brine. The capacity of the spray system is 30 1 water per hour.

For the air conditioning of the test bay an inforced air circulation system was installed. The heating system has a power rating of 9 kW. The boiling unit for the humidifying of the air has also a rating of 9 kW. Cooling and/or dehumidifying is achieved by a water cooler of 12 kW at 2OOC. The operating range of the test equipment is between +lOC and +60C and between 30 % and 95 % of relative air humidity.

The test cycles are controlled by a programmable micro processor unit. The different cycles to be applied are stored and can be started on request. The program is not restricted to controlling switches and valves of the climatic test cycles, it also starts in time the units of the climate control system and raises the test voltage to the set value. Test procedures may be performed over several days without the presence of any personnel.

For measuring and recording temperature, humidity, conductivity and RMS-value of test voltage 8 chanels are provided with a sampling rate of 1000 per second.

Leakage current and oscillation of test voltage can be recorded by 8 chanels with a bandwidth o f 100 kHz in order to evaluate the ohmic component of the current. For recording and evaluation a PC can be used.

Experience has shown that for Test Procedure A the full ratings of heaters, boilers and cooling units are required. To achieve 95 % of relative humidity at 5OoC a water content of 86 g per kg of air is necessary. Also the requested rate of change of temperature of 30 K/h make the large power ratings of the equipment necessary.

Test Procedure B can be performed easier. Cooling may be achieved by heat exchange through the walls of the test bay. Inforced cooling should not be applied since it results in considerable dehumidification. Thus temperature decay will be an exponential function curve.

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The Penetration Test is carried out with the same testing equipment as test procedure B. The main advantage lies in the far shorter test duration compared to the two ageing test procedures.

Environmental humidity stress on enclosed high-voltage switchqear in practical ~

service

From a modern design of high-voltage switchgear a user will expect that it withstands environmental influences of various kinds as well as fulfils all electrical requirements. The improved knowledge about fundamental physics of climatic influences and the new testing procedures described above will be important tools to achieve this target.

An analysis of failures of the last 20 years which occurred on switchgear applying solid insulating material to a high degree reveals the following main causes

- extreme climatic conditions with high rate of change of temperature, high humidity content of the air and con- densation on live parts and insulating components

- pollution on the surface of insulating components caused by solid or gaseous pollution of the air

- mechanical damages of solid insulating components, occassionally occurring as a consequence of mechanical tension stress.

In cases of correlation of such influences partial discharge currents may cause gradual deterioration and erosion of the surface and may finally result in disruptive discharges and short-circuit arcs.

Security and reliability of older switch- gear installations in service should be improved by influencing the climate of the live parts area, by determining any weak points and by preventive maintenance measures as required by the actual state. The improvement of climatic conditions can be achieved by constructional measures on the building (insulation, reduced ingress of outdoor climate) and by heating. In practical service good results were achieved with heaters controlled depending on temperature and humidity content of the surrounding air. In the course of preventive maintenance the switchgear should be cleaned at regular intervals and visual checks should be made of any critical components (bushings, cable terminations).

Recently a diagnostic system was developed * ) to investigate the dielectric state of a switchgear in service. The operating principle makes use of the fact that failures on insulating components are frequently announced by increased partial discharges (PD). Partial discharges result

in mechanical oscillations (noise) which can be recorded by means of accustic sensors. These shall be attached to the earth potential area of the insulating component, either to the insulating surface itself or to mechanically connected grounded metal parts.

The ideal way of attaching the sensor is via especially provided threads, but in order only to detect and localise partial discharges the sensor may be connected, even to live parts, via a sufficiently long glasfibre rod which is pressed against the surface of the component. By comparative measurements on identical components faulty pieces can be detected and eliminated. Practical application of this system will be continued to gain further experience.

With improved climatic condition, systematical checking for weak points and consequent maintenance measures, experience demonstrates, that safe and reliable service of a system can be achieved also with switchgear of older origin.

References

1. B. Muller, 1985, "Untersuchungen zum Oberflachenverhalten von stabformigen Isolatoren . . . I 1 , Thesis Techn. Univer- sity Darmstadt

2. D. Konig, P. Rasch, 1988, "Surface dis- charges on epoxy resin model post insu- lators ...I1, Conference Record of the IEEE International Symposium on Electrical Insulation, Boston, June 5-8, pp. 54-59

---------- * ) by the "Institut fur Energieversorgung", Dresden, GDR.

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Moisture layers on insulating surf aces

I I I Rise of micro partial discharges1 I I I

Generation of O3 and NOX

Formation of HN03

I 1

I Electrolytical partial 1 I discharge erosion I I I

Electrochemical Formation of dissolved nitrate

conductivity

I

Increase of the leakage current I I

Formation of thermally dried zones bridged by partial arcs, late stage of ageing

Figure 1 Ageing mechanism in the early stage

Abs. air humidity g/m

Figure 2 Clinatogram

I Air G Water

1 - Test bay 2 - Transformers 3 - Bushings 7 - Constant-level container 11 - Humidifier 4 - Atomizer Nozzles 8 - NaCl brine Figure 3 Test Equipment

5 - Deionizer 6 - Humidifier 10 - Heater

12 - Cooler 13 - Dehumidifier 14 - Water cooling unit

9 - Air circulation

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