Prefabricated Substation_IEP-SAC_ 2015-16

IEP-SAC JOURNAL 2015-16
Service Conditions for Prefabricated Substations according to IEC 62271-202
Abstract
Prefabricated substation is a prefabricated and type-tested assembly comprising an enclosure containing in general;
power transformers, high voltage and low voltage switchgear and controlgear, high voltage and low voltage
interconnections, auxiliary equipment and circuits. The IEC 62271-202 specifies normal and special service
conditions, rated characteristics, general structural requirements and test methods for prefabricated substations which
are cable connected, operated from inside (walk-in) or outside (non-walk-in), rated voltage above 1 kV and up to 52
kV, one or more transformers and outdoor installation in locations with public accessibility. The normal functioning
and life expectancy of components of prefabricated substations depend upon specification and selection of components
in view of service conditions. Therefore, it is necessary for the users to consider environmental conditions, factors,
parameters and service conditions carefully and transform in their technical requirement and specification. During
purchasing, pre-engineering, designing, operating and maintaining stages, these technical requirement and
specification shall be adhered by the users, contractors and suppliers. This paper discusses normal and special service
conditions for prefabricated substations according to IEC 62271-202.
Introduction
The prefabricated substations comprise a walk-in or non-walk-in enclosure containing power transformers, high
voltage and low voltage switchgear, high voltage and low voltage interconnections, auxiliary equipment and circuits
in various configurations. In Figure 1, electrical design, physical layout and final assembly of a non-walk-in
prefabricated substation is shown. Whereas in Figure 2, final assembly of a walk-in prefabricated substation with two
distribution transformers is shown.
Figure 1: Electrical design, physical layout and final assembly of non-walk-in prefabricated substation

Figure 2: Final assembly of walk-in prefabricated substation with two transformers
The normal safe operation and life span of components of prefabricated substations depend on the selection of
components based on service conditions such that all the adverse effects are compensated. The physical, chemical or
biological condition, external to a prefabricated substation, to which it is subjected at a certain time is called
environmental condition [8]. The environmental conditions are generally composed of environmental conditions
appearing in nature and environmental conditions generated by the product itself or by external sources [8]. A physical,
chemical or biological influence which, either singly or in combination with other influences, produces an
environmental condition (e.g. heat, vibration) is known as environmental factor. One or more physical, chemical or
biological properties characterizing an environmental factor (e.g. temperature, acceleration) is named environmental
parameter. A value of each quantity, characterizing the environmental parameter is known as severityof environmental
parameter. For example; the severity of sinusoidal vibration is defined by values of the acceleration (in m/s2) and
frequency (in Hz).
Appropriate engineering based on specifications (e.g. sizing, de-rating), design of enclosure, selection of right material
for all the metal parts, IP protection, coating and painting procedures, engineering practices and workmanship are
major elements for achieving the desired normal and special service conditions and maximizing component lifetime.
The enclosure can be made of different materials (concrete, metal, synthetic materials, etc.). The materials of the
enclosure be supposed to resist deterioration under the environmental conditions during its expected lifetime. The
major environmental issues related to enclosure design are ambient air temperature, humidity, altitude, pressure,
pollution, corrosive gases and salt spray. The corrosivity category regarding painting system, temperature class of
enclosure, mechanical impact i.e. IK protection, ingress protection degree and clearances are technical requirements
for the enclosure. The painting corrosion protection of enclosure be specified by the user to comply the requirements
of exterior environment like industrial areas and coastal areas with moderate salinity, desert with high temperature
and sand storms and interior environment like chemical plants and swimming pools etc. Expected durability without
considerable corrosion within these environments may be more than 20 years and up to 30 years. The other design
parameters influencing enclosure design are oil and winding temperature rise and winding hot spot temperature
specified for the transformer, temperature rise in low voltage panel at various locations like busbar connections and
IAC classification.
In Figure 3, several configurations of walk-in and non-walk-in prefabricated substations are shown. Some other kind
of customized configurations can also be built to fulfill the user’s requirement.

Figure 3: Different configurations of walk-in and non-walk-in prefabricated substations
Normal and Special Service Conditions for Prefabricated Substation
An example of service conditions in which prefabricated substation may operate in tropical and coastal areas is as
under:
1. The prefabricated substation will be installed outdoor at an elevation less than 1000 meters above sea level.
2. The site conditions under which the prefabricated substation is required to operate are as follows:
a. Summer outdoor design temperature +50 °C Maximum
b. Outdoor average temperature (24 hour period) +40 °C
c. Surface temperature (direct sun exposure) +80 °C Maximum
d. Relative humidity 100% Maximum
e. Blowing sand size 0.5 to 50 microns
f. Atmosphere Salt spray
The above service conditions fall under the category of special service conditions as per IEC 62271-202.
Figure 4: Corrosion of metallic parts in adverse environment due to selection of under specified material used in
prefabricated substations
In Figure 4, some metallic parts of prefabricated substation are shown which were subjected to marine environment
and badly affected. The substation enclosure and other metallic parts shall be corrosion resistant. Corrosion resistant
as defined by industry standards are constructed to provide a degree of protection against exposure to corrosive agents
such as salt spray. Stainless steel is the strongest of the corrosion resistant materials. It exhibits many of the same

resistances attributed to fiberglass materials as well as resistance to highly polar solvents such as acetone and MEK.
316 grade stainless steel is an option that provides improved resistance to salt, some acids, and high temperature. 316
grade stainless steel is a strongly recommended choice for marine environments which are within 5 miles of salt water
or otherwise subject to exposure to salt spray. It should be noted that 316 grade resistance to sulfates and chorine is
less than that provided by 304 grade stainless steel.
The normal and special service conditions for prefabricated substations are stated in IEC 62271-202:2014, sub-clauses
2.1 and 2.2 respectively and discussed below.
Normal Service Conditions for Prefabricated Substation [1]
The IEC 62271-202:2014, sub-clause 2.1 states that prefabricated substations are designed to be used under normal
service conditions according to following IEC standards:
· IEC 62271-1:2011 for indoor switchgear and controlgear
· IEC 61439-1:2011 for low-voltage switchgear and controlgear
· IEC 60076-1:2011 for power transformers
The service conditions stated in above IEC standards are as under:
IEC 62271-1:2007 for Indoor Switchgear and Controlgear, Sub-clause 2.1.1 [2]
High-voltage switchgear and controlgear, including the operating devices and the auxiliary equipment which form an
integral part of them, are intended to be used in accordance with their rated characteristics and normal service
conditions. If the actual service conditions differ from these normal service conditions, high-voltage switchgear and
controlgear and associated operating devices and auxiliary equipment shall be designed to comply with any special
service conditions required by the user, or appropriate arrangements shall be made as per IEC 62271-1:2007, sub-
clause 2.2.
The requirements for normal service condition are as under:
· The ambient air temperature does not exceed 40 °C and its average value, measured over a period of 24 hour
does not exceed 35 °C.
· The preferred values of minimum ambient air temperature are –5 °C, –15 °C and –25 °C.
· The influence of solar radiation may be neglected.
· The altitude does not exceed 1000 meters.
· The ambient air is not significantly polluted by dust, smoke, corrosive and/or flammable gases, vapours or
salt. The manufacturer will assume that, in the absence of specific requirements from the user, there are none.
The conditions of humidity are as follows:
o The average value of the relative humidity, measured over a period of 24 hour, does not exceed
95%;
o The average value of the water vapour pressure, over a period of 24 hour, does not exceed 2.2 kPa;
o The average value of the relative humidity, over a period of one month, does not exceed 90%;
o The average value of the water vapour pressure, over a period of one month, does not exceed 1.8
kPa.
For these conditions, condensation may occasionally occur. Condensation can be expected where sudden
temperature changes occur in periods of high humidity. To withstand the effects of high humidity and
condensation, such as breakdown of insulation or corrosion of metallic parts, switchgear designed for such
conditions should be used. Condensation may be prevented by special design of the housing, by suitable
ventilation and heating of the station or by the use of dehumidifying equipment.
· Vibrations due to causes external to the switchgear and controlgear or earth tremors are insignificant relative
to the normal operating duties of the equipment. In absence of specific requirements from the user,
manufacturer will assume that there are none.

IEC 61439-1:2011 for Low-voltage Switchgear and Controlgear, Sub-clause 7.1 [4]
Low-voltage assemblies are intended for use under the normal service conditions detailed below. If components, for
example relays, electronic equipment, are used which are not designed for these conditions, appropriate steps should
be taken to ensure proper operation.
· The ambient air temperature for indoor installations does not exceed +40 °C and its average over a period of
24 hour does not exceed +35 °C. The lower limit of the ambient air temperature is –5 °C.
· The ambient air temperature for outdoor installations does not exceed +40 °C and its average over a period
of 24 hour does not exceed +35 °C. The lower limit of the ambient air temperature is –25 °C.
· The relative humidity of the air for indoor installations does not exceed 50% at a maximum temperature of
+40 °C. Higher relative humidity may be permitted at lower temperatures, for example 90% at +20 °C.
Moderate condensation should be borne in mind which may occasionally occur due to variations in
temperature.
· The relative humidity for outdoor installations may temporarily be as high as 100% at a maximum
temperature of +25 °C.
· The pollution degree refers to the environmental conditions for which the assemblyis intended. For switching
devices and components inside an enclosure, the pollution degree of the environmental conditions in the
enclosure is applicable. For the purpose of evaluating clearances and creepage distances, the following four
degrees of pollution in the micro-environment are established.
o Pollution degree 1:
No pollution or only dry, non-conductive pollution occurs. The pollution has no influence.
Only non-conductive pollution occurs except that occasionally a temporary conductivity caused
by condensation is to be expected.
Conductive pollution occurs or dry, non-conductive pollution occurs which is expected to become
conductive due to condensation.
Continuous conductivity occurs due to conductive dust, rain or other wet conditions. Pollution
degree 4 is not applicable for a micro-environment inside the ASSEMBLY according to this
standard.
Low voltage assemblies for industrial applications are generally for use in a pollution degree 3
environment. However, other pollution degrees may be considered to apply, depending upon
particular applications or the micro-environment. The pollution degree of the micro-environment
for the equipment may be influenced by installation in an enclosure.
· The altitude of the site of installation does not exceed 2000 meters. For equipment to be used at higher
altitudes, it is necessary to take into account the reduction of the dielectric strength, the switching capability
of the devices and of the cooling effect of the air.
IEC 60076-1 for Power Transformers, Sub-clause 4.2 [3]
A transformer loaded with rated normal current inside an enclosure has a temperature rise, which is higher than in
open-field conditions and the temperature limits as defined in IEC 60076-2:2011 or IEC 60076-11:2004 can be
exceeded. The service conditions of the transformer will be determined according to the local outside service
conditions and the class of the enclosure as per IEC 62271-202, sub-clause 4.101.2. This will enable the transformer
manufacturer or user to calculate its possible de-rating according to IEC 62271-202, Annexure DD.
The service conditions set out in IEC 60076-1, sub-clause 4.2 represent the normal scope of operation of a transformer
outside an enclosure. For any unusual service conditions which require special consideration in the design of a
transformer, IEC 60076-1, sub-clause 5.5 is applicable. Such conditions include high altitude, extreme high or low
external cooling medium temperature, tropical humidity, seismic activity, severe contamination, unusual voltage or
load current wave shapes, high solar radiation and intermittent loading. They may also concern conditions for
shipment, storage and installation, such as weight or space limitations.

The normal service conditions according to IEC 60076 are as under:
· A height above sea-level not exceeding 1000 meters.
· The temperature of cooling air at the inlet to the cooling equipment not exceeding:
o 40 °C at any time;
o 30 °C monthly average of the hottest month;
o 20 °C yearly average.
and not below –25 °C in the case of outdoor transformers and–5 °C in the case of transformers where both
the transformer and cooler are intended for installation indoors. The user may specify a higher minimum
temperature of cooling medium in which case the minimum temperature of cooling medium shall be stated
on the rating plate.
· A sinusoidal wave shape of supply voltage with a total harmonic content not exceeding 5 % and an even
harmonic content not exceeding 1 %.
· Total harmonic content of the load current not exceeding 5 % of rated current. Transformers where total
harmonic content of the load current exceeds 5 % of rated current, or transformers specifically intended to
supply power electronic or rectifier loads should be specified according to IEC 61378 series. Transformers
can operate at rated current without excessive loss of life with a current harmonic content of less than 5 %,
however it should be noted that the temperature rise will increase for any harmonic loading and may exceed
rated rise.
· Symmetry of three-phase supply voltage
· Installation environment with a pollution rate that does not require special consideration regarding the
external insulation of transformer bushings or of the transformer itself.
· An environment not exposed to seismic disturbance which would require special consideration in the design.
· Where the transformer is installed in an enclosure not supplied by the transformer manufacturer remotely
from the cooling equipment, for example in an acoustic enclosure, the temperature of the air surrounding the
transformer is not exceeding 40 ºC at any time.
· Environmental conditions within the following definitions according to IEC 60721-3-4:1995:
o climatic conditions 4K2 except that the minimum external cooling medium temperature is –25 ºC;
o special climatic conditions 4Z2, 4Z4, 4Z7;
o biological conditions 4B1;
o chemically active substances 4C2;
o mechanically active substances 4S3;
o mechanical conditions 4M4.
For transformers intended to be installed indoors, some of these environmental conditions may not be
applicable.
Special Service Conditions for Prefabricated Substation [1]
IEC 62271-202:2014, sub-clause 2.2 states that when a prefabricated substation is used under conditions different
from the normal service conditions given in sub-clause 2.1 of IEC 62271-202:2014, the following applies.
· For installation of high-voltage switchgear and controlgear at an altitude higher than 1000 meters, IEC 62271-
1:2007 to be referred.
· For installation of low-voltage switchgear and controlgear at an altitude higher than 2000 meters, sub-clause
7.2 of IEC 61439-1:2011 to be referred.
· For installation of transformer at an altitude higher than 1000 meters, IEC 60076-2:2011 to be referred for
oil immersed transformer and IEC 60076-11:2004 for dry type transformer.
· In the case where there is exposed primary insulation inside the enclosure, the degree of pollution should be
selected having consideration that airborne salt or industrial pollution introduced via the enclosure ventilation
will not be washed off by rain. The degree of pollution inside the enclosure in such circumstances may be
more onerous than existing outside the enclosure.
· For substations that are intended to be installed in environments with the site pollution severity classes c, d
and e according to IEC/TS 60815-1:2008, the exposed insulation, if any, should be designed in order to
withstand these pollution levels. Alternatively, measures should be taken in order to prevent the build-up of
pollution on the exposed surfaces of the insulation.

· For installation in polluted ambient air, the degree of pollution may be specified according to the relevant
standards for the following equipment.
o High-voltage switchgear and controlgear sub-clause 2.2.2 of IEC 62271-1:2007 to be referred.
Metal-enclosed switchgear and controlgear, under the scope of IEC/TS 62271-304 and intended to
be used in service conditions more severe with respect to condensation and pollution than the normal
service conditions specified in IEC 62271-202 standard, can be classified with a "Design Class" 1
or 2 according to IEC/TS 62271-304 to demonstrate its ability to withstand such severe conditions.
o Low-voltage switchgear and controlgear, IEC 60664-1:2007 and sub-clause of 7.1.3 of IEC 61439-
1:2011 to be referred.
o Transformer, IEC 60076 series, in general, do not address pollution as a special service condition.
However, IEC 60067-11:2004 describes unusual service conditions for dry type transformers that
could give guidance in case of polluted environment.
· Temperature and humidity
For a prefabricated substation situated where the ambient temperature can be significantly outside the normal
service condition range for the enclosure stated in IEC 62271-202, sub-clause 2.1, the preferred ranges of
temperature to be specified can be:
–50 °C and +40 °C for very cold climates.
–5 °C and +50 °C for very hot climates.
If the service conditions at the intended installation site do not fall within the limits of “normal services conditions,
taking into account the class of the enclosure (refer IEC 62271-202, sub-clause 4.101.2), then the limits of the
temperature rise for the transformer shall be modified accordingly (refer Annex DD of IEC 62271-202).
According to IEC 60076-2:2011 and IEC 60076-11:2004, the rated power of a transformer is related to a 20 ºC average
yearly temperature. This accounts for a margin of 20 K to the maximum permissible ambient temperature of 40 ºC.
Different yearly temperatures and different substation enclosure classes result in different load factors, which can be
extracted from the Figures DD.1 to DD.9 of IEC 62271-202.
The maximum hot-spot temperature of the transformer should be maintained irrespective of the enclosure, and
therefore, it is necessary to de-rate the transformer to ensure that this hot-spot temperature is not exceeded.
The maximum hot-spot winding temperature and relative ageing rates due to hot-spot temperature for liquid-filled
transformers are given in IEC 60076-7 and maximum hot-spot winding temperature and insulation life consumption
for dry-type transformers in given in IEC 60076-12.
The influence of temperature and moisture content on the insulation system of transformer is shown in Table 1.
Table 1 – Life of paper under various conditions [6]
Paper Type / Aging Temperature
Life years
Dry and free
from air
With air and
2% moisture
Wood pulp at 80 °C 118 5.7
90 °C 38 1.9
98 °C 15 0.8
Upgraded wood pulp at 80 °C 72 76
90 °C 34 27
98 °C 18 12

The transformer corresponding to the rated maximum power of the prefabricated substation can have a different load
factor for different enclosure class and ambient temperature. Ambient air temperature is an important factor in
determining the load capability of a transformer since the temperature rises for any load must be added to the ambient
to determine operating temperatures. The annex DD of IEC 62271-202 gives a method of defining the load factor for
either liquid-filled or dry-type transformers. In Figure 5, load factors for liquid-filled transformer without enclosure
and in an enclosure of class 5, 10, 15, 20, 25 and 30 with oil and winding temperature rises 30-35K, 40-45K, 50-55K
and 60-65K and ambient air temperature from -30 °C to 60 °C are given.
Figure 5: Liquid-filled transformer load factor in an enclosure [1]
For dry-type transformers, the maximum hot-spot temperature is given in IEC 60076-12:2008 [7] and depends on the
temperature class of the insulating materials i.e. class A, E, B, F or H. In Figure 6, for dry-type transformer, load factor
outside of the enclosure is given. Whereas in Figure 7, load factor in an enclosure for insulation class F (155 °C) is
given.
Figure 6: Dry-type transformer load factor outside of the enclosure [1]

Figure 7: Dry-type transformers load factor in an enclosure; insulation class F (155 °C) [1]
In Table 2, load factors at 50 °C ambient temperature and oil and winding temperature rises for transformer 40-45K
and 50-55K and enclosure classes from class 5 to 30 are tabulated. It will be worth to realize that with the increase of
oil and winding temperature rises, there is drastic reduction in the load factor. Therefore, while specifying the
transformer oil and winding temperature rises; ambient air temperature and class of enclosure shall be considered for
achieving desired load factor.
Table 2 – Load factors at 50 °C ambient temperature and different enclosure class and O/W temperature
Enclosure Class
Load Factor at
40-45K O/W
Load Factor at
50-55K O/W
No enclosure 0.885 0.77
Class 5 0.835 0.7
Class 10 0.775 0.64
Class 15 0.71 0.57
Class 20 0.63 0.49
Class 25 0.56 ˂ 0.4
Class 30 0.49 ˂ 0.3
· Vibrations, shock or tilting
Sub-clause 2.2.4 of IEC 62271-1:2007 is applicable.
· Wind speed
Sub-clause 2.2.5 of IEC 62271-1:2007 is applicable.
· Other parameters
Sub-clause 2.2.6 of IEC 62271-1:2007 is applicable which states that when special environmental conditions
prevail at the location where switchgear and controlgear is to be put in service, they should be specified by the
user by reference to IEC 60721.

Conclusions
The prefabricated substations, in concrete or metallic enclosure, non-walk-in or walk-in are used in medium voltage
secondary cable distribution networks, in urban and rural areas, public and industrial areas, building sites, mining,
irrigation, agriculture and many others. In recent years prefabricated substations are being progressively used in
alternative energy production installations, such as solar energy, wind power, geothermal, biomass, biofuels and tidal
power.
The actual environmental conditions to which prefabricated substations are exposed are normally complex and
composed of a number of environmental factors and corresponding parameters [8]. Therefore, it is necessary that in
the user’s specifications the environmental factors involved to be listed and appropriate severities for each parameter
to be selected when defining the environmental conditions for prefabricated substations. The environmental influences
on a product in a certain application is a result of conditions of the surrounding medium, normally air, water and soil
in certain cases, conditions of the structure to which the product is connected and influences from external sources or
activities [8]. When selecting environmental factors and parameters for prefabricated substations, it is necessary that
these conditions and influences for single, combined and sequential environmental factors as they occur to be checked
by the user and specified in the technical requirements and specification.
The service conditions in the tropical countries fall under special service condition due to the high ambient air
temperature for example +50 or +55 °C. The other factors individual or combined like humidity, altitude, and ambient
air significantly polluted by dust, smoke, corrosive and/or flammable gases and saline environment, e.g. coastal areas
may worsen the service conditions. For example, at high altitudes due to decreased air density, the oil and winding
temperature rise of transformers and temperature rise of busbar connections and switchgears in low-voltage panels
will increase since they are dependent upon air for the dissipation of heat losses [9].
A transformer operating in an enclosure experiences an extra temperature rise which is about half the temperature rise
of the air in that enclosure [6]. The extra temperature rise to be determined by temperature rise tests, but when such
test results are not available, the values given in Table 6 of IEC 60076-7 for different types of enclosure may be used.
Since the temperature distribution in the transformer is not uniform, the part that is operating at the highest temperature
will normally undergo the greatest deterioration. Therefore, the rate of ageing is referred to the winding hot-spot
temperature. The ageing or deterioration of insulation system of transformer is a time function of temperature,
moisture content, oxygen content and acid content. The IEC 60076-7 presents model based only on the insulation
temperature as the controlling parameter.
The enclosure design, selection of material and other components like transformer, medium and low voltage
switchgear, protection relays, instruments and other accessories of prefabricated substation are intended to comply
normal service conditions and whenever needed to meet special service conditions as per IEC 62271-202, sub-clauses
2.1 and 2.2 respectively for achieving the expected lifetime of 25 to 30 years.
REFERENCES
1. IEC 62271-202, Edition 2.0, 2014, High-voltage switchgear and controlgear - Part 202: High-voltage/low-
voltage prefabricated substation.
2. IEC 62271-1, Edition 1.1, 2011, High-voltage switchgear and controlgear – Part 1: Common specifications.
3. IEC 60076-1, Edition 3.0, 2011, Power transformers – Part 1: General.
4. IEC 61439-1, Edition 2.0, 2011, Low-voltage switchgear and controlgear assemblies – Part 1: General
rules.
5. IEC 60076-11, First Edition, 2004, Power transformers – Part 11: Dry-type transformers.
6. IEC 60076-7, First Edition, 2005, Power transformers – Part 7: Loading guide for oil-immersed power
transformers.
7. IEC 60076-12, Edition 1.0, 2008, Power transformers – Part 7: Loading guide for dry-type power
transformers.
8. IEC 60721-1, Edition 2.2, 2002, Classification of environmental conditions – Part 1: Environmental
parameters and their severities.
9. IEEE standard C57.91:2011, IEEE Guide for loading mineral-oil-immersed transformers

Engr. Muhammad Hanif Muhammad (mhmy93@gmail.com) received his BEng degree in Electronics in
1983 from NED University of Engineering & Technology, Karachi and PGD in Total Quality Management
from University of Punjab in 2008. He has completed High Voltage Engineering Course from King Saud
University Saudi Arabia. He is Lean Six Sigma Black Belt from Asian Institute of Quality Management. He
has attended numerous seminars, conferences and trainings. He has worked in various public and private
sector organizations and presently works for ABB Electrical Industries, Riyadh in PPMV business unit as
the QA/QC Manager. Insulation lifetime, power and instrument transformers, on-load tap-changer, and
statistical quality control are his subjects of interest.

Prefabricated Substation_IEP-SAC_ 2015-16

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