IEC TS 63346-2-3:2025

IEC TS 63346-2-3 ®
Edition 1.0 2025-11
TECHNICAL
SPECIFICATION
Low-voltage auxiliary power systems -
Part 2-3: Design criteria - Low-voltage AC auxiliary power systems for
substations
ICS 29.240  ISBN 978-2-8327-0800-2

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CONTENTS
FOREWORD . 3
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 6
4 Abbreviated terms . 7
5 General requirements . 7
5.1 General . 7
5.1.1 System function . 7
5.1.2 Basic requirements . 8
5.2 Environmental conditions . 9
5.3 Electrical requirements . 9
5.3.1 Nominal voltage and nominal frequency . 9
5.3.2 Current rating . 10
5.3.3 System earthing . 10
5.3.4 Over-voltage protection and insulation coordination . 10
5.3.5 Harmonic distortion withstand capability . 11
6 Load . 11
6.1 General . 11
6.2 Load classification . 11
6.2.1 Classification by the degree of importance of the load . 11
6.2.2 Classification by load operation mode . 12
6.2.3 Classification by load operation duration . 13
6.3 Load calculation . 13
6.3.1 Station service transformer sizing . 13
6.3.2 Load calculated by parts . 14
7 System structure and wiring . 15
7.1 General . 15
7.2 Power supply of LV AC APS . 15
7.2.1 Power sources . 15
7.2.2 Selection of power sources . 16
7.3 Wiring of AC APS . 16
7.3.1 Configuration of power sources . 16
7.3.2 Wiring principles . 17
7.3.3 Typical wiring diagrams . 18
8 System protection and control . 21
8.1 Protection . 21
8.2 Control and signal . 21
8.3 Measurement and electric energy measuring . 22
9 Equipment selection . 22
9.1 General requirement . 22
9.2 Station service transformer . 22
9.2.1 General . 22
9.2.2 Electrical performance requirement . 23
9.2.3 Construction requirement . 23
9.3 AC distribution boards . 23
9.3.1 General . 23
9.3.2 Type selection . 24
9.4 Protective device . 24
9.4.1 General . 24
9.4.2 Electrical performance requirement . 24
9.4.3 Combination of protective devices . 24
9.5 Conductor . 25
9.5.1 General . 25
9.5.2 Conductor cross-section . 25
9.6 UPS . 25
9.7 Diesel generator . 26
9.7.1 General . 26
9.7.2 Electrical performance requirements . 26
10 Physical layout of APS . 26
10.1 General . 26
10.2 Station service transformer . 26
10.3 Diesel generator . 27
10.4 Power distribution room . 27
Annex A (informative) Typical AC loads in substations . 29
Annex B (informative) Applications of TN system in LV AC APS . 30
Bibliography . 31

Figure 1 – Diagram of basic single busbar arrangement . 18
Figure 2 – Diagram of sectionalized single busbar arrangement . 19
Figure 3 – Diagram of sectionalized single busbar with physical separation . 19
Figure 4 – Diagram of sectionalized single busbar with dedicated standby power
supply . 20
Figure 5 – Diagram of a typical wiring in France. 21

Table A.1 – Typical AC loads in substations . 29

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Low-voltage auxiliary power systems -
Part 2-3: Design criteria - Low-voltage AC auxiliary power systems for
substations
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
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9) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
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shall not be held responsible for identifying any or all such patent rights.
IEC TS 63346-2-3 has been prepared by IEC project committee 127: Low-voltage auxiliary
power systems for electric power stations and substations. It is a Technical Specification.
The text of this International Standard is based on the following documents:
Draft Report on voting
127/74/DTS 127/81/RVDTS
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts in the IEC 63346 series, published under the general title Low-voltage auxiliary
power systems, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
– reconfirmed,
– withdrawn, or
– revised.
1 Scope
This part of IEC 63346 specifies common rules and requirements for the design of low voltage
(LV) AC auxiliary power systems (APSs) intended to be installed in substations, mainly covering
the configuration of AC power sources, system wiring, protection, electric equipment selection
and physical layout.
For the purpose of interpreting this document, an AC APS in this document is considered as
follows:
– with a nominal voltage up to and including 1 kV AC;
– providing LV AC power to substation AC loads.
Though it is discussed where necessary, AC loads as well as high voltage (HV) part is beyond
the scope of this document.
Substations in this document refer to those which are part of an electrical system and contain
equipment that either receives and distributes electrical energy or transforms voltages to the
levels required by the loads they supply, or both.
This document does not apply to the design of any of the following:
– traction substation, which have different power supply requirements, such as unbalanced
load power supply;
– offshore substations, as factors such as waves, typhoons, salt spray, etc. need to be taken
into account, which have different requirements for power supply and equipment selection;
– the substation connecting a nuclear power plant to the grid and its associated LV APS
integrated with the nuclear power plant.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60038, IEC standard voltages
IEC 60076 (all parts), Power transformers
IEC 60076-3, Power transformers - Part 3: Insulation levels, dielectric tests and external
clearances in air
IEC 60364-1, Low-voltage electrical installations - Part 1: Fundamental principles, assessment
of general characteristics, definitions
IEC 60364-4-41, Low-voltage electrical installations - Part 4-41: Protection for safety -
Protection against electric shock
IEC 60364-4-43, Low-voltage electrical installations - Part 4-43: Protection for safety -
Protection against overcurrent
IEC 60364-4-44, Low-voltage electrical installations - Part 4-44: Protection for safety -
Protection against voltage disturbances and electromagnetic disturbances
IEC 60364-5-51, Electrical installations of buildings - Part 5-51: Selection and erection of
electrical equipment - Common rules
IEC 60364-5-53, Low-voltage electrical installations - Part 5-53: Selection and erection of
electrical equipment - Devices for protection for safety, isolation, switching and control and
monitoring
IEC TS 60815-1, Selection and dimensioning of high-voltage insulators intended for use in
polluted conditions - Part 1: Definitions, information and general principles
IEC 60909 (all parts), Short-circuit currents in three-phase AC systems
IEC 60947-2, Low-voltage switchgear and controlgear - Part 2: Circuit-breakers
IEC 60947-3, Low-voltage switchgear and controlgear - Part 3: Switches, disconnectors, switch-
disconnectors and fuse-combination units
IEC 61439 (all parts), Low-voltage switchgear and controlgear assemblies
IEC 61936 (all parts), Power installations exceeding 1 kV AC and 1,5 kV DC
IEC 61936-1, Power installations exceeding 1 kV AC and 1,5 kV DC - Part 1: AC
IEC 62040 (all parts), Uninterruptible power systems (UPS)
IEC 62271-202, High-voltage switchgear and controlgear - Part 202: AC prefabricated
substations for rated voltages above 1 kV and up to and including 52 kV
IEC TS 63346-1-1, Low-voltage auxiliary power systems - Part 1-1: Terminology
ISO 8528 (all parts), Reciprocating internal combustion engine driven alternating current
generating sets
ISO 8528-1, Reciprocating internal combustion engine driven alternating current generating
sets - Part 1: Application, ratings and performance
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC TS 63346-1-1 and
the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
– IEC Electropedia: available at https://www.electropedia.org/
– ISO Online browsing platform: available at https://www.iso.org/obp
3.1
main transformer
transformer in a substation that realizes energy conversion between power networks with
different voltage levels
3.2
single busbar
lines and transformers are connected to one busbar only
3.3
sectionalized single busbar
single busbar which is divided by switching devices (or disconnectors) into two or more sections
4 Abbreviated terms
The following abbreviations are always in capital and without dots.
APS auxiliary power system
DGS diesel generator set
EMI electromagnetic interference
GT grounding transformer
HV high voltage
HVAC heating, ventilation, air conditioning
LV low voltage
NPS normal power source
RCD residual current devices
SPS standby power source
SST station service transformer
SVC static var compensator
UPS uninterruptible power supply
VT voltage transformer
XLPE cross linked polyethylene
5 General requirements
5.1 General
5.1.1 System function
The LV APSs of substations play a crucial role in providing reliable electric energy to auxiliary
AC loads within substations, such as transformer forced oil cooling devices, mechanical
operation devices, pumping equipment, heaters, DC chargers, etc. The uninterrupted operation
of LV APS is vital for the safe and stable functioning of substations, as faults in the AC APS
can lead to power loss or even complete shutdown of the substation.
In designing an AC APS, it is essential to take into consideration the particularities of the country
or region where the substation is located; regulations and standards can apply. Initially, load
characteristics and power requirements of the substation, along with the available power
sources in the vicinity, should be assessed. Then the configuration and switching strategy of
the power supply, load distribution network, equipment selection as well as equipment layout
should be determined accordingly.
While implementing the design philosophies and practices outlined in this document,
adjustments should be made based on the specific environmental conditions and overall
requirements of the substation owner.
The design of the LV APS should take into account the design criteria of the HV substation as
outlined in 5.1.2.3 to minimize outages and common point of failures.
5.1.2 Basic requirements
5.1.2.1 General
Given the significance of LV APS to the operation of substations, factors such as system
stability, system reliability should be considered during the design process, details are
discussed in 5.1.2.2 to 5.1.2.6.
5.1.2.2 System stability
LV AC APS should maintain stability during external disturbances. Main measures shall be
taken in the system design phase to ensure system stability of LV AC APS including, but not
limited to, the following:
– adequate system sizing to prevent voltage and frequency fluctuations resulting from sudden
load changes, load statistics and capacity sizing of LV AC APS are specified in Clause 6;
– robust system configuration, such as balanced load distribution between phases and rapid
restoration of the network structure through power source switching as detailed in 7.2;
– implementation of protection systems capable of detecting and isolating faults to prevent
cascading failures as specified in 8.1.
5.1.2.3 System reliability
LV AC APS shall be designed to deliver a continuous and stable supply of electricity to auxiliary
AC loads. Main measures shall be taken in the design phase to ensure system reliability
including, but not limited to, the following:
– implementation of redundancy, such as the dual power supply line to critical loads in 7.3
and the configuration of main and standby power sources in 7.2;
– selection of appropriate equipment to minimize system failure rates;
– integration of technologies and practices to mitigate the impact of environmental factors on
the APS system as specified in 5.2;
– optimization of equipment layout to facilitate maintenance and operation as specified in
Clause 10;
– balancing considerations between safety and reliability, such as system nature point
earthing.
5.1.2.4 efficiency
System efficiency of LV AC APS is a measure of how effectively the system converts the input
energy into useful output energy. To enhance system efficiency, the following measures may
be implemented during the design phase:
– consider the system operating conditions in the environment to minimize energy waste;
– integrate energy-efficient technologies and practices into the system design;
– use high-quality materials and components to reduce energy losses, such as utilizing low-
resistance conductors to reduce line losses especially in long power distribution circuits as
outlined in 9.5.
5.1.2.5 Environmental compliance
The design of LV AC APS shall take into account the environmental impact throughout its
life-cycle, from manufacturing and installation to use and disposal. Measures to ensure
environmental compliance include the following:
– minimizing the use of hazardous materials and substances in the system design;
– choosing environmentally friendly materials and technology, such as using biofuel for diesel
generator, photovoltaic lighting;
– evaluating the environmental impact during system operation, such as noise pollution and
electromagnetic interference, and implementing measures to mitigate these effects.
NOTE Relevant environmental regulations and standards, such as those related to waste disposal, emissions, and
energy efficiency, can apply.
5.1.2.6 Safety of personal and equipment
Safety considerations for both personnel and equipment are paramount in LV AC APS design.
Protection measures shall be implemented to prevent personnel from exposure to hazardous
voltages and currents; these measures shall be in accordance with IEC 60364-1 and
IEC 61936-1. This can involve using insulation materials, protective barriers, and grounding
systems.
Protection measures shall be applied to minimize the risk of arc flash, overloads and short
circuits in the system; these measures shall be in accordance with IEC 60364-1，
IEC 60364-4-41，IEC 60364-4-43，IEC 60364-4-44 and IEC 61936-1. This includes equipment
grounding and the application of fuses, circuit breakers, and arc flash relays to minimize these
risks.
5.2 Environmental conditions
The design of AC APS shall take into account the environmental conditions it will encounter.
Environmental conditions which the system will be subjected to have a significant impact on the
performance and reliability of electrical systems. Some of the environmental conditions that
should be taken into consideration include:
– temperature: electrical systems can be affected by both high and low temperatures; extreme
temperatures can lead to component failure or degrade over time;
– humidity: moisture in the environment can result in corrosion and damage to electrical
components; high humidity levels also elevate the risk of electrical shock;
– altitude: higher altitude results in a decrease in air density, thereby weakening the insulation
medium strength of electrical equipment, especially impacting electrical clearances and
creepage distances;
– seismic activity: electrical systems subjected to seismic activity can suffer damage to
components and connections over time;
– EMI: EMI, originating from various sources such as other electrical equipment, radio signals,
and lightning, poses a threat to electrical systems and can lead to interference and damage;
– vermin proofing of the installation and equipment: they can enter the substations and
equipment and damage the cables and wiring.
In addressing these environmental conditions, the design of the AC APS should incorporate
measures to mitigate potential risks and ensure optimal performance and reliability in diverse
environmental settings. This includes selecting materials and components that can withstand
varying temperature and humidity levels, implementing protective measures against seismic
forces, and employing strategies to minimize the impact of electromagnetic interference on the
electrical system.
5.3 Electrical requirements
5.3.1 Nominal voltage and nominal frequency
The nominal voltage of the AC APS shall be consistent with the power distribution voltage of
the country or region and shall comply with IEC 60038.
The voltage fluctuation range of the bus during normal operation should consider factors such
as allowable highest (lowest) equipment voltage and cable voltage drop. This ensures that the
terminal voltage of the system remains within the normal operating voltage range of the
equipment.
The nominal frequency for AC APSs should be 50 Hz or 60 Hz.
NOTE Specific laws and regulations of the region or country can apply.
5.3.2 Current rating
5.3.2.1 Normal current rating
The rated current of the AC APS indicates its current-carrying capacity and shall be designed
to carry the expected loads safely without causing overheating or component damage. Current
rating of the AC APS is determined mainly by the following factors:
– nominal voltage of the system (see 5.3.1);
– load of the substation (see Clause 6).
5.3.2.2 Abnormal current rating
An important factor that shall be taken into consideration by the designer is the maximum
current that can flow through a circuit, especially under the overload or fault condition. Short-
circuit faults in all live parts shall be accounted for.
Electrical equipment used in the system shall be capable of withstanding thermal and
mechanical effects, and protective devices shall correctly operate to clear faults.
The short-circuit current level of a system, mainly based on system voltage and system
impedance, should be calculated according to IEC 60909 (all parts), which determines the
sizing of the SST and the selection of the short-circuit current ratings.
5.3.3 System earthing
The design of the earthing scheme of the equipment in LV APSs should be determined
according to the specific conditions of the equipment.
The system earthing arrangement should be in accordance with IEC 60364-1 and should also
comply with the relevant standards of the country or region.
TN system is generally applied in substations because of its good safety, reliability and
Annex B.
electromagnetic compatibility. More information is provided in
IT system is used in cases where the reliability of the power supply is relatively high; meanwhile,
the insulation level of the equipment in the system needs to be increased correspondingly and
insulation monitors or grounding monitors should be added accordingly.
TT systems are not applicable to substations. This is because the substation has a grounding
network covering the entire station, and the equipment in the substation is connected to the
grounding network to realize earthing.
5.3.4 Over-voltage protection and insulation coordination
Over-voltage protection and insulation coordination of the LV AC APS should be in accordance
with the IEC 60664 series. For more information about surge overvoltages and surge protection,
see IEC TR 62066.
5.3.5 Harmonic distortion withstand capability
An important factor that shall be taken into consideration by the designer is the harmonic
interference due to electronic converters or harmonics transferred from the HV network.
Electrical equipment used in the system shall be capable to withstand LV harmonic distortion
level.
6 Load
6.1 General
The AC loads of the substation are those that are supplied by the LV AC APS and include, but
are not limited to, the following:
– transformer forced cooling devices and auxiliaries;
– on-load tap-changers of transformers;
– motor operated mechanism for circuit breakers and disconnectors (if any);
– heater in distribution boards, in circuit breaker machinery box, terminal boxes and so forth;
– DC chargers and uninterruptible power supply (UPS, when it is used as a load);
– water pumps;
– heating, ventilation, air conditioning (HVAC) devices;
– test and maintenance equipment;
– lighting and power in the buildings, outdoor yards, equipment mechanism boxes and so
forth;
– fans;
– fire-suppression systems;
– microcomputer systems;
– protection and control equipment.
The AC APS shall exclusively supply power to the AC loads of the substation.
For the load which does not belong to the substation, it should not be electrically connected to
the APS, so as to maintain power supply reliability.
Load classification and calculation shall be carried out when designing APS in order to
determine the system capacity and supply all kinds of AC loads in an economical and effective
way.
6.2 Load classification
6.2.1 Classification by the degree of importance of the load
AC loads in substations are classified based on their importance and operational characteristics,
and should be categorized as class-I, class-II, and class-III loads or essential and non-essential
loads.
The classification of a load can vary in different countries or regions according to the specific
circumstances of the project, for example HVAC is considered as essential loads in South
Australia. It will be critical to have HVAC available during summer, but it is not the case in a
less hot region. Drainage pumps are considered as class III loads in general but they can be
essential loads for regions located in flood plains. Typical loads in substations are listed in
Annex A.
a) Class-I loads, also known as essential loads, are those whose power outage would
endanger personnel safety and equipment functionality, thus requiring continuous power to
maintain their services.Typical class-I loads includes, but are not limited to, the following:
1) transformer forced cooling device;
2) fire-fighting equipment;
3) protection and control.
The protection and control loads are supplied by DC power in most cases, but when supplied
by AC power they should be supplied as class-I loads.
Class-I loads require a backup power supply; for this purpose, a standby power source is
provided. A power interruption for such loads is permissible during the operation of
automation. However, there are certain loads that are sensitive to power quality and have
high requirements for reliability and stability of the power supply. These loads not only
necessitate backup power but also an uninterruptible power supply. Those loads are known
as UPS loads.
Typical examples of UPS loads includes, but are not limited to, the following:
4) computers, servers, and other microcomputer systems;
5) telecontrol equipment (remote terminal units or RTUs) and dispatch communication
systems;
6) fire protection systems;
NOTE 1 Fire protection systems can be supplied by UPS or emergency power supply (EPS).
7) cooling system for power electronics, SVC and static synchronous compensator.
b) Class-II loads are the ones that tolerate short power outages without endangering personnel
or equipment safety. Extended outages affect the substation operation. Weather class-II
loads belong to essential loads or non-essential loads depends on users preferences and
project specifics. Typical class-II loads includes, but are not limited to, the following:
1) motor operated mechanism for circuit breakers;
2) on-load tap-changer of transformer;
3) DC chargers;
4) UPS.
NOTE 2 When the UPS is used as an AC charging load, it can be powered as class-II load.
c) Class-III loads, also known as Class-II loads, are the ones that can withstand prolonged
power outages without directly affecting substation operations. Typical class-III loads
includes, but are not limited to, the following:
1) test and maintenance equipment;
2) drainage pumps;
3) lighting and general purpose power.
6.2.2 Classification by load operation mode
According to their operation frequency, AC loads should be classified as
a) frequently-used load, which generally refers to the load which is operated daily for normal
substation operation (e.g., transformer forced cooling devices, ventilating system), or
b) infrequently-used load, which generally refers to the load which is used only during
equipment maintenance, emergency or special conditions (e.g., firefighting pumps, test and
maintenance equipment).
6.2.3 Classification by load operation duration
According to their operation duration, AC loads can be classified as
a) continuous load, also known as standing load, which generally refers to the load
continuously operating for a long period of time, for example 3 h in American while 2 h in
China, such as forced cooling devices of transformers,
b) short-time load, also known as momentary load, which generally refers to the load
continuously operating for a short duration, for example between 10 min and 2 h, such as
rainwater pumps, firefighting pumps, or
c) intermittent load, which refers to the load operating repeatedly and periodically with a work
period less than that of the short-time load, such as on-load tap-changers of transformers,
motor operated spring system of circuit breakers.
6.3 Load calculation
6.3.1 Station service transformer sizing
The calculated KVA quantity of the station service transformer (SST) should be obtained with
Formula (1).
Q = L × F × F
(1)
demand load
where
Q is the calculated KVA quantity;
L is the total load;
F Is the demand factor;
demand
F is the load factor.
load
The size of the SSTs shall include the maximum estimated load under the worst ambient
temperature conditions.
a) Maximum demand calculation
The total load of a substation is the sum of the rated capacity of each AC load in the
substation and is calculated according to Formula (2).
n
LC=
(2)
∑ rated
i
i=1
where
L is the total load;
C is the rated capacity of each AC load in the substation.
rated
i
The total load shall be adjusted according to the actual situation of substation by means of
multiplying total load by the demand factor and the load factor.
b) Demand factor
The demand factor is the ratio of the maximum coincident demand of a system or part of a
system to the total load connected to the system or part of the system.
X
max
F =
demand (3)
L
system
where
F is the demand factor;
demand
X is the maximum coincident system demand;
max
L is the total load connected to the system.
system
c) Load factor
Load factor is the ratio of the average load over a designated time period to the peak load
occurring in that period.
L
av-t
F =
load (4)
L
peak-t
where
F is the load factor;
load
L is the average load over a designated time period;
av-t
is the peak load occurring in a designated time period.
L
peak-t
The length of the designated time period should be selected according to the overload capacity
of the SST.
6.3.2 Load calculated by parts
In contrast to 6.3.1, this load calculation method, based on load classification, involves adding
the calculated capacity of each type of AC APS load respectively as per Formula (5), to obtain
the calculated KVA quantity of the station service transformer.
n
Q=LF×× F
∑ (5)
i demand load
ii
i=1
where
Q is the calculated KVA quantity;
L is the total load of each type of AC APS load;
i
F
demand
i
is the demand factor of each type of AC APS load;
F
load
i
is the load factor of each type of AC APS load.
L is the sum of loads of a particular type, such as lighting loads or fire-fighting loads.
i
F
demand
i
Correspondingly, is the ratio of the maximum coincident system demand of this load
F
load
i
type to the total load of that type, and is the ratio of the average load of that type over a
designated time period to the peak load of that type occurring in that period.
F
demand
i
Specifications in the standards of the region or country can apply when determining
F
load
i
and .
If the kind of load which runs for a short period of time but requires a large amount of power
exits in the AC APS, for example fire-fighting equipment, the load calculation should be carried
out in a special way. For instance, when the calculated load of fire-fighting equipment exceeds
the cutoff load due to a fire, the fire-fighting load should be counted in the total load; otherwise,
it should not be counted.
7 System structure and wiring
7.1 General
The configuration of LV AC APS should align with the voltage level, operational mode, and
demands of the AC loads in the substation. The following aspects shall be taken into
consideration in the design of AC APS:
– requirement for independent multiple power sources of AC APS for crucial substations;
– reliability needs for load power supply, such as standby power supply for essential loads;
– operational factors;
– future expansion capabilities;
– financial constraints.
NOTE Country or regional standards and regulations can apply.
7.2 Power supply of LV AC APS
7.2.1 Power sources
The power source for LV AC APS typically includes the substation high-voltage bus, main
transformer tertiary winding, voltage transformer, grounding transformer, external power supply,
standby generator, renewable energy source, shunt reactor extra coil, and UPS.
Characteristics and relevant requirements of each power source are detailed below.
a) The LV side of the substation high-voltage busbar is preferred due to its high reliability and
easy availability.
b) Main transformer tertiary winding or auxiliary winding used as the power source should be
in compliance with the IEC 60076 series.
c) Voltage transformer (VT) also known as "loadable voltage transformer" can serve as a power
source for small substations.
d) In cases where the electric system is grounded by means of a grounding transformer (GT),
the extra coil of GT can be used as the power source.
e) An external power supply may be used as the power source by means of a dedicated line
or branch connecting to a power feeder, if the substation itself cannot provide sufficient
independent AC supplies for the APS. It can serve as the normal power source if it meets
long-term power supply requirements.
f) A standby power generator, typically a diesel generator set (DGS), may be installed in
substations lacking a reliable external power supply.
g) A mobile power generator, typically a diesel generator set (DGS), may supply essential
loads in case of emergency.
h) A renewable energy source may serve as the power source for LV AC APS if it prov
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