Presentation made at the Nigerian Institution of Electrical & Electronics Engineers (NIEEE) Lagos Chapter. This article gives the reader the basic knowledge of what sub-stations are, how they are designed and the factors considered at the design stage as well as the various protections used at sub-stations.

1. PRESENTATION AT NIEEE LAGOS CHAPTER
TITLE :- SUB-STATION DESIGN AND PROTECTION
AN OVERVIEW
BY :
AMEH MESSIAH EDACHE
(GRADUATE MEMBER NSE)

2. WHAT IS A SUB STATION?
A substation is an installation at which electricity is received from
one or more power stations for conversion from AC to DC,
transformation or switching before distribution by a low-tension
network.
•It is an assembly of electrical components such as bus bars, switchgear
apparatus, power transformers etc.
Sub-stations are an important part of the power system as the
form a link between generating stations, transmission stations
and distribution systems. It is therefore essential to exercise
utmost care while designing and building a sub-station.
•Substations are the convenient place to install various equipment for making
measurements to monitor the operation of various parts of the power system.

3. CLASSIFICATION OF SUB-STATIONS
• Service requirement and
• Constructional features.
There are several ways
of classifying sub-
stations. However, the
two most important
ways of classifying them
are according to
1. According to service requirement : sub-stations that fall under this class are used
to change voltage levels, improve power factor, convert AC power to DC power etc.
They are further classified into :
• Transformer sub-stations – these sub-stations transform voltage from
one level to another. Transformers are the main components in such
sub stations.
• Switching sub-stations – such sub-stations simply perform the switching
the switching operation of power lines. The voltage level at such sub-
stations remains the same i.e. incoming and outgoing lines have
same voltage. They are sometimes referred to as SWITCH YARDS.

4. CLASSIFICATION OF SUB-STATIONS…contd
• Power Factor correction sub-stations – such sub-stations are
generally located at the receiving end of transmission lines. These
sub-stations generally use synchronous condensers as the power
factor improvement equipment.
• Frequency Changer sub-stations – these sub-stations change the
supply frequency. Frequency change may be required for
industrial utilization.
• Converting sub-stations – these sub-stations receive AC power
and convert it into DC power.
Other examples under this classification are; Industrial sub-stations, etc.

5. CLASSIFICATION OF SUB-STATIONS…contd
2. Constructional Features : a sub-station has many components (e.g.
circuit breakers, instrument transformers, etc.) which must be housed properly
to ensure continuous and reliable service. They are further classified into:
• Indoor sub-stations : all the equipment of such sub-stations are
installed indoors i.e. within the sub-station building.
• Outdoor sub-stations : for voltages above 66kV, outdoor sub-stations
are generally used because adequate clearance is needed
between conductors, switches and other equipment.
• Underground sub-stations : in thickly populated areas where the
space is a major constraint, and cost of land is high, underground
sub-stations are used.
• Pole Mounted Sub-stations : this is an outdoor sub-station with
equipment installed overhead on a H pole or four (4) pole structure.
It is used for voltages not exceeding 11kV (and 33kV in some cases).

6. SOURCE : NATIONAL POWER TRAINING INSTITUTE OF NIGERIA (NAPTIN)

8. SUB-STATION DESIGN CRITERIA
Location – as far as possible, it should be located at the load center.
Capacity
Controllability
Reliability
Ease of Maintenance
Expansion (Load Growth)
Safety
Accessibility
Cost

9. SUB-STATION EQUIPMENT
1. TRANSFORMERS
2. CIRCUIT BREAKER
3. ISOLATING SWITCHES (ISOLATOR)
4. EARTH SWITCHES
5. INSTRUMENT TRANSFORMERS (CT & VT)
6. WAVE TRAP
7. REACTORS & CAPACITORS
8. INSULATORS & BUS BARS
9. SURGE ARRESTERS (LIGHTNING ARRESTERS)
10. METERING, INDICATING INSTRUMENTS AND
CONTROL PANELS
11. OTHERS (AUXILLIARIES)

10. A POWER TRANSFORMER
A transformer is used at a sub-
station to step-up or step-down
voltage to a required level.
Except at power stations, all
subsequent sub-stations use step-
down transformers.
The capacity of transformers in a
sub-station is chosen based on the
load requirement and anticipated
load growth.
Voltage control is done through
tap changers provided in power
transformers.

11. A circuit breaker is a device capable of
making or breaking a circuit under
normal (for operation and
maintenance purpose) and abnormal
(fault) conditions.
CB’s can be classified based on
• Rated Voltage (Low, Medium &
High voltage etc.)
• Medium of arc extinction
• Operating mechanism (Spring
charged, Pneumatic, Hydraulic,
combination of both)
• Construction (Outdoor CB –
fixed mount, Indoor CB – Draw
out mount)
• Structural form (Dead and Live
Tank).
A CIRCUIT BREAKER

12. AN ISOLATOR
An isolator or disconnecting switch
is essentially a knife switch which is
used to open a circuit under no load
condition.
Isolators help to ensure that an
electric circuit is completely de-
energized for service, test or
maintenance.
Isolators are generally used on both
ends of a circuit breaker to enable
personnel carry out repairs or
replacement of the breaker without
any danger.

13. EARTH SWITCHES
An Earthing switch is a device used to
discharge to earth any residual voltage
trapped on a line after it’s opening.
Earth switches are mounted on the
base of the line isolator. They are
normally vertical break switches and
have their contact arms aligned
horizontally in open position.
The sequence of operation while
opening and closing a circuit is
While opening:
 Open circuit breaker
 Open isolator
 Close earth switch
While closing:
 Open earth switch
 Close isolator
 Close breaker.

14. A CURRENT TRANSFORMER
A VOLTAGE TRANSFORMER
Instrument transformers are used in
power systems to
 Protect personnel and apparatus from
high voltage and large currents.
 Transfer voltages and currents in the
power lines to values which are
convenient for the operation of
protective devices and measuring
instruments.
They are classified as
• Protective transformers
• Metering Transformers
Normally both functions are usually
combined in one hence the general term
INSTRUMENT TRANSFORMERS.
There are majorly two main types namely
 Voltage transformers
 Current transformers

15. Wave traps are also known as a LINE TRAPS.
They are connected in series to transmission
lines and are a critical component of Power
Line Carrier Communication (PLCC) systems.
They are used to filter high frequency
communication signals sent on the line from
remote substations and divert them to the
telecom/tele-protection panels in the sub
station control room (through CCVT and Line
matching units LMU).
This is relevant in (PLCC) systems for
communication among various substations
without dependence on the telecomm
company network.
PLCC is the technology used for
communication, tele-monitoring and
protection between substations through high
voltage power lines.

16. A SHUNT REACTOR
SHUNT REACTOR
This are installed to control high
voltages that occur especially at
night due to the capacitive effect of
lightly loaded transmission lines and
also when a sudden loss of a block of
customer load occurs.
SHUNT CAPACITORS
These are installed to provide MVARS
to the system to help support voltage
levels. Shunt capacitors also help to
improve power factor by making
current to lead source voltage.
SERIES REACTORS
These are used as current limiting
reactors (decrease short-circuit
currents) to increase the impedance
of the line; thereby improving the
transmission capacity of power lines.
SERIES CAPACITORS
They are installed in very long
transmission lines to improve system
stability, reduce system losses and
optimize power flow.

17. Insulators : they are basically used to support conductors (or bus bars) and to confine the
current to the current to the conductors. The most commonly used material for the
manufacture of insulators is porcelain. There are several types of insulators (e.g. pin type,
disc type, suspension type, post insulator etc.) and their use in the sub-station depends on
the service requirement.
Bus Bars : this is a conductor or an assembly of conductors used to connect incoming and
outgoing lines in a sub-station. They are copper or aluminium bars and operate at constant
voltage.

18. Electrical equipment and devices in
power stations and sub-stations are
subject to over voltages which affect
their insulations. These over voltages
may be caused by system faults,
switching on and off of lines and
equipment as well as lightning
phenomena.
Over voltages are classified as
• Atmospheric Over voltages –
caused by lightning.
• Switching Over voltages – due to
system operations like switching off
transformers on no load etc..
• Temporary Over voltages – mainly
due to load rejection and or single
phase to ground faults.
Lightning arresters (LA’s) were basically used to protect systems from lightning in the past but in
recent times, due to development in power systems, EHV and UHV systems came into existence.
Thus switching over-voltages (opening and closing of breakers etc.) became so severe and since
LA’s also protect systems from switching over-voltages, the word Surge arresters is now been
widely used to refer to them.

19. There are several control,
metering and indicating
instruments being used at sub-
stations [e.g. relays,
voltmeters, ammeters, energy
analyzer (multifunction meter
MFM), frequency meter, etc.].
They are installed to enable
constant monitoring and
control of system parameters
for reliability of power supply.
A CONTROL ROOM

20. SUB-STATION CONFIGURATIONS
There are several bus bar configurations that can be used in a sub-station.
The choice of configuration depends upon various factors such as voltage
level, reliability, position of sub-station, cost etc.
Certain configurations may be more suited for a specific task hence the
equipment in each type of sub-stations may vary.
Typical Bus Configurations
 Single Bus
 Single Bus with sectionalisation
 Main and Transfer Bus
 Ring Bus
 Breaker and a-Half
 Double Breaker Double Bus

21. SINGLE BUS CONFIGURATION
This is the simplest type of bus bar configurations and has the least reliability. It consists
of a single bus bar and has both incoming and outgoing lines connected to this bus. It
can be constructed in either low or high profile arrangement depending on the space
available.
Advantages
• Low cost
• Less space
• Simplicity of operation and
maintenance
• Easily expandable
Disadvantages
• Low reliability
• Complete Interruption of supply is
needed for maintenance
• Can not be used for voltages
exceeding 33kV. (The indoor 11kV
sub-station often uses this bus
configuration)

22. SINGLE BUS WITH SECTIONALISATION
This is basically an extension of the single bus configuration. Here, two single bus
sections are connected together with a center circuit breaker (sometimes referred to
as bus-coupler) which maybe normally open or normally closed as the case maybe
and load is often equally distributed on both sections.
Advantages
• Flexible operation
• Isolation of bus sections for
maintenance
• Only a section is affected in event
of breaker failure or bus fault.
• Low maintenance cost.
Disadvantages
• Higher cost as compared to single
bus due to additional breaker.
• Maintenance without interruption
of supply is not possible.

23. MAIN AND TRANSFER BUS SYSTEM
This bus bar configuration system consists of two separate and independent buses; a
main bus and a transfer bus. Normally, all circuits (incoming and outgoing) are
connected to the main bus. For the purpose of repair or maintenance, the associated
circuit can be connected to the transfer bus (the circuit has been “transferred”) while
the main bus is isolated from the system.
Advantages
• Low cost
• Small area of land is required
• Maintenance can be carried out
without total loss of supply.
• Easily expandable
Disadvantages
• An extra circuit breaker is required
for bus tie.
• Relay protection is often
complicated
• Bus fault causes entire loss of
supply

24. RING BUS SYSTEM
This bus configuration is an extension of the sectionalized bus system. In this
system, there is a closed loop on the bus with each section separated by a
circuit breaker. This provides greater reliability and allows for flexible
operation.
Advantages
• Flexibility of operation
• High reliability
• Each circuit is double fed.
• Maintenance without circuit
disruption
Disadvantages
• An extra circuit breaker is required
for bus tie.
• Complexity of protection
• High cost

25. BREAKER AND A HALF SYSTEM
This bus configuration has two buses that are both energized during normal
operation unlike the main and transfer bus scheme. In this system, every two
circuits has three circuit breakers with each circuit sharing a common center
breaker. This enables maintenance to be carried out without total shut down.
Advantages
• Flexibility of operation
• High reliability
• Each circuit is double fed.
• Maintenance without circuit
disruption
Disadvantages
• An extra circuit breaker is required
• Complexity of protection
• Each circuit requires its own
potential source for relaying.
• High cost of installation

26. DOUBLE BREAKER DOUBLE BUS
This bus configuration also has two buses that are both energized during
normal operation like the breaker and a-half scheme above. In this system,
each circuit requires two breakers unlike the breaker and a-half that shares a
breaker. Incoming and outgoing feeders can be taken from any of the buses.
Advantages
• Flexibility of operation
• High reliability
• Each circuit is double fed.
• Maintenance without circuit
disruption
Disadvantages
• The high reliability comes with the
cost of additional breakers thus,
this bus scheme is usually used at
large generating stations.
• The most expensive bus
configuration.

27. SUB-STATION PROTECTION

28. WHAT IS SYSTEM PROTECTION?
Power system protection is the art and science that deals with the identification of
faults in the system and isolation of the faulty parts of the system.
The main purpose of a protection scheme is to ensure power system stability by
isolating the unhealthy part of the system (i.e. components under fault), whilst keeping
the healthy part of the system in operation.
The devices used to protect power systems from faults are called protection devices.
COMPONENTS OF PROTECTION SYSTEMS
Protection systems usually consists of the following components:
• Fuses
• Instrument transformers
• Relays
• Circuit breakers
• Batteries – DC source for operation of control circuits.
• Trip Circuits – the control circuit for opening and closing of switchgears.
• Communication channels/pilot cables.

29. WHAT IS A FAULT?
A fault could simply be defined as an abnormal condition that hinders the normal flow
of current in the power system.
Faults in the power system usually occur due to breakdown of equipment insulation.
Faults in a 3-phase power system can be classified into two main categories :
1. Symmetrical faults – these are faults that give rise to symmetrical fault currents
(i.e. faults with equal phase displacement of 1200). Three phase faults are
symmetrical faults. The most common example of symmetrical faults is when all
three conductors of a 3-phase line are brought into a short circuit condition.
2. Unsymmetrical faults – these are faults that give rise to unsymmetrical fault
currents (i.e. faults with unequal phase displacements). Unsymmetrical faults may
occur in such forms as; single phase to ground, two phase to ground, phase to
phase and three phase to ground.
The most common in occurrence of all the fault conditions listed above is the single
phase to ground fault which is about 70 - 80% of faults in the power system.
FAULTS IN POWER SYSTEM

30. CHARACTERISTICS OF FAULTS
A fault is characterized by :
1. Magnitude of the fault current
2. Power factor or phase angle of the fault current.
The magnitude of fault currents depends on the following factors :
a) The capacity and size (magnitude) of the generating sources feeding into
the fault.
b) The system impedance up to the point of fault or the source impedance
behind the fault.
c) The type of fault.
d) System grounding, number and size of overhead ground wires.
e) Fault resistance or resistance of the earth in the case of ground faults and
arc resistance in the case of both phase and ground faults.

31. CHARACTERISTICS OF FAULTS..contd
The phase angle (fault angle) of a fault current is the angle of the fault
current relative to it’s respective voltage.
The phase angle is dependent on :
a) For phase faults – the nature of the sources and connected circuits
up to the fault location
b) For ground faults – the type of system grounding and also the nature
of the sources and connected circuits up to the point of fault.
The current will have an angle of 80 to 850 lag for a phase fault at or near
generator units. The angle will be less out in the system, where lines are
involved.
The importance of this phase angle is only in distance relay applications.

32. FAULT CALCULATIONS
Fault calculations are done for the following reasons:
a) To select the appropriate protection scheme and relay settings.
b) To determine the maximum fault currents that can occur in sub-stations and hence
choose the appropriate rating of circuit breaker to be used.
c) To select the type of circuit breaker to be deployed based on the nature and type of
fault.
d) For proper co-ordination of the relays in the overall protection scheme of the system.
Fault are calculations are also done to meet future requirements such as:
a) Expansion schemes of the system when new generator units or power plants are
added to the system
b) Construction of new transmission lines to evacuate power.
c) Construction of new lines to meet load growth
d) Construction of interconnecting tie lines.
Basically, there are two approaches to fault calculations which are:
a) Actual reactance or impedance method
b) Percentage reactance or impedance method or per unit (p.u) reactance or
impedance method.

33. ZONES OF PROTECTION
Zones of protection are mapped out in power system to limit the extent of the
power system that is disconnected when a fault occurs in the system. Zones of
protection are made to overlap to ensure that no element or part of the
system is left unprotected.
Zones of protection guarantees total protection of power system sub circuits
and the mapping follows common logical boundaries to cover elements of the
system such as Generators, Transformers, Motors, Buses, Reactors, lines etc. as
well as combinations of these elements.
For zones of protection to be effective and genuine, there must be measuring
devices such as CT’s and VT’s and isolating devices such as breakers.
A zone boundary is usually defined by a CT and a CB.
 The CT provides the ability to detect a fault inside the zone
 The CB’s provide the ability to isolate the fault.

34. ZONES OF PROTECTION…contd
Other advantages of zones of protection includes:
1. Zones can have their unique protection schemes
2. Enables proper coordination of the relays in the system
3. Localization of faults
4. Effective back-up protection
POWER SYSTEM PROTECTION ZONES

35. ZONES OF PROTECTION…contd

36. ZONES OF PROTECTION…contd
The various types of protections used in the power system can be classified under two
major classes which are:
1. Primary protection
2. Secondary protection (backup protection)
1. Primary protection : - as the name implies, it is the main protection being used in a
zone of protection. The relays used for this kind of protection are called primary
relays. These relays are the first line of defense in the system and are generally high
speed relays. The primary protection scheme is expected to remove minimum
equipment from service.
2. Secondary protection : - secondary protection is generally referred to as backup
protection. This is basically an alternative protection system which operates when
the primary protection fails. The relays used here are referred to as secondary or
back up relays and their operation time is set to have a delay so as to allow primary
relays operate first. Backup protection isolates more equipment from service.
Backup protection comes in overlapping zones
Primary protection and backup protections are independent of each other.

37. TRANSFORMER PROTECTION ZONE
In this zone, the primary protection used is the differential
protection. Over-current and earth fault (unrestricted) protections
are the backup protections used in this zone. The usual faults that
can occur within this zone are :
Winding faults
Inter turn faults
Phase to phase faults
Phase to ground faults.
For the above mentioned faults, differential protection is used to
protect the transformer against such faults. Other forms of
protection used in this zone are Restricted earth fault protection,
Oil/Winding temperature relays, Bucholz relays (for oil filled
transformers) and Sudden pressure relays (for hermetically sealed
transformers) etc.

38. BUS PROTECTION ZONE
The usual protection used in this zone is the differential protection.
The most common fault that occurs within this zone is the phase to
ground fault which is generally a result of flash over on insulators
caused by lightning.
Buses usually do have several lines and feeders connected to them
hence the CT’s on the bus easily gets saturated due to these lines.
This makes differential protection the best form of protection for
effective protection of buses.

39. LINES PROTECTION ZONE
Lines are the means by which electric power is conveyed from generating
stations to the load points. Lines are used in the transmission and distribution
systems and these lines run into thousands of kilometers.
Trees, lightning, animals natural disasters, weather (wind, snow, ice) etc. are
responsible for most of the faults that occur in this zone.
The four major types of faults in this zone includes : line to ground, line to line,
double line to ground and three phase faults.
The following types of protection are used to protect transmission lines :
 Distance protection – this is the primary protection
 Differential protection (not used for long lengths)
 Instantaneous/inverse time over current (directional)
 Instantaneous/inverse time over current (non-directional)
 Current balance
 Pilot wire using communication channels
Distribution lines are effectively protected using over-current and earth fault
protection schemes as well as fuses.

40. SUB-STATION EARTHING
Earthing can simply be defined as the connection of an electrical system to the
general mass of earth. The importance of earthing in power systems can not be
overemphasized as it offers a high degree of safety to the system. This is why
earthing is done at every part of the power system (from generating stations to
consumer premises).
One of the main objectives of earthing electrical systems is to provide a
common reference potential for the power supply system, building structure,
plant steelwork (gantries), cable ladders and trays etc. To achieve this objective,
a suitable low resistance connection to the general mass of earth is required.
A low resistance connection is dependent on factors such as:
 Soil resistivity
 Stratification
 Moisture and chemical content of the soil
 Size and type of electrode used
 Depth to which the electrode is buried etc.

41. SUB-STATION EARTHING…contd
The subject of earthing in a power systems can broadly be classified into two
areas which are :
1. General earthing
2. Neutral earthing
1. General earthing : - this refers to the earthing of all metal parts (which do not
normally carry current) of apparatus and equipment used in electrical
systems. Examples of general earthing are; earthing of transformer tanks,
circuit breakers, gantries etc.
The main objectives of having general earthing in electrical systems are:
a) to provide protection for equipment and personnel due to
accidental grounding of equipment.
b) to provide a safe working environment for personnel while working so
as to prevent electrostatic and electromagnetic induction as well as
accidental contact with energized apparatus.

42. SUB-STATION EARTHING…contd
2. Neutral earthing : - this refers to the intentional connection of the star-point or
neutral point of a generator or transformer windings to the general mass of
earth.
The objective of having neutral earthing is to provide as near as possible a
surface under and around a sub-station which shall be at uniform potential.
The advantages of having neutral earthing at sub-stations includes :
a) General safety for personnel and equipment
b) Effective operation of Earth Fault Relays (EFR’s)
c) Unnecessary tripping of CBs are prevented hence improved system
reliability and stability.
d) Quick discharge path for over-voltages due to switching and lightning
hence limiting the effects of over-voltages in the system.
e) Stable neutral point

43. SUB-STATION EARTHING…contd
The various methods of neutral earthing are :
a) Direct Earthing or Solid Earthing – connection of a suitable conductor.
b) Impedance Earthing – neutral connection through an impedance path
(resistance or reactance). This could further be classified into the following
I. Resistance Earthing – connection of the neutral point through a resistive
element. E.g Solid-state resistor or Earth-liquid filled resistor.
II. Reactance Earthing – a reactance element is used here.
III. Resonant Earthing (Arc Suppression coil Earthing) – this makes use of a
tuned reactance or Peterson Coil.
Conductors used for earthing are – Copper, Mild steel, Aluminium and
galvanized iron rods.

44. SUB-STATION EARTHING…contd
Until recently, the concept of good earthing was simply to obtain an earth
resistance as low as possible. However in systems where the ground fault currents
are excessively high, keeping the ground potential within safe limits even with a
low resistance earth value is almost impossible.
Modern research brought up the concept of voltage gradient control under
ground fault conditions so as to keep the potential difference between safe limits
and prevent danger to personnel and equipment. This led to the development of
the grid-mesh method of earthing which is the present day practice used in sub-
stations.
This method is so useful in sub-stations because a multiple number of items can
be connected to the grid at various locations and the grid mesh provides a
good earth irrespective of the injection point of the fault current.

45. REFERENCES
 V.K.MEHTA, ROHIT MEHTA, Principles of Power System pg. 570-580.
 National Power Training Institute of Nigeria (NAPTIN) Graduate Skill Development
Programme (NGSDP) Power system protection course manual.
 "DIFFERENT TYPES OF ELECTRICAL CIRCUIT BREAKERS"
(http://electricalengineeringtutorials.com/different-types-of-electrical-circuit-
breakers/)
 “LINE TRAPS/WAVE TRAPS" (http://www.qualitypower.com/line-trap.html)
 "SERIES CAPACITORS" (https://www.scribd.com/document/292581330/NOKIAN-
Capacitors)
 GOPALA KRISHNA PALEPU ADE/MRT (PROTECTION), Different Bus bar Configurations
(https://www.slideshare.net/BoopathiThambusamy/busbar-configuarations)
 Dr.Prof. Mohammed Tawfeeq, Power System Protection Part 2 (Zones of Protection)
(http://www.philadelphia.edu.jo/academics/mlazim/uploads/Power%20System%20pr
otection%20-%20Part%2002.pdf)

46. REFERENCES
 "PROTECTIVE ZONES IN POWER SYSTEM"
(http://www.studyelectrical.com/2016/09/protective-zones-in-power-systems.html)
 "ELECTRICAL BUS SYSTEM AND ELECTRICAL SUBSTATION LAYOUT"
(https://electricalstudy.sarutech.com/electrical-bus-system-and-electrical-
substation-layout/index.html)
 "ELECTRICAL BUS-BAR AND ITS'S TYPES" (https://circuitglobe.com/electrical-bus-bar-
and-its-types.html)
 "UTILITY TRANSFORMER POWER SYSTEM PROTECTION"
(http://eecatalog.com/blog/2017/02/21/utility-transformer-power-system-
protection/)