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This is the maximum load on a system
during any specified period (half-hour
Since electrical energy can not be stored
on a significant scale it is also, by
definition the maximum output of the
system during that period.
Load Factor, L
It is always less than 1
A typical value for load factor, L is 0.35
Load factor, depending on the
requirements of consumers, is not under
the direct control of the supply authority
but it has a considerable effect on the
ultimate cost per unit of electricity.
Total capital expended on plant, as stated
previously forms a standing charge.
Average load factor in the UK?
The average load factor of generation
plants in the UK was 66.3% in 2004.
This means that, on average, the output
from power stations during 2004 was
66.3% of the total maximum possible
from every plant.
In ac circuits: Power (watts) = volts x amps x power factor
i.e. P = VI cos ϕ
With constant voltage V, it will be seen that a low power factor will
require a high current to flow for a given power, but current will be
reduced if power factor is improved, i.e. brought nearer to unity.
Thus a low power factor causes excessive current to be drawn for a
given power and this in turn will require plant and equipment of a
higher current rating. It is for this reason that plant and equipment
are rated in kVA and not kW.
To enable plant rating to be kept within rating and also have spare
capacity, the supply authority encourages consumers to improve
their power factor by having the maximum demand charge per kVA
instead of kW.
Thus for a given kW power demand, a low
power factor will give a high kVA
maximum demand charge.
This is the usual method of including
power factor into the tariff, but sometimes
the tariff will include a penalty clause for
poor power factor working and sometimes
a Varmeter will be installed and a charge
made for reactive kilovolt amperes.
Why improve Power Factor ?
Improving a systems power factor will reduce the total power
consumed by an electrical installation and will provide the following
Financial saving - By reducing power consumed electricity costs are
Extended equipment life - Reduced electrical burden on cables and
Increase load capacity - Provide additional capacity for other loads
to be connected
Environmental benefit - Reduced power consumption means less
“Greenhouse” gas emissions and fossil fuel depletion by power
Diversity Factor (D)
As stated earlier, the total charge is based on a standing charge and
a running cost charge, i.e. a two part tariff..
The standing is determined by plant size and that in turn, is
determined by the load connected by the consumers after applying
a diversity factor.
This factor takes into account the fact, that it is most unlikely that
all connected plant will be in use at any one time.
D = sum of consumer demands
Actual system maximum demand
The figure is always greater than 1 (typically 2.5)
The cost of building the scheme is obtained by
It is usually possible to borrow the money at a fixed rate
of interest. The cost per annum in interest is thus:
Capital cost (amount borrowed) x rate of interest (%)
Thus if 1M was borrowed at 10% p.a. the total cost in
interest p.a. would be £1,000,000 x 0.1 = £100,000
Depreciation Cost (1)
Since equipment will wear out and eventually
have to be replaced, a “sinking fund” must be
established to pay for repairs and replacement.
In order to arrive at a p.a. value for this, the
“lifetime” of the equipment must be estimated.
The cost p.a. in depreciation is thus:
Depreciation Cost (2)
Thus equipment costing £1 M with an estimated lifetime of 20 years
would require a “sinking fund” of p.a.
£1,000,000 = £50,000 p.a.
In this way, a sum of £1 M would be built up to enable either
(a) the equipment to be replaced, or
(b) the amount borrowed to be repayed, at the end of the equipment’s
Hence the total transformer costs per annum are:
Interest costs + depreciation costs
In the above example these amount to £150,000 p.a. or a total %
p.a. of 15% for interest and depreciation.
In UK, electricity is generated/produced at power generating stations at
25 kilovolt (KV) potential, in a 3 phase supply at 50 cycles per second
Thereafter it is processed by step-up transformers to 132, 275 or 400 kV
before connecting to the “National Grid”.
Vast and complex network of overhead lines and underground cables
carrying power at high voltages to centres of high load density.
Energy at high voltages transmitted by the National Grid system is then
fed into grid substations for the transformation of the transmission
voltage to secondary transmission voltages of 132 KV and 33 KV.
Secondary transmission is for large consumers
such as factories and those in areas of high load
Further substations reduce the secondary
transmission voltages to 11kV.
The term 'distribution' is usually used to refer to
the feeding of electrical energy through
overhead lines and underground cables to
supply small industrial, commercial and domestic
Comply with the requirements of the Electricity
This states that the supply authority must
constantly maintain the type of current (dc or
ac), the frequency and the declared voltage.
Frequency, however, may be varied by 1%; the
declared voltage can be allowed to vary by 6%.
The Electricity Supply Acts
The Electricity Supply Acts defines the voltages of
Extra-low voltage 30V (ac) or 50dc
Low Voltage 250V or less
Medium Voltage Between 251V and 650V
High Voltage Between 651 and 3000V
Extra-high Voltage Exceeding 3000V
The figure below summarises the pattern of electricity
generation, transmission and distribution today .
Introduction to Power
An electrical network initiates at the point of
Electrical power is generated by converting the
potential energy available in certain materials
into electrical energy.
This is either done by direct conversion of kinetic
energy, e.g. wind or water turbines, or creating
steam to drive the turbines, e.g. coal or nuclear
Generation of Electrical Powers
The electrical powers generated are either transferred onto a bus to
be distributed (small scale), or into a power grid for transmission
purposes (larger scale).
This is done either directly or through power transformers,
depending on the generated voltage and the required voltage of the
bus or power grid.
Power transmission, whereby the generated electrical potential
energy is transmitted via transmission lines, usually over long
distances, to HV substations.
HV substations will usually tap directly into the power grid, with two
or more incoming supplies to improve reliability of supply to that
substation’s distribution network.
Electrical transmission is normally done via high to extra high voltages, in
the range of 132 – 800 kV.
Mega volt systems are now being developed and implemented in the USA.
The longer the distance, the more economical higher voltages become.
Normally, the transmission voltage will be transformed at the HV substation
to a lower voltage for distribution purposes.
This is due to the fact that distribution is normally done over shorter
distances via underground cables.
The insulation properties of three-phase cables limit the voltage that can be
utilized, and lower voltages, in the medium-voltage range, are more
economical for shorter distances.
Schematical illustration of a
typical Power Grid.
Critical Medium Voltage
Critical medium-voltage (MV) distribution
substations will generally also have two or more
incoming supplies from different HV substations.
Main distribution substations usually supply
power to a clearly defined distribution network,
for example, a specific plant or factory, or for
Power distribution is normally done on the
medium-voltage level, in the range of 6.6
– 33 kV.
3-ph power is transferred, mostly via
overhead lines or 3-core MV power cables
buried in trenches.
Single-core-insulated cables are also used,
although less often.
LV distribution is also done over short
distances in some localized areas
Power Distribution Network
A power distribution network will therefore typically
include the following:
HV/MV power transformer (s) (secondary side)
MV substation and switchgear
MV power cables (including terminations)
MV/LV power transformer (s) (primary side)
The distribution voltage is then transformed to low
voltage (LV), either for lighting and small power
applications, or for electrical motors, which is usually fed
from a dedicated motor control center (MCC).
Typical Power Distribution
Voltage levels are defined internationally, as follows:
Low voltage: up to 1000 V
Medium voltage: above 1000 V up to 36 kV
High voltage: above 36 kV
Supply standards variation between continents by two general
standards have emerged as the dominant ones:
IEC governs supply standards
The frequency is 50 Hz and LV voltage is 230/400 V
In North America
IEEE/ANSI governs supply standards
Frequency is 60 Hz and the LV voltage is 110/190 V.
Overhead lines are far cheaper than underground cables for long
distances, mainly due to the fact that air is used as the insulation
medium between phase conductors, and that no excavation work is
Support masts of overhead lines are quite a significant portion of
the costs, that is the reason why aluminum lines are often used
instead of copper, as aluminum lines weigh less than copper, and
are less expensive.
Copper has a higher current conducting capacity than aluminum per
square mm, so once again the most economical line design will
depend on many factors.
Overhead lines are by nature prone to lightning strikes, causing a
temporary surge on the line, usually causing flashover between
phases or phase to ground.
Line insulators are normally designed to relay
the surge to ground, causing the least disruption
This is of short duration, and as soon as it is
cleared, normal operation may be resumed.
Sophisticated auto-reclosers are employed on an
increasing number of overhead lines.
Overhead Lines - Advantages
Less expensive for longer distances
Easy to locate fault.
Overhead Lines - Disadvantages
More expensive for shorter distances
Susceptible to lightning
High level of expertise and specialized
equipment needed for installation.
Underground Cable Installations
Underground (buried) cable installations are
mostly used for power distribution in industrial
They have the following properties:
Less expensive for shorter distances
Not susceptible to lightning
Not maintenance intensive.
Underground Cable Installations
Expensive for long distances
Can be difficult to locate fault.
A substation can be defined as any
premises or part of premises in which
electrical power is transformed or
converted to or from high voltage or
which contains high voltage switchgear.
Particular types of substation can be
identified as follows:
(1) Distribution substation (DSS)
A substation which has a function of
distribution only, with or without voltage
Any substation which controls an incoming
Supply of electrical energy from another
system or from the electricity company is
excluded from this category.
(2) Intake substation (ISS)
A substation, with or without voltage
transformation, which has the function of
controlling the incoming supply of
electrical energy from another system or
(3) Standby substation (SBSS)
A substation, with or without voltage
transformation, which has the function of
controlling the supply of electrical energy
from standby generators
Siting an Intake Substation
For large buildings or small sites where the ISS is the only substation its ideal location
is at the load centre.
In practice, difficult to achieve as account must be taken of: Physical and structural
Need for access for conveying and unloading (and replace at a later date) of heavy
equipment, e.g. switchgear and transformers
Fire and explosion risks, especially where oil immersed equipment is to be used.
In most large buildings, ISS will generally be located at ground floor, although with
the introduction of non-oil equipment high level substations are becoming more
Small sites a compromise for the ISS position has to be sought between the load
centre, access problems and the point of supply from the electricity company.
In large sites where the ISS performs its controlling function in association with one
or more DSS's its siting is less critical.
In the majority cases it will be near the site boundary and positioned to take account
of access and point of supply from the electricity company.
Siting a Standby substation
More complex than that of an ISS
depends mainly on whether there is to be a single large central standby or multiple
small individual standby supplies.
Where the decision is to utilise multiple small LV generating sets then these will be
sited at individual load and will not truly be standby substations.
With large central standby facilities the ideal situation, considering its control and
electrical protection, is to combine the ISS and SBSS.
However this may not be possible due to the following:
Noise problems associated with generators:
Desire to keep the two sources of supply (REC and STANDBY) electrically and
physically remote to ensure that the loss of any noise, however catastrophic and
cause, does not affect the other.
In determining the ideal position the 'factors to be considered are as follows:
As for ISS
The need to store and handle fuel for the prime mover
Siting a distribution substation
Ideal position for a Dist. Substation is at the load
centre of the area it is to serve.
Seldom possible to achieve and in practice a site
should be chosen that gives the best compromise
between the following:
Siting of the various loads
Access road for heavy equipment
Fire and explosion risks
The substation forms a node point in the electric
Substation equipment :
Transformer to change the voltage and current level.
Circuit breaker (CB) to interrupt the load and fault current. The fault
current automatically triggers the CB.
Disconnect switch to provide visible circuit separation. Permit CB
maintenance. No load operation.
Voltage and current transformers to reduce the current to 5 A,
and the voltage to 120 V, and to insulate the measuring circuit from
the high voltage
Surge arresters for protection against lightning and switching
overvoltages. They are voltage dependent, non linear resistance.
Arial View of a Substation
Substation Circuit Diagram
Supply S Transmission
CBA 4 CBA 1 switch
CBA 5 CBA 2
CBA 6 CBA 3 Disconnect
Bus 2 assembly (CBA)
A CB is a switching device built ruggedly
to enable it to interrupt/make not only the
relatively large load current, but also the
much larger fault current which may occur
on a circuit.
These are designed for asymmetrical
faults, which are more severe than the
symmetrical faults due to dc off-set of the
Circuit Breaker Concept
Moving Arc Fixed contact
SF6 Circuit Breaker
SF6 Circuit Breakers
The superior arc-quenching ability of SF6
gas can be attributed to the fact that it is
electronegative, which means that its
molecules rapidly absorb the free electrons
in the arc path between the breaker
contacts to form negatively charged ions
which are ineffective as current carriers.
Advantages of SF6 Circuit Breakers
Reliable current interruption, no restriking
Closed gas circuit keeps interior dry, so that
there is no moisture problems
Little erosion because of short arc time
No carbon deposit
As the CB is totally enclosed and sealed
from atmosphere, suitable for use in coal
mines; explosive hazard areas.
69 kV substation
Disconnect Circuit Disconnect
500 kV Circuit breaker
Shunt reactor protected by Lightning
Distribution line (4.2-45 kV)
Wood tower with cross arm. The Typical distribution line
wood is treated against rotting.
Simple concrete block foundation
or no foundation.
Small porcelain or plastic post
The insulators shaft is grounded
on important lines to eliminate
leakage current causing wood
Simple rod grounding.
Shield conductor is seldom used.
Radial Distribution system
Feeder 1 Feeder 4
Radial Feeder Fuse
Line Cable and
Sub-transmission and Distribution line
Fuse and disconnector
Distribution line 13.8 kV
Distribution Cable 13.8 kV