Medium Voltage
Content
The electrical performance of a distribution system is accessed
by energy loss and voltage drop from the input to the various
points from which the loads are taken . Loss is minimized by
distributing at high voltage as it is economic for the power
involved and the capital cost of the lines is also minimized by
using lines of smaller cross - section and greater resistance
than would be permissible with a lower distribution voltage .
As an example , the effect of increasing the voltage of a given
distribution m-times may be considered . To distribute the same
power , the current becomes (1/m) - times its former value and
the cross - section and weight of conductor . (1/m2) - times the
former value with a corresponding saving in capital cost : The
limiting factors are the disproportionately greater cost of
higher voltage overhead lines and cables and the fact that
power finally supplied to a consumer must be at consumer
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voltage , which is determined by safety considerations . The
most usual methods of distribution , the 3 - phase four - wire
system which does in fact show a higher ‘ copper efficiency‘
than distributing single - phase with two or even three wires .
The transmission and sub-transmission normally takes place
mainly at the 3-phase line voltage while the consumers are
supplied with single - phase at the phase voltage . This system
consists of three lines , at the line voltage , with a fourth wire
which is at or near earth potential , called the neutral .
Consumers are supplied with single - phase at the phase voltage
by connecting them between line and neutral. If these loads are
balanced between the phase , the lines carry equal currents , the
neutral carries zero current and the whole distribution takes
place at line voltage which is 3 - times the consumer voltage .
Even if the load balancing is not perfect there is only a small
neutral current , equal to the phasor difference of the three line
currents .
(7.2) Types of Distribution Systems :
An attempt to classify distribution systems must take
account of the fact that continual growth of demand has
necessitated corresponding extension of existing systems so the
initial plan may no longer be evident . However , it is
recognized that a distribution system may take one of two
forms , either radial or ring - main , depending on the layout of
112
feeders and distributors . Figure (7.1) shows four feeding points
supplied from a substation S . In (a) the feeders F supply the
feeding points in series , round a ring , while in (b) the feeders
go out radialy from the substation , each feeder supplying single
feeding point . In the former case the distributors may
themselves form rings or may be connected to a feeding point at
both ends . With (b) the distributors are normally radial from
the feeding points but it is sometimes possible for a distributor
to be connected between two feeding points and thus fed from
both ends which , as can be shown , gives minimum loss and
voltage drop for a given loading and therefore greatest
economies of copper by minimizing the necessary cross
sectional area of the conductor . It is possible , either with ring
feeders or ring distributors , to reduce the voltage drop at a
particular feeding point or load by establishing a direct
connection across the ring , between two feeding points or two
loads . This effect superimposes a radial arrangement on a ring
arrangement and makes an interconnected system which , for
purposes of calculation , must be dealt with as a single
network , it being no longer possible to deal with each branch
separately owing to the redundant branch which the
interconnector forms .
With an alternating current supply , the feeders in either a
ring or radial arrangement are often at a higher voltage than
the distributors , necessitating feeders , feeding points and
113
distributors . All the consumer voltage may have a high voltage feeder system associated with it , supplying the
consumer voltage network at a number of transformer
substations . In a large urban area , a three - voltage system is
normal with a 11 kV ring main. It consists of feeders supplying a
number of 11/0.380 kV transformer substations which are the
feeding points for the 380 V distribution system .
This system supplies the final voltage feeding points which
can be the distribution boxes in the streets , from which the
power , is conveyed by the distributors to the individual
consumers . Such a system would be true 3-phase (three lines or
cores only) for the 11 kV portions , with 3-phase four - wire for
the consumer voltage 380 V distributors to provide the
114
by energy loss and voltage drop from the input to the various
points from which the loads are taken . Loss is minimized by
distributing at high voltage as it is economic for the power
involved and the capital cost of the lines is also minimized by
using lines of smaller cross - section and greater resistance
than would be permissible with a lower distribution voltage .
As an example , the effect of increasing the voltage of a given
distribution m-times may be considered . To distribute the same
power , the current becomes (1/m) - times its former value and
the cross - section and weight of conductor . (1/m2) - times the
former value with a corresponding saving in capital cost : The
limiting factors are the disproportionately greater cost of
higher voltage overhead lines and cables and the fact that
power finally supplied to a consumer must be at consumer
111
voltage , which is determined by safety considerations . The
most usual methods of distribution , the 3 - phase four - wire
system which does in fact show a higher ‘ copper efficiency‘
than distributing single - phase with two or even three wires .
The transmission and sub-transmission normally takes place
mainly at the 3-phase line voltage while the consumers are
supplied with single - phase at the phase voltage . This system
consists of three lines , at the line voltage , with a fourth wire
which is at or near earth potential , called the neutral .
Consumers are supplied with single - phase at the phase voltage
by connecting them between line and neutral. If these loads are
balanced between the phase , the lines carry equal currents , the
neutral carries zero current and the whole distribution takes
place at line voltage which is 3 - times the consumer voltage .
Even if the load balancing is not perfect there is only a small
neutral current , equal to the phasor difference of the three line
currents .
(7.2) Types of Distribution Systems :
An attempt to classify distribution systems must take
account of the fact that continual growth of demand has
necessitated corresponding extension of existing systems so the
initial plan may no longer be evident . However , it is
recognized that a distribution system may take one of two
forms , either radial or ring - main , depending on the layout of
112
feeders and distributors . Figure (7.1) shows four feeding points
supplied from a substation S . In (a) the feeders F supply the
feeding points in series , round a ring , while in (b) the feeders
go out radialy from the substation , each feeder supplying single
feeding point . In the former case the distributors may
themselves form rings or may be connected to a feeding point at
both ends . With (b) the distributors are normally radial from
the feeding points but it is sometimes possible for a distributor
to be connected between two feeding points and thus fed from
both ends which , as can be shown , gives minimum loss and
voltage drop for a given loading and therefore greatest
economies of copper by minimizing the necessary cross
sectional area of the conductor . It is possible , either with ring
feeders or ring distributors , to reduce the voltage drop at a
particular feeding point or load by establishing a direct
connection across the ring , between two feeding points or two
loads . This effect superimposes a radial arrangement on a ring
arrangement and makes an interconnected system which , for
purposes of calculation , must be dealt with as a single
network , it being no longer possible to deal with each branch
separately owing to the redundant branch which the
interconnector forms .
With an alternating current supply , the feeders in either a
ring or radial arrangement are often at a higher voltage than
the distributors , necessitating feeders , feeding points and
113
distributors . All the consumer voltage may have a high voltage feeder system associated with it , supplying the
consumer voltage network at a number of transformer
substations . In a large urban area , a three - voltage system is
normal with a 11 kV ring main. It consists of feeders supplying a
number of 11/0.380 kV transformer substations which are the
feeding points for the 380 V distribution system .
This system supplies the final voltage feeding points which
can be the distribution boxes in the streets , from which the
power , is conveyed by the distributors to the individual
consumers . Such a system would be true 3-phase (three lines or
cores only) for the 11 kV portions , with 3-phase four - wire for
the consumer voltage 380 V distributors to provide the
114
