CHAPTER The unit scheme is totally based

                                              CHAPTER 1


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1.1 Radial
Feeder Protection

The modern age has come to heavily depend upon the continuous
and reliable flow of electricity because there are many appliances that need
electricity to carry out their work .Computer and telecommunication networks, industries,
railways ,Airports are the few applications that required the continuous and
high value of electricity to carry out their functions properly. No power
supply in this world is not design in such a way that it’s never fails . So
protection is required for the continuous flow of electricity.

In power distribution system there are mostly transformer,
shunt capacitors banks and protective devices which age over time and the
mostly the distribution system is over headed which could easily effected by
the weather, animals, etc. As the radial distribution system is characterized
by having only one single path of flow of power from source to consumer end .So
for continuous flow of power from source to load end a protection scheme is
required to installed on the whole system of distribution which result in fast
and accurate fault detection that could made the

1.1.0 Protection schemes can be divided into two major types.

(i) Unit Scheme

(ii) Non unit Scheme

 (i) Unit Scheme

This type of scheme only protects a specific area of the
distribution system, i.e. bus bar, distribution transformer, generators or
transmission line. The unit scheme is totally based on the Kirchhoff’s current
rule which states that the amount of total current entering in an area must be
equal to zero. Any deviation from Kirchhoff’s current rule must indicate an
abnormal condition. This scheme only respond to fault that occur in a clearly
defined zone and have fixed boundaries. The Restricted earth fault and Differential
protection are the certain types of unit scheme protection which mostly apply
on the transformer e.g. Earth fault protection is applied on the high voltages
of delta connected power transformer windings.

(ii) Non Unit Scheme

This type of scheme is also applied to a specific area of
power system but it have no fixed boundaries .This type of scheme can overlap into
other areas and can be used for backup purposes protection. This protection
scheme also measure current but work on inverse time characteristics which
allow the protection nearest to the fault to operate first.

 The non-unit
protection has following schemes.

a)   Distance protection

b)   Time graded over-current protection

c)   Current graded
over-current protection