Miniature Circuit Breaker or MCB
What is MCB ?
Nowadays we use more commonly Miniature Circuit Breaker or MCB in low voltage electrical network instead of fuse.The MCB has some advantages compared to fuse.
1. It automatically switches off the electrical circuit during abnormal condition of the network means in over load condition as well as faulty condition. The fuse does not sense but Miniature Circuit Breaker does it in more reliable way. MCB is much more sensitive to over current than fuse.
2. Another advantage is, as the switch operating knob comes at its off position during tripping, the faulty zone of the electrical circuit can easily be identified. But in case of fuse, fuse wire should be checked by opening fuse grip or cutout from fuse base, for confirming the blow of fuse wire.
3. Quick restoration of supply can not be possible in case of fuse as because fuses have to be rewirable or replaced for restoring the supply. But in the case of MCB, quick restoration is possible by just switching on operation.
4. Handling MCB is more electrically safe than fuse.
Because of to many advantages of MCB over fuse units, in modern low voltage electrical network, Miniature Circuit Breaker is mostly used instead of backdated fuse unit.
Only one disadvantage of MCB over fuse is that this system is more costlier than fuse unit system.
The female type contacts are fixed on the top of the other post
insulators which fitted at both sides of the central post insulator. The
female contacts are generally in the form of spring loaded figure
contacts. The rotational movement of male contact causes to come itself
into female contacts and isolators becomes closed. The rotation of male
contact in opposite direction make to it out from female contacts and
isolators becomes open.
Rotation of the central post insulator is done by a driving lever
mechanism at the base of the post insulator and it connected to
operating handle (in case of hand operation) or motor (in case of
motorized operation) of the isolator through a mechanical tie rod. 
One but major difficulty of these type of arrangement is that,
maintenance of equipment of any bay cannot be possible without
interrupting the feeder or transformer connected to that bay. 
If one section of the bus bar system is under maintenance, part load of
the substation can be fed by energizing the other section of bus bar.
By closing any of the isolators one can put the feeder to associated
bus. Both of the buses are energized and total feeders are divided into
two groups, one group is fed from one bus and other from other bus. But
any feeder at any time can be transferred from one bus to other. There
is one bus coupler breaker which should be kept close during bus
transfer operation. For transfer operation, one should first close the
bus coupler circuit breaker then close the isolator associated with the
bus to where the feeder would be transferred and then open the isolator
associated with the bus from where feeder is transferred. Lastly after
this transfer operation he or she should open the bus coupler breaker. 
In double breaker bus bar system two identical bus bars are used in
such a way that any outgoing or incoming feeder can be taken from any of
the bus similar to double bus bar system. Only difference is that here
every feeder is connected to both of the buses in parallel through
individual breaker instead only isolator as shown in the figure. By
closing any of the breakers and its associated isolators, one can put
the feeder to respective bus. Both of the buses are energized and total
feeders are divided into two groups, one group is fed from one bus and
other from other bus similar to previous case. But any feeder at any
time can be transferred from one bus to other. There is no need of bus
coupler as because the operation is done by breakers instead of
isolator. For transfer operation, one should first close the isolators
and then the breaker associated with the bus to where the feeder would
be transferred and then he or she opens the breaker and then isolators
associated with the bus from where feeder is transferred. 
This is an improvement on the double breaker scheme to effect saving
in the number of circuit breakers. For every two circuits only one spare
breaker is provided. The protection is however complicated since it
must associate the central breaker with the feeder whose own breaker is
taken out for maintenance. For the reasons given under double breaker
scheme and because of the prohibitory costs of equipment even this
scheme is not much popular. As shown in the figure that it is a simple
design, two feeders are fed from two different buses through their
associated breakers and these two feeders are coupled by a third breaker
which is called tie breaker. Normally all the three breakers are closed
and power is fed to both the circuits from two buses which are operated
in parallel. The tie breaker acts as coupler for the two feeder
circuits. 
This is an alternative of double bus system. The main conception of
Main and Transfer Bus System is, here every feeder line is directly
connected through an isolator to a second bus called transfer bus. The
said isolator in between transfer bus and feeder line is generally
called bypass isolator. The main bus is as usual connected to each
feeder through a bay consists of circuit breaker and associated
isolators at both side of the breaker. There is one bus coupler bay
which couples transfer bus and main bus through a circuit breaker and
associated isolators at both sides of the breaker. If necessary the
transfer bus can be energized by main bus power by closing the transfer
bus coupler isolators and then breaker. Then the power in transfer bus
can directly be fed to the feeder line by closing the bypass isolator.
If the main circuit breaker associated with feeder is switched off or
isolated from system, the feeder can still be fed in this way by
transferring it to transfer bus. 
This is combination of the double bus system and main and transfer
bus system. In Double Bus System with Bypass Isolators either bus can
act as main bus and second bus as transfer bus. It permits breaker
maintenance without interruption of power which is not possible in
double bus system but it provides all the advantages of double bus
system. It however requires one additional isolator (bypass isolator)
for each feeder circuit and introduces slight complication in system
layout. Still this scheme is best for optimum economy of system and it
is best optimum choice for 220KV system.
The schematic diagram of the system is given in the figure. It
provides a double feed to each feeder circuit, opening one breaker under
maintenance or otherwise does not affect supply to any feeder. But this
system has two major disadvantages. One as it is closed circuit system
it is next to impossible to extend in future and hence it is unsuitable
for developing system. Secondly, during maintenance or any other reason
if any one of the circuit breaker in ring loop is switch of reliability
of system becomes very poor as because closed loop becomes opened.
Since, at that moment for any tripping of any breaker in the open loop
causes interruption in all the feeders between tripped breaker and open
end of the loop. 
Along the secondary transmission lines, at load center, the secondary
transmission voltages are further stepped down for primary distribution
purpose. The stepping down of secondary transmission voltages to
primary distribution levels are done at secondary step down substation.
Outdoor type Substation are constructed in open air. Nearly all
132KV, 220KV, 400KV substation are outdoor type substation. Although now
days special GIS (Gas Insulated Sub – Station) are constructed for
Extra High Voltage system which are generally situated under roof. 
Consider a ring of ferromagnetic specimen of circumference L meter, cross - sectional area a m2 and N turns of insulated wire as shown in the picture beside,
The LV side of the transformer is short circuited . Now with help of
variac applied voltage is slowly increase until the ammeter gives
reading equal to the rated current of the HV side. After reaching at
rated current of HV side, all three instruments reading (Voltmeter,
Ammeter and Wattmeter readings) are recorded. The ammeter reading gives
the primary equivalent of full load current IL. As the
voltage, applied for full load current in short circuit test on
transformer, is quite small compared to rated primary voltage of the
transformer, the core 
Voltage drops in the windings occur due to impedance of transformer.
Impedance is combination of resistance and leakage reactance of
transformer. If we apply voltage V1 across primary of transformer, there will be a component I1X1 to balance primary self induced emf due to primary leakage reactance. (Here, X1
is primary leakage reactance). Now if we also consider voltage drop due
to primary resistance of transformer, then voltage equation of a
transformer can easily be written as,
Star Star Transformer is formed in a 3 phase
transformer by connecting one terminal of each phase of individual side,
together. The common terminal is indicated by suffix 1 in the figure
below. If terminal with suffix 1 in both primary and secondary are used
as common terminal, voltages of primary and secondary are in same phase.
That is why this connection is called zero degree connection or 0o - connection. 
If primary is HV side, then A1 will be connected to B2, B1 will be connected to C2 and C1 will be connected to A2.
Similarly in LV side 1 suffixed terminals of each phase winding will be
connected with 2 suffixed terminals of next phase winding. That means, a1 will be connected to b2, b1 will be connected to c2 and c1 will be connected to a2.
If transformer leads are taken out from primary and secondary 2
suffixed terminals of the winding, then there will be no phase
difference between similar line voltages in primary and secondary. This delta delta transformer connection is zero degree connection or 0o - Connection.
The delta connection in LV side is formed by connecting 1 suffixed
terminals of each phase LV winding with 2 suffixed terminal of next
phase LV winding. More clearly, a1 is connected to b2, b1 is connected to c2 and c1 is connected to a2.
The secondary (here it considered as LV) leads are taken out from 2
suffixed ends of the secondary windings of transformer. The transformer connection
diagram is shown in the figure beside. It is seen fron the figure that
the sum of the voltages in delta side is zero. This is a must as
otherwise closed delta would mean a short circuit. It is also observed
from the phasor diagram that, phase to neutral voltage (equivalent star
basis) on the delta side lags by − 30o to the phase to
neutral voltage on the star side; this is also the phase relationship
between the respective line to line voltages. This star delta transformer connection is therefore known as − 30o - Connection.
The heat dissipation rate can be still increased further if this oil
circulation is accelerated by applying some force. In OFAF cooling
system the oil is forced to circulate within the closed loop of
transformer tank by means of oil pumps. OFAF means "Oil Forced Air
Forced" cooling methods of transformer. The main advantage of this
system is that it is compact system and for same cooling capacity OFAF
occupies much less space than farmer two systems of transformer cooling.
Actually in Oil Natural cooling system, the heat comes out from
conducting part of the transformer is displaced from its position, in
slower rate due to convectional flow of oil but in forced oil cooling
system the heat is displaced from its origin as soon as it comes out in
the oil, hence rate of cooling becomes faster.