Traction Rolling Stock - Indian Railways Institute of Electrical ...

10. AC EMU DESCRIPTION OF
ELECTRICAL CIRCUITS
10.1 POWER CIRCUIT
10.1.1 Roof equipments (Ref. Fig.10.1.1)
The pantograph is connected to the roof bushing of the 25 kV cable via the air-blast
circuit breaker, the live side of which has a surge diverter connected between it and earth, At roofmounted
earthing -switch is provided, having two eathing blades in the closed position, these
blades contact both sides of the air-blast circuit breaker isolator arm, thereby earthing both the
pantograph and the primary winding of the transformer.
10.1.2 Transformer
The 25 kV cable from the roof bushing passes through. The emergency driving cab and
enters the transformer through another bushing where a connection is made to the primary
winding is brought out by an insulated cable which is connected to the traction motor earth
brushes coming on the four axles. The earth brushes are insulated from the motor frames. The
transformer return is therefore kept independent of the earthing of the equipment cases and under
frame.
Primary overload protection is provided by a relay operated through a current
transformer. The primary overload relay (OLP) is combined with the earth fault relay (EFR) (Ref.
fig 10.4), to form a double-element relay having a common set of interlocks and reset coil. Tripping
of either OLP or EFR causes the air-blast circuit breaker (ABB) to open. The two trip elements are
provided with flags to indicate which relay has caused the trip. The flag indication remains after the
relay has been reset electrically, the flag must be reset manually. The transformer secondary
consists of two separate windings each of 350V. One half has four taps dividing it into five sections
each 70V.
In the first half of the notching sequence, only the tapped portion of the winding is used,
while for the remaining notches the two sections are connected in series. The changeover is
effected by means of contacts W1 and W2 on the winding- grouping switch and are only operated
off load.
The tap changer contactors T1 to T9 carry out the switching of the transformer sections.
Contactors T1 to T6 are connected to the transformer tapings and select the required voltage. Tap
changing is carried out by means of reactor TL, in conjunction with the two contactors T7 and T8
to give alternate notches with and without the reactor in circuit. A resistor is also provided in series
with the reactor. But this is in circuit only during the actual tap changing operation, being
immediately cut out of circuit when contactor T9 closes. The purpose of this reactor is to ease the
duty on the opening contactor by reducing the circulating current.
The selected output voltage is fed through the overload relay trip coils OL5 and OL6 to
the main rectifier unit. The function of this relay is described more fully under the heading
TRANSFORMER OVERLOAD PROTECTION.