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i) Three post, centre post rotating double break type and
ii) Two post single break type.
These isolators are of horizontal break type on the secondary side i.e. 25 KV side, the
isolators used are of vertical break type. The rated normal current of these isolators is fixed to
match the rated current of circuit breakers and bus bars etc. The standard values of rated
normal current as specified in the IEC and IS specifications are the same as shown in para
5.5 for breakers.
4.7 CURRENT & POTENTIAL TRANSFORMERS
4.7.1 Bushing type CTs have been provided on primary and secondary side of the
traction transformers and are exclusively meant for differential protection. Separately mounted
132 KV and 25 KV CT and 25 KV PTs are of conventional type.
4.8 BUS BARS
4.8.1 Two types of bus bar have been used for traction substation viz. strung type
bus bar on the HV side and rigid type bus bar on LV side i.e. 25 KV side. The strung bus
used in the earlier electrification schemes was consisting of All Aluminium ‘spider’ conductor
of size 19/3 99 mm. The capacity of this conductor is adequate only to meet the maximum
fault current of above 17KA, but due to increase in the fault/level of grid system, the value
of fault current has exceeded 25KA. Therefore, to meet the higher requirement of fault
current in the system, use of ‘Zebra’ Acsr conductor has been adopted in the ongoing
electrification schemes, (size 61/3.18, dia 28.62 mm). This conductor is capable to withstand
fault currents of the order of 31.5 KA. The tension in the strung bus is kept between 500 to
900 kgf. To keep corona losses within limits, the minimum diameter of conductors (strung
bus) and jumpers shall not be less than 28 mm in case of 220 KV system.
4.8.2 Similarly the rigid type bus used in earlier electrification schemes was of
Aluminium Alloy of 36 mm O.D. (36/28 mm). This bus was capable to carry the normal
rated current of 960 Amps. But with the use of 20 MVA, transformers the requirement of
rated normal current has increased to 1500 Amps (for short duration). Therefore, the size of
the rigid bus bar has also been increased to 50 mm O.D. (50/39 mm). This bus is capable
to carry continuos current of 1530 Amps. The rigid type bus is supported on the support
insulators provided at a distance not exceeding 3 m. The minimum height of the 25 KV bus
bar has been kept as 3800 mm. Chances of failure of rigid bus are very remote, but its
installation is costlier than the strung bus.
4.9 LIGHTNING ARRESTERS
4.9.1 Lightning arrestors ar also called surge diverters. The primary purpose of a
lightning arrester is to protect the system from getting damaged by the over-voltages caused
due to lightning strokes and switching surges. Lightning arresters absorb the energy and
reduce the over voltage in the system. The ideal arrester is one which draws negligible
current at operating voltage by offering very high impedance and negligible impedance
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