GENERAL INFORMATION

1.1 INTRODUCTION

1.1.1 POWER TRANSFORMERS

Unlike shunt reactor and Neutral Grounding Reactors where standard specification is generally

followed by utilities, requirements of power transformers vary depending upon the system

design. Hence a tailor made product becomes the answer. Depending upon the requirements two

winding/ three winding/ auto connection/ split winding arrangement with ON LOAD or OFF CIRCUIT

tap changer, five limbs/ three limbs core construction, welded or bolted tank construction are

adopted. Variation in capacity, impedance and transport profile result in different sizes of

transformer. Refer VoI I for specific requirements of the equipment.

1.1.2 SHUNT REACTORS

Shunt Reactors are used in high voltage systems to compensate capacitive generation from long

lightly loaded overhead lines or extended cable systems and also for control of dynamic over

voltages.

Gapped core construction is preferred for high system voltages over coreless construction due to the

high energy density that can be achieved in gapped core construction. The core sections between

consecutive air gaps are moulded in epoxy resin to prevent movement between individual

laminations.

The spacers forming the air gaps are blocks of ceramics with a high modules of elasticity and the

whole stacking of core modules is cemented together during the assembly to form a solid column

without possibility of rocking , or rubbing between individual parts. The core segments are of

radial laminated configuration. The radial laminations prevent fringing flux from entering flat

surfaces of core steel which would result in eddy current overheating and hot spots. Five limbed

core construction is adopted to achieve high zero sequence impedance. In addition to the three

gapped core limbs with windings, there are two continuous outer return limbs. The two unwound

side limbs help in achieving zero sequence impedance approximately equal to the positive

sequence impedance. Other construction type can be a three limbed construction. For single

phase reactors e.g. 800 KV class middle leg wound with two return legs type of construction is

adopted. Interleaved disc winding has been used for rated voltages 220 KV and above. This type of

winding configuration provides better impulse voltage

distribution. For lower voltage classes a continuous disc winding or a multi

layer helical winding are used. The tank is rectangular in construction with flat

cover welded to the tank rim at top. The associated cooling control equipment

is housed in a tank mounted weather proof marshalling box. The reactors are

equipped with all standard measuring and controlling fittings and accessories

as described in Volume I.Prepared By Samvet Lahari, Bhopal. Phone- 5277412

Reactors have been designed for resting on concrete foundation as shown

in the foundation plan drawing. Rollers have been provided only for movement

of the reactor at site.

1.1.3 NEUTRAL GROUNDING REACTOR

When single pole reclosing is used on lines, the Neutral Grounding Reactor

is used.

The Neutral Grounding Reactors are connected between neutral point

of 400 kV/800 KV reactors and earth, where the neutral of shunt Reactor is

suitable for 145 kV class insulation.

The reactors are oil immersed type ONAN cooled (oil immersed with

natural air cooling) with continuous rating of 10/15 Amps and a 10 sec. rating

as indicated in the Rating Data sheet. As continuous losses of the reactors are

negligible, reactor tank surface is adequate for dissipation of these losses.

Hence radiators are not provided.

Considering linear impedance characteristic requirement upto rated short

time current a core less design with magnetic shielding of CRGO sheet has

been made. The magnetic circuit, therefore consists of rectangular frame of

CRGO steel lamination packets of adequate area. Area of magnetic circuit is

selected such that saturation does not take place under short time current. A

rigid clamping structure is provided for clamping of magnetic frame and a very

low flux density is used to minimize the vibration and achieve linear impedance

characteristic.

The graded disc type concentric winding suitable for a rated voltage of

145 kV and basic insulation level of 550 kVp has been provided without core

inside. Winding is held in position by way of special insulation structure inside

winding and finally kept under pressure between top and bottom yokes of

magnetic frame. Return path of flux is provided by way of two return limbs.

The line terminal is taken out through the tank cover via 145 kV, 800

Amps OIP condenser bushing provided with suitable terminal connector. The

neutral lead is taken out through the tank cover via a 36kV, 630 Amps porcelain

bushing.

The tank is of welded mild steel plate construction shot blasted on the

inside and outside to remove scales before painting. The tank is painted on the

inside with yellow paint and its outside surface is painted with two coats of

primer paint and finishing coat, of light grey paint to shade 631 of IS:5. This

is a standard painting scheme followed for all equipment .

The fittings include a conservator with a magnetic oil gauge, pressure

relief device, drain and filter valves, sampling valves, thermometer pockets,

Buchholz relay, temperature indicator for oil and silicagel breather .

These reactors are designed for mounting directly on plinth.

1.2 TRANSPORT OF TRANSFORMER/REACTOR

Power Transformers and Reactors hereinafter referred as "Transformers,"

depending upon the restriction imposed by transport weight and/or other

considerations, have to be transported either filled with oil or nitrogen as per

the description given below. The transformer is loaded on wagon as per loading

gauge drawing

1.2.1 WITH OIL

The Transformer is filled with oil such that about 10% of the tank

volume is left clear for expansion to limit the excess internal air pressure to

0.35 kg/cm 2 . Fittings dismantled before transport are packed in packing cases

in line with shipping list.

All openings resulting from removal of fittings for shipment are sealed

with suitable gasketted blanking plates during transport.

The balance oil quantity required for the complete filling of the reactor is

supplied separately.

A transformer despatched according to this method can be stored at

site or elsewhere for one year provided, that all insulating materials are covered

with oil and silicagel breather is mounted and quality of oil is maintained as per

IS: 1866. Measures to be taken on reception at site are given clause 2.1

1.2.2 WITH NITROGEN

Large oil filled transformers/reactors are not normally oil filled during

shipping due to weight limitations. To protect the active parts against moisture

the transformer tank is filled with dry Nitrogen at a positive pressure of 0.175

kg/cm 2 (2.5 psi) at BHEL works before despatch. The internal pressure at the

time of shipment is painted on the tank.

All openings resulting from the removal of fittings for shipment are

sealed with suitable gasketted blanking plates during transport.

A Transformer despatched according to this method should normally be

stored upto 3 months after arrival at site or elsewhere.

For control of gas pressure and maintaining the pressure during transport

and possible storage before assembling, the transformer is equipped with an

automatic device. This device is called two stage N

2

regulator and is kept in a

steel box and mounted on the frame provided on tank side for keeping two

back up Nitrogen cylinders. Nitrogen regulator is connected to two cylinders

by copper tubes and to tank by a rubber hose as shown in the Fig. 1.1.

N

2

regulator reduces cylinder high pressure of 120 to 140 kg/cm 2 to

required low pressure of 0.175 kg/cm 2 in two stages. One gauge provided on

the first stage indicates the cylinder pressure, whereas other gauge provided

on second stage indicates the tank pressure. Regulator is fixed at factory.

When tank pressure falls below 0.15 kg/cm 2 , due to leakage/fall of

ambient temperature, regulator automatically feeds the gas from cylinders, to

build up required pressure in the tank.

The gas consumption during transport and possible storage is difficult

to estimate as it depends on ambient temperature variation, possible leakages

as well as on the duration of transport and storage. However, two back up

cylinders are provided to meet the gas requirement. Normally one cylinder is

kept open and the other shut. When the pressure of first cylinder falls to 10

kg/cm 2 this should be considered as empty and immediately its valve should

be closed and the valve of second cylinder should be opened.

When cylinder is required to be removed for refilling, disconnect copper

tubing from cylinder valve.