Effect of vector Grouping :

(a) Vector Grouping : depending on phase difference of primary and secondary voltages, transformers are grouped as follows :

(i) Group-1, Zero phase displacement - Yyo_{, Dd}o

(ii) Group-2, 180o _{phase displacement - Yy}6_{, Dd}6

(iii) Group-3, lag phase displacement - Dy1_{, Yd}1

(iv) Group-4, 30o _{lead phase displacement - D}11_{, Yd}11

Vector Group in W.B.s.E.B. system :

Previously in W.B.S.E.B. system we used transformers of Yyo _{vector group. But gradually since}

1984 Yyo _{group is being replaced by Yd}1 _{group with the help of 100 KVA, 33/0 415 KVA, ZnY}

Earthing cum Station Service Transformers for earthing the delta secondary system as well as for station service. The question arises why this system was adopted inspite of use of empty earthing cum station service transformers. The reasons are as follows :_

(a) Yyo _{connection is most economical for small high voltage transformers because the number of}

turns per phase and the amount of insulation is minimum as phase voltage is only of the line voltage. This connection works satisfactory only if the load is balanced. In unbalanced loads, the neutral points shift, thereby making the line to neutral voltages (phase voltages) unequal. With the complicated loading pattern in W.B.S.E.B. system as well as need for increase of transformer capacity Yyo _{Group Required replaced.}

(b) In Yd1 _{system, no. difficulty is experienced from unbalanced loading as in case of Yy system.}

Voltage wave shape is not distorted due to the flow of third harmonic current in d delta winding. (c) Yyo _{transformer, if one of the line jumpering is burnt cut but not touching the tower, current in}

one phase will be zero. It sudh as case voltage in primary and secondary sides will be balanced but currents will be highly unbalanced and there will be quick high temperature rise in transformer which will damage the same. We faced such situation at Egra and Chandrakona Road 132 KV sub-Station and introduced special protection arrangements with 3 PTS connected in Open-delta- to sense one voltage sensitive relay to trip the transformer. But Y d transformer will not have such adverse eiffer.

(d) Yd1 _{tranformer is used alongwith an Earthing Transformer. The earthing tranformer has a very low}

impedance during its normal operation but offers a very high impedance during falt to act as a current limitor. It is found that fault current limiter. It is found that fault current in Yd. Transformer can be reduced to approximately 1/10th of Yy transformer with both the neutrals earthed,. (e) In Grid Sub-Stations burning / melting of clamps, connectors, jumpers in a chronic problem. though

we are using connector of phosphoer bronze, with the increase of faulti level, connectors of phospher bronze are also found to be not suitable. With the change over to Yd transformer alongwith earthing cum station service transformer, fault current is very much reduced. So we can use clamp and connectors of E.C. grade hard drawn extruded Almunium, which are less costly than phosper bronze and a considerable cost can be saved as well as frequent power interruptions due to replacement of clamps and connectors can be stopped. The cost of E.C. grade extruded

aluminium is nearly50% that of phospher broze. In W.B.S. E.B. system auto transformers or Yy Vector Group is in operation for voltage ratio 132/66/11 KV and 220/132/33V. In such cases Yy group has been chosen in order to avail of the economical advantage of auto-transformers discussed earlier.

10. Bushings :

(i) Bushings are provided with the transformer for entry of high voltage and exit of low voltage terminals and vice-versa. Bushings are very delicate part of a transformer and as such should be specified, procured, handled and maintained with care. Frequent bursting of bushing occurs in transformers due to various reasons.

(ii) At present, we are using bushings of 66 KV and above which are of oil filled condenser type hermatically sealed and 33 KV bushings are of porcelain oil filled type and 11 KV bushing are porcelain plain shed type. In general active part of the condenser bushing consists of a central metallic condoctor tube enclosed by wound bakalite paper or oil impregnated paper body or synthetic resin bond payer with aluminium foils in between the layers for field control. The innermost aluminium layer is connected to the conducting tube and the outermost to the fixing flange. A porcelain jacket is provided on the condenser bushing. The space between the paper body and the porcelain jacket is filled with highly viscuss oil with an air space below the protective hood of the bushing which is sufficient for expansion of oil. The entire space is hermatically sealed against the atmosphere. A slight glass is provided to indicate the top oil level. One red ball will be visible through the slight glass. After delivery of the bushings, it should be checked whether the red ball is at the centre or not when the bushing is vertically placed. If the red ball is displayed from the centre, the bushing is defective and the same shall be reported to the supplier or manufacturer immediately.

(iii) Bushing for 132 KV and above are equipped with a measuring tap at the bottom portion. It is used or checking the capacitance and power factor with the flange grounded. It can be used in conjunction with a special plug for measuring the voltage or for control purpose during operation. (iv) The bushings are suitable for vertical or inclined installations :

Vm 73 KV : 600 _{to vertical plane.}

Vm 100 KV : 30o _{to vertical plane.}

When Vm is the highest system voltage. Bushings are fitted in inclined position in order to maintain phase to phase clearance required for the voltage system without increasing appreciably the tank size.

(v) All bushings shall be provided with suitable shoulderless terminals of approved type and size and shall be suitable for bimetallic connection for terminal connectors.

(vi) The insulation class of the bushings shall be according to voltage classes for which they are used. The insulation class of the high voltage neutral bushing shall be properly co-ordinated with the insulation class of the neutral of high voltage winding. Each bushing shall be so co-ordinated with transformer insulation that all flash overs will occur outside the tank. This is because of the fact that a damaged bushing can be easily replaced by a spare one but flashover inside tank will involve the risk of fire in the oil and entire transformer may be burnt into ashes.

(vii) Special adjustable arching horns may also be provided for the bushings as per IS-371/1966, I.E.C. Publication No. 71A. This is very simple to look at, so it is generally neglected by site-Engineers. But arcing horns has tremendous importance and practical utility for the safety of the bushing as well as to reduce the length of the bushing. Without arcing horns, the length of the bushing would have been very high in order to provide sufficient creapage distance against any unanticipated

The bushing shall be suitable for normally polluted and heavily polluted atmosphere as well as under different weather conditions. In this respect both total and protected creapage distances are very important. To avoid failure by tracking following minimum creapage distances are specified.

Indoor insulators : 16 to 18 mm/KV

Outdoor moderately polluted : 18 to 22 mm/KV Outdoor hea vily polluted : 25 to 30 mm/KV

However, for normally polluted atmosphere minimum total creapage distance OK in mm can be found out from the equation approximately as

23

Lk = ——— Vm mm. 1.5

Protected creapage distance at an angle of :

45o _{= 61%}

900 _{= 43%}

For heavily polluted atmosphere the minimum creepage distance Lks in me can be found out as— Lks = 23 x Vm mm

Protected creepage distance at an angle of :

45o _{= 74%}

90o _{= 55%}

So as a thumb rule, total creepage distance shall not be less than 23 mm/KV for heavily polluted atmosphere.