La violencia contra la mujer: una cuestión de género

Failure of power transformers puts a difficult situation of Power interruption which is a time taking process for replacement of such a costly equipment. More over, even if a spare transformer is available, transportation, erection, commissioning testing etc. are all time taking process. Since the transformer is the most expensive single item and improved technique of manufacture and reliable testing, it is now considered a dependable equipment, as such keeping spare transformers to meet up such odd events is not at all economically viable.

Now, suppose a total power P is to be transmitted from a Grid Sub-Station. What will be the choice of transformer(s) to be installed in the Sub-Station, so that there will be minimum interruption of Power in case of failure of transformer. The choice may be in the following three ways. (i) I F C₁ = Cost of a single three phase unit of three phase capacity - P.

(ii) C₂ = Cost of two transformers each having capacity P₂ running in parallel. (iii) C₃ = Cost of 4 single phase units of each capacity P/3 having one spare unit.

Then, C : C₂ : C₃ : 0.7 : 1 : 1.2 -= 7 10 : 12.

It is evident that a single phase unit of capacity P will be less costly. Due to additional Civil work, assessors like OLTC, Bushings etc. Cost will gradually increase for two units and four units. The best choice will be two units of capacity P/2 running in parallel because of the fact that failure of one unit will maintain atleast 50% Power supply and by over loading 60-70% Power can be maintained for a short time, though more costly than a single three phase unit when entire Power supply will be interrupted.

In case of 4 Single phase units of each capacity P/3, advantages gained is that entire power system can be restored quickly by replacement of spare unit, but is much more costly, at the same time spare unit will be idle inventory and the accumulated cost of entire country will be a huge financial burden. However, single phase units have field application where transportation is a problem because of weight and dimensions etc.

3. LOSSES IN A TRANSFORMER :

(a Core and copper losses : In an ideal transformer, Power fed to the primary circuit is equal to

the Power received from the secondary circuit. But hardly can we meet this requirement. A considerable amount of Power fed to the Primary circuit is lost as (i) Core loss to maintain the magnetic circuit (ii) Copper less in the form of heat. So the Power received from the Secondary circuit is always less less than that fed to the Primary circuit. The core or iron loss of a transformer is more or less fixed but the cu-loss of a transformer is variable with the load as such sometimes called as load loss. The cost of a transformer can be adjusted appreciably with the ratio of iron and cu-losses, by choice of core materials, percentage reactance or core section and section of copper conductor. Moreover allocation of losses will decide the over load capacity of a transformer within specified temperature limit. So amplication of losses in a transformer is very important towards economic functioning as well as survival.

(b) Loss Capitalisation : No load loss and load loss in Distribution Transformer upto 100KVA rating

in 11KV System, has been fixed in I.S. at 75o_{C. So these transformers are to be manufactured}

TABLE - 1

KVA Rating of No load loss Load loss

11/0.415 KV 3-ph (Watts) with (watts) at Remarks

Transformer GRGO steel core 75oC

25 110 720 These losses are maximum allowable

63 200 1300 with no positive tolerance. No. weightage 100 290 1850 in price for offering lower losses. In case of Power Transformer, the losses are not fixed by the purchaser unless, of course, the new transformer is to run in parallel with the existing ones. In order to select technically best suited and economically lowest transformer the unique method of "Loss Capitalisation" for evaluation of losses are employed in tender evaluation.

In power transformer there are three losses viz (i) Iron (Core) loss, (ii) Load (Copper) loss (iii) Auxiliary losses. The auxiliary losses are the power losses of auxiliary equipment like cooling plants etc. and are considered to be a part of load loss. For the purpose of Comparison capitalised value of iron loss and load loss shall be mentioned in the specification of the purchaser. The tender shall state and guarantee the losses but shall not specify any tolerance limit for the same. Tolerance limit for no load loss is plus 10% of the guaranteed loss and that for load loss is also plus 10% of the guaranteed loss as per I.S.S. The main idea behind loss Capitalisation is that Capitalised value of the losses with I.S. tolerances shall be added with exworks price of the transformer for comparison and selecting lowest bidder and same shall be verified through testing before accepting the same.

4. PERCENTAGE IMPEDANCE :

The resistance of a transformer being bery small is unimportant as such discussion will be limited to percentage reactance only. For a given ratio and voltage the size and weight of a transformer is a function of its percentage reactance. The weight is a minimum for a particular reactance called "Economical Percentage Reactance". In the case of a 220KV transformer the cost decreases slightly when the percentage reactance is increased from 10% to 16% . This is because a small percentage reactance means a large main flux requiring large cross-section of the core. As reactance is increased the core section decrease and so the overall size. The iron loss is decreased but the copper loss is increased. The ratio of copper to iron loss is appreciably increased and the total loss is slightly increased. But when the reactance is still more increased the same argument does not hold, the cost increases becauses of high leakage flux. For every voltage there would be a normal range of percentage reactance within which the cost may not very appreciably. The ranger of values in Table-2 corresponds to usual practice.

TABLE - 2

Type of Transformer MVA Height System % Impedance

Range voltage (KV) Range

(i) Distribution Upto -1 - 4 to 5

(ii) Industrial S/S 6 to 20 36 to 100 6 to 10

While fixing percentage impedance of a transformer before procurement two points shall be kept in mind :

(a) If the newly procured transformer is not intended to run in parallel with any existing transformer the percentage impedance of the transformer shall be decided within the ranges shown in the Table-2, but matching with the system network impedance to which the said transformer is going to be connected.'

(b) In case the newly procured transformer is required to run in parallel with any existing transformer then the percentage impedance of the transformer to be procured shall be matched with the existing transformer.

From the above it is found that percentage impedance of a transformer is very important item for economical design of a transformer. So in the tender specification the value of percentage impedance shall be properly selected and clearly mentioned. Any abnormal value will make the transformer design un-economical and unnecessary cost thereby.

Considering all the facts percentage impedance of Power transformers in WBSEB system have been fixed in the range 10-12%.

5. TEMPERATURE STIPULATION :

Transformers are installed outdoor without any protection against sun and rains. The maximum temperature of the Hot-spot shall be limited to 105o_{C with class-A insulation. Each transformer}

shall be capable of operating continuously at its normal rating without exceeding the specified temperature rise limits as per I.S.S. The maximum temperature of top layer of the oil inside a transformer should not exceed 60o_{C above ambient temperature. The life of transformer oil is}

halved if its temperature is 10o_{C above normal. If average ambient temperature is 35}o_{C, top oil}

temperature should not exceed (35+60+10)o_{C or 85}o_{C for Power transformers. For Distribution}

transformers it is (85-10o_{C = 75}o_C.

6. COOLING :

The rating or power delivery capacity of a transformer can be increased by providing proper cooling arrangements, otherwise temperature limit will reach and or excessive temperature rise will damage the transformer as a hole. Natural cooling and forced cooling are employed stage by stage.

(a) Natural cooling, Radiators :

(i) Radiators : Transformers are generally filled with detachable radiators consisting of a series of separate circular or elliptical tubes welded at their top and bottom into header to be connected to the main tank by means of bolted, oil tight flanged joints. There are valves one at the top header, and other at the bottom one for circulation of all.

The main purpose of radiators is to provide increased cooling surface for circulating oil by increasing the total tank area without increasing appreciably the oil containing capacity. Moreover, thickness of the radiators is less than that of main tank, thus saving of iron materials, reduced weight for transportation, hence reducing the cost of transformers, and decreasing foundation cost etc. So main tank is subjected to vacuum Testing but the radiators to Pressure Testing only.

(b) Forced Cooling : The natural cooling is not sufficient as MVA rating of the transformers and

loading increases. So different types of forced cooling arrangement have to be employed. In WBSEB specification we are employing ONAN/ONAF/ODAF type of cooling upto 160 MVA transformers. ONAN rating shall be about 50% and ONAF is about 75% and ODAF is about 100% of the rated capacity without exceeding the temperature limits (ONAN-oil Natural & Air Natural, ONAF-Oil Natural & Air Forced; ODAF-Oil Directed & Air Forced).

The temperature setting of the cooler control contacts shall be generally as under :

Cooling Equipotent ON OFF

Fans (For forced air) 85o_{CV 60}o_C