High Voltage Distribution System

Single Phase Transformers in High Voltage Distribution Systems

Nature of Rural Loads
► Loads in rural India are predominantly pumpsets used for lift irrigation.
► These loads have low p.f., low load factor.
► Load density is low due to dispersal of loads.
► Existing system is to lay 11 KV lines, employ 3 phase DTRs 11kv / 433 volts and lay long LT lines.
► To fetch a load of one pump set of 5 HP (4 kw) ; two or three L.T. spans are to be laid.
► On each DTR 63 or 100 KVA 20 to 30 such pump sets are connected. If used for domestic services about 100 consumers can be connected.
► They run for about 1500 Hrs in an year of 8760 hours.

Disadvantages experienced with LVDS
► Poor tail end voltages.
► High quantum of losses.
► Frequent jumper cuts and fuse blow outs.
► Motor burn outs almost twice in each cropping period of 100 days.
► DTR failures due to frequent faults.
► Loss of standing crops due to inordinate delays in replacement of failed DTRs .

Solution ? H V D S ?
To improve quality of supply and reduce losses HVDS is recommended
► 11 KV lines are extended to as nearer to the loads as possible and erect small size single phase transformers 5, 10 or 15 KVA and release supply with NO or least LT line.
► Aim at “LT less” system starting from “Less LT”.
► Unavoidable short LT lengths to be covered by insulated wires like ABC (Aerial Bunched Cables).

Technical Superiority of HVDS
► For the distribution of same power, the comparison of voltage drop, losses etc., with 100 as base is illustrated below.
Parameter Single Phase 6.3 kv /
433 V HVDS
Conventional 3 phase LVDS
Current (A) 11 100
Losses (kw) 8.5 100
Voltage drop 12.7 100

► Smaller size conductors can be employed.
► Excellent voltage profile.
► Reduced losses.
► No frequent fuse blow outs.
► Negligible transformer failures.
► Two or three consumers get effected against 30 if DTR fails (even 100 domestic).
► Very easy and quick replacement.

HVDS – Single phase - Irrigation.
► Good quality single phase motors are available.
► High yield of water as pump set efficiency is high.
► Single phase motors are ‘capacitor start’ and ‘capacitor run’.
► Starters not required.
► Built in capacitor.

Additional advantages of HVDS
► Unauthorised hooking of loads is not possible as LT lines are short and insulated.
► System power factor improves (0.95) causing easy reactive power control.
► As only two or maximum (3) pump sets are connected on each DTR ; the consumers assume ownership and responsibility.
► High quality of power supply earns total consumer satisfaction.

Questions & Critical issues frequently faced on HVDS
A) Whether HVDS is for future areas to be electrified or to convert existing LVDS?
B) Can HVDS & LVDS coexist in the same area?
C) What is the linkage between load density and type of Distribution System.
D) Economics & payback period.
E) Policy on procurement of single phase transformers.

A&B
► H.V.D.S. to be implemented straight away where new villages are being electrified in view of its technical superiority.
► Where 3 phase pump sets are in use HVDS can be implemented by converting existing LT lines for 11 KV.
► Even three Nos. smaller size single phase transformers can be used for providing three phase LT supply capacity (3Xindividual capacity).
► Single phase transformers 6.3 kv / 233 Volts can be manufactured with single or two bushings.
► Three single phase transformers can be connected with HVs in star or delta.

                                                Comparision
Star – Star Delta – Star
Vector group connections outside Vector group connections outside
Single HV bushing Two 11 kv bushings
Graded insulation on HV Entire HV winding to be insulated for 11 kv
Current rating is high and hence more copper Current in HV is less in delta and hence less copper


(C) Linkage with current density
► In Metro areas with high load densities as high as 20 MW per sq km due to rise multistoried complexes enough load can be met with practically no LT.
► In smaller urban areas, single phase transformers can be employed selectively for single phase as well as three phase.
► Even if one limb fails, single phase loads on that phase can be distributed on the other two and the failed unit can be replaced very quickly as it is easy to transport and erect.
► In the case of 3 phase large capacity transformers, entire unit is to be replaced and down time is high.
► In rural areas of low load densities HVDS using one or three Nos. single phase transformers can be employed without any hesitation.
► Rural loads even village habitat portion are too low and mostly single phase.
► In villages getting electrified afresh; the load densities are bound to be too low.

(D) Economics
► Cost of three Nos. single phase transformers of capacity ‘K’ is equal to a three phase transformer of capacity (3K).
► Cost of employing three single phase transformers is 1.3 times the cost of a 3 phase transformer.
► There is no difference in no load losses and full load losses between one 3 phase transformer of 3 K and three Nos. single phase transformer of K.
► However current for same capacity is too low at 11 KV compared to LT.
► For 100 KVA current is 5 Amps at 11 KV and 140 Amps at LT.
► Line losses get reduced in the ratio 52:1402 (25:19600).
► Hence highly economical.

(E) Usage of three Nos. single phase against one three phase transformer in urban localities
► The schemes being implemented in many rural areas are mostly such as “Kutir Jyothi” and “A Lamp for each house” do not need high capacity transformers.
► There are single 5, 10, 15 / 16 capacity transformers also which can be used advantageously in these light load areas.
► This has many advantages as follows,
► Capacities of single phase units can be so selected to avoid laying LT lines as these units are available in ratings from 5 KVA to 15 KVA.
► Single phase loads can be connected on individual single phase transformers duly dividing them.
► In respect of extremely few three phase loads with connected loads of 4 kw or above (very rare in rural areas) three single phase transformers of smaller capacities can be employed.
► The loads are too small that they cannot contribute to high unbalances.
► In the event of one phase unit giving trouble, the loads can be distributed on the other two phase units and the defective unit can be taken out easily.
► Transport, replacement is very easy and can be done much faster.
►In fact a rolling stock of 4% can be maintained at each distribution section office for faster replacement.

PROS & CONS in Restructuring existing LVDS to HVDS
► 99.99% customers avail supply at 415 / 240 Volts thus operational performance of LV network is key to customer services.
► Losses in Indian Power System are around 20%. LV system is responsible for high loss scenario as LV line losses are 6 times of target limit and 3 times maximum tolerable limit. Switchover to HVDS alone can bring losses to international norms.
► Power loss for transmission of equal load in LVDS (415V) and HVDS (11000V) lines are in ratio 13:1.
► Maximum permissible voltdrop between DSS and customer premises is 10%. Hence compliance with IE rule. 56 on voltage drop is difficult and very expensive in LVDS whereas it is simple in HVDS.
► The investigation of typical LV feeders in LVDS indicate that 75% of LV feeders have voltdrop above 5% and is the cause of high losses whereas in HVDS losses on LV line are insignificant.
► The current for distribution of same power in L.V.D.S. is high and existing conductors get loaded beyond economic loading limits. It can be avoided by switch over to HVDS.
► The monitoring of feeders in LVDS is difficult compared to HVDS as number of feeders to be monitored is in ratio of 60:1.
► Unauthorised tapping of LV lines is simple and rampant in LVDS whereas it is very difficult in HVDS.
► ABC cables with tough insulation are indigenously available.
3 Phase HVDS
The work involved in restructuring distribution work are
► Conversion of existing low voltage lines to single phase 2 wire HV lines.
► Replacement of existing three phase distribution transformers with small capacity single phase transformers.
► Utilisation of existing three phase motors.

ADVANTAGES OF HVD SYSTEM
► The registered customers will feel ownership and take responsibility and not allow others to meddle with the L.T. Network.
► Prevention of unauthorized loads by the consumers themselves since the distribution transformer may fail if loaded beyond its capacity.
► Failure will be minimal because of no over loading and no meddling of L.T Lines.
► In the event of equipment failure only 2 or 3 customers will get affected instead of 25 to 30 customers in original system.
► High quality of supply since there is practically no voltage drop.
► Less burnouts of motors because of good voltage and less fluctuations.
► Considerable reduction in line losses and consequent savings in power purchase cost.
► Since Losses are reduced considerably ,Power can be supplied to additional loads without any further investment on infrastructure.
► No additional generation capacity needed for giving new loads due to reduction in power drawals.
► Accidents due to touching of snapped conductors reduced due to the fact that the breaker trips at substation since the line is at 11 KV potential.
 
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