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Present Procedure:  At present APERC approved new ISI methodology for estimating agricultural energy is being adopted which is summarized as follows:-

    • Energy meters are installed at LV Side of sampled (selected) Agl. DTRs of different capacities in each Mandal of the Circle
    • Monthly readings are taken manually and through MRI by an outsourcing agency till recently. Now it is instructed that AEs (Opn) shall take readings from here onwards.
    • Out of the sample DTR readings obtained only valid DTR readings which shall be at least above 50% as per APERC directions will be taken into consideration for estimation of Agl. Consumption. 10% of total sampled DTRs in all capacities are to be changed from month to month for better projection of sampled consumption.
    • LT line losses are deducted as per APERC guidelines communicated in which LT lines are classified into 9 groups  based on DTR capacities and no of LT circuits and losses are fixed for each groups as follows.

Group A B C D E
% LT loss 1.06 2.52 2.85 2.25 2.06
Group F G H I
% LT loss 2.88 5.42 5.39 6.46
  • Specific consumption of different capacity DTRs is calculated with total sampled DTRs consumption of a particular capacity and no of sampled DTRs of that particular capacity. The total estimated agl consumption is arrived by multiplying specific consumption with population of DTRs of different capacities in circle duly deducting APERC approved LT losses as above.
  • This new ISI methodology i.e. per KVA Consumption based on DTR capacity is superior than previous method i.e. per HP consumption as it is truing up the estimated agricultural consumption to more realistic level. But it is experiencing the following practical difficulties.
  • Meters fixed to LVDTRs are in open field and most of then are exposed to vagaries of nature. The rectification of defects will take considerable time much difficulty is being experienced to maintain 50% of valid DTRs as directed by APERC.
  • Operation and M&P wings are having problems to put consistent good efforts for maintaining accurate metering due to various works and emergencies arising in the field from time to time.LV DTR meters are spread in the field throughout the Mandal and readings cannot be taken as per fixed schedule. Only 4 hrs of Agrl. Supply during day time is also a constraint for taking readings as per schedule.
  • Loads on sample DTRs selected of different capacities varies from area to area in a district seasonally depending upon the nature of crops and different ground water levels.
  • As loads on sampled DTRs may increase / decrease in due course of time due to addition of services (or) erection of addl. DTR to relieve overload / release of new services. In such cases some percentage meters are to shifted to a some other DTRs which is again a additional burden for the field.

Direct method for measuring the agricultural energy:
A most direct method of measuring the agricultural energy in place of above statistical method is explored and the same is presented here under.

  • At present all the agriculture feeders emanating from 33/11KV Substations in APCPDCL are mixed feeders feeding 7 hours agriculture supply and 16 hours lighting supply to villages through single wire earth return (SWER) distribution practice via 6.3KV or 11KV/250V single phase DTRs.
  • Apart from the above, a small No. of 3-phase non-agriculture DTRs (for some RWS/industrial services) are also connected on agriculture feeders. And a very few 3-phase services like localized RWS water pumps, with loads below 10HP, a few  services like floor mills with loads below 10HP are connected on agriculture DTRs.
  • Ever since the single phasing system is adopted in AP DISCOMS back in 2002, most of the 3-phase non-agriculture services in villages as listed below are either converted into single phase (or) transferred to nearby MHQ / 24 hours (dedicated) feeders.
  • Example: (a) 3-phase services such as floor mills in villages:- Almost all of them were converted to single phase.  (b) RWS services:-  i) Composite RWS services are transferred to near by MHQ /  24 hours feeders and ii) Only local RWS pumps are incident on agriculture feeders (c) Telephone exchanges/Cell towers are  converted to single phase services.
  • Almost all the new services released in villages in the last 11 years are single phase services only.
  • With the above situation, only a handful of 3-phase non-agriculture services are incident on agriculture feeders at present which can be easily mapped feeder wise.
  • One of the predominant practice of providing single phase supply during non-agriculture supply periods is extending 6.3KV supply (one-phase) to one conductor (1/1 system) in which all the single phase DTRs are connected in a particular fixed conductor in a particular phase. The two remaining conductors (phases) are opened at substation end.
  • The other method of extending single phase supply during non-agriculture supply period through 6.3KV supply is by looping all the 3 conductors after isolating 2 phases at substation end ie., known as 1/3 system. In this method single phase DTRs can be connected to all the 3 conductors which will facilitate to extending single phase supply.
  • In either case ie., 1/1 or 1/3 arrangements, only one phase is loaded and only one CT in the feeder VCB is loaded.
  • At present all feeder VCBs have feeder meters with 3-phase 4-wire HT Trivector meters and records (a) Agricultural services consumption plus non-agriculture 3-phase services consumption during 3-phase supply period and (b) single phase lighting supply consumption for all non-agriculture services incident on the feeders during 3-phase and s-phase supply periods.
  • In the proposed attempt for measuring of energy supplied to agriculture consumers, another auxiliary HT Trivector meter is installed in series with the existing feeder meter with the following arrangement shown in schematic wiring diagram shown.
    Agl
  • As shown in the diagram, input currents for the auxillary meter are as follows. The two secondary currents from 2 Nos CTs other than from 3rd CT which is loaded during single phase mode are taken and they are looped and STAR jointed to create 3rd current for the meter assuming the 3-phase agl. load is balanced for all practical purpose.

The above arrangement will yield the following

(A)
The original feeder meter records total energy delivered by the feeder i.e. during 3-phase supply period and single phase supply period.

(B) The second meter i.e auxiliary meter  installed records the energy delivered during 3-phase supply period only.

(A-B)
gives the energy consumed by 6.3 KV S-ph DTRs.

  • The LT sales for 3-phase non-agrl. Services which are incident on the feeder shall be mapped for the feeder. Treating the sum of 3-phase non-agl. Services as ‘C’, (B-C) will give near accurate Agl. Supply delivered by the feeder.
  • To implement this, it must be ensured the prevailing 1/3 arrangements in the substations shall be dispensed and it must be switched over to 1/1 arrangement after ensuring all single phase DTRs are connected in single conductor only.

It must be also be ensured the following
Before proceeding further in the process of calculation of actual energy delivered to Agl. Consumer at the consumer terminals.

  1. It must be ensured that all the existing single phasing system is implemented in 1/1 mode and all 1/3 arrangement must be switched over to 1/1 mode.
  2. All the non-agl. 3-phase services are to be mapped accurately feeder wise.
  3. Feeder wise DTRs with exact capacity, structure code, phase,type of load of DTR Structure i.e.(a) pure Agrl. (b) Agrl. Mixed (ex: 3×15, 3×10 etc.,), (c) mixed (all S-ph DTRs) (d) any non-agrl. 3-ph. DTRs shall be updated in SAP and the same shall be replicated in EBS & E-AUDIT modules.
  4. The agl DTR structures with different capacities i.e. 16, 25, 3×10, 3×15, 63 & 100 incident on feeder shall be updated accurately.
  5. The readings of Agricultural feeder meters i.e. both A (feeder meter) &B(auxiliary meter) shall be read in first week or  before 10th of every month. MRI dumps are also be taken for the meters MD shall be made reset on the last day month automatically.
  6. All the parameters KWH, KVAH, MD(KVA) are to be noted for both meters. Power factor for the month (reset to reset) to be calculated. 
  7. Calculation of I2 losses for 11kv line requires the following

Load factor  = Average load in the month
                       Maximum load in the month

Shall be calculated as follows:

Load factor(LF)  =  KVAH recored in auxillary meter
                              Max demand X no of 3-ph supply hrs for the month

Load loss factor is the ratio between average power losses and losses during peak load .

LLF=∑(Dn)²/(MD²xN)
(where Dn is the average demand for nth half hour period and MD, the max demand and N , the total of half hour periods for the feeder in the month) (or)

Load loss factor(LLF)  =    K x LF + (I-K) x LF2
(where ‘K’ is coefficient based on the nature of loads on feeder)

I2 losses are calculated are as follows

Technical losses = 3 x I2 x R x L x LLF x 7 x 30 x 10¯9   in MU
(where I = Max load amps of the feeder (which can be be taken from MRI dumps)
R = Resistance of conductor in ohms / km ( depends upon type and size of the conductor of trunk line of the feeder
L = Length of feeder in km (length of line is limited to trunk line which can be  arrived for feeder on one time basis)
LLF = Load loss factor.( Can easily be calculated as the MD and average demand can be taken from MRI dumps) or
Can be calculated from if K is assumed as 0.8 (the average demand/peak demand for agl loads is 0.8 practically) from the empirical eqn  LLF=   K x LF + (I-K) x LF2

DTR Losses

  • For all practical purpose, 3x10KVA, 3×15 KVA can be equaled with 25KVA & 63KVA respectively for arriving losses.
  • Since the average loads on Agricultural DTR is around 80% and there is a mix of lot of age old DTRs in the system, the max allowable loss at 100% loading is considered for calculation of DTR losses.

Calculation sheet will be as follows

DTR Capacity DTR losses Agl. Supply (Hrs) Loss
16 480 210 101
25 695 210 146
63 1250 210 262.5
100 1800 210 378
3×10 146
3×15 262.5

DTR losses can also be calculated for the feeder from the equation

FLL=LLFx(1.3*CF)²X Cu X no of 3-ph supply hrs per month
(where, FLL=total full load DTR losses for month of the feeder
CF= capacity factor of feeder is ratio of peak demand(MD) and RL is sum of installed DTR capacity for the feeder=MD/∑(sum of DTR capacities)
Cu= total full load losses of the DTRs arrived as per the population of different capacity DTRs on the feeder and various capacities DTR losses as per the above table)

LT losses for DTRs can be calculated based on already approved LT losses by APERC.

Finaly the actual Agl consumption at consumer terminals can be tabulated in E-audit module as follows

SS Feeder Energy recorded Energy recorded
in Aux. meter
Town Feeder1 A B
1-phase Cons. Non-Agl. 3-phase sales Agl. Cons. at 11KV IR losses
C = A-B D E=B-D F
DTR losses LT losses Total losses Net Agl. Cons.
G H I=(F+G+H) J(E-I)


Advantages of this method and conclusion

  • This is not a statistical method and it is more direct method of measurement of Agl consumption.
  • The measurement is not approximated much at 11KV level and more realistic approximations are considered in the empirical equations for arriving I²losses and DTR losses.
  • There is no additional work involvement in the above method as MRI dumps for AGL feeders is in practice every month.
  • Accurate population of feeder wise Agl DTRs, mapping of non agl 3-ph services feeder wise , length of trunk line of the feeder is to be done on one time basis only and only proper updating of DTR population data from time to time in SAP and E-audit modules is required.
  • With the help of S-ph supply period consumption arrived as above E-audit can be performed for Rural feeders also.

References:
1) CEA guide lines on DTR specifications
2) Computation of Technical Power Loss of Feeders and Transformers in Distribution     System using Load Factor and Load Loss Factor
Sarang Pande and Prof. Dr. J.G. Ghodekar  Department of Electrical Engineering,       K.K. Wagh Institute of Engineering & Research, Nashik, India

By Er. P V Ramesh, M Tech
Divisional Engineer/Operation/Gooty

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