Transmission and distribution loss Electrical Utility Loss Calculation and identifying

?

Prepared by SHAN. B.S

aee, kseb

?

Abstract

Generally for an electrical utility it is easier to calculate the total AT&C loss than to calculate loss at component level. For finding the total loss, first find the total input energy which shall be the sum of net generated energy and purchased energy. Sold energy is available with some extra adjustments. The difference between these two shall be the total loss. But, if the objective is for reducing the energy loss, then it is necessary to find the loss on each components like feeders, transformers, substation etc. This paper explains a method to find the component wise loss of an electrical utility.

?

Transmission loss

Even though it is neglected, calculating Transmission loss can be done with higher accuracy and lesser approximation. This is because traditionally, transmission system is well metered and every year meter calibration works are done. All EHT feeders have sending end and receiving end meters and its readings are tabulated every hour. These readings are typically taken manually and recorded in log sheets. In some cases these readings are entered into a software.

Time synchronised readings

Some people tend to argue that these readings are not time synchronised and so cannot be used. But in reality, with a little analysis, it can be seen that readings are much useful and there is no need for a time synchronised metering system. Further with an in-depth analysis it can also be understood that the time synchronised readings is in reality a myth which cannot be achieved with 100% accuracy.

Typically it takes 5 minutes to 10 minutes to complete a meter reading session which is done every hour or half hour. But if the reading patterns is analysed it can be found that the time interval between readings of each meter does not vary more than 3 minutes. If the time variation is taken as taken as 5 minutes with additional two minutes as compensation then the percentage accuracy variation shall be

(5/60)*100

= 25/3 =8.33%

Thus it is seen that the percentage time deviation or in effect the error percentage is less than 10%.? Then if the loss is 15%, the maximum deviation or error in it shall be 2*(15/100)*(10/100), which is 3% or +/-1.5%.

In short if the calculated loss percentage is 15%, the average deviation will be 1.5% or the deviation shall be 13.5% to 16.5%.

Tolerance level is much higher as typically the loss values are not considered definitive instead as a comparing values. Loss is active power and its value is tabulated for different loads and the mean deviation is considered for arriving at the loss percentage. This being the case the tolerance levels accepted can be as high as 20%.

Hence it can be seen that the accuracy of the loss percentage is within 10% with manual readings. If the meters are time synced with internet connectivity or intranet connectivity then the response time and deviation of the response time for each meters, upon taking into consideration shall be 3 to 4% and communication time delays shall account to 4% and considering the other inevitable variations beyond our control then the total accuracy shall be near 10%. Considering the required technological cutting edge and cost incurred for reducing this accuracy variation is actually not justifiable nor is needed.

?

Loss calculation of EHT feeders

Sending end and receiving end meters of EHT feeders are located in different substations or generating station switch yards. Date-Time KW readings of these meters are needed for the calculation of loss. These are used to find the KW transmitted and received for over each interval of time. These values are then tabulated. Since EHT lines does not draw no load losses, let alone due to stray or shunt capacitance effect, it can be assumed that the only loss component is copper loss. But it may be noted that if the line length much higher, the loss due to capacitance effect shall be significant enough to fully neglect. In such cases the sending end energy with zero receiving end energy shall be noted. This can be found by simply disconnecting the receiving end. If this method is not practical, then the loss at 5% loading or less shall be taken as shunt capacitance loss. Since such loss is depending on the voltage levels and not on loading, this loss tend to remain constant and shall be considered as constant loss. It shall also remember that this loss varies with weather conditions such as rain and other factors such as corona.

?

General loss calculation method

General formula for finding the loss is very simple.

Loss = input energy in KWH/MU – receiving end energy in KWH/MU

The variable part of the loss shall be I2R loss. The constant loss shall be calculated by tabulating for different loads. Typical excel sheet tabulation is given below

Date/time interval

Sending end KWH (Ps)

Receiving end KWH

(Pr)

Loss

(Ps)- (Pr)

% loading

(Ps)*100/ (Ps max)

% loss

{(Ps)- (Pr)}*100

/ (Ps)

?

?

?

?

?

?

?

Calculation for the variable loss

Energy transferred in MU is found from the active power formulae of

?

Here it can be seen that ?are constant and as the deviation in the PF typically does not vary much it can be assumed to be constant. Thus

MU is proportional to I and

Loss is proportional to I2? ?or is proportional to MU2

Hence loss1 α MU1

Similarly loss2 α MU2

And??? loss1? ?????= ?????? MU1

loss2??????????????? MU2

Here MU1 & MU2 are total energy transmitted for different time intervals and loss1 is the calculated loss (in mu) occurred for MU1 while the loss2 shall be theoretical loss (in mu) at? MU2.

Difference between theoretical loss calculated and actual loss calculated by taking the difference between send energy and received energy shall be the constant loss.

These values shall be tabulated and by taking the mean average value the loss in the EHT feeders can be found.

Date-time interval

Energy transmitted

Total loss

Theoretical loss

(variable loss)

Constant loss

10.00 to 10.30

MU1

Loss1??

---

?

10.30 to 11.00

MU2

loss2???

loss2T?

loss2 - loss2T???????????

?

Difference between the no load loss or constant loss found with negligible load and the constant loss calculated from above table is very significant as it is the loss which can be controllable. This loss may be due to different rectifiable factors such as joint resistance, insulation leakage, bends, strains in the cable etc. joint resistance tends to rise with temperature or in effect with loading. So suitable analysis of the above figures with different loads and time slots shall be taken for rounding in on the exact loss point.

?

Loss Calculation for Substations

Loss calculation of substations depends on the feeding arrangement of that substation and so for calculating method for the substation loss differs with different substations. The different methods are given below.

1.???? Feeding from single tap without any export

2.???? Feeding from multiple tap without any export

3.???? Feeding from single/multiple tap with export

4.???? Feeding from single/multiple incomer and? exporting with different single/multiple feeders

5.???? Feeding with LILO without any export

6.???? Feeding with LILO with export from LV side

7.???? LV side generator and HV side exporting(generator Switch yards)

?

General principle to follow is to find out the total incoming active energy and sending out energy. It is also necessary to have thorough knowledge of the equipments/parts of the substation through which these energy actually flows.

?

1.???? Feeding from single tap without any export

This is the simplest substation configuration. Here energy flow from income to the LV side. LV side may have multiple feeders. The outgoing energy can be calculated by simple summation of the energy flowed out during the time interval considered. The difference between the incoming energy and the outgoing energy is the loss occurring in the substation.

If the auxiliary supply is taken from a transformer connected with dedicated incomer feeder and if the outgoing energy is calculated without considering this energy consumption, then the loss calculated shall include auxiliary consumption. In such cases auxiliary consumption has to be subtracted.

On the other hand if the auxiliary consumption is met from the auxiliary transformer connected to any of the outgoing feeders, then in such cases, outgoing energy includes auxiliary consumption and so it is not needed to be subtracted from the calculated loss.

?

2.???????????????? Feeding from multiple tap without any export

In such cases the calculation method to be followed is almost as above. The main difference is the additional calculation for finding the total input energy of the substation at any period of time. For that summation of all the input energy input at the HV side need to be found. Similarly the summation of the going total energy at the LV side also need to be found. The difference gives the energy loss of the substation.

?

3.???????????????? All other systems including -Feeding from single/multiple incomer and? exporting with different single/multiple feeders

?

For LILO and other systems, the substation may be located in between the major load centre and the feeding centre. In such cases all the energy coming through the incoming feeders does not pass through the whole substation arrangements but only through the HV bus bars. So the calculation approach to be taken for such system shall depend on the energy flow path. Generally for LILO arrangements, the summation of the total energy is to be calculated first. This is done by adding all the input energy and subtracting all the HV side exporting energy. This shall be the actual energy passing through the substation and loss is determined by subtracting that figure with the outgoing energy at LV side. Here this loss is occurring after the bus bar. The loss at the bus bar has to be calculated separately. This is very important, as any rectifiable loss occurring point exists in the busbar, shall contribute to very high loss, as the total energy passing through the busbar shall be very high and may be more than the substation capacity.( substation capacity is the total transformer capacity and not the actual energy flowing through the bus bars.

?

?

TRANSFORMER LOSS

Calculation of transformer loss is very simple, just find the input side energy from incomer feeder meters and subtracting the total output energy shall give the transformer loss. Whenever the transformer is switched ON, the no-load power need to be noted which shall be constant loss. Once in a while this KW reading shall be cross checked with the no-load energy meter readings. If the power calculated from the energy meter readings is not matching with that of the KW meter readings, the matter has to be reported to the transformer and meter testing wing.

It is always necessary to take the neutral current of the HV side while taking no-load readings. The no-load current of the transformer shall have high harmonics components and so the readings depend on the type of ammeter used for measurement and also the CT. If the ammeter is moving iron type and if it is connected to the CT, then the readings may be of no use. Still noting this readings is necessary, because it can be assumed that the current value obtained shall be the fundamental value alone. Generally for a transformer the no-load currents shall be well balanced and so the neutral current shall be zero.? Only the harmonics current shall flow through the neutral. If the fundamental current is present in the neutral wires during no-load condition, then it is due to winding unbalancing or due to external connecting cables, joints etc.??

Distribution loss

?

HT lines are frequently subjected to back feeding and so the 11KV feeder loss cannot be found correctly. So the LT side loss need to be found separately. 11KV section wise loss can also be found. And these data can be used to find the total distribution loss.

?

LT side loss

?

Backfeeding in LT side is possible but generally is not done frequently. It is true that such arrangements are done but very rarely. It is done by manually removing the jumper connections and so is not easy. Keeping this in mind, the strategy for determining the LT side loss can be found.

For this distribution transformers shall be connected with an energy meter preferably with AMR facility. The consumers connected to that transformer shall be identified and their billing meter readings shall be taken on the same day. Thereby the total energy through that transformer for a billing period can be found. The input energy of the transformer can be found from the transformer meter and subtracting the billed units from that energy gives the loss on the LT side. Generally the consumer readings if taken on the same day can be completed within 3 to 4 hours. This difference shall be contribute only less than 5% error which is tolerable. Both transformer meter reading and the consumer meter reading has to be taken on the same day and preferably during the same time interval.

Generally 100KVA transformers are majority in number and maximum of about 30 to 40 consumers are connected to each transformers and so arranging meter readers for taking readings of all consumers connected this transformers on the same day itself is possible by suitably arranging the walking order. Instead of galloping blindly for smart meters with very low success rate, the above method is viable and effective.

The loss calculated in this manner shall include the transformer loss. As the transformer losses are very small, ignoring the transformer loss shall not contribute much difference.

?

?

?

Conclusion

?

The method proposed is devised based on the ease of implementation. All the readings suggested is from the existing metering arrangements itself with very few exceptions if any. Suitable adjustments may be needed in the calculation procedure according to the actual situations. Component wise loss calculation is necessary for rectifications and adopting preventive measures.

The methods proposed can be implemented in software also. Input data filters shall be adopted with meaningful range, to be determined with due diligence. In fact statistical methods, mean averaging, propagation methods and other operation research mathematical tools if needed shall be done with software. In any case data range to be taken shall be given high importance else the end results may vary substantially. Considering this factors it is to be noted that the manual calculation methods yields much better results without the use of a special software.

?

?

References

1.???? Electrical Power Systems Quality, Third Edition 3rd Edition By Roger C. Dugan, Mark F. McGranaghan, Surya Santoso, H. Wayne Beaty Published: June 7, 2012

2.???? Electrical Transients in Power Systems, 2nd Edition Allan Greenwood

3.???? POWER SYSTEM ENGINEERING? Format: ebook? ?Rental Duration: Lifetime??? Author: D Kothari; I Nagrath Edition: 2ndPublisher:McGraw-Hill Education India

4.???? Power System Control and Stability, 3rd Edition Vijay Vittal, James D. McCalley, Paul M. Anderson, A. A. Fouad ISBN: 978-1-119-43369-9 October 2019 Wiley-IEEE Press

5.???? Renewable Integrated Power System Stability and Control Hêmin Golp?ra, Arturo Román-Messina, Hassan Bevrani ISBN: 978-1-119-68979-9 April 2021 Wiley-IEEE Press

6.???? J. Arrillaga, N.R. Watson, S. Chen: Power System Quality Assessment. New York :John Wiley, c2000

7.???? Ewald F. Fuchs, Mohammad A. S. Masoum :Power Quality in Power Systems and Electrical Machines. Elsevier Academic Press, c2008

8.???? Wilson E. Kazibwe and Mucoke H.Senduala : Electric Power Quality Control Techniques. New York: Van Nostrand Reinhold, c1993

9.???? Elias M. Stein, Timonthy S. Murphy : Harmonic Analysis: Real-Variable Methods,Orthogonality and Oscillatory Integrals.Princeton, N.J.: Princeton University Press,c1993.

10.??????????????????????? IssaBatarseh : Power Electronic Circuits.New York : John Wiley, c2004 Leonard L. Grigsby: Power Systems. CRC Press, c2007

11.??????????????????????? J. Arrillaga, N. R. Watson: Power System Harmonics. New York: John Wiley, c2003

12.??????????????????????? Arthur H. Seidman and H. Mahrous, Handbook of Electric Power Calculations. New York: McGraw-Hill, 1983 pp. 143-150

13.??????????????????????? P.W. Sauer and M.A. Pai, Power System Dynamics and Stability, New Jersey: Prentice-Hall, 1998 pp.161-180

14.??????????????????????? N. Cohn, Control of Generation and Power Flow on Interconnected Systems (New York: Wiley, 1971).

15.??????????????????????? L. K. Kirchmayer, Economic Operation of Power Systems (New York: Wiley, 1958).

?