Wednesday, May 14, 2014

Accuracy of Energy Meters

                       Reliable and accurate metering system is a vital link between a power utility and consumer, acquiring more significance day by day. In terms of Electricity Act 2003, CEA has notified a metering code for all the utilities to adopt appropriate metering technologies together with various associated methods to reduce commercial losses. Hence it is pertinent that the power utilities have to upgrade their metering system with the state-of-art technologies which are accessible, for reducing losses, improving financial status and better load management.
                        It is a standard utility practice to test consumer's metering equipments in situ and has many advantages over laboratory tests .The meters need not be de- installed and transported to other locations in order for the necessary tests to be performed. This is particularly important for transformerrated metering installations. If the measuring circuit is faulty the electricity meter receives voltages and currents which differ phase and/or amplitude from those it would receive. Faults may occur when metering equipments are under ongoing operation also. The surest way to find such faults is to check the meter as well as the associated instrument transformers.
                          The specification of LT, HT and EHT metering system in India are based on the guidelines of Central Electricity Authority (Installation and operation) Regulations (2006) which are already in practice all over India. Few area where electricity metering needs attention are
 1 3 phase 3 wire measurement system instead of 3 phases 4 wire meter.
2 Poor accuracy standards of Instrument transformers and Energy meters.
3. Installation of same rated CTs irrespective of contract demand.

1.  Effects of selection of higher burden values of   instrument transformers than actually required.

It can be seen that  at many instances the burden of CT is very high when compared to that of the associated metering equipment. (2-2.5VA).In EHT consumers, a slightly higher burden can be expected for pilot wires. However, a very high burden of instrument transformer gives a prima facie indication that the CT is not working in the defined accuracy range described in the IS. For eg. for a 20VA CT with a secondary current of 1A,  the load must be 20 Ohm. If only 3 Ohm loop resistance and digital relays with almost 0 Ohm impedance is connected, the accuracy may be outside specification since the accuracy is only guaranteed when the load is nominal (20 Ohm) or 1/4 of the nominal load (5 Ohm).Using CTs of burden values higher than required is unscientific since it leads to inaccurate reading (meter) or inaccurate sensing of fault / reporting conditions. Basically, such high value of design burden extends saturation characteristics of CT core leading to likely damage to the meter connected across it under overload conditions.
The CTs designed with a particular burden connected with lower burden application results to erroneous measurements. With the advent in technology, the burden of individual instrument has come down considerably.
                     It can be seen that for  indoor CTs, though the burden(VA) specified for the metering Core is 10-15 VA, it is unlikely to exceed even 3 VA.Hence, it is suggested that the rated Burden for the Metering Core of the CT may be a standard value as close to the connected Burden as possible. Even though a direct conversion of this abnormality into loss is not accurately estimated, a marginal error of 0.25 %   results into a substantial loss of revenue .

2) Adverse effect of using 3 phase 3 wire meters instead 3 phase 4 wire meters

Three phase three wire System of power measurement is in vogue in many utilities .This is two watt meter system of power measurement in which current of R& B phases are measured along with three line voltage. This system measures energy accurately both in balanced and unbalanced load conditions provided there is no neutral current flow. This system is just right so long as the conditions behind it are meticulously followed. However field conditions are entirely different. The present day practice of using star-star solidly earthed transformer permits the consumer to load on each phases heavily (for eg.single phase furnaces etc.) thus making erroneous reading, as current in Y phase of high voltage side is not recorded. If the consumer is intelligent enough to add single phase load deliberately at Y phase in above such condition, it is as good as a power pilferage situation. If a three phase four wire system is adopted, this tricky condition can be avoided. Morever, when CT/PT units of one phase becomes faulty, the total consumption of the system can be arrived more accurately. Hence it is high time for utilities to adopt three phase four wire system as already done elsewhere to plug the drain of revenue leakage.
3. Effects on adoption of Poor accuracy standards of Instrument transformer and Energy meters.

As per regulations 2, 5,8,12 and 16 of Central Electricity Authority (Installation and operation) Regulations (2006) and the schedules, the standards of measuring equipments have been specified. For HT consumers, class 0.5S or better is specified where as for EHT consumers its Class 0.2 S.The accuracy class of Current Transformers and Voltage transformers shall not be inferior to that of associated metes. The regulation states that the existing CTs and VTs not complying with these standards shall be replaced by new CTs and VTs. Thus the regulation is very specific in defining the accuracy class of both instrument transformer and Meters. However superior an energy meter may be, if the associated instrument transformers measure the actual energy with an element of error, the same will be replicated. The current transformer is Class 0.5 (HT) and Class 1.0 for EHT which is not in line with the standards of Central Electricity Authority, 2006.Though a direct conversion of this non compatibility is hard to achieve, revenue loss per month on installation of lower accuracy class CT and Energy meters on the EHT / HT consumers are sgnificantly high.

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