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 transformated 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. More ever, 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 significantly high.
