Failure Analysis Of Energy Meters In Testing Laboratory 

Energy meter is the key component of supply system whose reliable functioning is very essential for accurate registering, controlling and monitoring, of power consumption...- Deepa Warudkar, Priyamvada Chandel, B.A.Sawale, Neha Gupta, Shilpa Singh Yadav 

In earlier days the function of the energy meters was just to measure the power consumed by the user, but now the meters are smart enough not only to measure the energy consumption – but also to control load and giving signal to the utility to cut or retain the supply by analysing the data recorded by meters. Apart from this, meters have to detect tamper and fraud, TOD billings, load survey of last 30 days or more, maximum demand value and times, power failure events and its recordings, harmonics measurement in the supply system etc.

Energy meter is the key component of supply system whose reliable functioning is very essential for accurate registering, controlling and monitoring, of power consumption. Through energy meters utilities get their revenues for the electrical power they supply to its various consumers. Hence, it is the basic need of all utilities to get the accurate revenue for the electricity generated and supplied to the consumer.

Energy meter testing is an important way to determine their reliability, safety and acceptability, hence testing must be a part of the development process for manufacturing of the energy meter.

To prove the reliability and accountability of energy meters, they have to be tested as per relevant national and international standards in accredited testing laboratories. Apart from the type test, purchasers are also interested to ensure that the meters comply with the entire tender specifications.

CPRI being a testing organisation has carried out testing on various types of energy meters and of different manufacturers. Different Utilities from all over the country send meters for testing as per their tender specification.
The objective of testing is to ensure that the meters are designed to comply as per the relevant standard and the customer’s specifications. Apart from this the reliability in respect of electrical and mechanical design of the meters is also verified. In addition to the type tests the Indian meter Industry has to meet large nos. of anti-tamper features as per the purchaser’s tender specifications.

Type tests on meters as per relevant Indian standard comprises following tests:

  • Tests of insulation property
  • Tests of accuracy requirement
  • Tests of electrical requirement
  • Tests for electromagnetic compatibility
  • Tests for climatic influences
  • Tests for mechanical requirement…

Failure cases in laboratory

General observations on failure and some of the critical cases are discussed here. The major failure are in insulation properties test.

Failure in the tests of insulation properties

This test is carried out to check basic insulation between the live parts and body of meter to provide protection against electric shock.
General observations during the test in case of failure of the meter:

  • Display non functional
  • No pulse output
  • Accuracy of meter beyond the limit
  • Power supply circuit is affected
  • Error of meter is -33% in case of three phase meter. This indicates that, one phase circuit is affected.

Some of the reasons for failure are discussed in the following cases.

Case 1

Groove provided for screw to fix meter base and cover does not have sufficient insulation or the insulation at this place is weak – and hence when the base of the meter is placed on the conducting surface during application of 4kV High Voltage it was punctured.

Traces of insulation failure…

Case 2

Meter failed due to wire connected between neutral terminal and name plate. Clearance provided between name plate and the casing of the meter was not enough to withstand 4 kV High voltage.

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Wire connected between neutral and name plate…

Case 3

Screw projected out at the back side of the terminal block, which is touching the earth plate during test.

Screw projected out…

Case 4

Sealing wire of meter touching the name plate placed inside the meter. When conducting foil wrapped around the meter during test, the sealing wire came in contact with the conducting foil providing the path for leakage current.

Flash over between name plate and sealing wire…

Case 5

Protective device used to clamp the impulse signal across the input of the meter got punctured when impulse voltage applied. The accuracy of the meter is -33% after the test.

Failure in the tests of accuracy requirement under influence quantities

Purpose of these tests is to verify the performance of the meter under various power line abnormalities.
General observations during the test:

  • Meter didn’t start at specified starting current
  • Meter produced more than one pulse output during no load condition
  • Accuracy of meter found beyond the limit
  • Variation in % error under influence quantities found beyond the limits.

Case 1

Meter started recording energy corresponding to IMax under the influence of stray magnetic field i.e., 67 mT. However, recording of energy at current IMax is permitted only for abnormal magnetic field. This may be due to lack of interpretation of standard or the non uniformity of magnetic sensors used in meters.

Failure in the test of electrical requirements

Aim of the test is to verify design of the meter for electrical parameters in the field, to verify withstand capacity of the meter under fault condition.

General observation during test:

  • The accuracy of meter found beyond the limits
  • Variation in % error found beyond the limits
  • Over heating due to loose connections…

Case 1

Phantom load getting overloaded when Imax applied to the meter due to loose connection of the screw of the current circuit during heating test.

Screw connecting CT inside the meter is loose…

Failure in tests of electromagnetic compatibility

These tests are carried out to check meter immunity for the electromagnetic radiations and its interference.
General observation:

  • Meter produced pulses during no load test causing increment in kWh and results in change in information due to electromagnetic HF field.
  • Error of meter varies more than 2% during the sweep of frequency. Majority of failures are in the range of 80 to 400 MHz.
  • Failure can be due to picking up the radiated signal or the conducted signal through cables. Proper filter design can prevent meter from failure due to noise picked up by cables. Shielding and filtering of noise at processors would prevent meter failure from radiated noise.

Failure in test of climatic condition

This test is carried out to determine ability of the meter and its components at extreme non operating condition or during storage.

General observations:

  • Accuracy of the meter goes beyond defined class
  • Display of the meter not functional.
  • Meter records cover open tamper condition (if this feature provide in meter)
  • Date and time in the meter get drifted.

Case 1

Meter display not functional and meter did not produced pulse output after the test.

Case 2

% Error of the meter found beyond accuracy class.

Failure in test of mechanical requirement

Purpose of this test is to determine any mechanical weakness or degradation in the specified performance of the meter.

General observation:

  • Falling of components inside the meter during test
  • Accuracy of the meter found beyond the accuracy class
  • Meter did not read actual current/voltage applied
  • Backlit of meter found not functional
  • Meter logged cover open tamper
  • Components like battery or capacitor fallen inside meter.

Case 1

Capacitor of the meter fallen down inside the meter during vibration test.

Capacitor with broken leads fallen during vibration test…

Case 2

Power back-up battery fallen down inside the meter during test.

Battery with broken leads fallen due to vibration test…

Case 3

Meter did not measure voltage and current after the test indicating that the voltage or current circuit is disconnected.

Case 4

Meter was reading 2A current in R phase, however, the applied current to the concern phase was 10 A. Hence, the % error of the meter is – 31%.

Failure in tamper tests

Apart from the standard there are many tests, which are required as per the tender specifications of different utilities. Some of these tests and their results are discussed hereafter:

Failure in the test of abnormal frequency/high frequency of 35 kV spark discharge test:

This is more common test specified by almost all utilities in India. The meter is subjected to very high voltage high frequency spark. Evaluation criteria for this test is different for different utilities like:

  • Accuracy of the meter before and after the test shall not be affected
  • The accuracy of the meter, real time clock disturb, memory data corruption, meter functioning hang up etc., shall not occur.

General observations:

1) Meter stops functioning during/after the test
2) kWh reading of the meter changes
3) Date and time of the meter changes during/after the test
4) Meter starts recording energy corresponding to Imax. current.
5) Meter records cover open tamper or any other change of information.

Meter has to be shielded and earthed properly for compliance of this test.

Failure in the test of influence of permanent magnetic field:

General observations noticed during test:

  • Meter stopped functioning under the influence of 0.5T permanent magnet
  • Percentage error of the meter found beyond the limit.

Failure in the test of D.C. injection in neutral circuit through diode:

During the test, the meter should record energy as per voltage measured between incoming phase and neutral terminals when D.C. signal is injected in the neutral terminal of the meter though diode from load side as well as from source side.

General observations noticed during test:

  • This test was performed on meters of seven manufacturers. Results of four manufacturer were more or less same, but there was huge difference between the results of other three manufacturer:
  • Percentage errors obtained during test are summarised below:

Test condition 1

Diode is connected in incoming neutral circuit:

  • When reference meter is connected to meter terminal

  • When reference meter is connected to supply points

Test condition 2

Diode is connected in outgoing neutral circuit :

The large variation in meter accuracy is due different interpretation of the tender clause. As the test is nonstandard, the utilities do not specify test requirement properly and therefore the each manufacturer interpret the condition differently. This tamper condition is normally complied by the manufacturers under some assumptions and implementing the logics in software.

Failure during receiving of meters in the laboratory:

There are few discrepancies noticed during receipt of meter (before starting of the test) in the laboratory.

  • Current date and time displayed by the meters are not correct
  • Meters have already logged cover open tamper
  • kWh resolution of the meters are not sufficient to conclude some of the tests of the standard.
  • Percentage error of the meters found beyond the accuracy class because of current/ voltage read by the meter is not correct.

Statistical analysis of failure case: failure of energy meters in the laboratory (over the past five years that have been tested) are presented below:

Graphical representation of meter failure in the laboratory during 2008 to 2013 in the Energy Meter Testing Laboratory, CPRI, Bhopal…

Impulse voltage test

AC high voltage test

Test on limits of error

Voltage variation test

Test of DC magnetic induction 67 mT

Test of abnormal DC magnetic induction 0.27T

Test of influence of self heating

Immunity to electromagnet HF fields

Vibration test

Tamper test-permanent magnet 0.5 T

Tamper conditions as per various tender specifications

Tamper test- abnormal/high frequency 35 kv spark discharge test

Conclusion

Although testing process is considered as a tedious, time consuming and expensive, still it is important for deciding the reliability of energy meters. General observation and some of the failure cases are discussed here to understand that the minor mistaking during designing and manufacturing processes results in failure of meters. During ‘type test,’ failure in any one test results in non compliance with the requirement of standard, hence, each and every test is equally important.

Utilities are continuously adding various tests in their tender specifications to make the energy meter more reliable – as revenue collections of utilities depend upon the accurate functioning of the energy meters. Understanding their requirement and adding the logical features in the meters accordingly – along with making the meters compatible with the standard requirements is the most tedious, but important task for the manufacturer.


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