Calibratation and Testing of Energy Meters and Meters Laboratory

Energy meters form a vital instrument of revenue realization for the distribution utilities. Based on the meter reading, the utilities submit their bills to consumers for realization. Inaccurate/faulty recording may become catastrophic both to consumers and utilities. Thus, a need to ensure a high degree of performance and reliability from the meters has become important.The meters are tested and calibrated to ensure proper working and that the accuracy is to be maintained within permissible limits under IE Rules.As per Indian Standard specification 722, a meter should pass the following two types of tests:

• Type Tests

• Routine TestsMeters are calibrated such that the overall accuracy is within the percentage error limits. All initial calibrations of meters are performed in a laboratory or test house. The most common method for testing meters is to inject voltage and current from a stable power source in combination with a reference meter.

The main requirement in a test lab will be: Test bench, Reference standards, Power source, Software and Accessories.

Test Bench

 

The test bench could be for single phase or three phase meters,electromechanical or electronic. It could be fully automatic, semi automatic or manual. As per the Indian Electricity Rules, the meters in service need to keep to the prescribed accuracy range. With time the accuracy of the Ferrari’s meter degrades and the meter tends to slow down. For that, periodical testing of and recalibration of all meters installed in the field for all categories of consumers need to be carried out as per the meter standards. The test schedule as per the Central Electricity Authority Regulations on Installation and Operation of Meters is given in Table.

Test Schedule

Meter Type Schedule

The testing schedule for HT consumers should cover the entire Metering System including CTs, PTs and Pilot Wire. Detailed ratio testing of CT, PT and control Pilot/ wires is to be done. Testing through mobile vans/mobile cubicles through secondary injections kit and phantom loading may also be carried out by Measurement Unit. All new meters should meet the requirements of Accuracy Class spelt out in Table.

Requirements of Accuracy Class

Methods of Testing

Irrespective of purpose, testing is generally carried out

• on site, or

• in a Meter Testing Station (MTS) accredited as per IS/ISO/IEC 17025.

On-site testing may be carried out

i)under actual prevailing load; and

ii) by injection method after connecting a suitable phantom load or a resistive load.

i)Actual Loading Method
This method is employed generally for testing of single phase meters in laboratory. On a meter testing bench which is equipped with various taps to facilitate meter testing at different load conditions. The load is provided by standard resistance or lamps obviously power factor is unity. Load adjustments are done in steps of 1/10 th of full load at unity power factor.It is important to note in this connection that consumer’s power factor varies from 0.5 to unity (or 0.85). Therefore, meter must be calibrated at 0.5 PF. To achieve this condition, the current coil is energized by R phase and voltage coil by Y phase and the polarity of pressure coil is reversed.Under these conditions, PF becomes 0.5 because of 120 degree phase shift between voltage and current vectors.

ii) Phantom or Fictitious Loading Method
When the capacity of meters under test is very high, testing with actual load method would involve a considerable loss of power and would not beeconomical too. Therefore, to avoid wastage of power during testing,phantom load method is adopted. In this method, pressure circuit is supplied from a circuit of normal voltage source and the current circuit from a separate low voltage source. Thus, total power consumed during testing becomes very small due to small pressure coil current at normal voltage plus full load current at very less voltage for current circuit meter. However,the meter under test in this condition simulates the full load condition by such phantom load kit.

Testing of CT Operated Meters

The following tests should be carried out for CT operated meters:

i)Ratio test by primary injection kit,

ii) Polarity test,

iii) Continuity test of primary and secondary windings,

iv) Insulation resistance test, and

v) Terminal markings as per name plate details.

It should be noted that CT Secondary terminals should not be kept open. For CT and PT operated meters, it is preferable to use a Test Terminal Block(TTB). The TTB provides the facilities of shorting CT secondaries for testing purpose without any interruption to the consumer. The CT leads should be directly connected to the meter, while PT supply to the meter should be connected through the fuses of proper rating. To eliminate the possibility of tampering with the fuses, the fuses should be provided with protective cover and properly sealed.

Checking at Site

The routine testing of meters should be carried out once in 3 years. In addition, surprise checks should be conducted at the consumer’s installation and meters should be checked from time to time. The important points to be paid attention to while checking are given in Box .

Box : Checklist of Important Points for Checking at Site

i) Connections of meter are proper.

ii) Seals are not tampered with.

iii) No loose or direct connections exist.

iv) Meter is not damaged to record less.

v) CTs are properly connected. No reversal of any phase connection exists. Multiplying factor is properly calculated and applied.

vi) Meter is well protected from rain, vermin’s and dust etc.

vii) Links are connected.

viii) Terminal block is not damaged.

With the help of load current of known magnitude (preferably of resistive type) and a wrist watch having a second’s hand, you can off hand check at site the performance of the meter as also its accuracy to some extent.