By Ashok Kumar, Joint Director, Electronics Regional Test Laboratory (North) STQC Directorate, Department of IT, New Delhi
Metering management in power ensures the effectiveness of power monitoring, controlling, recording and data analysis resulting in optimum energy utilisation. Earlier when the energy cost was low and power quality requirements were less stringent, the performance consideration of instruments was secondary to the purchase price of instruments. The increase in the cost of energy and requirement for enhanced quality of power has led to sharpen the difference between commercial (less accurate) and expensive (accurate) instruments. To meet this challenge energy meters have been developed with the state of the art technology which are capable of measuring and registering data of loadings, reactive and apparent power, earth current, power quality including the harmonic contents, warning and alarms in real time with the non-volatile memory. These meters are compatible with the commercial GSM networks and with the latest information systems used in power networks.
The Energy Meter being a critical item in Power Systems, its correct functioning is essential for reliable operation of the System. The energy meters inside the sub-stations with state of the art technology are used to acquire data of loading, reactive and apparent power, earth current, and power quality including the harmonics. An utilities information centre uses this information to take the action in the most efficient and cost effective manner.
The limited availability of electrical energy has focused a great deal of attention on power generation, its application, monitoring and control. Power economics play an important role inside the sub-station with the high cost of power generation, transmission and distribution. It is of paramount concern to effectively register, monitor and control the use of energy. The measuring and monitoring of electrical power has also become more complex.
Safety of personnel and property is the most important factor in the operation of electrical system. The reliability of any electrical system depends upon design, preventive maintenance and also on the equipment used to monitor the system. It is therefore, essential to test the power monitoring equipment. Testing includes verification of the design, material and components. Ideally a perfect design with error free assembly, proven material and 100% component quality would eliminate testing altogether. Rapid changes in technology, design complexity for quality improvement advanced manufacturing techniques and process improvements, cost control to maintain competitiveness and customer satisfaction affects the quality of product and makes the testing essential.
TESTING OF ENERGY METER
Electronics Regional Test Laboratory (North) is an independent accredited Government laboratory under the Department of Information Technology providing services for testing of Energy Meters according to National (IS) and International (IEC) specifications to Indian and International manufacturers and utilities. We also provide services to other agencies like users, regulatory and certification bodies etc.
Testing may be based upon structural, functional/ parametric requirements and application of the equipment to ensure overall product performance and meet the requirements of specification. Prior to testing it is necessary to understand and interpret the specifications as well as functional, physical description, performance, modes of operations, description of methods of connections, position and associated accessories to function EUT. It is also equally important to select the test equipment for the relevant tests.
Classification of testing Broadly testing may be classified as:
- Developmental/Prototype testing
- Type testing
- Acceptance testing
- Routine testing
- Certification testing
- Third Party testing
- In-service check/On site testing
Specifications generally used for the testing of energy meters
Development testing is generally used as a part of investigations into the capabilities of prototypes. The testing should be started from least severe tests so that maximum possible information can be obtained before the test specimen is damaged.
Specification based type approval testing is carried out as per the test sequence so as to give the most significant effect and reveal the failures which may not come out if not tested sequentially.
After completion of testing, a test certificate containing the following information is issued:
- Any modifications or limitations applied to test requirements.
- Details of test equipments used for testing and their calibration status
- Test methods/procedures followed
- Uncertainty of test parameter
- Conformity to Specification statement and
- Opinion if required.
In India the National Accreditation Board for Test and Calibration laboratories is the authority to accredit the laboratories. This accreditation confirms that the laboratory meets the requirement of International Specification ISO/IEC 17025. This gives confidence to customers in the quality of testing of laboratory for acceptance and third party certification.
- Failure in insulation properties
After revision of IS 13779:1993, the test severity of impulse voltage changed as given above. Initially failure rate was 80%. It was due to change in source impedance from 500 Ohm to 2 Ohm and 12 Ohm. The energy transfer is more when source impedance is low. Whereas as per pre-revised IS and prevailing IEC the source impedance is 500Ohm and energy is limited to 0.5J. IS also specifies insulation resistance test in addition to impulse voltage and ac voltage tests. India being a tropical country the IR test is performed to reveal the deterioration in insulation properties after the impulse voltage and ac voltage test. To meet the requirement of IS for high energy transfer designs are modified and failure has been reduced to 5%.
- Severe conditions for mechanical tests
The severity for vibration and shock tests is high in IS as compared to the requirement in IEC. This has been done, keeping in view transportation conditions available in India. Test severity for vibration test is 0.15mm/4mm amplitude and acceleration 2/2.2gn in IS as compared to 0.075mm amplitude and acceleration 1gn in IEC. Severity of shock test in IS also 40gn as compared to 30gn in IEC. The meter meeting IS requirements are found well suitable for Indian transportation conditions.
- Severe conditions for influence of magnetic induction of external origin tests
The revised IS and CBIP has included the additional test for abnormal magnetic induction of external origin up to 0.2T for ac field and 0.27 T for dc field, as compared to IEC. Initially failure rate of meters was 30%. The modified design of meters has brought down the failure rate to 1%.
- Less severe conditions for harmonic tests
ISs specify test of the harmonic component to be carried out with 10% 3rd harmonic current injection in-phase and out of phase in current circuit only whereas as per IEC specs this test is carried out with 10% 5th harmonic voltage injection in-phase with voltage circuit and 40% 5th harmonic current injection in-phase with current circuit simultaneously. In addition, as per IEC the tests with phase fired (odd harmonics) and burst fired (sub-harmonics) waveforms injection in current circuit are also to be carried out which are not included in ISs. Nowdays most of the electricity used in domestic commercial and industrial installations is controlled by controllers which generate harmonics of higher orders in current circuits having significant amplitude of harmonics in comparison with fundamental and also distorts the waveform of the voltage circuit. This energy has to be registered by energy meters. To meet the requirements of installation/power supply conditions these tests may be included in National Specifications.
- Repeatability of error test
Repeatability of error test is performed as per IS to verify the short term stability of the energy meter with respect to time. This test is not included in IEC.
- Less severity for EMI/EMC tests
Severity for Test of immunity to electromagnetic RF field without current in current circuit in IS is with field strength of 10V/m as compare to 30V/m in CBIP (Indian) & IEC. In addition CBIP and IEC specify for test of immunity to conducted disturbances induced by RF field, damped oscillatory waves immunity test, surge immunity test. Keeping in view the existing environmental condition these tests may be included in IS.
- Test for output device
Test for optical output device (LED blinking) is covered in IEC. The wavelength, radiation strength, pulse frequency and transition time are verified to test the optimum pulse transmission. Ambient light intensity up to 16000Lux shall not affect the optical transmission of pulse. Poor quality of LED and very high pulse frequency may affect the optical transmission and add a lot to measurement uncertainties. This aspect is not covered in IS.
- Special test requirements
Some of the special test requirements by utilities/user are given below:
- Quantities to be measured and displayed
- MD reset
- MD integration period
- Communication/Data down loading capability
- TOD metering
- Load profiles
- Programming capabilities
- Self diagnostic features
- Monitoring and logging of system/installation abnormalities
- Battery back up
- Anti tamper and fraud protection tests.
The CBIP manual and other Utilities/State electricity boards also specify to test the meters against temper and fraud protection tests. Anti tamper features requirements are changing rapidly. Some of the utilities specify that the meter should be immune to 0.5T magnetic field obtained from permanent magnet. Some specifications specify that meter shall register the energy based upon the highest current flown through, either of two elements in case of single phase meter, irrespective of phase and neutral. In case of proper wiring and no leakage, the specification holds good. But some time it is observed that when neutral and ground wires inside the installation (ie load side) are shorted or there is any leakage current, the meter records energy even if there is no actual load connected to the meter, in some cases more energy than that of the maximum load connected to the meter. Recently evolved IS 15707:2006 gives thrust on in service check/ testing of consumer installation to protect the consumer against overbilling.
There are many parametric differences in IS and IEC for the similar type of energy meter. In the present scenario of globalisation we must harmonise the Specifications with national deviations if any.
Meter testing resulted in image building, statistical analysis, gaining confidence of customer, minimising customer complaints, meeting regulatory requirements and performance of meters in service. In-service/on-site testing is gaining momentum as a part of in-service compliance testing or to carryout metrological testing and functional verifications of meter on receipt of customer’s complaint or internal report. 15707:2006 covers on-site testing.
The primary objective of the meter to register the energy is becoming secondary. The importance is being given to detect tamper and fraud, monitor the quality of supply, time of use billing, demand analysis, outage management, remote metering, connect and disconnect. The existing meter test facilities are being upgraded to keep pace with the change in technology requirements. Anti tamper feature requirements are changing rapidly. To incorporate these features, the design of meter is becoming complex. This is affecting cost as well as reliability of the meter.
It is important to note that reliability aspect of electronic/ static meters is not covered in most of the specifications and no short-term test procedure is available which would fit into type test sequence to satisfactorily check the reliability. Revised CBIP manual 304:2008 describes a test procedure consisting of initial measurements, operational conditioning at maximum operating temperature for 1000hrs and final measurements. Similar test specification requirement may also be considered for inclusion in IS/IEC specification to ascertain the reliability of energy meters.
|Recommended type test sequence|
|Appropriate test sequence which is almost same for IS and IEC is given below:|
|Test of insulation properties
Impulse voltage test
AC voltage test
|Test the design and construction of meter for safety of personal / equipment in normal use and under normal conditions.|
|Test of accuracy requirements
Test of meter Constant
Test of starting condition
Test of no load condition
Tests of Influence quantities
|Verify the metrological requirements of energy meters under normal conditions of use and under the influence of influencing quantities.|
|Test of electrical requirements
Test of power consumption
Test of Influence of short time current
Test of Influence of self heating
Test of Influence of heating
Test of immunity to earth fault
|Determine the suitability of energy meters to meet various electrical requirements of mains supply as well load condition.|
|Test for electromagnetic compatibility (EMC)
Radio Interference suppression
Fast Transient burst test
Test of Immunity to HF field
Test of electrostatic discharges
Test of immunity to conducted disturbances induced by RF field
Damped oscillatory waves immunity test
Surge Immunity test
|Ensure electromagnetic compatibility of the energy meters in their operational environments.|
|Test of climatic influences
Dry heat test
Damp heat, cyclic test
Solar radiation test
|The effects of environment on energy meters which may be in the form of insulation failures, physical failures, chemical physical and photo chemical reactions, discoloring etc.|
|Test of mechanical requirement
|Principal effects of mechanical stress, fatigue, resonance resulting into mechanical failures, structural collapse etc.|
|Test of protection against penetration of dust and water||The sealing effect|
|Test of resistance to heat and fire &
Heat deflection test
|Personal and equipment safety against effect of excessive temperature and spread of fire.|