To prove the reliability, accuracy and accountability of energy meters Bureau of Indian Standard has brought out standards IS16444 (Part1) for whole current smart meters, IS16444 (Part2) for transformer operated smart meters and IS15884 for prepayment meters. Time to Time revisions also have been introduced by BIS to fulfill the latest scenario of AMI. Manufactures in India are preparing good quality of smart meters which has been proven by successful completion of pilot projects and implementation of AMI by many utilities[4].
Smart meters along with its associated systems Head and system (HES), Meter data management system (MDM), Billing system, communication technology provide the utilities the real time energy consumption data of each consumer for subsequent analysis and will pave the way for initiating various smart measures by DISCOMs like Time Of Day (TOD)/Time of Use (TOU) billing, prediction and management of peak demand, providing real time energy consumption data to consumer, prepaid billing facility, remote connection and disconnection of load, accurate billing, secure handling of the data etc. Installation of these meters will also obviate the need for the meter reader’s visit to each and every consumer.
To prove the reliability, accuracy, accountability and security of energy meters for the above task it is at most important that the utilities shall test the meters rigorously from the testing laboratories before purchasing so that its better performance can be ensured before putting to services to avoid breakdowns, malfunctioning.
Smart Meter Funtionalities
Smart meter being a smart intelligent component between distribution and consumption of electricity, many tasks to be performed by it. The different functions of smart meter is shown in the Figure.1 [5]
To perform all the functions mentioned in the figure below, smart meters have different features. These features of the smart meter are to be verified with reference to the Indian standards to confirm its performance. Table 1 given above illustrates the functionality of the smart meter and standard against which it has to be verified [1], [2],[3]
Metrological Testing: Testing in accordance with IS 16444
There are around 40 nos. of tests to verify the performance of the meters at different service conditions such as electromagnetic fields, climatic conditions, under various influence quantities such as voltage variation, frequency variation, harmonics, magnetic fields etc. and also under fault condition such as short circuits and earth faults, which may affect at the fields.
Meter Tariff Rate
In the smart prepayment meter, tariff of per kwh is programmed in the meter. For 1kWh consumption of energy, the credit deduction shall be as per tariff programmed in the meter. During installation it will be programmed as per utility requirement, but for the testing purpose it is not fixed. Each manufacture fixed the tariff rate as per their convenience, for example – Rs.1/kWh,Rs.5/kWh,Rs,10/kWh and so on. As mentioned in IS15884, the display resolution for monetary shall be 1 Rupee and that of energy consumption shall be 0.01kWh. After conducting each test or sequence of tests as per the standard, consumption of Energy and credit deduction with reference to consumption of kWh has to be verified[1].. Let us discuss the below cases.
Case 1: Tariff Rate – Rs.5/kWh
Here the monetary display is in Paisa and therefore the Energy consumption (kWh) is matching with the Credit deduction as per the tariff Rate i.e for 4.18 kWh it is 4.18 x 5 = Rs.20.90
Case 2 :Tariff Rate: Rs. 5/kWh
Here the monetary display is in Rupees and therefore the Energy consumption (kWh) is not matching with the Credit deduction as per the tariff Rate. In such case, test has to be continued till the kWh reading to reach round figure of 1 kWh, so that kWh consumption and exhausted credit can be exactly verified. This tariff mismatch may not have much effect in field for electricity billing, but the test results may vary in the lab during testing. In such case, the manufacturer has been asked to change the tariff to Rs. 10/kWh so that credit can be verified up to 0.1kWh. High resolution of kWh reading display is provided in the meter for the testing purpose, the credit deduction in high resolution means in paisa shall also be provided in the meter display for testing.
Maximum Load Setting (setting of Over Current and Load Control Limit)
Over current setting for the smart meter as per IS: 16444(Part 1) is 1.05 % of Imax and that of prepayment meters is not defined in the standard IS15884, generally it is taken as Imax. Standards specify that the load control limit to be programmed in the meter is to be defined by the utility. Test parameters for test of influence of heating are 115% of Vref. at Imax and UPF. As the voltage is 115% of Vref. at Imax and unity power factor during heating test [1]., the load will be 1.15 times higher than load at Imax with nominal reference voltage. If the load limit set by the utility/Manufacturer for the load w.r.t. Imax or 105% of Imax, the load switch will get opened/ disconnected due to over load and the meter will not comply with the requirement of standard for this test.
During final submission of Meter for testing or submission of samples for the tender, manufacturer shall take care of setting of overload current and load control limit to avoid discrepancies during testing in the lab.
Logging of tamper event during abnormal voltage test
During Abnormal voltage of test, voltage of 1.9 times Vref. [1.9( 240V) = 459V] applied to any two phases and neutral for 4 hours with 0.5Ibasic current. Test to be repeated 3 times to cover the pairs of phases, with a cooling period of 1 hour between each run[1].
Meter will detect this test condition as tamper event and the load switch will get opened if tamper of high voltage or neutral disturbance programmed in the Smart meter with logic of this condition.
Testing laboratory will not be able to perform the test on such meter, hence the tamper logic shall not be set for this condition.
Delay in closing of loads switch
Some load switches fitting in the smart meter take few milliseconds to close when power is switched ON. During testing of such meters on Phantom load, voltage has to be applied first and after getting load switch closed, current has to be applied. This is possible in manual mode of the test bench. In automatic mode, the test bench will over load due to opening of the load switch of the meter,
Load switching capability: Testing in accordance with IS 15884
The main purpose of the smart meters roll out is to manage supply through the reduction of demand during peak hour, energy saving through individual loads switching, accurate billing of consumers and the ability to disconnect user refusing to pay or abusing the use of electricity. This ability to disconnect/connect power is possible because of the use of latching relays (Load Switch) inside the meter and hence the quality, reliability and safety of the latching relays are an important aspect. There are tests stated in the standards IS 15888 to verify the load switch.
Closing of the load switch with the optical port during test of short circuit current carrying capacity
To verify the safety of the installation after experiencing short-circuit condition directly on meter terminals, Short circuit current carrying capacity is performed on the meters. Short circuit Test current of 4.5 kA for UC2 and 6kA for UC3 with power factor 0.65 to 0.80 inductive is to be applied for half cycle i.e.10 msec. as shown in the Figure 2. Test is carried out with load switch in closed condition [1].
Some of the meter as soon as switched off its load switch gets opened. Load switch has to be closed before start of the test which is done through Basic computer software (BCS) of the meters by giving command to load switch. For this proper synchronisation of closing of load switch with the operation of test set up is required. Delay in communication between BCS and meter results in the non application of short circuit current and test sequence has to be repeated till the application of test current.
Closing of the load switch with the optical port during Test of Fault current making capacity
To verify the capability of smart meter for making into fault currents and to check for welding of contacts caused by contact bounce at the point of closure into the test current.
Fault current making capacity test carried out at test current of 2.5 kA for UC2 and 3 kA for UC3 with power factor 0.85 to 0.90 inductive. It is required to close the load switch contacts at the above prospective test current and remain in the closed position [1].
The test current shall be maintained to flow up to the first zero point crossing of the current, at which point, the test equipment shall disconnect the voltage source as shown in figure 3.
As discussed earlier, the synchronization of closing of load switch with the operation of test set up is required. Delay in communication between BCS and meter results in the non application of Test current and test sequence has to be repeated till the application of test current.
Make and break operation Electrical Endurance test
To verify the load switching capability of load switch of the meter, Electrical Endurance test for 5000 operations of make and break have been carried out at maximum current, UPF and 0.5 lag each is to be carried out. ON and OFF operation of load switch is carried out by the BCS provided by the manufacturer through the optical port. The software has a program to switch ON the load switch for 10s and switch OFF the load for 20s with the counter to count no. of operation performed.
The ON/OFF operation of the load switch will not perform if there is communication failure and hence there will be possibility of heating/burning of load connected (Test set up) due to continuous supply.
Data Exchange Protocol: Testing in accordance with IS 15959
The main intention of introducing Meter protocol testing is to ensure interoperability of Energy Meters of various manufacturers for utilities and to support of AMI (Automatic Meter infrastructure).
DLMS (Device Language Message Specification) is important for achieving interoperability and hence changed the Power sector from monopoly to open and standardised market.
‘Tests for Data Exchange Protocol’ carried out for data verification on optical port to verify the association requirements, parameters and profiles in accordance with the standard. The ‘Tests for Smart Meter Communicability and Functional requirements’ carried out on meter for Compliance to DLMS/ COSEM (companion specification for energy metering) open protocol.
Scheduled activation of New Firmware:
Smart Meter shall have remote firmware upgrade feature as per IS:15959(Part-2),2016.The Firmware upgrade process is done in two steps, first step is image transfer(using IC18) which transfers new image file to the meter and second step is image activation which activates new image file that transferred earlier[3].
Image Activation can be done using two methods:
Immediate activation using image activate attribute of IC18 of smart meter –the firmware up gradation will be activated as soon as it is transfer to the
smart meter
Scheduled activation using image activate attribute of IC22 of smart meter – activation of firmware up gradation will be done at the schedule time which may be any time between 00:00:00 to 24.00.00.
If default scheduled activation time in the meter is set at 00.00.00, then activation of new firmware will be done at time 00.00.00 only irrespective of any other scheduled time for between 00:00:00 to 24.00.00. As per standard, the activation of firmware shall be done at any schedule time instead of fixed time of 00.00.00.
So, smart meter shall be programmed to activate the firmware image file at any scheduled time between 00:00:00 to 24.00.00 as per the requirement of standard instead of fixed/default setting.
Event Status Bit (Meter Load Connect and Disconnect)
Event status bit (ESW) is of 128 bit long in size where different bits are defined for some events as per standard. ESW is used to notify status change of any predefined event.
Event Bit no. 84 is defined for meter load connect and disconnect. As per IS15959 (Part 2) status of 84 bit is defines as [3]
- 84 bit value is “0” for Load Connect
- 84 bit value is “1” for Load Disconnect
Implementation of this bit may be done exactly opposite i.e.
- 84 bit value is “0” for Load Disconnect
- 84 bit value is “1” for Load Connect
due to the requirement of tender specification of Utilities or AMI systems, but it will not comply as per the standard.
Simultaneous Operation:
As per standard meter should not support simultaneous operation of both the ports, it should always give preference to optical port. To supports the operation through both the ports optical and RS232 ports at a time is not permitted by the standards [3].
Non availability on some Standard Objects:
List of standard object along with their capture parameter are defined in IS15959 for all categories of meter. In some cases it is noticed that meter does not have one or more object that are required by the standard.
For example
- Cumulative Power Failure Duration(0.0.94.91.0.255) is not present in one meter
- Cumulative Billing Count (0.0.0.1.0.255) is not present in one meter
In all such cases where any object is absent from meter, non compliance report is given.
Conclusion
Smart meters are a new generation of energy meters and heart of the AMI. we have discussed some of the issues noticed during testing of the Smart meters and prepaid smart meters which may results in non compliance with the National and International standards and those have to be taken care during construction and programming stage of the Energy Meters.
Performance of smart meter, operation of load switch of the smart meter, communicability and interoperability of the smart meters are the main parameters of the AMI for electricity metering, monitoring and management. It is the responsibility of meter manufactures, testing laboratories and the utilities to work together and contribute their best for the reliable operation of the whole system. With the upcoming technologies and system upgradation, many features will be add on. The results of testing laboratories and prototype projects will decide the success of the system.
Acknowledgement
The authors would like to thank the staff of Central Power Research Institute for their encouragement and support in their endeavors and also providing testing facilities of Energy Meter Testing Laboratory.
References
[1] IS 15884: AC direct connected static prepayment meter for active energyclass1and 2
[2] IS16444(Part1): AC static direct connected watt-hour smart meter, class1 and 2 [3] IS 15959(Part 1 & 2): Data Exchange for Electricity Meter – Reading Tariff and Load Control – Companion Specification
[4] ‘Testing challenges for DLMS Compliance tests of Smart Meters as per Indian Standards’ by Viji Bharathi, Jiju*, Dhanavath Shankar and P.V. Harinath Babu CPRI, Bangalore and *CDAC,Thiruvananthapuram
[5] https://www.electricalindia.in/role of smart meters in smart grid (visited on 22.02.23).
Deepa Warudkar completed graduation in Electrical Engineering from Nagpur University and M. Tech in HEE from MANIT, Bhopal. Presently she is working as a Test Co-ordinator in Energy Meter Testing Laboratory, CPRI, Bhopal. She has more than 20 years of experience in all types of Energy Meter certification testing and development testing.
Gyan Prakash Nirmal, completed graduation in Electrical Engineering from MMMUT Gorakhpur. He is now working as a Test Engineer in Energy Meter Testing Lab, CPRI, Bhopal. He has four years’ experience in the field of testing and certification on energy meters.
B.A.Sawle, Additional Director, Central Power Research Institute, BHOPAL
