Energy meters are used in every house and industry to measure the energy consumed. Large energy consumers want better data to make energy management decisions, and utilities need more data to improve their services. The thrust, therefore, is for more and more value-added features, along with compactness of size.
The increase in the use of electronics and microprocessors has led to the development of electronic meters with features like remote sensing, Internet connectivity, LCD displays for active as well as reactive energy, recording of tampering events with date and time, multi-tariff billing, maximum demand and many other power quality monitoring features.
These advances in technology have given rise to the problem of electromagnetic interference (EMI). The circuitry/subassemblies of equipment in use can emit electromagnetic energy, which can affect the performance and reliability of both the nearby equipment (i.e. intersystem interference) and the performance of the sensitive circuit within it (i.e. intrasystem interference).
Accurate billing of the power consumed is important to both utilities and consumers. It is necessary to ensure that meters function properly, particularly in the event of any EMI occurrence, which is a common feature. Events such as switching of inductive loads, electrostatic discharge, lightning, and the presence of telecom, radio and video broadcast signals will give rise to some type of EMI. It is thus essential to manufacture energy meters that are electromagnetically compatible – that is, they should be neither a source of EMI nor affected by any EMI.
This article outlines the various electromagnetic compatibility (EMC) tests according to International Electrotechnical Commission (IEC) standards, and some causes of and solutions to EMI.
NEED FOR ELECTRONIC ENERGY METERS
An electronic energy meter’s functionality and performance is superior to that of the traditional electromechanical energy meter. Electromechanical energy meters work on the Ferraris principle – they register energy on the basis of flux produced by voltage and current and integrate it through the mechanical movement of gears. These electromechanical energy meters have numerous drawbacks, such as lack of stability and accuracy drifts over long periods of time and with variations in the operating environment. Electronic energy meters, on the other hand, use electronic components and are capable of providing all the data needed by consumers and utilities in a reliable manner.
Electronic energy meters measure active/reactive energy by integrating active/reactive power with respect to time. A typical block diagram of an electronic energy meter is given Figure 1 below.
Figure 1: Operation of Electonic Energy Meter
Energy meters consist of a voltage processing circuit (VPC), a current processing circuit (CPC), an application specific integrated circuit (ASIC) and a counter or LCD display for registration of units. They incorporate a number of LEDs for phase, earth, and reverse connection, along with one LED to indicate impulses.
The VPC works as a potential divider, and the CPC functions as a current to voltage converter. The output of the VPC and CPC are fed to the analog to digital converter (ADC). The ADC samples the outputs of the VPC and CPC at a high frequency to translate a real-world waveform. These waveforms are then multiplied, filtered and integrated by microprocessors to extract the necessary information.
Standards set up a common protocol and similar structures which provide meter buyers with the ability to exploit the functionality of meters they buy. Standards provide a common vocabulary to describe meter capabilities.
Many countries have their own standards for energy meters, but most of them can trace their origin to either American Standards Institute (ANSI) or International Electrotechnical Commission (IEC) standards. ANSI standards originated in the U.S.A., while the IEC standards came from Europe. The largest markets for ANSI standards are in Canada, Mexico and the U.S. They are also used in parts of Asia, Central America and South America. To market the product in the European market, the ‘CE’ marking is required, which calls for testing as per the EN standards (which follow the IEC standards).
The IEC and ANSI series of standards for electricity metering cover both the electricity metering equipment and common protocols. In the ANSI series there are three active standards for metering equipment and three active standards for communication with the meter. These are given in Table 1A.
Table 1A : ANSI series of standards for energy meters
|ANSI C12-1||Code for electricity metering|
|ANSI C12-10||EM watthour meter|
|ANSI C12-15||Protocol specification for ANSI type 2 optical port|
|ANSI C12-19||Utility industry end device table|
|ANSI C12-20||Electricity meter 0.2 and 0.5 accuracy class|
|ANSI C12-21||Protocol specifications for telephone modem communication|
Depending upon the type of meter and the accuracy class, a number of IEC standards are available. The IEC series of standards for energy meters is given in Table 1B.
|Standard||Type of Meter
|IEC 62052-11||General requirements, tests and test conditions – electromechanical or static energy meters||General requirements|
|*IEC 62053-11||Electromechanical meters for active energy
(fitted with electronic functional devices)
|0.5, 1 and 2|
|*IEC 62053-21||Static meters for active energy||1 and 2|
|*IEC 62053-22||Static transformer-operated meters for active energy||0.2S and 0.5S|
|*IEC 62053-23||Static meters for reactive energy||2 and 3|
|IEC 62053-31||Pulse output devices for electromechanical and electronic meters ( two wires only)||-|
|IEC 62053-61||Power consumption and voltage requirements||-|
|IEC 62059 - 11||Dependability – general concepts||-|
|IEC 62059-21||Dependability – collection of meter dependability data from the field||-|
* All these specifications refer to IEC62052-11 for EMC tests
TESTING OF ENERGY METERS
The tests covered in the IEC standards for energy meters are broadly divided as follows:
- Mechanical requirements and tests
- Climatic conditions
- Electrical requirements.
The tests covered under electrical requirements are:
- Influence of supply voltage
- Immunity to earth fault
- Electromagnetic compatibility.
Electromagnetic compatibility tests
To ensure that the energy meter is electromagnetically compatible, EMC tests are divided into two categories:
- Emission tests
- Immunity tests.
There are two ways for EMI to travel – following a physical path through wire/cable (conducted) or through free space (radiated). Accordingly these tests are further divided (see below).
These tests ensure that the meter does not conduct or radiate EMI beyond a certain tolerable limit, as given in the standards – in other words, that it does not act as a source. Common sources of EMI in electronic systems are:
- Rectifying diodes
- Switching components
- High frequency transformers and chokes
- Circuit layouts (high dv/dt long wires, high di/dt wide loops, high current wires)
- Mechanical switching/bouncing
- Chassis design.
Depending upon the way the EMI escapes from the equipment, there are two emission tests as per IEC standards – the conducted emission (CE) test, which measures the EMI that passes on to the other equipment via the power lead of the meter under test, and covers the frequency range from 150kHz to 30MHz; and the radiated emission (RE) test, that measures the EMI which escapes into the free space from the meter under test. It covers the emission over the frequency range 30 MHz to 1000 MHz.
Table 2 gives the test methods and test setup for these emission tests.
|Name of test
||Test method description||Test setup
|Transducer used||Measuring Equipment|
|Conducted emission||CISPR22||Line impedance stabilization network||Test receiver/spectrum andalyser with quasi peak and average detecrot, anechoic chamber/OATS for radiated emission|
|Radiated Emission||CISPR22||Bi-conical and log periodic antennae|
These tests are done to ensure the proper functioning of the meter in the presence of EMI. In other words, these tests ensure that the meter does not act as a receptor. Again, depending upon how the EMI gets coupled, the two categories of the immunity tests are as follows:
1) Conducted immunity test
These tests ensure proper functioning of the meter under test, in case EMI generated from one or another source is passed into the meter via data or interface lines, power lines, or by body contact.
2) Radiated immunity test
This test is also known as the electromagnetic high frequency (EMHF) field test. It ensures the proper functioning of the meter under test, in case EMI is present in the surrounding environment.
During immunity testing different types of EMI are stimulated and are superimposed on the meter under test. The sources of EMI covered under each test are given in Table 3A, and Table 3B gives the applicable standards, test method and basic stimulation characteristics used for conducting these tests.
Table 3A : Sources of EMI covered in immunity tests
|Name of test||Sources of EMI covered|
|Electrical Fast Transient (EFT)||Switching transients (interruption of inductive loads, relay contact bounces etc.)|
|Electrostatic Discharge (ESD)||ESD generated owing to environment and installation conditions. such as low relative humidity, low conductivity carpets, vinyl garments etc.|
|Surge||Surges generated by over voltage from switching and lightening transients|
|Conducted susceptibility||Electromagnetic field coming from intended RF transmitters, in the frequency range of 9KHz to 80MHz.|
|Damped Oscillatory||Switching with restriking of the arc, typical of electrical plants, HV/MV stations as well as of heavy industrial installations.|
|Electromagnetic High Frequency Field (EMHF)||EM radiations generated by sources like small handheld radio transceivers, fixed station radio and TV transmitters, vehicle radio transmitters etc., and also spurious radiations caused by devices such as welders, thyristors, fluorescent lights, switches, operating inductive loads etc.|
Table 3B: Immunity tests for the static energy meters
|Name of Test||Standard
|Electrical fast transient||IEC61000-4-4||With voltage and auxiliary circuit energized with reference voltage and with basic current||Pulse : 1.2/50uS
Burst duration: 15mSec
Burst period: 300mSec
Polarity : +ve and –ve
Test voltage applied in common mode
Test voltage: 4KV (mains line)/ 2KV (auxiliary circuit)
Test duration : One minute at each polarity
|Electrostatic discharge||IEC61000-4-2||With voltage and auxiliary circuit energized with reference voltage and without any current in current circuit||Discharge : contact/air (If no metallic part outside)
Test voltage : 8 kV for contact discharge / 15 kV for air discharge
Number of discharges: 10 of each polarity
|Conducted disturbance induced by radio frequency fields||IEC61000-4-6||With voltage and auxiliary circuit energized with reference voltage and with Ib||Freq.: 150k to 80M Hz
Test voltage : 10 V
|Surge||IEC61000-4-5||With voltage and auxiliary circuit energized with reference voltage and without any current in the current circuit||Test voltage: 4 KV on mains / 1 KV on auxiliary circuit
Tested in differential mode
Generator source impedance : 2ï— for mains / 42ï— for auxiliary circuit
Number of surges : 5 of each polarity
Repetition rate : maximum 1/min.
|Damped oscillatory wave (only for transformer-operated meters)||IEC61000-4-12||With voltage and auxiliary circuit energized with reference voltage and with rated current||Test voltage : 2.5 V for common mode / 1 KV for differential mode
Voltage rise time : 75nsec
(a) 100 kHz, repetition rate 40Hz
(b) 1 MHz, repetition rate 400Hz
Test duration : 60 sec (15 cycles with 2sec on and 2 sec off for each frequency)
|Electromagnetic RF field||IEC61000-4-3||With voltage and auxiliary circuit energized with reference voltage and
|Frequency band : (80 to 2000) MHz
Carrier modulated with 80% AM at 1 KHz sine wave
Unmodulated field strength:
(a)With Ib – 10 V/m
(b) Without Ib – 30 V/m
* Basic stimulation characteristics are given here. For details refer to the relevant standard.
Ib – Basic current
EMI – CAUSES AND SOLUTIONS
Emission, immunity and path are the three constituents of the EMC. Of these, emission causes the most incompatibilities, while yielding the greatest number of solutions. Immunity, on the other hand, is more subtle in its effects and solutions. Finally the path can be the arbiter of both. Table 4A gives the sources of radiated and conducted emission and some solutions to avoid failure in these tests. Table 4B gives probable reasons for failure of immunity tests and their solutions.
Table 4A : Emission sources of EMI and solutions
|Emission sources||Solutions for compliance|
Clock frequencies and processing signals
|Use decoupling capacitor and ferrite beads, shielding meter, keep cable length short.|
|Conducted emission: SMPS, clock and signals||Put filter at input of SMPS, use ferrite beads and decoupling capacitors on VCC line of clock generating circuit and processing circuit.|
Table 4B : Reasons for failure of immunity tests and some solutions
|Test||Probable reasons for failure||Solutions|
|Electrostatic discharge||Improper grounding of cover, improper placement of components, non fixing of noise decoupling capacitor at input of deriver circuit of display||Make good conductive contact between top and bottom cover of meter, physically isolate the sensitive circuit from mains, use decoupling capacitor at the input of driver circuit of display|
|Electrical fast transient||AC power cord poorly filtered, power rails poorly decoupled||Use decoupling capacitor at input of driver circuit and separate the mains and sensitive circuit|
|Surge test||Poor insulation of internal wiring, low creepage distance and absence of surge protector||Use properly insulated internal wiring, provide adequate creepage distance, use surge protector (MOV)|
|Conducted susceptibility||Improper routing of the wires, use of unshielded wires||Use filter, shield cables with proper grounding of the shield, isolate power supply and signal lines|
|Damped oscillatory wave||Not using proper filter and MOVs at the input||Use MOV at input, use filters|
|Electromagnetic high frequency field||Openings, improper shielding and grounding||Improve shielding and grounding, add filter in input circuit. Ensure proper mating of top and bottom/ side covers and connector|
EMC tests are vital for energy meters fitted with electronic functional devices. Designing a meter that does not generate EMI is as important as designing it so that it will not be affected by EMI.
A unique solution for EMC does not exist. The solution can be as simple as adding an inductor to adding complex filters. Manufacturers often spend more money and lose contracts when their products do not meet the EMC standards at the time of final testing.
Two approaches are available for manufacturers:
- Test at designing stage
- Incorporate EMI fixes on the finished model.
The first approach is logical and reduces the time to market the products. The second approach will cause delay, frustration and money. Hence manufacturers should implement EMC facilities during the design phase to make their product robust and reliable in an EMI environment.
So – design it right, right in the beginning!