An energy meter is a tool that measures energy consumption of the customers based on their loading conditions. In both commercial and residential electric circuits, energy meters are used to determine power consumption. An energy meter measures electrical power and energy and enables us to charge customers for the use of electricity.
Types of energy meter and their brief descriptions
Static energy meter: A watt-hour meter in which current and voltage act on solid-state (electronic) elements to produce an output proportional to watt-hours. It is also known as a Static Energy Meter because it has no moving parts. The meter’s output is inversely proportional to its internal digital pulse.
Prepayment energy meter: Electricity meters with extra features that can be operated and managed to permit the flow of energy in accordance with established payment methods. In similar way as mobile phone balance recharge, you will get a recharge card and a usable energy unit for the remaining amount. Similar to prepaid cell phone recharges, the more energy you use, the more you recharge. When the zero balance limit is reached, the power will automatically cut off. However, depending on peak hours, energy consumption and deductions can be controlled accordingly.
Smart energy meter: An energy meter with time of use registers and two-way communication capabilities is known as a smart meter. Smart meters include first-generation smart meters (AMR meters) and second-generation smart meters (AMI meters). AMR meters self-check meter status, data communication using secure and open standard protocols, regular remote meter software updates over the transmission network, multi-utility meter functions, consumption data collection, and enables demand management and control. By comparison, AMI meters or smart meters provide effective use and management of meter data, automated meter management, two-way communication with meters, demand response capabilities, and as well as information on energy efficiency.
A few important definitions
- PCB: Printed circuit board.
- Creepage: the shortest distance between two conducting elements as measured across the insulation’s surface.
- Clearance: clearance is defined as the minimum distance through the air (line of sight) between two conductor traces.
- Wire wound resistor: Special construction of resistor done for limiting inrush current.
- Varistor: A metal oxide Varistor is a component that clamps the surge voltage at its limit.
Test for insulation properties
These tests are done to check the insulation strength of the meters. All-dielectric property tests must be performed for each terminal configuration when there are multiple terminal arrangements.
Only a fully assembled meter with its terminal cover and the terminal screws, tightened to the maximum relevant conductor installed in the terminals, may
be tested.
Following tests are there in standard to test the insulation properties of the energy meters:
- Test of Impulse Voltage
- Test of AC High Voltage
- Test of Insulation Resistance Measurement
Test of impulse voltage: To measure the impact of voltage stress on insulation caused by short-duration voltage pulses, impulse voltage testing is advised. An impulse is a type of overvoltage that can happen in real-world situations, like when lightning strikes insulation and creates a current route that permanently damages the insulating property. Alternatively, a voltage impulse on a line with an open MCB could jump across the contact and create an arc path, which could destroy a component of your apparatus.
With the help of impulse test, you can see how an electrical installation or component would be affected by a voltage surge caused by brief atmospheric disturbances. Overvoltage on overhead transmission lines can be caused by lightning, either directly or indirectly.
Systems for producing impulse voltages that mimic switching surges and lightning strikes are known as impulse testing systems. A charging rectifier, ‘Marx Circuit’ impulse stages, an impulse voltage divider, and an impulse voltage measurement system make up the entire test apparatus. Systems for impulse testing are created. Therefore, energy meters must adhere with high surge and impulse immunity levels. In India, some utilities demand 10 kV.
Various types of instruments are available for Impulse generators. Here is an example below.
Test procedure: This test is done to determine the effect of voltage surges due to atmospheric disturbances of very short duration on electrical installations and their individual parts.
Impulse voltage of +/- 6 kV; 1.2/50 microseconds.
This this done with both positive and negative polarities. The minimum interval between two impulses is 3 seconds.
Impulse voltage test of electric circuits relative to earth: The impulse voltage must be applied between the electric circuits and the earth.
Auxiliary circuits must be connected to earth whose ratings are less than 40V.
Impulse voltage tests for circuits and between the circuits:
- Between circuits (or group of circuits) that are isolated from one-another in normal use must be tested independently.
- The circuits which are not connected to impulse shall be earthed properly.
Set up of impulse voltage test: Impulse voltage test is recommended with the object to determine the effect of stress on insulation due to voltage surges of short duration. a surge is a form of over-voltage, which may be defined as an exceptional voltage over peak voltage to earth at a line or electrical system of which the line forms a part.
This impulse tester is designed to generate impulse voltage of 500v to 12kv & 6Kv for 0.5 Joule. The waveform generated has rise time of 1.2 micro second and fall time duration of 50 micro second duration as defined in IEC 60950-1:2005 and IS 13252 Part 1. The peak voltage can be adjusted continuously with the help of variac provided. The test sequence i.e., number of impulse for positive and negative polarity; time between two subsequent impulses can be programmed through key pad, with the help of dedicated PLC Module. The total sequence, for both positive and negative impulse is automatic. At the end of the test results are displayed on LED display.
Output terminal provided for connecting an oscilloscope for displaying the Waveform at the output Terminals.
Standard lightning impulse according to IEC 60060 is 1.2 µs ±30% / 50 µs ±20%. A tolerance of ±3% in peak values is permitted.
Test of AC high voltage
Test Procedure:
- Test of meter with Double insulation.
- Between all interconnected voltage and current circuits and auxiliary circuits having a reference voltage greater than 40 volts on the one point and earth on the other point: 4.0 kV
- Between circuits that are not connected together in normal operations. (Voltage and Current circuit in case of transformer operated meters): 2.0 kV.
- Test voltage is applied for 1 minute.
- The test voltage shall be sinusoidal, with a frequency between 45 Hz and 55 Hz.
- No flashovers, disruptive discharge, or punctures during testing.
- For any additional test, 80% of the test voltage shall be applied.
Test of insulation resistance measurement
Test Procedure:
- Test Voltage: DC 500 ± 50 V.
- Points of Application of the Test Voltage:
- Applied point of test voltage
- Between all interconnected current and voltage circuits and auxiliary circuits with a reference voltage > 40V on the one hand and earth on the other : 5 MΩ
- Between circuits not intended to be connected together in service (Current and Voltage circuit in case of transformer operated meters): 50 MΩ.
Case study in impulse test
Case Study 1: This test is done to make sure there is enough insulation between the live portions of the meter and the body of the meter to prevent against electric shock.
In case the energy meter malfunctions during the 10 kV Impulse test, following observation made after the test:
- A dysfunctional display after performing the impulse test.
- No pulse is produced from the Energy meter after performing the Impulse test.
- Meter accuracy exceeds the limit after performing the Impulse test.
- Affected power supply circuit.
Root cause analysis:
- After testing we found black marks on PCB, so we can identify the shortest current path for 10kV impulse.
- After test, we can analyse the failed PCB to trace the failure path by tracing the carbon spot carefully. See a small dot in marked area and damaged components.
- The shortest creepage is marked with a red arrow. So for the first time, failure was observed on this path.
- When we resolve this, the second-lowest creepage is marked by black.
- When we resolved both paths, the third-lowest creepage path was marked in blue.
- We can see that a slot is provided for increasing creepage, but its calculation is not done properly, and it is decreasing creepage instead of increasing it through components used in the second layer.
Solution:
- Measurement circuits used in the energy Meter should have a higher current/pulse rating or adequate isolation.
- Besides the components used for protection, PCB layout is very important to comply with immunity. If adequate creepage and clearance are not maintained in the PCB layout, then the energy meter will fail. Here we will discuss such practical examples.
- Follow the thumb rule to provide 1 mm of clearance for 1 kV between circuits where impulse will be applied. If clearance is less than the above criteria, a slot can be provided to increase creepage. But its calculation should be done properly from all directions.
- If the slot calculation is incorrect, it may also increase failure. In this case, we see that the slot reduced the creepage from second-layer components.
- We resolved all three issues, and now the PCB is passing the 10KV impulse test.
Case Study 2: After the impulse test, meter’s accuracy is found to be -33%. On further investigation it is noticed that meter’s one phase supply is not working properly.
Root cause analysis:
- The three-phase meter has a -33% meter error. This suggests that a single-phase circuit is compromised.
- When the impulse voltage was applied, the protective device that was being used to clamp the impulse signal across the meter’s input was pierced. In the wake of the impulse voltage test, the meter’s accuracy is -33%.
Solution:
- The components used for protection in energy meter must be of adequate rating to withstand impulse voltage.
- In energy Meters, components like wire wound resistors and Varistors are used for surge/impulse protection at the input stage.
- Use appropriate components, e.g., wire-wound resistors or Varistors, at the input stage.
Some failure cases of energy meters in the testing of insulating properties
- Case 1: When the base of the meter was put on the conducting surface during the application of 4 kV High voltage, it was punctured because the groove supplied for the screw to fix the meter base and cover does not have sufficient insulation, or the insulation in this area is poor signs of failed insulation.
- Case 2: Due to a wire connection between the nameplate and the neutral terminal, the meter cable failed. The name plate’s clearance from the meter’s housing was insufficient to sustain the 4 kV High voltage. Between the nameplate and the neutral is a wire.
- Case 3: The meter sealing wire is in contact with the nameplate inside the meter. During testing, the sealing wire came into contact with the conducting foil wrapped around the meter, creating a conduit for leakage current. Nameplate and sealing wire flash over.
Conclusion
Energy meters’ total life span should be higher than that of typical consumer products, so their design must be durable. Impulse testing, which checks the insulation properties of the energy meter, is a crucial technique for testing energy meters. The test measures the energy meter’s resistance to voltage spikes to guarantee its dependability and safety. The energy meter is subjected to a high-voltage impulse during the test, and its behaviour in reaction to the impulse is examined. The test findings are used to determine whether the energy meter satisfies the requirements for dependability and safety.
Even though testing is thought to be arduous, expensive, and time-consuming, it is nonetheless crucial for determining the dependability of energy meters, to comprehend that little error made during the development and manufacturing processes fails meters.
Rishiraj Meena completed his graduation in Electrical Engineering from REC College Dausa (RTU) Kota, Rajasthan. Currently he is working as an Engineering Officer in the Energy Meter Testing Laboratory at CPRI, RTL, NOIDA, and have four years of experience in the field of testing and certification of energy meters.
