Power consumption by energy meters
Electronic kWh meters offer many advantages over the electromechanical meter , such as higher accuracy across a wider range, additional features at very little or no extra cost, and smaller size.
The overall power consumption represents an important parameter. Similar to the electromechanical meter, it consists of a constant portion and a portion which depends on the current through the load.
The Voltage Circuit
The constant portion is the power dissipation in the voltage circuit. It is the circuit which is located between live and neutral or between two lives.
Meters with lower power consumption are preferred, because they cause less energy losses. IEC1036 allows for 2W for the active power consumption and 10VA for the reactive power consumption. If there are 30 million meters installed in a country, a constant supply of more than 60MW must be provided in order to power all the electricity meters. This is equivalent to the consumption of more than 30 000 households.
There is continuous pressure to reduce the power consumption in the voltage circuit. With state-of-the-art electronics, a power consumption of less than 0.2W is possible. Polarised latching relays are available for meters that feature disconnection and reconnection, such as prepayment meters. These relays require only a short current pulse through the coil in order to change the position. They do not contribute to the average power consumption of the meter and require very little overhead in the power supply. By comparison, a non-latching relay would require several watts continuous power. If required, the actual position of the contact can be detected easily for relays, which use a permanent magnet for holding the position.
The Current Circuit
The power consumption in the current circuit is a function of the load current and the total resistance in the current path. The IEC1036 limits are 4VA and 2.5VA (class 1 and class 2). This value is specified with the rated load current. Main contributors are the conductors through the meter and the current-sensor. Typical impedance values in the current circuit are from 100 micro Ohms up to 1000 micro Ohms, depending on meter rating and current sensor type.
A shunt is the most cost-effective solution for a single-phase meter. To achieve low power dissipation, low shunt resistor values are required. Connections (screw-type, brazing, soldering and welding) should be kept to a minimum, as they contribute to the power consumption in the current circuit. Assuming an average load of 2kW and 30 million meters installed, the total power loss is approximately 1500kW.
If a meter includes disconnection and re-connection functions, the switching device should have a contact resistance as low as possible. Newer latching relays with integrated shunt allow for lower power consumption in the current circuit, because they reduce the amount of connections (junctions) required.