Electromechanical meters have been long used to measure electricity consumed by residences, businesses and electrically powered systems. The most commonly used type of electromechanical meter is the induction watt-hour meter. The induction watt-hour meter comprises a rotatable metallic disc which rotates at a speed proportional to power consumption and thereby provides a measure of electricity consumption. The rotatable metallic disc is actuated by two coils: a first coil which is disposed in series with the current delivering conductors such that the coil produces a magnetic flux proportional to the current delivered; and a second coil which is disposed in parallel with the conductors over which the current is delivered such that the coil produces a magnetic flux proportional to the line voltage. A lag coil is used to delay the field of the second, line voltage measuring coil by ninety degrees whereby power measurement is based on the product of in phase current and line voltage signals.
More recently electronic meters have been introduced. Such electronic meters make use of advances in electronic technology by displaying electricity consumption on an LCD or LED display and providing for transmission of measurements to remote locations. Such electronic meters also provide for more sophisticated or extensive forms of measurement including the like of maximum demand, power factor, reactive power used and consumption during on-peak and off-peak hours. As the line voltage is often many times larger than the largest signal that can be safely or conveniently measured by the low voltage circuitry in electronic meters, a resistive potential divider is the most commonly employed means of sensing the line voltage. The resistive potential divider is disposed in parallel with the current delivering conductors and is operative to lower the voltage to be measured by the factor of the potential divider ratio. The potential divider ratio can be quite large. For example a potential divider ratio of approximately 2000:1 is required to reduce a 240V+/−20% RMS mains voltage to a signal that is <0.5V peak-peak in the worst case, which is the voltage that can be typically handled by a ground referenced divider without distortion arising from protection structures that may be present on the input of the voltage measurement chain.
Certain applications, such as metering of electricity consumption and generation, require measurement to high accuracy over extended periods of time. For example in North America the ANSI C12.20 standard specifies an accuracy of ±0.5% for Class 0.5 consumption meters and ±0.2% for Class 0.2 consumption meters. Standards applicable in Europe and elsewhere, such as IEC 62053, specify similar accuracy requirements. The potential divider ratio and the accuracy of the subsequent voltage measurement chain therefore needs to be known and of sufficient stability to meet the accuracy requirements of the power measurement application. Accurate line voltage measurement normally depends on the use of components with good temperature coefficients and known values. A lack of accuracy in the potential divider ratio or an error in the transfer function of the voltage measurement chain gain will cause an error in line voltage measurement. It is normal for this reason to perform a one-off factory calibration when the potential divider and the readout electronics are combined so that a factor related to the actual combined transfer function for line voltage to measurement value, which is determined largely by the potential divider and voltage measurement circuitry, can be stored and used in subsequent measurements to achieve the desired accuracy.
The components that make up the potential divider and the voltage measurement chain are required not to change significantly over the operating lifetime and the environmental conditions if there is to be no degradation in use that takes the measurement apparatus outside its targeted accuracy. The components used in the potential divider may be subject to various external stresses due to the like of ESD, surge and overvoltage conditions and may dissipate different wattages in normal operation. External stresses and dissipation of different wattages can lead to a change in potential divider ratio. It is therefore normal to use highly specified components in this application.
Potential dividers are also used in DC applications for voltage measurement where the voltage to be measured is outside the range that can be safely or conveniently handled by the voltage measurement chain voltage range. By way of example DC voltage measurement may be employed in battery monitoring.
High accuracy power calculation also requires accurate and stable relative phase and frequency response of the line voltage and current measurements in order to accurately determine the like of active and reactive power, differences between active and reactive power, power factor and harmonic content.
The present inventors have become appreciative of various shortcomings of known approaches to line voltage measurement and power measurement, such as the approaches described in outline in the preceding paragraphs. It is therefore an object for the present invention to provide improved voltage measurement apparatus which is configured to provide for accurate measurement of voltage, for example of line voltage in a circuit carrying mains current. It is another object for the present invention to provide an improved method of measuring voltage which provides for accurate measurement of voltage, for example of line voltage in a circuit carrying mains current. It is a further object for the present invention to provide power measurement apparatus which comprises improved line voltage measurement apparatus, whereby accurate power measurement may be achieved. It is a yet further object for the present invention to provide an improved method of measuring power which provides for improved measurement of line voltage, whereby accurate power measurement may be achieved.