There are a number of ways to implement a watthour meter, but all approaches require power to be measured, accumulated, and the results stored and displayed. As such, voltage and current for each electrical phase must be sensed (or approximated), voltage and current for each electrical phase must be multiplied, the resultant power must be accumulated, and the accumulated watthours must be stored and displayed.

For the electricity provider, the electricity meter (the watthour meter) is the cash register. As such, the meter must be very accurate and reliable over a variety of environmental conditions, and the meter performance must be certifiable to the energy provider, consumer, and any involved regulatory agencies.

A major challenge for the watthour meter manufacturer is to perform these functions economically. Each watthour meter approach has tradeoffs that are balanced by the meter manufacturer to meet the perceived market needs.

The best approach depends on how the user values the tradeoffs. Because of the care taken in their design and manufacture, and because of the long-wearing qualities of the materials used in them, modern watthour meters normally remain accurate for extended periods of time without periodic maintenance or testing. Probably no commodity available for general use today is so accurately measured as electricity.

The Two-Wire Single-Phase Meter
The two-wire meter is the simplest watthour meter and forms the basis for all other meters. The service this meter is used to measure has one voltage supplying the load and one current being used by the load. As such, the meter has one voltage sensor and one current sensor.

Because the voltage and current are changing with load conditions in real time, the voltage and current must be measured in real time. Regardless of DC, AC, or distorted waveforms, at each instant in time the following equation is true:

Watts = Vi x Ii

The real quantities in the electrical system are current, voltage, and (real) power and can be defined for each instance in time. Most other quantities reflect some average effect or are a mathematical convenience to more easily understand what is happening on a macro scale.

For example, it is common to speak of meters in terms of rms voltages, rms currents, and phase angles between these. However, in a real electrical system, the waveforms may be distorted and dynamically changing as loads are switched in and out of the service.

At one extreme, consider an electricity meter on an oil pump. During the up stroke of the pump, significant power is drawn from the service, during the down stroke, the pump motor turns into a generator. In between, there are significant current distortions.

If a revenue meter attempted to compute watts from the rms voltage, rms current, and phase angle between these, the meter would not be very accurate. Therefore, revenue meters measure watts in real-time and accumulate their effect to produce watthours. This can be accomplished with magnetic fluxes within an electromechanical meter disk or with electronic components.

The Three-Wire Single-Phase Meter
The three-wire meter has a voltage sensor, connected across the two line wires of a single-phase service, and two phase currents of the service usually passing through a single current sensor with a magnetic circuit. Each current passes through the magnetic circuit in such a way that the magnetic fluxes produced are additive.

In an electromechanical meter, the number of turns in each of the two current coils is one half as many as used in the current coil of a two-wire meter. According to Blondel’s theorem, two elements (stators) are required for accurate registration of energy flowing through a three-wire circuit. If the voltages between each line wire and the neutral are single phase and exactly equal, the single-stator, three-wire meter is accurate.

An imbalance in the voltage will cause accuracy proportional to one half the difference between voltages. Because modern systems are normally very closely balanced, any errors, usually less than 0.2%, are considered negligible.

With the improved voltage compensation on modern meters, some utilities use the standard three-wire, 240-volt, single-stator meter on two-wire, 120-volt services in place of the standard two-wire meter or the two- or three-wire convertible meters previously described.

By connecting the meter’s two current inputs in series, the meter Kh constant and registration are not changed on the two-wire service and the voltage compensation provides good performance at the 50% voltage operation.

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