Electric/Magnetic

Change for the Meter

July 1, 2005 By: Mark England, Richard Woodham Sensors


You know the drill: The utility truck drives up to your house; the meter reader gets out with a clipboard, checks the meter on your outside wall, notes down the new numbers, and drives off. Of course, if your meter is inside and you're away when the reader knocks (or the truck can't get up your road in winter), you'll be faced with an estimated bill, which makes no one happy. You don't want to be overcharged for your consumption, and no utility wants to supply free electricity. Handwritten numbers on a clipboard? Estimated bills? Do these make sense in 2005? A smarter approach to utility metering is needed, and meter manufacturers are making it possible with a new generation of solid-state meters.

 Electric meters must be read monthly by the utilities company—but before long the chore will be done remotely.
Electric meters must be read monthly by the utilities company—but before long the chore will be done remotely.

New Opportunities

Until recently, metering technologies were the Cinderellas of energy infrastructure, working hard as cash registers for the utilities, using electromechanical designs that have changed very little over the past 30 years. However, as the cost comes down for digital microelectronic devices, the opportunities offered by solid-state meters are looking up.

One key driver is the cost of reading meters manually, which has driven the development of a range of automated meter reading products. They also have a built-in data processing capacity that can add extra functions, such as demand management, to the meter. This feature allows utilities to reduce peak demand (and therefore the amount of investment needed in power stations) by signing up customers to a scheme where they allow noncritical loads such as pumps and water heaters to be turned off dynamically at peak times, in return for discounts or credits. The challenge is to develop a solid-state meter capable of supporting these communication devices without compromising accuracy, robustness, reliability, or lifespan.

The Single-Chip Solution

The energy delivered from an AC source is:



where:

V(t) and I(t) = time-varying voltage current

Both V and I alternate at the line frequency, but are not necessarily in phase. So simply measuring average voltage and current levels and multiplying the two does not give an accurate value for the energy delivered. For accurate billing, the product V × I has to be generated (with a bandwidth that is large compared with the line frequency; typically >1 kHz, to measure the harmonic energy content correctly) before the summation.

A particularly favorable approach is to digitize the current and voltage signals and to carry out the multiplication and summation in the digital domain. By using low-cost ΣΔ converters, you can reduce noise almost arbitrarily with progressive digital filtering. Even better, because ΣΔ converters can be implemented in CMOS technology, a complete energy metering function can be integrated into one chip.

Current and Voltage Sensing Requirements

To provide voltage sensing, simply divide down the line voltage using a standard potential divider circuit. Adding current sensing, by contrast, is rather more taxing. The main requirements for current sensing in a billing meter are:

  • 1. Capability of measuring high currents (up to 200 A) with low loss
  • 2. Linearity over a wide dynamic range
  • 3. Low offset (Who wants to pay for energy that's not used?)
  • 4. Stablity over time and temperature
  • 5. Insensitivity to currents in nearby conductors (Who wants to pay for the neighbor's energy?)
  • 6. Suitable bandwidth for accurate calculation of energy delivered in the presence of harmonics
  • 7. Low cost
  • 8. Isolation from line voltage, for extension to multiple phase supply installations

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