Change for the Meter

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

Typical meters have a nominal accuracy of 1% or 2%. Because the meter must maintain this accuracy over its whole service life, the initial accuracy required is often 0.1%. Furthermore, that accuracy must be maintained over several decades of current, over a very wide temperate range—from Alaska snow fields to Arizona deserts.

The Field Method

One of the most common approaches to sensing electrical current today is to detect and measure the magnetic field produced by the current flowing in a conductor. Traditionally, the preferred arrangement for current measurement has been a current transformer, a closely coupled transformer based on a toroidal core. A less common, but higher performance solution uses a Rogowski coil, an air-cored toroid around a straight conductor. The coil measures δI/δt and requires an integrator to recover current. To minimize sensitivity to external magnetic fields, a conventional Rogowski coil has a rigid coil former and symmetrical coil winding, as small errors in the positioning of the windings render the coil susceptible to fields from nearby conductors and have prevented it from being widely used in commercial applications.

The Circuit Board Solution

The latest generation of utility meters uses a unique solution to achieve the symmetry required in a Rogowski coil: fabricating the coil windings using a PCB. The geometries are tightly controlled, and the resulting response can be predicted with great accuracy. As an added benefit, the measurement electronics can be integrated on the same board.

As shown in Figure 1, each of the two mirror-image coil layers consists of two concentric sections wound in opposite directions, such that the inner and outer sections are equal. The current to be measured flows in bus bars formed in a plane above or below the plane of the coils (see Figure 2). Magnetic flux from the current flowing in the bus bars is coupled into the sensor coil. Other geometries of coils and bus bars are possible, tailored to fit the constraints of the application.

Figure 1. The typical layout of one layer of sensor coil features two concentric sections wound in opposite directions. U.S. Patent No. 6,414,475.
Figure 1. The typical layout of one layer of sensor coil features two concentric sections wound in opposite directions. U.S. Patent No. 6,414,475.

The sensor measures the rate of change of current, and this must be integrated to calculate the actual current. For maximum stability, this is best achieved digitally. For power measurement, a good choice is a chip such as the ADE7759 from Analog Devices (www.analog.com, which also carries out the multiplication necessary to compute power.

Figure 2. Current flowing in an adjacent conductor couples into the sensor coil.
Figure 2. Current flowing in an adjacent conductor couples into the sensor coil.

Figure 3 illustrates the performance of a 100 A current sensor. In this example, the sensor was interfaced using a low-noise pre-amp to an ADE7759 metrology chip. The measurements show the registration of power in a 60 Hz, 240 V circuit, with a typical error of <0.3% of reading over four decades of current. The performance is limited by the ancillary electronics, rather than the fundamental sensor characteristics. This technology has been adopted by Sensus Metering Systems (www.sensus.com) as the core of their iCon meter product family.

Figure 3. The performance of current sensor measuring power in a 240 V circuit shows a typical error of <0.3%.
Figure 3. The performance of current sensor measuring power in a 240 V circuit shows a typical error of <0.3%.

Broad Applications

There are many potential applications outside the metering industry for a wide dynamic range device to monitor electric current and power. Controlling and regulating the input power of devices allows their performance to be optimized for energy efficiency, which is of increasing environmental and financial importance. Some of these applications of current and power sensing include:

  • 1. Control system diagnostics
  • 2. Current supply fingerprinting (e.g., harmonic content)
  • 3. Incorporation of safety cutoff features and surge trips
  • 4. Charge integration and condition monitoring on rechargeable batteries
  • 5. Control of complex loads (e.g., electric motors)

Perhaps the best aspect of the PCB approach is that benefits can be added cost effectively. Using a small area of an existing PCB for current sensing incurs minimal costs, and the necessary signal processing can often be carried out using an existing microprocessor.



Dr. Mark England and Dr. Richard Woodham can be reached at Sentec Ltd, Cambridge, UK; + 44 1223 303800, info@sentec.co.uk, www.sentec.co.uk