All watthour metering approaches require power to be measured, accumulated, and the results stored and displayed. All approaches require that the voltage and current for each electrical phase be sensed (or approximated), voltage and current for each electrical phase must then be multiplied, the resultant power must be accumulated, and the accumulated watthours must be stored and displayed.
Electromechanical meters have evolved over many years and all manufacturers use very similar approaches. The same can not be said for totally electronic meters.
Significant design variations occur in every electronic meter on the market today. These variations even occur within a given manufacturer’s product line.
These reflect the individual trade-offs each designer felt were appropriate. Ulti-mately, it is the users or regulatory agencies that determine if the trade-offs are indeed appropriate. This is usually determined by detailed evaluation and qualification testing of each design. Following a brief review of the evolution of electronic watthour metering, the remainder of this chapter will deal with the most common approaches for performing the various metering subsystems.
EVOLUTION OF SOLID-STATE METERING The Watt/Watthour Transducer
Solid-state metering was introduced to the electric utilities in the early 1970s in the form of a watt/watthour transducer. See Figure 7-30. The advantages of solid-state electronic circuitry produced increased stability and accuracy surpassing the capabilities of the conventional electromechanical watthour meter, but at signifi-cantly higher costs. Consequently, the watt/watthour transducer was most suit-able to energy interchange billing and special applications where analog watt and digital watthour outputs were required. They are used in these applications today, although multi-function electronic meters are starting to replace them.
The watt/watthour transducer provides an analog (watt) output signal in the form of a DC current and also a pulse (watthour) output from a form C mercury-wetted relay or solid-state relay. The analog output may be used to drive a panel meter or strip-chart recorder, or telemetered to a supervisory control system. The pulse output may be used to drive a totalizing register, magnetic tape recorder, or a solid-state recorder.
Figure 7-30. Solid-State Watt/Watthour Transducer.
The Electronic Register
In the 1970s, the register function for solid-state transducers began to be provided with electronic components. In 1979, the first microprocessor-based electronic register was introduced as an addition to the electromechanical meter. This com-bination was referred to as a hybrid meter or as an electronic meter. Compared with mechanical registers, electronic registers were more reliable when perform-ing complex functions (demand) and could be provided at lower cost. In addition, electronic registers provided features not feasible with mechanical registers; such as, time-of-use measurements, sliding demand intervals, switchable registers, tamper detectors, and self-tests.
Today, automatic or remote meter reading is the most common application of electronic registers on electromechanical meters. These registers typically detect the disk rotation using some form of optical detector and communicate the energy consumption to a near-by meter reader or central system. Communication may be by radio frequency, power-line carrier, telephone, cable, or other appro-priate media.
Commercial Solid-State Meters
Totally electronic meters were originally used in high cost, high precision meter-ing applications. In the early 1980s there were a number of field tests to provide economical solid-state metering. By the mid-1980s, one manufacturer was pro-viding a totally electronic meter replacement for the electromechanical, four-wire, wye meter. By the late 1980s, multiple manufacturers had totally electronic re-placement meters for all electromechanical polyphase meter services. These still tended to be more expensive than the electromechanical meters, but provided more accuracy and greater functionality.
In 1992, polyphase metering changed dramatically with the introduction of a totally electronic meter that was highly accurate and cost competitive with the electromechanical demand meter. In addition, multiple service voltages and
multiple service wirings could be handled with the same physical meter. In the mid-1990s, additional functionality, such as instrumentation and site diagnostics, was added to the basic solid-state polyphase meter. Today, these features are the norm for polyphase metering.
Also in the mid-1990s a practical single phase solid-state meter was intro-duced for practical time-of-use and demand applications. By the late 1990s, other manufacturers had introduced more cost effective solid-state meters for lower-end single phase applications.
The Solid-State Watthour Meter Principle of Operation
A functional block diagram of an early watt/watthour transducer is shown in Figure 7-31. The watt section is an electronic multiplier which uses the time-division-multiplier (TDM) principle to produce a pulse train which combines pulse-width and pulse-amplitude modulation. The pulse initiator section receives a DC current signal proportional to power from the watt section. Output pulses, proportional to a convenient watthour-per-pulse rate, are fed from the KYZ output circuit to a register, tape recorder, electronic pulse counter, or other pulse-operated device. A complete description of the operation of the time-division-multiplier is included later in this chapter.
Figure 7-31. Functional Block Diagram Watt/Watthour Transducer.
Solid-State Watthour Meter
A typical meter consists of two sections: the multiplier and the register. In an electromechanical meter the multiplier consists of the voltage and current coils, and the meter disk; the register consists of the gears and dial indicators which count, store, and display the results of the multiplier. For clarity, the following definitions apply: a multiplier is a device which produces the product of a given voltage and current; a register is a device which counts and displays the results of the multiplier; a meter is an assembly which includes a multiplier and a register.
An electronic register is found on hybrid meters (meters with electromechan-ical multipliers and electronic registers), and on solid-state meters. Most registers use a microprocessor which follows instructions stored as firmware to control the counting, storing, and displaying of data received from the multiplier.
All solid-state meters must convert analog voltage and current signals into digital data. The digital data is sent to the register as serial or parallel data. Serial data is a series of pulses where each pulse has a predetermined value, such as 0.6 watthours per pulse. Parallel data is typically in bytes and represents a new value.
To implement an electronic multiplier, meter manufacturers use one of these four approaches: time-division multiplication, Hall-effect technology, transconductance amplifiers, or digital sampling techniques. Each method has advantages and disadvantages and some manufacturers offer more than one type of electronic multiplier.
Characteristics common to all electronic multipliers are: the original input signals are scaled down to lower voltages to be compatible with solid-state com-ponents, analog signals are converted to digital equivalents within the multiplier, and the phase angles between voltage and current are not measured directly.
CURRENT SENSING
All currents must be reduced to a signal level that the electronics can process.
The current sensor needs to accurately reflect the current magnitude and phase angle over the expected environmental and service variations. Because the current sensor measures the currents on lines that are at line voltage, current sensors must be isolated from each other on systems with multiple line voltages. The current sensor must also provide protection from power transients.
The most common current sensor circuits are typically current transformers.
Transformers allow the line voltages to be isolated from each other. A current transformer’s linearity is defined by the magnetic material used for its core.
Typically, a high permeability material is used to assure a linear performance, minimal phase shift, and immunity to external magnetic fields. Care must be taken to assure the material does not saturate under normal conditions. High permeability materials will saturate with DC currents, but these are not normally present on an AC electrical system.
A current sensor similar to the current transformer is the mutual inductance current sensor. This sensor uses air or a very low permeability material for the core because these materials are generally inexpensive, and will not saturate (as is the case with air) or require very high magnetic fluxes to cause the material to saturate. They also tend to have very good DC immunity. The drawback to this
sensor is that it is more susceptible to external magnetic fields, often have stabil-ity issues with time and temperature, and can not supply much current. As such, it tends to have large phase shifts that vary with sensor loading. Typically, a voltage is measured from the sensor instead of a current.
A common sensor used in two wire meters (particularly in Europe) is the cur-rent shunt. This sensor defines a geometry in the meter’s curcur-rent conductor that causes part of the total current to pass through a resistance so that a voltage will be developed that is proportional to the load current. This voltage is then meas-ured and represents the current. Because copper has a low resistance and a very large temperature coefficient, a special material is used for the shunt. The main disadvantages of this sensor is that it is not isolated from the line voltage and it is difficult to control the sensor’s performance over a wide temperature range.
A current sensor can be produced using the Hall effect. The Hall effect can best be explained as follows: When a current flows through a material which is in a magnetic field, a voltage appears across the material proportional to the product of the current and the strength of the magnetic field. This principle is illustrated in Figure 7-32. There is no magnetic field and no voltage appears across the materi-al. In Figure 7-32b, the magnetic field perpendicular to the path of the electrons displaces electrons toward the right side of the material. This produces a voltage difference side-to-side across the material. The voltage is proportional to the strength of the magnetic field and the amount of current flowing in the material.
The Hall effect device is usually inserted in a gap in a toroidal-shaped magnetic core. Because it measures the magnetic field of the current through a conductor, there is electrical isolation in the current sensor. The Hall effect device can also be used for multiplication with the line voltage, as discussed below. Depending on the voltage measurement approach, isolation may be lost. Historically, phase shift, and temperature and frequency output variations have been problems for Hall effect devices, but there have been significant improvements in the performance of meters using these sensors.
There are numerous variations of the above current sensors. Shunts can be combined with current transformers. Compensating wirings can be used on a cur-rent transformer made with a low permeability material. Generally, most of the performance issues related to a particular current sensor technology can be com-pensated in the associated electronics. How these issues are addressed will be unique to each meter design.
Figure 7-32. Hall Effect.