THERMAL (LAGGED DEMAND) ELECTRICITY METERS

Electromechanical
In the electromechanical thermal or lagged-type demand meter, the pointer is made to move according to the temperature rise produced in elements of the meter by the passage of currents. Unlike the integrating demand meter, the lagged meter responds to load changes in accordance with the laws of heating and cooling, as does electrical equipment in general.

Because of the time lag, momentary overloading, instead of being averaged out, will have a minor effect on the lagged meter unless the overloading is held long enough or is severe enough to have some effect on the temperature of equipment.

The demand interval for the lagged meter is defined as the time required for the temperature sensing elements to achieve 90% of full response when a steady load is applied. Like the integrating meter, the lagged meter is generally designed to register kilowatt demand.

The lagged type is essentially a kind of wattmeter designed to respond more slowly than an ordinary wattmeter. An important difference in these methods of metering demand is in the demand interval. In the integrating meter, one demand interval follows another with regularity, giving rise to the term block interval.

The thermal or lagged meter measures average load with an inherent time interval and a response curve which is based on the heating effect of the load rather than on counting disk revolutions during a mechanically timed interval.

Electronic
Electronic thermal demand emulation is the logarithmic average of the power used, with a more recent load being weighted more heavily than a less recent load, (approximated exponentially). The meter will record 90% of a change in load in 15 minutes, 99% in 30 minutes, and 99.9% in 45 minutes.

Because thermal demand emulation is the logarithmic average, the demand is not set to zero on a demand reset. On a demand reset, present demand becomes the new maximum demand.

AUTOMATIC METER READING (AMR) TECHNOLOGIES BASIC INFORMATION AND TUTORIALS

PURPOSE AND ADVANTAGE OF AMR
To eliminate meter access problems and data entry errors while improving overall meter reading efficiency, many utilities are implementing Automatic Meter Reading (AMR) technology. The AMR technology enables utilities to collect meter data without having to visit the meter. Nationwide, an increasing number of electric, gas, and water utilities are installing these remote data collection technologies.

Electric utilities are also beginning to deploy AMR technology for reasons that go beyond metering and customer billing. As the energy marketplace deregulates and becomes more competitive, utilities are seeking ways to use advanced metering data throughout their distribution operations to achieve a variety of other objectives.

These are; improved demand forecasting accuracy, increased energy distribution system efficiency, delivery of new rates and services, and successful management of customer choice and retail competition in their service territories.

Utilities have a wide choice of AMR technologies and companies from which to choose. These technologies include wired and wireless data collection systems that use both public and private communication networks, including broadband and cellular.

Some AMR vendors offer products that use existing power lines to transmit collected data while others are working to develop systems that rely on satellites and other more exotic technologies that have yet to prove themselves as either practical or cost effective. The AMR industry also features meter data collection technologies to serve residential, commercial, and industrial customers equipped with more advanced solid-state electric meters.

From radio-equipped handheld terminals to vehicle-based systems to large fixed networks, these automatic meter-reading systems offer a wide range of data collection functionality at a wide range of costs. The suitability and cost-effectiveness of a particular AMR technology depends on a number of variables; the type of service territory (urban, suburban, or rural), the type of customer (residential, commercial, or industrial), and the data collection needs of a particular utility.

These needs can vary greatly.

Many utilities are deploying integrated AMR solutions that combine different collection technologies (e.g., network, mobile, and telephone), to deliver the desired level of data collection functionality for different service areas and customer segments in the most cost-effective manner.

Each of these technologies— handheld computers, mobile vehicle-based systems, wireless networks, telephone-based systems and powerline carrier—has its own set of strengths depending on a utility’s operational and strategic objectives.


Radio-Based Mobile Automatic Meter Reading
With a radio-based system, each meter is adapted with a compact, low-power transmitter/receiver that upon request, broadcasts a radio signal containing metering and tamper data.

These meters are polled by a mobile handheld terminal or vehicle-based transmitter/receiver that collects meter readings from many meters and carries the readings back to the central office. At the end of the day, the readings are loaded directly into the utility’s billing computer.

The mobile transceiver can be in a handheld terminal, or a higher-powered unit installed in a truck that is driven down streets, polling meters and recording replies. Depending on the service environment, meter density, memory capacity, and the type of meter readings collected, handheld terminals are typically capable of reading several hundred meters or more in a day.

More powerful vehicle-based units raise meter reading efficiency even further by reading thousands of meters in a single day.  Radio-based meter reading is fast, efficient, and generally reliable. Some meter installations inside metal buildings create difficulties for radio transmission but these site-related problems can often be solved by re-locating the
meter antenna.

Network Meter Reading
Radio-based fixed networks offer the most advanced data collection functionality of any wireless meter reading technology. There are many variations in the network data collection products available from different vendors.

Most wireless network meter reading systems involve installing a fixed communications network over a population of meters equipped with radio transmitters to send the data through the network to the host processor. Companies apply a variety of communications strategies for their network products that utilize private dedicated wireless networks, public networks, or some combination of the two.

Though more costly than handheld and mobile automatic meter reading systems, these networks provide electric utilities with state-of-the-art automatic meter reading functionality, including consumption reads, on-request reads, tamper reporting, time-of-use, demand metering, load profile/interval reads, virtual connect/disconnect capabilities, outage detection and restoration reporting, consumption monitoring, and aggregation capabilities.

Until now the handful of electric utilities that have deployed network meter reading systems have done so mostly on a large-scale, territory-wide basis to spread the cost of the network over a large number of meters—typically more than 100,000. However, recently several new more scalable and flexible network products have emerged that enable electric utilities to deploy advanced network meter reading technology on a more selective and cost-effective basis to serve specific meter populations or specific customer segments (such as commercial and industrial customers with advanced solid-state meters).

To reduce both implementation and ongoing operations costs, these new networks combine private, dedicated RF communication networks to gather data from designated populations of automated meters and then use public communications networks to back haul the data from local collection points to the host processor. While its penetration has been limited thus far, deployment of network meter reading and data collection technology will likely accelerate in the years to come as costs come down and the need for utilities to gather more advanced metering data increases.

DIGITAL KILOWATT HOUR METER RESOLUTION AND ACCURACY

Resolution
The following example illustrates the resolution and accuracy of the digital-ramp A/D converter. Assume the following values for the A/D converter of Figure 6-1: D/A converter has a 10-bit input and a full scale analog output of 10.23 volts; the comparator can detect a voltage difference of 1 millivolt or greater; Vin is 3.728 volts.

Since the D/A converter has a 10-bit input, the maximum number of steps possible is (210 1)  1023. With a full-scale output of 10.23 volts reached in 1023 steps, the step size is 10 millivolts. This means Vout increases in steps of 10 mV as the counter counts up from zero.

Since Vin  3.728 volts and the comparator threshold is 1 mV, then Vout has to reach 3.729 volts or greater before the comparator switches Low. At 10 mV per step, this requires 373 steps.

At the end of the conversion, the counter holds the binary equivalent of 373, which is 0101110101. This is the digital equivalent of the analog input of Vin  3.728 volts. The resolution of this A/D converter is equal to the step size of the D/A converter which is 10 mV, or approximately 0.1% (.010/10.23  100  0.1%).

The resolution of an A/D converter is equal to the resolution of the D/A converter that it contains. The D/A output voltage Vout is a staircase waveform (digital ramp) that goes up in discrete steps until it exceeds Vin. Thus, Vout approximates Vin.

When the resolution (step size) is 10 mV, the accuracy we can expect is that Vout is within 10 mV of Vin. The resolution of the D/A converter is an inherent error, often referred to as a quantizing error. This quantizing error can be reduced by increasing the number of bits in the counter and in the D/A converter. It is specified as an error ± 1 least significant bit (LSB), indicating that the result can vary by that much due to the step size.

From another point of view, the input voltage Vin can take on an infinite number of values, from 0 to full scale. However, the output voltage Vout has only a finite number of discrete values. This means that similar values of Vin within a small range could have the same digital representation.

For example, if the counter goes through 1,000 steps from zero to full scale, any value of Vin from 3.720 to 3.729 will require 373 steps, thus resulting in the same digital representation. In other words, Vin must change by 10 millivolts (the resolution) to produce a change in the digital output.


Accuracy
The D/A converter accuracy is not related to the resolution. It is related to the accuracy of the components in its circuit such as the resistors in the D/A network, comparator, level amplifiers, and the reference power supply. If a D/A has an accuracy of 0.01% full scale, the A/D converter may be off by 0.01% full scale owing to non-perfect components.

This error is in addition to the quantizing error due to resolution. These two sources of error are usually specified separately, and for a given A/D converter are usually of the same order of magnitude.


In addition to the inherent errors noted above, the accuracy of an electronic instrument depends on proper selection of the meter range. Normally, the uncertainty of measurements is expressed as a percent of the reading plus the number of counts of the least significant digit (LSD) displayed for that range.

If the 1,000 volt DC range is selected to measure a 2 volt signal for a three-and-a-half digit digital multimeter with a nameplate accuracy of 0.5% of input voltage 1 LSD, this setup would result in a meter accuracy of 50.5%, as shown below.

Given: Meter Range Accuracy (MRA) is 0.5% of input voltage 1 LSD
Meter range set to 1,000 volts DC
Input voltage is 2 volts DC
Then: Meter Accuracy  [(MRA  input V  LSD)/Input Voltage]  100
 [(0.5%  2  1)/2]  100
 50.5%

However, selecting a meter range of 2 volts DC on the same digital multimeter would result in an accuracy of 0.60%, nearly 100 times better, as shown below.

Given: Meter Range Accuracy is 0.5% of input 1 LSD
Meter range set to 2 volts DC
Input voltage is 2 volts DC
Then: Meter Accuracy  [(MRA  input V  LSD)/Input Voltage]  100
 [(0.5%  2  0.002)/2]  100
 0.6%


Digital Display Resolution and Accuracy
Typical handheld digital instruments display from 3 to 5 digits. Laboratory digital instruments often offer 7 or 8 digits. The number of digits directly affects the available resolution of the reading.

For example, a full 4-digit display is capable of presenting numbers from 0 to 9999 (with a decimal point somewhere in the display depending on the range setting of the instrument). This display can provide 10000 different readings for a particular range setting, so its resolution is limited to 1 part in 10000, or 0.01%. You may see this display referred to as a 10000-count display.

A 6-digit display can present numbers from 0 to 999999. This display resolution would be 1 part in 1,000,000 or 0.0001%. It may be called a 1,000,000-count display.

Examples in the previous section used LSD, Least Significant Digit, to adjust accuracy calculations to the characteristics of the display.

The design of a digital instrument often further limits the display. A 4-digit display, by design, may display numbers from 0 to 3999, rather than to 9999. That is, the left-most digit is programmed such that it only displays the numbers 0 to 3.

This display is described as a 31/2-digit display or as a 4000-count display. This design does not further affect the accuracy of calculations. The value of the LSD is the same for a 31/2-digit display as for a 4-digit display.

ELECTRONIC DIGITAL INSTRUMENT USED IN POWER METERING

ELECTRICAL MEASURING INSTRUMENTS are necessary because the nature of most electrical phenomena is beyond the reach of our physical senses. Measurement of electrical quantities makes possible the design, manufacture, and maintenance of the innumerable electrical devices now in use.

The main purpose of any electrical instrument is to measure and indicate the value of an electrical quantity. The measurement may be indicated by a digital numeric value or by a pointer positioned on a scale. Some instruments provide additional functionality by recording measured values over time.

This recording may be in the form of a physical indication on a moving chart, as maximum and minimum values during a time frame, or as periodic data stored in electronic memory. The devices commonly used for such measurements are voltmeters, ammeters, and wattmeters.

The field of instrumentation is extensive and includes many classifications of instruments according to portability, type of indication or record, accuracy, design features, etc. We shall briefly discuss only those instruments commonly used in meter departments. These include displaying, indicating, electronic digital, and recording measuring devices.

A digital instrument is an electronic device that measures voltage, current, and/or resistance by converting the measured analog input signal into a digital representation that is then displayed as a digital readout.

Advances in technology have led to digital instruments that are capable of high degrees of accuracy in the measurement of voltages, currents, and resistances over a wide range of values. Analog instruments indicate measured quantities by the deflection of a pointer on a scale, requiring the user to “eye up” the reading.


Today’s digital instruments display measured results as discrete numbers (digits), removing much of the interpretation error from the act of reading an instrument. Typical display technologies include liquid crystal display (LCD), light emitting diode (LED), and gas discharge.

Some instruments offer the ability to send readings to other devices such as printers or computers or to be controlled by external computers. Interfaces built into the instruments, such as RS-232-C serial communications or the IEEE-488 bus standard, provide the data transmittal and external control capabilities.

The central component of a digital instrument is the digital DC voltmeter that uses electronic circuits to sense, process, and display the measured quantities. Input quantities other than DC voltages are converted to DC by transducers.

Examples of transducers include internal shunts used to measure current and ACto- DC converters to measure AC quantities. The transformed analog quantity (now in the form of an equivalent DC voltage) is then converted to a digital signal.

Active electronic components, such as transistors, operational amplifiers, and integrated circuit modules perform this analog-to-digital (A/D) conversion.


Analog-to-Digital Conversion
Electronic instruments employ several different A/D conversion processes. These include dual-slope integration, ramp-and-counter, successive approximation, and voltage-to-frequency conversion. Each of these techniques produces a digital output equivalent to the measured analog input.

Figure 6-1 shows a simple version of an A/D converter. In this example, a binary counter increments one count with each clock pulse, until Vout equals Vin. This type of A/D converter is called a “digital ramp and counter” because the waveform at Vout ramps up step-by-step, like a staircase.

It operates as follows:

1. A positive Start pulse is applied resetting the counter to zero. It also inhibits the AND gate so no clock pulses get through to the counter while the Start pulse is High.

2.With the counter at zero, Vout 0, so the comparator output is High.

3. When the Start pulse goes Low, the AND gate is enabled, allowing pulses to enter the counter.

Figure 6-1. Analog-to-Digital Converter