Electricity meter
Electric meter or energy meter measures the total power consumed over a time interval.
When energy savings during certain periods are desired, some meters may measure demand, the maximum use of power in some interval. "Time of day" metering allows electric rates to be changed during a day, to record usage during peak high-cost periods and off-peak, lower-cost, periods. Also, in some areas meters have relays for demand response load shedding during peak load periods.[1]
History
Direct current
As commercial use of electric energy spread in the 1880s, it became increasingly important that an electric energy meter, similar to the then existing gas meters, was required to properly bill customers, instead of billing for a fixed number of lamps per month.
DC meters measured charge in ampere hours. Since the voltage of the supply should remain substantially constant, the reading of the meter was proportional to actual energy consumed. For example, if a meter recorded that 100 ampere hours had been consumed on a 200-volt supply, then 20 kilowatt-hours of energy had been supplied.
Many experimental types of meter were developed.
An early type of electrochemical meter used in the United Kingdom was the 'Reason' meter. This consisted of a vertically mounted glass structure with a mercury reservoir at the top of the meter. As current was drawn from the supply, electrochemical action transferred the mercury to the bottom of the column. Like all other DC meters, it recorded ampere hours. Once the mercury pool was exhausted, the meter became an open circuit. It was therefore necessary for the consumer to pay for a further supply of electricity, whereupon, the supplier's agent would unlock the meter from its mounting and invert it restoring the mercury to the reservoir and the supply. In practice the consumer would get the supply company's agent in before the supply ran out and pay only for the charge consumed as read from the scale. The agent would then reset the meter to zero by inverting it.
In 1885
Alternating current
The first specimen of the
In 1894
Units
The most common unit of measurement on the electricity meter is the
Demand is normally measured in watts, but averaged over a period, most often a quarter- or half-hour.
Volt-amperes measures all power passed through a distribution network, including reactive and actual. This is equal to the product of root-mean-square volts and amperes.
Distortion of the electric current by loads is measured in several ways. Power factor is the ratio of resistive (or real) power to volt-amperes. A capacitive load has a leading power factor, and an inductive load has a lagging power factor. A purely resistive load (such as a filament lamp, heater or kettle) exhibits a power factor of 1. Current harmonics are a measure of distortion of the wave form. For example, electronic loads such as computer power supplies draw their current at the voltage peak to fill their internal storage elements. This can lead to a significant voltage drop near the supply voltage peak which shows as a flattening of the voltage waveform. This flattening causes odd harmonics which are not permissible if they exceed specific limits, as they are not only wasteful, but may interfere with the operation of other equipment. Harmonic emissions are mandated by law in EU and other countries to fall within specified limits.
In addition to metering based on the amount of energy used, other types of metering are available. Meters which measured the amount of charge (
Operation
Electricity meters operate by continuously measuring the instantaneous
Electromechanical
The most common type of electricity meter is the
On a single-phase AC supply, the electromechanical induction meter operates through electromagnetic induction by counting the revolutions of a non-magnetic, but electrically conductive, metal disc which is made to rotate at a speed proportional to the power passing through the meter. The number of revolutions is thus proportional to the energy usage. The voltage coil consumes a small and relatively constant amount of power, typically around 2 watts which is not registered on the meter. The current coil similarly consumes a small amount of power in proportion to the square of the current flowing through it, typically up to a couple of watts at full load, which is registered on the meter.
The disc is acted upon by two sets of
Different phase configurations use additional voltage and current coils.
The disc is supported by a spindle which has a worm gear which drives the register. The register is a series of dials which record the amount of energy used. The dials may be of the cyclometer type, an odometer-like display that is easy to read where for each dial a single digit is shown through a window in the face of the meter, or of the pointer type where a pointer indicates each digit. With the dial pointer type, adjacent pointers generally rotate in opposite directions due to the gearing mechanism.
The amount of energy represented by one revolution of the disc is denoted by the symbol Kh which is given in units of watt-hours per revolution. The value 7.2 is commonly seen. Using the value of Kh one can determine their power consumption at any given time by timing the disc with a stopwatch.
.
Where:
- t = time in seconds taken by the disc to complete one revolution,
- P = power in watts.
For example, if Kh = 7.2 as above, and one revolution took place in 14.4 seconds, the power is 1800 watts. This method can be used to determine the power consumption of household devices by switching them on one by one.
Most domestic electricity meters must be read manually, whether by a representative of the power company or by the customer. Where the customer reads the meter, the reading may be supplied to the power company by telephone, post or over the internet. The electricity company will normally require a visit by a company representative at least annually in order to verify customer-supplied readings and to make a basic safety check of the meter.
In an induction type meter, creep is a phenomenon that can adversely affect accuracy, that occurs when the meter disc rotates continuously with potential applied and the load terminals open circuited. A test for error due to creep is called a creep test.
Two standards govern meter accuracy, ANSI C12.20 for North America and IEC 62053.
Electronic
Electronic meters display the energy used on an
The meter has a power supply, a metering engine, a processing and communication engine (i.e. a microcontroller), and other add-on modules such as a real time clock (RTC), a liquid crystal display, infra red communication ports/modules and so on.
The metering engine is given the voltage and current inputs and has a voltage reference, samplers and quantisers followed by an analog to digital conversion section to yield the digitised equivalents of all the inputs. These inputs are then processed using a digital signal processor to calculate the various metering parameters.
The largest source of long-term errors in the meter is drift in the preamp, followed by the precision of the voltage reference. Both of these vary with temperature as well, and vary wildly when meters are outdoors. Characterising and compensating for these is a major part of meter design.
The processing and communication section has the responsibility of calculating the various derived quantities from the digital values generated by the metering engine. This also has the responsibility of communication using various protocols and interface with other addon modules connected as slaves to it.
RTC and other add-on modules are attached as slaves to the processing and communication section for various input/output functions. On a modern meter most if not all of this will be implemented inside the microprocessor, such as the RTC, LCD controller, temperature sensor, memory and analog to digital converters.
Communication methods
Remote meter reading is a practical example of telemetry. It saves the cost of a human meter reader and the resulting mistakes, but it also allows more measurements, and remote provisioning. Many smart meters now include a switch to interrupt or restore service.
Historically, rotating meters could report their metered information remotely, using a pair of
A KYZ interface is a
KYZ outputs were historically attached to "totaliser relays" feeding a "totaliser" so that many meters could be read all at once in one place.
KYZ outputs are also the classic way of attaching electricity meters to
EN 62053-31 (formerly DIN 43864) defines the S0 interface, which is a galvanically isolated
Many meters designed for semi-automated reading have a serial port that communicates by infrared LED through the faceplate of the meter. In some multi-unit buildings, a similar protocol is used, but in a wired bus using a serial current loop to connect all the meters to a single plug. The plug is often near a more easily accessible point.
In the European Union, the most common infrared and protocol is "FLAG", a simplified subset of mode C of
One protocol proposed for this purpose is
Electronic meters now also use
AMR (
Monitoring and billing methods
Commercial uses
Large commercial and industrial premises may use electronic meters which record power usage in blocks of half an hour or less. This is because most
Home energy monitoring
A potentially powerful means to
A study using a consumer-readable meter in 500 Ontario homes by Hydro One showed an average 6.5% drop in total electricity use when compared with a similarly sized control group. Hydro One subsequently offered free power monitors to 30,000 customers based on the success of the pilot.[21] Projects such as Google PowerMeter, take information from a smart meter and make it more readily available to users to help encourage conservation.[22]
Plug-in electricity meters (or plug load meters) measure energy used by individual appliances. There are a variety of models available on the market today but they all work on the same basic principle. The meter is plugged into an outlet, and the appliance to be measured is plugged into the meter. Such meters can help in energy conservation by identifying major energy users, or devices that consume excessive standby power. Web resources can also be used, if an estimate of the power consumption is enough for the research purposes. A power meter can often be borrowed from the local power authorities[23] or a local public library.[24][25]
Multiple tariff
This section possibly contains original research. (January 2018) |
Electricity retailers may wish to charge customers different tariffs at different times of the day to better reflect the costs of generation and transmission. Since it is typically not cost effective to store significant amounts of electricity during a period of low demand for use during a period of high demand, costs will vary significantly depending on the time of day. Low cost generation capacity (baseload) such as nuclear can take many hours to start, meaning a surplus in times of low demand, whereas high cost but flexible generating capacity (such as gas turbines) must be kept available to respond at a moment's notice (spinning reserve) to peak demand, perhaps being used for a few minutes per day, which is very expensive.
Some multiple tariff meters use different tariffs for different amounts of demand. These are usually industrial meters.
Domestic variable-rate meters generally permit two to three tariffs ("peak", "off-peak" and "shoulder") and in such installations a simple electromechanical time switch may be used. Historically, these have often been used in conjunction with electrical
Multiple tariffs are made easier by time of use (TOU) meters which incorporate or are connected to a time switch and which have multiple registers.
Switching between the tariffs may happen via ripple control, or via a radio-activated switch. In principle, a sealed time switch can also be used, but is considered more vulnerable to tampering to obtain cheaper electricity. [citation needed]
An Economy 10 meter is also available, which gives 10 hours of cheap off-peak electricity spread out over three timeslots throughout a 24-hour period. This allows multiple top-up boosts to storage heaters, or a good spread of times to run a wet electric heating system on a cheaper electricity rate.[26]
Most meters using
Smart meters
Smart meters go a step further than simple AMR (automatic meter reading). They offer additional functionality including a real-time or near real-time reads, power outage notification, and power quality monitoring. They allow price setting agencies to introduce different prices for consumption based on the time of day and the season.
Another type of smart meter uses nonintrusive load monitoring to automatically determine the number and type of appliances in a residence, how much energy each uses and when. This meter is used by electric utilities to do surveys of energy use. It eliminates the need to put timers on all of the appliances in a house to determine how much energy each uses.
Prepayment meters
The standard business model of electricity retailing involves the electricity company billing the customer for the amount of energy used in the previous month or quarter. In some countries, if the retailer believes that the customer may not pay the bill, a prepayment meter may be installed. This requires the customer to make advance payment before electricity can be used. [citation needed] If the available credit is exhausted then the supply of electricity is cut off by a relay.
In the UK, mechanical prepayment coin meters used to be common, both in private rented accommodation and residential customers of the electricity boards, the nationalised electricity sector. Disadvantages of these included the need for regular visits to remove the cash, and risk of theft of the cash in the meters by both customers and burglars.[citation needed]
The first automated pre-payment meters were introduced by London Electricity, in conjunction with the Schlumberger Metering based in Felixstowe, UK. They were initially called Key Meters and later renamed Budget Meters. They avoided the 60,000 disconnections for non-payment per annum and the many disadvantages of cash prepayment. They were also popular with customers who wanted a convenient payment method, especially in short term tenancies. well over 1 million such meters were installed across the Uk in the first few years after introduction. Modern solid-state electricity meters, in conjunction with
In South Africa, Sudan and Northern Ireland prepaid meters are recharged by entering a unique, encoded twenty digit number using a keypad. This makes the tokens, which may be electronically delivered or printed on a slip of paper at point of purchase, very cheap to produce.
Around the world, experiments are going on, especially in developing countries, to test pre-payment systems. In some cases, prepayment meters have not been accepted by customers. There are various groups, such as the Standard Transfer Specification (STS) association, which promote common standards for prepayment metering systems across manufacturers. Prepaid meters using the STS standard are used in many countries.[29][30][31]
Time of day metering
Time of Day metering (TOD), also known as Time of Usage (TOU) or Seasonal Time of Day (SToD), metering involves dividing the day, month and year into tariff slots and with higher rates at peak load periods and low tariff rates at off-peak load periods. While this can be used to automatically control usage on the part of the customer (resulting in automatic load control), it is often simply the customer's responsibility to control his own usage or pay accordingly (voluntary load control). This also allows the utilities to plan their transmission infrastructure appropriately. See also Demand-side Management (DSM).
TOD metering normally splits rates into an arrangement of multiple segments including on-peak, off-peak, mid-peak or shoulder, and critical peak. A typical arrangement is a peak occurring during the day (non-holiday days only), such as from 1 pm to 9 pm Monday through Friday during the summer and from 6:30 am to 12 noon and 5 pm to 9 pm during the winter. More complex arrangements include the use of critical peaks that occur during high demand periods. The times of peak demand/cost will vary in different markets around the world.
Large commercial users can purchase power by the hour using either forecast pricing or real-time pricing. Some utilities allow residential customers to pay hourly rates, such as in Illinois, which uses day ahead pricing.[32][33]
Power export metering
Many electricity customers are installing their own electricity generating equipment, whether for reasons of economy,
This exported energy may be accounted for in the simplest case by the meter running backwards during periods of net export, thus reducing the customer's recorded energy usage by the amount exported. This in effect results in the customer being paid for his/her exports at the full retail price of electricity. Unless equipped with a ratchet or equivalent, a standard meter will accurately record power flow in each direction by simply running backwards when power is exported. Where allowed by law, utilities maintain a profitable margin between the price of energy delivered to the consumer and the rate credited for consumer-generated energy that flows back to the grid.
Lately, upload sources typically originate from renewable sources (e.g.,
Location
The location of an electricity meter varies with each installation. Possible locations include on a
Current transformers permit the meter to be located remotely from the current-carrying conductors. This is common in large installations. For example, a substation serving a single large customer may have metering equipment installed in a cabinet, without bringing heavy cables into the cabinet.
Customer drop and metering equation
Since electrical standards vary in different regions, "customer drops" from the grid to the customer also vary depending on the standards and the type of installation. There are several common types of connections between a grid and a customer. Each type has a different metering equation. Blondel's theorem states that for any system with N current-carrying conductors, that N-1 measuring elements are sufficient to measure electrical energy. This indicates that different metering is needed, for example, for a three-phase three-wire system than for a three-phase four-wire (with neutral) system.
In Europe, Asia, Africa and most other locations,
Industrial power is normally supplied as three phase power. There are two forms: three wire, or four wire with a system neutral. In "three wire" or "three wire delta", there is no neutral but an earth ground is the safety ground. The three phases have voltage only relative to each other. This distribution method has one fewer wire, is less expensive, and is common in Asia, Africa, and many parts of Europe. In regions that mix residences and light industry, it is common for this to be the only distribution method. A meter for this type normally measures two of the windings relative to the third winding, and adds the watts. One disadvantage of this system is that if the safety earth fails, it is difficult to discover this by direct measurement, because no phase has a voltage relative to earth.
In the four-wire three-phase system, sometimes called "four-wire wye", the safety ground is connected to a neutral wire that is physically connected to the zero-voltage side of the three windings of the generator or transformer. Since all power phases are relative to the neutral in this system, if the neutral is disconnected, it can be directly measured. In the United States, the National Electrical Code requires neutrals to be of this type.[34] In this system, power meters measure and sum all three phases relative to the neutral.
In North America, it is common for electricity meters to plug into a standardised socket outdoors, on the side of a building. This allows the meter to be replaced without disturbing the wires to the socket, or the occupant of the building. Some sockets may have a bypass while the meter is removed for service. The amount of electricity used without being recorded during this small time is considered insignificant when compared to the inconvenience which might be caused to the customer by cutting off the electricity supply. Most electronic meters in North America use a serial protocol,
In many other countries the supply and load terminals are in the meter housing itself. Cables are connected directly to the meter. In some areas the meter is outside, often on a utility pole. In others, it is inside the building in a niche. If inside, it may share a data connection with other meters. If it exists, the shared connection is often a small plug near the post box. The connection is often
In 2014, networking to meters is rapidly changing. The most common schemes seem to combine an existing national standard for data (e.g.
Accuracy
Electricity meters are required to register the energy consumed within an acceptable degree of accuracy. Any significant error in the registered energy can represent a loss to the electricity supplier, or the consumer being over billed. The accuracy is generally laid down in statute for the location in which the meter is installed. Statutory provisions may also specify a procedure to be followed should the accuracy be disputed.
For the United Kingdom, any installed electricity meter is required to accurately record the consumed energy, but it is permitted to under-read by 3.5%, or over-read by 2.5%.[35] Disputed meters are initially verified with a check meter operating alongside the disputed meter. The final resort is for the disputed meter to be fully tested both in the installed location and at a specialist calibration laboratory.[36] Approximately 93% of disputed meters are found to be operating satisfactorily. A refund of electricity paid for, but not consumed (but not vice versa) will only be made if the laboratory is able to estimate how long the meter has been misregistering. This contrasts with gas meters where if a meter is found to be under reading, it is assumed that it has under read for as long as the consumer has had a gas supply through it.[37] Any refund due is limited to the previous six years.[38]
Tampering and security
Meters can be manipulated to make them under-register, effectively allowing power use without paying for it. This theft or fraud can be dangerous as well as dishonest.
Power companies often install remote-reporting meters specifically to enable remote detection of tampering, and specifically to discover energy theft. The change to smart power meters is useful to stop energy theft.
When tampering is detected, the normal tactic, legal in most areas of the United States, is to switch the subscriber to a "tampering" tariff charged at the meter's maximum designed current[citation needed]. At US$0.095/kWh, a standard residential 50 A meter causes a legally collectible charge of about US$5,000.00 per month. Meter readers are trained to spot signs of tampering, and with crude mechanical meters, the maximum rate may be charged each billing period until the tamper is removed, or the service is disconnected.
A common method of tampering on mechanical disk meters is to attach magnets to the outside of the meter. Strong magnets saturate the magnetic fields in the meter so that the motor portion of a mechanical meter does not operate. Lower power magnets can add to the drag resistance of the internal disk resistance magnets. Magnets can also saturate current transformers or power-supply transformers in electronic meters, though countermeasures are common.
Some combinations of capacitive and inductive load can interact with the coils and mass of a rotor and cause reduced or reverse motion.
All of these effects can be detected by the electric company, and many modern meters can detect or compensate for them.
The owner of the meter normally secures the meter against tampering. Revenue meters' mechanisms and connections are sealed. Meters may also measure VAR-hours (the reflected load), neutral and DC currents (elevated by most electrical tampering), ambient magnetic fields, etc. Even simple mechanical meters can have mechanical flags that are dropped by magnetic tampering or large DC currents.
Newer computerised meters usually have counter-measures against tampering. AMR (Automated Meter Reading) meters often have sensors that can report opening of the meter cover, magnetic anomalies, extra clock setting, glued buttons, inverted installation, reversed or switched phases etc.
Some tampers bypass the meter, wholly or in part. Safe tampers of this type normally increase the neutral current at the meter. Most split-phase residential meters in the United States are unable to detect neutral currents. However, modern tamper-resistant meters can detect and bill it at standard rates.[39]
Disconnecting a meter's neutral connector is unsafe because shorts can then pass through people or equipment rather than a metallic ground to the generator or earth.
A
Power thefts in the United States are often connected with indoor marijuana
Regulation and legislation
Following the
The company responsible for reading the meter may not always be the company which owns it. Meter reading is now sometimes subcontracted and in some areas the same person may read
The introduction of advanced meters in residential areas has produced additional privacy issues that may affect ordinary customers. These meters are often capable of recording energy usage every 15, 30 or 60 minutes. Some meters have one or two IR LEDs on the front: one used for testing and which acts as the equivalent of the timing mark on the older mechanical meters and the other as part of a two-way IR communications port for reading / programming the meter. These IR LEDs are visible with some night vision viewers and certain video cameras that are capable of sensing IR transmissions. These can be used for surveillance, revealing information about peoples' possessions and behaviour.[41] For instance, it can show when the customer is away for extended periods. Nonintrusive load monitoring gives even more detail about what appliances people have and their living and use patterns.
A more detailed and recent analysis of this issue was performed by the Illinois Security Lab.[42][further explanation needed]
See also
- Energy management software
- Energy monitoring and targeting
- Meter operator
- Utility submeter
- Zellweger off-peak
- Multimeter
Notes
- ^ E.g., Minnkota Power's Load Management System Archived 2006-06-16 at the Wayback Machine, accessed 22 August 2009.
- ISBN 0-521-43098-4, p 232–241
- ^ Whyte, Adam Gowans (1930). Forty Years of Electrical Progress. London: Ernest Benn. pp. 31, 159.
- ^ Eugenii Katz. "Blathy". People.clarkson.edu. Archived from the original on June 25, 2008. Retrieved 2009-08-04.
- ^ . Student paper read on January 24, 1896 at the Students' Meeting.
- ^ The Electrical engineer, Volume 5. (February, 1890)
- ^ The Electrician, Volume 50. 1923
- ^ Official gazette of the United States Patent Office: Volume 50. (1890)
- ISBN 0-521-53312-0, pages 1 and 258
- ^ U.S. Patent 388003
- ISBN 0-471-39484-X, page 875
- ^ "Shallenberger Integrating Wattmeter". watthourmeters.com. Archived from the original on 2008-06-25. Retrieved 2010-09-29.
- ISBN 1-55937-968-5page 47
- ISBN 978-0-7661-2648-0.
- ^ Fleming, J.A. (1914). Magnets and Electric Currents. New York: Spon & Chamberlain. pp. 335.
- ^ "Volume 3-10" (PDF). Archived from the original (PDF) on 2009-05-13. Retrieved 2009-08-04.
- ^ "What are KYZ Pulses?". SolidState Instruments. Retrieved 22 November 2012.
- ^ Handbook for Electricity Metering. EEI. Archived from the original on 2008-10-24.
- ^ "Get to know your Power Meter – What is Real, Apparent and Reactive Power". Chipkin Automation Systems.
- ^ "Do electric meters measure real or apparent power? – MVOrganizing". www.mvorganizing.org.
- ^ "CBPHydroOneReprint" (PDF). Archived from the original (PDF) on 2009-03-18. Retrieved 2009-08-04.
- ^ Verne Kopytoff; Ryan Kim (2009-02-22). "Google plans meter to detail home energy use". San Francisco Chronicle. Retrieved 2009-02-11.
- ^ "Residential — Home Energy Audit — Watts Up". Austin Utilities. Archived from the original on 2009-03-12. Retrieved 2009-08-04.
- ^ "Portable Energy Meter". Mge.com. Archived from the original on 2008-04-12. Retrieved 2009-08-04.
- ^ "LINKcat". Linkcat.info. Archived from the original on 2008-08-04. Retrieved 2009-08-04.
- ^ "Price comparison website". 19 March 2010. Retrieved 2010-12-15.
- ^ "EDF Energy Tariff Information Label checker". Retrieved 2017-07-28.
- ^ "EDF Energy Tariff Information Label checker". Retrieved 2017-07-28.
- ^ "Standard Transfer Specification Association NPC / 95/08496/08 > Home". www.sts.org.za.
- ^ "Genus Power Infrastructures Ltd". The Times Of India.
- ^ "Conlog". SAEEC. Archived from the original on 2011-01-10. Retrieved 2011-02-17.
- ^ "retail-energy". www2.ameren.com. Archived from the original on 2010-01-31. Retrieved 2009-08-04.
- ^ "Real Time Pricing". Thewattspot.com. Archived from the original on 2009-02-23. Retrieved 2009-08-04.
- ^ See the National Electrical Code, a large book, revised yearly, widely available for purchase.
- ^ Measuring Instruments (Active Electrical Energy Meters) regulations 2006, Schedule 1, Paragraph 15. These errors apply between +5°C to +30°C and a power factor of 0.8 leading to 0.5 lagging. Outside of these limits larger errors are permissible.
- ^ Electricity meter accuracy disputes
- ^ the Gas (Meters) Regulations 1983
- ^ Limitation Act 1980, Chapter 58, Part 1
- ^ Teridian Semiconductors Application Note, "Antitamper Features Enabled by the 71M6511" The 71M6511 is a single chip metering device widely used in computerised meters.
- ISBN 9780973892802.
- S2CID 41307271.
- ^ "Attested Metering". Illinois Computer Security Laboratory.
References
- "Handbook for Electricity Metering" by The Edison Electric Institute
External links
- Media related to Electricity meters (kWh) at Wikimedia Commons