Electricity meter
From Wikipedia, the free encyclopedia
An electric meter or energy meter is a device that measures the amount of electrical energy supplied to a residence or business. These are customers of an electric company.
The most common type is more properly known as a (kilo)watt-hour meter or a joule meter. Utilities record the values measured by these meters to generate an invoice for the electricity.
[edit] Unit of measurement
Demand is normally measured in watts, but averaged over a period, most often a quarter or half hour.
Reactive power is measured as "Volt-amperes reactive", (VARh) in kilovars-hours. It may help to think of reactive power as power that is "reflected" from a load, because the load cannot immediately use all the power provided by the distribution system. A "lagging" or "inductive" load such as a motor will have positive reactive power. A "leading" or "capacitive" load will have negative reactive power.
Volt-amperes measures all power passed through the distribution network, whether reactive or actual. This is equal to the product of root-mean-square volts and amperes. Alternatively, it is the square-root of the sum of the squares of watts and VAR.
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 negative value is a capacitive load, a positive is inductive. Current harmonics measure distortion of the wave form. For example, electronic loads often "cut off the peak" of the voltage to fill their power supplies. This flattening causes odd harmonics. Harmonics are often caused by tampering with meters (see below).
[edit] Operation
The electromechanical induction meter operates by counting the revolutions of a metallic disc which is made to rotate at a speed proportional to the power. The number of revolutions is thus proportional to the energy usage. The metallic disc is acted upon by three magnetic fields, one proportional to the voltage, another to the current and a third supplied by a permanent magnet and constant. One of the varying fields induces currents into the metallic disc which are then acted upon by the other varying field to produce a torque. This results in the torque being proportional to the product of the current and voltage, that is power. As the metallic disc rotates through the permanent magnetic field, eddy currents are again produced which dissipate energy (since the disc has some resistance) and act to slow the rotation. This drag is proportional to the rotation speed. The equilibrium between the applied torque and the drag results in a speed proportional to the power.
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. If the time in seconds taken by the disc to complete one revolution is t, then the power in watts is P = 3600×Kh/t. 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. [1]
The rotating disc in this type of meter is, in fact, an electric motor of a type called a reluctance motor or eddy current motor. It consumes a small amount of power, typically around 2 watts.
[edit] Demand meters
In its simplest form, a demand meter has a gauge whose pointer moves a marker. When the gauge falls back, friction keeps the marker in place. When a demand meter is read, its marker is reset, usually with a magnet from outside the sealed meter enclosure. Computerized demand meters usually find the fifteen-minute interval in the month with maximum demand. Often they also record a month worth of fifteen-minute averages. Some demand meters measure the temperature of a conductor, or simulate the heating of the conductor, in order to track "running" demand. Running demand meters usually log the times when a maximum demand is exceeded, or they log the times when the meter enters a different tariff rate (see below).
Electricity cannot be stored, so Electricity retailers need to arrange the necessary generators to meet the maximum demand. New generators are long-term capital investments, so demand also directly affects the retailers' and power-providers' accounting, and need for long-term debt. In particular, when interest rates are high, generating companies are reluctant to install new capacity, and want customers to reduce demand, so the retailers use meters to detect and surcharge high demand. Demand meters measure volt-amperes, which combine both reactive (reflected) and actual (consumed) AC power.
[edit] Multiple tariff (variable rate) meters
Electricity retailers may wish to charge customers different tariffs at different times of the day. This is because there is generally a surplus of electrical generation capacity at times of low demand, such as during the night (see supply and demand).
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. In the UK such tariffs are branded Economy 7 or White Meter and are commonly used in conjunction with electrical storage heaters. The popularity of such tariffs has declined in recent years, at least in the domestic market, due to the (perceived or real) deficiencies of storage heaters and the low cost of natural gas.
Domestic variable-rate meters normally only permit two tariffs ("peak" and "off-peak") and in such installations a simple electromechanical time switch may be used. 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 electricity grids have demand surges throughout the day, and the power company may wish to give incentives to large customers to reduce demand at these times. These demand surges often corresponding to meal times or, famously, to advertisements in popular television programmes.
Some multiple tariff meters use different tariffs for different amounts of demand. These are usually industrial meters.
The multiple tariff rates may also be dependent of frequency, also known as availability based tariff (ABT), deployed in gGrid substations and inter-utility transfer points for bulk transfer of energy. This is based on the premise that the system frequency is inversely proportional to the current load. This also causes self-regulation because the rates are higher when the system frequency is low, eventually bringing down the demand.
[edit] Means of reading
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.
Newer electronic meters can be read automatically. Remote meter reading is an application of telemetry. Often, meters designed for semi-automated reading have a serial port on that communicates by infrared LED through the faceplate of the meter. In some apartment 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 the mailboxes. In the European Union, the most common infrared and protocol is "FLAG", a simplified subset of mode C of IEC 61107. In the U.S. and Canada, the favoured infrared protocol is ANSI C12.18. Some industrial meters use a protocol for programmable logic controllers, Modbus. The most modern protocol proposed for this purpose is DLM/COSEM which can operate over any medium, including serial ports. The data can be transmitted by Zigbee, WiFi, telephone lines or over the power lines themselves. Some meters can be read over the internet.
[edit] Ownership
Due to the deregulation of electricity supply markets in many countries, the company responsible for an electricity meter may not be obvious. Depending on the arrangements in place, the meter may be the property of the electricity distributor, the retailer or for some large users of electricity the meter may belong to the customer.
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 gas, water and electricity meters at the same time.
[edit] Location
The location of an electricity meter varies with each installation. Possible locations include on a power pylon serving the property, in a street-side cabinet or inside the premises adjacent to the consumer unit / distribution board. Electricity companies may prefer external locations as the meter can be read without gaining access to the premises but external meters may be more prone to vandalism.
[edit] Connection
In North America, it is common for smaller electricity meters to clip into a standardised base unit. This arrangement allows the meter itself to be replaced without disturbing the supply and load cables which terminate in this base unit. Some base units may have a facility to bypass the meter whilst it 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.
In the UK, the supply and load terminals are normally provided in the meter housing itself, at least for smaller meters (up to around 100 A).
[edit] Tampering and security
Meters can be manipulated so as to make them under-register or even run backwards, effectively allowing power use without paying for it.
The owner of the meter normally secures the meter against such acts. Meters are usually sealed so that the connections and mechanism cannot be tampered-with without breaking the seal. 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. Antitamper techniques are well-known in the industry, but were not widely applied in developed coutries because tampering was rare.
When tampering is detected, the normal tactic, legal in most areas, is to switch the metering rate to the meter's maximum designed rate. Meter readers are trained to spot signs of tampering, and in this case, the maximum rate may be charged each billing period until the tamper is removed, or the service is disconnected.
Given the tamper resistance and sophistication of modern meters, some fraud perpetrators bypass the meter, wholly or in part, to use the power without it being recorded at their building's meter. This normally causes an increase in neutral current at the meter, which is detected and billed at normal rates by standard tamper-resistant meters.
Even if the meter's neutral connector is completely disconnected, and the building's neutral is grounded to the phantom loop, causing an unsafe house or building, metering at the substation can alert the operator to tampering. Substations and some interties and transformers have a high-accuracy meter for the area served. Power companies normally investigate discrepancies between the total billed and the total generated, in order to find and fix power distribution problems. These investigations are an effective method of discovering tampering.
Some newer meter types have counter-measures against many kinds of tampering. AMR (Automated Meter Reading) meters often have sensors that can report opening of the meter cover, magnetic anomalies, reversed or switched phases etc. These features are not available on all meters, though, and it could be catastrophic for grid operators and utilities if information about these weaknesses would be wide-spread, since the objective with AMR meters is that no visit at the meter is required, which would allow permanent tamperings to meters to not be noticed.
Power thefts are often connected with indoor marijuana grow operations. Narcotics detectives associate abnormally high power usage with the lighting such operations require. Indoor marijuana growers aware of this are particularly motivated to steal electricity simply to conceal their usage of it.
The introduction of advanced meters in residential areas has produced additional privacy issues that may affect even ordinary customers. These meters are often capable of recording energy usage very frequently, usually once every 15 minutes or hour. In an IEEE research paper published in 1989, it was shown that readings of this sort can reveal information about the electrical loads being used within the home. A more detailed and recent analysis of this issue was performed by the Illinois Security Lab, as discussed on the Attested Metering project website.
[edit] Prepayment meters
In the UK, mechanical prepayment meters used to be common in rented accommodation. Disadvantages of these included the need for regular visits to remove cash, risk of theft of the cash in the meter and the lack of a means of applying a standing charge is there.
Modern solid-state electricity meters in conjunction with smart card technology have removed these disadvantages and such meters are commonly used for customers considered to be a poor credit risk. In the UK, one system is the PayPoint network, where rechargeable tokens (Quantum cards for natural gas, or plastic "keys" for electricity) can be loaded with whatever money the customer has available.
The similar system applied with 2 way communication smart cards for more than 1 million meters in Turkey by Elektromed.
World over experiments are going on specially in developing countries to test the system. Lack of social acceptance has led to non-implementation of technology.
There are various technology such as STS which promotes platform across manufacturers use of prepayment dispensing system.
[edit] Power export
Many electricity customers are installing their own electricity generating equipment, whether for reasons of economy, redundancy or environmental reasons. Gas turbines, wind turbines and photovoltaic cells are all in common use. When a customer is generating more electricity than required for his own use, the surplus may be exported back to the power grid.
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. More sophisticated meters permit such exported energy to be recorded and accounted for separately.
See also Net metering.
[edit] High-end energy meters
Energy meters have come a long way from being just passive instruments determining consumption of energy during a predetermined period. The high-end meters today come packed with a plethora of features, with major breakthroughs in being non-tamperable and highly accurate at the same time.
[edit] Block diagram
Basic Block Diagram of an Electronic Energy Meter |
Residential electrical meter. |
[edit] Basic working of a modern electronic energy meter
As in the block diagram, the meter has a power supply, a metering engine, A processing and communication engine i.e a microcontroller, other add-on modules such as RTC, LCD display, communication ports/modules etc.
[edit] Power supply
The power supply has the responsibility of providing power to various electronic components in the meter. Meters usually use less than 50 milliwatts, in order to reduce the power company's costs (it is fraud for it to charge customers for the power meters' consumption). Normally the power supply uses a large capacitor, charged by the high voltage through a diode, and slowly drained through a resistor network and voltage regulator. This is far less expensive than a transformer, or the switching power supply used in a PC.
[edit] Metering engine
The metering engine is given the voltage and current inputs and has a voltage reference, samplers and quantisers followed by an ADC 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 such as powers, energies etc.
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 because most meters are outdoors. Characterizing and compensating for these is a major part of meter design.
[edit] Processing and communication section
This 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.
[edit] RTC and other add-on modules
These are attached as slaves to the processing and communication section for various input/output functions.
[edit] Time of use metering
They facilitate load control and planning on the part of utilities. This is effectively achieved using a concept called Time of Usage (TOU) metering. This 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 customers responsibility to control his own usage, or pay accordingly (voluntary load control). This also allows the utilities to plan their transmission infrastructure appropriately.
[edit] Communication technologies
Another development has been in the communication facilities that these high end meters boast of. They now come equipped with Low Power Radio, GSM, GPRS, Bluetooth, IrDA apart from the now conventional RS-232 and RS-485 wired link. They now store the entire usage profiles with time stamps and relay them at a click of a button. The demand readings stored with the profiles accurately indicate the load requirements of the customer. This load profile data is processed at the utilities and renders itself to a variety of representations, all sorts of graphs, reports et el.
[edit] Pre-payment electricity meter
Newer developments have also been in the field of Prepayment Electricity Supply, analogous to the Prepaid phone connection most of us are used to. These deploy Smartcards, both of conventional and the contact-less variety, to recharge the meter with a prestipulated amount of electricity, and optionally, to warn the utilities about any alarming signs in the usage behaviour of the customer by taking the data back to them at the next recharge.
[edit] AMR and RMR
AMR (Automatic Meter Reading) and RMR (Remote Meter Reading) describe various systems that allow meters to be checked by without the need to send a meter reader out. This can be effectively achieved using off-site metering, that is an electronic meter is placed at the junction point where all the connections originate, inaccessible to the end-user, and it relays the readings via the AMR technology to the utility.
[edit] Other types of electricity meter
In addition to the types of meter described above which directly measure the amount of energy used, other types of meter are available.
Meters which measure the amount of charge (coulombs) used, known as ampere-hour meters, were used in the early days of electrification. These were dependent upon the supply voltage remaining constant for accurate measurement of energy usage which is not a likely circumstance with most supplies.
Some meters measured only the length of time for which current flowed, with no measurement of the magnitude of voltage or current being made. These were only suited for constant load applications.
Neither type is likely to be found in electricity retail use today.
Meters for measuring single phase energy is also available. It also provides capabilities like Maximum demand with date&time a kWh Backups
[edit] Accuracy
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 creep test is when the meter is tested for the error due to creep.
[edit] External links
- [2] Paper describing in detail the mathematics and physics behind induction meters
- [3] PRI Ltd IEC standard Electricity meters, Prepayment solutions and Energy Management Services
- [4] Photos and descriptions of historic and modern North American electricity meters]
- EnergyICT: Energy Metering Hardware & Software
- The Meter Guy
- Metering International
- AMRA
- Handbook for Electricity Metering from EEI
History:
Electricity Metering Standards and Regulatory Bodies:
- ABNT Standards/Brazil
- ANSI Standards
- IEC Standards
- INMETRO/Brazil
- Measurement Canada
- IESO Ontario Canada
Meter's Manufacturers:
- Actaris Electricity Metering
- Echelon Networked Energy Services
- Elster Metering
- General Electric
- Itron Metering
- Landis+Gyr Metering
- PRI Metering Solutions
- Secure Meters Ltd
- Sensus Metering
Meter's Testing Equipments:
cs:Elektroměr de:Stromzähler es:Medidor eléctrico hr:Brojilo električne energije nl:Kilowattuurmeter pl:Licznik energii elektrycznej ru:Счётчик электрической энергии fi:Kilowattituntimittari
