| Electricity Meter |
Article Index for Electricity |
Website Links For Electricity |
Information AboutElectricity Meter |
| CATEGORIES ABOUT ELECTRICITY METER | |
| electric power | |
| electricity distribution | |
| measuring instruments | |
| electrical engineering | |
|
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. UNIT OF MEASUREMENT electricity meter, connected to 2 MVA electricity Substation . Remote current and voltage sensors. Capable of being read and programmed remotely by Modem and locally by Infra-red . The circle with two dots is the infra-red port. Tamper-evident seals can be seen.]]The most common unit of measurement on the electricity meter is the Kilowatt-hour which is equal to the amount of energy used by a load of one Kilowatt over a period of one Hour , or 3,600,000 Joule s. Some electricity companies use the SI Megajoule instead. Demand is normally measured in Watts, but averaged over a period, most often a quarter or half hour. Reactive power is measured as "volt-amps, reactive", (VARh) also in kilowatt-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-Amps measures all power passed through the distribution network, whether reactive or actual. This is equal to the product of root-mean-square volts and amps. Alternatively, it is the square-root of the sum of the squares of Watts and VARs. Distortion of the electric current by loads is measured in several ways. Power factor is the ratio of reactive to volt-amps. 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). 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). So, why would anyone care? 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 find and punish high demand. Since the need for new equipment is really set by how hot the existing equipment runs, demand meters are really an attempt to find and control the sources of overheating in the Electrical Grid . Heating is caused by current. Therefore, demand meters ideally measure Volt-amperes, which combine both reactive (reflected) and actual (consumed) AC Power , because both consumed and reflected power cause currents and therefore heating in distribution equipment. 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 Heater s. 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 Grid s 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 Programme s. Some multiple tariff meters use different tariffs for different amounts of demand. These are usually industrial meters. 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 Controller s, 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 Line s or Over The Power Lines Themselves . Some meters can be read over the internet. by Law . 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. 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 . s used as part of metering equipment for Three-phase 400 Amp electricity supply. The fourth neutral wire does not require a current transformer because current cannot flow in this wire without also flowing in one of the three phase wires.]]As stated above, the use of current transformers permits the meter to be located remotely from the current-carrying conductors. This arrangement is commonly used in larger installations, for example an outdoor Substation serving a single large customer may have metering equipment installed in a nearby cabinet without the need to bring the very heavy cables leading out of the substation into the cabinet. 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 amps). TAMPERING, SECURITY AND FRAUD Some customers attempt to manipulate the meter to cause it to under-register or even run backwards, effectively using power without paying for it. They excuse this by referring to the increasing costs of energy, perceived corrupt Profit s or actions of the electricity company, etc. This is Fraud , Illegal in most countries. Most companies however do not prosecute it, but rather charge the maximum possible rate to the service user under a "tampering" tariff. Tampering techniques vary from unsubtle means such as physically breaking the meter housing and jamming the mechanism to methods that apply Magnet s to the outside of the meter (to saturate the magnetic circuits) or alter the characteristics of the load (adding capacitance, rectified loads or instantaneous high currents) intending to alter the characteristics of the meter. The owner of the meter (That is, the supplier of the electricity) 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 customer refuses payment, ending service. 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. In regions where tampering is a widespread part of popular culture, operators normally shut down service to entire districts rather than lose money. For example, in some districts in India, quite expensive generating plants are idle for this reason, and local industry is developing computerised meters with the most thorough sets of anti-tampering features in history. PREPAYMENT METERS tokens, from a rented accommodation in the UK. The button labeled A displays information and statistics such as current tariff and remaining credit. The button labeled '''B''' activates a small amount of emergency credit should the customer run out.]]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 for whatever reason not pay the bill, a prepayment meter may be installed. This requires the customer to make advance payment before electricity can be used. If the available credit is exhausted then the supply of electricity is cut off by a Relay . 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. 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 rechargable tokens (Quantum cards for natural gas, or plastic "keys" for electricity) can be loaded with whatever money the customer has available. POWER EXPORT Many electricity customers are installing their own electricity generating equipment, whether for reasons of economy, redundancy or Environmental Reasons . Gas Turbine s, Wind Turbine s 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 . 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. Block Diagram |
|
|