Process of balancing the supply of electricity on a network
Daily load diagram; Blue shows real load usage and green shows ideal load.
Load management, also known as demand-side management (DSM), is the process of balancing the supply of electricity on the network with the electrical load by adjusting or controlling the load rather than the power station output. This can be achieved by direct intervention of the utility in real time, by the use of frequency sensitive relays triggering the circuit breakers (ripple control), by time clocks, or by using special tariffs to influence consumer behavior. Load management allows utilities to reduce demand for electricity during peak usage times (peak shaving), which can, in turn, reduce costs by eliminating the need for peaking power plants. In addition, some peaking power plants can take more than an hour to bring on-line which makes load management even more critical should a plant go off-line unexpectedly for example. Load management can also help reduce harmful emissions, since peaking plants or backup generators are often dirtier and less efficient than base load power plants. New load-management technologies are constantly under development — both by private industry[1] and public entities.[2][3]
Modern utility load management began about 1938, using ripple control. By 1948 ripple control was a practical system in wide use.[4]
The Czechs first used ripple control in the 1950s. Early transmitters were low power, compared to modern systems, only 50 kilovolt-amps. They were rotating generators that fed a 1050 Hz signal into transformers attached to power distribution networks. Early receivers were electromechanical relays. Later, in the 1970s, transmitters with high-power semiconductors were used. These are more reliable because they have no moving parts. Modern Czech systems send a digital "telegram." Each telegram takes about thirty seconds to send. It has pulses about one second long. There are several formats, used in different districts.[5]
In 1972, Theodore George “Ted” Paraskevakos, while working for Boeing in Huntsville, Alabama, developed a sensor monitoring system which used digital transmission for security, fire, and medical alarm systems as well as meter-reading capabilities for all utilities. This technology was a spin-off of his patented automatic telephone line identification system, now known as caller ID. In, 1974, Paraskevakos was awarded a U.S. patent for this technology.[6]
At the request of the Alabama Power Company, Paraskevakos developed a load-management system along with automatic meter-reading technology. In doing so, he utilized the ability of the system to monitor the speed of the watt power meter disc and, consequently, power consumption. This information, along with the time of day, gave the power company the ability to instruct individual meters to manage water heater and air conditioning consumption in order to prevent peaks in usage during the high consumption portions of the day. For this approach, Paraskevakos was awarded multiple patents.[7]
Advantages and operating principles
Since electrical energy is a form of energy that cannot be effectively stored in bulk, it must be generated, distributed, and consumed immediately. When the load on a system approaches the maximum generating capacity, network operators must either find additional supplies of energy or find ways to curtail the load, hence load management. If they are unsuccessful, the system will become unstable and blackouts can occur.
Long-term load management planning may begin by building sophisticated models to describe the physical properties of the distribution network (i.e. topology, capacity, and other characteristics of the lines), as well as the load behavior. The analysis may include scenarios that account for weather forecasts, the predicted impact of proposed load-shed commands, estimated time-to-repair for off-line equipment, and other factors.
The utilization of load management can help a power plant achieve a higher capacity factor, a measure of average capacity utilization. Capacity factor is a measure of the output of a power plant compared to the maximum output it could produce. Capacity factor is often defined as the ratio of average load to capacity or the ratio of average load to peak load in a period of time. A higher load factor is advantageous because a power plant may be less efficient at low load factors, a high load factor means fixed costs are spread over more kWh of output (resulting in a lower price per unit of electricity), and a higher load factor means greater total output. If the power load factor is affected by non-availability of fuel, maintenance shut-down, unplanned breakdown, or reduced demand (as consumption pattern fluctuate throughout the day), the generation has to be adjusted, since grid energy storage is often prohibitively expensive.
Smaller utilities that buy power instead of generating their own find that they can also benefit by installing a load control system. The penalties they must pay to the energy provider for peak usage can be significantly reduced. Many report that a load control system can pay for itself in a single season.
Comparisons to demand response
When the decision is made to curtail load, it is done so on the basis of system reliability. The utility in a sense "owns the switch" and sheds loads only when the stability or reliability of the electrical distribution system is threatened. The utility (being in the business of generating, transporting, and delivering electricity) will not disrupt their business process without due cause. Load management, when done properly, is non-invasive, and imposes no hardship on the consumer. The load should be shifted to off peak hours.
Demand response places the "on-off switch" in the hands of the consumer using devices such as a smart grid controlled load control switch. While many residential consumers pay a flat rate for electricity year-round, the utility's costs actually vary constantly, depending on demand, the distribution network, and composition of the company's electricity generation portfolio. In a free market, the wholesale price of energy varies widely throughout the day. Demand response programs such as those enabled by smart grids attempt to incentivize the consumer to limit usage based upon cost concerns. As costs rise during the day (as the system reaches peak capacity and more expensive peaking power plants are used), a free market economy should allow the price to rise. A corresponding drop in demand for the commodity should meet a fall in price. While this works for predictable shortages, many crises develop within seconds due to unforeseen equipment failures. They must be resolved in the same time-frame in order to avoid a power blackout. Many utilities who are interested in demand response have also expressed an interest in load control capability so that they might be able to operate the "on-off switch" before price updates could be published to the consumers.[8]
The application of load control technology continues to grow today with the sale of both radio frequency and powerline communication based systems. Certain types of smart meter systems can also serve as load control systems. Charge control systems can prevent the recharging of electric vehicles during peak hours. Vehicle-to-grid systems can return electricity from an electric vehicle's batteries to the utility, or they can throttle the recharging of the vehicle batteries to a slower rate.[9]
Ripple control
Ripple control is a common form of load control, and is used in many countries around the world, including the United States, Australia, Czech Republic, New Zealand, the United Kingdom, Germany, the Netherlands, and South Africa. Ripple control involves superimposing a higher-frequency signal (usually between 100 and 1600Hz[10]) onto the standard 50–60Hz of the main power signal. When receiver devices attached to non-essential residential or industrial loads receive this signal, they shut down the load until the signal is disabled or another frequency signal is received.
Early implementations of ripple control occurred during World War II in various parts of the world using a system that communicates over the electrical distribution system. Early systems used rotating generators attached to distribution networks through transformers. Ripple control systems are generally paired with a two- (or more) tiered pricing system, whereby electricity is more expensive during peak times (evenings) and cheaper during low-usage times (early morning).
Affected residential devices will vary by region, but may include residential electric hot-water heaters, air conditioners, pool pumps, or crop-irrigation pumps. In a distribution network outfitted with load control, these devices are outfitted with communicating controllers that can run a program that limits the duty cycle of the equipment under control. Consumers are usually rewarded for participating in the load control program by paying a reduced rate for energy. Proper load management by the utility allows them to practice load shedding to avoid rolling blackouts and reduce costs.
Ripple control can be unpopular because sometimes devices can fail to receive the signal to turn on comfort equipment, e.g. hot water heaters or baseboard electrical heaters. Modern electronic receivers are more reliable than old electromechanical systems. Also, some modern systems repeat the telegrams to turn on comfort devices. Also, by popular demand, many ripple control receivers have a switch to force comfort devices on.
Modern ripple controls send a digital telegram, from 30 to 180 seconds long. Originally these were received by electromechanical relays. Now they are often received by microprocessors. Many systems repeat telegrams to assure that comfort devices (e.g. water heaters) are turned on. Since the broadcast frequencies are in the range of human hearing, they often vibrate wires, filament light-bulbs or transformers in an audible way.[5]
The telegrams follow different standards in different areas. For example, in the Czech Republic, different districts use "ZPA II 32S", "ZPA II 64S" and Versacom. ZPA II 32S sends a 2.33 second on, a 2.99 second off, then 32 one-second pulses (either on or off), with an "off time" between each pulse of one second. ZPA II 64S has a much shorter off time, permitting 64 pulses to be sent, or skipped.[5]
Nearby regions use different frequencies or telegrams, to assure that telegrams operate only in the desired region. The transformers that attach local grids to interties intentionally do not have the equipment (bridging capacitors) to pass ripple control signals into long-distance power lines.[5]
Each data pulse of a telegram could double the number of commands, so that 32 pulses permit 2^32 distinct commands. However, in practice, particular pulses are linked to particular types of device or service. Some telegrams have unusual purposes. For example most ripple control systems have a telegram to set clocks in attached devices, e.g. to midnight.[5]
Greater loads physically slow the rotors of a grid's synchronized generators. This causes AC mains to have a slightly reduced frequency when a grid is heavily loaded. The reduced frequency is immediately sensible across the entire grid. Inexpensive local electronics can easily and precisely measure mains frequencies and turn off sheddable loads. In some cases, this feature is nearly free, e.g. if the controlling equipment (such as an electric power meter, or the thermostat in an air-conditioning system) already has a microcontroller. Most electronic electric power meters internally measure frequency, and require only demand control relays to turn off equipment. In other equipment, often the only needed extra equipment is a resistor divider to sense the mains cycle and a schmitt trigger (a small integrated circuit) so the microcontrollers' digital input can sense a reliable fast digital edge. A schmitt trigger is already standard equipment on many microcontrollers.
The main advantage over ripple control is greater customer convenience: Unreceived ripple control telegrams can cause a water heater to remain off, causing a cold shower. Or, they can cause an airconditioner to remain off, resulting in a sweltering home. In contrast, as the grid recovers, its frequency naturally rises to normal, so frequency-controlled load control automatically enables water heaters, air-conditioners and other comfort equipment. The cost of equipment can be less, and there are no concerns about overlapping or unreached ripple control regions, mis-received codes, transmitter power, etc.
The main disadvantage compared to ripple control is a less fine-grained control. For example, a grid authority has only a limited ability to select which loads are shed. In controlled war-time economies, this can be a substantial disadvantage.
The system was invented in PNNL in the early 21st century, and has been shown to stabilize grids.[11]
Examples of schemes
In many countries, including United States, United Kingdom and France, the power grids routinely use privately held, emergency diesel generators in load management schemes[12]
Florida
The largest residential load control system in the world[13] is found in Florida and is managed by Florida Power and Light. It utilizes 800,000 load control transponders (LCTs) and controls 1,000 MW of electrical power (2,000 MW in an emergency). FPL has been able to avoid the construction of numerous new power plants due to their load management programs.[14]
Australia and New Zealand
A ripple control receiver fitted to a New Zealand house. The left circuit breaker controls the water storage heater supply (currently on), while the right one controls the nightstore heater supply (currently off).
Since the 1950s, Australia and New Zealand have had a system of load management based on ripple control, allowing the electricity supply for domestic and commercial water storage heaters to be switched off and on, as well as allowing remote control of nightstore heaters and street lights. Ripple injection equipment located within each local distribution network signals to ripple control receivers at the customer's premises. Control may either done manually by the local distribution network company in response to local outages or requests to reduce demand from the transmission system operator (i.e. Transpower), or automatically when injection equipment detects mains frequency falling below 49.2Hz. Ripple control receivers are assigned to one of several ripple channels to allow the network company to only turn off supply on part of the network, and to allow staged restoration of supply to reduce the impact of a surge in demand when power is restored to water heaters after a period of time off.
Depending on the area, the consumer may have two electricity meters, one for normal supply ("Anytime") and one for the load-managed supply ("Controlled"), with Controlled supply billed at a lower rate per kilowatt-hour than Anytime supply. For those with load-managed supply but only a single meter, electricity is billed at the "Composite" rate, priced between Anytime and Controlled.
Czech Republic
The Czechs have operated ripple control systems since the 1950s.[5]
France
France has an EJP tariff, which allows it to disconnect certain loads and to encourage consumers to disconnect certain loads.[15] This tariff is no longer available for new clients (as of July 2009).[16] The Tempo tariff also includes different types of days with different prices, but has been discontinued for new clients as well (as of July 2009).[17] Reduced prices during nighttime are available for customers for a higher monthly fee.[18]
Germany
The distribution system operator Westnetz and gridX piloted a load management solution. The solution enables the grid operator to communicate with local energy management systems and adjust the available load for EV charging in response to the state of the grid.[19]
Rltec in the UK in 2009 reported that domestic refrigerators are being sold fitted with their dynamic load response systems. In 2011 it was announced that the Sainsbury supermarket chain will use dynamic demand technology on their heating and ventilation equipment.[20]
In the UK, night storage heaters are often used with a time-switched off-peak supply option - Economy 7 or Economy 10. There is also a programme that allows industrial loads to be disconnected using circuit breakers triggered automatically by frequency sensitive relays fitted on site. This operates in conjunction with Standing Reserve, a programme using diesel generators.[21] These can also be remotely switched using BBC Radio 4 Longwave Radio teleswitch.
SP transmission deployed Dynamic Load Management scheme in Dumfries and Galloway area using real time monitoring of embedded generation and disconnecting them, should an overload be detected on the transmission Network.
A power inverter, inverter or invertor is a power electronic device or circuitry that changes direct current (DC) to alternating current (AC). The resulting AC frequency obtained depends on the particular device employed. Inverters do the opposite of rectifiers which were originally large electromechanical devices converting AC to DC.
Power-line communication carries data on a conductor that is also used simultaneously for AC electric power transmission or electric power distribution to consumers.
Distributed generation, also distributed energy, on-site generation (OSG), or district/decentralized energy, is electrical generation and storage performed by a variety of small, grid-connected or distribution system-connected devices referred to as distributed energy resources (DER).
Automatic meter reading (AMR) is the technology of automatically collecting consumption, diagnostic, and status data from water meter or energy metering devices and transferring that data to a central database for billing, troubleshooting, and analyzing. This technology mainly saves utility providers the expense of periodic trips to each physical location to read a meter. Another advantage is that billing can be based on near real-time consumption rather than on estimates based on past or predicted consumption. This timely information coupled with analysis can help both utility providers and customers better control the use and production of electric energy, gas usage, or water consumption.
An electricity meter, electric meter, electrical meter, energy meter, or kilowatt-hour meter is a device that measures the amount of electric energy consumed by a residence, a business, or an electrically powered device.
Energy demand management, also known as demand-side management (DSM) or demand-side response (DSR), is the modification of consumer demand for energy through various methods such as financial incentives and behavioral change through education.
Grid energy storage is a collection of methods used for energy storage on a large scale within an electrical power grid. Electrical energy is stored during times when electricity is plentiful and inexpensive or when demand is low, and later returned to the grid when demand is high, and electricity prices tend to be higher.
Demand response is a change in the power consumption of an electric utility customer to better match the demand for power with the supply. Until the 21st century decrease in the cost of pumped storage and batteries electric energy could not be easily stored, so utilities have traditionally matched demand and supply by throttling the production rate of their power plants, taking generating units on or off line, or importing power from other utilities. There are limits to what can be achieved on the supply side, because some generating units can take a long time to come up to full power, some units may be very expensive to operate, and demand can at times be greater than the capacity of all the available power plants put together. Demand response seeks to adjust the demand for power instead of adjusting the supply.
A microgrid is a local electrical grid with defined electrical boundaries, acting as a single and controllable entity. It is able to operate in grid-connected and in island mode. A 'Stand-alone microgrid' or 'isolated microgrid' only operates off-the-grid and cannot be connected to a wider electric power system.
Electrical devices are considered grid friendly if they operate in a manner that supports electrical power grid reliability through demand response. Basic grid-friendly devices may incorporate features that work to offset short-term undesirable changes in line frequency or voltage; more sophisticated devices may alter their operating profile based on the current market price for electricity, reducing load when prices are at a peak. Grid-friendly devices can include major appliances found in homes, commercial building systems such as HVAC, and many industrial systems.
Advanced Distribution Automation (ADA) is a term coined by the IntelliGrid project in North America to describe the extension of intelligent control over electrical power grid functions to the distribution level and beyond. It is related to distribution automation that can be enabled via the smart grid. The electrical power grid is typically separated logically into transmission systems and distribution systems. Electric power transmission systems typically operate above 110kV, whereas Electricity distribution systems operate at lower voltages. Normally, electric utilities with SCADA systems have extensive control over transmission-level equipment, and increasing control over distribution-level equipment via distribution automation. However, they often are unable to control smaller entities such as Distributed energy resources (DERs), buildings, and homes. It may be advantageous to extend control networks to these systems for a number of reasons:
A smart meter is an electronic device that records information such as consumption of electric energy, voltage levels, current, and power factor. Smart meters communicate the information to the consumer for greater clarity of consumption behavior, and electricity suppliers for system monitoring and customer billing. Smart meters typically record energy near real-time, and report regularly, short intervals throughout the day. Smart meters enable two-way communication between the meter and the central system. Such an advanced metering infrastructure (AMI) differs from automatic meter reading (AMR) in that it enables two-way communication between the meter and the supplier. Communications from the meter to the network may be wireless, or via fixed wired connections such as power line carrier (PLC). Wireless communication options in common use include cellular communications, Wi-Fi, wireless ad hoc networks over Wi-Fi, wireless mesh networks, low power long-range wireless (LoRa), Wize Zigbee, and Wi-SUN.
A virtual power plant (VPP) is a cloud-based distributed power plant that aggregates the capacities of heterogeneous distributed energy resources (DER) for the purposes of enhancing power generation, trading or selling power on the electricity market, and demand side options for load reduction.
Dynamic Demand is the name of a semi-passive technology to support demand response by adjusting the load demand on an electrical power grid. It is also the name of an independent not-for-profit organization in the UK supported by a charitable grant from the Esmée Fairbairn Foundation, dedicated to promoting this technology. The concept is that by monitoring the frequency of the power grid, as well as their own controls, intermittent domestic and industrial loads switch themselves on/off at optimal moments to balance the overall grid load with generation, reducing critical power mismatches. As this switching would only advance or delay the appliance operating cycle by a few seconds, it would be unnoticeable to the end user. This is the foundation of dynamic demand control. In the United States, in 1982, a (now-lapsed) patent for this idea was issued to power systems engineer Fred Schweppe. Other patents have been issued based on this idea.
Zellweger is the brand name of an electric switching device also known as a Ripple Control Receiver used to control off-peak electrical loads such as water heaters by switching these loads OFF over peak energy use times of the day and switching them ON after peak energy use times of the day, hence the term 'off peak' control. It is an example of carrier current signaling. The Ripple Control Signal is generated at substations owned by Electricity Supply Authorities connected to the High Voltage transmission grid and injected into the Medium Voltage transmission grid at 11kV, 22kV, 33kV and 66kV, through a Coupling Cell consisting of a tuned L-C circuit. The Coupling Cell enables the Ripple Control Frequency to be superimposed on the 50 Hertz (Hz) mains frequency, which promulgates into the 415 V 3 phase power distribution lines providing energy to industrial and domestic customers of the Electricity Supply Authority. To avoid problems with other equipment connected to the distribution system ; i.e. industrial machinery and domestic appliances, the ripple frequency is selected to be offset from the third harmonic and its multiples, typically starting at 167 Hz and including, 217, 317, 425, 750, 1050, 1650. The choice of frequency depends upon the density of the load into which the ripple frequency is to be injected and the length of the distribution
Load balancing, load matching, or daily peak demand reserve refers to the use of various techniques by electrical power stations to store excess electrical power during low demand periods for release as demand rises. The aim is for the power supply system to have a load factor of 1.
A smart grid is an electrical grid which includes a variety of operation and energy measures including:
An electrical grid is an interconnected network for electricity delivery from producers to consumers. Electrical grids vary in size and can cover whole countries or continents. It consists of:
The term Smart Grid describes a next-generation electric power system, that is classified by the increased use of communication and information technology in the generation, delivery, and consumption of electrical energy. For individual consumers, smart grid technology offers more control over electricity consumption. Typically, the goal is overall greater energy efficiency.
Home idle load is the continuous residential electric energy consumption as measured by smart meters. It differs from standby power (loads) in that it includes energy consumption by devices that cycle on and off within the hourly period of standard smart meters. As such, home idle loads can be measured accurately by smart meters. According to Stanford Sustainable Systems Lab, home idle load constitutes an average of 32% of household electricity consumption in the U.S.
↑ U.S. Patent No. 3,842,208 (sensor monitoring device)
↑ U.S. patents Nos. 4,241,237, 4,455,453 and 7,940,901 (remote management of products and services) as well as Canadian Patent No. 1,155,243 (apparatus and method for remote sensor monitoring, metering and control)
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