Power system operations is a term used in
electricity generation
Electricity generation is the process of generating electric power from sources of primary energy. For electric utility, utilities in the electric power industry, it is the stage prior to its Electricity delivery, delivery (Electric power transmi ...
to describe the process of
decision-making
In psychology, decision-making (also spelled decision making and decisionmaking) is regarded as the Cognition, cognitive process resulting in the selection of a belief or a course of action among several possible alternative options. It could be ...
on the timescale from one day (day-ahead operation) to minutes prior to the
power delivery
Electricity delivery is the process that starts after generation of electricity in the power station, up to the use by the consumer.
The main processes in electricity delivery are, by order:
* Transmission
* Distribution
* Retailing
See also
*Ele ...
. The term power system control describes actions taken in response to unplanned ''disturbances'' (e.g., changes in demand or equipment failures) in order to provide reliable electric supply of acceptable quality.
The corresponding
engineering branch
Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad range of more speciali ...
is called Power System Operations and Control. Electricity is hard to store, so at any moment the supply (generation) shall be balanced with demand ("
grid balancing
Grid balancing ensures that electricity consumption matches electricity production of an electrical grid at any moment. Electricity is by its nature difficult to store and has to be available on demand, so the supply shall match the demand very cl ...
"). In an electrical grid the task of real-time balancing is performed by a regional-based control center, run by an electric utility in the traditional (
vertically integrated
In microeconomics, management and international political economy, vertical integration is a term that describes the arrangement in which the supply chain of a company is integrated and owned by that company. Usually each member of the supply ...
) electricity market. In the restructured
North American power transmission grid
The electrical grid, electrical power grid that powers Northern America is not a single grid, but is instead divided into multiple wide area synchronous grids. The Eastern Interconnection and the Western Interconnection are the largest. Three ot ...
, these centers belong to ''
balancing authorities'' numbered 74 in 2016,
the entities responsible for operations are also called
independent system operator
A regional transmission organization (RTO) in the United States is an electric power transmission system operator (TSO) that coordinates, controls, and monitors a multi-state electric grid. The transfer of electricity between states is considered i ...
s, transmission system operators. The other form of balancing resources of multiple power plants is a
power pool. The balancing authorities are overseen by
reliability coordinators.
Day-ahead operation
Day-ahead operation schedules the generation units that can be called upon to provide the electricity on the next day (unit commitment). The
dispatchable generation
Dispatchable generation refers to sources of electricity that can be programmed ''on demand'' at the request of power grid operators, according to market needs. Dispatchable generators may adjust their power output according to an order.
Non-disp ...
units can produce electricity on demand and thus can be scheduled with accuracy. The production of the weather-dependent
variable renewable energy
Variable renewable energy (VRE) or intermittent renewable energy sources (IRES) are renewable energy sources that are not dispatchable due to their fluctuating nature, such as wind power and solar power, as opposed to controllable renewable ener ...
for the next day is not certain, its sources are thus non-dispatchable. This variability, coupled with uncertain future power demand and the need to accommodate possible generation and
transmission
Transmission may refer to:
Medicine, science and technology
* Power transmission
** Electric power transmission
** Propulsion transmission, technology allowing controlled application of power
*** Automatic transmission
*** Manual transmission
*** ...
failures requires scheduling of
operating reserve
In electricity networks, the operating reserve is the generating capacity available to the system operator within a short interval of time to meet demand in case a generator goes down or there is another disruption to the supply. Most power sys ...
s that are not expected to produce electricity, but can be dispatched on a very short notice.
Some units have unique features that require their commitment much earlier: for example, the
nuclear power station
A nuclear power plant (NPP) is a thermal power station in which the heat source is a nuclear reactor. As is typical of thermal power stations, heat is used to generate steam that drives a steam turbine connected to a generator that produces elec ...
s take a very long time to start, while hydroelectric plants require planning of water resources usage way in advance, therefore commitment decisions for these are made weeks or even months before prior to the delivery.
For a "traditional"
vertically integrated electric utility the main goal of the unit commitment is to minimize both the
marginal cost
In economics, the marginal cost is the change in the total cost that arises when the quantity produced is incremented, the cost of producing additional quantity. In some contexts, it refers to an increment of one unit of output, and in others it r ...
of producing the unit electricity and the (quite significant for fossil fuel generation) start-up costs. In a "restructured"
electricity market
In a broad sense, an electricity market is a system that facilitates the exchange of electricity-related goods and services. During more than a century of evolution of the electric power industry, the economics of the electricity markets had un ...
a
market clearing
In economics, market clearing is the process by which, in an economic market, the supply of whatever is traded is equated to the demand so that there is no excess supply or demand. The new classical economics assumes that in any given market, assum ...
algorithm is utilized, frequently in a form of an
auction
An auction is usually a process of buying and selling goods or services by offering them up for bids, taking bids, and then selling the item to the highest bidder or buying the item from the lowest bidder. Some exceptions to this definition ex ...
; the
merit order
The merit order is a way of ranking available sources of energy, especially electrical generation, based on ascending order of price (which may reflect the order of their short-run marginal costs of production) and sometimes pollution, together w ...
is sometimes defined not just by the monetary costs, but also by the environmental concerns.
Unit commitment is more complex than the shorter-time-frame operations, since unit availability is subject to multiple constraints:
* demand-supply balance need to be maintained, including the sufficient
spinning reserve
In electricity networks, the operating reserve is the generating capacity available to the system operator within a short interval of time to meet demand in case a generator goes down or there is another disruption to the supply. Most power sys ...
s for contingency. The balance need to reflect the transmission constraints;
* thermal units might have limits on minimum uptime (once switched on, cannot be turned off quickly) and downtime (once stopped, cannot be quickly restarted again);
* "must-run" units have to run due to technical constraints (for example,
combined heat and power
Cogeneration or combined heat and power (CHP) is the use of a heat engine or power station to generate electricity and useful heat at the same time.
Cogeneration is a more efficient use of fuel or heat, because otherwise- wasted heat from elect ...
plants must run if their heat is needed);
* there is usually a single crew at the plant that needs to be present during a thermal unit start-up, so only one unit can be started at a time.
Hours-ahead operation
In the hours prior to the delivery, a system operator might need to deploy additional
supplemental reserve
In electricity networks, the operating reserve is the generating capacity available to the system operator within a short interval of time to meet demand in case a generator goes down or there is another disruption to the supply. Most power sys ...
s or even commit more generation units, primarily to ensure the reliability of the supply while still trying to minimize the costs. At the same time, operator must ensure that enough
reactive power reserves
Voltage control and reactive power management are two facets of an ancillary service that enables reliability of the transmission networks and facilitates the electricity market on these networks. Both aspects of this activity are intertwined (vo ...
are available to prevent the
voltage collapse
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to ...
.
Dispatch curve
The decisions ("
economic dispatch
The merit order is a way of ranking available sources of energy, especially electrical generation, based on ascending order of price (which may reflect the order of their short-run marginal costs of production) and sometimes pollution, together w ...
") are based on the dispatch curve, where the X-axis constitutes the system power, intervals for the generation units are placed on this axis in the ''
merit order
The merit order is a way of ranking available sources of energy, especially electrical generation, based on ascending order of price (which may reflect the order of their short-run marginal costs of production) and sometimes pollution, together w ...
'' with the interval length corresponding to the maximum power of the unit, Y-axis values represent the marginal cost (per-
MWh of electricity, ignoring the startup costs). For cost-based decisions, the units in the merit order are sorted by the increasing marginal cost. The graph on the right describes an extremely simplified system, with three committed generator units (fully dispatchable, with constant per-MWh cost):
[
* unit A can deliver up to 120 MW at the cost of $30 per MWh (from 0 to 120 MW of system power);
* unit B can deliver up to 80 MW at $60/MWh (from 120 to 200 MW of system power);
* unit C is capable of 50 MW at $120/MWh (from 200 to 250 MW of system power).
At the expected demand is 150 MW (a vertical line on the graph), unit A will be engaged at full 120 MW power, unit B will run at the dispatch level of 30 MW, unit C will be kept in reserve. The area under the dispatch curve to the left of this line represents the cost per hour of operation (ignoring the startup costs, $30 * 120 + $60 * 30 = $5,400 per hour), the incremental cost of the next MWh of electricity ($60 in the example, represented by a horizontal line on the graph) is called system lambda (thus another name for the curve, ''system lambda curve'').
In real systems the cost per MWh usually is not constant, and the lines of the dispatch curve are therefore not horizontal (typically the marginal cost of power increases with the dispatch level, although for the ]combined cycle power plant
A combined cycle power plant is an assembly of heat engines that work in tandem from the same source of heat, converting it into mechanical energy. On land, when used to make electricity the most common type is called a combined cycle gas turb ...
s there are multiple cost curves depending on the mode of operation, so the power-cost relationship is not necessarily monotonic
In mathematics, a monotonic function (or monotone function) is a function between ordered sets that preserves or reverses the given order. This concept first arose in calculus, and was later generalized to the more abstract setting of order ...
).
If the minimum level of demand in the example will stay above 120 MW, the unit A will constantly run at full power, providing baseload power
The base load (also baseload) is the minimum level of demand on an electrical grid over a span of time, for example, one week. This demand can be met by unvarying power plants, dispatchable generation, or by a collection of smaller intermittent en ...
, unit B will operate at variable power, and unit C will need to be turned on and off, providing the "intermediate" or "cycling" capacity. If the demand goes above 200 MW only occasionally, the unit C will be idle most of the time and will be considered a peaking power plant
Peaking power plants, also known as peaker plants, and occasionally just "peakers", are power plants that generally run only when there is a high demand, known as peak demand, for electricity. Because they supply power only occasionally, the power ...
(a "peaker"). Since a peaker might run for just tens of hours per year, the cost of peaker-produced electricity can be very high in order to recover the capital investment and fixed costs (see the right side of a hypothetical full-scale dispatch curve).
Redispatch
Sometimes the grid constraints change unpredictably and a need arises to change the previously set unit commitments. This system redispatch change is controlled in real-time by the central operator issuing directives to market participants that submit in advance bids for the increase/decrease in the power levels. Due to the centralized nature of redispatch, there is no delay to negotiate terms of contracts; the cost incurred are allocated either to participants responsible for the disruption based on preestablished tariffs or in equal shares.
Minutes-ahead operation
In the minutes prior to the delivery, a system operator is using the power-flow study
In power engineering, the power-flow study, or load-flow study, is a numerical analysis of the flow of electric power in an interconnected system. A power-flow study usually uses simplified notations such as a one-line diagram and per-unit system ...
algorithms in order to find the optimal power flow. At this stage the goal is reliability ("security") of the supply. The practical electric networks are too complex to perform the calculations by hand, so from the 1920s the calculations were automated, at first in the form of specially-built analog computer
An analog computer or analogue computer is a type of computer that uses the continuous variation aspect of physical phenomena such as electrical, mechanical, or hydraulic quantities (''analog signals'') to model the problem being solved. In c ...
s, so called ''network analyzers
Network, networking and networked may refer to:
Science and technology
* Network theory, the study of graphs as a representation of relations between discrete objects
* Network science, an academic field that studies complex networks
Mathematics ...
'', replaced by digital computers in the 1960s.
Control after disturbance
Small mismatches between the total demand and total load are typical and initially are taken care of by the kinetic energy
In physics, the kinetic energy of an object is the energy that it possesses due to its motion.
It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its accele ...
of the rotating machinery (mostly synchronous generator In an alternating current electric power system, synchronization is the process of matching the frequency of a generator or other source to a running network. An AC generator cannot deliver power to an electrical grid unless it is running at the sam ...
s): when there is too much supply, the devices absorb the excess, and frequency goes above the scheduled rate, conversely, too much demand causes the generator to deliver extra electricity through slowing down, with frequency slightly decreasing, not requiring an intervention form the operator. There are obvious limits to this "immediate control", so a ''control continuum'' is built into a typical power grid, spanning reaction intervals from seconds ("primary control") to hours ("time control").
Seconds-after control
The is engaged automatically within seconds after the frequency disturbance. Primary control stabilizes the situation, but does not return the conditions to the normal and is applied both to the generation side (where the governor
A governor is an administrative leader and head of a polity or political region, ranking under the head of state and in some cases, such as governors-general, as the head of state's official representative. Depending on the type of political ...
adjusts the power of the prime mover
Prime mover may refer to:
Philosophy
*Unmoved mover, a concept in Aristotle's writings
Engineering
* Prime mover (engine), motor, a machine that converts various other forms of energy (chemical, electrical, fluid pressure/flow, etc) into energy o ...
) and to the load, where:
* induction motors self-adjust (lower frequency reduces the energy use);
* under-frequency relays disconnect interruptible loads;
* ancillary services Ancillary services are the services necessary to support the transmission of electric power from generators to consumers given the obligations of control areas and transmission utilities within those control areas to maintain reliable operations of ...
are engaged (load is reduced as procured via reliability services contracts).
Another term commonly used for the primary control is frequency response (or "beta"). Frequency response also includes the inertial response of the generators. This is the parameter that is approximated by the frequency bias coefficient of the area control error
The utility frequency, (power) line frequency (American English) or mains frequency (British English) is the nominal frequency of the oscillations of alternating current (AC) in a wide area synchronous grid transmitted from a power station to t ...
(ACE) calculation used for automatic generation control.
Minutes-after control
The is used to restore the system frequency after a disturbance, with adjustments made by the balancing authority control computer (this is typically referred to as load-frequency control or automatic generation control) and manual actions taken by the balancing authority staff. Secondary control uses both the spinning
Spin or spinning most often refers to:
* Spinning (textiles), the creation of yarn or thread by twisting fibers together, traditionally by hand spinning
* Spin, the rotation of an object around a central axis
* Spin (propaganda), an intentionally b ...
and non-spinning reserves, with balancing services deployed within minutes after disturbance (hydropower plants are capable of an even faster reaction).
Tertiary control
The ''tertiary control'' involves reserve deployment and restoration to handle the current and future contingencies.
Time control
The goal of the time control is to maintain the long-term frequency at the specified value within a wide area synchronous grid
A wide area synchronous grid (also called an "interconnection" in North America) is a three-phase electric power grid that has regional scale or greater that operates at a synchronized utility frequency and is electrically tied together during no ...
. Due to the disturbances, the average frequency drifts, and a ''time error'' accumulates between the official time and the time measured in the AC cycles. In the US, the average 60 Hz frequency is maintained within each interconnection
In telecommunications, interconnection is the physical linking of a carrier's network with equipment or facilities not belonging to that network. The term may refer to a connection between a carrier's facilities and the equipment belonging to ...
by a designated entity, time monitor, that periodically forces a grid frequency change to bring the overall time offset within the predefined limits. For example, in the Eastern Interconnection
The Eastern Interconnection is one of the two major alternating-current (AC) electrical grids in the North American power transmission grid. The other major interconnection is the Western Interconnection. The three minor interconnections ...
the action (temporarily setting the frequency to 60.02 Hz or 59.98 Hz) is initiated when the time offset reaches 10 seconds and ceases once the offset reaches 6 seconds. Time control is performed either by a computer (''Automatic Time Error Correction''), or by the monitor requesting balancing authorities to adjust their settings.
References
Sources
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*
* {{cite book , last1=NERC , title=Balancing and Frequency Control , date=May 11, 2021 , publisher=North American Electric Reliability Corporation
The North American Electric Reliability Corporation (NERC) is a nonprofit corporation based in Atlanta, Georgia, and formed on March 28, 2006, as the successor to the North American Electric Reliability Council (also known as NERC). The original N ...
, url=https://www.nerc.com/comm/OC/ReferenceDocumentsDL/Reference_Document_NERC_Balancing_and_Frequency_Control.pdf
Electric power generation
Power engineering
Electric power infrastructure
Power station technology