Inertial response

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Inertial response is a property of large synchronous generators, which contain large synchronous rotating masses, and which acts to overcome any immediate imbalance between power supply and demand for electric power systems, typically the electrical grid. Due to the ever existing power imbalance between mechanical power supply and electric power demand the rotational frequency of the rotating masses in all synchronous generators in the grid either speed up and thus absorb the extra power in case of an excess power supply, or slow down and provide additional power in case of an excess power demand. This response in case of a synchronous generator is built-in into the design and happens without any external intervention or coordination, providing the automatic generation control and the grid operator with valuable time (few seconds) to rebalance the system [1] The grid frequency is the combined result of the detailed motions of all individual synchronous rotors in the grid, which are modeled by a general equation of motion called the swing equation .

Contents

In the US power systems, the grid operator is mandated to keep the frequency within a tight range, and can be financially responsible if the monitoring by the North American Electric Reliability Corporation detects a non-compliance. Furthermore, in order to protect the equipment, a portion of the load will be disconnected ("underfrequency load shedding", UFLS) if the frequency drops below a limit (59.5 Hz in most of the US, 59.3 Hz in Texas). [2] When an unexpected supply disruption occurs (for example, a generator failure), the primary frequency response kicks in automatically - a sensor detects the lower frequency and adjusts the power of the prime mover accordingly. For a typical synchronous generator, this adjustment involves manipulation of the mechanical devices (valves, etc.) and thus takes time. During this time, the power grid has to rely on the accumulated inertia to slow down the decrease in frequency. [3]

Synchronous generators

Inertia can be measured in power-time product units (say, gigawatt-seconds), [4] but is often normalized to the "size" (nominal electrical power) of the generator and thus can be described in the units of time (so called generator inertia constant [5] ). The faster spinning generators might store more kinetic energy (proportional to square of the rotational frequency), but are typically lighter and thus decelerate faster, causing more power to be injected early in the response ("front-loading") when compared to the slower and heavier machines; this is not necessarily better due to interaction between parts of the grid that can cause "bouncing" and instability. [6] Typical power plants have the inertia constant values from 2 seconds (hydropower) to 7 seconds (gas turbines). [5] Since the rotational speed and thus the kinetic energy of a synchronous generator does not depend on its current power level, the inertia of the overall grid is related to the inertia constants of the running generators; [7] at the time of lower power demand (say, at night) there might be less generators running, and thus a similar contingency might be harder to deal with. [8]

Load

The electrical load can have an inertia-like quality. For example, typical industrial electrical motors consume less power at lower frequencies, adding a small, but noticeable amount of inertia to the system, [9] this effect is diminishing due to switching to modern and efficient variable-speed controls that have much less inertia-like response.

The ULFS disconnects of the load lower the power demand thus slowing down the decrease in frequency, representing an equivalent to increasing the amount of inertia. [10]

Variable generation

Until the 21st century, conventional inertia in combination with primary frequency response was considered sufficient to reach the target reliability of the US electric grid. [11] High penetration of the variable renewable energy (VRE) created new challenges: [12]

The alternatives to the traditional inertia are therefore applied, and by the 2020s Texas (ERCOT) took the lead in the United States due its higher wind power penetration (almost double that of the Western Interconnection, WI) and its relatively small size that made the contingencies there larger in percentage terms (a single failure can take power equivalent to 6.4% of the average load in comparison to 2.6% for WI and 1.3% for the Eastern Interconnection). [13]

Addressing the decline in inertia

The following brute-force means are used to keep the grid reliability in the environment of reduced inertia:

Fast frequency response

Disconnection of the load can be done very quickly (half a second, including the frequency measurement). [17] Inverter-based resources (IBR), if not running at full available power, can also be ramped extremely quickly (25% per second for wind, 100% per second for photovoltaics), [19] limited amount of kinetic energy can be extracted from a wind turbine, providing an extra 10% of its capacity for about half a second (after a half a second delay). [20] Furthermore, the times when a lot of spare IBR capacity is available coincide with the times when the conventional inertia is at its lowest due to many synchronous generators being offline. These benefits of the new technology allow implementation of the fast frequency response (FFR) - frequency control using the dispatch of IBRs and load disconnects to achieve inertia-like reaction times, thus the alternative name for the FFR, synthetic inertia [19] (Eriksson et al. propose to use the term "synthetic inertia" for the units that react proportionally to the rate of change of frequency and reserve the FFR for the units that react to the effects of insufficient inertia, e.g. frequency deviation [21] ). Grid-scale batteries also can participate in FFR with ramp rate of 100% per second. [22]

Simulating power balancing with wind power

When the grid frequency is too high or too low, active power flow through the high-voltage direct current link will be ramped down or up. In turn, the wind generation will increase or decrease the blade angles to reduce or increase the captured wind power through pitch control. [23]

Related Research Articles

<span class="mw-page-title-main">Synchronous motor</span> Type of AC motor

A synchronous electric motor is an AC electric motor in which, at steady state, the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integral number of AC cycles. Synchronous motors use electromagnets as the stator of the motor which create a magnetic field that rotates in time with the oscillations of the current. The rotor with permanent magnets or electromagnets turns in step with the stator field at the same rate and as a result, provides the second synchronized rotating magnet field. A synchronous motor is termed doubly fed if it is supplied with independently excited multiphase AC electromagnets on both the rotor and stator.

<span class="mw-page-title-main">Utility frequency</span> Frequency used on standard electricity grid in a given area

The utility frequency, (power) line frequency or mains frequency is the nominal frequency of the oscillations of alternating current (AC) in a wide area synchronous grid transmitted from a power station to the end-user. In large parts of the world this is 50 Hz, although in the Americas and parts of Asia it is typically 60 Hz. Current usage by country or region is given in the list of mains electricity by country.

<span class="mw-page-title-main">Frequency changer</span>

A frequency changer or frequency converter is an electronic or electromechanical device that converts alternating current (AC) of one frequency to alternating current of another frequency. The device may also change the voltage, but if it does, that is incidental to its principal purpose, since voltage conversion of alternating current is much easier to achieve than frequency conversion.

Electrical devices are considered grid friendly if they operate in a manner that supports electrical 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.

<span class="mw-page-title-main">Texas Interconnection</span> Power grid providing power to most of Texas

The Texas Interconnection is an alternating current (AC) power grid – a wide area synchronous grid – that covers most of the state of Texas. The grid is managed by the Electric Reliability Council of Texas (ERCOT).

Doubly fed electric machines, also slip-ring generators, are electric motors or electric generators, where both the field magnet windings and armature windings are separately connected to equipment outside the machine.

An induction generator or asynchronous generator is a type of alternating current (AC) electrical generator that uses the principles of induction motors to produce electric power. Induction generators operate by mechanically turning their rotors faster than synchronous speed. A regular AC induction motor usually can be used as a generator, without any internal modifications. Because they can recover energy with relatively simple controls, induction generators are useful in applications such as mini hydro power plants, wind turbines, or in reducing high-pressure gas streams to lower pressure.

<span class="mw-page-title-main">DeWind</span>

DeWind was an internationally active producer of wind power plants, originating in Germany. The company was founded in 1995 as Private Limited Company (GmbH) in Luebeck, Germany by seven shareholders. Five of them actively worked in the company. In 2002 the founders sold 100% of the shares to FKI Plc, Loughborough, UK. Nowadays it is based in Irving, Texas, and Hamburg, Germany. DeWind has the legal form of a corporation, and, since September 2009, it has been a subsidiary company of Daewoo Shipbuilding and Marine Engineering (DSME), a large shipbuilder based in Korea. Manufacturing takes place in Lübeck and Round Rock, Texas.

Droop speed control is a control mode used for AC electrical power generators, whereby the power output of a generator reduces as the line frequency increases. It is commonly used as the speed control mode of the governor of a prime mover driving a synchronous generator connected to an electrical grid. It works by controlling the rate of power produced by the prime mover according to the grid frequency. With droop speed control, when the grid is operating at maximum operating frequency, the prime mover's power is reduced to zero, and when the grid is at minimum operating frequency, the power is set to 100%, and intermediate values at other operating frequencies.

<span class="mw-page-title-main">Electrical grid</span> Interconnected network for delivering electricity from suppliers to consumers

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:

<span class="mw-page-title-main">Wide area synchronous grid</span> Regional electrical grid

A wide area synchronous grid 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 normal system conditions. Also known as synchronous zones, the most powerful is the Northern Chinese State Grid with 1,700 gigawatts (GW) of generation capacity, while the widest region served is that of the IPS/UPS system serving most countries of the former Soviet Union. Synchronous grids with ample capacity facilitate electricity trading across wide areas. In the ENTSO-E in 2008, over 350,000 megawatt hours were sold per day on the European Energy Exchange (EEX).

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 the interconnected transmission system.

<span class="mw-page-title-main">Automatic generation control</span> System adjusting the output of electric generators

In an electric power system, automatic generation control (AGC) is a system for adjusting the power output of multiple generators at different power plants, in response to changes in the load. Since a power grid requires that generation and load closely balance moment by moment, frequent adjustments to the output of generators are necessary. The balance can be judged by measuring the system frequency; if it is increasing, more power is being generated than used, which causes all the machines in the system to accelerate. If the system frequency is decreasing, more load is on the system than the instantaneous generation can provide, which causes all generators to slow down.

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 closely at any time despite the continuous variations of both. In a deregulated grid, a transmission system operator is responsible for the balancing. In a wide area synchronous grid the short-term balancing is coupled with frequency control: as long as the balance is maintained, the frequency stays constant, whenever a small mismatch between aggregate demand and aggregate supply occurs, it is restored due to both supply and demand being frequency-sensitive: lower frequency increases the supply, and higher frequency increases the demand.

<span class="mw-page-title-main">Synchronverter</span> Type of electrical power inverter

Synchronverters or virtual synchronous generators are inverters which mimic synchronous generators (SG) to provide "synthetic inertia" for ancillary services in electric power systems. Inertia is a property of standard synchronous generators associated with the rotating physical mass of the system spinning at a frequency proportional to the electricity being generated. Inertia has implications towards grid stability as work is required to alter the kinetic energy of the spinning physical mass and therefore opposes changes in grid frequency. Inverter-based generation inherently lacks this property as the waveform is being created artificially via power electronics.

Capacity credit is the fraction of the installed capacity of a power plant which can be relied upon at a given time, frequently expressed as a percentage of the nameplate capacity. A conventional (dispatchable) power plant can typically provide the electricity at full power as long as it has a sufficient amount of fuel and is operational, therefore the capacity credit of such a plant is close to 100%; it is exactly 100% for some definitions of the capacity credit. The output of a variable renewable energy (VRE) plant depends on the state of an uncontrolled natural resource, therefore a mechanically and electrically sound VRE plant might not be able to generate at the rated capacity when needed, so its CC is much lower than 100%. The capacity credit is useful for a rough estimate of the firm power a system with weather-dependent generation can reliably provide. For example, with a low, but realistic wind power capacity credit of 5%, 20 gigawatts (GW) worth of wind power needs to be added to the system in order to permanently retire a 1 GW fossil fuel plant while keeping the electrical grid reliability at the same level.

Power system operations is a term used in electricity generation to describe the process of decision-making on the timescale from one day to minutes prior to the power delivery. The term power system control describes actions taken in response to unplanned disturbances in order to provide reliable electric supply of acceptable quality. The corresponding engineering branch is called Power System Operations and Control. Electricity is hard to store, so at any moment the supply (generation) shall be balanced with demand. 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 electricity market. In the restructured North American power transmission grid, these centers belong to balancing authorities numbered 74 in 2016, the entities responsible for operations are also called independent system operators, 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.

An inverter-based resource (IBR) is a source of electricity that is asynchronously connected to the electrical grid via an electronic power converter ("inverter"). The devices in this category, also known as converter interfaced generation (CIG), include the variable renewable energy generators and battery storage power stations. These devices lack the intrinsic behaviors and their features are almost entirely defined by the control algorithms, presenting specific challenges to system stability as their penetration increases, for example, a single software fault can affect all devices of a certain type in a contingency. IBRs are sometimes called non-synchronous generators. The design of inverters for the IBR generally follows the IEEE 1547 and NERC PRC-024-2 standards.

The power system reliability is the probability of a normal operation of the electrical grid at a given time. Reliability indices characterize the ability of the electrical system to supply customers with electricity as needed by measuring the frequency, duration, and scale of supply interruptions. Traditionally two interdependent components of the power system reliability are considered:

In an electrical grid, the short circuit ratio is the ratio of the short circuit apparent power (SCMVA) in the case of a line-line-line-ground (3LG) fault at the location in the grid where some generator is connected to the power rating of the generator itself (GMW). Since the power that can be delivered by the grid varies by location, frequently a location is indicated, for example, at the point of interconnection (POI):

References

  1. ↑ "AEMO: Wind Integration Study". AEMO. Archived from the original on 8 February 2012. Retrieved 9 December 2011.
  2. ↑ Denholm et al. 2020, p. 4.
  3. ↑ Denholm et al. 2020, pp. 4–6.
  4. ↑ Denholm et al. 2020, p. 9.
  5. 1 2 Denholm et al. 2020, p. 11.
  6. ↑ NERC 2021, p. 14, Inertial Response.
  7. ↑ Denholm et al. 2020, p. 12.
  8. ↑ Denholm et al. 2020, p. 18.
  9. ↑ Denholm et al. 2020, p. 13.
  10. ↑ Denholm et al. 2020, pp. 14–15.
  11. ↑ Denholm et al. 2020, p. 17.
  12. ↑ Denholm et al. 2020, p. 20.
  13. ↑ Denholm et al. 2020, p. 22.
  14. ↑ Denholm et al. 2020, p. 25.
  15. 1 2 Denholm et al. 2020, p. 26.
  16. ↑ Denholm et al. 2020, pp. 26–27.
  17. 1 2 Denholm et al. 2020, p. 27.
  18. ↑ Denholm et al. 2020, p. 31.
  19. 1 2 Denholm et al. 2020, p. 29.
  20. ↑ Denholm et al. 2020, p. 28.
  21. ↑ Eriksson, Modig & Elkington 2017.
  22. ↑ Denholm et al. 2020, p. 30.
  23. ↑ Miao, Zhixin; Lingling Fan; Osborn, D.; Yuvarajan, S. (Dec 2010). "Wind Farms With HVdc Delivery in Inertial Response and Primary Frequency Control ". Energy Conversions. 25 (4): 1171–1178. Bibcode:2010ITEnC..25.1171M. doi:10.1109/TEC.2010.2060202. S2CID   20573992.

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