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Lecture 10:Power Balance & Frequency Control of
Renewable Energy
Lecturers:
Syafaruddin & Takashi Hiyama
Time and Venue:
Wednesdays: 10:20 – 11:50, Room No.: 208
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http://www.cs.kumamoto-u.ac.jp/epslab/APSF/
Contents
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Frequency Response Services:•Wind power•Biofuels•Water Power•Photovoltaics
Frequency Control and Reliability:•Introduction•Aggregation of Sources•Value of Energy from the Wind•Impact on Balancing•Impact on Reliability•Discarded/Curtailed Energy•Overall Penalties Due to Increasing Penetration•Combining Different Renewable Sources•Differences Between Electricity Systems•Limits of Penetration from Non-dispatchableSources
Frequency Control & Reliability
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The introduction of variable RE generation into a network will have an impact and incurassociated costs in two main categories:
Both balancing and reliability involve statistical calculations , additional uncertainties when the value of electricity from RE sources is calculated.
•balancing impact relates to the management of demand fluctuations from seconds to hours. •reliability impact relates to the requirement that there is enough generation to meet the peak demand.
There is a widespread, but mistaken, belief that operation of an electricity system with renewable causes serious problems. •A common misconception is that significant additional plant must be held in readiness, to come on - line when the output from the wind plant ceases.
This would indeed be true in an island situation, with, for example, wind the principal source of supply.
• Modest amounts of variable renewable within an integrated electricity system pose, however, no threat whatsoever to system operation. •The reason for this is that these amounts do not add significantly to the uncertainties in predicting the generation to ensure a balance between supply and demand. •Therefore the risk of changes in the output from variable renewable sources has only a small influence on the needs for reserves.
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The smoothing benefit arising from aggregation is of vital importance to electricity utilities:•The more uncorrelated the demand among consumers, the more effective the overall smoothing.•As a consequence it is much easier to predict, and the generation required to supply this aggregate load can be scheduled and controlled very efficiently
Aggregation of Sources
•Integrated electricity systems benefit immeasurably from the aggregation of consumer demand•Fluctuating sources can benefit in the same way
The value of interconnection to form large power systems should now be clear: •It allows demand aggregation and the benefits that stem from this, primarily through the easier matching of supply and demand. •Some proponents of renewable energy suggest that national grids will become redundant once generators are located near to consumers, but this is a misconception
Unless an unprecedented breakthrough in energy storage technology is achieved.•Indeed, given the intrinsic variability of many dispersed renewable energy sources, interconnection may well prove to be even more valuable in the future
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The Monthly Distribution of Energy
The seasonal wind power availability from dispersed sites in the UK (Figure 3.11):It indicates limited production during summer and greater than average production during winter On average twice as much electricity is generated during the winter compared to the summer months. This pattern matches the seasonal demand pattern in the UK.
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The Daily Distribution of Energy
Figure 3.12 shows that, on average, wind power availability is higher during the daytime than at night in the UK“This trend is present irrespective of the time of year and is of benefit in a system where the demand peaks during the afternoon period when the wind power availabilityis near its maximum”
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Short Term Variability
The variability of wind power will cause changes in the power generated from one hour to the next. The maximum expected rate of change from hour to hour provides an indication of the reserves required to deal with shortfalls in supplying demand. Wind speed variations within the 15 – 25 m/s band will result in no change of power as the wind turbine will be operating at full output for winds in this range. However variations within the band 4 – 15 m/s will result in substantial power changes. The degree of dispersion of the resource will again be of advantage as increments of wind at one site will be compensated by decrements at another.
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The Capacity Factor
Figure 3.14 shows the results of a UK study on the relative capacity credit between seasons and between onshore and offshore resources as a function of wind power penetration (next slide)Due to the windiness of the UK offshore areas the credit is higher than 60% during winter, decreasing to less than 25% in the summer for small penetrations. The onshore picture is similar but scaled down.
As expected, the capacity credit decreases with increasing penetration.It may be concluded that, in the UK, because of the favorable weather conditions, a statistical correlation exists between the availability of wind power and demand. The capacity credit associated with wind power and therefore the contribution of wind towards reliability could be substantially higher than that calculated from the annual average wind capacity factor.
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Value of Energy from the Wind•The financial benefit of wind power can be calculated by determining the cost of determining the cost of supplying the supplying the total demandtotal demand and then subtracting the cost of supplying the residual demand subtracting the cost of supplying the residual demand ((after wind after wind power is added) from the power is added) from the existingexisting•In practice this calculation is extremely extremely difficult difficult to do to do and so, for simplicity, many analysts concentrate on fuel savings which are more straightforward to estimate. •Ideal fuel savings are simply those calculated from the cost of displaced generation and do not take account of any changes of operation forced on conventional plant by the time varying characteristic of the wind plant.
To estimate the fuel savings more realistically the following operating issues should be considered:If wind generation is subtracted from the gross demand, the residual demand will have more variability than the gross demand.
This means that the output of fossil fuel plant providing a continuous or frequency response service will have to be adjusted more frequently and to a greater extent. Additionally, with wind generation there is a need to ensure that the system can respond adequately to unpredicted changes over longer time periods. Extra balancing reserves provide greater headroom, but the lower loading level of thermal plant that this requires would result in lower operational efficiency of thermal plant. Finally, at higher penetrations (above 20%), some thermal plant may have to be shut down and started up to maintain adequate reserves This will incur what are known as cycling costs. All of the above effects make up the balancing balancing impact impact
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Wind generated electricity may increase the size of the system margin required to maintain the required level of reliability.
The The reason for this reason for this is that wind plants are less likely than thermal plants to be available to contribute towards times of peak demand. This effect makes up the reliability impact .
At high penetrations, energy may need to be spilled, discarded or curtailed because for operational reasons this energy cannot be safely absorbed while maintaining adequate reserves.
Value of Energy from the Wind…cont.
Impact on Balancing
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Thermal plant incur some costs when they are run in the frequency control mode. if variations in a variable source and in demand occur roughly independently, the total resulting variation in the net load to be met by the thermal plant is approximately a ‘ sum - of - squares ’ addition of the components:
•Thus, for example, when the average power variation of the added source equals that of the demand itself, the total variability is not doubled but increased by 40%. •This has some important implications. •The impact of impact of fluctuations fluctuations in variable sources at low penetrations in variable sources at low penetrations can be taken to be practically zero; in other words this impact is just noise added to demand fluctuations.
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Over longer timescales, the level of operating reserve required at any given time depends on two key factors:
uncertainties in demand prediction and the probability of loss of thelargest generation plant on the network.
•When wind power plant is introduced into the system, an additional source of variation is added to the already variable nature of demand. •To analyze the additional variation caused by the wind plant it is important to appreciate that the requirement is that the entire system must be balanced instead of balancing each individual load or resource. •The operator has to ensure that the average system reliability is maintained at the same level it would have been without the wind resource.
However, the crucial question is by how much does wind generation increase the balancing uncertainties? Intuitively it is known that minute - to - minute fluctuations in wind output are largely uncorrelated to load
This implies that the additional uncertainty introduced by wind power does not add linearly to the uncertainty of predicting the load.
As for the issue of variability dealt with above, it can be shown that when errors in predicting the output from variable sources occurs independently of those in predicting demand, the combined error is again a sum - of - squares addition
E.q (3.3):
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Demand prediction techniques are constantly being refined but there will always be occasions when unforeseen circumstances push up or depress the load. Equation (3.3) indicates that for small penetrations of variable sources the prediction errors are lost among load fluctuations.However, since demand is fairly predictable, forecasting errors from substantial penetration of wind will incur some penaltyincur some penalty.Analysis of the combined uncertainties of wind, demand and conventional generation based on the sum - of - squares calculation of Equation (3.2) make use of the standard error in predicting the generation/demand balance. On typical developed country networks, one hour ahead, this averages at around 1% of the demand. For four hours ahead, this figure rises to 3%.
Impact on Balancing…cont.
Impact on ReliabilityImpact on Reliability
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•In addition to the operating reserve, some system margin in excess of the system peak demand is required and this will be affected by the level of wind penetration. •Example: a power system with a peak demand of 70 GW•To guarantee long term security to an accepted level of reliability utilities in general would require that about 20% (14 GW) of that peak must be additionally available on the network as the system margin. •The system in question has a yearly energy demand yearly energy demand of 350 TW h. •If 10% of the system energy were to be generated by wind power it is possible to assess what the new plant margin should be. •To generate 10% of the energy, i.e. 35 TW h, a wind power capacity of 35 000/(8736 × 0.3) = 13 GW, where 8736 is the number of hours in the year and 0.3 is the average wind farm capacity factor. •The 35 TW h would be generated by 35 000/(8736 × 0.75) = 5.3 GW of conventional generation with a yearly load factor of 0.75.• Ideally the wind plant would Ideally the wind plant would directly substitute directly substitute for this conventional plantfor this conventional plant, but the variability of wind power means that not all of this conventional plant can be removed from the network. ••Statistical Statistical calculations by calculations by utilities indicate utilities indicate that 81 GW of conventional plant would be required to meet the same level of reliability.•Therefore only 3 GW (84 − 81 GW) can be removed from the network; i.e. 2.3 GW•(5.3 − 3 GW) is retained because of wind power variability, which is 17% of the wind power installed capacity.•In the UK context of a planned 20% penetration from wind and to maintain system reliability the associated cost lies within 0.03 and 0.05 p/kW h.
Discarded/Curtailed Energy
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•As the capacity of the variable sources injected into a system increases, there might be occasions when the available power from such sources cannot be used. •If the penetration is substantial, there might be periods when the available power from renewable exceeds demand, or cannot be accommodated by the transmission or distribution system. •However, even before this stage is reached, energy from variable sources will have to be shed because the power system would need to keep a minimum level of thermal plant generation in order to maintain adequate operating reserve.
Discarding energy from variable sources poses no particular operational difficulties:Output from wind turbines can be controlled through blade pitch variation,
from photovoltaics through inverter control and from hydro, wave and tidal schemes through similar control techniques.
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Discarded/Curtailed Energy…cont.
However, this discarding or curtailment of energy results in an curtailment of energy results in an economic penalty economic penalty on variable sources, which becomes increasingly important at high penetrations.
•This penalty is difficult to assess as it depends heavily on the flexibility of the base load units, i.e. the extent to which they could be operated in a stable regime at low power and upon how rapidly their output could be increased if required.
For the level of penetration expected over the next decade or so, the penalties due to the penalties due to discarded energydiscarded energy are unlikely to be of major significance. It should also be noted that curtailment may also be required due to distribution or transmission system constraints. Studies show that at a penetration of 20%, curtailed energy ranges from 0 to 7%. Most studies show that, with sensible design, curtailment due to local network capacity limitations would be would be rarely requiredrarely required.
Overall Penalties Due to Increasing Penetration
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•In previous sections, operational penalties due to increases in variable source penetration were reviewed. •A number of studies have been carried out to provide estimates of these penalties as the penetration of the penetration of renewable renewable increasesincreases. •The majority of these studies relate to wind wind powerpower, as this is the variable source with the largest installed world capacity to date.•The additional costs are tabulated in terms of the level of penetration.
Combining Different Renewable Sources
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The benefits of combining different variable sources:•As the capacity of a nondispatchable source increases, its marginal value declines, primarily because successive increments of capacity are correlated with those already on the system. •In contrast, combining capacity from renewable with uncorrelated or complementary outputs can therefore be of considerable benefit•Typically, a combination of wind and solar wind and solar could be beneficial.
In some circumstances, thermally driven winds can be strongest after sunset, so that the combination of wind and solar usefully covers periods of high demand.
•Other studies indicate that a combination of wind wind and tidal and tidal (two sources having statistical independence) increases their value compared with the case of having more of the same
A recent study indicates that a contribution from a mix of PV solar and wind contribution from a mix of PV solar and wind plus plus domestic domestic combined heat and powercombined heat and power has the potential to reduce significantly the overall variability that would have been experienced if only one renewable technology were to provide the total contribution.The potential synergies among different renewable sources are clearly much too important to ignore, and they may often make the combined exploitable potential larger than the sum of the parts considered in isolation.
Differences Between Electricity Systems
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It is appropriate here to stress that results from studies on one particular network do not necessarily apply elsewhere. The operational viability and costs of integrating renewable energy depend on a number of factors that characterize the local resource as well as the structure of the electricity network. These factors include:• the strength and temporal variability of the resource;• the possibility of geographical dispersion over a large area to gain the advantages ofaggregation;• the possible complimentarily between different types of renewable resources;• the correlation, if any, between availability of the resource and demand variation;• the extent to which the magnitude of the resources can be forecast, where some weatherpatterns are more predictable than others;• the robustness of the electricity network and the proximity of transmission lines to theareas of maximum resource availability;• the transmission links, if any, to adjacent networks;• the operating practices of the network, in particular how far in advance the system balancingreserve is planned;• the type of conventional plant in the network, for example, smaller and more modernthermal plants are more flexible than large base load plant such as nuclear.
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Figure 3.17 shows the output that would have been expected from 25 GW of dispersed wind capacity (middle graph) and 10 GW of tidal (lower graph) alongside the demand over a period of one month.
In this example 30 and 13% of the consumer energy would be supplied from wind and tidal respectively. The wind power varies less rapidly than demand, tidal more rapidly.
Limits of Penetration from Nondispatchable Sources
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Figure 3.18 shows the effect of subtracting the output from wind and tidal power from the demand, leaving a residual load (dashed curve) to be met by a conventionalthermal plant.
Limits of Penetration from Nondispatchable Sources…cont.
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More recently studies have considered the possibility of meeting the entire demand from a mix of renewable. Figure 3.19 taken from Streater ’ s work shows hour hour by by hour variations in the time hour variations in the time variable variable renewable. renewable. Penetration levels of such a high magnitude result in periods when the available power from the RE sources exceeds demand. Even before this stage is reached, for reasons of system reliability, the RE sources would have to be curtailed.
Limits of Penetration from Nondispatchable Sources…cont.
Limits of Penetration from Nondispatchable Sources…cont.
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Table 3.2 shows the capacities of the different renewable installed and their penetration, defined as the output divided by the total load (i.e. ignoring any curtailment
Limits of Penetration from Nondispatchable Sources…cont.
Limits of Penetration from Nondispatchable Sources…cont.
•Denmark and less so, Germany generate a high percentage of their total electricity needs from wind power and are planning further capacity. •A recent study from Elkraft, a Danish system operator, asserts that a wind penetration up to 50% is technically and economically feasible. •This is based on an increase of installed capacity of 3.1 GW in 2005 to 5 GW in 2025 and a substantial expansion of the Danish grid. •The study claims that even with this penetration, it will be necessary to shut down wind turbines only for a few hours in a year.•Similar studies by DENA in Germany indicate that up to 20% penetration by wind is possible with minor extensions of the grid and no need for construction of additional power stations.
Frequency Response Services from Renewable Energy
Frequency Response Services from Renewable Energy
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With the anticipated rise in the penetration of variable renewable sources, power systems will be required to accommodate increasing second to second imbalances between generation and demand requiring enhanced frequency control balancing services. Some renewable generation in principle may contribute to frequency regulation services, but this would require headroom in the form of part -loading. Technologies that could potentially provide such services are biomass, biomass, water power, water power, photovoltaicsphotovoltaics and variable speed wind turbines. and variable speed wind turbines.
Frequency Response Services from Renewable Energy…cont.
Frequency Response Services from Renewable Energy…cont.
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•With large penetrations from renewable there will be occasions, for instance during low demand days over summer, when the number of conventional generators needed to supply the residual load will be so few that an adequate level of response and reserve may be difficult to maintain. •Under such conditions renewable generators could be unloaded and instructed to take part in frequency regulation.
Such a provision has been made, for example in the Irish (ESB) code, for the connection of wind turbines. In a privatized system the opportunity benefits of running in this mode must more than compensate the loss of revenue from generating at less than the maximum potential.
Wind powerWind power
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Early wind power technology was mainly based on simple fixed one or two speed stall -regulated wind turbines with little control over the dynamic performance of the generator.However, over recent years active stall and pitch regulated variable speed wind turbines have been developed that are capable of increased conversion capable of increased conversion efficiencies efficiencies but also of but also of substantial control substantial control capabilities. capabilities. In principle, modern wind turbines are capable of providing a continuous response by fast increase in power from part loading through blade pitch control in response to drops in frequency and through the same mechanism provide high frequency response through fast reduction in power in response to increases in frequency.
•As wind power capacity has increased, to the extent that at times it is the dominant form of generation in parts of Denmark and Northern Germany, there is an increasing demand for wind capacity to be dispatchable and to behave more like conventional generation. •Very large wind farms are now expected to conform to connection standards that limit ramp rates for increase in power and also to contribute to frequency regulation under times of high network stress. •These requirements are increasingly included in the national grid codes that the national grid codes that regulate regulate access access to the public networks. •In these early days it is unclear to what extent this will result in wind power being curtailed, for example to comply with given ramp rates, and to what extent such constraints add value to the system operator.
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Conventional steam generation plant assist the network frequency stability at the onset of a sudden imbalance of demand over supply by slowing down. Wind turbines respond differently:
•The stored energy is in the rotor inertia and fixed speed turbines will provide a limited benefit from their inertia provided that the voltage and frequency remain within their operating limits. •Variable - speed wind turbines will not normally provide this benefit as their speed is controlled to maximize the energy production from the prevailing wind.
Wind power…cont.
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•Large wind turbines are now almost always of the variable speed type and as they increasingly displace conventional generation the total system inertia from such generation will decrease. •Consequently the rate of change of frequency and the depth of the frequency dip caused by a sudden loss of generation will both increase. •However, variable speed wind turbines could be controlled in principle to provide a proportionately greater inertial energy to the system than conventional plant of the same rating. •Such sophisticated control arrangements to support system functions are likely to be requested by utilities as wind penetration increases.
Finally, grid codes require wind turbines to maintain power infeeds to the system even under transient local voltage reductions. Such reductions are usually due to fault conditions in the vicinity of the wind farm. It can be shown that maintenance of power infeed from all generators is essential to ensure system recovery after a fault clearance.
Wind power…cont.Wind power…cont.
BiofuelsBiofuels
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Traditional thermal plant Traditional thermal plant could be described as capacity limited , i.e. capable of theoretically generating its rated output continuously, as gas, coal, oil or fissionable material is abundantly available on demand. In contrast, an energy crop based plant could be described as energy limited because the locally harvested fuel is limited in nature and may or may not be capable of sustaining all year round continuous plant generation at full capacity. Transporting biomass fuel from remote areas would not be economical.
A biomass plant would be expected to operate as a base - load generator running as far as possible at full output. Such plant would be able to contribute to continuous low or high frequency response services similarly to a conventional plant. For a low frequency response the plant would need to run part - loaded, a convenient strategy providing extra income if, say, due to a low crop yield year the stored fuel would not be capable of servicing continuous full output.
The land filled gas plant size is in the range of 0.5 – 1.5 MW and because of their small sizethey would not be suitable for the provision of frequency regulation services
Water Power
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Small and medium sized hydro schemes without significant storage capacity are characterized by substantial variability of output depending on rainfall. Because of this and their small size they are not suitable for frequency regulation duties.
control capabilities.
Tidal schemes could be very large indeed and their output would be highly predictable.Such plant would incur exceptionally high upfront capital costs and long payback periods.Operation revenue Operation revenue is vital to service the large loans and it is unlikely that frequency response revenue based on part - loading would be attractive enough. As such schemes are not yet in the planning stages, the jury is still out on their frequency control capabilities.The comments made above on wind power generally apply with reservations to futurewave power schemes. As the technology is still in its infancy and commercial schemes are not yet in existence, it is not known how their dynamics may be capable of responding to signals derived from frequency deviations.
Photovoltaics
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Here a distinction should be made between large concentrated PV installations and numerous roof top systems. For large installations, the comments on wind power apply albeit with some reservations. As the PV systems are interfaced to the grids through power electronic converters and as no mechanical inertia is involved, the speed of response in increments or decrements in power flow can be very fast indeed. On the other hand, solar radiation tends to solar radiation tends to vary more vary more slowly than windslowly than wind in the short term, and is fairly predictable. As with other renewables, in the future, PV systems may be required to operate at part load, thus providing ‘ headroom ’ for continuous or ‘ occasional ’ frequency control.
At this stage of PV technology, roof installations are not yet numerous enough to provide a credible frequency response service. However, in years to come if, as predicted, costs plummet and installations are numbered in millions, it is conceivable that the local the local inverters are fitted inverters are fitted with sophisticated controllers with sophisticated controllers to assist system frequency stability.to assist system frequency stability.
