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Wind Power SystemsEG2340
Wind Power –Network integration
Lennart SöderProfessor in Electric Power Systems
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Solved the problem –got the job
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A good engineer can not stop solving a problem
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Are there any problems concerningbalancing of large amounts of windpower?
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Are there any problems concerningbalancing of large amounts of windpower?
Yes
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Are there any problems concerningbalancing of large amounts of windpower?
Yes, but A good engineer can not stop solving a problem
Three challenges in a power system withlarge amounts of solar and wind power
C1: Keep the continuous balanceC2: Handle situatiuations with small amounts ofvariable production. C3: Handle situatiuations with large amounts ofvariable production.
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Swedish power production year 2011
Wind powerNuclearCHPInd. CHPHydro powerConv. therm.
Swedish Power production: Total 145,6 TWh(same as 2011)
Swedish Electric Supply 20XX
Wind power
Solar power
Combined Heat and Power
Industrial CHP
Hydro Power
Deficit
Report: Published 22 juni 2014
Studies:• Balancing from hour to hour
in ”isolated” Sweden!• High wind+solar / low
consumption• Low wind+solar / high
consumption• Transmission constraints• Can be downloaded from
KTH:s home page• EXCEL-file for calculations 50 100 150 200 250 300 350 400
0
0.5
1
1.5
2
2.5
x 104
MW
h/h
Consumption from 14 January to 30 January
ConsumptionHydro powerWind powerSolar powerCHP
High wind decrease CHP
Deficit situationlow solar+wind: January
Deficit situation (yearly basis)
Cost for this: 1,5 öre/kWh
0 500 1000 15000
1000
2000
3000
4000
5000
Number of hours with need of more production
Ene
rgy
leve
l [M
Wh/
h]
Max level: 5081 MW
Number of hours with need: 765 h
Energy: 1.259 TWh
a. Flexible demandb. Importc. Gas turbines (e.g. bio-fuelled)d. Use batteries or other storagee. Flexible electric vehicles or V2Gf. Extra CHP capacity (Top-spool-technology)
Possible ways to cover extra needs
Surplus situation (August)
Not OK: 83% limit, min-hydro, min-CHP
20 40 60 80 100 120 140 160 180 200 2200
2000
4000
6000
8000
10000
12000
14000
16000
MW
h/h
Consumption from 1 August to 10 August
ConsumptionHydro powerWind powerSolar powerCHP
Now OK: 83% limit, min-hydro, min-CHP
Surplus situation (August)
20 40 60 80 100 120 140 160 180 200 2200
2000
4000
6000
8000
10000
12000
14000
16000
MW
h/h
Consumption from 1 August to 10 August
ConsumptionHydro powerWind powerSolar powerCHP
Surplus during a year
0 200 400 600 800 1000 12000
1000
2000
3000
4000
5000
6000
7000
8000
9000
Number of hours with surplus/possible export
Ene
rgy
leve
l [M
Wh/
h]
Energy volume: 1.63 TWh
Max level: 9510 MWNumber of surplus hours: 860 h
Energy is ”produced” where the resource isThe energy has to be transported to consumption centerThe energy inflow varies, which requires storage and/or flexible system solutions
This is valid for hydro power, wind power, solar power
Renewable energy systems
ExampleNordic hydro power (inflow) can vary 86 TWh between different years (∆2001 to 1996)Transport from NV to SE + continent Energy balancing with thermal power in i Dk+F+Ge+Pl+NL+Ee
Sweden and our neighbors have had a need for cooperation since decades
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Future transport systems
• Power lines, 1 line ~ 10 TWh/year• Hydrogen from electrolysis driven by
renewable electric generation• Hydrogen in pipe lines (natural gas: 500
TWh/year, diameter = 1.5m)• Hydrogen in boat (natural gas, LNG: 0.8
TWh/boat)
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The Nordic Power System
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Aim of power system:
• Supply consumers with electricity when they want
• This is the same as keeping the continuous balance between production and consumption
• Keep the voltage for the consumers
• Energy = power · time• Power = current · voltage
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Wind power integration challenge
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Single-phase alternating voltage
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Keep voltage
• Kept by the grid owners
• There is only one player per costumer (only one connection point)
• Regulated monopolies (today)
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The Nordictransmissiongrid
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A synchronous power system• A synchronous power system is a power system where all
producers and consumers are connected to each other through transformers and AC transmission and distribution lines.
• Anything from a diesel generator set supplying a single load to a multi-national grid as the Nordel system (which connect Norway, Sweden, Finland and the eastern part of Denmark) can constitute a synchronous grid.
• An AC line has to have the same electric frequency at both ends of the line. If there were different frequencies at the ends then the voltage angle shift would increase until it reaches 180°, resulting in unacceptable large currents on the line. The same is valid for transformers. The conclusion is that in a synchronous grid the average electric frequency must be the same.
Power system challenge
Keep the balance:• Production = consumption• Electricity cannot be stored!• Exactly when a bulb is lighned
some generator will deliver the power• Exactly when a power plant is stopped, the
corresponding power will be delivered from another plant instead.
Keep the balance in a power system
The power system = a long bike Keep active power balanceBike• Pedal forces =
breaking forces• Otherwise changed
speed• Break bike => lower
speed
Power System• Total generation =
total load• Otherwise changed
electric frequency• Increase load =>
lower frequency
Speed control
Bike• Keep a constant speed• Measure the speed
(same on the wholebike)
• Reduced speed=> increase the force on the pedals.
Frequency controlBike• Keep constant speed• Measure speed (same
on whole bike)• Decreased speed =>
increase pedal force
Power System• Keep constant frequency• Measure frequency
(same in whole system)• Decreased frequency =>
increase generation
Keep the balance in a power system Real initial phase of a power system outage
Time steps:A. Disconnection of Swedish 1050 MW nuclear stationB. Primary control starts C. Primary control has increased with 1050 MW
Frequency drop after 3 real outages in Sweden
New unitOutage
Keep the balance in the power system
Different time steps:1. Inertia (seconds)2. Primary control (minutes)3. Secondary control(quarter)4. Tertiery control (quarter)5. Intra-day-trade (hours)6. Day-ahead-trade (day)7. Weekly planning (week)8. Yearly planning (year)
Security
Uncertianty Economy
Technology
1. Inertia: - In other power plants- Technically possible
in wind power plants
Contribution:• E.g. hydro power stations (larger) use synchronous machines
which are directly connected to the grid. This means an important contribution to the needed inertia.
Challenges:• More slimmed constructions may reduce the inertia
contribution.• A challenge in power systems with, e.g. large amounts of
solar power, wind power or HVDC infeed, which do not contribute with inertia.
2. Primary control:
Function:• Primary control means that one has to change the production fast
(within seconds). Both increase and decrease is needed. Thismeans that one has to keep margins. Hydro power is a very goodresource for this and the main one in the Nordic system.
Challenges:• At filled reservoirs there is a need to discharge as much as possible
and not keep margins. • During night Saturday-Sunday in June (Sweden) and high nuclear
and/or wind production, not so much hydro is needed, but still the reserve.
Efficiency:Depends on dischargeand head height
Consequence of a consumption increase(light a 60 W bulb in the Nordic system)
Speed droop = ”gain” = how much the powerstations reacts on frequency change. 1. Energy is obtained from the rotors (+shaft and turbine)
in the synchronous machines = inertia (60 W from Nordic synchronous machines)
2. The speed of the rotor then decreases and therebythe electric frequency. (frequency in the Nordic system decreases with 1·10-8 Hz)
3. Turbine control measures the frequency and changesthe power production in some power stations. (S: 25 W, N: 20 W, F: 12,5 W, Dk-E: 2,5 W)
Consequence of a consumption increase(light a 60 W bulb in the Nordic system)
= decrease wind power with 60 W
Electric frequency in Hz = nr of poles rotor speed in rpm2 60
·
Synchronous machineProblem 1: True or false?
a) With more inertia in the system, the frequency willnot drop so much after an outage
b) Primary control is used to correct the frequencyc) Primary control balancing implies that load
changes in Sweden can be balanced in Norway
Wind power and primary control
1) Wind power plants do not (normally) contribute to keepreserves. But they can!2) Wind speed changes between V-cut-in and V-rated3) Wind speed changes around V-cut-out
Wind power and primary control
1)Wind speed changes between V-cut-in and V-rated. In this region the changes in different wind power plants are nearlyindependent concerning fast changes. The result is low total variation.
2)Wind speed changes around V-cut-out. If a lotof wind turbines are hit at the same time witha storm front, then there could be a largeoutage. The probability for this is though low.
3)Conclusion: Primary control is not a dominant problem for wind power.
Primary control value of wind power
”True” value: Balancing of second to minute variations. A slightly negative value. Result from a Swedish study: 3530 MW wind power => 10 MW of extra reserves.Market value: In Sweden this is included in the ”balanceresponsibility”, where the system operator manage the variations within each hour. The cost for this is paid by the market actors.
Keep the balance in a power system
Secondary control, general function
• Adjust the frequency• The power system should be ready for a new load or
wind change• The power system should be ready for a new
disturbance.• AGC (Automatic Generation Control) implies an
economical reoptimization depending on new net load• Adjust the time deviation.
3. Secondary control:
Function details in Nordic system:• Secondary control implies that one at larger frequency
deviations changes the production in order to correct the frequency. This is in the Nordic system called ”LFC-LoadFrequency Control”. Decision from January 1 2013 todistribute at least 100 MW automatic LFC between the Nordic countries including 39 MW for Sweden.
• An automatic system. Challenges:• A new system (in the Nordic system), but needed.
Secondary control, wind power
• Wind power does not (normally) contribute to keepsecondary control margins. But possible!
• Wind power causes extra needs of secondarycontrol margins depending on not perfect windspeed forecasts.
• Secondary control is, as primary control, a part of the ”system responsibility”.
Secondary control value of wind power
”True” value: Balancing of minute to hour variations. A negative value. Result from a Swedish study: 3530 MW wind power => 230 MW of extra reserves (≠ ”new plants”).
Market value: In Sweden this is included in the ”balanceresponsibility”, where the system operator manage the variations within each hour. The cost for this is paid by the market actors.
4. Tertiery control:
Contribution:• Tertiery control is a manual system. The owners of controllable
power plants provide bids to the system operator before eachhour starts and offers to ramp production within 5-10 minutesduring the hour (at a certain price) if this is needed. The owners may/may not be paid to offer margins.
• The TSO can order changes if it is needed. Challenges:• There is some discussions whether there will be enough
bids/margins in the future.
5-6. Hourly/daily control (Nordic example):
Contribution:• The control from hour to hour is a manual system. The power plant
owners provide bids to Nordpool latest 12.00 the day before and 1 hourbefore Elbas. On Nordpool the price is set by ”price cross” and on ”Elbas with Elbas with”pay-as-bid”. The power plant owners are paid ifthe bids are accepted, otherwise not.
• In Sweden it is in reality the hydro power that makes all daily regulation.. • In reality 3 steps: a) Bid planning, b) Operation planning, c) Operation Challenges:• Hydro: Consider hydaulic decisions and hydraulic coupling. Bid level:
Store or sell?• Thermal: Consider start-up times, possible ramps, heating costs
Energy supply - 1
• The balance between total production-consumtion must be kept continously –Tramsmission System Operator (TSO) in Sweden Svenska Kraftnät
• Several possible producers/traders –Competition!
• Not deregulated market:Price = cost + (regulated) profit
• Deregulated market:Profit = price - cost
Energy supply - 2
• In regulated markets: Price is set by the mean cost. New expensive power plants were subsidised with old cheap ones.
• In deregulated markets: Power price is set by the marginal cost. The cost of the power plant with the highest operation cost sets the price for everyone.
Nordpool prices and price areas
21 January, 2015
Nordpool prices and price areas
21 January, 2015
Problem 2: Primary control: True or false?
Consider a system where the frequency is stable. Will the frequency increase or decrease after the following events?a) The load increases.b) A nuclear power plant is stopped due to a failure. The
nuclear power plant did not participate in the primary control.
c) A hydro power plant is started. The hydro power plant has a gain of 100 MW/Hz.
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Structure and trading model on the Nordic market
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Overview of balancing and markets
Contracts and price hedging
Bid to spot market
Price setting
Post trading
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Swedish “deregulation”(same type in many other
countries)New law, 1/1-96
• General idea was to “promote competition”
• All network at all levels must be open
• All network services must be unbundled from generation and electricity sales.
• A TSO, Transmission System Operator, is responsible for the short term power balance.
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Separation caused by deregulation
1.Network
2.Generation, production, trading
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Grid – 1
National gridowned bythe TSO
Svenska KraftnätRegionalgrid
Local grid
Local grid
Generator
Generator
Consumer
Consumer
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Grid-2 Measurements
• Measure loads connected to grid, MWh/h or estimate
• Measure generation connected to grid, MWh/h or estimate
• Measure transmission to/from other grids
• (Some other countries per half hour)
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Grid-3 Losses
Grid companies must• Measure their losses MWh/h or
estimate• Buy these losses on the market• Pay tariff to feeding grid• (other methods in other grids)
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Grid-4 Grid tariffs
Grid companies are financed with
• Power charges, SEK/MW• Energy charges, SEK/MWh• Extra charges for extra
investments.
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Grid-5 Grid tariff rules
• National grid, 220-400 kV, has a point of connection tariff that reflects marginal cost
• Regional and local grids must have same tariff per area and voltage level.
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Grid-6
• National grid congestions are handled by the TSO. Power is bought on one side and sold on other side.
• This implies that all grid costs are included in grid tariff. It does not matter to whom you sell or from whom you buy the power.
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Trading-1 Physical balance
• Svenska Kraftnät (SvK-TSO) is responsible for load-production short term balance.
• Handled by TSO who change production in units (not owned by TSO)
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Trading-2 Physical balance
• Primary control (time scale second-minute)
• Svenska Kraftnät have contracts with companies that have units participating in this. Nordic cooperation.
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Trading-3 Physical balance
• Secondary control (time scale minute-hour)
• The companies which offer the lowest price for increase/decrease of power are called when needed. Nordic cooperation
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Trading-4 Balance responsibility
A consumer is responsible to have a contract with a company (the balance provider) who is economically responsible for that the national power system is provided with the same amount of power (MWh/h) that the consumer consumes.
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Trading-5 Physical trading
• Bilateral, constant power• Nordpool exchange, bids• Elbas market, put/accept bids• Take & Pay contracts• Regulation market
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Nordpool spot prices [SEK/MWh]
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Trading-6
Producer
Retailer
NordpoolGrid owner
Consumer
Price insurance • per year
• per kW • per kWh
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Trading-7 Financial power market
• Future price uncertain• Buy price insurance!• Futures: Buy/sell power for constant
future price independent of market price
• Options: Buy an insurance concerning maximal buy price or minimum sell price.
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The value of a power source
• Operation cost value• Capacity credit• Control value• Loss reduction value• Grid investment value
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What is the value of the new source X in the power system?
Parameters that are important to be able to answer this question:
• How does the rest of the power system look like?– Transmission lines?– Which other power plants are present?– Power consumption level and distribution?
• What is the alternative to X?– Is the alternative to X gas power in Norway,
wind power in Northern Norway or import from Poland?
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What is the value of the new source X in the power system?
Two perspectives:1. What is the ”true value”, i.e., cost
reductions in the rest of the power system.2. Who is paid (pays) for what?
Important challenge:• Design the market regulation so the ones
that reduce the costs are paid for this.
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The Nordic Power System
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About operation cost value
Consider:• Actual operation costs• Possibility to store water• Possibility to replace most expensive plant• Market bidding
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Wind power operation cost value in Nordic countries• Wind power in Nordic countries normally
replaces coal power in Finland, Denmark, Poland or Germany, since these sources are the marginal ones.
• This means that the operation cost value is the operation cost in these sources
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Wind power operation cost value
• ”True” value: Each kWh of wind power normally replaces fuels in the units with highest operation costs. If there are ”externalities” in these fuels (e.g. CO2), then there is an extra value.
• Market value: In some countries there are subsidies. There may on the other side be problems to sell on a market if good forecasts are required.
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Trading on Nordic market
• Nordpool spotmarket: Put bids for a specific hour during a specific day not later than 12 am the day before. Price set by strike price.
• ELBAS market: Put bids for a certain hour not later than 1 hour before the trading hour starts. Bids distributed electronically. You sell you power if someone accepts the bid.
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Trading on Nordic market
MWh/h
SEK/MWhSupply bidsDemand bids
Strike price
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12-36 hours forecasts neededon Nordpool exchange
Real winds in Eastern Denmark, Real forecasts
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The control value of wind power
• Primary control• Secondary control• Daily planning and control• Seasonal planning• Multiyear planning
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Consequences of load increase(or lower wind power production)
1. Energy is obtained from the rotors (and connected shafts and turbinens) in the synchronous machines
2. The speed of the rotors decreases, since energy is taken from the rotational energy
3. This results in a decreased system frequency
4. The turbine governers measures the frequency and modifies the power production
5. This compensates the power imbalance
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The value of a power source
• Operation cost value• Capacity credit• Control value• Loss reduction value• Grid investment value
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Loss reduction value
• Real value: Production resources closer to consumers decrease losses in transmission and distribution systems.
• Market value: In Sweden the grid owner has to compensate the owner of local production for decreased losses and reduced grid charges.
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Loss reduction: Gotland case
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Loss reduction: Gotland case
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Öland case
• Marginal impact of 10 MW wind power plant at Utgrunden is studied.
• Lack of data is important• Estimated yearly production is 38000 MWh• Loss reduction on Öland 460 MWh/year,
corresponding to 1.2 %
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Comments to market value
• Local grid owner is not paid for increased losses but has to pay for decreased losses
• Regional tariffs are based on mean losses instead of marginal losses
• Not trivial to estimate wind power impact on system losses
