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Grid Integration of wind turbinesGrid Integration of wind turbines
Deepa Kurup, C-WET January 21,2011
Topics at a GlanceTopics at a Glance
Electrical systems in wind turbines
Grid system & properties
Power Quality characteristics
Integration & effects of wind farms on power systems
Electrical system in wind turbinesElectrical system in wind turbines
Electrical Schematic of a fixed speed wind turbine
Electrical system in wind turbinesElectrical system in wind turbines
Fixed speed wind turbine
Electrical system in wind turbinesElectrical system in wind turbines
Limited variable speed wind turbine
Electrical system in wind turbinesElectrical system in wind turbines
Variable speed wind turbine with partial scale frequency converter
Electrical system in wind turbinesElectrical system in wind turbines
Variable speed wind turbine with full-scale frequency converter
Wind Turbines based on generator types
Constant speed Doubly fed Variable speed
CWEL C30, 250 kW Repower MD 771500 kW
Enercon E48 & E 53, 800 kW
NORWIN 750 kW G58, 850 kW K82, 2000 kW
Pioneer P250, 250 kW LTW77, 1350 kW & 1500 kW
V39, 500 kW Vensys77, 1500 kW
Pawanshakthi, 600 kW
Siva 250/50, 250 kW
GWL225, 225 kW
Electrical system in wind turbinesElectrical system in wind turbines
Variable speed wind turbine◦Generator side converter , a diode based rectifier or a PWM Voltage Source Converter
◦Grid side converter, PWM Voltage source converter
Electrical system in wind turbinesElectrical system in wind turbines
Variable speed wind turbine
◦ Control of generator side converter
Load angle control technique Vector control technique
◦ Control of grid side converter Load angle control technique
Electrical system in wind turbinesElectrical system in wind turbines
Variable speed wind turbine◦ Load angle control technique of generator side
converter
Electrical system in wind turbinesElectrical system in wind turbines
Variable speed wind turbine◦ Vector control technique of generator side
converter
Torque is controlled by controlling the stator current
Electrical system in wind turbinesElectrical system in wind turbines
Constant speed Direct Drive Doubly fed
Simple & robust Aerodynamically efficient
Aerodynamically efficient
Electrically efficient Less mechanical stress
Less mechanical stress
Standard generator Large converter Small converter
Gearbox included No gearbox Standard generator
Power Electronic ConceptsPower Electronic Concepts
Soft starter
Frequency converters
Static Voltage compensator (SVC)
STATCOM
Power Electronic ConceptsPower Electronic Concepts
Soft starters
Improves the power factor of each turbine (loss reduction, voltage regulation)
Relatively inexpensive
Are switched in blocks
Reactive power compensationReactive power compensation
Capacitor banksFeaturesFeatures
•power factor and output power also fluctuates, ideal compensation will require variable reactive comp (SVC).• switching a big block of capacitance in and out can swing the voltage up or down and this variation is felt as an abrupt change in torque on the turbine gearboxes• are not good at addressingtransient events
Problems
Power electronic conceptsPower electronic concepts
Typical SVC Scheme
Power electronic conceptsPower electronic concepts
Static VAR compensators (SVC)
Thyristor (power electronics) switched capacitors and reactors
Continuous and do not cause sudden voltage changes on the system and are highly effective in regulating voltage.
Typically operate poorly at lower than nominal voltages.
Power electronic conceptsPower electronic concepts
STATCOM
Power electronic conceptsPower electronic concepts
STATCOM
Pure power electronic devices made from IGBT, GTO based converters to directly generate reactive currents.
Faster, smaller, and have better performances at reduced voltages as compared to SVCs.
Capability to address transient events
Grid system & propertiesGrid system & properties
Need to adapt the wind turbines (or the wind farm) and the rest of the power system in a manner to ensure optimal operation of the combined system.
Properties to be considered Short circuit power level (Ssc) Short circuit ratio (Ssc/Sr) Grid short circuit impedance angle (X/R) ratio
Grid system & propertiesGrid system & properties
Short Circuit Power level
Measure of electrical strength of a circuit
Determines the effect of the turbine on the steady state voltage
Voltage deviation at the wind turbine terminals is directly proportional to the ratio of rated apparent power of the turbine to the short circuit level.
Constant of proportion is 1, for steady state conditions 2, for transient conditions
Grid system & propertiesGrid system & properties
Short Circuit Ratio
The ratio (Rsc) of the short circuit level to the rated apparent power (Sr)of the turbine
Determines the stiffness of grid- weak grid or strong grid
Generally Rsc below 20 may mean a weak grid If weak grid system
Use appropriate reactive power compensation of fixed speed wind turbines.
Use of wind turbines with controllable power output. Reinforcement of the grid.
Grid system & propertiesGrid system & properties
Grid system & propertiesGrid system & properties
Limiting factors for weak grids
Voltage fluctuation due to wind turbines is a function of Rsc and X/R ratio.
With voltage fluctuation as a limiting factor,
For overhead lines, high X/R ratio and hence lower Rsc (Ssc) is possible.
For cable grid, low X/R ratio and hence higher Rsc (Ssc) is required
Power QualityPower Quality
Ability of a power system to operate loads, without damaging or disturbing them, a property mainly concerned with voltage quality at points of common coupling &
Ability of the loads to operate without disturbing or reducing the efficiency of the power system, a property mainly, but not exclusively, concerned with the quality of current waveform.
Power Quality characteristics of wind turbinesPower Quality characteristics of wind turbines
Causes for grid interferences of turbines
Meteorological and geographical conditions
Technical performance of the wind turbines
Power Quality characteristics of wind turbinesPower Quality characteristics of wind turbines
Voltage variations
These are defined as changes in the RMS value of the voltage during a short period of time, mostly a few minutes.
Reactive power
Power factors as high as 0.96 can be achieved by capacitor bank compensation of asynchronous generators
Variable speed turbines with inverters can achieve power factors close to unity
Power Quality characteristics of wind turbinesPower Quality characteristics of wind turbines
Harmonics
Integer multiples of the fundamental frequency
Inverters used in variable speed turbines are the major source of harmonics
Inverters with fixed clock frequency produce single inter harmonics which are multiples of the clock frequency
Inverters with variable clock frequency produce wide band of inter harmonics and integer harmonics.
Power Quality characteristics of wind turbinesPower Quality characteristics of wind turbines
Flicker
Voltage fluctuations in the frequency range of 35 Hz which causes annoying changes in the luminance of lamps which is referred to as ‘flicker’.
Caused by wake of the tower in fixed speed turbines Wind speed fluctuations have lower frequencies less
critical for flicker.
Integration & effects of wind farms on power systemsIntegration & effects of wind farms on power systems
Local area impactsImmediate vicinity of the wind farmDo not occur elsewhere in the grid
System wide impactsAffects the behavior of the system as a wholeDepend on the overall penetration of the wind farm in
the gridIndependent of the level of geographical dispersion of
the wind farms.
Integration & effects of wind farms on power systemsIntegration & effects of wind farms on power systems
Local ImpactsVoltage fluctuations at the point of connectionFlickerHarmonicsBehavior during fault conditions
System wide effects are due to the effect of wind turbines onReactive power Stability of the systemFrequency of the system
Integration & effects of wind farms on power systemsIntegration & effects of wind farms on power systems
Mitigating the local effects
Steps can be taken to mitigate these effects at the time of connection to the grid by a careful assessment of the grid system and properties at the point of connection.
Variable speed turbines show improved flicker behavior.
Harmonics which are mainly generated by power electronic devices can be kept within the regulatory limits
Integration & effects of wind farms on power systemsIntegration & effects of wind farms on power systems
Power quality Constant speed
Direct drive Doubly fed
Voltage fluctuations
Proper reactive power comp.
Converter compensation
Converter compensation
Harmonics Unimportant Important, can be kept within limits
Important, can be kept within limits
Flicker Important Unimportant Unimportant
Contribution to fault currents
Yes Turbine is quickly disconnected
Turbine is quickly disconnected
Integration & effects of wind farms on power systemsIntegration & effects of wind farms on power systems
Mitigating the system wide impacts
Adoption of suitable reactive power compensation schemes.
Allowable penetration limits of wind turbines in the grid have to be studied by setting up models of wind turbines and wind farms in the regional grid for simulation and subsequent validation.
Are not significant for low penetration levels.
Integration & effects of wind farms on power systemsIntegration & effects of wind farms on power systems
Power System Requirements Active power control To ensure stable frequency, prevent overloading of
transmission systems and avoid in- rush currents during start up and shutdown.
Frequency control Power output of a wind farm can vary up to 15% in 15
minutes and disturb the frequency. Voltage control To ensure voltage at PCC is maintained within acceptable
limits as required for utility and customer equipment.
Integration & effects of wind farms on power systemsIntegration & effects of wind farms on power systems
Power System Requirements Fault ride through recovery The wind turbines must be able to continue uninterrupted
operation under a transient voltage variation similar to the one illustrated below
85% voltage drop for 300 ms
Transient – 66 kVvoltage sag 75% -250 ms95% -0,5 sec.after fault 95%
Wind Power VariabilityWind Power Variability
Single wind turbine- 10 min average
Wind VariabilityWind Variability
RE regulationsRE regulations
Renewable Energy Certificates
EA 2003 set targets for distribution companies to purchase certain percentage of power from renewable energy sources-Renewable Purchase Obligations
States with lower RE levels could only meet lower RPO targets
The unit cost of RE being higher than conventional power hampers the growth of renewable energy.
RE regulationsRE regulations
Concept of REC mechanism
ForecastingForecasting
As per IEGC regulations 2010
Day ahead forecasts of wind power at 15 minutes interval for the next 24 hrs for aggregate generation capacity of 10 MW and above at 33 kV or above.
Wind generators shall forecast their generation with an accuracy of up to 70 %.
Wind generation shall bear the UI charges if the actual generation is not within these accuracy levels.
Host state shall bear the UI charges for variation within ±30 %.
Wind generator Purchaser
1. Schedule : 100 MW Actual generation : 100 MW
2. Schedule : 100 MW Actual generation : 70 MW
Wind generator Purchaser
Host state UI Pool
RRF
100 MW
70 MW 30 MW
30 MW
Wind generator Purchaser
3. Schedule : 100 MW Actual generation : 60 MW
4. Schedule : 100 MW Actual generation : 130 MW
Wind generator Purchaser
Host state UI Pool
RRF
RRF
UI PoolHost state40 MW
10 MW
70 MW 30 MW
10 MW
130 MW 30 MW
30 MW
30 MW
Wind generator Purchaser
5. Schedule : 100 MW Actual generation : 140 MW
6. Schedule : 100 MW Actual generation : 160 MW
Wind generator Purchaser
Host state UI Pool
RRF
RRF
UI PoolHost state10 MW
40 MW
130 MW 30 MW
10 MW
130 MW 30 MW
30 MW
60 MW
30 MW
20 MW- UI rate; 10 MW- UI rate for f < 50.02 Hz
SummarySummary
Some of the aspects to be considered for grid integration of wind turbines are Electrical system topology of the wind turbines Grid system and properties at the point of connectionPossible impacts of the wind turbine on the power
system Reactive power compensation schemes employedInterconnection standards, if any
Thank You