Grid Integration

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


Fixed speed wind turbine


Limited variable speed wind turbine


Variable speed wind turbine with partial scale frequency converter


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


Variable speed wind turbine◦Generator side converter , a diode based rectifier or a PWM Voltage Source Converter

◦Grid side converter, PWM Voltage source converter


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


Variable speed wind turbine◦ Load angle control technique of generator side

converter


Variable speed wind turbine◦ Vector control technique of generator side

converter

Torque is controlled by controlling the stator current


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


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.


STATCOM


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


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


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.



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


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


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.


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.


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


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


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


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.


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.


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



Host state UI Pool

RRF

100 MW

70 MW 30 MW

30 MW





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





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

Documents

Grid Integration