© P. Kundur
Outline Evolution of Wind Power Wind Turbine Characteristics Types of Wind Turbine Generator
Technologies Protection Systems Reactive Power Compensation and Voltage
Control Requirements Impact on Power System Dynamic
Performance Mitigation of Stability Problems
© P. Kundur
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One of the fastest growing primary sources
of energy for generating electricity in recent years In 2006, more than 20% of total energy
supply in Denmark was provided by wind power In Europe, the total installed capacity of
wind generation in 2011 was about 70 GW, and is expected to double by 2015 Global Wind Energy Council predicts that
global wind power capacity could reach 2,300 GW by 2030, providing nearly 20% of of the world’s electricity needs Introduces new challenges for ensuring
stable and reliable operation of power systems
© P. Kundur
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The building block to harvest wind power
is a wind turbine generating (WTG) unit A WTG includes four main components: Wind turbine Electric machine (generator) Power-electronic converter/conditioner WTG-level controller A Wind Power Plant (WPP) is a cluster of
WTG units that are collectively interfaced to the host power system at a point of interconnection (POI) WTG units are designed to collectively interact with the host power system so as to ensure satisfactory performance
© P. Kundur
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Most modern wind turbines have three
blades Based on the axis of rotation, wind turbines
can be classified into two basic types: Horizontal axis wind turbine (HAWT) Vertical axis wind turbine (VAWT) The HAWT are more efficient in extracting
the kinetic energy from the wind, and are widely used There are two types of HAWT rotor
configurations: upwind and downwind The dominant WTG technology, particularly
for applications in WPPs, is based on the horizontal axis, three bladed, upwind turbine structure
© P. Kundur
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Wind turbine components: wind turbine runs at low speed (0.5 Hz) mechanical drive train includes a gear box
converts low speed of turbine to high speed of generator
Mechanical speed regulation: blade pitch angle control
each blade rotated about longitudinal axis variable speed stall control
no pitch actuators required fixed speed
Types of generators induction generator synchronous generator doubly fed induction generator
WTG ratings range from 25 kW to 7.5 MW
© P. Kundur
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Figure below shows typical output versus wind Percentage Rated Output
speed characteristics of wind turbines:
cut-in
rated cut-out
wind speed (m/s)
The cut-in, rated and cut-out speeds shown are
typical for utility-scale WTGs
Generally, WTGs are designed to work at
maximum aerodynamic efficiency between cutin and rated wind speed
For wind speeds higher than rated and lower
than cut-out: blade pitching or blade stalling is used to maintain loading within the equipment’s rating
WTGs shut down for wind speeds higher than
cut-out speed to avoid excessive mechanical stress © P. Kundur
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Four major types of WTG Technologies used: 1. Squirrel Cage Induction Generators driven by
fixed-speed, stall-regulated wind turbines ((high mechanical stress) 2. Induction Generators with variable external
rotor resistance driven by a variable-speed, pitch regulated wind turbines 3. Doubly-Fed Induction Generators driven by
variable-speed, pitch regulated wind turbines 4. Synchronous or Induction Generators with full
converter interface (back-to-back frequency converter), driven by variable-speed, pitch regulated wind turbines (not synchronously connected to the grid. connection through frequency converters)
© P. Kundur
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Generator is an induction generator, which
is directly interfaced with the host utility network Rotor speed is determined by grid frequency, regardless of wind speed
Induction generator equipped with an
electronic starter and shunt capacitor banks for reactive power compensation Main features: simplicity, robustness of
components, and relatively low cost Drawbacks: excessive mechanical stress;
significant fluctuations in output quantities Widely used in the early 1990s;
not used for large-size WTGs and WPPs
© P. Kundur
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Generator is a wound-rotor induction
generator equipped with a rotor resistor adjustment device, and Enables slip control, typically up to 10% Shunt capacitor system for reactive power
compensation As compared to Type 1 WTG, slightly
aerodynamically more efficient and has modestly lower drive-train mechanical stress Not the preferred choice for present-day
large-size WTGs and WPPs
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WTG is composed of a pitch-controlled
wind turbine, a gear box, and a doubly- fed induction generator (DFIG) Stator of the DFIG is directly connected to
the host power system Three-phase rotor circuit is connected to
the grid through a back- to-back voltagesourced converter system Applies voltage across the rotor that is regulated by two rotor current controllers
Typically provides variable speed
operation from about –40% to +30% of the nominal power system frequency Aerodynamically more efficient;
lower drive-train mechanical stress; and lower power/voltage fluctuations Cont’d © P. Kundur
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Wound rotor induction generator with
slip rings Rotor is fed from a three-phase variable
frequency source, thus allowing variable speed operation reduction of mechanical stress; higher overall efficiency, reduced acoustical noise
The variable frequency supply to rotor is
attained through the use of two voltagesource converters linked via a capacitor Since the converter system handles only
the rotor quantities, its rating is significantly smaller (about 30%) than the generator rating Note: A more appropriate designation for this type of generator is: Doubly Fed Asynchronous Generator (DFAG) © P. Kundur
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DFIG
Grid
Grid side converter
DC Link
Reactor
Cbc
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Chopper
Rotor side converter
The converters handle ac quantities: rotor-side converter carries slip frequency current stator-side converter carries grid frequency current
Hence, they are controlled using vector-
control techniques:
based on the concept of a rotating reference frame and projecting currents on such a reference such projections referred to as d- and q-axis components
With a suitable choice of reference frame,
AC quantities appear as DC quantities in the steady state cont’d
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In flux-based rotating frames: changes in the d-axis component of current will lead to reactive power changes changes in the q-axis component will vary active power
This allows independent control of active
and reactive power of the stator Implemented through rotor-side converter control An important aspect of the DFAG concept !
Since rotor flux tracks the stator flux, air
gap torque provides no damping of shaft oscillations additional modulating signal has to be added
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Rotor current protection: Limits current in the rotor side converter If current rises above set value, a crowbar is activated
short-circuits the rotor winding at the slip rings with a static switch Generator operates as a squirrel cage induction motor In newer units, an “active crowbar” is used Typically, the case when the voltage at the terminals of the generator decreases rapidly, for example during a fault in the grid In order to avoid overspeeding of turbine, the speed reference for the pitch control is reduced simultaneously
increases pitch angle and reduces mechanical power
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Rotor speed protection: disconnects WTG from the grid if speed of rotor is higher or lower than set levels for a predefined time
Over/under voltage protection: disconnects WTG from the grid if voltage is above or below set values for a predefined time
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DFAGs have the ability to hold electrical
torque constant rapid fluctuations in mechanical power can be temporarily stored as kinetic energy improves power quality!
Performance for large disturbances
requires thorough analysis may lead to separation of the unit process may not be readily apparent from simplified dynamic simulations
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Large disturbances lead to large initial fault
currents, both at the stator and rotor will flow through rotor-side converter; voltage source converters are less tolerant of high currents further, additional energy goes into charging the dc bus capacitor and dc bus voltage rises rapidly
crowbar may be activated may lead to tripping of the unit
Need for a careful assessment and proper
design of controls to improve capability to ride through faults
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Temporary reduction of active power: Active Power is ramped down for a predefined time and then ramped up again to prefault value This stabilizes wind turbine during the fault and reduces the current in the rotor converter Disadvantage: rotor can speed up causing overspeed protection to trip turbine
handled by the pitch controller
Temporary reduction of active power with
reactive power boosting: Increases terminal voltage Improves system stability
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Generator is either a synchronous machine
or an induction machine Generator is connected to the utility
network through a power electronic converter system Most often composed of two cascaded AC-DC converters (back to back voltage source converters) Enables full rage of variable speed operation for the unit, and reactive power control at the point of connection
A pitch controlled wind turbine is
mechanically interfaced to the generator, either through a gearbox (conventional scheme) or directly (direct-drive scheme) For conventional structure, the generator
is a high speed (e.g. 4-pole) machine, and thus requires a gearbox © P. Kundur
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By contrast in the direct-drive structure,
the generator is a low speed (e.g., an 84pole) machine Generator is directly interfaced to the turbine rotor shaft; no need for a gearbox Generator can be a wound rotor (conventional) synchronous/induction machine or a permanent magnet synchronous machine (PMSG) Direct-drive units with permanent magnet
synchronous generators are increasingly being used for large-size WTGs Type 4 WTGs, like Type 3 WTGs, are
aerodynamically more efficient; have lower drive-train mechanical stress and lower power/voltage fluctuations
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Most modern WTGs are of:
- Type 3 configuration, or - Type 4 configuration with PMSG With a Type 3 unit, the the rating of power-
electronic converter system associated with the rotor circuit is about 30% of the generator rating Less expensive and have lower losses than the converter system for Type 4 units Main drawbacks of Type 3 configuration
are its requirement for slip ring and the need for special rotor current protection system Type 4 WTG with PMSG has the following
features: smaller and lighter, does not require slip-rings, and is structurally simpler since the rotor control system is not needed © P. Kundur
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Utility-scale wind power plants consist of
several tens to hundreds of WTGs Each unit with a pad-mounted transformer Connected to transmission network through a medium-voltage collector network A power transformer used to interface with the transmission grid Depending on the application and type of
WTG, shunt reactive power compensation may be added at one or more of the following locations: WTG terminals Collector system Substation interfacing with the Transmission grid
© P. Kundur
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Wind power plant output varies with wind
resource
Cannot be dispatched like conventional power plants System operators cannot control the rate of power decreases, i.e., ramp down due to falling wind speeds For ramping up, some manufacturers provide the option of controlling rate of power increase
As wind power capacity within a control
area increases, the variability of wind power can have a significant impact on:
the efficiency of unit commitment process, and the reserve requirements to meet reliability performance standards
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In areas with large amounts of wind
generation, wind variability can have a significant impact on voltage profiles may require switched capacitor banks and shunt reactors, and transformer tap changer control
Some wind power plants have the ability to
control/regulate voltage at or near the point of interconnection to transmission grid accomplished by installing separate devices such as SVCs and STATCOMS, alternatively, external controller may be added for adjusting the power factor of each individual WTG until target voltage is achieved
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The dynamics of individual WTGs and the
entire wind farms could have a significant impact on the stability of the bulk power system
“Rotor angle stability” is not an issue with
wind power plants because most WTGs are asynchronous units No equivalent concept of “rotor angle” or synchronizing and damping torques for such generators
Some studies have revealed that bulk
power system “transient rotor-angle stability” is improved if wind power plants, as compared to conventional power plants with synchronous generators, are added at the same location with WTGs, a smooth and non-oscillatory power delivery is re-established following a disturbance cont’d
© P. Kundur
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Wind power plants could have a significant
impact on “voltage stability” following a network fault
Induction generators absorb higher reactive power when voltage is low Even DFIGs may “crow-bar” during a fault, and act as an induction generator Increased reactive power consumption can lead to voltage instability if the transmission grid is weak Voltage stability related to characteristics of WTGs, as opposed to load characteristics A short-term phenomenon Adequate and fast control of reactive power and voltage required Overall solution requires coordinated control of wind farms, including use of external compensators such as SVCs and STATCOMS cont’d
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Type 3 and Type 4 WTGs do not contribute
to system inertia
May contribute to “frequency instability”, particularly in smaller power systems with high penetration of wind generation Special controls, such as “inertia control”, often used to address this problem
Detailed simulation studies using
appropriate WTG models essential for satisfactory integration of large WPPs into power grids includes EMTPDC/ PSCAD simulations, in addition to system stability studies
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A good source of reference addressing some of these issues is the CIGRE Technical Brochure on: “Modeling and Dynamic Behavior of Wind Generation As It Relates to Power System Control and Dynamic Performance” - CIGRE Technical Brochure 328, August 2007 - Prepared by CIGRE WG C4.601
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Wind field model describing wind speed Wind turbine model Model for internal grid of wind power plant For system studies aggregated
representation is sufficient
a single WTG model to represent the farm or a sub-group of WTGs Induction/synchronous generator
represented by a third order or fourth order model d and q axis rotor circuits and acceleration of rotor
Models for controls and protections
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Detailed models for WTGs developed by
manufacturers and consultants for grid integration studies and design of WPPs are considered as: Proprietary user-defined models Further, maintenance of numerous vendor-
specific models is unmanageable for regional reliability organizations and grid operators Efforts are underway for developing
“Generic” WTG models suitable for “system impact” studies and planning and reliability evaluation studies Joint report prepared by IEEE WG on Dynamic Performance of Wind Power Generation and WECC WG is attached
© P. Kundur
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In the past, wind power plants were
allowed to trip off for nearby transmission faults and system disturbances Early WTGs were not integrated in a way
that supported bulk power system operation Due to the significant increase in wind power capacity, this is no longer appropriate
Transmission operators and reliability
coordinators have begun to capture performance requirements for wind power plants in Grid Codes The Grid Codes are new and as such
evolving
Cont’d
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Grid Codes (Cont’d.) Grid codes typically include performance
requirements relating to: Active power and frequency control; Ramp rate control Reactive power and voltage control Fault tolerance/ ride-through capability
Requirements can vary depending on the
host power system characteristics, depth of wind power penetration With the anticipated significant growth in
wind power in North America and Europe, NERC and ENTSO-E have organized work plans to support the power industry’s integration of variable generation Wind power plants will be required to perform like conventional power plants from a terminal point of view
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Modern WTGs can contribute to the reliability and efficiency of grid operation by offering the following capabilities: Voltage and VAr control/regulation Fault ride-through: ability to ride through
specific low and high voltages Real power control, ramping, and
curtailment Primary frequency regulation Inertia response: special control Short-circuit duty control: inverter-based
WTGs have built-in capability to limit the fault current
Controllable wind power plants are the way of the future! © P. Kundur
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An effective way to integrate large
percentage of wind generation Permits interconnection with main
transmission network at relatively weak parts of the network Provides good dynamic response and
ability to comply with demanding grid code requirements Results in smaller “footprint” Ongoing development in power
semiconductor switches expected to address the problem of circuit interruption and switching Growing interest in the application for
interconnection of off-shore wind farms with the transmission network
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