Solutions for the grid integration of wind farms –

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					Solutions for the grid integration of wind farms – A survey
        Pierre Bousseau, Floriane Fesquet, Régine Belhomme, Samuel Nguefeu, Thanh Chau Thai
  Tel : +33 1 47 65 38 18, +33 1 47 65 54 94, +33 1 47 65 38 60, +33 1 47 65 36 51, +33 1 47 65 49 36
  Fax : +33 1 47 65 50 18, +33 1 47 65 50 18, +33 1 47 65 50 18, +33 1 47 65 41 76, +33 1 47 65 41 76
           Electricité de France, EDF R&D, 1 av du Général de Gaulle, 92141 Clamart, France
   pierre.bousseau@edf.fr, floriane.fesquet@edf.fr, regine.belhomme@edf.fr, samuel.nguefeu@edf.fr,
                                         thanh-chau.thai@edf.fr


                                                          In this paper, a survey of different possible technical
Abstract:                                                 solutions for grid integration of wind farms is
                                                          presented. In a first part, the main problems and
In the paper, a survey of different possible technical
                                                          constraints related to grid integration of wind farms
solutions for grid integration of wind farms is
                                                          are briefly described. In a second part are presented
presented. These solutions concern the main problems
                                                          the different solutions which may be used to cope with
encountered today and, whenever possible, examples
                                                          these problems or to meet the corresponding network
of actual implementation in wind farm projects are
                                                          requirements. Solutions at very different levels of
given. More specifically the following types of
                                                          development (feasibility study, prototyping, test on
solutions are considered:
                                                          pilot sites, commercial products, …) are considered.
- choice of an appropriate Wind Turbine Generator
                                                          Whenever        possible,    examples     of     actual
    technology,
                                                          implementation in wind farm projects are given.
- solutions based on coordinated wind farms and
    power system operation,
- reactive compensation and voltage control                   2       Problems and constraints
    systems,
                                                          The problems and constraints encountered when a
- Fault Ride Through solutions,
                                                          wind farm is connected to the grid may be related to
- energy storage systems,
                                                          the grid connection itself and therefore concern all
- current limitation devices,
                                                          kinds of DG plants whatever the generation process.
- directional protections, ...
                                                          They may also be more specific to wind energy and
                                                          the stochastic nature of wind. The main constraints are
Keywords: wind farms, grid integration, power
                                                          briefly described below [4].
quality, ancillary services, grid capacity
                                                              2.1      Grid capacity constraints
    1     Introduction
                                                              2.1.1       Steady-state thermal constraints and
Since the beginning of the 90's, many countries have                      network congestion
experienced a substantial growth of distributed
generation (DG) on their power systems. The grid          Over currents in grid equipments may appear in the
connection of DG gave rise to new problems and            vicinity of the connection point of the wind farms. In
constraints, especially on distribution networks. These   some cases of very large wind farms or for high
constraints led system operators, electric utilities,     penetration level of wind energy, problems of
governments or regulatory boards to define technical      congestion could occur in the transmission network
requirements for the grid connection of DG units.         and sometimes even on international interconnections
In the last decade, the expansion of wind energy on       with neighboring countries transmission systems.
distribution and transmission networks raised specific         2.1.2      Short-circuit powers and currents
problems related to this particular type of generation
                                                          The wind power plant may contribute to the fault
process (wind energy conversion with stochastic
                                                          currents on the network. Following the integration of a
generation) and to the technologies used. This led to
                                                          wind farm, the maximum short-circuit powers or
the definition of specific rules for the connection of
                                                          currents shall not exceed the maximum admissible
wind farms or to the adaptation of existing rules or in
                                                          values for the different pieces of grid equipment.
some cases to exemption.
At the beginning, the network requirements for wind            2.1.3      Steady-state voltage profile
farms were “softer” than for classical generating units   Generally, the voltage increases at the connection
but with the ever increasing development of wind          point of the wind farm and on the feeder which the
power, the impacts on the grids become more and           wind farm is connected to. It should be ensured that
more significant leading to the definition of more and    the voltage does not exceed the maximum limit values.
more “severe” requirements [1,2,3]. Therefore needs       Moreover, the grid connection of a wind farm affects
now appear for solutions which are more and more          not only the voltage profile on the network at the
complex, sophisticated and reliable … but also, as a      voltage level of the connection but also at lower
matter of fact, more and more expensive.                  voltage levels.
     2.2     Power quality issues                           Fault Ride Through (FRT) capabilities and so to
                                                            withstand specified voltage dips.
     2.2.1     Voltage fluctuations
Wind power plants may generate different kinds of                2.5      Ancillary services issues
voltage fluctuations : slow voltage fluctuations
(resulting from power variations due to changes in the           2.5.1     Reactive power compensation and
wind speed), and voltage step changes (when starting                       voltage control
up or when changing between generators, …).                 In power systems, different types of components such
                                                            as lines, transformers and loads generally consume
     2.2.2     Flicker                                      reactive power. To compensate this reactive
Voltage fluctuations linked to wind turbine switching       consumption and enable the power transits in the
operations, wind speed variations or shadow effect can      network, reactive power has to be supplied by
lead to flicker problems, especially in “weak”              generating plants or other types of equipment such as
networks (with low short-circuit power levels).             capacitor banks for instance. Wind farms like other
                                                            production means have to participate to this
     2.2.3     Harmonics
                                                            compensation.
   Power electronics converters generally produce
harmonics. However, harmonic currents emitted by a          Furthermore the network operation involves a grid
power electronics converter depend to a great extent        voltage control which is generally performed by
on its features. For instance, with the use of forced       generating units. Wind turbines may have to
switched inverters, better control is generally achieved    participate to this voltage control, even if they have
regarding the waveform of the electrical quantities at      rather limited reactive power consumption and
the inverter terminals. Nevertheless, the level of          production capabilities.
harmonic currents shall not exceed the limit values.
                                                                 2.5.2     Frequency control
     2.2.4      Disturbances of the remote control          The network operation involves too a frequency
                signal                                      control which is performed by the generating units.
This signal is used for instance to transmit information    Wind turbines have rather limited control capabilities
on the applicable price or the change in the price (from    concerning frequency control, mainly due to the
day to night…). The connection of a wind power plant        stochastic nature of wind energy. Furthermore, due to
modifies the network “impedances” and harmonic              the high variability of the active power they produce, a
resonance conditions, and the remote control signal         high penetration level of wind energy requires an
may thus be affected.                                       increase of frequency control capabilities and leads to:
                                                                 -    an increase of solicitations of the network,
     2.3     Protection issues                                   -    difficulties to define the operation program.

The integration of the wind farm shall not interfere
with the sensitivity and selectivity of the protection           3       Solutions for grid
scheme. It shall not entail for example : the non                        integration
detection of a fault, unexpected trippings of healthy
                                                            The commonly used solution to most grid integration
parts of the network, disturbances of fault locator, …
                                                            problems is grid reinforcement. However this solution
                                                            is often very costly. Moreover grid reinforcements
     2.4     Dynamic            behavior           and      don’t solve constraints linked to frequency control or
             stability                                      to the high variability of the wind farms production.
                                                            Other solutions envisaged to cope with the different
On the one hand, the wind farm operation shall not          constraints are presented in this section. An overview
affect or interfere with the grid dynamic behavior and      is given in Table 1. Each solution category is detailed
operation, as well as with the operation of special         later on. Note that in the present context, part of these
devices that may be connected to it. This has to be         solutions can be used only with the agreement of the
checked for the various configuration and operating         network operator.
conditions of the wind farms, e.g. for starting up, cut
off, wind speed variations, …
On the other hand, for severe disturbances on the
network such as faults and voltage dips, the
disconnection of the wind farm can worsen the
situation. This is especially true for three phase faults
on the transmission network with a high wind power
penetration or on island grids. If the wind farms are
disconnected this could lead to an important loss of
generation that can entail load shedding or even a
black out, in the worse case. For this reason,
depending on the voltage level at the connection point,
network operators may require wind farms to have
Table 1 :Overview of the solutions

    3.1     Wind turbine technology                      -   They are able to damp active power variations and
                                                             the resulting voltage variations by modifying the
Wind turbine technologies could be divided into 4            rotor kinetic energy. So they may have a limited
main types:                                                  impact on voltage quality.
- classical squirrel cage induction wind turbine         -   They can have a low short circuit contribution in
    generator (WTG),                                         case of the use of a back-to-back converter at the
- Induction WTG with dynamic slip control, i.e.              stator of the wind turbine. For other WTGs, an
    induction generator with a wounded rotor                 additional system is required to limit short-circuits
    connected through power electronics to a variable        (see Section 3.6).
    resistance.                                          -   They have low harmonic emission when they are
- Doubly-fed WTG, i.e. induction generator with the          equipped with IGBT or GTO based power
    rotor windings connected through a back to back          electronics at the stator of the generator.
    power electronics converter to the machine           -   They have sometimes a Fault Ride Through
    terminals.                                               capability (see Section 3.4) without need of an
- Induction or synchronous WTG connected to the              additional device. This FRT capability enable to
    grid through back-to-back power electronics              keep the stability of the Wind Farm in case of
    converters.                                              three phase fault. Furthermore, following a three
The wind turbines are very often equipped with a pitch       phase fault they may improve grid stability by
control system that enables to:                              supplying reactive power in priority over active
- partially damp the mechanical power variations             power during a few seconds.
    and the resulting voltage variations,
- limit the mechanical power on the main shaft of             3.2     Wind farms and                    power
    the WTG and so the active power delivered to the
    network, to the maximum active power the WTG                      system operation
    can withstand. It can also be used for frequency          3.2.1     Power curtailment or wind turbine
    control mainly when over frequencies occur and                      disconnection
    also during under frequencies providing that the
    active power was limited by the use of the pitch     In case of:
    control before the under frequency and providing     - grid capacity constraint resulting from a thermal
    that the wind speed doesn’t decrease when the            constraint in a grid component,
    under frequency occur.                               - or an operating constraint,
- limit the mechanical power on the main shaft of        - or security constraints,
    the WTG during voltage dips to help the wind         a possible solution would consist in limiting the active
    farm to withstand it.                                power delivered by the wind farm to the network. This
The wind turbines with power electronics converters      could be:
(induction WTG with dynamic slip control, doubly fed     - a permanent limitation defined prior to the
WTG, synchronous or induction WTG with full back-            connection and eventually modified during the life
to-back converters at the stator) :                          duration of the wind farm,
- are able to control the voltage and reactive power     - a periodic limitation for a given duration if the
    depending on the rating of their converters except       constraint appears only during a forecasted time in
    for induction WTG with dynamic slip control.             the year (due to seasonal maximal external
                                                             temperature, due to load and/or generation profile
    in the year). For example it could be a limitation
    during summer or winter, or a limitation during        This can be achieved for instance by:
    peak hours each day, or each day in a given time       - the use of a remote control signal to shut on or off
    period….                                                  loads on the network,
- A limitation determined one or several days or           - a automatic frequency control integrated directly
    hours a-head based on the assessment of the               at the load level,
    network conditions with appropriate simulation         - the use of an additional communication system.
    tools.
- A “real time” limitation based either on direct          Note that this requires that the loads can withstand an
    measurements on the network or again on the            intermittent functioning and that the service associated
    assessment of the constraints through simulation       to them is still acceptable.
    with an appropriate software tool. The                 For instance, different applications [5-8] are presently
    implementation of a real time limitation requires      developed and tested with water heating (used for
    an efficient communication system between the          heating purposes or for domestic consumption), with
    system operator and the wind farm.                     water pumping or with different kinds of domestic
Depending on the situation, this power limitation may      loads which present “inertia” characteristics or a low
be achieved for instance by:                               priority for the consumers.
- disconnecting the appropriate number of wind             In case of network congestions or when the power
    turbines in the wind farm, or,                         injected on the grid is limited, the wind power which
- adjusting the pitch control if existing.                 can not be supplied to the grid, can be used for other
Wind power curtailment is widely used for instance in      applications. For instance, applications are presently
USA as a solution to network congestions.                  considered with :
                                                           - desalination plants,
     3.2.2      Coordination with other generating         - hydrogen production,
                plants                                     - water pumping, …
The coordination of wind farms with one or several
other generating plants (using other primary sources            3.3     Reactive compensation and
than wind) may allow :
- to compensate for the short term, mid term or long                    voltage control
    term power variations of the wind farm. This                3.3.1      Wind turbine technology
    enable to limit the solicitation on the network, to
    facilitate the power system operation, and to          Depending on the technology, the wind turbines can
    improve the security.                                  directly (without need of additional device) participate
- to share the frequency control participation             to the reactive compensation and voltage control.
    between different generation units.
In case of grid capacity constraints corresponding to a    More precisely, doubly-fed and WTG connected to the
thermal constraint in a grid component, the true           grid through power electronics interfaces can be used
coordination of the wind farm with other generation        to control the reactive power and the voltage. The
plants (for instance hydro power plants) may provide a     reactive power provided or consumed by these WTGs
solution. The other generating units may adapt their       is limited by the current limitation of the converters
production in order to allow the wind farm to deliver      and so their rating. Depending on the technology, it
all the power it can provide while the grid thermal        generally enables to produce or consume a reactive
limits are respected. However this possibility is highly   power up to 30% of the nominal active power. To
dependent of the location of the other production units    perform the grid voltage control at the connection
regarding the grid equipment pieces under constraint.      point of the wind farm, a coordinated voltage control
In the same way, if a wind farm and an other               strategy between the different wind turbines in the
production unit have their connection point close to       wind farm has to be defined.
each other, it could be envisaged to share the reactive    Classical squirrel cage induction WTGs and induction
power production or the voltage control.                   WTGs with dynamic slip control can only consume
                                                           reactive power. They need additional devices to
Different applications have been developed for the         perform the reactive compensation and possibly
compensation of short term, mid term and long term         contribute to the grid voltage control.
power variations, for instance in :                             3.3.2     Thyristor-Switched Capacitor (TSC)
- isolated grids with diesel – wind coordination [5],
- the Nordic Power Pool with Hydro electric – wind         TSC are designed to insert the appropriate number of
    coordination [6].                                      multi-stage power capacitors into the network, without
                                                           transient effects. For that purpose, capacitors are
    3.2.3       Load control
                                                           switched into the circuit when the grid voltage and the
Local loads can be switched on or off to:
                                                           capacitor voltage are both equal to the positive or
- compensate slow active power variations of wind
   farms,                                                  negative peak voltage.
- perform a frequency regulation with a load control
   in order to facilitate the integration of wind farms    Tow types of TSC systems may be found : the first
   to the grid,                                            one is based on 2 thyristors connected in back to back
- solve a thermal constraint.
    and the second one consists of a thyristor and diode                                       SVC
    connected in antiparallel for each valve.                              PCC
    A microprocessor-based system manages the
    measurements, calculates the amount of reactive               GRID                                             WIND
    power to compensate and the gating signals to the                                TCR                           FARM
                                                                                                      C
    thyristors. TSC are often used to regulate voltage
    especially when it fluctuates slowly. The dynamic of a                          [-QL, 0]
    TSC is limited, as in any device including line-
                                                                                                       [+QC]
    commutated thyristors, at the double of the 50 Hz
    network frequency, i.e. 10ms. Practically one should
    add to this time interval, the computation delay of the         Figure 2: Implementation of a SVC across the grid and
    regulator (typically some milliseconds). Therefore the          a wind farm
    typical response time of a TSC is about 20
    milliseconds.                                                   In the simplest design, the star-connected 3-phase
                                                                    capacitor bank produces a fixed amount of reactive
                        U                                           power : +QC. Many capacitor banks are connected to
                            Detection
                                                                    the Point of Common Coupling (PCC) via small series
                                                                    inductances, resulting in a tuning of the LC dipoles on
                                                              t     harmonics 2, 3 and 4 or 5 generated by the switching
                                                                    of thyristors within a fundamental cycle. Passive
                       Uc               Detection
                                                                    filters are also designed with damping resistances in
       T1    T2
                                                                    order to avoid resonance with other grid components.
                                                                    Depending on the need for reactive power of the wind
                                                              t     farm, the TCR adjusts automatically its consumption,
U                                                                   hence it can perform a reactive power control or a
                        I
                                                                    voltage control at the PCC.
                                                                    In a variant of the SVC, the capacitor banks are
         I                                                          typically split into 4 steps that can be switched on or
                                                          t
       Uc                                                           off. The advantage lies in a division by up to 4 times
                                                                    the size of the TCR depending on the reactive power
                                                                    consumption capabilities required. The drawback is
                              T1        T2          T1              that the tuning of the filter on the low harmonics and
    Figure 1: Illustration of the zero current switching            the avoidance of resonance is slightly more difficult to
    principle                                                       achieve.
                                                                    A new type of SVC is the Voltage Controlled Static
    Among the available products, the following can be              VAR Compensator (VCSVC, Figure 3). It is made of a
    mentioned : ASVC by PQS (Power Quality System),                 capacitor connected to the network by a transformer
    Pureware AVC manufactured by S&C, Dynacomp                      with the secondary winding split into a fixed winding
    from ABB, Equalizer from ELSPEC. Some of those                  and a number of control windings. Consequently, the
    products are presently being tested in wind farms in            capacitor connected across the whole winding is seen
    France and elsewhere in the world.                              from the grid through variable ratios, allowing to keep
                                                                    the system voltage within the rated band [10].
          3.3.3    Static VAR Compensator (SVC)
    Over the last three decades, utilization of large pulsed
    type loads such as arc furnaces, welding equipment,                                                   TCR
    rolling mills has increased with the growing need of
    steel. The increasing number and size of those                           GRID
    fluctuating loads caused power quality problems such
    as flicker, harmonics and unbalance. Static Var
    Compensators have been used in the factories to
    reduce flicker and unbalance. Some 900 SVC
    applications are running to date around the world [9].
    The technology, based on thyristor-controlled reactors
    (TCR) is mature. Its exploitation to improve reactive
    compensation and grid voltage control in wind farms
    can be envisaged.
    In Figure 2, a SVC has been inserted between the grid
    and a wind farm, which can be seen from the grid as a           Figure 3 : Diagram of a VCSVC
    fluctuating user. The device utilizes a Thyristor
    Controlled Reactor (TCR) and a Fixed Capacitor (C).             The limits of SVCs are related to the inherent time
    Thanks to the semi-conductors, three branches of TCR            delay of the TCR, the dynamic interactions between
    are delta-connected to form a 3-phase variable                  the LC filter of the SVC and the TCR, and the SVC
    inductance capable of absorbing continuously a                  inability to compensate active power fluctuations.
    amount of reactive power from -QL to QC.
     3.3.4     STATCOM                                        and 4 MVar of switched capacitor banks, it is possible
The STATCOM (static compensator) and its variant,             to obtain a continuous compensation capability
the D-STATCOM, are based on voltage sourced                   ranging from -4Mvar to +8Mvar.
converters (VSC), and so are voltage sources where
amplitude, phase and frequency are entirely
controllable. The dc bus capacitor applies a dc voltage
at the input of the inverter. Its output is connected to
the AC grid via an inductance. While adjusting the
converter voltage (U2) with respect to the network
voltage (U1), the converter can very quickly supply or




                                                                                                                             to other loads or equipments
absorb reactive power thanks to the gating signals set
to the switches of the converter. The response time is
                                                                                                              Zero
mainly influenced by the switching frequency                                                                  current




                                                                    to loads or wind turbines
(typically 1 to 2 kHz) and by the size of the                                                                 crossing




                                                                                                   Order
inductance.                                                                                                   switches


                                                                                                D-VAR
                                                                                                           Capacitor banks


                                                              Figure 5: Principle diagram of a D-VAR and
                                   U1                         associated capacitor banks
                                           I
                                                              First of all, the D-VAR variable output is used to
                                                              regulate the voltage. When the D-VAR output level
                                                              reaches the size of an available capacitor bank, the
                                                              control system sends an order to this capacitor bank.
                                           U2
                                        ~=
                or wind turbines




                                                  Voltage
                                                              Simultaneously, the D-VAR adjusts its output, thereby
                to the loads




                                                  source      eliminating the step voltage change. The D-VAR then
                                                  converter
                                                              resumes its voltage regulation mode, dynamically
                                                              injecting or absorbing vars as required. Inversely, a
                                                              symmetrical operation mode is applied when the vars
                                        Statcom               decrease.
Figure 4: Connection of a STATCOM at the Point of                   vars generated by the D-VAR
Common Coupling of a grid and a wind farm                       8
                                                                0                                                   t
In theory, the STATCOM can help mitigating the                 -8
flicker due to variations of reactive power absorbed by
induction machine-based wind farms. The reactive               32
power required by the farm is evaluated and a
controller drives the STATCOM inverter so as to
generate the adequate quantity, permitting to reduce           16
drastically reactive power flows towards the grid and                                            Vars generated by the
therefore, the associated flicker. Unlike thyristor based                                           capacitor banks
solutions (TSC and SVC), VSC power electronic                  0                                                                                            t
systems are based on IGBT (Insulated Gate Bipolar              32
Transistor) technology, intrinsically faster than
thyristors. Continuous progress in the semi-conductor
                                                               16                                Vars generated by the
industry now makes it possible to build and operate
high power STATCOM (about a few tens of MVA),                                                    association of D-VAR
with a very good dynamic response, i.e. time constants                                            and capacitor banks                                       t
                                                               0
in the millisecond range.
                                                              Figure 6: Operating principle of a D-VAR and
Among the available products and wind farm
                                                              associated capacitor banks
references the following can be mentioned:
- Pureware DSTATCOM made by S&C,
                                                              Few STATCOMs have been used in wind farms
- D-VAR made by American Superconductor
                                                              probably because of their cost which is still high for
    (AMSC) [11].
                                                              this application.
The D-VAR is a "hybrid" system. It consists in a small
STATCOM that sends orders to adjacent switched                Some D-VAR wind farm references are given below:
capacitor banks. This coordinate operation of the D-          - Wyoming Wind Energy Project, PacifiCorp (USA):
VAR and the switched capacitor banks allows a wider             8 MVA D-VAR with 6 x 5.4 Mvar and 3 x 6.67
range of continuous supply and/or absorption of                 Mvar capacitor banks in 2002.
reactive power. For example, with a 4 MVA D-VAR
- Minot Wind Farm (South Dakota, USA) in 2003:             parallel. Optional switches allow disconnection of
  1 MVA D-VAR (without capacitor banks) was sold           each of those elements. When they are in service, the
  to Nordex-USA.                                           whole “RPS + impedance” system acts as an
- Mendota Hills wind farm (North Illinois, USA):           adjustable impedance element, thus allowing to absorb
  4 MVA D-VAR with 2 capacitor banks in 2003.              the excess of active power caused by the wind
- Summerview wind farm (Canada): 8MVA D-VAR                conversion process and to inject reactive power.
with 6 x 6.6 Mvar capacitor banks in 2004.
                                                           The output active power to the grid can be regulated
A synonym of STATCOM is STATCON, standing for              and kept constant owing to the absorption of the
Static Condenser. Its rotating ancestor, the               excess power by the resistor. If only the resistor is in
Synchronous Condenser can still help in wind farms.        service, line voltage is likely to vary because of the
                                                           increase reactive power consumption of the wind
                                                           farm. Operation of the capacitor helps maintaining
     3.3.5     Synchronous Condenser                       also the voltage in the rated range.
Synchronous condensers are synchronous generators          The RPS are still under development, so practical
without prime mover, that can be controlled to absorb      cases and field experiences can not been displayed yet.
or provide reactive power and can perform a voltage        An advantage with respect to the SVC is the capability
control. They have been partly supplanted by static        of handling active power and not only reactive power.
compensators. However developments for example on          Another feature to investigate, especially in the series
superconductor synchronous condensers (AMSC) can           configuration, should be its possible fault ride through
make them interesting for reactive compensation and        performance.
voltage control of wind farms.
                                                                3.4     FRT systems
      3.3.6     Rotary Phase Shifter (RPS)                 The goal of a Fault Ride Through system is to enable
By the end of the nineties, the study of a doubly-fed      the wind farm (WF) to withstand a severe voltage dip
adjustable-speed flywheel generating system was            at the connection point.
initiated in Japan, with prospective applications in the
stabilization of power in decentralized systems [12].      During the voltage dip, the active power provided to
Basically the system is an induction machine with a        the grid by the WF is instantaneously reduced. This
wound rotor. In the series type RPS with stator            power becomes at least temporarily lower than the
connected to the grid and rotor connected to a wind        mechanical power available, hence the rotor speed of
farm and no additional impedance element, thanks to        the generator increases. If the FRT capability is
the torque controller, the device works as a phase         required, the WTGs must not disconnect, for instance
shifter, capable (in the same way as a phase shifting      due to overspeed or under voltage protections. After
transformer) of controlling the power flowing between      the clearing of the fault that led to the voltage dip, the
the stator and the rotor. Thanks to the slip of the        voltage at the wind turbine bus increases. The WTGs
induction machine, it can also acts as a frequency         must resume their power supply to the network (and
changer. A first application could be to keep the          not lose stability). The risk of a loss of stability after
transmitted power constant (through a constant             the fault clearing is very limited for induction or
torque), hence mitigating flicker or other voltage         synchronous generators with full power electronics
fluctuations, despite the stochastically variable nature   converters, and limited for doubly-fed WTG, thanks to
of wind speed.                                             the control capabilities of the power electronic
                                                           converters.
The diagram of Figure 7 displays the conceptual
configuration of a shunt type RPS.                         For the WTGs not to disconnect in case of a voltage
                                                           dip, the main points are:
                                                           - to limit the increase of the rotor speed so that it
        PCC                                                    doesn’t exceed the maximal permitted value,
                                                WIND       - to keep the voltage at the generator bus in the
     GRID                                                      permissible range,
                                                FARM       - not to lose stability so as to be able to resume the
                                                               power supply after the fault clearing.
                  stator
                        rotor                              Depending on the technology, wind turbines may
           RPS                   C      R                  directly include FRT capabilities. These capabilities
                                                           can be achieved by an adapted control strategy:
                                                           - mechanical active power limitation with pitch
Figure 7: Insertion of a shunt type RPS with resistor          control,
and capacitor between the grid and a wind farm             - use of capacitor or breaking resistance on the DC
                                                               bus of the back-to-back converter (connected at
The stator windings of the doubly-fed induction                the stator or the rotor) to stock or dissipate the
machine are connected to the grid via an optional              active power during the voltage dip.
transformer. The wound rotor is connected to the           - disconnection and reconnection of stator.
stator circuit through a capacitor and a resistor in
For   example,   ENERCON,        GE,    NORDEX,            -   or guarantee the active power supply to the
REPOWER, VESTAS and other manufacturers                        network in case of the disconnection of the wind
propose WTGs with FRT capabilities [13-15].                    farm due to a voltage dip.

However, solutions based on additional devices may         Each energy storage system has to be chosen and
be proposed to enable FRT of wind farms. The               designed in function of the real constraints to cope
principle of these devices is generally to support the     with for the grid integration of a specific wind farm.
voltage at the generator terminals during the voltage      They are a way, with wind power prediction, to
sag by injecting reactive power into a decoupling          improve the reliability of wind power. Cost effective
reactance that could be permanently or temporarily         energy storage systems will be an important step for a
connected.                                                 high wind energy penetration in electric power
                                                           systems.
    To the
                                                     WF
   network
                      "decoupling"
                         reactor

                Voltage          TSC, SVC,
                control         STATCOM, ...

Figure 8: general principle of a FRT device

Note that it is important to smooth the voltage sag and
to send a reactive power burst through the reactor as
soon as possible after the beginning of the sag. In the
case of capacitor-based devices, the reactive power
varies with the square of the voltage, in case of
Voltage Sourced Converter-based devices, the reactive
power varies proportionally to the voltage. So these
latter devices can be more efficient, but are more
expensive too.                                                   BESS = batteries
                                                                 CAES = compressed air energy storage
Note that the use of a FRT device has to be considered
along with the reactive compensation itself because:             EZ+FC = hydrogen
- usually the FRT device can provide and                         FESS = flywheels energy storage system
    sometimes absorb reactive power,
- the decoupling reactance can affect the voltage                HAS = hydraulic accumulator system
    stability.                                                   PH = pumped Hydro
                                                                 S-CP = supercapacitor
Additionally, the use of a decoupling reactance can              SMES = superconducting magnetic energy
have a great impact on voltage quality.                          storage

     3.5     Energy storage systems                              Figure 9 : Energy storage systems

There are numerous types of energy storage systems
based on different technologies and for various time       3.6     Short circuit current limitation
scales (Figure 9, [16]). All these storage systems don’t   The internal wind farm network can be sized to reduce
necessarily fit the specificities of wind energy and       to a certain extent the contribution to the fault current.
don’t necessarily help for the grid integration of wind    For instance, for a wind farm connected at HV level,
power.                                                     this could be achieved by increasing the reactance of
                                                           the MV/HV transformer up to 20%. However the
However, depending on the technology and on the            efficiency is limited and highly depends on the
rating of the energy storage systems, they can enable      location of the constraint with respect to the wind
to:                                                        farm.
- improve power quality,                                   Moreover current limiters can be used, either on the
- smooth the voltage and power variations due to           wind farm grid or on the network operator grid, to
    wind turbulences,                                      reduce the fault current [17]. They can limit:
- smooth power variations in a time scale of               - Peak short-circuit current in case of a response
    minutes, hours or even days,                               time in the order of 3 ms (IGBT or GTO with an
- solve steady state current constraints,                      appropriate control system, fuse, superconductor-
- participate in voltage and frequency control,                based devices),
- either help wind farms to withstand voltage dips,        - Short-circuit       breaking    current     (previously
                                                               mentioned devices, thyristor, fast short-circuit
                                                               breaker-based devices).
                                                                  NWPC’04, Goteborg, Sweden, March 1-4, 2004.
Some of these current limiters insert a reactance in
series in the network during the short-circuit to limit      2.   E.on Netz GmbH, “Grid Code, High and extra
the short-circuit current. Other limit or cut the short-          high voltage”, Bayreuth, Germany, August 2003
circuit current without need of a series reactance.               [Online]. Available: http://www.eon-netz.com/
Current limiters are manufactured for example by             3.   R. Belhomme and Chantal Corenwinder, Wind
ABB, Siemens, FERRAZ, Hitatchi, Mitsubishi                        Power Integration in the French Distribution Grid
Electric…                                                         -Regulations and Network Requirements,
Several WTGs with power electronics include directly              Proceedings of NWPC’04, Goteborg, Sweden,
semiconductor-based current limiters.                             March 1-4, 2004.
                                                             4.   R. Belhomme, C. Naslin, G. Beslin, N. Albrieux,
      3.7     Other devices                                       Wind farms and networks – Main technical
     3.7.1     Directional protections                            issues, Proceedings of International Conference
                                                                  on Power Generation and Sustainable
In case of a connection to a radial network, depending
                                                                  Development, Liège, Belgium, 8-9 October 2001.
on the feeder where the WF is connected, a directional
protection may be required to enable the proper              5.   Per Lundsager, Henrik Bindner, Niels-Erik
operation of the protection system.                               Clausen, Sten Frandsen, Lars Henrik Hansen and
                                                                  Jens Carsten Hansen, Isolated Systems with Wind
    3.7.2      Filters
                                                                  Power, Main Report, Risø National Laboratory,
Two types of filters may be used for wind farms :                 2001

-    Anti-harmonic filters : generally, these are passive    6.   T. Key, Wind Power Integration Technology
     filters. Each filter is designed to eliminate a range        Assessment and Case Studies, EPRI Technical
     of specific frequencies.                                     Report, 2004
-    Additional passive or active filters : they are         7.   EDF, GESER (Génération d’Eau chaude
     designed in order to prevent disturbances of the             Synchronisée avec les Energies Renouvelables,
     remote control signal that may be generated by the           Water heating synchronized with renewable
     wind farm.                                                   energies), newspaper « L’INDEPENDANT », 16
                                                                  may 2004
      4     Conclusion                                       8.   P. Taylor, Econnect, Increased renewable energy
The solutions that have been presented here for the               penetration on island power systems through
grid integration concern the main problems and                    distributed fuzzy load control, International
constraints encountered today. Whenever possible,                 Conference Renewable Energies for Islands, 2001
examples of actual implementation in wind farm
                                                             9.   I.A. Erinmez, on behalf of CIGRE WG B4.19,
projects were given. More specifically, the following
solutions have been considered:                                   Static Synchronous Compensator (STATCOM)
                                                                  for Arc Furnace and Flicker Compensation,
- development at the level of the wind turbine itself
                                                                  Electra N° 211, 2003, pp 58-65
    and the coupling system to the grid,
- combined operation of wind farms and the grid or           10. J. Vithayathil, on behalf of CIGRE WG B4.35,
    grid components (curtailment, coordination with              Thyristor Controlled Voltage Regulators : Non-
    other generation means, or with controllable                 conventional static Reactive Power Regulation,
    loads),                                                      Electra N° 212, 2004, pp 74-83
- solutions to improve reactive compensation and
    grid voltage control,                                    11. N. Reddy, American Superconductor, D-VAR
- solution to improve wind farm capability to                    and SuperVAR Technology Presentation, Step
    withstand voltage dips (“fault ride through”)                Group Meeting, San Diego, California, March
- energy storage systems,                                        2004
- short-circuit current limiting devices, …                  12. H. Yamamoto, Y. Miyazaki, Y. Noro, T. Ishizuki,
Development of solutions for wind farm grid                      Y. Sato, R. Shimada, Application of Rotary Phase
connection is a rapidly evolving field which implies             Shifter on the decentralized Power System, IERE
highly confidential aspects and an intense competition           Zurick Workshop, June 2002
between the different wind turbine and equipment
manufacturers.                                               13. Sigrid M Bolik, R&D department Vestas Wind
                                                                 Systeme, Grid requirements challenges for wind
                                                                 turbines, fourth international workshop on large-
                                                                 scale integration of wind power and transmission
References                                                       networks for offshore wind, October 20-21, 2003
                                                                 Billund, Denmark
1.    Julija Matevosyan*, Thomas Ackermann*, Sigrid
                                                             14. Nicholas W. Miller, GE power systems, Power
      Bolik** and Lennart Söder*, Comparison of
                                                                 system dynamic performance improvements from
      International Regulations for Connection of Wind
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    Billund, Denmark
15. Stefan Hartge Enercon GmbH, Volker Diedrichs
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    Billund, Denmark
16. I. Cruz, F. Avia, F. Arias, L.M. Arribas, R.P.
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    integration,  EWEC      2001,    July    2001,
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    limiters as an alternative to substation upgrade
    when there is a need to increase the available
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    workshop on large-scale integration of wind
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    wind October 20-21, 2003 Billund, Denmark