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`TSO _ Western Part ofDenmar-k
`
`Fourth
`
`International Workshop on
`
`Large—Scale Integration of Wind Power and
`
`Transmission Networks for
`
`Offshore Wind Farms.
`
`October 20-21, 2003
`
`in Billund, Denmark
`
`Proceedings
`
`Edited by:
`
`Jllllja Matevosyan, Thomas Ackermann
`Royal Institute of Technology.
`Electric Power Systems
`3
`Stockholm, Sweden
`
`I
`
`_.7__
`
`GE 2007
`Vestas v. GE
`|PR2018-01015
`
`GE 2007
`Vestas v. GE
`IPR2018-01015
`
`1
`
`

`

`GRID REQUIREMENTS CHALLENGES FOR WIND TURBINES
`
`Sigrid M. Boiik
`
`Research and Development ananement
`Vesta: Wind Systems AIS. E. F. lacobsensvej 7. DK - 6950 Ringltabing
`phone: +45 97301446. fart: +45 97301308. e—mail: sibgd vesmsdl:
`
`ABSTRACT: The produced electrical power from wind has dramaticall} increased in the last years.
`Therefore today's wind turbines. which apically are centralised in wind parks. have a significant influence
`on the power production. Network operators have to ensure well function power transmission and supply to
`their customers. For this reason. there exist different requirements in the disrribution and transmission net.
`which deal with controlled power production. connect/ disconnect requirements and behaviour during faults
`in the grid. Several Network operators have also introduced smcial grid connection requirements for wind
`turbines. which are mainly based on existing requirements for power plants. Tnese requirements are a chal-
`lenge for wind turbine producers. New hardware and control su-ategies have to be developed and existing
`control strategies have to be changed. The important changes are due to pitch control. power control, grid
`connection and error handling. The changes should not afieCt the already existing advantages of the Wind
`turbines control Strategies.
`The paper contributes a discussion of the existing grid codes and furthermore a control strategy developed
`by Vectas is presented.
`
`Keywords: wind power generation. grid node. “ind turbine control
`
`1. INTRODUCTION
`
`the cost per
`in addition.
`the market demands.
`meet
`kilowatt should be low. The market for wind training
`
`it takes typically three to five years for the develop-
`ment ot'a new wind turbine from design to sale. As ex-
`ample the Vestas products over the past years are
`shown in Figure 1.
`A wind turbine is a state of the an product. which musr
`
`and the demands imposed on the wind turbines. are
`changing rapidl) and are usually only satisfied by next
`technical
`solutions. Tn:
`den-lands
`due
`to
`grid
`connection are especiall)
`challeng’ng. Riemann”)
`there were no demands. because the contribution into
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`right law (Title 17 U.S. Code)
`
`2
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`

`

`insignificant. Today wind
`the electrical grid was
`turbines are typically connected in wind farms or parks.
`Their power contribution to the grid is similar to a
`power plant. which is connected to the distribution or
`to the transmission net. For every grid exist connection
`requirements
`(grid
`codes.
`connection
`codes
`or
`distribution codes]
`to ensure a save operation and
`fulfilment of quality demands. These requirements are
`besides juristic terms concerning frequency. voltage.
`power production and many others characteristics of
`the system.
`Unfortunately are wind farms only similar to conven-
`tional power plants. Unlike power production from nu-
`clear power plants or coal power plants. where the pos-
`sible power production is dependent on the mass of raw
`material and the consumed power. wind turbines are
`dependent on wind forecasting to guarantee a predicted
`power production. This forecasung of power produc-
`tion from wind turbines is not a simple task. A wind
`turbine has thougr some advantages compared to a
`conventional power plant.
`However typical demands for the generating units do
`not meet wind
`turbine
`characteristics.
`Special
`connection conditions have to be discussed. A few
`
`countries are already working on grid codes especially
`for wind turbines or wind parks or a modification of the
`existing code. which
`also meets wind
`turbine
`characteristics 6/. Figure 2 is visualising the grid code
`development compared to the turbines development
`and approximate turbine development time.
`
`
`
`Figure 2 Wind Turbine development time and grid code
`demands
`
`Regrettably not all countries, where wind turbines are
`installed have specific or modified grid codes. This
`makes development of future wind turbines very diffi-
`cult. There is a big risk involved in developing new
`techniques to fulfill unspecified demands.
`An example how to tackle the grid requirements is the
`Vestas wind turbine V80. The V80 turbine is capable to
`expand their functionality to fulfill the most common
`grid requirements. One possible solution. which will be
`discussed later in this paper, is the system which was
`developed and introduced in the offshore park at
`“Horns Reef" in Denmark with a total capacity of 160
`MW.
`
`2. TO BE OR NOT TO BE -
`GRID CONNECTION DEMANDS
`
`of the existing grid codes it appears that there are very
`different
`in the most
`important characteristics. An
`example is the demand on the voltage tolerance of a
`system. Where in Australia the grid connected unit has
`to sustain 0% voltage for l75ms in Denmark it is only
`25% voltage for 100ms without disconnecting from the
`grid (see Figure 5). There is a huge difference for the
`turbine. if there is still some voltage available or not.
`However a voltage drop is always a very critical
`condition, because of c.g. the rising current, which can
`cause serious damage on the system. Usually the
`turbine gets disconnected to protect
`itself. A similar
`difference is
`seen in the active —
`reactive power
`capability and frequency fluctuation of the grid. (see
`Figure 3 and Figure 4)
`
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`Figure 3 active power (P) — reactive power (Q) de-
`mands
`
`typically
`The active — reactive power capability is
`described in one point.
`for a static reactive power
`exchange with the grid during 100% power supply. But
`why is the demand only for static condition and why
`only at one point? The answer lies in the hismry.
`
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`The most wind turbine producers today deliver wind
`turbines to an international marked and are faced with a
`
`various number of grid requirements. Looking at a few
`
`Figure 4 Frequencyfluctuations
`
`3
`
`

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`Germany ENE 1.8.2003
`Denmark
`
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`
`
`
`Figure 5 Voltage tolerance curves
`
`The connection demands. which are also base for draft
`
`versions of wind turbine connection requirements, are
`primary made for integration of "generating units" into
`the grid. Those “generating units" usually produces the
`full power after connecting to the grid and are slow in
`decreasing or
`increasing the power. The powar
`production of a wind farm can vary between 0 and
`100%. depending on the wind reserve and type.
`Because of the nature of wind power production from
`fluctuating wind. wind turbines have a fasr power
`control. Wind farms therefore are capable in a short
`time to adjust power production.
`Historical power plants or “generating units“ have big
`synchronous generators. Direct grid connected syn-
`chronous machines have stability problems due to load
`changes and frequency changes. Therefore an impor-
`tant issue is to operate in a wide range of frequencies
`(see Figure 4). A frequency change in a grid can be
`caused by loss of transmission and sudden demand
`changes or some problems of bigger power plans [loss
`of generation), which normally stabilise the grid fre-
`quency. Wind turbines with induction generators have
`no problem with asynchronous operation. Frequency
`operation range is more an issue of control strategy. A
`big frequency operation range for wind turbines and
`frequency control is therefore only interesting. if the
`net operator uses the wind farm for stabilising the grid
`frequency. Though wind turbines are able to tolerate
`big frequency changes they have difficulties to support
`frequency supporting operation in a wide range. hence
`the inertia in the system is very small. Anyhow it is a
`chance to include big wind farms into superior grid
`control, because of the fast control possibilities.
`For example it
`is possible to use the wind turbines
`reactive power capability for compensation or support
`of the grid in situation, where no active power is
`needed. In Figure 3 the possibility of the V80 to supply
`
`reactive power is shown. Hence Vestas is changing to
`star connection in small power
`ranges the reactive
`power capability is less as Well. But it
`is possible to
`stay in delta connection with a loss of some efficiency.
`Even the wind turbine is in smaller power capable of an
`excellent power factor control it will cost extra money
`to expand the range. especially at
`rated power to
`generate reactive power.
`The in Figure 3. Figure 4. Figure 5 visualised data
`can be found in Table 1. This table show some chosen
`
`demands from the grid codes with the purpose of
`comparison the mosr
`important demands. The last
`column shows data for a standard V80 turbine with the
`
`modifications towards technical solutions developed
`for the “Horns Reef' project.
`But what is important. missing. or improvable in grid
`codes for wind turbines?
`
`0
`
`.
`
`-
`
`I
`
`0
`
`Turbine should start automatically with a pre-
`defined time delay and power ramp
`Turbine should stop automatically with a pre-
`defined disconnection procedure (high wind
`or fault} and power ramp
`The continuous frequency range should be
`defined.
`
`synchronous
`A distinction between
`asynchronous generator should be made.
`rated
`A dynamical
`description
`during
`operation
`and
`fault
`situations
`regarding
`voltage, active power and reactive power
`should be made.
`
`and
`
`The new grid connecuon demands from Germany l5!
`have already included a lot of these points. Even they
`are not the best suited demands for wind turbines a big
`effort has been made to describe the expected behav-
`iour.
`
`4
`
`4
`
`

`

`3. CONTROL STRATEGIES
`
`A good example how to include wind farms control
`into the superior control can be shown in the “Horus
`Reef" wind farm.
`in this farm many of the lisued
`demands are taken into account. Especially challenging
`were the demands during grid fault. Vestas developed a
`feature, which is specially designed to tolerate short
`time voltage reductions (fault-ride—through capability).
`The grid support option limits the Lime before the
`turbine resumes pre-fault power production and limits
`the risk of voltage collapse due to a very low reactive
`power draw after the clearance of the grid fault. The
`method takes advantage of the benefits of having a
`pitch controlled variable speed wind turbine. The
`doubly fed induction generator fDFlG) is operated with
`variable speed and active and reactive power of the
`generator is controllable via the back to—back Vestas
`Convener System. The
`turbine will
`always
`be
`connected to the grid. even at 0 voltage for 200ms
`(Figure 5). To ensure a safe operation point the pitch
`system is optimised to keep the turbine within normal
`speed condition and a UPS backup system secures
`power to the essential units.
`In order to use the method. new hardware and control
`strategies had to he developed and existing control
`strategies had to be changed. The important changes
`are due to:
`
`0
`.
`
`-
`-
`-
`
`Pi tch control
`Power control
`
`Grid connecrion (Synchronisation)lDisconnection
`Error handling
`Backup
`
`the control
`simulated example for
`in Figure 6 a
`algorithm implemented in a V66 turbine is shown.
`After an occurrence of an error in the grid (such as a
`sudden
`voltage
`drop)
`the
`following
`steps
`are
`performed:
`
`1.
`
`2.
`
`The stator of the generator is disconnected from
`the grid
`The pitch reference is set to a no-load angle 90,
`where the rotor power is 0. 60 is continuously
`calculated based on the wind speed. generator
`speed and the Cp-curves.
`3. When the actual pitch position is close to 60 the
`controller is switched to speed control. The
`reference speed is set equal to the speed
`immediately before the error (although limited to
`the max. static speed).
`4. When the grid error is gone the normal procedure
`for connecting the Wind turbine to the grid is
`started.
`
`
`
`Figure 6 Contra.r example simulation (V66). 75 MW.
`wind speed I 0 m/s}
`I) grid error stare -— characterise the voltage drop in the grid,
`2) pitch angle [deg]. 3} generator speed {rpm}, 4) active
`power {kW}
`
`two
`during
`3 measurement
`shower
`7
`Figure
`subsequently occurring grid faults. Thereby the same
`control
`steps
`like in
`the
`simulated example are
`performed.
`
`
`
`Figure 7 Measured wind turbine response (V804 M W,
`wind speed 7 m/s)
`1) grid nus voltage {V}. 2) wind speed. 3lpr'rch angle [deg].
`4) generator speed [rpm]. 5) active power {kW}
`
`5
`
`

`

`Hc'ichstspannung, December 2001 (Germany)
`f5! EON Netz. Netzamchlmsregefn Bach- and
`Hochstspannung, August 2003 (Germany)
`lfil Eltra, Elitrafl System, Vindmoller rilrfurtet net med
`spandt'nger under 100 kV, Draft. May 2003
`(Denmark)
`I'll NECA. National Electricity Code, Version 1.0
`Amendment 8.0. 1999 - 2003 (Australia)
`1'8! Thomas Kruger. Jens Birk, Merode til konrrol af
`vindturbine under perioder med ajbrydelse of e!-
`nerret. Patent DK 174-411 Bl. February 2003
`{9/ Vestas intemai Documentation
`
`[10/ Sigrid M. Bolik. Jens Birl:1 Bjorn Andresen, Joh
`G. Nielsen, Vestas Handles Grid Requirements:
`Advanced Control Stradegyfor Wind Turbines.
`EWEC'03. Juni 2003, Madrid. Spain
`
`'
`
`4. CONCLUSION
`
`There exist a lot of different gn'd codes all over the
`world. All wind turbine producers have a numbered
`product range. which should be able to fulfil as many
`grid requirements as possible. With the lack of con-
`fotrnability wind turbines will be designed for the
`codes for the biggest marked and the outer limits. This
`is limiting the prize reduction of wind turbine technol-
`ogy. Therefore a close working relationship between all
`grid operators. costumers and wind turbines producers
`is required to find acceptable demands in the near fu-
`ture.
`
`5. APPENDDC
`
`REFERENCES
`
`fl/ SP Transmission & Distribution, Scottish Hydro
`Electric, Guidance Narefor the connection of
`Wind Farms, Issue No. 2.1.4, December 2002
`(Scotland)
`12! ESE National Grid. Wind Farm Connection Re—
`
`quirements. Draft Version 1.0, February 2002 (Ire-
`land)
`[3/ EON Netz. Ergdnzende Netzanschlussregelnfiir
`Windenergieanlagen, December 2001 (Germany)
`[4/ EON Netz. Netzanschlmsregeln Hoch- and
`
`Table 1: Grid Codes Overview
`
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