**A POWER REGULATION SCHEME FOR A CONVERTER FEEDING INTO AN ASYMMETRICAL GRID**

JP3697273 | ELECTROMAGNETIC PUMP DEVICE |

JPH0568375 | CONTROL METHOD FOR AIR-CONDITIONER |

JPH06205585 | THREE PHASE AC-TO-DC CONVERTER |

BO, Yin (Blk 417 Ang Mo Kio Ave 10#02-1009, Singapore 7, 56041, SG)

DENG, Heng (Lorong Selangat Sommerville Road, Singapore 5, 35872, SG)

LARSEN, Kim, Bo (Søhøjvej 18, Hadsund, DK-9560, DK)

BO, Yin (Blk 417 Ang Mo Kio Ave 10#02-1009, Singapore 7, 56041, SG)

DENG, Heng (Lorong Selangat Sommerville Road, Singapore 5, 35872, SG)

*;*

**H02M5/45***;*

**H02M1/084***;*

**H02M5/452**

**H02M5/458**US20070108877A1 |

BO YIN ET AL: "An Output-Power-Control Strategy for a Three-Phase PWM Rectifier Under Unbalanced Supply Conditions" IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, IEEE SERVICE CENTER, PISCATAWAY, NJ, USA, vol. 54, no. 5, 1 May 2008 (2008-05-01), pages 2140-2151, XP011204606 ISSN: 0278-0046 cited in the application

YONGSUG SUH ET AL: "A nonlinear control of the instantaneous power in dq synchronous frame for PWM AC/DC converter under generalized unbalanced operating conditions" CONFERENCE RECORD OF THE 2002 IEEE INDUSTRY APPLICATIONS CONFERENCE. 37TH IAS ANNUAL MEETING . PITTSBURGH, PA, OCT. 13 - 18, 2002; [CONFERENCE RECORD OF THE IEEE INDUSTRY APPLICATIONS CONFERENCE. IAS ANNUAL MEETING], NEW YORK, NY : IEEE, US, vol. 2, 13 October 2002 (2002-10-13), pages 1189-1196, XP010610031 ISBN: 978-0-7803-7420-1 cited in the application

HONG-SEOK SONG ET AL: "Dual Current Control Scheme for PWM Converter Under Unbalanced Input Voltage Conditions" IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, IEEE SERVICE CENTER, PISCATAWAY, NJ, USA, vol. 46, no. 5, 1 October 1999 (1999-10-01), XP011023580 ISSN: 0278-0046 cited in the application

CLAIMS: 1 A method of controlling a power converter to deliver an amount of active power and an amount of reactive power to a three-phase grid, said active and reactive power having a given power factor and a minimized power ripple, said method comprising: providing a wind-powered multi-phase generator, providing an AC-AC converter operating in a puise-width-modulation (PWM) mode, said AC-AC converter having a set of converter input terminals connected to said three-phase generator and a set of converter output terminals connected via a converter impedance to a set of grid input terminals of said three- phase grid, providing a contro! unit comprising a measurement unit for measuring current and voltage and a microcontroller running a control algorithm, and by performing the following steps. measuring the current and/or the voltage on said converter output terminals and/or on said grid input terminals by using said measurement unit and representing the current and/or the voltage by a current value and a voitage value respectively, generating a current reference by feeding said current value and/or said voitage value into said microcontroller, said current reference value corresponding to said amount of active power and said amount of reactive power having said given power factor and said minimized power ripple, and regulating the current on said grid input terminals to correspond to said current reference value by using said AC-AC converter 2 The method according to claim 1 , wherein said current value and/or said voltage value is/are measured/estimated on said grid input terminals. 3 The method according to claim 1 , wherein said current value and/or said voitage value is/are measured/estimated on said converter output terminais 4 The method according to claim 1 , wherein said current value and/or said voltage value is/are measured/estimated on said converter output terminals and on said grid input terminais. 5. The method according to any of the previous claims, wherein said current reference is derived from the following equation: 6. The method according to any of the claims 1-2, wherein said current reference value is calculated according to where 7. The method according to any of the claims 1 or 3, wherein said current reference value is calculated according to wher 8, The method according to any of the claims 1 or 4, wherein said current reference value is calculated according to where and where 9, The method according to any of the claims 1 or 4, wherein said current reference value is calculated according to and 10. The method according to any previous claim, wherein said control algorithm comprises a dual current control or alternatively a P+resonant controller or alternatively an iterative learning controller or yet alternatively a repetitive current controller. 1 1. The method according to any of the previous claims, wherein an active power, a reactive power and a power ripple is calculated by using said current value and/or said voltage value. 12. A system for controlling a power converter to deliver an amount of active power and an amount of reactive power to a three-phase grid, said active and reactive power having a given power factor and a minimized power ripple, said system comprising: a wind-powered three-phase generator, an AC-AC converter operating in a pulse-width-modulation (PWM) mode, said AC-AC converter having a set of converter input terminals connected to said three-phase generator and a set of converter output terminals connected via a converter impedance to a set of grid input terminals of said three-phase grid, a control unit comprising a measurement unit for measuring the current and/or the voltage on said converter output terminals and/or on said grid input terminals and representing the current and/or the voltage by a current value and/or a voitage value, respectively, and a microcontroller running a control algorithm generating a current reference corresponding to said amount of active power and said amount of reactive power having said given power factor and said minimized power ripple by feeding said current value and/or said voltage value into said microcontroller, said control unit regulating the current on said grid input terminals to correspond to said current reference value by using said AC-AC converter. 13 A converter system according to claim 12, further comprising any of the features of claim 2-1 1 14, An AC-AC converter comprising a set of electronic switches controlled by a control unit, said control unit comprising: a measurement unit for measuring the current and/or the voitage on one or more converter output terminals and/or on one or more grid input terminals and representing the current and/or the voltage by a current value and/or a voltage value respectively, and a microcontroller running a control algorithm generating a current reference corresponding to a specific amount of active power and a specific amount of reactive power having a given power factor and a minimized power ripple by feeding said current value and/or said voltage value into said microcontroller, said microcontroller regulating the AC-AC converter to produce a current corresponding to said current reference value. |

TECHNICAL FIELD OF THE INVENTION

Wind turbines are used to convert wind energy to electrical energy in a clean and efficient way. The electrical energy is typically supplied to an electric power transmission grid. Unlike "classical" power generation systems such as hydro electric systems, wind turbines typically operate with generators which are not able to support the grid in case of asymmetric grid voltages.. In the last decade, the worldwide penetration, i.e. installed capacity, of wind turbines for power generation has increased remarkably. As the penetration increases, the international standards are being oriented towards considering the wind turbines as "classical" generation systems, which should be able to support the grid when necessary.. This has developed a need for methods and systems for allowing wind turbines to operate under asymmetrical grid conditions.

Asymmetric grid voltages are a result from imbalances between the phases in a polyphase electric system. This phenomenon is quite common, in particular for weak AC systems The reason for the imbalances between the phases may be e.g. a load imbalance between the phases, asymmetric transmission line impedance or a fault such as a short circuit occurring in the grid. Asymmetrical grid voltages will typically yield a power ripple having a frequency twice the line frequency if the current is not properly controlled

BACKGROUND OF THE INVENTION

Technologies for instantaneous power regulation using a power converter has been described in "An output-power-control strategy for a three-phase PWM rectifier under unbalanced grid conditions" by Yin Bo et al., which has been published in IEEE Transactions on industrial electronics, vol. 55, No. 5 May 2008 In the above publication, a technology has been described where a PWM rectifier has been used to maintain a constant DC output voltage for a resistive load by drawing a certain amount of additional ripple power from the grid under unbalanced sinusoidal grid input voltage conditions. The above publication does not, however, describe any technology where power generated by a wind turbine, which is fed by a varying wind speed most of the time, is transferred to the grid under an unbalanced sinusoidal grid input voltage condition by maintaining constant DC input / DC-link voltages.

Other relevant prior art documents include H. S. Song and K. H. Nam, "Dual current control scheme for PWM converter under unbalanced input voltage conditions," IEEE Trans, on Industrial Electronics, vol 46, pp. 953-959, 1999, Yongsug Suh, Valentin Tijeras, Thomas A. Lipo. "A Nonlinear Control of the Instantaneous Power in dq Synchronous Frame for PWM AC/DC Converter under Generalized Unbalanced Operating Conditions" in Proceedings of the Industry Applications Conference, 2002. Pages: 1 189 - 1196 vol.2 and US 2007/0108771 A1. All of the above-mentioned publications are hereby incorporated in the present application by reference.

It is therefore an object according to the present invention to provide a method and a system for regulating the power of a full-scale converter-connected wind turbine under asymmetrical voltage conditions.

The above need and the above object together with numerous other needs, objects and advantages which will be evident from the below detailed description are according to a first aspect of the present invention obtained by a method of controlling a power converter to deliver an amount of active power and an amount of reactive power to a three-phase grid, said active and reactive power having a given power factor and a minimized power ripple, said method comprising: providing a wind powered multi-phase generator, providing an AC-AC converter operating in a pulse-width-modulation (PW M) mode, said AC-AC converter having a set of converter input terminals connected to said multi-phase generator and a set of converter output terminals connected via a converter impedance to a set of grid input terminals of said three- phase grid, providing a control unit comprising a measurement unit for measuring current and voltage and a microcontroller running a control algorithm, and by performing the following steps: measuring the current and/or the voltage on said converter output terminals and/or on said grid input terminals by using said measurement unit and representing the current and/or the voltage by a current value and a voltage value, respectively, generating a current reference by feeding said current value and/or said voltage value into said microcontroller, said current reference value corresponding to said amount of active power and said amount of reactive power having said given power factor and said minimized power ripple, and regulating the current on said grid input terminals to correspond to said current reference value by using said AC-AC converter

Although a wind turbine theoretically may be fed directly to the grid, typically an AC- DC to DC-AC converter operating in a pulse-width-moduiation (PWM) mode is employed between the generator and the grid. With a suitable DC output voltage, both the power and the current flowing in both the AC-DC and the DC-AC converter may be controlled using a control system with a proper control algorithm The current may be regulated such that it follows the sinusoidal asymmetry of the current reference. The current reference is preferably chosen to correspond to a specific power, which may be held constant over a period of time The voltages may be measured and used for current reference generation, in order to obtain a given power factor for a specific active power delivery, the current reference may be derived with measured or calculated voltages using the method according to the first aspect of to the present invention.

By applying the method according to the first aspect of the present invention, the generated wind power may be transferred to the grid instantaneously with a desired power factor, and the DC output voltage is maintained constant such that both the AC-DC and the DC-AC converter may perform well There are many ways to provide active power reference in a wind power system The active power value used for current reference generation may be the output from the DC output controller as will be further described in connection with fig 3 The active power value may be obtained from the rotor speed control Alternatively, a power plant control unit may give the active power value. Such power plant control units are typically available and further used for grid frequency control purposes

In a further advantageous embodiment according to the first aspect of the present invention, the specific power factor is substantially equal to unity Near unity power factor means that the reactive power is low in relation to the active power, The control algorithm may in this case be fed with the value zero to achieve a substantially unity power factor at the grid input terminals This means pure active power is delivered to the grid.

In a further advantageous embodiment according to the first aspect of the present invention, the specific power factor is substantially equal to the power factor required by the grid. By adjusting the power factor to be equal to the power factor required by the grid, reactive power compensation can be achieved and the wind turbine may be able to support the grid

In a further advantageous embodiment according to the first aspect of the invention, the current value and/or the voltage value is/are measured/estimated on the grid input terminals. Using the measured/estimated voltage on the grid input terminal, a current reference may be created for a specific active power, having a specific power factor required by the grid, to be transferred to the grid. By regulating the current to follow the current reference, a constant instantaneous power may be achieved on the converter output termina!.

In a further advantageous embodiment according to the first aspect of the invention, the current value and/or the voltage value is/are measured/estimated on the converter output terminals Using the measured/estimated voltage on the converter output terminal, a current reference may be created for a specific active power to be transferred having a specific power factor. By regulating the grid current to follow the current reference, a low power ripple may be achieved on the converter output terminal

In a further advantageous embodiment according to the first aspect of the invention, the current value and/or the voltage value is/are measured/estimated on the converter output terminals and the grid input terminals Using the measured/estimated voltage on both the converter output terminal and the grid input terminal, a current reference may be created for a specific active power to be transferred having a specific power factor. By regulating the grid current to follow the current reference, a low power ripple may be achieved on the converter output terminal.

In a further advantageous embodiment according to the first aspect of the invention, an active power, a reactive power and a power ripple can be expressed by using the current value and/or the voltage value The control algorithm is preferably fed with an active power, a reactive power and a power ripple as reference signals, The current reference can be calculated by solving the matrix equations as shown below in the description.

In a further advantageous embodiment according to the first aspect of the invention, the power ripple is nullified. With power ripple is meant the active power ripple stored in the converter inductance. It is preferably minimized, i e. the control algorithm may be fed the value zero.

In a further advantageous embodiment according to the first aspect of the invention, the current reference is derived from the following equation:

The current reference can be calculated by solving the generalized matrix equation shown above. The solution will be provided later in the description.

In a further advantageous embodiment according to the first aspect of the invention, the current reference value is calculated according to

where

Here, denotes the d-axis and the q-axis positive sequence components of the grid voltage in the positive sequence synchronously rotating frame (SRF), and d-axis and q-axis negative sequence components of grid voltage in the positive sequence SRF By calculating the reference current signals according to the above scheme, the power on the grid input terminal may be held constant, and average zero reactive power operation may be achieved

In a further advantageous embodiment according to the first aspect of the invention, the current reference value is calculated according to

where

By calculating the reference current signals according to the above scheme, the power ripple on the grid input terminal may be minimized,

In a further advantageous embodiment according to the first aspect of the invention, the current reference vaiue is calculated according to

where

and

" ) where

Here denotes d-axis and q~axis positives sequence components of the converter output voltages in the positive sequence synchronously rotating frame (SRF) and enotes d-axis and q-axis negative sequence components of the converter output voltage in the positive sequence SRF

Here, the converter output voltage signals can be estimated either from the DC output voltage together with the switch functions or from the grid AC voltages together with the inductor voltages.

By calculating the reference current signals according to the above scheme, the power on the grid input terminal may be held constant and at the same time the power ripple may be minimized,

In a further advantageous embodiment according to the first aspect of the invention, the current reference value is calculated according to

and

By calculating the reference current signals according to the above scheme, the power on the grid input terminal may be held constant, the power ripple may be minimized and the power factor may be adjusted to the power factor of the grid

The current reference may be different from the above if a different reactive power definition is used

In a further advantageous embodiment according to the first aspect of the invention the control system comprises a dual current controller or alternatively a P+resonant controller or alternatively an iterative learning controller or yet alternatively a repetitive current controller The dual controller is based on the Pl controller for continuously correcting any difference between the reference value and the measurements All the above methods are preferably implemented on a digital microcontroller.

The above need and the above object together with numerous other needs, objects and advantages which will be evident from the below detailed description are according to a second aspect of the present invention obtained by a system for controlling a power converter to deliver an amount of active power and an amount of reactive power to a three-phase grid, said active and reactive power having a given power factor and a minimized power ripple, said system comprising: a wind-powered multi-phase generator, an AC-AC converter operating in a pufse-width-modulation (PWM) mode, said AC-AC converter having a set of converter input terminals connected to said multi-phase generator and a set of converter output terminals connected via a converter impedance to a set of grid input terminals of said three-phase grid, a control system for measuring the current and/or the voltage on said output terminals and/or on said grid input terminals and representing the current and/or the voltage by a current value and/or a voltage value, respectively, and a microcontroller running a control algorithm generating a current reference corresponding to the specific power factor by feeding the current value and/or the voltage value into the control algorithm, the microcontroller regulating the AC-AC converter to produce the current reference value.

The above system may preferably be used together with any of the previously described methods The AC-AC converter preferably comprises an AC-DC to DC- AC converter comprising a number of electronic switches The converter impedance is used to suppress high frequency oscillations resulting from the switching operations of the electronic switches. The control system preferably includes a current measurement device and a voltage measurement device, such as a current transformer or Hall type sensor and a voltage transformer, respectively The measured current and voltage values may be used as input values for a microcontroller running a control algorithm The control algorithm may cafculate a current reference value by using e g. the power factor, which may be calculated from the current and voltage values.

The above need and the above object together with numerous other needs, objects and advantages, which will be evident from the below detailed description, are according to a third aspect of the present invention obtained by an AC-AC converter comprising a set of electronic switches controlled by a control system for measuring the current and/or the voltage on one or more converter output terminals and/or on one or more grid input terminals and representing the current and/or the voltage by a current value and/or a voltage value, respectively, a microcontroller running a control algorithm generating a current reference corresponding to a specific power factor by feeding the current value and/or the voltage value into the control algorithm, the microcontroller regulating the

AC-AC converter to produce a current corresponding to said current reference value

The above AC-AC converter is preferably used together with the previously described methods and systems The electronic switches may comprise e g IGBTs, GTOs or thyristors The microcontroller may control the electronic switches to achieve an output current equal to the calculated current reference value

It is evident that numerous variations of the system and method described above may be realized A detailed description of the figures of a presently preferred embodiment of the invention follows below.

The invention will now be further described by reference to the drawing, in which

fig 1 shows a basic topology of a full-scale converter-connected wind turbine 10, fig. 2 shows a zoomed view of the grid-connected converter, and fig 3 shows a block diagram view of a control system

Fig. 1 shows a basic topology of a full-scale converter-connected wind turbine 10 The wind turbine 10 is typically mounted in an elevated position such as on top of a tower due to the faster wind speed at higher altitudes The wind turbine 10 comprises a drive shaft 12 connected to one or more adjustable blades 14 Any number of blades 14 may be used, but typically three blades 14 are used. The blades 14 are preferably made of fiberglass or any other light and rigid material The wind turbine 10 may be rotated to allow the wind to strike the blades 14 in a substantially perpendicular direction The pitch of the blade 14 may be adjusted to increase or reduce the amount of wind energy captured by the blade 14 With pitch is understood the angle of which the wind strikes the blades 14 The drive shaft 12 is connected to the rotor of a generator 18 via a gear box 16 The gear box 16 steps up the low rotational speed of the drive shaft to a higher speed more suitable for the generator 16. For mechanical reasons, the rotational speed of the drive shaft 12 is typically in the range of 10-20 revolutions per minute The rotational speed of the generator 18 is typically significantly higher The most efficient rotational speed of the generator 18 is depending on the interna! characteristics and type of the generator 18 and may vary according to the number of poles of the generator 18 It is, however, possible to exclude the gear box 16 by using an appropriate multi-pole generator 18 suitable for a slow rotational speed.

To allow a variable speed of the drive shaft 12, an AC-AC converter 22 is included between the output terminals 20 of the generator 18 and the grid input terminals A The converter comprises a stator-connected converter 24 operating as an active pulse-width-modulated (PWM) rectifier comprising six electronic switches S _{1 }, S _{2 }, S _{3 }, S _{1 } ^{' }, S _{2 }', S _{3 }', The stator-connected converter 24 rectifies the AC from the generator to DC, which in turn feeds a DC link 26 The DC link 24 includes a capacitor C The capacitor C is used for smoothing the ripple otherwise occurring on the DC link 26 due to the switching operations The DC link 26 feeds the grid-connected converter 28 operating as an inverter The grid-connected converter 28 as well comprises six electronic switches S _{1 } , S _{2 }", S _{3 }", S _{1 } ^{'" }, S _{2 }'", S _{3 }" ^{1 } A controlling unit is controlling the individual eiectronic switches. The electronic switches S _{1 }, S _{2 }, S _{3 }, S _{1 } ^{' }, S _{2 }', S _{3 }', S _{1 } ^{" }, S _{2 }", S _{3 }", S _{1 } , S _{2 }'", S _{3 }'" may preferably comprise power semiconductor switches such as e g. IGBTs, GTOs, or thyristors The grid-connected converter may thus be used to provide a constant power to the grid The controlling unit is connected to a measurement unit, measuring the current and voltage on the grid input terminals A The grid choke 30 or alternatively converter impedance is located between the grid- connected converter 28 and the grid input terminals A The grid choke 30 consists of a converter inductance L and a converter resistance R The grid input terminals A are directly connected to the grid {not shown) The nominal frequency of the grid is 50Hz The instantaneous frequency at any time may vary a few percent in either direction of the nominal frequency

Fig 2 shows a zoomed view of the grid-connected converter 24, the capacitor C _{( } the converter resistance R and the converter inductance L If the generator has symmetrical three-phase windings and generates a ripple-free power for a given wind speed, constant power with sinusoidal line current and a substantially unity power factor would be delivered to the grid even under an asymmetrical grid voltage. Assuming the converter 24 is ideal, the instantaneous power at the DC side p _{dc } will be equal before and after the semiconductor switches P _{T }, i.e. P _{dC }=Pτ Ideal in the present case means at given time the semiconductor switches are behaving either like perfect conductors having no ohmic losses or as perfect insulators having an infinite resistance.

The instantaneous power balance at the DC side p _{dc } may thus be written as follows:

0)

where Pi _{n } representing the grid power and p _{L } representing the power in the inductance L. In a symmetric system with a three-phase balanced line current, the instantaneous power in the inductors, p _{t }, is zero.. However, in an asymmetric system, PL is not equal to zero due to the ripple power. When grid voitages are sinusoidal but asymmetric, each term in (1) will have three portions as indicated below:

(2)

where P _{z } is the constant portion of the instantaneous power p _{z } and P _{zc }, P _{25 } are coefficients of the second order harmonic power ripple power varying with cos2ωt and respectively. The subscript z may denote any of the symbols 'in', T, and 'L' from (1).

Therefore, the three power relationships below follow from (1 ):

For controlling power flow in the grid-connected converter 24, the average active power, average reactive power and instantaneous power ripple may be used either at the grid input terminals A or the converter output terminals B. in case both grid voltages and grid currents are sinusoidal and asymmetrical, the instantaneous active power p _{z } at any of the points of interest is given in (4) below,

where i _{a }, i _{b } and i _{c } represent the instantaneous input currents, and x _{a }, X _{b } and x _{c } represent the instantaneous voltages, x may stand for either the grid input voltage e, the converter output voltage V _{n }, the voltage across the converter inductance or the voltage across the converter resistance R _{1 } which is the parasitic resistance of the converter inductance L. The values may be averaged over a switching cycle,

For analysis and controller design purposes it would be more convenient to transform the three-phase variables into a stationary α-β frame (SF) or a synchronously rotating d-q frame (SRF). x _{α }, Xβ and i _{α }, i _{j }j then represent the voltages and currents projected onto the α-axis and β-axis, respectively, in SF, X _{d }, x _{q } and i _{d }, i _{q } represent the voltages and currents projected onto the d-axis and q-axis, respectively, in SRF.

where xf and if represent the positive sequence voltage and current vectors in the

SF, and

where nd represent the negative sequence voltage and current vectors in SF. Two different reactive power definitions may be used for reactive power calculation, either the conventional reactive power definition or the instantaneous reactive power definition

The conventional reactive power of a three-phase system is defined as follows:

(7)

wher

The instantaneous reactive power of a three-phase system is defined as follows:

Averaging q _{x }' in one fundamental cycle, the average reactive powers have the following relationship:

(9)

Substituting expressions of the positive- and negative- sequence voltage and current vectors into (7) and (9), the following equation system (10) in matrix form may be derived.

where the first equation determines the active power P _{z } delivered at the terminal z, the second and third equation determines the power ripple at the terminal z and the fourth equation determines the reactive power at the terminal z

In a generalized power regulation scheme, the second, third and fourth equation may be set to zero to nullify the power ripple and the reactive power at the terminal z. fn the fourth equation, either the instantaneous reactive power q _{z } or alternatively the average reactive power q' _{z } may be used. Both alternatives are shown in (10) separated by a comma.

It may be possible to use other reactive power definitions in the fourth equation in (10) for current reference generation.

The current commands may be obtained by solving (10) The generalized power regulation scheme (10) operating in a grid-connected converter will deliver constant power to the grid at a near unity power factor under asymmetrical grid voltages e

The second and third voltage equations in the matrix ( 12) may be easily replaced by switching functions due to the relationship x=uv _{dc }/2. In this case, the power variables on the left hand side of (10) will be changed into output current variables. In other words, instead of using x as the variable in the matrix of (10), switching functions u may be used to ensure a constant DC output current The changes do not appear to offer significant additional advantages. Such schemes are essentially the same as the power regulations schemes discussed here and hence they are not further addressed here

In a first control scheme of a first embodiment according to the present invention, the equation system (10) may be solved for the grid input terminals, i e. z representing the grid input terminals A and x representing the grid voltages e. P-, _{nQ } and P _{ins } are nullified at the grid input terminals A thus ensuring a constant instantaneous power The average reactive power is regulated to zero at the grid input terminals to obtain unity vector power factor Inserting the grid voltages e in (10) yields the following equation system (1 1a)

(11a)

Solving for the current commands, i _{d } ^{p } , iζ , i _{d }" and i" , the foϋowing solution to equation system 1 1a may be derived:

where

The first proposed power control scheme will ensure a constant power at the grid input terminals A with a sinusoidal line current at a substantial unity power factor. However, the instantaneous power ripple in the inductor L is not equal to zero and may exchange with the dc-iink capacitor C. This may result in a small voltage ripple on the dc-iink voltage ven when using a large capacitor C

In a second power control scheme according to a second embodiment of the present invention, the ripple power is nullified at the converter output terminals B for obtaining constant power p _{D }c _{» }- In the second proposed control scheme, z denotes the converter output terminals B and x denotes converter output voltages v _{n } in eq. ( 10).

The current commands, i _{d } ^{p } , i% , i _{d }" and i _{q }" , satisfying the above conditions may be derived as foilowing:

(12b)

where

Thus, (12b) assumes that the voltages at the rectifier-bridge input terminals are known. However, these voltages are not smooth but include a very large switching ripple due to the operation of the switches in the rectifier Thus, it is preferable to express (12b) in terms of supply input voltages rather than rectifier-bridge input voltages

In the second proposed scheme equation, (12b) determines the values of the current commands given the desired DC output power p ^{* } _{τ } and the converter voltages

Alternatively, the current commands may be derived given the grid voltages e:

Each component of the current commands in equation 12c has two parts, where the first part is proportional to the corresponding voltage component and the second part is proportional to the corresponding orthogonal voltage component. The first term contributes to the constant power portion of the power P _{1n } at the grid input terminals, while the second term contributes to the oscillatory power in the inductor L. The first term in each current component in equation 12c is the complete solution in the case of the first proposed power regulation scheme. Therefore, the second proposed current regulation scheme accounts for the ripple power in the inductors thereby ensuring that a constant power is transferred to the grid.

In a third power control scheme according to a third embodiment of the present invention, power ripple is eliminated at the converter output terminals B and at the same time zero average reactive power is maintained at the grid input terminals A. The following power condition should be met according to the third control:

2

The first and fourth equations in (13a) are satisfied at the grid input terminal A. The second and third equations in (13a) are satisfied at the converter output terminal B- The first equation in (13a) determines the active power P _{τ }, the second and third equations in (13a) are nullifying the ripple power P _{Tc } and P _{Ts } and the fourth equation in (13a) nullifies the reactive power q _{in }

The solution to equation system (13a) follows below:

where

and

wher

However, these voltages are not smooth but include a very large switching ripple due to the operation of the switches in the rectifier. Here, the converter output voltage signals may be estimated either from the DC output voltage together with the switch function or the grid AC voltages with inductor voltages as follows:

The previously shown power regulation schemes may improve both input performance and output performance with respect to constant power delivery and minimization of power ripple and reactive power.

In a generalized power regulation scheme according to a fourth embodiment of the present invention, the current references are derived under asymmetrical operation conditions of the grid.

Using the current commands obtained based on eq. (H b), ( 12c) or (14), a current controller may be constructed using dual current control in both positive sequence SRF and negative sequence SRF.

The proposed scheme can achieve reactive power compensation by adding the required reactive power value in the fourth equation of (10) as shown below in (15)

The first equation in (14.a) determines the active power P _{z } delivered. The second and third equation determine the power ripple, P _{zc } and P _{23 }, which should be nullified at the terminal z. The fourth equation is employed to regulate the average reactive power to the desired value corresponding to the reactive power of the grid. The current commands may be obtained by solving (14 a) This is a generalized power regulation scheme for delivering constant power with reactive power compensation for operating a grid-connected converter under asymmetrical grid voltages.

The solution to equation system (14. a) follows below:

Fig. 3 shows a block diagram view of a dual current controller 40 suitable for implementing the control schemes described above. The grid input voltage e and the grid input current i are measured and fed to a calculation unit 42 deriving the transformed current values and The current reference values k and are calculated by a current command generator 54 The transformed current values i _{d } and i _{q } and the current reference values i _{d } ^{* } and i _{q } ^{* } are fed to a set of four Pl controllers 46, where two Pl controllers are used for the positive sequence and the other two Pl controllers are used for the negative sequence to calculate a voltage control signal V ^{* }. The voltage control signal V* is fed to a PWM unit comprising a control block 48 and the grid-connected converter 24'.

The proposed control scheme may be implemented on a micro-controller/DSP based digital control system with suitable sampling and switching frequency. There are many ways to regulate current, e.g. dual current control, P+resonant controller, feedback + resonant controller and iterative learning controller.

The first, second and third power regulation scheme of the first, second and third embodiment of the invention may be used to obtain reactive power compensation by using (15) to calculate the current commands.. Once the current commands are obtained, the current controller may be constructed using dual current control in both positive sequence synchronously rotating frame (SRF) and negative sequence SRF.

Although the above embodiments have been described for a 50Hz system, the system and method may also be used for any other power frequency, such as 60Hz.

Alternatively, another controller than a PI controller may be constructed for the current control loop, e.g. an AC-regulator in a stationary cc-β frame or in a three- phase a-b-c frame

The wording "a full-scale converter-connected wind turbine" should be understood to mean a wind turbine connected to an asynchronous or synchronous generator where the stator of the generator is connected to a converter.

The converter setup described above makes an AC-DC-AC conversion and comprises 12 electronic switches. Alternatively, an AC-AC conversion may be implemented without the DC link, e.g. by using matrix converter. The generator can be multi-phase generator such as 3-phase generator or a 6- phase generator. Preferably a 3-phase generator is used, however, the same control scheme for the grid-connected converter can be applied using any type of multi-phase generator.

The above contra! algorithms are preferably implemented on a control unit such as a microcontroller or computer. The calculation unit performs the calculations Preferably digital technologies are used