Current controller and generator system

A current controller for controlling plural stator currents of plural stator windings of an electrical machine, in particular a generator, is provided, wherein the plural windings are separately connectable to a converter. The current controller includes a positive-sequence current controller configured to provide a first voltage command, in particular in a rotating dq+ frame, based on the plural stator currents; a negative-sequence current controller configured to provide a second voltage command, in particular in the dq+ frame, based on the plural stator currents. Further, the current controller includes a summation system for adding the first voltage command and the second voltage command to obtain a summed voltage command based on which the converter is controllable.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to European Patent Office Application No. 12170821.8 EP filed Jun. 5, 2012, the entire content of which is hereby incorporated by reference.

FIELD OF INVENTION

The present invention relates to a current controller for controlling plural stator currents of plural stator windings of an electrical machine and to a generator system comprising at least one current controller. The generator system further comprises a generator, in particular a wind turbine generator, which may have unbalanced impedance.

ART BACKGROUND

An electrical AC machine may generate or consume an alternating current power flow. The electrical AC machine may for example be a motor or a generator.

U.S. Pat. No. 8,009,450 B2 discloses a method and apparatus for phase current balance in active converter with unbalanced AC line voltage source, wherein individual phase voltage command values are compensated according to phase line voltage in balances to compensate for converter control to provide balanced phase currents in the present of unbalanced phase supply line voltages. Thereby, the line voltage has a fixed frequency.

There may be a need for a current controller and for a generator system, wherein machine and converter losses are reduced, wherein acoustic noise and vibration of the system are reduced, wherein no additional costs are involved and wherein no changes for the hardware is required.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention it is provided a current controller for controlling plural stator currents (in particular for each electrical phase) of plural stator windings (in particular at least one stator winding for each phase) of a (in particular variable frequency AC) electrical machine, in particular generator, wherein the plural windings are separately connectable to a converter, the current controller comprising: a positive-sequence current controller configured for providing a first voltage command, in particular in a dq-frame, based on the plural stator currents; a negative-sequence current controller configured for providing a second voltage command, in particular in a dq-frame, based on the plural stator currents; a summation system for adding the first voltage command and the second voltage command to obtain a summed voltage command based on which the converter is controllable.

The electrical machine may be in particular a variable frequency AC electrical machine, such as an electromotor or a generator, in particular a generator of a wind turbine. The generator may for example be a synchronous generator, in particular having a rotor with permanent magnets rotating around a stator, in particular comprising two or more stator windings for different phases, such as three phases, and the plural stator currents flow through the plural stator windings. The generator may be an inner rotor generators or outer rotor generators.

The stator may form a full circumference or may be a segmented stator comprising several (physically separated) stator segments. The generator can have multiple electrical-isolated three-phase windings (power output) even though the stator is not a segmented stator.

In most electrical AC machines impedance of the plural stator windings may be balanced. Thus, the machine impedance is equal for different phases, such that the impedance of the plural stator windings is equal or the same. However, under certain conditions the machine impedance may be unbalanced in some special electrical machines or under some certain conditions. As an example, especially for off-shore wind turbines, generators with plural stator windings are used for big wind turbines. Thereby, each set of the plural stator windings is connected to a separately controlled frequency converter which converts the variable frequency AC power stream received from the generator to a DC power stream and then from the DC power stream to a fixed frequency AC power stream which is then provided to a utility grid. When for example one or more windings of the generator or their converter is broken, mutual inductance between the remaining windings may be changed thereby causing an unbalanced impedance of the remaining stator windings. The unbalanced impedance in turn may cause unbalanced current at the machine stator. As a consequence it may not be possible to fully utilize the converter current rating with an unbalanced stator current. Further, the unbalanced currents may cause extra losses. Furthermore, unbalanced currents may cause high force ripples in the air gap between the permanent magnet rotor and the stator which may excite vibration and noise of the machine. Therefore, balanced machine currents are preferred such that the plural stator currents are all equal.

The current controller may be a component in a sequence of modules which finally control the converter for the plural stator windings. The number of the plural converters may equal the number of the one of more sets of stator windings each stator windings providing for e.g. three different phases. The one or more sets of plural windings supply the electric power flow to the converter(s) during a normal operation of the generator, such as a wind turbine generator. The plural windings (in particular of each set of windings) may for example correspond to three phases or 6 phases.

The plural stator currents may for example be represented by three phase currents, such as Ia, Iband Ic. The currents contain both positive-sequence component and negative-sequence component. While the positive-sequence component may be interpreted as a vector rotating in an anti-clockwise manner, the negative-sequence may be interpreted as a vector rotating in a clockwise manner. According to this embodiment of the present invention the positive sequence component and negative-sequence component are separately controlled. In particular, with the negative-sequence component a degree of unbalance of the plural stator currents is obtained and the unbalance is reduced, when the converter is controlled based on the summed voltage command

The current controller performs a method for controlling the plural stator currents of the electrical AC machine. Thereby, the current controller or the method performed by the current controller may be used both for machines with balanced impedance and for machines with unbalanced impedance. However, the current controller and the method performed by the current controller may be preferably used in variable speed wind turbines. However, it may be possible to use the method and the current control for other applications like motor drives and electrical cars.

The second voltage command may in particular be derived by the negative-sequence current controller, in order to reduce an unbalance of the plural stator currents. In particular, controlling the converter based on the summed voltage command may result in balanced plural stator currents.

The dq-frame is a particular frame or coordinate system to represent in particular three phase circuits or currents. Thereby, the three AC quantities, such as three phase currents are transformed to two (or three, if a dq0 frame is considered) quantities, which may be pure DC quantities if the three phase currents are balanced. In order to obtain the quantities in the dq-frame a 3×3 (or 2×3) transformation matrix is applied to the three phase currents. Thereby, the d-axis is perpendicular to the q-axis and the d-q-frame rotates with the frequency of the three phase currents according to the first order in an anti-clockwise manner. Thus, the first voltage command may comprise two (or three) components in the dq-frame and also the second voltage command may comprise two components (or three) in the d-q-frame. By the transformation, in particular Park's transformation, the three phase current Ia, Ib, Icare transformed into the quantities Id, Iq.

According to an embodiment of the present invention the positive-sequence current controller comprises a frame transformation module for transforming the plural stator currents into a current signal in a dq+ frame (also called dq+ current signal) based on an electrical angle of the electrical machine.

The plural stator currents may be represented e.g. by three-phase currents which comprise both a positive sequence component and a negative-sequence component. Both components are converted to a rotating frame (dq+) which rotates with electrical speed of the generator. The three-phase currents (both positive-sequence component and negative-sequence component) are converted to Id+ and Iq+ in the dq+ rotating frame. Thereby, the positive sequence component becomes a dc component in dq+ frame, while the negative sequence becomes a 2ndorder harmonic in dq+ frame.

The dq+ current signal may comprise at least two components, i.e. a positive sequence component and a negative sequence component, in the dq+ frame. The positive sequence component may be a pure DC signal in this frame and may indicate a magnitude of the plural stator currents, if the three phase current are balanced and without distortion(i.e. the plural stator currents, have the same peak value and have a relative phase difference of 120°. If three phase currents are unbalanced, then the negative current component may become 2ndorder harmonic in dq+ frame. The amplitude of the 2ndorder harmonic may be indicative for current unbalance. Thereby, the deviations may be detected and then modified.

Other high order harmonics in three-phase currents become high order harmonics in dq+ frame. For example, the 5thand 7thharmonic currents in abc frame become 6thharmonic in dq+ frame.

According to an embodiment of the present invention the negative-sequence current controller comprises a frame transformation module for transforming the plural stator currents, into a current signal in a dq− frame (also called dq− current signal) based on a negative of an electrical angle of the electrical machine.

The transformation may comprise a transformation into a dq− frame rotating in an opposite direction as the dq+ frame used by the positive-sequence current controller, in particular its frame transformation module.

Thus, while the frame transformation module of the positive-sequence current controller may transform into a dq+ frame rotating in an anti-clockwise manner, the frame transformation module of the negative-sequence current controller may transform into a dq− frame rotating in a clockwise manner

The dq− current signal may comprise at least a positive sequence component (which may be second order harmonic in the dq− frame) and a negative sequence component (which may be DC in the dq− frame). An unbalance of the plural stator currents may be indicated by the negative sequence component. Thereby, this unbalance may easily be detected. Furthermore, this enables to counteract the unbalance of the stator current by appropriately controlling the converter.

Similarly as in dq+ frame, there are other high order harmonics in dq− currents which are corresponding to high order harmonics in abc frame.

According to an embodiment of the present invention the positive-sequence current controller comprises a filter, in particular adaptive filter having as an input a electrical frequency of the generator, for generating a filtered dq+ current signal, in which AC-components, in particular AC-components corresponding to harmonics of the multiple of generator electrical frequency, are reduced in amplitude.

The filter may be or may be an electronic filter or software implemented filter. The filter may have transmission characteristics which may be adapted according to the frequency of the plural stator currents. The filter of the positive-sequence current controller may for example filter out or damp AC-components having a frequency of two times, three times, four times, five times, six times or more times the electrical frequency of the generator. Thereby, subsequent process of the dq+ current signal may be simplified and regulation may be enabled.

According to an embodiment of the present invention the negative-sequence current controller comprises a filter, in particular adaptive filter having as an input the electrical frequency of the generator, for generating a filtered dq− current signal, in which AC-components, in particular AC-components corresponding to harmonics of multiple generator electrical frequency, are reduced in amplitude.

Also the filter of the negative-sequence current controller may be an electronic filter or software implemented filter. The characteristics of the filter of the negative-sequence current controller may be adapted or changed depending on the electrical frequency of the generator. The filter of the negative-sequence current controller may for example filter out or damp AC-components in the dq− current signal which correspond or have frequencies of two times, three times, four times, five times, six times or more times the generator electrical frequency. Thereby, further process and regulation may be enabled.

According to an embodiment of the present invention a filter characteristics, such as a transmission characteristics, of the filter of the negative-sequence current controller and/or the positive-sequence current controller is adaptable according to the electrical frequency of the generator.

After filtering the filtered dq+ current signal and/or the filtered dq− current signal may be substantially a DC-signal which may more clearly indicate whether there is an unbalance of the plural stator currents.

According to an embodiment of the present invention the positive-sequence current controller comprises two regulators, in particular PI-controllers, for generating a dq+ voltage signal based on the filtered dq+ current signal and a positive-sequence current reference in dq+ frame received as further input. The regulator may generate the dq+ voltage signal so that the positive-sequence stator currents are following the positive-sequence current reference signal.

According to an embodiment of the present invention the negative-sequence current controller comprises two regulators, in particular PI-controllers, for generating a dq− voltage signal based on the filtered dq− current signal and a negative-sequence current reference in dq+ frame received as further input.

The regulator of the negative-sequence current controller may derive or generate the dq− voltage signal so that the negative-sequence stator currents are following the negative current reference signal. In particular, the negative current reference signal may be substantially zero. In particular, the negative current reference signal may comprise two components in the dq− frame which are both substantially zero.

According to an embodiment of the present invention the positive-sequence current controller comprises a voltage feedforward module for generating a feedforward dq+ voltage signal based on the dq+ current reference signal received as further input and based on the electrical frequency of the generator.

The voltage feedforward module may improve the dynamic performance of the controller and decouple components in the d-axle and the q-axle.

According to an embodiment of the present invention the positive-sequence current controller comprises an addition system for generating the first voltage command based on the feedforward dq+ voltage signal and the dq+ voltage signal output by the regulators.

According to an embodiment of the present invention the negative-sequence current controller comprises a voltage feedforward module for generating a feedforward dq− voltage signal based on the dq− current reference signal received as further input and based on the electrical frequency of the generator.

The feedforward module of the negative-sequence current controller may improve the dynamic performance of the controller and decouple components in the d-axle and the q-axle.

According to an embodiment of the present invention the negative-sequence current controller comprises an addition system for generating a summed dq− voltage signal based on the feedforward dq− signal and the dq− voltage signal output by the regulators.

According to an embodiment of the present invention the negative-sequence current controller comprises a reverse frame transformation module for generating the second voltage command, in particular according to reverse Park transformation with two times the electrical angle, signal based on the summed dq− voltage signal. The purpose of the reverse frame transformation is to convert voltage from dq− frame to dq+ frame.

According to an embodiment of the present invention the current controller further comprises a transformation module for transforming the summed voltage command from rotating dq+ frame to stationary three phase frame or two phase frame, and a modulator to modulate the three phase frame command according to the DC-link voltage of the converter.

The modulator may supply the modulated three phase command to the converter.

The current controller may receive the positive current reference signal and the negative current reference signal from a current reference calculation module which in turn receives as input or inputs a reference power, a reference torque, a voltage of the generator, a voltage of the DC-link, a frequency, a power and a torque.

According to an embodiment of the present invention it is provided a generator system comprising a generator having at least one (in particular two or more) stator segment annularly arranged, each stator segment having plural stator windings; at least one converters (one converter for each stator segment, if there are more than one stator segment), wherein the plural stator windings of each of the at least one stator segments are connectable to a corresponding converter of the at least one converter; at least one current controller (one for each converter) according to an embodiment as described above, wherein each current controller is connectable to a corresponding converter of the at least one converter, wherein the generator is in particular a variable frequency generator, wherein the converter is in particular a AC-DC-AC converter.

The generator may have a single stator forming a full circumference and may have one or more sets of plural stator windings, each set having e.g. three stator windings providing three phases.

In particular, the generator may have at least two plural stator windings or two or more sets of stator windings, each set in particular belonging to a particular stator segment or the two ore more sets belonging to a single stator forming the full circumference. Further, the generator system may comprise at least two converters, wherein the plural stator windings of each set of stator windings are connectable to a corresponding converter of the at least two converters. Further, the generator system may comprise at least two current controllers according to an embodiment as described above, wherein each current controller is connectable to a corresponding converter of the at least two converters. Thereby in particular, if one set of stator windings or its converter fails, it may be ensured that the remaining stator windings of the remaining set or sets have balanced stator currents. Thereby, the operation of the generator system may be improved, in particular regarding efficiency and noise reduction.

Embodiments of the present invention are now described with reference to the accompanying drawings. The invention is not restricted or limited to the illustrated or described embodiments.

DETAILED DESCRIPTION

FIG. 1illustrates in a schematic form a wind turbine100which provides electric energy to a utility grid101.

The wind turbine comprises a hub103to which plural rotor blades105are connected. The hub is mechanically connected to a main shaft107whose rotation is transformed by a gear box108to a rotation of a secondary shaft109, wherein the gear box108may be optional. The main shaft107or the secondary shaft109drives a generator111which may be in particular a synchronous permanent magnet generator providing a power stream in the three phases or windings113,115and117to a converter119which comprises a AC-DC portion121, a DC-link123and a DC-AC portion125for transforming a variable AC power stream to a fixed frequency AC power stream which is provided in three phases or windings127,129,131to a wind turbine transformer133which transforms the output voltage to a higher voltage for transmission to the utility grid101.

The converter119is controlled via a converter command135which is derived and supplied from a current controller150according to an embodiment of the present invention, which receives at least one input signal137, such as one or more reference values or one or more quantities indicative of the operation of the generator111or any component of the wind turbine100.

The generator inFIG. 1comprises a single three-phase stator winding. Thereby, the winding113carries the stator current Ia, the winding115carries the stator current Iband the winding117carries the stator current Ic.

The current controller115controls the converter119such that generator voltage and power follow commands and the stator current Ia, Iband Icare balanced.

FIG. 2illustrates in a schematic form another wind turbine200which is connected for energy supply to the utility grid201. If not otherwise stated components being equal in structure and/or function inFIGS. 1 and 2are denoted with the same reference sign differing only in the first digit. Thus, a description of elements not described with reference toFIG. 2may be taken from the description of these elements as described inFIG. 1.

Differing from the embodiment illustrated inFIG. 1the generator211of the wind turbine200illustrated inFIG. 2comprises the first set of three-phase windings210and the second set of three-phase windings212, wherein the first set of windings213,215,217provides the power flow to a first converter219. Further, the second set of windings214,216and218provides the power stream to a second converter220which may be similarly constructed as the first converter219. Thereby, the first converter219is controlled via a current controller250by providing a converter command235to the first converter219based on at least one input signal237. Further, the second converter220is controlled via a converter command236supplied by another current controller251according to an embodiment of the present invention which receives at least one input signal238.

The current controllers250,251control the first converter219and the second converter220, respectively, such that the currents, carried by the different windings213,215,217and214,216,218, are balanced.

FIG. 3illustrates graphs of the stator currents flowing in three stator windings of a generator with unbalanced impedance. An abscissa301denotes the time and seconds, and an ordinate303denotes the current flowing through the respective windings in Ampere. The curve305represents the stator current of a first phase, the curve307represents the stator current of a second phase and the curve309represents the stator current of a third phase. As is apparent fromFIG. 3the peak values of the curves305,307and309are different indicating that the currents or stator currents of the different phases are not balanced. Thereby, there may evolve losses and a deterioration of the efficiency. Further, oscillations may evolve increasing the noise emissions which may be undesirable.

FIG. 4shows in comparison curves of stator currents when the converter is controlled using a current controller according to an embodiment of the present invention.

An abscissa401denotes the time in seconds, while an ordinate403denotes the stator current in Ampere. The curve405represents the stator current of a first phase, such as the winding113or213as shown inFIG. 1or2. The curve407represents a stator current of a second phase such as of the winding115or215as shown inFIG. 1or2. The curve409represents a stator current of a third phase, such as of the winding117or217as shown inFIG. 1or2. As can be appreciated fromFIG. 4, the peak values of the curves405,407,409are at least approximately equal, indicating that the stator current of these different phases are balanced.

The balancing of the stator currents is achieved by controlling the respective converter using a current controller, such as current controller150,250or251, according embodiments of the present invention.

FIG. 5schematically illustrates a current control system550including a current controller560according to an embodiment of the present invention. The current control systems150,250and251illustrated inFIGS. 1 and 2may for example be implemented as the current control system550illustrated inFIG. 5.

The current control system550receives as input quantities537a reference power Pref, a reference torque Tref, the voltage Ugen of the generator, the voltage Udc at the DC-link of the converter, the electrical frequency ωeof the generator and the power P and the torque T of the generator. These input quantities537are supplied to a current reference calculation module551which calculates based on the input qualities537positive sequence current reference signals553in the dq+ frame and negative sequence current reference signals555in the dq− frame.

These references are provided to the current controller560according to an embodiment of the present invention which further receives the stator current Ia, Ib, Icand the electrical phase Θeand the frequency ωeof the generator as inputs557. The current controller560outputs a summed voltage command559having a component Vdand another component Vqin the dq+ frame. The summed voltage command559is provided to a transformation module561for transforming the summed voltage command (based on the received electrical phase Θe) into a three phase frame command563comprising the components Va, Vband Vc. The three phase frame command563is supplied to a modulator565which receives the voltage Udc of the DC-link of the converter and provides a modulated signal567to the converter519which is connected to the generator511, as is also illustrated inFIGS. 1 and 2. In particular, the current controller560comprised in the current control system550enables to control the converter519such that the stator currents flowing in the windings513,515and517to be balanced.

FIG. 6schematically illustrates the current controller560illustrated inFIG. 5in some more detail. The current controller560comprises a positive-sequence current controller601and a negative-sequence current controller603which separately regulate the positive-sequence component of the stator current and the negative-sequence component of stator current. The positive-sequence current controller and the negative-sequence current controller both receive the same three-phase currents, but the process and regulators are different.

The positive-sequence controller comprises a frame transformation module605and the negative-sequence current controller comprises a frame transformation module607. These frame transformation modules605and607are provided to transform the three phase input signal comprising the stator currents from stationary three-phase frame into rotationary dq+ and dq− frames, respectively, i.e. into a dq+ current signal609and a dq+current signal611, respectively. Thereby, the frame transformation module605of the positive-sequence current controller receives the electrical phase Θefor the transformation, while the frame transformation module607of the negative-sequence current controller receives the negative of the electrical phase, i.e. −Θe, i.e. the angle of generator back emf (The frequency of back emf is the same as generator current). Thus, the frame transformation module605transforms in a dq+ frame rotating in an anti-clockwise manner and the frame transformation module607transforms in a dq− frame rotating in a clockwise manner.

The dq+ current signal609is supplied to a filter system613comprising two filter components, one for each component of the dq+ current signal609. The filter system613is an adaptive filter which receives the frequency ωeof the generator and adapts according to this frequency its transmission behavior. In particular, the filter system613filters out AC-components of the dq+ current signal609which correspond to harmonics at the multiple of frequency ωeof the generator.

Also the negative-sequence current controller603comprises a filter system615comprising two filter components for each component of the dq− current signal611, in order to filter out higher harmonics of the frequency ωeof the generator.

The filtered dq+ current signal617is provided together with a positive-sequence current reference signal619to a regulator621, which may in particular comprise a PI-controller which adjusts a dq+ voltage signal623in order to regulate idq+ to comply with the positive-sequence current reference signal619.

Also the negative-sequence current controller comprises a regulator622(for each component) which receives a negative-sequence current reference signal620and the filtered dq− current signal618output by the filter system615in order to derive a dq− voltage signal624. Both, the positive-sequence current controller and the negative-sequence current controller comprise a voltage feedforward module625and626, respectively for generating a dq+ feedforward voltage signal627based on the positive-sequence current reference signal619and on the frequency ωeof the generator. The voltage feedforward module626of the negative-sequence current controller603generates a feedforward dq− voltage signal628based on the negative-sequence current reference signal620and the frequency ωeof the generator.

Further, the positive-sequence current controller comprises adding elements629for adding the feedforward dq+ voltage signal627and the dq+ voltage signal623to generate the first voltage command631.

The negative-sequence current controller603comprises adding elements633to generate a summed dq− voltage signal635based on the feedforward dq− voltage signal628and the dq− voltage signal624output by the regulator622. Further, the negative-sequence current controller603comprises a reverse frame transformation module637to transform (based on twice the negative of the electrical angle, i.e. −2Θe) the summed dq− voltage signal635to the second voltage command639.

Using adding elements641the first voltage command631generated by the positive-sequence current controller601is added to the second voltage command639generated by the negative-sequence current controller603, in order to obtain a summed voltage command643having component Vdand Vqwhich are then supplied to a converter, such as a converter519illustrated inFIG. 5or converter119illustrated inFIG. 1or converter219or converter220illustrated inFIG. 2.

In particular, the current controller560illustrated inFIGS. 5 and 6is a closed-loop feedback regulation of machine current in dq frame. For a better dynamic performance, voltage feedforward is also used in this block560. In particular, the current controller560comprises the fundamental positive-sequence current controller601which is configured for providing the first voltage command, i.e. voltage command V+d, V+qand any fundamental negative-sequence current controller603configured for using negative current reference, current feedback signal and machine speed in combination with Idregulator, Iqregulator and voltage feedforward to obtain the second voltage command639, i.e. the component V−d, V−q. Thereby, the positive-sequence current controller is in the synchronous dq+ frame +ωe. In contrast, to regulate the negative-sequence current, the three phase current Ia, Ib, Icis converted to a rotating frame (dq− frame) with −ωeby Park's transformation. The fundamental negative-sequence current is converted from AC to DC after the transformation. A PI-controller can be used to regulate the negative current following their references in this rotating frame. For achieving better dynamic performance, voltage feedforward is also used in this rotating frame. The output of the voltage feedforward and the feedback current controller are added together, and then they are converted to the synchronous dq+ frame.

The negative-sequence current is the second order harmonic in dq+ frame. The current regulators in positive-sequence current controller shall not give response to negative-sequence current. Therefore, the filter system613is used to remove harmonics in the current feedback in dq+ frame Similarly, positive-sequence current is second order harmonic in the dq− frame. Filter system615is then used to remove harmonics in the dq− frame. The two voltage feedforward blocks625and626in both the positive-sequence current controller and negative-sequence current controller are used to calculate decoupling voltages between the d-axis and q-axis at the two rotating frames based on generator models. Machine impedance values like resistance and inductance are used together with electrical speed and current references for this calculation.

The current controller of the present invention may contribute to minimizing unbalanced currents of an electrical machine and converter, may reduce machine and converter losses, may reduce acoustic noise and vibration of the machine, may not have any additional costs and may not require any changes in hardware.