System and method for reactive power regulation

A system and method are provided for performing reactive power control. The system includes a power converter and a controller coupled to the power converter. The power converter is configured to convert a first form of electric power generated from the power source to a second form of electric power suitable to be distributed by the electrical grid. The controller is configured to monitor the electric power transmitted between the power converter and the electrical grid. The controller is further configured to decouple a positive sequence component and a negative sequence component from the monitored electric power. The controller is further configured to perform a positive reactive power control and a negative reactive power control with respect to the decoupled positive and negative sequence components. The controller is further configured to transmit a control signal to the power converter based on the positive and negative reactive power control.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the invention relates generally to power regulation, and more particularly relate to reactive power regulation.

2. Description of Related Art

Power sources such as solar panels and wind turbines have received increased attention as environmentally safe and sustainable alternative power sources compared to traditional coal powered power sources. When the power output from the power sources is fed to an electrical grid for transmission and distribution, it is usually necessary to control the reactive power of the output power to fulfill electrical demand while providing stability for the electrical grid.

Conventional reactive power control is based on the assumption that the electrical grid is always symmetrical in three phases. Based on this assumption, the reactive power is regulated by directly adjusting the output power in positive sequence components without considering negative sequence components in the electrical grid. However, in an imbalanced electrical grid, the negative sequence components may lead to second order ripples in the output power. Therefore, the reactive power control is not accurate due to the lack of reactive power regulation with respect to the negative sequence components.

In addition, many countries now require that power sources stay connected with the electrical grid when the electrical grid experiences fault conditions. However, providing accurate reactive power control may be even more challenging during fault conditions.

It is desirable to provide a system and method for regulating reactive power to address the above-mentioned problems.

BRIEF SUMMARY OF THE INVENTION

In accordance with one embodiment disclosed herein, a system is provided for performing reactive power control. The system includes a power converter and a controller coupled to the power converter. The power converter is coupled between a power source and an electrical grid. The power converter is configured to convert a first form of electric power generated from the power source to a second form of electric power suitable to be distributed by the electrical grid. The controller is configured to monitor the electric power transmitted between the power converter and the electrical grid. The controller is further configured to decouple a positive sequence component and a negative sequence component from the monitored electric power. The controller is further configured to perform a positive reactive power control with respect to the positive sequence component. The controller is further configured to perform a negative reactive power control with respect to the negative sequence component. The controller is further configured to transmit a control signal to the power converter based on the positive reactive power control and the negative reactive power control to enable the power converter to adjust a reactive power of the electric power transmitted between the power converter and the electrical grid.

In accordance with another embodiment disclosed herein, a method is provided for performing reactive power control with respect to electric power transmitted between a power source and an electrical grid. The method includes monitoring the electric power transmitted between the power source and the electrical grid. The method further includes decoupling a positive sequence component and a negative sequence component from the monitored electric power. The method further includes performing a positive reactive power control with respect to the positive sequence component. The method further includes performing a negative reactive power control with respect to the negative sequence component. The method further includes adjusting a reactive power of the electric power transmitted between the power source and the electrical grid based on the positive reactive power control and the negative reactive power control.

In accordance with another embodiment disclosed herein, a system is provided for performing reactive power control. The system includes a power converter and a controller. The power converter includes a machine-side converter and a grid-side converter. The machine-side converter is electrically coupled to a power source for converting alternating current (AC) electric power to direct current (DC) electric power. The grid-side converter is electrically coupled to an electrical grid for converting the DC electric power to AC electric power for use by the electrical grid. The controller is operatively coupled to the grid-side converter and is configured to monitor the AC electric power transmitted between the grid-side converter and the electrical grid. The controller is further configured to decouple a first sequence component and a second sequence component from the monitored AC electric power. The controller is further configured to perform a first reactive power control with respect to the first sequence component to generate a first command signal and to perform a second reactive power control with respect to the second sequence component to generate a second command signal. The controller is further configured to transmit a control signal to the grid-side converter in response to the first command signal and the second command signal to enable the grid-side converter to adjust a reactive power of the AC electric power transmitted between the grid-side converter and the electrical grid.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments disclosed herein relate to a system and method for reactive power regulation. In one aspect, the system and method are implemented by decoupling positive sequence components and negative sequence components of the system output power. The system and method are further implemented by separately regulating reactive power with respect to the positive sequence and the negative sequence for controlling the reactive power more accurately and thereby stabilizing the electrical grid and mitigating grid imbalance. As the positive reactive power and negative reactive power are independently regulated, the terms “vector VAR control” or “vector VAR regulation” are introduced herein. These terms are not intended to limit the scope of the disclosure of reactive power control only as, in some implementations, “vector VAR control” may also include active power control or active power regulation.

FIG. 1illustrates a block diagram of a system100in accordance with an exemplary embodiment. In the illustrated embodiment ofFIG. 1, the system100generally includes a power source10, a power converter20, an electrical grid30, and a controller40. Each block of the system100will be described in further detail below.

The power source10is configured to generate a first form of electric power102from a variety of available energy sources. In one implementation of the disclosure, the power source10may include an electrical machine such as a wind turbine or a marine hydrokinetic energy turbine. A wind turbine is operable to transform mechanical wind power to mechanical rotational power and to convert the mechanical rotational power to generate three-phase alternating current (AC) electric power. Marine turbines are operable to transform mechanical tidal power to generate three-phase AC electric power. It should be recognized the three-phase AC electric power is one type of the first form of electric power102. In other embodiments, the first form of electric power may include poly-phase AC electric power or direct current (DC) electric power. In one implementation, the power source10may include a solar panel having a packaged assembly of solar cells. The solar panel is configured to generate DC electric power from the sun through photovoltaic effects.

The power converter20is coupled to the power source10for receiving the first form of electric power102from the power source10. The power converter20is configured to convert the first form of electric power102to a second form of electric power262. In one implementation of the disclosure wherein the power source10includes a wind turbine, the power converter20is designed to include a machine-side converter22, a grid-side converter26, and a direct current (DC) link24coupled between the machine-side converter22and the grid-side converter26. The machine-side converter22acts as a rectifier and is configured to rectify the three-phase AC electric power102to DC electric power222. The DC electric power222is transmitted to the DC link24. The DC link24may include one or more capacitors coupled in series or in parallel. The DC link24is configured to mitigate voltage variations across the DC link24with AC rectification. The DC electric power222is subsequently transmitted from the DC link24to the grid-side converter26. The grid-side converter26acts as an inverter, is configured to convert the DC electric power222from DC link24back to three-phase AC electric power262, and is controlled by the controller40. The three-phase AC electric power262is subsequently transmitted to the electrical grid30for transmission and distribution. In one embodiment, the machine-side converter22and the grid-side converter26may include a three-phase two-level topology with a series of semiconductor power switches fully controlled and regulated using a pulse width modulation (PWM) strategy. In alternative embodiments, the machine-side converter22and the grid-side converter26may include three-phase three-level topology. The semiconductor power switches may include any appropriate devices with several examples including insulated gate bipolar transistors (IGBTs), gate communicated thyristors (GCTs), and metal oxide semiconductor field effect transistors (MOSFETs). In embodiments wherein the power source10supplies DC power, the machine-side converter22may be omitted or may be configured as a DC to DC converter, for example.

In the illustrated embodiment ofFIG. 1, the system100further includes a voltage sensor32, a current sensor34, and a DC voltage sensor50. The voltage sensor32and the current sensor34are both electrically coupled to a joint connection between the grid-side converter26and the electrical grid30. The voltage sensor32is configured to measure a system voltage322of the three-phase AC electric power262transmitted to the electrical grid30, and in response thereto, to provide a feedback system voltage324to the controller40. In one implementation, the system voltage322may include three line voltages from the transmissions line. In another implementation, the system voltage322may include line-to-line voltages transmitted between two transmission lines. The current sensor34is configured to measure a system current342of the three-phase AC electric power262, and in response thereto, to provide a feedback system current344to the controller40. In one implementation, the system current342may include three currents flowing through the transmission lines. The DC voltage sensor50is configured to measure a DC voltage222across the DC link24, and in response thereto, to provide a feedback DC voltage502to the controller40.

The controller40operates in response to the feedback system voltage324, the feedback system current344, and feedback DC voltage502from DC sensor50and a variety of system commands to generate a control signal408for controlling the grid-side converter26. The system commands may include a positive reactive power command402, a negative reactive power command404, and a DC voltage command406. Although not a focus of this disclosure, controller40itself or an additional controller may be used to provide control signals for the machine-side converter22. Further details of the controller40will be described below.

FIG. 2illustrates a block diagram of the controller40shown inFIG. 1in accordance with an exemplary embodiment. As illustrated inFIG. 2, the controller40includes a voltage decoupling circuit42, a current decoupling circuit44, a power calculation circuit46, a positive power regulator48, a negative power regulator52, a current regulator54, and a PWM modulator56.

As illustrated inFIG. 2, the voltage decoupling circuit42is coupled to the voltage sensor32(FIG. 1) to receive the feedback system voltage324from the voltage sensor32. The voltage decoupling circuit42is configured to decouple positive and negative voltage components from the feedback system voltage324. In one implementation of the disclosure, the voltage decoupling circuit42may include a crossed-coupled phase lock loop (CCPLL) circuit58as shown inFIG. 3. In a synchronously rotating two-phase direct and quadrature (d-q) reference frame, the feedback positive sequence voltage component422decoupled from the CCPLL circuit58includes a d-axis positive voltage582and a q-axis positive voltage584. Similarly, the feedback negative sequence voltage424decoupled from the CCPLL circuit58includes a d-axis negative voltage586and a q-axis negative voltage588. The CCPLL circuit58is also configured to provide a positive phase angle426and a negative phase angle428. In one implementation, an example of a CCPLL circuit58can be found in commonly assigned Weng et al., U.S. Pat. No. 7,456,695, which is incorporated by reference herein.

As illustrated inFIG. 2, the current decoupling circuit44is coupled to the current sensor34(FIG. 1) to receive the feedback system current344from the current sensor34. The current decoupling circuit44is configured to decouple positive and negative current components from the feedback system current344according to the positive and negative phase angles426,428generated by the voltage decoupling circuit42. In one implementation illustrated inFIG. 3, in the d-q reference frame, the feedback positive sequence current442decoupled from the current decoupling circuit44includes a d-axis positive current622and a q-axis positive current624, and the feedback negative sequence current444decoupled from the current decoupling circuit44includes a d-axis negative current626and a q-axis negative current628. Further details of the current decoupling circuit44will be described below.

As illustrated inFIG. 2, the power calculation circuit46is coupled both to the voltage decoupling circuit42and the current decoupling circuit44for receiving the decoupled positive and negative sequence voltage components and positive and negative sequence current components to perform power calculation. In one implementation of the disclosure, the power calculation circuit46receives the feedback positive and negative sequence voltages422,424and the feedback positive and negative sequence currents442,444for use in calculating a feedback positive reactive power462and a feedback negative reactive power464. After the feedback positive and negative reactive powers462, and464are calculated, the controller40may perform reactive power control based on the positive reactive power command402and the negative reactive power command404. In another implementation, further referring toFIG. 3, the power calculation circuit46may be further configured to calculate a feedback positive active power466and a feedback negative active power468for facilitating performing active power control. Further details of calculating the feedback positive and negative reactive power will be described below.

As illustrated inFIG. 2, the positive power regulator48is coupled to the power calculation circuit46. The positive power regulator48is configured to receive the feedback positive reactive power462and to perform a positive reactive power control according to the positive reactive power command402. The positive power regulator48is further configured to receive the feedback DC voltage502and to perform a positive active power control according to the DC command406. By performing the positive reactive and active power controls, the positive power regulator48provides a positive current command482. Further details of performing the positive reactive power control will be described below.

As illustrated inFIG. 2, the negative power regulator52is configured to receive the feedback negative reactive power464and to perform a negative reactive power according to the negative reactive power command404. By performing the negative reactive power control, the negative power regulator52provides a negative current command522. Further details of performing the negative reactive power control will be described below.

As illustrated inFIG. 2, the current regulator54is coupled to the positive power regulator48and the negative power regulator52for receiving the positive current command482and the negative current command522. The current regulator54may also be coupled to the current decoupling circuit44for receiving the feedback positive and negative sequence current442,444. In one implementation, the current regulator54processes the feedback positive and negative sequence current442,444and the positive and negative current commands482,522to provide a voltage command540. The voltage command540is modulated in the PWM modulator56to provide the control signal408. The control signal408is applied to the grid-side converter26(FIG. 1) for driving the grid-side converter26to generate desired current output. The control signal408may include pulse signals having on and off states.

FIG. 4illustrates a block diagram of one embodiment of a positive current decoupling circuit441of the current decoupling circuit44for use in the embodiment inFIG. 3. The positive current decoupling circuit441is configured to decouple positive sequence current components from the feedback system current344. In one implementation, the positive current decoupling circuit441includes a positive rotating element45, a first positive low pass filter (LPF)47, and a second positive LPF49. The positive rotating element45is coupled to the current sensor34(FIG. 1) to receive the feedback system current344from the current sensor34. The positive rotating element45rotates the feedback system current344according to the positive phase angle426and outputs a d-axis positive current621and a q-axis positive current623. In one implementation, the positive rotating element45may rotate the three phase feedback system current344to two phase positive feedback current in the d-q reference frame according to the following matrix equation:

[Idp⁢⁢_⁢⁢fbk⁢⁢0⁢Iqp⁢⁢_⁢⁢fbk⁢⁢0]=⁢⁢[23⁢cos⁢⁢θp-13⁢cos⁢⁢θp+33⁢sin⁢⁢θp-13⁢cos⁢⁢θp-33⁢sin⁢⁢θp-23⁢sin⁢⁢θp13⁢sin⁢⁢θp+33⁢cos⁢⁢θp13⁢sin⁢⁢θp-33⁢cos⁢⁢θp]⁡[Ia⁢⁢_⁢⁢fbkIb⁢⁢_⁢⁢fbkIc⁢⁢_⁢⁢fbk],(1)
where Idp—fbk0, Iqp—fbk0are the d-axis positive current621and the q-axis positive current623respectively in the d-q reference frame, θpis the positive phase angle426, and Ia—fbk, Ib—fbk, Ic—fbkare three phase current components of the feedback system current344. The first positive LPF47removes high frequency components from the d-axis positive current621and outputs the d-axis positive current622. The second positive LPF49removes high frequency components from the q-axis positive current623and outputs the q-axis positive current624.

FIG. 5illustrates a block diagram of one embodiment of a negative current decoupling circuit443of the current decoupling circuit44for use in the embodiment inFIG. 3. The negative current decoupling circuit443is configured to decouple negative sequence current components from the feedback system current344. In one implementation, the negative current decoupling circuit443includes a negative rotating element51, a first negative low pass filter (LPF)53, and a second negative LPF55. The negative rotating element51is coupled to the current sensor34(FIG. 1) to receive the feedback system current344from the current sensor34. The negative rotating element51rotates the feedback system current344according to the negative phase angle428and outputs ad-axis negative current625and a q-axis negative current627. In one implementation, the negative rotating element51may rotate the three phase feedback system current344to two phase negative feedback current in the d-q reference frame according to the following matrix equation:

[Idn⁢⁢_⁢⁢fbk⁢⁢0Iqn⁢⁢_⁢⁢fbk⁢⁢0]=⁢⁢[23⁢cos⁢⁢θn-13⁢cos⁢⁢θn+33⁢sin⁢⁢θn-13⁢cos⁢⁢θn-33⁢sin⁢⁢θn-23⁢sin⁢⁢θn13⁢sin⁢⁢θn+33⁢cos⁢⁢θn13⁢sin⁢⁢θn-33⁢cos⁢⁢θn]⁡[Ia⁢⁢_⁢⁢fbkIb⁢⁢_⁢⁢fbkIc⁢⁢_⁢⁢fbk],(2)
where Idn—fbk0, Iqn—fbk0are the d-axis negative current625and the q-axis negative current627in the d-q reference frame, θnis the negative phase angle428, and Ia—fbk, Ib—fbk, Ic—fbkare three phase current components of the feedback system current344. The first negative LPF53removes high frequency components from the d-axis negative current625and outputs the d-axis negative current626). The second negative LPF55removes high frequency components from the q-axis negative current627and outputs the q-axis negative current628.

FIG. 6illustrates a block diagram of a first power calculation module461of the power calculation circuit46shown inFIG. 3in accordance with an exemplary embodiment. The first power calculation module461is configured to calculate the feedback positive reactive power462and the feedback positive active power466according to the feedback positive voltages582,584and feedback positive currents622,624. In one implementation, the first power calculation module461includes a first multiplication element11, a second multiplication element13, a third multiplication element15, a fourth multiplication element17, a first summation element19, a second summation element21, a first processing element23, and a second processing element25. The first multiplication element11multiplies the d-axis positive voltage582with the d-axis positive current622and provides a first multiplied signal112. The second multiplication element13multiplies the q-axis positive voltage584with the q-axis positive current624and provides a second multiplied signal132. The first summation element19sums the first multiplied signal112and the second multiplied signal132and provides a summation signal192. The summation signal192is processed by the first processing element23to provide the feedback positive active power466. In one example, the first processing element23multiplies the summation signal192by a coefficient or factor of 1.5. The third multiplication element15multiplies the d-axis positive voltage582with the q-axis positive current624and provides a third multiplied signal152. The fourth multiplication element17multiplies the q-axis positive voltage584with the d-axis positive current622and provides a fourth multiplied signal172. The second summation element21subtracts the third multiplied signal152from the fourth multiplied signal172and provides a subtracted signal212. The subtracted signal212is processed by the second processing element25to provide the feedback positive reactive power462. In one example, processing element25multiplies the subtracted signal212by a coefficient or factor of 1.5.

FIG. 7illustrates a block diagram of a second power calculation module463of the power calculation circuit46shown inFIG. 3in accordance with an exemplary embodiment. The second power calculation module463is configured to calculate the feedback negative reactive power464and the feedback negative active power468according to the feedback negative voltages586,588and the feedback negative currents626,628. In one implementation, the second power calculation module463includes a first multiplication element27, a second multiplication element29, a third multiplication element31, a fourth multiplication element33, a first summation element35, a second summation element37, a first processing element39, and a second processing element41. The first multiplication element27multiplies the d-axis negative voltage586with the d-axis negative current626and provides a first multiplied signal272. The second multiplication element29multiplies the q-axis negative voltage588with the q-axis negative current628and provides a second multiplied signal292. The first summation element35sums the first multiplied signal272and the second multiplied signal292and provides a summation signal352. The summation signal352is processed by the first processing element39to get the feedback negative active power468. The third multiplication element31multiplies the d-axis negative voltage586with the q-axis negative current628and provides a third multiplied signal312. The fourth multiplication element33multiplies the q-axis negative voltage588with the d-axis negative current626and provides a fourth multiplied signal332. The second summation element37subtracts the third multiplied signal312from the fourth multiplied signal332and provides a subtracted signal372. The subtracted signal372is processed by the second processing element41to get the feedback negative reactive power464.

FIG. 8illustrates a block diagram of a first positive regulation module120of the positive power regulator48shown inFIG. 2in accordance with an exemplary embodiment. The first positive regulation module120is configured to regulate the feedback DC voltage502from the DC sensor50as well as the DC command406and to provide a d-axis positive current command802. In one implementation, the first positive regulation module120includes a first summation element76, a DC voltage regulator78, and a current limiter80coupled in series. The feedback DC voltage502is subtracted from the DC command406by the first summation element76to provide a difference DC voltage command762. The difference DC voltage command762is regulated by the DC voltage regulator78to provide a d-axis positive current command782. The current limiter80limits the d-axis positive current command782, such that the resulting d-axis positive current command802does not exceed the capability of the grid-side converter26(FIG. 1).

FIG. 9illustrates a block diagram of a second positive regulation module140of the positive power regulator48shown inFIG. 2in accordance with an exemplary embodiment. The second positive regulation module140is configured to regulate the feedback positive reactive power462from the power calculation circuit46according to the positive reactive power command402and to provide a q-axis positive current command742. In one implementation, the second positive regulation module140includes a first summation element66, a VAR regulator68, a second summation element70, a voltage regulator72, and a current limiter74coupled in series. The feedback positive reactive power462is subtracted from the positive reactive power command402by the first summation element66to provide a difference positive reactive power command662. The difference positive reactive power command662is regulated by the VAR regulator68to provide a regulated voltage command682. A positive voltage magnitude110is subtracted from the regulated voltage command682by the second summation element70to provide a difference regulated voltage command702. The positive voltage magnitude110can be calculated by the following expression: VP—mag=√{square root over (Vdp2+Vqp2)} (3) wherein Vp—magis positive voltage magnitude110, Vdpis the d-axis positive voltage582, and Vqpis the q-axis positive voltage584. The difference regulated voltage command702is further regulated by the voltage regulator72to provide a q-axis positive current command722. The current limiter74limits the q-axis positive current command722, such that the resulting q-axis positive current command742does not exceed the capability of the grid-side converter26(FIG. 1).

FIG. 10illustrates a block diagram of a first negative regulation module260of the negative power regulator52ofFIG. 2in accordance with an exemplary embodiment. The first negative regulation module260is configured to regulate the q-axis negative voltage588and to provide a d-axis negative current command105. In one implementation, the first negative regulation module260emulates a L-R load in negative sequence, e.g., an inductance in negative sequence. The first negative regulation module260includes a multiplication element98, a filter102, and a limiter104coupled in series. The multiplication element98multiplies the q-axis negative voltage588by a q-axis gain signal230and provides a d-axis negative current982. The filter102filters the d-axis negative current982according to a q-axis signal250and provides a filtered d-axis negative current command1022. The q-axis signal250is a predetermined signal and is supplied for indicating a bandwidth of the filter102. The limiter104limits the filtered d-axis negative current command1022and provides the d-axis negative current command105.

FIG. 11illustrates a block diagram of a second negative regulation module280of the negative power regulator52ofFIG. 2in accordance with an exemplary embodiment. The second negative regulation module280is configured to regulate the d-axis negative voltage586and to provide a q-axis negative current command113. In one implementation, the second negative regulation module280also emulates a L-R load in negative sequence, e.g., an inductance in negative sequence. The second negative regulation module280includes a multiplication element106, a filter108, and a limiter112coupled in series. The multiplication element106multiplies the d-axis negative voltage586by a gain signal270and provides a q-axis negative current1062. The filter108filters the q-axis negative current1062according to a d-axis signal290and provides a filtered q-axis negative current signal1082. The d-axis signal290is also a predetermined signal and is supplied for indicating a bandwidth of the filter108. The limiter112limits the filtered q-axis negative current signal1082and provides the q-axis negative current command113.

FIG. 12illustrates a block diagram of the current regulator54shown inFIG. 2. The current regulator54is configured to control respective current errors of the feedback positive and negative current and the positive and negative current commands to zero in steady state. In one implementation, the current regulator54includes a positive current regulator128, a negative current regulator134, a first summation element132, a second summation element138, a sequence-rotating element136, and a two-to-three phase converter142.

As shown inFIG. 12, the positive current regulator128receives the d-axis positive current622, the q-axis positive current624, the d-axis positive current command802, and the q-axis positive current command742. The d-axis positive current622and the q-axis positive current624are regulated by the positive current regulator128according to the d-axis positive current command802and the q-axis positive current command742to provide a first d-axis positive voltage command1282and a first q-axis positive voltage command1284.

As shown inFIG. 12, the negative current regulator134receives the d-axis negative current626, the q-axis negative current628, the d-axis negative current command105, and the q-axis negative current command113. The d-axis negative current626and the q-axis negative current628are regulated by the negative current regulator134according to the d-axis negative current command105and the q-axis negative current command113to provide a d-axis negative voltage command1342and a q-axis negative voltage command1344. The d-axis negative voltage command1342and the q-axis negative voltage command1344in the negative sequence are rotated by the sequence-rotating element136to provide a second d-axis positive voltage command1362and a second q-axis positive voltage command1364in the positive sequence. In one implementation, the sequence-rotating element136may rotate the negative voltage components to positive voltage components in the d-q reference frame according to the following matrix equation:

As further shown inFIG. 12, the first d-axis positive voltage command1282and the second d-axis positive voltage command1362are summed by the first summation element132to provide a third d-axis positive voltage command1322. The first q-axis positive voltage command1284and the second q-axis positive voltage command1364are summed by the second summation element138to provide a third q-axis positive voltage command1382. The third d-axis positive voltage command1322and the third q-axis positive voltage command1382are converted by the two-to-three phase converter142to provide three-phase voltage commands542,544,546according to the positive phase angle426. In one implementation, the two-to-three phase converter142may convert the two phase voltage commands in the d-q reference frame to the three phase voltage commands according to the following matrix equation:

As described above, the controller40is operated to decouple positive sequence voltage and current components and negative sequence voltage and current components from the power transmitted to the electrical grid30. In one aspect of the disclosure, the controller40is further operated to calculate positive reactive power according to the decoupled positive sequence voltage and current components and to calculate negative reactive power according to the decoupled negative sequence voltage and current components. Because the positive reactive power and the negative reactive power are independently calculated, the controller40is further operated to perform positive reactive power regulation in the positive sequence and to perform negative reactive power regulation in the negative sequence. In this condition, both positive sequence reactive power and negative sequence reactive power are regulated, so that the reactive power of the power transmitted to the electrical grid30can be adjusted more accurately.

FIG. 13illustrates a block diagram of another embodiment of the current decoupling circuit44for use in the embodiment ofFIG. 3. In one implementation, the current decoupling circuit44includes a three-to-two phase converter63, a first summation element65, a second summation element67, a first positive rotating element69, a first positive low pass filter (LPF)71, a second positive LPF73, a second positive rotating element75, a third summation element77, a fourth summation element79, a first negative rotating element81, a first negative LPF83, a second negative LPF85, and a second negative rotating element87. The three-to-two phase converter63is coupled to the current sensor34(FIG. 1) to receive the feedback system current344from the current sensor34. The current decoupling circuit44is constructed in a cross-coupled manner. More specifically, in one aspect, two outputs of the second positive rotating element75are coupled to the third summation element77and the fourth summation element79respectively, and two outputs of the second negative rotating element87are coupled to the first summation65and the second summation element67respectively.

In one implementation, the three-to-two phase converter63converts the three-phase feedback system current344to two-phase feedback current, i.e., an α-axis feedback current632and an β-axis feedback current634. In one implementation, the three-to-two phase converter63may convert the three-phase feedback system current344to two-phase feedback current according to the following matrix equation:

[I⁢α⁢⁢_⁢⁢fbkIβ⁢⁢_⁢⁢fbk]=[23-13-13033-33]⁡[Ia⁢⁢_⁢⁢fbkIb⁢⁢_⁢⁢fbkIc⁢⁢_⁢⁢fbk],(6)
wherein Iα—fbk, Iβ—fbkare the α-axis feedback current632and the β-axis feedback current634respectively in the α-β reference frame, and Iα—fbk, Ib—fbk, Ic—fbkare three phase current components of the feedback system current344. As an α-axis negative feedback current872and an β-axis negative feedback current874are derived from the second negative rotating element87, the first summation element65subtracts the α-axis negative feedback current872from the α-axis feedback current632and outputs an α-axis positive feedback current652. The second summation element67subtracts the β-axis negative feedback current874from the β-axis feedback current634and outputs an β-axis positive feedback current672.

The first positive rotating element69rotates the α-axis positive feedback current652and the β-axis positive feedback current672according to the positive phase angle426and outputs a d-axis positive current622and a q-axis positive current624. In one implementation, the first positive rotating element69may rotate the two phase positive current in the α-β reference frame to the two phase positive current in the d-q reference frame according to the following matrix equation:

Further referring toFIG. 13, the third summation element77subtracts the α-axis positive feedback current752from the α-axis feedback current632and outputs an α-axis negative feedback current772. The fourth summation element79subtracts the β-axis positive feedback current754from the β-axis feedback current634and outputs an β-axis negative feedback current792. The first negative rotating element81rotates the α-axis negative feedback current772and the β-axis negative feedback current792according to the negative phase angle428and outputs a d-axis negative current626and a q-axis negative current628. In one implementation, the first negative rotating element81may rotate the two phase negative current in the α-β reference frame to the two phase negative current in the d-q reference frame according to the following matrix equation:

In alternative embodiments, the controller40of the system100may be further configured to have the capability of providing vector VAR control or vector VAR regulation even when the electrical grid30is subjected to voltage ride through conditions, such as low voltage ride through (LVRT), zero voltage ride through (ZVRT), and high voltage ride through (HVRT) conditions.

FIG. 14illustrates a block diagram of a first positive regulation module220of the positive power regulator48shown inFIG. 2in accordance with another exemplary embodiment. The first positive regulation module220is configured to provide a current command in consideration of voltage ride through conditions. In one implementation of the disclosure, the first positive regulation module220includes a first summation element76, a DC voltage regulator78, a second summation element96, a first current limiter80, a multiplication element88, a filter92, and a second current limiter94.

As shown in a lower part ofFIG. 14, the feedback DC voltage502is subtracted from the DC command406by the first summation element76to provide a difference DC voltage command762. The difference DC voltage command762is regulated by the DC voltage regulator78to provide a first d-axis positive current command782. As shown in an upper part ofFIG. 11, the multiplication element88multiplies the q-axis positive voltage584by a q-axis gain signal190and provides a d-axis positive current882. The filter92filters the d-axis positive current882according to a q-axis signal210and provides a filtered d-axis positive current command922. The q-axis signal210is supplied for indicating a bandwidth of the filter92. The second current limiter94limits the filtered d-axis positive current command922and provides a second d-axis positive current command942. The second summation element96sums the first d-axis positive current command782and the second d-axis positive current command942and provides a third d-axis positive current command962. The first current limiter80limits the third d-axis positive current command962and provides a limited d-axis positive current command802. The limited d-axis positive current command802is transmitted to the positive current regulator128.

FIG. 15illustrates a block diagram of a second positive regulation module240of the positive power regulator48shown inFIG. 2in accordance with another exemplary embodiment. The second positive regulation module240is configured to provide a current command in consideration of voltage ride through conditions. In one implementation of the disclosure, the second positive regulation module240includes a first summation element66, a VAR regulator68, a second summation element70, a voltage regulator72, a voltage limiter75, a third summation element77, a gain element82, a filter84, a first current limiter86, a fourth summation element73, and a second current limiter74.

As shown in an upper branch ofFIG. 15, the feedback positive reactive power462is subtracted from the positive reactive power command402by the first summation element66to provide a difference positive reactive power command662. The difference positive reactive power command662is regulated by the VAR regulator68and to provide a regulated positive voltage command682. In one implementation, the VAR regulator68may include a proportional integral (PI) controller. Other type of controllers can also be used, for example, proportional derivative (PD) controllers, and proportional integral derivative (PID) controllers. A positive voltage magnitude110is subtracted from the regulated positive voltage command682by the second summation element70to provide a difference positive voltage command702. The positive voltage magnitude110can be calculated by the expression (3) as discussed above with reference toFIG. 9. The difference positive voltage command702is further regulated by the voltage regulator72to provide a first q-axis positive current command722. In one implementation, the voltage regulator72may include a PI controller. Other type of controllers can also be used, for example, proportional derivative (PD) controllers, and proportional integral derivative (PID) controllers.

As shown in a lower branch ofFIG. 15, the voltage limiter75limits the d-axis positive voltage582and provides a limited d-axis positive voltage752. The limited d-axis positive voltage752is subtracted from the d-axis positive voltage582by the third summation element77to provide a difference d-axis positive voltage772. The gain element82multiplies the difference d-axis positive voltage772by a d-axis gain signal150and provides a q-axis positive current822. The filter84filters the q-axis positive current822according to a d-axis signal170and provides a filtered q-axis positive current command842. The d-axis signal170is a predetermined signal and is supplied for indicating a bandwidth of the filter84. The filtered q-axis positive current command842is limited by the first current limiter86to provide a second q-axis positive current command862. The fourth summation element73sums the first q-axis positive current command722and the second q-axis positive current command862and provides a third q-axis positive current command732. The second current limiter74limits the third q-axis positive current command732and provides a limited q-axis positive current command742. The limited q-axis positive current command742is transmitted to the positive current regulator128ofFIG. 12.

FIG. 16illustrates a block diagram of a second negative regulation module340of the negative power regulator52shown inFIG. 2in accordance with another exemplary embodiment. The second negative regulation module340is configured to regulate the d-axis negative voltage586and further to regulate the feedback negative reactive power464according to the negative reactive power command404to provide a q-axis negative current command1262. In one implementation, the second negative regulation module340includes a multiplication element106, a filter108, a first limiter112, a first summation element114, a VAR regulator116, a second summation element118, a voltage regulator122, a third summation element124, and a second current limiter126.

As shown in an upper branch ofFIG. 16, the multiplication element106multiplies the d-axis negative voltage584by a d-axis gain signal270and provides a multiplied d-axis negative voltage1062. The filter108processes the multiplied d-axis negative voltage1062according to a d-axis signal290and provides a first q-axis negative current command1082. The first limiter112limits the q-axis negative current command1082and provides a limited first q-axis negative current command1122.

As shown in a lower branch ofFIG. 16, the feedback negative reactive power464is subtracted from the negative reactive power command404by the first summation element114to provide a difference negative reactive power command1142. The difference negative reactive power command1142is regulated by the VAR regulator116and to provide a regulated negative voltage command1162. A negative voltage magnitude350is subtracted from the regulated negative voltage command1162by the second summation element118to provide a difference negative voltage command1182. The negative voltage magnitude350can be calculated by the following expression: Vn—mag=√{square root over (Vdn2+Vqn2)} (11), wherein Vn—magis the negative voltage magnitude350, Vdnis the d-axis negative voltage586, and Vqnis the q-axis negative voltage588. The difference positive voltage command1182is further regulated by the voltage regulator122to provide a second q-axis negative current command1222. The third summation element124sums the first q-axis negative current command1122and the second q-axis negative current command1222and provides a third q-axis negative current command1242. The second current limiter126limits the third q-axis negative current command1242and provides a limited q-axis negative current command1262. The q-axis negative current command1262is transmitted to the negative current regulator134for current regulation.

It is understood that the controller40may be implemented in a variety of ways. For instance, the controller40may hardwired or implemented as a set of computer programs operating on a general-purpose computer with appropriate interfaces to the voltage sensor32, the current sensor34, and the DC sensor50.

It is to be understood that not necessarily all such objects or advantages described above may be achieved in accordance with any particular embodiment. Thus, for example, those skilled in the art will recognize that the systems and techniques described herein may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.

Furthermore, the skilled artisan will recognize the interchangeability of various features from different embodiments. The various features described, as well as other known equivalents for each feature, can be mixed and matched by one of ordinary skill in this art to construct additional systems and techniques in accordance with principles of this disclosure.