# Patent

## Title
Wind turbine with parallel converters utilizing a plurality of isolated transformer windings

## Abstract

A system and method are provided to isolate outputs of parallel converter threads of a power system converter of a wind turbine generator by utilizing isolated power windings on the output transformer from the converter. Such isolation eliminates the circulating common mode current between the parallel converters of the wind turbine system and eliminates the need for a common mode inductor. System reliability is enhanced and total system cost is reduced.

## Background
BACKGROUND OF THE INVENTION
The invention relates generally to wind turbine generators and more specifically to a method of interconnecting parallel power converters for the wind turbine generators to eliminate a common mode current, which circulates between the parallel power converters.
Generally, wind turbines use the wind to generate electricity. The wind turns multiple blades connected to a rotor. The spin of the blades caused by the wind spins a shaft of the rotor, which connects to a generator that generates electricity. Specifically, the rotor is mounted within a housing or nacelle, which is positioned on top of a truss or tubular tower, which may be as high as about 100 meters. Utility grade wind turbines (e.g., wind turbines designed to provide electrical power to a utility grid) can have large rotors (e.g., 30 or more meters in diameter). Blades on these rotors transform wind energy into a rotational torque or force that drives one or more generators, rotationally coupled to the rotor through a gearbox. The gearbox may be used to step up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert mechanical energy to electrical energy, which is provided to a utility grid. Some turbines utilize generators that are directly coupled to the rotor without using a gearbox. Various types of generators may be used in these wind turbines.
Many devices, such as wind turbines, include power converter systems. A power converter system is typically used to convert an input voltage, which may be fixed frequency alternating current, variable frequency alternating current, or direct current, to a desired output frequency and voltage level. A converter system usually includes several power semiconductor switches such as insulated gate bipolar transistors (IGBTs), integrated gate commutated thyristors (IGCTs or GCTs), or metal-oxide semiconductor field effect transistors (MOSFETs) that are switched at certain frequencies to generate the desired converter output voltage and frequency. The converter output voltage is then provided to various loads. Loads as used herein are intended to broadly include motors, power grids, and resistive loads, for example.
FIG. 1 is a block diagram of a typical power system coupled to a wind turbine with synchronous wound-field or permanent magnet generator and implemented according to one aspect of the invention. The power system 10 is configured to provide AC output power to grid 21. A wind turbine 12 is configured for converting wind energy to mechanical energy. The wind turbine is coupled through a gear box 19 to generator 14 or alternatively coupled directly to generator 14. Wind energy is captured by the rotation of the wind turbine's blades, and generator 14 is configured by a power converter system 20 controlled by converter control system 24 for generating a variable frequency input power. The power is transformed to appropriate voltage by one or more transformers 22 and supplied to the power grid 21.
To accommodate the need for greater power from windfarms, individual wind turbine generators are increasingly being provided with higher power output capability. To accommodate the higher power output from the wind turbine generators, some wind turbine systems are provided with multiple parallel converters (also known as converter threads). Multiple parallel converters may also provide an advantage in wind converters due to the desire for high availability and low distortion
Typically, power converter systems use multiple power converter bridges in parallel with gating control to expand power-handling capability. In wind turbine applications, a power converter bridge usually refers to a three-phase converter circuit with six power switches. In order to meet both grid side and machine side power quality requirements, such systems generally use very large and costly filters to smooth out pulse width modulated waveforms. Such systems sometimes cause overheating of the generator and/or transformers and other distortion-sensitive equipment due to high harmonic components, when the large and costly filters are minimized.
FIG. 2 is a block diagram of a typical power system employing multiple parallel converters. Power system 10 is configured for supplying power to a load 21. A generator source 14 is configured to generate an AC input power. The AC input power is provided to power converter system 20. The power converter system 20 comprises converter 20-1 through 20-N. The converters are coupled in parallel and configured to receive the AC input power from the generator source 14. The power converter system 20 is configured to convert the AC input power to an AC output power. The AC output power is provided to load 21. Loads may include motors, power grids, and resistive loads, for example. Although grids are traditionally suppliers of power, in most wind turbine system embodiments, wind turbine power is supplied to a utility grid, which acts as a load.
The plurality of multiple parallel converters, each one of which (also called threads) has a fraction of the net system rating. These converter threads are tied together on both the input and output ends to form a net current/power rating on both the input and output that is directly related to the number of converter threads in parallel. Typically, one side of the converter is connected to a common power source (for example the grid) and the other to a plant (for example a generator). The circuit connecting the converter to the power grid will usually be referenced to ground. For cost and size reasons, each thread is connected to a common point on the grid and the plant with conductors that are sized in accordance with the rating of each thread and not the system rating.
Converter control system 24 is configured to provide control signals for the operation of the power converter system 20. The converter control system 24 is coupled to the power converter system 20 and is configured to drive the converter system according to predesignated switching patterns. The predesignated switching patterns provided by the converter control system 24 may provide for synchronous gating of the multiple parallel converters (20-1 to 20-n) or may provide an interleaved manner of control for each converter thread with phase displaced gating signals to reduce overall switching harmonic components due to cancellation of phase shifted switching waveforms.
FIG. 3 is a block diagram of a typical thread of a power converter system. Wind turbine embodiments, for example, typically comprise three-phase power converter systems. Converter 20-1 represents one thread of power converter system 20. Converter 20-1 comprises generator converter bridge 30 for AC-DC conversion, DC link 35, and load converter bridge 40 for DC-AC conversion at a suitable voltage and frequency. Generator converter bridge 30 may be implemented using six power semiconductor switches 45. Similarly, load side bridge 40 may be implemented using six power semiconductor switches 45. Generator side chokes 50 and load side chokes 55 may be sized to enable either non-interleaved or interleaved gating
Switching of power semiconductors in the converter threads causes a difference in voltage between the parallel converters, which creates a common mode current that flows between the converter threads, even without having a ground fault on the system. The common mode current will flow in a circular loop between the power converter threads, but not have any impact on the net current in either the grid or the plant. Common mode chokes 60 suppresses the high frequency (switching frequency range) common mode cross current that links both generator side converters and the load side converters.
FIG. 4 illustrates common mode current flow in a power system converter with n-paralleled converter threads (20-1 to 20-n) connected to a grid 21 and to a wind turbine generator 14. For example, it is possible that a current can flow into thread T1_L_Ia 110 and out T1_G_Ia 115 and return through thread T2_C_Ia 120 and T2_L_Ia 125. There are many combinations of loops for such current that will not affect the net current. However, these common mode currents, as well as normal mode circulating currents, force converter switching devices and other components to operate closer to thermal limits. Further, these common mode currents may cause a direct error in the measurement of ground fault currents of that loop, thereby making fault detection more difficult. Large common mode inductors are required to limit the amount of circulating common mode current between the converters, as well as, large normal mode reactors are required to limit circulating normal mode current where phase shifting is utilized to reduce net distortion.
Accordingly, there is a need to provide a structure and method for interconnecting the power converter in a manner so as to reduce or eliminate the common mode current that flows between the parallel converter threads, without the need for common mode inductors, coupled to a capability to phase shift multiple threads to reduce the need for bulky filters.
BRIEF DESCRIPTION OF THE INVENTION
Recently, wind turbines have received increased attention as an environmentally safe and relatively inexpensive alternative energy source. With this growing interest, considerable efforts have been made to develop wind turbines that are reliable and efficient. This invention relates to a specific method of interconnecting power converters in wind turbines, which allows optimization of system cost and reliability by eliminating the common mode current that circulates between parallel power converters.
According to a first aspect of the present invention, a power system for providing an output power to a load is provided. The power system includes a generator configured to generate an alternating current input power to a power converter system. The power converter system is coupled to the generator and interconnected to generate an output power and provide the output power to the load, wherein the converter system includes a plurality of parallel converter threads. A converter control system is coupled to the power converter system and configured to drive the converter system to reduce harmonic components in the output power or the alternating current input power. Isolation means for the output power from the converter to the load are adapted to prevent common mode current circulation between the parallel converter threads.
According to a second aspect of the present invention, a method is provided for eliminating common mode currents in a power system that supplies a power output to a load from a power converter system, including a plurality of parallel converter threads. The method includes operating the generator according to a controller to supply ac power to the power system converter and supplying the ac power to the power system converter. The method also includes operating the plurality of converter threads according to a controller for the power converter system; adapted to drive the power converter system to reduce harmonic components in the power output or the alternating current input power. The method further includes supplying the power output of the power converter system to the load through at least one set of a plurality of isolated power transformer windings, wherein at least one set of the isolated power transformer windings is fed by a corresponding converter thread of the plurality of converter threads.
According to a third aspect of the present invention, a wind turbine power system is adapted to prevent common mode current circulation among power system converter threads when providing output power to an electrical power grid. The wind turbine power system includes a wind turbine generator controlled by a wind turbine controller configured to generate an alternating current input power to a power converter system. The power converter system is coupled to the wind turbine generator and interconnected to generate an output power and provides the output power to the load. The power converter system includes a plurality of the parallel converter threads. A converter control system is coupled to the power converter system and configured to drive the parallel converter threads to reduce harmonic components in the output power or the alternating current input power. The isolation means for the output power from the converter to the power grid is adapted to prevent common mode current circulation between the parallel converter threads. A plurality of sets of isolated power windings on a power transformer, located between the converter and the load, is provided. Each set of the plurality of sets of isolated power windings on the transformer connects with only one of a corresponding parallel converter thread of the plurality of parallel converter threads.



BRIEF DESCRIPTION OF THE DRAWING
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
FIG. 1 illustrates a typical wind turbine generator power system for delivering an electric power output through a power system converter to a load from the wind turbine generator;
FIG. 2 illustrates a block diagram of a typical power system employing multiple parallel frequency and voltage converters;
FIG. 3 illustrates a block diagram of a typical thread one thread of a power converter system;
FIG. 4 illustrates common mode current flow in a power system converter with paralleled converter threads;
FIG. 5 illustrates the power output connections for non-isolated windings of a wind turbine generator power with converter threads of a power converter system;
FIG. 6 illustrates an embodiment of a power system including a power converter system incorporating a plurality of parallel converter threads supplying isolated power windings, in a wye configuration, for a load side transformer;
FIG. 7 illustrates another embodiment of a power system including a power converter system incorporating a plurality of parallel converter threads supplying isolated power windings, in a delta configuration, for a load side transformer; and
FIG. 8 illustrates a flowchart for a method of eliminating common mode currents in a power system supplying power output to a load from a power converter system including a plurality of parallel converter threads.



DETAILED DESCRIPTION OF THE INVENTION
The following embodiments of the present invention have many advantages, including eliminating the circulating common mode current between parallel converters threads. In order to eliminate the circulating common mode current between parallel converters of the wind turbine system, the new structure and method described isolate the outputs of the parallel converter threads on the side of the load-side, by utilizing isolated power windings on the input side of the main transformer. Such an arrangement eliminates the need for a common mode inductor, promoting system reliability and reducing the total system cost.
FIG. 5 illustrates the power output connections for prior art, non-isolated windings of a wind turbine generator power with generator-side parallel converter threads for a power converter system. The power system converter 20 includes parallel converter threads 20-1, 20-2, 20-3, and 20-n. The generator power windings 205 for wind turbine generator 14 may be configured in a wye arrangement, with power windings 210, 211, 212 tied at the wye neutral point 215. The opposite end of each phase of the power windings 210, 211, 212 (opposite from the wye neutral) may be tied to each input 250, 255, 260 of one corresponding phase (Phase A 230, Phase B 235, Phase C 240 for each of the parallel converter threads 20-1 to 20-n. Inputs of corresponding phase connections from each of the converter threads are tied together Inputs from the converter threads 20-1 to 20-n are similarly tied at points 270, 275 and 280 to converter side windings 25-1 for main transformer 25, permitting common mode circulating current 205 to flow in closed loop between converter threads.
FIG. 6 illustrates an embodiment of a power system including a power converter system incorporating a plurality of parallel converter threads supplied by isolated, wye-configured, power windings of the main transformer. A wind turbine generator 14 may supply inputs Phase A 230, Phase B 235, Phase C 240 to an exemplary four parallel converter threads 20-1 to 20-n of power system converter 20 from three sets of non-isolated three-phase windings 210, 215, 220 of generator power windings 205. In the instant example, non-isolated windings in a wye configuration are illustrated. However, the non-isolated wind turbine generator windings 405 may instead include any other transformer arrangement suitable for power application.
According to an aspect of the present invention, the converter side windings 25-1 of the main transformer 25 include a separate set of isolated three-phase power windings for each individual converter thread 20-1 to 20-n. Output connections 340, 345, 350 and 355 from of converter threads 20-1 to 20-n are tied to the isolated wye-configured power windings 305, 310, 315 and 320 of main transformer 25, respectively.
Because the converter side power windings 25-1 of main transformer 25 are isolated for the individual converter threads, the converter threads are no longer galvanically connected on the load side, thereby preventing the flow of common mode currents on the generator side between the converter threads. The flow path is isolated despite the connection at points 250, 255 and 260 between the same phase of each converter thread on the generator side. With the elimination of the common mode currents flowing between the converter threads, the common mode reactor may be eliminated, reducing cost, complexity, and size of the converter thread.
FIG. 7 illustrates another embodiment of a power system including a power converter system incorporating a plurality of parallel converter threads supplying isolated power windings, in a delta configuration, for a load side transformer. Isolated three-phase input transformer power winding 405, 410, 415 and 420 accept outputs 440, 445, 450 and 455 from individual converter threads 20-1 to 20-n, providing a similar isolation effect between the respective converter threads, thereby blocking common mode currents from flowing between converter threads.
While FIG. 6 or FIG. 7 illustrate wye-wye configured transformers and delta-delta configured transformers to be used to isolate circulation of common mode currents, it is contemplated that isolation for common mode circulating currents may be provided on the converter side of the load using a star-configured transformer or any other type of transformer suitable for power operation. Further, while a wye-wye transformer configuration and a delta-delta transformer configuration were illustrated in FIG. 6 and FIG. 7, respectively, wye-delta and delta-wye configurations may also be utilized, as well as other suitable types of power transformers
While this embodiment has described a power system and a power converter for a wind turbine generator, it is understood that the present invention may be generally applicable to other types of power systems, electric power generators and power system converters.
Isolation of the generator windings for providing electric power input to the converter may further be combined with isolation of the windings for the power output of the parallel converter threads of the converter on an output transformer.
In a further embodiment of the present invention, a method is provided for eliminating common mode currents in a power system supplying a power output to a load from a power converter system including a plurality of parallel converter threads. The method may include operating a wind turbine generator according to a wind turbine controller for providing an electrical output to a grid through a converter, including a plurality of parallel converter threads. The method also includes operating the parallel converter threads to supply electrical power to the grid according to a controller for the power converter system; adapted to drive the power converter system to reduce harmonic components in the power output or the alternating current input power using the electrical power supplied by the wind turbine generator. The method also includes supplying electrical power from each of the parallel converter threads through an isolated winding of the converter side transformer, adapted to electrically isolate a circulating path for common mode currents between converter threads.
FIG. 8 illustrates a flowchart of a method for eliminating common mode currents in a power system supplying a power output to a load from a power converter system including a plurality of parallel converter threads. Initial step 510, operates a wind turbine generator according to a wind turbine generator controller for supplying for an electrical output to a grid through a converter including a plurality of parallel converter threads. In step 520 the power system converter operates the parallel converter threads to supply electrical power to the grid using the electrical power supplied by the wind turbine generator. In operating the parallel converter threads, the controller for the converter provides gating signals to the semiconductor switches of the parallel converter threads to drive the power converter system to reduce harmonic components in the power output or the alternating current input power. Gating by the parallel converter threads may be performed in an interleaved or a non-interleaved pattern. In step 530, the parallel converter threads supplies power to the grid through isolated input power windings of the main transformer on the load side of the converter, preventing common mode current from circulating between converter threads.
While various embodiments are described herein, it will be appreciated from the specification that various combinations of elements, variations or improvements therein may be made, and are within the scope of the invention.

## Claims

1. A power system for providing an output power to a load, the system comprising;
a generator configured to generate an alternating current input power to a power converter system;
a power converter system coupled to the generator and interconnected to generate an output power and provide the output power to the load, wherein the converter system includes a plurality of parallel converter threads; each parallel converter thread including, in series, a semiconductor bridge ac-to-dc converter and a semiconductor bridge dc-to-ac conductor;
a converter control system coupled to the power converter system and configured to drive the converter system to reduce harmonic components in the output power or the alternating current input power; and
isolation means, for the output power from the converter to the load, comprising a single transformer, preventing common mode current circulation between the parallel converter threads as powered by voltage differences between the parallel converter threads generated by switching of power semiconductors of the semiconductor bridges of the parallel converter threads, the transformer including a plurality of sets of electrically isolated power windings between each of the converter threads and a load, wherein each set of plurality of sets of electrically isolated power windings of the transformer electrically interconnects only with an output from one corresponding parallel converter thread of the plurality of parallel converter threads and wherein the set of isolated power windings on the transformer comprises a three-phase power winding and each of the plurality of parallel converter threads comprise a three-phase power output.

2. The power system of claim 1, wherein electrical separation of the isolated power windings on the output transformer are maintained by physically interconnecting the isolated power windings of the output transformer with only one of a corresponding parallel converter thread of the plurality of parallel converter threads.

3. The power system of claim 2, wherein the three-phase power winding comprises:
wye-connected power input windings.

4. The power system of claim 2, wherein the three-phase power winding comprises:
delta-connected generator power input windings.

5. The power system of claim 2, wherein the three-phase power winding comprises:
at least one of delta-wye configured power windings and wye-delta power windings.

6. The power system of claim 2, wherein the generator comprises: a wind turbine generator.

7. The power system of claim 2, wherein the load comprises: an electric power grid.

8. The power system of claim 2, wherein the converter control system drives the parallel converter threads of the converter system according to an interleaved control scheme.

9. The power system of claim 2, wherein the converter control system drives the parallel converter threads of the converter system according to a non-interleaved control scheme.

10. A method for eliminating circulating common mode currents in a power system supplying a power output to a load from a power converter system including a plurality of parallel converter threads with each parallel converter thread including, in series a semiconductor bridge ac-to-dc converter and a semiconductor bridge dc-to-ac converter, wherein the circulating common mode currents between the parallel converter threads are powered by voltage differences between the parallel converter threads generated by switching of power semiconductors of the semiconductor bridges of the parallel converter threads, the method comprising:
operating the generator according to a controller to three-phase ac supply power to a power converter system;
operating the plurality of parallel converter threads to supply three-phase power to a load according to a controller for the power converter system adapted to drive the power converter system to reduce harmonic components in the power output or the alternating current input power;
supplying the three-phase ac power from the power system converter to a load through a plurality of sets of electrically isolated three-phase power windings of a single transformer on the load side of the power system converter, wherein each set of the electrically isolated three-phase power converter thread of the plurality of parallel converter threads feeds corresponding isolated three-phase power transformer windings; and
blocking the circulating common mode currents from flowing between the parallel converter threads.

11. The method for eliminating circulating common mode currents according to claim 10, comprising: supplying the ac power to the load from the power system converter through sets of isolated three-phase power windings on the transformer.

12. The method for eliminating circulating common mode currents according to claim 11, further comprising: supplying the ac power to the load from the power system converter through sets of isolated, wye-configured, three-phase power windings on the transformer.

13. The method for eliminating circulating common mode currents according to claim 11, further comprising: supplying the ac power to the load from the power system converter through sets of isolated, delta-configured, three-phase power windings on the transformer.

14. The method for eliminating circulating common mode currents according to claim 11, further comprising: supplying the ac power to the load from the power system converter through at least sets of isolated, wye-delta, three-phase power windings on the transformer and sets of isolated, delta-wye, three-phase power windings on the transformer.

15. The method for eliminating circulating common mode currents according to claim 11, wherein the generator comprises a wind turbine generator and the load comprises an electric power grid.

16. The method for eliminating circulating common mode currents according to claim 11 further comprising: driving the parallel converter threads of the converter system by the converter control system according to an interleaved control scheme.

17. The method for eliminating circulating common mode currents according to claim 11 further comprising: driving the parallel converter threads of the converter system by the converter control system according to a non-interleaved control scheme.

18. A wind turbine power system adapted to prevent common mode current circulation among power system converter threads, powered by voltage differences between the parallel converter threads generated by switching of power semiconductor bridges of the parallel converter threads, when providing output power to an electrical power grid, the system comprising;
a wind turbine generator controlled by a wind turbine controller configured to generate an alternating current input power to a power converter system;
a power converter system coupled to the wind turbine generator and interconnected to generate an output power and provide the output power to the load, wherein the power converter system includes a plurality of the parallel converter threads;
a converter control system coupled to the power converter system and configured to drive the parallel converter threads to reduce harmonic components in the output power or the alternating current input power; and
a plurality of sets of electrically isolated power windings on a single power transformer on the output side of the converter, wherein each set of the plurality of sets of electrically isolated power windings of the transformer interconnects with only one of a corresponding parallel converter thread of the plurality of parallel converter threads, blocking circulating common mode currents from flowing between the individual converter threads.

19. The wind turbine power system according to claim 18, wherein the sets of isolated power windings on the wind turbine generator comprise: three-phase power winding configured in at least one of a wye configuration, a delta configuration, a delta-wye configuration and a wye-delta configuration.