Due to the world-wide concerns about undesired consequences of using fossil fuels to produce energy, alternative or renewable energy technologies have been on the rise. Wind power is one popular renewable energy source. Using wind power involves conversion of wind energy into a more useful form of energy, such as electricity.
In a wind turbine, electricity is produced by the wind turning multiple blades connected to a rotor. The spin of the blades caused by the wind rotates a shaft of the rotor, which connects to a generator that generates electricity. The rotor may be mounted within a housing or nacelle, which is positioned on top of a truss or tubular tower (sometimes as high as 300 feet). Utility grade wind turbines (e.g., wind turbines designed to provide electrical power to a utility grid) may include large rotors (e.g., 100 or more feet in diameter). The blades of these rotors may transform the wind energy into a rotational torque or force that drives one or more generators, which are 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 then 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 wind turbines.
Wind turbines may include power converter systems. A power converter system is typically used to convert input electrical current, 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 on at certain frequencies to generate the desired converter output voltage and frequency. The converter output voltage is then provided to various loads. “Loads”, as the term is used herein, is intended to broadly include motors, power grids, and resistive loads, for example.
As the desired power level of a wind turbine increases, some wind turbine systems may require multiple power converters operating together in parallel to achieve the desired power rating.
FIG. 1 is a schematic representation of a conventional power system including multiple parallel converters. A power system 100 may be configured to supply power to a load 120. The power supplied may be generated by a generator 105 and subsequently provided to a power converter system 110. The power converter system 110 may comprise converters 110-1 through 110-N. The converters 110-1 through 110-N may be coupled in parallel and configured to receive power from the generator 105. The power converter system 110 may convert the received power and provide it to the load 120. The load 120 may include power grids, motors, resistive loads, and the like.
The power system 100 may also comprise a converter control system 115. The converter control system 115 may be configured to provide control signals for the operation of the power system 100. The converter control system 115 may be coupled to the power converter system 110 and configured to drive the power converter system 110 according to predesignated switching patterns. The predesignated switching patterns provided by the converter control system 115 may provide for synchronous gating of the multiple parallel converters 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.
Multiple converters operated in parallel within the power converter system 110 may provide high availability and low distortion. However, power systems utilizing multiple parallel converters may create common-mode currents flowing between the parallel converters, and this leads to a need for common-mode chokes, or otherwise isolating the converters at either the generator side or the line side to break the path of common mode currents between the converters.
As shown by FIG. 2, in some conventional power systems 200, common-mode chokes are used to protect the load 120 and the generator 105. In such existing power systems 200, the power converter system 110 includes generator side chokes 205, a Direct Current (DC) link 215, load side chokes 220, and parallel converters 210. The generator side chokes 205 and load side chokes 220 suppress common-mode current that links both converters 210.
However, this conventional solution may be costly and insufficiently reliable. Moreover, the possibility of arc flash events needs to be considered, since they become more prevalent as the power level of the system increases.
Therefore, with the increase of the desired power levels of power systems, it would be useful to provide power systems with multiple power converters operated together and, subsequently, there is a need for suitable means of optimizing and protecting such systems. According to the present disclosure, this may be achieved by combining multiple parallel converters with a multi-leg main transformer.