Offshore wind is a rapidly growing renewable energy resource. Harvesting offshore energy requires multimegawatt wind turbines and high efficiency, high power density, and reliable power conversion systems to achieve a competitive levelized cost of electricity. An integrated system utilizing one active and multiple passive rectifiers with a multi-port permanent magnet synchronous generator is a promising alternative for an electro-mechanical power conversion system. Deployment of the integrated systems in offshore wind energy requires maximum power point tracking (MPPT) capability, which is challenging due to the presence of numerous uncontrolled passive rectifiers. This paper shows feasibility of MPPT based on a finding that the active rectifier d-axis current can control the total system output power. The MPPT capability opens up opportunities for the integrated systems in offshore wind applications.
- Power conversion
- Ac-dc power conversion
- Dc power systems
- Wind energy
- Maximum power point trackers
- Wind energy generation
Figure 1. (a) Wind turbine power-point tracking architecture: the prime mover is a variable-speed wind turbine. The turbine shares a common shaft with the multi-port PMSG. Ac power is converted to dc by an integrated generator-rectifier system. The dc output is connected to a stiff dc interface. The integrated generator-rectifier system performs maximum power-point tracking to extract the turbine maximum power.
EXPECTED SIMULATION RESULTS:
Figure 2. (a) (Top plot) The active rectifier d-axis and q-axis currents track the reference command, presented by the dotted lines. The dc-bus current varies accordingly by changing the d-axis current, leading to a change in the dc-bus power (bottom plot). (b) The relationship between dc-bus power and active-rectifier d-axis current acquired from the simulation model (recorded by the markers) matches the theoretical analysis (plotted by the lines using equation (6)).
Figure 3. Waveforms to illustrate the system MPPT capability. (a) At each wind speed, the turbine speed (solid-blue line) successfully tracks the optimal speed to generate maximum power. (b) The dc-bus power and the turbine mechanical power versus time. (c) The d-axis and q-axis currents to achieve MPPT.
Figure 4. Generator phase-A back emf, phase-A current, and power of the passive and active rectifiers at different operating speeds. (a) Sinusoidal and phase-shifted back emfs at the rated generator speed. (b) The corresponding phase-A currents. (c) Sharing of PMSG input power between ac ports powering active versus passive rectifiers. (d) Back emfs at the minimum operating speed that is equal to 55% the rated speed. (e) Phase-A currents corresponding to the minimum speed. (f) Power sharing between the ac ports powering active and passive rectifiers at the minimum operating speed.
This paper presents an MPPT methodology for an integrated generator-rectifier system. An analytical relationship between the dc-bus power and the active rectifier d-axis current is established and validated using both simulation and experiment. A cascaded control architecture is proposed for practical implementation. The inner loop comprises PI current controllers with feed-forward terms, while the outer loop is a PI power controller. Satisfactory power tracking performance has been accomplished. The power flow control enables the wind turbine MPPT through controlling the dc-bus power. This capability opens up opportunities for the integrated generator rectifier systems in wind energy applications.
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