A NOVEL WIND-POWER TRAIN : PERMANENT-MAGNET GENERATOR WITH FLUX WEAKENING FEEDING A CONTROLLED REAL/REACTIVE POWER INVERTER

Abbas  A. FARDOUN

   Harnessing wind power has gained momentum due to new advances in technology and as global concerns have increased over safety and pollution produced by other energy sources such as nuclear energy or fossil fuels. Existing wind power plants utilize synchronous or induction generators in addition to gear boxes. In this thesis a direct permanent‑magnet drive without a mechanical transmission is introduced.

In the first part of the thesis two types of control schemes for permanent‑magnet machines with flux weakening are presented for a given rated output power (e.g., 20 kW). In the first one flux weakening is achieved via an additional rotor coil counteracting the permanent‑Magnet excitation as the speed increases resulting in a speed range of 1:10. In the second one (design # 4) flux weakening is controlled via the field orientation of the stator excitation resulting in a speed range of about 1:4. It is interesting to note that, while in the first type the additional rotor excitation is responsible for higher rotor losses (efficiency is below 70% at high speed) and a weight increase (1700 lbs.), in the second type the over‑excited stator magnetomotive force leads to higher stator losses at high speed and relatively larger stator slots for design # 4.

In the third Chapter design # 3 is investigated at a load of 300 kW. Two designs are also investigated with respect to the magnet demagnetization under short‑circuit conditions. The first design (design A) is the same as design # 3 while in the second design (design # B) the magnet is located within the rotor yoke.

In Chapter 4 an IGBT‑based inverter operating at a switching frequency of 5.76 kHz is presented. Opto‑couplers are used for isolation of the upper IGBTs, which permit a duty ratio of I and decrease the IGBT switching losses. The inverter output current as well as the inverter switching frequency are synchronized with that of the power system via a phase‑lock loop permitting the control of the output power factor and canceling noninteger harmonics. Thus no power factor correction capacitors are needed. The inverter output stage also acts as an active filter by being able to produce an output current of fundamental (identical to the power system frequency) and harmonic frequency. The inverter circuit has been built and tested at different load conditions up to 150 % of rated load, 30 kW.

In Chapter 5 a unity‑power factor, low total harmonic rectifier is utilized to extract maximum real output power from the permanent‑magnet generator. Since this type of rectifier utilizes one switch only, a simple control circuit is built to control the input power factor and the output load. The rectifier is built and tested at high power load.