Cycloconvertor-excited divided-winding doubly-fed machine as a wind-power convertor

Abstract

The paper describes a slip-ring induction machine operating as a doubly-fed generator above and below synchronous speed. A prime mover simulates a wind turbine by producing a torque increasing in proportion to the square of the speed. This torque is balanced in steady-state operation by the doubly-fed generator. The stator, or secondary of the generator is arranged in two electrically separate, magnetically coupled layers connected to a cycloconvertor operating with continuous circulating current. This reduces the harmonics injected into the supply and prevents line-to-line short circuits under shock conditions. The secondary frequency is locked to the difference between actual speed and synchronous speed. A deadband is built into the frequency demand loop to ensure steady-state synchronous rather than asynchronous performance of the generator. Control of the secondary voltage amplitude enables the system to generate power proportional to the cube of the speed over a wide speed range. Three alternative methods of secondary control are considered. These are: (i) secondary current modification, (ii) a voltage-speed function and (iii) constant secondary current feed. The efficiences and peak power capabilities of the methods of systems (ii) and (iii) are contrasted in operation as a quasi-wind-energy transducerss.