This thesis considers the use of a full-scale back-to-back pulse width modulated current source converter (BTB-PWM-CSC) as an interfacing converter for a variable-speed wind energy conversion system (WECS) that uses a permanent magnet synchronous generator (PMSG). It has been shown that the proposed current source converter based WECS can operate successfully under maximum power point tracking and meet all grid requirements such as ac fault ride-through and power quality aspects at the point of common coupling. This thesis presents three different CSC based WECSs. The first WECS is based on a back-to-back dual pulse width modulated current source converter (BTB-DPWM-CSC) as an interface for a dual three-phase PMSG, with two stator windings, shifted by 30°. Both the generator and grid side dual PWM rectifier and inverters (CSR and CSI) of the WECS are controlled using selective harmonic elimination (SHE) for low semiconductor loss operation. The grid side CSI is connected to the ac grid through a three-winding phase-shift transformer to benefit from cancellation of the low-order harmonics in the primary winding, which is connected to the ac grid side. The secondary and tertiary windings, which are connected to the upper and lower halves of the dual PWM-CSI, are delta and wye connected; thus, SHE is only needed to eliminate the 11th and 13th harmonics. The SHE modulation employed to control both the dual PWM-CSR and CSI has one unique pattern, which is characterised by continuous pulse angle changes and a fixed switching frequency over the full modulation index range. These attributes show that the proposed BTB-DPWM-CSC WECS is suited for multi-megawatt applications. The second and third WECSs are based on modified three-phase and dual three-phase BTB-PWM-CSCs. These configurations are developed to address the main drawbacks of the conventional CSC employed in the first proposal, such as transient over-voltages experienced by the converter switches during commutation and semiconductor loss reduction. The latter is achieved by using a dedicated high frequency synchronization method that ensures zero current switching at the CSI terminal; thus, nearly zero switching loss is achieved. The proposed WECSs offer the following additional advantages: reduced power circuit and control complexity; reduced switching frequency; applicable to fixed and variable frequency operation, hence, allowing maximum power point tracking with independent control of active and reactive powers delivered to the ac grid; and low-voltage ride-through capability. Viability of the proposed WECSs are assessed using simulations performed in PSCAD/EMTDC and confirmed experimental, with results from scaled down prototypes, assessed in steady-state and dynamically under different operating conditions. It is shown that continuous and discontinuous operation of the second and third configurations provide trade-offs between overall weight and size of the WECS and high current and voltage stresses in the semiconductor switches.
|Date of Award||24 Sep 2015|
- University Of Strathclyde
|Supervisor||Derrick Holliday (Supervisor) & Barry Williams (Supervisor)|