Modulation scheme investigation for high-power medium-voltage current source converter based drives

Abstract

Pulse width modulated (PWM) current source converter (CSC) based drives are commonly used in high-power (1-10 MW), medium-voltage (MV) (2.3-6.6 kV) applications. These drives feature a simple converter structure, inherent four-quadrant operation capabilities, motor-friendly waveforms, and reliable fuseless short-circuit protection. PWM CSC-based drives are generally constructed using symmetrical gate-commutated thyristors (SGCTs) with reverse voltage blocking capabilities. In order to avoid exceeding the thermal limits of these SGCT devices, and to minimize switching losses, the device switching frequency used by PWM CSC-based drives is typically kept below 500 Hz. There are three main modulation schemes used in MW-level MV PWM CSC-based drives: space vector modulation (SVM), trapezoidal pulse width modulation (TPWM), and selective harmonic elimination (SHE). Of these three modulation schemes, SHE possesses the best harmonic performance as it features the ability to eliminate a number of low-order harmonics, all while retaining a low switching frequency. However, due to its off-line implementation, SHE suffers from poor dynamic performance, and in certain cases, requires a large, memory-exhaustive look-up table. To address these issues, this research investigates ways of improving the dynamic performance of conventional SHE through on-line (i.e., real-time) implementation. Two new modulation schemes are proposed: on-line SHE for modulation of the grid-side PWM current source rectifier (CSR) and SHE-TPWM for modulation of the motor-side PWM current source inverter (CSI). The proposed online SHE scheme models the independent switching angles used in conventional SHE as polynomial functions by applying curve-fitting techniques. This method of implementation improves the dynamic performance of conventional SHE, as it enables real-time computation of switching angles, and eliminates the need for look-up tables. Conversely, the proposed SHE-TPWM scheme combines the principles, while retaining the respective advantages, of both conventional SHE and TPWM. This integrative approach enables SHE-TPWM to possess SHE-level harmonic performance, along with improved dynamic performance rivaling that of TPWM.