Stability issues in three-phase PWM current/voltage source rectifiers in the regeneration mode

Pulse-width modulated current-source and voltage-source rectifiers are becoming an interesting alternative as a source of variable DC power. Advantages include reduced line current harmonic distortion and complete displacement power factor control, including reactive power compensation. Consequently, literature has recently reported different control strategies, where the active power flows from the AC mains into the load. Unfortunately, when active power flows from the load into the converter, it is usually dissipated in the DC link by means of switched resistors. Such approach reduces the overall efficiency; specially, when regeneration is the normal operating mode as in down-hill belt conveyors and electrical shovels in mining applications. In these cases, the regenerative operating mode should be permanently sustained. This work reviews several important issues concerning such an operating mode; specifically, the stability of the converter-load system, and stability analysis of closed loop operation. The paper shows that PWM rectifiers controlled by a regular linear controller (such as PIs) may become unstable under the regenerative operating mode and analyses a control strategy to overcome such difficulties. Simulated results confirm the validity of theoretical considerations.