Search

In applications that require a three-phase induction motor (IM) to be driven by an inverter from a dc bus input of much higher voltage than needed, the option is to use a step down dc–dc converter before the inverter or use high-voltage rating switches in the inverter along with low modulation index. In either case, disadvantages include poor efficiency, high-voltage rated switches, etc. In open-end winding three-phase IM drives, the three separated windings are driven by three-phase inverters from both sides. The arrangement is equivalent to three one-phase inverters driving each winding separately from the same dc bus in parallel. This article proposes an inverter scheme for open-end IM drives, using a one-phase inverter for each winding supply, but all three inverters being connected in series by their dc bus to the higher applied dc voltage. Unlike existing techniques for interfacing an IM drive to a higher voltage dc bus, the proposed scheme requires lower voltage switches with lower voltage bus capacitors to drive the lower voltage motor from a higher voltage dc bus. The series-connected dc bus capacitor voltages are shown to remain balanced during normal operation.

A retrofit solution is proposed in this article for providing fault-tolerant capability in the existing three-leg grid synchronized converter operating at unity displacement factor, during open circuit fault of one of its phases. During such a fault, the scheme controls the current in the two operative phases of the converter for equal magnitude with a mutual phase shift of 60°. This 60° phase-shifted two-phase current is distributed as a balanced three-phase grid current at unity displacement factor, using an existing Δ-Y isolation transformer or a lower power-rated zig-zag autotransformer. Existing fault-tolerant schemes are applicable only if the device or its driver fails, with the addition of hardware in the form of the redundant leg for the total failure of one phase of the converter. In retrofit application, the proposed scheme modifies the control logic and does not require modification in the controller hardware circuit or addition of a leg in the converter power circuit. Hence, it can be conveniently implemented to add fault-tolerant capability to an existing three-leg voltage source converter in either an inversion or rectification mode of operation. In addition to semiconductor failure, the proposed scheme is applicable during an open-circuit fault in other parts of the circuit viz., line reactor, connecting wire, transformer, etc., which is not possible using the existing fault-tolerant techniques. Simulation and experimental results are provided to validate the proposed concept.

High-voltage dc is popular for transmission of large volume of power over long distances due to several advantages. Medium voltage dc transmission are becoming de facto standard in offshore wind generation and in similar cases where long cable lengths are required for transmitting power from the source to the usable destination. Several schemes have been proposed for generating the high dc bus voltage from the lower voltage ac generators at the offshore platform. However, size, weight, and maintenance are the major deciding factors. This article proposes a scheme of generating suitable dc voltage from a lower voltage synchronous generator without using transformers or boost converters. It uses an open end winding permanent magnet synchronous generator. The voltage outputs from each winding are converted to boosted dc by individual 1-phase pulsewidth modulation converters, whose dc bus are connected in series to convert to a higher dc bus voltage. This permits conversion to much higher voltage dc employing semiconductor switches of lower voltage rating. Simulation and experimental verification are presented to validate the proposed scheme.