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Pulsewidth modulation current source converter (CSC)-based drive is one of the widely used drives in high-power (megawatt level) medium-voltage (MV) (2.3–6.6 kV) applications. The switching frequency of the MV CSC is limited to around 500 Hz to satisfy thermal requirement and reduce switching loss. Under such a condition, the overall performance of the used modulation scheme, especially its harmonics performance, is an important consideration for CSC-based drives. To date, a couple of modulation schemes have been proposed for MV CSCs, and this paper aims to present an overview of these modulation schemes. Traditional modulation schemes are reviewed, the recent advances are illustrated and analyzed, and the challenges and trends are discussed.

Neutral-point (NP) potential imbalance is a crucial issue in five-level NP clamped (5L-NPC) full-bridge inverters. The basic idea of the conventional closed-loop NP potential balancing control methods is to sample the voltages of split dc-link capacitors first and then regulate the modulation waveform according to different voltages of split dc-link capactiors. In this paper, an open-loop NP potential balancing control for 5L-NPC full-bridge grid-tied inverters (FB-GCIs) is proposed. A balancing factor $k$ is introduced into the modulation signals and regulated according to the calculation results of the modulation index. Therefore, although the voltages of split dc-link capacitors have not been sampled and controlled, the dc-link capacitor voltages can be balanced over a wide modulation index range. Both the switching states and the NP potential balancing region of the 5L-NPC FB-GCI are analyzed in detail. The design guidelines of the balancing factor $k$ are also presented. A 5L-NPC FB-GCI prototype is built for the experimental tests, and experimental results validate the feasibility and effectiveness of the proposed control method.

Current source converter (CSC)-based wind energy conversion systems (WECSs) are recently gaining increasingly attention due to its reliable short-circuit protection. However, all existing CSC-based configurations are proposed for medium-voltage (MV) (3–4 kV) wind turbine systems, but not for low-voltage (LV) (690 V) systems which are dominating the market of WECS. The biggest challenge for using CSCs in LV systems is no proper switching devices in the market. This paper proposes a new configuration, where a proven modular LV voltage source converter is used as the generator-side converter to be compatible with an MV CSC. By such a design, MV CSCs are successfully used in LV systems with all advantages being inherited. In addition, this configuration features high reliability and reduced size and weight, thanks to the adoption of the modular converter. The performance of the configuration under normal and fault conditions are investigated. Apart from fulfilling traditional control objectives in WECS, a current balancing control is added to ensure safe operation. Additionally, limits and constraints of the configuration using in practice are discussed. Finally, simulation and down-scaled experimental results are presented.