In the medium voltage cascaded converter, the power imbalance problem among the converter modules will result in certain converter modules over-modulated. Reactive power control (RPC) scheme is an effective way to balance the cascaded converters. Many efforts, such as reactive power sharing control and apparent power sharing control have been used to solve the overmodulation issue. This paper investigates an optimization RPC scheme to solve the overmodulation problem by minimizing the total required reactive power. An improved minimum reactive power (IMRP) RPC scheme is presented, the feasible area for the active power imbalance is evaluated and compared with the reactive power sharing control scheme and the apparent power sharing control scheme. Finally, the control scheme is verified on a three-module scaled-down cascaded inverter.

As an innovative approach to improve the cooling efficiency of fluid, the use of nanofluids has attracted increasing attention in engineering applications. In this paper, the impact on the natural convective heat transfer in disc-type transformer windings due to transformer oil-based nanofluids is studied numerically. A low-voltage winding using nanofluid (SiC/oil) as the coolant is modelled two-dimensionally and simulated by computational fluid dynamics and the multi-phase mixture model. The numerical method is validated with the existing results of transformers using conventional oil cooling, and grid-independence study is carried out for the nanofluid flow. Compared with transformer oil cooling, the temperature of nanofluid cooled winding is significantly reduced, while the temperature trend along the flow direction remains essentially the same. Moreover, the effects of nanofluid on the mass flow rate and the coolant temperature have been taken into consideration in the heat transfer analysis.

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