This paper presents a model of an intelligent energy harvesting system from microbial fuel cells (MFCs) in the wastewater treatment process. The model consists of two direct current (DC/DC) converters connected in a cascade. One DC/DC converter is used to capture energy from MFC and store it in a supercapacitor. The other DC/DC converter is responsible for increasing the low output voltage to a higher voltage level. In the paper, the MFC is modeled by a DC voltage source instead of a real MFC that contains wastewater inside it. The experimental results demonstrate that the model of an intelligent energy harvesting system can increase the low output voltage of MFC up to 3.3 V and achieve intermittent output power at a high level that can use in practice.

-This paper presents an application of proportional-resonant (PR) current controllers in modular multilevel converter-high voltage direct current (MMC-HVDC) system under unbalanced voltage conditions. The ac currents are transformed and controlled in the stationary reference frame (αβ-frame). Thus, the complex analysis of the positive and negative sequence components in the synchronous rotating reference frame (dq-frame) is not necessary. With this control method, the ac currents are kept balanced and the dc-link voltage is constant under the unbalanced voltage fault conditions. The simulation results based on a detailed PSCAD/EMTDC model confirm the effectiveness of the proposed control method.

Abstract. This paper presents an application of doubleFourier series (DFS) algorithm for analyzing harmonics of a modular multilevel converter (MMC). By considering the effect of arm inductor, the application of DFS for calculating harmonics in the MMC is made. The DFS is then verified by comparing with simulation results used the fast Fourier transform (FFT) algorithm. Subsequently, the DFS will be used to analyze the effect of number of output voltage levels to the distribution of harmonics and total harmonic distortion (THD) in the MMC. The analytical results are performed in the Matlab programming in order to verify the theoretical analysis.

This paper presents a control method of the modular multilevel converter -high-voltage direct current (MMC-HVDC) system to regulate grid voltage on the basis of the Jeju Island power system. In this case, the MMC-HVDC system is controlled as a static synchronous compensator (Statcom) to exchange the reactive power with the power grid. The operation of the MMC-HVDC system is verified by using the PSCAD/EMTDC simulation program. The Jeju Island power system is first established on the basis of the parameters and measured data from the real Jeju Island power system. This power system consists of two line-commutated converter -high-voltage direct current (LCC-HVDC) systems, two Statcom systems, wind farms, thermal power plants, transformers, and transmission and distribution lines. The proposed control method is then applied by replacing one LCC-HVDC system with a MMC-HVDC system. Simulation results with and without using the MMC-HVDC system are compared to evaluate the effectiveness of the control method.

The Jeju Special Self-Governing Province is currently promoting the "Carbon-free Island by 2030" policy, which requires the use of renewable energy instead of fossil fuel so that the island will have no carbon gases generated by 2030. To implement this policy, the island plans to build a wind power plant capacity of 1.09 GW in 2020; this wind power plant is currently ongoing. However, when wind power output is greater than the power demand of the island, the stability of Jeju Island power system must be prepared for it because it can be a problem. Therefore, this study proposes a voltage source-type MMC-HVDC system linked to mainland Korea to expand the wind power penetration limits of Jeju Island under the stable operation of the Jeju Island power system. To verify the effectiveness of the proposed scheme, computer simulations using the PSCAD/EMTDC program are conducted, and the results are demonstrated. The scenarios of the computer simulation consist of two cases. First, the MMC-HVDC system is operated under variable wind power in the Jeju Island power system. Second, it is operated under the predicted Jeju Island power system in 2020.