Su-Seong Park

This paper proposes an adaptive hierarchical control strategy for a hybrid energy storage system (HESS) integrated within a hub station, aimed at mitigating output power fluctuations and enhancing grid stability under high renewable energy penetration. The proposed framework incorporates hierarchical structure based four control levels, combining optimal power control, adaptive power decomposition via a low-pass filter, adaptive ramp rate regulation, and SOC-based selective control. This structure enables effective coordinated management of HESS power output, SOC, and ramp rate in response to varying operating conditions. Simulation results demonstrate that the proposed strategy significantly reduces SOC variation and ramp rate violations compared to conventional methods. Furthermore, the proposed strategy achieves a substantial improvement in renewable energy hosting capacity, with the maximum allowable renewable generation increased by approximately 35% compared to the conventional method. System-level validation using the IEEE 14-bus test system further confirms that the hub station enhances grid voltage stability and increases the permissible RES penetration level by approximately 25%. These results verify the effectiveness and scalability of the proposed control strategy for stable renewable energy integration into power grids.

This paper proposes an adaptive hierarchical control strategy for a hybrid energy storage system (HESS) integrated within a hub station, aimed at mitigating output power fluctuations and enhancing grid stability under high renewable energy penetration. The proposed framework incorporates hierarchical structure based four control levels, combining optimal power control, adaptive power decomposition via a low-pass filter, adaptive ramp rate regulation, and SOC-based selective control. This structure enables effective coordinated management of HESS power output, SOC, and ramp rate in response to varying operating conditions. Simulation results demonstrate that the proposed strategy significantly reduces SOC variation and ramp rate violations compared to conventional methods. Furthermore, the proposed strategy achieves a substantial improvement in renewable energy hosting capacity, with the maximum allowable renewable generation increased by approximately 35% compared to the conventional method. System-level validation using the IEEE 14-bus test system further confirms that the hub station enhances grid voltage stability and increases the permissible RES penetration level by approximately 25%. These results verify the effectiveness and scalability of the proposed control strategy for stable renewable energy integration into power grids.

Recently, as the demand for LLC resonant converters has increased in various power electronics industry sectors, research on accurate analysis methods necessary for effectively designing converters has been conducted. This paper proposes an analysis model that can estimate the efficiency of the converter and the temperature of component parts with high accuracy while requiring a relatively low computational burden when analyzing LLC resonant converters. The proposed analysis model performs circuit and thermal analysis using advanced mathematical modeling methods based on time analysis, and uses a hybrid structure analysis model that analyzes magnetic components through an AI-based analysis model. Time analysis-based analysis is a method that accurately estimates the magnitude of voltage and current of resonant converters by equivalent modeling of circuit configurations for each operating state during a switching cycle using nonlinear circuit equations. The AI-based analysis model is a method that generates the relationship between design input variables and output variables of magnetic components through AI algorithms and uses this as an analysis model. To verify the effectiveness of the proposed method, a 1kW LLC resonant converter prototype is used, and the efficiency and temperature estimation accuracy of the proposed method are measured under various capacity conditions, input and output voltage conditions, resonant network conditions, and load conditions. The proposed method estimates efficiency with an error within 2% and estimates temperature with an error within 10% under various verification conditions. In terms of computation time, it takes about 7000[s] for 100 analyses, which shows relatively low computational burden compared to existing methods. Through this, the proposed method can effectively analyze LLC resonant converters with high accuracy and relatively low computational burden.

In this paper, a droop-based distributed control method is proposed to enhance the penetration rate of renewable energy sources and minimize low-voltage fluctuations within the DC local grid. The proposed method designs droop gains for state of charge (SoC)-based droop control considering line impedance and other parameters in the DC local grid, thereby facilitating precise SoC equalization among different energy storage systems (ESS) considering the dc bus voltage fluctuation and power sharing of each component. Further, the proposed technique applies droop control to the grid-connected converter considering the SoC of ESS, enabling effective power distribution between the AC grid and ESS based on the magnitude of SoC. Secondary voltage compensation control is performed by integrating the structure of ramp rate control into this control method for DC bus voltage control, which helps alleviate excessive output fluctuations of renewable energy sources. The proposed method can be applied based on the SoC of ESS, thereby achieving a 56.56 % improvement in the penetration rate of renewable energy. In addition, the DC bus voltage fluctuation is controlled within 1 %, and SoC equalization is achieved when operating more than two ESS units in DC local grid system. The validity of the proposed technique is verified using PSCAD/EMTDC simulations and shows improvement on both renewable energy penetration rate and voltage fluctuation.

An LLC resonant converter should have high efficiency and power density. Further, it is important to reduce the volume and loss of the transformer, which provides insulation and voltage conversion between the input and output. When a planar core is used to achieve high power density, the magnitude of the parasitic components and the transformer loss considerably depend on the winding arrangement within the limited window area. Therefore, analyzing the various winding arrangements is necessary. To this end, an interleaved serpentine winding method that reduces the winding loss attributed to the high current by maximizing the window area of the planar core and has advantages of low parasitic capacitance and excellent assembly is proposed. The proposed winding method uses a Litz wire to reduce the DC copper loss, and it does not require additional space for the winding arrangement compared to that required by the U-type winding method. The capacitive energy distribution of the proposed winding method is compared with that of the existing U-type winding method to confirm the parasitic capacitance reduction effect of the proposed winding method. The formulae for calculating the effective capacitance of each winding method are derived for accurately estimating the parasitic capacitance. Finally, the effectiveness of the proposed analysis method and prototype are verified through finite element analysis simulation and experiments and it shows improvement of current waveforms and the overall efficiency increases about 0.5%.

In this paper, impedance modeling of a DC microgrid system consisting of a source and load converter, including an input filter, is performed. Impedance-based modeling has been used to derive mathematical models of the output impedance of the source converter and the input impedance of the load converter. The correlation between the converter interaction and system stability is analyzed based on the mathematical model. An impedance-based stability analysis is used to determine the system stability by analyzing the interactions among the converters in the DC microgrid system. Middlebrook’s stability criterion, which uses the impedance transfer function, is applied to determine system stability. Moreover, in this paper, a stability enhancement control algorithm is proposed to resolve the system instabilities resulting from interaction among the converters and the distortion caused by the harmonics emanating from the AC input. The proposed stability enhancement control algorithm consists of a feed-forward type virtual impedance (VI) and a proportional-resonant (PR) controller. The validity of the proposed method is demonstrated by the results of the response characteristics in the frequency domain, and the effectiveness of the proposed control algorithm is verified via simulations and prototype experimental models.