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Recently the researches on modular multi-level converter (MMC) are being highlighted because high quality and efficient power transmission have become key issues in high voltage direct current (HVDC) systems. This paper proposes an improved pre-charging method for the sub-module (SM) capacitor of MMC-based HVDC systems, which operates in the nearest level control (NLC) modulation and does not need additional circuits or pulse width modulation (PWM) techniques. The feasibility of the proposed method was verified through computer simulations for a scaled 3-phase 10kVA MMC with 12 SMs per each arm. Hardware experiments with a scaled prototype have also been performed in the lab to confirm the simulation results.

A Voltage Sourced Converter (VSC) for High Voltage Direct Current (HVDC) operates in extremely high voltage and high-power conditions. Especially in case of Modular Multilevel Converter (MMC), hundreds of submodules are connected in series to meet the high voltage requirement. IEC 62501 suggests several tests such as the operational tests for a valve consisting of several submodules and the dielectric tests, etc. This paper proposes a new test circuit for the half bridge submodule in MMC-based HVDC system. Simulations are carried out to show the validity of the proposed test circuit.

Novel islanding detection method was proposed for the inverter-based distributed generation (DG). The proposed method detects the islanding state by inj ecting negligible amount of the 2nd order harmonic current and measuring the magnitude ratio of the 2nd order harmonic voltage to the fundamental voltage at the point of common coupling (PCC). The 2ndorder harmonic voltage is extracted from the load voltage at the PCC through the 2ndorder proportional resonant (PR) filter. The 2ndorder harmonic PR controller provides the voltage and current control for the inverter-based DG.

In this paper, the PSC PWM method is chosen as the optimal modulation method for a 20MW VSC HVDC, with consideration of the harmonic distortion of the output voltage, the switching frequency, and the control implementation difficulty. In addition, a new PSC PWM method is proposed in order to achieve an easy application and to solve the redundant control problems encountered in the previous PSC PWM method. To verify the proposed PSC PWM method, PSCAD/EMTDC simulations for an 11-level MMC RTDS HILS test and an 11-level MMC prototype converter test were performed. As can be seen from the results of these tests, the proposed PSC PWM method shows good results in an 11-level MMC with redundant sub-modules.

This paper proposes a new NLC (Nearest Level Control) scheme for MMCs (Modular Multilevel Converters), which offers voltage ripple reductions in the DC capacitor of the SM (Sub-Module), the output voltage harmonics, and the switching losses. The feasibility of the proposed NLC was verified through computer simulations. Based on these simulation results, a hardware prototype of a 10kVA, DC-1000V MMC was manufactured in the lab. Experiments were conducted to verify the feasibility of the proposed NLC in an actual hardware environment. The experimental results were consistent with the results obtained from the computer simulations.

The microgrid and demand response (DR) are important technologies for future power grids. Among the variety of microgrid operations, the multi-agent system (MAS) has attracted considerable attention. In a microgrid with MAS, the agents installed on the microgrid components operate optimally by communicating with each other. This paper proposes an operation algorithm for the individual agents of a test microgrid that consists of a battery energy storage system (BESS) and an intelligent load. A microgrid central controller to manage the microgrid can exchange information with each agent. The BESS agent performs scheduling for maximum benefit in response to the electricity price and BESS state of charge (SOC) through a fuzzy system. The intelligent load agent assumes that the industrial load performs scheduling for maximum benefit by calculating the hourly production cost. The agent operation algorithm includes a scheduling algorithm using day-ahead pricing in the DR program and a real-time operation algorithm for emergency situations using emergency demand response (EDR). The proposed algorithm and operation strategy were implemented both by a hardware-in-the-loop simulation test using OPAL-RT and an actual hardware test by connecting a new distribution simulator.

In remote islands PV (Photo Voltaic) panel with BESS (Battery Energy Storage System) supplies electric power to the customers in parallel operation with EG (Engine Generator) to save fuel consumption and to mitigate environmental load. BESS operates in voltage control mode when it supplies power to the load alone, while it operates in current control mode when it supplies power to the load in parallel with EG. This paper proposes a smooth mode change scheme from current control to voltage control of BESS by adding proper initial value to the integral part of voltage control, and a smooth mode change scheme from voltage control to current control by tracking the EG output voltage to the BESS output voltage using PLL (Phase-Locked Loop). The feasibility of proposed schemes was verified through computer simulations with PSCAD/EMTDC, and the feasibility of actual hardware system was verified by experiments with scaled prototype. It was confirmed that the proposed schemes offer a seamless operation in the stand-alone power system in remote islands.

This letter proposes a new scheme of smooth operation transition for the stand-alone power system which is composed of engine generator and battery energy storage system (BESS) with photo voltaic panels. The proposed scheme offers smooth operation transition of BESS controller from the current control to the voltage control and vice versa. The feasibility of proposed scheme was verified through experiments with a scaled prototype.

This paper proposes a new auxiliary resonant commutated pole (ARCP) inverter which has a modified auxiliary circuit. The proposed auxiliary circuit includes two auxiliary IGBT switches, an LC resonant circuit, and two clamping diodes. In order to analyze the performance of proposed ARCP inverter, computer simulations with PSCAD, and hardware experiments were carried out. Through analyzing the experimental results, it is known that the proposed ARCP inverter offers efficiency improvement of 1.5% compared with the hard-switching inverter.

This paper presents a scaling gain compensation method for current measurement of motor drive application under locked-rotor condition. In several applications such as elevators, cranes and so on, initial commissioning processes are performed under locked-rotor condition. To compensate the current scaling gain errors, the proposed method performs d-q axis current control according to two-phase sensing methods in the stationary reference frame. Thereafter, the three-phase scaling gains are estimated using the d-axis current command and the output d-axis voltage, and then, compensated. The proposed algorithm considers the inequality of the phase resistances so that the compensation performance can be improved. The validity of the proposed algorithm was verified through experiments.

Constant frequency torque controller-based direct torque control (CFTC-DTC) of AC motor drives is known to have limitation with torque loop bandwidth, since it uses sampling frequency available from a digital signal processor for generating triangular carrier-based waveform. The switching frequency of the control system is determined by the carrier frequency of CFTC. The worst case of CFTC-DTC drive exists at very low speed under light load where the zero voltage vectors are dominant, and the radial component of active voltage vectors are very short leading to flux droop at sector transitions. This results in the distortion of motor phase current and aggravation of speed waveform. This paper proposes a modification on the frequency carriers of CFTC to increase the torque loop bandwidth by replacing the triangular carrier-based with ramp carrier-based waveform. The proposed method is proven by the simulation results showing excellent performance of motor flux, phase current, and speed at low speed.

This paper proposes an offset voltage control scheme for the modular multilevel converter (MMC) operated in nearest level control (NLC) to improve the total harmonic distortion (THD) of AC phase voltages. Through the offset voltage control, the magnitude of each AC pole voltage maintains constant value with N+1 level in the range of whole modulation index (MI). The validity of the proposed scheme was confirmed by computer simulations for the MMC with 22.9kV/25MVA and experimental works for the scaled MMC with 380V/10kVA. It was confirmed that the proposed control scheme can generate constant pole voltages regardless the value of MI, and form phase voltages with improved THD.

This paper proposes an active islanding detection method for the BESS (Battery Energy Storage System) with 3-phase inverter which is connected to the AC grid. The proposed method adopts the DDSRF (Decoupled Double Synchronous Reference Frame) PLL (Phase Locked-Loop) so that the independent control of positive-sequence and negative-sequence current is successfully carried out using the detected phase angle information. The islanding state can be detected by sensing the variation of negative-sequence voltage at the PCC (Point of Common Connection) due to the injection of 2-3% negative-sequence current from the BESS. The proposed method provides a secure and rapid detection under the variation of negative-sequence voltage due to the sag and swell. The feasibility of proposed method was verified by computer simulations with PSCAD/EMTDC and experimental analyses with 5kW hardware prototype for the benchmark circuit of islanding detection suggested by IEEE 1547 and UL1741. The proposed method would be applicable for the secure detection of islanding state in the grid-tied Microgrid.

This paper proposes a new reduction scheme of circulating current when two units of BESS (Battery Energy Storage System) are operated in parallel with conventional droop control. In case of using conventional droop, the terminal voltage of each BESS are not equal due to the unequal line impedance, which causes the circulating current. The operation performance of BESS is critically dependant on the circulating current because it increases system losses which causes the increasement of required system rating. This paper introduces a new reduction scheme of circulating current in which the terminal voltage difference of each BESS is compensated by adding feed-forward path of line voltage drop to the droop control. The feasibility of proposed scheme was first verified by computer simulations with PSCAD/EMTDC software. After then a hardware prototype with 5kW rating was built in the lab and many experiments were carried out. The experimental results were compared with the simulation results to confirm the feasibility of proposed scheme. Two parallel operating BESS with proposed scheme shows more accurate performance to suppress the circulating current than those with the conventional droop control.

This letter suggests a large air-gap bidirectional wireless power transfer charger for electric vehicle. It is controlled by pulse width modulation with a self-resonant frequency formed by self-inductance and resonant capacitor so that constant frequency operation can be accomplished under large air-gap without additional current chopper. The feasibility of the proposed method has been verified with a 6.6-kW prototype with air-gap of 12-20 cm.

This paper proposes a DC microgrid operational strategy and control method for improved service reliability. The objective is to supply power to as many non-critical loads as possible, while providing an uninterrupted power supply to critical loads. The DC bus signaling method, in which DC voltage is an information carrier, is employed to implement the operational strategy in a decentralized manner. During grid-connected operation, a grid-tied converter balances the power of the microgrid by controlling the DC voltage. All loads are connected to the microgrid, and operate normally. During islanded operation, distributed generators (DGs), a backup generator, or an energy storage system balances the power. However, some non-critical loads may be disconnected from the microgrid to ensure the uninterrupted power supply to critical loads. For enhanced service reliability, disconnected loads can be automatically reconnected if certain conditions are satisfied. Control rules are proposed for all devices, and detailed microgrid operational modes and transition conditions are then discussed. Additionally, methods to determine control parameter settings are proposed. PSCAD/EMTDC simulation results demonstrate the performance and effectiveness of the proposed operational strategy and control method.

This letter proposes a new scheme of phase-shifted carrier pulse-width modulation for the modular multilevel converter with redundancy submodules (SMs). The proposed scheme can offer a smooth transition for the redundancy SM from the standby state to the startup operation when normally operating down. The operational feasibility of the proposed scheme was first verified through simulations with PSCAD/EMTDC, and then hardware-in-the-loop system tests with the real-time digital simulator.

Modular Multi-level Converter (MMC) has been widely researched in order to apply it for the back-to-back (BTB) high voltage direct current (HVDC) system. This paper proposes a new pre-charging scheme for the sub-module capacitors of MMC that is operated in nearest level control (NLC) modulation. The proposed scheme does not require any additional circuit or pulse width modulation (PWM) technique. The feasibility of proposed scheme was verified through computer simulations and experiments for a 3-phase 10kVA MMC with 12 sub-modules per each arm. The proposed scheme could be directly applied for the actual MMC-based HVDC system.

In this paper the operational characteristics of stand-alone microgrid was analyzed using RTDS simulation models. The accuracy of developed simulation models were verified by comparing with the analysis results using the PSCAD/EMTDC simulation models. The proper scenarios and operation algorithms were developed and analyzed in accordance with various situations that can occur in the actual system, so as to establish operation scheme for the stand-alone microgrid system. The developed simulation models can be effectively utilized to design a newly installed stand-alone microgrid and to develop various operation scenarios for stand-alone microgrid. And these models can be applied for analyzing the transient phenomena due to system fault so that system protection can be properly designed.

In this paper, a hardware prototype for the 10kVA 11-level MMC was built and various experimental works were conducted to verify the operation algorithms of MMC. The hardware prototype was designed using computer simulation with PSCAD/EMTDC software. After manufactured in the lab, the hardware prototype was tested to verify the modulation algorithms to form the output voltage, the balancing algorithm to equalize the sub-module capacitor voltage, and the redundancy operation algorithm to improve the system reliability. The developed hardware prototype can be utilized for analyzing the basic operation and performance improvement of MMC according to the modulation and redundancy operation scheme. It also can be utilize to analyze the basic operational characteristics of HVDC system based on MMC.Key Words : MMC (Modular Multi-level Converter), SM (sub-module), DC voltage balancing, Redundancy module, Harmonic reduction, EAM(Equal Area Modulation), SHE (Selective Harmonic Elimination), NLC (Nearest Level Control)*Dept. of Electrical Engineering, Myongji University, Korea**Hyosung Corporation and Dept. of Electrical Engineering, Myongji University, Korea†Corresponding Author : Dept. of Electrical Engineering, Myongji University, KoreaE-mail : erichan@mju.ac.krReceived : March 17, 2014; Accepted : July 14, 2014

This paper proposes a new bidirectional intelligent semiconductor transformer (BIST) for the smart distribution system and smart grid. The proposed BIST consists of high-voltage high-frequency ac/dc converter, bidirectional low-voltage dc/dc converter, and hybrid-switching dc/ac inverter. It features 1) input-to-output isolation with a high-frequency transformer; 2) bidirectional power flow; 3) small size and light weight; 4) capability of compensating voltage sag and/or swell; and 5) realization of three-phase structure based on single-phase module. The operational feasibility of proposed transformer was verified not only by computer simulation with PSCAD/EMTDC software but also by a hardware prototype with rating of 1.9 kV/127 V, 2 kVA, allowing a three-phase transformer of 3.3 kV/220 V, 6 kVA with three-phase construction.

Synthetic test circuit (STC) for thyristor valve test consists of a low-voltage high-current source to provide the forward current during turn-on state and a low-current high-voltage source to provide the reverse and forward voltage during turn-off state. This paper proposes a new low-voltage high-current source for STC that has a six-pulse thyristor converter and a two-phase chopper with an auxiliary switch. The operation of STC with a new low-voltage high-current source was verified through theoretical analysis and computer simulations with PSCAD/EMTDC. Based on computer simulation results, a scaled hardware system for the proposed circuit was built and tested to confirm the feasibility of implementing a real-size STC.

This paper introduces a coordinated droop control for the stand-alone DC micro-grid, which is composed of photo-voltaic generator, wind power generator, engine generator, and battery storage with SOC (state of charge) management system. The operation of stand-alone DC micro-grid with the coordinated droop control was analyzed with computer simulation. Based on simulation results, a hardware simulator was built and tested to analyze the performance of proposed system. The developed simulation model and hardware simulator can be utilized to design the actual stand-alone DC micro-grid and to analyze its performance. The coordinated droop control can improve the reliability and efficiency of the stand-alone DC micro-grid

This paper describes the operational analysis results of the bipolar DC distribution system coupled with the distributed generators. The energy management for AC/DC power trade and the operational principle of distributed generators and energy storages were first analyzed by computer simulation with PSCAD/EMTDC software. After then a hardware simulator for the bipolar DC distribution system was built, which is composed of the grid-tied three-level inverter, battery storage, super-capacitor storage, and the voltage balancer. Various experiments with the hardware simulator were carried out to verify the operation of bipolar DC distribution system. The developed simulator has an upper-level controller which operates in connection with the controllers for each distributed generator and the battery energy storage based on CAN communication. The developed hardware simulator are possible to use in designing the bipolar DC distribution system and analyzing its performance experimentally.

This paper proposes a new seamless operation scheme for a stand-alone power system, which has a battery energy storage system (BESS), photo voltaic (PV) panels, and an engine generator (EG) to supply electric power to the customers in remote islands. The proposed scheme offers smooth operation change of the BESS from the current control to the voltage control or vice versa when the EG is connected or disconnected to the load. The feasibility of proposed scheme was verified through computer simulations and experiments with a scaled prototype. It was confirmed that the proposed scheme offers a seamless operation with a simple approach to reduce the usage of EG in a stand-alone power system.

This paper proposes a new DC output voltage control for a battery energy storage system (BESS) with a lithium-ion battery based on the state of charge (SoC). The proposed control scheme was verified through computer simulations for a typical stand-alone DC microgrid, which consists of a BESS, photovoltaic (PV) panel, engine generator (EG), and DC load. A scaled hardware prototype for a stand-alone DC microgrid was set up in the lab, in which the proposed control scheme was loaded in a DSP controller. The experimental results were compared with the simulation results for performance verification. The proposed control scheme provides relatively lower variation of the DC grid voltage than the conventional droop control.

This paper proposes a new DC output voltage control for the battery energy storage system (BESS) based on the battery state of charge (SoC). The proposed control scheme was verified through computer simulations for a typical standalone DC microgrid, which consists of BESS, photo-voltaic (PV) panel, engine generator (EG), and DC load. A scaled hardware prototype for stand-alone DC microgrid was set up in the lab, in which the proposed control scheme was loaded in DSP controller. The experimental results were compared with the simulation results for performance verification. The proposed control scheme provides relatively lower variation of the DC grid voltage than the conventional droop control.

This paper describes a prototype of 3-phase 3.3kV/220V 6kVA modular semiconductor transformer developed in the lab for feasibility study. The developed prototype is composed of three single-phase units coupled in Y-connection. Each single-phase unit with a rating of 1.9kV/127V 2kVA consists of a high-voltage high-frequency resonant AC-DC converter, a low-voltage hybrid-switching DC-DC converter, and a low-voltage hybrid-switching DC-AC converter. Also each single-phase unit has two DSP controllers to control converter operation and to acquire monitoring data. Monitoring system was developed based on LabView by using CAN communication link between the DSP controller and PC. Through various experimental analyses it was verified that the prototype operates with proper performance under normal and sag condition. The system efficiency can be improved by adopting optimal design and replacing the IGBT switch with the SiC MOSFET switch. The developed prototype confirms a possibility to build a commercial high-voltage high-power semiconductor transformer by increasing the number of series-connected converter modules in high-voltage side and improving the performance of switching element.

This paper proposes a new grid-tied power converter for battery energy storage, which is composed of a 2-stage DC-DC converter and a PWM inverter. The 2-stage DC-DC converter is composed of an LLC resonant converter connected in cascade with a 2-quadrant hybrid-witching chopper. The LLC resonant converter operates in constant duty ratio, while the 2-quadrant hybridswitching chopper operates in variable duty ratio for voltage regulation. The operation of proposed system was verified through computer simulations. Based on computer simulations, a hardware prototype was built and tested to confirm the technical feasibility of proposed system. The proposed system could have relatively higher efficiency and smaller size than the existing system.

This paper describes a switching-level operation analysis of BTB(Back-To-Back) converter for HVDC(high voltage DC) application based on MMC(modular multi-level converter). A switching-level operation analysis for BTB converter is very important to understand the converter operation in detail and check the voltage and current transients in each components. However, the development of switching-level simulation model for the actual size BTB Converter is very difficult because the MMC normally has more than 150 sub-modules for each arm. So, a switching level simulation model for the 11-level MMC-based BTB converter was developed with PSCAD/EMTDC software, which has 12 sub-modules for the positive arm and another 12 sub-modules for the negative arm. The DC-voltage balance algorithm, the circulating-current reduction algorithm, the harmonic reduction algorithm, and the redundancy operation algorithm were included in this simulation model. The developed simulation model can be utilized to analyze the MMC-based BTB converter for HVDC application in switching level and to develop the protection scheme for the MMC-based BTB converter for HVDC application.