Byung-Geuk Cho

LCL filter design and control for grid-connected PWM converter

This paper describes LCL filter design and control for grid-connected PWM converter. To attenuate switching harmonics on grid side with an LCL filter, cost-effectiveness must be considered to select filter parameters. This paper proposes a filter design method based on a computer simulation tool. Depending on users optimal cost function, filter parameters are chosen without tedious and repetitive mathematical calculations. On the other hand, damping factors must be introduced in control of LCL filter to suppress the inherent resonant characteristics. Since the resonance phenomenon is minimized by increasing the ratio of switching frequency to fundamental grid frequency in most conventional researches, it is difficult to be employed in high power system due to the limited switching frequency of power semiconductors in the converter. In this paper, a novel current controller with generalized gain determination is suggested. Discrete time controller based on only grid voltages and currents information is introduced, which yields desirable dynamic response even in lower switching frequency of PWM converter. Simulations and experimental results are presented to validate the proposed design and control procedures.

Voltage injection method for boundary expansion of output voltages in three shunt sensing PWM inverters

This paper describes boundary conditions for phase current reconstruction of a three current-shunt inverter and suggests a voltage injection method to expand the measurable areas. The boundary conditions are derived on voltage vector plane for symmetric space vector PWM. For the expansion of the areas, different voltages are injected depending on the position of the voltage vector with double sampling. When voltage vector is located in the insensible area, additional voltage is injected to place the voltage vector in the measurable area during next half of switching period and compensation voltage is added in the other half period. Voltages with minimum magnitudes are chosen to minimize the current ripples. Simulation results are presented to support the effectiveness of boundary condition derivation and proposed voltage injection method.

LCL filter design for grid-connected voltage-source converters in high power systems

This paper deals with an LCL filter for the grid-connected converter. Active and passive damping methods are compared in a resonance suppression point of view and necessity of the passive damping is claimed, especially for high power systems in which switching frequency is limited lower than 2.5 kHz. Based on the analysis, LCL filter design procedure with passive damping for a two level voltage source converter is introduced. The filter components are optimally selected by investigating stored total energy in the filter and capacity of the converter. Then the resistors are inserted at several locations and comparisons are made to determine the most proper way of passive damping by examining the losses. The effectiveness of the study is supported by simulation and laboratory scale experiment.

Analysis of the Phase Current Measurement Boundary of Three Shunt Sensing PWM Inverters and an Expansion Method

To obtain phase currents information in AC drives, shunt sensing technology is known to show great performance in cost-effectiveness and therefore it is widely used in low cost applications. However, shunt sensing methods are unable to acquire phase currents in certain operation conditions. This paper deals with the derivation of the boundary conditions for phase current reconstruction in three-shunt sensing inverters and proposes a voltage injection method to expand the measurable areas. As the boundary conditions are deeply dependent on the switching patterns, they are typically analyzed on the voltage vector plane for space vector pulse width modulation (SVPWM) and discontinuous pulse width modulation (DPWM). In the proposed method, the voltage injection and its compensation are conducted within one sampling period. This guarantees fast current reconstruction and the injected voltage is decided so as to minimize the current ripple. In addition to the voltage injection method, a sampling point shifting method is also introduced to improve the boundary conditions. Simulation and experimental results are presented to verify the boundary condition derivation and the effectiveness of the proposed voltage injection method.

A power flow control strategy for optimal fuel efficiency of a variable speed engine-generator based series hybrid electric vehicle

This paper presents a new power flow control strategy for a variable speed engine-generator based series hybrid electric vehicle. The specific fuel consumption map of the internal combustion engine (ICE) has been obtained by off-line experiments to achieve optimal fuel efficiency. The proposed power flow controller makes it possible that the ICE operates at its optimal fuel efficiency points without degrading desired dynamic performance. Experimental results of the actual vehicle test as well as the laboratory based load test are shown to verify the proposed power flow control strategy.

Feasibility study of Integrated Power System with Battery Energy Storage System for naval ships

Korean navy is now investigating how new naval destroyer can be designed by Integrated Full Electric Propulsion (IFEP) system. The operating cost of the electric propulsion system is smaller than that of the mechanical propulsion system. However, the existing IFEP system is still spending an enormous fuel due to Navys conservative operating concept-Dual Generator Operation-for higher reliability. To improve the fuel-efficiency of the IFEP power system, the Single Generator Operation (SGO) is a rational solution because it could maximize the load factor of the generator. But the SGO is difficult to be accepted by the Navy as a reliable operating concept without full back-up of the entire electric power including propulsion power. This paper proposes a novel IFEP system having Battery Energy Storage System (BESS) which can guarantee reliable SGO. The feasibility, effectiveness and some design requirements of the proposed Integrated Power System (IPS) is discussed by computer simulations and reference data.

Consideration of active-front-end rectifier for electric propulsion navy ship

This paper proposes a control strategy of an active front end (AFE) rectifier to cope with voltage deviation at the shipboard AC-power system. The voltage sags in naval ships may occur when the step load such as a large pulse power weapon is applied or when faults happen in the grid or generators themselves. In this paper, the capability of AFEs to support voltage is presented for the voltage sags. In the proposed control scheme, reactive power is utilized to achieve voltage compensation. For the verification of the effectiveness, simulation using MATLAB/Simulink and PLECS has been conducted.

A separate double-winding 12-phase brushless DC motor drive fed from individual H-bridge inverters

This paper deals with a separate double-winding 12-phase BLDC motor fed from individual H-bridge inverters. The system is fault tolerant in that continuous operation is possible under fault conditions. Every phase is electrically independent from each other and doubly wound to intensify the redundancy of the system. Furthermore, each phase is asymmetrically 15 degrees apart to have the same torque ripple reduction effect as 24-phase counterpart. Two partial windings in each phase are taken control of by one inverter module. One module consists of two single phase H-bridge inverters and can be detached from other modules to be repaired or replaced while it is out of operation. In terms of motor drive, sinusoidal wave currents are applied to the stators for minimizing torque ripples and the operating efficiency at low rotational speed of motor is improved by series connection of the partial windings. Simulations and experimental studies are conducted to evaluate the performance of the system based on a prototype motor.

Non-iterative LCL filter design for three-phase two-level voltage-source PWM converters

This paper describe a design method of an LCL filter for the three-phase two-level voltage-source PWM converters. The grid current harmonics regulation is the primary condition for the operation of the grid-connected converters to satisfy. If the grid voltage is assumed to be ideally sinusoidal, PWM switching is the only factor to produce harmonics on the grid current, especially around the bands of the switching frequency. However, every single harmonic cannot be evaluated for the filter design. In this paper, it is found that there exists the most significant harmonic frequency at which the harmonic current is dominant. Theoretical description of the PWM waveform and the transfer function of the filters impedance are needed for the derivation of the most significant frequency. Once the most significant frequency is recognized, the total inductance to satisfy the grid current harmonic regulations is determined by selecting proper resonant frequency. Along with the information of the designed total inductance and preset resonant frequency, additional conditions such as the filters total energy, converter current ripple and converter capacity are utilized to specify the values of each component of the filter. The effectiveness of the proposed method is verified with simulation and experimental results.

Active damping method equivalent to series resistor effect for LCL filters in a grid-connected PWM converters

This paper proposes an active damping method for LCL filters of grid-connected converters. LCL filters are mainly utilized for the interface of distributed generation systems to the grid due to their small size and effectiveness of current harmonics attenuation. However, owing to the resonance of LCL filters, bandwidth of the current controller of the converters is usually limited far to the resonant frequency of the LCL filter. Otherwise, the system stability would be threatened, especially in high power systems where switching frequency and internal loss are relatively low. In this paper, an active damping method equivalent to the implementation of resistors in series to the filter capacitors is suggested and it is shown that the bandwidth of the LCL filter with the proposed damping method can be extended further than that with a conventional capacitor current feedback type damping method. The effectiveness of the proposed method is experimentally confirmed with a 5kW battery energy storage system connected to the grid.