Gab-Su Seo

Modeling and analysis of DC distribution systems

Research on modification and replacement of conventional AC distribution system to DC is widely undergoing. When DC distribution is applied in a system with DC-preferred appliances, it is possible to enhance energy consumption efficiency by reducing conversion losses. Moreover, renewable energy sources and storage devices can be fully utilized because they are more compatible with DC based system where generated energy can be directly used to loads and stored which results in less conversion loss. This paper presents modeling method to analyze DC based system to verify the feasibility of it. System component modeling is discussed and carried out including power converter models and load profile. Intelligent operation method can be applied to further improve system performance. Utilizing the modeling results, AC and DC based household power system is simulated. The results show the 4.7% efficiency improvement with DC transmission and 1.5% with DC distribution without renewable source usage showing the feasibility of DC system.

A New DC Anti-Islanding Technique of Electrolytic Capacitor-Less Photovoltaic Interface in DC Distribution Systems

This paper proposes a photovoltaic (PV) generation system interfaced with a dc distribution system. DC interface allows for the improvement of system efficiency by fully utilizing dc-based renewable sources and storage devices. In this paper, issues on PV interface for dc distribution systems are discussed for energy-efficient and reliable system implementation. AC and dc PV interfaces are mathematically analyzed. In dc distribution, eliminating electrolytic capacitors in PV interfaces improves system reliability, increases system efficiency, and reduces cost. In addition, this paper proposes a new anti-islanding technique for dc distribution as a system protection scheme. The operating principle is presented in detail and analysis shows that the proposed injected current perturbation technique is an effective solution for anti-islanding operation. A prototype converter features a simple structure with no electrolytic capacitor, which ensures a longer lifetime of the PV power circuit. Experimental results of the prototype circuit show a maximum efficiency of 98.1% and a European efficiency of 97.5%. The proposed anti-islanding technique shows fast response to the islanding condition in less than 0.2 s. It also shows that the average maximum power point tracking efficiency is 99.9% in normal conditions, which verifies the performance of the proposed scheme.

Selective flyback balancing circuit with improved balancing speed for series connected Lithium-ion batteries

In this paper, a cell balancing circuit for the Lithium-ion battery string based on the Flyback topology is proposed. The proposed circuit draws the charge out of the high voltage cell and recovers it to the overall battery pack. With the selectivity of the target cell, the proposed circuit uses the minimized power path to simplify the circuit and optimize the balancing efficiency. The circuit also employs the peak current mode control scheme to accelerate the balancing speed and reduce the balancing time. Experimental results with the laboratory prototype hardware for multi-cell battery string verify the feasibility of the proposed scheme.

Low-Common Mode Voltage H-Bridge Converter with Additional Switch Legs

H-bridge converter with additional switch legs (HA converter) and its offspring circuit are proposed in this paper with the intent to reduce the common mode noise. The proposed topology connects grounds of the input and output terminals, which gives zero common mode current in the ideal case. The operation of the proposed circuit is flexible and allows for the circuit to be capable of both ac–dc and dc–ac conversions. The proposed topology is especially advantageous when it is applied to the photovoltaic power conditioning system in dc distribution system or stand-alone power system because they include large stray capacitances and are prone to common mode EMI. In this paper, a 4-switch HA (HA4S) converter for both ac–dc rectification and dc–ac inversion is derived from the proposed HA converter as the implementation example. The experimental results based on the proposed and the conventional prototype circuits prove that the HA4S converter outperforms the conventional counterparts.

Digitally Controlled Current Sensorless Photovoltaic Micro-Converter for DC Distribution

In this paper, a digitally controlled photovoltaic (PV) micro-converter for dc distribution with sensing no current is presented. Current information for maximum power point tracking (MPPT) is estimated from system parameters, which leads to reducing power losses caused in sensing current and the number of components. Operation analysis of boundary conduction, quasiresonant, and discontinuous conduction modes is presented for its implementation. The high current estimation accuracy not only allows adaptive operation mode changes for efficiency optimization, but also guarantees achievement of high MPPT efficiency. Loss analysis of each operation mode is also presented to determine optimal transition points for mode change. A prototype PV micro-converter for a 120-W PV module is implemented by utilizing a digital signal processor. Experimental results verify that the proposed method is stable in both static and dynamic operations. European efficiency and MPPT efficiency are measured higher than 97.5% and 99.5%, respectively.

Arc protection scheme for DC distribution systems with photovoltaic generation

This paper presents a dc arc protection scheme for a dc distribution system. The location of arc fault detectors (AFD) and interrupting devices (ID) for system safety guarantee is discussed. Through the proposed scheme, the dc distribution system is protected from arc faults occurring in photovoltaic (PV) sources and load sides. Operation principle is discussed in detail. Experimental results verify the feasibility of the protection scheme.

A Simple Average Current Control With On-Time Doubler for Multiphase CCM PFC Converter

This paper proposes a new modulated carrier control method for an interleaved continuous conduction mode (CCM) power factor correction (PFC) boost converter. The proposed method allows precise sinusoidal line current shaping for a PFC boost converter operating in CCM. This is accomplished by simply comparing modulated carrier signal with switch current, such that inner current loop compensation design and rectified line voltage sensing are not required. The proposed control method greatly simplifies current sensing and control circuitry of multiphase converters since average current control is available by just sensing the switch current for each module. This also enables simple current balancing between parallel converters by employing modulated carrier signals with same amplitude. The principle of the control method is presented. The performance of the proposed control method is experimentally verified on a 600-W PFC converter. The measured power factor remains above 94.9% down to 20% of a full load for 110-Vrms and 220-Vrms line voltages. The line current harmonics also meet the IEC61000-3-2 Class D standard.

OCV hysteresis effect-based SOC estimation in extended Kalman filter algorithm for a LiFePO 4 /C cell

This work investigates the electric characteristics of a carbon-coated LiFePO4 cell with emphasis on their specific open-circuit voltage (OCV) characteristics, which include very flat OCV curves over the state-of-charge (SOC) and pronounced hysteresis phenomena. Examining discharging/ charging OCV measurement data of a LiFePO4/C cell elucidates these phenomena. A simple equivalent circuit model is newly derived and used in an OCV hysteresis effect-based SOC estimation in the extended Kalman filter (EKF) algorithm. The interval of the relationship between the OCV and SOC is decreased to 5% ΔSOC steps to improve the performance of the algorithm. Additionally, to compensate the model errors caused by the equivalent circuit model and variation in parameters, a measurement noise model and data rejection are implemented because the model accuracy is critical in the EKF algorithm in order to obtain a good estimation. All SOC estimation results of the proposed work satisfy the specification within ±5%.

Multilayer piezoelectric transducer design guidelines for low profile magnetic-less DC/DC converter

In this paper, multilayer piezoelectric transducer design guidelines are presented for a low profile magnetic-less DC/DC converter. Power capacity considerations are very important in designing magnetic-less power converters with piezoelectric devices, as it has different characteristics from conventional power devices such as magnetic inductors and transformers. Piezoelectric devices have fundamental limitations on power handling, such as maximum operation voltage and current capacity. Using an electro-mechanically coupled equivalent circuit model of a piezoelectric element, the power capacity is discussed and the current capacity equations, including material constants, are derived. In addition, benefits and trade-offs of a multilayer structure application to improve power capacity of piezoelectric transducers are discussed and design considerations for a piezoelectric power application are given. Customized piezoelectric transducers can be designed based on the design guidelines to meet system requirements. Experimental results with a 15W prototype magnetic-less DC-DC converter using a multilayered piezoelectric transducer verify the feasibility of the research.

Digitally controlled open-loop master-slave interleaved boost PFC rectifier

In this paper, a digital control method for open-loop interleaving technique for two-phase critical conduction mode (CRM) boost power factor correction (PFC) rectifier is proposed. The proposed control method guarantees the exact phase shift between master and slave module, as well as proper soft switching of slave MOSFET. The phase shift between the modules is maintained in any time including the transient operation, but slight time deviation of turn-on instant of slave MOSFET may occur as a trade-off. The control circuit requires simple logic elements and digital control ICs, and does not require any two-loop control calculation and phase-locked-loop. The performance of the proposed control method is verified by 500-W laboratory prototype hardware.