Dong-Jun Won

Power-Management Strategies for a Grid-Connected PV-FC Hybrid System

This paper presents a method to operate a grid connected hybrid system. The hybrid system composed of a Photovoltaic (PV) array and a Proton exchange membrane fuel cell (PEMFC) is considered. The PV array normally uses a maximum power point tracking (MPPT) technique to continuously deliver the highest power to the load when variations in irradiation and temperature occur, which make it become an uncontrollable source. In coordination with PEMFC, the hybrid system output power becomes controllable. Two operation modes, the unit-power control (UPC) mode and the feeder-flow control (FFC) mode, can be applied to the hybrid system. The coordination of two control modes, the coordination of the PV array and the PEMFC in the hybrid system, and the determination of reference parameters are presented. The proposed operating strategy with a flexible operation mode change always operates the PV array at maximum output power and the PEMFC in its high efficiency performance band, thus improving the performance of system operation, enhancing system stability, and decreasing the number of operating mode changes.

The DC link energy control method in dynamic voltage restorer system

Dynamic voltage restorer (DVR), which is installed between the supply and a critical load, can restore the load voltage to the pre-fault voltage during voltage sag. To restore load voltage, DVR should inject the equivalent of dropped voltage, which represents the voltage difference between pre-fault and fault voltage through series connected transformer. Voltage restoration of DVR needs to inject active power and energy from DVR to distribution system. However, the capability of energy storage that usually consists of capacitors in DVR is limited. Therefore, it must be considered how the injection energy can be minimized and the load voltage can be made close to the pre-fault voltage. This paper describes conventional restoration techniques like pre-sag compensation, in-phase compensation, and phase advance method. In addition, this paper proposes a new concept of restoration technique to inject minimum energy during DVR compensation. The proposed method is based on the definition of voltage tolerance of the load. In the proposed method, some particular disturbances can be corrected with less amount of energy discharge than those of conventional methods.

Optimal Number and Locations of Power Quality Monitors Considering System Topology

This paper presents a monitor positioning algorithm to determine the optimal number of power quality monitors and to decide the locations for a given distribution system. From graph theory, the system topology is formulated into a coverage matrix. The weighting factors of each component are newly defined from the guidelines of monitor positioning suggested. Weighting factors represent the degree of importance in the system topology. Ambiguity indices are suggested to evaluate the monitor locations in terms of topological ambiguity. The optimal number and locations are determined from the optimization routine utilizing the weighting factor and the ambiguity indices. The proposed approach can realize the experts knowledge on monitor positioning into a sophisticated automatic computing algorithm.

A new algorithm to locate power-quality event source with improved realization of distributed monitoring scheme

This paper presents a systematic algorithm to locate the power-quality (PQ) event source in the distributed monitoring system. Firstly, this paper proposes an improved realization of a distributed monitoring scheme for PQ diagnosis. The algorithm utilizes the topology of power system and the direction of PQ events. As a result, the coverage matrix and the direction matrix are constructed. The algorithm determines the candidate areas for an event source by manipulating these two matrices. For more accurate results, it calculates the distance from the PQ monitor to the location of the PQ event source. It is explained with an illustrative example and is applied to IEEE test feeder in order to validate the accuracy. The proposed algorithm can replace the experts analysis on the PQ event source with an automated computer program.

Mitigation of the Flicker Level of a DFIG Using Power Factor Angle Control

The flicker emission of the wind turbine is affected by wind speed, wind turbulence, generator type, and grid conditions (e.g., the short-circuit ratio, the grid impedance, and the load type). In this letter, an effective control scheme to mitigate flicker emission of a doubly fed induction generator (DFIG) is suggested. By controlling the power factor angle of the DFIG, the flicker is more effectively removed than any other flicker mitigation methods.

Development of power quality monitoring system with central processing scheme

Power quality monitoring system is the cornerstone of power quality analysis, diagnosis and improvement. Power quality (PQ) measurement concepts are evolving from instantaneous metering to continuous monitoring and developments in measurement technology make PQ monitoring system more powerful. That is, now it is possible to construct a more flexible, reliable, fast and economical PQ monitoring system. This paper presents an improved PQ monitoring system with central processing scheme. The proposed system basically consists of one PQ analyzer and multiple PQ meters. PQ meter only acts as raw data acquisition system and PQ analyzer performs all calculations and analysis algorithm. The proposed system is very economical especially for large-scale system because the price of PQ meter can be dramatically lowered in this scheme. PQ monitoring algorithms to catch steady-state trends and to detect PQ events are also proposed. The prototype of this system is constructed and real-time tests are performed using this system.

Determination of the relative location of voltage sag source according to event cause

Event source locating is important to improve the power quality level and to judge the responsibilities of power quality problems. One helpful way to find the location of the power quality event source is to determine the direction of event at each monitor. This paper presents a method to determine the relative location of event source that leads to voltage sag. For this, the relative location of the source is defined first. Then, line fault, induction motor starting and transformer saturation are classified as the main causes of voltage sag and their characteristics are discussed, respectively. From these investigations, the rules to determine the relative location of event source for each cause are proposed. At last, the overall algorithm to identify the relative location and the kind of event source is organized.

Power Sharing Method for a Grid connected Microgrid with Multiple Distributed Generators

In this paper, a grid connected microgrid with multiple inverter-based distributed generators (DGs) is considered. DG in FFC mode regulates the microgrid as a controllable load from the utility point of view as long as its output is within the capacity limit. The transition mode causes a change in frequency of microgrid due to the loss of power transferred between main grid and microgrid. Frequency deviation from the nominal value can exceed the limit if the loss of power is large enough. This paper presents a coordinated control method for inverter-based DGs so that the microgrid is always regulated as a constant load from the utility viewpoint during grid connected mode, and the frequency deviation in the transition mode is minimized. DGs can share the load by changing their control modes between UPC and FFC and stabilize microgrid during transition.

Development of power quality diagnosis system for power quality improvement

This paper represents the structure and functions of total power quality data management system including power quality monitoring system (PQMS), GUI system, and PQ diagnosis system. PQMS characterizes RMS value, harmonics, and frequency, and detects power quality (PQ) events. GUI system manages and stores power quality data received from a number of PQMSs and displays the power quality trends and events. PQ diagnosis system gives total solution of power quality problems such as statistical analysis, event prediction, power quality indexing, event identification, remedy suggestion, etc. Therefore, users can improve their power quality level according to advices provided by PQ diagnosis system. This paper explains the functions of PQ diagnosis system in detail. Moreover, a prototype of PQ diagnosis system as well as PQMS and GUI system is constructed and tested.

Operation Scheme for a Wind Farm to Mitigate Output Power Variation

Because of the nature of wind, the output power of wind turbines fluctuates according to wind speed variation. Therefore, many countries have set up wind-turbine interconnection standards usually named as Grid-Code to regulate the output power of wind farms to improve power system reliability and power quality. This paper proposes three operation modes of wind farms such as maximum power point tracking (MPPT) mode, single wind turbine control mode and wind farm control mode to control the output power of wind turbines as well as overall wind farms. This paper also proposes an operation scheme of wind farm to alleviate power fluctuation of wind farm by choosing the appropriate control mode and coordinating multiple wind turbines in consideration of grid conditions. The performance of the proposed scheme is verified via simulation studies in PSCAD/EMTDC with doubly-fed induction generator (DFIG) based wind turbine models.