Il-Yop Chung

Control Methods of Inverter-Interfaced Distributed Generators in a Microgrid System

Microgrids are a new concept for future energy distribution systems that enable renewable energy integration and improved energy management capability. Microgrids consist of multiple distributed generators (DGs) that are usually integrated via power electronic inverters. In order to enhance power quality and power distribution reliability, microgrids need to operate in both grid-connected and island modes. Consequently, microgrids can suffer performance degradation as the operating conditions vary due to abrupt mode changes and variations in bus voltages and system frequency. This paper presents controller design and optimization methods to stably coordinate multiple inverter-interfaced DGs and to robustly control individual interface inverters against voltage and frequency disturbances. Droop-control concepts are used as system-level multiple DG coordination controllers, and control theory is applied to device-level inverter controllers. Optimal control parameters are obtained by particle-swarm-optimization algorithms, and the control performance is verified via simulation studies.

Power-Sharing Method of Multiple Distributed Generators Considering Control Modes and Configurations of a Microgrid

This paper describes the active power and frequency-control principles of multiple distributed generators (DGs) in a microgrid. Microgrids have two operating modes: 1) a grid-connected mode and 2) an islanded mode. During islanded operation, one DG unit should share output generation power with other units in exact accordance with the load. Two different options for controlling the active power of DGs are introduced and analyzed: 1) unit output-power control (UPC) and 2) feeder flow control (FFC). Taking into account the control mode and the configuration of the DGs, we investigate power-sharing principles among multiple DGs under various system conditions: 1) load variation during grid-connected operation, 2) load variation during islanded operation, and 3) loss of mains (disconnected from the main grid). Based on the analysis, the FFC mode is advantageous to the main grid and the microgrid itself under load variation conditions. However, when the microgrid is islanded, the FFC control mode is limited by the existing droop controller. Therefore, we propose an algorithm to modify the droop constant of the FFC-mode DGs to ensure proper power sharing among DGs. The principles and the proposed algorithm are verified by PSCAD simulation.

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.

Slow coherency and Angle Modulated Particle Swarm Optimization based islanding of large-scale power systems

Power system islanding is an effective way to avoid catastrophic wide area blackouts, such as the 2003 North American Blackout. Islanding of large-scale power systems is a combinatorial explosion problem. Thus, it is very difficult to find an optimal solution within reasonable time using analytical methods. This paper presents a new method to solve this problem. In the proposed method, Angle Modulated Particle Swarm Optimization (AMPSO) is utilized to find islanding solutions for large-scale power systems due to its high computational efficiency. First, desired generator groups are obtained using the slow coherency algorithm. AMPSO is then used to optimize a fitness function defined according to both generation/load balance and similarity to the desired generator grouping. In doing so, the resulting islanding solutions provide good static and dynamic stability. Simulations of power systems of different scales demonstrate the effectiveness of the proposed algorithm.

Control parameter optimization for a microgrid system using particle swarm optimization

Microgrids are new concept of electric power networks consisting of distributed generators, renewable energy sources and sensitive loads. The goal of microgrid operation is to provide reliable and high-quality electric power regardless of faults or abnormal operating conditions. This paper presents control schemes for coordination of multiple microgrid generators, especially with voltage source inverter type interface, for both grid-connected and autonomous modes. To maintain required control performance and power quality during severe conditions, excessive trial-and-error-based repeated tuning process is required. This paper proposes an effective optimal control parameter-tuning method using the particle swarm optimization (PSO) algorithm. Requirements of power quality and load following performance during the island operation are considered in the PSO tuning.

Analysis of Power System Disturbances Based on Wide-Area Frequency Measurements

It is very important to understand system dynamics during disturbances in order to improve control measures to ensure power system security and reliability. Power system disturbances are examined in this paper based on wide-area frequency data from a nation-wide frequency monitoring network (FNET). Frequency characteristics of events in three major North America power grids are investigated. Typical frequency patterns of generation loss and load drop events are analyzed for Eastern Interconnection system, Western Electricity Coordinating Council system, and Electric Reliability Council of Texas system. The recovery patterns following disturbances are also studied to evaluate control performances of the interconnections.

The control and analysis of zero sequence components in DVR system

The magnitudes and phases of the compensating voltages in a dynamic voltage restorer (DVR) system depend on the voltage sag in the phases affected by the fault and on the influence of the zero sequence components. If the delta connection of the transformer is used, the zero sequence components do not appear on the load side. But nowadays, wye connected transformers with grounded neutral are usually used. Therefore the zero-sequence components occur during faults. The zero-sequence components result in the high insulation costs and the asymmetry of the phases and magnitudes of the terminal voltages. This paper proposes a method that can mitigate the zero-sequence under the unbalance faults causing voltage sags and phase angle jumps. Moreover, the energy optimizing method is considered in this paper.

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.

Integration of a bi-directional dc-dc converter model into a large-scale system simulation of a shipboard MVDC power system

To improve energy flexibility and deal with peak energy demand in shipboard power system, a bi-directional dc/dc converter is investigated for a notional U.S. Navy Medium Voltage DC (MVDC) shipboard power system. Surplus energy due to light electric load or ship-speed variation can be captured by energy storages distributed in 800 V load zones and during heavy load or black starting condition, supplied to the rest of the 5 kV MVDC system through the bi-directional dc/dc converters. This paper presents the controller optimization process using the particle swarm optimization for an isolated-type bi-directional dc-dc converter. The control performance of the proposed controller is evaluated using small-signal average models and a large-scale simulation of the notional U.S. Navy MVDC system using the real-time digital simulator.

Operating strategy and control scheme of premium power supply interconnected with electric power systems

This paper presents an inverter-interfaced distributed generation (DG) device, dubbed the premium power supply (PPS), which is designed to provide reliable electric power, and to improve power quality. This paper proposes an effective configuration for the PPS, using inverters, an inter-tie breaker, and a small generating unit. When the DGs are operated in parallel with the utility systems, some problems occur due to interconnections on such categories as reverse power flow and island protection. This paper advances both an operating strategy and a control scheme for the PPS, with respect to the interconnection standard, IEEE Standard 1547. Simulation results demonstrate the efficacy of the PPS.