Seon-Ju Ahn

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.

Power Scheduling of Distributed Generators for Economic and Stable Operation of a Microgrid

This paper is concerned with the power dispatch problem of distributed generators (DGs) for optimal operation of a microgrid. The objective is to minimize the fuel cost during the grid-connected operation, while ensuring stable operation after islanding. To achieve this goal, the economic dispatch (ED) problem and related constraints are formulated. The constraints considered in this study are: i) reserve for variation in load demand, ii) reserve for variation in the power outputs of non-dispatchable DGs, iii) flow limits between two adjacent areas, and iv) reserve for the stable islanded operation. The first three constraints, which have been employed in ED problem for conventional power systems, are modified to apply to Microgrids. We also provide a detailed formulation of the constraint for stable islanded operation in accordance with two power-sharing principles: i) fixed droop and ii) adjustable droop. The problem is solved using a modified direct search method, and the effect of the constraints on the operational cost is investigated via numerical simulations.

Coordinated Control of a DG and Voltage Control Devices Using a Dynamic Programming Algorithm

This paper presents a new control method, in which a distributed generator (DG) actively participates in steady-state voltage control, together with an under-load tap changer (ULTC) and shunt capacitors (Sh.Cs). In the conventional DG control method, the integration of DGs into a distribution power system increases the number of switching operations of the ULTC and the Sh.Cs. To solve this problem, this paper proposes that the DG output voltage be dispatched cooperatively with the operation of the ULTC and the Sh.Cs, based on load forecasts for one day in advance. The objective of the proposed method is to decrease the number of switching device operations, as well as to reduce the power loss in the distribution lines, while maintaining the grid voltage within the allowed range. The proposed method is designed and implemented with Matlab, using two different dynamic programming algorithms for a dispatchable and a nondispatchable DG, respectively. Simulation studies demonstrate that the objective can be achieved under various grid conditions, determined by factors such as the DG output power characteristics, the location of the DG-connected bus on the feeder, and the load profile of the feeder containing the DG.

Operation Schemes of Smart Distribution Networks With Distributed Energy Resources for Loss Reduction and Service Restoration

The integration of Distributed Energy Resources (DERs) into the future Smart Distribution Network (SDN) has challenging issues regarding the successful development of smart grids. The SDN offers new opportunities in the improvement of the efficiency of power distribution networks. The DERs will be distributed in the existing distribution networks, interconnected in customer areas and operated on its own schedule without communication to the control center of the existing distribution system. The DER units have both positive and negative effects regarding SDN operations. With the appropriate operation of the DER units in the SDN, losses can be reduced during normal operations and they can support local loads during abnormal conditions. Thus, the positive effects of the DER units need to be enabled in the SDN by adopting advanced operation schemes. In this paper, the smart control functions for the DER units in the SDN are defined and classified. In addition, the integration schemes for the SDN with DER units are introduced. The proposed operation strategies will be implemented into the Korean Smart Distribution Management System (KSDMS) as operation schemes used for loss reduction and service restoration. A sample case study shows the effectiveness of the proposed operation schemes to achieve smart operation functions for the SDN with DER units.

Economic scheduling of distributed generators in a microgrid considering various constraints

This paper deals with the economic dispatch problem in a microgrid to provide the optimal power reference of distributed generators. The objective function and the constraints related to the operation of the microgrid are formulated and the solution method for the problem is introduced. The constraints for this problem include i) additional reserve requirement due to the uncertainty in the output of renewable resources, ii) flow limit between the control areas, and iii) additional reserve for the stable operation during the islanded mode. In order to examine the effect of the constraints on the generation cost, some numerical experiments are conducted.

A modified sag characterization using voltage tolerance curve for power quality diagnosis

This paper newly defines a voltage sag duration considering the voltage tolerance characteristics of individual electrical device for power quality (PQ) diagnosis. The conventional sag characterizing method has the possibility to overestimate voltage sag in case of nonrectangular sag. Furthermore, it cannot take the voltage tolerance characteristics of individual device into account. The proposed method utilizes the monitored parameters (V/sub sag/,V/sub avg/,d) at PQ monitor and the minimum voltage (V/sub min/) of voltage tolerance curve. The voltage sag profile is approximated using k/sup th/ order radical root function and finally the sag duration is modified from this function. The proposed method is applied to the nonrectangular sag due to induction motor reacceleration. It effectively evaluates the actual effects of voltage sag on the customer equipments whether the sag is rectangular or not.

Simulation and Analysis of Existing MPPT Control Methods in a PV Generation System

Maximum Power Point Tracking (MPPPT) techniques are employed in PV systems to make full utilization of PV array output power which depends on solar irradiation and temperature. Recently, many MPPT algorithms and control schemes of PV system have been proposed, but different control methods and different PID controller parameters lead to different output efficiency and dynamic response. Therefore, in this paper, a PSCAD/EMTDC user defined module of the solar array is established to easily simulate its inherent characteristics with the basic specification data. Moreover, we also compare the advantages and weakness of the existing MPPT control methods by simulations to give a reference to the Grid-Connected Photovoltaic System (GCPS) designers.

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.