Soonman Kwon

Active Synchronizing Control of a Microgrid

A microgrid is an aggregation of multiple distributed generators (DGs), such as renewable energy sources, conventional generators, and energy storage systems that provide both electric power and thermal energy. Typically, a microgrid operates in parallel with the main grid. However, there are cases in which a microgrid operates in an islanded mode, or in a disconnected state. Islanded microgrid can change its operational mode to grid-connected operation by reconnection to the grid, which is referred to as synchronization. Generally, a single machine simply synchronizes with the grid using a synchronizer. However, the synchronization of microgrids that operate with multiple DGs and loads cannot be controlled by a traditional synchronizer. It is needed to control multiple generators and energy storage systems in a coordinated way for the microgrid synchronization. This is not a simple problem, considering that a microgrid consists of various power electronics-based DGs as well as alternator-based generators that produce power together. This paper proposes an active synchronizing control scheme that adopts the network-based coordinated control of multiple DGs. From the simulation results using Simulink dynamic models, it is shown that the scheme provides the microgrid with a deterministic and reliable reconnection to the grid. The proposed method is verified by using the test cases with the experimental setup of a practical microgrid pilot plant.

Solving Rank-Constrained LMI Problems With Application to Reduced-Order Output Feedback Stabilization

This paper presents an iterative penalty function method for solving rank-constrained linear matrix inequality (LMI) problems and illustrates its application to reduced-order output feedback stabilization. We propose a penalized objective function to replace the rank condition, so that a solution to the original nonconvex LMI feasibility problem can be obtained by solving a series of convex LMI optimization subproblems. Numerical experiments were performed to demonstrate the proposed method.

Test result of microgrid management function in KERI pilot plant

As usual, the microgrid operates in grid-connect mode, but, when a fault occurs in the upstream grid, it should disconnect and shift into islanded operation mode. In grid-connect mode, the controlling power flow at PCC is necessary management function rather than the frequency and voltage. However, the controlling the frequency and voltage is asked in islanded mode. To achieve these management goals appropriately, the cooperative control strategy among microsources is needed. This supervisory control action is executed at MMS. In this paper, the management function of MMS is presented and evaluated by HILS test system and microgrid pilot plant. The proposed HILS test system was composed of the RTDS and a communication emulator. The RTDS performed real-time simulation of component models of the microgrid and the communication emulator simulated communication functions of components of the microgrid. The pilot plant consists of the PV, PV/Wind Hybrid system, BESS, diesel generators, and RLC loads. The test results show that the proper control strategy can ensure stability of microgrid in islanded mode and maintain the power flow of PCC at desired value successfully.

Simultaneous stabilization by static output feedback: a rank-constrained LMI approach

This paper presents a linear matrix inequality (LMI) approach to the design of a static output feedback controller that simultaneously stabilizes a finite collection of linear time-invariant systems. The problem is formulated as a novel rank-constrained LMI feasibility problem with a nonconvex rank condition, and is solved using an iterative penalty function method. Numerical experiments are performed to illustrate the proposed method

Design of networked control system using RTT measurement over WSN

We design a networked control system (NCS) where the communication between sensors and controllers takes place over a wireless sensor network (WSN). In order to measure time delays between sensors and controllers in real time, we design an algorithm to measure round trip time (RTT) between WSN nodes, and implement it into TinyOS of WSN. By using the measured time delays, we construct the Smith predictor to compensate the time delays between sensors and controllers in real-time. For the real time experiment, we simulate the dynamic plant model, controller, and WSN interface using Real-Time Windows Target provided in MATLAB. The WSN interface in the Simulink model consists of serial ports, which connect the plant output and controller with WSN nodes. The experiment results show that the time delays between sensors and controllers are precisely measured in real time; the Smith predictor appropriately compensates the time delays; and the stability of the designed NCS is achieved.

Design of a structurally constrained suboptimal controller using an LMI method

This paper is concerned with an iterative linear matrix inequality (LMI) approach to the design of a structurally constrained output feedback controller such as decentralized control. The structured synthesis is formulated as a novel rank-constrained LMI optimization problem, where the controller parameters are explicitly described so as to impose structural constraints on the parameter matrices. An iterative penalty method is discussed to solve the rank-constrained LMI problem. Numerical experiments and comparisons with previous works are performed to illustrate the practicality of the proposed method.

Design of a pitch controller using disturbance accommodating control for wind turbines under stochastic environments

This paper describes a design of a wind turbine pitch controller based on the disturbance accommodating control (DAC) theory. Wind turbine systems generally operate under stochastic environments, such as random wind inputs and noise corrupted sensor signals. Especially wind inputs can be treated as persistent disturbances and DAC can play a role in reducing effects of wind disturbances. By doing this, wind turbines suffer less from mechanical fatigue loads and their lifespan can be increased. Because wind disturbances we must accommodate are stochastic, we design DAC for stochastic plants and compare with other controllers not considering stochastic conditions to verify the performances of our proposed controller.

RANK-CONSTRAINED LMI APPROACH TO MIXED H2/H∞ STATIC OUTPUT FEEDBACK CONTROLLERS

Abstract This paper deals with a numerical method for the design of mixed H 2 /H ∞ static output feedback controllers. We first formulate the problem as a new type of rank-constrained linear matrix inequalities (LMIs). Then, the LMI optimization problem subject to a rank condition is tackled by the recently developed penalty function method, where a linear penalty function is introduced for the nonconvex rank constraint. The overall procedure results in solving a series of convex optimization problems. With an increasing sequence of the penalty parameter, the solution of the penalized optimization problem moves towards the feasible region of the original nonconvex problem. Comparisons with previous research are performed to illustrate the proposed method.

Global Asymptotic Stability Analysis of FAST TCP Using Nonlinear Small Gain Theorem

We investigate the global asymptotic stability of FAST TCP based on a continuous-time model for FAST TCP network in which all of cross traffics, time-varying network feedback delays, and queuing delay dynamics at link are taken into consideration. We recognize the dynamic interaction between FAST TCP source and link as a feedback interconnection of two input-to-state stable(ISS) subsystems. Then, applying the Razumikhin-type nonlinear small gain theorem, we prove that FAST TCP network is inherently ISS and globally asymptotically stable without any specific conditions on the tuning parameter α or update gain γ for FAST TCP operation. The ISS property of FAST TCP guarantees the robustness to disturbances, and the approach using the small gain theorem provides a guideline to design a globally stable and robust congestion control algorithm. The simulation results demonstrate the validity of the global asymptotic stability of FAST TCP.

A Design and Implementation of a Fault-Tolerant Rod Control System for Nuclear Power Plants

The design and implementation of a fault-tolerant control rod control system for nuclear power plants is described. High reliability and safety are necessary for the instrumentation and control systems for safety-critical plants such as nuclear power plants. For a control rod control system that controls the nuclear reactivity inside the reactor by inserting or withdrawing the rods into or from the reactor, the reliability is more critical than the safety since its malfunction directly results in an unexpected shutdown of the power plant. This paper deals with a design and implementation practice to enhance the reliability of the control system. The reliability enhancement is basically achieved by adopting hardware redundancy in its structure. The reliability is evaluated quantitatively to check if the designed and implemented system is reliable enough to be applied to commercial plants. The ability of fault detection realized in the system is expected to give a further reliability enhancement by means of software. The bumpless control problem that can be arisen from adopting hardware redundancy is discussed here. Also a new algorithm for the rod movement detection is briefly introduced and demonstrated with a test result