Seog-Joo Kim

Synergistic Control of SMES and Battery Energy Storage for Enabling Dispatchability of Renewable Energy Sources

The use of renewable energy source can reduce greenhouse gas emission and fossil fuel pollution. Compared with fossil fuel energy, renewable energy is not stable and cannot supply firm electrical output (i.e., it is nondispatchable). Fluctuating power from renewables may result in grid power oscillation. To reduce grid swing, energy storage is necessary to smooth output from renewable energy. Energy storage with high energy density and fast response time or high power capacity is desired for compensation of fluctuating output. Generally, superconducting magnetic energy storage (SMES) has higher power capacity than battery energy storage, while battery provides higher energy density. Thus, this research proposes a hybrid energy storage system (HESS) composed of an SMES and battery. Novel and practical synergistic control is presented for firming power fluctuation by exploiting the strong power and energy capabilities of the SMES and the battery while within the efficient operating range of (i.e., state of charges of) HESS. Comprehensive case studies demonstrate the efficacy of the proposed HESS topology and control algorithm using PSCAD/EMTDC.

Enhanced frequency regulation service using Hybrid Energy Storage System against increasing power-load variability

Increasing penetration of wind energy resource raises concerns about system frequency regulation because wind turbines lack in control capability necessary to provide regulation in compensating generation-load imbalance. Inherent variability on every time scale, in fact contributes to a need for more sophisticated regulation; existing load-following spinning reserves may be too slow or limited in responding to the imbalance. One feasible solution for enhancing regulation capability is the use of Hybrid Energy Storage System (HESS) as proposed in this paper. In this research, the HESS is designed to be composed of a Li-ion battery and supercapacitor (SC), exploiting their respective high energy and power capabilities to fully handle long-term and short-term changes. This paper demonstrates the effectiveness of HESS for frequency regulation in meeting the grid code while smoothing the net variability (load-wind) for an isolated power system modeled using an electromagnetic transient program.

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 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.

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.

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

A COTS-based dependable excitation system design for 500 MW-class thermal power plants

This paper deals with an experience of the design and implementation of a dependable excitation system for large thermal power plants. Dependability enhancement in the design is mainly achieved by adopting hardware redundancy techniques. A pool, which is a set of component modules, is constructed by gathering commercial-off-the-self (COTS) products in market. Then, the authors derive a finite parameter space of possible values of the reliability for every functional block of the system. The parameter space is determined by the possible types of the configuration of each block. With a given constraint that is unique to the excitation system, they try to find a good configuration for the excitation system from the given COTS component pool. The chosen configuration from the above analysis is implemented as a real digital excitation system which is now in commercial operation at several power plants in Korea.

Grid-adaptive equivalencing of power system for generator tripping

This paper presents a practical yet accurate equivalencing method of power system for an online transient stability assessment. The proposed vector composition method effectively aggregates the multiple generators and transmission lines of the power system, and develops a grid-adaptive equivalent for which well-established analysis technique can be applied to assess the power system stability. It thus allows for activating preventive or corrective measures on time to stabilize the system in emergency. The parameters of the equivalent can be refined using the synchronized measurement data, and the transient stability margin can be updated. The efficacy and accuracy are validated for a multi-machine infinite bus system, and the utility planning model of Korea electric power system where an equal area criterion and the generator tripping are employed for assessing the stability and stabilizing action as specified in the special protection schemes for the Korea electric power system.

Blind identifiability analysis in a MIMO LTI system with inputs from a finite-alphabet set

Abstract A blind separation problem in a multiple-input-multiple-output (MIMO) linear time-invariant (LTI) system with finite-alphabet inputs is considered. A discrete-time matrix equation model is used to describe the input-output relation of the system in order to make full use of the advantages of modern digital signal processing techniques. At first, ambiguity problem is investigated. Then, based on the results of the investigation, a new identifiability condition is proposed for the case of an alphabet set which is widely used in digital communication. The proposed condition is compared to an existing condition in terms of the probability of satisfying each condition for an arbitrary input matrix. A probability bound such that an arbitrary input matrix satisfies the identifiability condition is derived. Monte-Carlo simulation is performed to demonstrate the fact that the identifiability conditions found so far are still very loose.

A Nonconvex Quadratic SDP for Fixed Order H∞ Control and its Application to Generator Excitation System Design

Abstract This paper proposes a computation method for fixed order controller synthesis which often arises in H ∞ or robust control. With the motivation that the fixed order control problem can be casted to a non-convex quadratic objective function minimization subject to linear matrix inequality (LMI) constraints, a quadratic semidefinite programming (SDP) problem is solved by primal dual interior point method (IPM). The advantage of the proposed method is two-fold. First, the existing SDP solving method can be used with a slight modification. Second, since the method directly computes the solution of the problem, much faster results can be obtained than the existing successive LMI relaxation method such as cone complementarity linearization algorithm. In order to demonstrate the algorithm presented, an excitation system for a synchronous generator connected to an infinite bus is designed and simulated. A proportional-integral type automatic voltage regulator and a power system stabilizer considering H ∞ performance are co-designed to provide appropriate damping for the system. The results show the practical applicability of the method.