Dynamics and stability of wind and diesel turbine generators withsuperconducting magnetic energy storage unit on an isolated power system

Dynamic system analysis is carried out on an isolated electric power system consisting of a diesel generator and a wind-turbine generator. The 150 kW wind turbine is operated in parallel with a diesel generator to serve an average load of 350 kW. A comprehensive digital computer model of the interconnected power system including the diesel and wind-power dynamics with a superconducting magnetic energy storage (SMES) unit is developed. Time-domain solutions are used to study the performance of the power system and control logic. Based on a linear model of the system, it is shown that changes in control-system settings could be made to improve damping and optimization of gain parameters and stability studies are done using the Lyapunov technique and eigenvalue analysis. The effect of introducing the SMES unit for improvement of stability and system dynamic response is studied     

Dynamics and stability of a hybrid wind-diesel power system

Dynamic system analysis is carried out on an isolated electric power system consisting of a wind turbine generator (WTG) and a diesel engine generator (DG). The 150 kW wind turbine generator is operated in parallel with the diesel generator to serve an average load of 350 kW. A comprehensive digital computer model of a hybrid wind-diesel power generation system, including the diesel and wind power dynamics for stability evaluation, is developed. The dynamic performance of the power system and its control logic are studied, using the time domain solution approach. A systematic method of choosing the gain parameter of the wind turbine generator pitch control by the second method of Lyapunov that guarantees stability is presented. The response of the power system with the optimal gain setting to the random load changes has been studied. Analysis of stability has further been explored using the eigenvalue sensitivity technique.     

Dynamics and control of isolated wind-diesel power systems

In this paper a comparative dynamic stability study has been carried out for wind-diesel systems with multiplicity of generation. The mathematical model considered for dynamic stability evaluation is based on small signal analysis. The Lyapunov technique is used to evaluate the optimum setting of gain parameters and comparison is made with the values given in the literature. Additionally the diesel unit equipped with a supplementary proportional integral controller is also considered and its effect on the dynamic stability of the system is investigated. It is shown that the wind-diesel system with multiple wind generation has the minimum first swing of oscillations and also higher damping of subsequent swings for disturbances due to load or due to variations in input wind power. Finally, the dynamic responses of the different power systems considered with optimal gain setting are also presented.     

Dynamic performance assessment of an isolated wind–diesel power system with superconducting magnetic energy storage unit under turbulent wind and load disturbances

This paper presents modelling and control aspects of an isolated wind–diesel system equipped with a superconducting magnetic energy storage (SMES) unit. The SMES unit is located at the induction generators' terminal bus, for exchanging real and reactive powers in four quadrants, with the wind–diesel system. The system components are modelled by non‐linear equations for accurate dynamic performance assessment and the SMES unit is modelled as a controllable current source. The control of the SMES unit is exercised through a multi‐input–multi‐output (MIMO) self‐tuning regulator (STR). The STR uses the local voltage and frequency measurements and generates appropriate signals for the control of the SMES unit. The SMES coil current deviation forms a part of one of the regulated variables of the STR for achieving a continuous control. The complete model of the hybrid system is developed and the parameters of the STR are adjusted for quality improvement of the power supply under turbulent wind. The scheme is then tested for load disturbances. The simulation results show the positive impact of the proposed scheme on the quality of the power supply both under turbulent wind as well as load disturbances. Copyright © 2002 John Wiley & Sons, Ltd.     

超导储能对并网型风电场运行性能影响的仿真分析

采用超导储能(SMES)可以改善风电场并网运行的稳定性,针对风电系统中出现的联络线短路故障和风电场的风速扰动,提出利用超导储能安装点的电压偏差信号作为超导储能有功控制器的控制策略。为了验证这种策略的有效性,建立了风电机组和超导储能装置的数学模型,并利用MATLAB/Simulink软件搭建了风电场接入电网后的仿真模型。仿真结果表明,采用该控制策略不仅可以在网络故障后有效地提高风电场的稳定性,而且能够在快速的风速扰动下平滑风电场的功率输出,降低风电场对电网的冲击。     

Power quality improvement of a stand-alone power system subjected to various disturbances

In wind-diesel stand-alone power systems, the disturbances like random nature of wind power, turbulent wind, sudden changes in load demand and the wind park disconnection effect continuously the system voltage and frequency. The satisfactory operation of such a system is not an easy task and the control design has to take in to account all these subtleties. For maintaining the power quality, generally, a short-term energy storage device is used. In this paper, the performance of a wind-diesel system associated with a superconducting magnetic energy storage (SMES) system is studied. The effect of installing SMES at wind park bus/load bus, on the system performance is investigated. To control the exchange of real and reactive powers between the SMES unit and the wind-diesel system, a control strategy based on fuzzy logic is proposed. The dynamic models of the hybrid power system for most common scenarios are developed and the results presented.     

Power flow control and damping enhancement of a large wind farm using a superconducting magnetic energy storage unit

A novel scheme using a superconducting magnetic energy storage (SMES) unit to perform both power flow control and damping enhancement of a large wind farm (WF) feeding to a utility grid is presented. The studied WF consisting of forty 2 MW wind induction generators (IGs) is simulated by an equivalent 80 MW IG. A damping controller of the SMES unit is designed based on the modal control theory to contribute proper damping characteristics to the studied WF under different wind speeds. A frequency-domain approach based on a linearised system model using eigen techniques and a time-domain scheme based on a nonlinear system model subject to disturbance conditions are both employed to validate the effectiveness of the proposed SMES unit with the designed SMES damping controller. It can be concluded from the simulated results that the proposed SMES unit combined with the designed damping controller is very effective in stabilising the studied large WF under various wind speeds. The inherent fluctuations of the injected active power of the WF to the power grid can also be effectively controlled by the proposed control scheme.     

The power regulation of a PWM type superconducting magnetic energystorage unit

This paper presents an analysis of the power regulation of a SMES unit under PWM switching control. The criteria for executing a power limit scheme to maintain power flows of the SMES unit within a controllable range are described. A general technique for design of power controllers based on the power limit scheme is developed. Two illustrative examples, being separate active and reactive power compensations of a model power system, are presented. The study indicates that active power priority control plays a dominant role in the power regulation of a SMES unit. Experimental results for two examples are also presented     

Techno-economic analysis of MJ class high temperature Superconducting Magnetic Energy Storage (SMES) systems applied to renewable power grids

High temperature Superconducting Magnetic Energy Storage (SMES) systems can exchange energy with substantial renewable power grids in a small period of time with very high efficiency. Because of this distinctive feature, they store the abundant wind power when the power network is congested and release the energy back to the system when there is no congestion. However, considering the cost and lifespan of SMES systems, there is an urgent demand to conduct a cost-benefit analysis to justify its role in smart grid development. This study explores the application and performs economic analysis of a 5 MJ SMES in a practical renewable power system in China based on the PSCAD/ EMTDC software. An optimal location of SMES in Zhangbei wind farm is presented using real power transmission parameters. The stabilities of the renewable power grid with and without SMES are discussed. In addition, a financial feasibility study is conducted by comparing the cost and the savings from wind power curtailment of deploying SMES and battery. The economic analysis tries to find the balance between SMES investment cost and wind farm operation cost by using real data over a calendar year. The technical analysis can help guide the optimal allocation of SMES for compensating power system instability with substantial wind power. Further, the economic analysis provides a useful indication of its practical application feasibility to fight the balance between cost and benefit.     

Experimental evaluation of nonlinear robust control for SMES to improve the transient stability of power systems

This paper presents a new approach and corresponding experiments for the nonlinear robust control of a superconducting magnetic energy storage (SMES) unit to improve the transient stability of power systems. Based on the result of SMES prototype experience, a new dynamic model with disturbances of SMES is adopted, and transferred to the per unit system for simplifying the dynamic analysis and controller design. Then, feedback linearization scheme and linear H/sub /spl infin// control theory are applied to design a novel SMES nonlinear robust controller in a one-machine infinite bus (OMIB) power system. In order to confirm such positive effects of the proposed control strategy, experiments are carried on a laboratory setup of SMES comparing that with a conventional proportional-integral (PI) controller. The results of the experiments demonstrate that the proposed nonlinear robust controller has more excellent performance to improve the transient stability of power systems than that of conventional PI controllers.     

Enhancement of transient stability of an industrial cogeneration system with superconducting magnetic energy storage unit

This paper has developed the coordination of load shedding scheme and superconducting magnetic energy storage (SMES) unit to enhance the transient stability of a large industry cogeneration facility. The load-shedding scheme and the tie line tripping strategy by using the frequency relays have been designed to prevent the power system from collapse when an external fault of utility power system occurs. An actual external fault case and a simulated internal fault case have been selected to verify the accuracy of the load shedding scheme by executing the transient stability analysis. To improve the frequency and voltage responses, an SMES unit with various control modes has been installed in the cogeneration system. The sensitivity analysis of the SMES unit with different parameters is applied to achieve better system responses. Besides, an SMES unit with active power deviation as feedback signal is also considered to improve the electric power fluctuation of the study plant with rolling mills. It is found that the SMES system will enhance the electric power quality and minimize the economic losses of the cogeneration facility due to unnecessary load shedding.     

Dynamic Stability Enhancement and Power Flow Control of a Hybrid Wind and Marine-Current Farm Using SMES

This paper presents a control scheme based on a superconducting magnetic energy storage (SMES) unit to achieve both power flow control and damping enhancement of a novel hybrid wind and marine-current farm (MCF) connected to a large power grid. The performance of the studied wind farm (WF) is simulated by an equivalent 80-MW induction generator (IG) while an equivalent 60-MW IG is employed to simulate the characteristics of the MCF. A damping controller for the SMES unit is designed by using modal control theory to contribute effective damping characteristics to the studied combined WF and MCF under different operating conditions. A frequency-domain approach based on a linearized system model using eigen techniques and a time-domain scheme based on a nonlinear system model subject to disturbance conditions are both employed to validate the effectiveness of the proposed control scheme. It can be concluded from the simulated results that the proposed SMES unit combined with the designed damping controller is very effective to stabilize the studied combined WF and MCF under various wind speeds. The inherent fluctuations of the injected active power and reactive power of the WF and MCF to the power grid can also be effectively controlled by the proposed control scheme.      

Sampled data automatic generation control with superconductingmagnetic energy storage in power systems

A discrete state-space model of a two-area interconnected power system with reheat steam turbine, governor deadband nonlinearity and superconducting magnetic energy storage is developed in this paper. The effect of a small-capacity superconducting magnetic energy storage (SMES) system is studied in relation to supplying sudden power requirements of real power load. The feasibility of using an IGBT power converter instead of a thyristor converter as a power conditioning system with the SMES is studied. Time domain simulation results are also presented which show the improvement of transient response with SMES     

Effect of Load Models on AC/DC System Stability and Modulation Control Design

This paper investigates important aspects related to the effect of load models on the modulation control design and stability of a modulated ac/dc system. Static load is modeled as a nonlinear function of load bus voltage and dynamic load is modeled by an equivalent induction motor. DC power and reactive power modulations are considered for the modulation controllers. A method for eigenvalue sensitivity calculation is developed to predict the effect of load characteristics on system stability. Eigenvalue sensitivity and simulation results show that static and dynamic load characteristics may have a considerable effect on the system stability. Figure 1 shows an ac/dc power system model used for studying the effect of nonlinear load on system stability. Reactive power modulation gain is obtained via optimal control theory. Figure 2 shows speed response of synchronous generator for a 5% change in reference current (Iref) of the rectifier terminal. Reactive power modulation by static var compensator improves system stability with constant impedance load model. However, reactive power modulation makes the system unstable when the modulation gain is based on constant impedance load model and the actual load is represented by induction motor. Important conclusions resulting from the computations and simulations performed for an integrated ac/dc system are listed below. 1. The dynamic behavior of induction motor load has a significant effect on the system stability. Induction motor in most cases reduces the overall system damping.     

An optimization technique of robust load frequency stabilizer for superconducting magnetic energy storage

As an interconnected power system is subjected to rapid load disturbances with changing frequencies in the vicinity of the inter-area oscillation mode, a system frequency may be heavily disturbed and oscillate. Under the circumstances, the stabilizing effect of the conventional load frequency control (LFC), i.e. a governor, cannot be expected. To compensate for such load disturbances and stabilize frequency oscillations, the active power controlled by superconducting magnetic energy storage (SMES) can be applied. In this paper, a new optimization technique of a robust load frequency stabilizer equipped with SMES is presented. To enhance the robustness of the load frequency stabilizer against system uncertainties such as various load changes, system parameters variations etc., the multiplicative uncertainty is included in the system modeling. As a result, the robust stability of the stabilized system can be easily guaranteed in terms of the multiplicative stability margin (MSM). The configuration of the load frequency stabilizer is practically based on a second order lead/lag compensator with a single feedback input. The control parameters are automatically optimized by a tabu search algorithm, so that the desired damping ratio of the target inter-area mode and the best MSM are achieved. The simulation study exhibits the high robustness of the load frequency stabilizer against uncertainties. Moreover, a SMES unit requires small power capacity for frequency stabilization.     

超导储能系统提高风电场暂态稳定性研究

应用超导储能系统(SMES)对提高风电场的暂态稳定性进行了研究。在深入研究超导储能系统运行原理的基础上,建立了基于电压型换流器(VSC)的超导储能系统模型,实现了有功功率和无功功率的解耦控制,并提出了有功、无功功率综合控制策略。利用PSCAD/EMTDC软件进行了仿真计算,结果说明超导储能系统不但能够在风速波动时平滑风电场的功率输出,而且能够提高风电系统的暂态稳定性。     

Damping subsynchronous resonance using superconducting magneticenergy storage unit

A novel damping scheme using superconducting magnetic energy storage (SMES) unit is proposed in this paper to damp subsynchronous resonance (SSR) of the IEEE Second Benchmark Model, system-1 which is a widely employed standard model for computer simulation of power system SSR. The studied system contains a turbine-generator set connected to an infinite bus through two parallel transmission lines, one of which is series-capacitor compensated. In order to stabilize all SSR modes, simultaneous active and reactive power modulation and a proportional-integral-derivative (PID) damping controller designed by modal control theory are proposed for the SMES unit. A frequency domain approach based on eigenvalue analysis and time-domain approach based on nonlinear model simulations are performed to validate the effectiveness of the damping method. It can be concluded from the simulation results that the proposed damping scheme can effectively suppress SSR of the studied system     

Application of 100 kJ/50 kW high-temperature superconducting magnet energy storage system in micro-grid

超导磁储能系统（superconducting magnetic energy storage,SMES）能够实现与电网之间的快速功率交换,对于增强电网稳定性,改善电能质量具有重要意义。本文针对一套100 kJ/50 kW高温超导磁储能系统,对其超导磁体设计与低温系统,功率调节系统的拓扑及控制策略设计、主监控系统的设计分别进行了阐述和分析。为了验证整个磁储能系统的性能,进行了相关的开环功率调节实验。实验结果表明,整套系统运行良好,SMES能够快速的响应主监控系统发出的功率指令,实现SMES与电网之间快速的功率交换。基于上述对SMES的分析和实验测试,结合云电科技园微电网的拓扑,给出了几种SMES在微电网中应用的试验方案,以验证SMES在微电网中应用的性能和作用。     

Application of superconductive magnetic energy storage in anasynchronous link between power systems

The concept that superconductive magnetic energy storage (SMES) can be incorporated into a back-to-back DC link is introduced. With an SMES-DC link, an SMES system can be shared between several neighboring power systems. This results in better economics for SMES usage for each participating power system. In addition to SMES operation, an SMES-DC link also allows asynchronous connection and interchange of power between the interconnected systems. It is demonstrated that an SMES-DC link can achieve significant economic benefits over pure power interchange or SMES operation alone. The basic principle of an SMES-DC link, which is able to interconnect any number of neighboring power systems with a single SMES unit, and various interconnected system operation modes are presented. A battery-DC link is discussed and compared with the SMES-DC link     

Prediction and evaluation of the performance of wind-diesel energysystems

A Monte Carlo based method for predicting the economic performance and reliability of autonomous energy systems consisting of diesel generators and wind energy converters (WECs), is proposed. Several technical constraints are applied, among them the most significant are the limitation of wind power penetration due to both the load demand and a minimum permissible power of the diesel generators. Start up-shutdown costs, two-fuel diesel units and reliability are considered. The proposed method divides the total simulation period into time intervals and for every time interval uses dynamic programming techniques to determine the diesel unit commitment. Results are presented for two Greek islands. It is shown that proper central control of WECs in a system increases significantly the wind energy penetration, which is strongly affected by the way commitment is made