Analysis and assessment of VSC excitation system for power system stability enhancement

The voltage source converter (VSC) excitation system is a novel excitation system based on pulse-width modulation (PWM) voltage source converter, which is proposed as improved alternatives to the conventional thyristor excitation systems. This paper aims to provide theoretical confirmation of power system stability enhancement by the VSC excitation system. The reactive current injected to generator terminals by the VSC excitation system can be controlled flexibly. Its capability of enhancing power system stability is investigated in this paper. The simplified model of VSC excitation system suitable for use in system stability studies is developed. An extended Philips–Heffron model of a single-machine infinite bus (SMIB) system with VSC excitation system is established and applied to analyze the damping torque contribution of the injected reactive current to the power system. This paper also gives a brief explanation on why the VSC excitation system can enhance the transient stability in light of equal area criterion. The results of calculations and simulations show that the injected reactive current of VSC excitation system contributes to system damping significantly and has a great effect on the transient stability. When compared with conventional thyristor excitation systems, the VSC excitation system can not only improve the small-signal performance of the power system, but also can improve the system transient stability limit.     

Nonlinear coordinated control design of excitation and STATCOM of power systems

This paper presents a nonlinear control scheme for designing the coordinated STATCOM and excitation controllers to enhance the transient stability of power system. The STATCOM installed at the midpoint of the transmission line in a simple power system (a single machine connected to infinite bus). The zero dynamics design theory and pole-assignment technique is employed to design the nonlinear controller. The controller's performance in a SMIB system is also examined using simulation studies and the results validate the efficacy of the proposed controller.     

A simplified nonlinear controller for transient stability enhancement of multimachine power systems using SSSC device

In this paper, a simplified nonlinear method is proposed to enhance the transient stability of multimachine power system by using a Static Synchronous Series Compensator (SSSC). The rate of dissipation of transient energy is used to determine the additional damping provided by a SSSC. The proposed algorithm is based on the direct Lyapunov method. The simplicity of the proposed scheme and its robustness with respect to large disturbances constitute the main positive features. Simulation results in the case of 3-machines power system show the effectiveness of the proposed method under large disturbances (3-phase and single phase short-circuits).     

Design of a nonlinear power system stabilizer using synergetic control theory

Electromechanical oscillations of small magnitude and low frequency exist in the interconnected power system and often persist for long periods of time. Power system stabilizers (PSSs) are traditionally used to provide damping torque for the synchronous generators to suppress the oscillations by generating supplementary control signals for the generator excitation system. Numerous techniques have previously been proposed to design PSSs but many of them are synthesized based on a linearized model. This paper presents a nonlinear power system stabilizer based on synergetic control theory. Synergetic synthesis of the PSS is based fully on a simplified nonlinear model of the power system. The dynamic characteristics of the proposed PSS are studied in a typical single-machine infinite-bus power system and compared with the cases with a conventional PSS and without a PSS. Simulation results show the proposed PSS is robust for such nonlinear dynamic system and achieves better performance than the conventional PSS in damping oscillations.     

Capability curve based enhanced reactive power control strategy for stability enhancement and network voltage management

Reactive power has become a vital resource in modern electricity networks due to increased penetration of distributed generation. This paper examines the extended reactive power capability of DFIGs to improve network stability and capability to manage network voltage profile during transient faults and dynamic operating conditions. A coordinated reactive power controller is designed by considering the reactive power capabilities of the rotor-side converter (RSC) and the grid-side converter (GSC) of the DFIG in order to maximise the reactive power support from DFIGs. The study has illustrated that, a significant reactive power contribution can be obtained from partially loaded DFIG wind farms for stability enhancement by using the proposed capability curve based reactive power controller; hence DFIG wind farms can function as vital dynamic reactive power resources for power utilities without commissioning additional dynamic reactive power devices. Several network adaptive droop control schemes are also proposed for network voltage management and their performance has been investigated during variable wind conditions. Furthermore, the influence of reactive power capability on network adaptive droop control strategies has been investigated and it has also been shown that enhanced reactive power capability of DFIGs can substantially improve the voltage control performance.     

Coordinated control for voltage regulation of distribution network voltage regulation by distributed energy storage systems

With more and more distributed photovoltaic (PV) plants access to the distribution system, whose structure is changing and becoming an active network. The traditional methods of voltage regulation may hardly adapt to this new situation. To address this problem, this paper presents a coordinated control method of distributed energy storage systems (DESSs) for voltage regulation in a distribution network. The influence of the voltage caused by the PV plant is analyzed in a simple distribution feeder at first. The voltage regulation areas corresponding to DESSs are divided by calculating and comparing the voltage sensitivity matrix. Then, a coordinated voltage control strategy is proposed for the DESSs. Finally, the simulation results of the IEEE 33-bus radial distribution network verify the effectiveness of the proposed coordinated control method.     

Coordinated control for voltage regulation of distribution network voltage regulation by distributed energy storage systems

With more and more distributed photovoltaic (PV) plants access to the distribution system, whose structure is changing and becoming an active network. The traditional methods of voltage regulation may hardly adapt to this new situation. To address this problem, this paper presents a coordinated control method of distributed energy storage systems (DESSs) for voltage regulation in a distribution network. The influence of the voltage caused by the PV plant is analyzed in a simple distribution feeder at first. The voltage regulation areas corresponding to DESSs are divided by calculating and comparing the voltage sensitivity matrix. Then, a coordinated voltage control strategy is proposed for the DESSs. Finally, the simulation results of the IEEE 33-bus radial distribution network verify the effectiveness of the proposed coordinated control method.     

Derivation and application of sensitivities to assess transient voltage sags caused by rotor swings

The paper introduces an approach to investigate voltage sags, which are caused by large generator rotor swings following a transient disturbance. Therefore, the method exploits sensitivities derived from the algebraic network equations. These provide information on the impact of a generator on the voltage magnitude at a load bus and the effect of load variation on the generator’s power injection. It is shown that these sensitivities give valuable information to identify critical generator–load pairs and locations for applying preventive control measures.     

A comparative study of voltage stability indices in a power system

The paper compares the effectiveness of voltage stability indices in providing information about the proximity of voltage instability of a power system. Three simple voltage stability indices are proposed and their effectiveness is compared with some of the recently proposed indices. The comparison is carried out over a wide range of system operating conditions by changing the load power factor and feeder X/R ratios. Test results for the IEEE 57 bus and IEEE 118 bus system are presented.     

面向直流受端新型电力系统暂态电压稳定的紧急控制策略

在直流受端新型电力系统中,新能源电源及直流的接入导致同步机开机减少,动态无功功率相对紧张,暂态电压失稳风险显著增大。为高效快速获取紧急控制策略,即直流电流控制方案,并使其适应不同的电网运行方式和故障场景,尤其是电网拓扑结构的变化,基于图卷积神经网络（GCN）对常规深度强化学习模型深度确定性决策梯度（DDPG）的网络结构进行改造,构建了GCN-DDPG融合模型。在此基础上,引入双评价网络机制和评价网络与动作网络非同步更新策略以提升算法效果。进一步地,在应用方面,基于GCN-DDPG融合模型构建紧急控制模型并将其下达至安控主站,安控主站将依据电网实际运行方式和故障等信息,对紧急控制策略进行在线量化计算,并发送至直流控保系统执行。最后,通过改造的IEEE 14节点系统验证所提方法的有效性和优越性。     

Dynamic preventive control strategies for power systems considering transient and midterm stabilities

This paper proposes a new dynamic preventive control strategy to maintain power system security, in terms of both transient and midterm stabilities. While the authors have already presented preventive control strategies for transient stability, a new problem has arisen in midterm instability in which some generators lose synchronism after 10 seconds or more from a fault clearance. It is clear that the cause of the instability is mainly post-contingency dynamic steady-state instability. This paper first shows a preventive control strategy for improving midterm stability using a dominant eigenvalue of post-contingency equilibrium point. This paper also presents a unified dynamic preventive control algorithm that can maintain both stabilities together. In this algorithm, the time domain for each stability problem is considered appropriately and the difference between the classical model and the detailed model is analyzed in detail. The effectiveness of the proposed method is examined with numerical examples for a model system.     

Rescheduling of power systems constrained with transient stability limits in restructured power systems

This paper investigates various approaches to relieve the transient stability constraint in restructured power systems. The approaches adopted fall into two broad categories: those based on eliminating the constraint in the least-cost way and those based on eliminating with the least possible rescheduling. The latter group can, on the other hand, emerge in the form of a pool-protected policy in which the bilateral contracts are rescheduled to maintain the stability or in the form of a contract-protected policy in which the realizable bilateral contracts are maximized while minimizing the rescheduling in pool market. Transient energy function (TEF) method is used as a tool to calculate the sensitivity of energy margin to the variations in the magnitude of generation and load. The effectiveness of the method is illustrated by case studies on Western System Coordinating Council (WSCC) 3-machine 9-bus power system and on the 10-machine 39-bus New England test system and the results are compared. The results are also verified by time domain simulations.     

Decentralized nonlinear coordinated excitation and steam valve adaptive control for multi-machine power systems

In this paper, a decentralized nonlinear coordinated excitation and steam valve adaptive control combined with a modified high-order sliding mode differentiator is designed for multi-machine power system stability enhancement. The proposed control scheme is based on Lyapunov’s direct method and requires only local information on the physically available measurements of relative angular speed, active electric power and terminal voltage with the assumption that the power angle and mechanical power input are not available for measurement. Each synchronous generator is considered as a classical fifth order model that includes turbine dynamics. The simplicity of the proposed scheme and its robustness with respect to large perturbations, change in operating point and parameter uncertainties constitute the main positive features. Simulation results in the case of the Kundur 4-machines 2-area power system show the effectiveness, robustness and superiority of the proposed scheme over the classical AVR/PSS.     

Transient stability enhancement of power systems via optimal nonlinear state feedback control

In this paper, the transient stabilization of power systems will be enhanced using optimal nonlinear feedback control. To this end, a partial differential equation called Hamilton-Jacobi-Belman (HJB), should be solved, which does not have a closed form solution in general. Nevertheless, there are some approximate solutions to solve HJB equations. In this paper, by using a Taylor Series expansion, a sub-optimal nonlinear control law is obtained. As a result, a transient controller for a single-machine infinite-bus power system is designed. Also, the superiority of the designed nonlinear controller in some grounds like increasing the region of asymptotic stability of the system or in other words, increasing the domain of validity is shown. Simulation results reveal the effectiveness of the proposed approach.     

Dynamic preventive control for power system using transient stability assessment based on pattern recognition

To maintain power system security, the authors are developing an integrated security monitoring and control (ISMAC) system which consists of the three main functions: security monitoring, preventive control and emergency control. This paper focuses on the dynamic preventive control which deals with the transient stability immediately after the contingency has occurred. The proposed method is based on the transient stability assessment using the pattern recognition with two-dimensional feature space. Therefore, a preventive control strategy can be obtained rapidly. An index which represents the severity of the contingency quantitatively (security index) is defined by the distance from a linear decision surface which divides a feature plane into a stable and an unstable region. Further, this method has also the advantage that it is possible to consider the effect of the control devices or damping to some extent and specify the operator demand for stabilization flexibly. The effectiveness of the proposed method is ascertained through numerical examples for model power systems.     

Robust excitation control design using sliding-mode technique for multimachine power systems

An output feedback controller is proposed to enhance the transient stability of nonlinear multimachine power systems considered as a classical model with flux decay dynamics. Combining high-order sliding-mode techniques with a robust high-order sliding-mode differentiator, a robust decentralized controller is obtained. Numerical results are presented to illustrate the performance of the proposed control scheme and its robustness properties.     

An adaptive neuro-control approach for multi-machine power systems

We investigate an adaptive neuro-control approach, namely goal representation heuristic dynamic programming (GrHDP), and study the nonlinear optimal control on the multi-machine power system. Compared with the conventional control approaches, the proposed controller conducts the adaptive learning control and assumes unknown of the power system mathematic model. Besides, the proposed design can provide an adaptive reward signal that guides the power system dynamic performance over time. In this paper, we integrate the novel neuro-controller into the multi-machine power system and provide adaptive supplementary control signals. For fair comparative studies, we include the control performance with the conventional heuristic dynamic programming (HDP) approach under the same conditions. The damping performances with and without the conventional power system stabilizer (PSS) are also presented for comparison. Simulation results verify that the investigated neuro-controller can achieve improved performance in terms of the transient stability and robustness under different fault conditions.     

Determination of voltage stability boundary values in electrical power systems by using the Chaotic Particle Swarm Optimization algorithm

A power system has critical values which are the limit values of voltage stability. These values are the highest active power taken by the load busses, voltage amplitude and the angle of the busses. In this research the critical values in electric power systems are defined with use of Chaotic Particle Swarm Optimization (CPSO) algorithm. In this study CPSO has been aimed to use logistic map and Henon map as chaotic maps to control the values of the parameters in velocity update formulation. Initially, critical values of voltage stability have been found by simple Particle Swarm Optimization (PSO). Then the same values have been found with CPSO. Accordingly, the results have been evaluated and observed that the stability critical values found by CPSO can be used to produce good potential solutions. Simulation results are promising and show the effectiveness of the applied approach.     

电力系统暂态稳定性实时预测与控制

利用外部实时同步测量的思想和能量原理,提出一种电力系统暂态稳定性实时预测方法,并在此基础上提出了暂态稳定紧急控制的方法。该预测和控制方法简单．概念清楚,无需要预先知道系统的网络结构和参数,计算速度快,能准确反映系统的真实运行情况,适于在线应用。