Flexible investment decisions in the European interconnected transmission system

Investment decisions in the power transmission system are vulnerable to substantial long-term uncertainties. Especially, in large interconnected power systems like in Europe, the impact of different influencing factors on the decision-making process is hard to estimate. In the presented approach, stochastic simulations incorporate uncertainties like the development of fuel costs and demand growth. The resulting operation plan of the available power plants and the respective utilization of the transmission network are obtained by calculating the (least-cost) optimal power flow. The performance of possible network upgrades in uncertain scenarios is evaluated by applying a real options approach based on Monte Carlo simulations. The focus of the presented work is on the strategic flexibility that FACTS devices can offer in order to appropriately cope with the uncertain development of the future. A case study for the cross-border connection between Germany and the Netherlands shows the applicability and practicability of the presented approach.     

Robust hierarchized controllers using wide area measurements in power systems

In this paper, we present a new method for robust centralized controller design using LMI’s with applications to power systems. This method makes possible to increase the damping rate of the decentralized system (that is, power system with PSS’s). It considers delays added to the signals that travel from the generators to the central controller. The robust hierarchized controller is composed by two control layers: the first one consists on decentralized controllers, that are applied independently to each machine of the system, what guarantees the stability and a minimum damping rate; the second one is composed by a centralized controller, that receives delayed information from all machines of the system, and it sends control signals to all generators. The centralized controller algorithm is robust, once it considers various operative conditions for the power system, and it also permits the choice of the communication delays.     

Impact of FACTS controllers on the stability of power systems connected with doubly fed induction generators

The increasing power demand has led to the growth of new technologies that play an integral role in shaping the future energy market. Keeping in view of the environmental constraints, grid connected wind turbines are promising in increasing system reliability. This paper presents the impact of FACTS controllers on the stability of power systems connected with wind energy conversion systems. The wind generator model considered is a variable speed doubly fed induction generator model. The stability assessment is made first for a three phase short circuit without FACTS controllers in the power network and then with the FACTS controllers. The dynamic simulation results yield information on (i) the impact of faults on the performance of induction generators/wind turbines, (ii) transient rating of the FACTS controllers for enhancement of rotor speed stability of induction generators and angle stability of synchronous generators. EUROSTAG is used for executing the dynamic simulations.     

电力市场下的互联系统自动发电控制改进模式

自动发电控制（AGC）作为能量管理系统（EMS）中最重要的控制功能之一,能有效地提高电网互联系统频率质量,提高经济效益和管理水平。但随着我国电力系统市场化体制改革的不断深入和完善,互联系统原先以联络线和频率偏差控制（TBC）模式为主的频率控制方式就成了电力市场环境下互联系统区域间交易的壁垒。为了适应现代电力市场环境的要求,提出一种新的以TBC为基础、满足二次辅助服务市场开放要求的区域间AGC改进模式。     

Structure and operation strategies of an automatic supervisory control system for the KEPCO UPFC

The Korea Electric Power Corporation (KEPCO) unified power flow controller (UPFC) is installed at the 154 kV Kangjin substation and is operated on the Kangjin–Jangheung transmission line. This UPFC has been operated by the manual set-point control method; however, it is very difficult for the operator to determine the operation point of UPFC in real-time. This paper proposes an automatic supervisory control system for the KEPCO UPFC to overcome the limitations of set-point control and increase the efficiency of UPFC. The functions and configuration of the system are described and the operation scheme that will be applied in the system is presented. In order to validate the proposed algorithm, a prototype program is developed and some simulations are conducted using the data collected from the load dispatch center in the KEPCO system. An erratum to this article can be found at     

Design and allocation of power system stabilizers using the particle swarm optimization technique for an interconnected power system

In this paper, three particle swarm optimization (PSO) based power system stabilizers (PSSs) are developed for three power systems. The system under study here is a power pool consisting of 3 power systems. System I represents the Egyptian power system, system II represents the Jordan and Syrian power systems, and system III for the Libyan power system, which are originally self standing and completely independent systems. As a matter of fact each of them should equipped with its own PSS. For this reason this work is started by designing an optimum power stabilizer for each of them standing alone. After which, the developed PSSs are firstly installed one at a time. Then the three PSSs are installed together in the interconnected power system and their effect on its dynamic performance is studied.As a test for stabilization efficiency, the detailed power system model is subjected to a forced outage of a 600-MW generator, which is the biggest unit in the pool, when it is fully loaded. This outage results in loosing of about 3% of the spinning capacity of system I and about 2% of the spinning capacity of the whole interconnected system. The obtained results show an improvement in the power pool performance accompanied with an improvement in the inter-area oscillation.     

Comprehensive modelling of the unified power flow controller for power system control

In this paper, comprehensive modelling of the unified power flow controller (UPFC) for power flow, voltage, angle and impedance controls is presented. The control modes include some thirteen different power flow, voltage, angle and impedance control functions. The similarities and differences between some of the control modes and those of traditional transformers and series compensation devices are also discussed. The control modes were successfully implemented in a Newton power flow algorithm. Numerical examples are given on the IEEE 30-bus system and the IEEE 118-bus system to illustrate the feasibility and the performance of the Newton power flow algorithm.     

Robust modeling of the interline power flow controller and the generalized unified power flow controller with small impedances in power flow analysis

The interline power flow controller (IPFC) and the generalized unified power flow controller (GUPFC) are two innovative configurations of the convertible static compensator (CSC) of FACTS. In this paper, direct modeling of the practical series or/and shunt operating inequality constraints of the IPFC and the GUPFC in power flow calculations are presented. Special initialization of a solution with the IPFC and GUPFC is also derived. Furthermore, an impedance compensation technique is proposed to deal with the numerical instability or the numerical difficulty of the IPFC and GUPFC models when either their coupling transformer impedances are too small or they are transformer-less controllers. Condition number analysis of the Newton power flow equations is given to get insights of the numerical instability of the voltage sourced models of the IPFC and GUPFC with small impedances. Numerical examples are given based on the IEEE 118-bus system, IEEE 300-bus system and a large scale system with 1000-buses.     

Comparison between two assessment modes for critical clearing times in electrical power systems

Two assessment modes for critical clearing times were analyzed in a power system taken as example: (a) with simultaneous tripping at both line ends; (b) with instantaneous tripping at one line end (zone-1 of distance relays), for computing the critical clearing times for the other end (zone-2 of distance relays). Both results are different, and an analysis of these results is included. The results of computing critical clearing times for zone-2 of distance relays when the other end operates in zone-1 have some advantages for the analysis of practical situations.     

联网环流问题及其可控移相器解决案例研究

结合2系统联网实例对电力系统联网中出现的环流问题进行分析，为避免通常采用的解网运行所带来的各种弊病．提出采用可控移相器控制联络线功率限制环流的方案，并对其可行性与经济性进行分析.     

Study of a major oscillations event in northeastern area of the Iranian power network

Iranian power system encountered major oscillations in January 2008 in the northeastern area with an amplitude of about 120 MW. Since not all the events and variables had been recorded, a scenario to simulate the recorded oscillations and results of studies conducted to reproduce the oscillations by simulation are discussed in the first part. Tuning of supplementary controllers, such as PSSs, on the generating units and the use of reactive power compensators in the long transmission line to enhance stability and eliminate severe oscillations between the north and eastern areas are investigated in the second part. Eigen-value analysis and participation factors are used to appreciate the nature of oscillations and the required PSS settings. The results show that, by using the properly tuned PSSs and accurate compensation of the reactive power, transient stability and damping of oscillations are considerably improved.     

Robustness of damping control implemented by Energy Storage Systems installed in power systems

An Energy Storage System (ESS) installed in a power system can effectively damp power system oscillations through controlling exchange of either active or reactive power between the ESS and power system. This paper investigates the robustness of damping control implemented by the ESS to the variations of power system operating conditions. It proposes a new analytical method based on the well-known equal-area criterion and small-signal stability analysis. By using the proposed method, it is concluded in the paper that damping control implemented by the ESS through controlling its active power exchange with the power system is robust to the changes of power system operating conditions. While if the ESS damping control is realized by controlling its reactive power exchange with the power system, effectiveness of damping control changes with variations of power system operating condition. In the paper, an example of power system installed with a battery ESS (BESS) is presented. Simulation results confirm the analytical conclusions made in the paper about the robustness of ESS damping control. Laboratory experiment of a physical power system installed with a 35 kJ/7 kW Superconducting Magnetic Energy Storage (SMES) was carried out to evaluate theoretical study. Results are given in the paper, which demonstrate that effectiveness of SMES damping control realized through regulating active power is robust to changes of load conditions of the physical power system.     

Performance comparison of several classical controllers in AGC for multi-area interconnected thermal system

This paper presents automatic generation control (AGC) of interconnected two equal area, three and five unequal-areas thermal systems provided with single reheat turbine and generation rate constraints of 3% per minute in each area. A maiden attempt is made to apply integral plus double derivative (IDD) controller in AGC. Controller gains in the two-area system are optimized using classical approach whereas in the three and five area systems controller gains and governor speed regulation parameters (R_i) are simultaneously optimized by using a more recent and powerful evolutionary computational technique called bacterial foraging (BF) technique. Investigations reveal on comparison that Integral (I), Proportional-Integral (PI), Integral-Derivative (ID), or Proportional-Integral-Derivative (PID) controllers all provide more or less same response where as Integral-Double Derivative (IDD) controller provides much better response. Sensitivity analysis reveals the robustness of the optimized IDD controller gains and R_i of the five area system to wide changes in inertia constant (H), reheat time constant (T_r), reheat coefficient (K_r), system loading condition and size and position of step-load perturbation.     

Unified power flow controller models for three-phase power flow analysis

In this paper, three models of the unified power flow controller (UPFC) suitable for three-phase power flow analysis in polar coordinates are presented. The symmetrical components control model can be used to control the positive-sequence voltage of the shunt bus and the total three-phase active and reactive power flows of the transmission line while the injected shunt voltages and the series voltages are balanced, respectively; the general three-phase control model can be used to control the three shunt phase voltages and the six independent active and reactive power flows of the transmission line; the hybrid control model can be used to control the positive-sequence voltage of the shunt bus and the six independent active and reactive power flows of the transmission line. The proposed UPFC models were successfully implemented in a three-phase Newton power flow algorithm in polar coordinates. In the implementation of these UPFC models, transformers of some common connection types, which connect the UPFC with the network, are explicitly represented. Numerical results based on a five-bus system and the modified IEEE 118-bus system are given to illustrate the UPFC control models and demonstrate the computational performance of the three-phase Newton power flow algorithm.     

Decomposition-coordination strategy to improve power transfer capability of interconnected systems

The maximum power transfer across critical corridors or interfaces is limited by various system constraints such as thermal, voltage, and stability limits. In an open transmission access environment, these constraints would be deeply influenced by the interactions among the path flows in different control areas. In particular, small signal stability, commonly in the form of low frequency oscillations, is considered a crucial factor since it limits the power transfer capability of transmission paths in interconnected multi-area systems. Based on such considerations, the focal point of this paper will be a new approach to coordinating the path transfers across multiple control areas, giving exclusive attention to the small signal stability. The differential eigenvalue method is used to derive the damping ratio constraints for satisfying the small signal stability criteria which are linear inequality constraints expressed in terms of the control parameter. Using Bender’s decomposition, the proposed methodology is formulated as a master problem and a set of sub-problems, each associated with one area motivated by the improvement of the overall computational efficiency via parallel processing. The performance of the decomposition-coordination method is illustrated with a 68-bus system from which it might be deduced that inter-area transfer margin could be improved by reasonable rescheduling of the neighboring tie-line flows.     

Comparative performance evaluation of SMES-SMES, TCPS-SMES and SSSC-SMES controllers in automatic generation control for a two-area hydro-hydro system

This paper presents the automatic generation control (AGC) of an interconnected two-area multiple-unit hydro-hydro system. As an interconnected power system is subjected to load disturbances with changing frequency in the vicinity of the inter-area oscillation mode, system frequency may be severely disturbed and oscillating. To compensate for such load disturbances and stabilize the area frequency oscillations, the dynamic power flow control of static synchronous series compensator (SSSC) or Thyristor Controlled Phase Shifters (TCPS) in coordination with superconducting magnetic energy storage (SMES) are proposed. SMES-SMES coordination is also studied for the same. The effectiveness of proposed frequency controllers are guaranteed by analyzing the transient performance of the system with varying load patterns, different system parameters and in the event of temporary/permanent tie-line outage. Gains of the integral controllers and parameters of SSSC, TCPS and SMES are optimized with an improved version of particle swarm optimization, called as craziness-based particle swarm optimization (CRPSO) developed by the authors. The performance of CRPSO is compared to that of real coded genetic algorithm (RGA) to establish its optimization superiority.     

Simulation studies on ECS application in a clean power distribution system

Dealing with power distribution system became one of the most important arts in the field of power system, especially in the rapid increase of the distributed generation (DG) penetration to the distribution level which is a vital and important part of the entire power system. In this paper, a very special power distribution system with a unique deployment of distributed generation, such as, photovoltaic and wind generation has been studied. Energy storage system is utilized to play the main role to control the system’s power quality and the system frequency, as load following operation (LFO) and automatic generation control (AGC), respectively. In this paper, a working criterion has been introduced followed by a case study focuses on two important conditions, one of them when the proposed system is connected to the electrical grid (upper system) and the other one when the system is completely islanded. In both cases, the crucial usage of the ECS gives a concrete result which made the system fully recommended to be applied in real life.     

基于电压稳定性对电压无功控制的改进

介绍变电站电压无功控制(VQC)的调节原理。从电压稳定性的角度出发,分析电力系统在扰动情形下VQC对变压器有载调压分接头(OLTC)的控制及对电压稳定的负面影响。结合广域测量系统(WAMS)和宽域网(WAN),对基于计算机监控系统的VQC提出改进方案。     

Modeling and stability analysis of hybrid power systems for the more electric aircraft

This paper presents a detailed modeling and a comprehensive assessment of small-signal stability for a “more-electric” vehicular power system consisting of a synchronous variable-frequency generator which supplies power electronic controlled loads via an 18-pulse autotransformer rectifier unit for AC-DC conversion. Functional models for key power system components and loads are derived. Analytical derivations employed for small signal stability analysis based on linearized equations are described, and the influence of leading design parameters on system stability is evaluated.     

Feature extraction of AR as a performance index for load frequency controls of interconnected power systems

Recently, the evaluation of control performances of interconnected power systems has become an important issue with respect to individual load frequency controls. Several performance measures are in practical use such as Area Control Error (ACE), Mutual Aid Criterion (MAC), and Control Performance Standard (CPS). Area Requirement (AR) is an alternative expression for ACE, where AR=?ACE. However, the characteristics of these indices have not been sufficiently studied as far as the authors know. In particular, this is the case for their dynamic behavior against random load disturbances. This paper investigates the performance index, AR, through analysis of two‐area systems. Major dynamic elements of interconnected systems affecting the frequency are taken into account to investigate responses of the system against load disturbances. The analysis shows that, when new systems are interconnected, the smaller system tends to require more effort of control against load disturbances to keep the same value of the performance indices. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 154(1): 20–28, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20233