Showing papers in "Iet Generation Transmission & Distribution in 2007"

Small-signal dynamic model of a micro-grid including conventional and electronically interfaced distributed resources

Abstract: A systematic approach to small-signal modelling of a micro-grid system that includes conventional (rotating machine) and electronically interfaced distributed resource (DR) units is presented here. The proposed approach incorporates fundamental frequency deviations in the overall system model and provides a methodology for the analysis of autonomous micro-grid, which inherently is more prone to frequency changes than the conventional utility grid. The model represents (i) electro-mechanical dynamics of the synchronous machine including the exciter and the governor systems, (ii) dynamics of the voltage-sourced converter and its real/reactive power controllers and (iii) the network dynamics. The model is intended for the controller design/optimisation, evaluation of angle/voltage stability, investigation of torsional dynamics, controller interactions of electronically interfaced DR units and low-frequency power quality issues. Typical results from application of the proposed modelling approach to a study system are presented. The results are qualitatively verified on the basis of the comparison with those obtained from time-domain simulation in the PSCAD/EMTDC environment

Small signal stability analysis and optimal control of a wind turbine with doubly fed induction generator

Abstract: A novel method using particle swarm optimisation (PSO) is proposed for optimising parameters of controllers of a wind turbine (WT) with doubly fed induction generator (DFIG). The PSO algorithm is employed in the proposed parameter tuning method to search for the optimal parameters of controllers and achieve the optimal coordinated control of multiple controllers of WT system. The implementation of the algorithm for optimising the controllers' parameters is described in detail. In the analysis, the generic dynamic model of WT with DFIG and its associated controllers is presented, and the small signal stability model is derived; based on this, an eigenvalue-based objective function is utilised in the PSO-based optimisation algorithm to optimise the controllers' parameters. With the optimised controller parameters, the system stability is improved under both small and large disturbances. Furthermore, the fault ride-through capability of the WT with DFIG can be improved using the optimised controller. Simulations are performed to illustrate the control performance.

Non-smooth/non-convex economic dispatch by a novel hybrid differential evolution algorithm

Abstract: This paper presents a novel stochastic optimisation approach to determining the feasible optimal solution of the economic dispatch (ED) problem considering various generator constraints. Many practical constraints of generators, such as ramp rate limits, prohibited operating zones and the valve point effect, are considered. These constraints make the ED problem a non-smooth/non-convex minimisation problem with constraints. The proposed optimisation algorithm is called self-tuning hybrid differential evolution (self-tuning HDE). The self-tuning HDE utilises the concept of the 1/5 success rule of evolution strategies (ESs) in the original HDE to accelerate the search for the global optimum. Three test power systems, including 3-, 13- and 40-unit power systems, are applied to compare the performance of the proposed algorithm with genetic algorithms, the differential evolution algorithm and the HDE algorithm. Numerical results indicate that the entire performance of the proposed self-tuning HDE algorithm outperforms the other three algorithms.

Generation curtailment to manage voltage constraints in distribution networks

Abstract: The issue of congestion management in distribution networks by considering a methodology for generation curtailment due to voltage constraints is addressed. The curtailment is based on the contribution of generators to the constraints as quantified by voltage-sensitivity factors. It has been shown that generation curtailment following a simple proportionality rule is not efficient and some alternatives, which give a more efficient outcome, have been proposed. Influence of network losses on the curtailment rules has also been shown. The considerations have been illustrated using two simple 11 kV networks

Transient stability constrained optimal power flow using particle swarm optimisation

Abstract: A novel approach based on the particle swarm optimisation (PSO) technique is proposed for the transient-stability constrained optimal power flow (TSCOPF) problem. Optimal power flow (OPF) with transient-stability constraints considered is formulated as an extended OPF with additional rotor angle inequality constraints. For this nonlinear optimisation problem, the objective function is defined as minimising the total fuel cost of the system. The proposed PSO-based approach is demonstrated and compared with conventional OPF as well as a genetic algorithm based counterpart on the IEEE 30-bus system. Furthermore, the effectiveness of the PSO-based TSCOPF in handling multiple contingencies is illustrated using the New England 39-bus system. Test results show that the proposed approach is capable of obtaining higher quality solutions efficiently in the TSCOPF problem.

Genetic-based algorithm for power economic load dispatch

Abstract: An improved genetic algorithm with multiplier updating (IGAMU) to solve practical power economic load dispatch (PELD) problems of different sizes and complexities with non-convex cost curves, where conventional mathematical methods are inapplicable, is developed. The improved genetic algorithm (IGA) provides an improved evolutionary direction operator and a migrating operator, enabling it to efficiently search and actively explore solutions. Multiplier updating (MU) is introduced to avoid deforming the augmented Lagrange function, which is adopted to manage the system constraints of PELD problems. The proposed IGAMU integrates the IGA with the MU. Two practical examples are employed to demonstrate that the proposed algorithm has the benefits of straightforwardness, ease of implementation, better effectiveness than previous methods, better effectiveness and efficiency than the genetic algorithm (GA) with MU (GA-MU), automatic adjustment of the randomly assigned penalty to an appropriate value and the requirement for only a small population when applied to real-life PELD operations.

Methods for customer and demand response policies selection in new electricity markets

Abstract: Different methodologies are available for clustering and classification purposes. The objective of the research is to prove the capability of self-organising maps (SOMs) to classify customers and their response potential from distributor, commercialiser, or customer electrical demand databases, with the help of load response modelling methodologies as support tools. The search for customer suitability is restricted to day-ahead and real-time products, in which interest is growing in developed countries. Therefore customer demand and response (demand and distributed generation policies) have been tested and compared with price curves. Both steps have been performed through SOMs. The results clearly show the capability of this approach to improve data management and easily to find coherent policies to accomplish cleared-demand offers in different prices scenarios.

Comparison of risk-based and deterministic security assessments

Abstract: The traditional N - 1 security criterion provides only a limited perspective on the actual level of security of a power system and a risk-based approach to security assessment provides considerably more information on which to base operating decisions. It further argues that this risk should be measured in terms of expected outage costs to the consumers. Furthermore, the risk calculation should not be limited to the consideration of a predefined set of contingencies but should factor in the actual probabilities of outages leading to load disconnections. A case study based on the IEEE-Reliability Test System shows that different operating points on the system's deterministic security boundary have very different levels of risk. For a given operating point, the risk level changes considerably between fair, average and bad weather conditions. Finally, the paper shows how, using adaptive deterministic security boundaries, it is possible to compare the cost and benefit of relaxing operating limits.

Harmonic and reactive power compensation as ancillary services in inverter-based distributed generation

Abstract: The introduction of distributed generation, DG, into low-voltage (LV) networks opens up the possibility of supplying ancillary services to aid network management and to maintain power quality. DG itself can cause voltage magnitude difficulties when injecting real power into networks with high R/X ratios, but control of reactive power injection can help overcome this. Continuous control and support only at the voltage limit conditions (to avoid unnecessary reactive power flows in normal conditions) are explored. Relatively high-impedance LV networks are prone to harmonic distortion from nonlinear loads. A variety of control methods that emphasise either harmonic-line flows or local-voltage distortion are examined, and a compromise method based on resistance emulation is shown to be effective. Experimental results from a single-phase laboratory network and 2 kVA inverter are used to illustrate how these additional control functions can be integrated into the existing control scheme for real-power management. Decomposition of observed voltages and currents into harmonic terms that are phasesynchronised to the grid voltage is a challenge in real-time systems. Kalman observers are used to achieve this with an additional advantage of avoiding explicit phase-locking while producing quadrature components useful in instantaneous calculation of reactive power and in providing feed-forward compensation terms

Next-day electricity-price forecasting using a hybrid network

Abstract: The paper proposes a novel method of forecasting short-term electricity price based on a two-stage hybrid network of self-organised map (SOM) and support-vector machine (SVM). In the first stage, a SOM network is applied to cluster the input-data set into several subsets in an unsupervised manner. Then, a group of SVMs is used to fit the training data of each subset in the second stage in a supervised way. With the trained network, one can predict straightforwardly the next-day hourly electricity prices. To confirm its effectiveness, the proposed model has been trained and tested on the data of historical energy prices from the New England electricity market.

Genetic algorithm-based approach for fixed and switchable capacitors placement in distribution systems with uncertainty and time varying loads

Abstract: Installation of capacitors in primary and secondary networks of distribution systems is one of the efficient methods for energy and peak load loss reduction. Also voltage profile in the feeder is improved and static voltage stability is enhanced. The main challenge is the determination of optimal location and size of fixed and switchable capacitors with respect to network configuration, distribution of load in the feeder, time variation of load and uncertainty in load forecasting or load allocation process. To solve this complex problem, an efficient method for simultaneous allocation of fixed and switchable capacitors in radial distribution systems is presented. Energy and peak load loss reduction, and capacitor cost are considered in the cost function. Time variation and uncertainty of load are also involved in problem formulation. Genetic algorithm with a new coding as two rows chromosomes is used for optimisation. Numerical studies show the effectiveness of the proposed procedure.

Study of differential evolution for optimal reactive power flow

Abstract: Differential evolution (DE) is studied in detail for optimal reactive power flow (ORPF) problems. The concept, mechanism, and parameter setting of DE are discussed. Based on the IEEE 14-, 30- and 57-bus system test cases, DE is compared with some basic or improved evolutionary algorithms that have been applied to ORPF. It is found that DE is generally a good algorithm for ORPF and worthy of more attention. However, it is also found that DE requires relatively large populations to avoid premature convergence. The impact of this shortcoming is made clear in the IEEE 118-bus system test case. The effectiveness of parallel computing technology for speeding up the computation of DE-based ORPF is also analysed.

Unit commitment computation by fuzzy adaptive particle swarm optimisation

Abstract: A fuzzy adaptive particle swarm optimisation (FAPSO) for unit commitment (UC) problem has been proposed. FAPSO reliably and accurately tracks a continuously changing solution. By analyzing the social model of standard PSO for the UC problem of variable resource size and changing load demand, the fuzzy adaptive criterion is applied for the PSO inertia weight based on the diversity of fitness. In this method, the inertia weight is dynamically adjusted using fuzzy IF/THEN rules to increase the balance between global and local searching abilities. Velocity is digitised (0/1) by a logistic function for the binary UC schedule. To improve knowledge, the global best location is also moved instead of a fixed one in each generation. To avoid the system to be frozen, stagnated/idle particles are reset from time to time. Finally, benchmark data and methods are used to show effectiveness of the proposed method.

Simulated annealing-based optimal wind-thermal coordination scheduling

Abstract: The rise of environmental protection and the progressive exhaustion of traditional fossil energy sources have increased the interests in integrating wind energy sources into existing power systems. Development of better wind-thermal coordination algorithms is necessary to determine the optimal proportion of wind generator capacity that can be integrated into the system for operating an isolated hybrid power system reliably and efficiently. A stochastic searching technique, which utilises a simulated annealing (SA) approach combined with an efficient constrained dynamic economic dispatch (CDED) method, is developed to coordinate the wind and thermal generation scheduling in isolated power systems with large integration of wind capacity. The SA algorithm is used for the scheduling of the generating units, whereas a direct search method routine carries out the ramp rate CDED when considering wind power generation. Several technique constraints are applied to determine the maximum proportion of wind generator capacity that can be integrated into the system. A constraint satisfaction technique for generating feasible neighbouring solution is also developed to improve the SA solution process. Numerical experiments are included to understand the wind generator capacity in the operating cost analysis and to provide valuable information for both the operational and planning problems.

Placement of FACTS controllers using modal controllability indices to damp out power system oscillations

Abstract: Flexible ac transmission systems (FACTS) controllers are being used to damp out the power system oscillations. The effectiveness of these controllers depends on their optimal location in the power system network. A controllability index has been proposed to find the optimal location of the FACTS controllers to damp out the inter-area mode of oscillations. Three types of FACTS controllers have been considered, which include static var compensator, thyristor-controlled series compensator and unified power flow controller. The proposed controllability index is based on the relative participation of the parameters of FACTS controllers to the critical mode. A simple approach of computing the controllability indices has been proposed, which combines the linearised differential algebraic equation model of the power system and the FACTS output equations. The placements of FACTS controllers have been obtained for the base case as well as for the critical contingency cases. The effectiveness of the proposed method is demonstrated on New England 39-bus system and 16-machine, 68-bus system.

Improved Grey predictor rolling models for wind power prediction

Abstract: A new technique for one step ahead average hourly wind speed forecasting and wind turbines' output power prediction based on using the Grey predictor models is presented. The required mathematical formulation for developing the Grey predictor models is also presented. The obtained results from the proposed models are compared with the corresponding results obtained when using the persistent model. Utilising the traditional Grey model, GM(1,1) was first investigated and showed good improvement over the persistent model. However, the generated results demonstrate the presence of intervals with overshoots in the predicted values. To reduce such overshoots, a modified version for the Grey predictor model referred to as the adaptive alpha GM(1,1) model is investigated and two new models are proposed, hereafter, referred to as the improved Grey model and the averaged Grey model. The presented results demonstrate the effectiveness, the accuracy and the superiority of the proposed averaged Grey model for wind speed and wind power prediction.

Active power-flow management utilising operating margins for the increased connection of distributed generation

Abstract: Active management provides a method to extend capacity for generation connections to distribution networks through the coordinated control of multiple network components in real-time. Identifying principles and strategies for active network management that hold for all situations will provide support to network operators and planners and form the basis for a new paradigm in utility strategy concerning the connection of distributed generation. The authors build upon previous work regarding the initial specification and economic evaluation of an active power- flow management (APFM) scheme concerned with facilitating increased generator connections. Operating margins have been identified as an essential element of the APFM scheme for the provision of network security and play a significant role in determining the economic viability of generation connected under the APFM scheme. An approach to the definition of the operating margins required to trigger generator output regulation (trimming) and tripping is introduced. These concepts are demonstrated through a case study using one of the UK generic distribution system models. The analysis considers technical, regulatory and economic factors, resulting in practical recommendations for future work and the identification of the factors that affect the implementation of such a scheme.

Analysis of measurement-set qualitative characteristics for state-estimation purposes

Abstract: The paper proposes a new tool for real-time assessment of measurement sets in the context of power-system-state estimation. This tool incorporates many functions including observability analysis and restoration, as well as identification of critical measurements and critical sets. Using only network-topology data, the proposed methodology updates the qualitative characteristics of the current available measurement set in real time. The identification of critical measurements and sets is done using only network topology and is completed before running any state estimation. Mapping the system states and using information concepts, a new state space is obtained where the relationship (information) between measurements and states is straightforward. This map is easily found via triangular factorisation of the Jacobian matrix. Starting from a base-case measurement set, every time a snapshot of measurements has to be analysed, only refactorisation has to be carried out. Sparse-matrix techniques speed up the process. The method was tested in the IEEE-14-bus system as well as in the Brazilian 383-bus system, proving to be reliable, fast, easy to implement and suitable for real-time operation.

Real-time monitoring of electromechanical oscillations in power systems: first findings

Abstract: Oscillations have been observed in power systems from the moment networks comprising multiple generators were interconnected in order to provide more power capacity and reliability. They are typically poorly damped, however, not dangerous as long as they do not become unstable. An algorithm for fast real-time detection and stability assessment of dominant electromechanical oscillations utilising Kalman filtering techniques, is presented. First results of its successful implementation on a recently developed industrial wide-area monitoring system are shown using measured data recorded during the resynchronisation of two major European power networks.

Advanced voltage sag characterisation ii: point on wave

Abstract: Two important but often neglected characteristics of voltage sags and short interruptions are: point on wave of sag/interruption initiation and point on wave of sag/interruption ending A critical review of existing approaches is presented and an appropriate definition of point-on-wave values proposed An identification of point-on-wave values in monitoring applications is discussed and problems related to identification/assessment of point-on-wave values highlighted Examples of equipment sensitivity to these phenomena are analysed The results related to identified ranges and relationships of point-on-wave values are reviewed, using examples of sags and interruptions recorded in power supply systems Adequate procedure for testing single-phase and three-phase equipment to sags and interruptions are discussed with respect to point-on-wave values

Voltage and frequency control of parallel operated synchronous generator and induction generator with STATCOM in micro hydro scheme

Abstract: Parallel operation of synchronous and induction generators in micro hydro scheme is presented. The synchronous generator has an exciter, which provides a fixed excitation to produce normal rated terminal voltage at full resistive load. On the other hand, the induction generator has neither exciter nor speed controller. Static compensator (STATCOM) is connected to the common bus for terminal voltage and frequency control. A resistive dump load is connected across the DC link capacitor of STATCOM through a chopper to control active power. Simulink model is developed to perform transient analysis of the proposed scheme. Experimental results are presented to compare with the simulation results. It is found that connection of an induction generator in parallel with the synchronous is much simpler than connecting two synchronous generators in parallel.

Out-of-step detection algorithm using frequency deviation of voltage

Abstract: Protection against transient instability and a consequent out-of-step condition is a major concern for the utility industry. An unstable system may cause serious damage to system elements such as generators and transmission lines, therefore out-of-step detection is essential to operate a system safely. The traditional out-of-step relays detect out-of-step conditions by using distance relays and timers. However, such a relay monitors only apparent impedance which is an indirect function of generator angle, and the relay cannot cope with the out-of-step situation for the more severe instability situation of very fast power swings which can also cause damage to transmission lines if not detected fast enough, as is the case with conventional detection techniques. Digital filters based on discrete Fourier transforms are used to calculate the frequency of a sinusoidal voltage, and then the generator angle is estimated using the deviation of the calculated frequency component of the voltage. The proposed out-of-step detection algorithm is based on the assessment of transient stability using the equal-area criterion. It is verified and tested by using ATP/EMTP MODELS, and the simulation results show that the out-of-step conditions are detected accurately employing the proposed algorithm.

Representative static load models for transient stability analysis: development and examination

Abstract: Load representation has a significant impact on power system analysis and control results. Currently, static load models are still popular in power industry for transient stability analysis. Dynamic load models are recommended in both industry and academia for possible improvement in analysis accuracy. The accuracy of using static (nonlinear) load models with suitably identified parameters for transient stability analysis is examined. Numerical studies conducted using on-line measurement data indicate that static load models, as compared with induction motor load models, are acceptable for modelling real power behaviours during disturbances and hence are adequate for transient stability analysis. Using multiple sets of on-line measurements, representative model parameters of five static nonlinear load models are derived and their performance in modelling dynamic behaviours of both real and reactive powers is compared. A method is presented to determine a representative set of parameters of static (nonlinear) load models for each loading condition.

Power transformer protection using S-transform with complex window and pattern recognition approach

Abstract: A new approach for power transformer protection using S-transform with complex window to distinguish between inrush current and internal fault is presented. The S-transform with complex window is used to extract patterns of transient current samples during inrush and faults. S-transform is a very powerful tool for non-stationary signal analysis giving the information of transient currents both in time and in frequency domains. The spectral energy is calculated for inrush and internal faults and an energy index is found out to distinguish between inrush magnetising current and internal faults. The simulation results and the results obtained using real-time data from a transformer in the laboratory environment indicate the robustness of the proposed technique.

Dynamic equivalencing of distribution networks with dispersed generation using Hankel norm approximation

Abstract: Nowadays the number of dispersed generators (DGs) is growing rapidly. This change will greatly influence power system dynamics. A distribution network, where DGs are connected to the grid, cannot be considered as passive anymore. Therefore, in the future it will not be possible to use simple equivalents of distribution networks for power system dynamic modelling, as it was done before. At the same time, the whole power system cannot generally be represented in a detailed manner for dynamic studies because of huge system dimension. Therefore special techniques have to be applied for aggregation and order-reduction of distribution networks with DG. A brief review of existing techniques is provided here, and the dynamic reduction using Hankel norm approximation is performed for a 10 kV distribution network, which includes DG of different types.

Composite load models based on field measurements and their applications in dynamic analysis

Abstract: The composite load model with an induction motor in parallel with a static load has been studied and applied in analysis of dynamics of power systems for a long time. However, the load parameters from field tests are still very limited. Based on the theoretical results of identifiability and estimation methodology of load parameters that have been achieved in the previous work, a project of load modelling based on field measurements is described. Eight sets of equipment for load modelling were installed in HeNan Electric Power Grid (HNEPG) Corporation, and the dynamics of the load were recorded and a dynamic index is proposed to evaluate the dynamic characteristics of the data measured. The load models derived from the field measurements are applied to stability analysis of HNEPG. The critical clearing time and power transfer capability analyses illustrate the enhancement of the stability analysis of the power grid using the load parameters derived from the field measurements and show the benefits of application of such model parameters in power system dynamic analysis.

Effect of TCPS on oscillations in tie-power and area frequencies in an interconnected hydrothermal power system

Abstract: The analysis of automatic generation control of a two-area interconnected hydrothermal power system considering a thyristor-controlled phase shifter (TCPS) in series with the tie-line is presented. It is possible to damp the system frequency and tie-power oscillations by controlling the phase angle of TCPS. A control strategy using TCPS is proposed to provide active control of system frequency. Gain settings of the integral controllers with and without considering TCPS are optimised using integral squared error technique following a step load disturbance in each of the areas by minimising a quadratic performance index. Analysis reveals that a TCPS is quite capable of suppressing the frequency and tie-power oscillations effectively under the occurrence of sudden load changes in any of the areas when compared with that obtained without TCPS.

Control strategy for dynamic voltage restorers to achieve minimum power injection without introducing sudden phase shift

Abstract: A power injection control strategy is proposed for dynamic voltage restorers (DVR) under voltage sag or swell conditions. Under such a strategy, the error between the voltage of the energy storage capacitor in the DVR and a given reference voltage is extracted to control the injected power such that during an upstream voltage disturbance, zero or minimum power injection can be achieved automatically, while sudden phase jump of the load-side voltage can also be avoided. The proposed control strategy is analysed in detail and numerical examples are provided to demonstrate the effectiveness of the scheme.

Reliability assessment of an automated distribution system

Abstract: Automation can greatly enhance distribution-network reliability by speeding up service restoration and thus significantly reduce customer-outage time. The paper presents an approach to assess quantitatively the adequacy of a particular automated distribution scheme designated as the `low interruption system' (LIS). Owing to the use of a high-speed communication system and line sensors, this automated scheme can reduce drastically the number of interruptions, the service interruption time and also the area affected by the fault. This scheme provides a simple and cost-effective way to automate distribution systems in which the remotely controlled switches speed up isolation of faulted sections and the restoration of healthy sections through alternative routes. The step-by-step calculation procedure is presented using a typical small automated distribution system. The proposed technique is then applied to a larger distribution system to examine the effectiveness of the technique and also to examine the level of reliability improvement achieved by automation

Time-frequency transform approach for protection of parallel transmission lines

Abstract: A new approach for protection of parallel transmission lines is presented using a time-frequency transform known as the S-transform that generates the S-matrix during fault conditions. The S-transform is an extension of the wavelet transform and provides excellent time localisation of voltage and current signals during fault conditions. The change in energy is calculated from the S-matrix of the current signal using signal samples for a period of one cycle. The change in energy in any of the phases of the two lines can be used to identify the faulty phase based on some threshold value. Once the faulty phase is identified the differences in magnitude and phase are utilised to identify the faulty line. For similar types of simultaneous faults on both the lines and external faults beyond the protected zone, where phasor comparison does not work, the impedance to the fault point is calculated from the estimated phasors. The computed phasors are then used to trip the circuit breakers in both lines. The proposed method for transmission-line protection includes all 11 types of shunt faults on one line and also simultaneous faults on both lines. The robustness of the proposed algorithm is tested by adding significant noise to the simulated voltage and current waveforms of a parallel transmission line. A laboratory power network simulator is used for testing the efficacy of the algorithm in a more realistic manner.