Decision tree-based fault zone identification and fault classification in flexible AC transmissions-based transmission line

Transmission line distance relaying for flexible AC transmission lines (FACTS) including thyristor controlled series compensator (TCSC), STATCOM, SVC and unified power flow controller (UPFC) has been a very challenging task. A new approach for fault zone identification and fault classification for TCSC and UPFC line using decision tree (DT) is presented. One cycle post fault current and voltage samples from the fault inception are used as input vectors against target output dasia1psila for fault after TCSC/UPFC and dasia0psila for fault before TCSC/UPFC for fault zone identification. Similarly, the DT-based classification algorithm takes one cycle data from fault inception of three phase currents along with zero-sequence current and voltage, and constructs the optimal DT for classifying all ten types of shunt faults in the transmission line fault process. The algorithm is tested on simulated fault data with wide variations in operating parameters of the power system network including noisy environment. The results indicate that the proposed method can reliably identify the fault zone and classify faults in the FACTs-based transmission line in large power network.     

Self-adaptiveness in particle swarm optimisation to enhance available transfer capability using thyristor-controlled series compensation (TCSC)

Available transfer capability (ATC) is one of the challenging criteria under the functioning of the deregulated power system. The high demand for improving ATC is generally met using flexible alternating current transmission system (FACTS) devices in the power system. However, it suffers from serious crisis during determination of the optimal location and compensation stage of FACTS. The present study uses thyristor-controlled series compensation (TCSC) devices in order to compensate for the limitation of FACTS. Further, a novel self-adapted particle swarm optimisation (SAPSO) algorithm is proposed in this study for enhancing ATC. Experiments are carried on three benchmark bus systems such as IEEE 24, IEEE 30 and IEEE 57. Performance and statistical analyses are carried out by comparing the proposed SAPSO with the conventional PSO. Eventually, the study proves the effectiveness of the proposed method in case of ATC enhancement.     

Structure-preserved power-frequency slow dynamics simulation of interconnected ac/dc power systems with AGC consideration

A structure-preserved power-frequency slow dynamics simulation model is suggested for interconnected ac/dc power systems with automatic generation control (AGC) consideration, which will be applied to study relevant emergency control in future so that the bulk system viability crisis caused by load-frequency slow dynamics can be released. In the model, the network structure of interconnected power systems is entirely preserved, and the multi-area dynamic load flow (DLF) is developed for simulation. The generator speed governor and rotor dynamics, load-frequency characteristics, simplified models for high voltage direct current (HVDC) transmission and flexible ac transmission systems (FACTS) device thyristor controlled series capacitor (TCSC) suitable for long-term dynamics are considered with their AGC interfaces kept for future emergency-AGC study. However, at this stage, the sub-problem of reactive power and voltage is neglected for modelling simplicity and dc load flow is thus used for network solution. The concept of area centre of inertia (ACOI) is used based on the assumption of uniform frequency in each control area similar to that of the conventional single-area DLF calculation. The application of ACOI concept is attractive because the signal can be obtained from wide-area measurement systems (WAMSs) in real time and used to enhance long-term frequency stability through advanced control in future. The computer test results from 2-area 4-machine and IEEE 30-bus power systems demonstrated the validity and effectiveness of the suggested model and corresponding algorithm.     

Robust low-order controller design for multi-modal power oscillation damping using flexible AC transmission systems devices

Damping of multi-modal oscillation through supplementary control of a single flexible AC transmission system (FACTS) device is illustrated here. This often requires multiple feedback signals in a centralised multi-input single-output framework for which extension of the classical control design approaches is not straight forward. Past contributions have either focused on decentralised design of low-order PSS in an SISO or MIMO framework; or alternatively, on robust control design techniques which of course, result in higher order controllers. An attempt to design a fixed (low)-order controller, which is robust and is able to damp multiple swing modes with a single FACTS device is presented. The control design problem is formulated as a multi-objective parameter optimisation and solved using a standard evolutionary optimisation technique. Possible post-contingency operating conditions are considered explicitly during the design phase itself to reduce the conservativeness. The present exercise is a step forward towards use of wide area measurement systems for closed-loop supplementary control (around the primary voltage and/or power flow control loop) of the FACTS devices to improve the transfer capacity of the existing corridors.     

Differential equation-based fault locator for unified power flow controller-based transmission line using synchronised phasor measurements

The fault location algorithm based on a differential equation-based approach for a transmission line employing a unified power flow controller (UPFC) using synchronised phasor measurements is presented. First, a detailed model of the UPFC and its control is proposed and then, it is integrated into the transmission system for accurately simulating fault transients. The method includes the identification of fault section for a transmission line with a UPFC using a wavelet-fuzzy discriminator. Features are extracted using a wavelet transform and the normalised features are fed to the fuzzy logic systems for the identification of fault section. After the identification of the fault section, the control shifts to the differential equation-based fault locator that estimates the fault location in terms of the line inductance up to the fault point from the relaying end. Shunt faults are simulated with wide variations in operating conditions and a pre-fault parameter setting. The instantaneous fault current and voltage samples at the sending and receiving ends are fed to the designed algorithm sample by sample, which results in the fault location in terms of the line inductance. The proposed method is tested for different fault situations with wide variations in operating conditions in the presence of a UPFC.     

Extended reliability model of a unified power flow controller

One of the most important aspects in evaluating system reliability is to have a complete understanding of the engineering implications of the system and its associated components. The next step involved in reliability assessment is to appropriately model the system components to represent their characteristics and functions in the overall system. The unified power flow controller (UPFC) is an important flexible AC transmission system controller that can be used to enhance power system reliability. A complete reliability model of a UPFC is developed. A UPFC consists of three subsystems, and a reliability model associated with each subsystem is developed. The individual equivalent models are then combined to form a complete state-space model that represents the UPFC. A sensitivity analysis is conducted to illustrate the impacts on UPFC availability of variations in the key component failure rates.     

Efficient and robust model predictive control for first swing transient stability of power systems using flexible AC transmission systems devices

A wide area measurement, nonlinear, model predictive control (MPC) approach is introduced to provide first swing stability protection of vulnerable power system transmission lines. The authors consider large disturbance events and focus on the avoidance of first swing angular separation using flexible AC transmission systems (FACTS) devices. The presented control strategy for FACTS devices is designed to be numerically efficient (due to the short MPC horizons) and robust (against complicated separation mechanisms). Simulation studies conducted on a three-machine system illustrate that the proposed MPC approach achieves first swing large disturbance performance that is near optimal and superior to existing transient stability controllers. Moreover, the proposed control approach is demonstrated to improve critical clearance times and to improve transfer capacity in simulation studies on the 39 bus New England system.     

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

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     

Neural networks for adaptive control coordination of PSSs and FACTS devices in multimachine power system

The paper develops a new design procedure for online control coordination which leads to adaptive power system stabilisers (PSSs) and/or supplementary damping controllers of flexible ac transmission system (FACTS) devices for enhancing the stability of the electromechanical modes in a multimachine power system. The controller parameters are adaptive to the changes in system operating condition and/or configuration. Central to the design is the use of a neural network synthesised to give in its output layer the optimal controller parameters adaptive to system operating condition and configuration. A novel feature of the neural-adaptive controller is that of representing the system configuration by a reduced nodal impedance matrix which is input to the neural network. Only power network nodes with direct connections to generators and FACTS devices are retained in the reduced nodal impedance matrix. The system operating condition is represented in terms of the measured generator power loadings, which are also input to the neural network. For a representative power system, the neural network is trained and tested for a wide range of credible operating conditions and contingencies. Both eigenvalue calculations and time-domain simulations are used in the testing and verification of the dynamic performance of the neural-adaptive controller.     

Self-tuning flexible ac transmission system controllers for power oscillation damping: a case study in real time

Design and real-time implementation of a self-tuning flexible ac transmission system (FACTS) controller is illustrated for power oscillation damping. Although the system model is not required for self-tuning control design, it is shown to perform similar to a model-based design. For parameter estimation, the classical recursive least square (RLS) is supplemented by a random walk (RW) with a switched structure and compared to standard variable forgetting factor (VFF) approach. It is shown that the RW improves the accuracy and convergence of the estimated system parameters, which is critical to self-tuning control following large disturbances. The performance is validated in real time using a commercial real-time simulation platform. The control computation time is shown to be considerably less than the typical sampling time used for power oscillation damping applications demonstrating the feasibility of self-tuning FACTS controllers in practice.     

Wavelet packet-based digital relaying for advanced series compensated line

A new approach for the protection of thyristor-controlled series-compensated (TCSC) line using wavelet packets transform (WPT) is presented. The proposed method uses one cycle post-fault-current samples just after fault inception, which is processed through WPT and decomposed into various decomposition levels. The decomposed components are grouped together to provide different frequency sub-bands. Then the phase selection signal (PSS) and section identification signal (SIS) are computed to identify the faulty phase and faulty section, respectively, involved in the fault process in transmission line including TCSC. A threshold value (THD) is selected for PSS, and PSS above THD describes the faulty phase involved, otherwise not. Similarly, another THD is selected for SIS, and SIS below THD describes fault that includes TCSC, otherwise fault that does not include TCSC. As PSS takes half cycle after fault inception to identify the faulty phase and then triggers SIS, the faulty phases and faulty sections are identified within one cycle of fault inception. The proposed WPT algorithm is also tested on physical transmission line model with TCSC, under wide variations in operating conditions and provides accurate results. Thus, the proposed method provides accurate and fast protection measures for TCSC-based line.     

N -21 security in optimal power flow control applied to limited areas

Blackouts in recent years have demonstrated that a reliable and secure power system is a key component of an efficient economy. Therefore control devices such as flexible AC transmission system devices (FACTS) are placed in the system and utilised to improve the security of the system. A method to determine appropriate settings for these devices is optimal power flow control. As the area of influence of a FACTS device is usually limited, it is sufficient to include only a reduced area in the optimisation problem. Here, such an optimal power flow problem is formulated where the considered area is defined using sensitivity analysis. To include N 2 1 security as an objective, a current injection method is applied, which facilitates the determination of the system state in the case of a line outage, without having to carry out a full-load flow simulation.     

Deterministic oscillatory search: a new meta-heuristic optimization algorithm

The paper proposes a new optimization algorithm that is extremely robust in solving mathematical and engineering problems. The algorithm combines the deterministic nature of classical methods of optimization and global converging characteristics of meta-heuristic algorithms. Common traits of nature-inspired algorithms like randomness and tuning parameters (other than population size) are eliminated. The proposed algorithm is tested with mathematical benchmark functions and compared to other popular optimization algorithms. The results show that the proposed algorithm is superior in terms of robustness and problem solving capabilities to other algorithms. The paradigm is also applied to an engineering problem to prove its practicality. It is applied to find the optimal location of multi-type FACTS devices in a power system and tested in the IEEE 39 bus system and UPSEB 75 bus system. Results show better performance over other standard algorithms in terms of voltage stability, real power loss and sizing and cost of FACTS devices.     

Recent advances in var compensators

Static var compensators have been, for many years, an essential component in the operation of power transmission systems. They are part of a family of devices known as Flexible AC Transmission System (FACTS) devices. The advent of large capacity force-commutated semiconductor switches allows many developments in power electronic converters to be applied to the implementation of high power compensators. This paper describes the principles of controlled reactive power compensation, particularly in the context of power systems. It focuses on active static power converter-based compensators and discusses issues related to the power circuit topology and control techniques, including the impact of Pulse Width Modulation (PWM) techniques. Compensators based on current and voltage source converters and onac controllers, both in the shunt and series configurations, are covered. Methods to enhance power capacity using multi-level and multi-pulse arrangements are discussed.     

Dynamic phasor based frequency scanning for grid-connected power electronic systems

Frequency scanning is a method of obtaining the frequency response of a system by injecting a small-amplitude wide-band signal as an input in a time domain simulation of the system. This is an alternative to analytical derivation of small-signal models, especially for complex grid-connected power electronic systems (PESs). These models are required for the study of adverse interaction of PES with lightly damped oscillatory modes in a power system. The use of the frequency scans for conventional small-signal stability analysis is predicated upon the time-invariance of the underlying model. Since PES are generally time-periodic, time-invariance may be achieved in some transformed variables. Although the DQ transformation is suitable in many situations, it is not so for systems with low-order harmonics, individual-phase schemes, unbalanced or single-phase systems, and PES with negative-sequence controllers. This paper proposes the use of dynamic phasor variables in such situations since the underlying model in these variables is time-invariant. The procedure for dynamic phasor based scanning is, however, intricate because wide-band signal injection results in the simultaneous presence of harmonic dynamic phasor components. The paper outlines this procedure and presents illustrative case studies of Thyristor Controlled Series Compensator (TCSC) and STATCOM. For the TCSC, a comparison of the frequency response obtained from the scanning method and the one obtained from an approximate analytical dynamic phasor model is also presented.     

Computation of the controllable swing mode spectrum of FACTS and HVDC in large power systems

This paper presents computation of swing modes of a large power system that could be significantly affected by power swing damping controllers in FACTS or HVDC devices at a given location. Modal controllability is a suitable measure to isolate these modes for analysis. Computation of the controllable swing mode spectrum is useful, especially in situations where the controller structure and feedback signals are not frozen (e.g., at the planning stage). This paper proposes two important steps that allow us to map the problem of finding highly controllable swing modes to the problem of finding the swing modes that have high transfer function residues (for which efficient algorithms are available). The steps are: (a) normalization of the eigenvectors corresponding to different modes and (b) identification of specific feedback signals for each type of FACTS/HVDC device such that the modal observability and modal controllability are tightly coupled. Once the mapping is done, a computationally efficient method like the Subspace Accelerated Dominant Pole Algorithm [16] (SADPA) can be adapted to find the highly controllable swing modes. The effectiveness of this approach is demonstrated by case studies of FACTS and HVDC devices in a 16-machine system and the Indian power grid.     

Feasibility of online collaborative voltage stability control of power systems

The authors present the results of feasibility study of a novel concept of power system online collaborative voltage stability control. Online collaboration between power system controllers is proposed in order to enhance their overall performance and efficiency to cope with the increasing operational uncertainty of modern power systems. The framework of the proposed online collaborative voltage stability control is first presented, which is based on the deployment of multi-agent systems and real-time communication for online collaborative control. Then, two of the most important issues in implementing the proposed online collaborative voltage stability control are addressed: (1) Error-tolerant communication protocol for fast information exchange among multiple intelligent agents; (2) Deployment of multi-agent systems by using graph theory to implement power system post-emergency control. Results of testing the proposed online collaborative voltage stability control in the case of the 10-machine 39-node New England power system are presented. Results of a feasibility study by means of a simulation are given that take into consideration low-probability cascading faults in the power system.     

Effect of transformer type on estimation of financial loss due to voltage sag - PSCAD/EMTDC simulation study

The fault in transmission and distribution lines of the power system creates voltage sag. This paper focuses on estimating the financial loss due to such voltage sags. The transformer in a system has great impact on the estimated number of voltage sags at the sensitive load due to faults in the system. So the financial loss in a system due to sag will be affected by the type of transformer in that system. The financial losses for different types of transformers are calculated in probabilistic manner for different types of load groups. Effect of the transformer in the system, on financial loss due to voltage sag is presented in this paper.     

Representation of line optimisation control in unified power-flow controller model for power-flow analysis

Drawing on constrained optimisation based on Newton's method, a systematic and general method for determining optimal reference inputs to unified power-flow controllers (UPFCs) in steady-state operation is developed. The method is directly applicable to UPFCs operation with a high-level line optimisation control. Through the selection of weighting coefficients used in the objective function which is formed from the weighted difference between the specified reference inputs and their optimal values, the method represents the priority assigned for any UPFC control function in constraint or limit resolution. Another key advance reported is that of combining the sparse Newton's method with the continuation technique for solving the nonlinear constrained optimisation. The composite algorithm extends substantially the region of convergence achieved with the conventional Newton's method. The method uses a general and flexible UPFC model based on nodal voltages developed. Any UPFC control functions together with operating limits can be included in the model. The steady-state formulation developed together with its software implementation is tested with a practical long-distance transmission interconnection where a UPFC is required.