Hjörtur Jóhannsson

Real-Time Remedial Action Against Aperiodic Small Signal Rotor Angle Instability

This paper presents a method that in real-time determines remedial actions, which restore stable operation with respect to aperiodic small signal rotor angle stability (ASSRAS) when insecure or unstable operation has been detected. An ASSRAS assessment method is used to monitor the stability boundary for each generator in real-time. The ASSRAS boundary represents the condition when a generator reaches the maximum steady state active power injection. The proposed control method exploits analytically derived expressions for the ASSRAS boundary and other characteristic curves in the injection impedance plane to determine an active power redispatch among selected generators to restore stable and secure operation. Since the method is purely based on analytically derived expression, the computation of the remedial actions is fast and well suited for real-time operation. The method was tested on the IEEE 14-bus and the Nordic32 test systems where results show that the method can efficiently determine the required active power redispatch to avoid an imminent instability.

Stabiliser Fault Emergency Control using Reconfiguration to Preserve Power System Stability

Abstract Stabiliser faults in multi-machine power systems are examined in this paper where fault-masking and system reconfiguration of the nonlinear system are obtained using a virtual actuator approach. Phasor Measurement Units, which can be integrated in wide-area transmission grids to improve the performance of power system stabilisers, are utilised when reconfiguring remaining stabilisers after a local failure has made one inoperable. A stability-preserving reconfiguration is designed using absolute stability results for Lure type systems. The calculation of a virtual actuator that relies on the solution of a linear matrix inequality (LMI) is detailed in the paper. Simulation results of a benchmark transmission system show the ability of the fault-tolerant reconfiguration strategy to maintain wide-area stability of a power system despite failure in one of the stabilisers.

Considering wind speed variability in real-time voltage stability assessment using Thévenin equivalent methods

In this paper, wind speed models are used to consider its variability in real-time voltage stability assessment using Thévenin equivalent methods. A stochastic differential equation was used to produce a set of wind speeds required for the simulation scenarios together with a very short-term forecast based on a probabilistic method, which provides the means for including anticipation in the stability assessment. This is achieved by representing the variation in the wind as an uncertainty in the Thévenin equivalent parameters, which are used for wide-area assessment, and studying corresponding changes in the stability boundary for a specific time horizon. The methodology was tested with time domain simulation a cigré benchmark system for network integrations of renewables, where the approach successfully represented the final Thévenin Equivalent parameters with a maximum error of 2.5% of the estimated variables, for a time horizon of 1 minute in the used forecast.

Super-Positioning of Voltage Sources for Fast Assessment of Wide-Area Thévenin Equivalents

A method for superimposing voltage sources is sought optimized by using a sparse triangular solver and multiprocessing. A revision to the method is suggested which exploits Schur’s complement of the network admittance matrix and optimal re-use of computations. The algorithm is implemented and parallelized for shared memory multiprocessing. The proposed algorithm is tested on a collection of large test systems and performance is found to be significantly better than the reference method. The algorithm will thereby facilitate a speed-up of methods relying on Thevenin equivalent representation such as the Thevenin equivalent method for contingency assessment.

An Improved On-line Contingency Screening for Power System Transient Stability Assessment

AbstractThis paper presents a contingency screening method and a framework for its on-line implementation. The proposed method carries out contingency screening and on-line stability assessment with respect to first-swing transient stability. For that purpose, it utilizes the single machine equivalent method and aims at improving the prior developed contingency screening approaches. In order to determine vulnerability of the system with respect to a particular contingency, only one time-domain simulation needs to be performed. An early stop criteria is proposed so that in a majority of the cases the simulation can be terminated after a few hundred milliseconds of simulated system response. The method’s outcome is an assessment of the system’s stability and a classification of each considered contingency. The contingencies are categorized by exploiting parameters of an equivalent one machine infinite bus system. A novel island detection approach, appropriate for an on-line application since it utilizes eff...

Real-time countermeasures preventing power system instability by using PMU data from RTDS simulation

This paper presents an innovative approach to apply wide-area control actions in real-time and prevent emerging instability. A software platform has been further developed, which utilizes Real Time Digital Simulator (RTDS) technology to prevent scenarios leading to power system instability. The software platform receives phasor measurement unit (PMU) data at a high repetition rate for full system observability. The PMU data serves as input for methods capable of analyzing the steady state torque balance for each individual generator and to determine available power reserves and possible remedial actions. The capabilities of the software platform were demonstrated by testing the methods on the Nordic32 test system and the results show that the methods can determine an active power re-dispatch and apply the countermeasures in real-time and prevent aperiodic rotor angle instability.

Unweighted betweenness centrality for critical fault detection for cascading outage assessment

This paper analyses the possible use of unweighted betweenness centrality instead of weighted betweenness centrality, for critical fault detection for assessment of cascading failures. As unweighted betweenness centrality is significantly faster to compute, the possible use of this will significantly improve the computation speed. The method is tested on four IEEE test systems, and the study finds that the unweighted analysis is not a good substitute if the method is only used naively, but it is possible to make graph modifications, and improve the unweighted analysis.

Improved Thévenin equivalent methods for real-time voltage stability assessment

An improved Thévenin equivalent method for real-time voltage stability assessment that uses wide-area information from synchrophasors is proposed. The improvements are a better modeling of the limited synchronous generators, and a processing that anticipates the effect of field current limiters, before the latter are activated. Several study cases using detailed dynamic simulations of the Nordic test system have been used to assess the performance of the proposed improvements. Their effectiveness is analyzed and, based on the results, their possible application in combination with the sensitivity-based voltage stability assessment method is explored.

Energy Optimization for Distributed Energy Resources Scheduling with Enhancements in Voltage Stability Margin

The need for developing new methodologies in order to improve power system stability has increased due to the recent growth of distributed energy resources. In this paper, the inclusion of a voltage stability index in distributed energy resources scheduling is proposed. Two techniques were used to evaluate the resulting multiobjective optimization problem: the sum-weighted Pareto front and an adapted goal programming methodology. With this new methodology, the system operators can consider both the costs and voltage stability. Priority can be assigned to one objective function according to the operating scenario. Additionally, it is possible to evaluate the impact of the distributed generation and the electric vehicles in the management of voltage stability in the future electric networks. One detailed case study considering a distribution network with high penetration of distributed energy resources is presented to analyse the proposed methodology. Additionally, the methodology is tested in a real distribution network.

On-line generation and arming of system protection schemes

This paper presents a new method to automatically generate system protection schemes in real-time, where contingencies are filtered using a method providing N - 1 system snapshots. With future power systems consisting largely of renewable distributed generation with time-varying production, highly fluctuating conditions throughout the day will be the result. This makes off-line design of extensive defense plans for power systems infeasible, forming the motivation for the presented method. It relies on the real-time identification of which disturbances that threatens a power systems integrity. The method is based on a recently proposed method of calculating post-contingency Thevenin equivalents, which are used to assess the security of the post-contingency condition. The contingencies that violate the emergency limits are contained by pre-determining event-based remedial actions. The instability mechanisms threatening the system are individually treated, such that appropriate controls are allocated. The procedure is illustrated through a case study using the Nordic32 benchmark system.