Structure Preserving Energy Functions Research Articles

Structure Preserving Energy Functions in Power Systems: Theory and Applications [Book Reviews

The author of this book has captured all the above aspects in a systematic way. What is interesting is that through this work it has opened the door for more research using structure preserving energy functions (SPEF) inclusion of renewable energy sources as well as a possible application in the WAMS area by using the PMU data to develop useful metrics for stability of the system. The book is divided into nine chapters. It is a thorough discussion of energy-based methods, starting from the equal area criterion to present-day complex structure preserving models with all types of devices included. At the end of each chapter there is good discussion of further research topics in the area. The bibliography is very comprehensive. The book on the whole provides a good platform for further research in using SPEF for stability and control in real time.

Energy function for an interline power-flow controller

An IPFC may be applied for steady-state power-flow and voltage control as well as for mastering dynamic phenomena like transient-stability margin enhancement, oscillation damping, etc. For these tasks the Lyapunov energy-function approach is frequently used as a convenient way to control or analyze the electric-power system (EPS). The basis for the implementation of such an approach is to know the energy function of the EPS. Currently, this is not possible for the EPSs that include IPFCs, because the already-known energy functions that proved to be suitable for an EPS do not include such a device. Therefore, in this paper, energy functions that consider the IPFC's action in the form of a supplement to the already-known structure-preserving energy functions were constructed. They are based on a structure-preserving frame and can be applied for an arbitrary number of IPFCs, which may consist of an arbitrary number of series branches. The developed energy functions were applied for a transient-stability assessment using the Lyapunov direct method, and they proved to be adequate.

The Control Strategy for an IPFC Based on the Energy Function

In order to be able to successfully apply interline power flow controllers (IPFCs) for power-system transient-stability improvement and for the damping of oscillations, proper control strategies should be applied to these devices. The basis for the implementation of such a strategy that is proposed in this paper is to know the energy function of a power system that includes IPFCs. Already-known energy functions that proved to be suitable for an electric-power system do not include such a device. Therefore, in this paper an energy function that considers the IPFC's action in the form of a supplement to the already-known structure-preserving energy functions was constructed. Furthermore, the strategy for implementing the newly developed IPFC energy functions in order to ensure a globally optimum control strategy in the Lyapunov sense is presented. The results of the numerical examples show that the magnitudes of the IPFC's series-injected voltages remain set to their maximum values and that only the angles of these voltages change, similar to the ldquobang-bangrdquo control strategies proposed for some other FACTS devices.

Phase-shifting transformers in a structure-preserving energy function

With some exceptions, direct methods have so far not been properly able to assess the transient stability of general power systems with phase-shifting transformers (PSTs) because there were no energy functions that would include their action in a post-fault situation. In this paper, we propose two additional potential-energy function parts for the two most common PSTs that can be added to any existing structure-preserving energy functions (SPEF). In this way, modularity is achieved and other FACTS devices can easily be taken into consideration. Using the proposed energy functions, it is possible to apply direct methods to any system that consists of an arbitrary number of phase-angle regulators (PAR) and quadrature-boosting transformers (QBT). These new energy functions assume a constant controllable parameter; however, we extended the use of the presented energy functions by a simple procedure that makes the direct method able to consider any parameter change in a post-fault system. The proposed energy-function extensions were tested in a longitudinal system, because only in such a system will direct methods and a simulation give the same results. Another demonstration was made with an IEEE nine-bus test system using a potential-energy boundary-surface (PEBS) direct method.

Direct stability evaluation of power systems with detailed generator models using structure-preserving energy functions

Energy-based direct methods for transient stability analysis are potentially useful both as offline tools for planning purposes as well as for online security assessment. In this paper, a novel structure-preserving energy function (SPEF) is developed using the philosophy of structure-preserving model for the system and detailed generator model including flux decay, transient saliency, automatic voltage regulator (AVR), exciter and damper winding. A simpler and yet general expression for the SPEF is also derived which can simplify the computation of the energy function. The system equations and the energy function are derived using the centre-of-inertia (COI) formulation and the system loads are modelled as arbitrary functions of the respective bus voltages. Application of the proposed SPEF to transient stability evaluation of power systems is illustrated with numerical examples.

Dynamic security assessment of power systems using structure-preserving energy functions

An application of direct methods to dynamic security assessment of power systems using structure-preserving energy functions (SPEF) is presented. The transient energy margin (TEM) is used as an index for checking the stability of the system as well as ranking the contigencies based on their severity. The computation of the TEM requires the evaluation of the critical energy and the energy at fault clearing. Usually this is done by simulating the faulted trajectory, which is time-consuming. In this paper, a new algorithm which eliminates the faulted trajectory estimation is presented to calculate the TEM. The system equations and the SPEF are developed using the centre-of-inertia (COI) formulation and the loads are modelled as arbitrary functions of the respective bus voltages. The critical energy is evaluated using the potential energy boundary surface (PEBS) method. The method is illustrated by considering two realistic power system examples.

A New Structure Preserving Energy Function Incorporating Transmission Line Resistances

A novel method to account for the transmission line resistances in structure preserving energy functions (SPEF) is presented in this paper. The method exploits the equivalence of a lossy network having the same conductance to susceptance ratio for all its elements to a lossless network with a new set of power injections. The system equations and the energy function are developed using centre of inertia (COI) variables and the loads are modelled as arbitrary functions of respective bus voltages. The application of SPEF to direct transient stability evaluation is presented considering a realistic power system example.