Tuomas Rauhala

Selecting wavelets for damping estimation of ambient-excited electromechanical oscillations

This paper discusses aspects related to a wavelet transform and random decrement technique based method developed for estimating power system electromechanical oscillation damping from ambient-excited oscillations. The main focus of this paper is in finding the optimal wavelet functions for the damping estimation method. General selection criteria for the wavelet functions are defined, the selection criteria are specified quantitatively for the Nordic power system and the wavelet functions fulfilling the criteria are presented. Damping of a simulated Nordic power system case is analyzed with the damping estimation method and with different wavelet functions. The damping estimation results show that when the mother wavelet is selected according to the specified criteria, the damping estimates are better than by using other wavelets.

Applications of phasor measurement units and wide-area measurement system in Finland

Wide-area measurement -project was started by Fingrid, the Finnish transmission system owner and operator (TSO), back in 2006. Nowadays the system deploys 12 phasor measurement units (PMU) and a phasor data concentrator (PDC) with applications. PMU measurements are also streamed from and to wide-area measurement systems in Norway and Denmark. From the very beginning, Fingrids focus of the Wide-area measurement system (WAMS) development has been on power oscillation monitoring, and Fingrid has actively worked for algorithm development. Although the original scope was to use WAMS as a monitoring tool in control center, during the project synchrophasor measurements have been successfully applied for various power system planning and analysis purposes. In addition to the WAMS, this paper presents also number of different applications of synchrophasor measurements in Finland. The practical experiences from different applications are described and the main projected challenges especially related to development of real-time stability monitoring applications suitable for Fingrids purposes are discussed in detail.

Performance of wavelet-based damping estimation method under ambient conditions of the power system

The paper discusses performance evaluation of damping estimation methods and specifically evaluates the performance of a wavelet-based damping estimation method. The focus is on the damping estimation under the ambient conditions of the power system. Damping estimation results are reviewed in case of the simulated data because then real damping is known and can be compared with the estimated damping. Various degrees of complexity of the simulation models are used: linear single and double pole pair models, and a detailed nonlinear system model of the Nordic power system. In the linear models, the real damping of the modes can be analytically calculated and compared with the estimated damping. Damping estimation performance is studied in different operating conditions of the studied simulation models. The main results are: frequency estimates are better than the damping estimates, other oscillation modes may affect the damping estimation, poor damping can be estimated more accurately than high damping, and the performance of the method is nearly similar regardless of the complexity of the simulation model.

Determining Subsynchronous Damping Based on PMU Measurements from Finnish 400 kV Transmission Network

In May 2006 two Phasor Measurement Units were installed in Finnish 400 kV transmission grid in order to study subsynchronous damping of 950 MVA generating unit located geographically close to 550 MW HVDC transmission system. Subsynchronous damping was analyzed based on post- disturbance data measured using PMU. In this paper the background of the study and the main results from the measurements over period between May 2006 and August 2006 are presented. The quality of measured subsynchronous oscillations is illustrated and the validity of approach on subsynchronous damping estimation is discussed based on the measurements. Also, amplitude of measured subsynchronous components related to different system disturbances are illustrated and the values determined for total subsynchronous damping based on the post-disturbance measurements are presented. Finally, the possibilities to use PMU output data for real-time monitoring of subsynchronous damping is discussed based on the obtained experiences.

Recent developments in modal estimation of power system electromechanical oscillations

This paper presents three data-driven methods to monitor electromechanical oscillations in power systems during ambient operation. The characteristics of the methods are compared and the differences in the characteristics are explained. The results indicate that all three methods are suitable for estimating the damping and frequency of electromechanical oscillations. The paper also studies the validity of the assumption that the load variations in power systems are Gaussian distributed. This is a common assumption in damping estimation methods. The assumption is shown to be valid for the studied loads.

On feasibility of SSDC to improve the effect of HVDC on subsynchronous damping on several lower range torsional oscillation modes

Subsynchronous damping controllers (SSDC) has been implemented as a part of the HVDC control systems basically since it was first recognized that HVDC LCC converter has adverse effect on subsynchronous torsional damping of especially the lowest subsynchronous torsional oscillation modes of turbine-generator units. Typically SSDC is designed to improve the effect of HVDC on subsynchronous damping on one or two lower range subsynchronous torsional modes. In this paper feasibility of SSDC to improve the effect of HVDC on subsynchronous damping on five torsional frequencies within torsional frequency range 5–20 Hz is evaluated. The feasibility study presented targets to answer the question if the adverse effect of HVDC can be improved on all the five modes using one common SSDC structure that may become of interest for example at the pre-specification stage of a HVDC project. In the paper study approach is presented, the structure of the study described and factors affecting SSDC feasibility is discussed. Finally, the contribution of presented SSDC structures to subsynchronous damping of multimodal lower range subsynchronous torsional oscillations is illustrated using on results of dynamic frequency scanning studies conducted using EMT tool.

Applying TCSC frequency response data derived using electromagnetic transient analysis in SSR frequency scanning studies

The traditional SSR frequency scanning technique can be generally considered as the most practical and straightforward approach to study the possible instabilities concerning torsional oscillations. However, as the number of controllable and highly nonlinear power system components is continuously increasing, their contribution to the frequency response characteristics of a power system will become more significant. Therefore, the traditional SSR frequency scanning technique can more often be found insufficient for modeling the torsional oscillation related characteristics of a power system with reasonable accuracy. In this paper approach for determination of the frequency dependent response of the Thyristor Controlled Series Capacitor (TCSC) to torsional oscillations is presented utilizing the generic signal injection approach. After analysis of the frequency dependent response characteristics of the TCSC with different study approaches suitability of the determined frequency response data in a SSR frequency scanning study is illustrated and analyzed. Finally, the possible benefits and limitations of different approaches to model the frequency dependent response characteristics of the TCSC for SSR frequency scanning purposes are discussed briefly.

Detection of subsynchronous torsional oscillation frequencies using phasor measurement

Summary form only given. This paper presents a non-invasive and easy to implement technique using phasor measurement units for accurate estimation of subsynchronous torsional frequencies. This information is relevant for the optimal design of HVDC subsynchronous damping controllers that enhance the effect of HVDC on subsynchronous damping. The method is rigorously justified using mathematical proofs as well as thorough EMT simulations. The method was implemented in the Finnish transmission network and proved to be effective.

Detection of Subsynchronous Torsional Oscillation Frequencies Using Phasor Measurement

This paper presents a noninvasive and easy-to-implement technique using phasor measurement units for accurate estimation of subsynchronous torsional frequencies. This information is relevant for the optimal design of high-voltage direct-current (HVDC) subsynchronous damping controllers that enhance the effect of HVDC on subsynchronous damping. The method is rigorously justified using mathematical proofs as well as thorough electromagnetic-transients simulations. The method was implemented in the Finnish transmission network and proved to be effective.