Mode-based Damping Torque Analysis in Power System Low-frequency Oscillations

The mode-based damping torque analysis(M-DTA)method for studying the effect of an external controller on power system low-frequency oscillations is proposed in this paper.First,based on the interconnection model between the system and the controller in the frequency domain,the oscillation loop corresponding to the electromechanical oscillation mode is built,and then the mode-based damping torque of the controller can be calculated.Then,the application of the M-DTA method in the power system is illustrated.The derivation shows that in the single-machine infinite-bus power system,the M-DTA method is completely equivalent to the classical damping torque analysis(C-DTA)method.In the multi-machine power system,the mode-based damping torque directily reflects the effect of the controller on the oscillation mode,overcoming the shortcomings of the C-DTA method in which there is no direct correspondence between the damping torque and the oscillation mode.By deriving the relationship with the residue index,the M-DTA method shows higher accuracy than the residue method in applications,such as controller parameter adjustment.Finally,two example power systems are presented to demonstrate the application of the proposed M-DTA method.Index Terms-Electromechanical oscillation mode,FACTS,interconnection model in the frequency domain,mode-based damping torque analysis(M-DTA),power system low-frequency oscillation,PSS,residue method.

Connection Between Damping Torque Analysis and Energy Flow Analysis in Damping Performance Evaluation for Electromechanical Oscillations in Power Systems

The damping performance evaluation for electromechanical oscillations in power systems is crucial for the stable operation of modern power systems.In this paper,the connection between two commonly-used damping performance evaluation methods,i.e.,the damping torque analysis(DTA)and energy flow analysis(EFA),are systematically examined and revealed for the better understanding of the oscillatory damping mechanism.First,a concept of the aggregated damping torque coefficient is proposed and derived based on DTA of multi-machine power systems,which can characterize the integration effect of the damping contribution from the whole power system.Then,the pre-processing of measurements at the terminal of a local generator is conducted for EFA,and a concept of the frequency-decomposed energy attenuation coefficient is defined to screen the damping contribution with respect to the interested frequency.On this basis,the frequency spectrum analysis of the energy attenuation coefficient is employed to rigorously prove that the results of DTA and EFA are essentially equivalent,which is valid for arbitrary types of synchronous generator models in multi-machine power systems.Additionally,the consistency between the aggregated damping torque coefficient and frequency-decomposed energy attenuation coefficient is further verified by the numerical calculation in case studies.The relationship between the proposed coefficients and the eigenvalue(or damping ratio)is finally revealed,which consolidates the application of the proposed concepts in the damping performance evaluation.

Power and Damping Torque of Electric-Quadrupole Radiation for Rotational Charged Rigid Bodies

Based on the basic theory of electrodynamics about electromagnetic radiation,the general formulae of radiation power and damping torque of electric-quadrupole moment for a steadily and uniformly charged rigid body with any periodic rotation are derived.The concise form of the formulae in some special symmetric cases are deduced and discussed,and the results of several common symmetric charged bodies are listed.

Design of Power System Stabilizer for DFIG-based Wind Energy Integrated Power Systems Under Combined Influence of PLL and Virtual Inertia Controller

Wind energy systems (WESs) based on doubly-fed induction generators (DFIGs) have enormous potential for meeting the future demands related to clean energy. Due to the low inertia and intermittency of power injection, a WES is equipped with a virtual inertial controller (VIC) to support the system during a frequency deviation event. The frequency deviation measured by a phase locked loop (PLL) installed on a point of common coupling (PCC) bus is the input signal to the VIC. However, a VIC with an improper inertial gain could deteriorate the damping of the power system, which may lead to instability. To address this issue, a mathematical formulation for calculating the synchronizing and damping torque coefficients of a WES-integrated single-machine infinite bus (SMIB) system while considering PLL and VIC dynamics is proposed in this paper. In addition, a power system stabilizer (PSS) is designed for wind energy integrated power systems to enhance electromechanical oscillation damping. A small-signal stability assessment is performed using the infinite bus connected to a synchronous generator of higher-order dynamics integrated with a VIC-equipped WES. Finally, the performance and robustness of the proposed PSS is demonstrated through time-domain simulation in SMIB and nine-bus test systems integrated with WES under several case studies.

Location methods of oscillation sources in power systems: a survey

The deployment of a synchrophasor-based widearea measurement system(WAMS) in a power grid largely improves the observability of power system dynamics and the operator’s real-time situational awareness for potential stability issues. The WAMS in many power grids has successfully captured system oscillation events, e.g. poorly damped natural oscillations and forced oscillations, from time to time. To identify the root cause of an observed oscillation event for further mitigation actions, many methods have been proposed to locate the source of oscillation based on different ideas and principles. However, most methods proposed so far for locating the oscillation source in a power grid are not reliable enough for practical applications. This paper presents a comprehensive review of existing location methods, which basically fall into four major categories, plus a few other methods. Their advantages and disadvantages are discussed in detail. Some trends and challenges on the problem of oscillation source location are pointed out along with potential future research directions. Finally, a practical, general scheme for oscillation source location using available location methods is suggested and analyzed.