Impact of electromechanical wave oscillations propagation on protection schemes

Major disturbances in power system can take place after power system elements such as generators, loads, or transmission lines are suddenly disconnected. Such disturbances can create so called “electromechanical wave oscillations” waves which propagate through transmission lines at much lower speed than speed of light. They can cause adverse effect on power system protective relays. In this paper, electromechanical wave oscillation propagation is modeled, and its impact on different power system protective relays, such as overcurrent, distance, and out-of-step relays is studied. Modified protection schemes are istance relay lectromechanical wave oscillation ut-of-step vercurrent relay ower swing presented for each protective device to avoid their malfunction under effects of electromechanical wave oscillations. The electromechanical model adopted in this study considers the dynamics of generator mechanical shaft as well as conversion of mechanical power to electrical. Simulations used for testing of the improved protection solutions are carried out in MATLAB considering 64-bus generator ring system and IEEE 118-bus system. rotective device . Introduction In the last two decades, Wide Area Measurement System WAMS) made synchronized measurements available from varius points across the power system. By analyzing such data, one an noticed the disturbances which propagate through the entire etwork at the speed much lower than the speed of light. They are alled the “electromechanical wave oscillation” propagations. Transmission line faults, load shedding or generator rejection an result in mismatch between the mechanical and electrical ower at the terminal of the generators [1]. As a consequence, enerator rotors start to move with respect to their synchronous eference frame. Due to the rotor inertia, re-synchronization of genrator with the rest of system (if it happens) occurs with certain elay. This re-synchronization delay can be seen as a disturbance n the voltage phase angle, which propagates through power sysem with limited speed. Such oscillations can trigger a series of ascade outages and finally a wide spread blackouts may occur as eported for some historical events [2–4]. For the first time, electromechanical wave oscillations were bserved and reported in July 1993 during a load rejection test n Texas [6]. In recent decades, substantial research was devoted ∗ Corresponding author. Tel.: +1 979 862 1097; fax: +1 979 845 9887. E-mail addresses: [email protected], [email protected] A. Esmaeilian). ttp://dx.doi.org/10.1016/j.epsr.2016.01.002 378-7796/© 2016 Elsevier B.V. All rights reserved. © 2016 Elsevier B.V. All rights reserved. to modeling the electromechanical disturbance propagation and understanding the dynamic behavior of power system [5–9]. Continuum approach is the most recognized method to model the propagation of electromechanical wave oscillations in power system. The continuum model is based on partial differential equation which offers a travelling wave description of power system dynamics and power system wide-area disturbances [6]. In [7], a continuum power system model is proposed to analyze the propagation of electromechanical disturbances in large power system with concentrated parameters. In this approach, power system is considered as a homogeneous system where transmission lines are represented by a reactance, and generators by a voltage source behind constant reactance. In [6], a more advanced continuum approach is proposed where the effect of loses is also included. Authors derived a nonlinear partial differential equation of the rotor angle with respect to time and two dimensional coordinates were introduced to model electromechanical disturbances propagation. In [9], the proposed continuum model is modified to take the geographical location of the elements of power system into the account. Gaussian smoothing method to deal with the spatially concentrated parameters of power system to represent the distribution of parameters in continuum model was deployed. Several studies have been done utilizing a non-uniform media [10–13] to characterize wave propagation. In [12], a general method for the solution of the linearized equations for both homogeneous and inhomogeneous media is developed. This method yields solutions which describe propagating waves such as pulses, rapidly 8 Power Systems Research 138 (2016) 85–91 c p t r a O s t g d ( o s c w t v a s