On the Impact of Discrete Secondary Controllers on Power System Dynamics
Abstract : This paper discusses the impact of discrete secondary controllers on the dynamic response of power systems. The idea of the paper originates from the observation that there is a range of values, from few tens of seconds to few minutes, of the execution cycles of conventional automatic generation control (AGC) that leads to a limit cycle. Below and above this range the system is stable. This is certainly not a problem in practice as the AGC updates the power set points of generating units every few seconds. However, this phenomenon has interesting consequences if one considers real-time electricity markets with short dispatch periods (i.e., 5 minutes) as these markets can be modeled as a sort of AGC. The paper first provides a formal analogy between conventional AGC and real-time electricity markets. Then it shows that the discretization-driven instability exists if the system includes a real-time electricity market modeled as secondary frequency controller. Finally, the paper discusses the impact of the combined effect of high wind generation shares and discrete secondary controllers on power system dynamics.
? Phasor Measurement Unit is based upon the synchrophasor measurement from the existing power systems. Synchrophasors are the expression of mathematical method by the phasor values of the sinusoidal waveforms in power system. The measurement derived from the synchrophasor is called synchronized measurement.
? Small signal stability problems exist in power system for many decades. The system oscillates with undamped or growing phenomenon because of lacking of damping or synchronism.
? Electromechanical oscillation in power system is studied in the area of rotor angle stability problem, the oscillation exists because of the behavior that the synchronous generators' power outputs are related to the variation of rotor angles.
? This is certainly not a problem in practice as the AGC updates the power set points of generating units every few seconds.
? Moreover, it is shown that the only effective solution to remove this issue is to keep as short as possible the AGC execution cycles.
? This is not a major constraint as, in practice, the AGC installed in the control centers of TSOs uses execution cycles that vary in the range of 2 to 6 s, which do not create instability issues.
? Moreover, and consider time periods up to a maximum of 14 seconds, which, as shown in this paper, are hardly an issue for secondary controllers.
• The generalization is motivated by the multitude of various representations of saturation that have appeared in the literature.
• Virtually all methods proposed to date involve the addition of one or more nonlinear terms to the model of (3.148)–(3.159).
• The notations and conventions used in this text are as standard as possible given the proliferation of models. It basically follows that of .
• Second, it is important to repeat that standard symbols have been defined through the model proposed in this text with a few noted exceptions.
? The scenarios with non-synchronous devices, i.e., the scenarios with inclusion of wind generation, worsen the performance of both AGC/MAGC controllers, and consequently the dynamic performance of the system.
? Regarding the differences with respect to the participation of WPPs in real-time electricity markets, the case study shows that this participation does not necessarily mean an improvement in the dynamic performance of the system.
? That integrating more wind power generation into power systems worsen the performance of both AGC controllers, and consequently the power system dynamic performance. Simulation results show that power systems that are based on real-time electricity markets should use shorter dispatch periods compared to the case without wind power
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