Abstract
In South Korea, the existing diesel generators are being replaced with renewable energy sources (RESs) in several stand-alone microgrids. However, their reliability and stability are not still guaranteed by fluctuations of the system caused by RESs, arising from unpredictable changes in environmental factors. Moreover, since the inertia that is originally provided by the diesel synchronous generators is reduced, the overall system becomes highly sensitive to frequency variations. As a result, the installation of RESs must be well planned while considering the upper limit of their installed capacity. In this paper, a new method for evaluating the maximum penetration capacity of photovoltaic (PV) generators to ensure the stable operation of a microgrid is proposed. For the analytical approach, two frequency stability indexes, which are the minimum instantaneous frequency and the quasi-steady-state (QSS) frequency after a disturbance, are used. The capacity of PV to be installed is limited by considering the characteristics of other generators. In addition, the efficiency of the energy storage system (ESS) is also analyzed to determine the optimal capacity of both PV and ESS. The effectiveness of the proposed method is firstly proven through the mathematical analysis. Then, case studies on a practical stand-alone microgrid in South Korea are carried out using a time-domain simulation based on the DIgSILENT PowerFactory® software.
Highlights
Most recently, stand-alone microgrids are involved in a great transition with the high penetration of renewable energy sources (RESs) by replacing conventional diesel synchronous generators
The maximum penetration capacity of PV was taken as the smaller value among the values obtained in the operable capacity range of PV from Equation (5) and the magnitude of credible contingency event calculated from Section 4.1.3
Time-domain simulations were carried out to determine whether the microgrid was stable, considering both QSS and instantaneous frequency in Sections 5.2.1 and 5.2.2
Summary
Stand-alone microgrids are involved in a great transition with the high penetration of renewable energy sources (RESs) by replacing conventional diesel synchronous generators. The second the stand-alone microgrid, the system frequency lasts at fQSS for a long duration since the stage is the control primaryacts frequency response (PFR), in which the power imbalance is eliminated by the secondary more slowly than in larger power systems. The minimum power output control of each generator, depending on the frequency deviation Both the deviation of n of generators in a power system utilize the droop control, the deviation of system frequency for the QSS and instantaneous frequency should not exceed their limits (∆fQSS,max and ∆fins,max , respectively).
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