Nonlinear Robust Control for Low Voltage Direct-Current Residential Microgrids with Constant Power Loads

Abstract

A Direct Current (DC) microgrid is a concept derived from a smart grid integrating DC renewable sources. The DC microgrids have three particularities: (1) integration of different power sources and local loads through a DC link; (2) on-site power source generation; and (3) alternating loads (on-off state). This kind of arrangement achieves high efficiency, reliability and versatility characteristics. The key device in the development of the DC microgrid is the power electronic converter (PEC), since it allows an efficient energy conversion between power sources and loads. However, alternating loads with strictly-controlled PECs can provide negative impedance behavior to the microgrid, acting as constant power loads (CPLs), such that the overall closed-loop system becomes unstable. Traditional CPL compensation techniques rely on a damping increment by the adaptation of the source or load voltage level, adding external circuitry or by using some advanced control technique. However, none of them provide a simple and general solution for the CPL problem when abrupt changes in parameters and/or in alternating loads/sources occur. This paper proposes a mathematical modeling and a robust control for the basic PECs dealing with CPLs in continuous conduction mode. In particular, the case of the low voltage residential DC microgrid with CPLs is taken as a benchmark. The proposed controller can be easily tuned for the desired response even by the non-expert. Basic converters with voltage mode control are taken as a basis to show the feasibility of this analysis, and experimental tests on a 100-W testbed include abrupt parameter changes such as input voltage.

Fractional-Order Approximation and Synthesis of a PID Controller for a Buck Converter

“…whereD ∈ [0, 1] is the average of duty cycle D. Considering that V c = V is the output of the system y(t) andD is the control input u(t), the transfer function of system (3) is described by, Parameters shown in Table 1 have been used to implement a Buck converter in [29]. The converter showed good performance operating at a switching frequency f s = 20 kHz in continuous conduction mode for microgrids with constant power loads.…”

Stabilization Method Considering Disturbance Mitigation for DC Microgrids with Constant Power Loads

“…Let us take R = 1 Ω and C 1 = 5 mF. According to (4), the operation point of the system can be obtained as (4.32, 115.68, 8.64, 57.84). According to the stability analysis, the system is stable if and only if (12) holds.…”

“…Usually, they can be worked in two modes: grid-connected mode, and islanded mode. Usually, the main control objectives of a DC microgrid include maintaining stability [4][5][6][7][8][9], minimizing cost [10][11][12], sharing power and regulating voltage [13]. For these objectives, there are mainly three control methods: decentralized control [10,12,[14][15][16][17], distributed control [18][19][20][21][22][23][24][25][26] and centralized control [11,27].…”

“…At present, domestic research on microgrids has primarily focused on AC microgrids [12][13][14][15][16][17]. However, DC microgrids have the following advantages: high transmission efficiency, high reliability, no frequency synchronization issues, easier integration of renewable energy, and ease of stabilization, therefore, DC microgrids are increasingly being used in applications such as aircraft, spacecraft, and electric vehicles [4,24].…”

“…In DC microgrids, the load is typically connected to the DC bus through a DC/DC or DC/AC converter. When the response of the load-end converter is rapid, the load exhibits a negative impedance, which is equivalent to a constant power load (CPL) [4]. To ensure stable and reliable operation of a DC microgrid, voltage stability is essential.…”