DC Microgrids–Part I: A Review of Control Strategies and Stabilization Techniques

Abstract

This paper presents a review of control strategies, stability analysis and stabilization techniques for DC microgrids (MGs). Overall control is systematically classified into local and coordinated control levels according to respective functionalities in each level. As opposed to local control which relies only on local measurements, some line of communication between units needs to be made available in order to achieve coordinated control. Depending on the communication method, three basic coordinated control strategies can be distinguished, i.e. decentralized, centralized and distributed control. Decentralized control can be regarded as an extension of local control since it is also based exclusively on local measurements. In contrast, centralized and distributed control strategies rely on digital communication technologies. A number of approaches to using these three coordinated control strategies to achieve various control objectives are reviewed in the paper. Moreover, properties of DC MG dynamics and stability are discussed. The paper illustrates that tightly regulated point-of-load (POL) converters tend to reduce the stability margins of the system since they introduce negative impedances, which can potentially oscillate with lightly damped power supply input filters. It is also demonstrated how the stability of the whole system is defined by the relationship of the source and load impedances, referred to as the minor loop gain. Several prominent specifications for the minor loop gain are reviewed. Finally, a number of active stabilization techniques are presented. Index Terms-DC microgrid (MG), local control, coordinated control, impedance specifications, stability. NOMENCLATURE Acronyms AVP Adaptive voltage positioning. BLDC Brushless DC. CC Central controller. CPL Constant power load. DBS DC bus signaling. DCL Digital communication link. DG Distributed generator. DPS Distributed power system. EET Extra element theorem. ESAC Energy storage analysis consortium. ESS Energy storage system. EV Electric vehicle. GM Gain margin.

Conformal mapping of impedance stability models for system-level dynamics assessments

“…The source impedance Z i (ω) is inductive at the low frequency range and capacitive in the high frequency range, and presents a wide and smooth magnitude peak at 5 Hz, which has been found to be highly dependent on the source converter controller. In the case of the load admittance, it should be noted that Y o (ω) is defined by a negative resistive behaviour in the low frequency range (∠Y o (ω) = 180 deg), as expected from a CPL [5], [6], [21], [22]. At the high frequency range, where the control action does not have an effect, Y o (ω) is inductive and has a low magnitude.…”

“…2. Subsequently, stability is assessed by the Nyquist criterion [5], [6]. This approach has been considered in different applications: ac/dc microgrids [5], [6], renewable energies [7]- [10] and traction [11].…”

“…A similar methodology is employed in low-voltage/mediumvoltage dc (LVDC/MVDC) systems [6], [12]. The source/load dynamic interactions of a possible MVDC electrical distribution system (see Fig.…”

“…More specifically, the criteria based on stability regions defined in the polar plot (mathematically, defined as the G-plane below) provide ambiguous interpretations when considering different modelling points (which, in principle, is only a conceptual issue) [5], [6]. In order to make the most information from the polar plot (obtained from modelling at any arbitrary point), this paper proposes a technique that exploits the conformal mapping property inherent to the Nyquist trajectories [14]- [16].…”

Stability Analysis of DC Distribution Systems with Droop-Based Charge Sharing on Energy Storage Devices

“…The values of the droop coefficients have a profound effect on system stability and current sharing accuracy. In general, the higher the droop coefficients, the more damped the system is, and better current sharing accuracy is achieved, although a trade-off is needed in order to maintain the voltage deviations at acceptable levels [13]. Assuming that droop control is slower than the primary outer-loop voltage control, and much slower than the inner-loop current control, and ignoring fast dynamics and using small signal analysis, it is shown [12] that stable operation of the DC-grid system is ensured.…”