Impedance-Based Modeling and Common Bus Stability Enhancement Control Algorithm in DC Microgrid

Abstract

In this paper, impedance modeling of a DC microgrid system consisting of a source and load converter, including an input filter, is performed. Impedance-based modeling has been used to derive mathematical models of the output impedance of the source converter and the input impedance of the load converter. The correlation between the converter interaction and system stability is analyzed based on the mathematical model. An impedance-based stability analysis is used to determine the system stability by analyzing the interactions among the converters in the DC microgrid system. Middlebrook's stability criterion, which uses the impedance transfer function, is applied to determine system stability. Moreover, in this paper, a stability enhancement control algorithm is proposed to resolve the system instabilities resulting from interaction among the converters and the distortion caused by the harmonics emanating from the AC input. The proposed stability enhancement control algorithm consists of a feed-forward type virtual impedance (VI) and a proportional-resonant (PR) controller. The validity of the proposed method is demonstrated by the results of the response characteristics in the frequency domain, and the effectiveness of the proposed control algorithm is verified via simulations and prototype experimental models.

A Stability Improvement Method of DC Microgrid System Using Passive Damping and Proportional-Resonance (PR) Control

“…As can be seen from the waveform, without applying the damping method in Figure 18, the gain margin exceeds 0 dB. This means that the system is unstable because it corresponds to the unstable condition of Middlebrook's stability criterion in Equation (12). On the other hand, when RC parallel, RL series, and RL parallel damping methods were applied, the gain margin was below 0 dB at the same point, which means the system was stable.…”

“…The harmonics decreased by 85.3% when RC parallel damping was applied, 93.3% when RL parallel damping was applied, and 85.6% when RL series damping was applied. The stability of the system was analyzed using T MLG , which is the ratio of input and output impedances in a Bode plot [12,13]. As can be seen from the waveform, without applying the damping method in Figure 18, the gain margin exceeds 0 dB.…”

“…Each converter is individually designed to operate seamlessly. However, in a microgrid system in which several converters operate simultaneously, unexpected interactions between converters may occur, which adversely affect other converters connected to the common bus, resulting in instability of the entire system [10][11][12]. Therefore, there is a need for a method to prevent malfunctions between converters and improve the stability of the system.…”

Two-Port Characterization and Transfer Immittances of AC-DC Converters–Part II: Applications

“…Most existing works on stability studies of dc microgrids and dc distribution systems, see e.g. [20] and [21], avoided these difficulties by treating the grid as an ideal voltage source, analogue to the assumption of ideal dc bus in the modeling of ac-port impedance. Under that assumption, dynamics of the converter associated with its three-phase circuit and control can be modeled and linearized in the dq reference frame [22].…”

“…The number of such converters in the grid has been increasing exponentially in recent years due to the rapid deployment of converter-based generation from renewable sources and its transmission over long distance by high-voltage dc (HVDC) [1]. Meanwhile, development of electrical vehicles [2], data centers and other types of large IT infrastructures [3], as well as dc distribution and microgrids [4] continues to drive up power electronics presence at the distribution level and on the load side. With plans announced by many countries and regions to achieve carbon neutrality by 2050 or sooner, future power systems will undoubtedly be built based on renewables and converters, and will be fundamentally different from traditional power systems based on synchronous generators.…”