Bangbang He

Immittance-determinant-based stability criteria are widely employed in multi-node AC/DC systems due to their lower computational complexity compared to the generalized Nyquist criterion. However, if the constructed immittance determinant does not satisfy the prerequisites of the argument principle, the corresponding frequency-domain stability analysis may lead to erroneous conclusion. To overcome these limitations, this article first derives an immittance determinant without right-half-plane (RHP) poles, such that system stability can be determined by checking whether this determinant has RHP zeros. Subsequently, to determine the presence of RHP zeros from the frequency‑domain perspective, the immittance determinant is reshaped by introducing left-half-plane (LHP) zeros/poles, enabling the application of the argument principle. On this basis, a phase-based stability analysis method is proposed, which facilitates the assessment of whether the reshaped immittance determinant encircles the origin. Finally, the effectiveness of the proposed approaches is demonstrated through case studies and experiments.

Immittance-based small-signal stability analysis methods are frequently employed in dc microgrids. However, most existing methods either suffer from right-half-plane (RHP) pole issues or depend on complete amplitude and phase information of all frequencies. To address these limitations, this letter constructs a novel immittance transfer function that combines the impedance/admittance of all converters, with introduced left-half- plane zeros to satisfy the prerequisites of the argument principle. Then, we rigorously prove that this immittance transfer function contains no RHP poles when each converter remains stable under standalone operating conditions. The stability of dc microgrids can therefore be determined by checking whether the immittance transfer function contains RHP zeros. Furthermore, this letter demonstrates that the immittance phases at zero and infinity frequencies directly indicate system stability, without counting the net number of times that the Nyquist contour encircles the origin. Finally, the proposed method is validated by case studies and experiments.

The existing immittance-based stability analysis approaches for dc microgrids rarely account for the line network, and they are always troubled by the non-minimum phase issues. In this article, we introduce a novel method for stability analysis based on the determinant of the constructed immittance matrix. Since the determinant's right-half-plane poles are explicitly proven absent, it is only necessary to ascertain the net number of times that the phase trajectory crosses the ±180° line in system stability analysis. Subsequently, based on the immittance matrix decomposition and sensitivity calculation, a participation analysis method is further presented to identify the critical converter that dominates system instability. Finally, the simulation case of a 750 V 30 kW dc microgrid and the experiment case of a 24 V 180 W dc microgrid are established to validate these methods.

The modular multilevel dc–dc converter emerges as a promising solution for the medium-voltage (MV) dc system due to its galvanic isolation, voltage conversion, and bidirectional power transmission capabilities. However, different from the traditional dc–dc converter, both the circulating energy and transmission power must be precisely controlled to achieve high dynamic and steady-state performance. In this article, the average value and small-signal models are developed for the series-arm modular multilevel dc–dc converter (SAMMDC) with quasi-square-wave modulation and phase-shifted control. A novel MV terminal current feedforward (MCFF) control strategy is proposed. First, the average value and small-signal models of the SAMMDC, which can describe the low-inertia circulating current characteristic of the modular multilevel structure, are rigorously derived utilizing the reduced-order method and the differential- and common-mode principles. Subsequently, the equivalent open-loop transfer function of the SAMMDC with dual-loop control is established, facilitating frequency analysis. Furthermore, the implementation of the proposed MCFF control strategy and design considerations are presented to increase crossover frequency for improved dynamic performance. Finally, experimental validation of the theoretical analysis and the control strategy is provided through results obtained from a 3.2-kW SAMMDC prototype.

The impedance-based stability analysis methods for ac or dc electronic power systems are always separately reported. Even in hybrid ac–dc systems, few studies adopt both ac and dc stability criteria simultaneously, let alone discuss their correlation and differences. In this article, the small-signal stability of grid-connected dc distribution power systems is investigated at both ac- and dc-sides. First, by modeling the system’s input-to-output transfer functions, two equivalent-minor-loop-gain-based stability criteria without right-half-plane (RHP) poles are proposed, and their equivalence is mathematically proved. Then, the existing subsystems-based stability criteria are further proven to be equivalent to each other and to the equivalent-minor-loop-gain-based stability criteria. Meanwhile, the differences of all the aforementioned stability criteria are analyzed and discussed. Finally, sufficient case studies and experiments validate the proposed methods and viewpoints.

The dc distribution power system (DPS) has witnessed rapid development in recent years, and it is gradually characterized by multiple voltage levels, multiple dc buses, and multiple converters. The complex topology of the system leads to some serious system stability problems. Compared with the cascaded dc DPS with multiple-voltage-level buses, the paralleled dc DPS with multiple-voltage-level buses is of outstanding advantage because of the higher efficiency in the case of more dc buses. To study the small-signal stability of the paralleled dc DPS with n dc buses, a simplified small-signal model of the system is first established in this article. Then, a stability criterion based on the equivalent loop gain of the system is proposed. Furthermore, the decentralized impedance specifications for the studied system are proposed to ensure the system's stability and improve the scalability of the system. First, the stability conditions for the design of impedance specifications are given. On this basis, the impedance specifications for the Z-type converter and dc transformers are proposed, which are only related to the dc bus voltage, the rated power, and the desired stability margin. Finally, sufficient case studies and experiments were conducted to validate the proposed criterion and impedance specifications.