The dc distribution power system has drawn widespread attention and rapidly developed in recent years; however, its stability issue is one of the urgent challenges, requiring much attention. In previous stability studies, the line impedance network is often overlooked, which is convenient for the geographically centralized systems, but inaccurate for the distributed systems. In this article, the modeling and stability criteria of these two systems are simultaneously studied. For the distributed systems, first, the transfer functions from each input voltage or current disturbance to the bus voltage are derived. On this basis, the sufficient and necessary condition for the system stability is discussed. Subsequently, a bus node impedance criterion (BNIC) is proposed to assess the small-signal stability of the dc distribution power system. For the centralized systems, by ignoring the line impedance network, its small-signal model can be deduced by the distributed system, and the proposed BNIC is proved to be still valid. In addition, some existing impedance-based stability criteria are analyzed and compared with the proposed BNIC. Finally, the correctness and effectiveness of the proposed BNIC are verified by the cases study and experiments.

Shunt active power filter is widely used for compensating harmonic current produced by nonlinear loads. As one of the critical technologies, the current control strategy has an important influence on the compensation precision and dynamic response performance. In this paper, the essential of the current-limiting demands for the comprehensive protection of SAPF are revealed, namely the limiting control objects: 1) RMS of the compensation current (mainly for the overheat protection of IGBTs and inductors); 2) the instantaneous wave of compensating current (mainly for the accurate current control and IGBT $I_{cnom}$ specification) and 3) the instantaneous wave of PWM-VSC modulation voltage (for the need of liner close-loop control). To avoid the over-current fault and over-modulation fault of SAPF, this paper proposes an improved current-limiting scheme based on particle swarm optimization (PSO) to achieve the two optimization targets: 1) the minimization THD for the grid-side current; 2) the maximization utilization ratio for the capacity of SAPF. The main advantage lies on the optimum limiting ratios of each harmonic order calculated in real time respectively to achieve the flexible and liner limiting control. In addition, taking the control structure consisting of selective harmonic detection under the multiple rotating reference frame and the single PI current loop for example, this paper gives the method for steady state error correction qualitatively and quantitatively, which employs the method based on the frequency response characteristics analysis. Finally, simulation and experimental results verify the effectiveness of the proposed strategy.

The stability analysis methods of single-bus dc distribution power systems (DPSs) have been widely studied; however, the multivoltage-level dc DPSs, which have been used in data center, all-electric ship, and so on, its stability analysis methods are rarely reported. The main focus of this article is to study the stability analysis methods, especially the impedance ratio of multivoltage-level dc DPSs. From the point of view of whole system, the two-port small-signal models of multivoltage-level dc DPSs are established and the equivalent loop gains are obtained and used to judge the system stability. On the other hand, the multivoltage-level dc DPSs can be regarded as conventional single-bus dc DPSs at different interfacing locations, and the impedance-ratio type stability criteria are obtained. And it is proved that the two stability analysis methods, namely the system equivalent loop gain and impedance-ratio type stability criterion at different interfacing locations, have the same stability results, which clarifies the stability status by considering different interfacing locations. Finally, the analysis is verified with simulations and hardware-based experiments of a three-converter three-stage dc DPS.

The current-limiting inductors (CLIs) are commonly used to suppress DC fault current to safely trip DC circuit breakers in MMC-HVDC systems, but it significantly jeopardizes DC dynamics and causes instantaneous power mismatch between AC and DC sides of islanded rectifier station (RS), exposing MMC internal states to large disturbance. This paper firstly provides a detailed analysis of the influence of CLIs on AC fault, establishes RS mathematical representation including AC side dynamic, and reveals the mechanism of DC voltage oscillation, modulation saturation and PCC waveform distortion. The analysis shows that the AC and DC fault characteristics are decoupled under unsaturated AC modulation, but coupled under excessively low submodule capacitor voltage, where AC modulation is saturated and PCC waveforms are distorted. To solve this issue, a novel RS AC fault ride through method for MMC-HVDC system with CLIs is proposed to stabilize RS DC voltage, desaturate AC modulation and decouple fault characteristics by regulating the zero-sequence modulation of circulating current suppression. In addition, a current reference disturbance and an AC energy dissipation device coordinated control are designed, which adaptively recovers the submodule capacitor voltage. Finally, the feasibility of the proposed method is verified by simulation results.

The large deployment of plug-in electric vehicles (PEVs) challenges the operation of the distribution network. Uncoordinated charging of PEVs will cause a heavy load burden at rush hour and lead to increased power loss and voltage fluctuation. To overcome these problems, a novel coordinated charging strategy which considers the moving characteristics of PEVs is proposed in this paper. Firstly, the concept of trip chain is introduced to analyze the spatial and temporal distribution of PEVs. Then, a stochastic optimization model for PEV charging is established to minimize the distribution network power loss (DNPL) and maximal voltage deviation (MVD). After that, the particle swarm optimization (PSO) algorithm with an embedded power flow program is adopted to solve the model, due to its simplicity and practicality. Last, the feasibility and efficiency of the proposed strategy is tested on the IEEE 33 distribution system. Simulation results show that the proposed charging strategy not only reduces power loss and the peak valley difference, but also improves voltage profile greatly.