It is well-known that the low-frequency negative input impedance of the constant power load (CPL) is the major cause of the cascaded system instability, and the heavier the power, the worse the stability. In this article, a power adaptive load-side parallel virtual impedance (PALPVI) control strategy is proposed to improve the stability of the cascaded dc system with buck-type CPL. First, a parallel impedance with power adaptability is derived followed up with the derivation of the corresponding compensation controller transfer function to realize it virtually. Considering that the compensation controller is highly dependent on the circuit parameters and needs extra current sensors to acquire the power information, a simplification of the compensation controller is made based on the open-loop characteristics of the buck-type CPL. The final PALPVI control strategy does not require the circuit parameters or any current sensor and has almost no side effect on the dynamic performance. Finally, a 48–24–12 V cascaded dc system is fabricated to verify the feasibility and effectiveness of the proposed PALPVI control strategy.

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.

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.

Two individually stable converters are well-known preconditions for discussing the stability of a two-stage cascaded system. However, for a multistage cascaded system, the relationship between its stability and subsystem stability is barely reported due to the lack of modeling method and in-depth stability analysis. In this article, the above problems are solved. First, the generic two-port small-signal model of the multistage cascaded dc–dc converters is established, and its equivalent minor loop gain is calculated, which can be used to assess system stability. On this basis, the stability condition of the triple-stage cascaded dc system is further deduced and compared with its two subsystems. Then, an interesting conclusion is obtained that the whole system may be stable even if some of its subsystems cannot operate stably individually. Besides, according to the deduced stability condition, some existing stability criteria of the triple-stage cascaded dc system are explained uniformly, and a bus equivalent impedance criterion (BEIC) is proposed. Finally, the case study and experiments are carried out to verify the correctness of the theoretical analysis.

During the past decades, the small-signal stability of the interconnected converters has been extensively researched, and several impedance/admittance-based criteria were reported and effectively verified. However, the equivalence and correlation among these criteria have not been clearly expounded. In this article, the internal relationship among the existing five stability criteria is firstly studied by modeling and stability analysis for different interconnected converters, and proving the equivalence of these stability criteria. Then, from the perspective of the application complexity, the convenience for system design and the reliability against zero-pole cancellation risk, the optimal stability criteria for different types of interconnected converter systems are recommended. Finally, based on a cascaded system composed of a boost and a buck converter and a paralleled system consists of two single-phase LCL inverters, the correctness of the proposed viewpoints is demonstrated by case studies and experiments.

This paper presents a modified power supply system based on the current alternating current (AC)-fed railways with neutral zones that can further improve the eco-friendliness and smart level of railways. The modified system complements the existing infrastructure with additional energy-storage-based smart electrical infrastructure. This infrastructure comprises power electronic devices with energy storage system connected in parallel to both sides of each neutral zone in the traction substations, power electronic devices connected in parallel to both sides of each neutral zone in section posts, and an energy management system. The description and functions of such a modified system are outlined in this paper. The system allows for the centralized- and distributed-control of different functions via an energy management system. In addition, a control algorithm is proposed, based on the modified system for regenerative braking energy utilization. This would ensure that all the regenerative braking energy in the whole railway electrical system is used more efficiently. Finally, a modified power supply system with eight power supply sections is considered to be a case study; furthermore, the advantages of the proposed system and the effectiveness of the proposed control algorithm are verified.