Parameter Design for Symmetrical CLLC-Type DC Transformer Considering Cascaded System Stability and Power Efficiency

The symmetrical CLLCCLLC resonant DC transformerDC transformer has gained popularity in DC microgrids due to its strong galvanic isolation and capability for bidirectional power transfer. However, there are certain issues with the CLLC resonant DCT’s design that could affect its overall performance. Firstly, when it operates alone, its voltage transfer ratioVoltage transfer ratio (VTR) is challenging to keep within a tolerable range when inductance or capacitance fluctuations are present in reality. Secondly, impedance interactions may cause instability phenomena when the CLLC resonant DCTDC transformer works in a cascaded system with a constant power load (CPL). The stability of this cascaded system will also be impacted by variations in inductance or capacitance. This work proposes a parameter design strategy for the CLLCCLLC resonant DCT to address these issues. A particular application of the Particle Swarm Optimization (PSO)Particle Swarm Optimization (PSO) technique is made to achieve extensive assessments of erratic parameter variations. Thus, even in the presence of circuit parameter variations, the developed CLLC resonant DCT can maintain a stable system and a strong VTR. A 1 kW prototype has demonstrated the viability of this paper’s proposed design strategy.

The DC side voltage stability is a prerequisite for the stability of the cascade system with voltage source converter (VSC). Compared with the small signal analysis method based on the state space model, the small signal analysis method based on the impedance model has a simple modeling process and clear physical meaning. The negative impedance characteristic of the constant power load (CPL) will reduce the effective damping of the system, resulting in system instability. In this paper, two-port small signal models of VSC and CPL are established respectively, and the small signal stability of the VSC and CPL cascade system in strong grid and weak grid is evaluated based on the impedance ratio criterion. In addition, the influence of CPL power and VSC DC side capacitance on the stability of the cascade system is analyzed. According to the stability analysis, the corresponding simulation case is designed to verify the accuracy of the analysis method.

This paper studies stability analysis of DC microgrids with uncertain constant power loads (CPLs). It is well known that CPLs have negative impedance effects, which may cause instability in a DC microgrid. Existing works often study the stability around a given equilibrium based on some nominal values of CPLs. However, in real applications, the equilibrium of a DC microgrid depends on the loading condition that often changes over time. Different from many previous results, this paper develops a framework that can analyze the DC microgrid stability for a given range of CPLs. The problem is formulated as a robust stability problem of a polytopic uncertain linear system. By exploiting the structure of the problem, we derive a set of sufficient conditions that can guarantee robust stability. The conditions can be efficiently checked by solving a convex optimization problem in polynomial time. The effectiveness and non-conservativeness of the proposed framework are demonstrated using a simulation example.

This paper investigates stability of a DC microgrid with integrated constant power loads (CPLs). The investigation is based on an interconnected model of the DC microgrid with two subsystems. One subsystem consists of a CPL under examination and the other subsystem is constituted by remainder of DC microgrid (ROM). Analysis is carried out to indicate when an oscillation mode of CPL subsystem is in the proximity of an oscillation mode of ROM subsystem on complex plane, stability of the DC microgrid decreases as caused by the CPL, leading to growing oscillations in the DC microgrid in the worst case. Hence, the investigation reveals the mechanism about why the CPL may cause instability of DC microgrid from a new perspective of modal proximity. An index is proposed to detect the instability risk by applying the modal analysis to subsystems. The proposed application extends the merit of widely used frequency‐domain analysis to the modal analysis, because instability danger can be detected by the proposed modal analysis in practice without having to establish parametric model of the DC microgrid. Two example DC microgrids with CPLs are presented to demonstrate and evaluate the analysis and conclusions made in the paper.

Energy management in DC microgrid with an efficient voltage compensation mechanism

Nowadays, cyber-attacks are immoral ways to disturb power grids and make them devitalized or unstable. To avoid such issues, detection and estimation of cyber-attacks are essential in modern power utilities. This paper investigates a new nonlinear Kalman filter, called Spherical Simplex Radial Cubature Kalman Filter (SSRCKF), as a reliable and accurate estimator to detect cyber-attacks in nonlinear Direct Current (DC) microgrids (MGs) with Constant Power Loads (CPLs). The developed nonlinear Kalman filter offers a high resilient State Estimation (SE) performance in presence of noise as well as desired accuracy, and low computational burden. The SSRCKF is modified for the attack detection issue by augmenting the occurring attacks to the original nonlinear dynamics. The developed SSRCKF approach is deployed to estimate the DC MG states in the presence of Denial-of-Service (DoS) and data integrity attacks. Simulation results show that the presented filter estimates the DC MGs states of the system immediately after the attack.

Stability analysis of islanded DC microgrid for the proposed distributed control strategy with constant power loads

A novel droop controller for DC microgrid systems, consisting of multiple paralleled sources feeding a constant power load (CPL), is proposed to achieve the desired voltage regulation and accurate load power distribution, while ensuring an overvoltage protection for each source. CPLs are well-known to exhibit negative impedance characteristic due to their nonlinear behaviour, which may cause instability to a DC microgrid if the necessary impedance inequality criteria are not satisfied. In this paper, a new droop control scheme is proposed to limit the voltage of each source below a desired bound, achieve tight voltage regulation and power sharing, and guarantee closed-loop system stability with the existence of a CPL. The upper limit of the voltage of each source is rigorously proven using ultimate boundedness theory, while after a suitable manipulation of the admittance matrix of the microgrid, analytic conditions of stability are obtained to guide the control parameters design. To validate the theoretical design and analysis, a detailed simulation is performed of a DC microgrid equipped with the proposed controller.

Grid connected DC Microgrid (DCMG) with wind turbine and solar PV distributed generations (DG), battery storage system and constant power loads (CPL) which are integrated via power electronic converter is presented in this paper. In DCMG interfaced with multiple parallel converters the stability of dc bus can be deteriorated by DG output power intermittency and microsources converter output impedance and load converter input impedance mismatches. Thus to solve these problems, this paper presents an autonomous control using decentralized control scheme based on dc bus voltage as means of communication is proposed in this paper. Droop control strategy for power sharing among the different sources and to stabilize bus voltage is employed. The stability analysis of the system is performed using Lyapunov stability criteria to determine the qualitative behavior of the system based on the simplified equivalent source transmission line. The control method verified via MATLAB/SIMULILK simulation.

In DC microgrid, it is common that constant power loads (CPLs) exhibit negative impedance characteristics, which degrades the stability margin of the whole system. This paper focuses on the stability problem of isolated DC microgrid with multiple DC voltage control units and constant power loads. Taking controllers into account, the detailed impedance model of DC microgrid is established, and it shows the interaction between converters through analyzing the minor loop gain by using Nyquist criterion. Meanwhile, resonance is examined using impedance model. Impedances-based stability analysis reveals the influence of different factors on the stability, such as the size of the CPLs and the droop coefficient. To verify the analysis results, the simulation model is built based on PSCAD/EMTDC. The simulation results confirm the validity of the method. With the developed dc impedance and stability analysis method, the DC microgrid system stability characteristics can be evaluated, which can be used for system design to maintain stability.

Direct Current (dc) microgrids due to their efficiency and energy savings are being deployed to provide power for servers in Internet of Things (IoT) data centers, in more electric aircrafts (MEA), electric ships and in rail systems round the word. In this paper, Takagi-Sugeno fuzzy inference method is used to establish a Lyapunov stability candidate for a 380 V ring bus dc microgrid modeled with Matlab. To determine suitability of using powerline communication (PLC) to monitor stability condition on the 380 V dc microgrid, impact of distortion caused by microgrid constant power loads (CPL) on signals transmitted over the dc microgrid PLC channel is examined. It is shown in this paper that while Lyapunov asymptotic stability is maintained on the dc bus, increasing CPL on the microgrid causes the dc microgrid PLC channel to experience growing signal distortion.

In a DC microgrid, DC-DC converters are widely used as linking interfaces between DC power sources (or energy storage units) and the DC bus. Typically, linear controllers are widely adopted for these source DC-DC converters to control the power generation of the sources. However, the system's stability can hardly be guaranteed when constant power loads (CPLs), whose input impedances are negative, are connected to the microgrid due to unexpected interactions between sources and CPLs. In addition, as the variation of DC bus voltage is limited to a specific range by electrical standards, which indicates constrained states of the DC-DC converters, conventional nonlinear controllers would cause deteriorated power quality in static and dynamic processes. This paper proposes a novel strict state-constrained sliding mode control strategy for the source DC-DC converters to address the stability and power quality issues of the DC microgrid. The design methodology, stability analysis, and implementation of the proposed control strategy are discussed in detail. A 3-kW buck/boost converter, which is widely used as the source DC-DC converter in a DC microgrid, is designed and established in the lab. The simulation results verify the effectiveness of the proposed control strategy.

Direct current (DC) microgrid is being increasingly investigated in modern power grid. An important issue in DC microgrid operation is to ensure proper current sharing among converters. While this has been addressed through droop control, the resulting voltage deviation in DC bus has to be compensated. To solve this problem, a new distributed secondary control scheme is presented in this paper for both current sharing and voltage restoration. A key part of the presented scheme is the integration of a new parameter `virtual voltage drop' defined from droop gain and line resistance. Since the DC bus voltage is not required as a feedback signal, the proposed secondary control is simple and easy to design and implement. In addition, as the proposed scheme has no requirement for the loads, it can handle both resistance loads and constant power loads (CPLs). Simulations as well as experimental studies are carried out to demonstrate the effectiveness of the proposed scheme.

This paper describes the behavior of voltage weak DC distribution systems. These systems have relatively small system capacitance. The size of system capacitance, which stores energy, has a considerable effect on the value of fault currents, control complexity, and system reliability. A number of potential definitions of voltage weak DC distribution systems are proposed. These definitions address the main characteristics of voltage weak systems. A small signal model of a general voltage weak DC distribution system is developed in order to observe sufficient conditions for system stability. This is achieved by analyzing the dominant poles. The source converters are modeled as droop-controlled current sources in parallel with their respective terminal capacitors. As constant power loads have incremental negative impedances, which affect the system stability, especially, in voltage weak system, ideal constant power loads with their terminal capacitors are included in the small signal model. A three-node voltage weak DC distribution grid is analyzed, as a case study, by implementing the developed small signal model. The effects on system stability by the values of system capacitance, cable inductance, and cable resistance are investigated using dominant pole placement. Likewise, the influence of proportional-integral regulators and droop coefficients of source converters on the stability of the system is examined. Finally, the three node DC distribution grid is developed in MATLAB/Simulink in order to demonstrate the influence of small capacitance on system stability. Moreover, effect of the rate of change of constant power loads on the system stability is simulated. These results are further compared and verified on a voltage weak DC experimental test bench with a 350 VDC bus voltage.

A novel distributed power consensus control approach with overvoltage protection is proposed and analysed for meshed direct current (DC) microgrids (MGs) with constant power loads (CPLs). The DC MG considered herein consists of source and load nodes connected over an undirected weighted graph induced by the electrical circuit network, namely the conductance matrix. When deploying, the proposed controller features a second graph, that models the communication network over which the source nodes exchange information such as the instantaneous powers, and which is used to adjust the power injection accordingly to achieve power sharing. Additionally, one aims to maintain the voltage at each source below operator-set limits. This feature is critical given the power and voltage dependency. By addressing the occurrence of abnormal voltage values at different nodes in the network, one would guarantee a relatively safer power consensus policy and microgrid operation. To accommodate both objectives, we developed a nonlinear power consensus-based control system, with a voltage-limiting component, by means of Lyapunov analysis and ultimate boundedness theory. Asymptotic closed-loop stability is also established around a set of equilibria. Finally, numerical simulations align with and validate our theoretical findings.