Large Signal Stability Criteria Research Articles

Large-Signal Stability Analysis for Islanded DC Microgrids with n+1 Parallel Energy-Storage Converters

In islanded DC microgrids, the negative impedance characteristics of constant power loads (CPLs) usually introduce instability influences; on the contrary, hybrid energy-storage systems (HESSs) constituted of batteries and supercapacitors (SCs) have stabilization advantages. To guarantee the large-signal stability of islanded DC microgrids with n+1 parallel energy-storage converters, an equivalent model is first constructed based on the control strategies of the converters. Then, according to the mixed potential function theory, a large-signal stability criterion, considering powers, inductors, capacitors, the DC bus voltage, the equivalent internal resistances of batteries, the proportional parameters of the inner current loop of n battery DC–DC converters, the proportional parameter of the outer power control loop of the SC DC–DC converter, and the proportional parameter of the inner current loop of the CPLs, is derived. Furthermore, the proposed large-signal stability criterion is optimized via the use of droop control for n battery converters, and coefficients related to the droop coefficients are also taken into account. These involved control parameters reveal the process of regulating the HESS and CPLs instead of ideal modeling and significantly reduce the conservatism of the criterion to some extent. In addition, on the basis of the large-signal stability criterion presented herein, the maximum CPL power that the islanded DC microgrids can stably support is obtained. Finally, simulation and experimental results verify the validity of the provided large-signal-stability criterion. The given procedure of analyzing large-signal stability is more consistent with planning and operating actual DC microgrids.

Large-Signal Stability Analysis of DC Distribution Systems With Cascading Converter Structure

DC-DC converters are major components of the DC distribution systems. The converters interface with external power inputs, internal DC buses and loadings of subsystems. The interfacing DC-DC converters should be stable locally and globally under mutual interactions through a DC bus within the system. Current research efforts have focused on the analysis of the stability of the DC distribution system subject to small-signal disturbance. However, in practice, the system routinely operates under large-signal disturbances, such as when an additional subsystem is turned on after being connected to the DC bus. In this scenario, the small-signal model may fail to fully describe the dynamics of the system. In this paper, we identify and analyze the bifurcation process when the system undergoes abrupt load changes. According to the nonlinear operation of the interconnected system, a large-signal stability criterion is derived. This criterion is simple, and can be easily extended to multiple connected converter systems. The criterion is also consistent with the result from bifurcation analysis. Finally, the validity of the proposed criterion is verified by the full-circuit simulations and the experimental works.

Mixed-potential-function-based large-signal stability analysis of DC microgrid with constant power loads

With the development of power grid technology, a large number of constant power loads (CPL) are introduced into the power grid system. The negative damping characteristic of CPL poses a great challenge to the stability of power grid under the condition of large signal disturbance. This paper focuses on the large signal stability and dynamic response characteristics of DC microgrid with CPL. The mixed potential function (MPF) method is adopted to model the large signal characterstic of load converter controlled by the traditional PI control strategy and the new virtual DC motor (VDM) control strategy. The large signal stability criteria and the asymptotic stability region of the DC microgrid system under the two control strategies are derived. The dynamic response characteristics of the DC microgrid are compared under three large signal operation conditions of the load step, load linear change and the load fault. The results show that VDM control strategy can effectively improve dynamic response characteristics of the DC microgrid system compared with those of PI control, and enhance the stability and anti-interference ability of the DC microgrid system.

Large Signal Stability Criteria Combined with a 3D Region of Asymptotic Stability Method for Islanded AC/DC Hybrid Microgrids

Large disturbances frequently happen in isolated AC/DC Hybrid Microgrids. Unfortunately, constant power loads (CPLs) with negative impedance characteristics are equivalent to positive feedback, resulting in an increase in large disturbances. The system can easily become unstable. Consequently, large signal stability criteria are proposed in this paper. Combined with a three-dimensional region of asymptotic stability (3D RAS) method for islanded AC/DC Hybrid Microgrids, important parameters to increase stability margins were determined. Firstly, mixed potential theory was used to derive a large-signal stability criterion. The criteria gave constraints on filtering parameters, CPL power, power of the battery to charge and discharge, AC resistive loads, and DC bus voltage. Then, Lyapunov functions were constructed, and the Lasalle invariance principle was adopted to achieve 3D RAS. When large disturbances emerged, and simultaneously voltage and current varied in 3D RAS, the system always obtained stability and reached new steady-state equilibrium points. Finally, according to comparisons, bigger capacitances of the DC bus capacitor and the AC capacitor, larger battery discharging power and smaller charging power could significantly increase stability margins of islanded Microgrids. Simulations and experimental data have shown that the large signal stability criteria and the 3D RAS work.

Delay-Based Phase-Locked Loop Parameters Design Based on Stability Region of Grid-Connected Single-Phase Inverter Under Grid Voltage Sags

For the grid-connected inverter, grid voltage sag can be a typical phenomenon. From the prospective of safe and robust operation, the inverter should be stable when the grid voltage sag (or large disturbance) occurs. Although the inverter stability in this case has attracted considerable attentions, few studies have given suggestions on the system stability boundary and parameter design. To fill this gap, this article deeply discusses the large-signal stability of the system under different situations and reveals the quantified impact of the phase-locked loop (PLL) on the large-signal stability of the system based on the fundamental current-source model. The large-signal stability criterion and boundaries for grid voltage sags are established, which are instructive to the design of PLL parameters. Besides, a novel design guideline for the PLL parameters is proposed to ensure the robust operation of the system both under the steady-state and grid voltage sags, as well as to achieve good dynamic performance after large disturbance occurs. The theoretical, simulation, and experimental results show that the proposed stability boundary has certain accuracy and the PLL parameters design guideline can ensure the system stability both under steady-state and grid voltage sag conditions.

Large signal stability criterion of AC–DC hybrid microgrids with constant power loads considering reference current limitation

Islanded AC–DC hybrid microgrids composed of new energy sources, constant power loads and energy storage system are typical non-linear systems, and guaranteeing large signal stability is a key issue. In this paper, the non-linear model of islanded AC–DC hybrid microgrids is established, and large signal stability criteria are obtained. Considering reference current limitation of the energy DC–AC converter, the derived large signal stability criterion gives important constraints on the constant power loads power, the power of energy storage unit, DC bus voltage, the inductor and equivalent resistance of the AC filter, DC capacitor, the proportional parameter of the inner current controller and the maximum instantaneous power of the DC–AC converter. The paper is summarized as follows. Firstly, the equivalent model of the AC–DC hybrid microgrids in DQ rotating frame is established. Then based on mixed potential theory, the mixed potential model is constructed, and large signal stability criterion is derived and optimized considering the reference current limitation of the DC–AC converter. Finally, the presented criterion is verified by simulation and experimental results.

Large-Signal Stabilization Method for Islanded DC Microgrids Considering Battery and Supercapacitor Hybrid Energy Storage Systems

Islanded DC microgrids composed of distributed generators (DGs), constant power loads (CPLs), parallel converters, batteries and supercapacitors (SCs) are typical nonlinear systems, and guaranteeing large-signal stability is a key issue. In this paper, the nonlinear model of a DC microgrid with a hybrid energy storage system (HESS) is established, and large-signal stability criteria are obtained. The HESS consists of batteries and SCs. The derived criteria reveal the influences of the filter parameters, CPL power, DG power and the proportional control parameters of the battery converter and the SC converter on the system large-signal stability. Furthermore, important large-signal stabilization methods for regulating the HESS converter’s control parameters can easily achieve the large-signal stabilization of islanded DC microgrids without extra equipment. The paper is summarized as follows: First, the topology of and control strategy for a DC microgrid with an HESS and CPLs are proposed. Then, according to the characteristics of the HESS, the DGs and the CPLs, the system is equivalently simplified. Finally, the nonlinear model and large-signal stability criteria are both derived using the mixed potential theory, and a large-signal stabilization design method for the HESS converter’s control parameters is proposed. The experimental and simulation results show the effectiveness of the proposed large-signal stabilization method.

Multi-terminal Medium Voltage DC Distribution Network Large-signal Stability Analysis

The Brayton-Moser’s mixed potential theory is a popular approach for large-signal stability analysis of DC-based systems with constant power load (CPL). The criterion from the conventional mixed potential theory contains multiple time-varying parameters which are complicated for convenient stability analysis hence the need for simplification. On the other hand, stability analysis of droop-controlled multiple source converter-based DC systems are scarce. Thus, the large-signal stability criterion of a scaled-down droop-controlled multi-terminal MVDC distribution network with CPL using Brayton-Moser’s mixed potential theory was simplified and derived to show that CPL power rating, PI control parameters, droop and damping coefficients have profound effects on the system’s large-signal stability. By way of MATLAB/Simulink simulations, the large-signal stability of the MVDC distribution system was observed to reduce with increase in CPL power rating and source converter droop coefficients as well as increase with increase in proportionate coefficient values and use of optimized unlike non-optimized damping coefficients. Therefore, the droop-controlled MVDC distribution network with optimized PI control parameters, droop and damping coefficients have enhanced dynamic response and large-signal stability margin proving the accuracy and validity of the simplified large-signal stability criterion.

Large-Signal Stability Criteria in DC Power Grids With Distributed-Controlled Converters and Constant Power Loads

The increasing adoption of power electronic devices may lead to large disturbance and destabilization of future power systems. However, stability criteria are still an unsolved puzzle, since traditional small-signal stability analysis is not applicable to power electronics-enabled power systems when a large disturbance occurs, such as a fault, a pulse power load, or load switching. To address this issue, this paper presents for the first time the rigorous derivation of the sufficient criteria for large-signal stability in DC microgrids with distributed-controlled DC-DC power converters. A novel type of closed-loop converter controllers is designed and considered. Moreover, this paper is the first to prove that the well-known and frequently cited Brayton-Moser mixed potential theory (published in 1964) is incomplete. Case studies are carried out to illustrate the defects of Brayton-Moser mixed potential theory and verify the effectiveness of the proposed novel stability criteria.

Large-Signal Stability Criterion for Parallel-Connected DC–DC Converters With Current Source Equivalence

The dc–dc converters are connected in parallel to feed the utility loads in many applications like the dc microgrid, the photovoltaic-battery hybrid power system, etc. Since the power sources usually connected to a dc bus through power electronics converters, the dynamics of the whole system is complex and the stability issues would be critical. In this brief, a large-signal modeling and stability analysis is performed to such system using the controlled current sources to represent the parallel-connected converters. The mixed potential theory-based stability criterion is then generated with the simplified model. The design-oriented analysis of the key system parameters is developed through the large-signal stability criterion and the power constrains. The analysis results are verified by the circuit simulations and the hardware-in-the-loop experiment of a dc–dc system with parallel-connected switching power converters.

Research on Large-Signal Stability of DC Microgrid Based on Droop Control

Ensuring the large signal stability of the DC microgrid is the premise of the safe operation of the DC microgrid, but the research on the large-signal stability of microgrids with multiple droop control micro-sources is still scarce. In this paper, a DC microgrid system model with multiple droop control micro-sources was established by appropriate simplification. Addressing the problem that most stability research methods cannot be quantitatively analyzed, the mixed potential function method was used to analyze the large signal stability of the system. However, the criterion obtained by the conventional mixed potential function method is complicated and contains multiple time-varying parameters, which is not convenient for analysis. Therefore, the simple form of the criterion was obtained through simplification and the analysis proved the rationality of the simplification. On this basis, a nonlinear droop control method was proposed to improve the anti-interference ability of the system. Finally, the accuracy of the large signal stability criterion and the effectiveness of nonlinear droop control on the system’s large signal stability were verified by simulation.

Large-Signal Stability Analysis of the Undersea Direct Current Power System for Scientific Cabled Seafloor Observatories

A large number of power electronic converters and long-distance submarine cables are an important part of the undersea direct current (DC) power system of the scientific cabled seafloor observatories (CSOs). However, the constant power load (CPL) characteristics of the converters and the distributed parameter characteristics of long-distance submarine cables greatly affect the stability of the CSO DC power system. This paper analyzes the large-signal stability of the CSO DC power system, and the equivalent circuits of long-distance submarine cables are established by theoretical analysis and computer simulation. A simplified computer simulation model and an equivalent experimental prototype model of a single-node CSO DC power system was built in the laboratory to study this issue. The mixed potential function method is used to analyze the large-signal stability of the CSO DC power system, and the large-signal stability criterion is obtained theoretically. The validity of the large-signal stability criterion is proved by simulations and experiments. The conclusion is that reducing the inductance of the submarine cable, increasing the capacitance of the submarine cable, increasing the output voltage of the shore station power feeding equipment (PFE) or reducing the power consumption of the undersea station, are beneficial to improve the large-signal stability of the CSO DC power system.

Bifurcation-Based Stability Analysis of Photovoltaic-Battery Hybrid Power System

Photovoltaic-battery hybrid power system (PBHPS) is a good candidate for rural electrification in remote areas. In this system, multiple energy sources are combined together to power the user loads under various system structures. We identify and analyze the bifurcation process when the system enters unstable operation states. Then, a small signal stability criterion is derived for the system based on reasonable simplification of the bifurcation analysis. Furthermore, according to the nonlinear dynamics of the interconnected system with closed-loop control, a large-signal stability criterion is derived with mixed potential theory. The criterion that is straightforward with the dual-loop control can be considered and easily extended to multiple connected converter systems. The criterion is also coincident with the result of bifurcation analysis. Finally, the validity of proposed criteria is verified by simulation and experimental works in a practical PBHPS of 1 kW.