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The DC solid-state transformer (DCSST) is a key component of the DC microgrid, and stability issues due to operation with constant power loads (CPLs) have become increasingly prominent. To stabilize the system of DCSST with CPLs, in this paper, a novel intelligent backstepping controller based on a nonlinear disturbance observer (NDO) is proposed, and the control coefficients of the backstepping control are automatically adjusted by an auxiliary deep deterministic policy gradient (DDPG) controller. More specifically, the NDO estimates the large-signal nonlinear disturbance of the load power variation, and a backstepping controller is designed to improve the stability of the system under the estimated large-signal disturbance. Then, an input voltage equalizing compensator is added to ensure the identical input voltage to each sub-module of DCSST. Furthermore, a DDPG sub-controller is introduced into the backstepping control to intelligently adjust the control coefficients to improve the operating efficiency of the system. The results of the simulation and experiment show that the system is more robust due to the learning ability of the proposed controller, and the proposed controller can improve the stability of the system under the large-signal disturbance and intelligently ensure fast dynamic response with accurate tracking over different operating environments. [ABSTRACT FROM AUTHOR]

This paper proposed a distributed uniform controlstrategy for parallel bidirectional interlinking converters(BICs) to realize resilient operation of hybrid ac/dc microgrid (HMG). The proposed control strategy can coordinate the control objectives of BICs in different HMG operation modes with a uniform control structure, thus, to eliminate any mode transition detections. In normal condition, the power coordination between ac and dc subgrids can be achieved, while ac or dc voltage regulation can be maintained when main voltage source(MVS) fails in ac or dc subgrid. The coordination between parallel BICs relies on low-bandwidth neighbourhood communication. With the communication planned, accurate power sharing can be achieved. Communication fault ride-through capability is enabled with less power sharing accuracy. The system reliability and resilience of HMG under various fault conditions can be improved by the proposed control strategy. To verify the effectiveness of the proposed control strategy, the controller hardware-in-the-loop experiment results have been provide in this paper. [ABSTRACT FROM AUTHOR]

In practical microgrids, the inhomogeneous initial values are widely appeared due to soft-starting operation. If traditional model order reduction approaches are applied, the input-output maps error between the original system and reduced-order system is large. To address this problem, this paper proposes a reduced-order aggregate model based on balanced truncation approach to provide the preprocessing approach for the real-time simulation of large-scale converters with inhomogeneous initial conditions in DC microgrid. Firstly, the standard linear time-invariant model with inhomogeneous initial conditions is established through non-leader multiagents concept. To end this, it is convenient for scholars to build complex system modeling with switched topology. Furthermore, the full system is divided into two components, i.e., the unforced component with nontrivial initial conditions and forced component with null initial conditions. Moreover, this paper presents an aggregated approach that involves independent reducing component responses and combining reducing component responses. Based on this, the input-output maps error is reduced. Then, the approximated error estimate of the reduced-order aggregate model regarding large-scale converters in DC microgrid is first provided, which provides prior knowledge and theoretical basis for DC microgrid designers. Finally, the simulation results illustrate the accuracy of the proposed approach. [ABSTRACT FROM AUTHOR]

Due to global climate change, the effect of extreme weather on power systems has attracted extensive attention. In the prior-art grid resilience studies, the hurricanes or wildfires are mainly defended in terms of expected line damages, while they are prone to trigger short-circuit fault (SCF) evolved with dynamic influence in reality. In this paper, a fragile model is developed to evaluate the nodal SCF probability considering the insulation aging of equipment and extreme weather condition. Then, a response framework for extreme weather events is developed for a transmission system to defend the cascading impacts of expected SCFs. Specifically, switches are shifted to restrain the out-of-range short-circuit currents (SCCs) so that to ensure the SCFs can be removed by circuit breakers, generation rescheduling and load shedding are arranged to maintain the post-fault system transient stability. The above measures are optimized simultaneously by an integrated Mixed-Integer Nonlinear Programming (MINLP). Considering the error or uncertainty of weather event forecasts, a multi-state model is established to provide the most cost-effective grid resilience enhancement scheme, in which the expected urgent adaptions of the initial scheme subject to weather state transition is included in the overall cost. The proposed model and techniques are validated using the IEEE 39-bus New-England test system and realistic meteorological data. [ABSTRACT FROM AUTHOR]

The droop control is an advantageous approach for stand-alone supply systems consisting of multiple batteries, allowing among various inverters without intercommunication. The droop coefficients of batteries always vary with their state-of-charge (SoC) and charge/discharge mode, resulting in small-signal instability. Nevertheless, the existing impedance-based approaches can only assess the droop coefficients stability point, but not the stability region. Therefore, this article proposes a droop coefficients stability region analysis approach. First, the charge/discharge SoC-based droop controlled battery, the P&Q controlled distributed generator and the constant power load are separately discussed. Meanwhile, the state matrix and return-ratio matrix are established, respectively. Furthermore, the novel forbidden region criterion based on the return-ratio matrix is constructed, which reduces conservatism compared with norm-based impedance criteria and partial forbidden region criteria. Such a forbidden region criterion is first switched to the Hurwitz identification problem regarding the equivalent return-ratio matrix. Combined the state matrix and the equivalent return-ratio matrix, the generalized incidence matrix is constructed to simultaneously identify subsystem stability and interactive stability. Based on the generalized incidence matrix, an adaptive step search strategy is proposed to obtain the droop coefficients coordinated stability region. Finally, the simulation and experimental results illustrate the validity of the proposed method. [ABSTRACT FROM AUTHOR]

The high penetration of renewable energy sources always leads to the fluctuation of the droop coefficients which are designed in inverse proportion to their rated capacity, and power electronics devices are prone to static instability. Although the impedance-based approaches have been widely studied to deal with this problem, the stability-oriented droop coefficients region identification due to the fluctuation of renewable energy sources is not provided. Thus, this article proposes an impedance-based approach to assess the droop coefficients stability region in the power system consisting of numerous distributed generators (DGs). First, the modified phase margin and opposing argument (MPMOA) forbidden criterion is constituted to acquire the complementary space of the droop coefficients stability region. The MPMOA forbidden criterion is first transformed into the condition that the generalized return-ratio matrix is Hurwitz through mirror, rotation, and translation mapping. In comparison with the previous simplified stability criteria, the conservatism of the proposed criterion is reduced a lot. Thereafter, the droop coefficients stability region is directly calculated by the generalized return-ratio matrix and guardian map theory. Eventually, the simulation and experimental results are provided to validate the conservatism and effectiveness of the impedance-based stability region identification approach. Therein, the simulation results illustrate that the proposed droop coefficients stability operation criterion has lower conservatism than these of the previous simplified stability criteria. Furthermore, the simulation and experimental results illustrate that the proposed stability-oriented droop coefficients region identification approach can provide an effective parameter stability region. [ABSTRACT FROM AUTHOR]

The hybrid fuel cell/supercapacitor (FC/SC) system is a promising onboard power supply system for more electric aircraft (MEA), where system stability is a critical issue due to the high penetration of constant power loads (CPLs) in MEA. This article proposes a composite finite-time controller for decentralized power sharing and stabilization of the hybrid FC/SC system with CPLs. It consists of an integral droop (ID) + finite-time controller for the SC converter and a proportional droop (PD) + finite-time controller for the FC converter. First, the coordination of PD and ID achieves decentralized power sharing between FC and SC such that SC only compensates fast fluctuations and FC provides smooth power at the steady state. Then, a finite-time observer is designed to provide feedforward compensation for the disturbances and enables accurate tracking with fast dynamics. Finally, a composite finite-time controller is constructed following a nonrecursive synthesis procedure with a rigorous large signal stability analysis. The proposed controller guarantees finite-time convergence even under large signal variations and can be easily implemented with a practical gain tuning procedure. Simulations and experiments are conducted to verify the proposed technique. [ABSTRACT FROM AUTHOR]

Due to the different characteristics of energy storages (ESs), proper dynamic power allocation to ESs in a hybrid energy storage system (HESS) is of high significance. To this end, integral droop (ID) controllers have been applied to a class of ESs (denoted as ES $_H$ s) with high ramp rates but low energy density. Conventional V-P droops have been adopted to regulate a class of ESs (referred to as ES $_L$ s) with high energy density but limited response speeds. Based on the cooperation of IDs and V-P droops, high/low frequency components of the load change can be autonomously responded to by ES $_H$ s/ES $_L$ s, thus possibly prolonging the HESS lifetime. However, existing works only focus on the small signal stabilization of HESSs, whose validity may only hold under certain equilibria. To address this issue, an original self-disciplined large signal stabilizing scheme is presented in this paper. Under this scheme, a compound stabilizer comprising disturbance observers and a backstepping controller is delicately designed for an ES-interfaced converter. The observers estimate the possible disturbances induced in the converter. Those estimations would be further neutralized by the backstepping controller, which simultaneously guarantees converter system stability in the large signal sense and drives the converter terminal voltage tracking the reference given by either the ID or V-P droop. Because of the proposed stabilizer, no communication happens in HESS, and the large signal stability of overall system can be spontaneously realized with rigorous mathematical proofs by Lyapunov techniques. This stabilizing scheme for HESS application is arguably reported for the first time in this article. A detailed parameter selection guideline is also provided. The effectiveness and feasibility of the compound stabilizer are verified by both simulations and hardware experiments. [ABSTRACT FROM AUTHOR]

The multilevel boost converter (MBC) has been widely adopted in the dc microgrid systems due to its high voltage gain and simple structure. In recent years, the power electronic loads, which usually behave as constant power loads (CPLs), are penetrating in microgrids. The incremental negative impedance of CPLs degrades the stability of microgrid systems. To ensure effective power flow and guarantee system stability, eliminating the undesired effects of CPLs is a necessity. In this article, a novel assorted nonlinear stabilizer, which is integrated with an extended nonlinear disturbance observer (NDO) and an adaptive backstepping controller, is proposed for stabilizing the MBC-fed microgrid system with CPLs. First, the reduced-order MBC model is transformed into the Brunovsky's canonical form using the exact feedback linearization technique. Second, owing to the NDO, fast system dynamic responses are achieved. Using the estimations of NDO, an adaptive backstepping controller is developed to strictly guarantee the microgrid bus voltage stability in the sense of a large signal. Simulation and experiment results are presented to verify the effectiveness and feasibility of the proposed stabilizer. [ABSTRACT FROM AUTHOR]

A hybrid ac/dc microgrid, which consists of alternating current (ac) subgrid(s) and direct current (dc) subgrid(s), can efficiently integrate ac and dc loads, distributed generators (DGs) with minimum conversion stages. The ac and dc subgrids are connected by the bidirectional power converter (BPC) to realize power interaction between them. This paper proposes a decentralized economic operation control for the hybrid ac/dc microgrid, which aims to realize economic power sharing among all DGs and economic power interaction between subgrids. For the DGs, the nonlinear droop controls based on ac frequency-incremental cost and dc voltage-incremental cost are proposed, which can equalize DGs’ incremental costs autonomously, and hence decrease the subgrids’ generation costs with the equal incremental cost principle. For the BPC, the economic power interaction control is proposed to optimize the power interaction between the two subgrids and further decrease the total generation cost (TGC). In addition, the economic operation is categorized into different modes according to the load conditions. The hybrid microgrid with nonlinear incremental cost droops is modeled, and the overall stability analysis is conducted to ensure stable operation in all load conditions and modes. The performance of the proposed control is verified by the real-time hardware-in-loop (HIL) test. [ABSTRACT FROM AUTHOR]

DC Microgrids have been widely used due to their high efficiency, high reliability and flexibility. A sine qua non condition for the correct operation of systems is the existence of a feasible power-flow solution. This paper analyzes the existence of the feasible power-flow solution of the DC microgrid under droop control. Firstly, the power-flow mathematical model of DC microgrid is established. Then, based on the nested interval theorem, we obtain the sufficient conditions of the existence of the feasible power-flow solution, and the uniqueness of the feasible power-flow solution is proved. Moreover, the iterative algorithm of the feasible power-flow solution is proposed, which is proved to be monotonically exponentially convergent. The proposed algorithm’s domain of attraction is derived, thus, the initial iterative value of which can easily be chosen to guarantee its convergence. Finally, case studies are given in this paper to verify the correctness and effectiveness of the proposed theorems. [ABSTRACT FROM AUTHOR]

Because of the low inertia of dc microgrids, system state variables are easily changed acutely after being disturbed. Hence, dc microgrids meet the serious transient stability issues especially for some stressed states. But the transient stability analysis is a very challenging problem since the dc microgrid system is high-order and nonlinear. To offer a new and more effective analysis framework, this paper proposes a nonlinear decoupling method to evaluate the transient stability of dc microgrids. The proposed nonlinear decoupling method takes full consideration of the nonlinearity of the dc microgrid system and approximately transforms the original nonlinear system into a series of decoupled first-order quadratic or second-order quadratic systems. For these decoupled low-order quadratic systems, their dynamics and stability can be analyzed easily, then the transient stability of the original system can be reflected indirectly. Also, the nonlinear decoupling based analysis framework can be extended to other power electronics dominated power systems to evaluate their transient stability. The accuracy of the proposed analysis method has been validated through related case studies. [ABSTRACT FROM AUTHOR]

Different from single-bus dc microgrids (MGs), a multibus MG normally bridge multiple dc buses via a complex line impedance network. This intricate configuration together with tickling constant power loads may cause severe stability issues. For system stability improvement, conventional ways include designing small-signal stabilizers in a decentralized way and constructing large-signal stabilizers by solving Lyapunov equation in a central controller. Note that centralized methods may be vulnerable to single point of failures, while decentralized patterns are commended to reinforce MG reliability and scalability. To implement large-signal stabilization in decentralized mechanisms, which has rarely been investigated in the existing literature, a novel uniform nonsmooth control scheme (UNCS) is proposed, in this article. UNCS has the following fourfold advantages. First, UNCS helps to express commonly used dc/dc converters, boost, buck, and buck–boost, into a uniform model. The model substantially facilitates a uniform stabilizer design that accommodates the preceding three converters despite their differences in topology. Second, UNCS assigns each converter based distributed generator (DG) with a composite large-signal stabilizer comprising a nonsmooth observer and a nonsmooth controller. The controller neutralizes the disturbances estimated by the observer, and it also rigorously stabilizes the internal states in DG uniform model. Third, the stabilizer also provides an additional control degree of freedom, a finetuning factor, to refine DG dynamic responses to meet high requirements for voltage and power qualities. Fourth, when numerous DGs with the proposed stabilizer congregate to build a multibus MG, the overall system large-signal stabilization is autonomously attained without communication links, which is mathematically justified by Lyapunov stability analyses. As such, DGs are allowed to randomly plug in/out without worrying about impairing system level stability. Simulations help to properly select relevant key parameters. Experiments show that UNCS outperforms the standardly designed PI controls, and it will stabilize the multibus MG in a large operation range. [ABSTRACT FROM AUTHOR]

The uncertainty of power system is intensified by the integration of large-scale renewable energy resources such as wind farm (WF). Considering the impact of system uncertainties, the probabilistic stability analysis methods have been used in the stability assessments of power system with the WF integration. However, in traditional probabilistic analysis methods, WF is normally considered as PQ bus or aggregated as one single-machine equivalent model regardless of wake effect, which might reduce the accuracy of probabilistic stability studies. In this article, a dynamic equivalent modeling method of WF for probabilistic stability assessments is proposed. The wake effect is considered in the modeling process and a practical four-machine clustering method is used in wind turbines clustering. Besides, the fisher discriminant analysis (FDA) is adopt to combine the similar WTs clustering results reasonably. Then, the WF is aggregated to a multi-machine equivalent model by capacity weighted method. Also, the established WF probabilistic model can be directly used in time-domain simulation based on Monte Carlo simulation and FDA. Finally, the efficiency of the proposed method is verified in an actual WF in China. [ABSTRACT FROM AUTHOR]

A virtual synchronous generator (VSG) control based on adaptive virtual inertia is proposed to improve dynamic frequency regulation of microgrid. When the system frequency deviates from the nominal steady-state value, the adaptive inertia control can exhibit a large inertia to slow the dynamic process and, thus, improve frequency nadir. And when the system frequency starts to return, a small inertia is shaped to accelerate system dynamics with a quick transient process. As a result, this flexible inertia property combines the merits of large inertia and small inertia, which contributes to the improvement of dynamic frequency response. The stability of the proposed algorithm is proved by Lyapunov stability theory, and the guidelines on the key control parameters are provided. Finally, both hardware-in-the-loop and experimental results demonstrate the effectiveness of the proposed control algorithm. [ABSTRACT FROM AUTHOR]

Bus voltage control is an essential issue in a DC microgrid (MG). In this paper, a decentralized composite controller (DCC) is proposed to unify two types of voltage controls, i.e., constant voltage mode and droop mode, and simultaneously realize global system large-signal stability of the MG. The main idea of the DCC is to partition the MG into dispatchable unit (DU) subsystems and the lumped load. A well-devised high gain observer is adopted to estimate the electrical coupling of a particular DU subsystem with other DUs and the load. Then, rather than dealing with the MG as a whole, the DCC is locally designed to offset the estimated coupling and stabilize the internal states of the DU subsystem. By doing so, each subsystem can be virtually decoupled from the MG and functions in an isolated fashion. When DU subsystems are interlinked, large-signal stabilization of the entire MG would be obtained by only guaranteeing the localized stability of individual subsystems. This stabilizing strategy has not been reported in the literature thus far. Relevant theoretical analyses are rigorously conducted by employing Lyapunov stability theorem. The effectiveness of the DCC has been verified by simulations. Experimentations show that the DCC enables faster system performance recovery and provides wider stability margin than the conventional PI controller. [ABSTRACT FROM AUTHOR]

Photovoltaic (PV) generators have continuously increased in recent years, whose power is usually controlled through the perturbation and observation (P&O) method. In essence, the P&O method is nonlinear and discontinuous. Hence, the conventional small-signal stability analysis is not suitable anymore when the influence of the P&O-based power control is considered. Focusing on this problem, this paper adopts the nonlinear describing function (DF) method to conduct the accurate stability analysis of PV generators with consideration of P&O-based power control. The detailed procedures about the DF method are introduced, and then the related influence factors like perturbation size, filters, and so on are analyzed quantitatively. Furthermore, the comparison with the conventional stability analysis methods is made, which suggests that the DF method can effectively enhance the accuracy of the stability analysis. All the conclusions are verified by the real-time hardware-in-loop tests. [ABSTRACT FROM AUTHOR]

Transportation electrification involves the wide utilization of power electronics based dc distribution networks and the integration of a large amount of power electronic loads. These power electronic loads, when tightly controlled, behave as constant power loads (CPLs) and may cause system instability when interacting with their source converters. In this paper, a composite nonlinear controller is proposed for stabilizing dc/dc boost converter feeding CPLs by integrating a nonlinear disturbance observer (NDO)-based feedforward compensation with backstepping design algorithm. First, the model is transformed into the Brunovsky’s canonical form using the exact feedback linearization technique, to handle the nonlinearity introduced by the CPL. Second, the NDO technique is adopted to estimate the load power variation within a fast dynamic response, serving as a feedforward compensation to increase the accuracy of output voltage regulation. Then a nonlinear controller is developed by following the step-by-step backstepping algorithm with strictly guaranteed large signal stability. The proposed controller not only ensures global stability under large variation of the CPL but also features fast dynamic response with accurate tracking over wide operating range. Both simulations and experiments are conducted to verify the proposed strategy. [ABSTRACT FROM AUTHOR]

Nowadays, with the highly penetrated wind generations, the accurate wind farm (WF) model is required for power system stability analysis. Due to the complexity on detailed WF model, the aggregated model, with a reasonable reduction of the detailed model meanwhile retaining the required level of accuracy is essential to be developed. In this paper, an improved WF aggregated modeling method for large-scale power system stability studies is proposed. To overcome the limitations of the traditional methods, a geometric template matching based time series wind turbines clustering method is developed. Moreover, a multiobjective optimization algorithm, which fully considered the wind speed disturbance and system fault together, is designed to identify both the generator and control parameters. Additionally, to shrink the size of the identified parameters and increase the modeling accuracy, a sensitivity and correlation analysis based key parameters selection scheme is also adopted. To verify the effectiveness of the proposed method, dynamic responses of the proposed aggregated model are compared against the responses of the traditional equivalent model for various wind scenarios through an actual case. [ABSTRACT FROM AUTHOR]

Due to the voltage fluctuation resulting from variable wind energy and reactive load change in an isolated wind-diesel hybrid system, an additional adaptive voltage compensation controller for static var compensator (SVC) by using double sliding-mode (SM) optimal strategy is proposed to maintain the system bus voltage stability. Considering the uncertainties of SVC model parameters and unobservable state variables, the SM algorithm is used to design the robust state observer and SVC adaptive voltage controller. The mathematical state model of isolated wind-diesel hybrid power system together with SVC is established, and then, the SM robust observer is constructed to estimate the SVC state variables. In order to improve the system bus voltage stability, an additional robust adaptive voltage controller is designed for SVC by selecting appropriate switching surface and adaptive reaching condition. The proposed control strategy is shown to be effective through opal-RT OP5600 simulator together with TIF28335 DSP for the isolated wind-diesel system under different operation point. [ABSTRACT FROM PUBLISHER]

When a short-circuit fault occurs in a distribution network, speed and reactive power consumption of an induction generator (IG) increase dramatically, which may cause system transient stability problems. This paper proposes an improved steady-state equivalent circuit method to determine transient stability of a distribution system or a micro-grid with multiple IGs. Interaction between IGs and distribution network during a fault is investigated. The relationship between network parameters and speeds of IGs is derived using the steady-state equivalent circuits of IGs. The critical speed and critical fault clearing time (CCT) for maintaining system stability are determined using the proposed technique. The factors, which affect transient stability of a multi-IG distribution system, are investigated. The correctness of the proposed method is verified using dynamic simulation. [ABSTRACT FROM AUTHOR]

Based on the decentralized sliding mode control, a load frequency controller is designed in this paper for multi-area interconnected power systems with matching and unmatched uncertainties. The proportional and integral switching surface is constructed for each area to improve system dynamic performance in reaching intervals. The robust controller is proposed by the reaching law method to assure that frequency fluctuation converges to zero after a load and operation point variation. A three-area interconnected power system is studied to illustrate the effectiveness of the proposed decentralized sliding mode control scheme. [ABSTRACT FROM PUBLISHER]

This paper concentrates on the design, control, and implementation of an LCL-filter-based shunt active power filter (SAPF), which can effectively compensate for harmonic currents produced by nonlinear loads in a three-phase three-wire power system. With an LCL filter added at its output, the proposed SAPF offers superior switching harmonic suppression using much reduced passive filtering elements. Its output currents thus have high slew rate for tracking the targeted reference closely. Smaller inductance of the LCL filter also means smaller harmonic voltage drop across the passive output filter, which in turn minimizes the possibility of overmodulation, particularly for cases where high modulation index is desired. These advantages, together with overall system stability, are guaranteed only through proper consideration of critical design and control issues, like the selection of LCL parameters, interactions between resonance damping and harmonic compensation, bandwidth design of the closed-loop system, and active damping implementation with fewer current sensors. These described design concerns, together with their generalized design procedure, are applied to an analytical example, and eventually verified by both simulation and experimental results. [ABSTRACT FROM PUBLISHER]

Reactive power plays a significant role in power system operation. However, in reliability evaluation, attention has seldom been paid to reactive power. In conventional power system reliability evaluations, the fixed maximum and minimum values are applied as the reactive power limits of generators. Failures of reactive power sources are rarely considered. The detailed causes of network violations for a contingency are also seldom studied. Real power load shedding is usually used to alleviate network violations without considering the role of reactive power. There are no corresponding reliability indices defined to represent the reactive power shortage in the existing techniques. Reactive power shortage and the associated voltage violations due to the failures of reactive power sources are considered in this paper. New reliability indices are proposed to represent the effect of reactive power shortage on system reliability. The reliability indices due to reactive power shortages have been defined and are separated with those due to real power shortages. Reactive power limits determined by real power output of a generator using P-Q curve have been studied. A reactive power injection technique is proposed to determine possible reactive power shortage and location. The IEEE 30-bus system has been modified and analyzed to illustrate the proposed technique. The results provide system planners and operators very important information for real and reactive power management. [ABSTRACT FROM AUTHOR]