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This paper proposes a novel imbalance-status (IS)-enabled autonomous global power-sharing scheme for a solid-state transformer (SST) interconnected hybrid AC / DC microgrid system. Firstly, the proposed scheme globally estimates instantaneous active power imbalances according to the existing droop regulation characteristics of each sub-microgrid, and normalizes these imbalances by introducing the definition of IS. Next, three novel IS-enabled controllers are designed for the SST’s three core converters, to autonomously share the power imbalances in the entire system. Then, the control parameters of the proposed scheme are optimized through small-signal analysis. The control effectiveness and flexibility of the proposed scheme in realizing global power-sharing function are verified by controller hardware-in-loop experiments in RT-LAB platform.

This article proposes a novel inertia emulation and fast frequency-droop (IEFF) control strategy for a point-to-point voltage source converter-based high voltage direct current (VSC-HVdc) transmission system. It allows asynchronous ac grid synchronization in forms of inertial and fast frequency-droop responses provided to 'both' sides of the VSC-HVdc interconnected grids, different from the historically reported schemes only providing these responses to one grid. Under the IEFF scheme, the power-regulating VSC (PR-VSC) station emulates a synchronous generator directly through its active power control, whereas the dc-voltage-regulating VSC (DR-VSC) station employs a dc voltage/frequency droop control to convey its connected grid frequency communication-free to the remote PR-VSC for inertial and fast frequency-droop oriented power variations. To avoid the clash of the PR-VSC IEFF and DR-VSC IEFF control under simultaneous frequency events in the asynchronous grids, a novel frequency support mode selection scheme is designed to effectively prioritize the IEFF support with a higher frequency gradient. The key parameters of the IEFF controller are optimized through small signal stability analysis. The effectiveness of the proposed IEFF scheme is verified through controller hardware-in-the-loop experiments. The results show that the proposed IEFF scheme effectively promotes synchronization and enhances overall frequency stability for the two interconnected asynchronous grids.

In this paper, a novel energy flow analysis (EFA) is proposed based on the signal reconstruction and decomposition to investigate the electromechanical oscillations. In contrast to the conventional EFA, the connection between the proposed EFA and modal analysis (MA) can be quantitatively revealed for arbitrary models of synchronous generators in multi-machine power systems. Firstly, the time-domain implementation (TDI) of the proposed EFA is designed. Specifically, the measurements at the terminal of a local generator are reconstructed through an exponential operator and then decomposed with respect to an angular frequency. Then, the mode-screened damping torque coefficient is defined to extract the damping feature with respect to an electromechanical oscillation mode. After that, the frequency-domain implementation (FDI) is derived. Specifically, the Parsevals Theorem is applied to transform the proposed EFA from the time domain to frequency domain. On this basis, the consistency between the proposed EFA and MA is strictly proved, which is applicable for arbitrary models of synchronous generators in multi-machine power systems. Additionally, the application procedure of the proposed EFA in investigating electromechanical oscillations is given. Finally, the proposed EFA is substantially demonstrated in multiple case studies.

This paper proposes a novel variable-inertia emulation control (VIEC) scheme that enables voltage-source-converter based high-voltage DC (VSC-HVDC) transmission systems to flexibly support AC grid frequency stability like synchronous generators. The VIEC scheme allows us to extract the energy from the augmented DC capacitance for inertia emulation by controlling the DC voltage without affecting the stability of the AC system connected on the remote side. In particular, with the proposed VIEC scheme, the inertia time constant can be flexibly emulated and its value can be automatically adjusted according to the rate of change of grid frequency as well as grid frequency deviations. This leads to more effective inertial support across different timescales. Modal analysis is carried out to investigate the impacts of inertia and DC capacitance, and to obtain the optimal control parameters. The effectiveness and advantages of the proposed VIEC scheme are demonstrated on an IEEE benchmark system in the presence of faults and load changes.

Hybrid HVDC system, which comprises a line commutated converter (LCC) at rectifier side and a voltage source converter (VSC) at inverter side, is drawing wide attention due to its high reliability and economic benefit. For such a new transmission technology, the system characteristics are not comprehensively studied, and inappropriate control parameters may be coherent to system instability. This paper puts forward a novel coherence analysis procedure which analyzes system stability based on a verified hybrid HVDC small-signal model. For the first time, the procedure is proposed to capture the coherence relationships between the system characteristics and control parameters. The analysis results reveal: the circuit and control parameters can highly affect the oscillatory modes and even cause instability. Moreover, the variation of grid Short Circuit Ratio (SCR) at LCC rectifier side trades off the response speed and damping of the oscillatory modes - under low SCR, large proportional and small integral coefficients of the LCC DC current control are suggested to enhance response performance. Also, the increase of VSC DC capacitance reduces the mode response speed - for a large capacitance, large proportional and small integral coefficients of the VSC DC voltage control shall be set to guarantee system stability.

The rapid growth in penetration levels of powerelectronics-interfaced renewable power generation into grids have been reducing system kinetic inertia, which results in degraded frequency stability and oscillation damping. In an attempt to address this challenge, this paper proposes a control scheme which realizes coordinated flexible damping mechanism (CFDM) with inertia emulation capability for modular multilevel converter based multi-terminal dc (MMC-MTDC) transmission systems. The proposed CFDM scheme allows the MMC-MTDC system to autonomously provide emulated inertial response with flexible damping effect to ac systems in a similar fashion of synchronous generator, without the need for communication. To ensure the precision of the total provided damping power, the power losses of the converter stations and MTDC networks are also compensated in the algorithm. The CFDM controller parameters are optimized by the small signal stability analysis. The effectiveness of the proposed CFDM scheme is demonstrated in a power system model simulated in MATLAB/Simulink which consists of a three-terminal MMC-MTDC transmission system and three asynchronous ac girds. It is verified that the proposed CFDM scheme can effectively minimize grid frequency deviation and damp angular oscillation in grid events of load changes and faults.

To mitigate the resonance between the full converter-based wind generation (FCWG) and the grids, their modal interaction is usually coordinated off-line and requires the full system model. In this paper, an on-line coordination strategy is proposed to achieve appropriate modal coordination in a model-free manner. Firstly, the dynamic interaction mechanism is examined to quantify the modal interaction impact, which provides theoretical foundation of how FCWG controller parameter tuning contributes to modal coordination. With phasor measurement unit (PMU) equipped in the power grid, the essential oscillation information (e.g. the critical mode that relates to modal resonance) is acquired without detailed modeling of the power grid. According to the frequency range of critical mode, the proper controller in FCWG is selected for parameter tuning. On this basis, we employ an on-line coordination strategy by adjusting relevant controller parameters to accommodate corresponding changes of system operational conditions and thus mitigate the potential risk of modal resonance. Case studies are carried out to demonstrate how FCWG adjusts the controller parameters timely to improve the oscillatory stability under various operating conditions, and thus in turn substantiate the robustness and adaptability of the proposed strategy.

Droop control has been well adopted for multi-source multi-load DC microgrids (MGs) due to its inherent modularity and reliability by only using local measurements, especially with the most commonly-used voltage-current droop (V-I) mode. While another implementation named current-voltage droop (I-V) mode also can be adopted. With the same value of droop gain in these two modes, a DC MG will have the same steady-state feature, but its dynamic performance of DC voltage control may be different. Our primary motivation is to investigate this phenomenon from the perspective of equivalent RLC circuits. This paper proposes a generic reduced-order modeling method suitable for exploring the dynamic stability of DC voltage control with these two modes. By ignoring fast inner current control dynamic, each droop based DC voltage control unit can be modeled as a RLC parallel circuit in these two modes, which is convenient for modular modeling and extension. In addition, if the inner current control with slow dynamic cannot be ignored in some specific scenarios, a modified RLC model can be still obtained to analyze its influence. Tthe effectiveness of the proposed reduced-order modeling method has been verified by detailed simulation and experiment results.

Increasing penetration level of renewable energy results in unprecedented operational challenges of AC grids due to generation uncertainty and reduced inertia. In grid emergencies, Fast active Power output Reduction (FPR) is demanded to drop wind turbine generator (WTG) power outputs faster than usual. However, the power mismatch between WTG mechanical power input and reduced electrical power output resulted from FPR can lead to rotor overspeed, which cannot be timely nullified by slow‐responding pitch control. Therefore, this paper proposes a novel FPR scheme for doubly‐fed induction generator (DFIG)‐based WTG through coordinated control of DC chopper and pitch angle. Specifically, the FPR command is executed by DFIG rotor‐side converter control, while rotor overspeed is restricted by triggering the DC chopper to dissipate the mismatched power. The pitch angle is actuated at a maximal rate to eliminate the mismatched power and switch off the DC chopper. The effectiveness of the proposed FPR scheme is verified through comparative simulation studies under FPR command execution and a grid fault. It shows the proposed scheme can achieve FPR in tens of milliseconds, avoid rotor overspeed and provide low‐voltage ride‐through capability in a fully‐controlled manner.

As a key application of smart grid technologies, the smart distribution network (SDN) is expected to have a high diversity of equipment and complexity of operation patterns. Situational awareness (SA), which aims to provide a critical visibility of the SDN, will enable a significant assurance for stable SDN operations. However, the lack of systematic evaluation through the three stages of perception, comprehensive, and prediction may prevent the SA technique from effectively achieving the performance necessary to monitor and respond to events in SDN. To analyze the feasibility and effectiveness of the SA technique for the SDN, a comprehensive evaluation framework with specific performance indicators and systematic weighting methods is proposed in this paper. Besides, to implement the indicator framework while addressing the key issues of human expert scoring ambiguity and the lack of data in specific SDN areas, an improved interval-based analytic hierarchy process-based subjective weighting and a multi-objective programming method-based objective weighting are developed to evaluate the SDN SA performance. In addition, a case study in a real distribution network of Tianjin, China is conducted whose outcomes verify the practicality and effectiveness of the proposed SA technique for SDN operating security.

The integration of full converter-based wind power generation (FCWG, e.g., permanent magnet synchronous generator (PMSG)) not only introduces the PMSG oscillation modes (POMs) but also might excite severe resonances with electromechanical oscillation modes (EOMs) of the power system. In this paper, a two-open-loop-subsystem dynamic model is firstly established to investigate the interactions between the PMSG and the rest of the power system. On this basis, a modal shift evaluation (MSE) method by using bilateral damping torque analysis is proposed to accurately quantify the interaction effect of POMs and EOMs on each other and effectively explain their complex interaction process. Then two important concepts, i.e., modal shift sensitivity (MSS) with respect to various PMSG controller parameters and resonance excitation index (REI) according to a per unit open-loop modal distance indicating the intensity of modal interactions, are derived to dig the essential modal resonance mechanisms. Furthermore, by using MSS and REI as two tools, the modal interaction optimization (MIO) is conducted through POM tuning in order to prevent potential system modal resonance and enhance resonance mode damping for the first time. The optimized modal interaction is validated to be beneficial and effective for the improvement of power system resonance stability.

This paper presents a flexible model and testing arrangement that can be used to mimic the fault infeed characteristics of power electronics converters and evaluate the performance of transmission protection schemes to faults when the system is “converter-dominated”. That is, sources (e.g. renewables) and infeeds (e.g. HVDC interconnectors) that are interfaced to the main power system via converters. Actual protection relays are injected, in real time, using the outputs of the various fault simulations. A range of potential protection issues, including slow tripping, erroneous discrimination and non-operation, are illustrated for particular scenarios. The main contributions of the paper include knowledge of how different protection schemes may be affected by converter-interfaced sources and guidance on possible solutions to the observed problems. Two solution options are highlighted: changes to the fault detection methods used within relays, and/or defining the fault-response elements of future grid codes (and therefore future converter fault responses) to ensure that existing protection schemes will not be adversely affected in future low-carbon, converter-dominated systems.

Uncertainties, which are introduced by environmental conditions in renewable systems, result in unstable energy supply and demand. For a solar heating system, the seasonal variation in the ambient temperature and radiation critically influence system performance. Meanwhile, the current design parameters for improving robustness, based on the empirical tolerance, are unreasonable, which can lead to an overestimation of the facility’s capacity. In addition, the effects of system control parameters, always assumed in the design stage, on the uncertainties of the system are not given more attention. In this study, a theoretical model was chosen to indicate the uncertainties of the design parameters in a complementary heating system by integrating solar energy and an air-source heat pump (ASHP), and a method was proposed to determine the design radiation by considering the uncertainties of the system. First, a Monte Carlo simulation was applied to propagate the input uncertainties of the solar fraction and system efficiency. Then, the input parameters were ordered by significance to the system performance based on sensitivity analysis, to improve the economic dependability of the design. Finally, the economic design radiation values of each district are decided under local calculated outdoor temperature of the enclosure structures in winter. Moreover, the proposed methodology could also be a guide for using uncertainties to determine the optimal design parameters for other renewable systems.

Learning from past accidents in railway operations is valuable for ensuring the future safety of railway operations. Railway operational accidents are of different types, such as collisions and derailments. Different types of railway operational accidents are related to each other due to the interactions between hazards leading to accidents. It is useful to explore the nature of accidents as a set. In this paper, a new network theory-based approach to understanding railway operational accidents is proposed, which aims to reveal latent patterns of hazards from an overall high-level perspective. This approach serves as a complement to conventional network theory-based analyses. Its originality is in the customization of a topological analysis for studying accidents, with several tailored indicators adapting to the characteristics of railway operational accidents. It also provides a practical way to extract and construct the accident causation network from numerous accident investigation reports. The outcomes of this approach could assist railway operators in formulating more targeted accident prevention strategies and approaches. The method has been applied to real railway operational accidents in the UK. The results show that the proposed approach is effective and practical in terms of capturing important causes of accidents and revealing latent rules of railway operational accidents.

A cluster with multiple AC or DC microgrids (MGs) based on flexible DC interlinking and distributed energy storage systems is studied in this paper, which has been identified to have the following issues in terms of its primary control scheme: coordinated power control among slack terminals in the MG cluster, seamless operation mode transition for the interlinking converters and the common DC bus voltage control. A novel flexible control scheme that unifies the control of all the MGs only using local measured signals without further communications is proposed in this paper, which aims to improve control stability, and operation reliability and flexibility. Theoretical introduction and analysis are presented to explain the proposed control. The detailed average value model and small-signal model of an MG cluster with two AC MGs and two DC MGs are simulated, respectively, with the proposed control in power systems computer-aided design/electro-magnetic transient design and control to verify the effectiveness in control stability and flexibility enhancement, and the impact of control parameters on the small-signal stability of the considered cluster is analysed through root locus.

Hybrid ac/dc microgrids (MGs) integrated with traditional diesel generators, distributed energy storage systems (ESSs), and high penetration of renewable energy sources (RESs)-based distributed generators (DGs) have become an attractive power supply solution for isolated remote areas and islands, which can effectively reduce environmental protection pressure and improve power supply reliability. However, in such inherent low-inertia systems, randomness and fluctuation of the output power of RESs and uncertain load consumption can easily incur dynamic stability issues, such as transient power impact, unacceptable frequency deviations, and operation mode transitions for security. To solve the above problems and enhance the dynamic stability of the system, an enhanced dynamic stability control (EDSC) scheme with locally measured signals only is proposed in this paper. In this control scheme, the bi-directional interlinking dc-ac converter uses the ac frequency in ac MG as the reference value of dc voltage and adopts the current feedforward control to control the dc voltage in the dc MG to be consistent with the ac frequency. By electrically coupling dc voltage and ac frequency, power disturbances in ac or dc sides will cause almost identical variation degrees in both ac frequency and dc voltage. Under the proposed EDSC scheme, the distributed ESSs in both ac and dc sides are then automatically coordinated and controlled by the unified droop control to balance transient power disturbances and smooth output of diesel generator under normal condition, which can effectively improve stability and controllability of such low-inertia systems. Furthermore, in an emergency such as failure of diesel generator, the operation mode can be switched seamlessly with the proposed EDSC scheme. The detailed theoretical analysis including control system design, small signal model analysis of key parameter influence on system dynamics, and simulation verifications in the PSCAD/EMTDC environment is presented to verify the effectiveness and practicality of the proposed EDSC scheme.

For a complex multi-terminal DC (MTDC) transmission system, controller parameter settings exert tremendous influence on the system dynamics and transient stability. The traditional droop control scheme, which aims to locally control the converter's power by regulating its DC voltage according to the pre-defined droop slopes, does not normally employ communications for coordination with other terminals. The droop scheme is designed for power sharing purpose and its controller gains' impact on the MTDC network voltage is usually ignored. Moreover, the droop controller parameters are selected without considering various operating states of the MTDC terminals. This paper proposes a droop controller parameters select the criterion to cater to MTDC network voltage stability. To do so, a detailed small-signal model for a four-terminal MTDC system candidate with typical network topology, considering each converter station's electrical characteristics and cascaded control loops, is established. In order to maintain the fidelity for DC network modeling, the model also takes account of the DC network electrical dynamics and losses. A time-domain simulation model is built in Matlab Simulink to verify the correctness of the small-signal model.

Multi-Terminal DC (MTDC) transmission systems typically operate under active power and DC voltage ( $P$ - $U_{DC}$ ) droop control for active power dispatch, and its design is greatly influenced by DC network characteristics. Hence, this paper firstly analyzes $P$ - $U_{DC}$ droop characteristics when implemented on Grid-Side Voltage-Source Converters (GSVSCs) of a MTDC network, in which variations of DC cables’ resistances with temperature are taken into account. The analysis establishes that the actual droop characteristics of GSVSCs are nonlinear, different from the widely assumption of linear relationships. This finding is the fundamental for the design of the accurate power dispatch for GSVSCs. Therefore, an improved Fuzzy Search based Droop Control (FSDC) scheme for MTDC systems is proposed, which performs real-time search to capture the accurate droop curves of GSVSCs over full range of permissible DC voltage and active power. The FSDC is implemented without the assumption of the prior knowledge of DC network parameters which are subject to variations and uncertainty, and instead the accurate droop curves are periodically obtained and updated. The technical viability of the FSDC and its superiority over the conventional linear droop scheme under the variations of DC cable resistances and power flow variations is verified by a four-terminal HVDC model established in MATLAB/Simulink.

This paper investigates power imbalance mitigations by grid impedances under multiple-point inertial responses of non-synchronous generation (NSG) with positive and negative power points. Firstly, an inertial normalization method is put forward to unify inertia time constants at the same base of kinetic energy for all generators. Then, a network reduction method is applied to converter complex network topology to simpler radial form with one center point for the ease of grid impedance losses analysis. Next, a novel theory is established to analyze total grid impedance losses under multiple-point synthetic inertial responses of NSG with different power points when being subject to same rate of change of grid frequency, which implies that heavier total inertia in the generation with negative power points (importation) results in grid impedance losses variations to counteract overall grid imbalance. This conclusion is effectively validated verified in the model of IEEE 3 machine 9-bus system.

This paper is devoted to tuning of proportional resonant (PR) controller in a three-phase four-leg voltage source inverter (VSI) by means of self-tuning method. The purpose of this tuning is to improve the quality of output voltage of the VSI which connects distribution generation (DG) units to micro grid along with that independently control each phase load voltage. The controller scheme is composed of an inner current control loop to control output current and an outer voltage control loop to control output voltage using proportional and practical swarm optimization (PSO) based PR regulator. PSO is one of the widely used intelligent searching algorithm that is applied for self-tuning of the power control parameters. The proposed method shows good tracking of the reference output voltage in the static and dynamic modes and satisfy the voltage quality requirements and proves the validity of the proposed strategy. Studies of different loads on VSI are carried out using MATLAB/SIMULINK, simulation results demonstrate the effectiveness of the proposed approach.

This paper investigates the stability of diode rectifier based HVDC (DR-HVDC) connected offshore wind farms. The impedance model of the DR-HVDC connected offshore wind farms in dq frame is analytically developed and is validated by comparing its impedances to those obtained from frequency scanning of time domain EMT model in PSCAD/EMTDC. Based on the impedance model, the impacts of the sizes of DR-HVDC DC smoothing reactors and AC filters on offshore system stability are analyzed. It shows reduced DC smoothing reactance degrades the d-axis stability margin and can potentially lead to system instability. Higher bandwidth of offshore AC voltage control of wind turbine line side converter is proposed to minimize such instability potential. The analysis also shows that reduced AC filters size helps to increase q-axis stability margin and leads to a more stable system. Moreover, stability analysis considering the interaction between WTs is conducted and it reveals that the WT P-V control has a significant impact on the stability of the DR-HVDC connected WTs.

As one key content of future grid development, the methodological rationality of power grid planning has crucial impact on the operational security and stability for the power grid. With rapid development of wind power and photovoltaics, the wind-solar-coal bundled transmission pattern is becoming an inevitable trend in order to balance the mismatch between renewable energy generation and load demand in China, which has caused great challenges to power gird planning. In order to address the uncertainty of wind power generation, a two-level planning model considering N-1 security constraint is established in this paper which divides the grid planning problems into four typical scenarios considering the seasonal characteristics of wind power outputs. The high-level model, comprehensively considering the operational efficiency, aims to minimize the costs of wind-solar-coal bundled power transmission under various operating conditions while the low-level model aims to minimize the load shedding based on N-1 security analysis. Both the high-level and low-level models take N-1 constraint as the primary constraint and the best planning scheme can be obtained through iterations. According to the given planning scheme and the actual operation scenarios, the final scheme can be optimized and selected.

With the rapid development of ultra-high voltage direct current (U-HVDC) transmission systems, the security and stability control system (SSCS) plays an increasingly important role in ensuring the reliable operation of the power grid as the second line of defense for power system protection. To analyze the impact of SSCS overall architecture on the system reliability, firstly, a reliability analysis method for the SSCS is proposed to correlate the hardware, software and communication channel of the U-HVDC's SSCS under different U-HVDC's SSCS operation modes by establishing a fault tree model of the SSCS. Secondly, the sequential Monte Carlo method is employed to analyze the reliability of the U-HVDC's SSCC model considering the failure rate and maintenance time for each part of the SSCS. The proposed fault tree model and reliability analysis method is used to simulate a real case of the U-HVDC's SSCS commissioned in China. The result compares the reliability of the system architecture under different operation modes and provides significant insights on the selection rules of the U-HVDC's SSCS operation mode for system reliability improvement.

Previous studies generally consider that the full converter-based wind power generation (FCWG) is a “decoupled” power source from the grid, which hardly participates in electromechanical oscillations. However, it was found recently that strong interaction could be induced which might incur severe resonance incidents in the electromechanical dynamic timescale. In this paper, the participation of FCWG in electromechanical dynamics is extensively investigated, and particularly, an unusual transition of the electromechanical oscillation mode (EOM) is uncovered for the first time. The detailed mathematical models of the open-loop and closed-loop power systems are firstly established, and modal analysis is employed to quantify the FCWG participation in electromechanical dynamics, with two new mode identification criteria, i.e., FCWG dynamics correlation ratio (FDCR) and quasi-electromechanical loop correlation ratio (QELCR). On this basis, the impact of different wind penetration levels and controller parameter settings on the participation of FCWG is investigated. It is revealed that if an FCWG oscillation mode (FOM) has a similar oscillation frequency to the system EOMs, there is a high possibility to induce strong interactions between FCWG dynamics and system electromechanical dynamics of the external power systems. In this circumstance, an interesting phenomenon may occur that an EOM may be dominated by FCWG dynamics, and hence is transformed into a quasi-EOM, which actively involves the participation of FCWG quasi-electromechanical state variables.