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This paper presents a novel cooperative control technique concerning fully-distributed AC/DC microgrids. Distributed generation based on inverters has two types, i.e., Current Source Inverter (CSI), also referred to as PQ inverter, and Voltage Source Inverter (VSI). Both inverter forms have a two-layer coordination mechanism. This paper proposes a design method for the digital Proportional-Resonant (PR) controller that regulates the current inside an inverter. The inverters will improve the voltage quality of the microgrid while maintaining the average voltage of buses at the same desired level. There is comprehensive detail on the computations specific to resonant and proportional gains and digital resonance path coefficients. The paper includes a digital PR controller design and its analysis in the frequency domain. The analysis is based on the w-domain. The main contribution of this paper is the proposed method, which not only focuses on the transient response but also improves the steady-state response which smoothens the voltage; furthermore, all inverters are effectively involved to increase the capacity of the microgrid for better power management. The suggested cooperative control technique is used on an IEEE 14-bus system having fully distributed communication. The convincing outcomes indicate that the suggested control technique is an effectual means of regulating the microgrid's voltage to obtain an evener and steady voltage profile. The microgrid comprises distributed resources and is used as the primary element to analyse power flow and quality indicators associated with a smart grid. Lastly, numerical simulation observations are utilised for substantiating the recommended algorithm. [ABSTRACT FROM AUTHOR]

With the rapid growth of renewable energy and power electronic loads, voltage source converters (VSCs) have been commonly applied in different VSCs-based power systems, which also bring in stability issues due to their power electronic characteristics. The impedance-based stability theory and corresponding reshaping method can be used to analyze and solve system stability issue. However, dc stability issue in VSCs-based power systems becomes more complicated when considering ac/dc coupling relationship. This paper proposes an improved reshaping control strategy based on the detailed impedance model considering grid impedance and control delay. The influence of grid impedance, control delay and control parameters of VSC on dc stability are further analyzed to identify the cause of potential resonance risk. The proposed impedance reshaping method considers the trade-off between stability improvement and control performance, and compensates the impacts of grid impedance and control delay at the same time. The validity of stability analysis and reshaping method is further verified through the simulation and experimental results. [ABSTRACT FROM AUTHOR]

A modular multilevel converter control system, based on converter energy storage, is proposed in this paper for two different control modes: active power and dc voltage. The proposed control system decouples the submodule (SM) capacitor voltages from the dc bus voltage. One of the practical applications is the management of active redundant SMs. A practical HVDC system with 401-level MMCs, including 10% redundancy in MMC SMs, is used for validating and demonstrating the advantages of the proposed control system. This paper also presents a novel capacitor voltage balancing control based on \max–\min functions. It is used to drastically reduce the number of switchings for each SM and enhances computational efficiency. [ABSTRACT FROM AUTHOR]

This paper establishes an instantaneous voltage model to illustrate the dynamic characteristics of grid voltage in electromagnetic time scale affected by power-electronic interfaced voltage regulatory devices with two typical categories of control systems, i.e., power-current cascading control and power synchronization control, exemplified by the STATCOM and virtual synchronous condenser (VSCON) respectively. In this paper, a linearized instantaneous voltage model is firstly established to analyze the impact paths of control loops in the STATCOM and VSCON. Based on the model, the dynamic process of grid voltage is divided into three stages. Various factors on the dynamic process of grid voltage are analyzed, especially cascading control loops prior to the functioning of AC voltage control. Moreover, stiffness characteristic is proposed to evaluate the effects of power electronic interfaced voltage regulatory devices with different control structures. In addition, the effectiveness of the stiffness compensation based on the VSCON is validated by the study cases. [ABSTRACT FROM AUTHOR]

The papers in this special section brings together papers focused on the recent advancements and breakthroughs in the technology of modeling and enhanced active/reactive power control of wind power conversion systems, ranging from components of wind turbines to wind farms. [ABSTRACT FROM PUBLISHER]

A converter monitoring unit (CMU) that monitors the power modules of the converter in a multimegawatt test wind turbine is presented in this paper. A method of calculating the virtual temperature ($T_{v}$) of the power semiconductors is laid out. Contrary to previous work, the method enables $T_{v}$ characterization of the semiconductors by means of sampled data during normal operation of the converter, without the need for special calibration routines. The dynamic operation of the wind turbine and the large amount of data sampled in the CMU enables a statistical approach to generate reference data for $T_{v}$ calculations. The method was tested on machine side converter diodes of the test turbine, based on samples of collector-emitter voltage ($vce_{\text{on}}$) and current ($i_{c}$). Other temperature sensitive electrical parameters may also be used as input to the method. The CMU has been operating for more than a year in the test turbine, showing consistent data throughout the whole period. While the monitoring method and results are based on measurements on a single test turbine, the paper also link these to converter failure data from a large fleet of operating turbines. [ABSTRACT FROM AUTHOR]

Urban power grids are facing many operational and expansion challenges to meet further demand growth and increased reliability requirements. Advanced transmission technologies have been considered by the electric utilities to effectively increase the utilization of existing infrastructure and operational flexibility. The focus of this paper is on VSC-HVDC technologies for urban power grid enhancement and modernization. First, a potential technical scheme is proposed for converting an existing AC circuit to DC operation, which could boost the power transfer capability of the critical transmission corridor and increase network operational flexibility. Second, this paper proposes three operation modes for the VSC-HVDC interties in urban power grids corresponding to normal, emergency and island operating conditions, respectively. An integrated, adaptive emergency control strategy (AEC) is proposed that can enable adaptive power flow responses of the VSC-HVDC intertie under varying system operating conditions and critical contingencies. The flexibility and effectiveness of the proposed operational principles of urban VSC-HVDC intertie and the corresponding control strategies are verified in PSCAD/EMTDC using a realistic urban power grid in China. [ABSTRACT FROM AUTHOR]

This work is an introduction of acoustic emission (AE) signals used in order to detect the malfunction of selected semiconductor elements. The authors proposed the use of internally generated signals (elastic waves) of acoustic emission leading to the detection of the pre-fail state of switching IGBT transistors. The analysis of the AE signals allows the creation of a reference pattern of properly working transistors and at the same time the identification of abnormal signals, which are generated by a defective element. Unlike many papers, this article shows experimental results demonstrating a comparison of undamaged, properly working and defective IGBT transistors which can be used, for example, as a reference for diagnostic tools. Analysis of the signal in the frequency domain obtained from the faulty transistor (overheated or with damaged casing) shows the presence of additional frequencies which can indicate the imminent occurrence of critical damage. [ABSTRACT FROM AUTHOR]

With the emergence of the hybrid multi-infeed high voltage direct current (HMIDC) system, there is complex dynamic interaction between the AC/DC system and converter stations, while the interaction mechanism of the HMIDC system is unclear. For this purpose, this paper establishes the HMIDC linearized model based on the motion equation concept. Then, after decoupling and reconstructing the interaction path of the HMIDC system, the damping coefficients are derived. The AC system strength and voltage source converter (VSC) terminal voltage controller parameters are selected to analyze the impacts on damping coefficients, and the interaction mechanism of the HMIDC system is analyzed. In order to solve the potential instability issue, an additional damping controller in the VSC terminal voltage loop is proposed to enhance the voltage stability of HMIDC system, which is realized by adjusting the reactive power/amplitude coupling damping coefficient. Finally, the RT-LAB-based hardware-in-the-loop experiments verify the accuracy of the interaction mechanism analysis of the HMIDC system and the effectiveness of the proposed controller. [ABSTRACT FROM AUTHOR]

Frequency-domain modeling is an effective technique in the dynamic analysis of power electronic converters-based power systems. In this paper, a unified single-input single-output (SISO) loop gain modeling for the three-phase grid-tied voltage source converters (VSCs) under both symmetric and asymmetric AC grids is presented, which facilitates the physical measurement and stability analysis. Based on the linear-time-periodic (LTP) modeling technique, the harmonic admittance model of the three-phase grid-tied VSC is developed in the stationary (αβ)-frame, which accurately captures the coupling dynamics. This model can be transformed into other existing models through coordinate transformations. According to the idea of mathematical induction, a two-by-two recursive admittance matrix (RAM) model is derived, which is easy to include frequency coupling components of arbitrary order. It is illustrated that the VSC admittance is coupled with the grid admittance due to the frequency coupling effect (FCE). Furthermore, the RAM is converted to its equivalent SISO models following the concept of loop gain. The system stability is thus assessed by the SISO stability criteria (e.g., Nyquist stability criterion). In addition, the loop gain allows the traditional SISO perturbation and measurement scheme to be used for detecting the stability margin information. Finally, simulation results verify the feasibility and correctness of the theoretical analysis presented above. [ABSTRACT FROM AUTHOR]

The existing research on voltage-source converter HVDC (VSC-HVDC) system stability mostly focuses on small-signal stability analysis methods. However, when large-signal disturbances occur, the intrinsic nonlinearity of the VSC-HVDC system becomes inevitable so that the small-signal studies are no longer valid. In this paper, a large-signal stability analysis method of the VSC-HVDC system is proposed based on the estimation of domain of attraction (DOA). This method can not only analyze the asymptotic stability of the operating point, but also answer the question of how large deviations from the operating point can be tolerated by the system. By using Takagi-Sugeno (T-S) fuzzy model and Lyapunov stability theorem, the DOA of the VSC-HVDC System at the operating point is estimated, and the large-signal stability of VSC-HVDC System is analyzed. On this foundation, the influence of system parameters on the large-signal stability is studied. The results can be used to guide the adjustment of system parameters to solve the problem of system instability. The time-domain simulations conducted in Matlab/Simulink are used to test the stability of the system under large-signal disturbance in the range of DOA. The simulation results verify the feasibility and effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Inductive power transfer (IPT) technology offers a promising solution for electric vehicle (EV) charging. It permits an EV to charge its energy storage system without any physical connections using magnetic coupling between inductive coils. EV inductive charging is an exemplary option due to the related merits such as: automatic operation, safety in harsh climatic conditions, interoperability, and flexibility. There are three visions to realize wireless EV charging: (i) static, in which charging occurs while EV is in long-term parking; (ii) dynamic (in-motion), which happens when EV is moving at high speed; and (iii) quasi-dynamic, which can occur when EV is at transient stops or driving at low speed. This paper introduces an extensive review for IPT systems in dynamic EV charging. It offers the state-of-the-art of transmitter design, including magnetic structure and supply arrangement. It explores and summarizes various types of compensation networks, power converters, and control techniques. In addition, the paper introduces the state-of-the-art of research and development activities that have been conducted for dynamic EV inductive charging systems, including challenges associated with the technology and opportunities to tackle these challenges. This study offers an exclusive reference to researchers and engineers who are interested in learning about the technology and highlights open questions to be addressed. [ABSTRACT FROM AUTHOR]

This paper presents a new control strategy for enhancing the dc voltage regulation and the transient response in the multi-terminal VSC-HVDC system by ensuring swift power-sharing among converters when encountering large disturbance such as converter loss or ac fault at one of the converter stations. The quick power sharing can be realized using direct power control (DPC) since it has a merit of fast power dynamic response. On the other hand, to limit the converter current during ac fault, a new coordination strategy between DPC and vector current control (VCC) is proposed. Furthermore, a new simplified model of the MTDC system is developed to study the performance of DPC in enhancing the dc voltage dynamics response analytically. The time-domain simulation of the detailed model of the four converter stations MTDC system verify that the proposed control strategy significantly reduces the overshoot in the transient dc voltage response following an outage or ac fault. As a result, together with the droop gain selection for the maximum permissible steady-state dc voltage deviation, the proposed control strategy can help to reduce the size of the dc chopper in the MTDC grid or even avoiding it. Moreover, the comparative analysis based on time-domain simulation depicts that the proposed simplified model has the same dc voltage response compared to the detailed model of the MTDC system except for a steady-state error of 0.47% and a little bit longer settling time (1.5 ms) at transient. [ABSTRACT FROM AUTHOR]

This paper investigates the applicability of averaged-value models (AVMs) for modular multilevel converters (MMCs) operating in a voltage-sourced converter-based-high-voltage dc (VSC-HVDC) grid. The AVM models are benchmarked by comparison with a detailed electromagnetic transient model of the grid, including a fully detailed MMC model. Analysis results show that the AVM is only effective as long as the capacitors are large enough to maintain nearly constant voltage across each MMC submodule. This paper also shows that previously developed MMC averaged models are not able to accurately simulate the transients under dc fault conditions. This paper introduces topology changes to a previously proposed averaged model that results in much improved simulation for such conditions. This paper also shows that the model can be used effectively to study HVDC grids with significant time savings. [ABSTRACT FROM AUTHOR]

The insulation condition of HVDC grids consisting of cable systems, GIS, and converters should be monitored by partial discharge (PD) analysers using artificial intelligence (AI) tools for efficient insulation diagnosis. Although there are many experiences of PD monitoring solutions developed for the supervision of the insulation condition of HVAC grids using PD analysers, there are no standardised requirements for their qualification available yet. The international technical specification TS IEC 62478 provides general rules for PD measurements using electromagnetic methods but does not define performance requirements for qualification tests. HVDC and HVAC PD analysers must be tested by unambiguous test procedures. This paper compiles experiences of using PD analysers with HFCT sensors in HVAC grids (cable systems, GIS, and AIS) to define a qualification procedure for HVAC systems. This procedure is applicable to HVDC grids (cable systems, GIS, AIS, and converters) because the particularities related to the insulation behaviour under HVDC voltage are also considered. Representative PD sources are discussed in HVAC and HVDC positive and negative polarity. The PD pulse trend of representative insulation defects in HVDC cable systems is quite different from that of HVAC grids. Special attention should be paid to the acquisition of PD signals in HVDC grids since few pulses appear in solid insulations, mainly during voltage changes (polarity reversals or surges), but rarely in continuous operation with constant direct voltage. A synthetic PD simulator has been developed to reproduce trains of PD pulses or noise signals, similar to those that can appear in the power network. A set of three functionality tests has been developed for qualification of the diagnostic capabilities of PD analysers working up to 30 MHz addressed to HVDC or HVAC grids: (1) PD recognition test, (2) PD clustering test, and (3) PD location test. This qualification procedure has been validated by means of a round-robin test performed by five research institutes (RISE, FFII, TUDelft, TAU, and UPM) using commercial and in-development AI PD recognition and clustering tools to demonstrate its robustness and applicability. Applying this qualification procedure, two PD methods for electrical detection and prevention of insulation defects have been approved, one for HVAC and the other for HVDC grids. [ABSTRACT FROM AUTHOR]

This article classifies, describes, and critically compares different compensation schemes, converter topologies, control methods, and coil structures of wireless power transfer systems for electric vehicle battery charging, focusing on inductive power transfer. It outlines a path from the conception of the technology to the modern and cutting edge of the technology. First, the base principles of inductive coupling power transfer are supplied to give an appreciation for the operation and design of the systems. Then, compensation topologies and soft-switching techniques are introduced. Reimagined converter layouts that deviate from the typical power electronics topologies are introduced. Control methods are detailed alongside topologies, and the generalities of control are also included. The paper then addresses other essential aspects of wireless power transfer systems such as coil design, infrastructure, cost, and safety standards to give a broader context for the technology. Discussions and recommendations are also provided. This paper aims to explain the technology, its modern advancements, and its importance. With the need for electrification mounting and the automotive industry being at the forefront of concern, recent advances in wireless power transfer will inevitably play an essential role in the coming years to propel electric vehicles into the common mode of choice. [ABSTRACT FROM AUTHOR]

Series-connected-based offshore wind farms can eliminate bulky central offshore converters, transformers, and platforms. In a series-connected-based offshore wind farm, the dc voltage of the converters should be changed within a large range to control the dc-link voltage (grid side) and maximize wind power acquisition (generator side). In this paper, the full-bridge-based modular multilevel converter (FB-MMC) is proposed as grid- and generator-side converters. The dc voltage of the FB-MMC can vary in a large range because the full-bridge submodule (FBSM) is used. The control method of the FB-MMC in a series-connected-based offshore wind farm is different from those of traditional ones. The dc current control and capacitor voltage balance control are proposed in this paper, with which the FB-MMC can operate normally in the series-connected offshore wind farm. Moreover, the capacitor ripple voltage in the generator-side FB-MMC is studied. The result shows that the capacitor in the FBSM can be designed in rated power and frequency, and it will be feasible when the power and frequency decrease. Simulation and experiment results verify the feasibility of the proposed configuration and the corresponding control algorithm. [ABSTRACT FROM AUTHOR]

DC technology has gained considerable interest in the medium voltage applications due to the benefits over the AC counterpart. However, to utilize the full capacity of this development, the selection of a suitable power electronic converter topology is a key aspect. From the pool of voltage source converters (VSC's), it is unclear which topology is suitable for multi-megawatt applications at medium voltage dc (MVdc) levels. To address this, the paper proposes a selection guideline based on reliability and optimum redundancy levels of VSCs for MVdc applications. This will be combined with other functional factors such as operational efficiency and return-on-investment. Three candidate multi-level topologies namely three-level neutral point clamped converter (3L-NPC), modular multi-level converter (MMC) and cascaded 3L-NPC (which is being used for the first MVdc link in the U.K.) have been evaluated over two-level-VSC from $\pm$ 10 kV to $\pm$ 50 kV. Results show that with the increase of MVdc voltage level MMC shows better performance whereas at low MVdc voltage levels 3L-NPC is the prominent topology. [ABSTRACT FROM AUTHOR]

The converter is an important component of a wind turbine, and its control system has a significant impact on the dynamic output characteristics of the wind turbine. For the double-fed induction generator (DFIG) converter, the control parameter identification method is proposed. In this paper, a detailed dynamic model of DFIG with the converter is built, and the trajectory sensitivity method is used to study the observation points that are sensitive to the change of control parameters as the observation quantity for control parameter identification; the Whale Optimization Algorithm (WOA) is used to study the converter control system parameters that dominate the output characteristics of DFIG in the dynamic full-process simulation. To validate the proposed method, four classical test functions are used to verify the effectiveness of the algorithm, and the control parameters are identified by setting a three-phase grounded short-circuit fault under maximum power point tracking (MPPT), and the identification results are compared with particle swarm optimization (PSO) and chaotic particle swarm optimization (CPSO) to show the superiority of the proposed method. The final results show that the proposed WOA can identify the control system parameters faster and more accurately. [ABSTRACT FROM AUTHOR]

In this paper, a new dc switchyard structure for a multiterminal dc (MTDC) grid is proposed. Fast fault current breaking capability, which is essential for a dc grid, can be achieved with available HVDC breaker solutions (e.g., ABB's hybrid HVDC breaker). However, the hybrid HVDC breaker concept also opens up possibilities for unconventional (relative to ac) switchyard solutions which may be more cost effective. The main aim of this paper is to expound how the proposed dc switchyard can be built with hybrid HVDC breakers in a configuration where the number of components is much less compared to a conventional switchyard structure. After that, the switchyard operation, fault current handling flexibility, and actions against component failure are described conceptually. Finally, the performance of the proposed switchyard is validated and compared with that of a conventional double breaker double busbar switchyard structure for selected cases through time-domain simulations [ABSTRACT FROM PUBLISHER]

This paper proposes a startup sequence for a modular multilevel converter (MMC) HVDC-connected offshore wind farm (OWF) and demonstrates how it can be simulated using an electromagnetic transient-type tool. The objective is to analyze inrush currents of transformers and cables (HVAC and HVDC). Modeling accuracies and computational performances are examined using three existing modeling methods for the MMCs. [ABSTRACT FROM PUBLISHER]

The dc–dc modular multilevel converter (MMC), which originated from the ac–dc MMC, is an attractive converter topology for the interconnection of medium-/high-voltage dc grids. This paper presents design considerations for the dc–dc MMC to achieve high efficiency with reduced component sizes. A steady-state mathematical model of the dc–dc MMC in the phasor domain is developed. Based on the developed model, a design approach is proposed to size the components and to select the operating frequency of the converter to satisfy a set of design constraints while achieving high efficiency. The design approach includes sizing the arm inductor, submodule capacitor, and phase filtering inductor along with selection of the ac operating frequency of the converter. The accuracy of the developed model and the effectiveness of the design approach are validated based on the simulation studies in the PSCAD/EMTDC software environment. The analysis and developments of this paper can be used as a guideline for designing the dc–dc MMC. [ABSTRACT FROM PUBLISHER]

Series compensation is a cost-efficient way to enhance the system reliability and the power transfer capabilities of long transmission lines. As a result of series compensation, the sub-synchronous oscillation (SSO) causes a severe risk of torsional interactions (TI). Therefore, SSO becomes a serious risk factor in grid-integrated renewable energy systems. Numerous researchers have evaluated SSO instances in several types of asynchronous generators in power systems. In this paper, the categorization and the overview of the SSO phenomena have been vital for the different mechanisms, sophisticated systems, analytical techniques, and multiple reviews that have been propagated. This study provides SSO analysis for various types of renewable energy power plants. Finally, while dealing with conventional and new power systems, this study summarizes recent SSO-damping and alleviation techniques for practical perception and future perspectives. [ABSTRACT FROM AUTHOR]

A novel approach to synchronization of shore and ship power systems is presented in this paper, providing a blackout-free supply from the shore. An application of the shore-to-ship synchronization strategy, proposed in the paper, instead of the commonly used ship-to-shore synchronization, enables a fuller exploitation of the high dynamic offered by power electronic converters. Converters are increasingly being used in shore-to-ship supply arrangements in order to ensure the required levels of voltages and frequencies, specified in shore-supply related standards. Simulation results, supported by preliminary tests in small-scale experimental arrangements, have confirmed that shore-to-ship applications might significantly improve the dynamics of the synchronization as well as the parameters of the load transferring process, especially under distorted ship voltage conditions. [ABSTRACT FROM AUTHOR]

Due to the anticipated proliferation of HVDC links up to and beyond 2020 into transmission systems, it is expected that in some locations, two or more HVDC links will be installed with a short electrical distance between them and will form a multiinfeed (MI)-HVDC scenario for the system. The control and operation of MI-HVDC systems are of particular concern for weak grids such as that likely to be found in the North of Scotland network. It is shown that the ac voltage regulation in the VSC-HVDC link may adversely affect the dynamic performance of the host weak ac system under the MI-HVDC scenario. To overcome this identified limitation associated with the ac voltage controller in the VSC-HVDC link, a new controller modification conforming to the grid code is proposed in this paper. Furthermore, this paper presents a robust control design method for ac voltage control using the fractional order (FO) control design theory. The influences of the proposed FO-based ac voltage controller are assessed alongside various voltage controllers for the representative North Scotland system. It is shown that the FO-based ac voltage controller has favorable effects on the dynamic behavior of weak ac systems with MI-HVDC. [ABSTRACT FROM AUTHOR]

For a hybrid ac/dc power system incorporating voltage source converter-based multi-terminal direct current (VSC-MTDC) grids, it is essential to solve the power flow problems efficiently for reliable operation. This paper presents an efficient ac/dc power flow algorithm considering the complementary constraints of VSC's reactive power limit and ac node voltage. It can be distinguished from the existing works in terms of both modeling and algorithm aspects. First, the VSC model is formulated, in which the interactions between the reactive power limit and the ac node voltage of VSC are viewed as complementary relations. The complementary relations are expressed using a smooth Fischer-Burmeister function. Then, power flow models of the ac grid and dc grid with a constant Jacobian matrix are formulated based on the VSC model, respectively. To efficiently solve the power flow of an ac/dc grid, the structure characteristic of the Jacobian matrix of ac/dc power flow computation is analyzed. It shows that the Jacobian matrix can be approximated as a block-upper triangular one. Based on this characteristic, a decoupled ac/dc power flow computation algorithm is provided. Test results on two hybrid ac/dc systems under different scenarios validate the correctness, convergence, and efficiency of the proposed power flow algorithm. [ABSTRACT FROM AUTHOR]

The increasing application of voltage source converter (VSC) based high voltage direct current (VSC-HVDC) technology in power grids has raised the importance of incorporating DC grids and converters into the existing transmission network. This poses significant challenges in dealing with the resulting optimal power flow (OPF) problem. In this paper, a recently proposed nonconvex distributed optimization algorithm—Augmented Lagrangian based Alternating Direction Inexact Newton method (aladin), is tailored to solve the nonconvex AC/DC OPF problem for emerging voltage source converter (VSC) based multiterminal high voltage direct current (VSC-MTDC) meshed AC/DC hybrid systems. The proposed scheme decomposes this AC/DC hybrid OPF problem and handles it in a fully distributed way. Compared to the existing state-of-art Alternating Direction Method of Multipliers (admm), which is in general, not applicable for nonconvex problems, aladin has a theoretical convergence guarantee. Applying these two approaches to VSC-MTDC coupled with an IEEE benchmark AC power system illustrates that the tailored aladin outperforms admm in convergence speed and numerical robustness. [ABSTRACT FROM AUTHOR]

The increasing use of voltage-source converters (VSCs) in modern power systems has led to extensive studies on their frequency response and stability, which are characterized by power-frequency oscillations and overshoots following a disturbance. However, the existing small-signal analysis models do not adequately describe the impact of the types of disturbance on the torque-frequency dynamics of the multi-VSC parallel system. In this paper, based on the principle of electromechanical analogy, a unified equivalent admittance modeling method is proposed by analogizing the rotor dynamic model of a synchronous generator (SG) to the circuit network, which has the advantages of simple modeling, clear physical meaning and easy expansion. Firstly, the equivalent admittance model of a single SG/VSC is established, and then easily extended to the multi-VSC system. The proposed modelling method not only applicable to a variety of VSCs, but also can handle different types of disturbance. More importantly, the circuit theory can be applied to analyze the torque-frequency dynamic characteristics. Subsequently, the equivalence between the proposed model and the state-space model under different damping terms is compared. In addition, the response characteristics of two different damping terms are compared. Next, the dynamic response characteristics of single VSC and multi-VSC parallel systems under different disturbances are analyzed. Finally, related simulations and experiments are executed to verify the accuracy and effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

In this paper, we model and analyze the power losses of push–pull converters. The proposed model considers conduction and dynamic power losses, as well as transformer and inductor losses. Transformer and inductor models include skin and proximity effects, as well as power losses in the core. Moreover, the model includes the diode recovery time losses. We derived the equations for both continuous and discontinuous current operating modes. All model parameters can be obtained either from the datasheets of the used components or by simple measurement techniques. The model is verified experimentally by measuring the efficiency of the 500 W push–pull converter prototype. Simulations and experimental validation are conducted using the assumption that the converter is used in a permanent magnet (PM) wind turbine generator. [ABSTRACT FROM AUTHOR]

In recent years, it is getting attention for renewable energy sources such as Fuel Cell (FC), batteries, ultracapacitors or photovoltaic panels (PV) for distributed power generation systems (DG) or electrical vehicles. This paper proposes a DC/DC converter and DC/AC inverter scheme to combine the Fuel Cell Stack (FC). The power system consist of a FC stack, a DC/DC converter, inverter and load. A FC mostly could not produce necessary output voltage, the DC/DC boost converter is used for obtaining the desired output voltage. The inverter is used for many applications such as electrical vehicles (EV) and distributed generation systems (DG). In this paper, sinusoidal pulse modulation technique and space vector pulse width modulation technique have been simulated and compared for electrical vehicles applications. [ABSTRACT FROM AUTHOR]

This paper extends the previous virtual synchronous generator (VSG) control for voltage-source converters to modular multilevel converters (MMCs). Compared with the ac synchronous generator (SG), the VSG-based MMC is embedded with a power-angle characteristic. Starting from this feature and with reference to the existing ac system coherency-based equivalence methods, this paper proposes a novel coherency equivalence method for MMC used in large-scale renewable energy systems connected to the ac grid. This novel method mainly focuses on the coherency criterion and parameter aggregation methods which have significant differences compared with the ac system methods. The proposed equivalence method is validated on PSCAD/EMTDC and the study results indicate that: 1) the VSG-based MMC model does show the power-angle characteristic similar to the SG model; 2) the proposed coherency criterion can accurately and efficiently classify the paralleled MMC into coherent groups; and 3) the dynamic behaviors of the external systems are preserved after the aggregations of all previous coherency MMC groups. [ABSTRACT FROM AUTHOR]

In this paper, a static synchronous compensator (STATCOM) operation scheme of the modular multilevel converter adopting clamp double submodules (CDSM-MMC) during a pole-to-pole dc fault is proposed. By analyzing the current paths of a CDSM, this paper proposes three new operation states of the CDSM, which allow the CDSM to provide bipolar voltages when the arm current that flows through it is negative. Through control of the CDSMs to operate in these new states along with the blocked state, the arms of the CDSM-MMC can operate alternately in two different modes, that is, the conducting mode and blocked mode even when the dc-link voltage drops to zero. Thus, the CDSM-MMC is capable of working as a STATCOM during a dc fault. Moreover, a control strategy of the dc-fault STATCOM operation mode is designed to control the reactive power provided by the CDSM-MMC, to limit the dc fault current, and to reserve and balance the energies stored in capacitors. Simulation results conducted in PSCAD/EMTDC demonstrate the validity of the proposed STATCOM operation scheme and control strategy of the CDSM-MMC during a pole-to-pole dc fault. [ABSTRACT FROM AUTHOR]

This paper describes a novel and high-performance phase-locked loop (PLL) resolver converter for the measurement of mechanical angles. The proposed method does not require the use of demodulators, voltage-controlled oscillator, digital-to-analog converter, counter, and lookup tables, that are associated with conventional PLL converters. The proposed method employs full-wave rectifiers to obtain the absolute values of the modulated resolver signals. This scheme results in a triangular output voltage and two binary outputs that represent a measure of the input angle. The required sine and cosine of the estimated angle, which are needed for PLL converters, are determined using simple approximation techniques. The proposed method can be used to measure angles in the full 360° range, is simpler than the available techniques, and may be easily implemented using digital or basic analog electronic circuitry. This paper describes the proposed method, full details of its implementation using standard electronic components, and experimental results. The performance of the proposed converter is comparable with that of a commercial product. [ABSTRACT FROM AUTHOR]

This paper presents the modular multilevel cascade converter based on double-star chopper-cells, which is intended for grid connection to medium-voltage power systems without using line-frequency transformers. The converter is characterized by a modular arm structure consisting of cascade connection of multiple bidirectional pulsewidth modulation chopper-cells with floating dc capacitors. This arm structure requires voltage-balancing control of all the dc capacitors. However, the voltage control combining an averaging control with an individual-balancing control imposes certain limitations on operating conditions. This paper proposes an arm-balancing control to achieve voltage balancing under all the operating conditions. The validity of the arm-balancing control as well as the theory developed in this paper is confirmed by computer simulation and experiment. [ABSTRACT FROM AUTHOR]

In this paper, the control of a grid-connected wind energy conversion system (WECS), based on an induction machine fed by a matrix converter (MC), is presented. The MC is controlled using a space vector modulation algorithm with zero displacement factor at the input. Stability issues related to the operation of the WECS for the whole power and speed range are fully analyzed in this paper, using a simplified small-signal model. The implementation of a model reference adaptive system (MRAS) observer, for sensorless control of the proposed WECS, is also presented in this paper. The MRAS observer is implemented using the output voltage demand, avoiding the use of voltage transducers to measure the machine voltage. From the speed estimated by the MRAS observer, the electrical torque of the induction generator is regulated in order to drive the WECS to the operating point where the aerodynamic efficiency is maximized. Experimental results obtained with a 2.5-kW prototype are presented and fully discussed in this paper. [ABSTRACT FROM PUBLISHER]

This paper presents a novel dc–dc converter incorporating a distributed impedance-source network ( Z-source network, or Z-network) to achieve the buck (step-down) and boost (step-up) function of a transformer-isolated dc–dc converter. In this paper, the distributed impedance source network composed of an array of inductors and capacitors is coupled between power source and main switching devices. The great and unique feature about the distributed impedance source network dc–dc converter is that unlike the traditional V-source or I-source converters, it can be open- and short-circuited without damaging switching devices. Therefore, the desired buck and boost function can be achieved. Moreover, converter reliability can be greatly improved. A dc–dc converter using this proposed concept is built and tested. Its performances are verified with experimental results. [ABSTRACT FROM PUBLISHER]

This "Special Section on New Technique Trends for Power Converters in Distributed Power Generation Systems" of the IEEE Transactions on Industrial Electronics (TIE) 35 papers selected among 89 submitted papers. Therefore, this Special Section will be published in two parts. In Part I, all the papers are related to dc/ac grid-tied inverters. [ABSTRACT FROM PUBLISHER]

Medium to long-distance underground ac lines and both short- and long-distance HVdc are frequently integrated into network expansion plans. In conjunction with growing numbers of power-electronic converters in the grid, there is a rising risk related to the effects of harmonic currents. Resonance analysis should therefore be part of power system expansion planning. In this paper, network harmonic modeling and analysis are discussed, in particular, power transformer and power converter frequency models are proposed. Frequency scanning and resonance mode analysis are compared and discussed. The changes in resonance response due to transmission network expansion are drawn. The methods are applied to the investigation of the impact of one link reinforcement, both ac and ac solutions, as well as the impact of the power transformer disconnection. The performed analysis enables finding the resonance location at each studied case. The frequency analysis methods presented in this paper enable forseeing network frequency behavior that is valuable for both network analysis and network expansion planning. [ABSTRACT FROM AUTHOR]

This paper proposes an active switching harmonic current reduction method within the dc-link capacitor of two-level, three-phase, back-to-back converters. This method is based on the derived analytical solution for switching harmonic currents in the dc-link. It is shown that by controlling the pulse width modulation carrier waveform's phase angles, the harmonics in the second carrier band of the rectifier and the inverter can be synchronized such that cancellation occurs in the dc-link capacitor. This synchronization is provided by harmonic phase feedback control. The feasibility of the proposed approach has been verified experimentally and results are presented in this paper. [ABSTRACT FROM PUBLISHER]

Every electric vehicle requires fast, safe, and reliable charging with optimal performance. Conventional plug-in charging systems have several issues that wireless charging eliminates. Wireless charging is also considered the most environmentally friendly and user-friendly option. It does not require bulky connectors and safety equipment that add cost and take up space. The majority of work focuses on presenting an overview and basic infrastructure of wireless power transfer systems. This paper describes the latest technological advancements in both far and near-field wireless charging methods. It then examines the near-field inductive charging method for both static and dynamic electric vehicle applications. The paper also discusses compensation networks' role in inductive power transfer systems, delving into the advancements made in compensation networks that enhance coil utilization and efficiency. Additionally, the paper presents various magnetic coupler designs and their optimization to improve overall magnetic coupling efficiency, pulsations, and power null region to support their suitability in static and dynamic applications. The paper also stresses the importance of safety when installing wireless power transfer systems, along roads or at home. As a result, updated standards from IEEE/SAE and IEC, analyzing system and equipment requirements, are suggested in this review. Based on the presented discussion, the DD with DDQ receiver coil is considered the most useful solution with LCC-S compensation, offering the highest electrical performance characteristics and a recorded efficiency of 93% for modern dynamic charging. Finally, double-sided DD coils with LCC-S compensation with voltage source output characteristics are recommended to reduce control complexity.

Bipolar VSC-MTDC is a promising solution to increase power transmission capacity and system reliability for large-scale renewable energy integration. However, traditional interpolar active power control methods cannot comprehensively explore the maximum transmission capability of the bipolar VSC-MTDC under abnormal operating conditions. Considering the flexibility of the bipolar VSC-MTDC that the positive-pole and negative-pole DC networks can be operated independently, a novel fault self-recovering control strategy is proposed in this paper to provide automatic, global and timely recovery solutions under various fault conditions: (a) Firstly, a coordinated time-driven and event-driven security monitoring, optimization and recovering control framework is proposed to guarantee the accuracy and speed of the corresponding calculation and control, hence to meet the timeliness requirement of fault self-recovery; (b) Secondly, a global optimal power allocation calculation method is proposed to automatically calculate the optimal power allocation between the positive-pole and negative-pole DC networks; (c) Thirdly, an interpolar active power dispatch method is proposed to realize the fault self-recovering control strategy based on primary control strategies of bipolar VSC-MTDC. Simulation results using MATLAB and PSCAD/EMTDC show that the proposed control strategy can timely and effectively enhance the overall power transmission capability for large-scale renewable energy integration after the fault. [ABSTRACT FROM AUTHOR]

Grid connected voltage source converters are inevitable means now for expanding and supporting power systems. Nevertheless, their harmonic injection and high frequency switching may cause degradation of power quality and detraction of the equipment reliability. Therefore, the converters injected harmonics must observe the corresponding standards, while lower switching frequencies and rapid transient responses are desirable. In this paper, a new combined control method with a flexible switching table is presented for the converters to enhance the power quality and transient response. The method employs a virtual flux in connection with a combined control for the first time. In comparison with the conventional virtual flux direct power control, the proposed method provides better power quality, reduced THD and faster transient response, in addition to less grid parameters dependency. Compared to the conventional combined control method, it determines more accurately the converter voltage vectors, while avoiding grid voltage measurement and cumbersome controller tuning. [ABSTRACT FROM AUTHOR]

This paper proposes a novel admittance model for the stability analysis of grid-connected voltage-source converters (VSCs) which features a symmetric structure. On this basis, the grid-connected VSC system can be represented by a reciprocal two-port circuit to explore the oscillation mechanism of the system. Furthermore, the grid-connected VSC system can be modelled as a single-input single-output (SISO) system rather than a multi-input multi-output (MIMO) system. We analytically evaluate the influences of the VSC control parameters and the power factor on the stability of the grid-connected VSC by employing the Nyquist criterion and the stability margin. In addition, we demonstrate that the VSC's current output is in inverse proportion to the system's gain margin. Further, an analytical expression of the current limit constrained by small-signal stability is derived in closed form so as to calculate the admissible current output. All of analysis results are verified by simulations based on Simulink and hardware-in-the-loop (HIL) platform. [ABSTRACT FROM AUTHOR]

Rooftop and building-integrated distributed photovoltaic (PV) systems are emerging as key technologies for smart building applications. This paper presents the design methodology, mathematical analysis, simulation study, and experimental validation of a digital twin approach for fault diagnosis. We develop a digital twin that estimates the measurable characteristic outputs of a PV energy conversion unit (PVECU) in real time. The PVECU constitutes a PV source and a source-level power converter. The fault diagnosis is performed by generating and evaluating an error residual vector, which is the difference between the estimated and measured outputs. A PV panel-level power converter prototype is built to demonstrate how the sensing, processing, and actuation capabilities of the converter can enable effective fault diagnosis in real time. The experimental results show detection and identification of ten different faults in the PVECU. The time to fault detection (FD) in the power converter and the electrical sensors is less than 290  $\mu$ s and the identification time is less than 4 ms. The time to FD and identification in the PV panel are less than 80 ms and 1.2 s, respectively. The proposed approach demonstrates higher fault sensitivity than that of existing approaches. It can diagnose a 20% drift in the electrical sensor gains and a 20% shading of a solar cell in the PV panel. [ABSTRACT FROM AUTHOR]

The articles in this special issue focus on the latest achievements of modular multilevel converters regarding the development of new circuit configurations, converter models, modulation strategies, and control schemes. [ABSTRACT FROM AUTHOR]

The IEEE 1584 Guide does not address arc-flash hazard in dc systems. The interest in this subject is evidenced by current publications. The National Fire Protection Association (NFPA) 2012 methodology has added a small description of the calculations of arc-flash hazard in dc systems. However, there are serious inaccuracies in the application of the methodology that is advocated in the NFPA. A review of the current literature indicates that there is no comprehensive document on the subject and that some of the assumptions made in the current literature are not accurate, e.g., with respect to the short-circuit current profiles and protective devices in dc systems, which are very fundamental to the accurate calculations of arc-flash hazard in dc systems. This paper is presented in two parts. This part provides equations with practical examples of short-circuit calculations in dc systems. The second part provides the analysis of arc-flash hazard. Accurate calculations of the short-circuit currents and ascertaining their profiles is the first step in the arc-flash hazard analysis. [ABSTRACT FROM PUBLISHER]

This paper focuses on the modelling and control of an interline current flow controller (CFC) for meshed HVDC grids. The operation states of the CFC are presented, and an average model is derived. The average model is used to perform steady-state analysis on a 3-terminal meshed grid, showing the current change capabilities and the benefits on the operation area. The converter control is designed using a linearized model. The system performance of the CFC is tested by means of simulation in a 3-terminal grid and in a 5-terminal grid. [ABSTRACT FROM AUTHOR]

This paper proposes a modular multilevel converter (MMC) topology for high-voltage dc (HVdc) system applications. The proposed MMC employs two half-bridge converters, a cascade of networks that consist of electronic switches, and multiple capacitors. Thus, charged capacitors are inserted in series with the arm current path, for a desired level of the output ac voltage. The switches that must be turned on are judiciously selected for the most efficient current path. The proposed MMC also offers dc-side fault handling capability. The paper compares the proposed MMC with the half-bridge submodule-based MMC, full-bridge submodule-based MMC, and clamp-double submodule-based MMC technologies. It is shown that the proposed MMC is more efficient than the three aforementioned MMC technologies, while it offers the same fault handling capability as that of an equivalent full-bridge-based MMC. Time-domain simulation studies confirm the effectiveness of the proposed MMC under various normal and faulted operating scenarios, using detailed as well as reduced models. [ABSTRACT FROM AUTHOR]

A new class of modular multilevel dc/dc converters has recently emerged that eliminates the traditional full-rated intermediate ac stage inherent to two-stage dc/dc converter structures, that is, two cascaded dc/ac modular multilevel converters (MMCs). This paper introduces the first averaged dynamic state-space model for this new class of single-stage dc/dc converter, called the DC-MMC, which captures all input and output terminal dynamics as well as internal capacitor dynamics. Based on the developed analytical model, a dynamic phasor model is derived to accommodate the multifrequency components that exist at steady state. The phasor model enables: 1) solving for the full steady-state solution of the DC-MMC under arbitrary loading and 2) evaluation of DC-MMC open-loop stability via eigenvalue analysis. These analyses are not possible with existing DC-MMC models. Simulations of a comprehensive switched model verify the accuracy of the presented analytical models, which represent valuable study tools for the analysis of emerging modular multilevel dc/dc converters containing multiple frequency components. [ABSTRACT FROM PUBLISHER]