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This paper proposes an adaptive self-tuned current controller design for an LCL-filtered LC-tuned single-phase shunt hybrid active power filter (HAPF), which offers adaptability and flexibility of the control system to filter parameters changes. System modelling and online identification of parameters are presented. Then, a systematic self-tuned controller (STC) design is proposed to calculate the controller parameters and obtain the final control law adaptively. The exact HAPF parameters design is described and the overall closed-loop stability evaluation with considering digital control delay is discussed in this paper. To confirm the theoretical achievements, simulations and experimental tests on a real prototype system are performed. The results are in good accordance and demonstrate the effectiveness and superior performance of the suggested control technique. Furthermore, a comprehensive comparison between the proposed control technique and other controllers is provided to show the superiority of the proposed system from different aspects. [ABSTRACT FROM AUTHOR]

This paper analyzes the unsymmetrical operation of the dc/dc MMC caused by parametric uncertainty. The unsymmetrical operation is referred to as the condition in which parameters involved in ac power generation are not equal in the phase-legs of the converter. Because existing models in the literature cannot exhibit the unsymmetrical operation of the dc/dc MMC, new detailed and simplified steady-state models are developed in this paper. In the developed models, the connection between the operation of different phase-legs shows itself when the dc-link impedance is considered. Using the developed models, the parameters that can force the converter to operate in unsymmetrical conditions are identified, and the maximum variation that the converter can tolerate is found. It is shown that even a small parameter variation leads to the leakage of the ac circulating current into the dc-link, which is not acceptable. By conducting various case studies, the impacts of the dc-link length, the transmitted power, and the conversion ratio on the unsymmetrical operation of the dc/dc MMC are characterized. The results showed that the converter would be more sensitive to the parameter variation as these parameters increase. Comparative analysis and simulation results confirm the validity and effectiveness of the presented models and analysis method. [ABSTRACT FROM AUTHOR]

A novel single-stage doubly grounded transformerless inverter configuration which can buck-boost the output voltage is proposed in this paper. In this transformerless inverter configuration grid neutral is directly connected to PV panel negative rail which suppresses/eliminates the PV leakage currents. In addition, the tracking of maximum power point is possible for wide variation of weather conditions due to buck-boost voltage capability of inverter. Moreover, the proposed inverter topology has symmetrical operation in both half cycles hence injects negligible DC offset current in to the grid. Here the symmetrical operation of inverter is achieved due to one inductor is used for both half cycles of the grid. The proposed topology is realized using five switches, two of which work at 50 Hz and remaining three switches at high frequency which reduces overall semiconductor losses. Further, the proposed inverter topology is controlled through a newly developed modulation technique and possible working modes are analyzed and described thoroughly. For performance validation, the experimental analysis of grid connected proposed inverter is performed on a 1 kVA system and results show that proposed inverter feeds power in to the grid with lesser THD while achieving greater efficiency. [ABSTRACT FROM AUTHOR]

When a fault occurs on the dc side of the voltage source converter based high voltage direct current (VSC-HVDC) transmission system, the fault current rises quickly due to the low damping of the dc system. Using a dc fault current limiter (DC-FCL) to limit the rise of the fault current can significantly reduce the breaking requirements of the DCCB. A topology of a bidirectional active dc fault current limiter (BA-FCL) based on coupled inductor is proposed in this paper. No expensive full-controlled power electronic devices are used in this topology. Only a few half-controlled power electronic devices are used to achieve active control of the current limiting unit. Compared with many topologies of current limiter at present, the fault current limiting effect of BA-FCL is improved significantly while the cost, volume, topology complexity, and control complexity are reduced. The stress analysis and parameter design of the proposed current limiter are carried out. Finally, the simulation case study and hardware experiment are implemented to verify the good economy and current limiting performance of the proposed topology. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC), owing to the presence of a large number of submodules (SMs) and energy storing elements, requires a large footprint and initial investment. To address these issues, this paper proposes the switched modular multilevel converter (SMMC). It consists of two major elements; the chain-links (CLs), consisting of cascade connection of SMs as in MMC, and the single-pole-triple-throw (SPTT) switches, which can be realized using either the T-type or NPC-type switch strings. The SMMC works on the principle of connecting all the phase-legs in series and their respective positions are mutually interchanged. It offers a compact structure as the SMs count is reduced to 33% of MMC, which results in a much lower energy storage requirement. Furthermore, it does not have inter-phase circulating current problem and hence the CL inductors can also be avoided. The SMMC structure, operating principles, control, and modulation schemes are explained in detail. A detailed comparison of SMMC with the other major existing converters is carried out to highlight the important features of SMMC. The efficacy of SMMC is validated using PSCAD simulation studies with a 19-level grid-connected SMMC model. A 3-phase 19-level SMMC laboratory prototype is developed to further validate its feasibility. [ABSTRACT FROM AUTHOR]

The current-limiting inductors (CLIs) are commonly used to suppress DC fault current to safely trip DC circuit breakers in MMC-HVDC systems, but it significantly jeopardizes DC dynamics and causes instantaneous power mismatch between AC and DC sides of islanded rectifier station (RS), exposing MMC internal states to large disturbance. This paper firstly provides a detailed analysis of the influence of CLIs on AC fault, establishes RS mathematical representation including AC side dynamic, and reveals the mechanism of DC voltage oscillation, modulation saturation and PCC waveform distortion. The analysis shows that the AC and DC fault characteristics are decoupled under unsaturated AC modulation, but coupled under excessively low submodule capacitor voltage, where AC modulation is saturated and PCC waveforms are distorted. To solve this issue, a novel RS AC fault ride through method for MMC-HVDC system with CLIs is proposed to stabilize RS DC voltage, desaturate AC modulation and decouple fault characteristics by regulating the zero-sequence modulation of circulating current suppression. In addition, a current reference disturbance and an AC energy dissipation device coordinated control are designed, which adaptively recovers the submodule capacitor voltage. Finally, the feasibility of the proposed method is verified by simulation results. [ABSTRACT FROM AUTHOR]

Conventionally, the current flow control and DC fault protection issues of HVDC grids are supposed to be solved by the DC current flow controller (CFC) and DC circuit breaker (DCCB) separately, which may result in a high capital cost. This paper proposes a CFC topology with DC fault current limiting and interrupting capabilities. The topology and operating principle of the CFC are presented with theoretical analysis. The control strategies under normal and fault conditions are described. In order to reduce the use of IGBTs, an H-bridge inter-line CFC with fault current limiting capability is further proposed based on the proposed CFC. The proposed CFCs are tested in PSCAD/EMTDC. Simulation results show that the proposed two CFCs can effectively control the current flow of two lines during normal operation and limit and interrupt DC fault currents. [ABSTRACT FROM AUTHOR]

The paper examines the possibility to exploit the inductance of wound aluminum foil capacitors in order to realize an integrated component. Objective is the design and construction of an integrated inductor/capacitor to filter harmonic modulations as an alternative to the conventional passive filter, consisting of capacitor and inductor as discrete components with the prospect of reducing size, material and consequently costs. The principle functioning of the integrated component resulting from the position the supply terminals are placed on the aluminum foils is being demonstrated. An analysis of technical and geometric parameters leads through a calculation scheme to the design of such an integrated component. Finally, the construction of a manual winding machine enables the wrinkle free winding of the four foils on a central core to produce a pilot model operational at 400 V with multiple access terminals providing hence diverse experimentation possibilities. Most significant result is the measured current frequency characteristic of the pilot model revealing clearly its series resonant behavior with a quality factor close to ten, well comparable to values of conventional industrial filters indicating thus at this stage reasonable prospects for such an integrated component to operate as a filter in the grid absorbing harmonic modulations. [ABSTRACT FROM AUTHOR]

Modular multilevel converter (MMC) based high voltage direct current (HVDC) transmission system is one of the important development trends of DC power grid. After fault isolation, the traditional reclosing method of MMC-HVDC system is directly reclosing DC circuit breaker (DCCB). Once the fault point still exists, DCCB will be tripped again. This method will make the DC system suffer from the secondary strike of fault in a short time, and seriously endanger the service life of DCCB. In order to overcome the drawbacks of the traditional reclosing schemes, this paper proposes a novel adaptive reclosing scheme based on pulse injection from parallel energy absorption module. When dc fault current is interrupted by DCCB, the fault energy stored in current-limiting reactors (CLR) and dc lines is transferred to energy absorption module automatically. By switching on IGBTs embedded in absorption module for a short time, a voltage pulse is injected directly into the faulty line without interference from CLR. Then the nature of fault is identified and the fault distance can also be calculated in the case of permanent faults. The simulation results verify the effectiveness of the proposed reclosing scheme. [ABSTRACT FROM AUTHOR]

Recently the hybrid multi-port DC circuit breaker (MP-DCCB) is becoming popular in protecting HVDC grids, thanks to their reduction of power electronics devices. In this paper, an enhanced multi-port current-limiting DCCB (MP-CLCB) for multiple line protection is proposed. The integrated fault current limiter (FCL) inside the MP-CLCB can clear the fault faster with slightly increased costs. To reduce the energy dissipation requirement for the surge arresters caused by the newly added current-limiting path, an energy transfer path which provides a loop with the inductors during the current decay stage is designed. The theoretical analysis of the pre-charging, current-limiting, fault interruption and energy dissipation of the MP-CLCB is carried out. Moreover, the design principles of the energy dissipation and the key parameters of the MP-CLCB are provided. The proposed approaches are verified through simulations in PSCAD/EMTDC. The results show that the MP-CLCB can replace multiple DCCBs, accelerate the fault current interruption and reduce the energy dissipation requirement for the surge arresters. [ABSTRACT FROM AUTHOR]

With the development of the MMC-HVDC, the lightweight requirements of the converter valves have become increasingly urgent. The reduction of submodule (SM) capacitance is difficult because the ripple of capacitor voltage cannot be too large, which is one of the key factors that affect the weight and the volume of the MMC. On the basis of the fundamental frequency and double fundamental frequency components are the critical factors for voltage ripples, this paper proposes an arm inductor switching control (AISC) strategy to reduce the voltage ripples of SM capacitor by controlling the inductance value of the bridge arm. The proposed strategy and the existing circulating current suppression controller (CCSC) are researched in terms of mathematical principles. A two terminal DC transmission model is built in PSCAD/EMTDC simulation environment, then the capacitor voltage fluctuation curves of non-capacitance-reducing control, CCSC, and proposed strategy with the same parameters are simulated. Afterwards, an experiment prototype is built to verify the simulation results. Simulation and experimental results are presented to demonstrate the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

Multi-terminal HVDC grid has extremely high demand for protection speed of DC lines, consequently, the sampling time window of the main protection is very short, which affects its selectivity and reliability. To make up for the insufficiency of the non-unit main protection, this paper proposes a pilot protection scheme based on the voltage polarities of current-limiting reactors at the ends of DC lines. Analysis shows that the voltage polarities of terminal reactors at the two ends of the DC lines are both positive when internal fault occurs, while they are opposite for external faults. Based on the aforementioned characteristic difference, a pilot protection criterion is built to discriminate between internal and external faults. To avoid mal-operation of the protection on healthy pole generated by the coupling effect, the polarity mutation of reactor voltage on healthy pole due to coupling effect is analyzed when single-pole-to-ground fault occurs. Based on the analysis, the fault voltage of current-limiting reactor is integrated, and its polarity can be identified based on the integration value. At last, a four-terminal modular multilevel converter (MMC)-based HVDC grid model is built in PSCAD/EMTDC, and the effectiveness of the protection scheme is verified by extensive simulations. [ABSTRACT FROM AUTHOR]

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]

Isolation of the fault dc lines during the initial short period after fault is a major challenge in the multi-terminal dc (MTDC) system based on a voltage-source converter (VSC). Existing protection schemes are mainly designed based on the numerical simulations, which lacks the theoretical analysis. In this paper, a high-frequency equivalent model of a VSC-based MTDC system is first proposed for the fault current calculation. Based on the dc fault analysis, it is validated that the high-frequency fault current components flow in the fault line and then dramatically decay in the healthy line. Accordingly, a novel dc fault detection method for the VSC-based MTDC system is proposed. In the proposed approach, the primary protection utilizes the transient high-frequency energy of line current and the fault line can be identified quickly without communication. In addition, the influence of the transient traveling-wave appearing on the long-transmission line on the proposed protection scheme is also investigated. Numerous simulation studies carried out in PSCAD/EMTDC have demonstrated that the proposed high-frequency equivalent model can be utilized for the initial dc fault analysis of the VSC-based MTDC system and the proposed protection scheme is effective in different time windows, different fault locations, and high fault resistances. Compared with the existing rate of change of current-based protection scheme, the proposed primary protection requires relatively low sampling frequency, is insensitive to the parameter changes, and has high robustness with respect to the outside noises and data missing. [ABSTRACT FROM AUTHOR]

The voltage ratings for dc transmission and distribution are not currently standardized because cable technology is advancing rapidly and therefore a low step-ratio dc–dc converter at multi-megawatt scale is a crucial technology for combining point-to-point dc links into multi-terminal networks. This paper presents a modular multilevel dc–dc converter configuration with only a single stack of half-bridge sub-modules (SM) and a resonant tank. The power processed through the stack is a small fraction of the throughput power if the step ratio is small ($1 - {\boldsymbol{v}_{\boldsymbol{L}}}/{\boldsymbol{v}_{\boldsymbol{H}}}$). Detailed analysis shows the required semiconductor ratings are much smaller compared to the front-to-front MMC and modular converters derived from standard dc–dc topologies. Further, the requirement for SM capacitive energy storage is also significantly less than those converters. The theoretical analysis and the operating principles have been verified by full-scale simulation examples and experiments with a down-scaled prototype. The SMs are operated with an adjustable phase shift to create a high frequency excitation across the resonant tank for further volume reduction. The SM capacitors self-balance without the need for switching pattern adjustments and the resonant operation provides soft switching for all power switches to increase efficiency. [ABSTRACT FROM AUTHOR]

Power electronic transformer (PET) composed of modular dual active bridge (DAB) blocks has a promising future in ac–dc hybrid distribution network because of its energy routing ability. The key element that restrains the development of PET is the vulnerability of power electronic devices during faults, especially dc faults. DC faults (mostly line faults) have very short transient process and no obvious fundamental frequency, which means fault location is difficult to be realized for dc lines in distribution network. Considering that fault location plays an important role in realizing fast fault clearance and recovery, this paper analyzed fault characteristics of dc line faults with respect to the basic structure of modular DAB block and proposed a two-terminal fault location method based on transient voltages measured in dc line boundaries. To evaluate the validity of proposed method, numerical simulations with different fault conditions were implemented in PSCAD/EMTDC. The simulation results indicate that the proposed fault location method located the faulted point quickly and correctly. [ABSTRACT FROM AUTHOR]

This paper proposes a transformerless dc–dc converter, which features a large voltage ratio, for medium-voltage dc (MVDC) applications. The proposed converter combines a boost converter with a series-parallel resonant converter which enables soft switching of the power switches and diodes. Further, the power switches and diodes of the proposed converter experience lower voltage stresses compared to the other topologies. The proposed converter is modular and, therefore, permits series and/or parallel connection of multiple units for the desired voltage and/or power ratings. This paper discusses the principles of operation, analysis, and design of the proposed converter. Simulation and experimental results are presented to demonstrate the effectiveness of the proposed converter. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive analysis and a novel solution for the discharge problem on the half-bridge submodules’ (HBSMs’) capacitors in the hybrid modular multilevel converter (MMC)-based high-voltage direct current transmission systems under a long-term (e.g., several minutes or longer) pole-to-ground fault condition. The mechanism of this issue is analyzed, and then a solution based on fundamental-frequency reactive circulating current (FFRCC) injection is proposed. The influence of FFRCC injection is analyzed in terms of the fluctuated arm capacitor energy and the current stress of semiconductors. Moreover, an enhanced arm voltage modulation method based on HBSMs and full-bridge submodules coordinate control is proposed to accelerate the capacitor voltage balance of HBSMs. Finally, the performance of the proposed control strategy is verified by the simulation results. [ABSTRACT FROM AUTHOR]

This paper presents a transformerless high-voltage dc–dc converter based on cascaded sub-modules. It is intended for interconnecting high-voltage or medium-voltage dc grids. The proposed dc–dc converter consists of two phase legs, each of which consists of an upper arm and a lower arm with their middle nodes crossly connected through a branch of active sub-modules. By properly controlling the output voltage of the cross-connected branch, a trapezoidal ac current is induced to interact with the ac voltage components in the upper and lower arm for rebalancing the power amongst converter arms. The features of modular design, single-stage power conversion, and transformerless structure make the proposed dc–dc converter gain the outstanding merits of wide voltage ratio range, high system efficiency, and light converter weight. A control scheme is also elaborated for guaranteeing the normal operation of the proposed dc–dc converter. A 100-kV 100-MW simulation model performed in MATLAB/Simulink verifies the feasibility of the proposed dc–dc converter. Experimental results obtained from a 100-V 1-kW laboratory setup also confirm the validation of the proposal. [ABSTRACT FROM PUBLISHER]

This paper presents fault analysis and protective inductors design for an offshore wind farm, where the power collection system in the wind farm and the power transmission link to the grid adopt high-voltage direct-current (HVDC) technology. This paper focuses on dealing with short-circuit faults in the HVDC link between the offshore station and the onshore station. The transient characteristics of the transmission system are analyzed in detail. The criteria of selecting protective inductors are proposed to effectively limit the short-circuit current and avoid the damage to the converters. A dc grid offshore wind farm is simulated, and the results demonstrate the effectiveness of the proposed protective inductors design. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC or M2C) is an emerging attractive multilevel topology for medium-voltage high-power applications. The capacitor voltage balancing control and the improvement of output current rating are two challenging issues of MMC. This paper proposed a new parallel-connected diode-clamped M2C (DCM2C), in which each arm consists of two parallel-connected clusters. In this topology, the capacitor voltages can be balanced automatically without any balancing control algorithms. Theoretically, no voltage sensors are needed to measure the capacitor voltages theoretically. Thus, the control of the converter can be simplified greatly. With a simple current-sharing control, an equal current distribution can be achieved between the two clusters. Furthermore, an extended parallel-DCM2C is proposed for larger current applications, and the corresponding current-sharing control method is developed. Experimental results validated the voltage self-balancing capability of parallel-DCM2C and the effectiveness of the proposed current-sharing control. [ABSTRACT FROM PUBLISHER]

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]

A novel apparatus based on variable inductors to control the insertion compensating voltage of the transmission line is proposed in this paper. The apparatus can change the alternative voltage into the desired alternative voltage directly by using the control strategy proposed in this paper and insert the voltage into the line. Since the amplitude and the phase angle of the voltage supplied by the apparatus can be adjusted continuously and independently, the apparatus can control the real- and reactive- power flow independently and realize the direct voltage regulation (DVR) and the phase angle regulation (PAR) of the transmission line. The control methods suited for the proposed apparatus about the independent control of real and reactive power flow in the transmission line and the DVR and the PAR are also proposed. The control strategy and methods are verified with experimental results and simulation results. [ABSTRACT FROM PUBLISHER]

This paper shows two applications for using vacuum circuit breakers (VCBs) in flexible ac transmission systems in the high-voltage (HV) power grid. This is first a mechanically switched reactor, which is series connected to a thyristor-controlled series compensator in order to enhance the inductive working range. Nevertheless, the switching frequency of VCBs is limited by their mechanical properties. In order to achieve a higher switching frequency, more VCBs are switched in parallel to each other. Second, a device is investigated, which consists of mechanically switched capacitors and a mechanically switched reactor. It is connected to a node in the HV system in order to enhance the voltage quality and to avoid a voltage collapse. Since VCBs are mainly available in the medium-voltage range, a transformer is used to connect the mechanically switched device to the HV system. In this paper, it could be shown that the mechanically switched series reactor is able to enhance the damping progress of power oscillations and that the mechanically switched device using VCBs can stabilize the voltage of the node in case of disturbances, such as faults in the power system. Furthermore, the electrical stress on the VCBs has been assessed for different simulation cases with PSCAD. [ABSTRACT FROM PUBLISHER]

A continuously variable series reactor (CVSR) is a novel application for power-flow control in meshed electric power networks. CVSR provides continuous modulation of line reactance by controlling the magnetization in a ferromagnetic core, which occurs by altering the direct current in a winding to bias the magnetization in the core. The deployment of CVSR in commercial power systems requires careful planning to determine if the equipment will meet design objectives and how the equipment responds during power system faults. This paper presents a modeling method that enables simulation of the magnetic interactions within a prototype device simultaneously with the voltages and currents of an externally connected electric circuit. Simulations of the model are compared to finite-element analysis and available experimental results. The simulations are performed on a simple single-phase system under nominal and fault conditions. The simulation results demonstrate the effectiveness of the method to represent CVSR transient behavior from the perspective of the ac power system. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) has become an attractive option for dc/ac conversion in many power system applications, in particular, for HVDC transmission. Due to its parallel arrangement of phase legs, the MMC is known to be a wye-configured architecture. In this paper, the equivalent delta formulation of the conventional wye architecture is introduced. The delta-configured MMC, enabled by series-stacking phase legs, is an alternative realization to the wye-configured MMC and has significantly different operating characteristics. The introduction of the delta MMC enables greater flexibility in converter voltage/current scaling for a given set of submodules, and may be an appealing option for dc/ac applications requiring a power tapoff from a high-voltage dc rail. The single-ended and differential delta topologies are derived, with the latter seen as the preferred topology due to its reduced energy-storage requirements. The principle of operation of the differential delta-configured MMC is described in detail. A dynamic controller is developed that regulates grid power injections while suppressing unwanted circulating ac currents. Comprehensive simulation results validate the differential delta MMC operation and the proposed control scheme. [ABSTRACT FROM AUTHOR]

In this paper, a novel solid-state fault current-limiting circuit breaker (SSFCLCB) based on a series resonance LC tank is proposed. Since the series configuration of the resonance structure (a capacitor and a reactor) of the SSFCLCB is invisible during normal operation mode, it shows negligible impedance in the line. In fault conditions, the SSFCLCB topology changes to a rectifier bridge and feeds the resonance structure with rectified ac voltage. With the rectified voltage, the series capacitor is charged and, as a result, the faulty line is opened and the fault current is seized. Hence, the suggested SSFCLCB cannot only limit the fault current but can also open the faulty line and acts as a circuit breaker. For confirmation of these aspects, simulations are performed using MATLAB software. Also, a prototype structure is designed and built for results confirmation. This experimental setup is established to prove that SSFCLCB has excellent performance in its startup, normal operation, and current-limiting/breaking conditions. The results show that the SSFCLCB has the ability to improve distribution network reliability and it can decrease the network fault current level successfully. [ABSTRACT FROM PUBLISHER]

For a large multiterminal HVDC system, it is important for a dc fault on a single branch to not cause significant disturbance to the operation of the healthy parts of the dc network. Some dc circuit breakers (DCCBs), for example, mechanical type, are low cost and have low power loss, but have been considered unsuitable for dc fault protection and isolation in a multiterminal HVDC system due to their long opening times. This paper proposes the use of additional dc passive components and novel converter control combined with mechanical DCCBs to ensure that the healthy dc network can continue to operate without disruption during a dc fault on one dc branch. Two circuit structures, using an additional dc reactor, and a reactor and capacitor combination, connected to the dc-link node in a radial HVDC system, are proposed to ensure that overcurrent risk at the converters connected to the healthy network is minimized before the isolation of the faulty branch by mechanical DCCBs. Active control of dc fault current by dynamically regulating the dc components of the converter arm voltages is proposed to further reduce the fault arm current. Simulation of a radial three-terminal HVDC system demonstrates the effectiveness of the proposed method. [ABSTRACT FROM PUBLISHER]

This paper investigates a new family of converters for high-power transfer between dc buses with different voltage levels. The proposed converters achieve high-voltage dc-dc conversion without an intermediate ac conversion stage. Three converter circuits are investigated, all based on the use of a common switching cell module. The first is a step-down converter, the second is a step-up converter, and the third is an inverting step-up/downconverter. For each converter, the structure is presented, its operation principle is explained, and the design equations are derived. A comparative analysis between the converters is carried out to highlight the merits and limitations of each topology. [ABSTRACT FROM PUBLISHER]

Capacitor switching transients are harmful to the capacitor and the switching device. For reducing these transient phenomena, this paper proposes a symmetrical structure transient limiter (SSTL). The proposed SSTL can automatically provide high impedance at the instant of capacitor energization, and, thus, the switching transients can be effectively suppressed. After the restriction, owing to the compensation of all power losses caused by the diodes and reactors, the SSTL acts as a short circuit during the steady state and, thus, has no effect in the circuit. As a result, the capacitor voltage and current waveforms will not be distorted. Moreover, due to the freewheeling effect in the SSTL, no transient overvoltage will appear across the switch contacts at the instant of capacitor de-energization even though the reactors are inserted into the circuit. Theoretical analyses of the proposed SSTL for the energization state, steady state, and de-energization state have been completely carried out. Also, the selections of the related components are depicted. Finally, the experimental results obtained using different methods of capacitor switching verify the feasibility and performance of the proposed SSTL. [ABSTRACT FROM AUTHOR]

With the increasing use of high-voltage ac cables at transmission levels, phenomena, such as current zero missing, start to appear more often in transmission systems. Zero-missing phenomenon can occur when energizing cable lines with shunt reactors. This could considerably delay the opening of the circuit breaker (CB), leaving the system unprotected and vulnerable to failures. Methods to prevent zero-missing phenomenon are still being studied and compared in order to identify effective countermeasures. This paper contributes to these efforts by presenting several countermeasures that can be applied to reduce the hazards of zero-missing phenomenon. The authors discovered that this phenomenon can be eliminated, merely by using an extra CB or a preinsertion resistor. [ABSTRACT FROM PUBLISHER]

The multiterminal dc wind farm is a promising topology with a voltage-source inverter (VSI) connection at the onshore grid. Voltage-source converters (VSCs) are robust to ac-side fault conditions. However, they are vulnerable to dc faults on the dc side of the converter. This paper analyzes dc faults, their transients, and the resulting protection issues. Overcurrent faults are analyzed in detail and provide an insight into protection system design. The radial wind farm topology with star or string connection is considered. The outcomes may be applicable for VSCs in the multi-VSC dc wind farm collection grid and VSC-based high-voltage direct current (HVDC) offshore transmission systems. [ABSTRACT FROM PUBLISHER]