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The capacitor condition monitoring is an important issue for the reliable operation of modular multilevel converters (MMCs) in high-voltage applications. The numerous capacitors make the monitoring for the capacitor condition complex and computation-consuming. This paper proposes a hierarchic capacitor condition monitoring strategy to assure the reliability of high-voltage MMCs. In the strategy, the switching times of the switching signals are counted to detect the condition of the capacitor in each submodule (SM). Then, the subsequent capacitance calculation method is only conducted to the abnormal capacitors indicated by the above condition detection algorithm. Meanwhile, the variations of the measured and reference capacitor voltages are employed to calculate the capacitance in the proposed method. The condition detection algorithm shrinks the scope where the abnormal capacitors locate, which avoids calculating the total capacitances in MMCs. In the proposed capacitance calculation method, the complex operations are further saved. Both the two measures reduce the computation burden effectively. The implementation of the whole monitoring strategy is simple and fit for high-voltage MMCs. Moreover, the strategy has no adverse influence on the normal operation of MMCs. The effectiveness of the proposed strategy is verified by the simulation studies with the professional tool Matlab/Simulink. [ABSTRACT FROM AUTHOR]

This paper summarizes the work performed by the Task Force —within Performance Characteristic Subcommittee of IEEE PES Transformers Committee— to investigate potential revisions to IEEE Std C57.142-2010. Although the scope of this document does not implicitly exclude HV and EHV systems, the existing document primarily includes examples and information on distribution voltage systems. A Task Force was formed to investigate the need to include information about power system transient events on HV systems more explicitly in this existing document. The Task Force was charged with the following tasks:-Establish system voltages encompassed by C57-142-2010 Guide. -Gather field data, reports, and literature on HV and EHV failures related to substation transients and transformer interaction. -Solicit input from the other technical committees on this subject. -Review IEC and CIGRE work in this area. -Recommend course of action on revision of the present Guide. -Recommend high level changes to the Guide. -Prepare final paper for the Subcommittee. In the course of this work several failure scenarios were identified. These are categorized and the failures are described in this paper. The final recommendations to the Subcommittee are summarized and recommendations are presented for the course of action on the revision/addition of HV and EHV material to the current C57.142 Guide. [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]

Modular multilevel converters (MMCs) in high- voltage applications contain large numbers of power switches, which increases the fault probability inside MMCs. This paper proposes a detection method for the faulty arm containing switch open-circuit failures in high-voltage MMCs to improve the reliable operations of MMCs. The proposed method uses the periodicity of the deviation between the maximum and minimum capacitor voltages to provide the detection reference. To yield a reliable reference, the repetitive signal generator is employed. Then, the fault is identified by the difference between the referenced and measured capacitor voltage deviations. An auxiliary criterion is also designated to avoid the false detection caused by the mutations in MMCs. The proposed method can reduce the difficulty in the selection of thresholds. Moreover, the proposed method is robust in high-voltage MMCs with inferior capacitor voltage balancing performance, which ensures detection rapidity and accuracy. Finally, the feasibility of the method is verified by simulation results in Matlab/Simulink. [ABSTRACT FROM AUTHOR]

Different from typical circuit breakers, breakers used to switch AC filters in convertor stations have to operate frequently according to the real-time operation mode and daily load changes. Therefore, the performance of filter breakers decreases very quickly, which leads to frequent accidents. Both preinsertion resistors and controlled switching devices can suppress switching transients. In this paper, a comparative analysis of whether a controlled switching device and preinsertion resistor should be adopted is carried out using PSCAD. The transient is calculated as the combined result of the switching angle and preinsertion resistor value, and the optimal configurations are proposed. After determining the resistor value and whether a controlled switching device should be included, the reference voltage of the arrester should be reevaluated. [ABSTRACT FROM AUTHOR]

Voltage source converters (VSCs) have been extensively used in power systems for interfacing renewable energies and other modern loads. Because of their nonlinear nature, a proper harmonic model of VSC devices must be established when conducting harmonic analyses in power systems. This is especially true for low-order harmonics, which are routinely encountered in present power systems. To this end, an analytical harmonic model that represents the low-order harmonic characteristics of VSCs is developed in this paper. The new model takes both the control loop and power loop (i.e., DC-link capacitor) of the VSC into account through rigorous mathematical derivations. The final result has shown that the VSC can be represented by a harmonically coupled impedance matrix at harmonic frequencies, which can be easily used for harmonic analyses of large-scale power systems. It also shows that the DC-link capacitor has a more noticeable impact on the harmonic impedance of VSCs with a smaller DC-link capacitance and low-order harmonics. The appropriateness of the new model was verified using EMT simulations and lab experiments. A complete procedure to construct the proposed VSC model is also established based on the research findings. [ABSTRACT FROM AUTHOR]

This paper proposes an adaptive model of modular multilevel converter (MMC) for electromagnetic transient (EMT) simulations. The model is applicable to MMCs with arbitrary numbers of half-bridge and full-bridge submodules. The proposed design includes average value model, arm equivalent model, and detailed equivalent model. It allows smoothly transitioning from one model to another during time-domain simulations depending on the desired accuracy and execution time constraints. Modifications required in conventional MMC models to achieve smooth transitions are presented in the paper. Time-domain initialization methods are developed for each constituting model, including initialization of the appropriate control system blocks. Validity and effectiveness of the proposed adaptive model is demonstrated using EMT simulations of 401-level MMC-HVDC system. [ABSTRACT FROM AUTHOR]

The missing capability of DC fault current limitation is an inherent disadvantage in the typical half-bridge modular multilevel converter (MMC), which restricts its application in long-distance high voltage direct current (HVDC) systems. The full-bridge submodule (FBSM) and clamped double submodule (CDSM) topologies allow for fault current self-clearing, but also suffer from additional hardware cost and power losses. In this paper, an improved submodule topology utilizing the new reverse-blocking insulated gate bipolar transistor (RB-IGBT) is proposed to improve system economy without compromising its performance. The operating principles and DC fault characteristics of the proposed topology are analyzed in details, including comparison with various topologies in terms of cost, losses, fault current clearing speed and thermal. Compared with the traditional CDSM topology, the proposed RBSM can clear DC faults faster with reduced cost of switching devices. A DC fault protection strategy for MMC-HVDC system is also presented. Simulation results under normal operating, permanent and transient fault conditions are carried out to verify the effectiveness of the proposed topology. [ABSTRACT FROM AUTHOR]

This paper presents a novel oscillation wave test system (OWTS) to detect the partial discharge (PD) of mounting defects in power cable insulation. In the proposed system, a charging capacitor is added to the circuit topology based on the traditional OWTS. The charging capacitor is charged to the desired voltage by a high-voltage DC power source. After the charging process, the test cable sample is charged by the capacitor. The resonant circuit is formed by the charging capacitor, the test cable, and the resonant reactor to create an oscillating decay voltage. With this novel method, no space charge is injected into the cable insulation. The effect of the charging capacitance on the charging efficiency is calculated with the Multisim software. This paper describes the working principle and the assembly of a laboratory prototype for the PD detection of a 500 m cable circuit. The test results show the feasibility of the novel OWTS method. [ABSTRACT FROM AUTHOR]

The integration of overhead lines in future MTDC grids leads to a higher fault rate compared to cables. Most of the faults are considered to be non-permanent, so that the line can be reconnected after dielectric recovery. The auto-reclosing procedure in HVDC systems reveals new challenges since multiple restrikes are possible within a short period of time. This paper proposes an auxiliary circuit for DC Circuit Breakers (DCCB) which enables to use the DCCB internal capacitor for the line pre-energization. A comprehensive description of the switchgear operations during the pre-energization sequence is provided and the fault property identification method is defined. The method is validated by simulations in a MTDC grid and a sensitivity analysis is carried out using EMTP-RV. [ABSTRACT FROM AUTHOR]

Aging of power system assets challenges utilities to maintain reliable service while minimizing their maintenance costs. Although malfunction of devices in the transmission or distribution networks can have catastrophic consequences, their failure is seldom unforeseen. Typically, the device aging occurs gradually over time until its health condition reaches a point of complete breakdown. The overall objective of this paper is to devise a non-invasive, online condition monitoring technique of circuit switchers for shunt capacitor banks. It aims to detect abnormal operation of these devices based on features derived from raw voltage and current waveforms, such that a predictive maintenance strategy may be employed before a complete failure occurs. The data analyzed in this paper are collected at a transmission capacitor bus, where the banks are equipped with pre-insertion impedance. Through a baseline range of values for selected parameters, the proposed technique is able to detect missing pre-insertion impedance during the capacitor energizing and failure of pre-insertion impedance switch out a few cycles after the energizing operation. [ABSTRACT FROM AUTHOR]

DC transformer is an essential equipment for formation of multi-voltage-level DC grid. This paper proposes a novel non-isolated Boost-type alternate arm DC transformer (Boost-AADCT) for medium- and high-voltage level DC grid application. The Boost-AADCT is comprised of three submodule-cascaded arms, two thyristor-cascaded director switches, and two buffer capacitors. Bidirectional power transmission is achieved by alternatively charging and discharging of the intermediate arm following the Boost principle, while the pulsating power component caused by intermittent transmission is actively smoothened by grid-side buffer subcircuits formed by the buffer capacitors and grid-side arms. The active power buffering method increases the tolerance range of voltage ripple of the buffer capacitor, and the required energy storage capacity is resultantly reduced. Circuit configuration, operation principle, and mathematical model are first introduced. Then, the control strategy is proposed, the main circuit parameters are designed, and bidirectional fault-blocking capability is analyzed. Comparison with other DC-DC converters shows that the Boost-AADCT significantly reduces the requirement of semiconductors and has relatively lower synthetic cost. At last, performance of the Boost-AADCT and the proposed control strategies are evaluated by simulation tests conducted in MATLAB/Simulink environment. [ABSTRACT FROM AUTHOR]

This paper analyzes the effects of shunt capacitors installed on the low voltage sides of 10/0.4 kV distribution transformers on the operation of these transformers. Using the results of an extensive measurement campaign, this paper compares: real and reactive power losses, secondary-side current, and primary-side apparent power before and after the installation of transformer capacitors. The following four cases of reactive power compensation are considered and discussed: adequate, conditionally adequate, and total compensation, as well as overcompensation. The presented analysis helps to identify transformer substations where the initial compensation should be changed or adjusted, in order to improve transformers’ operational performance. Voltage dependencies of real and reactive power demands of the transformer-supplied loads are taken into account, based on the previously identified load characteristics in the considered networks as well as voltage dependence of reactive power of shunt capacitors. The result with and without including these voltage dependencies in the analysis is compared and discussed. [ABSTRACT FROM AUTHOR]

This paper introduces an averaging control strategy of the phase-unit losses of modular multilevel converters (MMCs). The quantitative analysis of the MMC phase-unit loss distribution under unbalanced ac conditions is carried out by investigating the individual component of the MMC arm currents. It shows that the unbalanced arm currents will result in unbalanced phase-unit losses. To address this issue, this paper proposes a predictive closed-loop-control-based zero-sequence voltage injection method that results in much averaged loss distribution under unbalanced ac conditions. The essence of the new method is that it adjusts the three-phase sinusoidal references so that the unbalanced arm currents are fully utilized to average phase-unit losses. This paper shows the effectiveness of the new approach by electromagnetic transient simulations on PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

This paper proposes a new modulated model predictive control method for the control of modular multilevel converters (MMCs) in voltage source converter-high voltage dc systems. The proposed method retains the advantages of the conventional finite control set-model predictive control methods by programing the nonlinear properties of the MMC into the design calculations while minimizing the ac line and circulating current ripples and steady-state error by generating modulated switching signals with a fixed switching frequency. In this paper, first, the predictive modeling of the MMC is provided. Next, the proposed control method is described. Then, the application of the proposed method to an MMC system is detailed. Finally, experimental results from an MMC system connected to a three-phase grid are provided to validate the theoretical outcomes. [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]

Modular multilevel converter based high voltage direct current (MMC-HVDC) transmission is the prospective technology for large-scale wind farms integration. In this paper, a comprehensive coordinated control strategy for DC fault ride-through (FRT) of wind farms connected to the grid through bipolar MMC-HVDC transmission is proposed. To reduce the requirement of breaking speed and capacity of DC breakers, an active current-limiting control method is proposed to limit the rise rate of DC fault current, and the principle of parameter selection is designed. After DC breakers isolate fault, the control mode switching methods of bipolar MMC are designed to make the fault pole MMC operate as a STATCOM to support the AC grid and improve the active power transmission capability of the healthy pole MMC, which can reduce the impact range of unbalanced power between wind farms and converter station. In addition, a coordinated control strategy for wind farms load-shedding considering speed constraints is designed to realize the optimal distribution of load-shedding power among wind turbines, which can regulate unbalanced power together with the healthy pole MMC to realize DC FRT. Finally, the effectiveness and feasibility of the proposed control methods are verified by simulation results. [ABSTRACT FROM AUTHOR]

The emerging voltage source converters (VSCs) such as hybrid cascaded multilevel converters (HCMCs) are promising converter technologies which enable higher converter efficiency, compactness, and DC fault resilience for high voltage direct current (HVDC) transmission. However, the large number of switching events due to the converters’ submodules introduces a heavy computational burden for electromagnetic transient (EMT) simulations. Efficient and accurate EMT-type converter models play a vital role in the control and design of HVDC converters. Several computationally efficient models for the HCMCs are proposed in this paper, including the detailed equivalent model (DEM) and several average value models (AVMs) of varying accuracy. Additionally, a combined model is proposed which allows the users to switch among the DEM and AVMs during dynamic simulations. The proposed models are validated against the DEM for dynamic transients. The simulation results demonstrate good modeling accuracy of the proposed models. The AVMs are also shown to significantly improve simulation efficiency compared to the DEM, which is very desirable especially for HCMCs with a large number of submodules. [ABSTRACT FROM AUTHOR]

Pole-to-pole dc short circuit fault blocking capability is a major concern when modular multilevel converters (MMCs) are applied in high voltage direct current (HVDC) transmission systems. Traditional MMCs based on half-bridge submodules (HBSMs) cannot block the fault current, thus needing expensive direct current circuit breakers (DCCBs). There are MMCs based on improved submodule topologies that can block dc faults, but usually with significantly increased device cost. This paper proposes an improved HBSM-based MMC (HB-MMC) which needs fewer extra semiconductor devices. The improved HB-MMC is more competitive than other schemes when it comes to investment cost and loss. An H-bridge circuit composed of two director switches and two diode valves is installed around one arm of the HB-MMC, which will not change the direction of current during the normal operation, whereas it can change the direction of the fault current when a pole-to-pole dc short circuits fault is detected. After two director switches have been turned off, a reverse-biased voltage provided by one arm can block the fault current. The topology can be further improved to provide reactive power during faults. Simulation results are presented to validate the features of the proposed improved HB-MMC. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is one of the most attractive topologies for high-voltage and high-power applications. Reliability is an important challenge for MMCs due to large amounts of power switches in the phase leg of the MMC. To enhance the reliable performance of MMCs, an effective and accurate fault detection and localization strategy for MMCs with the nearest level modulation is proposed in this paper. According to the general characteristics of the MMC, the capacitor voltage of the faulty submodule (SM) is higher than the healthy ones’. Based on the sorting algorithm, the faulty SM will have the maximum voltage and it will be identified by the proposed fault detection method. The auxiliary monitoring is supplemented to the detection algorithm for the cases of faults occurring in multiple SMs. The faulty SM localization strategy is based on the differences of capacitor voltages. The faulty switches are located by the distinctions of capacitor voltage in different current intervals. The proposed strategy could be applied under the cases of single or multiple SMs failures and identifies three kinds of the power switches faults in the faulty SM. The validity of the proposed strategy is verified by the simulation results in MATLAB/Simulink. [ABSTRACT FROM AUTHOR]

Transient voltage and current stresses are essential for the design and protection of multi-terminal DC (MTDC) transmission systems, especially under the short-term dynamics after severe grid faults. In this paper, we propose a discrete-time model for the modular multilevel converter based MTDC (MMC-MTDC) system considering the DC-side disturbances in the system. Based on the piecewise linear modeling, the state variables such as the transient voltage on the sub-module (SM), arm current, etc., can be calculated analytically for the critical short term (within 10 ms) after the severe DC fault while the system protection is yet to be activated. Parameter sensitivity analysis is also performed to give a better system design guideline. The estimation results are validated by detailed electromagnetic (EMT) circuit simulations and the scaled-down experimental laboratory prototype. [ABSTRACT FROM AUTHOR]

Fast and detailed calculations of harmonics, generated by three-phase uncontrolled rectifiers with finite dc smoothing, can be performed by using detailed analytical procedures (DAPs). Several DAPs are developed for continuous- and discontinuous-conduction modes (CCM and DCM) when balanced operation of the system is assumed. The authors of this paper have recently proposed a new DAP (model 1a) in which nonideal conditions, such as unbalances and even harmonic voltages on the ac side, are taken into account. However, only CCM is assumed in model 1a. The main objective of this paper is to overcome this limitation of model 1a so that a new DAP (model 1b) for DCM is presented, which is capable of considering unbalanced operation. Moreover, the boundary of the DCM is also estimated for model 1b under nonideal conditions. [ABSTRACT FROM PUBLISHER]

This paper describes a method of fast commutation of DC current into a capacitor. Theoretical study is provided, which enables evaluation of commutating DC current for the given contact velocity, capacitance, and dielectric strength. It is concluded that a non-zero contact velocity at separation is required, and a corresponding switch design is proposed. Experimental results on a laboratory setup illustrate successful DC current commutation up to 400 A, with voltages rising to 1.3 kV. Further experiments demonstrate that parasitic parameters reduce the magnitude of the current that can be commutated. A detailed non-linear PSCAD model and a linear model for the parasitic circuit are presented to enable prediction of the success of commutation. The model accuracy is confirmed with experimental tests. The DC current commutation in the proposed method occurs 5–10 μs after the contact separation, which is much faster than with other methods employing moving contacts. A further benefit of the extremely short arcing is the elimination of thermal issues on contacts, and possible simplified design of the mechanical switch. [ABSTRACT FROM AUTHOR]

The modular multilevel converter is the state-of-the-art topology for voltage source converter HVDC. Despite its advantages, this converter handles large internal low-frequency energy ripples, and the capacitance that supports these dynamics is a key design parameter that affects the operating region of the converter. Different strategies can be found in the literature to increase the feasible region of operation of the converter. Nevertheless, they are typically open loop in nature and use precalculated control references. This paper presents an alternative based on model predictive control that steers the system through optimal control trajectories that are calculated online. This provides feedback and corrective control action in real time. The predictive controller used for this purpose is presented and a linear time-varying approximation is used to reduce the computational burden of the algorithm. The feasible boundaries of the converter are sought and the final performance of the control algorithm is evaluated through detailed simulations using a switching model of the converter. [ABSTRACT FROM AUTHOR]

Switching of shunt capacitors can cause transient overvoltage. The voltages are traditionally evaluated with electromagnetic transients (EMT) simulation programs. It is a very time-consuming process mainly due to the need to convert a power-flow (or short-circuit) case to the EMT case. This paper proposes an idea to conduct capacitor switching studies directly inside commercial power-flow/short-circuit tools such as PSS/E, thus avoiding the data-conversion process. The idea is to conduct switching studies in the frequency domain. The frequency response of the study network is determined using the short-circuit solution engine of the host commercial software. This paper presents the theory, unique issues, and implementation details of the idea. The effectiveness of the proposed method is verified by comparing the results with those of the EMT simulations. [ABSTRACT FROM PUBLISHER]

The standard alternative-arm converters (AAC) with the sweet spot or short overlap operation have faced a challenge of balancing submodule (SM) capacitor voltages, where this issue has not been paid much attention so far. In this paper, a new control strategy for the SM-capacitor voltage balancing of the AAC applied in high-voltage dc transmission systems is proposed. The suggested control technique is composed of three regulators, where a control for the average voltage of all SM capacitors, the voltage balancing among the converter arms, and the voltage balancing of the individual SM capacitor are performed. Based on the operation principle of the standard AAC, which activates only three of six converter arms simultaneously, the energy balancing of the three activated arms is performed every one sixth of the fundamental cycle by the converter common offset voltage. Band-stop filters with the central frequencies at the fundamental, second-, and third-order components of the line frequency are applied to obtain the average values of the SM capacitor voltages for the feedback controllers. The simple proportional and integral controllers are employed for this scheme and the voltages of SM capacitors are mostly balanced during the steady states and kept closely to the rating with overshoot of less than 7% under the transient conditions. The proposed control method is verified by simulation results for a 33-level AAC under both normal and fault conditions. [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]

This paper presents an accurate steady-state initialization method for electromagnetic transient (EMT) models of modular multilevel converters (MMCs). The proposed method uses steady-state arm voltages and currents to initialize internal electrical variables and control systems of the converter, including capacitor voltage and gating signals of each sub-module. In addition to typical voltage source converter upper level control, MMC specific controls are also considered, including circulating current control, dc ripple control, and capacitor voltage balancing algorithm. EMT simulations on 101-level and 401-level MMC-HVDC systems confirm the accuracy of the proposed method. [ABSTRACT FROM AUTHOR]

A novel scheme is proposed to monitor the operation of capacitors scattered in MV distribution systems. It first codes each capacitor with a distinctive switching character by setting a unique combination of a three-phase switching order and time interval. As a result, the transient of the three-phase reactive power during capacitor switching will appear in a unique way. Then, the reactive power disturbances monitored in the substation are decoded to identify which capacitor is being switchedonandoff. A detailed decoding method is developed to realize this identification without being impacted by the switching interference of loads and other reactive power compensation devices. Moreover, the implementation of the proposed scheme in the field is elaborated, and the resultant three-phase voltage unbalance is analyzed in this paper. Compared with existing methods, this scheme only requires one power quality monitor located at the substation to monitor the operations (energizing or de-energizing) of downstream distribution capacitors without extra communication devices. The operation information is valuable for tackling power quality problems caused by capacitor switching and optimizing the distribution grid operation. [ABSTRACT FROM AUTHOR]

This paper presents a new concept and approach for topology control and switching in distribution systems by extending the traditional circuit switching to laterals and single-phase loads. Voltage unbalance and other key performance indicators including voltage magnitudes, line loading, and energy losses are used to characterize and demonstrate the technical value of optimizing system topology on a per-phase basis in response to feeder conditions. The near-optimal per-phase topology control is defined as a series of hierarchical optimization problems. The proposed approach is applied to IEEE 13-bus and 123-bus test systems for demonstration, which included the impact of integrating electric vehicles (EVs) in the test circuit. It is concluded that the proposed approach can be effectively leveraged to improve voltage profiles with electric vehicles, the extent of which depends on the performance of the base case without EVs. [ABSTRACT FROM AUTHOR]

For lack of the appropriate global distribution of dc currents, the conventional suppression method to suppress dc bias based on capacitive dc blocking device (BD) redirects current to the ground as much as possible, which predisposes to the exceeding neutral current of other transformers in the regional power grid and leads to the contradiction between the power grid corporation and other public enterprises. Therefore, this paper presents a flexible suppression method for dc bias based on a novel dc-bias suppression device. First, a current balancing device (CBD) based on adjustable parallel resistances is designed. The mathematical model for global optimal switching of CBDs is established by a field-circuit coupling method with the equivalent resistance network of an ac system along with the location of substations and ground electrodes. The optimal switching scheme to minimize the global maximum dc current is obtained by gravitational search algorithm. Based on the aforementioned work, we propose a suppression strategy considering electro-corrosion of metal pipelines. The effectiveness and superiority of suppression methods are verified by comparative case studies of the Yichang power grid. [ABSTRACT FROM PUBLISHER]

An effective measure to mitigate a shunt capacitor caused harmonic resonance is to add a C-type damping unit in series with the capacitor. If there are multiple switchable capacitors, each of them requires a C-type damping unit. The cost and space requirement for implementing these changes, therefore, increase with the number of switchable capacitors. In view of this disadvantage, a scheme that allows multiple switchable capacitors to share a common damping unit is proposed in this paper. The damping unit is designed in such a way that it will provide adequate damping for all possible on/off combinations of the capacitors involved. In addition and proven methodically, the design method can further guarantee resonance free for all possible network impedance conditions. Performance and usefulness of the proposed scheme has been demonstrated through comparative studies on actual capacitor application cases. [ABSTRACT FROM PUBLISHER]

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]

Modular multilevel converters (MMCs) have become one of the most promising topologies for high-voltage dc–ac conversion. DC-side fault blocking capability of the MMC has prompted significant research in recent years. In this paper, a new switched capacitor submodule (SCSM) for MMCs is proposed which provides operation with DC fault blocking capability. In addition, successful voltage balancing is achieved with half the number of voltage sensors used with existing MMC cells. Generally, conventional sensor-based balancing techniques require a significant amount of measurements, 2m(N-1) voltage sensors, and 2m current sensors for an N-level m-phase MMC. The proposed cell will thus aid in reducing the complexity of the control system. A detailed illustration of the operational concept of the proposed architecture is presented in this paper. A comparison between the proposed SCSM and other existing MMC cells has been included to highlight the benefits of the proposed SCSM. A simulation model for an MMC-based HVDC system along with the proposed cell has been built to test its performance during normal as well as abnormal operating conditions. The simulation results show the effectiveness of the proposed architecture. [ABSTRACT FROM PUBLISHER]

This paper highlights an immediate need for renewable generation developers, manufacturers, utilities and industry standardization institutes to jointly address a challenge of capacitor switching transient immunity in inverter-based wind or solar farms. The need was identified from wide-area operation records which showed partial or full generation loss in coincidence with routine utility-owned capacitor switching. The lack of a clear industry guideline has caused confusion to all involved parties. This paper recommends all entities to address this challenge collaboratively through enhanced equipment specification, design, and test before a well-defined industry guideline becomes available. [ABSTRACT FROM PUBLISHER]

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]

A parallel hybrid modular multilevel converter (PHMMC) belongs to the class of modular multilevel converters, which have become potential candidates for high-voltage direct-current (HVDC) transmission systems. Due to the circuit topology of a PHMMC, the dc bus voltage contains low-order harmonics and cannot be fully regulated at a constant dc voltage. The dc bus voltage, if not properly controlled, leads to improper power transfer and increases the magnitude of dc current ripple on the dc transmission line. This paper proposes a zero-sequence voltage injection (ZSVI)-based model predictive control (MPC) strategy to control the dc current/power flow and simultaneously minimize the dc current ripple. The proposed strategy takes advantage of a cost function minimization technique to determine and inject the optimal zero-sequence voltage components into the dc- bus voltage of a PHMMC system. This paper derives a discrete-time dynamic model of the dc transmission-line current and, correspondingly, develops a predictive model. The predictive model is used to inject the appropriate amount of zero-sequence voltage components to the dc bus reference voltage waveform. Compared with the existing triplen harmonics injection method, the proposed ZSVI-MPC strategy improves the performance of a PHMMC system in terms of minimization of the dc current/voltage ripple. Performance of the proposed strategy for a 21-level PHMMC-based HVDC station system is evaluated based on time-domain simulation studies in the PSCAD/EMTDC software environment. The reported results demonstrate superior performance of the PHMMC-HVDC station operating based on the proposed ZSVI-MPC strategy, under various operating conditions, as opposed to the existing triplen harmonics injection method. [ABSTRACT FROM AUTHOR]

This paper proposes a model predictive control (MPC) method for modular-multilevel-converter (MMC) high-voltage direct current (HVDC). To control the MMC-HVDC system properly, the ac current, circulating current, and submodule (SM) capacitor voltage are taken into consideration. The existing MPC methods for the MMC-HVDC system utilize weighting factors to configure the cost function in combinations of the SM capacitor voltage balancing algorithm, ac current control, and circulating current control. Because all combinations of the switch states are considered in order to minimize the cost function, their possible combinations increase geometrically according to the increase of the level of the MMC, which is a significant disadvantage. This paper proposes a new MPC method with a reduced number of states for ac current control, circulating current control, and the SM capacitor voltage-balancing algorithm. The proposed cost functions are divided into three types according to their control purposes. Each cost function determines the minimum number of states for controlling the ac current, circulating current, and SM capacitor voltage. The efficacy of the proposed controlling method is verified through simulation results using PSCAD/EMTDC. [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]

The model of a modular multilevel converter (MMC) determines the extent of critical circuit information that electromagnetic transient simulations can reveal. In this paper, two MMC models are proposed for efficient real-time hardware-in-the-loop (HIL) emulation on the field-programmable-gate-arrays (FPGA). The nonlinear switch-based model employing the insulated-gate bipolar transistor (IGBT) dynamic curve-fitting model considers factors affecting its transient performance so that device-level behavior such as power loss and junction temperature can be reproduced accurately in the electro-magnetic-thermal simulation of a power converter for its design evaluation. Meanwhile, regarding the MMC submodule as a transmission line stub achieves faster computation speed and enables the formation of a hybrid arm to save FPGA hardware resources. As the large network that the MMC presents is burdensome for real-time execution with a small time-step, circuit simplification based on partitioning and merging is conducted. Hardware implementation of a three-terminal high-voltage direct-current system containing an MMC-based dc–dc converter is carried out and the efficacy of proposed models is validated by comparing HIL emulation results with the offline simulation tool PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

Shunt power capacitors are essential to system operation and reliability as a critical reactive power source to increase system capacity, support voltage magnitude, improve power quality, and reduce reactive current flow. To mitigate capbank switching transients, the synchronous switching with preinsertion resistor technology is developed and used at an extra-high voltage transmission system. In this paper, the coupling relationship between several key factors that contribute to switching transients is studied. Optimization solutions are developed to improve the switching process. Field operations with an updated setting are compared with original setting records to prove the correctness of the study. [ABSTRACT FROM PUBLISHER]

This paper studies the dc fault development stages and analyzes the fault dynamics in a point-to-point multilevel modular converters (MMC)-based dc connection. First, the effect of the dc grid configuration on the normal operation was investigated by comparing an asymmetric monopole with metallic return and a symmetric monopole. Then, the main parameters that affect the dc fault response of a grid, such as the fault type, impedance and converter blocking, were examined. Compared to previous studies, which are based on simulation results, the analysis is performed hereby both in theory, deriving the equations that describe the dc fault stages, as well as using experimental results obtained in the designed laboratory setup. The setup consists of two MMC terminals connected to two ac sources representing independent ac grids. These terminals are connected using a simple dc link based on pi-section equivalent of dc cables. The obtained results, which verified the theoretical analysis, can be used to determine the protection function thresholds of the MMC, as well as to estimate the developed stresses on dc lines during fault conditions and to define the design requirements for dc breakers. [ABSTRACT FROM PUBLISHER]

The current-carrying capability of dc lines is limited by their thermal and electric stress limits. Thus, the line current must be maintained within the permissible operational region to protect the lines from damage. In a dense dc grid, control over each line current cannot be achieved without including additional control devices. In this paper, a dual H-bridge current flow controller (2B-CFC) is used to manage the dc grid line power flow by providing dc voltage compensation in series with dc lines. A centralized hierarchical control system is proposed to coordinate the operation between multiple CFCs. A novel voltage-sharing control scheme is demonstrated. It is shown that such a scheme reduces the workload on a single CFC by sharing the required control voltage between multiple CFCs, and, in addition, can be used to avoid control conflicts among active CFCs during communication failure. An experimental platform consisting of a three-terminal dc grid and small-scale 2B-CFC prototypes has been developed to validate the concepts. For completeness, the CFC performance has been analyzed for overload conditions and when no communication exists. Small-scale dc circuit breakers have been developed to study the CFC performance under a pole-to-pole fault. [ABSTRACT FROM PUBLISHER]

The dc tap or dc transformer will play an important role in interfacing different voltages of dc links in dc grids. This paper presents an isolated resonant mode modular converter (RMMC) with flexible modulation and assorted configurations to satisfy a wide variety of interface requirements for medium-voltage dc (MVdc) networks. The transformerless RMMC, as introduced in the literature, implemented a restricted modulation scheme leading to a very limited range of step ratio and the diode rectifier resulted in unidirectional power flow. Both of these limitations are removed in this proposal and galvanic isolation has also been added. Moreover, this new RMMC approach can serve as a building block for variety of configurations. Two such derived topologies are given, which inherently balance the voltage and current between different constituent circuits and realize the high power rating conversion for very low or very high step-ratio application. The theoretical analysis is validated by a set of full-scale simulations and a downscaled experimental prototype. The results illustrate that this isolated RMMC and its derivatives have promising features for dc taps or dc transformers in MVdc applications. [ABSTRACT FROM PUBLISHER]

Recently, a modular multilevel converter (MMC) has become one of the most popular converters in medium/high-power applications. In this paper, a new technique is presented to decrease the computational time of the MMC by using switching state grouping for the predictive controller. The proposed technique has the ability to control the output current, minimize the circulating currents, and balance all capacitor voltages, simultaneously. For a predefined number of MMC submodules, the number available switching states is known. The proposed technique uses two stages of predictive modules. In the first stage, the switching states of the MMC are divided equally into a number of groups, controlled simultaneously by the same number of predictive modules. The optimal number of the switching state groups in the first stage is obtained. In the second stage, only one predictive module is used to obtain the optimal switching state of the MMC without any other complexities or additional sorting controllers. The effectiveness of the proposed control technique has been verified by using MATLAB/SIMULINK software considering steady state and dynamic analysis. The results of the proposed technique are compared with the conventional one in terms of computation time, verifying better performance of the proposed method. [ABSTRACT FROM PUBLISHER]

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]

The large number of switching elements in the modular multilevel converter (MMC) is a challenging problem when modeling the MMC-HVDC systems for the computation of electromagnetic transients. The modeling complexity increases even further when a multiterminal (MT) MMC-HVDC system is used to integrate offshore wind farms (OWFs) with power-electronics-based wind energy converters, such as doubly fed induction generators (DFIGs). This paper compares modeling accuracy and computational performances for various combinations of MMC and OWF models. Onshore and offshore ac fault simulations are performed for an OWF system composed of DFIG-type wind turbines and connected to a practical ac grid through an MT MMC-HVDC system. The OWF system model includes the detailed representation of the offshore collector grid and the associated overcurrent protection. The offshore MMC controls include an offshore fault current limiter and fast OWF power generation reduction-based fault-ride-through function. [ABSTRACT FROM PUBLISHER]

This paper reveals the existence of poorly damped resonant modes in the modular multilevel converter (MMC) that can result in harmonic instability. A small-signal model based on dynamic phasors is introduced which takes into consideration the dynamics of internal variables, such as circulating arm currents, which have a tendency for a large second harmonic component and module capacitor voltage ripple, which has other harmonic components. The small-signal model is validated by comparison with the electromagnetic transient simulation of a detailed model. It is shown that the use of an active circulating current suppressing scheme can effectively improve the damping of internal harmonic modes in comparison with a passive filter used for the same purpose. The damping of internal harmonic modes has been shown to be significantly impacted by the gains of the MMC's outer-loop controller but is less sensitive to the ac system strength or the gain of the phase-locked loop. A larger arm resistance also increases the damping of these internal modes. [ABSTRACT FROM PUBLISHER]

This paper proposes an optimal design method of modular multilevel converter (MMC) levels to approach the reasonable compromise between accuracy and computational efficiency. The entire procedure is benchmarked by a Thévenin equivalent 451-level MMC model. First, the calculation method of the critical maximum simulation time step is proposed to faithfully represent each voltage level of the staircase waveforms. Second, the previously developed improved analytic hierarchy process is adopted to integrate the relative errors of all the main electrical parameters into one comprehensive relative error at each MMC level. Third, the steady state, ac, and dc transient state relative error results as well as the central-processing unit times of all models are normalized and curve fitted based on the benchmark model. Finally, the optimal MMC level is obtained by the plotted curves and the given subjective weights considering the accuracy and efficiency. And the effectiveness and robustness of the proposed method are validated by various simulation scenarios on PSCAD/EMTDC. [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]