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This paper presents a reduced-stage three-phase isolated AC/DC converter topology used for auxiliary power units (APUs) in avionics and electric vehicle (EV) applications. Traditional two-stage cascaded AC/DC converters are widely used for these applications due to their ease of implementation and simple control schemes. With the objective of improving the power density of the APUs, the proposed converter achieves reduction in the number of active power devices along with a smaller DC link capacitance by implementing an unconventional structure with a bandpass filter, to allow only the required frequency components to transfer the rated power. To accurately characterize the proposed converter, a detailed formulation of the filter design for the circuit along with mathematical derivation of the control scheme is presented. In terms of achieving enhancement in the overall efficiency, a comparative study is presented that focusses on the conduction and switching losses incurred in the proposed topology as compared to the state of the art (SOA) two-stage AC/DC conversion topologies. The analyses portray efficiency improvement of 0.8% for a 6 kW rated load, with significant reduction in active device losses. Additionally, to support the formulations and derivations, simulation studies and experimental verifications are also presented. The results show an input power factor of 0.9918 (lagging) with a conversion efficiency of 97.6%, a tightly regulated output voltage with less than 1% peak-peak ripple and the input current total harmonic distortion (THD) of as low as 1.57%. [ABSTRACT FROM AUTHOR]

Currently, HVDC is in the spotlight for long-distance transmission and power connection between countries. Among them, MMC-HVDC is a new conversion technology that can solve the problems of switching loss and harmonics. MMC consists of many submodules connected in series. And the one submodule is composed of IGBTs, Diode, and Capacitor. At the transient state, the charged capacitor is discharged and added to the fault current. It will generate tens of times the surge current during the delays all IGBTs turn-off. This causes semiconductor device destruction. To solve this problem, we applied SFCL with fast response characteristics to the MMC. The response speed of the resistive SFCL is faster than that of the MMC system protection device, which can effectively limit the capacitor discharge current before the protection device operates. In this paper, AC grid, MMC-HVDC, DC cable and SFCL were modeled using PSCAD/EMTDC. After that, we confirmed the capacitor discharge current, current limiting rate, and interruption characteristics through DC line short circuit failure simulation. As a result, SFCL effectively limits the capacitor discharge current and continuously lowers the level of the fault current. [ABSTRACT FROM AUTHOR]

Two-stage AC-DC converters are considered as a prominent solution for DC-type electric vehicle (EV) chargers. However, this kind of architecture suffers from high switching losses with large heatsink and DC-link capacitor volume. To relieve this issue, this paper presents a new hybrid modulation for DC-type EV chargers, where a two-phase clamped discontinuous pulse-width-modulation (DPWM) in the front-end circuit is cooperated with the variable frequency triangular-current-mode (TCM) zero voltage switching (ZVS) or its simplified implementation, i.e., boundary-ZVS (B-ZVS) strategy, in the back-end circuit. The former can stop the switching actions in the front-end stage during two-thirds of the grid period, while the AC currents are at their highest values, which can yield to the best switching loss reduction and deliver high power factor operation. Besides, TCM-ZVS or B-ZVS modulations can achieve ZVS turn-on action for all semiconductors during all operating range in the DC-DC stage to further reduce the power losses on the semiconductors. With such characteristics, the proposed strategies can reduce the switching losses of the system to the best extent, and thus allow an enhancement of the system power density by improving the power conversion efficiency. The proposed strategy is described, analyzed, validated, and benchmarked in a 5 kW SMD SiC MOSFET-based two-stage AC-DC converter. A 99% power efficiency can be achieved with the solution implementing the TCM-ZVS strategy at an output voltage of 400 V and rated power. [ABSTRACT FROM AUTHOR]

The line commutated converter (LCC) based HVDC projects generally use passive AC filters for harmonic cancellation, which has disadvantages of large footprint, poor filtering effect due to the offset of the resonance point, etc. This paper introduces a high-power LC hybrid active power filter (HAPF), which can meet the requirements of filtering and dynamic reactive power compensation. First, the passive part of the HAPF adopts two parallel single-tuned LC filters, which has low impedance at 12th and 24th. Second, considering the effects of the time delay of the high-voltage sensors, current controller and valve controllers, the possible negative impedance of the HAPF may bring a great risk of resonance and thus endanger the stability of the system. Third, an impedance model of the HAPF is established and the impact of the time delay on system stability is analyzed. Further, an adaptive resonance damping strategy based on least mean square (LMS) algorithm is proposed. Finally, the overall performances of the HAPF are verified by simulations on PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

Both the local and concurrent commutation failures (LCF and CCF) are concerned threats to the secure operation of multi-infeed LCC-HVDC (MIDC) systems, which will be largely impacted by the voltage stability. However, this impact has not been clearly revealed yet by earlier works due to the lack of the related analytical evaluation method. Thus in this paper, the analytical evaluation method related to the LCF and CCF with the voltage stability in MIDC systems is firstly proposed considering the complicated inter-inverter interactions. This is realized by deriving the analytical expressions of the LCF and CCF immunity indices (LCFII and CCFII) as the functions of the voltage stability factor (VSF) based on the system quasi-steady-state model. Moreover, the proposed method is used to clearly evaluate the impact of the voltage stability on the LCF and CCF by comprehensively conducting the parametric dependence analysis of the LCFII, CCFII, and VSF versus various system parameters and operation variables. The analysis results indicate that in some certain situations, increasing the voltage stability of MIDC systems however imposes an unexpected negative impact on the LCF and CCF. Finally, the simulation results of the PSCAD/EMTDC program based on a tri-infeed LCC-HVDC system validate the theoretical developments. [ABSTRACT FROM AUTHOR]

Hybrid dc circuit breakers (HCBs) are recognized as suitable devices for protecting high-voltage direct-current (HVDC) systems, along with other dc circuit breakers (DCCB). However, compared to mechanical circuit breakers, HCBs exhibit higher conduction losses. Such losses are inevitable under no-fault conditions as current may flow through some of the semiconductor switches. An integrated circuit breaker (ICB) minimizing these losses is presented in this paper, and this is achieved by replacing semiconductor switches by mechanical components in the current path. For completeness, the topology design, operating sequence and a mathematical analysis for component sizing of the device are provided. In addition, an estimation of the conduction losses is quantified. It is estimated that the power losses of an ICB are 2 to 30% of an HCB only. The ICB has been implemented in PSCAD/EMTDC to demonstrate its effectiveness for isolating dc faults, with simulations conducted on a three-terminal HVDC grid. [ABSTRACT FROM AUTHOR]

Inertia requirement is the paramount challenge in future power systems with a significant share of renewable energy generators. A promising solution to this issue is applying Distributed Virtual Inertia (DVI) concept, i.e. releasing energy preserved in the DC–link capacitors of employed power converters in the grid following a frequency disturbance. Nonetheless, small–signal stability analyses affirm that a local mode associated with the control system is prone to become unstable when the grid–interactive converter augmented with the DVI operates in weak grids. To overcome this problem, an efficient compensator is proposed in this paper. This compensator introduces one degree–of–freedom to the direct–axis current controller in synchronous reference frame, which eliminates the adverse impact of DVI function on converter stability. Finally, the efficacy of the proposed control framework is depicted by simulations in MATLAB. The results illustrate that the grid frequency rate of change following a step–up load change is improved by 30% compared to the case in which the DVI loop is nullified. [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]

Modular multilevel converter (MMC) has been widely used in the multi-terminal overhead line high-voltage direct current (HVDC) system due to its outstanding performance. However, the AC side and the DC side of MMC-HVDC have a strong interaction and will be affected by each other's faults, which brings the risk of system instability. In this paper, a modified MMC named active MMC with embedded energy storage in submodules (SMs) is proposed to isolate the impact of faults as a firewall. Firstly, the topologies of the active MMC and SMs with energy storage are introduced. Then the operation principle of the active MMC under different faults is analyzed in a multi-terminal MMC-HVDC system. Thirdly, the complete control scheme of the active MMC is proposed for bidirectional fault isolation. Finally, two main application scenarios of the active MMC are simulated in a three-terminal MMC-HVDC system and a particularly designed AC/DC hybrid transmission system respectively in PSCAD/EMTDC. The simulation waveforms of the active MMC are compared with the conventional MMC. Simulation results show that the active MMC can effectively protect the AC system and DC system from each other's faults. [ABSTRACT FROM AUTHOR]

This paper discusses the analysis and experimental validation of bidirectional Z-source breaker and bidirectional Z-source breaker using coupled inductor for low-voltage dc microgrid application. The proposed topologies use an SCR as a switch to isolate the fault from the system, and LC network to generate required commutation conditions for the SCR under fault conditions. Thus, both topologies are able to isolate the fault naturally within a few microseconds without any fault detection and sensing circuit. In this paper, the breaker operating principle is discussed during fault condition and breaker design criteria are derived. When the breaker is connected in series with power electronic converter, Z-source components offer filtering characteristic. The filtering characteristic offered by the breaker components is analyzed. Functional prototypes are developed for both configurations to validate the breaker performance and experimental results are presented. [ABSTRACT FROM AUTHOR]

This paper deals with the modeling, hardware results and model validation by measurements of a VSC assisted resonant current (VARC) dc circuit breaker (CB) and the application within a future network by simulation. The newly emerging VARC dc CB can be used as a solution for the protection of offshore multi-terminal HVDC (MTDC) grids. In this paper, the proposed VARC dc CB is modeled in detail in a PSCAD environment, by taking into account dielectric strength of the vacuum gap, high-frequency current quenching ability and parasitic components. The PSCAD-model is then verified by data from the testing of a 27 kV VARC dc CB prototype with maximum current interruption capability of 10 kA. Additionally, the initial transient interruption voltage and current slope at zero-crossing during the interruption are analyzed. With respect to scaling to a higher voltage level, three types of series connected modules are presented and the performances are compared. The performance of the series connected modules is simulated in a model of a 4-terminal HVDC grid. The obtained results validate the VARC dc CB as a promising solution for the dc fault isolation in MTDC grids. [ABSTRACT FROM AUTHOR]

DC shipboard power systems have been considered as a promising solution for stricter environmental regulations on ships due to their main benefits in fuel savings with variable speed engines and easy integration of energy storage systems. In order to employ the dc solution in the shipboard power systems, the dc power systems have to be protected from a system fault with protection selectivity to minimize impacts of the fault or to avoid other undesirable situations in the system. For low-voltage dc shipboard power systems, a three-level protection has been proposed: 1) fast action (first)—bus separation by solid-state dc bus-tie switch; 2) medium action (second)—feeder protection by high-speed fuse; and 3) slow action (third)—generator–rectifier fault controls. This paper proposes a new method by means of additional bus capacitance added in main dc buses to help reliable operation of the three-level protection. The principle of the proposed method is introduced and the sizing of the additional bus capacitance is addressed in this paper. With the modeling of the dc shipboard power systems, the analyses of voltage drops for the bus separation failure and fault clearing time for the feeder protection are carried out to verify the proposed method. The results show that the proposed method not only mitigates the voltage drop for the bus separation failure, but also achieves the selectivity and the sensitivity for the feeder protection. [ABSTRACT FROM AUTHOR]

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

Due to different solar radiation, temperature, and other reasons of modules in the three-phase cascaded H-bridge (CHB) photovoltaic (PV) inverter, the output power among PV modules will be unequal and lead to unbalanced grid currents, which cannot meet the requirements of grid codes. On the other hand, the second-order voltage ripple is aroused in the dc-link capacitor since each phase-leg of the CHB inverter is made up of a single-phase inverter. Concerning these two issues, this paper proposes an optimized third harmonic injection method based on fundamental frequency zero sequence injection (FFZSI), which is a conventional method to handle the slight power imbalance problem. In this paper, the FFZSI is adopted to redistribute the power among three phases, and then the third harmonic is injected into the three-phase combined reference waveforms according to the two requirements: all modules are free from overmodulation and the dc-link voltage fluctuation is mostly reduced. Therefore, the proposed method can reduce the dc-link voltage fluctuation and extend the power balance range of FFZSI at the same time. The validity and effectiveness of the proposed method are verified by simulation and experimental results. [ABSTRACT FROM AUTHOR]

This paper proposes a new three-phase to five-phase transformer, with a 30-pulse AC–DC converter for power quality improvement in vector-controlled induction motor drive. Here, three isolated transformers (T1, T2, and T3) are used and each transformer is designed to convert three-phase AC-supply into five-phase AC supply, to achieve ten-pulse AC–DC conversion. A unique configuration and angle among these three transformers is designed in such a way that the resultant input current of this transformer shows 30 pulses in it. A hybrid multilevel inverter is proposed in this paper, which is cascading a three-level neutral point clamped inverter with a two-level voltage source inverter to drive an induction motor (IM). A modified level shifted carrier-based sinusoidal pulsewidth modulation and a 3rd harmonic injection techniques are utilized to modify the modulating signal, to enhance the fundamental magnitude, and to reduce the harmonics than traditional technique. The responses of proposed medium voltage induction motor drive (MVIMD) are evaluated by simulating in MATLAB-Simulink. The proposed MVIMD performance is then verified using a scaled down laboratory prototype under various operating conditions such as steady state, starting, load perturbation, speed control, field weakening, varying load conditions, and over a wide speed range of IM. [ABSTRACT FROM AUTHOR]

This paper proposes a family of single-stage converters consisting of three different converters, each of which is able to produce a variety of waveforms through the waveform-subtraction technique. We brand this converter family as waveform-subtraction based single-stage converter (WSSC). By using only a single-stage converter, the proposed waveform generation technique is able to realize dc–dc–ac conversions and suppress input current ripple, meeting the needs of modernized power grid with renewable energy sources. At the same time, converters in this WSSC family are capable of generating a range of shapes, such as triangular, rectangular, and sawtooth waveforms, with simple structures, demonstrating its suitability of being applied in the electrochemical industry, e.g., in the electroplating process. Furthermore, the proposed family converters also realize substantially better input current ripple suppression compared to other conventional converters. In order to prove the validity and efficacy of the proposed WSSC and the theory underpinning it, in this paper, extensive simulations and analyses are conducted to verify the theoretical foundation of the proposed WSSC strategy, and the prototype of a converter from the proposed converter family is built and tested in our laboratory, which validates the functionality of the WSSC. [ABSTRACT FROM AUTHOR]

This paper proposes a new control scheme to eliminate the 3rd harmonic in the output currents of grid-following inverters under unbalanced grid conditions. Unbalanced grids adversely affect the performance of grid-following inverters due to the oscillations appearing on the DC-link voltage with a frequency twice the line frequency. The paper is based on instantaneous active reactive control (IARC) technique due to its advantages over other existing methods. However, the presence of severe asymmetrical 3rd harmonic distortions in the inverter output currents is the main challenge with IARC method, which impairs the power quality requirements. This paper enhances the IARC scheme by proposing a 3rd harmonic elimination approach by adopting a current reference generation method using the symmetric sequence components concept. Furthermore, the proposed scheme complies with the grid code requirements by injecting the requested reactive current. Finally, the proposed approach is evaluated by theoretical and simulation analysis and validated experimentally using a hardware setup. [ABSTRACT FROM AUTHOR]

This paper proposes a novel polygonal voltage space vector structure (SVS) having 30 sides, for a star-connected induction motor drive. The SVS eliminates the presence of harmonics up to 25th order from motor phase voltage throughout the entire modulation range, providing a torque profile devoid of lower order pulsations. Linear modulation is extended till 99.63% of base speed without exceeding the motor phase voltage rating. Topology consists of a dc-link fed primary inverter and two equal low voltage modular capacitor fed secondary inverters. Here the harmonics generated by the primary inverter is canceled by the secondary inverter which acts as a switched capacitive filter. Detailed description of the SVS generation and timing calculations are provided in this paper. Effectiveness of the proposed scheme is validated using experimental results, inverter loss calculations, and harmonic analysis. [ABSTRACT FROM AUTHOR]

Modular multilevel converters (MMCs) can be configured to perform ac/ac conversion, which makes them suitable as railway power supplies. In this paper, a hierarchical control scheme for ac/ac MMCs for railway power supplies is devised and evaluated, considering the requirements and the operating conditions specific to this application. Furthermore, admittance models of the ac/ac MMC are developed, showing how the suggested hierarchical control scheme affects the three-phase and the single-phase side admittances of the converter. These models allow for analyzing the stability of the interconnected system using the impedance-based stability criterion and the passivity-based stability assessment. Finally, the findings presented in this paper are validated experimentally, using a down-scaled MMC. [ABSTRACT FROM AUTHOR]

The series bridge converter (SBC) is a modular multilevel converter (MMC) recently developed to enhance power density in high-voltage high-power applications. The MMC is a well-established solution, widely researched, and exploited in practical HVdc connections, thanks to its high power quality and high efficiency. However, the main limitation of the MMC is the relatively large energy storage, also due to the fact that power ripples in the submodule capacitors include a component at the fundamental ac frequency. As a result, volume becomes critical in applications such as offshore or city center in-feeds, where space is restricted and expensive. The SBC offers a more compact footprint by exploiting a series connection on the dc side and by operating the submodules with rectified waveforms, thus moving the minimum component of the instantaneous power to twice the ac fundamental and reducing capacitors size. The drawback of the converter is a more complex energy control compared to the MMC. This paper proposes the first experimental validation of the SBC, using a 2-kW laboratory-scale prototype. Since the basic converter design has been discussed in previous papers, the focus of this paper is on converter control design and experimental validation. [ABSTRACT FROM AUTHOR]

This paper proposes a pulsewidth modulation (PWM) control strategy for the efficient operation of the three-phase three-level neutral-point-clamped (NPC) inverters based on a bootstrap gate drive circuit. The bootstrap circuit used in the three-level NPC inverter has advantages in terms of the cost reduction and the compact circuit design. However, the bootstrap circuit has a drawback due to the low gate-emitter voltage during the low-frequency and variable load condition of the inverter system. This problem leads to lower efficiency and reliability of the inverter. Therefore, this paper analyzes the causes of the voltage decrease in bootstrap capacitors and proposes PWM control strategy to increase the voltage level of bootstrap capacitors for the efficient operation of the inverter. The proposed PWM control strategy provides efficient and stable operation of the inverter system by selecting the unipolar, dipolar, or partial-dipolar modulation methods according to the fundamental output frequency and load current of the inverter system. Furthermore, this paper proposes a design method of the size of bootstrap capacitors for the optimal circuit design. The effectiveness of the proposed PWM control strategy is verified using the experimental results. [ABSTRACT FROM AUTHOR]

In this paper, a dynamic model based on the sampled-data method is presented for a wireless power transfer (WPT) system. The orders of the derived model are equal to the orders of a system. The modeling is different from the existing dynamic modeling for WPT systems, which almost double the model's order. Furthermore, the systematic analysis process is suitable for any WPT topologies and without the higher harmonics neglected and low-frequency approximation. Meanwhile, the state variables are allowed to vary up to twice the switching frequency, since the half-period symmetry property is exploited. In this paper, the series–series (SS) compensated WPT system is chosen as an example and the large-signal model and the steady-state operation point are determined. The small-signal responses perturbed by the phase-shift angle, switching frequency, and input voltage are obtained. The derived model is more adequate with simulation and experimental validation. [ABSTRACT FROM AUTHOR]

Active damping is a common way to stabilize the current control of LCL-filtered converters. In this paper, the stable region of −180° phase crossing is first identified within a predefined range of grid impedance and LCL parameter variations. Once the phase of the current control loop is in the identified region, a stabilization control can be attained. Subsequently, digital filters can be adopted to achieve active damping by reshaping the open-loop phase. Various digital filters are selected and benchmarked in this paper. It is confirmed that the all-pass filter has a unity gain and adjustable lagging phase before the Nyquist frequency, thereby being a promising solution to the phase reshaping. Therefore, the all-pass filter is employed to move the phase of the open-loop control (i.e., −180° phase crossing) into the targeted region for active damping. Notably, the current controller and the all-pass filter-based active damping can be separately designed, indicating the easy implementation of the active damping. Experimental tests demonstrate that the proposed method can ensure the system stability over a wide range of parameter variations (e.g., grid impedance changes and LCL-filter parameter drifts) while maintaining fast dynamics with the grid-side current control. [ABSTRACT FROM AUTHOR]

Four-level hybrid-clamped inverter is a newly proposed topology that can operate under a wide voltage range without switches connected in series. However, when it is applied in medium voltage high power conversions, the flying capacitors in each phase will occupy a huge volume and a high switching frequency is required to restrain the voltage ripples. In order to overcome this drawback, a four-level active neutral-point clamped inverter is discussed in this paper, which consists of only six switches and no diodes or flying capacitors are required. In order to balance the neutral-point voltages under the full power factor and modulation index range, a capacitor voltage balance method based on carrier-overlapped pulsewidth modulation is proposed in this paper. The upper and lower dc-link capacitor voltages are balanced by zero-sequence voltage injection and the central dc-link capacitor voltage is balanced by adjusting the duty cycles of switching signals slightly. Simulation and experimental results are presented to confirm the validity of this method. [ABSTRACT FROM AUTHOR]

This paper presents a 5-level inverter topology for open-end induction motor drives by using a single dc source. The open stator windings of the drive are supplied with a 3-level flying capacitor inverter from one end and capacitor-fed 2-level inverter from another end. The voltage ratio of the dc link to the capacitor in 2-level inverter is maintained at 4:1 ratio to generate five-level voltage output. The capacitor in 2-level inverter is balanced by the switching redundant vector combinations from both the inverters while the floating capacitors in the 3-level inverter are balanced by using redundant switching states. The proposed topology gives 5-level operation with less number of floating capacitors and power semiconductor switches compared to other existing topologies. Also, the balancing of the capacitors is independent of load power factor and modulation index. Further, the generalization of the proposed dual inverter scheme for any n-level inverter is also included in this paper. The experimental results and required analysis are also presented to validate the inverter scheme. [ABSTRACT FROM AUTHOR]

The modular multilevel DC converter (M2DC) exploits the interleaving between the three legs of an MMC to realize a promising uninsulated DC/DC converter to interconnect HVDC grids. This paper details a current and energies decoupled model of the M2DC. The major idea proposed in this paper is focused on the full energy control generating optimal current references to minimize the internal currents magnitude. The energy sum and difference models are fully detailed. Both current and energy control loops are based on the model inversion principle in order to control all the state variables. The proposed control is based a dynamic control developed with the model inversion principle associated with an optimization of the current magnitude deduced from a quasi-static analysis. All dynamics of the system are then explicitly controlled, which guarantee a good dynamic behavior during the transient. Therefore, current and energy controls are presented in detail. Simulation results show the dynamic behavior of the converter for various operating points. [ABSTRACT FROM AUTHOR]

This paper presents a measurement-based method for real-time determination of the wind farms or wind turbines that contribute most significantly to a subsynchronous resonance (SSR) event. The method is based on the concept of subsynchronous power (SSP). This paper has shown that it is the SSP that drives the SSR. By monitoring the direction and amount of SSPs produced by different wind farms or turbines, the most critical wind farms or turbines can be identified and quantified. A dq0 transform-based algorithm is developed to calculate the SSP in real time. The method overcomes the interference of fundamental frequency components and non-steady-state SSR components on the extraction of the SSP component. Several test cases have demonstrated the usefulness of the proposed concept and method. [ABSTRACT FROM AUTHOR]

In this paper, the modeling of storage elements and a unified theory liberated of the I–V characteristics of a specific technology is developed. First, the definition of an ideal storage element with limited energy capacity is presented, then the semiideal storage properties are discussed. The addition of power conservative two-port network denoted POPI (Pout = Pin) is added in order to define a flexible storage element in terms of characteristics. The paper demonstrates the theory by examples of storage elements integrated in power electronics systems. The ideas and methodology presented in this work can be applied to power electronics applications as well as power systems, smart grids, and microgrids, where storage is an important factor. [ABSTRACT FROM AUTHOR]

A new single stage three-phase ac–dc converter with four switches and galvanic isolation is proposed in this paper. The new converter is simple and uses fewer switches than previously proposed ac–dc converters of the same type. It is a bridgeless converter that can operate with continuous input current and with any pulsewidth modulated method suitable for a standard three-phase six-switch voltage source rectifier. In this paper, the operation, control, analysis, and design of the proposed converter are explained and its features are discussed. Experimental results obtained from a prototype that confirm the feasibility of the converter are presented as well. [ABSTRACT FROM AUTHOR]

In this paper, a new modular and active snubber cell, easily applicable, and attractive for converters, especially have many switches proposed. This snubber cell is implemented on the single phase of a grid-connected three-phase T-type three-level inverter (T-3LI). In this new converter, the main switches turn-on under zero-voltage transition and turn-off under zero-voltage switching (ZVS). Also, auxiliary switches turn-on under zero-current switching and turn-off under ZVS. All snubber diodes operate under soft switching (SS). There are no additional current and voltage stresses on the main switches. During an SS operation, the switching energies are transferred to a dc voltage source. Then, these energies are processed to input with an independent dc–dc converter. Thus, all switching energies are recycled. In this paper, the principle operation and steady-state analysis of the modular and active snubber cell for a single-phase T-3LI are presented and experimental results, rated 1 kW and 100 kHz, are provided to verify theoretical analysis. [ABSTRACT FROM AUTHOR]

This paper presents a unified second-stage LC filter applied in the three-state switching cell buck–boost converter, which provides current and voltage with low ripple and low noise. The unified second-stage filter promotes a reduction in the number of devices while maintaining the filtering performance of the converter, with second-stage LC filters at the input and at the output. Moreover, the unified filter, when associated with a three-state switching cell buck–boost converter, allows a size reduction of filter devices, especially for designs with duty cycle range around 1/2. Furthermore, the unified filter arrangement provides nondissipative clamping of switching devices and reduce the system order when compared to the converter with second-stage LC filters. In this paper, the static and dynamic characteristics of the proposed converter in continuous conduction mode (CCM) are reported. Experimental data were obtained from a laboratory prototype with an input range of 32 $\sim$ 72 V, constant output of 48 V, a load of 150 W, and switching frequency of 45 kHz. The measured efficiency for the rated load was 97% for maximum input voltage. A comparison of the size and performance of the proposed filter with the topology that uses second-stage LC filters at the input and at the output is also presented. [ABSTRACT FROM AUTHOR]

The main technical challenge of the medium-voltage motor drives with modular multilevel converter (MMC) is the serious low-frequency fluctuation on the capacitor voltages, and the existing suppression methods inevitably inject high-frequency circulating currents in MMC. However, the output currents are evidently distorted by the large injected components, while little research has been done to study and solve this problem. This paper analyzes and proves the essential interactions between the output current distortion and the injected components, and the specific dominant output harmonic groups are derived and summarized, revealing the nature of the problem, and it can be taken as a favorable reference for the controller designing. Based on the derivations, this paper proposes a novel suppression method for the greatly improved harmonic characteristics of the output currents during the low-frequency operation. The output harmonics can be accurately measured by the proposed control and effectively suppressed for much lower current total harmonic distortion, benefiting for the superior output control performances with improved efficiency. The simulation results on PSCAD/EMTDC and experimental verifications on a 2.2 kW MMC-based motor drive prototype prove the validity of the presented harmonic analysis and suppression method. [ABSTRACT FROM AUTHOR]

In order to increase the efficiency of modern microprocessor power supplies used in data centers, 380-V dc power distribution systems have been attracting attention due to their high efficiency and high reliability. This paper presents an innovative single-stage approach for 380-V voltage regulator modules (VRMs) based on a quasi-resonant multilevel topology constant on-time operation. The proposed topology inherently integrates the multiphase approach, providing fast phase shedding and flat high-efficiency curves even at light-load conditions. This is a unique advantage, which is not possible to establish in the two-stage approach, which is very important in server architectures, and where high efficiency is required even at light-load conditions. This paper analyzes the circuit topology and proposes a control architecture for fast transient response, including current-sharing capabilities. The digital controller has been implemented in 0.16- $\mu$ m lithography together with a digital pulsewidth modulation with a 195-ps resolution and a 40-MS/s 7-bit analog-to-digital converter. Experimental results show an efficiency of 93% for a 120-A 380–1.8-V VRM power supply. [ABSTRACT FROM AUTHOR]

In this paper, many impedance-source inverters with coupled inductors have been investigated to obtain a high step-up boost ratio. However, the leakage inductors in these topologies induce great voltage spikes at the dc link, which will increase the voltage stress of switches. Thus, the power level of inverters is limited. Furthermore, the efficiency of the inverters can be degraded by the losses associated with leakage inductors. To address abovementioned issues, this paper proposes a family of low-spike high-efficiency Y-source inverters. The proposed inverters have the ability of eliminating voltage spikes at the dc link and recycling the leakage energy via the additional diode and capacitor. In order to show the excellent characteristics of the proposed topologies, this paper compares the performances of the proposed inverters and the improved-Y-source inverter in many aspects. Simulation and experimental results have verified the abilities of the proposed inverters. [ABSTRACT FROM AUTHOR]

A space vector modulation (SVM) technique of a dual-inverter system for an open-end winding motor drive is described in this paper, where one inverter has a battery power source and the other has an only capacitor across the dc bus. The SVM must be achieved to operate the motor with field-oriented control and simultaneously to control the capacitor voltage at a constant value by using redundant switching states of the dual-inverter system. The control of the capacitor voltage is carried out by selecting a charging or a discharging mode in each redundant switching state, taking the instantaneous motor power factor into account. In addition, it is also required to reduce the error voltage pulses, which are generated in output multilevel voltage waveforms during the dead time. The compensation method of the existing dead-time scheme and the improved SVM sequence to reduce the error voltage vectors are proposed in this paper. The proposed methods are examined through several experimental tests and are confirmed to generate superior output voltage waveforms from the viewpoint of the measured total harmonic distortion and dv/dt. [ABSTRACT FROM AUTHOR]

This paper presents a new series capacitor based frequency scan method to assess the subsynchronous resonance risk including induction generator effect (IGE) and subsynchronous control interaction (SSCI). Most existing frequency scan methods are generator based and calculate the impedance as viewed from the neural bus of the study generator. In this paper, the generator-based frequency scan method is discovered to be inadequate or inapplicable to deal with a cluster of inverter-based generation resources under some circumstances. The proposed method scans the impedance as viewed from the series capacitor to assess the IGE/SSCI risk. With nearby inverter-based generation resources modeled by their frequency-dependent impedances, the proposed method is capable of assessing the IGE/SSCI risk for a cluster of inverter-based generation resources. The electromagnetic transient simulation was conducted to validate the proposed method. The proposed method also successfully demonstrates the nonlinear impact of the number of in-service units on the SSCI performance for a cluster of inverter-based generators. [ABSTRACT FROM AUTHOR]

This paper proposes a detailed equivalent model for electromagnetic transient simulation of a modular multilevel converter with embedded battery energy storage in its submodules. The model offers an accuracy identical to that of a detailed switching model (DSM), while it markedly reduces the computational complexity of simulations. This is achieved by modeling each multivalve as a Thevenin equivalent considering the full dynamics of each constituent submodule, which results in a significant reduction in the number of switchable nodes in the converter model and hence the dimensions of the system's admittance matrix. The paper presents the mathematical development of the model and validates it against detailed switching models through several case studies. Experimental results from a scaled-down laboratory setup are also presented for further verification. [ABSTRACT FROM AUTHOR]

Various methods have been presented in the past to emulate the electrical behavior of modular multilevel converters (MMCs). To meet the demands for the reliability aspect study of MMCs, this paper proposes a viable setup to emulate the thermal behavior and to investigate its feasibility for reliability testing and thermal model validation of the power modules in the MMC. The proposed mission profile emulator has three distinctive features: 1) capable of emulating and measuring the thermal stresses of power modules; 2) capable of implementing practical switching profile as a full-scale MMC; and 3) having significantly reduced requirement for dc power supply compared to existing setups used for electrical studies. Theoretical discussions, and simulations as well as the experimental results are presented to demonstrate the capability of the mission profile emulator both electrically and thermally. Moreover, this paper is accompanied by a video demonstrating how to measure the junction temperature of the power devices using the optical fiber and the thermal camera. [ABSTRACT FROM AUTHOR]

The paper considers the problem of sub-synchronous resonances (SSR) in doubly fed induction generators connected to the grid through series-compensated lines. The existence of oscillations is observed on a laboratory test-bed in various conditions, with instabilities developing in the worst-case scenario. Despite the limitations of a small-scale test-bed, the ability to investigate problems and solutions in ways that would not be possible on a full-scale system is very valuable. In particular, the paper demonstrates the significant effect of the choice of dq reference frame on the severity of SSR. Stator-aligned algorithms are found to be significantly more resonant than grid-oriented algorithms, as evidenced in experiments as well as using an analysis method proposed in the paper. From this observation, an algorithm is proposed to emulate a grid-alignment using stator voltages and currents, but without measurements of the grid and series-capacitor voltages. Two control laws are then presented to regulate the active and reactive powers generated, resulting in well-damped transient responses. SSR oscillations are found to be eliminated in experime, as well as simulations of a full-scale system and using a frequency response analysis of the closed-loop system. The effectiveness of the proposed control schemes is verified through experiments for different compensation levels and varying speeds. Within the assumptions made in the design, the proposed controllers are global. [ABSTRACT FROM AUTHOR]

This paper presents a two-timescale multi-objective coordinated volt/var optimization system for active distribution networks with renewable energy resources (RESs). In the hourly timescale, power loss and the number of control actions of on-load tap changers and (switchable) shunt capacitors for 24 h are minimized by coordinating on-load tap changers, (switchable) shunt capacitors, and reactive power outputs of RESs. A three-step method consisting of global search, user preference, and local search is adopted to solve the corresponding multi-objective mixed-integer nonlinear optimization problem. A rolling volt/var optimization is executed every 6 h, considering the day-ahead RES forecast errors. In the 15-min timescale, the RES reactive powers are optimized with RES uncertainties explicitly taken into consideration to minimize the pilot-bus voltage deviation under all representative scenarios. The pilot buses are those buses whose voltage magnitudes capture the network voltage profile. In this paper, a linear method is applied to quickly identify a set of pilot buses, and the impact of the identified pilot bus on the entire network voltage profile is evaluated. The IEEE 33-bus distribution network is used to evaluate the proposed two-timescale volt/var optimization formulations and methods with promising results. [ABSTRACT FROM AUTHOR]

Multilevel inverters (MLIs) are playing a pivotal role in the power sector with potential applications, such as interfacing renewable energy sources with the grid and several industrial drive applications. MLIs with a smaller number of switching devices are more promising due to their compact size, reduced cost, and higher efficiency compared with their traditional counterparts. This paper, therefore, presents a new three-phase seven-level inverter. This topology is a combination of two cascade-connected two-level voltage-source inverters (VSIs) and H-bridge cells with flying capacitors (FCs). This paper presents the operating principle and the balancing technique for the dc-link capacitors and FCs. The generation of various output voltage levels and the limitation of the sinusoidal pulsewidth modulation control for FC voltage balancing is also presented. The number of components in the proposed circuit configuration and their voltage ratings are considerably lower compared with the recently proposed topologies. The behavior of the proposed circuit configuration is first assessed with simulation studies and is then tested with a laboratory prototype. The simulation and experimental results validate the effectiveness of the proposed topology and the voltage balancing technique. [ABSTRACT FROM AUTHOR]

With the rapid development of photovoltaic and fuel cell stacks, high voltage gain converters are urgently demanded. Conventional boost is limited by the exaggerated duty cycle, it is impossible to obtain a high voltage gain while ensuring efficiency. Therefore, a zero-voltage switching (ZVS)-modified SEPIC converter that combines a voltage multiplier cell with traditional SEPIC is proposed in this paper. With detailed parameters design process, this presented converter works at zero voltage switching (ZVS), while obtaining high voltage gain without extremely high duty cycle. Besides, voltage stress is also reduced and low-voltage component with lower parasitic parameters can be used, thus efficiency is maintained. In order to further improve the power density, an integrated inductor scheme is adopted in this paper without changing intrinsic working states of the two inductors. A 36-W prototype is built, and experiments are conducted to verify the advantages of the proposed converter. The efficiency is up to 91.5% at full load. [ABSTRACT FROM AUTHOR]

This paper presents a novel integrated-stage light-emitting diode (LED) driver based on Flyback and resonant converters. The Flyback converter realizes power factor correction in the discontinuous conduction mode. The resonant converter is operated as a dc–dc converter to support the LED loads. Moreover, the resonant converter can clearly decrease the drain–source voltage of the active switch and exhibits zero-voltage-switch characteristics. Therefore, a cost-effective MOSFET can be chosen as the main control component that can decrease the cost of the whole system. Compared to a single-active-switch converter (such as Class-E converter), the number of passive components in the resonant converter can be limited to four. Therefore, the power density of the system can be improved drastically. In this paper, a 100-W prototype is presented under the experimental condition to validate the proposed integrated-stage LED driver. [ABSTRACT FROM AUTHOR]

In this paper, a new family of impedance source inverters is presented. It employs high-frequency electrical isolation between the inverter bridge switches and the load along with voltage clamping across the dc-link voltage. Conventional Z-source inverters (ZSIs) employs an impedance network that consists of inductors and capacitors. It has unique features that realize both step-up/step-down functions and eliminates the need of dead/overlap times. This paper extends this novel concept by using electrical isolation in impedance source inverters. High-frequency isolation has many advantages in terms of immunity and reliability; when applied with impedance source inverters this makes ZSIs a preferable choice for industrial applications. In photo-voltaic systems, the addition of the high-frequency transformer provides safety by avoiding the injection of dc circulating current into the grid, without the need of an external bulky line frequency transformer. The gain of the proposed inverter design can be accurately selected by choosing the turns ratio of the high frequency transformer (HFT) or by adjusting the shoot-through duty cycle (STDC) to the inverter. This allows for greater freedom especially when utilizing a higher modulation index, with the STDC allowing dynamic gain adjusts to be done speedily during operation of the inverter. Additionally, a dc-rail voltage clamping technique for the proposed class of isolated ZSIs is also discussed. This technique provides benefits not only in improving the output voltage quality, but also in reducing voltage stress of the active and passive components by minimizing the voltage spikes across the switching devices. In this paper, several high-frequency isolated ZSIs are presented, and an example isolated improved ΓZSI design is shown and discussed in detail. Simulations are provided for the proposed class of isolated inverters to verify their working. Further experimental investigation has been done for which results for the isolated improved ΓZSI are reported. These empirical results have largely confirmed the expected benefits that were determined through simulation and accurate model-based testing. [ABSTRACT FROM AUTHOR]