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A modular multilevel converter (MMC) is one of the next-generation multilevel converters intended for high- or medium-voltage power conversion without transformers. The MMC is based on cascade connection of multiple bidirectional chopper-cells per leg, thus requiring voltage-balancing control of the multiple floating dc capacitors. However, no paper has made an explicit discussion on voltage-balancing control with theoretical and experimental verifications. This paper deals with two types of pulsewidth-modulated modular multilevel converters (PWM-MMCs) with focus on their circuit configurations and voltage-balancing control. Combination of averaging and balancing controls enables the PWM-MMCs to achieve voltage balancing without any external circuit. The viability of the PWM-MMCs, as well as the effectiveness of the voltage-balancing control, is confirmed by simulation and experiment. Index Terms--Medium-voltage power conversion, multilevel converters, voltage-balancing control.

Renewable energy sources such as wind turbine generators and photovoltaics produce fluctuating electric power. The fluctuating power can be compensated by installing an energy storage system in the vicinity of these sources. This paper describes a 6.6-kV battery energy storage system based on a cascade pulsewidth-modulation (PWM) converter with focus on a control method for state-of-charge (SOC) balancing of the battery units. A 200-V, 10-kW, 3.6-kWh (13-M J) laboratory system combining a cascade PWM converter with nine nickel metal hydride (NiMH) battery units is designed, constructed, and tested to verify the validity and effectiveness of the proposed balancing control. Index Terms--Active-power control, battery energy storage system (BESS), cascade converter, nickel metal hydride (NiMH) battery, state-of-charge (SOC) balancing.

This paper addresses shaft end-to-end and shaft-to-frame voltages that appear in the 400-V, 15-kW induction motor driven by a voltage-source pulsewidth modulation (PWM) inverter. A shaft-to-frame voltage can be observed at either shaft end with respect to the grounded motor frame. A shaft end-to-end voltage can be observed as a voltage difference between the shaft-to-frame voltage at the drive end (DE) and that at the nondrive end (NDE). Experimental waveforms lead to the following interesting observations: motor internal coupling and parasitic capacitance, along with the high-frequency common-mode voltage generated by the PWM inverter, cause a shaft-to-frame voltage with a peak of 8 V at both DE and NDE. When the shaft-to-frame voltage at either DE or NDE exceeds a dielectric breakdown voltage of thin bearing lubricating grease films, a shaft end-to-end voltage with a peak of 2 V and a width of 30 ns occurs along the motor shaft. This paper makes experimental discussions on the shaft end-to-end voltage generation. Installing a differential-mode filter and/or a common-mode filter on the motor drive system gives a hint on the mechanisms of the occurrence of the shaft end-to-end voltage. Index Terms--Bearing current, electromagnetic interference (EMI), ground leakage current, motor drives, pulsewidth modulation (PWM) inverters, shaft voltage.

This paper considers the transient performance and stability of a recurrent discrete Fourier transform (DFT) based control method for a series active filter integrated with a 12-pulse diode rectifier. The control method targets specific harmonics and/or the negative sequence fundamental component of the supply current, and is intended for use with nonsinusoidal/unbalanced supply voltages. The proposed control method is based on DFTs instead of the dq-method, and a simple approach is used to account for small frequency variations found in practical power systems. Index Terms--Active filters, digital control, discrete Fourier transforms (DFTs).

This paper presents a comparative analysis between results from applications of the p-q and the p-q-r theories in shunt active power filters for three-phase four-wire systems, discussing aspects related to the influence of the system voltage in the control methods that calculate the compensating currents. It is shown that in some cases, a preprocessing of the system voltage is required if the goal is to achieve sinusoidal compensated currents. On the other hand, when the goal is to compensate zero-sequence current, the need of energy storage elements in the active filter is discussed. In this case, if zero-sequence components are present simultaneously in the system voltage and load current, they produce zero-sequence power flow, and the control methods based on both theories must contain additional calculations to allow the elimination of energy storage elements in the active filter. A control strategy based on the p-q theory is proposed to eliminate the neutral current without the need of energy storage elements, with the advantage of avoiding the extra transformation from [alpha][beta]0 to pqr coordinates that is needed in the p-q-r theory. Simulation results are presented for the purpose of comparing the performance of both control methods. Index Terms--Active filters, p-q theory, p-q-r theory.

This paper describes a 6.6-kV adjustable-speed motor drive for pumps and blowers without transformer. The power conversion system consists of a front-end diode rectifier, a five-level diode-clamped pulsewidth modulation (PWM) inverter with a voltage balancing circuit, and a hybrid active filer for harmonic-current mitigation of the diode rectifier. The control of the inverter is characterized by superimposing a third-harmonic zero-sequence voltage on each of the three-phase reference voltages to achieve the so-called overmodulation and reduce the switching stress of insulated gate bipolar transistors (IGBTs). A 200-V 5.5-kW downscale model is designed, constructed, and tested with focus on the five-level PWM inverter and the voltage balancing circuit. Experimental results obtained from the 200-V downscale model verify the viability and effectiveness of the 6.6-kV adjustable-speed motor drive, showing that the four split dc capacitor voltages are well balanced in all the operating conditions and that the switching stress of the IGBTs is reduced at low modulation indexes. Index Terms--Energy savings, medium-voltage drives, modulation indexes, multilevel inverters, voltage balancing.

This paper discusses a transformerless hybrid active filter integrated into the 6.6-kV, 1-MW adjustable-speed motor drive having a three-phase diode rectifier at the front end. The hybrid filter consists of an active filter using a three-level diode-clamped pulsewidth modulator converter rated at 60 kVA, and a 250-kVA passive filter tuned to the seventh harmonic frequency. They are directly connected in series without a transformer. This circuit configuration enables one to use 1.2-kV insulated gate bipolar transistors because the dc voltage of the three-level converter is 1.32 kV (20 % of 6.6 kV). Voltage balancing control characterized by superimposing a sixth harmonic zero-sequence voltage on the active filter voltage reference in each phase is introduced to the three-level converter with triangle carrier modulation. Experimental waveforms obtained from a 400-V, 15-kW downscaled system verify the viability and effectiveness of the proposed hybrid filter, keeping the two dc capacitor voltages well-balanced. Index Terms--Active filters, diode rectifiers, harmonics, hybrid filters, passive filters, three-level converters.

Electric double-layer capacitors (EDLCs) have attributes that feature high power density, quick charge/discharge time, long life cycle, and environmental friendliness. These attributes accord for increased appeal in employing the EDLCs as energy-storage devices in renewable energy systems, industrial applications, and hybrid electric vehicles as compared to other mature static energy-storage devices. This paper describes the construction of a 320-V, 12-F EDLC energy-storage bank connected to a bidirectional isolated dc-dc converter. Two types of EDLC bank configurations are considered with emphasis on their voltage-balancing circuits. Subsequently, this paper proposes a voltage-balancing circuit based on a center-tapped transformer, and includes its experimental verifications. It also discusses the charge-discharge and self-starting operation of the EDLC energy-storage system. During the charge-discharge operation, a low ripple current flowing in the EDLC bank is observed, leading to a theoretical analysis. The EDLC bank is also successfully charged to its rated voltage without any external dc charging circuit. Index Terms--Bidirectional isolated dc-dc converter, electric double-layer capacitor (EDLC), voltage balancing.

This paper describes a transformerless energy storage system based on a cascade multilevel pulsewidth modulation converter with star configuration. The system is intended for power leveling of renewable energy sources, as well as for improving power quality and reliability of a power distribution system. This paper pays attention to active-power control and voltage-balancing control that are indispensable for proper operation of the energy storage system. A 200-V 10-kW 8.8-kJ downscaled laboratory system is designed, constructed, and tested, replacing electric double-layer capacitors with large-capacity electrolytic capacitors. Experimental results obtained from the laboratory system verify the viability and effectiveness of the 6.6-kV energy storage system. Index Terms--Active-power control, cascade multilevel pulsewidth-modulation (PWM) converter, electric double-layer capacitor (EDLC), energy storage system, output-power leveling, renewable energy sources, voltage-balancing control.

This paper addresses a transformerless mediumvoltage adjustable-speed motor drive consisting of two five-level diode-clamped PWM converters connected back-to-back. It is followed by designing, constructing, and testing a 200-V downscaled model to verify the validity and effectiveness of the medium-voltage motor drive. This downscaled model has four split dc capacitors equipped with a voltage-balancing circuit using two bidirectional buck-boost choppers. The two five-level converters are based on sinusoidal pulsewidth modulation with a carrier frequency of 3 kHz. The motor tested in this paper is a three-phase four-pole induction motor rated at 200 V, 5.5 kW, and 60 Hz. Experimental waveforms show that the four split dc-capacitor voltages are well balanced in any operating conditions and that the total harmonic distortion values of the input line current and the output motor current are 3.9% and 3.5%, respectively. Index Terms--Medium-voltage drives, multilevel inverters, neutral-point-clamped converters, voltage balancing.

This paper deals with a dynamic voltage restorer (DVR), or a voltage-sag compensator, which consists of a set of series and shunt converters connected back-to-back, three series transformers, and a dc capacitor installed on the common dc link. The DVR is characterized by installing the series converter on the source side and the shunt converter on the load side. This system configuration allows the use of an extremely small dc capacitor intended for smoothing the common dc-link voltage. This paper provides a design procedure of the dc capacitor under a voltage-sag condition and proposes a control method for the series converter, which is capable of reducing the voltage ratings of both the series converter and the series transformers. Experimental results obtained from a 200-V 5-kW laboratory system are shown to confirm the validity of the design procedure and the effectiveness of the control method. Index Terms--Compensating energy, compensating methods, dc capacitor banks, dynamic voltage restorers, series converters, shunt converters, voltage sags, zero-sequence voltages.

This paper proposes a control method for eliminating dc magnetic flux from the series transformer of a dynamic voltage restorer. The method is characterized by intentionally injecting no compensating voltage during a one-sixth line cycle (=3.3 ms at 50 Hz). The control method is independent of an initial phase angle of a voltage sag, thus requiring no flux detection. The period, during which no compensating voltage is injected, is one-third as short as that of an existing method. This paper confirms the effectiveness and viability of the proposed method. A 200-V 5-kW laboratory system verifies experimentally that the proposed method brings no dc magnetic flux to the series transformer. Moreover, the proposed method brings a smaller dc voltage drop to the dc load of a three-phase diode rectifier than the existing method does in the same conditions. Index Terms--Compensating voltage, dc magnetic flux, dynamic voltage restorers (DVRs), magnetic saturation, series transformers.

This paper deals with a 50-MW self-commutated Back-To-Back (BTB) system intended for power-flow control between transmission networks. It focuses on the dynamic behavior of the BTB system during single-line-to-ground (SLG) faults that often result in voltage sags. A non-negligible amount of dc magnetic flux deviation appears in the converter-transformers of the BTB system just after the occurrence and restoration of an SLG fault. The dc deviation may bring magnetic flux saturation and an excessive magnetizing current to the transformers. This paper derives theoretical equations related to the dc deviation with reasonable approximation. The theoretical results are confirmed by computer simulation. Index Terms--Back-To-Back (BTB) system, converter-transformer, dc magnetic flux deviation, multilevel converter, power flow control, single-line-to-ground (SLG) fault.

This paper addresses a specific filter for integration into a motor drive system with an active front end. Three circuit configurations are investigated with focus on the high-frequency leakage current flowing out of a common grounded heat sink for two insulated-gate bipolar transistor modules. The motor drive system used in this experiment consists of a voltage-source pulsewidth modulation rectifier and inverter with the same carrier frequency as 14 kHz, together with an interior permanent magnet synchronous motor (IPM-SM) rated at 190 V and 5.5 kW. Experimental waveforms, along with putting an interpretation on their filtering performance, lead to the best filter configuration among the three. Index Terms--Common-mode voltage, converters, leakage current, motor drives, pulsewidth modulation (PWM).

The dramatic growth of power electronics applications has led to serious harmonic pollutions in the power system. The phenomenon of harmonic resonance resulting in severe voltage distortion has been reported. Various suppression techniques have been proposed to resolve this problem. The distributed active-filter system (DAFS) is a more effective solution by deploying multiple shunt active filter units (AFUs) in the system to cooperatively damp the harmonic resonance. This paper proposes a dynamic tuning scheme for the DAFS to enhance its filtering performance. Based on the measured voltage harmonic distortion at the installation point, each AFU can dynamically adjust its own filtering volt-ampere (VA) capacity to maintain the total harmonic distortion of the line voltage at an allowable level. The proposed strategy can be integrated with the droop control between the conductance command and VA consumption so that the harmonic-filtering workload is evenly shared among various AFUs. The operation principles and dynamic analyses are presented in this paper, and test results based on simulations and experiments validate the proposed approach. Index Terms--Active filters, distributed active-filter systems (DAFSs), harmonic damping, harmonic resonance.

This paper discusses the potential of a transformerless STATic synchronous COMpensator (STATCOM) intended for installation on the 6.6-kV industrial and utility distribution systems. The STATCOM consists of a five-level diode-clamped pulsewidth-modulation converter rated at 1 MVA and a voltage-balancing circuit rated at 6 kVA. Moreover, this paper designs, constructs, and tests a 200-V 10-kVA STATCOM based on a five-level converter with a carrier frequency of 3 kHz and a voltage-balancing circuit with a switching frequency of 3 kHz. The STATCOM and its experimental waveforms verify the viability and effectiveness of the 6.6-kV transformerless STATCOM. Index Terms--Multilevel converter, reactive power, split dc capacitors, static synchronous compensator (STATCOM), voltage-balancing circuit.

This paper provides theoretical and experimental discussions on conducted electromagnetic interference (EMI) emissions from an inverter-driven motor rated at 400 V and 15 kW. It focuses on a line EMI filter and its combination with a motor EMI filter, along with their effects on attenuation of conducted emission voltage. When no EMI filter is connected, the motor drive cannot meet the conducted emission limits prescribed by Category 3 in the IEC61800-3 regulations. The reason is that the common-mode voltage generated by a voltage-source pulse width modulation (PWM) inverter causes a common-mode leakage current flowing into the ground wire lead through parasitic capacitors inside the motor. When the line EMI filter is connected, the motor drive can meet Category 3. The motor EMI filter eliminates the common-mode voltage from the motor terminals, thus bringing a drastic reduction to the leakage current. The combination of the two EMI filters can comply with the limits prescribed by Category 2, which are much stricter than those by Category 3. Index Terms--Electromagnetic interference (EMI), leakage currents, motor drives, pulsewidth modulation (PWM) inverters.

This paper addresses a bidirectional dc-dc converter suitable for an energy storage system with an additional function of galvanic isolation. An energy storage device such as an electric double layer capacitor is directly connected to a dc side of the dc-dc converter without any chopper circuit. Nevertheless, the dc-dc converter can continue operating when the voltage across the energy storage device drops along with its discharge. Theoretical calculation and experimental measurement reveal that power loss and peak current impose limitations on a permissible dc-voltage range. This information may be useful in design of the dc-dc converter. Experimental results verify proper charging and discharging operation obtained from a 200-V, 2.6-kJ laboratory model of the energy storage system. Moreover, the dc-dc converter can charge the capacitor bank from zero to the rated voltage without any external precharging circuit. Index Terms--Bidirectional isolated dc-dc converter, electric double layer capacitors, energy storage system, loss analysis, starting procedure.

This paper deals with the dynamic behavior of a 21-level (line-to-line) back-to-back (BTB) system based on series connection of 16 converter cells under a single-line-to-ground (SLG) fault condition. The BTB system is an ac-dc-ac power conversion system intended for achieving asynchronous power-flow control between transmission networks. When the SLG fault happens, an imbalance in the supply voltage brings an amount of negative-sequence current to the BTB system. The negative-sequence current should be mitigated because it would result in an increased supply current and a dc-capacitor voltage fluctuation. This paper intends for reducing both the negative-sequence current and the dc-voltage fluctuation to acceptable levels. A laboratory system rated at 200 V and 20 kW confirms the validity of the reduction scheme. In addition, this paper addresses the issue of magnetic saturation in the 16 three-phase converter--transformers. Index Terms--Back-to-back (BTB) system, multilevel converter, negative-sequence current, power-flow control, single-line-to-ground (SLG) fault.

This paper addresses a transformerless shunt hybrid filter integrated into a three-phase diode rectifier. The hybrid filter consists of a three-phase passive filter tuned to the seventh-harmonic frequency, and a small-rated active filter based on a three-phase voltage-source pulsewidth modulation converter. The validity of the system design is confirmed by experimental results obtained from a 400-V, 15-kW laboratory system. The experimental results show that the active filter plays an essential role in reducing supply harmonic currents. The total harmonic distortion of the supply current gets less than 5%. Moreover, the hybrid filter provides satisfactory transient performance as a result of having achieved an optimal design of circuit and control parameters including the dc capacitor voltage and feedback gains of the active filter, along with the cutoff frequency of high-pass filters for harmonic extraction. Index Terms--Active filters, diode rectifiers, harmonics, hybrid filters, passive filters, power conversion.

This paper presents a three-phase transformerless cascade pulsewidth-modulation (PWM) STATic synchronous COMpensator (STATCOM) intended for installation on industrial and utility power distribution systems. It proposes a control algorithm that devotes itself not only to meeting the demand of reactive power but also to voltage balancing of multiple galvanically isolated and floating dc capacitors. The control algorithm based on a phase-shifted carrier modulation strategy is prominent in having no restriction on the cascade number. Experimental waveforms verify that a 200-V 10-kVA cascade PWM STATCOM with star configuration has the capability of inductive to capacitive (or capacitive to inductive) operation at the rated reactive power of 10 kVA within 20 ms while keeping the nine dc mean voltages controlled and balanced even during the transient state. Index Terms--Cascade connection, multilevel converters, reactive power, STATic synchronous COMpensator (STATCOM), voltage balancing.

This paper discusses a shunt active filter intended for installation on a power distribution system. The active filter has an additional capability to regulate the distribution-line voltage by means of adjusting reactive power. Theoretical analysis investigates the dynamic performance of combined harmonic damping and voltage regulation. As a result, harmonic damping makes it possible to improve the stability of the control loop for voltage regulation, and the combined harmonic damping and voltage regulation lead to good voltage regulation performance. Experimental results are shown to verify the effectiveness of the combined harmonic damping and voltage regulation. Index Terms--Digital control, distributed generators, power distribution systems, shunt active filters, voltage regulation.

This paper describes a bidirectional isolated dc--dc converter considered as a core circuit of 3.3-kV/6.6-kV highpower-density power conversion systems in the next generation. The dc--dc converter is intended to use power switching devices based on silicon carbide (SiC) and/or gallium nitride, which will be available on the market in the near future. A 350-V, 10-kW and 20 kHz dc--dc converter is designed, constructed and tested. It consists of two single-phase full-bridge converters with the latest trench-gate insulated gate bipolar transistors and a 20-kHz transformer with a nano-crystalline soft-magnetic material core and litz wires. The transformer plays an essential role in achieving galvanic isolation between the two full-bridge converters. The overall efficiency from the dc-input to dc-output terminals is accurately measured to be as high as 97%, excluding gate drive and control circuit losses from the whole loss. Moreover, loss analysis is carried out to estimate effectiveness in using SiC-based power switching devices. Loss analysis clarifies that the use of SiC-based power devices may bring a significant reduction in conducting and switching losses to the dc--dc converter. As a result, the overall efficiency may reach 99% or higher. Index Terms--Bidirectional isolated dc--dc converter, mediumvoltage power conversion systems, power density, wide-band-gap semiconductors.

This paper presents a practical approach to eliminating both bearing current and ground leakage current from an inverter-driven motor rated at 400 V and 3.7 kW. When the shaft voltage with respect to the motor frame exceeds the dielectric breakdown voltage of thin lubricating grease films in two metal bearings at the drive and non-drive ends, an electrical discharge machining (EDM) current flows through the bearings. A passive electromagnetic interference (EMI) filter can keep the shaft voltage in check, as a result of having eliminated high-frequency common-mode voltage from the motor terminals. Hence, no dielectric breakdown occurs in the grease films, so that no EDM current flows in the bearings. Experimental results verify the viability and effectiveness of the passive EMI filter designed in this paper. Index Terms--Bearing current, common-mode voltage, electromagnetic interference (EMI), ground leakage current, inverters, motor drives, shaft voltage.

This paper presents dynamic control and performance of a unified power flow controller (UPFC) intended for installation on a transmission system consisting of two sets of three-phase transmission lines in parallel. When no UPFC is installed, interruption of either three-phase line due to a fault reduces an active power flow to half, because the line impedance becomes double before the interruption. Installing the UPFC makes it possible to control an amount of active power flowing through the transmission system. The validity of the theoretical analysis developed in this paper is verified by experiments using a 10-kVA laboratory setup, as well as a computer simulation. Index Terms--Line interruption, power swings, transmission systems, unified power flow controller (UPFC).

The design optimization procedure for a new active resonant snubber topology, specifically suited for high power insulated gate bipolar transistor (IGBT) converters, is introduced. After the basic operation principles and certain implementation consideration are discussed, the optimization strategy, based on an analytical loss evaluation, is described. Experimental results obtained on an IGBT phase-arm fitted with an optimally designed snubber are presented. Index Terms--Active resonant snubber topology, insulated gate bipolar transistor (IGBT).

This paper focuses on the high-frequency leakage current flowing through the grounded heat sink of a voltage-source pulsewidth-modulation (PWM) inverter in an adjustable-speed motor drive system. A passive electromagnetic interference (EMI) filter is proposed, designed, and tested for a 200-V 3.7-kW drive system. This filter does require access to the ungrounded motor neutral point. An equivalent common-mode circuit for the drive system equipped with the EMI filter is derived from experimental waveforms along with theoretical discussions. Experimental results verify that the EMI filter is viable and effective in eliminating both heat-sink and motor-frame leakage currents from the drive system. Index Terms--Common-mode voltage, electromagnetic interference (EMI), inverters, leakage current, motor drives, shaft voltage.

A voltage-source back-to-back system for the purpose of achieving power flow control and/or line-frequency change in transmission systems has the attractive features of reliable and continuous operation, even during power line faults. However, an overvoltage appearing across the dc link during the faults should be limited to as low a value as possible because it does affect the power device ratings. This paper proposes a dc-link voltage regulator for effectively suppressing the overvoltage during power line faults. This regulator is characterized by compensating for a time delay inherent in each current controller, and for a power flow imbalance occurring during power line faults. The validity of the proposed regulator is confirmed by theory and computer simulation. Index Terms--Back-to-back (BTB) system, dc-link voltage regulation, line-frequency change, power flow control, power line faults.

This paper presents a fully-digital-controlled shunt hybrid filter for damping of harmonic propagation in power distribution systems. The harmonic propagation is caused by resonance between line inductances and power capacitors installed for power factor correction. A possible solution to damping out harmonic propagation is based on installation of a shunt pure active filter at the end of a feeder. This paper proposes a shunt hybrid active filter characterized by series connection of a seventh-tuned LC filter per phase and a small-rated three-phase active filter. Like the pure filter, the hybrid filter is connected to the end bus of a feeder. The capacitor of the LC filter imposes a high impedance to the fundamental frequency, so that the fundamental voltage appears across the capacitor. This unique feature allows us to directly connect the hybrid filter to the 6.6-kV power line without step-down transformers. Furthermore, the capacitor used in this hybrid filter is lighter, cheaper and smaller than the transformer used in the pure filter. Theoretical analysis, along with experimental results obtained from a 200-V, 20-kW laboratory system, verifies the viability and effectiveness of the proposed hybrid filter. Index Terms--Active filters, harmonic propagation, hybrid filters, power distribution systems, power filters.

This paper deals with integrating a small-sized passive electromagnetic interference (EMI) filter with a voltage-source pulse-width modulated (PWM) inverter. The purpose of the filter is to eliminate both high-frequency common-mode and normal-mode voltages from the three-phase output voltages of the inverter. A laboratory system consisting of a 5-kVA inverter, a 3.7-kW induction motor, and a specially-designed passive EMI filter was constructed to verify the viability and effectiveness of the filter. As a result, both line-to-line and line-to-neutral output voltages look purely sinusoidal as if the inverter were an ideal three-phase variable-voltage, variable-frequency power supply when viewed from the motor terminals. This results in a complete solution to serious EMI issues related to high-frequency common-mode and normal-mode voltages produced by the PWM inverter. Index Terms--Common-mode voltage, electromagnetic interference (EMI) filters, shaft voltage, three-phase voltage-source pulse-width modulated (PWM) inverters.