A combined system of shunt passive and small rated series active filters has already been proposed by the authors. The operating principle and steady compensation characteristics have been presented also. The combined system can greatly reduce problems of using only shunt passive or shunt active filters and is suitable for harmonic compensation for large VA rated loads in power systems, because the required VA rating of the series active filter used here is considerably smaller than that of a conventional shunt active filter.In steady states, the combined system behaves like an ideal compensator, because the extraction of the harmonics easy. However, some loads such as cycloconverters fluctuate with rapid speed and cannot be regarded as steady loads. It is difficult for a conventional shunt passive filter to compensate for these fluctuating loads because they generate various frequency harmonics. So it is necessary to discuss transient compensation characteristics of the combined system when it is applied to compensation for fluctuating loads.In this paper, focusing on transient states, stability and compensation characteristics of the combined system are analyzed and discussed theoretically and experimentally in detail. The stable and unstable regions of the combined system and the problems faced by improving transient compensation characteristics are revealed. Then, to improve transient characteristics of the system, a new control method is proposed, and the validity is experimentally demonstrated.
This paper describes the control and operating performance of a modular multilevel PWM inverter for a transformerless medium-voltage motor drive. The inverter is prominent in the modular arm structure consisting of a cascaded stack of multiple bidirectional chopper-cells. The dominant ac-voltage fluctuation with the same frequency as the motor (inverter) frequency occurs across the dc capacitor of each chopper-cell. The magnitude of the voltage fluctuation is inversely proportional to the motor frequency. This paper achieves theoretical analysis on the voltage fluctuation, leading to system design. A downscaled model rated at 400 V and 15 kW is designed and built up to confirm the validity and effectiveness of the nine-level (17-level in line-to-line) PWM inverter for a medium-voltage motor drive.
This paper presents a modular push-pull PWM converter (MPC) for a battery energy storage system, which is intended for grid connections to medium- or high-voltage power systems. The converter per phase consists of a center-tapped transformer and two arms based on a cascade connection of multiple bidirectional PWM chopper-cells with floating dc capacitors. This paper discusses the operating performance and control method of the MPC, focusing on voltage balancing of all the floating dc capacitors. Moreover, a comparison is made between the MPC and a modular multilevel converter (MMC), under an assumption that both are used as the same battery energy storage system. The validity of the operating performance and control method is confirmed by both computer simulation using the "PSCAD/EMTDC" software package and experiment using a three-phase, 200-V, 5-kW downscaled model.
This paper focuses on fault-tolerant control for a battery-energy-storage system based on a multilevel cascade pulsewidth-modulation (PWM) converter with star configuration. During the occurrence of a single-converter-cell or single-battery-unit fault, the fault-tolerant control enables continuous operation and maintains state-of-charge balancing of the remaining healthy battery units. This enhances both system reliability and availability. A 200-V, 10-kW, 3.6-kW·h laboratory system combining a three-phase cascade PWM converter with nine nickel-metal-hydride battery units is designed, constructed, and tested to verify the validity and effectiveness of the proposed fault-tolerant control.
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 PWM inverter with a voltage-balancing circuit, and a hybrid active filer for harmonic-current mitigation of the diode rectifier. A 200-V 5.5-kW downscale model is designed, constructed and tested with focus on the five-level PWM inverter. 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 capacitors are well balanced in all the operating conditions with improvements of operating performance at low inverter modulation indices.
The combined system of a series active filter and a shunt passive filter has been proposed by the authors. It has the ability to eliminate such a disadvantage of a shunt passive filter as a harmonic amplifying phenomina. The series active filter needs much smaller kVA rating than a conventional shunt active filter does. As a result, the combined system shows good filtering characteristics and high efficiency.This paper presents an optimum design of the shunt passive filter for a great reduction of the required kVA rating of the series active filter. It can minimize the peak voltage across the series active filter, and reduce the required kVA rating of the series active filter to 60%. The optimized system is compared in compensation characteristics with the combined system using a conventional shunt passive filter by digital computer simulation, paying attention to practical applications to large rated three-phase twelve-pulse thyristor rectifiers. Experimental results obtained by a laboratory model are shown to verify the design theory.
This paper deals with a self-commutated BTB (Back-To-Back) system for the purpose of power flow control and/or frequency change in transmission systems. Each BTB unit consists of two sets of 16 three-phase voltage-source converters, and their ac terminals are connected in series each other via 16 three-phase trans-formers. Hence, the BTB unit uses totally 192 switching devices capable of achieving gate commutation. This results in a great reduction of voltage and current harmonics without performing PWM control. Simulation results verify the validity of the proposed system configuration and control scheme not only under a normal operating condition but also under a single-line-to-ground fault condition.
Harmonic interference problems generated by bulk semiconductor power converters become more serious in power systems with their wide use in industrial applications. Shunt passive filters consisting of LC tuned filters or shunt active filters using PWM inverters have been used to suppress harmonic currents in distribution systems. To install the passive or active filters, it is necessary to find exactly a dominant harmonic source in distribution feeders. A harmonic power on each phase and order at a receiving terminal has been investigated by using a digital power meter including an FFT analyzer. However, the conventional detection method has a problem in that it is difficult to discuss exactly harmonic power flow because the detected harmonic power is much smaller compared with a fundamental active power at the receiving terminal. In this paper, the harmonic power in three phases, which can be detected on real time, is defined by applying the pq theory. Next, it is verified by a digital simulation that the proposed method can discuss harmonic flows more easily than the conventional method. As a result, it is more simple and precise to find the harmonic source. Moreover, measurement errors of the harmonic power are revealed quantitatively, and the validity of the proposed method is demonstrated by digital simulation.
No abstract is provided for this article.
This paper discusses a 6.6-kV next-generation BTB system in which bidirectional isolated dc/dc converters are combined with modular multilevel cascade converters. The system consists of N converter cells cascaded in each phase at both front ends. Each converter cell consists of a bidirectional isolated dc/dc converter and two voltage-source H-bridge PWM converters. This circuit configuration can be adopted to significantly reduce harmonic voltages and currents because the voltage steps are extremely low. In a conventional power conversion system, bulky line-frequency transformers are employed. On the other hand, in the present system, light and compact high-frequency transformers are used for providing galvanic isolation. Thus, the overall physical size and weight are reduced. The authors design, construct, and test a single-phase 120-V, 3.3-kW downscaled model with N=3, in order to verify the feasibility and effectiveness. The downscaled model helps to develop an operable 6.6-kV system with focus on control and performance.
A combined system of shunt passive and small rated series active filters, as well as its operating principle and steady compensation characteristics, was previously presented by the authors (1988). The combined system can greatly reduce problems of using only the shunt passive or shunt active filters and is suitable for harmonic compensation for large VA-rated loads in power systems. Transient compensation characteristics and the stability of the system are discussed and analyzed theoretically and experimentally in detail. A control method enables the combined system to be applied to cycloconverter compensation. Some results obtained with the experimental model are presented to demonstrate and confirm the method's validity.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">></ETX>
A neutral-point-clamped voltage source inverter (NPC-VSI), which is one of double series connected inverters, is able to output 5 level step-shaped line to line voltage without output transformers or reactors, thus reducing harmonic currents. The NPC-VSI is suitable to high voltage applications, because the required voltage rating of the switching devices used in the NPC-VSI is only 1/2 of the DC link voltage. However, the DC link voltage may be applied to the switching devices in the worst case if the potential of neutral point the DC link varies because of the flow-in or flow-out of the neutral point current.In this paper, the authors discuss the variation of the neutral point potential theoretically, and propose a suppression method of the variation. This is based on the control of a zero-sequence voltage that is a voltage difference between the center of the DC link and the neutral point of a load. The analysis is done for the vector control drive system of an induction machine without or with the proposed suppression method. Consequently, it is shown that the variation of the potential can be completely suppressed by the method except for a special operating condition. Moreover, the relation between the potential variation and the capacity of the DC capacitor is clarified to lead to the desing of the DC capacitor.
The paper describes a new family member of the modular multilevel cascade converter (MMCC), with the given name, triple-star bridge-cells (TSBC). It is a direct ac-to-ac power converter with the capability of bidirectional power flow with three-phase sinusoidal input and output currents. Therefore, it is suitable to a medium-voltage motor drive requiring regenerative braking. This paper has an intensive discussion on a general control of the TSBC intended for the motor drive. Theoretical analysis and computer simulation confirm the effectiveness and viability of the TSBC.
This article, based on modeling and scaling as well as simulation and experiment, provides a comprehensive discussion on a real-time real-power emulator for a medium-voltage, high-power, high-speed motor drive. A three-phase modular multilevel double-star chopper-cell (DSCC) inverter under test is connected in front-to-front (FTF) with the emulator. The DSCC inverter is the same in circuit configuration as the DSCC rectifier. This emulator is characterized by integrating mechanical dynamics of both motor and load into it. The power-electronic circuit of the emulator consists of a three-phase DSCC rectifier, three ac inductors, and a single two-winding common-mode inductor at the dc side. This article starts with modeling and scaling of a three-phase 6.6-kV 10.9-MW four-pole 9000-r/min induction motor drive. Then, the authors design, build, and test a three-phase 200-V 10-kW downscaled test bench. Experimental waveforms from the 10-kW test bench, as well as simulated waveforms from the 10.9-MW full-scale and 10-kW downscaled systems, lead to the following conclusion: The emulator can reproduce both electrical and mechanical dynamics of the three-phase 200-V, 10-kW, four-pole, 9000-r/min induction motor coupled with a centrifugal compressor.
