John W. McKeever

A multilevel voltage-source inverter with separate DC sources for static VAr generation

A new multilevel voltage-source inverter with separate DC sources is proposed for high-voltage, high power applications, such as flexible AC transmission systems (FACTS) including static VAr generation (SVG), power line conditioning, series compensation, phase shifting, voltage balancing, fuel cell and photovoltaic utility systems interfacing, etc. The new M-level inverter consists of (M-1)/2 single phase full bridges in which each bridge has its own separate DC source. This inverter can generate almost sinusoidal waveform voltage with only one time switching per cycle as the number of levels increases. It can solve the problems of conventional transformer-based multipulse inverters and the problems of the multilevel diode-clamped inverter and the multilevel living capacitor inverter. To demonstrate the superiority of the new inverter, a SVG system using the new inverter topology is discussed through analysis, simulation and experiment.

A power line conditioner using cascade multilevel inverters for distribution systems

A power line conditioner (PLC) using a cascade multilevel inverter is presented for voltage regulation, reactive power (VAr) compensation and harmonic filtering in this paper. The cascade M-level inverter consists of (M-1)/2 H-bridges in which each bridge has its own separate DC source. This new inverter: (1) can generate almost an sinusoidal waveform voltage with only one time switching per line cycle; (2) can eliminate transformers of multipulse inverters used in the conventional static VAr compensators; and (3) makes possible direct connection to the 13.8 kV power distribution system in parallel and series without any transformer. In other words, the power line conditioner is much more efficient and more suitable to VAr compensation and harmonic filtering of distribution systems than traditional multipulse and pulse width modulation (PWM) inverters. It has been shown that the new inverter is specially suited for VAr compensation. This paper focuses on feasibility and control schemes of the cascade inverter for voltage regulation and harmonic filtering in distribution systems. Analytical, simulated and experimental results show the superiority of the new power line conditioner.

Low-cost sensorless control of brushless DC motors with improved speed range

This paper presents a low-cost position sensorless control scheme for brushless dc motors. Rotor position information is extracted by indirectly sensing the back EMF from only one of the three motor-terminal voltages for a three-phase motor. Depending on the terminal voltage sensing locations, either a low-pass filter or a band-pass filter is used for position information retrieval. This leads to a significant reduction in the component count of the sensing circuit. The cost saving is further increased by coupling the sensing circuit with a single-chip microprocessor or digital signal processor for speed control. In addition, a look-up-table-based correction for the nonideal phase delay introduced by the filter is suggested to ensure accurate position detection even at low speed. This extends the operating speed range and improves motor efficiency. Experimental results are included to verify the proposed scheme.

Cascade multilevel inverters for utility applications

Cascade multilevel inverters have been developed by the authors for utility applications. A cascade M-level inverter consists of (M-1)/2 H-bridges in which each bridge has its own separate DC source. The new inverter: (1) can generate almost sinusoidal waveform voltage while only switching one time per fundamental cycle, (2) can eliminate transformers of multipulse inverters used in conventional utility interfaces and static VAr compensators, and (3) makes possible direct parallel or series connection to medium- and high-voltage power systems without any transformers. In other words, the cascade inverter is much more efficient and suitable for utility applications than traditional multipulse and pulse width modulation (PWM) inverters. The authors have experimentally demonstrated the superiority of the new inverter for reactive power (VAr) and harmonic compensation. This paper summarizes features, feasibility, and control schemes of the cascade inverter for utility applications including utility interface of renewable energy, voltage regulation, VAr compensation, and harmonic filtering in power systems. Analytical, simulated, and experimental results demonstrate the superiority of the new inverters.

A multilevel voltage-source converter system with balanced DC voltages

In this paper, a multilevel voltage-source converter system is proposed for high-voltage, high-power applications such as back-to-back interconnection of power systems, large induction motor drives, and electrical traction drives. Multilevel voltage-source converters have a voltage unbalance problem in the DC capacitors. The problem may be solved by use of additional voltage regulators or separate DC sources. However, these solutions are found not to be practicable for most applications. The proposed converter system can solve the voltage unbalance problem of the conventional multilevel voltage-source converters, without using any additional voltage balance circuits or separate voltage sources. The mechanism of the voltage unbalance problem is analyzed theoretically in this paper. The voltage unbalance problem of multilevel converters in the DC capacitors has been solved by the proposed internal connections of the AC/DC and DC/AC converters. The validity of the new converter system is demonstrated by simulation and experiment.<<ETX>>

Experimental verification of optimal flux weakening in surface PM machines using concentrated windings

A design approach is presented for achieving optimal flux weakening operation in surface PM synchronous machines by properly designing the machines stator windings using concentrated, fractional-slot stator windings. This technique makes it possible to significantly increase the machine inductance in order to achieve the critical condition for providing wide speed ranges of constant-power operation. The conditions for optimal flux weakening can be achieved while simultaneously delivering sinusoidal back-emf waveforms and low cogging torque. A closed-form analytical model is described that can be used to design SPM machines to achieve optimal flux weakening conditions. This technique is applied to design a 6 kW SPM machine that achieves constant-power operation over a wide speed range. Performance characteristics of this machine are compared using both closed-form and finite element analysis.

A novel resonant snubber based soft-switching inverter

A new soft-switching inverter has been developed to overcome overvoltage and overcurrent problems in the existing resonant link inverters. This inverter employs a single auxiliary switch and a resonant inductor per phase to produce a zero voltage across the main switch so that the main switch can turn on at a lossless condition. Both the auxiliary switch and the resonant inductor are operating at a fractional duty, and thus are small in size compared to the main inverter circuit components. Although the resonance can be obtained with the use of the stray capacitance across the main switch, typical implementations require a small capacitor paralleling with the main switch to obtain nearly lossless turn-off. This paper describes both three-phase and single-phase circuit configurations. Operation modes in a complete zero-voltage switching cycle for the single-phase soft-switching inverter are described in detail with graphical explanation. Because the zero voltage condition is predictable by the selected circuit components, the implementation of this inverter does not require any voltage or current sensors. The circuit operation was first verified by a computer simulation and then further tested with a 1 kW power MOSFET based single-phase inverter. Both simulation and experimental results are presented to show the superiority of the proposed soft-switching inverter.<<ETX>>

A delta-configured auxiliary resonant snubber inverter

A delta (/spl Delta/) configured auxiliary resonant snubber inverter is developed to overcome the voltage floating problem in a wye (Y) configured resonant snubber inverter. The proposed inverter is used to connect auxiliary resonant branches between phase outputs to avoid a floating point voltage which may cause overvoltage failure of the auxiliary switches. Each auxiliary branch consists of a resonant inductor and a reverse blocking auxiliary switch. Instead of using an anti-paralleled diode to allow resonant current to flow in the reverse direction, as in the Y-configured version, the resonant branch in the /spl Delta/-configured version must block the negative voltage, typically done by a series diode. This paper shows single-phase and three-phase versions of /spl Delta/-configured resonant snubber inverters and describes in detail the operating principle of a single-phase version. The extended three-phase version is proposed with nonadjacent state space vector modulation. For hardware implementation, a single-phase 1 kW unit and a three-phase 100 kW unit were built to prove the concept. Experimental results show the superiority of the proposed topology.

Modified Vector Control Algorithm for Increasing Partial-Load Efficiency of Fractional-Slot Concentrated Winding Surface PM Machines

This paper presents a modified vector control algorithm for a fractional-slot concentrated-winding surface permanent magnet (SPM) machine that has been developed to maximize the machines partial-load efficiency over a wide range of operating conditions. By increasing the amplitude of the negative d-axis current, the resulting increase in the stator copper losses can be more than offset by the reduction in the iron core losses achieved by lowering the stator d -axis flux amplitude. The effectiveness of this technique has been demonstrated using both analytical models and finite element analysis for a 55-kW (peak) SPM machine design developed for a demanding set of traction drive performance requirements. For this example, the modified control strategy increases the partial-load efficiency at 20% of rated torque by > 6% at 2000 r/min compared to the maximum torque/ampere algorithm, making the machine much more attractive for its intended application.

Efficiency modeling and evaluation of a resonant snubber based soft-switching inverter for motor drive applications

This paper establishes an analytical model for a resonant snubber based soft-switching inverter. The model adopts the loss separation method to evaluate losses in individual components. Because of symmetry of the inverter circuit, the developed model is suitable for both single-phase and three-phase inverters. A single-phase inverter has been built and tested with a single-phase induction motor driving a fan load to verify the developed model. The equivalent single-phase induction motor model was curve-fitted from experimental results. The analytical results demonstrated reasonable agreement with the experimental results. The same efficiency evaluation method was then applied to a nonzero-voltage turn-on hard-switching inverter, and the results were compared with that of the soft-switching inverter. The resonant snubber based soft-switching inverter shows substantial efficiency improvement over the hard-switching inverter, especially in low speed operation.<<ETX>>