B. P. McGrath

Optimized space vector switching sequences for multilevel inverters

Previous work has shown that space vector modulation and carrier modulation for two-level inverters achieve the same phase leg switching sequences when appropriate zero sequence offsets are added to the reference waveforms for carrier modulation. This paper presents a similar equivalence between the phase disposition (PD) carrier and space vector modulation strategies applied to diode clamped, cascaded N-level or hybrid multilevel inverters. By analysis of the time integral trajectory of the converter voltage, the paper shows that the optimal harmonic profile for a space vector modulator occurs when the two middle space vectors are centered in each switching cycle. The required zero sequence offset to achieve this centring for an equivalent carrier based modulator is then determined. The results can be applied to any multilevel converter topology without differentiation. Discontinuous behavior is also examined, with the space vector and carrier based modulation methods shown to similarly produce identical performance. Both simulation and experimental results are presented.

Regions of Active Damping Control for LCL Filters

The control of a grid-connected voltage source inverter with an inductive-capacitive-inductive (LCL) filter is a very challenging task, since the LCL network causes a resonance phenomenon near to the control stability boundary. While many active damping methods have been proposed to overcome this issue, the role that pulse width modulation transport delay plays in the effectiveness of these strategies is still not fully resolved. This paper presents a theoretical discrete time-analysis framework that identifies three distinct regions of LCL filter resonance, namely, a high resonant frequency region where active damping is not required, a critical resonant frequency where a controller cannot stabilize the system, and a low resonant frequency region where active damping is essential. Suitable controllers are then proposed for the two stable regions, with gain calculations that allow for the greatest system bandwidth and damping. Simulation and experimental results verify the presented analysis.

Power converter line synchronization using a discrete Fourier transform (DFT) based on a variable sample rate

Line synchronization of grid connected power converters is a well recognized problem when the grid is weak, or derives from a remote area power supply with poor frequency regulation. Such systems can suffer significant line voltage distortion due to notches caused by power device switching and/or low frequency harmonic content, which can easily corrupt the output of a conventional zero crossing detector. This paper presents a method of filtering the incoming grid voltage using a recursive discrete Fourier transform (DFT). The filter provides a high degree of noise immunity but does produce a phase shift between the incoming grid voltage and the filtered output voltage when the DFT time window does not match the grid period. Two methods of compensating this phase shift are presented, based on tracking the drift in the phase predicted by the recursive DFT. The first method makes a deadbeat adjustment to the time window (thereby changing the sampling rate) while the second approach calculates the phase error based on the linear phase response of the DFT. These compensation algorithms can correct for discrepancies of at least 25% between the DFT time window and the system period, and can track grid frequencies with slew rates as high as 40 Hz/s with negligible phase shift (<2/spl deg/) between the grid voltage input and the filtered output waveforms.

Current Regulation Strategies for Vector-Controlled Induction Motor Drives

Dynamically accurate torque control is an essential prerequisite for higher performance motor drive systems. For ac induction motors (IMs), the two most established strategies are direct torque control (DTC) and vector or field orientated control. DTC directly switches the inverter to regulate torque without requiring explicit stator current regulation. However, it suffers from variable switching frequency and is more challenging to implement in digital controllers. Vector control separately regulates the “torque” and “flux” producing components of the motor stator current and is readily suited to a digital implementation with a constant switching frequency. However, it requires accurate current control to be effective, typically achieved using a linear current regulation system. The principles of linear current regulation are well established and have been researched intensively over many years. However, their quantitative design is still an uncertain mix of theory and practice, including in particular how to best set the regulator gains. This paper addresses this issue, by presenting a precisely matched comparative analysis of three alternative PI, and a hysteresis-based, current regulation strategies, suitable for use in a “standard” vector control IM drive. The results show that properly tuned, all four strategies have essentially the same performance, suggesting that the choice between them needs really only be made on the basis of convenience of implementation and/or cost.

Dual AC-drive system with a reduced switch count

A dual current-regulated pulsewidth-modulated voltage-source inverter based on multiple two-phase PWM inverters, also called a B4 topology, requiring a dual AC-drive system with reduced switch count is proposed. The drive utilizes a total of only eight switches to produce two sets of three-phase or two-phase sinusoidal output currents that can be employed to feed three-phase or two-phase induction motors. A suitable control strategy of this new scheme is shown to minimize the single-phase current now through the DC-link capacitors, which is a common problem in reduced-switch-count topologies. In order to verify the performance of the motor drive system, an application on traction of an electric vehicle is carried out. Results show that the AC current through the DC link can be minimized, and when utilizing two-phase motors on the proposed dual drive, the reduced voltage gain problem, also common in B4 topologies feeding three-phase motors, can be solved.

An Improved Three-Phase Variable-Band Hysteresis Current Regulator

This paper presents an improved variable-band hysteresis current controller for a two-level three-phase voltage source inverter (VSI). The controller takes the average voltages of the phase-leg switched outputs as an approximation to the load back-EMF voltages, and uses these results to vary the hysteresis bands so as to maintain constant phase-leg switching frequencies. The switching frequency control process is then further refined by fine tuning the hysteresis band variations to synchronize the zero crossings of the phase-leg current errors with a fixed reference clock so as to achieve a nearest space vector switching sequence, which further ensures that the switched output spectrum has been optimized. Finally, a technique is proposed to replace the third phase-leg current regulator with a fixed-frequency open-loop pulse-width modulator, where its commanded reference is generated from the average switched output voltages of the other two phase legs. This avoids the hazard of the three independent hysteresis current regulators adversely interacting with each other in a conventional system, resulting from an overconstrained control problem with only two degrees of freedom. Additionally, this approach allows the linear modulation range to be increased by adding a common-mode third-harmonic component to the third phase-leg reference command signal.

A Decentralized Controller Architecture for a Cascaded H-Bridge Multilevel Converter

Despite its inherently modular hardware structure, the control system of most cascaded H-bridge converters is usually highly centralized and relies on a high-bandwidth intraconverter communication system to transmit time critical control signals to the module controllers. In contrast, this paper presents a decentralized control strategy for a modular cascaded converter, where each module controller determines its own switching actions based on local sensors, a local current regulator, and a local modulator. The system achieves the same performance as an optimized centralized control system while only requiring a low intraconverter communication bandwidth.

High-Performance Current Regulation for Low-Pulse-Ratio Inverters

AC current regulation for a low pulse ratio inverter is particularly challenging because the controller bandwidth is often comparable to the target fundamental frequency, which can lead to a poor transient response. This paper presents an improved current regulation strategy to overcome this issue, using state feedback concepts to deterministically compensate for PWM transport delay so that the linear controller gains can be significantly increased. Design principles for the controller are presented using discrete Linear Quadratic Regulator theory, including strategies to make the controller robust to plant parameter variations and PWM saturation. Experimental results for both a 50 Hz and 400 Hz system confirm the theoretical concepts and demonstrate the superior transient performance of the proposed strategy.

High-Performance Voltage Regulation of Current Source Inverters

Current source inverters (CSI) offer advantages of voltage boost, short-circuit protection, reduced electromagnetic interference, and direct regeneration. While CSI control strategies are less developed than for voltage source inverters (VSI), the topologies are functional duals and have much in common in a control sense. In particular, since CSI voltage regulation is the dual of VSI current regulation, current regulation control strategies for a VSI can be readily implemented as equivalent voltage control strategies for a CSI. This paper shows how a high-performance proportional-integral stationary frame and P+Resonant CSI voltage regulator can be analytically designed and optimized, while in particular taking into account the second-order response of a CSI filter/load combination.

A Novel Three-Level Hysteresis Current Regulation Strategy for Three-Phase Three-Level Inverters

This paper presents a new hysteresis current regulation strategy for the neutral point clamped and flying capacitor (FC) three-level inverters. The strategy uses the measured average of the switched phase leg output voltage to adjust the controller hysteresis band as the load back EMF varies to maintain a near constant phase leg switching frequency. The phase leg switchings are then fine tuned to a fixed frequency clock to further improve frequency regulation. Next, the zero-crossings of the measured phase leg average voltages are used to select between positive and negative switched output voltage levels, so that only one hysteresis current regulator is required for the full inverter switched output voltage range. For the FC inverter, a state machine is then added to select between redundant switching states to maintain balanced capacitor voltages. Finally, the controller is extended to a three-phase system by subtracting the common mode interacting current from the total phase leg current error before making any switching decision. The resulting controller achieves a line-to-line harmonic performance that is very close to open-loop phase disposition pulse width modulation, while retaining all of the dynamic benefits of hysteresis current regulation.