Nikolaos Oikonomou

Synchronous Optimal Pulsewidth Modulation and Stator Flux Trajectory Control for Medium-Voltage Drives

Employing synchronous optimal pulsewidth modulation (PWM) techniques permits operating the PWM inverter of medium-voltage drives at very low switching frequency. The switching losses of the power semiconductor devices are thus reduced. The benefit is that a given inverter produces higher fundamental power. The optimal pulse patterns are determined by offline calculation, assuming steady-state operation of the drive machine. Dynamic modulation errors and high overcurrents, as a consequence, are therefore encountered when the operating conditions change. To overcome this problem, the harmonic components of the stator flux linkage vector are subjected to closed-loop control. The target trajectory is derived from the respective pulse pattern in use, while the actual stator flux trajectory is estimated. The approach is insensitive to parameter variations. It eliminates excessive transients when the operating conditions change. Experimental results obtained from an industrial 1-MVA 4.16-kV three-level inverter ac drive are presented

Model Predictive Pulse Pattern Control

Industrial applications of medium-voltage drives impose increasingly stringent performance requirements, particularly with regards to harmonic distortions of the phase currents of the controlled electrical machine. An established method to achieve very low current distortions during steady-state operation is to employ offline calculated optimized pulse patterns (OPP). Achieving high dynamic performance, however, proves to be very difficult in a system operated by OPPs. In this paper, we propose a method that combines the optimal steady-state performance of OPPs with the very fast dynamics of trajectory tracking control. A constrained optimal control problem with a receding horizon policy, i.e. model predictive control (MPC), is formulated and solved. Results show that the combination of MPC with OPPs satisfies both the strict steady-state as well as the dynamic performance requirements imposed by the most demanding industrial applications. This is achieved without resorting to complicated structures such as observers of the state variable fundamental components of the electrical machine, which are required by state-of-the-art methods. A further advantage of the MPC method is the use of a receding horizon policy to provide feedback and a high degree of robustness.

Direct Model Predictive Control: A Review of Strategies That Achieve Long Prediction Intervals for Power Electronics

Direct model predictive control (MPC) strategies that achieve long prediction horizons with a modest computational complexity are reviewed in this article, focusing on power electronics applications. In many MPC problems, a long prediction horizon is required to ensure an adequate closed-loop performance in steady state and to avoid stability issues. However, the computational effort of solving the optimization problem underlying MPC problems with long prediction horizons is often very great, making the implementation of such schemes in real time a difficult and challenging task. To overcome this difficulty, three established methodologies are surveyed that yield long prediction horizons with a modest computational burden. Case studies are investigated to substantiate the merits of these schemes. More specifically, for dc?dc boost converters, a move blocking strategy is reviewed, and for ac medium-voltage (MV) drives, both an extrapolation and an event-based horizon strategy are examined.

Optimal Control of a Dual Three-Level Inverter System for Medium-Voltage Drives

Actual developments in medium-voltage drives aim at increasing the power capability of pulsewidth modulated (PWM) inverters. Parallel connection of power semiconductor devices permits increasing the output current, whereas series-connected devices increase the output voltage. In either case are additional means required for balancing the current or voltage stress of the devices. The three-level neutral-point-clamped inverter topology offers an inherent means to ensure dynamic voltage balancing of a maximum of two series-connected devices. Higher operating voltages can be achieved by series connection of entire inverter topologies. Two circuit topologies that behave as five-level units are considered. Synchronous optimal PWM is applied for their control. This permits reducing the switching frequency to the 100-Hz region without sacrificing on harmonic distortion.

Closed-Loop Control of Medium-Voltage Drives Operated With Synchronous Optimal Pulsewidth Modulation

Inverters for medium voltage drives operate at reduced switching frequency so as to restrain the dynamic losses of the power semiconductor devices. The resulting current harmonics can be reduced by synchronous optimal pulsewidth modulation (PWM), provided that steady-state conditions prevail. Transient conditions, however, interfere adversely with the optimal modulation patterns. Such conditions necessarily occur when the modulator forms part of a conventional closed-loop control scheme. Trajectory tracking control is employed to achieve high dynamic control in conjunction with synchronous optimal PWM. An optimal trajectory of the stator flux linkage vector is derived from the pulse pattern in actual use. The stator flux linkage vector is forced to follow this target trajectory. Modifying the target trajectory in transient conditions enables closed-loop torque control in a deadbeat fashion while conserving optimal modulation. Experimental results obtained from a 30-kW prototype drive operated at only 200 Hz switching frequency demonstrate the effectiveness of the approach.

Fast Dynamic Control of Medium Voltage Drives Operating at Very Low Switching Frequency—An Overview

Medium voltage AC machines fed by high-power inverters operate at a low switching frequency to restrain the switching losses of power semiconductor devices. Particular care is thus required in the design of the drive control system. The signal delay caused by low switching frequency operation increases undesired cross-coupling effects in vector-controlled schemes. These are not sufficiently compensated by established methods like feedforward control. Improvements are achieved by a more accurate modeling of the machine and the inverter. An adequate controller is introduced, having a transfer function with complex coefficients. The high harmonic distortion due to the low switching frequency is a tradeoff. Using synchronous optimal pulsewidth as an alternative permits reducing the switching frequency without increasing the harmonics. The detrimental effects of conventional control methods are eliminated by forcing the harmonic components on an optimal spatial trajectory. Deadbeat behavior and complete decoupling are thus achieved. The performance of the aforementioned schemes is compared based on mathematical analyses and experimental results.

Estimation of the Fundamental Current in Low Switching Frequency High-Dynamic Medium Voltage Drives

The switching frequency of medium voltage ac drives is limited to low values to restrain the dynamic losses of the power devices. This favors the use of synchronous optimal pulsewidth modulation schemes that minimize the harmonic current. It is a drawback, though, that optimal algorithms do not have a means to extract the fundamental component of the load current. High-performance torque control is therefore difficult to obtain. The paper proposes a method to identify the instantaneous fundamental component of the stator currents. A novel observer is developed for this purpose. The approach enables fast torque control at very low switching frequency. Experimental results from a 30-kW induction motor drive are presented.

Neutral point potential balancing algorithm at low modulation index for three-level inverter medium voltage drives

Three-level inverters produce low harmonic distortion of the ac currents even when operated at moderate switching frequency. This makes them the preferred candidates for high-power medium-voltage applications. To improve the utilization of the semiconductor devices, synchronous optimal pulsewidth modulation is employed. This permits reducing the switching frequency to very low values. Carrier modulation is maintained in the lower range of the modulation index. Operation at very low switching frequency increases the steady-state ripple of the neutral point potential. An intrinsic natural balancing mechanism of the neutral point clamped inverter topology eliminates long-term neutral point potential offsets. Transient conditions, however, may create successive increments of the offset to high values, which requires fast compensation. The novel method of selecting the appropriate redundant inverter sub-bridge meets this requirement without incurring additional penalties. The effectiveness of the approach is documented by experiments obtained from a medium-voltage motor drive fed by a 1-MVA three-level inverter

Synchronous optimal pulsewidth modulation and stator flux trajectory control for medium voltage drives

High-power PWM inverters for medium voltage drives operate at low switching frequency to maintain the switching losses of the power semiconductor devices at an acceptable level. For these applications, synchronous optimal pulsewidth modulation methods permit a good trade-off between switching losses and harmonic distortion of the machine currents. The optimization of the pulse patterns is done off-line on the assumption of steady-state conditions. Dynamic modulation errors, and high overcurrents as a consequence, are encountered when the operating conditions change during operation. To overcome this problem, the stator flux linkage vector, including its harmonics, is subjected to closed loop control. A stator flux trajectory controller is employed for this purpose. Experimental results obtained from an industrial 1-MVA, 4.16 kV three-level inverter ac drive are presented.

Model predictive control of the internal voltages of a five-level active neutral point clamped converter

In this paper, model predictive control (MPC) is introduced to control the internal voltages of an active neutral-point clamped five-level converter (ANPC-5L). The proposed control scheme aims to keep the neutral point and phase capacitors voltages of the converter within given hysteresis bounds while at the same time minimizing the switching frequency. An additional benefit of the controlled voltages is a reduced level of output current distortion. The large number of redundant states that exist in multi-level converters makes it possible for all the objectives to be achieved. A short horizon is employed in order to ensure a manageable level of complexity. At the same time extrapolation is used to bring the performance to the desired level. Simulation results that substantiate the effectiveness of the proposed approach are presented.