Switching frequency reduction for efficiency optimization in two paralleled UPS systems

An improved data-driven based model predictive control for zero-sequence circulating current suppression in paralleled converters

Parallel inverters are widely used in the micro-grid systems to meet the demand for increased power capacity and enhanced reliability. However, unbalanced connections between the parallel inverters may cause a spike in the zero-sequence circulating current (ZSCC), leading to performance degradation. To address this issue, a virtual voltage vector-based sequential model-free predictive control method is proposed in this paper for parallel converters. Specifically, extended state observer (ESO) and virtual voltage vectors are introduced into the finite control-set model predictive control (FCS-MPC) architecture to cope with the multi-objective including ZSCC inhibition and robustness performance improvement. Next, to prevent the adjusting of weighting factors, a sequential model-free predictive control solution is constructed. The main feature of the proposed control methodology is that the reliability and robustness of the controlled system can be significantly enhanced without knowing the accurate mathematical models and parameters. Finally, the effectiveness of the suggested approach is verified by performance evaluation.

This paper proposes a new control scheme for parallel three-phase pulse width modulation converters under generalized unbalanced operating conditions. An average model of the parallel system in positive-sequence synchronous reference frame is derived to analyze the influence of generalized unbalanced operating conditions in ac side. It is seen that the unbalance factors in filter inductance will not only give rise to negative-sequence circulating current, but also contribute to generating zero-sequence circulating current (ZSCC). The negative-sequence circulating current can be inhibited by suppressing the negative-sequence components in ac output currents of parallel modules with a proportional integral resonant (PIR) controller. An improved feed-forward strategy and a PIR controller for ZSCC control are proposed for unbalanced operating conditions. The disturbances in ZSCC caused by unbalance factors in filter inductance can be rejected with feed-forward strategy. Because the disturbance in ZSCC is the fluctuation in grid frequency which can be suppressed by a resonance controller, therefore, a PIR controller is adopted in ZSCC controller. The proposed scheme can effectively suppress the circulating currents between the parallel modules and as a result, the distortions in output currents can be greatly reduced. Experimental results confirm the performance and effectiveness of the proposed method.

In this paper, a Lyaupnov-based finite control set model predictive direct torque control for the permanent magnet synchronous machine (PMSM) is proposed. In the proposed control scheme, the finite control set prediction and the Lyapunov theory are combined to minimize the torque ripple. The eight voltage vectors of the two-level converter are utilized as a finite control set for the torque prediction of the PMSM. A cost function considering the torque error, the maximum torque per ampere operation and the current limitation is introduced. Comparing to the conventional finite control set predictive control, the dominant part of the cost function is utilized as a Lyapunov function to estimate the duty cycle of each voltage vector. An optimum voltage can be obtained by the optimum voltage vector from the eight vectors and their duty cycles. A small sampling frequency and a fixed switching frequency can be realized when compared to the conventional finite set model predictive control. In the end, the simulation and experimental results validate the performance of the proposed control scheme.

This paper investigates the application of Finite Control Set Model Predictive Control (FCS‐MPC) for high‐performance speed regulation of Brushless DC (BLDC) motors under dynamic operating conditions. The study addresses the challenge of precise tracking for complex references, including sinusoidal, square, triangular, and sawtooth waveforms essential for advanced applications such as robotic and hydraulic actuator systems in aerospace applications. A key contribution is the analytical investigation of the predictive cost function, which elucidates the trade‐off between inverter switching frequency and tracking accuracy. This analysis identifies an optimal weighting factor that enables a ∼55% reduction in switching frequency while maintaining dynamic tracking errors below 0.5 RPM. For experimental validation, a custom BLDC motor drive was designed and implemented using an ESP32 processor, incorporating a novel bidirectional torque‐limiting strategy for operational safety. Both simulation and experimental results confirm that the proposed approach achieves accurate dynamic speed control for frequencies up to 0.3 Hz. Furthermore, a comparative study demonstrates the superiority of the method, showing an 85% reduction in amplitude error compared to a conventional PWM‐ON strategy for a 0.3 Hz sinusoidal reference.

Finite control set model predictive control (FCS‐MPC) has been widely used in the control of grid‐connected converters with the advantages of fast dynamics, multi‐objective control, and easy implement. However, the conventional FCS‐MPC bears with variable switching frequency, high current ripple and computational burden. An improved current model predictive with the cost function‐based modulation scheme (CFM‐MPC) is proposed for a three‐phase three‐level VIENNA rectifier to improve the power quality. First, the mathematical model and voltage vector are given according to the principle of deadbeat control. Then, the voltage vector of different voltage vectors are selected according to the location of the voltage vector reference, and the switching action time of the selected are directly calculated by the inversely proportional with cost function value of the selected vectors. It remains the merits of both the conventional MPC and space vector pulse width modulation schemes to track the optimum voltage vector without increasing the computational burden. Finally, a comparative study with the proposed CFM‐MPC and conventional FCS‐MPC has been conducted to verify the superiority of the proposed scheme. The results show the proposed CFM‐MPC has the advantages of lower power ripple, fixed switching frequency, lower total harmonic distortion and neutral point potential balance.

A direct torque control (DTC) with a modified finite set model predictive strategy is proposed in this paper. The eight voltage space vectors of two-level inverters are taken as the finite control set and applied to the model predictive direct torque control of a permanent magnet synchronous motor (PMSM). The duty cycle of each voltage vector in the finite set can be estimated by a cost function, which is designed based on factors including the torque error, maximum torque per ampere (MTPA), and stator current constraints. Lyapunov control theory is introduced in the determination of the weight coefficients of the cost function to guarantee stability, and thus the optimal voltage vector reference value of the inverter is obtained. Compared with the conventional finite control set model predictive control (FCS-MPC) method, the torque ripple is reduced and the robustness of the system is clearly improved. Finally, the simulation and experimental results verify the effectiveness of the proposed control scheme.

Efficiency optimisation is one of the critical issues concerned with the electric vehicle. It has a significant influence on the vehicle performance parameter - milage or running distance per battery charge. Modification of the existing structural design and minimising the component losses are two possible solutions. Minimising the component and its interaction losses by modifying the control strategy has less impact on the existing system architecture. Direct torque control (DTC) is often preferred for electric vehicle thanks to its fast dynamic response. However, inverter switching losses are very high in DTC. Along with this, harmonics in the inverter output increases the electric motor losses. Switching frequency reduction can help to minimise inverter losses. But reduced switching frequency pushes the lower order harmonic component of the inverter output voltage to the high-frequency side and demands increased size of the filter to be employed. These two conflicting objectives addressed with the help of finite control set-model predictive control (FCS-MPC). Simulation results of both control strategies for constant velocity and drive cycle tests confirm the effectiveness of this proposal. The performance measures ascertain a strong argument for FCS-MPC as a future electric vehicle control strategy.

This work introduces the control concept of variable sampling time finite control-set model predictive control (FCS-MPC). The new control concept is introduced in theory based on a review of the conventional FCS-MPC concepts performed with a constant sampling time. Based on the partitioning of the sampling instant in multiple smaller sampling instants it is possible to optimize, besides the switching states, the switching states turn-on times. Therefore, the proposed variable sampling time FCS-MPC sets both: the switching state and the related turn-on times. To utilize the available calculation power for longer sampling instants an adaptation of the control- and prediction horizon to the sampling time is proposed. The theoretical control concepts are applied to the control of a grid connected two-level voltage-source converter where a simple L-type line-filter is used to demonstrate the control performance of the variable sampling time FCS-MPC algorithms in the laboratory environment.

In the post-industrial digital-economy, motors powering manufacturing have been replaced by computers powering ecommerce - with significant financial losses attributable to poor power quality (PQ). At the same time, increasing penetration of distributed generation and constant power loads complicate the job for the electric utilities to deliver high-quality electrical power. Traditionally, voltage source converters provide PQ compensation; the backbone of these converters, however, are large electrolytic capacitors (e-caps), which have well-known failure rates. A D-STATCOM that does not use e-caps was shown to increase the converter service life and increase the reliability of the power system; finite control set model predictive control (MPC) was shown to achieve high fidelity tracking for multi-objective cost functions. However, high fidelity (to achieve low total harmonic distortion) results in high switching frequency, which increases losses and device stresses, and reduces the overall converter reliability. This paper improves the capacitorless D-STATCOM by investigating an adaptive MPC that trades-off high-fidelity performance and switching frequency, subject to IEEE 519 THD standards, to achieve good-enough performance. The method is verified experimentally in a 7.5 kVA D-STATCOM hardware prototype. The results show a reduction in switching frequency by over 30% compared with the MPC methods that prioritize high-fidelity alone.

Increasing tendency in online businesses and work from home, attracted many businesses and individuals to consider single-phase uninterruptible power supplies (UPSs). However, low power UPSs are normally incapable of operating for extended time periods. In order to extend UPS ranges, researchers integrated auxiliary energy sources to UPSs. However, integrating auxiliary power sources require additional power electronics active and passive components. This will result in low power density and bulky UPSs. To overcome this issue, multi-port converters have been proposed as an alternative to employing multiple converters. Nevertheless, using multi-port converters require more advanced control methods. This paper proposes a novel multi-port UPS with a finite control set model predictive controller. Simulations are conducted to demonstrate the effectiveness of the proposed converter. The results show the load current THD of 0.77%, resilient transient response, and efficiencies up to 97.5% using SiC MOSFETs.

Deadbeat control (DBC) has been known for decades due to its conceptual simplicity and good performance. Recently finite control set model predictive control (FCS-MPC) is emerging as a powerful strategy for control of power converters. Different from DBC, which uses a modulator such as space vector modulation (SVM), FCS-MPC directly obtained the optimal switching states of power converters by minimizing a cost function. Several improved versions of FCS-MPC have been proposed to improve the steady state performance by introducing two or three vectors during one control period. This paper investigates the inherent relationship between SVM-DBC and multiple-vector FCS-MPC. It is found that with the aim of tracking error minimization, the two kinds of predictive methods can be unified in one frame, where the vector sequence and duration in FCS-MPC can be easily obtained from SVM-DBC. In other words, under some special cases, FCS-MPC and its improved versions can be considered as special cases of SVM-DBC. This is the main contribution of this paper, because DBC and FCS-MPC are previously studied separately. The theoretical study is confirmed by the analysis on state-of-the-art DBC and FCS-MPC and supported by the selected simulation study and experimental tests from a three-phase two-level voltage source PWM rectifier.

This paper adopts a parallel system where two three-level neutral-point-clamped (NPC) inverters connect both ac and dc sides directly considering compact size, low cost, simplicity and easy expandability. But there is a potential zero-sequence circulating current (ZSCC), when the space-vector modulation method is used in the non-isolated system, then a control strategy for the ZSCC is present. The balancing problem of the DC-link capacitor voltage is inherent in the NPC inverter, it is solved throug zero-sequence voltage injection. A coordinate control strategy is then proposed to solve the two problems in NPC inverter at the same time. Experimental results validate the developed model and the proposed control scheme.

One of the main drawbacks with the use of finite control set model predictive control (FCS-MPC) in power converters is the variable switching frequency. When not taken into consideration, it can vary randomly depending mainly on the operating point of the system and the sampling time. This will produce a wide distributed voltage and current spectrum, causing audible noise, resonances, and poor steady-state behavior. To address this issue, a new algorithm, based on the control of the switching period within the cost function, is proposed in this paper. The algorithm shows to be very effective in achieving a regular commutation pattern, similar to pulse width modulation (PWM) techniques, which is evidenced even by similarities in the sideband harmonics generated in the voltage spectrum. The algorithm is easy to design and implement, demanding very low computation power, and enables a combination of the advantages of FCS-MPC with the benefits of a PWM-like power quality. Simulations and experimental results are presented to validate the proposed method, which are compared to previously reported FCS-MPC techniques dealing with the same challenge.

Parallel connections of three-phase Vienna rectifiers have been widely utilized to increase power ratings of charging stations. However, paralleled operations will introduce a path for the zero sequence circulating current (ZSCC) circulations. This problem will become worse especially for these rectifiers are operated under interleaved modes with unbalanced filter inductors. To mitigate these circulating currents, an integrated control scheme is proposed by mitigating the low frequency component of the ZSCC by PIR control in the inner-loop control and the high frequency component of the ZSCC by discontinuous PWM. Both numerical simulations and hardware experiments are performed to validate the effectiveness of the proposed control scheme.