The development of microgrids is an advantageous option for integrating rapidly growing renewable energies. However, the stochastic nature of renewable energies and variable power demand have created many challenges like unstable voltage/frequency and complicated power management and interaction with the utility grid. Recently, predictive control with its fast transient response and flexibility to accommodate different constraints has presented huge potentials in microgrid applications. This paper provides a comprehensive review of model predictive control (MPC) in individual and interconnected microgrids, including both converter-level and grid-level control strategies applied to three layers of the hierarchical control architecture. This survey shows that MPC is at the beginning of the application in microgrids and that it emerges as a competitive alternative to conventional methods in voltage regulation, frequency control, power flow management and economic operation optimization. Also, some of the most important trends in MPC development have been highlighted and discussed as future perspectives.

Model predictive control of microgrids – An overview

Starting from the principle of instantaneous power theory, this article explores various direct power control (DPC) strategies for three-phase two-level pulsewidth modulation (PWM) converters. After summarizing the fundamental power formula of PWM rectifiers, this article studies the operating principle of the conventional table-based approach and its related improvements. It further looks into the advanced counterparts employing space vector modulation and different nonlinear control strategies. The emphasis is put on the prevailing predictive DPC. Besides, the voltage-sensorless and robust DPC methods based on the virtual flux concept and the state observer or estimator are investigated. Critical issues, including the sample delay, constant switching frequency, duty cycle optimization, objective function, and unbalanced operation are examined.

A Review on Direct Power Control of Pulsewidth Modulation Converters

A modulated model-free predictive control with minimum switching losses (MSL-MMFPC) is proposed to improve the steady-state performance and reduce the switching losses for a permanent magnet synchronous motor (PMSM) drive system. Firstly, two adjacent current vectors are determined based on the predefined first-level cost function, and then, make the current vector at the next control period equal to the reference current vector by modulating the selected current vectors properly. Additionally, in order to keep optimal control performance also in the over-modulation region, a new rotating coordinate frame is used to adjust the optimal voltage vector. Then, the second-level cost function is designed to select the optimal voltage vector sequence, so that the switching of a VSI leg does not happen during the phase-current maximum, which can reduce the switching losses of the inverter. The simulation and experimental results verify the effectiveness of the proposed control method.

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Modulated Model-Free Predictive Control With Minimum Switching Losses for PMSM Drive System

A model predictive control (MPC) strategy for grid-connected power converters is proposed in this article to reduce the current ripples without using ac voltage sensors. First, a sliding-mode observer is designed to estimate the grid voltage with an adaptive compensation algorithm. Thus, grid-voltage sensors are removed. The novelty of the proposed grid-voltage observation method is that it is inherent frequency adaptive without using the accurate grid angular frequency. By implementing the MPC strategy using the estimated grid voltage, the current ripples are reduced, especially under distorted grid-voltage conditions because of the low-pass filter used in the observer. Next, to further reduce the current ripples, a double-voltage vector-based MPC strategy is proposed based on the principle of the modulated MPC. Detailed theoretical analyses are also carried out to show its effectiveness for the first time. Finally, experimental studies are carried out to verify the validity of the proposed ac voltage sensorless MPC strategy.

A Model Predictive Control Method for Grid-Connected Power Converters Without AC Voltage Sensors

The model predictive direct power control (MPDPC) has attracted significant attention due to its outstanding dynamic response and high power factor. However, the variable switching frequency makes the design of alternating current (ac) filter more challenging, and the heavy computational burden limits the application of MPDPC. This paper proposed a new cost function and four steps’ MPDPC (FSMPDPC) scheme for T-type inverters. The proposed cost function can reduce the number of division operations and does not require calculating the duty cycles of all vectors. Meanwhile, the four steps’ calculation process is divided into four steps to reduce the number of cycle calculations. The first three steps are assigned to adjust the active and reactive powers, and the neutral-point (NP) voltage is balanced in the fourth step. An experimental platform of a T-type inverter is established to demonstrate the superiorities of the proposed FSMPDPC. The results show that FSMPDPC improves the steady-state performance of the T-type inverter with lower current total harmonic distortion (THD) and lower ripples in the active and reactive powers. In addition, the proposed algorithm eases the computational burden of the digital signal processor (DSP).

Model Predictive Direct Power Control With Fixed Switching Frequency and Computational Amount Reduction

A voltage sensorless control of low-complexity model predictive direct power control (LC-MPDPC) for pulsewidth modulation (PWM) rectifier is proposed. The conventional LC-MPDPC adopts one or two voltage vectors during one control period, which achieve good steady-state performance and quick dynamic response. In addition, based on the mathematical model of the real system, the conventional method only requires one prediction to find the optimal voltage vector, which reduces the control complexity and computational burden. However, the use of one or two vectors during one sampling interval presents abundant current harmonics and high power ripples, and the switching frequency is variable. In order to solve these problems while preserving all the advantages of the conventional LC-MPDPC, this paper presents a novel control scheme, with the aim of operating at a constant switching frequency and obtaining an excellent steady-state performance at a low switching frequency. The proposed method is based on an optimal application of three voltage vectors in a symmetrical way, which takes advantage of advanced PWM techniques. Furthermore, the virtual flux-based control scheme is introduced to achieve voltage sensorless control. The proposed strategy is compared with the conventional MPDPC methods and its effectiveness is confirmed by both simulation and experimental results from a three-phase PWM rectifier under 1000-W operation condition.

Three-Vector-Based Low-Complexity Model Predictive Direct Power Control Strategy for PWM Rectifier Without Voltage Sensors

A novel strategy of model predictive direct power control (MPDPC) is proposed in this paper. In the conventional MPDPC, the instantaneous active and reactive power is simultaneously controlled with a single cost function, which consists of active and reactive power errors. If either of the two error terms has a large variation, the control weight is concentrated on one side, which generates mutual interference problem. To solve this problem, the proposed control strategy reorganizes the single cost function by adding weighting factors to the two error terms. Moreover, the weighting factors are obtained by fuzzy logic modulator, which works based on the absolute value of active and reactive power errors. Simulation results prove that, compared to the conventional MPDPC, the proposed control scheme can realize the decoupling control of the active and reactive powers.

Fuzzy Logic Based Model Predictive Direct Power Control of Three Phase PWM Rectifier

A novel strategy of model predictive direct power control (MPDPC) for PWM rectifier is proposed in this paper. Previous studies have proposed one-vector-based and two-vector-based MPDPC methods for PWM rectifier, which achieve good steady-state performance and quick dynamic responses. However, due to the limited switching states in the three-phase two-level PWM Rectifier, the sampling frequency should be high to obtain the satisfactory control performance. Besides, the switching frequency is variable, and this makes it difficult to design the filtering inductance. To deal with those problems, while maintaining all the advantages of the conventional MPDPC, this paper presents a new control strategy. The proposed MPDPC is based on an optimal application of three voltage vectors in a symmetrical way, which is known as symmetrical 3+3 vectors' sequence. The simulation and experimental results prove that, compared to conventional MPDPC, the proposed MPDPC achieves the steady-state harmonic spectrum at lower level and remains quick dynamic responses.

Three-Vector-Based Model Predictive Direct Power Control Strategy for PWM Rectifier

The electrolytic capacitor is the key component that not only lowers the reliability of single-phase to three-phase power converter for drive system, but also makes high current harmonics at the AC grid. Recently, the drive system with a film capacitor has been researched for its long lifetime and the characteristics of small volume and low cost. However, the input power factor is reduced and the AC input current waveform is distorted when the motor is driven at light load conditions. In order to deal with this problems, the direct input power control method that related to the AC input voltage and current is proposed in this paper. Furthermore, in order to keep the dc-link voltage higher than the inverter output line-to-line voltage, the flux-weakening control that varies synchronously with the double frequency of the AC input voltage is adopted. The results verified the correctness and effectiveness of the proposed method.

Hui Li

Papers

Three-Vector-Based Low-Complexity Model Predictive Direct Power Control Strategy for PWM Rectifier Without Voltage Sensors

Fuzzy Logic Based Model Predictive Direct Power Control of Three Phase PWM Rectifier

Three-Vector-Based Model Predictive Direct Power Control Strategy for PWM Rectifier

Direct Input Power Control for Drive System of Single-Phase to Three-Phase Power Converter Without Electrolytic Capacitor