Explore the vast range of titles available.

In this paper, a new carrier phase-shift (CPS) modulation method based on switching the displacement angle (SDA) is proposed to compromise the harmonic content of the output voltage and the circulating current. It can be used in medium- and low-voltage applications where the AC-side voltage and DC-side current of the modular multilevel converter (MMC) are required to have low harmonic content simultaneously. In this proposed SDA-based CPS modulation, the carrier displacement angle of the MMC with N submodules in each arm is periodically switched between the values of 0 and π/N degrees, so that the harmonic content of the output voltage and the harmonic content of the circulating current will not be in extreme conditions, which occurs when the displacement angle is set to 0 or π/N degrees. The effectiveness of this method has been verified by simulation and experimental results.

Modular multilevel converter (MMC) circulating current suppression is an effective method in improving power conversion quality. However, due to the complex composition of circulation under a three-phase imbalanced state, as well as the limitations of existing sliding mode circulation suppression methods that can easily cause high-frequency oscillation in the system, the circulation suppression effectiveness still needs to be further improved. Firstly, this article clarifies the phase sequence law and key control factors of the circulation by separating the MMC circulation phase sequence and decoupling the model. Secondly, the generalized proportional integral sliding surface and the hyperbolic tangent convergence law are introduced into sliding mode control to improve the system’s ability to flexibly suppress the circulating current. Then, the proposed method is evaluated using the system power quality and insulated-gate bipolar transistor (IGBT) junction temperature amplitude. The results show that the proposed method reduces the total harmonic distortion of the bridge arm current to 1.28% and 1.03%, respectively, under three-phase balanced and imbalanced states, and effectively smooths the IGBT junction temperature fluctuation of submodules. It also improves the stability and robustness of MMC system to circulation suppression gain variations, sudden load changes, and switching failures. This article provides an effective method for the synchronous implementation of MMC circulation suppression and IGBT junction temperature smoothing under complex and variable operating conditions.

In electrical machinery, the rotor windings’ internal short-circuit faults are addressed by the instantaneous over-current protection of the power electronic excitation device, which has low sensitivity and has difficulty meeting the safety requirements. In this paper, a rotor windings’ internal short-circuit fault protection method is proposed based on the harmonic characteristics of the circulating current between stator branches. The magnetomotive force distribution of the short-circuit coils in the rotor windings is theoretically deduced, and the characteristic frequencies of the circulating current between stator branches are analyzed. On this basis, the protection criterion of the rotor windings’ internal short-circuit fault is constructed by using the harmonic component of the circulating current. Then, an analytic model of the variable-speed pumped storage unit is established based on the multi-loop method, and the finite element method is used to verify the correctness of the proposed modeling method. An actual large variable-speed pumped storage unit is taken as an example, and the possible faults under different slip ratios are simulated. In the simulation results, the stator branch circulation has the obvious characteristic frequency harmonic components, which is consistent with the theoretical analysis. It verifies the effectiveness of the proposed protection method. Finally, it is analyzed and verified that the proposed protection has a strong maloperation prevention ability under other kinds of faults.

This paper presents a novel approach to reduce harmonic distortion and mitigate zero-sequence circulating current (ZSCC) in parallel active power filters (APFs). By employing Dual-Comparison One-Cycle Control (DC-OCC), this method effectively reduces harmonics. Carrier asynchronization among inverter modules in parallel configurations leads to the generation of ZSCC, which distorts output waveforms and reduces system efficiency. A mathematical model is developed to decompose ZSCC into low-, medium-, and high-frequency components, revealing how these components are influenced by carrier-phase deviations. Based on this model, a ZSCC extraction and compensation scheme is proposed. This method enables effective suppression of ZSCC without requiring additional components, communication links, or sensors. Simulation and experimental results demonstrate that the proposed approach achieves significant harmonic suppression, improved power factor, and a peak efficiency of 98.7%, confirming the effectiveness of the control strategy in practical applications.

In order to simultaneously reduce the input line current harmonic and the output voltage ripple of the 12-pulse rectifier, a hybrid harmonic suppression method (HHSM) with high harmonic suppression performance and low harmonic suppression cost based on the active-tapped inter-phase reactor (AT-IPR) is proposed in this paper. The AT-IPR is connected in series with the load to generate a passive circulating current (PCC) directly modulating the output current of the 12-pulse star diode-bridge rectifier (SDBR); the compensation circuit (CC) connected in parallel with the load is used to inject the active compensation circulating current (ACC) into the AT-IPR to indirectly modulate the output current of the SDBR. The optimally designed compound circulating current consisting of PCC and ACC first modulates the output current of the SDBR. Based on the current relationship between the ac and dc sides and the voltage relationship between the dc sides, the input line current of the SDBR is further shaped nearly the sinusoidal current and the pulse number of output voltage is doubled to 24. Moreover, the kVA ratings of AT-IPR and CC are only about 1.8% and 1% of the output power of the proposed rectifier using HHSM, respectively. Therefore, the proposed method is suitable for high-power rectification where the requirements for power quality and reliability are relatively high. The detailed analysis and relevant simulation results are given to validate the proposed method.

Compared with other topologies, the modular multilevel converter (MMC) has the advantages of higher scalability and lower harmonic distortion. When carrier-based pulse-width modulation approaches are used for the MMC, the number of carriers increases for more sub-modules, and the complexity of the control and the memory required increases as a result. In addition, the synchronization of several carriers is another issue. Due to the unique constructional characteristics of the MMC, circulating currents will be generated internally, causing distortions in the arm currents and, thus, unnecessary converter losses. In this paper, an improved 2N+1 pulse-width modulation approach with low control complexity and a circulating current suppression strategy are proposed. Firstly, the conventional carrier phase-shifted 2N+1 pulse-width modulation approach is improved so that the number of carrier signals adopted in each arm is always two. Secondly, the redundant switching states are used to suppress the circulating current. Finally, the effectiveness of the proposed strategy is verified experimentally. The results show that the proposed method reduces the control complexity while retaining the output performance. Meanwhile, the circulating current can be suppressed.

In recent years, modular multilevel converters (MMCs) have been increasingly used in the field of electric vehicle charging and discharging due to their unique performance advantages. However, the unique cascade structure of MMCs raises the problem of the circulating current. Due to chemical processes, aging effects, etc., the capacitance parameters of the upper and lower bridge arms will be asymmetric, which will introduce odd harmonics into the circulating current, increasing system losses and threatening the system reliability. To address this phenomenon, this paper proposes a circulating current suppression strategy with additional virtual impedance (VI) based on a repetitive controller (RC). The corresponding simulation models were built for the comparative study of different circulating current suppression strategies. The results show that the VI-RC circulating current suppression strategy can significantly reduce the odd and even harmonics in the circulating current under asymmetric conditions, and the total harmonic distortion (THD) of the bridge arm current is only 0.98%, which verifies the effectiveness of the proposed strategy.

In order to reach better results for pulse width modulation (PWM)-based methods, the reference waveforms known as control laws have to be achieved with good accuracy. In this paper, three control laws are created by considering the harmonic components of modular multilevel converter (MMC) state variables to suppress the circulating currents under nonlinear load variation. The first control law consists of only the harmonic components of the MMC’s output currents and voltages. Then, the second-order harmonic of circulating currents is also involved with both upper and lower arm currents in order to attain the second control law. Since circulating current suppression is the main aim of this work, the third control law is formed by measuring all harmonic components of circulating currents which impact on the arm currents as well. By making a comparison between the switching signals generated by the three proposed control laws, it is verified that the second-order harmonic of circulating currents can increase the switching losses. In addition, the existence of all circulating current harmonics causes distributed switching patterns, which is not suitable for the switches’ lifetime. Each upper and lower arm has changeable capacitors, named “equivalent submodule (SM) capacitors” in this paper. To further assess these capacitors, eliminating the harmonic components of circulating currents provides fluctuations with smaller magnitudes, as well as a smaller average value for the equivalent capacitors. Moreover, the second-order harmonic has a dominant role that leads to values higher than 3 F for equivalent capacitors. In comparison with the first and second control laws, the use of the third control-law-based method will result in very small circulating currents, since it is trying to control and eliminate all harmonic components of the circulating currents. This result leads to very small magnitudes for both the upper and lower arm currents, noticeably decreasing the total MMC losses. All simulation results are verified using MATLAB software in the SIMULINK environment.

The harmonic injection method is widely used in modular multilevel converters (MMCs) to suppress the significant amplitude of the submodule (SM) capacitor voltage fluctuation under low-frequency operation. A method is required to inject high-frequency common-mode voltages and circulating currents, which both occupy modulation space and are prone to over-modulation. This paper focuses on the modulation signal required for circulating current injection, which has been neglected in previous studies. In addition, this paper redefines the amplitude references of the injected harmonics to reserve modulation space for the injected circulating current. By analyzing the characteristics of an MMC system with the harmonic injection method, the relationship between the modulation signal for the circulating current and other variables (such as the output modulation index, output frequency and injection frequency) is obtained. Using the obtained relationship, a strategy for selecting the amplitude and frequency of the injected harmonics is presented to avoid over-modulation under variable conditions while suppress the SM capacitor voltage fluctuation. The theoretical analysis, the proposed injection method and its parameter selection strategy are verified by experimental results.

Modular multi-level converters (MMCs) are often used for high and medium voltage applications. However, to reduce losses and costs, many researchers prefer a half-bridge converter. In addition, the half-bridge-based MMC is vulnerable in the event of an error, so the full-bridge MMC is used here to work with faulty network states. The losses and harmonics in the system could be reduced by using an appropriate arm voltage and circulating current control model. In order to operate the MMC in a grid-tied renewable system, both outer and inner loop control were performed. In order to realize outer-loop control, a fractional-order proportional–integral–derivative controller using a deep learning technique is proposed. An active power filter-based fractional-order proportional resonant controller with improved pulse width modulation achieves arm balancing with harmonic mitigated circulating current regulation. The simulation shows that the proposed method reduced the current and voltage harmonics to 71.56% and 10.42% through an improved control strategy based on pulse width modulation.