Dual inverter drive system for permanent magnet synchronous motor based on intermediate hexagonal modulation

This paper investigates fault line selection in small current grounding systems, focusing on the zero sequence current phase characteristics. Small current grounding systems, prevalent in medium voltage distribution networks, face challenges in fault detection due to minimal fault currents during single status phase grounding faults. Traditional methods relying on manual line switching and zero sequence voltage monitoring are inefficient and can induce overvoltage. This study categorizes fault line selection techniques into three approaches: steady-state fault information, transient information, and modern signal processing technology. The analysis of zero sequence currents in non-grounded systems reveals that fault line zero sequence currents equal the sum of system capacitive currents, flowing opposite to those in non-fault lines. Additionally, transient signal analysis shows that these signals provide more robust information for fault detection. MATLAB simulations validate the theoretical models, demonstrating that zero sequence current direction and amplitude effectively identify fault lines. The results confirm the efficacy of combining steady-state and transient signal analyses for accurate fault line selection, contributing to enhanced safety and reliability in power distribution networks. This research offers practical insights and tools for engineers, improving fault management and reducing potential economic losses in small current grounding systems.

Torque ripple caused by zero-sequence current (ZSC) and relatively high switching frequency severely worsens the operating performance of an open-winding permanent-magnet synchronous machine (OW-PMSM) system with common dc bus. In this paper, a novel q-axis current injection method based on the angle-shift-based voltage distribution is proposed for the OW-PMSM system with common dc bus. With the proposed method, torque ripple caused by ZSC can be counteracted by the torque component generated by the injected q-axis current. Meanwhile, the corresponding modulation range is analyzed to investigate the availability of the proposed method. Moreover, in order to reduce the switching frequency, zero vector (111) is no longer necessary to compensate the zero-sequence component and conventional space vector pulsewidth modulation (PWM) with zero vectors (000) and (111) can be simplified. The switching actions in one PWM period can be reduced from 12 to 8. The comparison of losses shows that this strategy can effectively improve the efficiency under both heavy load and high switching frequencies. Finally, the effectiveness of the proposed method is validated through an OW-PMSM experiment with common dc bus.

Zero-sequence current (ZSC) is inevitable in the structure of an open-end winding permanent-magnet synchronous machine (OW-PMSM) system with a common dc bus due to the simultaneous existence of the ZSC loop and the zero-sequence voltage (ZSV) caused by the inverters and third electromotive force. To suppress the ZSC, a novel strategy with a reduced switching frequency is proposed in this paper. The main contribution of the proposed method is that the ZSC is suppressed totally with a reduced switching frequency. The duration time of the active voltage vectors is redistributed to generate the reference voltage in a zero-sequence loop, which leads to the suppression of the ZSC. Different from the conventional method, only the active voltage vectors instead of the zero voltage vectors are employed to suppress the ZSC. Consequently, the switching frequency in the modulation process can be reduced. In addition, the effective modulation range of the ZSV is analyzed, which further reveals the relationship among the effective modulation of the ZSV, the modulation index, and the ratio of the third flux linkage component to the fundamental flux linkage component. Finally, the effectiveness of the proposed method is validated by an OW-PMSM experimental setup with a common dc bus. Moreover, the proposed method is experimentally compared with the existing methods.

Direct torque control (DTC) systems for open-end winding permanent magnet synchronous motors (OEW-PMSM) with common DC bus generate zero sequence currents (ZSC) and large torque ripple, resulting in increased system loss and poor operating characteristics. In this paper, a DTC based on five level torque hysteresis (FLTH) is proposed to improve the system performance. Firstly, an online calculation and selection strategy based on switch table is designed to suppress ZSC. By analyzing the composition of zero sequence voltage and using the double vector method to suppress it, the ZSC can totally be suppressed in theory, without affecting the system performance. On this basis, this paper proposes to add FLTH to DTC to reduce torque ripple. FLTH can not only effectively use many voltage vectors, but also reduce torque ripple. In the case study, the torque ripple rate has been reduced by 53%. Through simulation and experiment, the conventional DTC and the FLTH DTC are compared, and the results prove the validity of the proposed strategy.

Dual three-phase permanent-magnet synchronous machines (DTP-PMSM) connected with a single neutral point provide a loop for zero-sequence current (ZSC). This paper proposes a novel space vector pulse width modulation (SVPWM) strategy to suppress the ZSC. Five vectors are selected as basic voltage vectors in one switching period. The fundamental and harmonic planes and the zero-sequence plane are taken into consideration to synthesis the reference voltage vector. To suppress the ZSC, a non-zero zero-sequence voltage (ZSV) is generated to compensate the third harmonic back-EMF. Rather than triangular carrier modulation, the sawtooth carrier modulation strategy is used to generate asymmetric PWM signals. The modulation range is investigated to explore the variation of modulation range caused by considering the zero-sequence plane. With the proposed method, the ZSC can be considerably reduced. The simulated and experimental results are presented to validate the effectiveness of the proposed modulation strategy.

Zero sequence impedance is an important parameter for accurate estimation of unbalanced fault current in power systems. In case of high voltage cable systems, there are various kind of bonding schemes resulting in more complicated zero sequence circuit compared to that of overhead transmission systems. Besides, the zero sequence circuit can be much more complicated when the cable system has a single-point bonding because of the rather complicated distribution of zero sequence current between cable sheaths grounding and the earth. In this paper, sequence impedances of 345 kV 4 circuits cable system with various bonding schemes is analyzed using ATP (Alternative Transient Program). The calculation results showed that the conventional method of zero sequence calculation can be erroneous in case of underground transmission lines mainly caused by its complicated and nonlinear return path of ground fault current.

Detection of high resistance earth faults in medium voltage networks is an important problem due to ineffectiveness of traditional earth fault protections. Such short circuits can be detected by the criterion of a reactive power of higher harmonics for zero sequence current and voltage. The main problem is determination of the power setting value in the protection, which depends on the asymmetry of phase-to-earth capacitances and higher harmonics in supply voltages, which are generated by non-linear loads. The intensive tests of the asymmetry of the zero sequence currents and voltages for harmonics and their reactive power have been carried out in 15 kV compensated network as a function of all relevant parameters, i.e.: maximum capacitance deviation of the network and protected line, percentage content of harmonics in supply voltages, capacitive current of the network and the line. It has been shown that third harmonics of the zero sequence voltage and current are the best suited for practical use, since the asymmetry reactive power of these components is the smallest among the considered harmonics and the protection sensitivity will be the highest.

The voltage unbalance between the upper and lower capacitors is an important problem in the three-level PWM converter system, and it is known that the voltage unbalance is directly related to the zero sequence. In this paper, we provide a zero sequence current path by connecting the neutral point of the source transformer to the midpoint of the DC link capacitors. Further, we control the unbalanced voltage with the zero sequence current. The zero sequence current is controlled by the zero sequence voltage made from sinusoidal PWM. The effectiveness of the proposed scheme is verified by computer simulations and experiment.

In the open-winding permanent magnet synchronous motor (OW-PMSM) system fed by dual inverter with common dc bus, suitable control strategy should be adopted to suppress the torque ripple generated by the third harmonic flux and 0-axis current. However, the mathematical model will change when the motor is under open-phase fault so that the coordinate transformations and control strategy should be redesigned accordingly in order to suppress the torque ripple. In this article, the postfault OW-PMSM system is remodeled considering the third harmonic flux. Based on the postfault model, new coordinate transformations are designed to obtain a concise torque and decoupling voltage expressions for the simple and accurate control of torque. Moreover, the torque ripple is suppressed by modifying the current reference. In addition, a simple decoupling modulation is designed to directly obtain the switching signals for the dual inverter. Experimental results confirm the effectiveness of the proposed fault-tolerant control strategy.

This paper presents and compares control strategies for three-phase open-end winding drives operating in the flux-weakening region. A six-leg inverter with a single dc link is associated with the machine in order to use a single energy source. With this topology, the zero-sequence circuit has to be considered since the zero-sequence current can circulate in the windings. Therefore, conventional overmodulation strategies are not appropriate when the machine enters in the flux-weakening region. A few solutions dealing with the zero-sequence circuit have been proposed in the literature. They use a modified space vector modulation or a conventional modulation with additional voltage limitations. This paper describes the aforementioned strategies, and then, a new strategy is proposed. This new strategy takes into account the magnitudes and phase angles of the voltage harmonic components. This yields better voltage utilization in the dq frame. Furthermore, inverter saturation is avoided in the zero-sequence frame, and therefore, zero-sequence current control is maintained. Three methods are implemented on a test bed composed of a three-phase permanent-magnet synchronous machine, a six-leg inverter, and a hybrid digital signal processor /field-programmable gate array controller. Experimental results are presented and compared for all strategies. A performance analysis is conducted as regards the region of operation and the machine parameters.

This paper presents a closed-loop control strategy for dual inverter-fed, permanent magnet synchronous motors (PMSM) with non-sinusoidal back electromotive force (back-EMF). The inverters are connected to both ends of the machine winding but use only one power source, thus providing a path for zero sequence currents to circulate in the motor windings. Due to the non-sinusoidal back-EMFs and circulation of zero sequence currents, the machine electrical torque is now a function of the rotor position that leads to torque ripple oscillations. Therefore, a closed-loop control strategy to suppress the circulation of zero sequence currents and amplitude of the dominant current harmonics caused by the non-sinusoidal back-EMFs is developed and demonstrated by experiment. The zero sequence component is considered as one additional degree of freedom that has to be managed. Therefore, the control scheme to regulate the ac signals in the regulators is developed and an approach to minimizing the torque ripples is presented.

To improve the parameter robustness of the open-winding permanent magnet synchronous motor (OW-PMSM), this paper proposes an improved model-free predictive current control (MFPCC) method. First, the first-order ultra-local model of OW-PMSM is established, which does not contain any motor parameter. Secondly, a sliding mode observer (SMO) is introduced into the model to estimate the current in d-q-0 axis. Then, to suppress the torque ripple generated by the zero-sequence current (ZSC), the central hexagonal modulation strategy and the q-axis current injection method are combined, and the compensation current that needs to be injected into the q-axis is calculated according to the estimated value of ZSC. Finally, after considering the one-step delay, the voltage reference at the next instant is calculated based on the estimated values of d-q axis current and compensation current. Simulation and experimental results indicate that the proposed method can improve the parameter robustness of OW-PMSM, and the performance of current and torque has been improved under various parameter mismatch conditions.

Three-phase open-end winding permanent magnet synchronous motor (OEW-PMSM) with common dc bus, which has advantages of good controllability, high driving power and strong fault-tolerant ability, is becoming more and more popular. Direct torque control (DTC) has simple structure and rapid dynamic response, which is a competitive control strategy. However, DTC of this drive system is difficult to implement for that the zero-sequence current suppression should be also taken into consideration besides the control of electromagnetic torque and stator flux amplitude. To address this problem, a novel DTC strategy based on zero-sequence current suppression is proposed in this paper. Different from the conventional DTC, one more hysteresis controller is introduced to suppress the zero-sequence current. In addition, the optimal switching table has been modified based on the synthesized virtual voltage vectors. Simulation research on this strategy has been done, and the results show that the electromagnetic torque, stator flux amplitude and zero-sequence current can be controlled at the same time.

The zero-sequence current (ZSC) in the zero-sequence loop (ZSL), which is inherent in open-end winding permanent magnet synchronous machine (OW-PMSM) drives with a common dc bus, increases the torque ripple and the system loss. The simultaneous control of dual inverters and the ZSL complicates the pulse width modulation (PWM) method. This article proposes a double-symmetrical-voltage placement PWM (DSVP-PWM) method based on phase-voltage-independent control. This method significantly reduces the calculation time of the modulation and achieves the equivalent effect of the traditional space vector pulse width modulation (SVPWM). Furthermore, the effect of different voltage placements on the current ripple is discussed. Compared to the traditional SVPWM method, the proposed DSVP-PWM method leverages the characteristic of phase-voltage-independent control in OW machines and demonstrates enhanced universality for various OW machines. Finally, the effectiveness of the proposed method is verified experimentally.

Zero sequence voltage suppression control for a modular multilevel matrix converter is investigated, which is required for applications to power systems and high power motor drives. The schemes of power flow control and capacitor voltage balancing control with voltage space vector modulation have been proposed. However, these control schemes have the problem they cause zero sequence current that affects protection for power systems and motors. To reduce the zero sequence current, the control scheme that suppresses zero sequence voltage by compensating voltage difference between input and output systems is proposed in this paper. Numerical simulation of a 9-level modular matrix converter and experiments using a 2-kW 5-level modular matrix converter are conducted to verify the control scheme.