Impedance-based method is an effective method to analyze the system stability. Based on the impedance stability theory, a doubly fed induction generator (DFIG) based wind power generation system can be decoupled into a positive- and a negative-sequence subsystem. Then, under small-signal operation, each subsystem is assumed to have linear characteristic in the frequency domain. Therefore, a single-in-single-out stability criterion can be applied for each subsystem to analyze the stability of the interconnected system consisted of the DFIG system and the grid. However, when there is frequency coupling in the DFIG system, this linear assumption may lead to wrong conclusion of the system stability analysis. The frequency coupling of DFIG can be caused by the PLL and the asymmetric rotor current control. In this paper, the frequency coupling characteristic of DFIG caused by these two causes will be studied, and an analytical model that describes the frequency coupling of DFIG will be developed and validated by simulations. Based on this derived model, the system stability and the influencing factors of frequency coupling will be analyzed.

Fault detection and location method are two significant techniques to recognize and clear the fault timely. This paper proposes a novel fault detection and location method for mesh-type DC microgrid. The key of this method is to implement similarity analysis on the sampled lines current and the lines current reference by employing an improved Pearson correlation coefficient. Firstly, the fault detection can be implemented by comparing the sampled lines current with steady-state lines current in a movable time window. And the fault line and fault type can be recognized based on the analysis on the correlation coefficient. Secondly, the fault location can be obtained by comparing the sampled lines current with calculation results of transient lines current, which corresponds to the estimated fault location scheme, within a fixed time window. The estimated location scheme, including the fault position and fault resistance, is updated by genetic algorithm, and will be determined when the correlation coefficient is higher than the threshold. Moreover, both numerical simulation and experiment results validate that the proposed method can recognize the fault line and fault type timely, while obtain the fault position and fault resistance accurately.

The zero-sequence loop inherently exists in the structure of three-phase open-end winding permanent-magnet synchronous machine (OW-PMSM) system with common dc bus, which provides the circulating path for zero-sequence current (ZSC) and consequently causes the torque ripple and extra system loss. Besides, the common mode voltage (CMV), which can cause many negative effects, such as the bearing current and the failure of motor bearing, is another concern in the driving system. However, the simultaneous suppression of ZSC and the CMV control of the devices is investigated mainly in the application of induction machine system and multi-phase OW-PMSM system, while it's ignored in the application of three-phase OW-PMSM system, in which the suppression of ZSC is more complex considering the effect of flux linkage harmonic components. On this basis, this paper proposes a strategy which can not only suppress the ZSC but also eliminate the ripple of CMV in three-phase OW-PMSM system. In the proposed method, the suppression of ZSC is achieved by synchronously adjusting the duration time of each phase within every control cycle based on the reference voltage of zero-sequence control loop, which can simplify the conventional re-arrangement-based scheme.

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.

This paper presents an improved virtual synchronous generator (VSG) control strategy of doubly-fed induction generator (DFIG) during symmetrical voltage fault. Based on VSG control, DFIG can behave like a conventional synchronous generator (SG), which can support frequency and voltage in grid-connected mode. Moreover, when microgrid disconnects from grid, DFIG can provide voltage and power for local loads. However, during symmetrical voltage fault, the transient components in DFIG cannot be controlled properly by VSG. To solve this problem, the air-gap flux feedback is designed in VSG control to accelerate the decay of transient components, and rotor current limiting method is also introduced in the article. In the proposed method, there is no need to switch the control structure or control targets. DFIG always works in the VSG control. Therefore, DFIG can operate in the standalone mode when the connection to grid is cut off. The decay time constant, the voltage control, current limiting control and parameter variation of the improved VSG control are analyzed in detail. Finally, the simulation and experimental results in both normal and fault grid condition are given to validate the effectiveness of proposed VSG control strategy.

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.