Neutral-point (NP) voltage imbalance is an inherent problem of three-level neutral-point clamped (NPC) converter. To solve this problem, an improved virtual space vector pulse width modulation (VSVPWM) is proposed in this paper. A new repartitioning of the space vector sectors is proposed for three-level NPC converter by considering the amplitude variations of small vectors under NP voltage imbalance. The factor k of the NP voltage imbalance is introduced. The small vector favorable to the NP voltage balance is selected based on the factor k and phase current direction to achieve precise control and balancing of NP voltage. The proposed method can effectively maintain NP voltage balance and eliminate the low frequency oscillation of the NP voltage over the whole modulation index range. The improved VSVPWM also has advantages of good dynamic performance. The method has been tested and verified by simulations and experiments.

In this paper, a fast and robust diagnostic method for multiple open-circuit (OC) faults is presented for voltage-source inverters. Only two line voltages are used as diagnosis variables, which is economic, simple, and can reduce the influence caused by the failure rate of information sources. Possible magnitudes of these line voltage vectors are characterized in detail both for the healthy and faulty operations. Then, several voltage features are extracted for diagnosis. Single-switch open-circuit (SSOC) faults and double-switch open-circuit (DSOC) faults can be accurately and quickly located through the proposed method. The fastest diagnostic process can be finished in 1/20 of the fundamental period. The diagnostic results of the proposed method are not affected by certain load OC faults, which may lead to false diagnosis of the existing methods. Moreover, the proposed method is robust to different control strategies, filter components, and carrier frequencies. Experiments are carried out on a dSPACE platform, and the effectiveness of the proposed method is verified.

With the increase of photovoltaic penetration, the power-electronic converter (PEC)-interfaced generators with traditional control strategy may cause severe power oscillations after being disturbed due to lack of inertia and damping. The photovoltaic power plant controlled by virtual synchronous generator (VSG) technology, while introducing virtual inertia and virtual damping to improve the stability of the system, will also inherit its oscillation characteristics and cause a new oscillation mode. Aiming at this problem, this paper analyzes the difference between the system low-frequency oscillation (LFO) modes before and after the VSG is connected to the two-area system by the modal analysis. It is pointed out that the (PEC)-interfaced renewable energy generators based on VSG control technology could change the oscillation mode of the original system. The influence of VSG's main parameters and capacity on system dynamics is analyzed. Finally, a synchrophasor-based inter-area oscillation suppressed comprehensive control strategy is proposed to suppress the LFO of active power on the tie line, and the superiority of the strategy is verified by DIgSILENT /PowerFactory.

Open-circuit (OC) fault diagnosis of voltage source inverters on trains still has two problems to be improved. First, it is usually forbidden to change the system structures of trains or to add any extra sensor. Therefore, diagnostic methods which need pulse width modulation control signals or extra output voltage sensors are no longer suitable for trains. Second, loads on the train vary in a wide range and have significant influence on output currents. In order to solve these problems and increase the maintenance efficiency, an online diagnostic method for single switch OC (SSOC) faults and double switches OC (DSOC) faults is proposed. Supposing that three phases are balanced, it is proved that the trajectory of two output currents is an ellipse in the Cartesian coordination, and each OC fault has a unique trajectory. SSOC faults and DSOC faults can be isolated by detecting abnormal slopes and directions of several consecutive points on the trajectory. The fastest diagnostic process takes less than half a period. Load variation has impact on the size and shape of the trajectory but does not affect the diagnostic method. Experimental tests are carried out on dSPACE platform. Results show the accuracy of the mathematical model and the effectiveness of the online diagnostic method proposed in this paper.

The safety and reliability of inverters play a significant role in stable power electronic system operations. Considering a railway vehicle air-conditioning inverter as a case study, this paper presents a generic open-circuit fault diagnosis method that can be applied to various inverter and control schemes. First, a detailed model of three-phase output currents under both normal and fault status is derived, and then the concept of space trajectory is introduced based on the three-phase output currents. The open-circuit faults of the inverter lead to output phase current distortion. Therefore, different types of faults have different space trajectories, and the corresponding shape of the space trajectory is considered to be a fault feature. Then, the fault is located by the direction vector and center-of-mass coordinate. The effectiveness and reliability of the diagnosis method are verified and analyzed using a hardware-in-the-loop test platform.

DC-link capacitor is one of the significant parts of traction converters. Due to the impact of electrical stress, high temperature and humidity, its capacitance degrades faster than expected. Accurate capacitance estimation is critical to the condition monitoring of DC-link capacitors, which is also the basis of online capacitor life prediction. Currently, there is a big challenge for capacitor capacitance estimation in railway applications. The noise fluctuation of the voltage sensor may be nearly equal to that of ripple voltage, leading to the considerable errors of the existing capacitor condition monitoring methods. Therefore, a capacitance estimation method based on pre-charging model and noise evaluation is proposed in this paper. The capacitance is estimated by improved recursive extended least square (RELS). The influence of the noise can be effectively reduced and both the calculation accuracy and convergence speed can be well guaranteed through an independent noise estimation model. The proposed method has good immunity in a certain range of signal-noise ratio (SNR) or signal bias. It shows good performance in experiments.