The fault of transmission main shaft often originates from cracks. Traditional condition monitoring for main shaft crack is often based on detecting the abnormal vibration and image caused by cracks, which cannot identify early slight shaft crack in time. In this paper, we use the inherent characteristic, resonant frequency of the main shaft, to monitor main shaft crack and propose an active condition monitoring scheme for monitoring the slight main shaft crack based on the chirp torque injection. First, we study the physical meaning of each order resonant frequency of the shaft system by a complex frequency domain-space shaft torsional vibration model, and determine the main shaft resonant frequency as monitoring indicator. Then, we obtain main shaft resonant frequency and monitor early slight cracks of the main shaft by chirp torque injection. Through theoretical derivation and experimental analysis, it is proved that our active monitoring scheme can be used to monitor early slight main shaft crack of the mechanical system in time to avoid the occurrence of severe faults.

Given the reverse distribution of energy supply and electricity demand in China, the Line-Commuted Converter High Voltage Direct Current (LCC-HVDC) delivery system has emerged as the primary solution to transmit renewable energy over long distances. The evaluation on potential damage of the wide-band oscillations to the grid stability requires the creation of a mathematical model of the LCC-HVDC delivery system to comprehensively analyze its interaction with renewable power stations. Traditionally, it has been assumed that the long DC transmission lines would separate the sending-end and receiving-end systems, leading to the simplification of the system into a single local rectifier station connected to a DC voltage source. This oversimplification, however, lacks a theoretical basis and hence, the scope of application of the equivalent model remains undefined. This study utilizes double Fourier transform to establish the impedance model of the LCC-HVDC delivery system, taking into account the receiving-end system which includes a remote inverter station and the AC grid. The internal harmonics distribution and propagation characteristics are then explored. Finally, the influence of the receiving-end AC grid on the complete system impedance is analyzed, providing insights into the dominant factors that determine the applicability of the equivalent model, which is limited to the rectifier station only.

The grid-connected photovoltaic (PV) power generation system which has multiple parallel DC modules is popularly applied in industry. In each DC-DC converter, the DC-link capacitor is the weak component which is subject to electrical property degradation. Most existing methods hardly achieve on-line capacitor diagnosis in economy and efficiency manners, considering the cost of measurement units and the effect of PV cables. In this paper, a high-efficiency diagnosis method for DC-link capacitors is proposed using two readily-available currents. The novelty lies in the linear relationship between the proposed current gain and the degradation degree at a certain characteristic frequency. The DC-link capacitor in each parallel branch can be diagnosed separately. Based on the built simulation model, the performance of the proposed DC-link capacitor diagnosis method is clarified with effectiveness and the effects of inherent harmonics and control strategies are investigated. The proposed current gain is not affected by voltage and current harmonics, as well as the varying load under the specified inverter control strategy. However, the proposed current gain is dependent on temperature and irradiance since the duty cycle of DC module is dynamically adjusted in the operation.

Leakage current is an effective indicator for groundwall (GW) insulation condition of power transformers. The existing sensing technology based on magnetic shielding design and differential measurement principle enables on-line leakage current measurement from strong magnetic noises caused by load currents. The technology has been verified in the application of the traditional power system. In this paper, an improved method is proposed for harmonic leakage current measurement, with the goal of on-line GW insulation monitoring of converter-connected transformers in microgrids. First, the simulation model of modular multilevel converter system is built in MATLAB/ Simulink. The amplitude-frequency characteristics of the load current and the leakage current of the converter-connected transformer are analyzed. Second, the sensing performance of the improved method is investigated in converter harmonic frequency band, by considering the effect of the frequency-dependent core permeability. Third, the methodology is clarified to select the appropriate monitoring frequency with the highest sensitivity. Finally, a sensor unit with the dual-core structure is manufactured and its performance is verified by off-line and on-line experiments. On-line leakage current measurement is enabled with 1-mA sensitivity and 100-kHz bandwidth from strong noises caused by load currents up to 400-A.

On-line aging monitoring is the basis for high reliability operation of converters because many converter failures are related to aging defects. Conventional aging monitoring methods mainly focus on a single semiconductor device or module, and their industrial applications are limited. First, several aging monitoring circuits are required in a converter, which raises the cost. Second, the converter structure has to be modified, which is impractical. To solve this problem, a converter-level aging monitoring method is proposed in this paper, which is based on the on-line extraction of DC-link bus leakage current. The monitored aging mode of the proposed method is Si material degradation, which is one of the chip-related aging defects. First, the DC bus leakage current is analyzed theoretically as an indicator of the Si material degradation. A corresponding description parameter dSiconis proposed. Second, an on-line measuring method is proposed for the DC bus leakage current, which is called the state-selection method. Third, a converter-level Si material degradation monitoring strategy is proposed. On-line experiments based on a constant-voltage-constant-temperature (CVCT) accelerated aging test verify the effectiveness of the proposed method.

The permanent magnet synchronous generator (PMSG) connected to pulse width modulation (PWM) rectifier is widely used in the renewable energy applications, such as direct–drive wind turbines and marine current turbines. The reliability of the PMSG is crucial due to the high maintenance cost of the turbines. The stator winding insulation is one of the main reasons for the PMSG failures. However, the current technologies can only detect the insulation failure when the short-circuit failure has already happened, which cannot avoid the corresponding downtime losses. Therefore, the online monitoring method for stator winding insulation is performed on the PMSG-PWM rectifier system in this paper for the first time. The factors affecting the voltage harmonics on the stator winding insulation are studied and their effects on the monitoring results are analyzed. Based on the flow path of the insulation leakage current and the phase-to-ground voltage, the insulation impedance can be measured online. The effects of the rotor position and the working condition are also analyzed. The effectiveness of the online condition monitoring method for PMSG-PWM rectifier system under various working conditions is verified using offline and online tests.