Pollution flashover accidents are still happening in areas with severe environmental conditions, and the existing pollution flashover warning methods are still very simple. In this article, the space electric field characteristics of silicone rubber insulators during pollution flashover were revealed, and the application of the space electric field signal (SEFS) for pollution flashover prewarning was proposed for the first time. During the research, artificial pollution tests were conducted to obtain the SEFS waveform, and then its time-frequency characteristic parameters under different pollution flashover stages were extracted and analyzed. Research results indicate that, according to the SEFS amplitude variation, the development of pollution flashover can be divided into three zones: safe zone, dangerous zone, and critical zone. The four characteristic parameters, the rms increase factor, maximum value increase factor, peak factor, and third harmonic factor change obviously during different flashover stages. Combining the SEFS parameters and the pattern recognition algorithm, the recognition of pollution flashover zones is realized, which can be directly used to form the pollution flashover prewarning strategy.

Partial discharge (PD) measurement is necessary for insulation diagnosis and usually a required item of the maintenance procedure for most high voltage equipment assets. optical fiber sensor (OFS) provides an innovative and nondestructive approach to convert PD-induced acoustic signal into optical parameter, but the optical probe is challengeable since it is the key component to pick up the extremely weak PD-induced acoustic signal. A universal three-dimensional skeleton-type fiber probe is designed. To optimize the specific parameters of the optical probe, the multi-parameter sensitivity model of tiny skeleton-type optical fiber probe is proposed and established, involving in the material, physical height, radius, and the sensing fiber length. A skeleton-type probe with a wingding length of 60 m, the skeleton height of 12 mm, and peek material is designed for PD ultrasonic sensing. Theoretical modeling, mathematical simulation, sensitivity calibration, and comparative PD tests with regard to the sensing probe and the corresponding specific optical PD measurement system. PD results show that the average detection amplitude of the developed optical fiber sensor is 1.27 times that of commercial piezoelectric transducer (PZT). The flexible installation and the enhanced sensitivity make the Sagnac ultrasonic sensing system an excellent approach to detect and locate weak PD signals inside the power apparatus.

Through spatial electric field monitoring, it is expected to realize insulator pollution condition monitoring and contamination flashover warning in a non-contact way. Therefore, in this paper, the spatial electric field distribution characteristics of 110 kV composite insulators are simulated, where the effects of different surface states and their discharge levels on the spatial electric field of insulators are analyzed. On this basis, a non-contact monitoring method for composite insulator pollution based on the spatial electric field is proposed. The results show that there are significant differences in the spatial electric field of the composite insulator among three conditions, namely cleaning, pollution layer wetting, and dry band arcing. Increases of pollution layer wetting and dry band arcing would lead to an increase of the amplitude of the spatial electric field of the insulator. Verification experiments well indicated that it is feasible to identify the degree of pollution layer wetting as well as dry band arcing of the insulator string by fixed-point monitoring, the spatial electric field signal at the cross-strand of d = 0.5 m and directly opposite the last three positions. Research results can provide references for the online monitoring of overhead line polluted insulators and its flashover warning.

Distributed energy sources are usually interfaced to the grid using power electronic converters, and lack of inertia in inverter dominated microgrids can affect the system stability. This paper presents a new method for virtual synchronous generator (VSG) control in order to solve the low system inertia and support the grid frequency problem. In this paper, the VSG based on electromagnetic transient characteristics is improved and an adaptive command filter back-stepping controller is designed. Firstly, the rotor swing equation and power part are modeled to complete the controller design for achieving system stability in the islanded, grid-connected and transition modes. In addition, a limited-amplitude command filter is used to deal with computational complexity and nonlinear saturation problems in the design process. Secondly, projection operator, and adaptive inertia and damping control are introduced to reduce the modeling error and disturbance caused by changing parameters. This ensures the boundedness of the estimated value and further improves the frequency response, especially in the transition mode. Finally, simulation results show that the proposed controller is more effective than the traditional control method for achieving power stability and frequency improvement.