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.

Grid-forming converters (GFCs) offering sinusoidal voltage supply have been widely employed in a lot of applications. In this article, the dual-loop voltage–current control for GFCs is comprehensively approached. First, optimum tuning of the current loop is addressed to achieve the maximum enhanced active damping from the prospect of the developed equivalent plant, instead of the current loop, which has been the focus of the existing literature. Then, effectiveness of resonant controllers (RCs), i.e., proportional resonant and vector proportional integral, which are very popular for voltage regulation of GFCs, is addressed. It is found that for GFCs with a reasonably designed output filter, especially for high-power applications with a low switching frequency, a weekly damped region exists in the closed-loop frequency response with either of the two RCs employed, which can further result in constrained bandwidth and slow transient response. To solve this issue, a discrete RC is developed, which avoids the weekly damped region, and a fast response of the output voltage can be achieved. Experimental results have verified the effectiveness of the proposed controller for voltage regulation of GFCs.

Effective sinusoidal voltage regulation is of permanent importance for grid-forming converters. Usually, the following two types of schemes are employed to achieve a high level of performance: 1) single-loop voltage and 2) dual-loop voltage-current control. The performance of both schemes has comprehensively been evaluated in this work, based on the developed discrete-time model of the LC-filtered grid-forming converter. The challenges of insufficient stability margin, constraint bandwidth, and high sensitivity to parameter variation faced by the single-loop control scheme have been addressed, if the high-performance resonant controllers are employed for voltage regulation. Alternatively, the dual-loop control does not experience such issues with the inclusion of inner-current loop which provides active damping for the overall system. The essence of the inner-current loop is identified based on the discrete root locus analysis. Also, to obtain the highest damping and most enhanced stability, the criterion for current loop design has been addressed and a method for optimal tuning of the inner-current loop is developed, where the original plant with the one-sampling delay and the current gain are considered as the equivalent plant for the voltage controller. Experimental results have verified the effectiveness of the developed method for regulation of grid-forming converters.

The new-type synchronous condenser uses the uncontrollable and irreversible freewheeling grid-connection mode during the start-up process, so that considerable rates of grid-connection failure could be observed in case of strict grid-connection criterions. Therefore, this paper proposes an optimized start-up process for the new-type synchronous condenser, with the effort to improving its grid-connection reliability and safety. This optimized process involves two new approaches, namely freewheeling speed ratio regulation and starting freewheel point setting, to adjust the covered range of the freewheeling speed over time, which indirectly expands the allowable initial phase angle difference and thus achieves an increased grid-connection success rate under strict grid-connection conditions. The effectiveness of the proposed optimized process incorporating the two new approaches is validated using statistical analysis in Matlab, as compared to the conventional freewheeling process without optimization. The results show that the successful grid-connection operations of the synchronous condensers can be guaranteed in strictest cases at the cost of a slight increase of the starting freewheeling point (from 105% to around 107%) with 99% rated freewheeling speed ratio.

Voltages sags have become the major issue that prevents the customers from getting high-quality power supply, and the dynamic voltage restorer (DVR) is considered an effective way to solve this issue. In this work, a method for voltage sag detection implemented in the time-domain is firstly addressed, which features highly accurate and fast response. Then, the dual-loop voltage–current control for the DVR is intensively investigated. Specifically, the optimal tuning of the inner current loop to achieve the maximum active damping is approached, and the voltage controller implemented in the discrete-time is developed. Tuning of the voltage loop based on critical damping is also approached, which features reduced settling time and avoidance of overshoot. The simulation and experimental results have verified the effectiveness of the proposed method for detection and management of voltage sags.