Fault Handling for Medium-voltage (MV) Grids

This thesis provides an overview of the fault detection and protection methods for medium- voltage grids. First, it discusses and review the evolving direct-current medium-voltage (MVDC) grids and their application for various on-shore and off-shore cases. It then explores provided techniques and solutions in the literature to study challenges related to the short-circuit faults that a grid might be prone to them. Advantages and disadvantages of each technique are investigated. This is done with the ultimate goal to propose a new and fast fault detection, classification and location control method to be implemented for any given medium-voltage grid for prompt fault analysis. The so called proposed Grid Transient Classifier-Active Impedance Estimation (GTC-AGIE) provides a two-step fault detection, classification and location method based on the artificial neural network (ANN), wavelet transform (WT) and active high-frequency signal injection. The GTC part decomposes voltage and current signals using WT to extract feature vectors. Then, by the aid of two separate ANN, fault type and an estimation of its location (zone and side where fault has occurred) can be identified. The AGIE plays a complementary role to calculate fault resistance and its distance in a particular zone and side, which are identified by GTC. The AGIE performs its function by injecting a small duration high-frequency signal into the grid and then calculates corresponding impedance to retrieve fault distance and resistance. Shipboard MVDC system is considered as the case study to investigate applicability of the proposed method. Shipboard grid includes several power and voltage stages with various interconnections and load zones in a compact structure with small distances. Compared to alternative current (AC) system, fault current rises quickly in DC ones and a very fast fault analysis is required. Hence, shipboard MVDC is considered as a good case study to examine the effectiveness of the proposed GTC-AGIE. In the following, some solid-state fault current limiter (FCL) topologies for grid protection are reviewed and their advantages and disadvantages are assessed to identify potential areas for improvements. Finally, a novel intelligent multi-functional fault current limiter (IMFCL) topology is proposed to provide protection over short circuit faults and also address any voltage sag/swell by operating as dynamic voltage restorer (DVR) in a hybrid medium voltage alternative current (MVAC) and MVDC grid. A simple fault disturbance detector is proposed to quickly identify voltage sag/swell or fault current conditions. Furthermore, in case of any fault occurrence, control system in IMFCL injects a short duration high-frequency signal into the grid to quickly calculate system impedance in new condition. By knowing the impedance, it is possible to calculate fault resistance and estimate fault location. Both simulation and hardware-in-the-loop (HIL) results are presented throughout the thesis to evaluate performance of the proposed GTC-AGIE and IMFCL.