Current Based Protection Scheme Research Articles

Active Cable Sheath Current-Based Protection Scheme for an Offshore HVAC Wind Transmission System

Due to the complex sending terminal structure of offshore wind transmission systems, conventional on-line monitoring-based protection cannot work well. The electromagnetic transient faults are also difficult to locate due to a short time span with fast characteristics that are affected by various power electronic devices and control algorithms. To solve the aforementioned problems, a new offshore wind transmission line protection scheme based on active detection of submarine cable sheath current is proposed. This method uses a normalized coding cooperation to realize the risk levels and failure locations of the transient defects, and then the sheath currents become a characteristic input data source, which can build a clear system protection boundary. Case studies using MATLAB and ATP simulations are carried out, where three types of transient faults represented by sheath currents are studied, i.e., loss of electrical continuity of grounding device, short circuit of segmented metal sheath of cable joint, and water immersion of junction box. Testing results illustrated that the proposed method could achieve fast fault detection and precise fault location of the electromagnetic transients. Moreover, compared with conventional wind farm protection techniques, it only needs to add few signal injection modules with high sampling frequency into the submarine optical fibers.

A novel complex current ratio-based technique for transmission line protection

With respect to sensitivity, selectivity and speed of operation, the current differential scheme is a better way to protect transmission lines than overcurrent and distance-based schemes. However, the protection scheme can be severely influenced by the Line Charging Capacitive Current (LCCC) with increased voltage level and Current Transformer (CT) saturation under external close-in faults. This paper presents a new UHV/EHV current-based protection scheme using the ratio of phasor summation of the two-end currents to the local end current, instead of summation of the two-end currents, to discriminate the internal faults. The accuracy and effectiveness of the proposed protection technique are tested on the 110 kV Western System Coordinating Council (WSCC) 9-bus system using PSCAD/MATLAB. The simulation results confirm the reliable operation of the proposed scheme during internal/external faults and its independence from fault location, fault resistance, type of fault, and variations in source impedance. Finally, the effectiveness of the proposed scheme is also verified with faults during power swing and in series compensated lines.

DC Fault Detection in MTDC Systems Based on Transient High Frequency of Current

Isolation of the fault dc lines during the initial short period after fault is a major challenge in the multi-terminal dc (MTDC) system based on a voltage-source converter (VSC). Existing protection schemes are mainly designed based on the numerical simulations, which lacks the theoretical analysis. In this paper, a high-frequency equivalent model of a VSC-based MTDC system is first proposed for the fault current calculation. Based on the dc fault analysis, it is validated that the high-frequency fault current components flow in the fault line and then dramatically decay in the healthy line. Accordingly, a novel dc fault detection method for the VSC-based MTDC system is proposed. In the proposed approach, the primary protection utilizes the transient high-frequency energy of line current and the fault line can be identified quickly without communication. In addition, the influence of the transient traveling-wave appearing on the long-transmission line on the proposed protection scheme is also investigated. Numerous simulation studies carried out in PSCAD/EMTDC have demonstrated that the proposed high-frequency equivalent model can be utilized for the initial dc fault analysis of the VSC-based MTDC system and the proposed protection scheme is effective in different time windows, different fault locations, and high fault resistances. Compared with the existing rate of change of current-based protection scheme, the proposed primary protection requires relatively low sampling frequency, is insensitive to the parameter changes, and has high robustness with respect to the outside noises and data missing.

Operation and performance of a medium-voltage DC link

The operation of a voltage source converter (VSC)-based medium-voltage (MV) direct current (DC) link is described. The use of a DC link in an MV distribution network is presented, considering salient similarities and differences to a high-voltage DC link. A PSCAD/EMTDC model was used to demonstrate the dynamic operation of an MVDC link, under normal and faulted network conditions. The study shows that MVDC can provide flexibility in a distribution network by decoupling real (P) and reactive (Q) power flows through the link, under normal operation. During a fault, however, the current injected from the converter could interfere with the detection and discrimination of AC system faults, in distribution networks with current-based protection scheme.