The application of high-power voltage-source converters (VSCs) to multiterminal dc networks is attracting research interest. The development of VSC-based dc networks is constrained by the lack of operational experience, the immaturity of appropriate protective devices, and the lack of appropriate fault analysis techniques. VSCs are vulnerable to dc-cable short-circuit and ground faults due to the high discharge current from the dc-link capacitance. However, faults occurring along the interconnecting dc cables are most likely to threaten system operation. In this paper, cable faults in VSC-based dc networks are analyzed in detail with the identification and definition of the most serious stages of the fault that need to be avoided. A fault location method is proposed because this is a prerequisite for an effective design of a fault protection scheme. It is demonstrated that it is relatively easy to evaluate the distance to a short-circuit fault using voltage reference comparison. For the more difficult challenge of locating ground faults, a method of estimating both the ground resistance and the distance to the fault is proposed by analyzing the initial stage of the fault transient. Analysis of the proposed method is provided and is based on simulation results, with a range of fault resistances, distances, and operational conditions considered.

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Short-Circuit and Ground Fault Analyses and Location in VSC-Based DC Network Cables

The multiterminal dc wind farm is a promising topology with a voltage-source inverter (VSI) connection at the onshore grid. Voltage-source converters (VSCs) are robust to ac-side fault conditions. However, they are vulnerable to dc faults on the dc side of the converter. This paper analyzes dc faults, their transients, and the resulting protection issues. Overcurrent faults are analyzed in detail and provide an insight into protection system design. The radial wind farm topology with star or string connection is considered. The outcomes may be applicable for VSCs in the multi-VSC dc wind farm collection grid and VSC-based high-voltage direct current (HVDC) offshore transmission systems.

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Multiterminal DC Wind Farm Collection Grid Internal Fault Analysis and Protection Design

High-voltage dc (HVDC) transmission based on the line-commutated converter (LCC) is the most established and widespread technology around the world. However, HVDC based on the voltage source converter (VSC) has emerged as the best option to integrate renewable energy sources, e.g., offshore wind farms. This work investigates the feasibility of using a conventional LCC-HVDC transmission in combination with a VSC to integrate offshore wind farms. Such integration results in a hybrid HVDC connection, i.e., the connection of a VSC with an LCC through a dc cable. The operational features of a model of three-terminal hybrid HVDC, two LCC stations and one VSC station, is investigated using PSCAD/EMTDC. The simulations include an aggregate model to emulate the wind farm. The corresponding control strategies are proposed for each terminal and verified under various conditions including wind speed variations and ac faults.

Offshore Wind Farm Grid Integration by VSC Technology With LCC-Based HVDC Transmission

This paper discusses the issues concerning industrial power systems with large capacitor banks and the energization of large furnace power transformers. The application of the capacitor banks could tune the system to a frequency that is excited by the furnace transformer energization current. Examples of situations occurring in industrial systems are shown with field-measured data. Transient and harmonic simulations are also shown describing the phenomena. The effects on the capacitors caused by the overvoltages that are created due to the resonant conditions are discussed. The paper also addresses relevant capacitor standards and how they are related to this type of system phenomena. >

Damaging long-term overvoltages on industrial capacitor banks due to transformer energization inrush currents

As modern equipment is more sensitive to power quality phenomena, transients introduced by capacitor switching become an increasing concern. The paper addresses the identification of switched capacitor position such that on-line or post-fault measures can be taken. The position is given in terms of proportional line impedance seen from the load bus. Perturbation methods are used to find an asymptotic expression for capacitor position as a function of the transient frequency. This paper presents analytical and simulation results for a standard radial distribution system.

Identification of capacitor position in a radial system

The protection of power factor correction capacitors used to enhance electrical system efficiency is addressed. Different considerations in the selection of capacitor fuse applications and the philosophies behind them are reviewed. Individual and group fusing are discussed. Continuous current, transient current, fault current, tank rupture curve coordination, voltage on good capacitors during a fault, energy discharged into a failed unit, outrush current, and coordination with unbalance detection schemes are topics covered. Examples of actual applications of capacitors in industrial distribution systems are presented. >

Overcurrent protection of capacitors applied on industrial distribution systems

Many industrial facilities apply power correction capacitors to enhance their electrical efficiency. Power factor improvement, power loss reduction, release of system capacity, and voltage improvement can all be achieved by applying capacitors in industrial plants. Protection of these capacitor banks against excessive overcurrents is a critical part of the safe and reliable operation of the bank. This paper reviews different considerations in the selection of capacitor fuse applications and the philosophies behind them. Individual and group fusing are discussed. Continuous current, transient current, fault current, tank rupture curve coordination, voltage on good capacitors during a fault, energy discharged into a failed unit, outrush current, and coordination with unbalance detection schemes are topics covered. Examples of actual applications of capacitors in industrial distribution systems are presented throughout the paper. >

M.T. Bishop

Papers

Overcurrent protection of capacitors applied on industrial distribution systems

Capacitor overcurrent protection for industrial distribution systems