Thomas Leibfried

Multi-terminal HVDC control strategies applied to the Cigré B4 DC Grid Test System

Further developments in voltage source converter technology have triggered an increase in HVDC projects across Europe. To master future challenges within the energy sector, a continental DC overlay grid is discussed and seems to be feasible from a technical perspective. Therefore existing offshore wind and inter market DC links are proposed to be connected to each other in a first step. Supplemented by additional branches the resulting meshed DC system offers a higher degree of flexibility and reliability if adequate converter and grid control schemes are utilized. Especially in case of a converter outage or line fault, proposed droop and DC grid control methods with distributed back ups offer significant advantages compared to centralized techniques since power sharing between different converters distributes the burden caused by a sudden power variation. This paper presents a framework to evaluate DC grid and converter control methods. First, following the introduction of the underlying VSC model, basic converter and grid control schemes are introduced. Furthermore, advanced approaches such as dead-or undead-band droop and distributed grid voltage control to deal with grid contingencies are described and evaluated. Finally, results based on a MATLAB Simulink environment show adapted variations of the presented strategies which are applied to the Cigré B4 DC Grid Test System to examine suitability, performance and drawbacks of selected methods.

Considerations on energy absorption of HVDC circuit breakers

This article deals with possible solutions for energy absorption in HVDC transmission systems. Depending on the converter topology high current slopes can appear in case of DC short circuits. Besides the handling of high currents itself, HVDC Circuits Breakers have to decrease the energy stored in line inductances in order to interrupt. Although different breaker concepts have been proposed, energy absorption is yet generally based on high voltage surge arresters. Isolation and over-voltage protection level have to be well adapted to expected surges and metal oxide surge arresters fit this problem. However, validating the maximum switching action count of those breakers, lifetime considerations of ZnO varistors have to be studied critically. In this paper a comparison on dimensioning needs and voltage stresses regarding an HVDC environment shall be carried out for existing solutions and furthermore a new proposal using resistors in addition to freewheeling diodes shall be made. This is verified by simulations with MATLAB Simulink.

Short circuit detection in HVDC grids

Diverse grid structures in HVDC systems are generally possible. Though HVDC fault detection methods from overcurrent to derivative detection have recently been proposed, a broad discussion about reasonable fault localization is mostly absent. A MATLAB Simulink model with four different transmission line scenarios was developed for application-oriented considerations of short circuit fault detection and resulting tripping orders. Simulations presenting peak derivative value curves over distance and the influence of branching points support discussion.

Active DC fault management of a bipolar full-bridge MMC-HVDC scheme with metallic return

Besides offshore applications, MMC-HVDC is discussed for embedded onshore structures. In contrast to their maritime counterparts, these systems shall be carried out with overhead transmission lines to minimize investment costs. To avoid a full outage in case of a dc fault, bipolar topologies seem advantageous considering reliability aspects. While ground currents during asymmetric operation are unwanted, an additional return path needs to be introduced. This paper presents an active clearing sequence of a bipolar full-bridge MMC-HVDC system with metallic return. Subsequent to a pole to ground fault, detection, short term asymmetric operation and full system restoration are investigated. If blocking of the IGBT modules can be avoided to maintain the converters in a controllable mode, the midpoint voltage shift at the ungrounded terminal has to be explicitly considered in system controls. Transient simulations validate the developed methodology.

HVDC grid protection with integrated fault categorization for selective tripping

HVDC Circuit Breakers require adequate fault detection for selective breaking. In terms of tripping commands, general fault categorization is necessary and, where appropriate, even short circuit localization. Wave propagation phenomena from switching operations, blocking MMCs, lightning strikes and short circuits must be clearly distinguished, which is fundamentally examined here. Analyses have been undertaken by accompanied simulations in MATLAB Simulink.

Planning and design of a European HVDC grid divided into feasible protection zones

While recent advances in voltage source converter technology boost the number of installed offshore DC links and interconnectors, the possibility to build HVDC grids only arises, if existing technical barriers can be removed. In this work the planning of an HVDC overlay grid is introduced. Using a European transmission network model, a future scenario of 2020 is considered to identify HVDC terminals and connect them in an optimal way. Based on the determined HVDC network, strategies are applied to split the network into protection zones subsequent to contingencies. Transient simulations proof that DC faults can be handled and a global outage is avoided.