Technical feasibility of complex multi-terminal HVDC systems
By Gregor Czisch
One of the "problems" often mentioned is the misinformation that HVDC circuit breakers were not available and therefore faults in complex HVDC systems could not be cleared (solved).
This is absolutely wrong.
HVDC circuit breakers have been built and tested in the mid 1980's. The highly regarded HVDC expert Karl-Werner Kanngießer who was an employee of Calor Emag Schaltanlagen later ABB Calor Emag Schaltanlagen and who also worked for Siemens wrote in [Kan99] about the HVDC circuit breakers.
"Untersuchungen  haben gezeigt, dass durch den Einsatz von HGÜ-Leistungsschaltern in parallel geführten Gleichstromleitungen die Klärung von transienten oder auch permanenten Leistungsfehlern in etwa 100 ms möglich ist, eine störende Auswirkung auf die angeschlossenen Drehstromsysteme somit praktisch vermieden wird. Nachdem der Prototyp eines 500 kV-HGÜ-Leistungsschalters gebaut und in einer bestehenden HGÜ-Anlage erfolgreich getestet worden ist , darf auch hier die Ausführbarkeit vorausgesetzt werden."
My translation in English:
"Research  has shown that trough the use of HVDC power circuit breakers in parallel operated direct current lines, clearing transient as well as permanent faults in about 100 ms is possible, a disturbing effect on the connected AC systems thus can practically be avoided. After the prototype of a 500 kV HVDC-breaker has been built and successfully tested in an existing HVDC system , the executability can be seen as being assured."
So there is no question that HVDC circuit breakers are available since about 1985. It just has not been necessary to use them in point to point systems. However, they furthermore would not be absolutely necessary even in a complex HVDC system. This can be understood reading the following explanations of Dr Uwe Radke a former member or E.ON Netz (Grid) and participant of highly relevant feasibility studies about complex HVDC systems which have been elaborated under participation of many big utilities and experts in the field of electricity transport (see e.g. [BDE+98], [BDE+00], [Rad00a], [Rad00b]): Following statement lately was written by Dr Uwe Radtke about HVDC circuit breakers:
"The HVDC circuit breakers are able to interrupt a short circuit power flow in the range of milliseconds to separate a disturbed section of HVCD overhead lines or HVDC cable links with a minimum influence of the undisturbed transmission system. This kind of circuit breakers is not installed in any HVDC point-to-point links because it is not necessary.
Therefore, the alleged unavoidability of HVDC circuit breakers for multi-terminal HVDC systems as well as the alleged unavailability of HVDC circuit breakers are both fakes. They are simply not true. There is no real problem with HVDC circuit breakers.
There is a second myth about multi-terminal HVDC systems. It is the myth about missing control methods. However, there are methods to control even complex meshed HVDC systems. One was developed by Franz Karlecik-Maier an expert for control systems when he was working for Siemens. He invented the so-called Combined and Coordinated Control Method (CCCM), which makes it possible to run big complex HVDC Systems. It is a method, which permits operation of the system without a "master controller" (central control). At the least after its invention, it could now be assumed that there is barely a technical limit in the number of converter stations in complex HVDC systems. This knowledge as well has been around for many years now. It is for example published in [BDE+00] [Rad00b]. (Some related patents of Franz Karlecik-Maier are [K-M08] [K-M97])
So, whoever wants to know about the existing - end tested methods - can find all necessary information about it.
Without CCCM - or another operation method with similar properties - the HVDC system would have to be divided into individual less complex subsystems, for example, ring systems, as investigated in [ABE +93], or in linear systems, such as described in [BHK +98] . This would increase the number of required converters. In the extreme and unnecessarily simplified case with just two converter stations at the two ends of each point to point HVDC subsystem the number of required converter stations would nearly be doubled. Even this would not make the Supergrid unfeasible. But those speculations are in fact unnecessary because the control methods for complex multi-terminal HVDC systems are available as mentioned before.
Summarizing it can be stated that there is no reason why a complex multi-terminal HVDC system should no be feasible. Multi-terminal HVDC systems are feasible with existing technology.
[ABE+93] ASAL, H. ; BRUMSHAGEN, H. ; EROULT, M. ; HASSON, B. ; KULA, M.; MOGUIREV, V. ; PIRET, J.-P. ; RIPPAR, G. ; SEIPEL, E. ; SMIRNOV, I. ; SOULIS, E.; VERGELLI, L.:
[BDE+98] BELENERGO ; DC BALTIJA ; EESTI ENERGIA ; LATVENERGO ; LIETUVOS ENERGIJA ; PPGC ; PREUSSENELEKTRA ; RAO EES ROSSIJ ; VEAG: TEN-Energy Study East-West High Power Electricity Transmission System - Baltic Route - Phase I / European Commission: Trans-European Energy Networks, Gelsenkirchen, Germany, April 1998. - Executive Summary.
[BDE+00] BELENERGO ; DC BALTIJA ; EESTI ENERGIA ; LATVENERGO ; LIETUVOS ENERGIJA ; PPGC ; PREUSSENELEKTRA; RAO EES ROSSIJ ; VEAG,: TEN-Energy Study East-West High Power Electricity Transmission System - Baltic Route - Phase II / European Commission: Trans-European Energy Networks, Gelsenkirchen, Mai 2000. - Executive Summary.
[BHK+98] BRUNTT, M. ; HANSSON, B.; KNUDSEN, L. ; NURMINEN, H. ; RADTKE, U.; STØVRING-HALLSSONAND, S.: Baltic Ring Study, Power System Analysis Report: Multiterminal HVDC Systems / Baltic Ring Electricity Co-operation Committee (BALTREL). 1998
[Kan99] KANNGIESSER, K.-W.: Nutzung regenerativer Energiequellen Afrikas zur Stromversorgung Europas durch Kombination von Wasserkraft und Solarenergie. In: BRAUCH, H. (Hrsg.) ; CZISCH, G. (Hrsg.) ; KNIES, G. (Hrsg.): Regenerativer Strom für Europa durch Fernübertragung elektrischer Energie. Moosbach, Germany: AFES-Press, September 1999 (1). - ISBN 3-926979-71-2, S. 111-122
[K-M08] Karlecik-Maier, F.:
[K-M97] Karlecik-Maier, F.:
[Rad00a] RADTKE, U.: TEN-Energy Study East-West High Power Electricity Transmission System - Baltic Route: Main Results and Conclusions of the Final Report Phase II, Executive Summary Working Group "Environmental and Routing" / European Commission: Trans-European Energy Networks. 2000.
[Rad00b] RADTKE, U.: TEN-Energy Study East-West High Power Electricity Transmission System - Baltic Route: Main Results and Conclusions of the Final Report Phase II, Working Group 1 "Technical Aspects" / European Commission: Trans-European Energy Networks. 2000.
 Kanngießer, K.W.; Wolters, I.: "Comparative performance of an HVDC transmission system with d,c, breakers for clearing d.c. line faults". CIGRE-Symp. 200-05, Boston 1987.
 Bachmann, G.; Mauthe, G.; Ruoss, E.; Lips, H.P.; Porter, J.; Vithayathil, J.: "Development of a 500 kV airblast HVDC circuit breaker". IEEE trans. power app. a, syst. PAS-104 (1985) S.2460