BACK REFLECTION Introductory Remarks by José Chesnoy, Editor of Back Reflection
P
ower feeding of a submarine cable is often a discrete part of the system. The operation staff has identified from the beginning that it is a high voltage equipment needing specific training and that manipulation of itt can immediately impact the transmission. Any change has to be done carelully, usually by the after sales team of the supplier. Evelrybody has not realised that the PFE is a very specific equipment that feed the return current of the cable through the earth. It is why it deserve a specific high power earthing technique that is well separated from the station grounding earth.
Cable powering is an important part of the system treated in a “power budget” as for transmission itself. It is properly treated in technical texbooks about submarine cables [*] , but since transmission is now capped by the Shannon limit, powering is catching a new interest in our world since power feeding of the cable become nowaday the limiting factor of the achievable capacity of future submarine cables. The topic of Powering being of such a paramount importance, we shall dedicate at least a session of the Subtelforum magazine. For this purpose I am very proud to invite Michel Martin and Antoine Rey for their expertise of the field at the operational and after sales aspect, with a pronounced curiosity for the historic backaground. [*] Updated text book of submarine cable technology can be found in the reference book “Undersea fiber communication systems ed.2, Elsevier, José Chesnoy editor”.
HISTORICAL OVERVIEW OF SUBMARINE CABLE POWERING BY MICHEL MARTIN AND ANTOINE REY FOREWORD
As long as the early submarine cables were only designed for telegraphic communications, no active component was inserted (repeater, amplifier) on the cable. In fact, there were a few exceptions on some British cables, where one repeater was inserted at the edge of the continental shelf, but powering of that single element was not a real problem. Things changed with long distance systems designed for telephony. The first one of this type, the Florida – Cuba system, deployed in 1950 was equipped with three repeaters, and TAT1, (1956), required 51 repeaters. As we will see in this article, the key requirements have not changed in more
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SUBMARINE TELECOMS MAGAZINE
than 150 years, even though the transmission medium changed regularly: The in line elements ( repeaters, active equalizers, branching units nowadays) are: • fed by a constant DC current, • electrically connected in series The resulting DC voltage needed to power the line is higher than the standard 48V available from the cable station power plant, since the voltage drops of each wet plant element add. Powering a system is an integral part of the design of a submarine system [Reference 1, Reference2]. However, little information is to be found in the literature, compared to the transmission performance, cable
technology and marine installation. This is changing nowadays, and the « electrical power budgets » are nearly as important as the « optical power budgets » on the very long distance high capacity systems. But, let’s go back to the early age of the repeatered systems.
THE COAXIAL SYSTEMS POWER/SIGNAL SEPARATION
The transmission medium being a coaxial, the obvious solution was to share the conductors between the AC transmission signal and the DC power. This requires combining AC and DC in the station and separating /re-