Back Reflection: Historical Overview of Submarine Cable Powering
By Michel Martin and Antoine Rey with Introductory Remarks by José Chesnoy
May 18, 2020
Power 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 it can immediately impact the transmission. Any change has to be done carefully, usually by the after sales team of the supplier.
Everybody 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 textbooks 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 nowadays 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 Subtel Forum 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 background.
[*] Updated text book of submarine cable technology can be found in the reference book “Undersea fiber communication systems ed.2, Elsevier, José Chesnoy editor”.
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 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.
2. The coaxial systems
a. 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 /recombining them in each repeater, by the means of a Power Separation Filter (PSF). See Figure 1.
b. Transmission Band separation
While not directly related to the powering, we will see that the band split had an impact on the powering.
As early as 1943 (in Holyhead – Port Erin between the Wales and the Isle of Man), the British used a mono-container repeater. This repeater (this will also be the case later for the French design) could accommodate the electronics and filters to amplify two frequency bands, one for each direction.
Because of its bulkiness, laying a repeater required the cable ship to nearly stop at this time [Reference 3]. In addition, the maximum frequency to be transmitted was higher than for a uni-directional amplifier, therefore increasing the number of repeaters in the system. But the clear advantage was the ability to handle bi-directional communications with only one cable.