In some high current, full static excitation systems for which high availability is required, it is necessary to have a redundant power section or some sort of backup to allow the unit to stay online in case of a failure.
There are different approaches such as the Master/Backup technology or sharing the load between bridges.
The master/backup is probably the safest (as long as it has the warm standby configuration) but as it needs to basically double the whole power section it can quickly build up cost and size for the high range of excitation currents, generally above 1000A. For that reason there is an approach called N+1 reliant on sharing the excitation system’s load. The idea of the N+1 is to divide the load in small parts and accordingly multiply the power sections (smaller rectifiers are used) and allow for one additional power stage on top of the full capacity. Depending on the quality of sharing and the number of power sections used, the reserve part can be as little as 10-20% in opposition to the master/backup of 100% reserve.
There are different way to share the load with various results.
In most systems of the past, rectifier bridges used balancing reactors to force current sharing between bridge rectifiers, but today, electronic current sharing is more commonly used.
Although one would be inclined to believe that rectifier bridge DC current sharing would be the best solution, it does not take into consideration the heat affecting the life of the power SCR rectifier bridges.
Take the example of stacked bridges into a single cabinet section. Although they could be designed to share current perfectly, the buildup of heat on these multiple bridge systems would be greater on the upper bridge. The higher SCR temperature heating effect experienced on the uppermost bridge could lead to a shorter life or require derating of the entire system to the current capability of the uppermost bridge.
An alternative solution is to increase the output of each bridge, through a temperature sharing method in lieu of the traditional current sharing approach. In balancing the bridges by temperature it is possible to achieve a higher current rating for the static exciter or higher temperature margins on the SCRs, resulting in longer operating life.