In EHV/HV system where more than 1 -in coming power supplies are received from Two of more Power sources (synchronized). Supposing one source is in one direction of the station where as the other is on the opposite direction of the station. If feeding from one source fails the supply to all the feeders on that particular bus shall get affected and shall stop supplying the load. In such cases the bus tie breaker plays a vital role of feeding the load through the live remaining bus and interruption of supply is averted.
If you have a substation with multiple incoming power supplies, and 2 or more sections of busbar, that is supplying customers, or important loads that require a firm supply (that is one which does not fail due to one of the incoming power supplies failing, you need to run it with the bus tie / bus section / bus coupler circuit breaker(s) closed. This means that when an incoming circuit trips out, the substation load will be maintained by the remaining incomers with no break in supply (maybe just a voltage dip). The pre-conditions are:
- The resulting busbar fault level (kA) must not exceed the breaking capacity of the local circuit breakers and also all downstream circuit breakers.
- The maximum load demand on the substation must not exceed the rating of the remaining incoming feeders / transformers (N-1, where N = total no of incomers).
- For incoming feeder / transformers, the voltage control scheme controlling the on load tapchangers must ensure that all transformers are approx. in step.
The disadvantage is that since there is a single bus, error in one switchboard can affect another. In offshore applications classification societies allow this if there is advanced supervision and control of power plant. If this system detects quick faults quickly and isolates generators or switchboards. Kongsberg Maritime offers a product for offshore and marine applications called AGS. It is the most only system in its market fully integrated to power management system requiring no extra hardware, thus reducing failure modes. It has been successfully delivered on many different types of vessels. This is an affective way to have a safer and more efficient power plant in ships.