The 2-breaker scheme is employed to service a single motor bus from two alternate sources. The normal source feeds the motor bus through the Main breaker, while the alternate source feeds the motor bus through the tie breaker.
A typical example is that of a thermal power plant, where the unit auxiliaries, such as boiler feed pumps, forced draft and induced draft fans, cooling water pumps, etc., are supplied through unit boards. The configuration in Fig. 1 shows a single unit board, although higher capacity units typically have two or more unit boards.
The unit board can be fed from two sources. The Unit Auxiliary Transformer (UAT) (normal source) supplies locally generated power to run the auxiliaries when the unit incoming breaker (UAT I/C) is closed. The station board (alternate source) supplies power to the auxiliaries from the grid when both tie breakers (TIE-1 and TIE-2) are closed, and UAT I/C is open.
During startup, the generator transformer breaker (GTB) is open until the generator is synchronized with the grid. Until then, the station board supplies the unit board. After the generator is synchronized, the unit board is transferred to the UAT so that the unit feeds its own auxiliaries. Such a transfer is referred to as a Station-to-Unit transfer. There are several prioritized and categorized unit tripping conditions such as generator trip, load throw off, turbine trip, boiler trip etc. along with UAT / GT transformer trips on differential, winding temperature, oil temperature etc. under which it is required to automatically transfer the unit board from the UAT to the station board. These transfers are referred to as Unit-to-Station transfers. Automatic transfers on unhealthy bus conditions determined by different auto-initiation criteria are also employed in order to constantly provide a healthy supply to the motor bus. Manual transfers are commonly conducted during planned start-ups and shutdowns.
Typical breaker-failure logic safeguards the unit board from a permanent paralleling condition. TIE-2 is a normally closed (NC) breaker, used as a backup measure to safeguard the unit from a dangerous generator back-feed condition, in case both TIE-1 and UAT I/C fail to open.