Battery DOD and station service schemes

Depth of discharge (DOD)

Depth of discharge (DOD) is the fraction of a battery's rated capacity that has been taken out, expressed in percent:

$$\text{DOD} = \frac{\text{Ah discharged}}{\text{Rated Ah capacity}} \times 100\,\%$$

It is the complement of the state of charge: $\text{SOC} = 100\,\% - \text{DOD}$. For example, taking 60 Ah from a 200 Ah battery gives a DOD of 30 %.

Importance:

• A deeper DOD shortens battery life (fewer cycles). Lead-acid batteries are usually designed for 50–80 % maximum DOD, Ni-Cd and Li-ion can go deeper.
• Battery sizing for a station uses the allowed DOD and the end-of-discharge voltage (e.g. 1.75–1.8 V/cell for lead-acid).

Station service (auxiliary supply) schemes

Station service supplies the auxiliaries: pumps, cooling, governors, cranes, lighting, battery chargers. Common schemes:

1. Unit auxiliary transformer (UAT) tapped from the generator bus, with station transformer backup

 Gen G1 --+-- GSU T1 --+-- HV bus (132 kV)
          |            |
         UAT1     Station service tr (from HV bus)
          |            |
  Unit aux board 1 ----+---- Common station board
                                  |
                          DG set (standby / black start)

2. Common station service board fed from two sources (main bus and tertiary/grid)

 11 kV gen bus --> SST-1 --+
                           |-- 415 V board --> loads
 33/11 kV feeder -> SST-2 -+   (bus coupler)
                               |
               DG set --> Emergency board
                               |
           Battery charger --> DC board

Key features:

• At least two independent sources with automatic changeover (bus coupler).
• An emergency diesel generator for black start and essential loads.
• Unit boards for each generating unit's own auxiliaries, plus a common board for shared loads.
• Essential loads moved to the emergency board; DC system with its own battery for vital loads.