Unit generator-transformer schemes

In a power plant, the generator is connected to the grid through a step-up transformer. Two common schemes are (1) the unit scheme, one generator to one transformer, and (2) the grouped (combined) scheme, two generators to one transformer.

Scheme 1: Unit connection (one generator – one transformer)

   G1          G2
   |           |
  GCB         GCB
   |---UAT     |---UAT
  GSU1        GSU2
   |           |
  HV CB       HV CB
   |           |
 ===================== HV bus

• Fault level: a fault on the generator bus is fed by one generator plus the grid through one GSU impedance; LV fault level is lower, so GCB and bus duct ratings are smaller.
• Maintenance outage: a GSU fault or maintenance takes out only its own unit; other units keep running. Each unit is independent.
• Efficiency: each transformer runs close to full load when its unit runs; when a unit is off, its transformer can be disconnected, so no-load losses are saved. But total no-load and load loss of several small transformers is higher than one large transformer, and cost is higher.
• Reliability: highest; widely used for large units.

Scheme 2: Grouped connection (two generators – one transformer)

   G1          G2
   |           |
  GCB1        GCB2
   |           |
 ================ LV (11 kV) bus
         |---- Station transformer
        GSU (2 x unit rating)
         |
        HV CB
         |
 ================ HV bus

• Fault level: a fault on the LV bus is fed by both generators plus the grid, so the LV fault level is much higher; GCBs and bus need higher breaking capacity.
• Maintenance outage: a GSU fault or maintenance takes out both units, so energy loss is larger. A generator can be taken out by its own GCB while the other runs.
• Efficiency: one large transformer has lower cost per MVA and lower total losses at full load; but when only one unit runs (dry season), the transformer runs at half load and its full no-load loss remains.
• Cost: fewer transformers, HV breakers and bays, so cheaper and smaller switchyard; common for small and medium hydro in Nepal.

Comparison