Reactors in stations: merits and demerits

A reactor is an inductive coil with high reactance and negligible resistance. In generating stations and substations it is used to limit short-circuit current, localise faults, and keep bus voltage up on healthy sections. Air-core (dry, concrete-supported) reactors are common because their reactance does not fall by saturation at high fault current.

1. Generator reactors

  G1      G2      G3
  |       |       |
 [X]     [X]     [X]
  |       |       |
 =+=======+=======+===  bus
     |       |
  feeders

• Merits: protects each generator from heavy fault current; useful for old machines with low reactance.
• Demerits: always carries full load current, so constant voltage drop and $I^2R$ loss; a bus or feeder fault still affects all generators; gives no protection between feeders.
• Application: rarely used now, since modern generators have enough leakage reactance.

2. Feeder reactors

  G1      G2
  |       |
 =+===+===+===+===  bus
      |       |
     [X]     [X]
      |       |
   feeder1  feeder2

• Merits: a fault on one feeder hardly affects bus voltage, so other feeders keep supplying; breaker ratings of feeders reduced.
• Demerits: no protection for bus-bar faults; constant drop and loss on every feeder; poor regulation.
• Application: distribution substations with many radial feeders.

3. Bus-bar reactors

(a) Ring system

  G1          G2          G3
  |           |           |
 =S1===[X]===S2===[X]===S3=
  |                        |
  +----------[X]-----------+

(b) Tie-bar system

  G1      G2      G3
  |       |       |
 =S1=    =S2=    =S3=   sections
  |       |       |
 [X]     [X]     [X]
  |       |       |
 =+=======+=======+=  tie-bar

• Merits: in normal operation little current flows through bus reactors, so loss and drop are small; a fault is confined to one section; tie-bar system lets sections be added easily, and fault current from other sections passes through two reactors in series.
• Demerits: a fault on a section gets full fault current from its own generator; tie-bar needs an extra bus; large voltage difference between sections if loads are unbalanced.
• Application: large generating stations with several sections and many generators.

Summary