P-f and Q-V control loops

A hydro generating unit has two main control loops. The P-f loop (automatic load frequency control, ALFC) controls real power and frequency through the turbine gate. The Q-V loop (automatic voltage regulator, AVR) controls reactive power and terminal voltage through the field excitation.

 P-f loop (ALFC)

 f_ref -->(+)--> speed governor --> hydraulic
           ^ -                      amplifier
           |                            |
           |                            v
   frequency sensor               wicket gates
           ^                            |
           |                            v
           +----- speed ------ turbine-generator

 Q-V loop (AVR)

 V_ref -->(+)--> amplifier --> exciter
           ^ -                    |
           |                      v
   PT + rectifier          generator field
           ^                      |
           |                      v
           +------ Vt ----- generator terminals

P-f loop

1. A speed (frequency) sensor measures the shaft speed.
2. The speed governor compares it with the reference and, through the speed droop, gives a command to the hydraulic amplifier (pilot valve and servomotor).
3. The servomotor moves the wicket gates (or needle), changing the water flow and the turbine power $P_m$.
4. Real power balance decides frequency: if $P_m < P_e$ frequency falls. The loop restores balance. Primary control uses droop; secondary control changes the load reference to restore 50 Hz.
5. It is slow (seconds) because of large mechanical and water time constants.

Q-V loop

1. A potential transformer measures terminal voltage; it is rectified and filtered.
2. The comparator finds the error $V_{ref} - V_t$.
3. The amplifier boosts it and drives the exciter, which changes the field current of the generator.
4. Field current changes the generated emf and hence the reactive power output and terminal voltage.
5. It is fast (fraction of a second) because electrical time constants are small. Stabilising feedback is used to avoid oscillations.

Interaction

Real power depends mainly on rotor angle and frequency, while reactive power depends mainly on voltage magnitude. Also, the AVR loop is much faster than the P-f loop. So the two loops are weakly coupled and are analysed and designed separately (decoupled control).