Isochronous governor and parallel operation problem

An isochronous governor is a speed governor that keeps the turbine speed (frequency) exactly constant at the set value for every load. "Isochronous" means "same speed". It uses integral action: the gate keeps moving as long as there is a speed error.

Closed loop control system

 w_ref +   error  +------+ dY +-------+ dPm
 ---->(+)-------->| K/s  |--->|turbine|---+
       ^ -        +------+    +-------+   |
       |                                  v
       |                         -dPL -->(+)
       |      +-----------+               |
       +------| 1/(2Hs+D) |<--------------+
        speed +-----------+

Governor law:

$$\Delta Y(s) = -\frac{K}{s}\,\Delta\Omega(s)$$

Operation:

1. Load increases, $P_e > P_m$, so the rotor slows down.
2. The speed error $(\omega_{ref} - \omega)$ is integrated; the integrator output opens the gate.
3. $P_m$ increases; once it exceeds $P_e$ the rotor accelerates back.
4. The integrator stops changing only when the error is zero. So in steady state $\omega = \omega_{ref}$ and $\Delta P_m = \Delta P_L$: zero steady-state frequency error.

 speed
  w0 |-----.         .---------- w0 (restored)
     |      \       /
     |       '.___.'
     +-------------------------> t
     Pm rises until Pm = Pe

Why it is not suitable for units in parallel

• Units in parallel are tied to the same frequency. Each isochronous governor tries to hold the frequency at its own set value.
• The set points can never be exactly equal. If unit 1 is set at 50.01 Hz and unit 2 at 50.00 Hz, unit 1 keeps opening its gate to raise frequency while unit 2 keeps closing, so they fight each other. One unit is driven to full load and the other to zero (or motoring).
• With integral control there is no unique load sharing: the load can be divided in any ratio and drifts. The operating point is not stable.
• Therefore only one unit in a system (or an isolated unit) may run isochronous. Other units use a speed droop characteristic, which gives a definite sharing of load in proportion to $1/R$.