1.3.4 Closed-loop control of turbine electrical load
Closed-loop control of turbine electrical load is added to the basic governing system to give accurate load droop characteristics and facilitate the provison of variable-droop settings. The basic governor incorporates a speed droop system where the trimming signal is derived from steam valve position.
Because the steam flow/position relationship is non-linear, the basic speed governor incorporates an inverse function so that all upstream gover¬nor processing has an almost linear effect on the steam demand at one nominal set of steam input conditions. However, if the conditions vary, the significant non-linearities will give rise to imperfections. Closed-loop load control overcomes these imperfections by superimpos¬ing a trimming signal onto the governor speed reference. A block diagram of the load loop is shown in Fig 2.8 and the characteristic it produces in Fig 2.9.
The frequency range over which the load loop is operative is limited so that, even when the load loop droop is set to high or infinite values, the unit responds to any large frequency errors in a manner matched to the characteristics of the normal 4% droop of the basic speed governor. By limiting the load range over which the load loop is operative, the magnitude of sudden load changes on the boiler system is limited if a low speed droop is used. For obvious reasons (Fig 2.9), speed and load variations over which variable-droop is permitted are known as the box characteristics.
Consider a sudden fall in network frequency on a number of machines operating in parallel with different droops but similar box size. Because the load loop is slow-acting, through the governor speed reference, all machines initially contribute an increase in demanded load. Then those machines with high or infinite load loop droops will slowly reduce in load, returning to a value at or near their original load. This allows the other machines with low droop to pick-up load at a rate compatible with their boilers. The advantage of this arrangement is that some units can be designated for rapid load pick-up, whilst those unable to sustain such a transient can still contribute usefully to the control of falling frequency.