1.4 Electronic governing - part 4
Having now outlined the basic structure of a three-channel analogue governing system, it is now appropriate to examine in more detail some of the functional blocks shown in Fig 2.12. The provision of separate narrow range and wide range speed detectors stems from the requirement (for a turbine driving a generator) for accurate control at the network frequency.
The narrow range speed detector uses either the characteristics of a tuned circuit or pulse timing techniques to determine the deviation from the reference speed corresponding to the nominal network frequency. The detector circuit provides a linear frequency to voltage conversion over a limited range only. The reason for providing a separate narrow range signal is that, by using such techniques, better long-term accuracy can be obtained. However, neither technique will function correctly when there is a substantial deviation (e.g., >10%) from the reference frequency. In the tuned circuit, this is because of a loss of linearity away from the null point to the extent that gross errors in the sense of the output will occur at sub-harmonic frequencies. The timing circuit measures the time for a number of teeth on a speed-sensing wheel to pass the detector and compares this with a fixed accurate time reference. The number of teeth passing in one revolution of the wheel and the number of teeth taken for measuring purposes are determined to provide a compromise between accuracy of measurement (the more teeth the better) and adequate transient response, particularly of the derived acceleration signal (the more measurements per second the better). The accuracy with which the teeth are cut on the wheel and the combination of speed detector and trigger circuitry for registering the passage of teeth are also considered carefully in the design.
Some of the latest governors employ a pulse timing technique for speed sensing with auto-ranging to determine the optimum number of teeth which are to be timed. This gives an adequate range of the speed signal over the whole operating regime of the turbine from approximately barring speed to the overspeed range.