1.4.1 Digital processing
Two types of digital processing are employed on the latest governing systems for CEGB machines. Both provide highly accurate measurement of turbine speed and have precise functions, such as steam valve linearisation characteristics, resident in the software for accuracy and ease of setting-up.
One type uses the triplex, modular redundant configuration very similar to that previously shown in Fig 2.13. Each single channel uses a combined processor and memory module which is an independent speed-computing element in its own right. The module also includes all the built-in features to permit its use in the triplex system. Parallel bus-links are used for the communication with peripherals, whilst secure serial links provide interchannel communication for channel harmonisation and fault detection purposes.
As with an analogue governor, '2 out of 3' majority voting is carried out at key points in the system. Control variables are entered by means of setpoints which can be ramped up or down by the user. High priority setpoints such as the speed and load setpoints are avail¬able to the operator by direct access to the processor. Lower priority setpoints, which alter the control char¬acteristics (e.g., droop variation), are accessed via a serial communications link to the interface processor. Control modifiers, such as vacuum unloading, are sent to the channels from the interface processor via serial links. The system timing, the synchronisation between channels and the fault monitoring process are chosen to suit the needs for adequate transient response and to allow for abnormal operation, such as operation with a faulty channel and the subsequent channel reinstatement.
As with the analogue governor, individual valve amplifiers accept the three triplex speed channel outputs in analogue form and process them to drive the servo-valve coils on each steam valve.
The second type of governor employing digital processing uses a completely different configuration. A separate single-channel, basic speed governor is pro¬vided for each steam valve. At this level, termed the base level governor (BLG), there is high integrity due to the multiple redundancy and built-in error checks which initiate closure of the related steam valve in the event of a valve channel fault.
At the upper control level (UCL), a single Unit Pro¬cessor provides all the other functions required in a governing system. All interfaces to the operator and plant, except the basic signals connected directly to the BLG, are processed by the UCL. Output commands from the UCL to alter the speed, load or operating mode take effect through the BLG, but only if they meet the acceptability criteria stored at base level. This form of digital governor is shown diagrammatically in Fig 2.17.
The BLG hardware-and software are totally standard for all applications whereas, at the UCL, the hardware is a standard minicomputer but the software incor¬porates all functions and parameters special to the application. A particular feature of the design is the ability of the UCL to provide communications to the operator via a multi-character display panel. This permits the interactive prompting of the operator for every mode of operation and enables him to see the response to his actions. Additionally, precise details are given of any governor fault alarm which might occur. The op¬erator may also select particular forms of display, for example, to facilitate turbine run-up.
The BLG channel includes all the hardware and signal routes from the speed probe signal-bus to the electrohydraulic servo-valve of its associated steam valve relay. As well as validating data inputs from the UCL, the BLG incorporates extensive self-checking facilities. In the event of a fault, the steam valve is closed by an independent means through its own failsafe watchdog hardware.