3.3 Electrically-signalled trips - part 1
The electrical tripping scheme is also shown in outline on Fig 2.41. Any electrical trip function is signalled to the front and rear trip solenoids via duplicated relay contact systems. These relays are segregated into Category A and Category В trips. Separate contact sets on the same relays trip the circuit-breakers directly in the case of the Category A trips and via low forward-power relays in the case of Category B.
To enable the mechanically-signalled trips, e.g., the overspeed trip, to signal opening of the circuit-breakers, loss of protection fluid pressure is sensed by sets of pressure switches that provide further inputs to initiate Category В trips via the low forward-power relays. The pressure switches may also be used to trip the boiler and auxiliary devices, e.g., bled-steam non-return valves, dependent on the application.
The low forward-power relays use power sensing via voltage and current transformers to determine when the power generated is less than about 1%. This ensures that the steam valves are very nearly closed and that an overspeed cannot result when the circuit-breakers are opened, even if no further steam valve closure occurs.
The above general principles may be implemented in different ways with either '1 out of 2' or '2 out of 3' redundancy. Although the hydraulic tripping system previously described is a '1 out of 2' system, it will be seen that compatibility with a three-channel electrical system is still possible. Each of the two systems is now described in detail.
The redundancy of the '1 out of 2' system is more complex than a simple interpretation of this title suggests. Additional transducers are incorporated in each of the two channels so that at least two transducers per channel, or four transducers in total, are employed. Series-connection of the two trip-initiating contacts in each channel permits any single transducer to fail without causing a turbine trip. However, no single failure of an electrical component will prevent a genuine trip from occurring. Identification of faulty components is picked up either by supervision circuits or by routine on-load testing of the trip-initiating transducers of the 'front' and 'rear' systems in turn. Figure 2.42 shows a simplified trip circuit for one Category A trip, one Category В trip and the operator's emergency pushbuttons. The 'front' and 'rear' circuits are completely independent being powered by two different DC battery supplies. It is necessary to energise the turbine trip solenoid SOL to initiate a turbine trip via the emergency trip valve (circuit 2). As a back-up, circuit 3 shows a duplicate set of contacts operating the auxiliary relay OP. Individual sets of contacts in this relay then energise the solenoid dump valves of each steam valve relay, these being of Type 2.
Circuit 1 shows a typical Category A tripping function, high LP turbine exhaust pressure, sensed by pressure switches PS6 and 7. These are shown in the normal running position; when there is a trip condition, PS6 closes and energises flag relay AXR1.1. Providing PS7 also closes, TR3 is energised through contacts AXR1.1 and AXR1.2. If the 'front' system is being tested, the test switch will be in the Tl position and, instead of a trip initiation, lamp LP1 will illuminate when PS6 and PS7 close. Test facilities, interlocked with the test switch, enable atmospheric pressure to be supplied to pressure switches PS6 and PS7, thus comprehensively testing the functioning of all components through to the lamp.