3.3 Electrically-signalled trips - part 2
Circuit 4 is very similar, in this case PS1 and PS2 provide the Category В low relay fluid pressure trip shown in Fig 2.41. Auxiliary relay TPR2.1 is used to provide other interlocks as well as trips. One contact, TPR2.1 is connected in parallel with other Category В trip contacts to energise trip relay TR7. In turn, contacts on this relay, which are not shown, initiate circuit-breaker tripping via the low forward-power relay.
The '1 out of 2' tripping concept is provided by the duplication in the rear channel circuit, where completely independent sets of transducers initiate tripping. Because of the configuration of the turbine hydraulic tripping system, a trip occurs as soon as any one of the turbine trip solenoids is energised.
The examples chosen to illustrate typical Category A and В trips have each used two pressure switches in each channel. For other tripping functions some other form of redundancy may be appropriate. Thus, in some applications, high LP exhaust temperature is used for tripping, the sensing being carried out in each flow of each exhaust. For a six-exhaust machine, 12 transducers would be used, six in each channel combined in three pairs to provide immunity from spurious tripping.
Figure 2.43 shows the second method of implementing the turbine-generator electrical tripping scheme using a '2 out of 3' majority voting system. Using this technique, it is virtually impossible for any single component or transducer failure to cause a spurious trip. Likewise, a single failure will not prevent a trip from occurring. Auxiliary contacts, not shown in the figure, give the operator an alarm if there is a failure and inhibits onload testing until the fault is rectified.
The circuit is very simple, with the main functional blocks being triplicated or duplicated as necessary. The transducers are connected in three similar auxiliary tripping circuits. In auxiliary tripping circuit 1, PS7 provides a typical Category A tripping function such as high LP turbine exhaust pressure. It operates auxiliary relay RL7 through a flag relay. The equivalent auxiliary relays in channels 2 and 3 are relays RL8 and RL9. On the right-hand side of the diagram there are four circuits each using a set of contacts from relays RL7, RL8 and RL9 connected in a '2 out of 3' voting fashion. The first two sets operate the front and rear turbine trip solenoids, while the second two sets operate trip relays TR3 and TR4 to give a direct trip of the circuit-breaker.
A feature of this system is that on-load testing can be carried out on each transducer in turn and circuit operation checked right through to the operation of the turbine trip solenoid. Thus, if the front solenoid is hydraulically isolated and test switch contacts TF are closed, transducer PS7 can be vented to atmosphere so that it gives contact closure, energising RL7 and the front turbine trip solenoid.
Actual operation of this can be indicated to the operator via a relay fluid pressure switch monitoring the protection fluid pressure at an appropriate point on the front emergency trip valve. During testing, the rear turbine trip solenoids provide protection in the event of a genuine trip. Similar test facilities are provided for most of the transducers, the test inputs being applied by a solenoid valve adjacent to each transducer. This enables all on-load testing to be carried out from the trip gear cubicles, with a full set of indications to the tester so that faults can readily be traced and rectified.
The Category В circuit is very similar to that described for Category A. Here the tripping contacts from all the tripping parameters are connected in parallel. Thus several similar sets in channel 1 are in parallel with PS1 and RL11 and they all operate relay RL1, which is one of the auxiliary tripping relays forming the '2 out of 3' contact sets to operate the turbine trip solenoids and the Category В tripping relays TR7 and TR8.