3.2   Protection scheme

 

The trip-initiating devices as described in the previous paragraph will be of two types:

  • Devices where an electrical changeover contact operates
  • Devices capable of direct tripping of the hydraulic fluid system

The result of a trip initiation will be to close the turbine steam valves and trip the generator circuit-breaker, having regard to the division into Categories A and B. The tripping system includes redundancy, so that the failure of any one element in the system does not prevent tripping, and is also designed to avoid spurious tripping as far as possible.

Hidraulic trip unit - simplified block diagram

Figure 2.41 shows schematically the main functions of the hydraulic tripping system much simplified. The interfaces with the electrical tripping system and the stop and governor valve relays are also shown. The diagram does not show the redundancy associated with the electrical tripping system, which is described later in this section. The heart of the unit comprises two sets of emergency trip valves and trip plungers, each associated with a set of overspeed-trip bolts mounted in the main turbine shaft. Fluid at high pressure is supplied to the emergency trip valves so that if either is caused to trip, i.e., move to the left, protection fluid is connected to drain via pipe A or pipe В and closure of all the turbine steam valves results. Control fluid pressure is maintained to avoid excess fluid consumption from the FRF supply unit.

The emergency trip valves are operated by spring-loaded trip plungers. When in normal operation, the spring is held compressed by a spigot held in place by the Y-shaped trip arm and latch. Initiation of the overspeed trip, the manual trip or the solenoid trip causes the spring-loaded latch to be released. High pressure protection fluid in the chamber at the left-hand end of the emergency trip valve is thus released to drain and causes them to move to the tripped condition. It should be noted that when the interlock unit is in its normal mid-position, operation of any one overspeed trip will cause both emergency trip valves to move to the tripped condition.

The trip plungers and latches must be reset by additional hydraulic units before the subsequent turbine run-up. For clarity, these have not been shown on the diagram.

To exercise the system and ensure that any faults in the tripping system are identified and rectified without delay, regular on-load testing is recommended. To test the front emergency trip valve, the interlock unit is shifted so that the left-hand ends of the trip cylinders are isolated from each other. The same operator action also blocks port A on the front emergency trip valves. While in this condition, any solenoid, manual or over-speed trip acting on the front trip valve will cause the emergency trip valve to move to the tripped position. Due to the isolation, protection fluid pressure is not lost and the unit will run undisturbed throughout the test. A genuine trip occurring during the test period will be initiated by the rear trip valve and will result in the unit tripping in the normal way.

The detailed implementation of the hydraulic tripping system by each turbine manufacturer will vary. GEC Turbine Generators, mounts the complete unit in the front pedestal, with a short direct connection to the overspeed trip units. In the case of NEI Parsons, the overspeed trip, latch lever and trip plunger are in the front pedestal of necessity. However, all the other units including the emergency trip valves, test interlock, isolating and reset facilities are implemented by means of proprietary components mounted on a large manifold block outside the turbine casing.

 

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