3 Turbine protective devices
3.1 Possible hazards
Abnormal operating conditions, including those where the turbine and its systems are not working as designed, will cause damage to the plant and possibly to personnel if allowed to persist. The dangers likely to be encountered are:
- Lubricating oil failure
- High turbine exhaust pressure (low condenser vacuum)
- Governor failure
- Water ingress to blading
- Thrust bearing failure
- Excessive vibration
- Excessive temperature differentials
- Excessive eccentricity
If the plant is manned by responsible operating staff on a continuous basis, the last four items can be monitored by supervisory equipment having preset alarm levels. This allows the operator freedom to manoeuvre the operating conditions to reduce the dangerous condition. If unable to do this, the operator must trip the plant manually.
The other dangers have more immediate effects on the plant and are generally detected by special trip gear which releases the protection system hydraulic pressure, and thus closes all the steam valves and disconnects the generator.
The above list only includes those dangers which are special to the turbine. Other hazards specific to the boiler, generator, transformers and high voltage connections may also initiate a turbine-generator trip. The system described here can be tripped by signals from these devices, which are described in the relevant volume of this series. The arrangements to trip the field switches and the high and low voltage circuit-breakers are described in detail in Volume D.
Overspeed would have serious consequences for both plant and personnel, therefore the protective arrangements have been designed to eliminate any possibility of a dangerous overspeed.
The turbine governing system provides the first line of defence against overspeed and the second line of defence is provided by separate overspeed trips. The turbine cannot overspeed when the generator is connected and in synchronism with the electrical power system network. The turbine is at risk during run-up and also when a disconnection occurs on-load. Since disconnection can occur as a result of another trip function, e.g., loss of lubricating oil, it is desirable to reduce the potential for overspeed by co-ordinating the tripping of the circuit-breaker and the closure of the steam valves. An overspeed will only occur if the steam passing through the turbine exceeds that required to match the load. Thus, if possible, tripping of the turbine-generator should first close the turbine steam valves. When the steam flow has reduced nearly to the no-load value, the generator may be disconnected from the power system network without risk of overspeed. This is known as a Category В trip and is particularly important in the case of water-ingress trips or a manual trip following such an incident. If the event caused distortion of the steam valves and they failed to close, simultaneous disconnection of the generator would promptly lead to destructive overspeed. However, if the generator maintains its connection, the unit can be safety shut down by closing the boiler stop valves. All tripping conditions where this sequence of events is desirable are made Category В trips. These include governor failure and lubricating oil failure as well as overspeed, water-ingress trips and the operator's emergency stop button.
Other tripping functions, such as turbine high exhaust-pressure trips and certain electrical trips, require immediate generator disconnection. As these trips are unlikely to have a cause common with any condition which is likely to lead to jamming of the turbine steam valves, this can be done without any significant degradation to the overspeed integrity of the unit. These are referred to as Category A trips.
The high exhaust-pressure trip is, like the overspeed trip, the last line of defence against an abnormal operating condition. The danger is that if the exhaust pressure is excessive, the last-stage LP turbine blading will become overheated and damaged. Since the prime cause of poor exhaust-pressure is likely to be loss of CW flow, it is usual practice to provide high exhaust pressure unloading gear as part of the governing system in an effort to reduce turbine load and avoid tripping. LP exhaust temperature sprays are often provided as another stage of protection, coming into operation when a high temperature is sensed or when the turbine load falls below a predetermined value. Note that there is a greater potential for heating the blading at very low loads as there is no steady flow through the turbine and recir-culation can occur. The final line of defence is provided by bursting diaphragms fitted to the LP turbine casing. These are normally set to operate at slightly above atmospheric pressure.
Although a continuous supply of lubricating oil is practically ensured by separate AC and standby DC pumps, there is always the possibility of a fractured pipe. The turbine is then tripped on low lubricating-oil pressure.
In the event of a complete failure of the turbine governing system, i.e., failure of more than one channel or a failure which renders the governor operative, the governor is designed to send trip-initiating signals to the protection scheme.
Water ingress to the turbine can occur due to malfunction of boiler or feed controls, especially during a change in load. In this case, wet steam or even slugs of water could be introduced through the HP steam line. The degree of hazard and the type of protection required depends on the boiler type and steam conditions at the inlet to the turbine. When a drum boiler is used to supply superheated steam, turbine protection may not be necessary, providing loss of boiler firing is adequately detected and alarmed so that the operator can take the necessary corrective action. In the case of a once-through boiler supplying superheated steam, it is necessary to trip the turbine on low steam temperature before saturated steam conditions reach the turbine. The danger is that, if not tripped, severe distortion could occur, causing seizure of the turbine steam valves or fracture of the turbine blading. Once-through boilers supplying dry saturated steam represent less of a hazard due to chilling. However, the sudden ingress of wet steam could result in severe thrust loadings on the turbine and blading, so the plant must be designed to accept this or be protected against it. Water ingress may also occur from feedheating plant via back-flow to the tapping points on the main turbine. The arrangements to protect against this are discussed in Chapter 3 on feedheating plant.