Steam Turbine - Protective Devices

Steam Turbine - Protective Devices

Over-speed Trip

Two Over-Speed trips MAY10 AA001 and 002 are provided to trip/shut down the turbine in the event of overspeed.

Each trip device consists of an eccentric bolt/striker fitted in the emergency governor shaft with its center of gravity displaced from the axis of rotation and held In position against centrifugal force by a spring up to an adjustable preset speed of 10 to 12 % above the normal turbine operating speed.

At the preset overspeed, centrifugal force overcomes the spring force and the eccentric bolt/striker flies outwards into its extended position.

In doing so it strikes the pawl which releases the piston of the overspeed trip release device KA01.

Through combined spring force and fluid pressure, the piston opens the auxiliary trip fluid circuit to the main trip valves MAX51 AA005 and MAX51AA006.

Thrust-Bearing Trip

Thrust bearing trips MAD12CY011/012/013 are tripped electrically in the event of excessive axial displacement of the turbine shaft.

Pressure Switch

Installed in the trip fluid circuit are two pressure switches MAX51CP011 and MAX51CP012

which bridge the longtime delayed relays of the reverse-power protection system in such a way that the generator is shut down by response of the short-time delayed relays as soon as it begins to motor.

The annunciation Turbine trip initiated is transmitted simultaneously to the control room.

Remote Solenoid Trip

Remote solenoid trip is activated via solenoid valves MAX52 AA001 and MAX52 AA002.

The remote solenoid trip may be initiated

1. manually from the control room by push button,

2. by the electrical low-vacuum trip or

3. the thrust bearing trip

4. or other protective devices.

Low-Vacuum Trip for Turbine Protection

An increase of pressure in the condenser causes the valve of low-vacuum trip MAG01 AA011 to move downwards from its upper position under the force of the pre-tensioned spring.

This action depressurizes the space below the right-hand valve.

The right-hand valve is moved into its lower position by a spring and thus opens the auxiliary trip fluid circuit.

Opening the auxiliary trip fluid circuit depressurizes the fluid below the differential pistons of main trip valves MAX51AA005 and MAX51AA006 and the differential pistons are activated by a spring.

This closes the control fluid inlet to the trip fluid circuit and at the same time opens the main trip fluid circuit to drain, causing the trip fluid pressure to drop and all stop and control valves of the turbine to close.

Limit switch MAG01CG011B signals to the control room that the low-vacuum trip is not in its normal operational position.

Limit switch MAG01 CG011C indicates in the control room that turbine trip has been initiated by the low- vacuum trip.

To make it possible to latch-in the trip devices and thus to build up trip fluid pressure for adjusting and testing the control loop or similar purposes when the turbine is shut down and no vacuum exists, the low- vacuum trip has an auxiliary piston which is loaded with primary oil pressure above the adjustable compression spring.

When the turbine is shut down there is no primary oil pressure and so the auxiliary piston is unable to tension the adjustable compression spring arranged above the diaphragm system.

The spring below the diaphragm system lifts the valve, closing the auxiliary trip fluid circuit so that the trip devices can be latched in.

As soon as the turbine is started up and brought up to speed, primary oil enters the space above the auxiliary piston, forcing in into its lower end position at a turbine speed far below rated speed.

Thus the low-vacuum trip is reset for initiation of turbine trip before the turbine has reached rated speed.

Solenoid Valves for Load Shedding Relay

Solenoid valves MAX45 AA001 and MAX46 AA011 are provided to prevent the turbine from reaching trip-out speed in the event of a sudden load rejection.

These solenoid valves are actuated by the load shedding relay if the rate of load drop relative to time exceeds a predetermined value.

Solenoid valve MAX45AA001 opens the IP secondary fluid circuit directly.

Solenoid valve MAX46 AA011 opens the auxiliary secondary fluid circuit.

Pilot valve KA07 of hydraulic converter MAX45BY011 moves upward and allows the control fluid to flow to the area below piston KA08 of the converter.

Piston KA08 moves to its upper end position, thereby depressurizing all secondary fluid circuits.

Since the reheat IP secondary fluid circuit opens directly, the IP control valves (which control the major portion of the power output) close without any appreciable delay.

A small delay is involved in closing all other control valves by depressurizing the auxiliary secondary fluid circuit, but his action is still performed before an increase in turbine speed causes the speed controller to respond.

At the same time, the extraction check valves, which are dependent on secondary fluid via extraction valve relay MAX51AA011, close.

After an adjustable interval, the solenoid valves are reclosed, permitting secondary fluid pressures corresponding to the reduced load to build up again.

Turbine Trip Gear

The trip fluid is taken from the control fluid  via main trip valve MAX51AA005 and MAX51AA006 and flows both to the secondary fluid circuits and to the stop valves MAA10+20AA001 and MAB10+20AA001.

The main trip valves serve to rapidly reduce the fluid pressure in the trip fluid circuit.

If the pressure below the differential piston of main trip valves MAX51 AA005 and MAX51AA006 drops below a preset adjustable value, the piston in each valve is forced downwards by the spring, opening the drain passage for the trip fluid and closing the control fluid inlet.

If the pressure in the trip fluid circuit drops below a predetermined value, spring loading separates the upper and lower pistons of main stop valves MAA10+20 AA001 and reheat stop valves MAB10+20 AA001, and all the stop valves close very rapidly.

At the same time, the control valves and extraction check valves also close, as the secondary fluid circuits are fed from the trip fluid circuit.

Thus on trip initiation, all turbine stop and control valves close.

Manual local Trip Method of Initiating Turbine Trip

Manual local initiation of turbine trip is performed by way of local trip valve MAX52 AA005.

The valve must be pressed downwards manually, thus opening the drain passage for the auxiliary trip fluid.

The two limit switches MAX52CG005C and MAX52 CG005E indicate in the control room that trip has been initiated locally by hand.