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Steam Turbine

Membahas pembangkit listrik rasanya belum lengkap tanpa membahas masalah turbine generator. Sengaja saya sajikan dalam bahasa inggris untuk sekedar variasi bahasa saja. 🙂

In steam power plant, water is superheated in boiler to produce steam. The superheated steam which is contained enthalpy energy is converted into mechanical energy. This mechanical energy is used for rotating turbine blades that are attached to turbine shaft then it is coupled in line with generator to generate electricity based on electromagnetic principle.

How does a Steam Turbine work? Well, Steam from boiler flow into turbine trough governing control valve then enters the nozzle in the form of a steam jet, directing its force to rotate turbine blades. In general, turbine rotates at 3000rpm or 6000rpm depend on its specific design.

In turbine start up process, as emergency stop valve is fully opened, turbine speed increases and at this point governing control valve will take over. Superheated steam flow into turbine then is controlled by governing control valve. Set point of governing control valve output is based on MW set point or speed set point.  The set point later will determine the output control of governing control valve.

Soon after steam enters turbine, expansion is taken place to convert steam enthalpy energy to mechanical energy through several stages. The expansion takes place through a series of fixed blades (nozzles) that are concentrically arranged within the circular turbine casing and moving blades which is attached on the central turbine rotor. Each row of fixed blades and moving blades is called a stage. Steam pressure and temperature will dramatically reduced when it reach the last stage. This steam will be condensed by surface condenser after steam turbine. Condensation will rapidly occur in lower pressure, to achieve thus condition steam ejector is equipped after condenser to provide vacuum pressure. Finally, condensate will pump to boiler deaerator/feedwater tank and continue the rankine cycle.

Basically, there are several system equipped in turbine generator such as oil system, condensing system, turning gear, seal steam, generator cooling system and etc.

The oil system supplies the turbo set, consisting of turbine and alternator with lubricating oil and the control and emergency trip system with control oil. The turning gear operated after turbine stopped, speed of turbine decrease by remain the turbine rotor rotation turning gear auto start if turbine speed below 4 rpm, the function of turning gear to equal rotor temperature during cooling down, also easy turbine rotor rotation during turbine start up.

There are some common problems of turbine generator:

1. Turbine Trip

Turbine trip is an abrupt shutdown of turbine generator caused by safety interlocking system includes fault in electrical distribution system (short circuit, transformer blow out, etc), mechanical failure and low oil pressure.

2. Bad Quality of Steam

Bad quality of water that contains silica could lead to bad steam quality that carries over into turbine blade. This will cause deposits on turbine blade that will lead imbalance in turbine, reduce turbine efficiency and in worst situation can cause turbine damage.

3. Vibration

Vibration can be caused by misalignment, imbalance of the turbine rotor and turbine reach its critical speed. Critical speed is the angular speed at which a rotating shaft becomes dynamically unstable with large lateral amplitudes, due to resonance with the natural frequencies of lateral vibration of the shaft (Wiki Answer). High vibration occurs when turbine reach its critical speed.

4. Turbine over speed

Turbine over speed refers to over speed of turbine rotation, generally turbine rotate at 3000 rpm or 6000 rpm. Due to safety consideration, when turbine reach its over speed limit, protective devices that actuated by mechanically or electrically will trip the turbine because there is a danger risk of an accidental turbine over speed.

5. Oil Moisture

This is the water content of the lubricating oil measured as a percentage of the total volume. Excessive water in the oil will defeat the purpose of lubrication. Water in the oil is continuously removed by the used of an oil purifier. However on extreme case, heavily contaminated oil has to be replaced with fresh oil.

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