As a result, the steams flow rate into the turbine increases which will bring the speed of the turbine to the normal range. When speed of the turbine will come to its normal range, fly balls will come into its normal position. Now, sleeve and control valve rod will back to its normal position. It is another interesting method by which turbine's speed can be controlled.
Nozzle control governing of steam turbine is basically used for part load condition. Some set of nozzles are grouped together may be two, three or more groups and each group of the nozzle is supplied steam controlled by valves.
Every valve is closed by the corresponding set of nozzle. Steam's flow rate is also controlled by these nozzles. Actually, nozzle control governing is restricted to the first stage of turbine whereas the subsequent nozzle area in other stage remains constant. According to the load demand, some nozzles are in active and other inactive position.
Suppose turbine holds ten numbers of nozzles. Skip carousel. Carousel Previous. Carousel Next. What is Scribd? Explore Ebooks. Bestsellers Editors' Picks All Ebooks. Explore Audiobooks. Bestsellers Editors' Picks All audiobooks. Explore Magazines. Editors' Picks All magazines. Explore Podcasts All podcasts. Difficulty Beginner Intermediate Advanced. Explore Documents. Steam Turbine Governor. Uploaded by Mohammad Ibnul Hossain. Document Information click to expand document information Description: Steam turbine Governor.
The high-frequency gain of the PID is limited by a first-order low-pass filter with time constant Td s. Specifies the input of the feedback loop: gate position set to 1 or electrical power deviation set to 0. The initial mechanical power Pm0 pu at the machine's shaft.
This value is automatically updated by the load flow utility of the Powergui block. Reference mechanical power in pu. This input can be left unconnected if you want to use the gate position as input to the feedback loop instead of the power deviation.
Machine actual electrical power in pu. It also demonstrates the use of the Machine Initialization tool of the Powergui block to initialize machine currents and initial mechanical power of the HTG block. A three-phase generator rated MVA, The system starts in steady state with the generator supplying MW of active power. An electro hydraulic interface is used to connect the electronic set point signal into a hydraulic oil flow from a hydraulic servo valve system which determines the position of the turbine control actuators.
This is a PID controller. Examples of Analogue electronic type hydraulic governor are as follow and a typical block diagram is shown in figure 6. Fig: 6. This is also generally PID controller. Digital control hardware running an application programme accomplishes the required control function with this system.
Digital controllers used for turbine governing system are very flexible and can be used for functions not directly related to the turbine governing control function. Present day practice is to use digital governing control system in hydroelectric units.
The major advantages of microprocessor based system over the earlier analogue governors based on solid state electronic circuitry are higher reliability, self diagnostic feature, modular design, flexibility of changing control functions via software, stability of set parameters, reduced wiring and easy remote control through optical fiber cables.
Microprocessor based governor control system are capable of carrying out the following control functions in addition to speed control during idle run , operating in isolated grid; interconnected operation and islanding operation.
Digital controls can be designed in any configuration because software changes or additions can be made easily without requiring additional hardware. A typical digital governor specified for a Kaplan turbine with control of wicket gates and blades is shown in figure 6. Figure 6. The governor drive should not be affected by variations in the voltage or current of the main generator or exciter or of the power system to which the main generator is connected.
Following speed sensing methods are used. PMG output frequency and voltage are proportional to unit speed. In mechanical governor a permanent magnet generator PMG mounted on the rotor shaft drives a synchronous pendulum motor as explained the Para 6. The ball head mechanics of mechanical governors is the oldest speed sensing devices. In electro-hydraulic analogue governors the speed sensing was by permanent magnet generator PMG mounted on the rotor shaft delivers frequency signals to the governor circuits.
Also, it is a source of an output to operate speed relays for various sequence controls and of power to drive the transistor amplifiers. This requires that the speed be sensed at about every 10 mili second or better. Further pulse sensing is preferred for direct use in digital governing.
Modern speed sensing speed signal generator is achieved generally by magnetic or fiber-optic sensors operated in conjunction with toothed wheels or other devices directly connected to the generator shaft speed signal generator-SSG.
A typical common device in which speed is sensed electrically by speed signal generator SSG is shown in figure 6. A centrifugal over speed trip and a means for detecting creep is provided. The signal from magnetic pick up is fed to the pulse generator card which is used for speed switches and speed as well for tachometer.
The creep is sensed by detecting with a photo cell, the interruption of a beam of light passing through the tooth of the speed signal gear. Creep equivalent to 1 tooth is generally required to activate a relay for alarms etc. Modern practice for speed sensing mostly used for large units is toothed wheel with a magnetic pick up. The toothed wheel generally encircles the turbine shaft.
The output frequency of the pickup is measured by speed sensor to determine shaft speed. The transformer must be capable of operating at very low residual voltages in absence of field excitation. Speed is measured from PT voltage transformer by measuring frequency. Pair of Poles The sensitivity with this method is lower about 0. This method is used in small units. Speed for control is measured from output voltage potential transformer secondary by measuring time period of the output wave.
At Bahkra power plants combined capacity of the pump per minute was three times the total oil volume of the servomotor of the turbines. Normal operating oil pressure was specified For Beas project power house oil pressure was specified in the same range. Present day practice is to specify higher oil pressure. Higher pressures allow using smaller valves, smaller pressure tanks, piping and servomotors for the same effort and may be less expensive. However hazards of leakage increases.
Typically two pumps are used one as lead and the other as back up lag. Lead and back up pumps are frequently interchanged for uniform wear and tear. At Bhakra the capacity of the tank was specified not less than 20 times the volume of the servomotor cylinders of the turbine. It may require heating or cooling system to maintain usable oil temperature range for normal governor operation.
Normal value for maximum velocity is 5 meter per second. In Bhakra power plant it was about 4. A typical hydraulic schematic diagram of 3 MW Sobla Francis turbine with digital governor and servomotor is shown in figure 6.
Overall strategy to control turbine speed in modern electronic governors is given below. This signal is based on the results of PID algorithm, given speed droop curve and governing dead-band. This signal interfaces with the proportional valve electronics.
Similarly number of nozzles operating in Pelton turbine and nozzle needle opening and closing is in accordance with the given curves between speed and nozzle opening. Some of the end points are hard coded in the ROM memory and other points are extrapolated in software. This controller has a term that is proportional to the error, and a term that is proportional to the integral of the error, and a term that is proportional to the rate of change derivative of the error.
The general form of this controller is shown in equation 1. The second term in the controller provides a component of the output that is equal to the integral of the error. In other words, as long as the error differs from zero, the controller output will continue to change. However, it should be noted that there are physical limits beyond which the controller output cannot change even though this algorithm potentially could require it.
Without integral action, the process output never reaches the set point following the changes unless the proportional constant is made unrealistically large. The last term in the controller provides a component of the output when the rate of change of error is non-zero.
The proportional term is the same. However, the integral symbol is replaced by a summation symbol and derivative operation is replaced by a first order difference approximation.
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