Droop and isochronous mode operation of steam Turbine

 Droop and synchronous modes are two operating modes commonly used in turbine control systems, particularly in the context of electrical power generation. These modes help regulate the speed and power output of the turbine to maintain stability in the electrical grid.

 Droop Mode Operation:

In droop mode, the turbine operates with a speed or frequency droop characteristic. Speed droop refers to the decrease in turbine speed as the load increases, while frequency droop refers to the decrease in electrical frequency. This mode allows multiple turbines or generators to share the load in a grid.

 In droop mode, the turbine's governor control system adjusts the fuel supply to the turbine based on the difference between the actual speed/frequency and a reference speed/frequency. As the load on the turbine increases, the speed or frequency decreases slightly, which causes the governor to open the fuel valve and increase the steam flow, compensating for the increased load. Similarly, when the load decreases, the speed or frequency increases, resulting in a reduction in fuel supply.

 Droop mode operation allows for load sharing among multiple turbines or generators. Each unit operates at a slightly different speed or frequency, which helps balance the load in a grid. The speed or frequency difference between units is known as the droop setting, and it determines how the load is shared between them.

 Key features of droop mode operation:

 Speed Control: The turbine's speed is adjusted to maintain a stable power output as per the grid's load demand. As the load increases, the turbine's speed decreases.

 Frequency Regulation: The frequency of the electrical output from the turbine is dependent on the load. As the load increases, the frequency decreases, and vice versa.

 Load Sharing: Multiple turbines operating in droop mode share the load in proportion to their capacities. Each turbine adjusts its speed based on the droop characteristic to contribute its fair share to the overall power demand.

 Load control in droop mode

 While STG is connected with grid this mode becomes active.If, STG is connected to other STG, but without grid paralleling then also this mode can be made active.

Generally 4 to 6% of droop is set for electro hydraulic control system.

By taking 4% droop as an example, 1% droop corresponds to 25% load, 2% droop is equivalent to 50% & 4% refers to 100% load.

In such controllers mode,load will be input & based on it speed will be adjusted.

 For example:

A 25 MW turbine has 8500 RPM and has droop set 4%.If Turbine is operating at 12.5 MW then controller speed set point is

 8500 + 2% X 8500 = 8670 RPM

 If it is operating on full load, then speed setting will be

 8500 + 4% X 8500 = 8840 rpm

 Isochronous mode operation:

 In isochronous mode, the turbine operates at a constant speed or frequency regardless of the load variations. In this mode, the governor control system works to maintain a steady speed or frequency by adjusting the fuel supply to the turbine.

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 The governor closely monitors the speed or frequency and makes minute adjustments to the fuel valve to counteract any changes caused by load fluctuations. As a result, the turbine operates at a constant speed or frequency, providing a stable power output.

 Isochronous mode is typically employed when maintaining a constant frequency is critical, such as in certain industrial applications or when connected to a sensitive electrical grid that requires precise frequency control.

 Key features of isochronous mode operation:

Read Generator and Turbine inter tripping

 Speed Control: The turbine's speed is regulated to remain constant, regardless of the load demand. As the load increases or decreases, the turbine adjusts its power output while maintaining a constant speed.

 Frequency Regulation: The turbine's output frequency is maintained at a constant level, typically the nominal frequency of the electrical grid. The turbine adjusts its power output to match the load demand while keeping the frequency stable.

 Load Balancing: In isochronous mode, each turbine connected to the grid contributes to the load based on its power capacity. The turbines collectively adjust their power outputs to meet the total load demand while maintaining a constant speed and frequency.

 It is to be noted that,when STG runs in parallel mode, it remains in droop mode.If the STG is connected to to grid, as soon as STG comes out from grid (Island mode), auto changeover occurs from droop mode to synchronous mode.Then STG controls speed only

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Why do turbines go into over speed trip???

Effect of moisture content in steam Turbines

Effect of moisture content in steam Turbines

 When steam passes through a turbine, it undergoes expansion and releases energy, which is harnessed to generate power. However, if the steam contains water droplets or moisture, it can have detrimental effects on the turbine blades. As the steam expands and flows through the turbine stages, the droplets can impinge on the blades, leading to erosion, pitting, or damage.

 The churning effect typically occurs in the last few stages of the turbine, where the steam is at lower pressure and velocity. At these stages, any remaining liquid particles in the steam are prone to separation from the gas phase and can cause erosion on the turbine blades.

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 Moisture content in steam  leads into the following problems:

 Erosion: The high-speed impact of liquid droplets on the turbine blades can cause erosion, leading to damage and reduced efficiency over time. Erosion can wear down the blade surfaces, affecting their aerodynamic shape and performance.

 Vibrations: The churning effect can induce vibrations in the turbine rotor and other components. Excessive vibrations can cause mechanical stress and fatigue, leading to increased wear and potential failures.

 Loss of efficiency: The presence of liquid droplets in the steam reduces the effective energy transfer from the steam to the turbine blades. This can result in a decrease in overall turbine efficiency and power output.

 To mitigate the this effect and protect the turbine blades, several measures can be implemented:

 l Proper design of steam Turbine & blading to ensure proper expansion of steam in blades.Blade Design: Turbine blades can be designed to minimize the impact of churning. This can include using erosion-resistant materials, shaping the blades to minimize droplet impingement, and providing protective coatings.

 l Drainage Systems: Proper design and implementation of drainage systems within the turbine help remove condensed water and moisture effectively

 l Proper operation of the Turbine & maintaining steam parameters as per design

 l To mitigate the moisture content in steam and its negative consequences, steam turbines are equipped with various mechanisms and components, including steam separators, moisture separators, and steam dryers. These devices help remove or reduce the moisture content in the steam before it enters the turbine, ensuring better steam quality and minimizing the chances of churning.

 Proper design, operation, and maintenance practices, such as regular inspection and cleaning of turbine components, are essential to prevent or minimize the churning effect and maintain optimal turbine performance and longevity.

10-Difference between fixed nozzle and Variable nozzle de-super heating

  

 De-superheating is the process of reducing the temperature of superheated steam. This is typically achieved by injecting a cooling medium, such as water, into the steam flow. The nozzles used in the desuperheating process can be classified as either variable nozzle or fixed nozzle de-superheaters. Here's

 The differences between these two types are:

Sl No.	Variable nozzle de-super heater	Fixed nozzle de-super heater
1	Variable nozzle desuperheating systems have adjustable nozzles that allow for controlling the flow rate of the cooling medium injected into the steam flow.	Fixed nozzle desuperheating systems have non-adjustable nozzles, meaning the cooling water flow rate and the degree of desuperheating are fixed
2	The nozzle opening can be adjusted to vary the amount of cooling water injected, thereby controlling the degree of desuperheating and achieving the desired steam temperature.	Separate control valve is required to adjust the water flow
3	More flexibility in adjusting the cooling water flow rate and achieving precise temperature control.	Not much accuracy in temperature control
4	Variable nozzle desuperheating systems are often used in applications that require tight temperature control,	USed where there is much tolerance in temperature control, Ex: In process industries They are commonly used in applications where a constant degree of desuperheating is sufficient, such as in industrial processes with steady steam loads or in small-scale power plants.
5	Complex design	Simple design
6	More costlier than fixed nozzle de-super heaters	Less costlier
7	Little bit complicated operation	Simple operation
8	Can be used for variable inlet flow & temperature	Used only for fixed flow & temperature
9	Size of nozzle is variable	Size of nozzle is fixed
10	Maintenance is difficult & costlier	Maintenance is simple & cheaper

 The choice between a variable nozzle and fixed nozzle desuperheater depends on factors such as the required temperature control accuracy, steam flow variability, plant operating conditions, and budget considerations. Variable nozzle desuperheaters are often preferred in applications where precise temperature control and flexibility are crucial, while fixed nozzle desuperheaters can be suitable for applications with relatively stable operating conditions and lower cost requirements.

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Why pressure reducing of steam is being done before de-super heating?

 Following advantages we can have, if pressure reducing is done before de-super heating

1.  Reducing the pressure before de-super heating helps in controlling the steam temperature with great accuracy.High-pressure steam entering the de-superheating station can be challenging to control accurately due to its high energy content. By reducing the pressure, the energy content decreases, allowing for better control of the de-superheating process and ensuring that the desired steam temperature is achieved.
2.  Heat transfer between water & steam is more better at lower pressure
3.  Helps is protecting the down stream equipment if desuper heating is done at lower pressure since there will not be no any much chances of low or high temperature steam entry into the Turbine or process.
4.  Erosion of down stream pipe lines can be minimized due to high pressure & high velocity steam
5.  Cost of high pressure lines can be reduced.
6.  Total head required for de-superheating water will be reduced, thereby reducing feed pump pumping power.
7.  Related cost of control valves & pipe lines will be reduced
8.  Damage to the de-super heating equipment can be reduced by avoiding thermal shock by avoiding direct mixing of high pressure steam and high pressure water
9.  Pressure reduction before the de-superheating station helps maintain safe operating condition.High-pressure steam carries a greater risk of causing damage or injury in the event of a malfunction or sudden pressure surge. By reducing the pressure beforehand, the risk of high-pressure steam reaching the de-superheating station is mitigated, enhancing overall system safety.
10.  Pressure reduction can also contribute to overall system efficiency. Lowering the pressure before the de-superheating station helps control the temperature more effectively, resulting in more efficient heat transfer and reduced energy consumption.
11. Reducing the pressure before de super heating will also helps in reducing the temperature of steam, which helps in reduction of water quantity required for desuper heating

 

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