Power plant and calculations: 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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