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Input-delayed load-frequency control using discrete sliding mode with fast output sampling

Load-frequency control is type of control in power system where we try to follow load demands from consumers by controlling the generation from power plants in real time while keeping the system frequency at constant value of 50 Hz (60 Hz in the North America). Main problems in the LFC are:

  • constantly changing consumption of power,

  • time delays in the controller’s signal due to large distance between controller and power plants.

 

 Power system with LFCi   

Consumption uncertainties and the presence of delay makes it more difficult to achieve the satisfactory performance and stability of the system.

Input to the controller is called ACE (Area Control Error) and it is calculated from the deviations of the power flow in the tie lines between areas and frequency deviation from 50 Hz. Areas define desired power flow one day in advance.

ACE signal has the following form:

,

where  is constant value.


To overcome difficulties caused by uncertainties and time delay we use sliding mode control. It is widely used robust control technique and it’s proven to be very effective dealing with those kind of difficulties.

 Output sampling with state estimationi

Sliding mode control is based on the assumption that all system states are known. But, in the LFC, states are unknown and we have to estimate them from the area’s output signals: frequency and tie line deviations. Estimation is done by multiple sampling of the output signal during one period of the input signal.

Output signal from sliding mode controller depends on the future value of system states and future value of distance from the sliding surface:

Because of that, it is necessary to have some sort of predictor for prediction of the system states.

 Parts of the controller


 

 

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