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Frequency Control/Protection Engineering Services


One of the chief requirements, for operation of power system is to ensure acceptable quality of system frequency and to protect the equipments from abnormal supply frequency. Specifically an abnormal frequency in generator can result in shaft fatigue and failure of the shaft. The generator shaft is not a uniform mechanical shaft but has different masses on the same shaft that has different time response for any changes in the input mechanical energy and output electrical energy. Consequent to different time responses of the masses on the shaft, the shaft is subjected to torsional fatigues.

 An industrial plant may employ its own captive power plant to partly or wholly meet its plant load demand. Most industrial units operate in parallel with the grid and may import/export power from/to the grid. The following scenarios may arise 

-       Loss of part of the in plant generation due to some fault conditions, resulting in excessive load demand on the remaining generators within the plant

-       Loss of generation in the external grid, which diverts excessive electrical demand on the industrial plant

 Either of the above scenarios will result in fall in industrial plant frequency, thus affecting the quality of power supply and subjecting the remaining generators to torsional fatigue/stress.

 Typically generator manufacturers specify the band width of the frequency within which their units should function. They also state the limits on acceptable rate of change of frequency, which when exceeded, results in generator shaft fatigue. The control and protection of the generator must take these limits into account, while designing the suitable protection. Depending on the situation, the following protection functions may be considered for an industrial plant 

-       Load Shedding

-       Isolation from The grid

-       Isolation of the generation plant itself from the network.

 

The frequency response of the generator unit is dependent on the following factors

 

-       Type of disturbances

-       Electrical power output from the units under consideration

-       The response of the generator prime-mover governor system

-       The response of the generator excitation system that influences the electrical power output

-       The power swings between the various units and the grid

 

Evidently the frequency response is not identical in all the cases and needs to be studied in detail for various operational and disturbance scenarios.  Such analysis can be performed using a transient stability analysis tool such as the one available with the PowerApps product.  The importance of the transient stability analysis of the system is as follows

 

-    To identify the critical operating and disturbing scenarios which result in unacceptable generator frequency and thereby taking appropriate control and protection measures. The most important thing is that the control and protection scenario may not be necessary for all operating conditions and disturbance scenarios and may be necessary for specific conditions only.

-    To recommend the sequence of control and protection scenarios under specific operating condition and disturbance scenarios and to suggest the necessary measurements and specifications to be implemented for a PLC based control and protection scheme.

 

Thus a more precise and dependable solution for the protection and control of generator units can be provided by a detailed transient stability analysis, considering the characteristics of the generators, excitation system, governor/prime mover systems, external grid, plant operating conditions and so on.

 

The simulation from transient stability analysis can also be used for setting conventional relays such as underfrequency and under voltage based relay settings.


PLC based Load Shedding Scheme



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