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Chapter: Flexible Alternating Current Transmission System - Co-Ordination of FACTS Controllers

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SVC-SVC Interactions

The Effect of Electrical Coupling and Short-Circuit Levels

SVC – SVC Interactions

 

The Effect of Electrical Coupling and Short-Circuit Levels

 

Ø   The interaction phenomena are investigated as functions of electrical distance (electrical coupling) between the SVCs and the short-circuit level at the SVC buses.


 

1. Uncoupled SVC Buses

 

Ø   A simplified test system shown in Fig. is considered for the interaction analysis performed through eigenvalue analyses and root-loci plots.

 

Ø   All the generating units are represented by infinite buses. If the transfer reactance between buses 1 and 2 is high, making the buses electrically uncoupled, then the SVCs connected to those buses do not interact adversely.

 

Ø   Increasing the proportional gain of SVC 1 connected to bus 1, even to the extent of making the SVC unstable, does not affect the eigenvalues of SVC 2—implying that the controller designs of SVCs can be done independently for multiple SVCs in a power system if the transfer reactance between their connecting buses is high.

 

 

 

2. Coupled SVC Buses

 

Ø   If the reactance between the two SVC buses is low, it constitutes a case of high electrical coupling between the SVCs.

 

Ø   Here again, two ossibilities exist with respect to short-circuit capacity of the region where the SVCs are installed: the SVC region with a high shortcircuit capacity and the SVC region with a low short-circuit capacity.

 

Ø   For high short-circuit capacity conditions in the same system as Fig. reveal that by increasing the proportional gain of one SVC, the eigenvalues of the other SVC are impacted very slightly. Almost no control interaction exists between the two SVCs irrespective of their electrical coupling, as long as they are in a high short-circuit-level region, that is, when the ac system is stiff.

 

Ø   The reason for this condition is that the interlinking variable between the two SVCs is the bus voltage.

 

Ø   Thus the controls of both SVCs can be independently designed and optimized, but if the short-circuit capacity of the SVC region is low, varying the proportional gain of SVC 1 will strongly influence the eigenvalues associated with SVC 2.

 

Ø   Therefore imperative that a coordinated control design be undertaken for both SVCs.

 

Ø   Despite simplifications in the study system and in the analysis approach, the aforementioned interaction results are general, for the phenomena investigated are independent of the number of buses, transmission lines, or generators.


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