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PARAMETERS

 

General

 

 

Generator

 

 

1)   Unom of the node. Also with a step up transformer Unom must equal Unom of the node.

 

Type

The type list contains all asynchronous generators from the component type database with a Unom between the 80 and 120% of the Unom of the node. See also: Type.

 

Cos-phi nom

After a modification of the nominal power factor (cos phi) the curve has to be adapted. If the curve is not adapted, the model parameters of the machine can not be determined with sufficient accuracy. Also it can occur that the curve fitting process can not find the correct model parameters (P-cos-curve does not fit).

 

Curve

 

The curve describe the behaviour of the machine (cos phi) for other then nominal loads. Each modification is directly graphically reflected in the curve.

 

 

The asynchronous machine parameters are determined using the power factor curve. Using curve fitting, the Heyland-diagram will be constructed, from which the internal impedances follow. For more information, see: http://www.phasetophase.nl/pdf/Asynchronous_machine_model.pdf.

 

To simplify the addition of an asynchronous generator, default values are given in the form for most parameters. These values will be adequate in most cases. As the function cos(phi) = f(Pe,ref/Pe,nom) is known for an asynchronous generator, this can be given if necessary. At least three points must be given for this function. After exiting the form via OK, curve fitting is performed on the entered points. In case the curve fitting fails, a message will pop-up to indicate this.

 

 

Connection

 

 

 

Dynamics

 

 

 

Reliability

 

 

 

MODELLING

 

Load flow

The Heyland-diagram is used as a basis for load flow calculations. This is determined from the curve Cos(phi) = f(P) via curve fitting. The actual power is kept constant:

 Pload = - Pe ref

 Qload depends on the Heyland-diagram and the node voltage.

 

IEC 60909

In IEC 60909 calculations, an asynchronous generator is represented as a passive impedance in the form of R + jX to earth.

 

This impedance is determined using the nominal voltage, the starting current, the rated power and the number of pole pairs. The generator impedance is then determined in accordance with the following:

 Zgenerator = (Unom generator)² / (Ia/Inom * Pe nom / cos(phi)nom)

 

The R/X ratio depends on the power per number of pole pairs:

 Pnom / (number of pole pairs) = Pnom * ( speed / nmax )

where:

 nmax = 3000 r/min. at 50 Hz

 

 

After which the following is determined using the nominal generator voltage and the power per number of pole pairs R and X:

Unom generator <= 1 kV:

 Xgenerator = 0.992 * Zgenerator

 Rgenerator = 0.42 * Zgenerator

Unom generator > 1 kV:        

 Pnom / (number of pole pairs) < 1 MW:

         Xgenerator = 0.989 * Zgenerator

         Rgenerator = 0.15 * Xgenerator

 Pnom / (number of pole pairs) >= 1 MW:

         Xgenerator = 0.995 * Zgenerator

         Rgenerator = 0.10 * Xgenerator

 

For rotating machines, in contrast to static network components, normal impedance generally differs from inverse impedance (Z2 not equal to Z1). For the asynchronous generator, however, Z2 is approximately equal to Z1. In accordance with IEC 60909, Vision applies Z2 = Z1.

 

The zero sequence impedance Z0 is assumed to be infinite (floating neutral point).

 

Fault analysis

In sequential fault analysis, the asynchronous generator is represented as a Norton equivalent circuit. The source impedance of this equivalent is determined in the same way as in an IEC 60909 calculation.