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PARAMETERS

 

General

 

 

Motor

 

 

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.

 

Efficiency and Cos-phi nom

After a modification of the efficiency or the nominal power factor (cos-phi) the curves must be adapted. If the curves are 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-efficiency curve does not fit).

 

R/X and Cos-phi during motor start

The cos-phi during a motor start is calculated from the R/X ratio and vice versa.

 

Is/Inom

The quotient of the starting and nominal currents is used for the motor impedance calculation. This value will not be used for the motor starting calculation.

 

Curves

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

 

 

The asynchronous machine parameters are determined using the efficiency and power factor curves. 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 motor, 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 motor, 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

 

 

Starter

There are three possible configurations:

•direct on line (DOL): Is/Inom has the same value as specified on the Motor input tab; the motor always has a contribution to a short-circuit

•soft starter: I/Inom is smaller than the value of Is/Inom as specified on the Motor input tab; the motor always has a contribution to a short-circuit

•Converter (VSDS, variable speed drive system): Is/Inom has a value of about 1; the motor contribution to a short-circuit is optional

 

Is/Inom

The quotient of the starting and nominal currents is used for the motor starting calculation. This value may differ from the quotient Is/Inom from the motor data, e.g. for modelling a soft starter.

 

No SC contribution

This prohibits an asynchronous motor to contribute to a short circuit. If enabled, the motor is modelled as a load in the short circuit calculation.

 

 

Dynamics

 

 

 

Reliability

 

 

Harmonics

Application of a harmonics type. See: Harmonics, Calculation.

 

 

MODELLING

 

Load flow

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

 Pload = - Pe ref

 Qload is dependent on the Heyland-diagram and the node voltage.

 

IEC 60909

Asynchronous motors contribute to the initial symmetrical short-circuit current Ik", to the peak short-circuit current ip, to the symmetrical short-circuit breaking current Ib and, for unbalanced short circuits, also to the steady-state short-circuit current Ik.

Reversible static converter-fed drives are considered for three-phase short circuits only, if the rotational masses of the motors and the static equipment provide reverse transfer of energy for deceleration at the time of the short circuit. Then they contribute only to the initial symmetrical short-circuit current Ik" and to the peak short-circuit current ip. They do not contribute to the symmetrical short-circuit breaking current Ib and the steady-state short-circuit current Ik.

The next table summarises the contribution to a short-circuit current.

 

 

In IEC 60909 calculations, an asynchronous motor with generating capacity (enabling possible contribution to a short circuit) is represented as a passive impedance in the form of R + jX to earth. (IEC 60909, clause 3.8)

 

This impedance is determined using the nominal voltage, the starting current, the nominal (mechanical) power and the number of pole pairs. The power is given in electrical terms in Vision, so that the following applies for electrical power:

 Pe nom = Pm nom / Efficiency

 

The motor impedance is then determined in accordance with the following:

 Zmotor = (Unom motor)² / (Ia/Inom * Pe nom / cos(φ)nom)

 

The R/X ratio is dependent on the (mechanical) power per number of pole pairs:

 Pm nom / (number of pole pairs) = Pm nom * ( speed / nmax )

where:

 nmax = 3000 r/min. at 50 Hz

 

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

 

Unom motor <= 1 kV:

 Xmotor = 0.992 * Zmotor

 Rmotor = 0.42 * Zmotor

Unom motor > 1 kV:

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

         Xmotor = 0.989 * Zmotor

         Rmotor = 0.15 * Xmotor

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

         Xmotor = 0.995 * Zmotor

         Rmotor = 0.10 * Xmotor

 

For rotating machines, in contrast to static network components, normal impedance generally differs from inverse impedance (Z2 not equal to Z1). For the asynchronous motor, 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 motor is represented as a Norton equivalent. The source impedance of this equivalent is determined in the same way as in an IEC 60909 calculation.