Transformers; Unit Transformers; General Information About Calculating The Required Apparent Power Of A Unit Transformer - Siemens SINAMICS G130 Engineering Manual

Fundamental Principles and System Description
Engineering Information

1.3 Transformers

This section describes the process for selecting and dimensioning mains transformers.
1.3.1

Unit transformers

Unit transformers supply only a single converter and are specifically rated for its output power.
If a converter is supplied by a unit transformer, a line reactor does not generally need to be installed provided that the
relative short-circuit voltage v
Exceptions:
·
With S120 Smart Infeeds, a line reactor must be installed unless the relative short-circuit voltage of the
transformer (per unit impedance) is v
·
In the case of converters with rectifiers connected in parallel, line reactors are required to provide balanced
current distribution in 6-pulse operation (SINAMICS G150 with 2 parallel-connected power units or S120
Basic Infeeds or S120 Smart Infeeds with parallel-connected power units).
1.3.1.1

General information about calculating the required apparent power of a unit transformer

The required apparent power S of the transformer is calculated according to the power balance of the drive which
must be supplied by the transformer.
The line-side active power P of the drive and the associated line-side active current I
of the mechanical shaft power of the motor plus the power losses of the motor and converter.
Another factor to be considered is that a phase displacement φ
V = V /√3) and the line-side fundamental current I
1 Line
Smart Infeed). Therefore a line-side fundamental reactive current I
Q occurs in addition to the line-side active current I
1
It must also be noted that additional harmonic currents I
the drive. These increase the rms value of the line current, generate a distortive (reactive) power D and cause stray
losses in the transformer. The spectral composition and amplitudes of the individual harmonic currents I
depend on the type of SINAMICS Infeed used and on the line-side impedance. For further information about the
harmonic currents of different SINAMICS Infeed types, please refer to sections "Harmonic effects on supply system"
and "SINAMICS Infeeds and their properties".
The line-side apparent power S of the drive thus comprises three components according to the relation
Key to formula:
·
S......... Apparent power (total apparent power)
·
P......... Active power:
This is calculated from the active component of the fundamental current (I
It comprises the mechanical shaft power of the motor plus the power losses of the motor and
converter.
·
Q ......... Fundamental reactive power:
1
This is calculated from the reactive component of the fundamental current (I
·
D......... Distortive (reactive) power:
This is calculated on the basis of the harmonic currents I
The distortive (reactive) power is also referred to as "harmonic reactive power".
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of the transformer (per unit impedance) is ≥ 4 %.
k
≥ 8%.
k
with the line-commutated SINAMICS Infeeds (Basic Infeed and
1
.
1 act
= + +
2 2
S P Q
1
P = 3 • V • I • cosφ
1 1
Q = 3 • V • I • sinφ
1 1 1
= ¥
h
å
= × ×
D 3 V
1
=
h 2
develops between the line voltage (phase voltage
1
and thus also a fundamental reactive power
1 react
are superimposed on the line-side fundamental current I
h
2
D
.
.
1
.
1
:
h
2
I
.
h
are calculated on the basis
1 act
essentially
h
= I • cosφ ):
1 act 1 1
= I • sinφ ):
1 react 1 1
of
1