Voltage Drop Calculation: Voltage Change During Drive Start
Voltage Drop Calculation: Voltage Change During Drive Start
Voltage Drop Calculation: Voltage Change During Drive Start
Power Quality
Krzysztof Piatek
AGH-University of Science & Technology
S 16 MVA
u 11
20 kV PPC 2
S 5 MVA
u 7
6 kV PPC 1
S 1 MVA S 1 MVA
u 7 u 7
Power Quality
P 1450 kVA
U 800 V
Power Quality
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Problem
A converter drive is connected as in figure. The voltage change coefficient during the drive start shall be determined
at PCC 1 and PCC 2.
The converter drive data:
- starting overload coefficient: kr =1.7 kVA/kW
- reactive power reduction due to the sequence control: q = 0.8
S 16 MVA
u 11
20 kV PPC 2
S 5 MVA
u 7
6 kV PPC 1
S 1 MVA S 1 MVA
u 7 u 7
P 1450 kVA
U 800 V
Solution
The relative voltage change coefficient can be determined using the formula for the voltage drop across the power
system components R, X, expressed in terms of the active and reactive power P, Q transmitted in the system
PR + QX
ΔU =
UN
Assuming R=0 we obtain
ΔU Q
Ku = ≈ 1.1 max
UN SSC
where Qmax is the maximum reactive power, Ssc is the short-circuit capacity at the point for which the voltage change
coefficient is calculated. In this formula the resistances of components are disregarded, what can lead to large error
in low-voltage systems. The voltage change coefficient calculated this way is lower than that obtained from the
accurate calculation.
For the sake of simplicity all coefficients will be calculated from the parameters reflected to the 6kV side.
The first step is to determine short-circuit capacities at all points by calculating the components impedances and
subsequently the short-circuit capacity at the required point.
The power system impedance
UN2 62
Z SEE =1.1 =1.1 = 247.5 mΩ
SSC 160
2
Voltage drop calculation: voltage change during drive start
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