Voltage drop calculation: voltage change during drive start

Guide for electrical design engineers

Power Quality Krzysztof Piatek AGH-University of Science & Technology

Voltage drop calculation: voltage change during drive start 110 kV

S SC 160 MVA S 16 MVA u 11

20 kV

PPC 2 S 5 MVA u 7

6 kV

S 1 MVA u 7

PPC 1

S 1 MVA u 7

Power Quality

P 1450 kVA U 800 V

Power Quality http://www.leonardo-energy.org

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

110 kV

S SC 160 MVA S 16 MVA u 11

20 kV

PPC 2 S 5 MVA u 7

6 kV

PPC 1

S 1 MVA u 7

S 1 MVA 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 ΔU =

PR + QX UN

Assuming R=0 we obtain Ku =

Q ΔU ≈ 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 Z SEE =1.1

UN2 62 =1.1 = 247.5 mΩ SSC 160

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Voltage drop calculation: voltage change during drive start http://www.leonardo-energy.org

The Tr1 110/20 kV transformer impedance Z Tr1 =

u% U 2 62 = 0.11 = 247 mΩ 100 STr1 16

The Tr2 20/6 kV transformer impedance Z Tr2 =

u% U 2 62 = 0.07 = 507 mΩ 100 STr2 5

The short-circuit capacity at PCC 1 we obtain from the formula SPPC1 =1.1

UN2 ZPPC1

where ZPPC1 is the total impedance at PCC 1, i.e. ZPPC1 = Z SEE + Z Tr1 + Z Tr2 =1Ω thus, we obtain

62 = 39.6 MVA 1 Similarly, we calculate the impedance at PCC 2 SPPC1 =1.1

ZPPC2 = Z SEE + Z Tr1 = 0.494 Ω and then the apparent power SPPC2 =1.1

UN2 62 =1.1 = 80.2 MVA ZPPC2 0.494

The reactive power during start we calculate from the given coefficients and the rated drive power. Qmax = qkr PN =1.7⋅0.8⋅1450 =1972 kVAr Using the simplified formula we calculate the voltage change coefficient, for PCC 1 we obtain Q 1.972 K u, PPC1 ≈ 1.1 max =1.1 = 0.059 , K u, PPC1 ≈ 5.9% 39.6 SPPC1 whereas for PCC 2 we obtain K u, PPC2 ≈ 1.1

Qmax 1.972 =1.1 = 0.027, K u, PPC2 ≈ 2.7% 80.2 SPPC2

The obtained coefficients should be compared with it’s permissible levels. For example, if the planned number of starts is 20 per hour, the permissible level of the voltage change coefficient is 3%. According to this limit value, the voltage change is to large in PCC 1, therefore the drive must not be connected directly to the network. The above calculations suggest also the solution – reactive power of the drive during start should be decreased in order to satisfy the limit value at PCC 1, or sensitive equipments should be feed from PCC 2, where the voltage change satisfy the limit.

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