Medium Voltage Application Guide
Medium Voltage Application Guide
Medium Voltage Application Guide
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SWITCHGEAR<br />
Line inductor sizing<br />
Various software tools and calculation methods are used to determine the inductance value required for a specific<br />
soft starter application. In most cases, the required inductance per phase will be at least 100 µH. The required<br />
inductance increases as the mains supply voltage increases.<br />
The following rating information is usually provided with line inductors:<br />
<strong>Voltage</strong> must be at least equal to system voltage<br />
Frequency must equal system frequency<br />
Current must at least equal motor FLC<br />
Inductance (as calculated)<br />
Sizing guidelines for AuCom MV soft starter applications<br />
For AuCom MV soft starter installations with output motor cable runs exceeding 100 metres, compensation<br />
inductance may be required. Consult AuCom for advice on these applications.<br />
As a general guideline, compensation inductance required (per phase) can be calculated as:<br />
Where:<br />
L COMP = compensation inductance per phase (µH)<br />
U s = line supply voltage (V)<br />
didt = SCR maximum didt rating (A/µs)<br />
NOTE<br />
This calculation involves many assumptions. Double the required compensation inductance before<br />
selecting the output inductors.<br />
Exercise:<br />
Calculate the compensation inductance required for a 4.2 kV MVS soft starter installation. Assume the maximum<br />
SCR didt rating to be 100 A/µs.<br />
This calculation should be doubled to use a minimum compensation inductance of 102 µH per phase.<br />
710-12280-00A <strong>Medium</strong> <strong>Voltage</strong> <strong>Application</strong> <strong>Guide</strong> Page 131
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