# Per unit calculations on transmissionlines

Discussion in 'High Voltage' started by rune haferkamp, Feb 29, 2016.

1. ### rune haferkampNew Member

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Hey! We are writing out electro engineer bachelor this semester, and my task is to "directstart a subsea motor". Usually you start the motor by a frequencyconverter, but we are trying to figure out if it is possible to start it without one. Therefore we are making a scaled down model in out labaratorium. The cirquit is basicly like this:

Generator - Transformator - Transmissionline(Pi-elements) - Transformator - Motor

By scaling down the model we are going to need to use the Per Unit system, but we can't figure out what base to use. Do any of you have any experience with this system and understand what base to use?

Thank you so much for any help at all

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2. ### strantorActive Member

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https://en.wikipedia.org/wiki/Per-unit_system

I would choose the subsea motor's power (kW) and voltage (kV) as the base units because the rest of the system is to designed around (designed to accommodate) those parameters.

3. ### strantorActive Member

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Also, when using a generator (assuming a dedicated generator, not shipboard power), it is still possible to ramp up to full power, same as with a frequency drive. You do not need to throw a switch and apply full line voltage and frequency from topside and "hope" that the motor starts. You can start the motor with the generator at idle, and then increase engine RPM to full RPM gradually. Further (if starting at idle is still a problem), you can employ the use of a clutch between the engine and the generator head to soft-start all the way from 0Hz to idle speed.

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5. ### rune haferkampNew Member

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Hey! Thank you so much for the reply! We will try to use your idea on base.

Yeh, i wassent really sure if i needed the switch, but i think your right. The main problem is the enormous ressistance when starting it (3-15 MVA motors).