Optimization of Capacitor for Sub Sea Motor Drive Application

Abstract

Optimization of Capacitor for Sub Sea Motor Drive ApplicationStudent: Umesh ThapaSupervisor: Prof. Lars NorumContact: Espen HauganProblem DescriptionSubsea drives are very large and expensive equipments. Normally subsea drives are encapsulated in thick walled tanks holding 1 atmosphere. As new gas fields are being discovered continually at deeper waters, this type of solution is becoming more costly. So interest in investigating the possibility of minimizing the size of electric motor drives and exposing the power electronics equipment to a high ambient pressure at the bottom of the sea had been dramatically increased. The whole power electronics will be merged in dielectric oil which has same pressure as surrounding sea water. One of the important components of an electric motor drive is capacitor. It is used to maintain good quality DC voltage and lesser ripples in DC link of the drive. It means DC link Capacitor must withstand very high pressure of 200-300 bar. Often it is DC link capacitors that account for a major fraction of the volume of an electric drive, weight and also cost of an inverter. Electrolytic Capacitor has been dominant in power drives in the Market because of its high capacitance. But the Capacitor is often bulky, low current handling capacity, moderate life hrs (around 10,000hrs) and high ESR. Electrolytic capacitors are weak and less rigid in construction to tolerate such a high pressure. An alternate could be film capacitors which normally are big in size but long life hrs (>100000 hrs), low ESR, high current handling capacity and most important thing is it has shown tendency to tolerate a very high pressure. Now a million dollar question is, Is it really possible to find a capacitor calculation method which will reduce the capacitor size drastically compare to aluminum electrolytic capacitor? SummaryThis master thesis works mostly deal with capacitor theory and different methods to calculate the capacitance of DC link capacitor of an electric motor drive application. What are the factors those need to be considered before selecting a capacitor has been greatly highlighted. How and which factors will affect in sizing capacitors has also been discussed. Why analysis of the harmonics or ripple components of the input current and voltage of PWM inverters is very important for the DC filter specification is also discussed. Simple methods of calculating capacitor ripple current and voltage ripple to optimize capacitor is clearly shown. A simulation has been done in Mathlab to support analyzed results. This master thesis has tried to investigate a suitable capacitor which satisfies both electrical properties and mechanical properties for subsea motor drives. The Thesis also compares important electric and mechanical properties of electrolytic and film capacitors with an example. If current pulses are no more limiting factor for selecting capacitors, then voltage ripple factors will be the only parameter to be taken into consideration while selecting the capacitors. Calculation in example shows that size of film capacitors chosen is 10 times smaller than aluminum electrolytic capacitor. Pressure test of such film capacitors done by SINTEF ENERGY RESEARCH [Enabling pressure tolerant power electronic converters for subsea applications by Magnar Hernes and Riccardo Pittini] shows that film capacitors tolerate a very high pressure. Some capacitors have been subjected to a 100 bar nitrogen atmosphere pressure and others up to 300 bar pressure (passive test) in Midel®7131. The capacitors were tested in long time stressed with full DC voltage rating and full capacitor current ripple limited by temperature. The test did not show any observerable damage. Film capacitors do cost more per uF/volume than electrolytic capacitors. But because of its high current handling capacity, long life hours, low ESR & ESL, rigid in construction; it is getting more attention in inverter application. Thesis works has shown with an example that the amount of capacitance needed for an inverter bus link capacitor design is much less for a film capacitor than an electrolytic capacitor because the film capacitor is not limited by ripple current rating like the electrolytic capacitor is. Hence the size of DC link capacitor can be drastically minimized by using film capacitors. Finally film capacitors can be considered as strong candidates in subsea electric motor drives and direct uF replacement in an electric drives.