Electrical Transmission System with Variable Frequency Through Long Length Cable
Authors: Oliveira J.C. , Paulsen R.J. , Amaral M.A. , Andrade D.A. , Araujo S.C.N. 
Published: October 2008

In face of its oil reserves characteristics, deep and ultra-deep exploitation have become a vital issue for Brazil 1, In those cases where the oil field does not have enough pressure to ensure natural oil flow from the bottom of the sea to the platform, the process will require the use of variable speed inverter-fed sub sea electrical motors placed fat away from the electrical energy source. This implies in a new operational condition for the electric system, which is the electrical energy transmission with variable frequency, through long length cable, in the sub sea environment. The behavior of such systems has not yet been extensively studied, and in face of the coats involved, careful analysis is mandatory. The paper presents a power simulation tool developed to the analysis of such electrical systems. The analysis is carried out in time domain. Harmonics components of voltages and currents and their effects on the system operation are analyzed. Comparison of simulated and measured results for an actual working installation is shown Simulation results for a long length cable transmission system are discussed.

Introduction

Offshore oil exploitation in shallow waters is a well established technology world-wide, However, when it comes to the oil extraction in waters deep more than 400m, experience starts to become restrict, and for depths over 1000m. mature technology is stall under development.

Brazil has particular interest in developing deep water exploitation schemes, as 64°/0 of all its assured oil reserves, and 65% of all the potential discoveries are found to be in deep and ultra-deep waters, i. e., depths over 10OOml. PETROBRAS is already successfully exploring a well completion at 1027 m V/D (water depth), but in face of the challenge imposed by the characteristics of its reserves it understands new technologies must be developed.

This has called for a comprehensive and well coordinated development program which involves all aspects concerned with ultra-deep water exploitation and has been set up by PETROBRAS through the PROCM-200($. Among the several objectives of this program stands the research and development of technologically innovative projects aiming to provide technical capability in electrical systems to exploit deep and ultra-deep oil marginal fields using sub sea hosting systems, reduce investment and operational exploitation costs, and the extension of the useful life of wells located at deep and ultra deep waters. In this context, directions for reducing or even eliminating the number of platforms by using sub sea boosting systems has gained particular attention.

These sub sea boosting systems comprehend the use of electrical submersible pumps placed directly in the wells, sub sea separation and multiphase flow pumping arrangements placed at distances of 25 km or more from the Production Unit. Under the operational point of view this indicates that electrical Energy is to be transported along these distances via sub sea cables. Furthermore, for the efficient operation of the production systems, adjustable speed motors located at the bottom of the sea are necessary, thus electrical energy transmission through long sub sea cables with variable frequency is required.