Power System Analysis (Non-Typical) for Offshore

Abstract

The interconnected power system is considered as the largest and most complex machine. Power system analysis of interconnected or islanded system is concerned with understanding and operation of the system as a whole. The advancements in power systems and computerized analysis of them have taken a leap in the recent years. Power System Analysis is used for regular operational studies like load balancing, load forecast as well as for pro-active and planning studies, where they can alleviate/ prevent catastrophe.

With loads which could be more diverse equipments, the challenging problem in Power System Analysis is to appropriately model all the system components to get the aggregate effect. This article focuses on certain non-typical Power System studies encountered by PowerQ over the past one and half decades of experience primarily with oil and gas industry, mobile and fixed offshore systems. Apart from steady state analysis of power systems, in practice they undergo changes due to change in loads, outages of equipments due to maintenance or disturbances, such as equipment failures, live faults, lightning strikes or any number of other events. Extremely fast Electro-Magnetic transients, undetectable leakage currents, persistent harmonics, protective relay settings for different operations on site during commissioning are few of the peculiar experiences which have been discussed briefly in this article.

This will give an insight of the practical problems faced along with reasonable solutions that may guide/ be helpful to students, prospective engineers with an urge to elevate in this field of engineering/ power system analysis.

INTRODUCTION

This document presents few of the practical problems in power systems experienced on site. Electro Magnetic transients, harmonics, TRVs are to name a few. It is essential for systems located in a place with high lightning activity to have appropriate lightning and earthing protection and electronic surge protection.

These systems should also thoroughly be referred against standards for their design and installation. Similarly, effects of harmonic voltages and harmonic currents are significantly more pronounced on marine generators due to their source impedance being typically three to four times that of utility transformers. The major impact of voltage and current harmonics is the increase in machine heating caused by increased iron losses, and copper losses, both frequency dependent In addition, there is the influence of harmonic sequence components, both on localized heating and torque pulsations.

Harmonic, in addition to causing additional heating, can create mechanical oscillations on the generator shaft. Another important phenomenon in a power system is Transient recovery voltage which is contingent upon the circuit conditions and parameters of the circuit, thus making it essential to study them for application of circuit breakers.

Generally, circuit breakers in a system are applied based on available short circuit capability at that point in the circuit. But, when circuit is interrupted it results in a TRV, this has deleterious effects on the circuit breaker. Thus TRV is a decisive parameter that limits the interrupting capability of a circuit breaker.

LIGHTNING PROTECTION

A review study for the west Natuna Sea, Indonesia records a relatively high lightning activity, where the gas field is situated. Weather is hot and humid and conditions are favorable for the development of lightning producing cumulonimbus clouds.

The destructive power of lightning is so tremendous that apart from igniting forest fires, damaging electronic equipments, disrupting electrical power and telecommunication system, they may cause fatality either directly or indirectly. Hence, the gas field in west Natuna Sea needed a detailed review of the design, construction and installation of the lightning and earthing protection system to be deployed for it. A clear practical recommendation was also needed based on the existing project status.

The platform built for drilling was built of steel with all major structures firmly welded to the deck. Earth bars were bonded to platform legs. Our experience with other projects added value in assessing this platform. The general observations from our experience shall be revealed that certain structures and equipments are usually not provided with lightning protection. The list of observations is as follows:

1. The external metal parts such as crane flare stack and generator exhaust stacks and telecom antennae are usually not provided with any air termination, down conductor or lightning arrestors.

2. The main earth bars are welded to structure while the panels and equipment are bonded to deck or earth bar by means of compression gland.

3. Surge Protection devices are not applied to Low voltage distribution panel supplying lighting and power socket outlets.

4. The cranes are not provided with any lightning arrestor or air terminals.

5. The CVT is not installed.

6. Also in our experience on other offshore projects involving drillships, semi-submersible vessels and jack up rigs, we have not observed any lightning protection system on certain projects.

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