Electrical System Studies for Large Projects Executed at Multiple Engineering Centres

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2009 Bechtel Corporation. All rights reserved.

INTRODUCTION

Electrical system studies are carried out toverify that major electrical equipment isadequately rated, determine the conditions

for satisfactory and reliable operation, and

highlight any operational restrictions required

for safe operation.

The various units within the Jamnagar Export

Refinery Project (JERP) were engineered at

Bechtel engineering centres (London, Houston,

Frederick, Toronto, and New Delhi), the Bechtel/

Reliance Industries Limited joint venture (JV)

office in Mumbai, and the sites of non-Bechtel

engineering contractors. The core Electrical

group based in Bechtels London office (the

London core group) was tasked with preparing

a combined model of the electrical system and

with conducting the system studies.

The system studies for this project presented

unique challenges because of the sheer size of the

captive power generation (with a new 800 MW

power plant operating in parallel with an existing

400 MW power plant), the Jamnagar plants

extensive power distribution network, and the

engineering work distributed amongst various

Bechtel engineering centres and non-Bechtel

engineering contractors around the world.

System studies are normally conducted o

selected set of study cases, and their res

are used to determine the system behavi

under all operating conditions. For this proj

it was difficult to select the cases to simulate

study because of the large number of possoperating configurations for such a comp

industrial electrical network. The system stu

themselves were a challenge because so m

study cases (particularly transient stabi

analysis studies) had to be evaluated.

This paper presents an overview of system st

execution on the complex electrical network

the JERP, along with a brief report on the vari

studies conducted as part of this project.

OVERVIEW OF THE JERP

Reliance Industries operates the JamnaDomestic Tariff Area (DTA) oil refinand petrochemical complex located in Guja

India. The complex processes 650,000 bar

per stream day (650 kbpsd) of crude

and produces liquefied petroleum gas (LP

naphtha; gasoline; kerosene; diesel; sulph

coke; polypropylene; and numerous arom

products, including paraxylene, orthoxyle

Issue Date: December 2009

AbstractElectrical system studies are carried out to verify that major electrical equipment is adequarated, determine the conditions for satisfactory and reliable operation, and highlight any operational restrictirequired for safe operation.

The system studies for the Jamnagar Export Refinery Project (JERP) presented unique challenges becausthe sheer size of the captive power generation (with a new 800 MW power plant operating in parallel withexisting 400 MW power plant), the plants extensive power distribution network, and the engineering wdistributed amongst various Bechtel engineering centres and non-Bechtel engineering contractors around world. The large number of system study cases (particularly transient stability analysis studies) to be evalua

also made the task challenging.This paper presents an overview of system study execution on the complex JERP electrical network, along wa brief report on the various studies conducted as part of this project.

Keywordsanalysis, electrical system studies, Electrical Transient Analysis Program (ETAP), JamnaExport Refinery Project (JERP)

ELECTRICAL SYSTEM STUDIES FOR

LARGE PROJECTS EXECUTED AT

MULTIPLE ENGINEERING CENTRES

Rajesh NarayanAthiyarath

[email protected]



Bechtel Technology Journal2

and benzene. The original project, which

Bechtel designed and constructed, was the

worlds largest grassroots single-stream

refinery. The complex includes a captive power

plant (CPP) designed to produce 400 MW of

power (backed up by a 132 kV grid supply) to

meet the refinerys power demands.

The JERP comprises a new export-oriented

refinery located in a special economic zone

(SEZ) adjacent to the DTA site. The project aimsto almost double the capacity of the Jamnagar

refinery to more than 1,200 kbpsd; add crude

distillation, associated secondary conversion

facilities, and an 800 MW CPP; and modify the

existing refinery to ensure the efficient operation

of both it and the new refinery.

On completion of the JERP, the Jamnagar

complex will be the worlds largest refinery,

surpassing Venezuelas 940 kbpsd Paraguana

refining complex.

ENGINEERING THE JERP

The JERP required approximately 6 millionengineering jobhours within a short andchallenging project schedule. Hence, project

engineering was split up amongst the various

Bechtel offices, headed by the London core

group (Figure 1). The key task of conducting

overall system studies on the JERP and DTA

electrical networks was handed over to the

London core group.

POWER GENERATION AND DISTRIBUTION

Asimplified depiction of the JERP powergeneration and distribution system isportrayed in Figure 2.

JERP Power System

As the JERP power source, the CPP consists of six

125 MW, 14.5 kV gas turbine generators (GTGs),

with space allocated for three future GTGs. The

GTGs are connected to the 220 kV switchyard

bus via their dedicated 14.5/231 kV, 161 MVA

step-up transformers. Eight 220/34.5 kV, 174 MVA

refinery service transformers (RSTs) connectedto the 220 kV switchyard feed the JERP plant

substations through 33 kV switchboards in two

main receiving stations (MRS-1 and MRS-2). Two

11 kV, 25 MW steam turbine generators (STGs)

are connected to the switchboards in MRS-1

via 11/34.5 kV, 38 MVA step-up transformers.

Finally, a pair of 220/132 kV, 107 MVA auto-

transformers are provided as the interconnecting

The key task of

conducting overall

system studies on

the JERP and DTA

electrical networks

was handed over

to the London

core group.

ABBREVIATIONS, ACRONYMS, AND TERMS

AVR automatic voltage regulator

BSAP Bechtel standard applicationprogram (a software applicationthat Bechtel has determinedto be suitable for use tosupport functional processescorporate-wide)

CPP captive power plant

DTA Domestic Tariff Area

DWI discipline work instruction

EDMS electrical distributionmanagement system

EMS energy management system

ETAP Electrical Transient AnalysisProgram (a BSAP)

FEED front-end engineeringand design

GTG gas turbine generator

HVDC high-voltage direct current

ICT interconnecting transformer

IEC International ElectrotechnicalCommission

IEEE Institute of Electrical andElectronics Engineers

IHD individual harmonic distortion

JERP Jamnagar Export Refinery Project

JV joint venture

LMS load management system

LPG liquefied petroleum gas

LV low voltage

MRS main receiving station

MV medium voltage

OLTC on-load tap changer

PC personal computer

RST refinery service transformer

SEZ special economic zone

STG steam turbine generator

THD total harmonic distortion

VSD variable-speed drive



December 2009 Volume 2, Number 1

transformers (ICTs) between the JERP and DTA

electrical systems.

The JERP electrical system incorporates an energy

management system (EMS) that comprises an

electrical distribution management system

(EDMS) to control and monitor the electrical

network and a load management system (LMS) to

carry out load shedding, if required, within the

JERP and DTA electrical networks.

DTA Power System

The DTA CPP consists of nine 28 MW GTGsand six 25 MW STGs that feed the five

33 kV switchboards, from which power is further

distributed to the DTA plant substations.

ELECTRICAL SYSTEM STUDIES

System studies is the generic term for awide range of simulations conducted on

System studies

is the generic ter

for a wide range

of simulations

conducted on

a model of an

electrical system

under various

operating

conditions

encountered or

anticipated

during operation

of the network.

BECHTEL HOUSTON/SHANGHAI

CFP, Crude &

Alkylation Units

BECHTEL FREDERICK

Captive Power

Plant (CPP)

JAMNAGAR

ENGINEERINGOFFICE (JEC)JAMNAGAR (SITE)

Merox, ATU, SWS, PRU Units

FCC/VGO Units (Balance)

DTA Revamp

BECHTEL NEW DELHI

Captive Power

Plant (CPP)

THIRDPARTY ANDLICENSOR OFFICES

Coker (FWHouston)

Sulphur & TGTU

(BVPIKansas City)Sulphur Granulation

Hydrogen (LindeMunich)Acid Regeneration

(MECSSt. Louis)

BECHTELRELIANCE JVMUMBAI

Offsites & Utilities

CFP, Crude & Alkylation

Units (Balance)

BECHTEL LONDON

DTA Expansion Group

DTA Revamp FEED

FCC Group

FCC/VGO Units

BANTREL TORONTO

Aromatics Unit

CNHT/KHT Units

BECHTELLONDON

COREFUNCTIONS

Interconnection

to DTA

Four Sets 33 kV

Main Receiving Switchgear

6.6 kV/415 V Distribution

Network for Each Unit

220 kV Switchgear

(1 Breaker Scheme)

Two 25 MW STGs

Captive Power Plant

(Six 125 MW GTGs)

Figure 1. Project Execution Locations

Figure 2. JERP Power System Generation and Distribution

ATU amine treating unit

CFP clean fuel plant

CNHT cracked naphtha hydrotreater

CPP captive power plant

DTA Domestic Tariff Area

FCC uid catalytic cracker

FEED front-end engineering and design

FW Foster Wheeler

JEC Jamnagar Engineering O fce

KHT kerosene hydrotreater

Merox (mercaptan oxidation process)

PRU propylene recovery unit

SWS sour water stripper

TGTU tail ga s treatmen t unit

VGO vacuum gas oil




a model of an electrical system under various

operating conditions encountered or anticipated

during operation of the network. System

studies analyse the behaviour of the electrical

network components under various steady-state,

dynamic, and transient conditions, and the

results are used to predict the networks

behaviour under actual operating conditions.

System studies are conducted at different

stages of a project. The results of system studiesperformed during the front-end engineering and

design (FEED) and detailed engineering stages

enable proper selection of equipment ratings,

identification of the electrical system loading

and operational modes for maximum reliability

and safety, and selection of the control modes for

major equipment. These early system studies can

also assess the ability of the electrical network to

meet present and future system energy demands.

System studies conducted after the power

system network is operational generally study

the feasibility or effects of system expansion,

check conformance with any changes in codes

and standards, or analyse system behaviour

to identify the underlying causes of a network

disturbance or equipment failure.

In the case of the JERP, the electrical system is

planned to operate in parallel with the existing

DTA electrical system and the grid supply from

the local electricity utility. The JERP electrical

system also has to be adequate for the addition

of future units and high-voltage direct current

(HVDC) links to the local electricity utility supply.

Hence, this combination of large-scale greenfield

project/major expansion of an existing networkbecomes a special case for system studies.

The sheer size of the JERP and DTA electrical

networks (with a combined power generation

of 1.2 GW), the extensive power distribution

network within the JERP and DTA plants, and

the crucial need to ensure reliability of the power

supply under all operating conditions make

it important to conduct reliable and accurate

system studies. Further, the study results

can help in the design of a reliable electrical

system suitable for the projects present and

future requirements.

Three key elements are at the heart of a proper

system study:

A dependable and versatile system study

software program

A reliable model of the electrical network

Selection of studies to be conducted and

study cases to be simulated

ELECTRICAL SYSTEM STUDY

SOFTWARE PROGRAMS

System studies entail the analysis of theinteractions amongst the various componentsof the electrical network to determine the power

flows between elements and the voltage profile

at the various buses in the network. Many

mathematical computations are required to

analyse even a small network, precluding the use

of manual calculation techniques to conduct anybut the most rudimentary system studies.

These circumstances have led to an effort

since the late 1920s to devise computational

aids for network analysis. From about 1929 to

the 1960s, special analogue computers in the

form of alternating current network analysers

were used for system studies. These network

analysers contained scaled-down versions of the

network components, such as power sources,

cables, transmission lines, and loads, that were

interconnected using flexible cords to represent

the system being modelled. Although limited in

scope and complexity, the network analysers were

used to study power flows and voltage profiles

under steady-state and transient conditions.

The next stage in the evolution of system study

software programs was the use from the late

1940s of digital computers to conduct system

studies. These programs were initially limited

in scope due to the programming methods

used (punched-card calculators). However, the

availability of large-scale digital computers

from the mid-1950s gave a boost to the use

of computer programs for system studies.

Although these programs originally required

mainframe computing power and specialised

programming techniques, the growth in the

computing power of desktop PCs and laptops

has seen these programs become an essential

tool for the electrical engineer. Current system

study programs offer flexible and easy-to-use

techniques for system modelling, analysis,

and presentation.

One of the more commonly used system study

software programs is Operation Technology,

Inc.s (OTIs) Electrical Transient Analysis

Program (ETAP), which has been qualifiedas a Bechtel standard application program

(BSAP). The offline simulation modules of

ETAP 6.0.0, the most current release at the time

of project execution, were used to conduct the

JERP power system studies.

The sheer size of

the JERP and DTA

electrical networks,

the extensive

power distribution

network within

the JERP and DTA

plants, and the

crucial need to

ensure reliability

of the power

supply under

all operating

conditions make

it important to

conduct reliable

and accurate

system studies.




MODEL OF THE JERP ELECTRICAL NETWORK

The various Bechtel and third-partyengineering centres prepared models of theelectrical networks for the individual JERP units.

The London core group integrated these various

submodels into a composite model of the overall

plant electrical network.

It was necessary to ensure that all the engineering

centres used uniform modelling principles to

prepare the individual models, to speed the

process of integrating them. The London core

group issued specific discipline work instructions

(DWIs) to the engineering centres and held a

series of conferences to explain the modelling

principles to be followed to ensure uniformity.

These work instructions covered key points such

as model structure, division of responsibility

for preparing and using the model, key data

required, instructions for dealing with cases of

incomplete/missing data related to network or

equipment required for the model, and use of

library data (accompanied by a common librarydatabase to be used to populate the model).

The major items modelled were the GTGs/STGs

along with their control systems (governors,

exciters, and power system stabilisers), plant

loads, and interconnecting power cables. The

modelling of certain complex portions of the

GTG control system required software such

as Simulink, a specialised program used to

model and simulate dynamic control systems.

OTI constructed the models, which were later

integrated with the overall model.

ELECTRICAL SYSTEM STUDIES AND

STUDY CASES

Awide range of system studies can beconducted on electrical networks to studythe behaviour of the system under steady-state

conditions as well as conditions in which it is

subjected to disturbances in normal operation

(e.g., step loading or load sharing amongst

generators) or unplanned events (e.g., electrical

fault, generators tripping). Because it is not

possible to analyse every expected operating

condition, it is very important to select the studycases whose results can be used to predict the

system behaviour under all operating conditions.

As a result, the studies are usually conducted on

the most onerous conditions expected during the

refinery operation.

The following system studies were carried out

to analyse the behaviour of the JERP and DTA

electrical networks. In line with the specification

requirement for the JERP, the engineering centres

used International Electrotechnical Commission

(IEC) standards as the basis for evaluating the

results of all studies except harmonic analysis.

Load Flow Analysis

Once the refinery is commissioned and fully

operational, the electrical system is expected to

operate in a stable condition. Load flow analysis

is a steady-state analysis that calculates the active

and reactive power flows through each elementof the network and the voltage profile at the

networks various buses. A balanced load flow

analysis is adequate because the vast majority

of loads in the refinery are inherently balanced

(e.g., three-phase motors).

Load flow analyses help identify any abnormal

system conditions during steady-state operation

that can be harmful for the system in the long

run. They also provide the initial basis for other

detailed analyses such as motor starting and

transient stability. It is also to be noted that the

results of the load flow analysis affect theseother analyses. For example, an electrical system

operating under steady-state conditions is more

likely to satisfactorily survive a transient event

such as the step-loading or tripping of one of

the operating generators if its initial operating

conditions are favourable (e.g., voltages within

limits, sufficient margin in the loading of various

network elements).

Some of the main parameters examined in a

load flow analysis are presence of overvoltage

or undervoltage at any point in the electrical

network, overloading of any network element,and very low system power factor.

To study system behaviour at the JERP under all

expected operating conditions, the London core

group carried out load flow analyses under these

three sets of conditions:

Normal system configuration, i.e., with

redundant power feeds, where available,

to various plant switchboards that simulate

the normal operating condition of the

electrical network

Loss of redundant power feed, i.e., with single

power feed to the various plant switchboards

(This condition of single-ended operation

can occur in the electrical network under a

contingency like loss of plant transformers.)

No-load conditions (This study case

was selected to assist in identifying any

dangerous overvoltage that may occur

when the network is operating under

no-load or lightly loaded conditions [e.g.,

plant startup conditions]).

It is

very important

to select the

study cases

whose results

can be used

to predict the

system behaviou

under all operatin

conditions.




The results of these analyses revealed some

instances in which the bus voltages exceeded

the acceptable limits. The London core group

recommended that the tap settings of the

upstream transformers associated with these

switchboard buses be changed to bring the

voltages within the specified limits. The core

group also highlighted cases of potential

overloading of transformers under loss of

redundant power feed (second case) forobservation during actual plant operation.

Short Circuit Analysis

A short circuit condition imposes the most

onerous short-time duty on the system electrical

equipment. This fault condition arises as a result

of insulation failure in the equipment or wrong

operation of the equipment (e.g., closing onto an

existing fault or closing circuit breakers when the

associated earth switch is closed), leading to the

flow of uncontrolled high currents and severely

unbalanced conditions in the electrical system.

The four main types of short circuits are:

Three-phase short circuit with or without

earthing (This is usually the most severe

short circuit condition.)

Line-to-earth (single-phase-to-earth) fault

(In certain circumstances, the short circuit

current for a line-to-earth fault can exceed

the three-phase short circuit current.)

Line-to-line (phase-to-phase) fault

Double line-to-earth fault

The electrical equipment has to be rated

for the short circuit level of the system, which

basically requires all of the following conditions

to be met:

The electrical equipment must be able to

withstand the short circuit current until

the protective equipment (relays) detects

the fault and it is cleared by opening circuit

breakers (i.e., thermal withstand short

circuit current). The IEC standards specify a

standard withstand duration of 1 second or

3 seconds. The JERP used switchgear rated

for 1-second withstand time.

The circuit breakers must be suitable to

interrupt the flow of the short circuit current

(i.e., breaking duty).

The circuit breakers must be suitable to close

onto an existing fault (i.e., making duty).

Additionally, the protective system of the

network has to be set to enable reliable detection

of any short circuit condition (minimum and

maximum short circuit conditions).

The calculation of the short circuit current

for these conditions is made more complex

by the behaviour of the short circuit current

immediately after the fault. Depending on

the network characteristics, behaviour of the

generators in the network, and the exact instant

of the fault, the short circuit current may contain

significant amounts of transient alternating

and direct current components, which decay to

zero over time, depending on the characteristicsof the network and the rotating machines. It

is very difficult to account for the effects of

these phenomena through manual calculation

methods. This is particularly true because the

presence of a large direct current component in

the short circuit current imposes a very stringent

breaking duty on the circuit breakers, since a

natural current zero may not be achieved.

The results of the short circuit analysis calculated

the following various components of the short

circuit current at each bus:

ipPeak current in the first cycle after theshort circuit

IdcDirect current component at the instant

the circuit breaker opened

Ib sym and Ib asymSymmetrical and

asymmetrical root mean square currents at

the instant the circuit breaker opened

IthThermal withstand short circuit current

for 1-second rating

These results were cross-checked with the

equipment ratings to verify that the equipment

short-time ratings were suitable for the shortlevel of the system.

Stability Analysis

It is relevant to note the concept of stability as

defined in standards such as Standard 399 of

The Institute of Electrical and Electronics

Engineers, Incorporated (IEEE). [1] IEEE 399

states that a system (containing two or more

synchronous machines) is stable, under a

specified set of conditions, if, when subjected

to one or more bounded disturbances (less

than infinite magnitude), the resulting system

response(s) are bounded.

System stability requirements can be generally

categorised into steady-state stability, dynamic

stability, and transient stability. [2]

Steady-State Stability Analysis

Steady-state stability is the ability of the system

to remain stable under slow changes in system

loading. The power transfer between two

synchronous machines (generator G and motor

A short circuit

condition imposes

the most onerous

short-time duty

on the

system electrical

equipment.




Mrefer to Figure 3) with internal voltages of

EGand EM, respectively, and a phase angle of

between them is represented in Equation 1: [1]

(1)

The maximum power that can be transferred

occurs when = 90 degrees, per Equation 2:

(2)

For particular values of EG and EM, the

machines lose synchronism with each other if

the steady-state limit of Pmax is exceeded. The

steady-state stability study determines the

maximum value of machine loading that is

possible without losing synchronism as the

system loading is increased gradually.

Dynamic Stability Analysis

A steady-state scenario never exists in actual

operation, however. Rather, the state of the

electrical system can be considered dynamic,

whereby small, random changes in the system

load constantly occur, followed by actions of the

generator governor, exciter, and power system

stabiliser to adjust the output of the machine and

match the load requirement. The system can be

considered stable if the responses to these small,

random disturbances are bounded and damped

to an acceptable limit within a reasonable time.

The dynamic stability analysis of any system

is only practical through specialised computer

programs such as ETAP.

Transient Stability Analysis

Transient stability is the ability of the system to

withstand sudden changes in generation, load,

or system characteristics (e.g., short circuits,

tripping of generators, switching in large

bulk loads) without a prolonged loss of

synchronism. [1] Traditionally, transient stability

analysis focused on the ability of the system

to remain in synchronism immediately after

the occurrence of the transient event (i.e.,

the first swing of the machines, generally

within 1 second of the event). Also, the

traditional transient stability analysis ignored

the action of the machine governor, exciter,

and automatic voltage regulator (AVR) because

they were slow-acting compared with the

duration of the analysis.

This approach to transient stability analysis

has been modified in recent times since the

advent of governors, exciters, and AVRs basedon fast-acting control systems. It has also been

seen that different sections of an interconnected

network may respond at different times to a

transient event that sometimes may be outside

the traditional 1 second window for transient

analysis. Also, the behaviour of different sections

of the network may be different for the same

transient event. Hence, to verify whether system

stability is retained, the transient stability

analysis needs to be carried out for a longer

duration (preferably over a range of transient

events having varying severities and durations).This kind of analysis was not possible in earlier

years due to the high complexity of modelling

and limited computing power, but today such

an analysis can be performed because of the

availability of practically unlimited computing

power on desktop and laptop computers,

coupled with specialised computer programs

such as ETAP. Hence, the dividing line between

dynamic stability analysis and transient stability

analysis has been virtually eliminated.

A range of stability analyses was carried out

on the JERP refinery system. They covered the

operation of the JERP electrical network while

in a standalone condition as well as in parallel

operation with the DTA electrical network. The

stability analyses can be broadly classified into

the following categories.

Transient and Extended Dynamic

Stability Analysis

Fault withstand study: This study entailed

simulation of single-phase and three-phase

faults at various locations in the electrical

network. It analysed the behaviour of the

power system in the pre-fault stage, during

the fault, and after the fault was cleared bythe systems protective devices.

Load throw-off study: A load throw-off

condition can cause the machines to over

speed. Temporary overvoltage conditions

can also occur in the system. Hence, the

behaviour of the electrical system was

studied for all probable cases of load throw-

off in which a substantial portion of the

operating load was suddenly tripped.

Stability analyse

carried out

on the JERP

refinery system

covered the

operation of

the JERP electric

network in

standalone condit

as well as in

parallel operatio

with the DTA

electrical networ

G

EG

EM

jX M

Figure 3. Power Transfer Between Machines

sinM=E GE

PX

M=

E GE

P Xmax


http:///reader/full/electrical-system-studies-for-large-projects-executed-at-multipl


Load sharing on tripping of tie circuit

breaker between JERP and DTA electrical

systems: The behaviours of the JERP and

DTA electrical systems were studied on

disconnection of the JERPDTA tie line.

This study was carried out for various

combinations of operating GTGs/STGs in

the JERP and DTA electrical systems (i.e.,

various power flow scenarios between JERP

and DTA systems). When system stabilitycould not be achieved by load sharing

amongst the operating GTGs/STGs in each

individual network, load shedding was

simulated to try to achieve a stable system.

Contingency Analysis

Load sharing on tripping of JERP/DTA

GTG: The behaviour of the power system

was studied when one or more of the

operating GTGs/STGs tripped, causing a

loss of generation. This study was carried

out for various combinations of operating

GTGs/STGs in the JERP and DTA electricalsystems. When system stability could not

be achieved by load sharing amongst the

remaining operating GTGs/STGs, load

shedding was simulated to try to achieve

a stable system.

Operational Analysis

Step-load addition study:A sudden addition

of load on operating machines can cause loss

of stability. A step-load addition scenario can

occur in a variety of ways in an electrical

system, the most probable being loss of one

of the operating machines, which can cause

a sudden increase in the load demand on the

other operating GTGs/STGs. The behaviour

of the system was studied for all probable

scenarios of step-load addition.

The results of these stability analyses helped

define the limits of safe operation of the

power system under various generation/

load scenarios.

Motor-Starting Study

At the instant of starting, synchronous and

induction motors draw a starting current that is

several times the full-load current of the motor.

In the absence of assisted starting, this starting

current is typically between 600% and 720% of

the normal full-load current. This high current

causes a voltage drop in the upstream electrical

network, as well as in the motor feeder cable.

The effects of this voltage drop include:

The combined voltage drop in the supply

network and the motor cable reduces the

voltage available at the motor terminals

during the starting period. Because the motor

torque is directly proportional to the square

of the applied voltage, excessive voltage

drops can mean that insufficient torque is

available to accelerate the motor in the face

of the load torque requirement, leading to

very long starting times or a failure to start.

The voltage drop at the switchboard buses

can affect the other operating loads, mainlyin the form of nuisance tripping of other

loads on the network (e.g., voltage-sensitive

loads or contactor-fed loads where the

control voltage for the contactor is derived

from the switchgear bus). There can also be

cases in which the reduction in the terminal

voltage for the operating motors causes the

motor-torque curve to shift downwards.

This reduction in the motor torque can cause

the running motors to stall.

For the other operating loads, a reduction

in the motor terminal voltages causes the

current drawn by the motors to increase

as they strive to produce the power the

process demands of them. This condition

exacerbates the voltage problem because the

increased current gives rise to an increased

voltage drop in the system.

Depending on the size of the motor being started

and the generating capacity available, motor

starting can impose a very high short-term

demand on the operating generators.

Studying motor starting can help identify these

voltage-drop-related problems at the designstage. Usually, the worst-case motor-starting

scenario is the starting of the highest-rated motor

(or the highest-rated standby motor) at each

voltage level with the operating load of the

plant as the standing load. However, other

worst-case scenarios may require evaluation in

certain situations:

Motors with an unusually long supply cable

circuit

Motors fed from a weak power supply (e.g.,

starting on emergency power supplied from

a diesel generator set of limited rating) Simultaneous starting of a group of motors

In the event of an unfavourable outcome from

the motor starting study, various improvement

measures are available, including:

Specifying that motors be designed with

a lower value of starting current, which is

particularly feasible for the larger medium-

voltage (MV) motors

The results

of stability

analyses help

define the limits of

safe operation of

the power system

under various

generation/load

scenarios.


http:///reader/full/electrical-system-studies-for-large-projects-executed-at-multipl


Specifying lower impedance for the upstream

transformer after verifying the suitability

through a short circuit analysis

Starting the largest motors in the network

with a reduced standing load

Using larger cable sizes for the motor feeder

to improve the motor terminal voltage

Providing assisted starting, if required, for

the larger HV/MV motors instead of direct

on-line starting

Using motor unit transformers to feed power

to large MV motors, which ensures that the

effect of the voltage drop on the rest of the

electrical system is reduced

Increasing upstream bus voltage temporarily

(e.g., through on-load tap changers [OLTCs])

before starting large motors

Various motor starting scenarios were modelled

for the JERP, and the results indicated that the

motors could be started satisfactorily.

Transformer Energisation Studies

The inrush phenomenon in transformers can

inflict a very severe, albeit short-term, effect

on the voltage profile at the refinerys various

switchboard buses. The inrush current taken

by the transformers is due to the behaviour of

the magnetic circuit. The constant flux linkage

theorem states that the magnetic flux in an

inductive circuit cannot change suddenly. Hence,

the magnetic flux immediately after energisation

(t = 0+) should be equal to the magnetic flux

immediately before energisation (t= 0).

When a transformer is switched on, the

magnetic flux immediately after energisation

depends on the following factors that are

essentially random:

The point on the sine wave voltage

waveform where the transformer is switched

on, which decides the amount and direction

of the flux requirement

The amount and direction of the remnant

flux, which depends on the point on the

sine wave voltage waveform where the

transformer was last switched off

As explained by the constant flux linkage

theorem, the magnetic flux after energisation

retains a sinusoidal shape that is biased by the

flux requirement at the point of energisation

and the remnant flux. Depending on the design

of the transformer, this condition can cause the

flux requirement to be well above the knee-point

voltage on the transformer magnetising curve,

leading to very high excitation currents that may

reach large multiples of the full-load current

of the transformer. The inrush current decays

substantially within a few cycles.

Although modern protection systems are well-

equipped with algorithms to distinguish the

transformer inrush current from the short circuit,

the inrush current still causes a severe voltage

dip at the other switchboards in the network.

This voltage dip can cause nuisance tripping of

other network loads.

The London core group studied various probable

transformer energisation scenarios (including

group energisation of transformers) to confirm

that the network voltages recover without

tripping system operating loads.

Because ETAP could not directly model

transformer behaviour under inrush conditions,

the impact of the transformer inrush current

was simulated by switching a series of low

power-factor loads in and out at intervals of

5 milliseconds. The load values were selectedas exponentially decreasing to simulate the

inrush current decay. To ensure accurate

modelling, the inrush current data was based on

transformer manufacturers data supplemented

by the measurements recorded during site

testing and commissioning.

The results of the transformer energisation

studies established the network conditions

under which the JERP transformers can be safely

energised. This finding was crucial because in

certain scenarios, the JERP main transformers

were to be energised from the DTA electrical

system and any disruption to the DTA operating

loads could lead to tripping of the DTA refinery.

Harmonic Analysis

The amount of periodic waveform distortion

present in the power supply is one of the most

important criteria for measuring power quality.

Periodic waveform distortion is characterised by

the presence of harmonics and interharmonics

in the power supply.

Harmonics are sinusoidal voltages and currents

with frequencies that are integral multiplesof the fundamental frequency of the system.

Interharmonics are sinusoidal voltages and

currents with frequencies that are non-integral

multiples of the fundamental frequency of

the system.

For the JERP:

f1= fundamental frequency = 50 Hz

fharmonic= nxf1(n= 2, 3, 4, ) (3)

finterharmonic= mxf1(m> 0 and non-integral) (4)

The results of

the transformer

energisation

studies

established the

network condition

under which the

JERP transformer

can be safely

energised.


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Periodic waveform distortion is caused by non-

linear loads, which are loads that do not draw a

sinusoidal current when excited by a sinusoidal

voltage. The non-linear loads act as sources

of harmonic currents in the power system,

which cause a voltage distortion at the various

buses because of the harmonic voltage drops

across the impedances of the network. Hence,

the quantum of voltage distortion depends on

the harmonic currents injected into the systemand the impedance of the system (the voltage

distortion in a weak system, characterised by a

high system impedance, is higher).

The presence of excessive harmonics can lead to

premature aging of electrical insulation due to

dielectric thermal or voltage stress in equipment

such as motors, cables, and transformers. Other

possible effects of harmonics include reduced

power factors, incorrect operation of protection

systems, interference with communication

networks, and occurrence of series and parallel

resonant conditions that can lead to excessivecurrents and voltages in the system. Hence, it

is important to carry out a harmonic analysis

wherever the non-linear load forms a significant

portion of the total load.

The JERP electrical network includes a large

number of harmonic-generating loads, mainly

22 kW and 37 kW low-voltage (LV) variable-speed

drives (VSDs) that act as sources of harmonic

currents. The London core group carried out a

harmonic analysis of the JERP electrical network

to verify that the voltage distortion at the

networks various switchboards caused by these

harmonic-generating loads is within the limits

specified in Table 11-1 of IEEE 519 (Table 1).

All harmonic-generating process loads were

modelled in the ETAP model used for harmonic

analysis. The power sources in the JERP network

(GTGs and STGs) were assumed to have no

harmonic distortion. As a worst-case scenario,

the harmonic analysis was carried out with

the minimum generation configuration under

normal operating conditions because this

configuration corresponds to the maximum

system impedance.

The results of the harmonic analysis highlighted

the switchboards whose power quality needs to

be monitored further during plant operation.The London core group recommended that any

corrective action (such as adding harmonic filters)

to reduce the harmonics at these JERP plant

switchboards be undertaken after measuring the

actual harmonic levels at the various 6.6 kV/415 V

switchboards when the plant is operating.

CONCLUSIONS

The results of the system studies of the JERPelectrical network verified the adequacy ofthe ratings for the systems major equipment.The results also helped determine the conditions

for satisfactory and reliable system operation

and highlighted any operational restrictions

required for safe operation.

LESSONS LEARNT

Conducting electrical system studies ona complex project such as the JERP andworking with execution centres and non-Bechtel

engineering contractors located across the globe

have highlighted three major areas, discussedbelow, where existing Bechtel project procedures

can be improved or fine-tuned to increase

operating efficiency.

Distributing Work

The work distribution amongst the execution

centres, non-Bechtel engineering contractors,

and the London core group for carrying out

ETAP modelling must be clearly defined

through proper DWIs. Amongst other things,

the instructions should include the structure of

the model, the extent of modelling required, the

data required to be populated in the model, themethodology of populating the data, the use of

assumptions and approximations, the common

library to be used to populate the standard data

in the model, and the tests that must be carried

out to ensure that sections of the model meet

all requirements before they are transferred to

the London core group for integration into the

overall model.

Table 1. Harmoni c Limits as Define d by IEE E 519

Rated

Bus Voltage

Individual

Harmonic

Distortion

(IHD), %

Total

Harmonic

Distortion

(THD), %

69 kV and less 3 5

Greater than 69 kV

up to 161 kV1.5 2.5

161 kV and greater 1 1.5

For shorter periods, during startups or unusual conditions,

these limits may be exceeded by 50%.

Conducting

system studies on

a complex project

such as the JERP

has highlighted

areas where

existing Bechtel

procedures

can be improved

or fine-tuned.


http:///reader/full/electrical-system-studies-for-large-projects-executed-at-multiple- December 2009 Volume 2, Number 1

Handling Model Revisions

Proper work procedures for handling ETAP

model revisions need to be furnished to the

execution centres/non-Bechtel engineering

contractors so that the London core group

can integrate the revised models or revised

sections of same into the overall model without

causing rework or loss of data.

Identifying Study CasesTo increase engineering efficiency, it is essential

to optimise the types of studies to be conducted

on a project and the number of cases to be

analysed for each study. At the same time,

it is essential to ensure that the number

and types of study cases allow the engineer

to determine system behaviour under all

operating conditions.

This opportunity is particularly valuable because

the projects that Bechtel is bound to take up

(in the role of engineering contractor or as a

project management consultant or member ofa project management team) are more likely

to be of the scale of the JERP, and it is highly

likely that the engineering work for such

projects will be divided amongst various

execution centres.

ACKNOWLEDGMENTS

The author wishes to express his gratitude to

R.H. Buckle (chief engineer), R.D. Hibbett (lead

electrical engineerJERP), and David Hulme

(project engineering manager) for their support

and guidance during execution of the JERP

system studies. The author also wishes to

thank V. Shanbhag, B.S. Venkateswar, and

M.A. Mujawar from Reliance Industries for

their support and encouragement.

TRADEMARKS

ETAP is a registered trademark of Operation

Technology, Inc.

IEEE is a registered trademark of The Institute

of Electrical and Electronics Engineers,Incorporated.

Merox is a trademark owned by UOP LLC,

a Honeywell Company.

Simulink is a registered trademark of The

MathWorks, Inc.

REFERENCES

[1] IEEE 399-1997, IEEE Recommended Practicefor Industrial and Commercial Power SystemsAnalysis, The Institute of Electrical andElectronics Engineers, Inc., 1998, pp. 79,209214, access via http://standards.ieee.org/colorbooks/sampler/Brownbook.pdf.

[2] D.P. Kothari and I.J. Nagrath, Power SystemEngineering, 2nd Edition, Tata McGraw-HillPublishing Company Ltd., 2008, Chapter 12,pp. 558560, access via http://highered.mcgraw-hill.com/sites/0070647917/information_center_view0/.

ADDITIONAL READING

Additional information sources used to develop

this paper include:

P.M. Anderson and A.A. Fouad, PowerSystem Control and Stability, 2nd Edition,IEEE Press Series on Power Engineering,

John Wiley & Sons, Inc., 2003, pp. 510,access via http://www.amazon.com/Power-System-Control-Stability-Engineering/

dp/0471238627#noop. Systems, Controls, Embedded Systems, Energy,

and Machines, The Electrical EngineeringHandbook, 3rd Edition, Richard C. Dorf, ed.,Chapter 5, 2006, CRC Press/Taylor & FrancisGroup, LLC, Boca Raton, FL, pp. 5-1 5-3,access via http://www.amazon.com/Controls-Embedded-Machines-Electrical-Engineering/dp/0849373476.

BIOGRAPHY

Rajesh Narayan Athiyarathis a senior electrical engineer

in Bechtels OG&C GlobalBusiness Unit. He has 16 yearsof experience in engineeringoil and gas, petrochemical,and GTG power plant projectsworldwide. During his 3 yearswith Bechtel OG&C (London),Rajesh has contributed to

system studies and relay coordination studieson the JERP and to FEED for the Ruwais refineryexpansion project. He has also acted as theresponsible engineer for the energy managementsystem and load shedding system on the JERP.

Rajesh received a performance award for his work onthe JERP system studies.

Rajesh holds a BE from Mumbai University, India,and is a chartered electrical engineer (CEng,member of Institution of Engineer ing and Technology[MIET], UK). He is a Six Sigma Yellow Belt.


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Electrical System Studies for Large Projects Executed at Multiple Engineering Centres