hv industrial network design
TRANSCRIPT
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n 169
HV industrialnetwork design
Georges THOMASSET
Graduated as an engineer from theIEG (Grenoble Electrical EngineeringInstitute) in 1971.Since then, he has been designingcomplex industrial networks in theframework of Merlin Gerincompany's Technical Division.
After having headed the "MediumVoltage public distribution andhydroelectric installations"engineering department, he hasbeen in charge of the engineeringsection of the Contractingdepartment's industrial unit since1984.
E/CT169 first issued october 1994
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lexicon
flicker: periodic fluctuations the lightoutput of lamps.
HTA and HTB: categories of mediumvoltage defined by a French decreedated 14 November 1988.
Voltage levels are classified by differentdecrees, standards and other particularspecifications such as those of utilities.AC voltages greater than 1000 V aredefined by:s the French decree of 14 November1988 which defines two categories ofvoltage:HTA = 1 kV < U 50 kV,HTB = U > 50 kV.s CENELEC (European committee forelectrotechnical standardisation), in acircular dated 27 July 1992, specifies:MV = 1 kV < U 35 kV,HV = U > 35 kV.
s the IEC publication sets forth thehighest voltage ranges for equipment:
protection system used, the choice ofdevices and device settings.
network structure: overall networkarrangement, often represented in asingle-line diagram, which indicates the
relative arrangement (interconnections,separation of circuits, etc.) of thedifferent sources and loads.
plant: grouping (in a location) ofseveral energy consumers.
private network: electrical networkwhich supplies power to one or moreuser sites (plants) which generally havethe same owner.
utility supply: electrical network whichbelongs to the national or local energydistributor and serves severalindependent consumers.
sensitive loads: loads for which noabsence of power supply is tolerated.
range A = 1 kV < U < 52 kV,range B = 52 kV U < 300 kV,range C = U 300 kV.A revision is pending, which will includeonly two ranges:range I = 1 kV < U 245 kV,range II = U 245 kV.s the French national utilities EDF nowuses the classification given in thedecree cited above.
network dynamic stability: thecapacity of a network, which includesseveral synchronous machines, toreturn to normal operation after asudden disturbance that has caused atemporary change (i.e. short-circuit) orpermanent change (line opening) in thenetwork configuration.
network protection plan: overall
organization of electricalprotection gear, including: the
appendices
appendix 1: extension of an hypotheses p. 19
approximate calculation of a p. 19
current limiting reactor
appendix 2: computerized means calculation software programs p. 20
expert system for evaluating p. 20
electrical network design quality
appendix 3: general principle of compensation p. 20
appendix 4: choice of the earthing system for a HV industrial network p. 21appendix 5: network voltage drops p. 22
appendix 6: stages in industrial network design p. 22
appendix 7: bibliography p. 24
existing industrial network
used for network analyses
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Faced with increasingly fiercecompetition, industrialists must employhighly rigorous management and theirproduction facilities must have a high
level of availability.Electrical networks supply the energyrequired to run the production facilities.The provision of a continuous powersupply to loads is strived for from thestart of the network design, in particularduring preliminary choices in the single-line diagram.Reductions in electrical installation andoperating costs, together with reliablefailure-free operation, are vitalconditions for profitability. Thistechnical and economic optimizationcalls for detailed and global preliminary
analyses, which include:s specific needs and constraintsrelated to the type of industry,s integration of the limits andconstraints of the public distributionnetwork,s standards and local practices,s particularities of the operatingpersonnel, facilities manager andmaintenance personnel.The scope of this study is limited to theanalyses involved in the design of HighVoltage ("HTA" and "HTB") high powerindustrial installations which have the
following main characteristics:s installed capacity in the 10 MVArange,s autonomous electrical energyproduction (when applicable),s power supplied by a nationaltransmission or distribution network( 20 kV),s private Medium Voltage distribution.
HV industrial network design
contents
1. needs and main constraints to be met needs to be met p. 4
main constraints p. 5
2. main rules of industrial network load survey, load and diversity p. 7
design factors
choice of voltages p. 7reactive power compensation p. 7
backup and replacement sources p. 7
autonomous electrical energy p. 7production
division of sources p. 7
overall electrical diagram p. 8
3. validation and technical and choice of the earthing system p. 9
feeder definition p. 9
insulation coordination analysis p. 9
protection system definition p. 9
short-circuit currents calculation p. 10
calculation of voltage fluctuations p. 11under normal and disturbedoperating conditions
choice of motor starting method p. 11
network dynamic stability p. 12
synthesis p. 13
standard network structures p. 14
concrete example of a structure p. 16
choice of equipment p. 16
optimal operation p. 16
5. conclusion p. 18
appendices see page on left
economic optimization
4. choice of optimal network structureand operation
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sinitial investment cost,soperating and maintenance costs,scost of production losses associatedwith the network design and protection
plan (protection system used, choice of
devices and settings).
Optimization of electrical energy
When a plant includes electrical energy
generators, it is necessary to manage
the energy supplied by the utility and
the energy produced locally in the best
manner possible.A control and monitoring system makes
it possible to optimize the cost of plant
power consumption in accordance with:
sthe contract with the utility (billingrates according to the time, day and
season);
sthe availability of the plantgenerators;
sindustrial process requirements.Network changes and future
extensions
When designing an industrial network,
it is of primary importance to make acareful assessment of the future
development of the plants, especially
when extensions are foreseeable.
Changes that are liable or due to be
made in the future should be taken intoaccount:sin sizing the main power supplycomponents (cables, transformers,switching devices),sin designing the distribution diagram,sin calculating the areas to be setaside for electrical rooms.This forward planning will result in
increasingly flexible energymanagement.
Network upgrading
Electrical energy consumptionincreases as extensions are made tomeet the needs of new types ofmanufacturing and ever more powerfulmachines. This makes it mandatory toupgrade and/or restructure the network.
Greater care must be taken in networkupgrading analyses than in analyses ofnew installations since additionalconstraints are involved, i.e.:
1. needs and main constraints to be met
sovercurrent (short-circuits andoverloads),
sovervoltage.The solutions that are used must
ensure at the least the following:
squick fault clearance and continuedpower supply to the fault-free sections
of the network (discrimination),
ssupply of information on the type ofinitial fault, for quick servicing.
Continuous power supply to loads
Continuous power supply to the loadsis necessary for the following reasons:
ssafety of people, e.g. lighting;ssustained production performance,e.g. glass wire drawing;
sproductivity;soperating convenience, e.g.simplified machine or workshop restart
procedure.
Loads are divided into three groups
according to their operating
requirements:
s
"normal" loads,
s"essential" loads,s"sensitive" loads for which noabsence of power supply is tolerated.
Network operating ease
In order to carry out their tasks safely
and reliably, network facilities operators
need the following:
sa network that is easy to operate inorder to act correctly in the event of a
problem or a manoeuvre;
ssufficiently sized switchgear andequipment, which require littlemaintenance and are easy to repair
(maintainability);sefficient means of control andmonitoring which facilitate remotecontrol of the network by real timecentralization in a single location of allthe information relating to the state ofthe "electrical process", under normaland disturbed operating conditions.
Minimum electrical installation cost
The minimum electrical installation costdoes not necessarily mean theminimum initial cost, but the sum ofthree costs:
Industrial electrical networks must
supply power to all the plant loads, at
optimal investment, operating and loss
of production costs, taking into account:
sthe needs to be met:sssafety of people,sssafety of property,sscontinuous power supply,ssnetwork operating ease,ssminimum installation cost,ssoptimization of electrical energy(cost / quality),
ssnetwork changes and futureextensions,
ssnetwork upgrading;sconstraints linked to:ssthe industrial process,ssthe electrical process,ssthe utility,ssthe climate and geography of the site,ssstandards, regulations and localpractices.
It is clear that not all the needs can be
optimally met, which means that the
network designer must endeavour to
find the best compromise.
needs to be met
Safety of people
Even if not all countries have standards
and rules, certain obvious principles
must be adhered to:
sprohibiting access to energized parts(protection against direct contact),
sa system to protect against the rise inpotential of metal structures (protectionagainst indirect contact),
sprohibiting on-load line disconnectingswitch operations,sprohibiting earthing of live conductorssquick fault clearance.Safety of property
Electrical installations should not besubmitted to stress that they are notable to withstand. The choice ofmaterials and equipment is therefore ofprime importance. Two electricalphenomena are to be considered inorder to avoid fire and to limitdestructive effects:
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s insufficient electrodynamic anddielectric strength of existing devices orequipment;
s capacity to supply power to large
loads (starting current, dynamicstability, etc.);
s area and height of existing electricalrooms that cannot be changed;
s imposed geographical location ofequipment and loads.
Example of upgrading with the additionof a new transformer: the installation ofa three-phase reactor between the oldinstallation and the transformer makesit possible to limit the increase in short-circuit current and therefore to continueusing the existing equipment (see
appendix 1).Quality of analyses
The design analysis and detailedanalysis must be carried outmethodically and precisely. ISO 9001certification entails setting upprocedures which describe theessential steps involved in design anddetailed analyses in the view ofmastering these analyses and meetingthe clients' needs in the best waypossible.
Merlin Gerin's Contracting Departmentwas granted ISO 9001 certification
in 1992.
main constraints
Constraints linked to the industrialprocess
Apart from the need for a high level ofcontinuous power supply in certainindustrial processes, those veryprocesses also impose constraints:s power supply and, in particular,starting of very large motors whichdrive crushers, grinders, fans, pumpsand conveyor belts (a major cause ofvoltage drops during starting);s power supply to arc furnaces, the arcoften being unstable, which causesbrief but repeated unbalanced voltagedrops (resulting in flicker), and cancreate harmonics;s power supply to high capacityelectronic devices (rectifiers, thyristors,etc.) which create major voltage wavedeformations (harmonics) in thenetwork and lower the power factor;this is the case for potlines, DC electricfurnaces and variable speed drives.
In addition, some industrial processes
pollute their environment. They produce
substances (dust, gas) that are
sometimes corrosive and can jam
mechanisms or degrade the
performance of electrical devices
(e.g. dielectric strength) or even cause
explosions in the presence of electric
arcs.
Constraints linked to the electrical
process
During the analysis, it is necessary to
take into consideration various
"electrotechnical" conditions that all
electrical networks must fulfil, in
particular:
s limitation of short-circuit currents and
their durations starting or restarting of large motors
without excessive voltage drops,
s stability of alternators after a
disturbance has occurred.
Constraints imposed by the utility
electrical power distribution network
s short-circuit capacity
The short-circuit capacity of the
upstream network supplying the private
network is a decisive element when
choosing:
ss the structure of the privatedistribution network,ss the maximum load,ss particular loads that are sensitive tovoltage drops.
s utility earthing system
The same type of earthing system, as
used by the utility, is often used for the
private network but it is sometimes not
compatible with particular loads. Insuch cases, it is necessary to create a
separate system with:
ss a type of electrical protection forphase-to-earth faults,
ssa special network operating method(fault tracking and/or clearing by
operating personnel in the case of
isolated neutral systems).
s momentary dips or substantialtransient single-phase or multiphase
voltage drops.
The presence of such phenomena can
cause disturbances, or even production
shutdowns and machine destruction.
These phenomena may result in:
ss operating errors and/or the loss ofdata of the industrial process computer
system (central computers, PLCs), andmanagement control system as well asscientific calculation errors.ss mechanical destruction of motors,motor coupling and/or the drivenmachines. This occurs duringmomentary dips (re-energizing ofmotors that are still in motion), sincecoupling may take place with phaseopposition between the mains voltageand the residual voltage generated bythe motor, this resultat in motorcurrents being very high , i.e. 4 to5 times the rated current, and causeexcessive electrodynamic stress.
s overvoltage of external origin, inparticular lightning strokes (refer to"Cahier Technique" n168).
s value and quality of the supplyvoltage:ss the value of the supply voltagedictates to a certain extent theorganization of private networkvoltages. If the power supply is in HighVoltage, it may be of benefit to use thatvoltage level for the plant's maindistribution system;ss the quality of the supply voltage.Various voltage fluctuations may botheror even prevent production equipmentoperation. The chart in figure 1 showssuch faults, the causes and
consequences, as well as the mainremedies.
Remarks:In terms of supply voltage quality,frequency deviations must bewithin 2% and harmonic voltagesmust be less than 3%.
A major European national utility hasinstalled more than 2000 recordingdevices on the power supply system formajor industrial sites in order to assessthe quality of the electrical power beingsupplied.The devices measure:s rms voltage and current;s active and reactive power,s short and long power outages,s voltage dips,s harmonic voltages and currents,s voltage unbalance.They also detect 175 and 188 Hzremote control signals.
Climatic and geographicalconstraintsIn order to determine the bestspecifications for equipment anddevices according to the types of
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voltage fluctuations a few % 5 to 25% 5 to 25% 100%
duration 1/100 to 1 s 0.5 to 20 s 20 s to 1 hour 0.1 to 0.3 s or 10 to 30 s
periodicity YES NO NO NO
causes presence of electric s starting of large motors absence of voltage fast and/or slow HV linearc furnace s single- or multiphase regulation in primary reclosing system
fault substations
consequences variations in incandescent s risk of network instability s motor overload momentary suspension oflighting (Flicker) s high risk of network power supply to all loads
instability
main remedies use static var s change the starting equip the transformers use shunt circuit breakerscompensation equipment method with on-load tap changers
s increase protectionplan rapidity
fig. 1: voltage fluctuations, causes ,consequences and main remedies
installations, it is necessary to know thefollowing:s average and maximum dailytemperatures,s relative humidity rate at maximumtemperature,s maximum wind speed,s presence of frost, ice, sand-bearingwinds,
s environment (corrosive atmosphere
or risk of explosion),
s altitude,
s frequency of lightning strokes in the
region,
s difficulties in gaining access to the
site (for the transport of materials and
also for maintenance).
Compliance with standards and localpracticesIn this regard, it is especially importantto be familiar with:s national and/or internationalswitchgear and installation standards,sregulations and rules which applyspecifically to the industrial complex,s local practices.
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network. The division of sources (seefig. 3) is used to isolate the fluctuatingloads and it offers two additionaladvantages as well:
s improved discrimination between theprotection devices, thereby increasingcontinuous power supply to the otherloads;s adaptation of the earthing system tosuit the loads.
overall electrical diagramThe network designer uses the variouselements described earlier in thischapter to establish a preliminarystructure, which he then fine-tunes inaccordance with the constraints of theindustrial site to obtain the "overall
single-line diagram" of the plant'selectrical distribution system. This is thestarting point for technical andeconomic network optimization.The next chapter presents the differentchoices that are available and thecalculations that are required to find theoptimal solution.
fig. 3: the division of sources is a means of separating the "fluctuating" loads from the other
loads.
M M M
normal
loads
"sensitive"
loads
"fluctuating" loads
lighting
computers
high power
motors
variable
speed motor
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3. validation and technical and economic optimization
choice of the earthingsystemStandards and regulations make
protection against direct and indirect
contact compulsory for all electrical
installations. In general, protection
functions automatically interrupt the
power supply (upon the first or second
phase-to-earth fault according to the
earthing system used). There are also
special protection devices designed for
specific situations. High Voltagenetwork earthing systems may be
chosen according to the criteria given inappendix 4.
However, it is very often of benefit to
use different earthing systems in the
same industrial network, each of which
provides a predominant advantage.
feeder definitionFeeders account for a large portion of
electrical installation investment. It is
therefore important, for safety and costreasons, to:
s choose the best type of equipment(cables),
s make the most accurate calculations
of the minimum cross section, whiletaking into account short-circuit and
starting currents, voltage drops, losses,
etc.
insulation coordinationanalysisThe coordination of insulation provides
the best technical and commercial
compromise in protecting people and
equipment against the overvoltage that
may occur in electrical installations,
whether such overvoltage originates
from the network or from lightning.
Three types of overvoltage may cause
flashovers and hence insulation faults,
with or without destruction of the
equipment:
s power frequency overvoltage (50 to
500 Hz),
s switching surges,
s atmospheric overvoltage (lightning
strokes).
Insulation coordination contributes to
obtaining greater electrical power
availability. Correct insulation
coordination entails:
s knowing the level of overvoltage that
is liable to occur in the network;
s specifying the desired degree of
performance or, more explicitly, the
acceptable insulation failure rate;
s installing protection devices that aresuited to both the network components
(insulation level) and the types of
overvoltage;
s choosing the various network
components based on their level of
overvoltage withstand, which must
meet the constraints described above.
"Cahier Technique" n 151 describes
voltage disturbances, ways of limiting
them and the measures set forth in the
standards to ensure dependable,
optimized electrical energy distribution,
thanks to insulation coordination.
protection system
definitionWhen a fault appears in an electrical
network, it may be detectedsimultaneously by several protection
devices situated in different areas ofthe network. The aim of selective
tripping is to isolate as quickly aspossible the section of the network that
is affected by the fault, and only that
section, leaving all the fault-free
sections of the network energized. Thenetwork designer must first of all selecta protection system that will enable him
to specify the most suitable protection
devices. He must also create a relay
tripping plan, which consists ofdetermining the appropriate current andtime delay settings to be used to obtain
satisfactory tripping discrimination.
Different discrimination techniques are
currently implemented, which use or
combine various data and variablessuch as current, time, digital
information, geographicalarrangements, etc.
Current discrimination
Used (in Low Voltage) when short-circuit current decreases rapidly as thedistance increases between the source
and the short-circuit point being
examined, or between the supply and
load sides of a transformer.
Time discrimination
Used in Low Voltage and very often in
High Voltage as well. Protection device(circuit breaker) tripping time delays areincreased as the devices becomecloser to the source. In order to
eliminate the effects of transient
currents , the minimum time delay at
the furthest point from the source(directly upstream from the load) is
0.2 s or even 0.1 s if the current settingis high, with a maximum time delay
of 1 s at the starting point of the privatenetwork.
These time delays may be definite time
or short-circuit current dependent.
Discrimination by logical data
transmission, or logical
discrimination
This type of discrimination consists ofthe transmission of a "blocking signal"
of a limited duration, by the firstprotection unit situated directly
upstream of the fault, and which is
supposed to open the circuit, to the
other protection units situated furtherupstream.
This technique, which has been
developed and patented by Merlin
Gerin, is described in detail in "CahierTechnique" n 2.
The tripping time delay is short and
definite, wherever the fault may be
situated in the network. This helpsincrease to the dynamic stability of the
network and minimizes the damagingeffects of faults and thermal stress.
Discrimination using directional or
differential protection
This type of discrimination provides
specific protection of a portion orparticular element of the network.
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Examples: transformers, parallel
cables, loop systems, etc.
Discrimination using distance
protectionThis type consists of splitting the
network into zones. The protectionunits locate the zone area in which the
fault is situated by calculating the circuit
impedance.
This technique is seldom used except
when the private HV network is very
widespread.
An example of a protection system that
uses several discrimination techniques
is given in figure 4.
N.B. The criteria used for selecting
protection systems, relays andprotection settings are not discussed in
this section. A number of other
publications deal with those matters,
e.g. "Cahiers Techniques" n 2 and
158.
short-circuit current
calculationIn order to find the best technical and
economic solution, it is necessary to
know the different short-circuit values
so as to determine:
s making and breaking capacityaccording to the maximum peak and
rms short-circuit current;
s equipment and switchgear resistanceto electrodynamic stress, according to
the maximum peak short-circuit current;
s protection tripping settings in
discrimination analyses according to
the maximum and minimum rms short-
circuit current.
"Cahier Technique" n 158, after
reviewing the physical phenomena to
be taken into consideration, reviews the
calculation methods set forth in thestandards.
When rotating machines (alternators or
motors) are included, the form of short-
circuit current can be broken down into
three portions:
s subtransient,
s transient,
s steady state.
The subtransient and transient stages
are linked to the extinction of the flux
0.5 s
0.5 s
0.2 s
0.2 s
0.2 s
pilot wire
pilot wire
discrimination by data
transmission
time discrimination
current discrimination
discrimination by data
transmission
fig. 4: protection system using several discrimination techniques: current, time and logic.
built up in the synchronous and
asynchronous rotating machines.In both of these stages, it is necessaryto deal with the aspect of an
asymmetrical component, also referredto as a DC component, the damping ofwhich depends on the R/X ratio of theupstream network and the timing of the
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fault with respect to the phase of the
voltage.
In addition, it is necessary to take intoaccount motor contribution to short-
circuit current. When a three-phase
short-circuit occurs, asynchronous
machines are no longer supplied with
power by the network, but the magnetic
flux of the machines cannot suddenly
disappear.
The extinction of this flux creates a
subtransient and then transient current
which intensify the short-circuit current
in the network.
The total short-circuit current is the
vectorial sum of two short-circuit
currents: source short-circuit current
and machine short-circuit current.
calculation of voltagefluctuations under normaland disturbed operatingconditions
Normal operating conditions
The calculation of voltage fluctuations
under normal operating conditions is a
design task which serves to verify thevoltages throughout the network.If the voltages are too low, the network
designer should verify whether:s the active and reactive power flowsare normal,s the feeders are correctly sized,s the transformer power ratings aresufficient,s the reactive power compensationscheme is appropriate,s the correct network structure is beingused.
Disturbed operating conditions
It is necessary to calculate voltagefluctuations under disturbed operating
conditions in order to verify whether thefollowing phenomena will result inexcessive voltage drops or rises:s starting of large motors (see fig. 5),
s downgraded network operation (e.g.2 transformers operating instead of the3 intended for normal operation),s no-load network operation, with orwithout reactive power compensation.Appendix 5 gives the mathematicalexpression and vector diagram of a
network voltage drop.
choice of motor starting
methodThe starting method used (delta-star,
autotransformer, resistors or statorreactors, etc.) must obviously provide
sufficient accelerating torque
(accelerating torque is generally greater
than 0.15 times the rated torque), and,
in addition, must only cause acceptable
voltage drops (< 15%).
Reminder: the equation governing the
interaction of the motor with the driven
mechanism is:
T'm - Tr= Jd
dt
T'm = motor torque when energized atactual supply voltage (Ur),
Tr = braking torque of the drivenmachine,J = inertia of all the driven masses,dw/dt = angular acceleration,
T'm = TmUr
Un
2
Tm = motor torque when energized atrated voltage (Un).Ta = T'm - TrTa = accelerating torque.
1200 kW= 5 nI
4.5 kV4.75 kV Un = 5 kVUn = 5 kV
plant network
motor during
starting phase
57 kVUn = 60 kV
Un = 60 kV
30kmline
I
StartI
StartI
StartI
Start
fig. 5: example of the determination of voltage dips during motor starting for the installation of a
1200 kW motor supplied by a 30 km long, 60 kV line. It should be noted that separation of the
"plant network" power supply and the motor power supply have made it possible to have a
voltage drop of only 250 V in the "plant network".
xx kV = voltage during motor starting
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The curve showing Ta as a function ofspeed is given in figure 6 below.
The chart in figure 7 shows the mostfrequently used starting methods (forfurther details, please refer to "CahierTechnique" n 165).
network dynamic stabilityUnder non-disturbed operatingconditions, all the rotating machines(motors and alternators) included in theinstallation form a stable system withthe utility network.This equilibrium may be affected by aproblem in one of the networks (utilityor private), such as: major loadfluctuation, change in the number of
transformers, lines or supply sources,multiphase fault, etc.This results in either short-livedinstability, in the case of a well-designed network, or a loss of stabilityif the disturbing phenomenon is veryserious or if the network has a lowrecovery capacity (e.g. short-circuitcapacity too low).
Asynchronous motor performance inthe presence of a three-phase fault
Here, as an example, is the analysis ofasynchronous motor performance inthe presence of a three-phase fault(see fig. 8).After the fault has been cleared, twosituations may occur:
s the motor torques are greater than
the braking torques, in which case themotors can reaccelerate and return to
their stable state;
s the motor torques are less than thebraking torques, in which case the
motors continue to slow down, drawinglarge currents which are detected by
the motor and/or network protectiondevices, which trip the related circuit
breakers.
Motor reacceleration, and hencenetwork dynamic stability, are favouredby:s a suitable load shedding plan(tripping of normal, non-essentialmotors in the event of serious faults);
s a powerful network (correctlyregulated alternators and low voltagedrop);
s fast-acting protection systems whichreduce the duration of motor slowdown(e.g. logical discrimination);s a suitable network structure with:
ss separation and regrouping inseparate circuits of normal, non-essential loads and essential andsensitive loads, which facilitates loadshedding,ss minimal impedance connections foressential and sensitive loads so as to
limit voltage drops.The duration of the return to normalmotor speed depends on:
s motor accelerating torque, and hence
voltage drop,
s rotating machine inertia.
Calculation software may be used tosimulate the dynamic performance of
electrical networks, making it easier to
fig. 7: the most commonly used starting methods
T'm = motor torque when energized at theactual supply voltage (Ur),Tr = braking torque of the driven machineTm = motor torque when energized at therated voltage (Un)Ta = accelerating torque
fig. 6: diagram of different motor torquesaccording to speed, with a voltage drop ofabout 15% (hence T'm = 0.7 Tm) andbraking torque TR = 0 during starting.It should be noted that this voltage dropwould not allow the motor to be started witha Tr > 0.6 Tn when started, since Tr > T'm.
0
TTn
torque
1
0
speed ( )
Tm
T'm
Tr
TnTa
0.6
fig. 8: performance of asynchronous motors
in the presence of a three-phase fault
three-phase fault
voltage drop
increase
in current
consumed
by motors
decrease inmotor torque
(proportional tovoltage
squared)
slowing down
of all motors
application application starting advantagesneeds characteristics method drawbacks
continuous or virtually machines that require direct simplicity, reducedcontinuous process strong starting torque investment.starts 1 per day upon starting:
sstrong torque,frequent motors with low inrush direct shigh inrush current,starts > 1 per day current or low power sstrong mechanical stress.
pumps, fans, machines that start with stator- reactor reduction of torque andcompressors, weak torque inrush current upon startingfrequent starts (adjustment possible).
optimization of when starting current stator - optimization of torquestarting needs to be reduced autotransformer (reduced) and inrush currentcharacteristics while maintaining the upon starting (adjustment
torque needed for possible).starting
optimization of strong the most difficult starts rotor low inrush current andtorque starting strong starting torque.characteristics
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make the correct choices, in particular,when establishing the procedures to beused to separate power supply sources(creation of "isolated systems"), non-
essential load shedding plans andprotection systems (logicaldiscrimination for very short trippingtimes). All of these factors contribute tomaintaining the dynamic stability of thenetwork when disturbances occur.
To summarize:
If the disturbance is minor (two-phase
short-circuit far from the load), stability
is restored by speed and voltage
regulators. When there is a high risk of
instability, it is necessary to include a
protection device that will clear the fault
within a very short time (0.2 to 0.3 s)
and/or a device to divide the network so
as to sustain it (load shedding) and
avoid the risk of a complete installation
shutdown (see fig. 9).
synthesisAll of the information presented in this
chapter is summarized in a logic
diagram in appendix 6.
M1 M2 M3 M4 M5
shed circuit
G(rad/s)
300
250
0.15 1 2 3 t (s)
alternator
motor M1, with load shedding
motor M1, without load shedding
clearing of the short-circuit
G
fig. 9: presentation of the dynamic performance of a network with and without load shedding
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Different network structures are
possible, the most common of which
are described in this chapter together
with the main areas in which they are
used.
The choice of a network structure,
which is always a decisive factor in
terms of energy availability, is often a
difficult one to make.
The most rational method consists ofmaking a quick comparison of the
unavailability of voltage at a particular
point in the network for differentstructures and using a very interesting
expert system (see appendix 2).
standard network
structures
Open or closed loop, also called
"primary loop system"
(see fig. 10)
Recommended for very widespreadnetworks, with major future extensions.
Open loop operation is advisable.
Double radial feeder, also called
"primary selective system" (manualor automatic)
(see fig. 11)
Recommended for very widespread
networks with limited future extensions
and which require a high level of
continuous power supply.
Radial feeder, also called "single
power supply"(see fig. 12 on the opposite page)
Recommended when continuous power
supply requirements are limited. It is
often used for cement plant networks.
Dual power supply
(see fig. 13 on the opposite page)
Recommended when a high level of
continuous power supply is required or
when the operating and maintenance
teams are small.
It is very often used in the steel and
petrochemical industries.
4. choice of optimal network structure andoperation
fig. 10: diagram of a primary loop.
fig. 11: diagram of a primary selective network.
HV
LVLVLV
HV
LV
LV
LV
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G
fig. 15: diagram of a network with local generation.
mainsource
replacementsource
essential
loads
(network with backup)
non-essential
loads
G
fig. 16: diagram of a network with a replacement source and load shedding.
With replacement source and loadshedding(see fig. 16)This is the typical case for an industrial
network in which a very high level ofcontinuous power supply is requiredusing a single power supply source,i.e. the utility.
LV
HV
subdistribution
board
LV
HV
fig. 12: diagram of a single power supply
network.
HV
HV
LV
fig. 13: diagram of a dual power supply
network.
Dual busbar(see fig. 14)Recommended when a very high levelof continuous power supply is required
or when there are very strong load
fluctuations. The loads may be
distributed between the two busbars
without any break in the power supply.
With energy generating sets(see fig. 15)
This is the simplest structure, and is
very often used.
fig. 14: diagram of a dual busbar network.
feeders
incomers
busbar B
busbar A
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concrete example of astructureThe diagram in figure 17 was designed
and developed for a multi-metal mine inMorocco. It includes different networkstructures that were described earlier.The power supply to the variousworkshops is provided by a loop, aduplicate feeder, or a main source andreplacement source.
choice of equipmentWhatever the structure that is used, theequipment must comply with:s standards in effect;s network characteristics:
ss rated voltage and current,ss short-circuit current (making andbreaking capacity, electrodynamic andthermal withstand);s the required functions (fault breaking,breaking in normal operation, frequencycontrol, circuit isolation, etc );s continuous power supplyrequirements (fixed, disconnectable,withdrawable switchgear);s operating and maintenance staffqualifications (interlocks, varyingdegrees of automatic control, breakingtechnique requiring maintenance or
maintenance-free);s requirements related to maintenanceand possible extensions (extra spacefor future use, modular system, etc.).
N.B. At this stage of the analysis, allthe network characteristics result fromcalculations that are often carried outusing scientific calculation software.
optimal operationOptimal electrical distribution systemoperation means finding:s the best level of continuous power
supply,s minimum energy consumption cost,s optimization of the operating andmaintenance means which contribute tocorrect network operation, both steadystate and transient, and in the presenceof a fault.The solution lies in the implementationof an electrical Energy ManagementSystem for the entire network.
The Energy Management Systems thatare currently offered by manufacturers,such as Merlin Gerin, make full use ofmicroprocessor performance. These
fig. 17: structure of the electrical network of a Moroccan mine (Merlin Gerin).
G
crushingpit head
63 kV
10 MVA2 MVA10 MVA
replacement source
5 kVLV LV
20 kV
level - 300
pit n 1
5 kV
treatment grinders
level - 500
pit n 2
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components, integrated in local andremote management centres, and inthe protection and control gear installedat the actual energy consumption sites,
are at the origin of the concept of"decentralized intelligence". The term"decentralized intelligence" means thatthe control centres and devicesautonomously carry out theirassignments at their levels (without anyhuman intervention) and do not callupon the "upper" level except whenfailures occur. The remote monitoringand control system continuouslyinforms the network manager or user ofchanges.This explains the importance of clearlyspecifying the network architecture.
Description of an Energy
Management System
(see fig. 18)
Energy Management Systems are set
up in four levels:
s level 0: sensors (position, electrical
variables, etc.) and actuators (trip units,
coils, etc.);
s level 1: protection and control units,
e.g. HV cubicle;
s level 2: local control, e.g. a HV/LV
substation of a plant or LV switchboard
in a workshop:
s level 3: remote control of an entire
private network.
All this equipment, particularly levels 1
to 3, is linked by digital communication
buses (networks via which theinformation is conveyed).
Assignments carried out by EnergyManagement Systems
s managing energy supply andconsumption according to:ss subscribed power,ss utility billing rates,ss availability of the private generatingstation,ss industrial process requirements.s maintaining continuous power supplythrough:ss quick, discriminating protection (e.g.logical discrimination system),ss automatic power supply sourcechangeover,
fig. 18: example of Energy Management System architecture.
level 3
remote control of the entire
private electrical network
(remote control and
monitoring system and
data loggers)
level 2
local management of
"plant-substation" networks
(network control centre)
level 1
protection and control units(HV cubicles, workshop main
LV switchboards)
level 0
sensors and actuators
(trip units, limit switches,
instrument transformers, etc.)
control-monitoring station
n1
control-monitoring station
n2
feeder 2: trigged, ground faulter 2: Overload 150A, Load shedding
I 1 = 136 A
Welding works branch: intensity 32A
I 1 = 96 ADef A1
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ss efficient load shedding/reconnection,the parameters of which can be set viathe man/machine interface with theestablishment of load shedding criteria
(load shedding/reconnection plan);ss sequential workshop restarting,ss adjustment of voltage, power factor,etc.ss maintaining power supply toessential loads during outages of theutilily supply the local alternators.
s enabling man/machine dialogue:ss real time display of network andequipment status via mimic diagrams(single-line diagrams, detail diagrams,curves, etc.);
ss remote control of switching devices;ss
data and measurement logging,ss chronological recording of faults andalarms (10 ms),ss filing of events,ss metering, statistics,ss archiving.
All this data is used, in particular, toorganize preventive maintenance.
s drawing up "User Guide" procedureswhich, for example:ss prohibit the starting of particularmotors according to the poweravailable from the generating station,the time, or the degree of priority of themotors,ss prohibiting the use of particular HighVoltage switchboard power supplyconfigurations (paralling of supplies),ss proposing the most suitable backuparrangement for serious faults in a mainfeeder or a generator,ss proposing operating instructions andmaintenance operations (electrical,mechanical, etc.).
Advantages of an EnergyManagement System
The development of level 1 digitalprotection and control systems and therapid rise in the performance/cost ratio
of level 2 hardware and softwareprovide industrialists with technical andeconomic benefits, in particular:
s increased operating dependability;
s a broader range of accessiblefunctions, especially data logging,preventive maintenance and remotecontrol,
s easier commissioning and more
efficient operation.
The richness of the functions that areoffered by these systems, new
possibilities for self-testing or even self-
diagnosis and monitoring, as well as
the user-friendliness of the userdialogue interfaces, naturally make the
facilities manager's role more effective
and interesting. The facilities manageris better able to assess the operation ofhis network and to optimize, apart fromremote monitoring and control,maintenance and renewal of hiselectrical equipment.
investment, operating costs andproduction losses.
The best operating conditions provide a
level of continuous power supply to
loads which is compatible with
installation requirements, in the aim of
obtaining maximum productivity and
maximum safety of people andproperty.
The new generations of electricalswitchgear and equipment aredesigned to communicate, via digitalcommunication buses, with one ormore control centres. And it is thecombination of both networks, theenergy network and the informationnetwork, at an acceptable investmentcost, which provides optimal fulfilmentof users' needs.
Well-mastered electrical networkdesign makes it possible to ensureoptimal operation under normal and
disturbed network operating conditions.
The best cost does not necessarilymean the minimum initial investment,
but rather the design of an electrical
network which proves to be the most
economical from the viewpoint of initial
5. conclusion
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The extension of an existing industrialnetwork by adding a transformer thatcan be connected parallel to theexisting transformer has the drawbackof increasing short-circuit currentstrength and also of making itnecessary to increase:s the breaking and making capacity ofthe existing devices,sthe old installation's resistance toelectrodynamic stress.
The installation of a three-phasereactor between the old and newfacilities eliminates these difficulties(see fig. 19).
hypothesess short-circuit current of the existinginstallation: 17 kA (Isc1),s short-circuit current in the existingbusbar must be limited to 21 kA (Isc2),XTr = 0.63 .
approximate calculation ofa current limiting reactor(resistors neglected)
The current flowing through the reactorshould be equal, in the firstapproximation, to:
fig. 19: extension of an existing industrial network by addition of an extra transformer.
existing installation new installation
new
20 MVA
transformer
Usc = 12%
36 MVA
Ucc = 12%
17 kA
Isc 1 X = 0.81
4 kA21 kA
limiting
reactor
10 kV 10 kV
appendix 1: extension of an existingindustrial network
21 - 17 = 4 kA = IscL
scL = current limited by the reactor
=V
X
X = total reactance (20 MVAtransformer and limiting reactor)
X = VIscL
= 10,000
3 x 4,000= 1.44
X = Xreact + XTrXTr = 0.63X react = 1.44 - 0.63X react = 0.81
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Here is a list of the main softwareprograms used in Merlin Gerin'sdifferent departments that areresponsible for analyzing and/ordesigning electrical networks.
calculation softwareprogramss load flow study,s short-circuit currents,s voltage drops,
s network dynamic stability,
s harmonic currents and voltages,s lightning and switching voltagesurges,s transformer and capacitor switching,s unavailability of electrical powersupply.
expert system forevaluating electricalnetwork design qualityAn expert system called ADELIA has
been developed and is used by
Merlin Gerin. It is used to make a quick
comparisons of the unavailability of
voltage at a particular point in the
network using distribution schemes.
It offers the advantage of requiring
fewer calculations than the Markov
graph method, and also of providing
both qualitative information (graph of
combinations of events which lead to
system failures), and quantitative
results (network unavailability
calculations).
The principle of compensation usingcapacitors may be illustrated by the twofigures opposite.
s figure 20 shows the vectorialcomposition of the different currentsand for a given active current, thereduction in the total current in theconductors.
Ia = active current consumedIt1 = total current prior to compensationIr1 = reactive current supplied via thetransformer prior to compensation
It2 = total current after compensation
Irc = reactive current supplied by the
capacitorIr2 = reactive current supplied by thetransformer after compensation
(Ir2 = Ir1 - Irc)
s figure 21 illustrates the localexchange of reactive power whichtakes place between the load and thecapacitor. The total current supplied bythe network It2 is reduced and the
appendix 2: computerized means used fornetwork analyses
appendix 3: general principle ofcompensation
fig. 20: phasor diagram of the different
currents and the effect of compensation.
fig. 21: diagram depicting the energy
exchange in a consumer circuit and the
advantage of compensation.
installlation output is thereby improvedsince losses due to the Joule effect areproportional to the square of thecurrent.
It1
Irc
It2
Ir1
Ir2
Ia1 2
before
compensation
after
compensation
increased
available
power
reactive
power
output by the
transformer
reactive
power
supplied by
the capacitor
active
power
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The choice of the earthing system for aHigh Voltage industrial networkinvolves the following criteria:s general policy,s legislation in effect,s constraints related to the network,s constraints related to networkoperation,
s constraints related to the type ofloads,s etc.Five diagrams may be considered:s directly earthed neutral,s reactance- earthed neutral,s resonant- earthed neutral,s resistance- earthed neutral,
s isolated neutral.
Each of them offers advantages and
drawbacks with which the network
designer should be familiar before
making his final choice. They are listed
in the chart below (see fig. 22).
appendix 4: choice of the earthing systemfor a HV industrial network
directly earthed neutral advantages drawbacks in practicediagram
directly earthed neutral sfacilitates earth fault causes high earth fault currents not useddetection and protection (dangerous for personnel and riskdiscrimination, of major equipment damage)slimits overvoltage
reactance-earthed limits earth fault currents srequires more complex protection applicable without any particularthan direct earthing, precautions only if the limitingsmay cause severe overvoltage, impedance is low with respect todepending on installation configurations the circuit's earth fault resistance
resonance-earthed is conducive to self-extinction requires complex protection ssometimes used in Eastern(Petersen coil) of earth fault current (directional devices that are difficult countries
to implement) snot used in France
resistance-earthed slimits earth fault currents, the most beneficial for industrialsfacilitates earth fault current distribution: it combines all thedetection and protection advantages
discrimination,slimits overvoltage
isolated from earth limits earth fault currents srisk of overvoltage no tripping when the first earth faultsrequires the use of over-insulated occurs entails:equipment (line voltage between phase s being granted specialand earth when a zero impedance dispensation (French law)fault occurs), s that the capacitance betweens overvoltage protection advisable, active network conductors ands insulation monitoring obligatory earth do not cause earth fault(French law), current that is dangerous fors complex discrimination between personnel and machines.earth fault protection devices
fig. 22: advantages and drawbacks of the different earthing diagrams used for a HV industrial network.
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fig. 23: phasor diagram of network voltage drop
Voltage drops in a network may becalculated using the followingmathematical expression:V = R I cos + X I sin The electrical diagram and vectordiagram which correspond to thisequation are given in figure 23.
V
F
ECD
B
A
I
Va
Vd
XI
RI
RL
load
VaVd
source
I
cable
AD = RI cos
DE = BC sin + CF sin
DE = BF sin = XI sin
This logic diagram comprises twoloops:s the first one, the analysis and
selection loop, starts at "Needs andconstraints to be met" and leads to the
appendix 6: stages in industrial networkdesign
organization of a network structure inan "Overall single-line diagram";s the second is aimed at optimization
of the structure.
analysis and selection loopleading to the organization ofa structure
structure optimization loop
appendix 5: network voltage drops(mathematical expression and vector diagram)
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Needs andconstraints to
be met
NO
Major needsmet and/orconstraints
mastered
load survey$ active$ reactivebased on$ load factors$ diversity factors
Choice of the motor starting methodThis depends on:$ permissible network voltage drop$ braking torques and motors$ inertia of rotating machines
N.B. The maximum voltage drop and starting timevalues currently encountered are:$ voltage drops = 10 to 15% of rated voltage$ starting times: pumps = 0.5 to 2 s grinders = 5 to 10 s conveyor belts = 5 to 30 s fans = 10 to 200 s
Choice of voltagesAccording to:$ the function to be performed: transmission distribution consumption$ power available and powerto be supplied$the distance between sourcesand loads$ local practices and habits
Reactive energy compensationUsing the following main criteriafor choosing the type ofcompensation:$ feeder length and crosssection$ desired increase in availableactive power
Backup sources$ standards$ laws
Replacement sources$ maintaining of production facilitiesoperation$ power supply convenience
Autonomous electrical powerproductionIf the industrialist:$ has available vapour produced
by the process$ has fuel available at a low cost$ is unable to obtain a strong orreliable utility supply
Division of sourcesTo delimit areas disturbed by:$ starting of large motors$ power electronic systems$ etc.
Calculation of voltage fluctuations under normaland disturbed operating conditions
Short-circuit current calculation
Protection system definition
Feeder definitionMain criteria to be used:$ steady state current (or equivalent)$ installation method$ exposure to sun$ ground temperature$ air temperature$ ground conductivity$ permissible voltage drop under normal and disturbedoperating conditions$ short-circuit current withstand$
cable screen withstand to single-phase and/ormultiphase faults$ contact voltage$ etc.
Insulation coordination analysis
Choice of earthing systemsamong the following possible diagrams:$ isolated neutral$ directly earthed neutral$ neutral earthed via a reactor or Petersen extinction coil$ neutral earthed via a resistor
YES
Overallsingle-line
diagram
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Merlin Gerin Cahiers Techniques
s Protection of electrical distributionnetworks by the logic selectivity systemCahier Technique n 2F. SAUTRIAU
s Mise la terre du neutre dans unrseau industriel haute tensionCahier Technique n 62F. SAUTRIAU
s Gestion de l'nergie dans lesprocessus industriels
Cahier Technique n 133C.G. POUZOLS
s Surtensions et coordination del'isolement,Cahier Technique n 151D. FULCHRION
s Calcul des courants de court-circuit,Cahier Technique n 158R. CALVAS, B. DE METZ NOBLAT,A. DUCLUZAUX and G. THOMASSET
s Lightning and HV electricalinstallations,Cahier Technique n 168B. DE METZ NOBLAT
Miscellaneous publications
s Overvoltage when clearing short-circuits from networks with a reactance-earthed neutral system.Bulletin de la socit Franaise desElectriciens, series 8, Volume 1, n 4(April 1960)LE VERRE
appendix 7: bibliography