hv industrial network design -...

n° 169HV 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

Cahier Technique Merlin Gerin n° 169 / p.2

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: the French decree of 14 November1988 which defines two categories ofvoltage:HTA = 1 kV < U ≤ 50 kV,HTB = U > 50 kV. CENELEC (European committee forelectrotechnical standardisation), in acircular dated 27 July 1992, specifies:MV = 1 kV < U ≤ 35 kV,HV = U > 35 kV. 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 therelative 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. 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: overallorganization of electricalprotection gear, including: the

appendices

appendix 1: extension of an hypotheses p. 19

approximate calculation of a p. 19current limiting reactor

appendix 2: computerized means calculation software programs p. 20

expert system for evaluating p. 20electrical network design quality

appendix 3: general principle of compensation p. 20

appendix 4: choice of the earthing system for a HV industrial network p. 21

appendix 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


Faced with increasingly fiercecompetition, industrialists must employhighly rigorous management and theirproduction facilities must have a highlevel 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 preliminaryanalyses, which include: specific needs and constraintsrelated to the type of industry, integration of the limits andconstraints of the public distributionnetwork, standards and local practices, 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 thefollowing main characteristics: installed capacity in the 10 MVArange, autonomous electrical energyproduction (when applicable), power supplied by a nationaltransmission or distribution network(≥ 20 kV), private Medium Voltage distribution.

HV industrial network design

contents

1. needs and main constraints to be met needs to be met p. 4main constraints p. 5

2. main rules of industrial network load survey, load and diversity p. 7design factors

choice of voltages p. 7

reactive power compensation p. 7

backup and replacement sources p. 7autonomous electrical energy p. 7productiondivision of sources p. 7

overall electrical diagram p. 8

3. validation and technical and choice of the earthing system p. 9

feeder definition p. 9insulation coordination analysis p. 9protection system definition p. 9short-circuit currents calculation p. 10calculation of voltage fluctuations p. 11under normal and disturbedoperating conditionschoice of motor starting method p. 11network 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


initial investment cost, operating and maintenance costs, cost of production losses associatedwith the network design and protectionplan (protection system used, choice ofdevices and settings).

Optimization of electrical energyWhen a plant includes electrical energygenerators, it is necessary to managethe energy supplied by the utility andthe energy produced locally in the bestmanner possible.A control and monitoring system makesit possible to optimize the cost of plantpower consumption in accordance with: the contract with the utility (billingrates according to the time, day andseason); the availability of the plantgenerators; industrial process requirements.

Network changes and futureextensionsWhen designing an industrial network,it is of primary importance to make acareful assessment of the futuredevelopment of the plants, especiallywhen extensions are foreseeable.Changes that are liable or due to bemade in the future should be taken intoaccount: in sizing the main power supplycomponents (cables, transformers,switching devices), in designing the distribution diagram, in calculating the areas to be setaside for electrical rooms.This forward planning will result inincreasingly flexible energymanagement.

Network upgradingElectrical 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

overcurrent (short-circuits andoverloads), overvoltage.

The solutions that are used mustensure at the least the following: quick fault clearance and continuedpower supply to the fault-free sectionsof the network (discrimination), supply of information on the type ofinitial fault, for quick servicing.

Continuous power supply to loadsContinuous power supply to the loadsis necessary for the following reasons: safety of people, e.g. lighting; sustained production performance,e.g. glass wire drawing; productivity; operating convenience, e.g.simplified machine or workshop restartprocedure.

Loads are divided into three groupsaccording to their operatingrequirements: "normal" loads, "essential" loads, "sensitive" loads for which noabsence of power supply is tolerated.

Network operating easeIn order to carry out their tasks safelyand reliably, network facilities operatorsneed the following: a network that is easy to operate inorder to act correctly in the event of aproblem or a manoeuvre; sufficiently sized switchgear andequipment, which require littlemaintenance and are easy to repair(maintainability); efficient 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 costThe minimum electrical installation costdoes not necessarily mean theminimum initial cost, but the sum ofthree costs:

Industrial electrical networks mustsupply power to all the plant loads, atoptimal investment, operating and lossof production costs, taking into account: the needs to be met: safety of people, safety of property, continuous power supply, network operating ease, minimum installation cost, optimization of electrical energy(cost / quality), network changes and futureextensions, network upgrading; constraints linked to: the industrial process, the electrical process, the utility, the climate and geography of the site, standards, regulations and localpractices.It is clear that not all the needs can beoptimally met, which means that thenetwork designer must endeavour tofind the best compromise.

needs to be met

Safety of peopleEven if not all countries have standardsand rules, certain obvious principlesmust be adhered to: prohibiting access to energized parts(protection against direct contact), a system to protect against the rise inpotential of metal structures (protectionagainst indirect contact), prohibiting on-load line disconnectingswitch operations, prohibiting earthing of live conductors quick fault clearance.

Safety of propertyElectrical 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:


insufficient electrodynamic anddielectric strength of existing devices orequipment; capacity to supply power to largeloads (starting current, dynamicstability, etc.); area and height of existing electricalrooms that cannot be changed; 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 (seeappendix 1).

Quality of analysesThe 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 certificationin 1992.

main constraintsConstraints linked to the industrialprocessApart from the need for a high level ofcontinuous power supply in certainindustrial processes, those veryprocesses also impose constraints: power supply and, in particular,starting of very large motors whichdrive crushers, grinders, fans, pumpsand conveyor belts (a major cause ofvoltage drops during starting); power supply to arc furnaces, the arcoften being unstable, which causesbrief but repeated unbalanced voltagedrops (resulting in flicker), and cancreate harmonics; 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 processespollute their environment. They producesubstances (dust, gas) that aresometimes corrosive and can jammechanisms or degrade theperformance of electrical devices(e.g. dielectric strength) or even causeexplosions in the presence of electricarcs.

Constraints linked to the electricalprocessDuring the analysis, it is necessary totake into consideration various"electrotechnical" conditions that allelectrical networks must fulfil, inparticular: limitation of short-circuit currents andtheir duration starting or restarting of large motorswithout excessive voltage drops, stability of alternators after adisturbance has occurred.

Constraints imposed by the utilityelectrical power distribution network short-circuit capacityThe short-circuit capacity of theupstream network supplying the privatenetwork is a decisive element whenchoosing: the structure of the privatedistribution network, the maximum load, particular loads that are sensitive tovoltage drops. utility earthing systemThe same type of earthing system, asused by the utility, is often used for theprivate network but it is sometimes notcompatible with particular loads. Insuch cases, it is necessary to create aseparate system with: a type of electrical protection forphase-to-earth faults, a special network operating method(fault tracking and/or clearing byoperating personnel in the case ofisolated neutral systems).

momentary dips or substantialtransient single-phase or multiphasevoltage drops.The presence of such phenomena cancause disturbances, or even productionshutdowns and machine destruction.These phenomena may result in: operating errors and/or the loss ofdata of the industrial process computer

system (central computers, PLCs), andmanagement control system as well asscientific calculation errors. 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.

overvoltage of external origin, inparticular lightning strokes (refer to"Cahier Technique" n°168).

value and quality of the supplyvoltage: 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; the quality of the supply voltage.Various voltage fluctuations may botheror even prevent production equipmentoperation. The chart in figure 1 showssuch faults, the causes andconsequences, 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: rms voltage and current; active and reactive power, short and long power outages, voltage dips, harmonic voltages and currents, 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


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 starting of large motors absence of voltage fast and/or slow HV linearc furnace single- or multiphase regulation in primary reclosing system

fault substations

consequences variations in incandescent risk of network instability motor overload momentary suspension oflighting (Flicker) high risk of network power supply to all loads

instability

main remedies use static var change the starting equip the transformers use shunt circuit breakerscompensation equipment method with on-load tap changers

increase protectionplan rapidity

fig. 1: voltage fluctuations, causes ,consequences and main remedies

installations, it is necessary to know thefollowing: average and maximum dailytemperatures, relative humidity rate at maximumtemperature, maximum wind speed, presence of frost, ice, sand-bearingwinds,

environment (corrosive atmosphereor risk of explosion), altitude, frequency of lightning strokes in theregion, difficulties in gaining access to thesite (for the transport of materials andalso for maintenance).

Compliance with standards and localpracticesIn this regard, it is especially importantto be familiar with: national and/or internationalswitchgear and installation standards,regulations and rules which applyspecifically to the industrial complex, local practices.


2. main rules of industrial network design

The aim of this chapter is to explainhow the industrial network designprocess takes into consideration all theobligations (needs and constraints)described in the previous chapter.Since plants are designed to operatenon-stop, any break in the electricalpower supply should be assessedduring the analysis phase and theconsequences examined in order todetermine the measures to be taken.The method put forth in this chaptercomprises two phases:- 1 - endeavouring to achieve anappropriate technical balance betweenneeds and constraints (see chapter 1),- 2 - technical and economicoptimization through the correct use ofcertain calculations and the conceptspresented below.Appendix 2, without being exhaustive,lists the main calculation softwareprograms that are used by specializednetwork analysis engineers.

load survey, load anddiversity factorsThis is the first essential step indesigning a network.It should define and geographicallylocate the power requirements.

Load surveyIt is necessary to: distinguish active, reactive andapparent power; group the power demands bygeographical area (3 to 8 areas)according to the size of the site; identify, for each area, the "normal" -"essential" and "sensitive" loads.

Load and diversity factors (see fig. 2)

choice of voltagesThe choice of voltages is determined bythe function that is to be performed:transmission, distribution orconsumption. In HV, the distributionvoltage is not necessarily the same asthe consumer voltage. For example,20 kV may be the optimal distributionvoltage in a plant in view of the powerflow and the distance of workshops

from the main substation, even if theten largest motor loads require anominal voltage of 6.6 kV.

reactive powercompensationThe local utility generally establishesthe minimum power factor (p.f.) for theclient's supply point.Reactive power compensation is oftennecessary to meet this requirement andmay be carried out at two levels: at the substation (or mainswitchboard) level: globalcompensation; at the load level: distributedcompensation.The general principle of compensationusing capacitors is presented inappendix 3.Remark : strong compensation bymeans of fixed capacitor banks maycause overvoltage. A particular case ofthis is the phenomenon of the self-excitation of asynchronous machines:the capacitors that are associated withan asynchronous motor (distributedcompensation) may give rise to veryhigh overvoltage when there is a breakin the power supply. This phenomenoncan occur when compensation isgreater than 90% of the magnetizingcurrent, which is approximately equal tomotor no-load current.

backup and replacementsourcesBackup sources are installed to protectpeople (standards and laws),e.g. emergency exit route lighting.Replacement sources are installed inorder to maintain production facilities

operation or to provide greateroperating flexibility.

autonomous electricalenergy productionA plant may have its own means ofgenerating electrical power to supply its"sensitive" loads, for electricity billingrate reasons or when the plant'smanufacturing process producesenergy (thermal or mechanical), e.g. inthe form of vapour.If the utility network has a sufficientshort-circuit capacity and voltage andfrequency quality, it is preferable tooperate the autonomous sources andthe mains in parallel, since the mainshelp to stabilize the performance of theplant's alternators (voltage and speed).When parallel operation is used, asystem for distributing active andreactive power between the differentalternators and the mains should beincluded.When serious electrical problems occurin the private network, or in the utilitynetwork near of the plant, instabilitymay result. It may be necessary toseparate the private network from theutility network (creation of an isolatednetwork supplied by the alternators)within an extremely short time (about0.2 seconds) so as not to risk a fullfacilities shutdown. Separation of thenetworks is generally accompanied bythe disconnection of non-essentialloads from the private network in orderto avoid overloads.

division of sourcesCertain loads cause a high degree ofinterference in the utility supply

motors lighting powerheating outlets

load factor 0.75 1 (**)

diversity factor 0.70 (*) 1 0.1 to 0.3

* Depends on the process** Depends on the destination

fig. 2: general indication of load and diversity factors.


network. The division of sources (seefig. 3) is used to isolate the fluctuatingloads and it offers two additionaladvantages as well: improved discrimination between theprotection devices, thereby increasingcontinuous power supply to the otherloads; 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 "overallsingle-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 otherloads.

M M M

normal loads

"sensitive"loads

"fluctuating" loads

lightingcomputers

high powermotors

variable speed motor


3. validation and technical and economic optimization

choice of the earthingsystemStandards and regulations makeprotection against direct and indirectcontact compulsory for all electricalinstallations. In general, protectionfunctions automatically interrupt thepower supply (upon the first or secondphase-to-earth fault according to theearthing system used). There are alsospecial protection devices designed forspecific situations. High Voltagenetwork earthing systems may bechosen according to the criteria given inappendix 4.However, it is very often of benefit touse different earthing systems in thesame industrial network, each of whichprovides a predominant advantage.

feeder definitionFeeders account for a large portion ofelectrical installation investment. It istherefore important, for safety and costreasons, to: choose the best type of equipment(cables), make the most accurate calculationsof the minimum cross section, whiletaking into account short-circuit andstarting currents, voltage drops, losses,etc.

insulation coordinationanalysisThe coordination of insulation providesthe best technical and commercialcompromise in protecting people andequipment against the overvoltage thatmay occur in electrical installations,whether such overvoltage originatesfrom the network or from lightning.Three types of overvoltage may causeflashovers and hence insulation faults,with or without destruction of theequipment: power frequency overvoltage (50 to500 Hz), switching surges,

atmospheric overvoltage (lightningstrokes).

Insulation coordination contributes toobtaining greater electrical poweravailability. Correct insulationcoordination entails: knowing the level of overvoltage thatis liable to occur in the network; specifying the desired degree ofperformance or, more explicitly, theacceptable insulation failure rate; installing protection devices that aresuited to both the network components(insulation level) and the types ofovervoltage; choosing the various networkcomponents based on their level ofovervoltage withstand, which mustmeet the constraints described above.

"Cahier Technique" n° 151 describesvoltage disturbances, ways of limitingthem and the measures set forth in thestandards to ensure dependable,optimized electrical energy distribution,thanks to insulation coordination.

protection systemdefinitionWhen a fault appears in an electricalnetwork, it may be detectedsimultaneously by several protectiondevices situated in different areas ofthe network. The aim of selectivetripping is to isolate as quickly aspossible the section of the network thatis affected by the fault, and only thatsection, leaving all the fault-freesections of the network energized. Thenetwork designer must first of all selecta protection system that will enable himto specify the most suitable protectiondevices. He must also create a relaytripping plan, which consists ofdetermining the appropriate current andtime delay settings to be used to obtainsatisfactory tripping discrimination.

Different discrimination techniques arecurrently implemented, which use orcombine various data and variablessuch as current, time, digital

information, geographicalarrangements, etc.

Current discriminationUsed (in Low Voltage) when short-circuit current decreases rapidly as thedistance increases between the sourceand the short-circuit point beingexamined, or between the supply andload sides of a transformer.

Time discriminationUsed in Low Voltage and very often inHigh Voltage as well. Protection device(circuit breaker) tripping time delays areincreased as the devices becomecloser to the source. In order toeliminate the effects of transientcurrents , the minimum time delay atthe furthest point from the source(directly upstream from the load) is0.2 s or even 0.1 s if the current settingis high, with a maximum time delayof 1 s at the starting point of the privatenetwork.These time delays may be definite timeor short-circuit current dependent.

Discrimination by logical datatransmission, or logicaldiscriminationThis type of discrimination consists ofthe transmission of a "blocking signal"of a limited duration, by the firstprotection unit situated directlyupstream of the fault, and which issupposed to open the circuit, to theother protection units situated furtherupstream.This technique, which has beendeveloped and patented by MerlinGerin, is described in detail in "CahierTechnique" n° 2.The tripping time delay is short anddefinite, wherever the fault may besituated in the network. This helpsincrease to the dynamic stability of thenetwork and minimizes the damagingeffects of faults and thermal stress.

Discrimination using directional ordifferential protectionThis type of discrimination providesspecific protection of a portion orparticular element of the network.


Examples: transformers, parallelcables, loop systems, etc.

Discrimination using distanceprotectionThis type consists of splitting thenetwork into zones. The protectionunits locate the zone area in which thefault is situated by calculating the circuitimpedance.This technique is seldom used exceptwhen the private HV network is verywidespread.

An example of a protection system thatuses several discrimination techniquesis given in figure 4.

N.B. The criteria used for selectingprotection systems, relays andprotection settings are not discussed inthis section. A number of otherpublications deal with those matters,e.g. "Cahiers Techniques" n° 2 and158.

short-circuit currentcalculationIn order to find the best technical andeconomic solution, it is necessary toknow the different short-circuit valuesso as to determine: making and breaking capacityaccording to the maximum peak andrms short-circuit current; equipment and switchgear resistanceto electrodynamic stress, according tothe maximum peak short-circuit current; protection tripping settings indiscrimination analyses according tothe maximum and minimum rms short-circuit current."Cahier Technique" n° 158, afterreviewing the physical phenomena tobe taken into consideration, reviews thecalculation methods set forth in thestandards.

When rotating machines (alternators ormotors) are included, the form of short-circuit current can be broken down intothree portions: subtransient, transient, steady state.The subtransient and transient stagesare 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 andasynchronous 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


fault with respect to the phase of thevoltage.In addition, it is necessary to take intoaccount motor contribution to short-circuit current. When a three-phaseshort-circuit occurs, asynchronousmachines are no longer supplied withpower by the network, but the magneticflux of the machines cannot suddenlydisappear.The extinction of this flux creates asubtransient and then transient currentwhich intensify the short-circuit currentin the network.The total short-circuit current is thevectorial sum of two short-circuitcurrents: source short-circuit currentand machine short-circuit current.

calculation of voltagefluctuations under normaland disturbed operatingconditionsNormal operating conditionsThe calculation of voltage fluctuationsunder normal operating conditions is adesign task which serves to verify thevoltages throughout the network.If the voltages are too low, the networkdesigner should verify whether: the active and reactive power flowsare normal, the feeders are correctly sized, the transformer power ratings aresufficient, the reactive power compensationscheme is appropriate, the correct network structure is beingused.

Disturbed operating conditionsIt is necessary to calculate voltagefluctuations under disturbed operatingconditions in order to verify whether thefollowing phenomena will result inexcessive voltage drops or rises: starting of large motors (see fig. 5), downgraded network operation (e.g.2 transformers operating instead of the3 intended for normal operation), no-load network operation, with orwithout reactive power compensation.Appendix 5 gives the mathematicalexpression and vector diagram of anetwork voltage drop.

choice of motor startingmethodThe starting method used (delta-star,autotransformer, resistors or statorreactors, etc.) must obviously providesufficient accelerating torque(accelerating torque is generally greaterthan 0.15 times the rated torque), and,in addition, must only cause acceptablevoltage drops (< 15%).Reminder: the equation governing theinteraction of the motor with the drivenmechanism is:

T'm - Tr = J dωdt

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 = Tm UrUn

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 duringstarting phase

57 kVUn = 60 kV

Un = 60 kV

30 k

m li

ne

I

Start I

Start I

Start I

Start

fig. 5: example of the determination of voltage dips during motor starting for the installation of a1200 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 avoltage drop of only 250 V in the "plant network". xx kV = voltage during motor starting


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 oftransformers, 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 faultHere, 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:

the motor torques are greater thanthe braking torques, in which case themotors can reaccelerate and return totheir stable state; the motor torques are less than thebraking torques, in which case themotors continue to slow down, drawinglarge currents which are detected bythe motor and/or network protectiondevices, which trip the related circuitbreakers.

Motor reacceleration, and hencenetwork dynamic stability, are favouredby: a suitable load shedding plan(tripping of normal, non-essentialmotors in the event of serious faults); a powerful network (correctlyregulated alternators and low voltagedrop); fast-acting protection systems whichreduce the duration of motor slowdown(e.g. logical discrimination); a suitable network structure with: separation and regrouping inseparate circuits of normal, non-essential loads and essential andsensitive loads, which facilitates loadshedding, minimal impedance connections foressential and sensitive loads so as tolimit voltage drops.

The duration of the return to normalmotor speed depends on:

motor accelerating torque, and hencevoltage drop, rotating machine inertia.

Calculation software may be used tosimulate the dynamic performance ofelectrical 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

0speed ( )ω

Tm

T'm

Tr

TnTa

0.6

fig. 8: performance of asynchronous motorsin 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 downof 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:

strong torque,frequent motors with low inrush direct high inrush current,starts > 1 per day current or low power strong 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


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-phaseshort-circuit far from the load), stabilityis restored by speed and voltageregulators. When there is a high risk ofinstability, it is necessary to include aprotection device that will clear the faultwithin 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) andavoid the risk of a complete installationshutdown (see fig. 9).

synthesisAll of the information presented in thischapter is summarized in a logicdiagram in appendix 6.

M1 M2 M3 M4 M5

shed circuit

Gω(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


Different network structures arepossible, the most common of whichare described in this chapter togetherwith the main areas in which they areused.The choice of a network structure,which is always a decisive factor interms of energy availability, is often adifficult one to make.The most rational method consists ofmaking a quick comparison of theunavailability of voltage at a particularpoint in the network for differentstructures and using a very interestingexpert system (see appendix 2).

standard networkstructures

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 widespreadnetworks with limited future extensionsand which require a high level ofcontinuous power supply.

Radial feeder, also called "singlepower supply"(see fig. 12 on the opposite page)Recommended when continuous powersupply requirements are limited. It isoften used for cement plant networks.

Dual power supply(see fig. 13 on the opposite page)Recommended when a high level ofcontinuous power supply is required orwhen the operating and maintenanceteams are small.It is very often used in the steel andpetrochemical 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


G

fig. 15: diagram of a network with local generation.

mainsource

replacementsource

essentialloads

(network with backup)

non-essentialloads

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 industrialnetwork in which a very high level ofcontinuous power supply is requiredusing a single power supply source,i.e. the utility.

LV

HVsubdistribution

board

LV

HV

fig. 12: diagram of a single power supplynetwork.

HV

HV

LV

fig. 13: diagram of a dual power supplynetwork.

Dual busbar(see fig. 14)Recommended when a very high levelof continuous power supply is requiredor when there are very strong load

fluctuations. The loads may bedistributed between the two busbarswithout any break in the power supply.

With energy generating sets(see fig. 15)This is the simplest structure, and isvery often used.

fig. 14: diagram of a dual busbar network.

feeders

incomers

busbar B

busbar A


concrete example of astructureThe diagram in figure 17 was designedand 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: standards in effect; network characteristics: rated voltage and current, short-circuit current (making andbreaking capacity, electrodynamic andthermal withstand); the required functions (fault breaking,breaking in normal operation, frequencycontrol, circuit isolation, etc …); continuous power supplyrequirements (fixed, disconnectable,withdrawable switchgear); operating and maintenance staffqualifications (interlocks, varyingdegrees of automatic control, breakingtechnique requiring maintenance ormaintenance-free); 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: the best level of continuous powersupply, minimum energy consumption cost, 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 - 300pit n° 1

5 kV

treatment grinders

level - 500pit n° 2


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 EnergyManagement System(see fig. 18)Energy Management Systems are setup in four levels: level 0: sensors (position, electricalvariables, etc.) and actuators (trip units,coils, etc.); level 1: protection and control units,e.g. HV cubicle; level 2: local control, e.g. a HV/LVsubstation of a plant or LV switchboardin a workshop: level 3: remote control of an entireprivate network.All this equipment, particularly levels 1to 3, is linked by digital communication

buses (networks via which theinformation is conveyed).

Assignments carried out by EnergyManagement Systems managing energy supply andconsumption according to: subscribed power, utility billing rates, availability of the private generatingstation, industrial process requirements.

maintaining continuous power supplythrough: quick, discriminating protection (e.g.logical discrimination system), automatic power supply sourcechangeover,

fig. 18: example of Energy Management System architecture.

level 3remote control of the entire private electrical network (remote control and monitoring system and data loggers)

level 2local management of "plant-substation" networks (network control centre)

level 1protection and control units (HV cubicles, workshop main LV switchboards)

level 0sensors and actuators (trip units, limit switches, instrument transformers, etc.)

control-monitoring stationn°1

control-monitoring stationn°2

feeder 2: trigged, ground fault er 2: Overload 150A, Load shedding

I 1 = 136 A

Welding works branch: intensity 32A

I 1 = 96 ADef A1


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); sequential workshop restarting, adjustment of voltage, power factor,etc. maintaining power supply toessential loads during outages of theutilily supply the local alternators.

enabling man/machine dialogue: real time display of network andequipment status via mimic diagrams(single-line diagrams, detail diagrams,curves, etc.); remote control of switching devices; data and measurement logging, chronological recording of faults andalarms (10 ms), filing of events, metering, statistics, archiving.

All this data is used, in particular, toorganize preventive maintenance.

drawing up "User Guide" procedureswhich, for example: prohibit the starting of particularmotors according to the poweravailable from the generating station,the time, or the degree of priority of themotors, prohibiting the use of particular HighVoltage switchboard power supplyconfigurations (paralling of supplies), proposing the most suitable backuparrangement for serious faults in a mainfeeder or a generator, proposing operating instructions andmaintenance operations (electrical,mechanical, etc.).

Advantages of an EnergyManagement SystemThe 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: increased operating dependability; a broader range of accessiblefunctions, especially data logging,preventive maintenance and remotecontrol, easier commissioning and moreefficient operation.The richness of the functions that areoffered by these systems, newpossibilities for self-testing or even self-diagnosis and monitoring, as well asthe user-friendliness of the userdialogue interfaces, naturally make thefacilities manager's role more effectiveand 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 alevel of continuous power supply toloads which is compatible withinstallation requirements, in the aim ofobtaining maximum productivity andmaximum 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 anddisturbed network operating conditions.The best cost does not necessarilymean the minimum initial investment,but rather the design of an electricalnetwork which proves to be the mosteconomical from the viewpoint of initial

5. conclusion


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: the breaking and making capacity ofthe existing devices, the old installation's resistance toelectrodynamic stress.

The installation of a three-phasereactor between the old and newfacilities eliminates these difficulties(see fig. 19).

hypotheses short-circuit current of the existinginstallation: 17 kA (Isc1), short-circuit current in the existingbusbar must be limited to 21 kA (Isc2),XTr = 0.63 Ω.

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 transformerUsc = 12%

36 MVAUcc = 12%

17 kAI sc 1

X = 0.81 Ω

4 kA21 kA

limitingreactor

10 kV 10 kV

appendix 1: extension of an existingindustrial network

21 - 17 = 4 kA = IscLΙscL = current limited by the reactor

= VX

X = total reactance (20 MVAtransformer and limiting reactor)

X = VIscL

= 10,000

3 x 4,000= 1.44 Ω

X = Xreact + XTrXTr = 0.63X react = 1.44 - 0.63X react = 0.81 Ω


Here is a list of the main softwareprograms used in Merlin Gerin'sdifferent departments that areresponsible for analyzing and/ordesigning electrical networks.

calculation softwareprograms load flow study, short-circuit currents, voltage drops, network dynamic stability,

harmonic currents and voltages, lightning and switching voltagesurges, transformer and capacitor switching, unavailability of electrical powersupply.

expert system forevaluating electricalnetwork design qualityAn expert system called ADELIA hasbeen developed and is used by

Merlin Gerin. It is used to make a quickcomparisons of the unavailability ofvoltage at a particular point in thenetwork using distribution schemes.It offers the advantage of requiringfewer calculations than the Markovgraph method, and also of providingboth qualitative information (graph ofcombinations of events which lead tosystem failures), and quantitativeresults (network unavailabilitycalculations).

The principle of compensation usingcapacitors may be illustrated by the twofigures opposite. 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 compensationIt2 = total current after compensationIrc = reactive current supplied by thecapacitorIr2 = reactive current supplied by thetransformer after compensation(Ir2 = Ir1 - Irc) 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 differentcurrents and the effect of compensation.

fig. 21: diagram depicting the energyexchange in a consumer circuit and theadvantage of compensation.

installlation output is thereby improvedsince losses due to the Joule effect areproportional to the square of thecurrent.

It1

Irc

It2

Ir1

Ir2

Iaϕ1 ϕ2

before compensation

after compensation

increased available power

reactive power output by the transformer

reactive powersupplied bythe capacitor

active power


The choice of the earthing system for aHigh Voltage industrial networkinvolves the following criteria: general policy, legislation in effect, constraints related to the network, constraints related to networkoperation,

constraints related to the type ofloads, etc.Five diagrams may be considered: directly earthed neutral, reactance- earthed neutral, resonant- earthed neutral, resistance- earthed neutral,

isolated neutral.Each of them offers advantages anddrawbacks with which the networkdesigner should be familiar beforemaking his final choice. They are listedin 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 facilitates earth fault causes high earth fault currents not useddetection and protection (dangerous for personnel and riskdiscrimination, of major equipment damage) limits overvoltage

reactance-earthed limits earth fault currents requires more complex protection applicable without any particularthan direct earthing, precautions only if the limiting may 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 sometimes used in Eastern(Petersen coil) of earth fault current (directional devices that are difficult countries

to implement) not used in France

resistance-earthed limits earth fault currents, the most beneficial for industrial facilitates earth fault current distribution: it combines all thedetection and protection advantagesdiscrimination, limits overvoltage

isolated from earth limits earth fault currents risk of overvoltage no tripping when the first earth fault requires the use of over-insulated occurs entails:equipment (line voltage between phase being granted specialand earth when a zero impedance dispensation (French law)fault occurs), that the capacitance between overvoltage protection advisable, active network conductors and insulation monitoring obligatory earth do not cause earth fault(French law), current that is dangerous for 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.


fig. 23: phasor diagram of network voltage drop

Voltage drops in a network may becalculated using the followingmathematical expression:∆V = R I cos ϕ + X I sin ϕThe electrical diagram and vectordiagram which correspond to thisequation are given in figure 23.

V

F

EθCD

B

Aϕ

I

ϕVa

Vd

XI

RI

RL

loadVaVd

sourceω

I

cable

AD = RI cos ϕDE = BC sin ϕ + CF sin ϕDE = BF sin ϕ = XI sin ϕ

This logic diagram comprises twoloops: the first one, the analysis andselection 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"; the second is aimed at optimizationof the structure.

analysis and selection loop leading to the organization of a structure

structure optimization loop

appendix 5: network voltage drops(mathematical expression and vector diagram)


Needs and constraints to

be met

NO

Major needs met and/or constraints

masteredload 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 time values 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 power to be supplied$ the distance between sources and loads$ local practices and habits

Reactive energy compensationUsing the following main criteria for choosing the type of compensation:$ feeder length and cross section$ desired increase in available active power

Backup sources$ standards$ laws

Replacement sources$ maintaining of production facilities operation$ power supply convenience

Autonomous electrical power productionIf the industrialist:$ has available vapour produced by the process$ has fuel available at a low cost$ is unable to obtain a strong or reliable utility supply

Division of sourcesTo delimit areas disturbed by:$ starting of large motors$ power electronic systems$ etc.

Calculation of voltage fluctuations under normal and 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 disturbed operating conditions$ short-circuit current withstand$ cable screen withstand to single-phase and/or multiphase faults$ contact voltage$ etc.

Insulation coordination analysis

Choice of earthing systems among the following possible diagrams:$ isolated neutral$ directly earthed neutral$ neutral earthed via a reactor or Petersen extinction coil$ neutral earthed via a resistor

YES

Overall single-line

diagram


Merlin Gerin Cahiers Techniques Protection of electrical distributionnetworks by the logic selectivity systemCahier Technique n° 2F. SAUTRIAU

Mise à la terre du neutre dans unréseau industriel haute tensionCahier Technique n° 62F. SAUTRIAU

Gestion de l'énergie dans lesprocessus industrielsCahier Technique n° 133C.G. POUZOLS

Surtensions et coordination del'isolement,Cahier Technique n° 151D. FULCHRION

Calcul des courants de court-circuit,Cahier Technique n° 158R. CALVAS, B. DE METZ NOBLAT,A. DUCLUZAUX and G. THOMASSET

Lightning and HV electricalinstallations,Cahier Technique n° 168B. DE METZ NOBLAT

Miscellaneous publications Overvoltage when clearing short-circuits from networks with a reactance-earthed neutral system.Bulletin de la société Française desElectriciens, series 8, Volume 1, n° 4(April 1960)LE VERRE

appendix 7: bibliography

Réal.: Sodipe - ValenceEdition: DTE - Grenoble10/94 - 2500 - Printing: Leostic - Seyssinet - ParisetPrinted in France

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