Estimation of Plant Electrical Load

Estimation of Plant Electrical Load

One of the earliest tasks for the engineer who is designing a power system is to estimate the normaloperating plant load. He is also interested in knowing how much additional margin he should includein the final design. There are no 'hard and fast' rules for estimating loads, and various basic questionsneed to be answered at the beginning of a project, for example,

o Is the plant a new, 'green field' plant?

o How long wil l the plant exist e.g. 10,20,30 years?

o Is the plant old and being extended?

o Is the power to be generated on site, or drawn from an external utility, or a combination of both?

o Does the owner have a particular philosophy regarding the 'sparing' of equipment?

o Are there any operational or maintenance difficulties to be considered?

o Is the power factor important with regard to importing power from an external source?

o If a generator suddenly shuts down, will this cause a major interruption to the plant production?

o Are there any problems with high fault levels?

1.1 PRELIMINARY SINGLE.LINE DIAGRAMS

In the first few weeks of a new project the engineer will need to roughly draft a key single-linediagram and a set of subsidiary single-line diagrams. The key single-line diagram should show thesources of power e.g. generators, utility intakes, the main switchboard and the interconnections tothe subsidiary or secondary switchboards. It should also show important equipment such as powertransformers, busbars, busbar section circuit breakers, incoming and interconnecting circuit breakers,large items of equipment such as high voltage induction motors, series reactors for fault currentlimitation, and connections to old or existing equipment if these are relevant and the main earthingarrangements. The key single-line diagram should show at least, the various voltage levels, systemfrequency, power or volt-ampere capacity of main items such as generators, motors and transformers,switchboard fault current levels, the vector group for each power transformer and the identificationnames and unique 'tag' numbers of the main equipment.

The set of single-line diagrams forms the basis of all the electrical work carried out in aparticular project. They should be regularly reviewed and updated throughout the project and issued

Handbook of Electrical Engineering: For Practitioners in the Oil, Gas and Petrochemical Industry. Alan L. Sheldrake@ 2003 John Wiley & Sons, Ltd ISBN: 0-471-49631-6

HANDBOOK OF ELECTRICAL ENGINEERING

Thble 1.1. Voltages used in different countries for generation,distr ibution and transmission

Low voltagegeneration andthree-phaseconsumers (volts)

High voltagegeneration and

distribution(kilovolts)

High voltagetransmission

less than 75 kV(kilovolts)

9 1 0660600*525500480460440*420415*400*380*346277260) \ a++

240**230**220**208200190

l 8l 614.413 .8 *13.2*12.612.512.4712.4t 21 1 . 5lt.4I l -10.41 098.98.48.387 .3

7.2-6.96.6*6.56.36.246.5.554.84.16r4J . J

J

2.42.3

lo 24.569* 2466* 2365 2260 2050 t946 154544383635? / < +

J J

3 1 . 53027.627.5272524.9

Notes* Commonly used voltages in the oil industry.Notes** Commonly used as single-phase voltages.

in their final form at the completion of the project. They act as a diary and record the developmentof the work. Single-line diagrams are also called 'one-line diagrams'.

At this stage the engineer can begin to prepare a load schedule for each subsidiary switchboardand motor control centre, and a master schedule for the main switchboard. The development of thesingle-line diagrams during the project is discussed in sub-section 1.7.

The master load schedule will give an early estimate of the total power consumption. Fromthis can be decided the number of generators and utility intakes to install. The kW and kVA ratingsof each generator or intake will be used to determine the highest voltage to use in the powersystem. Table 1.1 shows typical voltages used throughout the world for generation, distribution andtransmission of power at oil industry plants, see also sub-section 3.7.

1.2 LOAD SCHEDULES

Each switchboard will supply power to each load connected to it and in many cases it will also supplypower to switchboards or distribution boards immediately downstream. Hence the input power to a

ESTIMATION OF PLANT ELECTRICAL LOAD

switchboard will have the possibility of two components, one local and one downstream. Hereinafterthe term switchboard will also include the term motor control centre, see sub-section 7.1.

Each local load may be classified into several different categories for example, vital, essentialand non-essential. Individual oil companies often use their own terminology and terms such as'emergency' and 'normal' are frequently encountered. Some processes in an oil installation mayhandle fluids that are critical to the loss of power e.g. fluids that rapidly solidify and therefore mustbe kept hot. Other processes such as general cooling water services, air conditioning, sewage pumpingmay be able to tolerate a loss of supply for several hours without any long-term serious effects.

In general terms there are three ways of considering a load or group of loads and these maybe cast in the form of questions. Firstly will the loss of power jeopardise safety of personnel orcause serious damage within the plant? These loads can be called 'vital' loads. Secondly will the lossof power cause a degradation or loss of the manufactured product? These loads can be called the'essential' loads. Thirdly does the loss have no effect on safety or production? These can be calledthe'non-essential ' loads.

Vital loads are normally fed from a switchboard that has one or more dedicated generatorsand one or more incoming feeders from an upstream switchboard. The generators provide powerduring the emergency when the main source of power fails. Hence these generators are usuallycalled 'emergency' generators and are driven by diesel engines. They are designed to automaticallystart, run-up and be closed onto the switchboard whenever a loss of voltage at the busbars of theswitchboard is detected. An undervoltage relay is often used for this purpose. Testing facilities areusually provided so that the generator can be started and run-up to demonstrate that it is ready torespond when required. Automatic and manual synchronising facilities can also be provided so thatthe generator can be loaded during the tests.

Low voltage diesel generators are typically rated between 100 and 500 kW, and occasionallyas large as 1000 kW. High voltage emergency generator ratings are typically between 1000 and2500 kW. The total amount of vital load is relatively small compared with the normal load and, inmany situations, the essential load. Consequently the vital load is fed from unintemrptible powersupplies (UPS), as AC or DC depending upon the functions needed. The vital loads are usually fedfrom a dedicated part of the emergency switchboard. The UPS units themselves are usually providedwith dual incoming feeders, as shown in Figure 17.3.

Some of the vital and essential loads are required when the plant is to be started up, and thereis no 'normal'power available. In this situation the starting up of the plant is called'black starting'.The emergency generator must be started from a source of power, which is usually a high capacitystorage battery and a DC starter motor, or a fully charged air receiver and a pneumatic starter motor.

In many plants, especially offshore platforms, the vital and essential loads operate at lowvoltage e.g. 380, 400, 415 volts. Large plants such as LNG refrigeration and storage facilities requiresubstantial amounts of essential power during their start-up and shut-down sequences and so highvoltage e.g.4160,6600 volts is used. The vital loads would sti l l operate at low voltage. Tables 1.2and 1.3 shows typical types of loads that can be divided into vital and essential loads.

All of the vital, essential and non-essential loads can be divided into typically three duty categories:

r Continuous duty.

o Intermittent duty.

o Standby duty (those that are not out of service).

4 HANDBOOK OF ELECTRICAL ENGINEERING

Table 1.2. Vital and essential AC loads

Vital AC loads Essential AC loads

UPS supplies Diesel fuel transfer pumpsEmergency lighting Main generator auxiliariesEmergency generator auxiliaries Main compressor auxiliariesHelicopter pad lighting Main pump auxiliariesControl room supplies Diesel fire pump auxiliariesVital LV pumps Electric fire pumps

Living quartersAir compressorGeneral service water pumpsFresh water pumpsEquipment room HVAC suppliesLife boat davitsAnti-condensation heaters in

panels and switchboardsSecurity lighting suppliesControl room suppliesUPS suppliesRadio suppliesComputer suppliesBattery chargers for engine

starting systemslnstrumentation supplies

Table 1.3. Vital DC loads

Public address systemPlant alarm systemsSystem shutdown systemTelemetry systemsEmergency radio suppliesFire and gas detection systemNavigation aids

Hence each switchboard will usually have an amount of all three of these categories. Callthese C for continuous duty, 1 for intermittent duty and S for the standby duty. Let the total amountof each at a particular switchboard j be C;rr., fru- and S;rr-. Each of these totals will consist ofthe active power and the corresponding reactive power.

In order to estimate the total consumption for the particular switchboard it is necessary toassign a diversity factor to each total amount. Let these factors be Dri for Cru.;, Dii for lsumy andDr1 for S.u-;. Oil companies that use this approach have different values for their diversity factors,largely based upon experience gained over many years of designing plants. Different types of plantsmay warrant different diversity factors. Table 1.4 shows the range of suitable diversity factors. Thefactors should be chosen in such a manner that the selection of main generators and main feeders froma power utility company are not excessively rated, thereby leading to a poor choice of equipment interms of economy and operating efficiency.

Type of project


Table 1.4. Diversity factors for load estimation

D" for Cru* D; for /ru. D" for S.u-

Conceptual design of a newplant

Front-end design of a newplant (FEED)

Detail design in the first half ofthe design period

Detail design in the second halfof the design period

Extensions to existing plants

1 .0 t o L l

1 . 0 r o l . l

1 .0 t o l . l

0 .9 to 1.0

0.9 to 1.0

0.5 to 0.6

0.5 ro 0.6

0.5 to 0.6

0.3 to 0.5

0.3 to 0.5

0.0 to 0.1

0.0 to 0.1

0.0 to 0.1

0.0 to 0.2

0.0 to 0.2

The above method can be used very effectively for estimating power requirements at thebeginning of a new project, when the details of equipment are not known until the manufacturers canoffer adequate quotations. Later in a project the details of efficiency, power factor, absorbed power,rated current etc. become well known from the purchase order documentation. A more accurate formof load schedule can then be justified. However, the total power to be supplied will be very similarwhen both methods are compared.

The total load can be considered in two forms, the total plant running load (TPRL) and thetotal plant peak load (TPPL), hence,

TPRL: f {D.c,u*; * Dil,u*.r) kw

TPPL: I{D.C,'-; .| Di1,o-j * D"S,u*j) kwi - l

Where n is the number of switchboards.

The installed generators or the main feeders to the plant must be sufficient to supply the TPPLon a continuous basis with a high load factor. This may be required when the production at the plantis near or at its maximum level, as is often the case with a seasonal demand.

Where a plant load is predominantly induction motors it is reasonable to assume the overallpower factor of a switchboard to be 0.87 lagging for low voltage and 0.89 lagging for high voltagesituations. If the overall power factor is important with regard to payment for imported power, andwhere a penalty may be imposed on a low power factor, then a detailed calculation of active andreactive powers should be made separately, and the total kVA determined from these two totals. Anynecessary power factor improvement can then be calculated from this information.

1.2.1 Worked Example

An offshore production and drilling platform is proposed as a future project, but before the detaildesign commences it is considered necessary to prepare an estimate of the power consumption. Theresults of the estimate will be used to determine how many gas-turbine driven generators to install.

n


Table 1.5. Subsidiary load schedule for the low voltage process switchboard

Description of load No. ofunits

Namepla te Cont inuousratings of powereach unit consumed

(kw) (kw)

Intermittent Standbypower power

consumed consumed(kw) (kw)

Production area lightingGlycol pumpsGlycol reboilersGlycol transfer pumpRefrigeration compressorDeareator vacuum pumpsWater injection booster pumpsDeareator chem. injection pumpsSea drain sump pumpsWater inj. chem. injection pumpsReclaim oil pumpsTreated water pumpsOil transfer pumpsElectric gas heatingHP gas comp. preJube pumps2 x HP gas comp. auxiliariesLP gas comp. pre-lube pumps2 x LP gas comp. auxiliaries4 x water inj. pump auxilianesCorrosion inhibitor pumpsTrace heating

2/2l32A

A

I622/z

2222A

I2

150A

75l 5

32030

2704

1 08

J I

13230

3005

355

35'70

J I

80

75z

I J

I J

16030902

l 02

) t

t321 0

3005

,40

0000000000

) t

t J z

l 000

350

357000

0I

750

1603090404000

3005

1005

100200

00

Sub-totals for the switchboard 1652 319

Normal running load for the switchboard : ( l .0 x 1652) * (0.5 x 319) + (0.1 x lO77) : 1920 kW

Table 1.6. Subsidiary load schedule for the low voltage utilities switchboard

l0'7'7


Nameplateratings ofeach unit

(kw)

Continuouspower

consumed(kw)

Intermittent Standbypower power

consumed consumed(kw) (kw)

Utilities area lightingPotable water pumpsLiving quarters feeder-A(on)Living quarters hot water pumpsControl room suppliesComputer suppliesRadio suppliesInstrument air compressorInstrument air driersPlant air compressorsHVAC fansHVAC main air handling unitHVAC standby air handling unitHVAC refrigeration unitCas turbo-generalor auxi l iar iesTrace heating

r505

300l 5l 51 530901 09088300

l 510060

75

500l 51 5l 53090l 090l l30l 01 5

30

22I

2I2222z

l 6

A

2

00000

t 53090l 090880

l 00

3000

05

200l 50000000000

1000

Sub-totals for the switchboard

Normal running load for the switchboar6: (1.0 x 1013) * (0.5 x

1013 320

320) + (0.1 x 633) : 1236 kw600


This in turn will enable an initial layout of all the facilities and equipment to be proposed. Since this isa new plant and the preliminary data is estimated from process calculations, mechanical calculationsand comparisons with similar plants, it is acceptable to use the following diversity factors, D,:7.0,Di : 0 .5 and D" : 0 .1.

Tables 1.5, 1.6, 1.'l and 1.8 show the individual loads that are known at the beginning ofthe project.

The total power is found to be 12,029 kW. At this stage it is not known whether the plant iscapable offuture expansion. The oil and gas geological reservoir may not have a long life expectation,and the number of wells that can be accommodated on the platform may be limited. The 4000 kWof power consumed by the drilling operations may only be required for a short period of time e.g.one year, and thereafter the demand may be much lower.

Table L.7. Subsidiary load schedule for the low voltage emergency switchboard

Description of load No. of Nameplateunits ratings of

each unit(kw)

Continuouspower

consumed(kw)

Intermittentpower

consumed(kw)

Standbypower

consumed(kw)

Emergency lightingChlorine generatorDesalination unitPotable water pumpsInstrument air compressorsInstrument air driersLiving quarters feeder-B(off)Living quarters emergency feederLiving quarters hot water pumpsDiesel fuel transfer pumpEmergency diesel eng. sump heaterEmergency diesel gen. auxiliariesEmergency diesel eng. bat. chargerEmergency diesel eng. room fansControl room fansComputer UPS supplyEmergency radio suppliesNavigation aids UPS supplyLife boat davit suppliesLife boat diesel heater suppliesFire pump engine battery chargersSeawater washdown pumpAnti-condensation swbd heatersAnti-condensation motor heatersPortable lighting supplies

r 7 5l * 30l * 75l 51 9 0l l 0l* 500l 100l t 5l 51 3t 2l l2 1 12 2 2l 5l l 01 1 0

2 42 2 2l * 37

) \25

l l

1530755

90l00

50l55J

00

l l

2250

1 00i

220

25l 0I

0000000

500000I00000

504

22370

t 50

0000000000020000

l00

) 4

000000

Sub-totals for the switchboard

Normal running load for the switchboard: (1.0 x 468) + (0.5 x

For black start delete loads marked (*)

Black start sub-totals for the switchboard: (1.0 x 363) * (0.5 x

468 119

179) + (0.1 x 58) :563 kw

142) + (0.1x 0) :434 kW

58


Table 1.8. Master load schedule for the hish voltase main switchboard


Nameplateratings ofeach unit

(kw)

Continuouspower

consumed(kw)

Intermittentpower

consumed(kw)

Standbypower

consumed(kw)

HV motor loadsMain oil expert pumpsGas compressorSeawater lift pumps

LV motor loctdsFeeder to drillingFeeder to LV process MCCFeeder to LV utilities MCCFeeder to LV emergency MCC

3A

A

I2I1

650500450

0000

I 3001500I 350

270016521 0 1 3168

000

2400319320179

650500450

1000t07'763358

Sub-totals 9983 3218 4368

T o t a l s f o r t h e m a i n g e n e r a t o r t o s u p p l y - ( 1 . 0 x 9 9 8 3 ) + ( 0 . 5 x 3 2 1 8 ) * ( 0 . 1 x 4 3 6 8 ) : 1 2 , 0 2 9 k W

During the detail design phase of the project the load schedules will be modified and additionalloads will inevitably be added. At least 707o extra load should be added to the first estimate i.e.1203 kW. The total when rounded-up to the nearest 100 kW would be 13,300 kW.

Sufficient generators should be installed such that those that are necessary to run should beloaded to about 80 to 857o of their continuous ratings, at the declared ambient temperature. Thissubject is discussed in more detail in sub-section 1.3. If four generators are installed on the basis thatone is a non-running standby unit, then three must share the load. Hence a reasonable power ratingfor each generator is between 5216 kW and 5542 kW.

1.3 DETERMINATION OF POWER SUPPLY CAPACITY

After the load has been carefully estimated it is necessary to select the ratings and numbers ofgenerators, or main incoming feeders from a power utility company. Occasionally a plant may requirea combination of generators and incoming feeders e.g. refinery, which may operate in isolation or insynchronism with the utility company.

Usually a plant has scope for expansion in the future. This scope may be easy to determineor it may have a high degree of uncertainty. The owner may have strong reasons to economiseinitially and therefore be only willing to install enough capacity to meet the plant requirements inthe first few years of operation. If this is the case then it is prudent to ensure that the switchgear inparticular has adequate busbar normal current rating and fault current rating for all future expansion.The main circuit breakers should be rated in a similar manner. If the switchgear is rated properly atthe beginning of a project, then all future additions should be relatively easy to achieve in a practicaland economical manner. Such an approach also leads to a power system that is easy to start up,operate and shut down.

The supply capacity normally consists of two parts. One part to match the known or initialconsumption and a second part to account for keeping a spare generator or feeder ready for service.

ESTIMATION OF PLANT ELECTRICAL LOAD 9

Any allowance required for future load growth should be included in the power consumption calcula-tions. This two-part approach is often referred to as the 'N - I philosophy', where N is the numberof installed generators or feeders. The philosophy is that under normal operating conditions in a fullyload plant N * I generators or feeders should be sufficient to supply the load at a reasonably highload factor.

Let P1 - power consumption required at the site ambient conditionsPe : rated power of each generator or feeder at the site ambient conditionsF, : overload power in Vo when one generator or feeder is suddenly switched out of servrce

4 : load factor in Vo of each generator or feeder before one is switched out of serviceN : number of installed generators or feeders. N is usually between 4 and 6 for an

economical design of a generating plant and 2 or 3 for feeders.

P1 and P, are usually the known variables, with d and Fo being the unknown variables.Several feasible ratings of P, may be available and the value of N may be open to choice. A goodchoice of P, and N will ensure that the normally running load factor is high i.e. between 70Vo and857o, whilst the post-disturbance overload on the remaining generators or feeders will not be so highthat they trip soon after the disturbance, i.e. less than 1257o.

The initial load factor can be found as,

, _ 100P, ,-' ' - P | ( N - l )

The post-disturbance overload can be found as,

n': =Y\-voPs(N -2 )

If it is required that Fi is chosen for the design such that F :7O0Vo and no overload occursthen let F be called F;1ss and so,

( N - 2 ) 1 0 0f i t o o :

N _ , f o r n o o v e r l o a d i n g .

Table 1.9 shows the values of F; against N for the no overloading requirement.

Table 1.9. Selecting N and Firoo on thebasis of N - I capacity with overloading nottolerated

No. of installed Value of F;1ss togenerator or ensure no overloadingfeeders N Firc1Vo

2 Not practical3 50.04 66.615 75.006 80.007 83.338 86 .71


Table 1.10 shows the values of the load factor F; and the overload factor F, for a range oftypical power consumptions Py. The values of P, are the site requirements and relate approximatelyto the ratings of gas-turbine generators that are available and used in the oil industry i.e. 2.5 to40.0 Mw.

The table was compiled by constraining { and F, to be within good practical limits,

66.'7Vo < 4 < 90.07o

and8 0 < f , { 2 5 V o

Table L.L0. Selecting generator ratings on the basis of N - 1 capacity with tolerance of overloading

Powerconsumption(Mw) &

Initialload factor

(vo)F,

Number ofinstalled

generatorsN

Generator rating atsite conditions

(Mw)D

Turbine ISO ratingsfor a site amb. temp

of 40"C (MW)4so40

Final loadfactor(vo)

F

l 0l 0l 01 0l 01 01 01 5l 51 5l 51 51 5202020202525252530303030404040404040

66.774.183.371.483.366;l80.066.175.083 .366.775.085.766.766.780.088.969.483.383.366.771 .483.375.080.066.-774.183.3-7 1.483 .366.'7

5 .0A <

4.03 .53.03.02.57.55.04 5

4.03.5

r0 .0'7.5

5.0A <

12.010 .57.57.5

14.012.010.07.5

20.018 .016.014.012.0t2.o

5.95.34.74 . 13 .53 .52.98 .85.95 .35 .34.74 . 1

I 1 . 88 .85.95.3

l4. l1 1 . 88.88.8

16 .514.11 1 . 88 .8

23.52 r . 21 8 . 816 .514.114.1

A

4A

56f)

A

55666A

56644564456A

i

+556

100.0l l l . l125.095.2

l l l . l83 .3

100.0100.0100.01 1 1 . 183.393.8

107. I100.089.9

100.0l l l . lr04.2r25.0l l l . l83.3

to-l.rr25.0100.0100.0r00.0l l l . l125.095.2

1 1 1 . 183.3


Table 1.10. (continued)

11

Powerconsumption(MW) Pi

lnitialload factor

(vo)Fi

Number ofinstalled

generatorsN

Generator rating atsite conditions

(Mw)Ps

Turbine ISO ratings Final loadior a site amb. temp factor

of 40"C (MW) (Vo)Piso4o Fo

40606060606060606060808080808080808080

t00100100100100100100

80.066.772.180.066.775.083.366.775.085.766.716.266.772.780.088.97 l . l80.088.983.371.483.366.772.780.088.9

10 .030.027.525.022.520.018 .01 8 . 016.014.040.035.030.027.525.022.522.520.018 .040.035.030.030.027.525.022.5

I 1 .835.3) L - +

29.426.s23.52 t . 221.218 .816 .54'7.141.235.332.429.426.526.523.52 t . 24'7.14 r . 235.335 .332.429.426.5

100.0100.01 09.1120.088.9

100.01 1 1 . 183.393.8

107.1100.0114.388.997.0

106.7I 18.588.9

100.01 1 1 . 1125.095.2

I t 1 . l83.39r.9

100.0l l l . l

6A

a

1

555666A

A

55556664556666

In practice if F; is too high the operator of the plant will become nervous and will oftenswitch into service the spare generator. If F; is too low then there will be too many generators inservice and it should be possible to withdraw one. Gas turbines have poor fuel economy when theyare lightly loaded.

High values of F, should be avoided because ofthe risk ofcascade tripping by the gas turbines.The margin of overload that a gas turbine can tolerate is relatively small and varies with the turbinedesign. The higher the normal combustion temperature within the turbine, the lower the toleranceis usually found to be available. A high overload will also be accompanied by a significant fall inelectrical system frequency, caused by the slowing down of the power turbine and the relativelylong time taken by the speed governing system to respond. Many power systems that use gas-turbinegenerators are provided with underfrequency and overfrequency protective relays, and these maybe set to trip the generator when a high overload occurs. The initial rate of decline in frequency isdetermined by the moment of inertia of the power turbine, plus the generator rotor, and the magnitudeof the power change seen at the terminals of the generator. See Reference 1. This subject is discussedand illustrated in sub-section 12.2.10 and Appendix D.

L2 HANDBOOK OF ELECTRICAL ENGINEERING

If F, is designed to be less than approximately 105Vo then the generators will be able toabsorb the overload until some corrective action by an operator is taken e.g. puts the spare generatorinto service.

However, it is also possible to introduce a high-speed load shedding scheme into the powersystem when F, is found to be above 1057o. Such a scheme will compute in an anticipatory mannerhow much consumption should be deleted in the event of a loss of one generator. The designer willbe able to predetermine enough low priority consumers to achieve the necessary corrective action.See Chapter 16.

The application of the N - I philosophy is less complicated with incoming feeders e.g. under-ground cables, overhead lines. N is usually chosen as 2 because it is not usually economical to usethree or more feeders for one switchboard. Both feeders are usually in service and so the 'spare'

does not usually exist. However, each feeder is rated to carry the full demand of the switchboard.Therefore with both in service each one carries half of the demand, and can rapidly take the fulldemand if one is switched out of service. This approach also enables a feeder to be taken out servicefor periodic maintenance, without disturbing the consumers.

1.4 STANDBY CAPACITY OF PLAIN CABLE FEEDERSAND TRANSFORMER FEEDERS

In sub-section 1.2 the three ways of considering consumers were discussed, and the terms, vital,essential and non-essential were introduced. Because of the sensitive nature of the vital and essentialconsumers with regard to personnel safety and production continuity, it is established practice tosupply their associated switchboards with dual, or occasionally triple, feeders. For non-essentialswitchboards it may be practical to use only one feeder.

For switchboards other than those for the generator or intake feeders it is established practice toadd some margin in power capacity of their feeders so that some future growth can be accommodated.The margin is often chosen to be 25Vo above the TPPL.

If the feeders are plain cables or overhead lines then it is a simple matter to choose theircross-sectional areas to match the current at the l25Vo duty.

For transformer feeders there are two choices that are normally available. Most power trans-formers can be fitted with external cooling fans, provided the attachments for these fans are includedin the original purchase order. It is common practice to order transformers initially without fansand operate them as ONAN until the demand increases to justify the fan cooling. Thereafter thetransformer is operated as ONAF, see sub-section 6.5. Adding fans can increase the capacity ofthe transformer by 257o to 35Vo, depending upon the particular design and ambient conditions. Thealternative choice is simply to rate the ONAN transformer for the l25vo duty, and initially oper-ate it at a lower level. The decision is often a matter of economics and an uncertainty about thefuture growth.

When standby or future capacity is required for transformers it is necessary to rate the sec-ondary cables or busbars correctly at the design stage of the project. Likewise the secondary circuitbreakers and switchgear busbars need to be appropriately rated for the future demand. The decisionto over-rate the primary cables or lines may be made at the beginning of the project or later whendemand increases. Asain this is a matter of economics and forecastins demand.


1.5 RATING OF GENERATORS IN RELATIONTO THEIR PRIME MOVERS

1.5.1 Operation at Low Ambient Temperatures

In some countries the ambient temperature can vary significantly over a 24-hour period, and itsaverage daily value can also vary widely over a l2-month period. The power plant designer shouldtherefore ascertain the minimum and maximum ambient temperatures that apply to the plant. Themaximum value will be used frequently in the sizing and specification of equipment. The minimumvalue will seldom be used, but it is very important when the sizing of generators and their primemovers are being examined.

Prime movers will produce more output power at their shafts when the ambient temperatureis low. The combustion air in the prime mover is taken in at the ambient temperature. Gas turbinesare more sensitive to the ambient air temperature than are piston engines.

If the ambient temperature is low for long periods of time then the power plant can generateits highest output, which can be beneficial to the plant especially if a seasonal peak demand occursduring this period of low temperature. In some situations a generator may be able to be taken out ofservice, and hence save on wear and tear, and fuel.

With this in mind the generator rating should exceed that of the prime mover when power isrequired at the low ambient temperature. A margin of between 5Vo and l07o should be added to theprime mover output to obtain a suitable rating for the generator. It should be noted that when theoutput of a prime mover is being considered, it should be the output from the main gearbox if oneis used. Gearbox losses can amount to 7Vo to 27o of rated output power.

1.5.2 Upgrading of Prime Movers

Some prime movers, especially new designs, ale conservatively rated by their manufacturer. As theyears pass some designs are upgraded to produce more power. As much as l\Vo to 757o can beincreased in this manner. If the power system designer is aware of this potential increase in ratingthen the generator rating should be chosen initially to allow for this benefit. At the same time thecables and switchgear should be rated accordingly.

Situations occur, especially with offshore platforms, where no physical space is available toinstall an extra generator and its associated equipment. Sometimes the main switchrooms cannotaccept any more switchgear, not even one more generator circuit breaker. Therefore the potential forupgrading a prime mover without having to make major changes to the electrical system is an optionthat should be considered seriously at the beginning of a project.

1.6 RATING OF MOTORS IN RELATIONTO THEIR DRIVEN MACHINES

The rating of a motor should exceed that of its driven machine by a suitable margin. The selectionof this margin is often made by the manufacturer of the driven machine, unless advised otherwise.The actual choice depends on various factors e.g.

l4 HANDBOOK OF ELECTRICAL ENGINEERING

Table 1.11. Ratio of motor ratins to the driven machine ratins

Approximate rating of themotor or machine (kW)

Margin of the motor ratingabove the machine rating (Vo)

Up to 1516.0 ro 55Above 55

t251 1 5l l 0

o The absolute rating of either the motor or the driven machine i.e. small or large machines.

o The function of the driven machine e.g. pump, compressor, fan, crane, conveyor.

o Expected operating level e.g. often near to maximum performance, short-term overload-ing permitted.

e Shape of the operating characteristic of the machine e.g. pressure (head) versus liquid flow rate ina pump.

o Change in energy conversion efficiency of the machine over its working range.

o Machine is driven at nearly constant speed.

o Machine is driven by a variable speed motor.

o Harmonic currents will be present in the motor.

o The nearest standard kW rating available of the motor.

o Ambient temperature.

Some rule-of-thumb methods are often stated in the purchasing specifications of themotor-machine unit, see for example Table 1.11, which applies to low voltage three-phaseinduction motors.

Where the driven machine is a centrifugal type i.e. pump or compressor, the shaft powermay be taken as that which occurs at the 'end of curve' operating point. This rule-of-thumb pointis defined as being l25Vo of the power required at the maximum operating efficiency point on thedesigned curve of pressure (head) versus fluid flow rate, at the rated shaft speed.

These rule-of-thumb methods can be used to check the declared performance and ratings froma machine manufacturer.

1.7 DEVELOPMENT OF SINGLE.LINE DIAGRAMS

Single-line diagrams are the most essential documents that are developed during the detail designphase of a project. They identify almost all the main items of power equipment and their associatedancillaries. Initially they define the starting point of a project. Finally they are a concise record ofthe design, from which all the design and purchasing work evolved.

The final single-line diagrams should contain at least the following information. Complicatedpower systems may require the single-line diagrams to be sub-divided into several companion dia-grams, in which aspects such as protection, interlocking and earthing are treated separately. Thisensures that the diagrams are not overly congested with information. The end results should beunambiguous and be easily read and understood by the recipient.


1.7.1 The Key Single Line Diagram

Switchboards and motor control centres:

o All switchboards and motor control centre names, bus-section numbers, line voltages, number ofphases. number of wires. frequency, busbar continuous current rating.

o Identification of main incoming, bus-section, outgoing and interconnecting circuit breakers, includ-ing spare and unequipped cubicles.

o Some diagrams show the cable tag number of the principal cables.

Generators:

o Names and tag numbers.

o Nominal ratings in MVA or kVA and power factor.

r D-axis synchronous reactance in per-unit.

r D-axis transient reactance in per-unit.

o D-axis sub-transient reactance in per-unit.

e Neutral earthing arrangements, e.g. solid, with a neutral earthing resistance (NER), with a commonbusbar, switches or circuit breakers for isolation.

o Current and time rating of the NER if used, and the voltage ratio of the earthing transformerif used.

Transformer feeders:


o Nominal ratings in MVA or kVA.

o Leakage impedance in per-unit.

o Symbolic winding arrangement of the primary and secondary.

r Line voltage ratio.

High voltage and large low voltage motors:


r Nominal ratings in kW.

General notes column or box:

Usually several notes are added to the diagram to explain unusual or particular features, such asinterlocking, limitations on impedance values for fault currents or voltdrop.

1,7.2 lndividual Switchboards and Motor Control Centres

o Switchboards and motor control centre name and tas number.

o Bus-section numbers or letters.o Cubicle numbers or letters.


. Line voltage, number of phases, number of wires, frequency, busbar continuous current rating.

o Busbar nominal fault breaking capacity in kA at 1 or 3 seconds.

o Identification of all circuit breakers, fuse-contactor units, and their nominal current ratings.

o Neutral earthing arrangements, e.g. connections to the incomers.

o Protective devices of all incomers, bus-section circuit breakers, busbars, and outgoing circuits.

o Interlocking systems in schematic form.

r Local and remote indication facilities.o Details of special devices such as transducers, automatic voltage regulators, synchronising schemes,

fault limiting reactors, reduced voltage motor starters, busbar trunking.

o Rating, ratio and accuracy class of current and voltage transformers.

o Identification of spare and unequipped cubicles.

o References to other drawing numbers, e.g. continuation of a switchboard, associated switchgear,drawing in the same series, legend drawing, cables schedule and protective relay schedule.

o Column or box for detailed notes.o Column or box for lesend of svmbols.

1.8 COORDINATION WITH OTHER DISCIPLINES

At the earliest practical time in a project the engineers will need to identify areas of engineeringand design where interfaces are necessary. An efficient system of communication and exchange ofinformation should be established and implemented at regular intervals. Meetings should be arrangedto discuss problem areas and short-falls in information. The following generally summarises what isneeded, particularly during the feasibility and conceptual stage of a project.

In order to be able to engineer an economical and efficient power system it is desirable forthe electrical engineer to have:

o A basic understanding of the hydrocarbon and chemical processes and their supporting utilitiese.g. compression, pumping, control and operation, cooling arrangements.

o A procedure for regular communication with engineers of other disciplines, e.g. instrument, process,mechanical, safety, telecommunications, facilities, operations and maintenance.

o An appreciation of the technical and economical benefits and shortcomings of the various electricalengineering options that may be available for a particular project.

o The technical flexibility to enable the final design to be kept simple, easy to operate and easyto maintain.

1.8.1 Process Engineers

The process engineers should be able to inform the electrical engineers on matters relating to theproduction processes and supporting utilities e.g.:

o Oil, gas, condensate and product compositions and rates, and their method of delivery to and froma plant.

o Variation of production rates with time over the anticipated lifetime of the plant.

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Fuel availability, rates and calorific values, pollution components e.g. sulphur, carbon dioxide,alkali contaminants, particle size and filtration.

Electrical heating and refrigeration loads, trace heating of vessels and piping.

Make available process flow diagrams, process and instrumentation diagrams, utilities and instru-mentation diagrams.

1.8.2 Mechanical Engineers

The mechanical engineers will normally need to advise on power consumption data for rotatingmachines, e.g. pumps, compressors, fans, conveyors, and cranes. They will also advise the poweroutput options available for the different types and models of prime movers for generators, e.g. gasturbines. diesel engines. gas engines.

In all cases the electrical engineer needs to know the shaft power at the coupling of theelectrical machine. He is then able to calculate or check that the electrical power consumption isappropriate for the rating of the motor, or the power output is adequate for the generator.

The mechanical engineer will also advise on the necessary duplication of machinery, e.g.continuous duty, maximum short-time duty, standby duty and out-of-service spare machines. He willalso give some advice on the proposed method of operation and control of rotating machines, andthis may influence the choice of cooling media, construction materials, types of bearings, ductingsystems, sources of fresh air, hazardous area suitability, etc.

The electrical engineer should keep in close 'contact' with the progress of machinery selectionduring the early stages of a project up to the procurement stage in particular, so that he is sure theelectrical machines and their associated equipment are correctly specified. Likewise after the purchaseorders are placed he should ensure that he receives all the latest manufacturers' data relating to theelectrical aspects, e.g. data sheets, drawings, changes, hazardous area information. See also Chapter 19and Appendix E.

1.8.3 Instrument Engineers

The process and instrument engineers will generally develop the operation and control philosophiesfor individual equipments and overall schemes. The electrical engineer should then interface to enablethe following to be understood:

Interlocking and controls that affect motor control centres and switchboards, generator controls,control panels, local and remote stations, mimic panels, SCADA, computer networking, displaysin the CCR and other locations.

Cabling specifications and requirements, e.g. screening, numbers of cores, materials, earthing,routing, segregation and racking of cables.

Power supplies for control systems, AC and DC, UPS requirements, battery systems.

Symbolic notation, e.g. tag numbers, equipment names and labels, cable and core numberingsystems.

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1.8.4 Communication and Safety Engineers

The communication and safety engineers will be able to advise on power supply requirements for:

o Radar, radio, telecommunications and public address.

o Aids to navigation, e.g. lamps, beacons, foghorns, sirens; also alarms, lifeboat davits, etc.

o Emergency routing and exit lighting systems.

o Supplies for emergency shut-down systems.

1.8.5 Facilities and Operations Engineers

These engineers do not normally contribute any power consumption data, but their input to the workof the electrical engineer is to advise on subjects such as equipment layout, access to equipment,maintainability, maintenance lay-down space, emergency exit routing, operational philosophies ofplant and systems, hazardous area classification.

REFERENCE

l. J. L. Blackburn, Applied protective relaying. Westinghouse Electric Corporation (1976). Newark, NJ 07101,USA. Librarv of Consress Card No. 76-8060.

Estimation of Plant Electrical Load

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