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Substation Design can be broken down Substation Design can be broken down

into the following partsinto the following parts

� Planning

� Engineering

� Construction

� Operation

Substation Design is not independent Substation Design is not independent

from the rest of the T & D systemfrom the rest of the T & D system

� Has to interface with the transmission

system power levels and voltage

� Has to be compatible with the distribution

equipment and design philosophy.

PlanningPlanning

� Perhaps the most critical as it will determine

need, location, how it is connected to the

distribution and transmission system, etc.

� Let’s look at the planning steps

PlanningPlanning--GeneralGeneral

� General planning Philosophy

Substation Voltage Range

MVA

Location How many square miles per substation

Indoor or out door

Insulation type Air, SF6

SCADA Controlled

Reliability expected/ what types of customers will it serve

Economics- How much is a customer willing to pay for his electric?

Substation VoltageSubstation Voltage

� Usually determined already. From past history

and now what voltages are available.

� Usually these voltages are transmission type

voltages: 115,138,230,345,500,765kV etc.

� Or Subtransmission type voltages

23,25,34.5,46,69kV etc.

� Distribution voltages 4, 12.47,13.8,23,25,34.5kV

Substation SizeSubstation Size

� How big is your standard design going to

be. I.E. how much load do you want to

serve off of this substation. Will it be

10MVA, 20MVA, 30MVA, 40MVA etc.

� This will depend on the area to be served

and the type of customers to be served, and

reliability you want to achieve

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Indoor/ Outdoor,Air, SFIndoor/ Outdoor,Air, SF66

� Space requirements will generally

determine this. If space is small and the

location is in an urban area then you may

want to consider an indoor design possibly

using SF6.

� If space or location is not a concern then

most probably an outdoor air substation is

the most economical

SCADASCADA

� Is SCADA required? Many substation do

not have SCADA and many do. It depends

on what level reliability is expected of the

substation.

Reliability and EconomicsReliability and Economics� Probably the most concern in today’s society that depends almost

entirely of having electrical service.

� Everyone wants 100% continuous service, but are they willing to pay for it.

� You hear about 9 nines of reliability or they want electric service 99.9999999% of the time or want it off only about .000000001 or 1.9 cycles/year

� Typically we can give about 99.95% or about 5 hours that the electric is off per year. This would be composed of about 2 to 3 outages per year lasting longer than 1 minute. About 30 momentary interruptions lasting about 2 seconds each and about 150 to 200 voltage sags less than 90% voltage per year of which 60 are less than 80% voltage per year. This is per an EPRI study. And is an average.

� But the cost for this service is about $0.07/kWH. If we double the rate to $0.14/kWH we can increase reliability to about 99.97% generally so is the customer willing to pay for that increase?

� Pretty much our designs have evolved to the reliability we have based on what the customer is willing to pay.

Planning Planning --High SideHigh Side

� High Side Configuration

� Breakers or Airswitch types of Design

� Loop or Radial

� Protection and Relaying

� Reliability

� Loading

� Fault Current

� Maintenance

High Side PlanningHigh Side Planning

� Since the substation will be serving a lot of

distribution circuits an outage of the substation

will affect a lot of customers. Therefore you will

have to decide how to feed the substation to

provide the most reliability at the most economical

cost. Usually this means a looped line. Also can

an air switch be used to provide protection of the

substation instead of a circuit breaker?

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Example of a Radial Substation design

Example of a looped design

We also try to be economical We also try to be economical

with looped lines with looped lines

� Breakers are expensive, but they are the

only thing that can interrupt fault currents

switch can not. Switches can only be

opened when the line is de energized.

� But switches are cheaper than breakers, so

we have devised a way in which we can use

switches on our looped lines to save money

A B C

12X Y

For a fault breaker 1 and breaker 2 clear. During the time the line is

de energized switch X and Y open. Breakers 1 and 2 close. If the

fault is still there Breaker 1 opens and stays open. Switch Y detects

voltage so it closes. Substation B is restored.

This is called a Sectionalizing Station

XFault

A B C

12X Y

XFault

In this station design Switch X is closed and switch Y is open under normal

conditions. For a fault between switch X and Breaker 1. Breaker 1 opens Breaker 1

may try to close to see if the fault is still there if so it will open and stay open then

Switch X opens as the line is de energized and Switch Y closes restoring restoring

substation B

This is called a Transfer Station

� We use only sectionalizing stations on the

transmission system

� We use both sectionalizing and transfer stations on

the Sub transmission system as a transfer station is

cheaper. Substation C is less reliable as it has

only one feed in a transfer scheme.

� Transmission system looped lines are also more

reliable as they have overhead shield wires above

the phase wires which intercept lightning strokes.

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Planning Planning --TransformerTransformer

� Transformer bank configuration

3 single Phase or 1 three phase

High side connection (Delta or Wye)

Low side Connection (Delta or Wye)

Ratings (OA, FA, FA)

Voltage regulation

Fault Current

Protection

Maintenance

PlanningPlanning-- Low SideLow Side

�Low Side Configuration

�Breakers or Recloser

�Loop or Radial

�Protection and Relaying

�Reliability

�Loading

�Fault Current

�Maintenance

Need StandardsNeed Standards

� To have cost effective designs there needs to be a standard configuration that is done for every substation.

� For this to happen you need to have Standards.

A. Engineering Standards

B. Construction Standards

C. Material Specification

D. Operation and Maintenance Standards

E. Control Standards

Conductor

a. Calculating impedance

b. Ampacity

c. Underground type

d. Fault current calculations

e. Connected kVA

f. Voltage

Transformers

a. 2 bushings

b. 1 bushing

c. 1 phase & 3 phase

d. Polarity

e. Delta vs Wye

Standards need to include

f. Construction

g. Tank Heating

h. Ferro resonance

I. Grounding banks

J. Ungrounded Wye problems

k. Different types of connections and the

advantages/disadvantages of each

l. Economic evaluations

m. Standards

Voltage regulators and LTC.

a. Vars and Power equations

b. Vector diagrams

c. Regulator construction

d. Regulator connections

e. Line drop compensation

f. Loss evaluation

g. Standards

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Capacitors

a. Construction

b. Where to put

c. Current limiting fuses

d. Capacitor switching

e. Back to back capacitor switching

f. Loss evaluation

g. Series capacitors

Circuit protection

a. Breakers versus reclosers

b. Construction

c. Fault current calculations

d. Minimum fault currents

e. Overloads

f. Fuses

g. Inrush

h. Asymmetry and X/R

I. Standards

Over voltage protection

a. Sizing arresters

b. Insulation Coordination

c. Separation distance

d. Arrester connections

e. Arresters for PQ

f. Shielding of wires

g. Mechanism of lightning

Power quality and the distribution line

a. PQ disturbance categories

b. Harmonics

c. Cause of sags

d. Motor drives and customer equipment

e. Grounding

Distributed generation

So now you want to build a So now you want to build a

Substation Substation

� Need a plan

� Need a Single Line

� Need a Three Line

� Need Construction Prints

Deliver power to customersDeliver power to customers

� Many customers

� At their locations

� On Demand

� Ready to use

� High Reliability

� Stable Voltage

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On Demand and ready to useOn Demand and ready to use

� Must provide power at the instant of

demand—Can’t wait Can’t average

� Power must be provided at Utilization

voltage

- Usually 120V nominal in the United

States. Between 220 and 250 in most other

countries.

High Reliability, low voltage High Reliability, low voltage

fluctuationfluctuation� Incredible availability expectation

� Nine nines of availability required in some industries or about 1 cycle per year. We in the US average about 99.9% or about 9 hours per year.

� Stable voltage

-Power Quality anomalies are unacceptable

-Voltage flicker unacceptable

Therefore the mission of Therefore the mission of

Transmission and Distribution Transmission and Distribution

is tois to� Get electrical energy to the customer

� Have capacity to meet the instantaneous

demand

� Availability somewhere between 4 and 9

nines.

� Voltage regulation to between 3%

� And do it at the lowest possible cost!!

Some laws of T&DSome laws of T&D

� Power is most economically produced at central stations

� Power must be distributed to many small loads.

� Utilization voltage is worthless for moving power

� It is more economical to move power at high voltage

� High voltage equipment has a greater cost but much greater capacity.

� It is costly to change voltage levels

Power is most economically Power is most economically

produced at large central stations:produced at large central stations:

� Despite all the hype about “dispersed generation” a

tremendous economy of scale still exists in favor of large

generation.

� DG is popular because

- there is a fringe always ready to welcome a new idea.

- DG has some merit: close to the customer. It has to beat

only the efficiency of generation/TD combination.

- Small hi tech generators(45% efficient) can easily beat

older central generation stations(35% efficient)

� Large hi tech generators(52% efficient) can

still beat everything else, even when T&D

costs are added.

� But utilities are stuck with “sunk costs” of

older units.

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Power must be distributed to Power must be distributed to

many small load points.many small load points.

� At Allegheny Energy we have about 1.5

million electrical customers and an average

peak load of about 8kW

� We have a generation capacity of about

12000MW

Utilization Voltage is worthless Utilization Voltage is worthless

for moving power any distancefor moving power any distance

� 120-250 volt single phase can move power

only a few hundred meters before conductor

and loss cost, and voltage drop becomes

unacceptable.

� European systems with 416V(p-p)

secondaries, are another matter. Here the

secondary basically replaces the single

phase laterals in American systems.

It is more economical to move It is more economical to move

power at high voltage.power at high voltage.

� The higher the voltage the lower the cost

per kilowatt-mile.

� However, higher voltage equipment has

greater minimum costs and greater

minimum capacity. Therefore you must

arrange for that kilowatt to be part of a large

amount of power being moved as one block.

It is costly to change voltage It is costly to change voltage

levelslevels

� “Transformation” accomplishes nothing in

moving power

� It can be afforded, but increases costs.

� Its purpose is to permit splitting power more

economically.

Lower voltage and split Lower voltage and split

path.The fundamental rule of path.The fundamental rule of

power system layoutpower system layout� Every time voltage is reduced the pathway

is split.

� Idea is to keep splitting power into smaller

and smaller units as it is moved nearer the

customer, all the while keeping it at a

voltage level that is most economical for

that amount of power being moved.

� The way it has happened over the years is

that you started at utilization voltage then

you started to have problems as the lines

became longer so you looked at the cost of

building another generator verses raising the

voltage and the economics associated with

that.

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This structure gives rise to This structure gives rise to

T&D levelsT&D levels� Levels are:

- High voltage grid(transmission)

-Switching stations

-Subtransmission

-Substations

-Primary Feeder

-service transformers

-Secondary circuits

Levels of T&D systemsLevels of T&D systems

� Each covers the entire system

� Each is indispensable in service

� Each has more units than the level above it.

� On average, units of lower capacity than the level

above it

� A total capacity greater than the level above it.

� Each level divides the system into “service areas”

Reliability problems are Reliability problems are

usually on the distribution usually on the distribution

systemsystem

Distinguishing between Distinguishing between

Transmission and DistributionTransmission and Distribution� By voltage class- Transmission is above 34kV,

distribution is below it(Niagara Mohawk)

� Transmission is above 69kV(Allegheny power)

� By function: Distribution is anything feeding service transformers(Central Maine Power)

� By configuration: transmission is a network distribution is radial(Houston Light and Power)

� By purpose: Transmission is everything built at least partly for stability and operating requirements, distribution is everything built solely to distribute power to customers(ABB)

Distribution system Distribution system

componentscomponents� Transmission

� Subtransmission

� Distribution Substations

� Primary feeders

� Laterals and Branches

� Service transformers

� Secondary circuits

� Service drops

Transmission levelsTransmission levels

1.1MV 1.1MV –– 115kV115kV

� A network- many paths between any two

points

� Provides voltage stability, and dispatch

ability functions, as well as power delivery

� Power from any generator can be moved

anywhere.

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Subtransmission 23kV to Subtransmission 23kV to

230kV230kV

� Transmission level equipment that exists

solely to route power to distribution

substations.

� Occasionally radial, and hence relatively

unreliable

substationssubstations

� Lower voltage to the primary distribution

level, and split power routing amoung

feeders

� Typically, power system configuration

changes from transmission network to radial

feeders here.

Primary feeder levelPrimary feeder level

� Three Phase network as far as

construction(at least in urban areas)

� Usually operated radially by proper

positioning of open/closed switches

� Single phase laterals

Service transformers and Service transformers and

secondarysecondary

� Radial secondary system in most cases

� Transformer to customer ratio varies from

about 1:2 to 1:12 depending on the utility’s

system.

Sometimes all equipment Sometimes all equipment

including lateral service including lateral service

transformer secondary is transformer secondary is

identified as the service levelidentified as the service level

� Although different voltages, common

operating character: an outage must be

repaired to restore service.

� Quite common designation in Europe

At some utilities, system is operated as At some utilities, system is operated as

three levels distinguished by different three levels distinguished by different operating/outage characteristics.operating/outage characteristics.

� Grid: relatively small number of circuits, complicated

electrical behavior, outages do not necessarily cause

interruptions, repairs are very involved.

� Distribution: High number of circuits, radial simple

electrical behavior, outages cause interruptions, restoration

is usually by switching or repair.

� Service: very high number of circuits, radial, simple

electrical behavior, outages always cause interruptions,

restoration requires repairs, repairs are generally quick and

easy.

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A brief History lessonA brief History lesson

� Why do we use 50 hertz or 60 hertz?

� Why do we use three phase?

� Why do we have the voltage levels we

have?

50 hertz/ 60 hetz50 hertz/ 60 hetz

� The original power systems started out as DC. As you varied the voltage or speed of the generator you increased or decreased the power. However you were limited to the distance traveled before voltage drop became a problem. Therefore, you had to increase the size of the wire or put additional generators along the way. You could not change voltage levels as transformers do not work with DC. Voltage levels at the point of utilization in customers house were about 100 volts in the US. But do to voltage drop voltages were supplied 10 % higher at about 110volts.

� Because you could not distribute power very far with DC at 110 volts, AC became an attractive alternative. There were many articles about the effect of using AC as it was felt at the time it was very deadly. The reason AC was so attractive was because you can transform it from one voltage level to another using transformers. AC requires the use of synchronous generators and these generators must work at a constant speed. Transformers could be built to operate at about 25 Hz. And motor speeds could be matched to give motion to the prime mover at about 1500 RPMs. But with 25Hz you had objectionable light flicker. In the US they were building engines that operate at about 1800RPMs and in Europe they were building engines that operated at 1500RPMs. So if you build a 4 pole generator you get 50 and 60 hertz respectively.

� Now as you interconnect generators together you have to match frequencies so you now have systems in the US that are operating at 60 hertz and systems in Europe that are operating at 50 Hertz. If you were building a power system today you may consider again DC as you now have power electronics to do voltage conversions or you may consider AC systems operating in the 100’s of hertz as transformer size would be reduced. Airplanes use 400 Hz systems

Why 3 phase Why 3 phase � Why do we use 3 phase instead of 2 phase 4 phase, etc.

� Well three phase originated from the fact that generators were originally single phase. But for an induction motor to operate you needed more than one phase to give you a rotating field. If you put a winding on the generator at 90 degrees to the original winding you can get 2 phase. The same size generator can be used and you get 1.414 the amount of power out. You need three wires to take the power out of the machine.

� If you put three windings on the generator you can get 1.5 the amount of power out of the machine without increasing the size(of course the prime mover has to be sized accordingly) and you can take the power out with 3 wires.

� If you go to 4 phase you get 1.53 times the power

out and you need 4 wires.

� If you go to infinitely many phase you get 1.57

times the power out and you need infinitely many

wires.

� So it appears that the best choice is either two or 3

phase as you have 3 wires, but with 3 phase your

get 1.5 the amount of power out of a generator so

3 phase became the number of phases we use.

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Why the voltage levels we Why the voltage levels we

have.have.� In the US you had 110 volts, this then became 115

because of voltage drop and finally became 120 volts and that became the standard. If you multiple 115 times two you get 230v and take that times 10 you get 2300V. This became an industrial voltage for a delta system. Take this times 10 and you get 23kV a subtransmission voltage(if you take 2300 time 15 you get 34.5kV). Take this times 3 and you get 69kV and times 2 you get 138kV. If you take 120 times 2 and times 10 you get 2400V. Then times 3 you get 7200V times the 1.732 you get 12.47 times 2 you get 25kV.

� The 230kV is 2 time 115kV, 345 is 3 time

115kV

Delta verses WyeDelta verses Wye

� Delta requires only three wires

� Delta- one wire can be connected to ground and and the system can keep on functioning.

� Detecting line to ground faults can be a problem.

� Arcing faults can be a problem

� Wye systems can be grounded or ungrounded

� Wye systems have good fault sensing for line to ground faults

� Insulation levels only have to be 57.7 % of that needed for Delta systems for solidly grounded wye systems.

� Therefore most of the systems used these days are multigrounded wye.

What can you do when you What can you do when you

plan a Substationplan a Substation� If you are starting out with an entirely new system

and you are going to manufacture the equipment

yourself, you can choose frequency, voltage,

Number of phases, and whether it is delta or wye

connected if you chose 3 phase.

� If it is an existing system all you can do is pretty

much locate the new circuit route and where the

substation goes. But there is a lot of work

involved in that.

Radial Distribution SystemRadial Distribution System

� Majority of distribution is radial primary/secondary

� One source: Voltage and power flow downhill

� Often built as a network, but radialized by open Switches

� Contingency backup achieved by “transfer Zones”

switching segments to other feeders during an outage.

� Advantages

-Traditional: equipment available, understandable

-Economy: It is the cheapest in many ways

-Easy to engineer so it works well.

Loop Distribution SystemsLoop Distribution Systems

� Operates either as open or closed loop.—open

loop requires more expensive protection

� Either way, has an open, or at least “Zero flow”

point at all times

� Real concept is: contingency back up comes from

other side of the loop.

� Used through out Europe, Africa, and much of

Asia

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Feeder 1

Feeder 2

Substation

Loop support. The simplest

possible contingency backup

shown here is a loop feeder

layout. This involves

building feeders in pairs and

operating them with an open

tie between their ends.

Additional switches located

along the way permit

isolation of outage segments

Network DistributionNetwork Distribution

� Many types, all expensive compared to

radial

� Capacity cost is actually less than for radial

� Protection and control cost is much greater

than for radial

� Usually installed for reliability issues

� Most popular type is secondary network.

Interlaced Secondary NetworkInterlaced Secondary Network

� Radial feeders, network secondary

� Can be engineered to be incredibly robust

� Multiple radial feeders, feeding alternate

service transformers

A Wise Rule for problems on A Wise Rule for problems on

the Electrical Systemthe Electrical System� A wise old engineer once told me that

� 1. 80% of the problems experienced are cause by moisture.

� 2. 15% of the problems experienced are caused by a bad ground

� 3. The remaining problems are cause by exotic stuff such as ferroresonance, tank heating etc.

Service AreasService Areas

� Usually are designed to correspond to each

distribution circuit as it leaves the

substation

Service areas are dynamic Service areas are dynamic

from a planning standpointfrom a planning standpoint

� Service area transfers are an important

element of expansion planning.

� You can transfer load to another substation.

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T&D System CostsT&D System Costs

� Land

� Preparation

� Equipment

� Installation & Setup

� Losses

� Maintenance

Cost to upgrade usually Cost to upgrade usually

exceeds cost to build:exceeds cost to build:

� One mile of new 600KCM 3 phase overhead feeder cost about $186K to build, 14MVA capacity or $13,300/MVA/mile

� One mile of new 336KCM 3 phase overhead feeder cost about $140K to build, 9.3MVA capacity or $15,000/MVA/mile

� On mile of 336 can be upgraded to 600KCM for 226K or $48,000/MVA/mile

Reliability is assured through Reliability is assured through

Quick Service restorationQuick Service restoration� A majority of the equipment in the system

is in radial configuration– any failure causes some customer interruptions.

� The equipment is usually simple, and relatively inexpensive and easy to repair.

� Restoration time is predominately a function of identifying the problem and travel to the outage site.

Distribution: the most Distribution: the most

important part of the power important part of the power

systemsystem� Connected to the customer

� 52% on the investment

� 66% of the losses

� 90% of the reliability and power quality

issues

The System’s ApproachThe System’s Approach

� Concept: the various levels of the system are connected to one another

-transmission

-substation

-feeder

-secondary

As a result the layout and design at one level influences requirements of the other levels

MultiMulti--System LevelsSystem Levels

� Each level has costs constraints and

interactions within it, that are unique to it.

� But it also depends on the levels connected

to it, for example the feeders all must start

at the substation

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Thus the planner’s goal is not to Thus the planner’s goal is not to

optimize any one level, but to optimize any one level, but to

optimize the combination of levelsoptimize the combination of levels––

The whole systemThe whole system

� The way one level is designed impacts the electrical and economic performance of the other levels connected to it.

� In many cases, interactions with the other levels are the most important aspect of the design!!

� Planner’s goal is to design the best combination of levels: the best system!!

Example:Example:

The following four design The following four design

questionsquestions� How far apart should substation be?

� What size should substations be?

� What is the best transmission voltage?

� What is the best primary voltage?

� All are versions of the same question!

How far apart should How far apart should

substations be?substations be?

� If the substations are moved farther apart

then each will have a larger service area

� If the stations are 4 km apart then each

covers about 16 square km

� If the stations are 6 km apart then each

covers about 36 square km

If the substation has a larger If the substation has a larger

area each will have a larger area each will have a larger

load to serveload to serve� Need larger more expensive substations

� But fewer will be needed

Is it better to have fewer, but larger Is it better to have fewer, but larger

and more expensive substations?and more expensive substations?

� Certainly part of the answer depends on the cost of

the substations and how it varies depending on

size etc.

� Generally the answer considering only the

substation is yes. For example, it is usually

cheaper to build 5 50 MVA substations than 11

25MVA substations.

� However substation cost is not the only issue

Substation size influences Substation size influences

Transmission designTransmission design� The system design with larger substations need

more power on average delivered to each

� Hence, the transmission system must be capable of delivering larger amounts on power to each substation

� But there will be fewer lines needed, because with larger substations, there are fewer lines.

� Are fewer but larger transmission lines less expensive?

� Even when you answer this, this is not the complete answer.

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Impact of DistributionImpact of Distribution

� Fewer, larger substations means that feeders

must move power farther.

� Requires higher voltage

� May require more feeders

� May require reinforced feeders

� This is often then a major expense

Fewer but larger substations Fewer but larger substations

require feeders to move power require feeders to move power

fartherfarther� Any way you look at it, the larger, fewer, farther

apart substations are will require a stronger

distribution system- one that moves power farther

� This means a more expensive feeder system but

this greater expense might be justified by the

savings in substation and transmissions systems

costs due to their greater “economy of scale”

The point:The point:

� The economics and electrical behavior of

the substation, of the feeder, and of the

transmission levels need to be added

together to determine cost and performance

as a whole.

� This is what planners must concentrate on.

This concept is the key to the design This concept is the key to the design

of low cost, workable T&D systemsof low cost, workable T&D systems� In order to determine what is the best transmission system

voltage to use you need to do a couple of cases: Price 138kV using 4/0 wire and 954kCM wire and determine the load it can carry: Price 230kV using 4/0 wire and 954kCM wire and determine the load it can carry.

� Price the cost of a 230 to 12.5kV substation 12/16/20MVA and 18/24/30MVA substation. Price the cost of 138kV to 12.5kV Substation using the same loading. Do all four cases again for 34.5kV distribution.

� Price the cost of 12.5 kV distribution using 336kCM wire and then with 795kCM wire

� Price the cost of 34.5kV distribution using 336kCM wire and then with 795kCM wire. Determine how much MVA each will carry.

Don’t forget reliability Don’t forget reliability

� As feeder get longer reliability goes down.

� Reliability problems are the sum of failures

anywhere in the chain from generation to

customer.

� If one needs to improve reliability where is

the lowest cost fix that can be made.

System ApproachSystem Approach

� Important points are:

- interactions of levels means costs and

performance depend on all levels, not any one.

-Operating interaction is often more important in

optimal design than actual economic performance

at that level

-the goal is to design the best overall system

taking into account all levels

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Need to determine the appropriate Need to determine the appropriate

information for the distribution planning information for the distribution planning engineer to useengineer to use

� Standards Group

1. Determine the equipment that is going to be

used.

2. Make a construction standard to install the

equipment.

3. Determine the cost for construction.

4. Determine the special engineering

requirements for the most economical system.

Need to have a Financial Planning Need to have a Financial Planning

Group to develop a consistent way to Group to develop a consistent way to

evaluate projectsevaluate projects� A computer program is the usual method to

do the evaluation and the group provides the

financial parameters to put into the program

Distribution System ReliabilityDistribution System Reliability

� Reliability

� Service Quality

� Ways to improve both

What is Reliability?What is Reliability?

� Reliability analysis involves quantitative

measures of system performance regarding

interruption of services, through historical

data analysis and theoretical predictions

Why Bother?Why Bother?

� The purpose of reliability engineering is to maintain service quality standards with limited capital investments

� How much is reliability worth?

-repair and emergency crew expense

-loss of revenue

-public image

-loss of customers

Outages and InterruptionsOutages and Interruptions

� An outage is what happens to equipment

when not in service.

� An interruption is what happens when

insufficient equipment exists to serve the

customer

� Outages cause interruptions

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Two Aspects of Service Two Aspects of Service

ReliabilityReliability

� Frequency of interruptions- How often is

the power interrupted.

� Duration of interruptions- How long do

interruptions last.

Frequency and Duration are Frequency and Duration are

somewhat independent somewhat independent

aspects of reliabilityaspects of reliability� Frequency of interruptions is mostly a

function of engineering factors, equipment

selection, layout, design of protection, etc.

� Duration of interruption is mostly a function

of operating factors: number and location of

repair crews, speed in handling trouble

calls, and dispatching, etc.

� Customers react differently to frequency and duration of interruptions.

� To some customers, a short (2 second) interruption is nearly as serious as a longer one: computers, robotic control, synchronous motors.

� To others short interruptions create few problems

Types of interruptionsTypes of interruptions

� Instantaneous: An interruption restored

immediately by completely automatic

equipment. It is caused by a momentary

fault that produces no reaction from

protective equipment. According to IEEE

1250-1995 it is .5 to 30 cycles in duration

� Momentary: An interruption restored by

automatic operation of protection

equipment. From 30 cycles to 2 seconds

� Temporary: An interruption restored with

supervisory control usually between 2

seconds and 2 minutes

� Sustained: Any interruption that is not

instantaneous, momentary, or temporary.

Normally more than 2 minutes. Usually

involves manual switching and/or repair

work

07/07/2013

18

Interruptions are also Interruptions are also

distinguished by whether they distinguished by whether they

were planned:were planned:� Scheduled: The utility scheduled the

interruption for maintenance purposes, and

the customer was given advanced warning

� Forced: The interruption was not expected

and not scheduled. You may have to open a

line to make a repair.

Reliability IndicesReliability Indices

� Combine frequency, duration, and other factors into a single value, a measure of reliability on the system.

� There are lots of them

� SAIDI: System Average Interruption Duration Index The average total duration of interruptions per customer during a period (month, year)

� This is the total number of interruption minutes divided by the number of customers.

� SAIFI: System Average Interruption Frequency Index The average number of interruptions per customer during a period(month, year)

� This is the total number of customer interruptions divided by the number of customers.

CAIDI CAIDI

� CAIDI: Customer Average Interruption

Duration Index- The average total duration

of interruptions per customer that had an

interruption during a period (month, year)

� This is the total number of interruption

minutes divided by the number or

customers who had at least one interruption

during the period.

CAIFICAIFI

� CAIFI: Customer Average Interruption

Frequency Index-The average number of

interruptions per customer during a

period(month,year)

� This is the total number of customer

interruptions divided by the total number of

customers who had at least one interruption

during the period.

MAIFIMAIFI

� MAIFI: Momentary Average Interruption

Frequency Index-The average number momentary

interruptions per customer during a period(month,

year)

� This is the total number of customer interruptions

divided by the total number of customers.

� This is a necessary index because momentary

interruptions are not counted among interruptions

by many utilities.

Reliability Reporting can be Reliability Reporting can be

misleadingmisleading

� Tremendous difference in reliability levels

reported by utilities just because of

differences in definition.

� Cannot compare reliability statistics

reported by different utilities(at least across

US State Lines because of different

commissions)

07/07/2013

19

Variations in definitionsVariations in definitions

� Interruption length. Most utilities don’t report interruptions below some minimum duration. This is because they have no way of knowing what happened and to which customers.

� Customer- Some utilities report master metered customers as one customer, others estimate the impact on a household

� Start time- Some utilities estimate interruption duration as starting when the outage occurred, others when the interruption was reported to dispatching

Determining reliability levelsDetermining reliability levels

� Comparison of years method

� Gather interruption data for the past years, the more the better. Ten years would be good.

� Plot SAIDI verses SAIFI

� You should see a pattern and any years that fall outside of that pattern my be considered bad years.

ValueValue--based planning is not based planning is not

that simplethat simple

� Data is difficult to find

� Customer costs are nearly impossible to

determine except for large industries

� Takes lots of work

� Functions are often discontinuous.

Building reliability into a Building reliability into a

Substation DesignSubstation Design

� Remember that engineering, design and

layout influence mainly the number of

interruptions. Good design can reduce the

number. But good design has little

influence on the duration of the interruption.

Engineering’s influence on Engineering’s influence on

ReliabilityReliability� Design and layout can reduce frequency of outage:

-Using equipment that fails less often(correctly sized, etc.) conductors slapping

-Coordination of equipment(insulation coordination, etc.) so equipment is coordinated with design and layout. Electronic reclosers with sequence coordination.

-using layout that reduces the extent of equipment outages: Cutouts, etc.

Engineering can influence Engineering can influence

duration only in one respectduration only in one respect� Remember we are talking about sustained outages

� Feeder layout and switching locations can be

coordinated so that interruptions can be restored

quickly by switching.

� This provides the potential for shorter duration

interruptions but requires competency and

preparation on the part of the Operations

Department to use this capability.

07/07/2013

20

Building reliability into a Building reliability into a

distribution systemdistribution system

� Use good equipment

� Maintain it well

– By schedules as recommended

– RCM- Reliability Centered Maintenance, O&M

controlled by analysis of importance, diagnostic

data

� Operate it well

Layout makes a big differenceLayout makes a big difference

� Can apply concepts manually in day to day

planning

� You now have to remember you are adding

another layer of criteria/constraints on top of

voltage standards, operation guidelines, etc.

� It would be nice to have a computer program to

help, but we haven’t found one yet.

Reliability versus cost is really Reliability versus cost is really

the only issuethe only issue� The type of system makes a big difference

� Non-Linear: a little reliability costs little. More costs a lot

more.

� Reliability is easy to build into a power system

� The problem is doing it economical

reliability

co

st

Secondary networks with Secondary networks with

interlaced feeder systeminterlaced feeder system� Using secondary networks, parallel feeders, serve

every other service transformer from each

� If properly designed, the resulting system can

tolerate a lose of a feeder without an interruption

European 11kV loop systemEuropean 11kV loop system� All feeders operate as closed loops

� Essentially two radial feeders with closed tie, no

laterals

� Outage interrupts flow only to customers on

segment

� Expensive: requires close to two times conductor

capacity, and protection is expensive.

Normally

closed

Normally Closed

Open LoopOpen Loop

� Most typical European design

� Outage in this example drops ¼ to ½ of the

customers

� Still requires 2 times capacity, but

protection is simple

� Switching restores some service.

Normally

closed

Normally closed

Normally open

07/07/2013

21

Loop Feeder Systems are Loop Feeder Systems are

good design but expensivegood design but expensive

� Every feeder must be built to support twice

the load and twice the distance

� Planners can reduce number of customers

affected by outages by adding sectionalizers

Normally

closed

Normally

closed

Feeder must support all the load with either segment out at the substation

Loop Zonal Transfer SchemeLoop Zonal Transfer Scheme

Normally

closed

Normally closed

Normally

closed

Normally closed

Normally

closed

Normally closed

Normally

closed

Normally closed

Normally

closed

Normally closed

Requires less capacity but lower reliability because more

involved switching needed to balance loads during outages

American Radial SystemAmerican Radial System

� Basic concept is transfer of radial, not loop,

zones.

� The feeder is built from sections or zones,

each switchable to at least two other zones

� Support is given by switching as needed

after an outage

� Least expensive

ValueValue--based Planningbased Planning� This is probably the best way, but it is

complicated.

� Utilities cost increases as reliability increases

� Customer’s loss dollars decrease as as reliability increases.

� Add the two together and the low point in dollars is the optimal point.

� But who pays? The utility or the customer?

� Using smaller feeders.

� Improving sectionalizing and protection– Fuses and sectionalizers protect the rest of the feeder

from outages behind them

– Segmenting feeder into many protection sections avoids outages spreading interruptions to many customers

Adding more switching zones Adding more switching zones

generally does not improve generally does not improve

reliability greatlyreliability greatly

� Dividing each feeder up into five switchable

zones, instead of three will not improve

reliability greatly. May make it more

difficult to switch

� It will reduce cost as neighboring sections

need less contingency margin to pick up

load. They are picking up 1/5 of the feeder

instead of 1/3

Automation and ReliabilityAutomation and Reliability

� Automated distribution offers two

capabilities that improve reliability

1. Monitoring- you can see trouble coming and

know what equipment can take during

contingencies.

2. Automated switching(verses manual) is

faster, more flexible and can do more

involved switching

07/07/2013

22

Automation and where it Automation and where it

improves reliabilityimproves reliability� Monitoring of conditions by

– RTUs

– On-line trouble analysis

� Remote Switching– Fast, cuts outage duration, but not frequency of the

outage

– May include resetting protection for a new configuration

� Computerized operations management– On line analysis of restoration

– Crew tracking, optimization

Three Levels of Automation Three Levels of Automation

regarding Distribution regarding Distribution

SystemsSystems� Static systems- Fuses, manual switching.

� Automatic system- automatic sectionalizers,

and reclosers

� Automated system-remote control of

sectionizers, and switches

Measures to improve Measures to improve

Reliability and Service QualityReliability and Service Quality� Need new tools and methods: cannot achieve a

goal unless you can measure and direct progress toward it.

� Understand customer’s need for and valuation of reliability and service quality

� Shift from standard-driven to performance based design

� Optimize reliability within budget: reliability centered planning and reliability centered maintenance.

Capacitors, Reactive Power Capacitors, Reactive Power

and Feeder Planningand Feeder Planning

Power Flow on the Feeder Power Flow on the Feeder

System is ComplexSystem is Complex

� The two components of complex power

flow

4.8MW

3 .6MVAR

6 MVA

Here, a flow of 4.8MW is desired and

3.6MVAR is undesired creates a total

of 6MVA of flow

Real Power MW, useable, sellable,

does work. Requires capacity to

move it, creates both losses and

voltage drop as it moves

Reactive Power MVAR, unusable,

unsellable, does no work. Requires

capacity to move it, creates lines

losses, X & R component of line

creates voltage drop.

Power Factor Refers to the Power Factor Refers to the

Ratio of Real to Total FlowRatio of Real to Total Flow

� Total flow is the magnitude of complex

power flow:

Total = Real + Reactive2 2

Power Factor = Real/Total

The Power Factor=4.8/6.0 =80%

07/07/2013

23

Reactive Flow is commomly called Reactive Flow is commomly called

“VARs” (Volt Amperes Reactive) and it is “VARs” (Volt Amperes Reactive) and it is unwantedunwanted

� VARS take up capacity on the line as

shown the line has to be able to carry

current equivalent to 6MW(6MVA), but is

only delivering 4.8MW because of the VAR

Flow

� VARs create voltage drop- so they use up

economic reach

Reactive Power Flows(VARs) Reactive Power Flows(VARs)

are caused by Loadsare caused by Loads� Many loads=particularly wound devices like

motors and solenoids create a lag between voltage

and current, in effect, is the source of VARs.

Only purely resistive loads like incandescent

lights, etc. are free of reactive loads.

� VARs that flow at the load from the current

lagging the voltage by 90 degrees are considered +

+

-V I

-P

+Q

+P

+Q

-P

-Q

+P

-Q

Reference

Voltage

I

I I

I

V

Shunt Capacitors are used to Shunt Capacitors are used to

produce produce ––VARs or anti VARsVARs or anti VARs� A shunt capacitor installed anywhere on a feeder

will produce VARs, satisfying the load

downstream of it.

� In the example we have been using a 1MVAR

capacitor produces .985kVAR reducing the VAR

load to 2.65MVAR. As a result real power flow

can be increased to 5.4MW( at 12.5%

improvement) while keeping the total flow under

its 6MVA capacity loading

Shunt Capacitors are voltage Shunt Capacitors are voltage

sensitive devicessensitive devices� They produce their rated VAR output at their rated

nominal voltage

� The 1 MVAR capacitor only produced .985kVAR

because the voltage was less than 1.0 P.U.

� The VARs produced flow downstream to serve the

VAR demand farther out. If there are less VAR

demand than the capacitor’s output the VARs

move upstream toward the feeder source.

VAR Flow DiagramsVAR Flow Diagrams

5

4

3

2

1

0

MVAR

0 1 2 3

Miles

5

4

3

2

1

00 1 2 3

Voltage Drop

•A useful tool for the study of

VARs and VAR correction is the

VAR flow diagram, which is a

profile of the VAR flow on the

feeder

•Here a 3 mile long feeder trunk

has a uniform VAR load of

2MVAR per mile. The diagram

shows VAR flow at all points along

the feeder

•Also shown is the voltage drop

along the feeder(not uniform

because real load and conductor

size are not all uniform.

07/07/2013

24

The Effect of a Shunt Capacitor is The Effect of a Shunt Capacitor is

easy to plot and seeeasy to plot and see5

4

3

2

1

0

0 1 2 3

3000kVAR

CAP bank

Here a 3000 kVAR shunt capacitor

bank has been installed at a point 1.5

miles out of the feeder

It satisfies all the VAR demand needs

after it.

Impact on VAR flow is as shown. The

substation is cut in half and there is a

point of zero VAR flow just before the

capacitor location.

5

4

3

2

1

00 1 2 3

Flow diagrams can be used to study Flow diagrams can be used to study

how to improve capacitor utilizationhow to improve capacitor utilization

5

4

3

2

1

0

0 1 2 3

3000MVAR

CAP bank

5

4

3

2

1

0

0 1 2 3

3000MVAR

CAP bank

Here we see what happens when

you move the capacitor farther out

the feeder.

-the VAR-miles of flow in the

unshaded area are removed and the

VAR-miles of flow shaded yellow

are added.

-the capacitor feeds some of its

VARs back toward the substation

-In total, VAR-miles of flow are

reduced.

This is an improvement, because the total This is an improvement, because the total

VARVAR--miles of flow have been reduced miles of flow have been reduced substantiallysubstantially

6

5

4

3

2

1

0

0 1 2 3

VAR-miles of

flow reducedThe reduction is equivalent to the

unshaded area

-This represents VAR-miles of flow

that no longer exist.

-Voltage drop is improved.5

4

3

2

1

00 1 2 3

With cap

No cap

Voltage Drop

The Capacitor can be moved until the amount The Capacitor can be moved until the amount of MVARof MVAR--miles gained and lost from any farther miles gained and lost from any farther

movement is the samemovement is the same

5

4

3

2

1

0

0 1 2 3

3000MVAR

CAP bank

This occurs for the 3000KVAR bank when it reaches a location at 2.25

miles from the source. At this point the MVAR-miles on the feeder are at a

minimum.

Voltage drop will be even better.

Similarly, the impact of changing Similarly, the impact of changing

capacitor size can be studiedcapacitor size can be studied

5

4

3

2

1

0

0 1 2 3

4000kVAR

CAP bank•By increasing the size of the capacitor

to 4500kVAR, the VAR-miles shown

in the unshaded area are removed and

those in the shaded yellow area are

added

•The result again is an improvement

more were removed than added.

5

4

3

2

1

0

0 1 2 3

4500kVAR

CAP bank

The TwoThe Two--Thirds Rule for Thirds Rule for

Capacitor ApplicationCapacitor Application

5

4

3

2

1

0

0 1 2 3

4000kVAR

CAP bank

If this method is applied to determine both the optimum size and location of a

single capacitor, what capacitor size and location results in the minimum VAR-

miles- The result will be a 4000kVAR capacitor located at 2 miles from the

source.

This is a graphically derived two thirds rule for capacitors. Among all single

capacitor applications this minimizes the resulting VAR-miles.

07/07/2013

25

TwoTwo--thirds rule, cont.thirds rule, cont.

A traditional rule-of-thumb for capacitor application to a feeder is to place a capacitor equal to 2/3’s the VAR load of the feeder at a point 2/3’s of the distance from the substation.

The graphical method of capacitor impact analysis or an algebraic equivalent, can be used to confirm that this minimizes the total MVAR-miles, provided the VAR loading is uniform.

This rule is one of the most widely used guidelines in all power distribution. Most power engineers are aware of it and apply it. Most do not know however, that it can be applied by way of a simple graphic method shown here and that it can be generalized to more than one capacitor

Generalized Two Thirds RuleGeneralized Two Thirds Rule

� The impact of two capacitor banks can be

similarly represented graphically.

A bit of experimentation will show

that the best two capacitor solution is

two capacitors equal to 2/5’s of the

VAR load located at 2/5’s and 4/5’s

of the distance out from the

substation

5

4

3

2

1

0

0 1 2 3

2400MVAR

CAP bank

2/5*6000kVAR=2400kVAR

Generalized twoGeneralized two--thirds rule cont.thirds rule cont.

Inspection of such graphs, or algebraic

manipulation that accomplishes the same

can establish that for a feeder uniformly

loaded with a VAR load of Q, the optimal

size for each of N equally sized capacitors

is: Size of each of N banks=2Q/(2N+1) at

evenly spaced locations L=n*2l/(2N+1)

where n=1,2,….,N l = length of feeder

Generalized twoGeneralized two--thirds rule cont.thirds rule cont.

As a result the MVAR-miles on a feeder are reduced from Q*l/2(the amount without any capacitors ) to

Total MVAR-miles=(Q*l/2)/(2N+1)

Therefore the best two capacitor solution is two equally sized banks of 2/5 of the VAR load of the feeder, located 2/5’s and 4/5’s of the way from the substation, which reduces MVAR-miles to 1/5 of their previous level. The best three capacitor bank solution are banks of 2/7’s the VAR load, at locations of 2/7’s, 4/7’s, and 6/7’s out from the source and they will reduce the VAR-miles to 1/7 of their uncorrected level,etc.

The ultimate end of this series would be for N to be very large, which would distribute a very large number of very small capacitors, equal to the total VAR load of the feeder, spaced uniformly along its length. This would reduce

VAR-miles to zero.

A Lot of feeder loads aren’t A Lot of feeder loads aren’t

exactly uniformly distributedexactly uniformly distributed

5

4

3

2

1

0

0 1 2 3

A lot of feeders may have non uniform

VAR load.

For one thing, loading both real and

reactive is discrete, not continuous

However, modeling loading as a

continuous distribution adds little error,

but the wrong distribution along the

trunk adds considerable error.

At the left is a more representative

loading distribution which represents a

large trunk feeder serving a triangular

area of uniform area VAR load.

When distribution is not longer uniform, When distribution is not longer uniform,

the best capacitor location is not longer the best capacitor location is not longer given by the 2/3’s rule.given by the 2/3’s rule.

Here, a capacitor equal to 87% of the feeder

VAR load located 2.25 miles out is about

optimum for the triangular area loading.

5

4

3

2

1

0

0 1 2 3

07/07/2013

26

Many feeders have express portionsMany feeders have express portions-- they they

have no load on their initial lengthhave no load on their initial length

5

4

3

2

1

0

0 1 2 3

5

4

3

2

1

0

0 1 2 3

Shown here is the distribution of

VAR-miles for a feeder with a 1.66

mile express portion then with

6MVAR loading evenly spread along

the remaining 1.33 miles.

Optimum reduction in VAR miles is

from a capacitor equal to 100% of the

VAR loading located 77% of the way

out the feeder

Generalized 2/3’s ruleGeneralized 2/3’s rule

� The concepts outlined here-including the graphical

method of deriving a recommended size and

location for capacitors on a feeder, based on its

VAR load distribution will be called the

generalized 2/3’s rule

� When loading is not uniform, the capacitors in a

multi capacitor application are not necessary the

same size, nor are they evenly spaced.

General Capacitor Utilization Guidelines General Capacitor Utilization Guidelines

Based on the Generalized 2/3’s RuleBased on the Generalized 2/3’s Rule

� The MVAR-miles minimization method used above is basically a more flexible application of the 2/3’s rule, which can accommodate uneven VAR loadings. Therefore the following guidelines can be thought of as corollaries to the 2/3’s rule, applicable to situations distribution planners are more likely to face:

� On some typical feeders, the best single capacitor solution is a bank sized 7/8 of the feeder VAR load, located ¾ of the way out the feeder. The best 2 capacitor application is 45% of the VAR load at .3 the length and 50% load at .90 the length of the feeder

In cases where an express feeder trunk is used, the best single capacitor bank application is usually the bank size equal to the VAR load of the feeder, located at the halfway point of the VAR load in the loaded section. The best two capacitor solution is banks equal to ½ the VAR load located at ¼ and ¾ on the loaded portion length.(this is assumed to be uniform loaded)

Any large VAR load should be corrected at its location. In most cases a large or special load will create a very large VAR load at one point on the circuit. Analysis using graphical methods will show the best strategy for minimizing the impact is to install a capacitor equal to its VAR load at its location(ie cancel the VARS at their source)

The twoThe two--thirds rule works as thirds rule works as

well for feeders with brancheswell for feeders with branches

� It may be more difficult to apply, but it

works as well.

Incremental sizes and Incremental sizes and

maximum sizesmaximum sizes� Capacitors are usually available only in standard

unit sizes(100kVAR /phase units at 12.47kV) and

there is a limit to the size of bank that can be

installed at any one location. Typically no more

than 5 to six units per phase. In addition the fault

duty of capacitors must be considered. Large

banks may have to high of outrush current.

Therefore a planner may be limited to no more

than 1800kVAR.

07/07/2013

27

Using more capacitors generally improves Using more capacitors generally improves

the results(and increases the cost of the the results(and increases the cost of the capacitors)capacitors)

Expected reduction in MVAR-mile flow from the application of the

generalized 2/3’s rule as a function of the number of capacitors in %

# of Caps. % Reduction in MVAR-miles flow on a trunk

A uniformly loaded trunk A typical Feeder

1 66 77

2 80 87

3 86 93

4 89 95

5 91 96

6 93 97

In addition, power factor as seen at the In addition, power factor as seen at the

substation is improvedsubstation is improved

Corrected Power Factor at the substation After the application of the

generalized 2/3’s rule as a function of the uncorrected power factor

Uncorrected

PF

P.F. at the substation after the application of the caps

With one Capacitor With two Capacitors

90 99 100

80 97 99

70 95 98

60 91 97

50 87 94

40 80 91

Power Factor ProfilesPower Factor Profiles

0 1 2 3

1.0

0.9

0.8

0.7

A. Evenly loaded at 2MW and

2MVAR/mile 70%PF

0 1 2 3

1.0

0.9

0.8

0.7

C. Corrected with 2/3’s rule 2 caps

2400kVAR at 1.2 and 2.4 mile

0 1 2 3

1.0

0.9

0.8

0.7

B. Corrected with the 2/3’s rule 1

cap. 4000kVAR at 2 miles

0 1 2 3

1.0

0.9

0.8

0.7

D. Typical feeder cap 5400kVAR

at 2.25 miles

Power Factor ProfilesPower Factor Profiles

� Another useful tool is the power factor profile.

Among other things, it will amply demonstrate

that even when capacitors are used power factor is

not uniformly corrected.

� This demonstrates why about the best that can be

done is an average 90% power factor. It can be

corrected to better than that in some places, but

over the entire feeder an average of about 90% is

the best that can be done.

Shortcomings of the 2/3’s ruleShortcomings of the 2/3’s rule

The graphical MVAR-mile minimization method used in the examples

above is a very useful mechanism for illustrating the basics of VAR

capacitor interaction, and for deriving approximation guidelines, such

as the 2/3’s rule, for capacitor application. A number of important

factors are not considered however:

Complex power flow. Actual power flow is complex. The MVAR-mile

analysis deals only with VARs without recognizing that the impact and

importance is somewhat a function of real power flow too.

Economics. The value of VAR reduction depends on the cost of losses

and the need for additional capacity and reach released by the

improvement in power factor. Capacitor application ought to be based

on economic benefit versus cost analysis.

Shortcomings of the 2/3’s rule cont.Shortcomings of the 2/3’s rule cont.

Line impedance. Both the response of a feeder to changes in VAR flow

and the importance of reducing VAR flow vary depending on the

impedance of various line segments, whereas the approximate method

essentially treats all portions of the feeder as equivalently important.

Discontinuous Load. Actual kW and kVAR load on a feeder is

discontinuous, whereas we represented it as continuous.

Detailed analysis of capacitor interaction for each specific feeder, taking

in all the above, is necessary to optimize capacitor application.

Usually, application involves so many variables and is so complex and

complicated that computer analysis is necessary to produce any

improvement over intelligent application of the generalized rule

described.

07/07/2013

28

Switched CapacitorsSwitched Capacitors

� Many times, all or some of the capacitor

banks on a feeder will be switched-

connected some of the time and

disconnected some of the time.

� The reason is that the VAR load changes

over time and thus the need for the

capacitors to change.

Seasonal and daily variation in Seasonal and daily variation in

loadloadShown here are the daily MW and

MVAR loads for a feeder in the

southwest of the US

Both MW and MVAR vary by season

and time of day.

Note that the MVAR load varies more

than the MW. In summer, power factor

off peak is about .83 and on peak it is

.71

Therefore VAR requirements change

over time.

Mid Noon Mid

6

4

2

0

Summer peak day

Mid Noon Mid

6

4

2

0

Autumn Day

MW

MVAR

MW

MVAR

Overboosting VoltageOverboosting Voltage

In some cases when the VAR load is quite low, shunt capacitors can boost voltage above permitted levels, in such cases, they must be switched off when the load is low. The voltage boost at the end of the feeder, due to a capacitor, can be estimated as

Voltage rise(120volt scale)=.12(CkVA*X)/KV**2

Where X is the line reactance to the capacitor location and CkVA is the capacitor’s capacity. For example, 4000 kVAR at three miles on a 12.47kV feeder with X=.63, would boost voltage about 6.31volts

During periods of Low VAR Demand, leaving During periods of Low VAR Demand, leaving Capacitors connected to the feeder increases Capacitors connected to the feeder increases

VARVAR--mile flowmile flow

5

4

3

2

1

0

0 1 2 3

3-1800kVAR

CAP bank

Peak Conditions

5

4

3

2

1

0

0 1 2 3

Minimum Conditions

Here three capacitors minimize

VAR-miles at peak(top), but during

minimum load time, creates a

tremendous VAR flow back towards

the substation(bottom)

Power Factor Correction and Power Factor Correction and

X/R RatioX/R Ratio

0 1 2 3 4 5 6 7 8 9 10 11 12

Peak Load -MW

1.0

.8

.6

.4

.2

0

PW

Cost

$ m

illi

onIf VAR-miles were reduced to

zero on a feeder then there

would be no voltage drop

from the reactive component

of the impedance

In that case, voltage drop like

losses depends only on R and

big conductor would have a

longer reach

Power Factor correction

impacts larger conductors

more than small ones because

its impedance is mostly X not

R

70%

80%90%

100%

SummarySummary

� VAR flow on feeders uses capacity and shortens the economic reach

� VARs can be reduced by the installation of shunt capacitors

� The generalized two thirds rule permits analysis and understanding of VAR flow and corrective issues

� Power Factor can be corrected at best to about 90% on average, which improves voltage drop(economic reach) of conductors.

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Multi Feeder System PlanningMulti Feeder System Planning

� We will look at

� Feeders as part of a feeder system

� Upgrading configuration not line capacity

� A formula that estimates feeder system cost

� Guidelines to reduce cost

The systems Approach:The systems Approach:

Feeders are only part of the SystemFeeders are only part of the System

� A power delivery system consists of many levels of equipment, including sub-transmission, substations, feeders and service

� The recommended perspective for planning is to always view each level as part of a larger whole and plan it using a system’s approach. This means that when laying out a particular feeder, the goal is not to minimize its cost, but to plan it so it contributes to achieving the lowest overall total system cost.

Avoid “feederAvoid “feeder--atat--aa--time” myopiatime” myopia

This has been mentioned before, but this is the most common mistake made in distribution planning methodology.

It is common for distribution planners to focus on the study of one feeder at a time. Usually a feeder that has a new load, a voltage problem, etc. and a solution needs to be found. This feeder at a time focus is necessary in certain phases of planning, but it can lead to a kind of design myopia which is responsible for missed opportunities for savings and service quality improvement.