MINIMUM DISTRIBUTION SYSTEM CONCEPTS AND APPLICATIONS

Larry Vogt Manager, Rates

Minimum Distribution System

What is MDS? MDS is an analysis module of the cost-of-service study in which distribution investment is classified between demand-related and customer-related cost components. Why is MDS important? MDS is key to determining the monthly fixed costs of providing basic electric service. It provides a cost justification basis for the Customer Charge portion of the rate structure.

Basic Cost Components

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The classification step of the cost-of-service study assigns all of the functionalized cost elements to the cost causation components of Customer, Demand, and Energy.

Energy-related costs – variable costs which are dependent on kWh requirements.

Demand-related costs – fixed costs which are dependent on kW requirements.

Customer-related costs – fixed costs which are independent of load or energy requirements.

Cost Classification Categories

• CUSTOMER COSTS

• DEMAND COSTS

• ENERGY COSTS

Minimum Distribution

The Access Function Of The Distribution System

5 5

All primary and secondary customers are connected to a distribution voltage source, i.e., a local substation.

There is a physical path which brings voltage to the customer’s premise.

Maintaining the voltage path ensures customer access to electrical power.

SUB

The Capacity Function Of The Distribution System

6 6

Primary and secondary distribution system facilities and lines must be sized to adequately handle the customers’ demand for power.

Electric service facilities are rated in terms of kVA capacity (conductors rated in terms of ampacity).

Customer Load

Feeder Load

Distribution System Lines and Facilities

7 7

FERC Description 360 Land and Land Rights 361 Structures 362 Station Equipment 363 Storage Battery Equipment 364 Poles, Towers & Fixtures 365 OH Conductors & Devices

Switches Reclosers & Sectionalizers

366 UG Conduit 367 UG Conductors & Devices 368 Line Transformers

Regulators Capacitors Cutouts Arresters

369 Services 370 Meters 373 Street Lighting

SUB

S R

N.O.

N.C.

N.C. N.C.

1,500 cKVAR

333-333-333

50-50-50

75

15

25 37.5

336.4 MCM ACSR

4/0

CU

NO. 2 AL C/N

1/0 CU

M

Objective of the Minimum Distribution System Analysis

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To assess each device utilized in the distribution system in terms of its “mission” in order to determine if its function is: Dependent on kW load requirements and therefore demand related,

or

Independent of kW load requirements and therefore customer related.

Customer or Demand?

9

MW

TIME

Capacitor-Based Voltage Control

SUB FEEDER

120 V

108 V

132 V +10%

-10%

DISTANCE

10

Customer or Demand?

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Protection Scheme Temporary Fault Condition

R CB SUB

12

Protection Scheme Permanent Fault Condition

R CB SUB

13

Protection Scheme Permanent Fault Condition – No Load

R CB SUB

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Customer or Demand?

Classification of Distribution Plant for the Cost-of-Service

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Demand Customer

Distribution Substations X

Primary Lines* X X

Line Transformers* X X

Secondary Lines* X X

Other Line Equipment* X X

Service Lines X

Meters X * Minimum Distribution System facilities.

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Zero-Intercept Methodology

THE Y-AXIS INTERCEPT IS THE UNIT COST OF ZERO CAPACITY

CAPACITY

UNIT COSTS

× ×

× ×

× COST OF A STANDARD SIZE UNIT

Applied to: Line Transformers Conductors Poles

Line Transformers

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Weighted Linear Regression For Distribution Line Transformers

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N = Total number of all transformers of a given type, e.g., 59,800 7.2 kV - 120/240 V, single-bushing, pole- mount units n = Number of a given size transformer, e.g., 9,935 15 kVA X = Transformer size in kVA, e.g., 5, 7.5, 10, 15, etc. Y = Transformer unit cost in $ per unit, e.g., $724.48 (cost of a 15 kVA unit)

[ ] [ ][ ] [ ]22 ∑∑

∑ ∑∑−×

×−×=

nXnXN

nYnXnXYNm

×−= ∑∑

NnX

NYn

mb

SLOPE

y-INTERCEPT

Zero-Intercept Example

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Single-Phase Overhead Transformers

1. ZERO-INTERCEPT: $463.975/transformer Based on various kVA sizes of 7.2 kV - 120/240 V, single bushing, pole-

mount transformers

2. TOTAL NUMBER OF OVERHEAD TRANSFORMERS: 98,278 CUSTOMER COMPONENT = $ 463.975 × 98,728 = $45,807,307

3. TOTAL OVERHEAD TRANSFORMER COST: $109,960,813 DEMAND COMPONENT = $109,960,813 - $45,807,307 = $64,153,506

CUSTOMER COMPONENT = 41.7%

DEMAND COMPONENT = 58.3%

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Zero-Intercept Analysis The Problem With Vintage Costs

$0

$200

$400

$600

$800

$1,000

$1,200

$1,400

$1,600

$1,800

$2,000

0 10 20 30 40 50 60 70 80 90 100

Analysis of Pad-Mount Line Transformers Based on Booked Installed Costs

kVA

Uni

t Cos

t

1Φ

3Φ

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Zero-Intercept Analysis Use of Current Costs

3Φ

1Φ

Analysis of Pad-Mount Line Transformers Based on Rebuild Costs

kVA

Uni

t Cos

t

$0

$1,500

$3,000

$4,500

$6,000

$7,500

$9,000

$10,500

$12,000

$13,500

$15,000

0 10 20 30 40 50 60 70 80 90 100

Primary and Secondary Conductors and Poles

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PRIMARY

NEUTRAL

SECONDARY

Overhead Conductors Relative Frequency Distribution

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0%

10%

20%

30%

40%

50%

60%

70%

80%

CU BARE CU WP AL BARE AL WP

477 AAC 795 AAC 1,351 AAC0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

50%

4 ACSR 2 ACSR 1/0 ACSR 4/0 ACSR 336 ACSR 477 ACSR 795 ACSR

Weighted Linear Regression For Distribution Conductors

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N = Total feet of all conductors of a given type, e.g., 47,557,568 ft of ACSR conductors

n = Number of feet of a given size conductor, e.g., 26,194,939 ft of #2 ACSR

X = Conductor size in MCM (a #2 wire is 66.36 MCM), e.g., 26.24, 41.74, 52.62, 66.36, etc.

Y = Conductor unit cost in $ per feet, e.g., $0.659/ft (cost of a #2 ACSR conductor)

[ ] [ ][ ] [ ]22 ∑∑

∑ ∑∑−×

×−×=

nXnXN

nYnXnXYNm

×−= ∑∑

NnX

NYn

mb

SLOPE

y-INTERCEPT

Zero-Intercept Example

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Primary Overhead Conductor

1. ZERO-INTERCEPT: $0.396/ft Based on various MCM sizes of bare ACSR conductors

2. TOTAL LENGTH OF PRIMARY CONDUCTORS: 15,708,000 ft PRIMARY CIRCUIT LENGTH: 15,708,000 × 2 = 31,416,000 ft CUSTOMER COMPONENT = $0.396 × 29,898,000* = $11,827,081 * Minimum Distribution System Length

3. TOTAL PRIMARY CONDUCTOR COST: $56,416,253 DEMAND COMPONENT = $56,416,253 - $11,827,081 = $44,589,172

CUSTOMER COMPONENT = 21.0%

DEMAND COMPONENT = 79.0%

Determination Of Overhead Circuit Lengths For The MDS

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SUB

PRIMARY

SECONDARY UNDERBUILD

PRIMARY NEUTRAL COMMON NEUTRAL SECONDARY NEUTRAL

SECONDARY TAPS

TOTAL POLE MILES

UNDERGROUND PRIMARY CABLE

CONDUCTOR

CONCENTRIC NEUTRAL

CONDUIT FOR UNDERGROUND CABLES

RIGID PVC FLEXIBLE

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Distribution Poles Types Of Materials

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

ALUMINUM CONCRETE FIBERGLASS STEEL WOOD

0.0%

0.1%

0.2%

0.3%

0.4%

0.5%

0.6%

0.7%

0.8%

0.9%

1.0%

ALUMINUM CONCRETE FIBERGLASS STEEL

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Pole Heights Relative Frequency Distribution

0%

5%

10%

15%

20%

25%

30%

35%

40%

30' 35' 40' 45' 50' 55' 60' 65' 70' 75' 80' 85' 90' 95'POLE HEIGHTS

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Pole Line Routing

SUB

Clearance Requirements

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Poles lines must be designed to ensure proper safety clearances, such as specified in the National Electric Safety Code (NESC), Section 23.

The NESC provides specific minimum clearances of power lines located over:

Roadways, parking lots, driveways, pedestrian areas, railroad track rails, water ways, etc.

Other electric conductors and services, trolley/electric train cables, communications cables, etc.

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Pole Line Grading

IMPROPER GRADING: POLES ALL HAVE THE SAME HEIGHT

PROPER GRADING: POLES WITH VARYING HEIGHTS

Distribution Pole Classification Conclusion On Pole Height

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Pole lines are built to connect customers to the power source, i.e., the substation. The routing of these lines is predominantly a function of where customers are located.

Pole height requirements are predominantly a function of clearances and line grading, which are related to safety and mechanical design.

Thus, pole height is not a major function of load.

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Pole Class

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Standard Pole Classes Example: 35’ Wood Pole

No. 1 39.0”

No. 2 36.5”

No. 3 34.0’

No. 4 31.5”

No. 5 29.0”

No. 6 27.0”

No. 7 25.0”

MINIMUM CIRCUMFERENCE OF SOUTHERN YELLOW PINE POLES (@ GROUND LINE)

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Pole Classes Relative Frequencies By Height

0%

10%

20%

30%

40%

50%

60%

1234567POLE CLASS

45 FT POLES 40 FT POLES 35 FT POLES 30 FT POLES

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Pole Class Requirements Based On Transformer Capacity

0

1

2

3

4

5

6

7

10 15 25 37.5 50 75 100 167 250TRANSFORMER kVA

POLE

CLA

SS

1 TRANSFORMER 2 TRANSFORMERS 3 TRANSFORMERS

Distribution Pole Classification Conclusion On Pole Class

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The physical sizes and weights of line transformers and wires are related to their current carrying capabilities.

Pole class must be increased (i.e., going from 7 to 1) to carry heavy mechanical loads caused by large line transformers and conductors (3Φ lines are indicative of greater electrical load density than 1Φ lines).

Thus, pole class is predominantly a function of load.

Pole Capacity

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Poles have no electrical capacity component, but they do have a mechanical capacity (strength) component that can be viewed as a proxy for electrical loading.

Pole class (or circumference) can represent loading capability for wood poles, but it does not work for steel or concrete poles since different classes can have the same physical dimensions.

Ground line moment capacities do differ by class for all poles.

Transverse Wind Load of 1,200 lb

Example: 35’ 5-C Pole

GLMC = 33,000 ft-lbs = 33 kips

Weighted Linear Regression For Distribution Poles

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N = Total feet of all poles of a given type, e.g., 55,642 ft of 40 ft wood poles n = Number of feet of a given size pole based on its GLMC, e.g., 21,137 ft of 76.80 kilopounds (kips) poles X = Ground Line Moment Capacity in kips, e.g., 48.0, 60.8, 76.8, 96.0, etc. Y = Pole unit cost in $ per feet, e.g., $12.05/ft (cost of a 76.8 kips pole)

[ ] [ ][ ] [ ]22 ∑∑

∑ ∑∑−×

×−×=

nXnXN

nYnXnXYNm

×−= ∑∑

NnX

NYn

mb

SLOPE

y-INTERCEPT

Zero-Intercept Example

43

Wood Poles

1. ZERO-INTERCEPT: $7.883/ft Based on various kip ratings of 40’ southern pine poles

2. TOTAL LINEAR FEET OF WOOD POLES: 5,579,390 ft CUSTOMER COMPONENT = $7.883 × 5,579,390 = $43,982,773

3. TOTAL WOOD POLE COST: $66,254,744 DEMAND COMPONENT = $66,254,744 - $43,982,773 = $22,271,971

CUSTOMER COMPONENT = 66.4%

DEMAND COMPONENT = 33.6%

Example MDS Analysis Results Poles, Transformers, and Conductors

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Customer Demand Poles • Wood 66.4% 33.6% • Concrete 47.3% 52.7% • Steel 57.5% 42.5%

Transformers • 1Φ OH* 41.7% 58.3% • 1Φ UG** 61.5% 38.5% • 3Φ UG** 34.2% 65.8% * Basis for classifying transformer vaults ** Basis for classifying transformer pads

Customer Demand Conductors Primary • Bare ACSR OH 21.0% 79.0% • 15 kV CN UG* 57.4% 42.6%

Secondary • WP AL OH 38.4% 61.6% • Duplex OH 31.4% 68.6% • 1-Conductor UG* 60.7% 39.3%

* Basis for classifying conduit

Example MDS Analysis Results Distribution Line Devices

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Primary Secondary Customer Demand Customer Demand Regulators & Capacitors 100%

Reclosers and Sectionalizers 100%

Cutouts & Arresters • Line Transformers (OH) 41.7% 58.3% • Regulators & Capacitors 100% • Reclosers & Sectionalizers 100% • Line Protection 100%

Bypass Switches • Regulators 100% • Reclosers & Sectionalizers 100%

OH Line Switches* 21.0% 79.0% UG Line Switches* 57.4% 42.6% * Based on conductors

Larry Vogt

Q&A