Minimum Distribution System Concepts and Applications

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
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.
3

Cost Classification Categories
• CUSTOMER COSTS
Minimum Distribution
• DEMAND COSTS
• ENERGY COSTS

The Access Function Of The
Distribution System
• All primary and secondary
customers are connected to
a distribution voltage
source, i.e., a local
substation.
SUB
• There is a physical path
which brings voltage to the
customer’s premise.
• Maintaining the voltage path
ensures customer access to
electrical power.
5

The Capacity Function Of The
Distribution System
• 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
6

Distribution System
Lines and Facilities
FERC
Description
360 Land and Land Rights
361 Structures
362 Station Equipment
S
336.4 MCM ACSR
SUB
M
N.C.
R
363 Storage Battery Equipment
364 Poles, Towers & Fixtures
365 OH Conductors & Devices
• Switches
4/0 CU
NO. 2 AL
C/N
75
N.C.
N.C.
1,500 cKVAR
• Reclosers & Sectionalizers
366 UG Conduit
367 UG Conductors & Devices
333-333-333
50-50-50
368 Line Transformers
• Regulators
• Capacitors
• Cutouts
15
1/0 CU
• Arresters
369 Services
370 Meters
25 37.5
N.O.
373 Street Lighting
7

Objective of the Minimum
Distribution System Analysis
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.
8

Customer or Demand
9

Capacitor-Based Voltage Control
MW
SUB
FEEDER
132 V
+10%
120 V
108 V
-10%
TIME
DISTANCE
10

Customer or Demand
11

Protection Scheme
Temporary Fault Condition
SUB
CB
R
12

Protection Scheme
Permanent Fault Condition
SUB
CB
R
13

Protection Scheme
Permanent Fault Condition – No Load
SUB
CB
R
14

Customer or Demand

Classification of Distribution
Plant for the Cost-of-Service
Demand
Customer
Distribution Substations
X
Primary Lines* X X
Line Transformers* X X
Secondary Lines* X X
Other Line Equipment* X X
Service Lines
Meters
X
X
* Minimum Distribution System facilities.
16

Zero-Intercept Methodology
Applied to:
• Line Transformers
• Conductors
• Poles
THE Y-AXIS
INTERCEPT
UNIT
COSTS
×
×
×
×
IS THE UNIT
COST OF
ZERO CAPACITY
×
COST OF A STANDARD
SIZE UNIT
CAPACITY
17

Line Transformers
18

Weighted Linear Regression
For Distribution Line Transformers
m
=
[ N×
∑nXY] − [ ∑nX×
∑nY]
2
[ N×
nX ] − [ nX] 2
∑
SLOPE
∑
N = Total number of all transformers of
a given type, e.g., 59,800 7.2 kV -
120/240 V, single-bushing, polemount
units
n = Number of a given size transformer,
e.g., 9,935 15 kVA
b
y-INTERCEPT
⎡ ⎤
∑nY
⎢∑nX
= − m×
⎥
N ⎢ N ⎥
⎣ ⎦
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)
19

Zero-Intercept Example
Single-Phase Overhead Transformers
1. ZERO-INTERCEPT: $463.975/transformer
Based on various kVA sizes of 7.2 kV - 120/240 V, single bushing, polemount
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%
20

Zero-Intercept Analysis
The Problem With Vintage Costs
$2,000
$1,800
$1,600
$1,400
Analysis of Pad-Mount Line Transformers
Based on Booked Installed Costs
Unit Cost
$1,200
$1,000
$800
$600
$400
$200
3Φ
1Φ
$0
0 10 20 30 40 50 60 70 80 90 100
kVA
21

Zero-Intercept Analysis
Use of Current Costs
$15,000
Analysis of Pad-Mount Line Transformers
Based on Rebuild Costs
$13,500
$12,000
3Φ
Unit Cost
$10,500
$9,000
$7,500
$6,000
$4,500
$3,000
$1,500
1Φ
$0
0 10 20 30 40 50 60 70 80 90 100
kVA
22

Primary and Secondary
Conductors and Poles
PRIMARY
NEUTRAL
SECONDARY
23

Overhead Conductors
Relative Frequency Distribution
50%
45%
40%
80%
70%
60%
50%
35%
30%
25%
20%
15%
40%
30%
20%
10%
0%
CU BARE CU WP AL BARE AL WP
10%
5%
0%
4 ACSR 2 ACSR 1/0 ACSR 4/0 ACSR 336 ACSR 477 ACSR 795 ACSR
477 AAC 795 AAC 1,351 AAC
24

Weighted Linear Regression
For Distribution Conductors
m
=
[ N×
∑nXY] − [ ∑nX×
∑nY]
2
[ N×
nX ] − [ nX] 2
∑
SLOPE
∑
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
b
y-INTERCEPT
⎡ ⎤
∑nY
⎢∑nX
= − m×
⎥
N ⎢ N ⎥
⎣ ⎦
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)
25

Zero-Intercept Example
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%
26

Determination Of Overhead
Circuit Lengths For The MDS
TOTAL POLE MILES
PRIMARY
SUB
PRIMARY NEUTRAL COMMON NEUTRAL SECONDARY NEUTRAL
SECONDARY
UNDERBUILD
SECONDARY
TAPS
27

UNDERGROUND
PRIMARY CABLE
CONCENTRIC
NEUTRAL
CONDUCTOR

CONDUIT FOR
UNDERGROUND CABLES
RIGID PVC
FLEXIBLE

Distribution Poles
Types Of Materials
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
1.0%
0.9%
0.8%
0.7%
0.6%
0.5%
0.4%
0.3%
0.2%
0.1%
0.0%
ALUMINUM CONCRETE FIBERGLASS STEEL
0%
ALUMINUM CONCRETE FIBERGLASS STEEL WOOD
30

Pole Heights
Relative Frequency Distribution
40%
35%
30%
25%
20%
15%
10%
5%
0%
30' 35' 40' 45' 50' 55' 60' 65' 70' 75' 80' 85' 90' 95'
POLE HEIGHTS
31

Pole Line Routing
SUB
32

Clearance Requirements
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.
33

Pole Line Grading
IMPROPER GRADING:
POLES ALL HAVE THE SAME HEIGHT
PROPER GRADING:
POLES WITH VARYING HEIGHTS
34

Distribution Pole Classification
Conclusion On Pole Height
• 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.
35

Pole Class
36

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)
37

Pole Classes
Relative Frequencies By Height
60%
50%
40%
30%
20%
10%
7
6
5
4
3
2
1
0%
POLE CLASS
45 FT POLES 40 FT POLES 35 FT POLES 30 FT POLES
38

Pole Class Requirements
Based On Transformer Capacity
0
TRANSFORMER kVA
10 15 25 37.5 50 75 100 167 250
1
2
POLE CLASS
3
4
5
6
7
1 TRANSFORMER 2 TRANSFORMERS 3 TRANSFORMERS
39

Distribution Pole Classification
Conclusion On Pole Class
• 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.
40

Pole Capacity
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.
Example: 35’ 5-C Pole
Transverse Wind
Load of 1,200 lb
GLMC = 33,000 ft-lbs = 33 kips
41

Weighted Linear Regression
For Distribution Poles
m
=
[ N×
∑nXY] − [ ∑nX×
∑nY]
2
[ N×
nX ] − [ nX] 2
∑
SLOPE
∑
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
b
y-INTERCEPT
⎡ ⎤
∑nY
⎢∑nX
= − m×
⎥
N ⎢ N ⎥
⎣ ⎦
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)
42

Zero-Intercept Example
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%
43

Example MDS Analysis Results
Poles, Transformers, and Conductors
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%
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 transformer vaults
** Basis for classifying transformer pads
* Basis for classifying conduit
44

Example MDS Analysis Results
Distribution Line Devices
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
45

Q&A
Larry Vogt