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
366
367
368
369
370
373
Description
Land and Land Rights
S
Structures
Station Equipment
Storage Battery Equipment
Poles, Towers & Fixtures
OH Conductors & Devices
 Switches
 Reclosers & Sectionalizers
UG Conduit
UG Conductors & Devices
Line Transformers
 Regulators
 Capacitors
 Cutouts
 Arresters
Services
Meters
Street Lighting
SUB
R
M
N.C.
336.4 MCM ACSR
N.C.
4/0 CU
FERC
360
361
362
363
364
365
75
N.C.
1,500 cKVAR
NO. 2 AL
C/N
333-333-333
50-50-50
15
1/0 CU
25
37.5
N.O.
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
FEEDER
MW
SUB
+10%
132 V
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
X
Meters
X
* Minimum Distribution System facilities.
16
Zero-Intercept Methodology
Applied to:
 Line Transformers
 Conductors
 Poles
UNIT
COSTS
×
THE Y-AXIS
INTERCEPT
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
N = Total number of all transformers of
SLOPE
[
N × ∑ nXY ]− [∑ nX × ∑ nY ]
m=
[N × ∑ nX ]− [∑ nX]
a given type, e.g., 59,800 7.2 kV 120/240 V, single-bushing, polemount units
2
2
n = Number of a given size transformer,
e.g., 9,935 15 kVA
y-INTERCEPT
 ∑ nX 
nY
∑

b=
− 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
Analysis of Pad-Mount Line Transformers
Based on Booked Installed Costs
$2,000
$1,800
$1,600
Unit Cost
$1,400
$1,200
$1,000
$800
3Φ
$600
$400
1Φ
$200
$0
0
10
20
30
40
50
60
70
80
90
100
kVA
21
Zero-Intercept Analysis
Use of Current Costs
Analysis of Pad-Mount Line Transformers
Based on Rebuild Costs
$15,000
3Φ
$13,500
$12,000
Unit Cost
$10,500
$9,000
$7,500
$6,000
1Φ
$4,500
$3,000
$1,500
$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
80%
50%
70%
45%
60%
40%
50%
35%
40%
30%
30%
20%
25%
10%
20%
0%
CU BARE
CU WP
AL BARE
AL WP
15%
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
N = Total feet of all conductors of a
SLOPE
[
N × ∑ nXY ]− [∑ nX × ∑ nY ]
m=
[N × ∑ nX ]− [∑ nX]
2
2
y-INTERCEPT
 ∑ nX 
nY
∑

b=
− m× 
N


N


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)
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%
1.0%
0.9%
70%
0.8%
0.7%
60%
50%
0.6%
0.5%
0.4%
40%
0.3%
0.2%
30%
20%
0.1%
0.0%
ALUMINUM
CONCRETE
FIBERGLASS
STEEL
10%
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
No. 2
No. 3
No. 4
No. 5
No. 6
No. 7
39.0”
36.5”
34.0’
31.5”
29.0”
27.0”
25.0”
MINIMUM CIRCUMFERENCE OF SOUTHERN YELLOW PINE POLES (@ GROUND LINE)
37
Pole Classes
Relative Frequencies By Height
60%
50%
40%
30%
20%
10%
0%
7
6
5
4
3
2
1
POLE CLASS
45 FT POLES
40 FT POLES
35 FT POLES
30 FT POLES
38
Pole Class Requirements
Based On Transformer Capacity
TRANSFORMER kVA
10
15
25
37.5
50
75
100
167
250
0
1
POLE CLASS
2
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.
Example: 35’ 5-C Pole
Transverse Wind
Load of 1,200 lb
GLMC = 33,000 ft-lbs = 33 kips
Ground line moment capacities
do differ by class for all poles.
41
Weighted Linear Regression
For Distribution Poles
N = Total feet of all poles of a given
SLOPE
[
N × ∑ nXY ]− [∑ nX × ∑ nY ]
m=
[N × ∑ nX ]− [∑ nX]
2
2
y-INTERCEPT
 ∑ nX 
nY
∑

b=
− m× 
N


N


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)
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
• Concrete
• Steel
Transformers
• 1Φ OH*
• 1Φ UG**
• 3Φ UG**
66.4%
47.3%
57.5%
41.7%
61.5%
34.2%
Customer Demand
33.6%
52.7%
42.5%
Conductors
Primary
• Bare ACSR OH
• 15 kV CN UG*
21.0%
57.4%
79.0%
42.6%
58.3%
38.5%
65.8%
Secondary
• WP AL OH
38.4%
• Duplex OH
31.4%
• 1-Conductor UG* 60.7%
61.6%
68.6%
39.3%
* Basis for classifying transformer vaults
** Basis for classifying transformer pads
* Basis for classifying conduit
44
Example MDS Analysis Results
Distribution Line Devices
Regulators & Capacitors
Reclosers and Sectionalizers
Cutouts & Arresters
• Line Transformers (OH)
• Regulators & Capacitors
• Reclosers & Sectionalizers
• Line Protection
Primary
Customer
Demand
100%
Secondary
Customer
Demand
100%
41.7%
58.3%
100%
100%
100%
Bypass Switches
• Regulators
• Reclosers & Sectionalizers
100%
OH Line Switches*
UG Line Switches*
21.0%
57.4%
100%
79.0%
42.6%
* Based on conductors
45
Q&A
Larry Vogt