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