Short T.A. Electric Power Distribution Handbook
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3.2.3 Neutral or Shield
3.2.4 Semiconducting Shields
3.2.5 Jacket
3.3 Installations and Configurations
3.4 Impedances
3.4.1 Resistance
3.4.2 Impedance Formulas
3.4.3 Impedance Tables
3.4.4 Capacitance
3.5 Ampacity
3.6 Fault Withstand Capability
3.7 Cable Reliability
3.7.1 Water Trees
3.7.2 Other Failure Modes
3.7.3 Failure Statistics
3.8 Cable Testing
3.9 Fault Location
References
4
Transformers
4.1 Basics
4.2 Distribution Transformers
4.3 Single-Phase Transformers
4.4 Three-Phase Transformers
4.4.1 Grounded Wye – Grounded Wye
4.4.2 Delta – Grounded Wye
4.4.3 Floating Wye – Delta
4.4.4 Other Common Connections
4.4.4.1 Delta – Delta
4.4.4.2 Open Wye – Open Delta
4.4.4.3 Other Suitable Connections
4.4.5 Neutral Stability with a Floating Wye
4.4.6 Sequence Connections of Three-Phase Transformers
4.5 Loadings
4.6 Losses
4.7 Network Transformers
4.8 Substation Transformers
4.9 Special Transformers
4.9.1 Autotransformers
4.9.2 Grounding Transformers
4.10 Special Problems
4.10.1 Paralleling
4.10.2 Ferroresonance
4.10.3 Switching Floating Wye – Delta Banks
4.10.4 Backfeeds
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4.10.5 Inrush
References
5
Voltage Regulation
5.1 Voltage Standards
5.2 Voltage Drop
5.3 Regulation Techniques
5.3.1 Voltage Drop Allocation and Primary Voltage Limits
5.3.2 Load Flow Models
5.3.3 Voltage Problems
5.3.4 Voltage Reduction
5.4 Regulators
5.4.1 Line-Drop Compensation
5.4.1.1 Load-Center Compensation
5.4.1.2 Voltage-Spread Compensation
5.4.1.3 Effects of Regulator Connections
5.4.2 Voltage Override
5.4.3 Regulator Placement
5.4.4 Other Regulator Issues
5.5 Station Regulation
5.5.1 Parallel Operation
5.5.2 Bus Regulation Settings
5.6 Line Loss and Voltage Drop Relationships
References
6
Capacitor Application
6.1 Capacitor Ratings
6.2 Released Capacity
6.3 Voltage Support
6.4 Reducing Line Losses
6.4.1 Energy Losses
6.5 Switched Banks
6.6 Local Controls
6.7 Automated Controls
6.8 Reliability
6.9 Failure Modes and Case Ruptures
6.10 Fusing and Protection
6.11 Grounding
References
7
Faults
7.1 General Fault Characteristics
7.2 Fault Calculations
7.2.1 Transformer Connections
7.2.2 Fault Profiles
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7.2.3 Effect of
X/R
Ratio
7.2.4 Secondary Faults
7.2.5 Primary-to-Secondary Faults
7.2.6 Underbuilt Fault to a Transmission Circuit
7.2.7 Fault Location Calculations
7.3 Limiting Fault Currents
7.4 Arc Characteristics
7.5 High-Impedance Faults
7.6 External Fault Causes
7.6.1 Trees
7.6.2 Weather and Lightning
7.6.3 Animals
7.6.4 Other External Causes
7.7 Equipment Faults
7.8 Faults in Equipment
7.9 Targeted Reduction of Faults
References
8
Short-Circuit Protection
8.1 Basics of Distribution Protection
8.1.1 Reach
8.1.2 Inrush and Cold-Load Pickup
8.2 Protection Equipment
8.2.1 Circuit Interrupters
8.2.2 Circuit Breakers
8.2.3 Circuit Breaker Relays
8.2.4 Reclosers
8.2.5 Expulsion Fuses
8.2.5.1 Fuse Cutouts
8.2.6 Current-Limiting Fuses
8.3 Transformer Fusing
8.4 Lateral Tap Fusing and Fuse Coordination
8.5 Station Relay and Recloser Settings
8.6 Coordinating Devices
8.6.1 Expulsion Fuse–Expulsion Fuse Coordination
8.6.2 Current-Limiting Fuse Coordination
8.6.3 Recloser–Expulsion Fuse Coordination
8.6.4 Recloser–Recloser Coordination
8.6.5 Coordinating Instantaneous Elements
8.7 Fuse Saving vs. Fuse Blowing
8.7.1 Industry Usage
8.7.2 Effects on Momentary and Sustained Interruptions
8.7.3 Coordination Limits of Fuse Saving
8.7.4 Long-Duration Faults and Damage with Fuse
Blowing
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8.7.5 Long-Duration Voltage Sags with Fuse Blowing
8.7.6 Optimal Implementation of Fuse Saving
8.7.7 Optimal Implementation of Fuse Blowing
8.8 Other Protection Schemes
8.8.1 Time Delay on the Instantaneous Element
(Fuse Blowing)
8.8.2 High-Low Combination Scheme
8.3.3 SCADA Control of the Protection Scheme
8.8.4 Adaptive Control by Phases
8.9 Reclosing Practices
8.9.1 Reclose Attempts and Dead Times
8.9.2 Immediate Reclose
8.9.2.1 Effect on Sensitive Residential Devices
8.9.2.2 Delay Necessary to Avoid Retriggering
Faults
8.9.2.3 Reclose Impacts on Motors
8.10 Single-Phase Protective Devices
8.10.1 Single-Phase Reclosers with Three-Phase Lockout
References
9
Reliability
9.1 Reliability Indices
9.1.1 Customer-Based Indices
9.1.2 Load-Based Indices
9.2 Storms and Weather
9.3 Variables Affecting Reliability Indices
9.3.1 Circuit Exposure and Load Density
9.3.2 Supply Configuration
9.3.3 Voltage
9.3.4 Long-Term Reliability Trends
9.4 Modeling Radial Distribution Circuits
9.5 Parallel Distribution Systems
9.6 Improving Reliability
9.6.1 Identify and Target Fault Causes
9.6.2 Identify and Target Circuits
9.6.3 Switching and Protection Equipment
9.6.4 Automation
9.6.5 Maintenance and Inspections
9.6.6 Restoration
9.6.7 Fault Reduction
9.7 Interruption Costs
References
10
Voltage Sags and Momentary Interruptions
10.1 Location
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10.2 Momentary Interruptions
10.3 Voltage Sags
10.3.1 Effect of Phases
10.3.2 Load Response
10.3.3 Analysis of Voltage Sags
10.4 Characterizing Sags and Momentaries
10.4.1 Industry Standards
10.4.2 Characterization Details
10.5 Occurrences of Voltage Sags
10.5.1 Site Power Quality Variations
10.5.2 Transmission-Level Power Quality
10.6 Correlations of Sags and Momentaries
10.7 Factors That Influence Sag and Momentary Rates
10.7.1 Location
10.7.2 Load Density
10.7.3 Voltage Class
10.7.4 Comparison and Ranking of Factors
10.8 Prediction of Quality Indicators Based on Site Characteristics
10.9 Equipment Sensitivities
10.9.1 Computers and Electronic Power Supplies
10.9.2 Industrial Processes and Equipment
10.9.2.1 Relays and Contactors
10.9.2.2 Adjustable-Speed Drives
10.9.2.3 Programmable-Logic Controllers
10.9.3 Residential Equipment
10.10 Solution Options
10.10.1 Utility Options for Momentary Interruptions
10.10.2 Utility Options for Voltage Sags
10.10.2.1 Raising the Nominal Voltage
10.10.2.2 Line Reactors
10.10.3.2 Neutral Reactors
10.10.2.4 Current-Limiting Fuses
10.10.3 Utility Options with Nontraditional Equipment
10.10.3.1 Fast Transfer Switches
10.10.3.2 DVRs and Other Custom-Power Devices
10.10.4 Customer/Equipment Solutions
10.11 Power Quality Monitoring
References
11
Other Power Quality Issues
11.1 Overvoltages and Customer Equipment Failures
11.1.1 Secondary/Facility Grounding
11.1.2 Reclose Transients
11.2 Switching Surges
11.2.1 Voltage Magnification
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11.2.2 Tripping of Adjustable-Speed Drives
11.2.3 Prevention of Capacitor Transients
11.3 Harmonics
11.3.1 Resonances
11.3.2 Telephone Interference
11.4 Flicker
11.4.1 Flicker Solutions
11.4.1.1 Load Changes
11.4.1.2 Series Capacitor
11.4.1.3 Static Var Compensator
11.4.1.4 Other Solutions
11.5 Voltage Unbalance
References
12
Lightning Protection
12.1 Characteristics
12.2 Incidence of Lightning
12.3 Traveling Waves
12.4 Surge Arresters
12.4.1 Ratings and Selection
12.4.2 Housings
12.4.3 Other Technologies
12.4.4 Isolators
12.4.5 Arrester Reliability and Failures
12.5 Equipment Protection
12.5.1 Equipment Insulation
12.5.2 Protective Margin
12.5.3 Secondary-Side Transformer Failures
12.6 Underground Equipment Protection
12.6.1 Open Point Arrester
12.6.2 Scout Arresters
12.6.3 Tapped Cables
12.6.4 Other Cable Failure Modes
12.7 Line Protection
12.7.1 Induced Voltages
12.7.2 Insulation
12.7.2.1 Practical Considerations
12.7.3 Shield Wires
12.7.4 Line Protection Arresters
12.8 Other Considerations
12.8.1 Role of Grounding
12.8.2 Burndowns
12.8.3 Crossarm and Pole Damage and Bonding
12.8.4 Arc Quenching of Wood
References
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13
Grounding and Safety
13.1 System Grounding Configurations
13.1.1 Four-Wire Multigrounded Systems
13.1.2 Other Grounding Configurations
13.2 System Grounding and Neutral Shifts During Ground Faults
13.2.1 Neutral Shifts on Multigrounded Systems
13.2.2 Neutral Reactor
13.2.3 Overvoltages on Ungrounded Systems
13.3 Equipment/Customer Grounding
13.3.1 Special Considerations on Ungrounded Systems
13.3.2 Secondary Grounding Problems
13.4 Ground Rods and Other Grounding Electrodes
13.4.1 Soil Characteristics
13.4.2 Corrosion and Grounding Electrodes
13.4.3 Resistance Measurements
13.5 Shocks and Stray Voltages
13.5.1 Biological Models
13.5.2 Step and Touch Potentials
13.5.3 Stray Voltage
13.5.4 Tree Contacts
13.6 Protective Grounding
References
14
Distributed Generation
14.1 Characteristics of Distributed Generators
14.1.1 Energy Sources
14.1.2 Synchronous Generators
14.1.3 Induction Generators
14.1.4 Inverters
14.1.5 Modeling Small Generators
14.2 Islanding Issues
14.2.1 Effect of Transformer Connections on Overvoltages
14.2.1.1 Overvoltage Relays and 59G Ground Fault
Detection
14.2.1.2 Effectively Grounding a Grounded-Wye –
Grounded-Wye Transformer Connection
14.2.1.3 Sizing a Neutral Grounding Reactor on a
Grounded-Wye – Delta Connection to Maintain
Effective Grounding
14.2.2 Anti-Islanding Protection
14.2.3 Active Anti-Islanding
14.2.4 Relaying Issues
14.2.5 Self-Excitation
14.2.6 Ferroresonance
14.2.7 Backfeed to a Downed Conductor and Backfeed
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Voltages
14.3 Protection Issues
14.3.1 Tradeoff Between Overvoltages and Ground Fault
Current
14.3.2 Fuse Saving Coordination
14.4 Power Quality Impacts
14.4.1 Voltage Regulation
14.4.2 Harmonics
14.4.3 Flicker
14.4.4 Other Impacts on Power Quality
14.4.5 High Quality Power Configurations
14.5 Generator Reliability
References
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1
Fundamentals of Distribution Systems
Electrification in the early 20th century dramatically improved productivity
and increased the well-being of the industrialized world. No longer a luxury
— now a necessity — electricity powers the machinery, the computers, the
health-care systems, and the entertainment of modern society. Given its
benefits, electricity is inexpensive, and its price continues to slowly decline
(after adjusting for inflation — see Figure 1.1).
Electric power distribution is the portion of the power delivery infrastruc-
ture that takes the electricity from the highly meshed, high-voltage trans-
mission circuits and delivers it to customers. Primary distribution lines are
“medium-voltage” circuits, normally thought of as 600 V to 35 kV. At a
distribution substation, a substation transformer takes the incoming trans-
mission-level voltage (35 to 230 kV) and steps it down to several distribution
primary circuits, which fan out from the substation. Close to each end user,
a distribution transformer takes the primary-distribution voltage and steps
it down to a low-voltage secondary circuit (commonly 120/240 V; other
utilization voltages are used as well). From the distribution transformer, the
secondary distribution circuits connect to the end user where the connection
is made at the service entrance. Figure 1.2 shows an overview of the power
generation and delivery infrastructure and where distribution fits in. Func-
tionally, distribution circuits are those that feed customers (this is how the
term is used in this book, regardless of voltage or configuration). Some also
think of distribution as anything that is radial or anything that is below 35 kV.
The distribution infrastructure is extensive; after all, electricity has to be
delivered to customers concentrated in cities, customers in the suburbs, and
customers in very remote regions; few places in the industrialized world do
not have electricity from a distribution system readily available. Distribution
circuits are found along most secondary roads and streets. Urban construc-
tion is mainly underground; rural construction is mainly overhead. Subur-
ban structures are a mix, with a good deal of new construction going
underground.
A mainly urban utility may have less than 50 ft of distribution circuit for each
customer. A rural utility can have over 300 ft of primary circuit per customer.
Several entities may own distribution systems: municipal governments,
state agencies, federal agencies, rural cooperatives, or investor-owned utili-
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ties. In addition, large industrial facilities often need their own distribution
systems. While there are some differences in approaches by each of these
types of entities, the engineering issues are similar for all.
For all of the action regarding deregulation, the distribution infrastructure
remains a natural monopoly. As with water delivery or sewers or other
utilities, it is difficult to imagine duplicating systems to provide true com-
petition, so it will likely remain highly regulated.
Because of the extensive infrastructure, distribution systems are capital-
intensive businesses. An Electric Power Research Institute (EPRI) survey
found that the distribution plant asset carrying cost averages 49.5% of the
total distribution resource (EPRI TR-109178, 1998). The next largest compo-
nent is labor at 21.8%, followed by materials at 12.9%. Utility annual distri-
bution budgets average about 10% of the capital investment in the
distribution system. On a kilowatt-hour basis, utility distribution budgets
average 0.89 cents per kilowatt-hour (see Table 1.1 for budgets shown relative
to other benchmarks).
Low cost, simplification, and standardization are all important design
characteristics of distribution systems. Few components and/or installations
are individually engineered on a distribution circuit. Standardized equip-
ment and standardized designs are used wherever possible. “Cookbook”
engineering methods are used for much of distribution planning, design,
and operations.
Distribution planning is the study of future power delivery needs. Plan-
ning goals are to provide service at low cost and high reliability. Planning
requires a mix of geographic, engineering, and economic analysis skills.
New circuits (or other solutions) must be integrated into the existing distri-
bution system within a variety of economic, political, environmental, elec-
trical, and geographic constraints. The planner needs estimates of load
FIGURE 1.1
Cost of U.S. electricity adjusted for inflation to year 2000 U.S. dollars. (Data from U.S. city
average electricity costs from the U.S. Bureau of Labor Statistics.)
1920 1940 1960 1980 2000
0
20
40
Cost of electricity
Cents per kilowatt-hour
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