1. Impacts of Distributed Generation

2. Overview
 Brief Bio
 Distributed Generation
• What is Distributed Generation (DG)?
• Types and Operating Characteristics of DG
 DG Impacts
• Islanding
• Power Quality
• Fault Current
• Peak Load Shaving
• Subtransmission
 Mitigation Methods
 Future of Distribution Systems
 Summary
 Q&A
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3. 3
Brief Background
 George Sey, Jr.
• Education
 BSEE, Pennsylvania State University (Dec. 2004)
 MESE, Pennsylvania State University (Dec. 2011)
• Certifications
 PA Professional Engineer as of June 2015
• Professional Experience
 Testex, Inc. (June 2005 – June 2006)
o Field Engineer
 PECO Energy (June 2006 - Present)
o Distribution System Dispatcher (June 2006 – July 2010)
o Distribution Capacity Planning Engineer (July 2010 – Present)

4. 4
Distributed Generation
 What is Distributed Generation?
• Distributed Generation - the production of electrical energy
at, or close to, the location where it is to be used.1
 Typically produces power in the form of AC.
 Nominal frequency dictated by residing country (60 Hz in US).
 Customer-owned and maintained facilities.
 Distributed Generation, or DG, systems classified by type of utility
interface.
1IEEE 1547-2003 Draft 6.0 – Draft Guide to Conducting Distribution Impact Studies for
Distributed Resource Interconnection

5. 5
Types of Distributed Generation
 Static or Solid-State Inverter
• Power electronic device that converts DC power from the primary
energy source to AC power suited to the grid.2
• Typically powered by photovoltaic (PV) generation, micro turbines,
wind turbines, etc.
 Output current is limited or controlled real-time
 Output voltage is regulated by utility (voltage following)
• Power factor is generally fixed near unity
• Available as utility-interactive or stand alone
• Like models capable of multi-inverter, parallel operation
• Single, large units or micro-inverter units available
2IEEE 1547-2003 Draft 6.0 – Draft Guide to Conducting Distribution Impact Studies for
Distributed Resource Interconnection

6. 6
Types of Distributed Generation
 Inductive Generation
• Identical design to the AC induction motor
• Multi-phase armature windings within solid core stator
• Electromagnetically driven squirrel-cage rotor
• Reactive power is consumed during motor mode operation
• Active power is produced during generator mode operation
• Operates at rotor rotational speeds greater than synchronous
speed (rpm)
• Generally found in variable-speed applications
Renewable Systems Interconnection Study: Advanced Grid Planning and Operations – EPRI,
Feb. 2008

7. 7
Types of Distributed Generation
 Synchronous Generation
• Similar in design to inductive generator
• Solid core stator designed with armature windings within laminated
slots
• Rotor creates rotating magnetic flux via three methods
 Permanent Magnets
 DC-powered electromagnets via slip rings/commutator system
 Brushless exciter alternator
• Induced current imposed upon armature windings
• More costly than inductive generators due to:
 Increased cost of permanent magnets
 Higher complexity in comparison to inductive motors
 Maintenance required for commutator and slip ring adjustments, replacements,
etc.

8. 8
Operating Characteristics
 Static or Solid State Inverter
• Typical DC source to inverter-based systems are photovoltaic (PV)
panels
• Performance has direct correlation to dynamic solar irradiation
• Momentary cloud cover causes erratic output
• “Rule of Thumb” fault current = 2 x normal rated current3
Renewable Systems Interconnection Study: Advanced Grid Planning and Operations – EPRI, Feb. 2008
3Understanding Fault Characteristics of Inverter-Based Distributed Energy Resources – NREL,
Jan. 2010

9. 9
Operating Characteristics
 Inductive Generation
• Field excitation requires leading VARS supplied by capacitors
• Difference between actual rotor driving speed and utility-dictated
synchronous speed is known as slip
• Generator rotational speeds above synchronous speed, or
negative slip, achieves power production
• Prime mover required to achieve negative slip
• Voltage and frequency regulated by utility
• Generation decays to zero steady-state post fault isolation
Understanding Fault Characteristics of Inverter-Based Distributed Energy Resources – NREL,
Jan. 2010

10. Operating Characteristics
 Synchronous Generation
• Driving speed (rpm) determined by number of poles and desired
frequency
• Variable rotor torque dictates real power generation
• Variable excitation level dictates reactive power generation
• Power factor can be controlled via torque and excitation level
• Generation generally persists longer than induction generation due
to rotor inertia and residual magnetic field
10
Understanding Fault Characteristics of Inverter-Based Distributed Energy Resources – NREL,
Jan. 2010

11. Benefits of Distributed Generation
 Customer Benefits
• Reduced apparent power consumption visible to utility, resulting in
reduced electric bill
• If net metering is employed, excess energy may be exported back
to utility in exchange for billing credits
 Utility Benefits
• Reduced apparent power consumption may reduce overall losses
• Depending upon characteristics of DG and high voltage feeder,
service transformer, and DG system, voltage support possible in
some cases
• Dispatchable DG provides alternate sources of power generation
to provide system redundancy and hardening
 Increased renewable energy DG systems (PV, Wind, Geothermal, etc.)
supports AEPS Act 213 initiatives
11

12. 12
Impacts of Distributed Generation
 Secondary Overvoltage
• Voltage rise on secondary service cables
 ANSI Service Voltage Range A, or ANSI Range A, dictates service voltage to
remain within +/- 5% of nominal4
 Most inverter-connected systems operate in voltage-following mode
 Injected power causes voltage rise in correlation to impedance of conductor
between inverter and point of common coupling (PCC)
4Renewable Systems Interconnection Study: Advanced Grid Planning and Operations – EPRI,
Feb. 2008

13. 13
Impacts of Distributed Generation
 Voltage Fluctuation
• Voltage may become highly variable within short periods of time
 Voltage “rise” induced by inverter-based DG varies with variable DC input (i.e.
PV, Wind)
 Voltage swing between DG inverter “maximum output” and “minimum output”
induced voltages measured at customer can produce significant voltage dips
 Resultant voltage dips, or “flicker”, must be evaluated against IEEE 519-1992
flicker curve
 Large voltage swings may also cause sporadic capacitor bank switching and/or
regulator voltage “hunting”
 Generally severity increases with DG penetration and/or concentration level, as
well as system strength
Renewable Systems Interconnection Study: Advanced Grid Planning and Operations – EPRI,
Feb. 2008

14. 14
Impacts of Distributed Generation
 Primary Overvoltage
• DG service transformer is delta primary, grounded-wye secondary
• Occurs when substation breaker isolates multi-grounded-wye
source from faulted facilities and DG continues to operate
• During line-to-ground faults, “neutral shifting” occurs affecting
single-phase loads on non-faulted phases
Renewable Systems Interconnection Study: Advanced Grid Planning and Operations – EPRI,
Feb. 2008

15. 15
Impacts of Distributed Generation
 "Effectively Grounded"
• "Effectively grounded" is defined as:
 “Grounded through a sufficiently low impedance (inherent or intentionally
added, or both) so that the COG does not exceed 80%. This value is obtained
approximately when, for all system conditions, the ratio of the zero-sequence
reactance to the positive-sequence reactance, (X0/X1),is positive and ≤3, and
the ratio of zero-sequence resistance to positive-sequence reactance,
(R0/X1), is positive and < 1”5
 Key ratios to obtain “effectively grounded"
o Coefficient of Ground (COG): (VL-G,fault / VL-L, nominal pre-fault) < 0.8 p.u. or 80%
o R0/X1 < 1 and X0/X1 < 3
• Grounding compliance is maintained by utility source, but not
necessarily by DG during islanding conditions
• "Effectively grounded" must be maintained by DG source
 Compliance limits potential damage incurred by temporary “neutral shift”
 DG point of common coupling interconnection typically indicates severity of
grounding concern
5IEEE Std. 62.92.1-2000: Guide for the Application of Neutral Grounding in Electric Utility
Systems – Part 1 - Introduction

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Impacts of Distributed Generation
 Unintentional Islanding
• Condition established when a section of the grid is isolated from
the substation supply while the load continues to be maintained by
DG within the isolated section that continues to provide power.6
• Results in potentially non-phasing automatic reclosing between
utility and island
6IEEE 1547-2003 Draft 6.0 – Draft Guide to Conducting Distribution Impact Studies for
Distributed Resource Interconnection
Renewable Systems Interconnection Study: Advanced Grid Planning and operations

17. 17
Impacts of Distributed Generation
 Unintentional Islanding
• Island remains energized at a different frequency, voltage, and/or
power factor
• DG generally has insufficient regulatory controls to maintain
frequency and voltage within ANSI Range A limits
• Possible damaged conductors contacting ground could remain
energized
 Poses risk to public of additional source of electrical injury
 Poses additional risk to line crews performing restorative work
Renewable Systems Interconnection Study: Advanced Grid Planning and Operations – EPRI,
Feb. 2008

18. 18
Impacts of Distributed Generation
 Unintentional Islanding
• IEEE 1547 standard and utility interconnection guidelines require
DG systems to have anti-islanding protection employed
• Anti-islanding protection must comply with UL 1741 testing
standards
• 15% penetration threshold provides a “rule of thumb” indication of
potential for interconnection issues
Understanding Fault Characteristics of Inverter-Based Distributed Energy Resources – NREL,
Jan. 2010

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Impacts of Distributed Generation
 Reduced LTC Effectiveness
• Load Tap Changer (LTC) voltage regulation can be negatively
impacted
 LTC regulation is accomplished by current monitoring of regulator terminals
 Line drop compensation performed as a function of current measured and total
impedance from source to a desired PCC for downstream load
 Voltage is increased by LTC to comply to ANSI Range A limits at the end of
feeder(s) served
 High penetration DG installation(s) near LTC can alter “visible current”
measured by LTC
Renewable Systems Interconnection Study: Advanced Grid Planning and Operations – EPRI,
Feb. 2008

20. 20
Impacts of Distributed Generation
 Fault Current Contribution
• Excessive fault current
• Coordination issues due to sympathetic tripping
 Unnecessary tripping of additional protective devices on unrelated feeder(s)
due to a fault on an adjacent feeder
 High penetration levels required for sufficient fault current for multi-feeder
scenario
 Similar condition exists for lower penetrations on a single feeder
 Protective devices may operate out of intended coordination scheme
Renewable Systems Interconnection Study: Advanced Grid Planning and Operations – EPRI,
Feb. 2008
F1
F3
F2

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Impacts of Distributed Generation
Impact of Solar Distributed Generation on System Peak
[Based on Existing Requests]
5000
6000
7000
8000
9000
0:00
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Time
SystemPeakMW
0
75
150
SolarPeakMW
System Peak MW Solar Reduced System Peak MW Solar MW
Typical Solar
Output Curve
Solar Generation
has little impact on
System Peak
 Solar DG peak load shaving

22. 22
Impact of All Distribution Generation on System Peak
[Based on Existing Requests]
5000
5500
6000
6500
7000
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9000
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Time
SystemPeakMW
0
75
150
225
300
CogenerationMW
System Peak MW Congen Reduced System Peak MW Total Cogen MW
Solar Output plus
Other Cogen at 100%
0800 to 2000
Impacts of Distributed Generation
 All DG peak load shaving

23. 23
Mitigation Methods
 "Effectively Grounded"
• Utility-interconnected DG systems must comply with “effectively
grounded" criteria
 Resolution: If system is found non-compliant, system zero sequence
impedance or positive sequence reactance must be modified.
• Ground reference absent due to delta primary, grounded-wye
secondary interconnection
 Resolution: Installation of isolation transformer or grounding bank (i.e. -
grounded-wye, delta), which provides neutral ground/zero sequence reference
to stabilize voltage.
 Excessive Fault Current
• Aggregate utility and DG fault current contribution exceeds
allowable limits
 Resolution: Current limiting equipment may be installed (i.e. series line
reactors) to increase impedance path to utility interconnection, reducing fault
current. Reduction in DG system generating capacity also a potential solution.

24. Mitigation Methods
 Potential Islanding Condition
• Aggregate DG approaching or exceeding 15% Line section peak
 Resolution: If found to be >15%, supplemental analysis should be completed.
Transfer-trip relaying scheme is a viable solution.
24
Renewable Systems Interconnection Study: Advanced Grid Planning and Operations – EPRI,
Feb. 2008

25. 25
Mitigation Methods
 Secondary Overvoltage
• Voltage rise of utility-interactive inverter seen by customer is
greater than ANSI Range A upper limit
 Resolution: Reduce conductor impedance through placing inverter in closer
proximity of PCC, and/or use lower impedance conductor path.
o Alternative option to reduce current injection (power output) of inverter onto system will also
provide a reduction of overvoltage.
 Reduced LTC Effectiveness
• DG interconnected in close proximity of LTC of feeder may lower
monitored downstream current
 Resolution: For aggregate lower penetrations, effects are typically insignificant.
Higher power output systems, or increasingly high aggregate penetration
levels, may require relocation, greater dispersion, or reduced power output.

26. 26
Additional Impacts of Distributed Generation
 Subtransmission Concerns
• Ground fault overvoltage
 Analogous to distribution level concerns, excessive voltage on non-faulted
phases may occur during phase-to-ground faults.
 Due to higher capacity of subtransmission system, significant penetration
levels must exist for this condition.
Renewable Systems Interconnection Study: Advanced Grid Planning and Operations – EPRI,
Feb. 2008

27. 27
Future Distribution System Design
 Interactive volt/var regulation
 Adaptive protective relaying schemes
 Advanced islanding control
 Improved grounding compatibility
 Energy storage
Renewable Systems Interconnection Study: Advanced Grid Planning and Operations – EPRI,
Feb. 2008

28. Summary
 Distribution system is not designed to accommodate
multiple sources other than the substation
 Future adaptations to the standard distribution system
design must be considered for proper integration with high
DG penetration such as:
• Interactive volt/var regulation
• Transfer/trip communication
• Communication amongst LTCs, regulators, inverter-based
systems, etc.
 Effects upon local utility system varies upon characteristics
of the system, proposed DG, and its interconnection site
 Severity of impacts are generally proportional to
penetration level
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