IEEE Rural Electric Power Conference
Distributed Generation System Impact
Analysis with Computer Modeling
Tools
Sean A. Kufel, P.E.
Power System Engineering, Inc.
www.powersystem.org
April 20, 2015
Session Summary
Goal: Developing a reliable, efficient process for
performing DG system impact studies.
• Data Requirements
• Modeling Generation
– Types of generator models & adding generation
– Types of analysis
– Common modeling errors
• Focus on steps where errors or confusion are common
• NOT: Instructions on using any modeling program
• DISCLAIMER(S): I am not a programmer. I am not
endorsing any particular modeling application.
© 2015 Power System Engineering, Inc.
2
System Impact Analysis Goals
Common impacts of DG that can be identified with
computer modeling:
• Voltage rise
• Conductor/equipment overload
• Inadequate device interrupt rating
• Reverse power flow
• Potential for islanding
© 2015 Power System Engineering, Inc.
3
Data Requirements
• Utility side:
– Electrical model of distribution system area where DG is
proposed, including:
• Source impedance, including substation power transformer
• System conductors
• Major system equipment – transformers, regulators, capacitors &
protective devices
• Substation area/feeder peak demand
– Historical minimum load in area/on feeder
• Often estimated at around 25% of peak if historical hourly data is not
available
– Protective device settings
– Regulator/LTC settings
© 2015 Power System Engineering, Inc.
4
Data Requirements
• Applicant/Developer/Generation side:
– Number of generators to be installed and total aggregate
capacity
– Proposed facility one-line/three-line diagram
– Expected peak generator output & how the generation will be
used (back-up only, intermittent operation, on-site load service,
power export, etc.)
– Location of proposed interconnection (preferably in reference to
distribution system)
© 2015 Power System Engineering, Inc.
5
Data Requirements
• Applicant/Developer/Generation side:
– Generator data:
• Operating voltage
• Ratings: kW, kVA, power factor
• Fault information
– Steady-state, transient & subtransient reactance values or generator
equivalent circuit for rotating machines
– For PV with inverter(s), fault current is typically a multiple of rated output
(150% is often used)
– Other equipment data:
• Inverter ratings, solar panel data for PV installations
• Ratings & impedances of any generator step-up transformer units (GSU)
© 2015 Power System Engineering, Inc.
6
Generator Fault Information
Impedance Data
© 2015 Power System Engineering, Inc.
Equivalent Circuit
7
Modeling Programs
Milsoft WindMil®
Eaton CYME
• SynerGEE, eTap, Dapper, etc.
© 2015 Power System Engineering, Inc.
8
Types of Generator Models
• Generic Generator
• Two operating modes:
– Negative load (constant
kW output)
– Swing kVAR (hold
desired voltage by
adjusting kVAR output)
• Fault output based on
generator impedances
© 2015 Power System Engineering, Inc.
9
Types of Generator Models
• Specific Generators
• Common types:
– Synchronous & Induction
– Wind turbine
– Solar array
– Others
• Operating modes &
fault contributions
dependent upon type of
generation
• Mostly the same as
generic for synchronous
machines
© 2015 Power System Engineering, Inc.
10
Generic Generator Pros/Cons
• Con:
– Sometimes need to perform impedance calculations when only
fault duty is available
• Pro:
– Conversion utilities are often built in
© 2015 Power System Engineering, Inc.
11
Specific Generator Pros/Cons
Pro:
• Possible to create extremely
detailed generator models
with LOTS of specific data
Con:
• Hyper-specific data
generally not needed for
snapshot analysis
• Possible (?) to run highly
specific analysis depending
upon program modules &
capabilities
• Lots of specific data can be
overwhelming
© 2015 Power System Engineering, Inc.
12
Information Overload?
© 2015 Power System Engineering, Inc.
13
Add Generation to the Model
© 2015 Power System Engineering, Inc.
14
Voltage Drop/Load Flow Analysis
Peak Load
© 2015 Power System Engineering, Inc.
Minimum Load
15
Voltage Drop/Load Flow Analysis
Common Error: Generators not included in analysis
© 2015 Power System Engineering, Inc.
16
Collecting Results
• From on-screen result boxes:
© 2015 Power System Engineering, Inc.
17
Collecting Results
• Via custom reports:
© 2015 Power System Engineering, Inc.
18
Voltage Drop/Load Flow Analysis
• Other things to check:
– Substation/source power factor, before & after generation is
added
– Native loads in all protective zones upline of the generation
(with generation offline)
• Aid in determining if islanding is possible
– Current flow through equipment with generation online
– Reverse power flow through equipment and/or substations when
generation is operating (more likely and higher at minimum
load)
© 2015 Power System Engineering, Inc.
19
Checking Protective Zone Loads
© 2015 Power System Engineering, Inc.
20
Turning on Power Flow Arrows (If Available)
© 2015 Power System Engineering, Inc.
21
Short-Circuit Analysis
© 2015 Power System Engineering, Inc.
22
Short-Circuit Analysis: General
Gen Z: Steady-state or
“None”
© 2015 Power System Engineering, Inc.
Gen Z: Subtransient
23
Coordination Analysis
© 2015 Power System Engineering, Inc.
24
Device-Device Coordination Setup
CYME TCC Settings 
© 2015 Power System Engineering, Inc.
Device Coordination Check
25
Short-Circuit Analysis: Fault Flow
Generator
© 2015 Power System Engineering, Inc.
End of Protective
Zone 1
Feeder Recloser
26
Short-Circuit: Reverse Fault Flow
Initiate fault
immediately upline
of protective device
© 2015 Power System Engineering, Inc.
27
Short-Circuit: Fault Flow
• Locations to check during fault flow:
– End of protective device zones
• Especially in direct path between generation and source
• Especially for electronically-controlled reclosers
– Check ground pickup setting versus minimum fault flowing through device
with generation contributing
– Source side of devices in direct path between generation and
source
– Other feeders on substation
© 2015 Power System Engineering, Inc.
28
System Impact Study Model Use Keys
• Develop a process and stick to it
• Double-check entered data
• Document analysis results clearly
• Keep track of model changes made to improve system
conditions
– Probably not a good idea to alter the working model of your
existing system if you are a utility, especially if it is an
enterprise or shared model
• Step back and sanity-check results
© 2015 Power System Engineering, Inc.
29
QUESTIONS &
DISCUSSION
© 2015 Power System Engineering, Inc.
30
Thank You:
IEEE 2015 REPC
Attendees!
Power System Engineering, Inc.
Name: Sean A. Kufel, P.E.
Title: Electrical Engineer
Direct: (740) 568-9220 x11
Mobile: (216) 544-8614
Email: kufels@powersystem.org
www.powersystem.org
© 2015 Power System Engineering, Inc.
31