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© 2011 EnerNex. All Rights Reserved. www.enernex.com
Integration issues and simulation challenges of high‐penetration PV
March 6, 2014
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EnerNex Capabilities
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Power Systems ModelingTransient Analysis Our preferred tools: EMTP‐RV, PSCAD
Steady‐State & Quasi Steady‐State Analysis Our preferred tools: OpenDSS, GridLAB‐D
Smart Grid EngineeringAdvanced Metering Infrastructure (AMI)Distribution Automation (DA)Demand ResponseMicrogrids
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UVIG
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Established by 6 utilities in 1989 with support from EPRI and DOE/NREL
Utility members from IOU, public power, and rural electric cooperative sectors along with RTOs/ISOs
Includes associate members from development, IPP, equipment, and consulting community
Non‐profit corporation governed by board of directors from utility and ISO/RTO members
Has over 180 members from US, Canada, Europe, Asia, and Australia/New Zealand
Focus on technical issues related to wind and solar generation
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Motivation
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Renewable generation is being incentivized in the United States and globally. For instance, California has 1.6 GWs of installed distributed solar generation as of March 2013.
Cooperatives starting to see high PV penetration levels (Kauai Island, Arizona, Colorado).
Increasing numbers of residential‐, community‐, and utility‐scale photovoltaic (PV) installations. What are the potential issues for utilities and utility customers?
Simulations to predict & fix issues
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Learning Objectives
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Impact of PV on distribution systemsWhat are the potential issues?
Value (and challenges) of computer simulations to predict & fix issues
Case study on residential feeder What are the actual issues?
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Distribution System Impact of High‐Penetration PV
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What are the potential issues?
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Category ImpactReverse
Power FlowOvercurrent protection gets “confused” ‐> false trips, no tripsLine regulators get “confused” ‐> high/low voltage on DG side
Voltage Fluctuation
Capacitor switching, Load Tap Changer (LTC) operation, and line Voltage Regulator (VR) operation caused by cloud shading.Flicker caused by cloud transients.Capacitor switching transients (synchronous closing, pre‐insertion impedance, point‐on‐wave)
Feeder Section Loading
Low/medium PV penetration ‐> PV offsets load thereby decreasing section loadingHigh PV penetration ‐> PV may exceed base load, capacity sufficient to distribute surplus power?
Power Losses PV changes loading (see row above). Impact on losses
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… more potential issues.
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Category ImpactFault Current PV increases fault current. Impact on relay protection.Unintentional Islanding
Utility system reclosing into live island may damage switchgear and loads.
Ground Fault Overvoltage Single‐phase fault ‐> TOVs on unfaulted phase.
Harmonics Harmonics caused by PV inverter
DynamicsEffect of fast transients caused by cloud shading and system disturbances. Dynamic interaction of transients with other conventional and non‐conventional control devices.
Feeder Imbalance
Imbalance caused by uneven distribution of PV causing Neutral‐to‐Earth voltages, Overloaded Neutrals
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Computer Simulations
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Why simulations?
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Possible Simulation Outcomes
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Simulation Challenges: Tool Selection
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Simulation Challenges: No single tool can do it all
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Best choice
Can be done, but not preferred choice
Cannot be done
Load Flow,balanced
Load Flow,unbalanced
Short Circuit
Relay Coordination
Arc Flash
Harmonics TransientAnalysis
Dynamic Analysis
Quasi Steady‐State Analysis
ATP, EMTP‐RV, Simulink, PSCAD
Aspen, Cape
DesignBase,PowerFactory,Gridiant
NexHarm
PSLF, PSS/E
OpenDSS
GridLAB‐D
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Simulation Challenges Modeling PVsPV generators are complex devices.Many different types of inverters out there – difficult to get information needed for modeling them in detail.
Need to fit the complexity of the model to the problem.
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Simulation Challenges Variability happens on many scales.
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MonthlyVariation (Season)
Variation over Hours and Minutes (time of day, clouds)
Variation over Decades(Solar cycle, insignificant for power generated by PV)
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Simulation Challenges Variability due to clouds (often oversimplified)
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Simulation Challenges Building the system
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Residential Feeder with Rooftop PV
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Simulation Challenges Translating issues to costs
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Utility‐Scale PV
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Utility‐Scale PV
Transmission‐Connected
Large (>1 MVA) Three Phase Bulk
GenerationVery Few Systems
No distribution system issues (transmision connected).Positive Sequence tool (e.g., PSLF or PSS/E) for technical study.
Economic impact evaluated in an integration study (using, e.g., Promod).
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Community PV
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Community PV
Distribution‐Connected
Medium(a few 100 kVA) Three Phase Somewhat
Distributed Few Systems
Distribution system issues listed previously apply.Positive Sequence tool (e.g., PSLF or PSS/E) or OpenDSS for technical study.
Need to “translate” simulation output (e.g., operation of voltage regulators) to costs.
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Rooftop PV
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Rooftop PV
Distribution‐Connected
Small(a few kVA) Single Phase Widely
DistributedMany Systems
Distribution system issues listed previously apply.Distribution software required (e.g., OpenDSS).Disaggregation of load/generation for accurate results => biggest simulation challenge.
Need to “translate” simulation output (e.g., operation of voltage regulators) to costs.
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Case Study:Impact of Residential PV
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Distribution Systems Characteristics
Mostly residential feeder with some commercial load.
Lots of rooftop PVs (around 5 kW each).Two large 1 MW PVs.
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Model Validation, Power Flow
0 10 20 30 40 50 600
2000
4000
6000
8000
10000
12000
Distance, kft
Act
ive
Pow
er, k
W
SynergeeOpenDSS
0 10 20 30 40 50 60-1000
-500
0
500
1000
1500
2000
2500
3000
3500
4000
4500
Distance, kft
Rea
ctiv
e P
ower
, kVA
r
SynergeeOpenDSS
Active Power
Reactive Power
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Model Validation, Short Circuit
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Comparison of Short Circuit Currents
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Simulation Scenarios
1. Low (actual) penetration of small PV w/ 2 MW PV
2. Low (actual) penetration of small PV w/o 2 MW PV
3. High penetration of small PV w/ 2 MW PV
4. High penetration of small PV w/o 2 MW PV
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Distribution Feeder Topology
Two large 1 MW PVs Simulations run with and
without large PVs Effect of centralized PV
vs. distributed PV
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Simulation Scenarios
1. Low (actual) penetration of small PV w/ 2 MW PV
2. Low (actual) penetration of small PV w/o 2 MW PV
3. High penetration of small PV w/ 2 MW PV
4. High penetration of small PV w/o 2 MW PV
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Distribution Feeder Topology
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Simulation Scenarios
Case # Load Aggregated
PV Aggregated
Resolution Sky Condition
0 Yes Yes 1 h Cloudy to Overcast1 Yes Yes 30 sec Cloudy to Overcast2 No Yes 30 sec Cloudy to Overcast3 Yes No 30 sec Cloudy to Overcast4 No No 30 sec Cloudy to Overcast5 No No 1 h Cloudy to Overcast6 No Yes 30 sec Clear
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Accounting for moving clouds
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System A, Overvoltages
PV raises voltage over permissible limit (at some locations, at some times).
5% voltage limit
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System A, Tapchange Operations
PV significantly increases tap changing operations. Case 2 (PV aggregated) vs. Case 4 (no PV aggregation). Cases 0 and 5, 1 hour simulation step size (vs. 30 seconds for other cases). Case 6, clear day.
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What does it all mean for utilities?
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General Observations
Line Losses
Tap Changes
Real Power Use
Reactive Power Use
IncreasingPV on a Feeder
Impact on tap changing operations discussed today. Documentation and discussion of other observation in report. Report publicly available.
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Effect of Aggregation
IncreasingPV on a Feeder
Model predictions based on aggregated models exaggerate the actual tap changing operations for high-PV penetration scenarios PV-caused wear & tear on voltage regulators less
than predicted by most models
Actual tap changes
Tap changes predicted from models that use aggregated PV generation
Model predicted tap changes =Actual tap changes
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Conclusions, System A (Residential Feeder)
PV caused overvoltage => additional voltage regulators required
PV increased tap changing operation of voltage regulators increased => loss‐of‐life and increased maintenance
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Conventional mitigation– Set relays in bidirectional mode to account for reverse fault current flow
– Add voltage regulators– Use Current Transformers (CTs) that can sense bi‐directional current flow.
Advanced technologies– PV with Volt/VAr capability– PV with communication interface– Storage (PV with storage or utility‐scale storage)
How to mitigate PV‐caused issues
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Information Sources
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More Information atUVIG Website: http://variablegen.org
– Library of wind and solar literature– Wind and solar FAQs– News about upcoming workshops and other events
UVIG DG Toolbox: http://variablegen.org/toolbox/– FERC and Flicker screening– Feeder Simulator– Economic Analysis
UVIG Wiki: http://wiki.variablegen.org/– Wind Turbine and Plant Modeling– PV Modeling (work in progress)
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Discussion Points
• Have you seen any PV‐caused problems on your system?
• Are you worried about problems future PV may cause on your system?
• Any thoughts on how to translate simulation results to cost to utility?
• Availability of detailed system information and data (solar and load data) needed to accurately predict the issues?
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BACKUP
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Picking the right tool for the job
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Operational ToolsOnline operationFacilitate real‐time operational decision regarding voltage regulation, transformer loading, PQ, etc.
Gridiant’s GRIDview, PowerAnalytic’s Paladin Live, Paladin SmartGrid
Planning/Analysis Tools (this is what we are using)Offline simulations Facilitate planning/design decisionsLook at ‘what if’ scenarios
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Steady‐State Analysis 1/2
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Changing generation levels: second time frame (clouds), minute/hour time frame (time of day) =>Increased equipment wear (tap changes, cap switching)
What changes for protection coordination (fuse blowing/saving, reduction of reach)?
Reverse current flow affects protection coordination and “confuses” voltage regulators.
DG ground sources act like a current divider in the zero sequence path causing some ground current to bypass CT.
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Steady‐State Analysis 2/2
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Modification of Feeder Section Loading. Capacity sufficient to distribute surplus power?
Impact on losses/economics.Impact of changed loading and power quality issues on equipment (such as transformers and conductors) rating, sizes, and life cycle.
Employ PV and storage as backup during system outages.
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How do we know the steady‐state models are right?
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Make the simulation as realistic as possible!!!– Disaggregating Generation:Modeling each individual PV system and use local irradiance data at each PV location
– Disaggregating Loads:Model each device in the building and turn them on/off stochastically (GridLAB‐D can do this)
Model validation– Benchmark our simulation tools (OpenDSS, GridLAB‐D) against results from utility tools (e.g., CYME, Synergee)
– Validate simulation results with measured data
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Automatic System Conversion
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Centralized Data Format In MATLABBased on OpenDSS
OpenDSS File
CYME EMTP‐RV
SynerGEE Electric
OpenDSS
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Reverse Power Flow
5 10 15 20-5
0
5
10
Time of Day
Pow
er
Total Power (Case 1)
MW (PV)MW (No PV)
- 2.1 MW
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Ideally building accurate models works like this:
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BUT, measured data not always available.Use engineering judgement in the absence of data.
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OpenDSS
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OpenDSS
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OpenDSS
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