Integration issues and simulation challenges of high penetration PV

Integration issues and simulation challenges of high penetration PV March 6, 2014

EnerNex Capabilities Power Systems Modeling Transient Analysis Our preferred tools: EMTP RV, PSCAD Steady State & Quasi Steady State Analysis Our preferred tools: OpenDSS, GridLAB D Smart Grid Engineering Advanced Metering Infrastructure (AMI) Distribution Automation (DA) Demand Response Microgrids 2

UVIG 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 3

Motivation 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 4

Learning Objectives Impact of PV on distribution systems What are the potential issues? Value (and challenges) of computer simulations to predict & fix issues Case study on residential feeder What are the actual issues? 5

Distribution System Impact of High Penetration PV 6

What are the potential issues? Category Reverse Power Flow Voltage Fluctuation Feeder Section Loading Power Losses Impact Overcurrent protection gets confused > false trips, no trips Line regulators get confused > high/low voltage on DG side 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, preinsertion impedance, point on wave) Low/medium PV penetration > PV offsets load thereby decreasing section loading High PV penetration > PV may exceed base load, capacity sufficient to distribute surplus power? PV changes loading (see row above). Impact on losses 7

more potential issues. Category Impact Fault Current PV increases fault current. Impact on relay protection. Unintentional Utility system reclosing into live island may damage Islanding switchgear and loads. Ground Fault Single phase fault > TOVs on unfaulted phase. Overvoltage Harmonics Harmonics caused by PV inverter Effect of fast transients caused by cloud shading and system Dynamics disturbances. Dynamic interaction of transients with other conventional and non conventional control devices. Feeder Imbalance caused by uneven distribution of PV causing Imbalance Neutral to Earth voltages, Overloaded Neutrals 8

Computer Simulations 9

Why simulations? 10

Possible Simulation Outcomes 11

Simulation Challenges: Tool Selection 12

Simulation Challenges: No single tool can do it all Load Flow, balanced Load Flow, unbalanced Short Circuit Relay Coordination Arc Flash Harmonics Transient Analysis Dynamic Analysis Quasi Steady State Analysis ATP, EMTP RV, Simulink, PSCAD Aspen, Cape DesignBase, PowerFactory, Gridiant NexHarm PSLF, PSS/E OpenDSS GridLAB D Best choice Can be done, but not preferred choice Cannot be done 13

Simulation Challenges Modeling PVs PV 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. 14

Simulation Challenges Variability happens on many scales. Variation over Decades (Solar cycle, insignificant for power generated by PV) Monthly Variation (Season) Variation over Hours and Minutes (time of day, clouds) 15

Simulation Challenges Variability due to clouds (often oversimplified) 16

Simulation Challenges Building the system Residential Feeder with Rooftop PV 17

Simulation Challenges Translating issues to costs 18

Utility Scale PV Utility Scale PV Transmission Connected Large (>1 MVA) Three Phase Bulk Generation Very 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). 19

Community PV 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. 20

Rooftop PV Rooftop PV Distribution Connected Small (a few kva) Single Phase Widely Distributed Many 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. 21

Case Study: Impact of Residential PV 22

Distribution Systems Characteristics Mostly residential feeder with some commercial load. Lots of rooftop PVs (around 5 kw each). Two large 1 MW PVs.

Model Validation, Power Flow 12000 10000 Active Power Synergee OpenDSS Active Power, kw 8000 6000 4000 4500 4000 3500 Reactive Power Synergee OpenDSS 2000 0 0 10 20 30 40 50 60 Distance, kft Reactive Power, kvar 3000 2500 2000 1500 1000 500 0-500 -1000 0 10 20 30 40 50 60 Distance, kft

Model Validation, Short Circuit Comparison of Short Circuit Currents 25

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

Distribution Feeder Topology Two large 1 MW PVs Simulations run with and without large PVs Effect of centralized PV vs. distributed PV

Distribution Feeder Topology

Simulation Scenarios Case # Load PV Resolution Sky Condition Aggregated Aggregated 0 Yes Yes 1 h Cloudy to Overcast 1 Yes Yes 30 sec Cloudy to Overcast 2 No Yes 30 sec Cloudy to Overcast 3 Yes No 30 sec Cloudy to Overcast 4 No No 30 sec Cloudy to Overcast 5 No No 1 h Cloudy to Overcast 6 No Yes 30 sec Clear

Accounting for moving clouds

System A, Overvoltages 5% voltage limit PV raises voltage over permissible limit (at some locations, at some times).

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.

What does it all mean for utilities?

General Observations Tap Changes Real Power Use Reactive Power Use Line Losses Increasing PV on a Feeder Impact on tap changing operations discussed today. Documentation and discussion of other observation in report. Report publicly available.

Effect of Aggregation 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 Model predicted tap changes = Actual tap changes Increasing PV on a Feeder Tap changes predicted from models that use aggregated PV generation Actual tap changes

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

How to mitigate PV caused issues Conventional mitigation Set relays in bidirectional mode to account for reverse fault current flow Add voltage regulators Use Current Transformers (CTs) that can sense bidirectional current flow. Advanced technologies PV with Volt/VAr capability PV with communication interface Storage (PV with storage or utility scale storage)

Information Sources 39

More Information at UVIG 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) 40

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? 41

BACKUP 42

Picking the right tool for the job Operational Tools Online operation Facilitate 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 decisions Look at what if scenarios 43

Steady State Analysis 1/2 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. 44

Steady State Analysis 2/2 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. 45

How do we know the steady state models are right? 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 46

Automatic System Conversion CYME SynerGEE Electric EMTP RV Centralized Data Format In MATLAB Based on OpenDSS OpenDSS OpenDSS File 47

Reverse Power Flow 10 Total Power (Case 1) MW (PV) MW (No PV) 5 Power 0-2.1 MW -5 5 10 15 20 Time of Day 48

Ideally building accurate models works like this: BUT, measured data not always available. Use engineering judgement in the absence of data. 49

OpenDSS 50

OpenDSS 51

OpenDSS 52