integration issues and simulation challenges of high ... · 06/03/2014 · line regulators get...

© 2011 EnerNex. All Rights Reserved. www.enernex.com

Integration issues and simulation challenges of high‐penetration PV

March 6, 2014


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


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


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


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?


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


… 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


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

© 2011 EnerNex. All Rights Reserved. www.enernex.com14

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.


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)


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


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).


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.


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.


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.


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


Model Validation, Short Circuit

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Comparison of Short Circuit Currents


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



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



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


Accounting for moving clouds


System A, Overvoltages

PV raises voltage over permissible limit (at some locations, at some times).

5% voltage limit


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

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.


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


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


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


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)


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?


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


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.


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.


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


Automatic System Conversion

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Centralized Data Format In MATLABBased on OpenDSS

OpenDSS File

CYME EMTP‐RV

SynerGEE Electric

OpenDSS


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


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.


OpenDSS

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OpenDSS

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OpenDSS

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integration issues and simulation challenges of high ... · 06/03/2014 · line regulators get...

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