Hawaiian Electric Grid Support Utility Interactive (GSUI) Inverter Pilot – Preliminary Findings

Andy Hoke, PhD, PE May 23, 2018

Presentation to the Hawaiian Electric Advanced Inverter Function Working Group

NREL | 2

NREL: Peter Gotseff, Nick Wunder, Julieta Giraldez, Courtney Pailing HECO: Earle Ifuku, Tom Aukai, Reid Sasaki, Reid Ueda, Vivian Paek, Claudia Rapkoch, Wil Lum/Meter Shop, Dixson Lau, Brendan Bailey

Contributors

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• Hawaii has more distributed PV than any other U.S. state. • DERs play a major part in the plan for 100% renewables by 2045 • Current levels of PV result in steady-state voltage issues in some cases • Communication to DERs is typically proprietary or non-existent • Near-term solution: autonomous inverter-based grid support • Pilot focus: Volt-var, fixed power factor, and volt-watt for steady-state

voltage regulation

Motivation

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• Objective: Field validation of inverter-based voltage regulation functions o Investigate impacts on feeder voltages (mainly secondaries) o Investigate curtailment impacts on PV kWh production o Validate VROS project feeder models with field data o Confirm that there are no undesired interactions among inverters or between inverters and utility

equipment

• Functions under test: o Volt-var control (VVC) o Volt-watt control (VWC) o Non-unity fixed power factor (FPF) operation o Combination of VVC and VWC

• NEM customers offered ability to connect without waiting for secondary system upgrade in exchange for participating in the pilot

Project Overview

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• Hawaiian Electric Company – Oahu • NREL • Inverter manufacturers selected based on the choices of the customers who volunteered:

o SolarEdge o Enphase o Both manufacturers provided valuable technical support

• PV installers selected based on the choices of the customers who volunteered: o Alternate Energy o Creative Energy o ECO Solar o Hawaii Energy Connection o Island Pacific Energy o Sunetric

• Customers o 15 active, plus others providing data only o Two “cluster” sites (i.e., more than one advanced inverter plus legacy PV on a single secondary transformer) o In process of adding additional ~20 CGS customers in 2nd phase expansion o Four customers identified to continue to 2nd phase expansion

Entities involved

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• > 100 “Stuck” NEM customers invited in January 2017 o Most did not have to pay for an upgrade so had little motivation

• 15 customers participating • All residential, on various Oahu feeders

o Not enough customers on any one feeder to see feeder-level voltage impacts • All were legacy NEM customers that required a secondary upgrade and agreed to switch to

Advanced Inverters • 2 clusters of customers on same transformer

o Each cluster has multiple legacy PV systems and 2-3 new advanced inverter systems o Irradiance sensors installed at each cluster

Customers

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• Irradiance: 1-second intervals (selected locations only) • Inverter measurements: V, P, Q, pf, frequency, etc at inverter terminals

o String inverters: 1-second intervals; Microinverters: 5-minute intervals • AMI: V, P, Q at customer meter; 1-minute intervals • Grid2020: V, P, Q on LV side of transformer; 1-minute intervals • Feeder SCADA data (V, P, Q, PF, LTC setting)

Data collection points

Distribution transformer

~

Inverter AC & DC Irradiance AMIGrid2020Substation SCADA

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• Developed a remote inverter control and data collection system • Required significant custom effort • External power source required

• Various communication issues requiring multiple field visits • Communication intermittency • Not all PV systems capable of remote configuration

• Operational costs to set up communications systems are significant; the methods used here may not be economically scalable

Communications

Cradlepoint cellular router with LAN (CAT5) connection to inverter. Allows remote TCP/IP access to inverter

using SunSpec Modbus TCP

120 V power for modem RTU interfaces between cell router and rooftop solar irradiance sensor

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Data collection, storage, and visualization platform

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Example of site with high correlation between irradiance and voltage - 4/23/18

• Cluster 1: Volt-var + volt-watt mode

• This is the second-highest voltage location in the pilot

• M3 circuit - Long shared overhead secondary

• Voltage peaks below 1.06 pu this day (i.e. volt-watt not active)

• Minimal impact on PV production

• DC/AC ratio is 1.2 (fairly typical)

• Vars absorbed when V>1.03, as expected.

• Inverter power still ~3.8 kW (rated P)

Solar clearly driving

secondary voltages

• V peaks at 1.056 pu • Irradiance at 1.1

kW/m2, 110% of “maximum”

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Example of site with low correlation between irradiance and voltage - 4/23/18

• Cluster 2: Volt-var + volt-watt mode

• This is a more typical location

• Circuit A - Newer underground secondary

• Two of three GSUI inverters shown

• Issue is the transformer loading and not voltage

No impact on PV production

Voltage stays about the

same regardless of

PV output

V peaks at 1.033 pu

Volt-var barely active

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Secondary voltage rise, as modeled in VROS report

• For customers with highest voltage, rise often occurs mostly on secondary conductor

• Shown is a worst-case extremely high-penetration secondary from VROS simulations. (Highest voltage in pilot is about 1.08 pu.)

• Volt-var (or FPF) can only do so much on such a secondary.

• In such cases, distributed VAr sinks/sources (e.g. Varentec) will have similar limitations on effectiveness

Voltage rise across transformer

Voltage rise on secondary conductor

Volt-var is great for transformer voltage rise, but not resistive rise on conductor

Volta

ge, p

.u.

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Initial output: quantify behind-the-meter voltage rise

• Current from inverter creates voltage change as it flows to meter: ΔVbtm = I*Zbtm • When in volt-var or volt-watt mode, most DER today respond to the voltage at their terminals (PoC)

• May differ from voltage at meter (PCC) • When in volt-var or volt-watt mode the change in output (real or reactive power) will be affected by

voltage rise behind the meter • VArs won’t mitigate ΔVbtm much because wires are mainly resistive (not inductive)

• This will affect the impact of volt-var and volt-watt both on the feeder and on PV energy production

InverterDistribution primary (and rest of grid)

Distribution transformer

Z2nd ~ ZDCZgrid

Meter

Zbtm

PV / DC source

Wires between meter and

inverter AC terminals

- +ΔVbtm

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Sample ΔVbtm for a string inverter

Data from 2 fairly sunny days:

2017 timeAug 05 Aug 06 Aug 07

0100200300400500600700800900

10001100120013001400

Irrad

ianc

e (W

/m2

)

Customer PN-15-02262

2017 timeAug 05, 00:00 Aug 05, 12:00 Aug 06, 00:00 Aug 06, 12:00 Aug 07, 00:00

236

238

240

242

244

246

Volta

ge (V

olts

)

Customer PN-15-03853

Inverter V

AMI V

Aug 06, 2017 time12:00 12:30 13:00 13:30 14:00 14:30 15:00 15:30 16:00

238

240

242

244

246

Volta

ge (V

olts

)

Customer PN-15-03853

Inverter V

AMI V

SE2

SE1

SE1

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PV system with worst-case voltage rise

• Microinverter system; higher ΔVbtm typical due to longer AC wire run

• Bi-modal: PV arrays on two different roof pitches

• One array peaks at average of 2 V rise (0.8%)

• Other array peaks at average of 5 V rise (2.1%)

• 2.1% is the highest array-level voltage rise seen in all 15 systems in the project

0 0.2 0.4 0.6 0.8 1 1.2

Inverter Real Current (Amps)

-4

-3

-2

-1

0

1

2

3

4

5

6

7

8

9

10

Del

ta V

olta

ge (V

)

Customer PN-15-02416

Total inverters : 34

EN2

Voltages on 240 V basis

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Daily maximum and minimum voltages – weak secondary location

• Cluster 1 – circuit M3 – long shared OH secondary (slide 10)

• Inverter voltage typically peaks around 1.04 pu, sometimes higher

• Volt-var active most days, volt-watt typically not

• Based on 15-minute average voltages. 1-minute and 1-second data shows greater variation

Higher voltages due to temporary primary configuration

Aug 2017 Sep 2017 Oct 2017 Nov 2017 Dec 2017 Jan 2018 Feb 2018 Mar 2018

time

0.94

0.96

0.98

1

1.02

1.04

1.06

1.08

V (p

u)

AMI 15min avg

AMI Daily Max

AMI Daily Min

Inv Daily Max

Inv Daily Min

Inv Avg Daily Max

Inv V avg = 1.026

Inv V avg daily max = 1.040

Inv V avg daily min = 0.992

AMI Vavg = 1.014

AMI V avg daily max = 1.039

AMI V avg daily min = 0.991

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Daily maximum and minimum voltage histogram – strong secondary location

• Cluster 2 – circuit A – newer UG secondary (slide 11)

• Impact of PV on voltage distribution is minimal

• Minimal behind-the-meter voltage rise

• Volt-var is rarely active and volt-watt is never active

• Estimated curtailment of PV production due to volt-var and volt-watt at this location is zero. Volt-var

threshold Volt-watt threshold

Mean daytime AMI voltage

Mean inverter voltage

ami night <06:00 >18:00

ami day 9:00-15:00

eni day 9:00-15:00

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Daily maximum and minimum voltage histogram – weak secondary location

• Cluster 1 – circuit M3 – long shared OH secondary

• 2nd-highest voltage location in pilot

• Impact of PV on voltage distribution is clear

• Minimal behind-the-meter voltage rise

• Estimated curtailment of PV production due to volt-var and volt-watt at this location is < 0.5% of monthly energy production.

Volt-var threshold

Volt-watt threshold

Mean AMI voltage Mean inverter voltage

ami night <06:00 >18:00

ami day 9:00-15:00

eni day 9:00-15:00

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0.94 0.96 0.98 1 1.02 1.04 1.06 1.08

Vpu

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

Pro

babi

lity

ami night <06:00 >18:00

ami day 9:00-15:00

eni day 9:00-15:00

ENIday Vavg = 1.040

AMI Vavg = 1.018

AMInight Vavg = 1.011 AMIday Vavg = 1.033

AMIday Vsd = 0.009

AMIday Vskew = -0.510

AMIday Vkurt = 3.84

AMI Total Sample Days = 111.6

AMI Total Samples = 2679

AMI Perc daytime >= 1.03Vpu = 64.2

AMI Perc daytime >= 1.06Vpu = 0.0

ENI Total Sample Days = 111.0

ENI Total Samples = 2664

ENI Perc daytime >= 1.03Vpu = 76.2

ENI Perc daytime >= 1.06Vpu = 6.2

Daily maximum and minimum voltage histogram – highest voltage location

• Significant voltage rise both on secondary and behind-the-meter

• Volt-var active about 76% of the time between hours of 9am-3pm

• Volt-watt active about 6% of the time between hours of 9am-3pm

• Expected curtailment is difficult to quantify exactly without irradiance data at this location o Based on available data,

estimated curtailment is < 5% of monthly energy production

o Re-examine in 2nd phase of pilot with irradiance sensor installed Volt-var threshold Volt-watt threshold

Mean AMI voltage Mean inverter voltage

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The table below was included in a letter sent to the customer. The table summarizes findings for that location for the first phase of the pilot.

Sample customer report

Customer name -Site -PV inverter -Application ID -

Typical daily maximum Typical daily minimum

Percentage of time above 103% of nomimal

Percentage of time above 106% of nomimal

Typical daily maximum

Typical daily minimum

125 volts 119 volts 38% 0.4% 124.5 volts 119 volts < 0.5% <0.5%

Note:Voltages are based on 15-minute average voltage measurements, and are shown on a nominal 120-volt basisThe typical daily maximum (15 minute average) is 124.5 volts as measured at the utility meter. The allowed voltage range is +/- 5% or 126-volts (maximum) to 114-volts (minimum).The volt-var curve is designed to provide reactive power above 123.6 volts to 127.2 volts (between 103% to 106% of nominal) for approximately 38% of the time in your location.The volt-watt curve curtails the PV output when voltages are above 127.2 volts (between 106% to 110% of nominal) for approximately less than <0.4% of the time in your location.

Recommendation:For safety reasons and to comply with interconnection standards, we will require the continued activation of the volt-watt Grid Support Function for your PV system to remain interconnected.With the additional panels added to your PV system, we are recommending continued monitoring to ensure the continued safe operation of your PV system.

Inverter voltage on a nominal 120-volt basis Meter voltage on a nominal 120-volt basis Estimated energy curtailment impact due to

volt-var:

Estimated energy curtailment impact due to

volt-watt:

Given the current voltage conditions observed are higher than the normal range of 114 volts to 126 volts, the PV output may be experiencing curtailment of less than < 1.0% of the total PV energy

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• In general, maximum field voltages measured do not appear to be as high as would be expected from interconnection reviews, likely for several reasons: o Without detailed load and secondary voltage information available during PV application review,

worst-case assumptions must be made o Limited sample size

• Communicating with inverters and sensors in the field is challenging o Cellular communications generally more reliable than customer internet

• Correlation between voltage and irradiance o In some locations, PV is very clearly driving secondary voltages - high correlation between voltage

at inverter and irradiance o In other locations, correlation between voltage and irradiance is moderate or low

• For voltage rise due to small secondary conductors, effectiveness of reactive power (volt-var or fixed PF 0.95) is limited

• Initial data confirms VROS finding that impact of volt-var and volt-watt on PV production is minimal o Measuring impact exactly is difficult without irradiance data – 2nd phase focuses on this (as well

as increasing pilot sample size, additional modeling/simulation/analysis)

Summary of findings so far

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• Additional recruitment of customers to avoid possible sample bias • Upfront effort to determine “customers of interest” for targeted sensor

deployment • What is the most critical data needed?

o Grid2020 valuable for analyzing problem cases, and for extrapolating to other scenarios via simulation

o Inverter manufacturer data is very valuable and available at relatively low effort compared to deploying dedicated communications and sensing

o AMI data very valuable for multiple purposes • Importance of AMI metering data

o Resolution of 1-minute vs. 15-minute – depends on use case o Availability of data for all customers tied to a node is useful for extrapolating

to other scenarios via simulation

Lessons Learned – Field Validation

www.nrel.gov

NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Thank you

Publication Number

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Example of high correlation between irradiance and voltage

Cluster 1: One of two advanced inverters operating, in 0.95 PF mode This is the second-highest voltage location in the pilot Long shared overhead secondary

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Worst-case ΔVbtm for string inverter

0 5 10 15 20 25 30 35 40 45

Inverter Real Current (Amps)

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Del

ta V

olta

ge (V

olts

)

Customer PN-15-03415

y = 0.083x1 + -0.796x 0

Est elec length 26/41/66 ft (AWG 12/10/8)

• 6 kW inverter • Note ~0.8 V

difference in inverter voltage and AMI voltage even at zero current

• Potentially due to voltage sensor mismatch and house wiring configuration

• Accounting for mismatch, max voltage rise would be ~2 volts (0.8%)

SE3