Lessons Learned from Design, Building, Commissioning and...

Lessons Learned from Design, Building, Commissioning and Operation of Mount Holly

Microgrid

Aleksandar Vukojevic, P.E.Manager, Emerging Technologies - Duke EnergyNorth Carolina State University, PhD Candidate

2019 Duke Energy - All Rights Reserved

Emerging Technology and Grid Solutions

TECHNOLOGY EXPLORATION

▪Opportunities/risks of new technologies

▪New technology standards

▪Applied technology maturityEmer

gin

g Te

chn

olo

gy

GRID INFRASTRUCTURE & TECHNOLOGY APPLICATION

▪Modern grid capabilities

▪Scaled deployments

Gri

d

Solu

tio

ns

GRID & TECHNOLOGY OPERATION

▪Operation and maintenance

Op

era

tio

ns

• Pre-scale Deployments

• Prototypes• Capability Maturity Models

• Scale Deployments • Business requirements


Research Objectives – Reliable and Secure Microgrid P&C Design

DER Design

1

Grounding

2

Seamless Islanding

3

Rate of change of frequency

(ROCOF)

4

Inrush current mitigation

5

Microgrid Operating

Modes6

Grid re-synchronization

9

Seamless Islanding vs. black-start

8

Future research

10

7

P&C in islanded

mode


Duke Energy DER Pilot Feeder

Substation with battery & super-capacitor

storage system

Recloser1.2MW Solar

Farm

Voltage Regulator

Mount Holly Microgrid


Mount Holly Microgrid - video


Mount Holly Microgrid – Switchyard

4-WAY Switchgear

Grounding Transformer

BESS

PCC

PCC Relay

Inverter PLC & PCS

DI/DO


Mount Holly Microgrid – Solar Farm

150kW PV Farm

100kVA PV Inverter

DC-coupled BESS



POI

9.3kW DC system

EVCS


Mount Holly Microgrid – Generator and Microturbines

2-65kWmicroturbines

450kVA generator


Mount Holly Microgrid –

One Line Diagram


1. DER Design

• During the literature review that consisted of reading more than 500 publishedpapers, masters thesis, PhD thesis, conference presentations, books on microgridsand system protection and control, three major microgrid design flaws werediscovered in vast majority of the literature:

a. PV systems as the only DER within the microgridb. Microgrid DERs with Yg – Δ transformersc. Lack of Grounding Transformer


1. DER Design

Simulink 100kW PV Farm Model2019 Duke Energy - All Rights Reserved

100kW PV Farm Model – Simulation after PCC Opens

1. DER Design


100kW PV Farm – What happens in the field after loss of AC on PV inverter side

1. DER Design


100kW PV Farm Model – Properly designed PV inverter controls

1. DER Design


1. DER Design

Model of typical feeder with 1LG fault – no DER


1LG fault on the feeder as seen from the substation relay

1. DER Design


Model of typical feeder with 1LG fault - with DER connected with Yg – Delta Transformer

1. DER Design


Fault current as seen by the substation breaker relay – DER with Yg-Delta Transformer

Fault current seen by the substation relay reduces

with increased DER rating

1. DER Design


Fault current in neutral of the Yg-Delta Transformer

Fault current seen by the neutral CT in the Yg side of DER transformer increases with increased DER rating

1. DER Design


Model of typical feeder with 1LG fault - with DER connected with Yg – Delta Transformer

1. DER Design


Model of typical feeder with 1LG fault - with DER connected with Yg – Y Transformer

Fault current seen by the substation relay does not

changes if DER is connected to Yg – Y transformer (1633A)

1. DER Design


2. Grounding

Purpose:

1. To provide solid groundreference for the microgrid

2. To detect the faults within themicrogrid faster and morereliable and secure

3. To enable grid re-synchronization

Neutral CT

Relay

Yg Δ

Grounding Transformer 2019 Duke Energy - All Rights Reserved

2. Grounding Transformer - Design

Parameters for designing the grounding transformer:

1. Construction type: Yg – Δ or “Zig-Zag”

2. Primary and secondary voltage: 12.47kV Yg – 480V Δ

3. Impedance – designed so that the voltages on un-faulted phases during the ground fault are within the temporary over-voltage capability of the transformer and other primary equipment:

a. IEEE 142 – challenging approach because it is hard to define positive and negative sequence impedances for DER

b. IEEE 1547.8 – more applicable to this application


2. Grounding Transformer - Design

4. Steady-state circulating current – maximum level of zero-sequence current seen in theneutral of the grounding transformer

5. Neutral fault current withstand rating – the most critical design parameter since onlyzero-sequence current is present in the grounding transformer neutral during the fault

6. kVA rating:

a) Grounding transformer has no loadb) For solid grounding, desired X/R ratio should be greater than 4c) Higher kVA ratings have higher X/R ratio (500kVA transformers have typically X/R

ratio greater than 5)d) However, transformer with higher kVA rating has higher inrush current, so BESS

might not be able to support this inrush therefore causing the microgrid blackout


3. Seamless Islanding

Islanding can be:

a. intentional

b. unintentional

In both cases, islanding can be achieved by:

1. 52a switch from PCC directly wired to the battery controller

2. Battery inverter detects the 27/59/81 state without direct 52a status

3. Battery inverter or PCC relay detects the 27/59/81 state with direct 52a status


3. Seamless Islanding – voltage response (simulation)

Voltage during the Grid to Island transition – different simulations2019 Duke Energy - All Rights Reserved

3. Seamless Islanding – voltage response (microgrid)

Voltage during the Grid to Island transition – actual microgrid response2019 Duke Energy - All Rights Reserved

3. Seamless Islanding – microgrid response

Grounding Transformer Energization Effect2019 Duke Energy - All Rights Reserved

3. Seamless Islanding – frequency response (simulation)

Frequency during the Grid to Island transition – simulation in MATLAB/Simulink2019 Duke Energy - All Rights Reserved

3. Seamless Islanding – frequency response (microgrid)

Frequency during the Grid to Island transition – actual microgrid response


3. Transition from Grid to Island

HMIMicrogrid controller

PCC relayIED #1

Microgrid relayIED #2

PCC Breaker

Battery Inverter

DI/DO

1. Initiate Islanding

2. Trip PCC

3. Go to VSI V/F Mode

4. Trip Islanding switch (PCC)

5. PCC Open

5a. Close Grounding

Transformer

1. Grid Disturbance

27/59/81

27/59/81U/O, df/dt


3. Seamless Islanding


3. Mount Holly Microgrid – Islanding Detection Scheme


3. How fast is current Mount Holly islanding detection scheme?

Scheme can detect islanding for PL=PG

Island TRIP due to LROV


4. Transformer inrush current mitigation


5. Microgrid Operating Modes

objective: minimize kW WAY 1

subject to:

kW WAY 1 = kW WAY 2 + kW WAY 3 + kW WAY 4

kW WAY 4 = kW VSI BATTERY + kWAUX. BATT + kWAUX. INVERTER

kW WAY 2 = kWPV + kW CSI BATTERY + kWLOAD BANK + kWBLDG

PCC closed

SOCmin < SOC < SOCmax

kWPV = f(MPPT) (i.e. no curtailing)

kWLOAD BANK is in [0 - kWMAX] range

V min < V < V max, for VSI Battery

f min < f < f max, for VSI Battery

kW CHARGE RATE VSI BATTERY is in [0 – kWMAX CHARGE] range

kW DISCHARGE RATE VSI BATTERY is in [0 – kWMAX DISCHARGE] range

objective: minimize time for PCC = CLOSED

subject to:

kW WAY 2 + kW WAY 3 + kW WAY 4 = 0

kW WAY 4 = kW VSI BATTERY + kWAUX. BATT + kWAUX. INVERTER

kW WAY 2 = kWPV + kW CSI BATTERY + kWLOAD BANK + kWBLDG

SOCmin < SOC < SOCmax

kWPV = f(MPPT) (i.e. no curtailing)

kWLOAD BANK is in [0 - kWMAX] range

V = 1.0 p.u. for BESS

f = 1.0 p.u. for BESS

Net-0 Mode Auto Mode

This operating mode was implemented within the microgrid controller at the actual utility’s microgrid

and it ran seamlessly and without any interruption for over 4 months without any user input!!!


5. Microgrid Operating Modes - Net-0 and Manual


6. Seamless islanding vs. Black start

Source: S&C Electric – Selection Guide for Transformer Primary Fuses in Medium and High Voltage Utility and Industrial Substations


6. Seamless Transition vs. Cold-Load Pick-up Operating Mode

Source: S&C Electric – Selection Guide for Transformer Primary Fuses in Medium and High Voltage Utility and Industrial Substations


6. Seamless Transition vs. Cold-load Pick-up Operating Mode


6. Seamless Transition vs. Cold-load Pick-up Operating Mode

• If we are designing the microgrid for seamless transition with inverter-based DERs, DERrating (BESS) should be based on the peak microgrid loading (unless there is volt/VARcontrol mechanism, demand response system or load shedding system implemented,which can reduce the load within the microgrid within 20ms);

• If we are designing the microgrid for cold-load pick-up with inverter-based DERs, DERrating (BESS) should be based on the total transformer kVA/MVA rating within themicrogrid and historical cold-load pick-up quantities. This might require the addition ofrotating mass generation as additional DER.

• Using single pole operation device with controller that switches based on the residualmagnetism can significantly reduce the inrush current, which is a proposition also forgrounding transformer as well as cold-load pick-up operation

• Using line reclosers can further improve the reliability of this scheme by sectionalizing thefeeder and energizing it section-by-section


7. Grid re-synchronization

HMIMicrogrid controller

PCC relay

Microgrid relayIED #2

PCC Breaker

Battery Inverter

DI/DO

1. Initiate grid-syncsequence

2. Enable grid auto-connect 3. Grid voltage

stable for 5 min

4b. Start grid-synch procedure& switch to VSI PQ mode

5. Grid-synch completed

6. Close Islanding switch (PCC)

7. PCC Closed

8. Stop grid-synch procedure

7. PCC Closed

6a. Open Grounding

Transformer


7. Seamless Grid Reconnect


Microgrid Operation – as seen by IEEE C37.118 enabled relay data


Lessons Learned from Implementation of Rate-of-change of Frequency (81R) and

Synchophasor-Based Islanding Schemes



How to initiate islanding?

Islanding can be:

a. intentional

b. unintentional

In any case, islanding can be achieved by:

1. 52a switch from PCC directly wired to the battery controller (McAlpine microgrid – 6 cycles to detect the islanding)

2. Battery inverter detects the 27/59/81U/81O state without direct 52a status (Mount Holly microgrid – 2016)

3. Battery inverter or PCC relay detects the 27/59/81U/81O state with direct 52a status (Mount Holly microgrid – 2017)


How fast is current Mount Holly islanding detection scheme?

Scheme can detect islanding for PL=PG

Island TRIP due to LROV


Microgrid islanding detection for different PL/PG ratios

PL/PG = 0.4PL/PG = 1.0


81U vs. 81R – Unintentional islanding


81R vs. 81U/O pick-up & time-out


Percentage islanding detection improvement - 81R over 81U/O (with 81R set at ±2.5Hz/s)


Security testing for 81R


Synchophasor-based islanding detection


Synchophasor-based islanding


Field settings

y=-5x+2.5

y=-(25/7)x-2.5

Synchophasor-based islanding detection is based on:

1. Voltage angle difference –set-point: ≥10°

2. Slip frequency and acceleration


Synchophasor-based islanding vs. 81R


Implementation of IEC 61850 Based Protection and Control Techniques within Duke Energy’s

Mount Holly Microgrid



Emerging Technology and Grid Solutions

TECHNOLOGY EXPLORATION

▪Opportunities/risks of new technologies

▪New technology standards

▪Applied technology maturityEmer

gin

g Te

chn

olo

gy

GRID INFRASTRUCTURE & TECHNOLOGY APPLICATION

▪Modern grid capabilities

▪Scaled deployments

Gri

d

Solu

tio

ns

GRID & TECHNOLOGY OPERATION

▪Operation and maintenance

Op

era

tio

ns

• Pre-scale Deployments

• Prototypes• Capability Maturity Models

• Scale Deployments • Business requirements



4-WAY Switchgear

Grounding Transformer

BESS

MU #1 & #2

MU #3 MU #5

MU #6


Mount Holly Microgrid – Solar Farm

150 KW PV

100 KVA PV Inverter

DC-Coupled Battery

250 KW DC

240 KW / 122 KWhBattery


Mount Holly Microgrid – Generator and Microturbines

2-65kWmicroturbines

450kVA generator

MU #4

MU #7


Mount Holly Microgrid – Islanding Detection Scheme

Traditional Protection

Merging Unit #1

Merging Unit #2

Selector Switch



Merging Unit #7

Traditional

Selector Switch


Lessons Learned from Implementation of IEC 61850 Protocol

5 phases of commissioning:

1. Individual vendor merging unit and relay set-up and operability

2. Vendor-to-Vendor set-up and interoperability

3. Microgrid settings implementation

4. 85RIO scheme implementation

5. Load shedding transfer-trip scheme



A. Strategic deign approach

B. PTP network traversing

C. Vendor interfaces

D. Fiber switch interface speed

E. Fiber switch partition➢ Install two fiber network switches – one for process bus and one for station bus)

➢ Install one fiber network switch and use VLAN zero

➢ Install one fiber network switch and separate process and station bus within the switch


IEC 61850 Protocol – proposed solution architecture


IEC 61850 Protocol – current solution architecture



F. Global synch vs. Local synch

G. Interoperability


Duke Energy Emerging Technology Office

Battery Energy Storage System Integration on the DC Bus of the PV

Farm Inverter



Mount Holly Microgrid – PV FarmPV System Characteristics at Mount Holly:

1. South Facing system2. 20˚ - tilt ground mount system3. PAC = 100kW; PDC = 149.50kW4. PV irradiance > 700 W/m2

5. GPS Coordinates: 35.29˚ Latitude

DC/AC Ratio Annual Energy AC Production Energy Lost to “Clipping”

1.0 163.06 MWh 0.0 MWh

1.3 193.86 MWh 1.8 MWh (0.9%)

1.5 217.24 MWh 11.0 MWh (4.9%)

*) Table obtained from blog.aurorasolar.com


Mount Holly Microgrid – DC Coupled Battery

100 kVA Parker Hannifin PV

Inverter

150kW PV Farm

240kW/122kWhSAFT Battery

250kW DynaPower

DC-DC Converter

RecombinerBox

Combiner Box

Blocking Diode

480V - 315VXMFR

277V – 120VXMFR


Mount Holly Microgrid – One Line Diagram

SAFT Mini – E240kW

122kWh

RCB

DynaPower 250kWDC-DC Converter

DPS - 250

ABB 200A DC Breaker

Blocking Diode & CT

25 strings with 19 panels in series (total 150 kW)


Battery DC Bus Integration with PV –Design Considerations


1. Typical ILR are in [1.2 – 1.6] range2. New solar panels were different from the original ones3. PV system is grounded inside PV inverter and battery is

typically floating4. DC-DC Converter has no isolation transformer5. Battery will always report “Negative Grounded” alarm6. Blocking diode installed to prevent the battery “backfeed”

to solar panels7. Additional CT installed to signal the reverse power flow in

case of blocking diode failure8. PV timer needs to be disabled for MPPT algorithm9. Grounding leakage current setting needs to be increased


Efficiency = 86.2% = .95 * .984 * .99 * .99 * .984 * .984 * .99Efficiency = 89.2% = .95 * .982 * .982 * .984 * .99

DC-COUPLED

HIGHER EFFICIENCY

DC-Coupled Solar + Storage

• 3 power electronic conversions

• 1 battery charge and discharge

• 1 transformer conversion

AC-Coupled Solar + Storage

• 3 power electronic conversions

• 1 battery charge and discharge

• 3 transformer conversions

1

~

=52 G

MV Step-up

transformer

~

=52 2

=

=

~

=

1

2

G

MV Step-up

transformer

Assumed efficiencies:

PV inverter = 98.4% transformer = 99%

Battery inverter = 97.5% batteries = 95% round trip

DC-DC = 98.2%


DynaPower DC-DC Converter – DPS 250

Basic

Constant Current

Constant Power

Constant Voltage

Advanced

Clipping

Early Morning/Late Evening Capture

Capacity Firming

Ramp Rate Control

PV Time Shifting

Operating Modes


DPS 250 DC-DC Converter

On-board controller in converter communicates directly with the battery BMS


Operating Modes - Advanced


Estimated energy that can be captured from clipping with 1.5 ILR: ~5%Early morning/Late evening capture: DC-DC Converter operates in MPPT mode, while PV inverter is OFF




Capacity Firming enables the fixed user defined AC inverter output, regardless of the PV DC production




PV Time Shifting: battery is charged during the peak time and discharged during the non-peak time


Blocking Diode & CT Transducer

http://www.electronics-tutorials.ws/diode/bypass-diodes.html

CT Transducer

Blocking Diode



PV Inverter

OFFDC- DC Converter

OFF

MPPT

CLIPPING OFF

OFFMPPT

MPPT

OFF PVEMUL

MPPT OFF

OFF

Early MorningCapture

Late EveningCapture

OFF


[email protected]


mailto:[email protected]

[email protected]

“It’s a progress…not a perfection!”


mailto:[email protected]

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