INTELLIGENT CONTROL FOR MICROGRIDXianyong Feng, PhD, PECenter for ElectromechanicsThe University of Texas at Austin

Outline• Microgrid Overview

• MPC-based Microgrid Energy Management

• DC Microgrid Protection

• Summary

2

Introduction

• Microgrid Definition: A group of interconnected loads and distributed energy resources within clearly defined electrical boundaries that acts as a single controllable entity with respect to the grid. A microgrid can connect and disconnect from the grid to enable it to operate in both grid-connected or island-mode.

3

PV system

ES system

~

Distributed Generator M

~

Microgrid

Communication link

Central controller

Local controller

Local controller

Local controller

Local controller

Distributed Generator 1

s1s2s3s4s5s6

s7

s8

s9

PCCUtility Grid

...

Controllable load 2

Critical load Controllable

load N

Controllable load 1

Local controller

Local controller

Local controller

Local controller

...

PCC controller

• Utility electricity price signal• Weather forecast

Introduction• MW dc microgrid at CEM in UT-Austin

4

~ ~

~ ~

M ~D

480 V AC source 3-ph

AC breaker

480 V AC source 3-ph

AC breaker

490 kVA480 V/808 V transformer

DC disconnector

AC breaker

Diode rectifier 3.2 MVA

DC disconnector

DC cable

DC disconnector

DC disconnector

DC disconnector

DC disconnector

1 MVA motor drive

2 MVA motor

5 MW dynamometer

DC disconnector

1.2 MVA480 V/808 V transformer

1.2 MVA controlled rectifier

2 MW DC chopper

1.3 MW resistive load

1120 VDC

1120 VDC

Zone 1

Zone 2

DC bus 1

DC bus 2

Challenge and Opportunity

1. High Penetration DER• Accommodate high penetration

intermittent DERs2. Energy Efficiency

• Reduce operational cost• Reduce emission

3. Reliability and Resilience • Improve system stability• Reliable fault ride-through and

protection• Seamless mode transitions

5

Time (sec)

Frequency (Hz)

60larger inertia

smaller inertia

LP∆ is the same

1f

2f

∑ =⋅

∆−= n

i i

L

HfP

dtfd

1

rated

2Total inertia

Stability and Resilience

PV generation (partial cloud day) Wind generation (daily curves)Renewable Intermittency

10 hrs.

Microgrid Control

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Tertiary control

Secondary control

Primary control

Information flow

Decision signals

Economic dispatchUnit commitmentOptimal power flowVoltage var controlTime frame: seconds to minutes

Real-time load managementSecondary load-frequency controlSecondary voltage control Automatic generation controlTime frame: 100s milliseconds

Droop control Protection control Time frame: milliseconds

MPC-based Microgrid EMSObjective and Approach

1. Objective• Enable economic and secure

steady-state operation• Seamless integration with local

controllers

2. Approach• Look-ahead operational planning:

Optimize DER schedule for the next 12-24 hours

• Online operation: Use ED/OPF to determine DER set-points

• Planned outage control

3. HIL Test• Test the EMS performance• Test the EMS and fast control

integration …

Mixed Integer Linear Programming (MILP)

problem

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1. Opal-RT simulator• Simulated a 13.8 kV

microgrid (step: 100 us)• Local controllers

communicate with Opal-RT simulators

2. Microgrid EMS• Improve the economics

and reliability• EMS is deployed on

central controllers• Dispatch signals are

transferred to local controllers through LAN

MPC-based Microgrid EMSHIL Test

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PXIe Real-Time/FPGA HIL System Opal-RT Simulator

High speed communication link

Opal-RT OP5600• eMEGAsim solver – 900 nodes at 50 µs• IEC61850 GOOSE and Sample Values• C37.118 interface• 32 Analog I/Os• 64 Digital I/Os

NI PXI HIL simulator• 2 eHSx64 solver – 144 switches at 250 ns• 48 Analog inputs• 100 Analog outputs• 364 Digital inputs• 48 Digital outputs

DC microgrid circuit in Opal-RTSimulated switching devices in NI PXI simulator

Potential Control, Protection and Monitoring Hardwares

Digital

/analo

g I/O

inter

face

MGCCTime Sensitive Network

NI cRIO controller

Novel dc microgrid EMS

...Local fault detection

and location

Stability control

...

MPC-based Microgrid EMSPreliminary Test Result

Low utility price

High utility price

Heuristic EMS

EMS optimization results for a day

Central EMS vs Heuristic Control

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1. DC microgrid• AC generator is interfaced with dc

grid through converter• Re-dispatch ac generators for

efficiency improvement

2. Hybrid approach• Use ESS to shift energy to operate

generator at the maximum efficiency point

• Develop comprehensive microgridEMS to improve the overall system efficiency

MPC-based Microgrid EMSImprovements

rotating speed [rad/s]

120 140 160 180 200 220 240 260 280

Torq

ue [N

-m]

300

400

500

600

700

800

900

0.1

0.15

0.2

0.25

0.3

Original operating point

Optimal operating point

125 kW contourRated power

time (hour)

0 5 10 15 20 25

spee

d (p

u)

0.5

0.6

0.7

0.8

0.9

Fuel efficiency map for a 250-kW fossil-fuel engine (unit in legend: kg/kWh)

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DC Microgrid Protection

AC fault current

DC fault current

=

L L

=~ =

DC Power Supply

...

...

Fault 1 Fault 2

Fault 3...

L L...

...

DC distribution system example

DC microgrid protection challenges1. No fault current zero-crossing2. Lower line impedance3. High di/dt4. Power electrics device can not tolerate

high fault current5. Fast capacitor discharge

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Fast DC Fault Localization Algorithm

Inductance-based dc fault location*1. Estimate fault inductance with local

measured v(t) and i(t)2. Use estimated L to locate fault

=

L L L

Zone 1 (20 m)

Zone 2 (65 m)

Zone 4 (1.5 m)

=~ =

DC Source

...

...

Fault 1 Fault 2

Fault 3...

L L L...

Zone 3 (10.2 m)...

...

Equivalent inductance

Distance

level 1 level 2 level 3

L1

L2

L3

Line inductance distribution

RF

LRi

v

+

-

Equivalent fault circuit

*X. Feng, et.al., “A novel fault location method for dc distribution protection,” IEEE Trans. Industrial Applications, vol. 53, no. 3, May-June, 2017.

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DC Protection Control PrototypeProtection strategy design

1. Online moving-window least square method2. Digital di/dt approximation3. Algorithm on embedded controller

ADC ADC

v (1) i (1)

PRUs read data sequentially and store

them in memory

Main program executes the fault detection and

location routine

Locate fault?

YSend tripping

Request new data once

finishing the previous cycle

N

v i Fault detection and location routine

Start

Go to next time intervalFault detected?

Yes

No

if k < M

No

Yes

k = 0

No

k = k + 1

Read in measurements v(k) and i(k)

=

)()(

)1()1(

)0()0(

kikdtdi

idtdi

idtdi

A

=

)(

)1()0(

kv

vv

B

+−+−

+−+−

=

)()(

)2()2(

)1()1(

kikdtdi

MkiMkdtdi

MkiMkdtdi

A

+−+−

=

)(

)2()1(

kv

MkvMkv

B

( ) BAAARR

L TT

F

1−=

+

if 0 < L < Lth

Yes

Send tripping signal

End

if t < Tmax

Yes

No

di/dt estimation

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DC Microgrid Protection Test

14

LVDC fault test circuit

DC power supply

30 kW converter with FCL

Downstream circuit

NI sbRio control board

==

Line impedance 1

Load Source 2Source 1

Line impedance 2

Current sensor

Controller

DC Protection Test Results*: measured inductance value□: estimated inductance value

CASE #

1 2 3 4 5 6 7 8 9Es

timat

ed in

duct

ance

(uH

)0

10

20

30

40

50

CASE #

1 2 3 4 5 6 7 8 9

Estim

atio

n er

ror (

%)

-30

-20

-10

0

10

20

30

Case # RF (mΩ) L (µH)

1 33 12.5

2 33 24

3 33 42

4 50 12.5

5 50 24

6 50 42

7 100 12.5

8 100 24

9 100 42

Test Scenarios

*5 tests in each case

Test Circuit:• Single dc source • One line impedance

connecting source and load• Fault is on load side

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Summary1. The look-ahead EMS approach fully utilizes the most

recent load and renewable forecast to improve the predictive control accuracy

2. The decoupled DER schedule and real-time ED approach significantly reduces the computational complexity

3. DC prot. is enabling tech. for large-scale deployment of dc systems

4. Extra-fast fault location and restoration are keys for grid resilience

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Thanks for your attention

Contact information:Xianyong FengCenter for ElectromechanicsThe University of Texas at AustinEmail: x.feng@cem.utexas.eduPhone: 1-512-232-1623

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