1Jian Sun 10-10-2013

Dynamic Microgrid Stability and Control

Jian Sun, Professor and DirectorNew York State Center for Future Energy Systems

jsun@rpi.edu; (518) 276-8297

2

Introduction• Conventional Grids

– Central, Dispatchable Generation– Central Coordination Control– (Almost) Fixed Topology

• Dynamic Microgrids– Dynamic Configurations; Variable Topology– Dynamic, Distributed Generation– Control of Loads is as Important as Controlling

Sources

Jian Sun 10-10-2013

3

Control Partitioning

• Energy Balance– Production– Load Management

• Energy Efficiency Optimization

• Communication with Certain Devices

• Energy Balance– Production– Load Management

• Energy Efficiency Optimization

• Communication with Certain Devices

• Power Balance• Autonomous Control

with Global Effects– Responsive to System

Conditions – Require No/Minimum

Communication

• Power Balance• Autonomous Control

with Global Effects– Responsive to System

Conditions – Require No/Minimum

Communication

Jian Sun 10-10-2013

1 Hz0.1 Hz 100 Hz 1 kHz1 mHz10 Hz

1 sec1 min1 hr1 daySystem ControlSystem Control Local ControlLocal Control

4

Impedance-Based Approach

)()(

1

1)()(

)()()(

sZsZsZsZ

sZsIsI

in

ggin

in

in

g

Grid Inverter +

Zg

ZinVg

Small-Signal Model

Iin

Ig

sZsZg

in

_

Ig(s)Iin(s)Inverter Modeled as a Current Source

with Finite Output Impedance

“Minor Loop” due to Impedance Coupling; Harmonic Resonance as a Stability Problem

Jian Sun 10-10-2013

J. Sun, IEEE Transaction on Power Electronics, Nov. 2011

5Jian Sun 10-10-2013

Local Control Stability

c

1

Phase Margin

Gain Margin

Im

Re

• System is Stability if Zg/Zin Meets Nyquist Stability Criterion– Grid Impedance Should be Low; Weak Grid Issues– Inverter Output Impedance as High as Possible

• Lack of Stability Margin Leads to Under-Damped Resonance

)()(

1

1)()(

)()()(

sZsZsZsZ

sZsIsI

in

ggin

in

in

g

+

Zg

ZinVg

Small-Signal Model

Iin

Ig

6

Three-Phase Device Modeling• Decomposition of Three-Phase Voltage Perturbation

by Symmetrical Components

• Calculation of Current Responses to Each Voltage– Zero-Sequence Voltage Produces No Current– Positive & Negative Sequence Responses May be Coupled

Jian Sun 10-10-2013

+ +

n

p

in

n

p

nnnp

pnpp

n

p

V

V

V

V

YY

YY

I

IY

n

p

in

n

p

nnnp

pnpp

n

p

V

V

V

V

YY

YY

I

IY

7

Three-Phase System Model• Sequence Representation of Grid Impedance

• “Minor Loop” Gain in a Matrix Form– Application of Generalized Nyquist Stability Criterion

• Uncoupled: Symmetrical Grid, Balanced Operation

Jian Sun 10-10-2013

nnnp

pnpp

gZZ

ZZZ

nnnp

pnpp

gZZ

ZZZ

gingin ZZZY 1 gingin ZZZY 1

8

Three-Phase Wind Inverter

ib (4 A/div.)ia (4 A/div.) ic (4 A/div.)

Jian Sun 10-10-2013

Negative Seq.Positive Seq.

ipin

0.00 0.01 0.02 0.03

-2.00

2.04.0

9

-3 -2 -1 0 1 2 3 4

0

1

2

3

100 Hz 1 kHz 10 kHz-200-150-100-50

050

100150

100 Hz 1 kHz 10 kHz10

20

30

40

50

Imag

inar

y Pa

rtImpedance-Based AnalysisNyquist Plot of Zg/Zin

← =170°

←Hz

Sequence Gain Margin Phase Margin

Positive 2.7 dB @ 420 Hz 10° @ 440 Hz

Negative 7.5 dB @ 350 Hz 23° @ 440 HzPh

ase

(DEG

)M

agni

tude

(dB

)

f

ZpZn

Zg

Jian Sun 10-10-2013

Lack of Phase Margin Causes Strong Harmonics near Intersection Frequency

10

Grid Impedance Measurement

CurrentControl

FPGADSP

• Single or Sequence of Current Impulse Injection• Fourier Analysis to Extract Grid Impedance

Grid

Jian Sun 10-10-2013

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Form of Injection

-20

-10

0

10

20

30

40

Cur

rent

(A)

Current a Current b Current c

-100

0

100

200

300

Vol

tage

(V)

Voltage a Voltage b Voltage c

10 20 30 40 50 60-40

-30

-20

-10

0

10

20

Cur

rent

(A)

10 20 30 40 50 60

-300

-200

-100

0

100

200

Vol

tage

(V)

Single Impulse Injection

Binary Sequence Injection

Jian Sun 10-10-2013

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Zg = 0.06 pu, Current Injection

102 103

10

20

30

40

Mag

nitu

de(d

B)

102 103

-100

0

100

Frequency (Hz)

Pha

se(d

eg)

ReferenceImpulseMLBS

0.6 pu Single Impulse0.1 pu MLBS

MLBS = Maximum-Length Binary Sequence

Jian Sun 10-10-2013

13

Zg = 0.01pu; 0.1 pu Injection

102 103-20

0

20

40

Mag

nitu

de(d

B)

102 103

-100

0

100

Frequency (Hz)

Pha

se(d

eg)

MLBSDIBSReference

DIBS = Discrete Interval Binary Sequence

Jian Sun 10-10-2013

14

Summary• Dynamic Microgrid Requires New Methods

and Tools for Local Control Design• Impedance-Based Approach Provides a

Theoretical Framework for Such Design• Online Measurement of Grid Impedance and

Other Parameters Enables Local Control That can Adapt to Changing System Conditions– Supplemented by System Coordination Through

(Slow) Communication

Jian Sun 10-10-2013

15

Sim

ulat

ed G

rid w

ith P

rogr

amm

able

Vo

lt/Fr

eq/Im

peda

nce

PV Simulators

Central Inverters (3)

In

verte

rs (2

0)=

~

4th Gen Wind Turbine Simulator

Electronic Loads

~= MG

=~

=~

=~

Utility Grid

=~

Grid Simulator

System Test-Bed