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© 2010

Microgeneration and Microgrids(modeling, islanding operation, black start, multi-microgrids)

J. Peças Lopes

Power Systems Unit

SEVILLA, APRIL 2010

2© 2010 2

PV

Wind Gen

MicroGrid: A Flexible Cell of the Electric Power System

Microturbine

Fuel Cell

Storage DeviceMGCC

MC

MC

MC

MC

MC

LC

LC

LC

LC

LC

MG Hierarchical Control:

• MGCC, LC, MC

• Communication infrastructure

3© 2010

The MicroGrid Concept

• A Low Voltage distribution system with small modular generation units providing power and heat to local loads

• A local communication infrastructure

• A hierarchical management and control system

Operation Modes:

• Interconnected Mode

• Emergency ModeMV

LV

MGCCMC

LC

Fuel CellMC

PV

MC

MC

LCLC

LC

MC

LC

ACDC

ACDC

ACDC

ACDC

Microturbine

Wind Generator

MC StorageACDC

Microturbine

PV

ACDC

MC

4© 2010

Microgeneration technologies: Micro-wind turbines

5© 2010

Microgeneration technologies: Micro-wind turbines

6© 2010

Micro-wind turbines

• Vertical axis micro-wind turbines

7© 2010

Microgeneration - Solar Photovoltaic (PV)

I

ISC

Imax

M N

O

A

P

SVVmax VOC

1/Ropt

1/R

8© 2010

Microgeneration technologies: BIPV

Other solutions: surfaces coating (Glasses, Roofs, etc.) with thin films.

9© 2010

Microgeneration - Microturbines

• Microturbine of 80 kW

In general the microturbine is

connected to the grid through an

electronic converter.

1,5 kHz to 4kHz

(single shaft)

10© 2010

Micro CHP (Stirling engines)

• Packaged as a domestic boiler for mass market

11© 2010

Fuel-Cells

• Different Types (PEM, SOFC, Alkaline, PAC…)

12© 2010

Energy storage - flywheels

• Key element for the operation of a microgrid

13© 2010

MicroGrids – Modes of Operation

• MicroGrids can operate:

– Normal Interconnected Mode :• Connection with the main MV grid;• Supply, at least partially, the loads or injecting in the MV grid;

– Emergency Mode :• In case of failure of the MV grid;• Possible operation in an isolated mode as in physical islands:

– Moving to island mode;– Load following;

• In this case, the MGCC:

– Changes the output control of generators from a dispatch power mode to a frequency mode;

– Primary control – MC and LC;

– Secondary control – MGCC;

– Eventually, triggers a black start function.

Requires dynamic behavior analysis

14© 2010

Emergency operation requires specific studies

• Development of models for microgenerators:

– Inverters

– Microturbines (single shaft and split-shaft);

– Fuel cells (SOFC);

– PV arrays;

– Wind generators;

– Flywheels;

– Frequency and voltage controls.

– Controllable loads

• Identification of possible control strategies (load shedding included)

15© 2010

Development of Models of Microsources (MT)

• Turbine modeling

+-

LVgate 2

11 T s?

3

11 T s? TK

turD

mP

rw

1

11 T s?

maxV

minV

maxL

+

+ +

-

inP

16© 2010

Development of Models of Microsources (FC- SOFC)

• Nerst equation plus the Ohm law

refP

demP

infcV

Limit

max

2 r

UK

min

2 r

UK

11 eT s

infcI r

fcI

2 rK rK r

+-

+-

+-

_

1

H Or1

1 fT s

2 r

opt

KU

rfcI

2

2

11

H

H

Ks

2

2

11

O

O

Ks

2

2

11

H O

H O

Ks

2 2

2

1 2

0 0 ln2

H O

H O

p pR TN EF p

+

-

X

2

inHq

2

inHq

2

inHq

2

inOq

2Hp2H Op

2Op

rfcV

rfcI

FP

eP

eQ

2 2

2

1 2

0 0 ln2

H Or rfc fc

H O

p pR TV N E r IF p

Chemical response of the fuel processor

Electrical response of the FC

Dynamic responseof the flow

17© 2010

Inverter control types

• PQ inverter control:

– the inverter is used to supply a given active and reactive power set-point.

• Voltage Source Inverter control logic:

the inverter is controlled to “feed” the

load with pre-defined values for voltage

and frequency. Depending on the load,

the Voltage Source Inverter (VSI) real and

reactive power output.

Microsource Vdc

DC

AC

Vdc ref -PI

i acti react

xi act

i reactx

Set Point

u= ugrid + k(iref - i)

iref

u, i

P

Q

P vs f droop

Q vs V droop

Decoupling References U

Current controlled voltage source

18© 2010

When in islanding mode, micro generators participate in voltage and frequency regulation using the proportional concept of frequency and voltage droops.

u

voltage droop

u

0 1-1 QQ N

-4%u

0

f

f 0

0 1-1

-1%

PPNfrequency droop

f

Frequency and voltage control

19© 2010 19

MicroGrid Islanded Operation

• The MicroGrid can operate autonomously in case of

– Failure in the upstream MV grid – forced islanding

– Maintenance actions – intentional islanding

– In this case the MGCC:

• Performs frequency and voltage control in close coordination with the local

controllers in order to not jeopardize power quality

• Triggers a black start function for service restoration at the low voltage level if

the MicroGrid was unable to successfully move to islanded operation and if the

main power system is not promptly restored after failure removal

MicroGrid flexibility will contribute to the improvement of the energy system reliability and quality of service

20© 2010

Islanding operation modes

• Single Master Operation:

– A VSI or a synchronous machine directly connected to the grid (with a diesel engine as the prime mover, for example) can be used as voltage reference when the main power supply is lost; all the other inverters can then be operated in PQ mode;

• Multi Master Operation:

– More than one inverter is operated as a VSI, corresponding to a scenario with dispersed storage devices; other PQ inverters may also coexist.

ACDC

VDC

ElectricalNetwork

Loads

AC

DCPrimer Mover

VSI

VSIControl Controller

PQControl

Q Set Point

VDC

V, I V, I

P

MGCCDroop Settings P&Q Settings

ACDC

VDC

ElectricalNetwork

Loads

AC

DCPrimer Mover

VSI

VSIControl

MGCC

ControllerPQ

ControlQ Set Point

Droop Settings P&Q Settings

VDC

V, I V, I

VDC

AC

DCPrimer Mover

V, I

VSIControl

P

Controller

P

21© 2010 21Microgeneration: Changing the Paradigm of the Electric Power System

Proving the Technical Feasibility of the MicroGrid Concept

• Microgrid Islanded Operation

• Development of control strategies

• Dynamic behavior in the moments subsequent to MicroGrid islanding

• Seamless transition to islanding operation

• MicroGrid Black Start

• Identification of rules and conditions for service restoration at the LV level

after a general blackout

• Evaluation of fast transients associated with the initial stages of the

restoration procedure

• Synchronization with the main power system

Development of simulation tools

Assessment of system performance in laboratorial tests

22© 2010

LV Test System

23© 2010 23

Test System in the MATLAB/Simulink Simulation Platform

SSMT

PV

LOAD

WIND GENERATOR

VSI + STORAGE

SOFC

24© 2010 24


Frequency Control

Microturbine

Grid Side Converter

25© 2010 25


Frequency control

Microturbine

Grid Side ConverterSSMT Mechanical Part

PMSG

Machine Side Converter

26© 2010 26

Results from Simulations

• MG Frequency and VSI Active Power

0 50 100 150 200 25049.2

49.4

49.6

49.8

50

50.2

Freq

uenc

y (H

z)

0 50 100 150 200 250-20

-10

0

10

20

30

40

50

Time (s)

VSI A

ctiv

e Po

wer

(kW

)

27© 2010 27

Results from Simulations

• Controllable Microsources Active Power

0 50 100 150 200 2500

5

10

15

20

25

30

Time (s)

Act

ive

Pow

er (k

W)

SSMT1 & SSMT2

SSMT3

SOFC

28© 2010 28

Improving MicroGrid Robustness Regarding Islanding

• When the MicroGrid is disconnected from the upstream MV network, several key issues must be considered in order to guarantee system survival in the moments subsequent to islanding:

– Is the energy available in storage devices enough for a seamlesstransaction to islanded operation?

– How much load must be shed?– How much dump loads must be connected?– How much power reduction should be performed in the islanded MG?

On-line evaluation of system robustness and fast determination of remedial actions

29© 2010 29

Evaluating MicroGrid Security in case of Islanding

• Preventive Control Strategy – Load Shedding:

40 60 80 100 120 140 160-1

0

1

2

3

4

5

MicroGrid Total Load (kW)

Ener

gy In

ject

ed b

y th

e FE

SS (M

J)

Emax

30© 2010 30

Using MicroGrids for Service Restoration

• DG maturation can offer ancillary services, such as the provision of Black Start in low voltage grids

• Black-Start is a sequence of events controlled by a set of rules

– A set of rules and conditions are identified in advance and embedded in a MGCC software module

– These rules and conditions define a sequence of control actions to be carried out during the restoration stages

– The electrical problems to be dealt with include:

• Building LV network• Connecting microsources• Connecting controllable loads • Controlling frequency and voltage• Synchronization with the MV network (when available)

31© 2010 31

MicroGrid Black Start

MV

LV

Storage Device

Microturbine

PV

Fuel Cell

Wind Gen

Fault in the upstream MV network followed byunsuccessful MG islanding

32© 2010 32


Storage Device

Microturbine

PV

Fuel Cell

Wind Gen

33© 2010

MV

LV

33


Storage Device

Microturbine

PV

Fuel Cell

Wind Gen

34© 2010 34


Storage Device

Microturbine

PV

Fuel Cell

Wind Gen

35© 2010 35


Storage Device

Microturbine

PV

Fuel Cell

Wind Gen

36© 2010 36


Storage Device

Microturbine

PV

Fuel Cell

Wind Gen

42© 2010 42

Results from Simulations – Long Term Dynamics

• An Overview of the Service Restoration Procedure

90 100 110 120 130 140 150 160 170 180 190 200 210 22049.6

49.8

50

50.2

50.4

Freq

uenc

y (H

z)

90 100 110 120 130 140 150 160 170 180 190 200 210 220-20

0

20

40

Act

ive

Pow

er (k

W)

90 100 110 120 130 140 150 160 170 180 190 200 210 220

0

20

40

60

Time (s)

Act

ive

Pow

er (k

w)

MG main storage

SSMT 1SSMT 2SSMT 3

load connection

PVs connectionWG connection

Motor load start up

43© 2010 43

Laboratorial Tests: INESC Porto, University of Kassel and ISET - Institut fürSolare Energieversorgungstechnik

46© 2010 46

Frequency Control After Islandinghttp://www.iset.uni-kassel.de/abt/FB-A/publication/2006/2006_Napa_Strauss.pdf

47© 2010 47

Load Disconnection and Frequency Controlhttp://www.iset.uni-kassel.de/abt/FB-A/publication/2006/2006_Napa_Strauss.pdf

48© 2010

Evolution of the MicroGrid Concept

• Microgrids

– DFIM

– Fuel Cell

– Microturbine

– Storage(VSI)

– PV

• Large VSI

• Large DFIM

• Hydro

• CHP

• Small Diesel

• Sheddable Loads

HV Network

VSI

Diesel

DFIM

MicroGrid

MicroGrid

MicroGrid

CapacitorBank

Hydro

CHP

SheddableLoads

49© 2010

• New concept Multi-Microgrids

• Requires a higher level structure, at the MV level, consisting of LV Microgrids and DG units connected on several adjacent MV feeders

• Microgrids, DG units and MV loads under DSM control can be considered as active cells, for the purpose of control and management

• An effective management of such a system requires the development of a hierarchical control architecture, where intermediate control will be exercised by a Central Autonomous Management Controller (CAMC) to be installed at a HV/MV substation

250

kVA

400

kVA

400

kVA

250

kVA

160

kVA

160 kVA

250 kVA

160 kVA

160 kVA

G

Evolution of the MicroGrid Concept

49

50© 2010

New Control Architectures (Distribution Grid)

DMS – Distribution Management SystemCAMC – Central Autonomous Management ControllerMGCC – MicroGrid Central ControllerRTU – Remote Terminal Unit

MV

Micro-Turbine

LV

MGCC

MC

LC

Fuel CellMC

MC

CHP

PV FlywheelMC

MC

LC LC

LC

MC

ACDC

ACDC

ACDC

ACDC

DCAC

DMS

50

52© 2010 52

Conclusions

• The feasibility of the MicroGrid concept was proved:

– Flexibility to operate autonomously under emergency conditions

– Demonstration by laboratorial tests

– Using Low Voltage MicroGrids for service restoration

The MicroGrid is a very flexible cell of the Electric Power System and can contribute to enhance the quality of service by reducingthe number and duration of interruptions.

Smartmetering can be used to foster and support the development of microgrids and Smartgrids

microgeneration and microgrids (modeling, islanding ...catedraendesa.us.es/documentos/seminario joao...

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