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Campus da FEUPRua Dr. Roberto Frias, 3784200 - 465 PortoPortugal
T +351 222 094 000F +351 222 094 050
© 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
Test System in the MATLAB/Simulink Simulation Platform
Frequency Control
Microturbine
Grid Side Converter
25© 2010 25
Test System in the MATLAB/Simulink Simulation Platform
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
MicroGrid Black Start
Storage Device
Microturbine
PV
Fuel Cell
Wind Gen
33© 2010
MV
LV
33
MicroGrid Black Start
Storage Device
Microturbine
PV
Fuel Cell
Wind Gen
34© 2010 34
MicroGrid Black Start
Storage Device
Microturbine
PV
Fuel Cell
Wind Gen
35© 2010 35
MicroGrid Black Start
Storage Device
Microturbine
PV
Fuel Cell
Wind Gen
36© 2010 36
MicroGrid Black Start
Storage Device
Microturbine
PV
Fuel Cell
Wind Gen
37© 2010 37
MicroGrid Black Start
Storage Device
Microturbine
PV
Fuel Cell
Wind Gen
38© 2010 38
MicroGrid Black Start
Storage Device
Microturbine
PV
Fuel Cell
Wind Gen
39© 2010 39
MicroGrid Black Start
Storage Device
Microturbine
PV
Fuel Cell
Wind Gen
40© 2010 40
MicroGrid Black Start
Storage Device
Microturbine
PV
Fuel Cell
Wind Gen
41© 2010 41
Results from Simulations – Initial BS Stages
MG main storage
SSMT1
MG main storage
SSMT1
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
44© 2010 44
Pre-islanding Scenariohttp://www.iset.uni-kassel.de/abt/FB-A/publication/2006/2006_Napa_Strauss.pdf
45© 2010 45
Micro-Grid Islandinghttp://www.iset.uni-kassel.de/abt/FB-A/publication/2006/2006_Napa_Strauss.pdf
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
51© 2010
SmartMetering infrastructure - the platform for developing Grids
••
ICTs
51
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