Control of Low Voltage Microgrid in Autonomous Operation Mode · the microgrid delivers electricity...
Transcript of Control of Low Voltage Microgrid in Autonomous Operation Mode · the microgrid delivers electricity...
Control of Low Voltage Microgrid
in Autonomous Operation Mode
Michał Małaczek, Irena Wasiak
8th EPNet, Szklarska Poręba, 19-21 September 2016
Presentation Outline
Introduction
Objective of the paper
Microgrid operation modes
Proposed control strategy
Microgrid simulation model
Simulation results
Conclusions
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Typical structure of the Microgrid
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Grid
Energy Storage
Wind
Turbines
Generators
Photovoltaic
Panels
Loads
Microgrid
DC
AC
AC
DC
Variable speed
source
Distributed generation units installed in
Microgrid
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High speed source
Microgrid
DC
AC
AC
DC
Photovoltaic Panel
Microgrid
DC
AC
Objective of the paper
It could be expected from the microgrid to ensure sufficient
generation capacity, controls and operational strategy to supply
loads after being disconnected form the supplying network even if
the microgrid delivers electricity with less power and lower quality.
With an appropriate control strategy of the sources there is a
possibility to provide power supply to local customers even if any
disruption occurs in the power system.
The major purpose of the work is to demonstrate the
control strategy for autonomous mode of microgrid operation.
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Grid-connected mode
When a microgrid operates in grid-connected mode:
No direct voltage and frequency control is required
Utility grid constitutes a reference voltage and frequency source
Energy sources are controlled as current sources
Interface converters are Voltage Source Converters (VSC)
operating in current controlled mode (CC-VSC) in
synchronization with the suppling voltage, according to P-Q
control strategy
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Grid-connected mode
Control algorithms can be delivered in various coordiante systems,
such as:
αβ0 – stationary reference frame
abc – natural reference frame
dq0 – synchronous reference frame
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Q
Controller
abc
dq
PWM
InverterSource
iabc
uabc PLLΘ
Pref
P
Qref
idref
iqrefController
id
iq
abc
dq
abc
dq Θ
uqud
iq
id
Θ
Controller
Controller
udref
uqref
uqud
iq
idP
Q
Power
Calculation
Θ
Block diagram of P-Q control in dq reference frame
Autonomous mode
The objectives of control strategy in autonomous microgrid
operation is to:
Ensure power balance
Maintain the required values of voltage amplitude and its frequency
at the point of connection with the supplying network
In order to meet these requirements it is necessary to have “grid-
forming” unit within the microgrid which has adequate power reserve to
cover power demand changes.
Such unit is assigned to regulate the voltage at the PCC and set
the system frequency. It employs voltage controlled mode for voltage
source converter (VC-VSC) and follow U-f strategy.
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Autonomous mode
Control algorithm for VC-VSC according to U-f control strategy
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PWM
InverterSource
Urms
Urms refController
miReference
Voltages
Determination
uaRef ubRef ucRef
Virtual
PLL
Θset
Θset
Block diagram of U-f control
Autonomous mode
Under autonomous operation mode microgrid can be controlled
according to two possible control strategies:
Single Master Operation (SMO)
Muliti Master Operation (MMO)
In the case of SMO strategy when two or more energy units are
connected to the microgrid operating autonomously, only one of
these can be the source of reference voltage. This unit follows the
U-f control, while the other sources implement the P-Q control.
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Autonomous mode
MMO strategy is applied if two or more sources actively
participate in microgrid voltage and frequency control.
In such a case a frequency-droop and voltage-droop control is
derived that share active and reactive powers between sources.
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Proposed control strategy
The proposed control strategy assumes that:
All energy sources within the microgrid are CC-VSC controlled
independently on the operation mode.
In autonomous microgrid operation SMO strategy is applied and
the source of reference voltage is the energy storage.
The control of energy storage changes from P-Q in grid-
connected regime to U-f in autonomous operation.
Operation of the microgrid is coordinated by central controller
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Proposed control strategy
The central controller can performs the following functions:
Keeps constant active power exchange with the network during
grid connected mode of microgrid operation (optional)
Identifies the conditions for island creation
Changes the storage control from P-Q to U-f
Determines set points for MT during autonomous operation
Identifies the condition for reconnection of the microgrid with the
network
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Microgrid Simulation Model
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Microturbine
Energy Source
Energy
Storage
Load
PV
Panels
VSC
VSC
VSC
Utility
Grid
YKY 5x 50 mm2
L = 300 m
Transformer63 kVA
15 / 0,4 kV
500 MVA
15 kV
YKY 5x 25 mm2
L = 50 m
YKY 5x 25 mm2
L = 100 m
YKY 5x 50 mm2
L = 150 m
Energy SourceCentral
Controller
PCC
Microgrid Simulation Model
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Model of PV source in PSCAD program
a b c n
A
B
C
iARef
iBRef
iCRef
3 P
hase
RM
S
URMS
I0Ref
Reference currents
forming
uA
URMS
iARef iBRef ICRef
Pget
uA
Active Power
Determination
Pgen
uB
uC
uB
uC
Microgrid Simulation Model
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Microturbine model in PSCAD program
abc
IcF
IbF
IaF
1
2
3
2
5
2
4
2
6
2
2
2
G1 G3 G5
G4 G6 G2
IFdc
1 [uF]
1 [uF]
1 [uF]
1.0
[o
hm
]V
icin
v
Iac
ibin
v
iain
v
UaF
UbF
UcF
uabFLcouple
+
Lcouple
+
Lcouple
+
Control System DQ
uAccsuBccsuCccsiAccs iBccsiCccs
G1 G2 G3 G4 G5 G6
Firing pulsePset
Qset
udref uqref uqud
udref
uqref
uq
ud
uAccsuBccsuCccs
Simulation results
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Active power flows within microgrid
Simulation results
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Reactive power flows within microgrid
Simulation results
RMS voltage at the PCC (pu)
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Simulation results
Voltage frequency within the microgrid
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Conclusions
The idea of proposed control allows to continue load supply after
microgrid disconnection from the utility network
The applied strategy assumes that energy storage maintains the
reference voltage in the microgrid. Thus, the control of energy
sources can remain the same as in grid-connected mode and
uninterrupted transition from grid-connected to islanding
operation is possible.
The central controller is designed which determine set points for
the controllable source to cover load power changes in the
microgrid. As a result the storage power is maintained close to
zero which makes the autonomous operation of the microgrid
possible in longer-term
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Thank you for your
attention
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