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© Imperial College LondonPage 1
A Review and Proposal on Controller Design for a DC/AC Power Converter
Xinxin Wang
Control and Power Group
© Imperial College LondonPage 2
Outline
1. Background
2. The two-loop control system review
3. A new discrete voltage controller design and switching procedure design
4. DSP implementation
5. Conclusion
© Imperial College LondonPage 3
1. Background
• The distributed generation (DG) is developing rapidly.
• Power converters, such as IGBTs, are used as the interfaces between DGs and local loads.
• H∞ repetitive control theory is used to design a controller for the DC/AC power converter.
© Imperial College LondonPage 4
• System Modelling• Two-loop control system• Repetitive control system• Formulation of the H-infinity problem• Calculation of active and reactive power• Active and reactive power controller
2. Two-loop control system review
The work in this section has been done before. Two published papers, ‘H∞ Repetitive Control of DC-AC Converters in Microgrids’, G. Weiss, et. al. (2004) and ’Decoupling control of the active and reactive power for a grid-connected three-phase dc-ac inverter’, J. Liang, et. al. (2003), are related to this section.
© Imperial College LondonPage 5
• System Modelling
PWMIGBT bridge
DC source
u
DCV
fu
fR fL
fr
1i fi ci
cV cS
C
outV
di
L
Rr
i3i
local loads
harmonicdistortion
grid
gV
2i
gL
grgR
gi
grid interface inductor
micro-grid
neutral
filterinductor
inverter
gS
back
© Imperial College LondonPage 6
Decoupling power controller Plant
Repetitive voltage controller
Calculation of P and Q
PLL
refP
refQ
gPgQ
di gv
refv
u
ci
e
gi
gv
quadv
gVwtwt ),cos(),sin(
• Two-loop control system
© Imperial College LondonPage 7
• Repetitive control system
plant
Stablizing compensator
sdesw )(
e
+
ic
P
C
p
u
id
Vg
Vref
winternal model
M
© Imperial College LondonPage 8
• Formulation of the H-infinity problem
1v
2vw~
w
u
+
+
z~
y~
P~
ci
e
P
C
WuW
1~z
2~z
1~y
2~y
a b
© Imperial College LondonPage 9
• Calculation of active and reactive power
],cos)2[cos()cos(2cos2)()()( wtVIwtIwtVtitvtp
T quad
T
VIdttitvT
Q
VIdttitvT
P
0
0
,sin)()(1
,cos)()(1
We assume that the grid voltage is the reference phasor, with angle zero:
, , gygxggg IjIIVV
, , ggyggx VIQVIP
,)( ggygxggggzggref ZIjIVZIVVVV
From the figure of the system modeling,
© Imperial College LondonPage 10
,sincosgg zz
g
g jZ
Zn
Assume that the we know the angle of the equivalent impedance ,gz
, , gg
Qgg
P ZV
QVZ
V
PV
,nVjnVVV QPgref
wtjVVwtVVVvgggg zQzPzQzPgref cos))cos()sin((sin))sin()cos((
,gg
gg
gref ZV
QjZ
V
PVV
Here, Zg is the equivalent impedance of the grid interface inductor and short distance of the transmission line.
© Imperial College LondonPage 11
• Active and reactive power controller
PIform
reference voltages
PI
Pref
Qref
QgPg
vref
+-
+-
))((
))((
s
KKQQV
s
KKPPV
IQPQrefQ
IPPPrefp
© Imperial College LondonPage 12
• Shortcoming of the previous voltage controller
• Formulation of H∞ problem for the new controller
• Simulation results of the new discrete controller• Switching procedure design and simulation
results
3. The new discrete voltage controller design and switching procedure design
© Imperial College LondonPage 13
• Shortcoming of the previous voltage controller
back
© Imperial College LondonPage 14
• Formulation of H∞ problem for the new controller
1v
2vw~
w
u
+
+
z~
y~
P~
ci
gi
e
P
C
WuW
gW
1~z
2~z
3~z
1~y
2~y
3~y
a b
The new block is Wg which is of PI type.
© Imperial College LondonPage 15
• Simulation results
0 0.5 1 1.5 2 2.5 3 3.5-250
-200
-150
-100
-50
0
50
100
150
200
250
1.85 1.9 1.95 2 2.05 2.1
-3
-2
-1
0
1
2
3
There is no DC component in the tracking error. Compared to the previous tracking error, this new controller has a better performance.
© Imperial College LondonPage 16
• Switching procedure design
loads
converter
Sc Sg Vg
In some cases, Sg and Sc should be switched on and off.
This is the simplified circuit.
© Imperial College LondonPage 17
• Grid disconnected and connected while the converter is working
• ‘Grid up’ : The breakdown is over. Then set the reference voltage Vref = Vg
1, which is the fundamental component of the grid voltage Vg . Now, Sc is closed and Sg is open.
• ‘Grid connected’ : Set active and reactive power reference Pref = 0, Qref = 0. Connect the grid to the micro-grid. Now both Sc and Sg are closed.
• ‘Delayed grid connected’ : Set Pref and Qref to desired values.
© Imperial College LondonPage 18
• Simulation results
control signals
tracking error
© Imperial College LondonPage 19
• Converter disconnected and connected while the grid is working
• ‘Converter up’ : Assume the converter is not connected. Now, Sg is closed and Sc is open. Measure the active and the reactive power Pm, Qm.
• ‘Converter connected’ : Set active and reactive power reference Pref = Pm, Qref = Qm. Connect the converter to the micro-grid. Now both Sc and Sg were closed.
• ‘Delayed converter connected’ : Change Pref and Qref to desired values.
© Imperial College LondonPage 20
• Simulation results
control signals
tracking error
© Imperial College LondonPage 21
• Two DSPs introduction. • The controller structure with two DSPs.• Parallel communication between the two DSPs.• Layout of the printed circuit board in Protel.
4. DSP implementation of the controller
© Imperial College LondonPage 22
• Two digital signal processors (DSPs) from TITM, a fixed-point DSP LF2407A and a high speed floating-point DSP C6713, are used.
• The power controller, voltage controller and neutral-point controller are implemented by C6713. The LF2407A is used to implement PLL, monitor the protection and transmit the values of voltages and currents.
• External memory interface (EMIF) of LF2407A and host port interface (HPI) of C6713 are used to do the parallel communication between the two DSPs.
• Introduction of the two DSPs
© Imperial College LondonPage 23
• The controller structure with two DSPs.
EVM LF2407A
Power Reference Start/Stop
Control
DSK 6713
HP
I
EM
IF
SCI
A/D
PC
Power Control
Neutral-Point Control
H_inf Repetitive
Control
PLL
IGBT, VDC & Overcurrent
Protection and Internal Monitoring
Switch Logic Control
Digital Output Board
15VPWM
Sc, Sg and Sd
Transducersand
Conditioning Board
VDC
VgABC
VCABC
IgABC
ICABC
f, V, I, P, Q Signals
PC
© Imperial College LondonPage 24
• Parallel communication between the two DSPs
GND
TMS320C6713Host Port Interface (HPI)
TMS320LF2407 (host)External Memory Interface
HCNTL[1:0]
HHWIL
HR/W
HD[15:0]
HDS1
HDS2
HCS
HAS
HRDY
HINT
A[3:2]
A[1]
R/W
D[15:0]
RD
WE
STRB
READY
VIS_OE
EDA-144
Vcc
Vcc
© Imperial College LondonPage 25
• Layout of the printed circuit board in Protel
Three connectors are used. One is for the connection to HPI, and the other two are for the connection to EMIF.
© Imperial College LondonPage 26
• Summary of the work to date• Future work
5. Conclusion
© Imperial College LondonPage 27
• Review the voltage and power controller design.
• Design a new discrete voltage controller for the converter system.
• Design the switching procedures of the grid or converter disconnected and connected to the local loads.
• Do the parallel communication between the two DSPs.
• Summary of the work to date
© Imperial College LondonPage 28
• Finish DSP implementation of the control system in the experiment.
• Design a power controller using dq transformation.
• Design control system for converters in parallel.
• Future work
Short term:
© Imperial College LondonPage 29
Gear
Doubly-fedinduction generator
ACDC
DCAC
Grid
Pitch
Rotor-sideconverter
Grid-sideconverter
Wind
sP
Stator power
• Finish the embedding of the whole system which includes the wind turbine, the DFIG, the back to back converters and the grid.
A wind turbine blade experiences a variety of loads which occur at specific frequencies, leading to the output power variations. Such as:
• Tower shadow;
• Frictions due to the gearboxes and drive train.
Long term:
© Imperial College LondonPage 30
P
C
Mw
u
e p
y
. . . . ,1 ,)(1
1)(
,1 and 0 ,11
NnsWe
sM
MM
sn
N
nnn
N
nnn
n
• Generalization of the internal model principle. Such model can reject (track) different disturbances (references) with different fundamental frequencies.
© Imperial College LondonPage 31