Download DSP-controlled Photovoltaic Inverter for Universal

1
DSP-controlled Photovoltaic Inverter for Universal
Application in Research and Education
Fredrick Ishengoma, Member, IEEE, Fritz Schimpf, Non-Member, IEEE, and Lars Norum, Member, IEEE
Abstract—This paper presents a setup for a universal inverter
board to be used for teaching and research on photovoltaic (PV)
power systems. The control of power conversion components is
done by a DSP which offers the advantage of great flexibility.
Depending on the control strategy, the converter can be operated
as a stand-alone PV system, hybrid PV system, grid-tie PV
system and mixtures of these configurations. A description of
the hardware and software setup is given and sub-modules to
operate the board in different modes are presented.
Index Terms—Batteries, Converter, Digital control, DSP, gridtied, Inverter, Microgrid, Photovoltaic, Pulse width modulation,
Solar-home-system
which restrict its usage in the lab. The most severe one is the
(completely unnecessary) risk of electric shock when touching
parts of the board which are connected to the grid voltage
(used for synchronization). Another problem which prevents
the board to be used in a standalone system is the missing
power supply from the DC-side.
Our goal is to develop a board which does not have these
shortcomings. It can be used for laboratory experiments as well
as for stand-alone-operation without any additional hardware.
The efficiency does not have to be extremely good, but
should be high enough for building a small, fully functional
demonstration system.
I. I NTRODUCTION
U
NIVERSITIES and colleges worldwide nowadays offer
courses and research on photovoltaic power systems.
Main topics cover PV panels and their characteristics, battery
charging and discharging, PV power conversion elements
(inverters and dc-dc converters) and control of components
in the PV systems.
Students need a platform to do practical work in order to
illustrate different theories covered in modeling and control of
PV systems. Researchers also need a platform to do experiments in order to improve various aspects of PV systems.
The way the experiments are done differs from one university to another depending on what is installed in the labs.
Some use commercial balance of system (BOS) components
such as inverters, charge controllers, and DC-DC converters.
Even if such commercial devices become more accessible
and cheap, they have disadvantages regarding the use in a
lab, like incomplete documentation and closed firmware. So
experimenting with such devices becomes difficult.
An interesting approach of offering a freely programmable,
universal inverter board is the C2000 Renewable Energy Developer’s Kit [1] from Texas Instruments. The kit is designed
to work with Texas Instruments C2000 microcontrollers. The
board allows implementing all the major functions of a solar energy system such as boost DC to DC conversion for
maximum power point tracking (MPPT), single phase inverter
operation, synchronizing inverter output with the AC line, and
DC to DC buck operation for possible battery charging. But
there are a number of shortcomings associated with this board
All authors are with the department of Electric Power Engineering, Norwegian University of Science and Technology, O.S. Bragstads plass 2E, NO-7491
Trondheim, Norway.
F. Ishengoma: [email protected]
F. Schimpf: [email protected]
L. Norum: [email protected]
Manuscript received February 28, 2011.
II. H ARDWARE DESCRIPTION
A. Overview of the system
Fig. 1 gives an overview of the system. It consists of three
power processing stages which are all connected to a common
DC-link. Two of them are DC/DC-converters, one connected
to the PV-input allowing MPPT; the other one is bi-directional
and is connecting a battery bank to the DC-link. The third
stage is an inverter for generating an AC-output. The AC
output is connected to a filter and a transformer for feeding
power to the utility grid or for supplying a load in an island
grid. Using a low-frequency transformer increases the safety
of the setup, because of the galvanic isolation from the grid,
even though this is not the most efficient topology [2].
The setup is flexible and can be programmed to operate
in several different modes, like shown in Fig. 2. Possible
operating modes are:
1) Battery charge controller from PV: In this mode, the
system acts as a battery charge controller and MPPT. DCloads can be connected to the battery. This mode forms a
P
P
LF−Transformer
11
00
00
11
00
11
PV−Panel
Boost−DC/DC
MPPT
~
common
DC−link
Inverter
LC−Filter
(Island−)
grid
P
111
000
000
111
DC/DC for battery−
(dis−)charging
Fig. 1.
Block diagram of the converters
Battery
fused and
switchable
Terminal for
DC−loads
2
2
3
TABLE I
R ATINGS
4
1
1
11
00
00
11
00
11
2
3
2
3
4
PV−Panel
Boost−DC/DC
MPPT
~
DC-link voltage
(Island−)
grid
Max. input voltage (PV)
35 V
MPP tracking range
10..35 V
Inverter
Legend:
1: Solar home system (only DC−loads)
2: Grid tied inverter
3: Battery−inverter with island−grid
4: MPPT for DC−loads
1
000
111
111
000
000
111
1
3
35 V
Max. input current (PV)
10 A
Battery voltage range
10..30 V
Max. battery current
10 A
Continuous power all stages
150 W
Transformer
pri.: 24 V; 6,25 A; sec: 230 V; 0,7 A
DC−load
Vbat
RS232
To relays
Relays
Drivers
JTAG
JTAG connector
PC (GUI & CCS)
Block diagram of inverter board
+3.3V
+12V
Additional
digital I/O
EPWM5A
EPWM4B
EPWM4A
EPWM3B
PWM
+5V
SCI
DAC
PWM_INV4
PWM_INV1
PWM_INV2
PWM_INV3
PWM_DISC
PWM_CHG
PW_BOOST
ADC
EPWM3A
EPWM2A
EPWM1A
MOSFET
DRIVERS
EPWM2B
PWM_CHG
PWM_DISC
Analog signals
conditioning, scaling and
offset
0-3V signals
Fig. 3.
Grid
DC
Load
Ibat
Iac
Vac
Ibat
Vac_inv
Vpv
± 10V
Local ac Loads
Relay2
Battery
C. Hardware structure and block diagram
A block diagram of the complete board is shown in Fig.
3. Basically it consists of the power electronic part, a power
supply, measurement circuitry and the DSP.
Vac
Iac
PWM_INV1/2/3/4
Bi-directional
DC/DC
Fuse
Additional analog
signals
LF Transformer
Inverter
Status LEDs
Ipv
Vac_inv
Vdc_link
Relay3
Boost DC/DC
Relay1
Vpv
PV array
B. Electrical ratings
For easy and safe use, all voltages on the board are limited
to ”Safety extra low Voltage” (SELV) which is below 42 V
[3]. The DC-link-voltage is set to a maximum of 35 V. Since
the PV-modules are connected through a boost-converter, the
maximum voltage of the PV-modules should be in the same
range. The converter for the battery has a boost-characteristic
from the battery to the DC-link and a buck-characteristic in the
other direction. Reasonable system-voltages for the battery are
therefore 12 or 24 V. The power rating of all converter stages
is 150 W. This is still small enough to make a cheap board
without large heatsinks, but it is enough power for reasonable
operation of a small one-family-PV-system which could supply
lamps, a laptop, and charge mobile devices. A summary of the
technical data is given in table I.
+12V
± 10V
+5V
+3.3V
Power
Supply
Vbat
system also known as solar home system. (PV → Battery →
DC-load).
2) Grid-tie-inverter with MPPT: In this mode, only the
MPPT-converter and the inverter are used. A battery is not
connected (PV → AC-Load → Grid).
3) Battery-inverter for island grid: Instead of the grid, an
AC-load is connected and is supplied from PV-power and/or
the battery (PV → AC-Load).
4) MPPT for DC loads: This scenario supplies power from
the PV array directly to the DC load. No battery is connected.
A typical application is water pumping (PV → DC-load).
5) Combinations of the above: In addition there are other
possible combinations. Some examples are: Grid-tie-inverter
with short term energy storage (supplying peak loads from
the battery for grid stabilization) and a battery charge controller from the grid (useful for hybrid systems with a diesel
generator).
1) DSP: A central part of the board is the DSP. It is
controlling the power electronic stages by pulse-width modulation (PWM) and receives feedback via several analog
measurements. For our board the TMS320F2808 from Texas
Instruments is used. It is a fixed-point DSP, running at
100 MHz. For avoiding problems during the manual soldering
we decided to use a controlCARD from TI, which is a small
PCB with the DSP, some power-supply components and an
isolated RS232-port. The controlCARD can be plugged into
a 100-pin DIMM-connector. TI offers some controlCARDs,
which are pin-compatible; therefore we could easily change
to the alternative DSPs TMS320F28035 and TMS320F28027.
Sadly we found out that it is very hard to get the required
100-pin DIMM-socket. Even TI itself seems to be unable to
deliver it. Therefore we plan to put the DSP directly on our
PWM_BOOST
Possible operation modes
Vdc-link
Fig. 2.
Battery
Ipv
DC/DC for battery−
(dis−)charging
I/O
3
board in the next revision.
2) Power supply: One important design criterion for the
board is that it should be possible to operate it as a standalone unit. That means it should be independent from external laboratory power supplies or AC-adaptors. Therefore
the power supply for the board is taken from the DC-link
of the power stage. That means that a connected PV-panel,
a battery or any other source connected to the terminals of
the converters can be used for supplying the logic circuits
and the DSP. The minimum input voltage required is 15 V
and the maximum voltage is 35V. The output voltages of
the power supply are +12 V (mainly relay-drivers and gatedrivers), +5 V (Logic, DSP), ±10 V (analog measurements)
and 3.3 V (mainly JTAG and clamping of the analog inputs).
The different supply voltages are also shown in Fig. 3.
3) Analog measurements: Several analog parameters are
measured via the ADC of the DSP for feedback:
•
•
•
•
•
Voltage and current at PV input,
voltage at DC-link,
voltage and current at battery,
inverter voltage and
grid voltage and current measured on low-voltage side of
transformer.
All analog signals are conditioned and scaled to be within
the range of 0..3V which is the voltage range which can be
converted by the ADC inputs of DSP. Currents are converted
to voltages through shunt resistors and then amplified. For ac
quantities which have positive and negative values, an offset is
added in order to represent negative values by voltages from
0 to 1.5 V. Scaling and offsetting is done through operational
amplifiers.
III. S OFTWARE AND C ONTROL
A. Digital control
The system is controlled digitally. This increases the flexibility of the converter board, because the control becomes
a question of software only. For example, between the configurations for an off-grid-system and a grid-tied inverter no
hardware changes will be needed (except for the sources/sinks
of the power). For a closer review of advantages of digital
control refer to textbooks like [4].
B. Software Framework
The software for the DSP is written in C. Fig. 4 shows the
flow diagram of the software. At startup, the main subroutine
initializes the DSP and all peripheral units like PWMs, ADCs
and GPIOs. Then the background loop is entered. It is used
to execute non time-critical tasks like communication via the
RS232 port and switching of relays and LEDs. The background loop can trigger different tasks on a specific timing.
There are tasks which are executed every 1 ms, 10 ms, 100 ms
and every second.
Currently, five interrupt service routines (ISRs) are used:
• End of conversion ADC ISR is triggered when the ADC
module completes ADC conversion. This ISR is the most
important because it is the time-base for all real-time
calculations. In this ISR, values of measured signals (in
digital form) are extracted, and scaled. Then all digital
controllers which run at the PWM-frequency (50 kHz) are
Main
4) PWM and gate drive: All power stages are controlled by
PWM-signals generated from the ePWM-modules of the DSP.
The DC/DC-converter for MPPT needs only one PWM-signal,
the battery-charger needs two inputs with interlock and the
inverter requires four PWM-signals with pair-wise interlock.
Integrated gate drivers IRS2183 are used. They have an
integrated interlock between the upper and lower switch of
a leg and insert a fixed deadtime of 400 ns between turn-off
and turn-on.
Initialization
28x device level
Peripheral level
System level
ISR, ADC
7) Digital Outputs: Some digital output signals are used for
controlling status LEDs and relays. There is also an additional
connector for future extensions of the board or for debugging
(e.g. trigger output for oscilloscope).
Get measured
ADC and scaling
Calculate
averages
Background loop
Update
parameters for
controllers and
PWMs
1ms task
10 ms task
100 ms task
1 s task
Data logging
enabled?
5) Serial communication: For communication between the
DSP and PC the board has an isolated RS-232 port. The
communication is used to transmit commands from a PC to
the inverter. Later on we will implement a Graphical User
Interface (GUI) on the PC for sending commands, configuring
the converter and also to visualize logged data on the PC.
6) JTAG: The DSP can be programmed and debugged
via a JTAG port. Texas Instrument’s Code composer studio
software is used for software development, debugging and
online visualization of the desired variables from the running
code in the DSP.
EOC ADC ISR
Y
update
datalogger
buffers
Background loop
Every
100µs
Timer0 ISR
Update
counters for
timing of
tasks in
background
loop
Fig. 4.
Exit EOC ADC
ISR
If Vdclink>35V
Trip Zone
(TZ) ISR
Disable
PWMs
outputs and
turn ON
error LED
EOC ADC
ISR
SCIRx ISR
Enable logic
for serial
communicati
on manager
Flowchart of the control program
SCITx ISR
Transmit
character
to PC
N
4
•
•
•
•
calculated/serviced. This ISR is also used for servicing a
data-logger which is very useful for debugging.
Timer 0 ISR occurs at every 100 µs and is used to update
various program counters used for timing of different
tasks in the background loop.
Trip zone (TZ) ISR is used for protection purposes of
the semiconductors on the board. Whenever the DC link
voltage exceeds 35 V, the PWM modules are disabled
from outputting the PWM signals. This ISR is only used
for stopping the inverter and turning on a trip-LED.
SCIRx ISR is triggered when there is a character received
from the PC GUI through the serial port.
SCITx ISR is triggered when there is a character to
transmit from the DSP to PC GUI for visualization.
CT 1,T 4 = d
(1)
CT 2,T 3 = Ptimer − d
(2)
Where C is the new compare value and d is the desired
duty cycle. For negative output voltage, d has negative values. Ptimer is the period of the timer. Fig. 6 illustrates the
generation of the PWM waveforms for T1/T4 and T2/T3. The
frequency of the timer is exaggeratedly low in the figure for
simplicity.
PRD
Compare 1
Timer 1
0
PRD
C. PWM generation for the inverter stage
Timer 2
The used DSP has a flexible hardware unit for generating
PWM waveforms. For the generation a compare value is
compared with a counter. When the compare value matches the
counter, the unit can fulfill an action like clearing or setting
the corresponding output pin. It is possible to differentiate
between a compare match at up-count and at down-count.
Clearing or setting of the output is also possible when the
counter reaches zero or its programmed maximum value. For
detailed documentation of the PWM unit refer to [5].
Unipolar switching of the inverter bridge is used at the
moment, but we plan to program bipolar switching for comparison, too. In the unipolar method the inverter output is switching between the positive DC-link voltage and a zero vector
during the positive half wave of the desired output and between
the negative DC-link voltage and zero during the negative half
wave. Compared to bipolar switching which always switches
between positive and negative DC-link voltage, unipolar PWM
creates less distortion and reduces the losses in the AC-filter.
Fig. 5 shows the switching during positive and negative half
wave. When the desired output voltage is positive, only T1
and T4 switch synchronously with HF, while T2 and T3 stay
off permanently. For a negative output, only T2 and T3 are
switching with HF, while T1 and T4 stay off.
T1
T3
+
_
T2
Fig. 5.
T4
Compare 2
0
1
PWM
T1 / T4
0
1
PWM
T2/T3
0
Fig. 6.
PWM generation
D. Control sub-modules
Since the setup allows quite complicated combinations of
the power processing stages, the control will be developed in
sub-modules. These can be operated separately or in connection with each other.
As an example, Fig. 7 shows the control structure for
grid-tied operation [6]. Here the sub-modules MPPT, PLL
and inverter control are used.
In the following paragraphs some of the required modules
are described:
1) MPPT module: The output of the photovoltaic module/array varies with the amount of solar radiation available
and ambient temperature. The solar radiation and ambient
temperature depends on the time of year, time of the day and
the amount of clouds. There is one point on the P-V curve
where the PV array generates the maximum possible power
output for a given irradiance, ambient temperature and loading
condition [7]. The MPPT is used to track this point in order
to utilize the power from PV array to the maximum.
Illustration of unipolar switching
V DC,ref
For generating the required signals, two PWM-units are
used. The timers of the two units are synchronized and are
counting up and down, creating a discrete sawtooth carrier
signal. Each of the power transistors has its own compare
value which determines its duty cycle. The distinction between
positive and negative half wave is done by using an “inverted“
compare value for transistors T2 and T3. Whenever the
compare registers are updated, they are calculated from the
desired duty cycle:
I PV
MPPT
V PV
D
PWM
V DC
DC/DC
−
DC−link voltage− I r
controller
PV
Ir
V AC
PLL
x
I g,ref
−
current
controller
PWM
Ig
Fig. 7.
Control structure for grid-tie operation
DC/AC
~
grid
5
2) Battery charging/discharging module: The bi-directional
DC/DC converter operates as a buck converter when charging
the battery and as a boost converter when discharging the
battery. The charging/discharging algorithm can be developed
to cater for different types for batteries since each type of
battery has different characteristics. We will start with implementing a charging algorithm for a lead-acid battery, since
this is a commonly used type of battery in PV systems. The
algorithm to be developed will conform to ones recommeded
by battery manufacturers, i.e. a four state charge algorithm
with temperature compensation.
3) Inverter current-control module: This module includes
a resonant controller for controlling the output-current of the
inverter. It is optimized for a sinusoidal reference input.
4) Grid synchronization: A software-PLL is used to generate the phase-angle of the grid voltage from a voltage
measurement. This is very helpful for generating a sinusoidal
current, even if the grid-voltage is distorted.
5) Frequency and voltage thresholds, anti-islanding: This
module checks if the grid voltage is within the allowed tolerances defined in the standards. Also it checks, if the connection
to the grid is active or not. The information generated by the
module can be used to change the mode of operation from
grid-tied to island-operation or vice versa.
6) Data logging module: This module is responsible for
logging the values of interested variables in the DSP RAM.
Logging is initiated through configurable trigger conditions
which are specified by the user through the GUI on a PC
connected to the board. By downloading the logged data to
a PC, further visualization and analysis can be done using
software such as MATLAB, LabView and Microsoft Excel.
7) Extra features module: Extra features can be added. For
example, the loads can be run in a prioritized mode depending
on the source of power. In a stand-alone system, low-priority
loads can be switched off in case there is not enough power
from the battery and PV to supply them all. This is done
automatically by the controller.
Another feature is running the system in hybrid mode where
an additional power source like a diesel generator can be
connected to the AC-terminal of the inverter. Then the batteries
can be charged while the generator is running.
For such additional features the board has four general
purpose relay outputs which can be used for switching loads,
the generator or the connection to the grid.
E. Graphical user interface
For debugging the software, the JTAG-connection and Code
Composer Studio is used. But in order to get an easy to use
environment which also can be operated by less experienced
users, a GUI will be programmed.
It can be used for changing parameters of the converter software, controlling relays, and logging and displaying/plotting
data. We plan to use MATLAB’s GUIDE environment for
programming the GUI. From MATLAB it can be compiled to a
stand-alone application which is not dependent on a MATLAB
installation any more.
IV. R ESULTS AND FUTURE WORK
The hardware setup, i.e. the board is finished and tested.
A photo of the complete board is shown in Fig. 8. In the
background the power stage with the MOSFETs, inductors
and DC-link capacitors is visible. More in the foreground the
DSP-card and the analog circuitry can be seen.
The basic software framework is coded and tested, too. At
the moment we are at the stage of implementing the controlsubmodules and the GUI. We will continue the development
and will demonstrate the setup in different configurations at
Powertech 2011 in Trondheim.
V. C ONCLUSION
The universal PV-Inverter is useful for teaching and research, because it allows an easy process of getting started
with DSP-control of power electronic converters. The board
is also fully operational without additional hardware and can
be used in actual PV-systems. This is a feature which is not
offered by any commercial board on the market. Since digital
control is used for control of the converters on the board,
it is anticipated that students and researchers will be able to
implement various control algorithmms for PV systems and
hence improve the techniques for harvesting electricity from
PV panels.
Fig. 8.
Photo of the complete board
6
R EFERENCES
[1] Texas Instruments, C2000 Renewable Energy Developer’s Kit
(TMDSENRGYKIT),
http://focus.ti.com/docs/toolsw/folders/print/
tmdsenrgykit.html, retrieved 22 October 2010.
[2] F. Schimpf, L. E. Norum, Grid Connected Converters for Photovoltaic,
State of the Art, Ideas for Improvement of Transformerless Inverters,
Nordic Workshop on Power and Industrial Electronics, 2008, 9-11 June
2008, Espoo, Finland.
[3] European standard IEC 60664-1:2007, Insulation coordination for equipment within low-voltage systems - Part 1: Principles, requirements and
tests.
[4] S. Buso, P. Mattavelli, Digital Control in Power Electronics (Synthesis
Lectures on Power Electronics), Morgan & Claypool, 2006.
[5] Texas Instruments, TMS320x280x, 2801x, 2804x Enhanced Pulse Width
Modulator (ePWM) Module - Reference Guide, SPRU791F, November
2004, revised July 2009
[6] R. Theodorescu, V. Benda, P. Rodriguez, D. Sera, T. Kerekes, Course
material from Industrial/PhD-course Photovoltaic Power Systems - in
theory and practice, October 25-28 2010, Department of Energy Technology, Aalborg University, Aalborg, Denmark
[7] F. M Ishengoma, L. Norum, Design and implementation of a digitally controlled stand-alone photovoltaic power supply, in Proc. 2002
NORDIC Workshop on Power and Industrial Electronics.
Fredrick Ishengoma received his MSc degree
in Control Engineering from Bradford University,
Bradford, England in 1988. Since 1988, he is a member of academic staff at the department of Electrical
and Computer Systems Engineering, University of
Dar es Salaam, Tanzania. Currently he is a PhD
student at Norwegian University of Science and
Technology, Norway. His research focus is on Control and Energy Management of hybrid PV systems
for microgrid applications.
Fritz Schimpf finished his MSc degree in Electrical Power Engineering 2004 at the University
of Technology Berlin, Germany (TU Berlin). From
2004 to 2007 he worked in the R&D-department
of SMA Solar Technology AG in Kassel, Germany
on transformer-less grid-tied inverters for PV applications. Since 2008 he is a PhD-student at the
Norwegian University of Science and Technology
(NTNU) in Trondheim, Norway with research focus
on Power Electronic Converters for PV-applications.
Lars Einar Norum received his MSc and PhD
degrees in Electrical Engineering from the Norwegian Institute of Technology (NTH), Trondheim in
1975 and 1985 respectively. From 1975 to 1980
he was a member of the research staff at the
Norwegian Electric Power Research Institute. He
then joined the Norwegian University of Science
and Technology (NTNU), where he is currently a
Professor in Industrial Electronics at the Faculty of
Information Technology, Mathematics and Electrical
Engineering.
He served as Head of the Department of Electrical Power Engineering at
NTNU (1996-2000) and as a Scientific Advisor to SINTEF Energy Research.
He has been involved in many research and industrial development projects in
the field of power and industrial electronics. Presently his research activities
are within the field of digital and analogue signal processing, mathematical
modelling and control of electrical energy conversion in renewable energy
systems.
Prof. Norum is a member of IEEE, ACM and ISES. He has served as
President for the Board of Science and Technology at the Norwegian Society
of Chartered Engineers (1997-2001).