Electrical Power Quality and Utilisation, Journal Vol. XII, No. 2, 2006
Digitally Controlled Interface Between
Supercapacitor Energy Storage and DC Link
Mietek NOWAK, Juliusz HILDEBRANDT, Jacek RABKOWSKI,
Roman BARLIK, Member IEEE
Warsaw University of Technology, Poland
Summary: In the paper specification of a complex converter designed as an interface between a
supercapacitor energy storage battery and other DC auxiliary links, with emphasis on its control
system, and some laboratory investigation results, is presented. The converter applied in the
solution contains a DC/DC power controller and a DC/AC/DC medium frequency intermediate
link with a transformer, permitting good voltage matching. Specific control of both cooperating
converters in accordance with the proposed solution gives the possibility of synchronized and
phase controlled operation of DC/DC and DC/AC converters resulting in the impact on the AC
current wave shape. In this way AC link switching and conducting loss minimize and improving
of interface efficiency have been emphasized.
Key Words:
supercapacitor energy
storage,
DC-AC-DC converter,
digital control
1. INTRODUCTION
2. CONVERTER DESCRIPTION
Energy storage is actually one of the most important issues
regarding developing small autonomic energy systems such
as for automotive vehicles as well as large power grid
applications. Converters operating as interfaces between
electrochemical or electromagnetic energy storage equipment
are intensively developed and applied. The main feature of
these converters is bi-directional energy flow and control,
permitting storage in the case where the supply or generator
disposes energy reserves and recovery of stored energy to
the system when necessary. A new and attractive method of
energy storage, because of its direct electromagnetic
character, is the use of supercapacitors. The main problem
with the application of supercapacitors is that maximum
voltage of each cell in the stack (2,5V) should not be exceeded.
It is probably reasonable to limit the number of cells in series
in batteries, and to match voltages of interconnected DC
links using a converter containing an AC medium frequency
link with transformer. In recent papers [2,4,6] some proposals
of converters suitable as bi-directional interfaces for
applications have been under consideration. One of these
solutions [2], the subject of this paper, is presented in Figure
1. The control of power for both energy flow direction is
obtained by control of a bi-directional DC/DC converter Td1Td2. Proper action of the circuit requires synchronized
switching of both, being in-series converters, i.e. DC/DC
chopper and inverter/rectifier. It gives the possibility for
steady and smooth operation of the inverter input circuit and
additionally to affect the current wave shape in intermediate
AC link, thus decreasing energy losses. For the developed
converter, a suitable digital control has been prepared using
TMS320F2812 DSP controller with all the main control
functions implemented. In the laboratory model an AC link
frequency of 20 kHz was selected, but of course in this case
a double DC/DC chopper frequency was necessary. The
laboratory test results obtained and presented in the paper
confirm the expected features of the converter and designed
controller.
As shown in Figure 1 the complex converter designed as
interface between the supercapacitor storage battery and
other DC links contains two main blocks. On the
supercapacitor side a simple bi-directional buck-boost DC/
DC converter (Td1, Td2) is used as a power controller.
Depending on current id sign the power direction flow in AC
link changes. This AC link contains two voltage type
inverters connected through a transformer. In accordance
with the power sign, transistors of only one inverter are active.
At positive power and energy storage Tp1 and Tp2 are
switching and Ts1-Ts4 are working as a diode bridge. During
energy recovery (power negative) transistors Ts1-Ts4 are
active and on the other side of transformer only diodes
integrated with switches Tp1 and Tp2 are conducting. As
mentioned above, only if current pulses are synchronous
with the AC wave or if the chopper switching frequency is
much higher than the AC frequency, can steady operation of
the converter be obtained. The second case is of course not
desirable because of increased switching losses, so for a
developed solution synchronized control of the converter
has been designed.
The functional scheme of the proposed control structure
is shown in Figure 2. It should be noticed that in this idea,
phase shifting between AC wave and DC chopper pulses is
Fig. 1. Scheme of developed bi-directional converter with intermediate
AC transformer link
M. Nowak et al.: Digitally Controlled Interface Between Supercapacitor Energy Storage and DC Link
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gives AC current wave-shape in transformer:
i Lk (x − x k ) = I dk + (U pk − U Cik ) sin wr (x − x k ) +
KKKK .... + ( I ik − I dk ) cos wr (x − x k )
(2)
and voltage on the intermediate capacitor Cd:
u Cdk (x − x k ) = U pk − (U pk − U Cik ) cos wr (x − x k )K
Fig. 2. Functional scheme of control structure permitting for
synchronous switching of DC/DC and DC/AC converters
possible. For the analysis of AC current waveform in the
transformer and both DC/AC and AC/DC converters a simple
equivalent circuit as given in Fig.3a. is useful. Input capacitor
Cd builts together with transformer stray inductance Ls is an
oscillating circuit excited by current pulses delivered from
the DC chopper. In each time interval during the whole halfperiod of the AC voltage wave, this same equation and its
solution respectively of intermediate transformer current iL
and its secondary voltage Udp can be used, taking into
account changing current values in particular time intervals
The solution of the differential equation ignoring the
remeining resistance:
Up − Xs
di L
− 1 (i L − I d )d x = 0
dx YC
∫
where:
YC = Cd(wB2Ls); x = wB t
(1)
KKKKK +
I ik
sin wr (x − x k )
YC
(3)
where:
k
— number of sequential interval,
wr = (wr / wB) = 1 / sqrt (XsYC),
UCik, Iik — capacitor voltage and inductance current initial
values on beginning of k-th interval.
The complex solution of the above given equations of the
whole half-period is rather complicated and an important
parameter needs to be taken into account, i.e. phase shifting
of current pulses provided to the AC link from the DC chopper.
Simulations seemed in this case a quite reasonable method of
solution. In Figure 3c an example bunch of curves representing
the AC link current half-period waves is shown. As a parameter,
a phase shifting angle y related to p is used. Another important
set of parameters which have to be considered regarding
current wave shape are the values of leakage inductance of
the transformer and capacitance of Cd. By selecting the value
of this capacitance to obtain a resonant frequency close to the
switching frequency and matching the value of phase shifting
depending on power value and sign, it is possible to minimize
power losses in the AC link.
Fig. 3. Converter equivalent circuit (a), typical AC current (is) and AC voltage waveforms(us) and dc-pulse current (id) – (b) and diagrams of
current waves depending of y and Yc – (c)
&
Power Quality and Utilization, Journal • Vol. XII, No 2, 2006
3. MICROPROCESSOR IMPLEMENTATION
OF CONTROL
According to the functional scheme of developed interface
control (Fig. 2.) presented above, a TMS320F2812 DSP–based
controller has been designed. Power control is related to
storage capacitor voltage and is obtained through duty ratio
D control of the bi-directional DC/DC converter. The value of
commanded current is calculated taking into account the
supercapacitor voltage. It is very important not to exceed the
maximal acceptable value as determined by the limited value
of supercapacitor cells. If the capacitor voltage reaches the
permitted value the loading current must be set to zero. The
switching frequency of the AC intermediate link is equal to
half of the chopper switching frequency. Only transistors of
one of both inverters that are coupled through transformer
inverters must be switched at the same time – always on the
side which delivers energy. Transistors comprising the other
side of the converter stay off, so this complementary inverter
is working as a simple diode rectifier. In the control algorithm
and hardware arrangement, matching of phase shifting y
between chopper current pulses and AC voltage wave is
connected with the dc- current value control parameter duty
ratio D, obtained as the output of the PI- controller.
These two quantities which define current pulse width
(D) and its front edge position (y) have the same
representation related to the AC-half-period and chopper
switching period Ts. Execution of D and y settings is done
using a controller built-in modulator with a complex function.
Three programmable counters are necessary for generation
of control signal patterns of chopper and inverters. The way
by which this has to be achieved is explained with time
diagrams in Figure 4.
Both cases represented by diagrams a) and b) are
differentiated using condition: D+y <1 or D+y >1 . The
complete flow-chart of the developed control program is
shown in Figure 5a, and in the Figure 5b the functional scheme
of hardware with logical interface circuit is presented.
Fig. 4. Time diagrams explaining modulator output pattern at two
possible conditions: y + D < 1 —(a) and y + D > 1 —(b)
4. LABORATORY MODEL TESTS
The careful investigation of a constructed 4,5 kW
laboratory model of converters and its microprocessor control
(Figure 6) have confirmed all expectations in accordance with
the main features of the proposed solution.
An especially effective method of affecting the current
wave-shape in AC intermediate link has been proved. In
Figure 7 appropriate records of current waves at three different
DC converter current pulse phase shifting values are presented.
It is noticeable that changing current waveform results also in
significant changing of switching current values in AC/DC/
AC converters. This interesting property of the designed
control shows the possibility of adaptive control of phase
shifting. The curves in Figure 8 measured for different values
of y and both power flow directions prove this thesis.
An important part of the laboratory investigation
concerned the dynamic properties of the current control loop.
A set of records showing fast step response of digitally
controlled current are given in Figure 9.
Fig. 5. Flow chart of control algorithm (a) and scheme explaining mP
interface for transistor drives
Fig. 6. Photo of laboratory bunch converter model and its mP control board
M. Nowak et al.: Digitally Controlled Interface Between Supercapacitor Energy Storage and DC Link
'
a)
a)
b)
b)
Fig. 8. Diagrams of measurem efficiency of converter at different y
values and storage (a) and recovery (b) process
c)
Fig. 7. AC circuit current (is) and voltage (us) registered at different y
values: y = 18°el, (a), y = 54°el , (b), y = 126°el (c)
5. CONCLUSIONS
For the proposed and laboratory constructed converter
interface with a medium frequency intermediate link, a full
digital control of power and current control has been
successfully tested. Thanks to precise and controllable
synchronizing of a DC/DC bi-directional converter with DC/
AC inverters, a possibility of AC current shaping and
efficiency optimization has been demonstrated. The
!
laboratory measurements have shown efficiency
improvements of 2–5 percent. The DSP processor
TMS320F2812 gives a possibility of complete implementation
of speedy power control including energy flow reverse.
Additionally, a method of energy optimization has been
proved
APPENDIX
Basic laboratory stand parameters:
— UCS = 12,5 – 25 V
— Udp = 300V
— fs = 20 kHz /40 kHz
— Cd = 80 mF
— LS = 0.75 mH
ACKNOWLEDGMENT
The job is sponsored by Polish State Committee for
Scientific Research (Project No. 4T10A 050 25) .
Power Quality and Utilization, Journal • Vol. XII, No 2, 2006
2. N o w a k M . , H i l d e b r a n d t J . , B a r l i k R . : Bidirectional Converters as Power Interfaces for Supercapacitor
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Converters with AC Transformer Intermediate Link Suitable as
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M . H .: A three phase soft switched, high power density dc-dc
converter for high power applications. IEEE Trans Ind. Appl.
1991, 27. pp. 63–73.
5. W a l t e r J . , D e D o n c k e r R . : High power bi-directional
DC/DC Converter Topology For Future Automobiles. Conf. Rec.
CD-ROM EPE 2003 Tolouse.
6. R u f e r A . : Power Electronic Interface For a SupercapacitorBased Energy Storage Substation in DC Transportation Networks.
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7. TMS320F2812 Users Manual.
a)
Mieczys³aw Nowak
Assistant-professor in the Institute of Control and
Industrial Electronics Warsaw University of
Technology. His main interest field are converters
for drive applications and power conditioning. As
author and co-author published over 100 papers, also
28 patents and 6 books in power electronics and
converter control. As research team leader he took
part and realized about 50 R&D projects ( also as
national grants) many of this direct for industry applications. Since
1996 is a head of University Promotion Centre in Power Electronics
where he has organised many short courses and tutorials for industry.
He spent about 3 years on West-European Universities (RWTH Aachen,
TU Graz) and electrical companies (BBC/ABB- Swiss).
Juliusz Hildebrandt
was born in Warsaw, Poland in 1976. Graduated from
WUT in 2001. He is currently a PhD-student in the
Institute of Control and Industrial Electronics and
now is finishing his PhD thesis . His main interest
area is topology and control of bi-directional
converters, and supercapacitor energy storage.
b)
Jacek Rabkowski
received the MSc.and Ph.D. degrees from the Warsaw
University of Technology in 2000 and 2005
respectively. He is currently a Assistant Professor in
Electrical Engineering Faculty at the Warsaw
University of Technology. He is author and c-author
of over ten papers. His main interest area is power
electronics converters for renewable energy sources
and energy storage.
Roman Barlik
c)
Fig. 9. Dynamic features of investigated converter with digital control:
step current command at storage (input and output converter current)
— a) current command at recovery — b) current reverse — c)
REFERENCES
was born in Bydgoszcz, Poland, in 1947. He received
the MSc., Ph.D. and Sc.D. degrees from the Warsaw
University of Technology in 1972, 1977 and 1987
respectively. He is currently a Professor in Electrical
Engineering Faculty at the Warsaw University of
Technology. From 1999 to 2005 he was a Dean of
this Faculty. In 1982/1983 he was a Guest Researcher
with the Faculty of Electrical Engineering of the
Aachen University of Technology, Germany. He has carried out research
in the field of static converters, electrical drives and industrial
automation and has authored and co-authored more than 150 papers,
published in refereed international and domestic journals and conference
proceedings. He is co-author of the Handbook of Power Electronics.
He has directed numerous research projects granted by the University,
industrial companies and State Research Committee. He holds 20 Poland
patents. He is a Senior Member of the IEEE.
1. N o w a k M . , H i l d e b r a n d t J . , B a r l i k R . :
Investigation of bi-directional converters with intermediate
transformer link designed as interfaces for supercapacitor energy
storage. in Polish ,Conf. Rec. Modeling and Simulation Zakopane
2004.
M. Nowak et al.: Digitally Controlled Interface Between Supercapacitor Energy Storage and DC Link
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