Design of Soft Switching Converter with Digital Signal Processor Based MPPT for Solar Hybrid Applications

ACEEE International Journal on Control System and Instrumentation, Vol. 1, No. 1, July 2010

Design of Soft Switching Converter with Digital
Signal Processor Based MPPT for Solar Hybrid
Applications
N.SenthilMurugan 1 , C.Sharmeela 2 , K.Saravanan3
1
Sri Venkateswara College of Engineering , Dept. of Electrical & Electronics Engineering,
Sriperumbudur-602105, Tamilnadu, India.
Email: nsm@svce.ac.in
2
Anna University, Department of Chemical Engineering, Chennai -25,Tamilnadu, India.
3
MNC- Power Electronics Division, Chennai, Tamilnadu, India.
{Email: sharmeela20@yahoo.com, saravanank96@gmail.com}

Abstract: This paper is based on the design of soft supplied by solar arrays depends upon the insolation,
switching converter (ZVS-ZCS resonant action) with temperature and array voltage, it's necessary to draw
digital signal processor (DSP) based maximum power the maximum power of the solar array. A DSP based
point tracking (MPPT) algorithm for solar hybrid simple MPPT algorithm that adjusts the solar array /
applications. The converter aims to get the regulated
output voltage from several power sources like wind wind voltage with a discrete PI control to track the
turbines, photovoltaic (PV) arrays and energy from these MPP for the converter system is used in this paper to
sources are simultaneously transferred to the load. The achieve the maximum power transfer and high
input stage circuits for different energy sources are put in efficiency for the solar energy system. The tracking
parallel using a coupled inductor and the converter to efficiencies are on firmed by simulations and
prevent power coupling effect it acts in interleaving experimental results. If the solar energy system
operating mode. As the buck/boost converter input range provides power to a load, the system often operates
is restricted interleaved ZVS-ZCS converter with low away from maximum power points of the solar array.
switching loss and conduction loss and efficiency of more
Fig.1 shows the solar array I-V characteristics and the
than 92% can be easily achieved. DSP based MPPT
algorithm adjusts solar array voltage (equal to battery load curve, together with constant power curves (P =
voltage) with a digital compensator technique and VI = const). It is observed that the delivered output
discrete PI control to track the MPP with high tracking power, which is represented by the operating point 1, is
efficiency. Hence the proposed work gives a novel idea in significantly smaller than the maximum output power,
the modern hybrid energy system. which is represented by point 2. In order to ensure a
maximum power transfer, DC/DC converters are used
Index Terms -- Zero Voltage Switching (ZVS), Zero to adjust the voltage at the load to the value of Vr = √
Current Switching (ZCS), Maximum power point (Pm.R), r – equivalent resistance of the load. In the
tracking (MPPT), Solar Wind Hybrid systems (SWHS).
districts where solar energy and wind energy are
naturally complementary, the application of solar-wind
I. INTRODUCTION hybrid generation systems (SWHS) can reduce the
In late years, the problem of energy crunch is more storage capacity of batteries and the total cost of the
and more aggravating. Very much exploitation and system compared with stand-alone PV or wind
research for new power energy are preceded around the generation system.[1][2]. This paper presents a cost-
world. In particular, the solar energy attracts lots of effect controller system for 1kW to 5kW SWHS with
attention. In recent years, the development of power low DC voltage input (18V DC or 48V DC) and high
semiconductor technology results in easier conversion output AC sine wave voltage (220V AC).
between AC and DC. Therefore, the use of solar
energy is emphasized increasingly and regarded as an
important resource of power energy in the next
century. [1]The solar modules have a long lifetime (20
years or more)and their best production efficiency is
approaching 20%.
Solar energy can be utilized in two ways: solar
heating/cooling and solar electricity. Some appliances
can be connected directly because they work on dc at
the system voltage. Solar arrays were developed for Fig. 1 The operation of the MPPT
power satellites in the space program. In high power This control system includes features like: 1) PWM
applications, parallel connected converters are often technique in charging control of batteries, which can
used to provide electrical power. As the power make wind turbine and solar array operate at maximum

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power point so that the overall system efficiency can normally large air-gap, while the leakage inductance
be improved greatly. 2) Constant voltage and limited increases and magnetization inductance decreases. This
current two-loop control of battery charge, which can causes low power conversion efficiency. To overcome
make batteries in float charging state, enhance the such problems, a series resonant converter is widely
cycle rate and prolong the life of the batteries. 3) used [4] However, the series resonant converter for the
SPWM conversion with front-end high frequency DC- power supply generally operates with higher switching
DC modules in parallel and special DSP control frequency than resonant frequency to achieve soft
technique with high performance-cost, which can switching under continuous resonant current mode. In
accomplish sine wave output voltage at the final stage this case, the main switches can achieve zero voltage
to feed the grid with high reliability and high load switching (ZVS), but it has disadvantage that the
efficiency. secondary side diode converter cannot achieve zero
current switching. Furthermore, due to the higher
switching frequency operation than resonant frequency,
it has low voltage gain and high power loss since a
large primary side circulating current flows [5]. Since
the proposed power supply using LLC converter
operates with lower switching frequency than the
resonant frequency, it can achieve high voltage gain,
which, in turn, offers low turns ratio for the
Fig. 2 Proposed Hybrid system block diagram transformer and high efficiency due to discontinuous
resonant current.
Fig. 2 gives the introduction to overall hybrid system
with soft switching converters and DSP based MPPT III. POWER MODULE IN PARALLEL
algorithm implementation which is a novel idea in The DC-DC boost converter stage, used to convert
modern renewable energy system. The impact of air 18V DC or 48V DC to 350 DC, is designed as 1kW
pollution and global climate change are becoming module that can dynamically adjust its output current
increasingly important topics throughout the world and in terms of load current. In this way, 3kW and 5kW
international organizations are fighting to reduce the system can be derived from the 1kW system with the
carbon emissions produced by fossil fuel. [3] Energy parallel operation of multi independent modules in
conservation has become a priority. Table. I give, using 1kW. These multiple independent power modules can
hybrid systems how C0 2 emissions reduced drastically. be paralleled such that each module supplies only its
Over a five year operating period, corresponding
approximately to the life duration of the Ni-Cd battery
and of the gen set in such a hybrid application, the
reduction in the C02 emission for a 2 kW mobile site
with this hybrid power system will be higher than 200
tons. This 200 tons should be compared with about
only 2.7 tons of equivalent C0 2. The addition of a wind
turbine or possibly solar panels will emphasize this
reduction in C02 with an increase in cycling time.

TABLE. I
EXAMPLE OF CO2 SAVINGS WITH HYBRID AND GEN SET
Fig. 3 Simplified circuit of proposed ZVS-ZCS boost converter
Operational 2.5 KW 6 KW 12 KW 24 KW
Details System System System System TABLE. II
Daily SPECIFICATION OF HALF BRIDGE CONVERTER
Generator 8.25h 9h 9h 11.75h
Operation
Daily
Battery 15.75h 15h 15h 12.25h
Operation
CO2
savings 58 Tons 56 Tons 58 42 Tons
Per year Tons

II. SOFT SWITCHING CONVERTER OPERATION
In this paper, a hybrid power generation system proportionate share to total-load current. This sharing
using soft switching technique is proposed as shown in is accomplished by controlling each module’s power
Fig 3. The proposed system consists of a ZVS-ZCS stage with a command generated from a voltage
boost converter, a half-bridge LLC resonant converter feedback amplifier whose reference can be
and transformer. However, since the transformer has independently adjusted in response to a common-share-

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DOI: 01.ijcsi.01.01.03


bus voltage. By monitoring the current from each where Kp, is the proportional gain, and K i, is the
module, the current share bus circuitry determines integralgain.Rearranging equation in finite-
which paralleled module would normally have the difference form
highest output current and with the designation of this [Y(n+l)-Y(n)]/ T = Ki,U(n)+Kp[U(n+1) - U( n)/ T]
unit as the master, adjusts all the other modules to where T is the sampling time .
increase their output current to within 2.5% of that of
Taking the Z-transform of equation yields
the master. The 1kW DC-DC stage with load sharing
controller UC3907 and PWM controller SG3525 is Y(Z) / U(Z) =Kp+KiT / Z-1
shown in Fig.4 Equation can be expressed in state variable form as
X(n+1)= AX(n)+BU(n)
Y(n) = Cx(n)+Du(n)
where A=l , B=KiT, C=l , D=Kp, and X(n) is the
state variable. Fig. 7 shows the block diagram of the
compensator for digital implementation.

Fig. 4 Inverter stage and controller in Hybrid system Fig.7 Implementation of the digital compensator

A. DSP Algorithm
Fig. 8 shows the simplified MPPT control block
diagram. In Fig. 8, D(j), V ref(j) and vcell(j) are
respectively the converter switching duty ratio, the
Fig. 5 Block diagram of inverter stage demanded cell voltage and the actual cell voltage in the
jth MPPT controller cycle, where j = k, k+1. The MPPT
IV. MPPT CONTROL OF SWHS controller calculates the new cell voltage set point
based on the converter switching duty ratios and the
The basic block diagram of the MPPT control is measured cell voltages in the past and at present. The
shown inFig. 6. The proposed control consists of Proportional-Integral (PI) controller forces the cell
two loops, the maximum power point tracking loop voltage to follow the demanded cell voltage signal. In
is used to set a corresponding ‘Vref’ to the charger the practical design of the control software, the
input, the regulating voltage loop is used to regulate threshold ε j (j = 1,2,3 ), which is a small positive
the solar array output voltage according to ‘Vref’ number close to zero, is used to determine whether the
which is set in the MPPT loop.The functions of the MPP has been reached and CV is used as a positive
two loops are performed by a DSP based controller. increment in the demanded cell voltage. The variable δ
The controller senses the solar array urrent and j ( j = 1, 2, 3 ) can be defined as:

voltage to calculate the solar array output power, δ1 = V cell(k+1)-V cell(k)
power slope and ‘Vref’ for maximum power control δ2 = D(k+1)-D(k)
δ3 = D(k+1)+V cell(k+1) δ 2 / δ 1
When | δ1 | > ε1, the MPPT controller can be
simplified
as: V ref(k+1) = Vref (k) + Cv, δ3 > ε3
V ref(k+1) = Vref (k), |δ3 | > ε3
Fig.6 Basic block diagram of the control loop V ref(k+1) = Vref (k) - Cv, δ3 < -ε3
When | δ1 | < ε1, the MPPT controller can be
The equation can be expressed as the following simplified
as: V ref(k+1) = Vref (k) + Cv, δ2 > ε2
Vref(k+1) = Vref(k) ± C
V ref(k+1) = Vref (k), |δ2 | < ε2
C is the amount of disturbance and the sign of C is V ref(k+1) = Vref (k) - Cv, δ2 < -ε2
determined by the power slope. In the voltage loop,
the PI compensator is used to make the system
stable.Therefore, the discretization of the
compensator transfer function is required for system
implementation.The transfer function of a traditional
compensator is
Y(S) / U(S) = Kp + Ki / S
Fig. 8 Simplified MPPT control block diagram

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The insolation changes much more rapidly than the
temperature does. As the cell MPP voltages only vary
slightly under different insolation levels, the PI
controller runs at a much faster frequency than the
MPPT to force the cell voltage to follow the demanded
voltage. The MPPT controller executes once every 16
PI control cycles. The PI and the MPPT controllers in
Fig. 9 are implemented by the digital signal processor,
which is designed for 1.8V minimum supply voltage. Fig. 12 Efficiency characteristics of the proposed hybrid system
As the cell voltage may fall below 1.8 V under rapid
insolation changes, the switching control signals for the
MPPT converter will be suspended by the software CONCLUSION
once the under voltage happens to allow the cell
The design of novel soft switching converter with
voltage to recover and the digital signal processor to
digital signal processor based maximum power point
function properly.
tracking algorithm for solar wind hybrid applications is
presented in this paper. The novelty in the proposed
circuit is the method of generation of the auxiliary
voltage needed to reset the ZVS circuit including the
advantages of both ZVS and ZCS resonant action. The
method of analysis is outlined for a boost converter
with coupled inductor. Simulation results are presented
Fig. 9 MPPT device circuit diagram for a 33W, 400 kHz boost converter. In this paper, a
simple MPPT algorithm based on a DSP is presented to
V. PSPICE SIMULATION deliver the highest possible power to the load from the
solar arrays DC-DC converter were used in the solar
The Figure 10 represents the simulated boost converter wind hybrid energy system to investigate the
with coupled inductor using PSpice. Simulated results performance of the converters. The simulated results
of the 33 W, 400 kHz boost converter with auxiliary show excellent performance (efficiencies over 92% for
switch are presented in this section. The 33 W 400kHz ZVS-ZCS converters) and further analysis can be done
boost converter with auxiliary switch is simulated in on improving the converter efficiency with high power
PSpice environment. rating in MW to electrify rural areas.

REFERENCES
1.Youjie Ma, Deshu Cheng and Xuesung Zhou, “ Hybrid
Modeling & Simulation for boost converter in Photo Voltaic
system”, IEEE Computer Society International Conference
on Information and Computer Science, January 2009,
pp. 85-87.
2.S.M.Mousavi, S.H Fathi, “Energy Management of Wind/PV
Fig. 10 Simulated boost converters with coupled inductor
and battery hybrid system”, IEEE Proceedings on Industrial
Electronics, August 2009, pp. 630-633
3.Joel Brunarie, George Myerscough, “Delivering Cost
The main switch is turned on with a delay of 0.3 µs Savings & Environmental Benefits with hybrid power”,
after the auxiliary switch is turned on. IEEE Transactions on Industrial Electronics, January 2008,
pp. 1203-1212.
VI. SIMULATION INPUT/OUTPUT 4. M. Z. Youssef, H. Pinheiro, and Praveen K. Jain,
“Analysis and Modeling of a Self-Sustained Oscillation
WAVEFORMS Analytical Technique,” Proceedings of the IEEE
International Energy and Telecommunications
Conference, INTELEC, Oct. 2003.pp.282-289
5. Laszlo Huber, Kevin Hsu and Milan M. Jovanovic,
“1.8-MHz, 48-V resonant VRM: Analysis, Design and
Performance Evaluation” IEEE transactions on Power
Electronics Vol. 21 No 1,January 2006. pp. 79-88

Fig. 11 Output DC

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DOI: 01.ijcsi.01.01.03

Design of Soft Switching Converter with Digital Signal Processor Based MPPT for Solar Hybrid Applications

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Design of Soft Switching Converter with Digital Signal Processor Based MPPT for Solar Hybrid Applications