PV systems meant for acting as a stand alone power supply require an auxiliary power source in form of a battery. PV systems meant for small or low power applications generally exhibit relatively high power capacity at high level of irradiation and very low power capacity at low level of irradiation. So an economic design of a PV based power generation system requires an energy storage element in the form of battery or super capacitor. Household applications generally involve AC power driven appliances while output power of PV or battery is DC in nature. Hence, there is a need for efficient and reliable electrical power conversion system. The scope of the present paper involves integration, analysis and simulation of such an efficient, reliable & reduced stage topology.
Past work on stand alone PV based applications have dealt with many different configurations for integrating PV modules with battery and load. Previous literature existing in this area of application involves large number of stages of power conversion and is having serious limitation of unregulated battery charging and discharging. Stand Alone PV based units require to produce stable single phase ac supply. Conventional VSls require a large dc link, of 400V order to have satisfactory performance. Hence the challenge is to design the
Kishore Chatterjee Electrical Engineering Department
Indian Institute of Technology Bombay Mumbai, India
dc stage of such a scheme to provide 400V dc bus. The configuration from [1],[3] shown in Fig. 1, has four power stage converters with two DC links whose voltages are to be controlled for proper operation. This kind of configuration requires at least five controlled switches on the DC side. The input dc stage to inverter is generally having a high step up gain (transformer coupled). It is having two controlled switches to provide a high voltage DC link to inverter. The bidirectional battery converter also requires two controlled switches and one switch is required for PV converter. Controlling two DC link voltages in such cases makes the system slower in response.
Fig. 1. Four stage configuration with two controlled DC links
The configuration shown in Fig. 2 requires a transformer coupled configuration for high step up dc to dc converter which requires three switches on the DC side (including PV converter) along with four separate switches for the inverter, but this configuration has cascaded battery connection, which is not a preferable scheme as the battery current remains uncontrolled leading to frequent unwanted charging and discharging of battery, thereby exerting considerable stress on the battery and thereby reducing its life [I ],[2].
The configuration presented in Fig.3 shows a topology, which requires either PV array to consist of several series connected modules to sustain a high voltage over a wide operating range, together with a high gain bidirectional battery charger, or a boost inverter configuration on the AC side with lower DC side voltage. The problem associated with boost inverter is that ground connection is not available at the
output, so a coupling power transformer is required for providing a ground terminal on the load side [7].
PV DCfDC boost
BATTERY
High Step-up DC/DC
LOAD & FILTER
Fig. 2. Three stage configuration with cascaded battery
Fig. 4 shows a three stage configuration, where two high step up transformer coupled dc-dc converters, are required to maintain the 400V DC link[I], [2], [3], [6]. Tn this case, both the high step up converters, mainly of transformer coupled configuration, requires the battery charger to be bidirectional with four switches and two switches for PV step-up converter. It demands six switches for the DC stage along with four inverter switches.
V Inverter
PV Load
230 V. 50 Hz -
Bidirectional
==ttl DC-DC
48V BATTERY
Fig. 3. Two stage configuration with high boost inverter or high PV voltage
High Step-up DCIDC
High Gain Bidirectional
DCIDC LOAD & FILTER
Fig. 4. Three stage configuration with high step-up DC-DC converters
Fig. 5 shows a three port transformer based implementation. In this configuration, minimum six switches are required for transformer coupled converters in addition with four inverter switches. The power flow in this system is controlled by controlling the phase angle between the terminal voltages across the three windings of the transformer [4], [5]. This
converter allows efficient operation of the system but increases the number of switches as controlling power flow using phase angle shift technique requires controlled switches connected to all the three high frequency converters.
PV Boost
High Fre DC/AC L-_.JII"--_....I
Bidirectional High Freq
DC/AC
High frequency
ACfI)C INVERTER
LOAD & FILTER
Fig. 5. Three port high frequency transformer with four converters
Tn the above configurations, it is found that either higher number of converter stages and controlled switches are required for proper operation or there is unregulated battery performance or higher PV voltage or requirement for isolation transformer. Tn order to limit these drawbacks by reducing the number of converter stages and to reduce the number of controlled switches while having a better control on the battery current a configuration using transformer coupled halfbridge boost converter, a bidirectional battery controller and a single phase inverter has been proposed in this paper. Using half bridge boost converters with voltage-doubler rectifier and high frequency transformer provides high step up gain [8] and use of multiple energy sources on the primary side of DC link of half bridge boost converter is also viable [9]. The operating principle of the proposed scheme is briefly presented in the next section. Schemes pertaining to the sizing of the battery and the PV array are discussed in Section TIT and the details of the controller configuration and its operating issues are dealt with in Section IV. The viability of the proposed scheme is validated by performing detailed simulation studies.
IT. CONFIGURATION OF THE PROPOSED SCHEME
The proposed configuration using half bridge boost converter, bidirectional converter and single phase inverter is shown in Fig. 6. For PV based systems, it is required to charge the battery by PV and feed the load. Hence the power flow from PV is controlled through unidirectional half bridge converter and the charging/discharging current of the battery is controlled by the bidirectional buck-boost converter connected to the dc link existing in the primary side. The single phase VST for feeding ac loads is connected to the dc link of the secondary side. The output voltage of the half bridge converter, which is the input to the dc link of the . nVpv mverter can be expressed as V DC = V C3 + V C4 = -- (l) (1-0)
The half bridge boost converter operates under constant
1139
(2)
switching frequency (10 kHz) operating the PV at MPP or at some other point of V-T curve depending on the load demand.
I
Bidirectional
DC·OC buck- I boost I
I
Transformer coupled half bridge boost
S,
I : Inverter with filter & load
I I I
--
Fig. 6. Proposed converter configuration using half-bridge boost converter.
+ Vo
The bidirectional battery converter maintains the battery current as specified by the controller using hysteresis current control technique. During battery charging, PV converter dumps energy on primary side of the dc link and the battery converter draws the charging current from it. While during battery discharge, the battery converter dumps energy on the primary side of the dc link and the switches T3, T4 are controlled to transfer this energy to the secondary side of the transformer. The single phase VST is controlled to operate at 10 kHz to maintain the load voltage at nov, 50 Hz using conventional SPWM technique.
TIT. PV AND BATTERY SIZING
In order to obtain an economic design of the system, a typical pattern of rural household load characteristics has been derived from the load characteristics presented in [10] for determining the PV and Battery sizing. In rural households, the load requirement is higher during night than during day time when the solar radiation is high. So optimum sizing necessitates the energy discharged by the battery over a whole day should be returned to the battery by the PV modules during the daytime.
A. Load Characteristics and Solar Radiation Variation A typical load characteristic from [10] has been referred as
a baseline for determining the load characteristics. The load characteristic explained in [10] has a maximum load of 3.8 kV A. For this PV power generating unit, the maximum load demand has been specified as 1 kVA, hence an approximated scaled down version of the characteristic has been taken as shown in Fig. 7.
While determining PV sizing, the worst month of the year having the lowest solar radiation needs to be considered. From [11], the solar radiation data over Mumbai for the month of August has been found to be lowest for the year. The variation
of peak power variation over a day for 36V (mpp), 280Wp module from [12], for lowest radiation is shown in Fig. 8.
1100 1000
.00
Mean lo"" 600 Demo"" (IIlVA'J
'00
200
00 10 15 ,. TIme of the day (in hours) --+
Fig.7. Household Load Variation over a typical day in Volt-Amps
To determine the number of PV modules to be used, the total energy supplied by the PV modules over the day, should be sufficient enough to charge the battery for each day requirement. Let K be the number of PV modules required. The following equation is formed for determining the number of PV modules, taking into consideration all the losses in circuit and battery.
f (KPpv - Pout - PcirJoss - IbJoss )dt = f LlQbdt (2)
K = number of PV modules Ppv = output power of a single PV module
Pout = output power PcirJoss = 0.15 * Pout (circuit loss) PbJoss= I (KPpv - Pout - PcirJoss)I * 0.1 (3) LlQb = correspond to change in battery energy level or SOC
m,------,------,-------,------,-----,
200
150
_ 100 variation
%�-----+--J---�10�-----1�5-----����---.. Trne of ihtdayrlnhotn) ---+
Fig. 8. Peak Power variation of solar module over the day
The above integral over a 24 hr period must equate out to zero or greater for proper charging of the battery. By solving the above equation over the 24 hr variation of load and solar radiation as shown in Fig. 7 and Fig. 8, value of K = 10 satisfies for f LlQbdt > O. Hence the optimum value of K is equal to 10. The function f LlQbdt is plotted over a 24 hr scale as shown in Fig. 9.
As evident from the plot of f LlQbdt, there is a surplus of 490 Watt-hr of energy for the PV system designed. Hence the
1140
(3)
choice of K = 10, provides the optimum PV capacity to supply the Watt-hr requirement of the battery for a single day.
�oo,------,------,-------,------,-----,
4000
T 3()00
I woo Change :���) 1000
·1000
-woo
.3(lOOO�--------!----------+"0,------------c,J,-5 -------f.cW---------' Time of the day(in hours)----+
Fig_ 9_ Plot of change in SOqin kWhr) level of the battery over 24 hr period_
B. Battery Sizing Stand alone PV based systems often require the battery
capacity to sustain for extra few hours without solar radiation, To select an optimum size of battery an additional 6 hours of back up for the battery has been considered. Having lower battery voltage leads to higher current rating or higher Amp-hr rating. In present case it is needed that the PV voltage and battery voltage be kept close to each other as they are connected to same DC-link capacitor bank through boost converters (Fig.6). The battery nominal voltage is chosen to be 48 Volt. Considering 10 PV modules of 36V (mpp), 280Wp and the load characteristic as depicted in Fig. 10, the battery discharge requirement is 5980 Watt-hr( daily) and is having 6 hours of backup(around 2250 Watt-hr). The Amp-Hr discharge requirement is given as follows
Amp-Hr discharge requirement = (discharge requirement in Whr)
(nominal voltage ofbattery=48V) (4)
The Amp-hr requirement comes out to be 171 Amp-hr. The regular Lead-Acid Batteries can be discharged till 50% of SOc. Hence using a deep cycle battery, the Amp-hr requirement of the battery is (171/0.5) = 342 Amp-hr. The efficiency of the battery is considered to be 90%, which gives Amp-hr rating of 381 Amp-hr. For convenience, we can choose a battery of 400 Amp-hr rating.
IV. CONTROL SCHEME FOR PV, BATTERY AND INVERTER
A. DC side controlfor PV and Battery
Tn PV based power generation with battery as an energy storage element, the PV and the battery should handle together the power flow between them such that the load demand is met by controlling the dc link voltage VDe, and by controlling
the battery current. Moreover, during charging, battery current should not exceed its maximum limits specified by SOC. The PV and battery can be operated in the following two ways.
MPPT mode:
P_pv + P_bat = PJoad
where, P _pv = Power supplied by PV at MPP P _bat= Power supplied(positive) or taken (negative) by
battery P Joad = Power taken by load
(5)
Tn this mode, P _ bat is negative when the excess power is fed to the battery provided that the battery current does not exceed its maximum limit specified by SOC. P _ bat is positive when battery is discharging to meet the load demand. The control scheme for MPPT mode is depicted in Fig. 10.
Here, TBref = (TOref - Tpvref/K ;where K=I.25. Torefindicates the load component of the current as reflected to the PV side. Here PV current follows the reference from MPPT controller, and the battery controller maintains the dc link voltage by following the current reference (TOref - Tpvref ), representing the difference between the power demanded by the load and the power fed by PV while maintaining MPP. The system operates in MPPT mode, as long as battery charging current TB is less than the maximum value specified by SOC of the battery.
VOCref
Vp'l Ipv
Hysteresis controller
t---IB •• f �r:hl---+ T2
IB =:tYJ--+ T1
PWM
Fig_ 10_ Block diagram for MPPT Control Mode Operation
Non-MPPT mode:
In Non-MPPT mode the same relationship P _pv + P _bat = P Joad is valid, however in this case, P _ bat always
assumes a negative value and the battery charging current is maintained at its reference value as derived from SOC. PV is having excess power in this case if operated at Maximum Power Point, hence PV is operated at some other operating point (on the negative slope of P vs V characteristic) to supply the load demand and charge the battery with full charging current.The control scheme for Non-MPPT mode is given in Fig. 11.
1141
(4)
VOCref
v. I.
Fig. 11. Block diagram for Non- MPPT Control Mode Operation
Here, TPVref= TOref - K* TBref, where K=1.25. Tn this mode Battery current follows the reference tram SOC controller and PV maintains the V DC by following the current reference from the difference of TOref and K*TBref, representing the load component of the required current. The system operates in the Non-MPPT mode, as long as the operating point does not
cross the Maximum operating point (�< 0) or VDC does not dv fall below some specific value (380). The lower limit for VDC is provided to allow the battery in having a faster control while restoring back V DC, Any drastic change in load will cause V DC to fall below 380, the controller will shift from Non-MPPT mode to MPPT mode while entrusting the battery with the responsibility to maintain VDC• Efficient operation of the system requires the two conditions for MPPT mode and NonMPPT mode, to be checked consistently and shift the operating mode trom one to the other and disabling the other mode. This is achieved by generating two control signals Ml and M2 (complimentary to each other) as shown in Fig. 12.
10
Monoshot Multivibrator
Fig. 12. Generation of signals Ml, M2 for control mode changes
Using the signals Ml, M2 the controller shifts from one mode of control to another mode as depicted in the Fig. 13.
VOCrof
Fig. 13. Full control scheme for operating in the MPPT and Non-MPPT states
V. SIMULATION RESULTS
Tn order to ascertain the viability of the proposed scheme detailed simulation studies are carried out on MA TLAB/SIMULlNK platform to emulate different operating conditions. Switching frequency for PV operation is taken as 10 kHz. For the configuration shown in Fig 6, there are two boost converters on the DC side, practical boost converters can provide a maximum gain of 2 to 2.5. PV operating voltage is 36 to 44 volts. Open circuit voltage = 44 V, MPP Voltage = 36 V, battery nominal voltage = 48 V. Hence for MPP, the primary side DC link voltage = 36 x 2 for maximum boost at MPP. The transformer turns ratio for this case turns out to be 1 :5.5 to get a 400 V dc at the secondary side. Primary and secondary side DC link capacitors are chosen to be 500 IlF, while the secondary side filter capacitor for the entire DC link is chosen to be 2000 IlF. The filter inductor associated with the half bridge boost converter is chosen as 0.5 mH, and the inductive filter associated with the bidirectional converter of the battery is chosen as 3 mH. The cut-off frequency of the inverter side filter is chosen around 700 Hz. For simulation purpose the maximum allowable battery charging current is taken as 20A.
Case 1: The system operates at steady MPPT mode, supplying a battery current less than the reference value trom SOC controller. Solar radiation S = 0.5 kW/m2, Load Demand = 1000 W, 100 V AR. Maximum allowable battery charging current (trom SOC) is taken as 20A.
In Fig. 14, the secondary side dc link voltage is maintained constant at 400V by the battery which takes a charging current of 8 Amps(approx.) as shown in Fig. 15. Tn Fig. 16, the current delivered by PV at MPP is depicted.
It can be inferred from Fig. 15, that the battery current ripple is maintained at a very low value by the hysteresis current control technique. In this case PV delivers a surplus of power while operating at MPP, the battery takes a charging current less than that specified by SOC to maintain the secondary side dc link at 400V. Fig. 17 and Fig. 18 show the output ac voltage and current of the inverter representing a
constant load being fetched.
Tlme(secs) _
Fig. 18. Plot of Output Current against time.
Case 2: PV initially operates at MPPT , while a step change in solar radiation takes place at t=5 sec from 0.4 kW/m2 to 0.7 kW/m2 at t=5 sec and from 0.7 kW/m2 to 0.3kW/m2 at t=9 sec .
Tn Fig. 19, initially the DC link voltage is undisturbed and the battery current is less than maximum permissible value as shown in Fig. 20. PV operates at MPP delivering a constant current as evident from Fig. 21. At t=5 sec, a step change occurs in solar radiation, battery current increases, taking excess power delivered by PV. As battery current hits the maximum limit of 20A, the battery converter maintains the charging current at 20A. PV current increases to a new value supplying the power required by both the load and the battery.
It can be thus inferred that the controller shifts from MPPT mode to Non-MPPT mode at the instant battery current hits 20A.
At t=9 sec, solar radiation decreases largely by a step. VDC of secondary side DC link falls drastically below 380(nearly 300) as evident in Fig. 19. The battery comes out of constant charging mode, delivering power required to restore VDC and finally settles down to a new value(Fig. 20).
It can be thus concluded that the controller shifts back from Non-MPPT mode to MPPT mode after VDC falls below 380V. PV current experiences transients due to changes in solar radiation, but maintains MPPT at new operating condition. Fig. 22 and Fig. 23 show the output ac voltage and current of the inverter under the changing solar radiation.
Vo (volt)
10 lamp)
Tlme{secs) _
Fig. 22. Plot of Output voltage against time.
Tme(lees) _ Fig. 23. Plot of Output current against time.
CONCLUSION
The proposed configuration along with the associated control scheme is found to have a stable performance for all possible operating conditions to be encountered. In the proposed scheme it is possible to control the boosted dc link voltage and the battery current while assuring MPP operation for the PV. The controller is able to maintain the charging and discharging current of the battery within an acceptable percentage of ripple. The efficacy of the proposed scheme is ascertained by performing detailed simulation studies.
REFERENCES
[I] H. Wang, D. Zhang, 'The Stand-alone PV Generation System with Parallel Battery Charger", in Int. Coni Electrical and Contr. Engineering (ICECE), Jun. 2010, pp. 4450 -4453.
[2] R. Gules, P. D. Pellegrin, H.L. Hey, 1. Imhofl "A Maximum Power Point Tracking System with Parallell Connection for PV Stand Alone Applications", IEEE Trans. Ind. Electron., vol. 55, no. 7, pp. 2674 -2683, .luI. 2008.
[3] D. B. Candido, .l.R.R. Zientarski, R.C. Beltrame, .I.R. Pinheiro, H.L. Hey, "Implementation of a Stand Alone Photovoltaic System Based on Decentralized DC-DC Converters", in Brazilian Power Electron. Con/, COBEP '09, Sep. 2009, pp. 174 - 180.
[4] L. Shengyong, Z. Xing, G. Haibin, Xie Jun, "Multiport DCIDC Converter for Stand-Alone Photovoltaic Lighting System with Battery Storage", in Int. Coni Electrical and Control Engineering (ICECE), Jun. 2010, pp. 3894-3897.
[5] C. Y-Ming, 1. Y-Chuan, W. F-Yu, "Multi-input DC/DC Converter Based on the Multiwinding Transformer for Renewable Energy Applications", iEEE Trans. ind. Appl., vol. 38, no. 4, pp. 1096 - 1104, .luI! Aug. 2002.
[6] M.1.y. Vazquez, 1.M.A Marquez, F.S. Manzano, "A Methodology for Optimizing Stand-Alone PV System Size Using Parallel Connected DCIDC converters", IEEE Trans. Ind. Electron., vol. 55, no. 7, pp. 2664 -2673, Jul. 2008.
[7] R.O. Caceres, I. Barbi, "A Boost DC-AC Converter: Analysis, Design, and Experimentation", IEEE Trans. Power Electron., vol. 14, no. 1, pp. 134 - 141, Jan. 1999.
[8] F. Khairy, W. L. Hyun, T. Mishima, M. Nakaokal, "Boost-Half Bridge Single Power Stage PWM DC-DC Converter for Small Scale Fuel Cell Stack", in IEEE Int. Power and Energy Coni, PECan '06, Nov. 2006, pp. 426 -431.
[9] H. Tao, A. Kotsopoulos, .I. 1. Duarte, M. A. M. Hendrix, "Multi-Input Bidirectional DC-DC Converter Combining DC-Link and MagneticCoupling for Fuel Cell Systems", in Ind. Appl. Con/, Fourtieth lAS Annual Meeting, Oct. 2005, vol. 3, pp. 2021-2028.
[10] E. S. Sreeraj, K. Chatte�iee, S. Bandyopadhyay, "Design of isolated renewable hybrid power systems", ScienceDirect, Solar Energy, .luI. 2010, vol. 84, pp. 1124-1136.
[11] A. Mani, S. Rangarajan, "Handbook of Solar Radiation over India", Allied Pub. 1982.
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