13 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 6. NO. I, JANUARY 1991 ~ 000 000 000 000 000 000 Combined Low-Cost , High-Efficient Inverter, Peak Power Tracker and Regulator for PV Applications _- - 771 ,L --/77YPPr = -/ J. H. R. Enslin, Member, IEEE, and D. B. Snyman, Member, IEEE Abstract-A novel photovoltaic converter system is proposed, imple- menting a new maximum power point tracking technique. The three functions, battery regulation, inverting and maximum-power-point- tracking, needed for photovoltaic systems with battery back-up, are integrated in a single cost effective converter. This converter charges the battery, operates close to the maximum power point of the photo- voltaic array and forms a dc to ac inverter for a complex power load. The step down charger allows the combination of high-voltage PV ar- rays with low-voltage batteries. A full description of the circuit and practical measured results with efficiencies are presented. INTRODUCTION HE purpose of this work is to introduce a novel compound T power converter that serves as a dc to ac inverter, maximum power point tracker (MPPT) and battery charger for stand alone photovoltaic (PV) power systems. A theoretical analysis of the proposed converter is performed and compared with experi- mental results obtained from a 1.5-kW prototype. The overall cost of PV systems can thus be reduced by employing load man- agement control [ 11 and efficiency optimization techniques. This will have a positive effect on the viability of renewable energy systems in general [l], [2], [3], [5], [14], [IS]. The overall efficiency of a cascade system comprises the product of the individual efficiencies of the subsystems. For this reason grid-connected PV systems have normally higher total system efficiencies because of the lack of series power stages [2], [7], [15]. The total cost of the energy system comprises the sum of the subsystem costs, which has a negative effect on the viability of PV energy systems. Photovoltaic power systems, used as remote area power supplies (RAPS), have to make use of battery back-up, regulators, MPPT's and inverters. Fig. 1 shows a PV system with a PV generator, a maximum-power- point-tracker (MPPT) [SI, a voltage regulator, battery back-up and an inverter. When assumed that all the energy is stored in the battery, and then supplied to the load, the overall efficiency (?le) from the PV array output to the load input, is given by ve = qrnppt . ?r . ?b ' 171. With an assumed 90% efficiency in each building block, i.e., MPPT ( vmppt), regulator ( qr), battery (vb) and inverter ( vl), the effective efficiency ve = 66%. However, when cascaded power conditioning systems are replaced with paralleled power conditioning systes, the total power system efficiency ( qe) is improved [9], [ 161. (1) Manuscript received May 16, 1989; revised May 4, 1990. This paper was presented at the 1989 IEEE Power Electronics Specialists Conference, Milwaukee, WI, June 26-29. J. H. R. Enslin is with the Department of Electrical Engineering, Uni- versity of Pretoria, 0002, Pretoria, Republic of South Africa. D. B. Snyman is with Alternative Energy, ESKOM, Nelspruit, Republic of South Africa. IEEE Log Number 9038024. Fig. 1. Traditional cascaded PV power conditioning system. THE NOVEL MPPT TOPOLOGY A large amount of work has been done on the topic of max- imum power point tracking and maximum-power-point-trackers (MPPT's) [l], [5]. A new proposed technique for maximum power point tracking employs a capacitor in series with the PV array and the battery bank. Fig. 2 shows this technique sche- matically. Most of the time this capacitor C, is the decoupling capacitor at the input of the converter and no extra capacitor is needed. Maximum power point tracking is performed by con- trolling the converter output voltage to load the PV array in such a manner as to keep the PV output voltage constant [5]. When the capacitor voltge is kept for instance equal to the bat- tery voltage, the power delivered to the battery equals the power delivered to the capacitor. Under this condition, half of the PV power is supplied directly to the battery at an efficiency of nearly loo%, if the conduction losses of the cabling is ignored, while the other half is converted to the battery by means of the power converter. Analysis of MPPT Eficiency Improvement When the converter in Fig. 2 operates at an efficiency of q,% , the overall PV converter efficiency (qpvc) can, in general, be written as follows (Note that I and V refer to average values): Pout input power to battery Pi, vpvc = - = output power from PV array "" + rlcvczc [when V, = constant] - - (vc + Vb)zpv voltage is kept constant] 0885-8993/91/0100-0073$01 .OO 0 1991 IEEE74 I, vc c :: ‘I - IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 6, NO. I. JANUARY 1991 . MPPT Control I-+-- PEC 7- +- L - IC IC - -7,/ TC = + c - Ib Ib-Ic Y 0 a ESTIWTED PU CONUERTER EFFICIENCY WITH PEC q, F\S PF\METER , El 1 2 3 4 5 UJU, RclT IO Fig. 3. Graph of efficiency improvement as function V,/V,. Thus, for TJ, = 90% and V, = vb, qpvc = 95%. This shows that the total efficiency of the PV converter system is highet than the efficiency of the power electronic converter (PEC). Fig. 3 shows the efficiency improvement as function of the capacitor voltage (V,) and battery voltage (vb) ratio. Note that the asymptote for each curve, where V,/vb ap- proaches infinity (thus vb = o), the efficiency of the power electronic converter is equal to the efficiency of the total PV power conditioning system. Power Rating Enhancement of MPPT Converter versus the power rating of the PV array can be written as The power rating of the power electronic converter (PEC) power rating of PEC - VJpv - power rating of PV array (V, + Vb)Ipv Thus, for the capacitor voltage equal to battery voltage, the power rating of the power electronic converter (PEC) needs to ‘IC = 4c = ‘IC = 9c = 4c = 95X 98x 85X 8@/. 75x be half the power rating of the PV array. It should however be noted that the current ratings for the PEC and the PV array are the same. This is an encouragement for using higher voltage PV arrays. It is thus clear that a smaller power electronic con- verter, with a lower power rating than the power rating of the PV array, may be used to perform MPPT. Applications in Standard MPPT’s The
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