Renewable Enerq) Vol. 3, No. 8, pp. 841 848, 1993 096~1481~93 $6.00+.00 Printed in Great Britain. ,~ 1993 Pergamon Press Lid AN EXPERIMENTAL EVALUATION OF MPPT CONVERTER TOPOLOGIES FOR PV INSTALLATIONS D. B. SNYMAN and J. H. R. ENSHN Department of Electrical and Electronic Engineering, University of Stellenbosch, Stellenbosch 7600, South Africa (Received 4 July 1992 ; accepted 20 April 1993) Abstract--Maximum Power Point Tracking (MPPT) converter topologies are evaluated in practice using the same basic power components. Extensive experimental verification of the operation of these converter topologies is included. A new MPPT converter topology, based on the Parallel Power Conversion Technique (PPCT), which enhances the energy conversion efficiency in the PV system, is described, analyzed and experimentally evaluated. INTRODUCTION A typical stand-alone photovoltaic (PV) energy sys- tem comprises an input section, an energy storage section and an output section. The energy from the Sun is converted to electrical energy by means of a photovoltaic (PV) array with a typical efficiency of 14%. The cable efficiency is not negligible, especially in lower battery voltage systems with direct PV match- ing, typically 94%. PV generated electric energy can be passed through a Maximum Power Point Tracker (MPPT), with a battery lifetime optimization algo- rithm at a typical efficiency of 90%. Some of this energy passes directly to the inverter or load, while the other portion is used to charge the battery. The typical battery effÉciency is 80%. The load receives energy from the battery storage and/or directly from the energy system, through an inverter or converter with an average efficiency of 60%, since the inverter is normally oversized to handle peak loads. [em+ bE l rh = q~r/klTmpptr#'[ E~+E,,, J < 6%' (1) The total efficiency of the power system is thus the cascaded product of the individual efficiencies, and can be as low as 4-6% for a typical remote PV energy system. Figure 1 shows the structure of such a system [1-3]. Equation (1) relates the total system efficiencies and indicates the influence of the energy supplied directly to the load (without storage in the battery) on the total system efficiency [2]. MAXIMUM POWER POINT TRACKERS Since the output characteristics of a PV array, wind and hydro turbines and even diesel generators, show peak power points with, respectively, solar insolation, cell temperature, wind speeds, water flow rates and torque as parameters, self-adaptive control algo- rithms should be used to utilize the input power source to its fullest capability. Several MPPT systems have been introduced with reasonable reliability and efficiency [2, 4-8]. A secondary, but no less profitable advantage of using a MPPT is the ability to control the power flow. This feature is used to regulate battery charging in order to prevent excessive gassing due to overcharge of batteries. Battery lifetime is therefore optimized by using the same MPPT converter. Other functions normally available from standard regulators are also available from new generation MPPTs, for example, load control, monitoring, metering, etc. 1. Standard topologies used as MPPTs Maximum power plant trackers utilize standard switch-mode power supply technologies, incor- porating switching transistors, diodes, capacitors and control algorithms. The three basic topologies are the Buck (Down), Boost (Up) and Buck-Boost (UI~ Down) converters. (a) Down or buck converter. Figure 2 shows a sche- matic diagram of the down chopper [4, 7]. For this converter the output (battery) voltage is always lower than the input PV array voltage (down chopper). Power flow from the PV array to the battery is con- trolled by means of the on/off duty cycle of the swit- ching transistor. This converter topology can be used in conjunction with high array voltages and lower battery voltages. This topology is normally used for MPPT systems. Note that one extra series panel in 841