改进MPPT算法AdvancedalgorithmformpptcontrolofPhotovoltaicsystems

Canadian Solar Buildings Conference Montreal, August 20-24, 2004 Refereed Paper ADVANCED ALGORITHM FOR MPPT CONTROL OF PHOTOVOLTAIC SYSTEMS C. Liu, B. Wu and R. Cheung Department of Electrical W. Xiao et al., 2004), open- and short-circuit method (T. Noguchi et al., 2002), incremental conductance algorithm (C. Hua,1998), and fussy logic (N. Patcharaprakiti et al.,2006) and artificial neural network (A. Torres et al.,1998). Perturb-and-observe (P W. Xiao et al., 2004). This is essentially a “ trial and error ” method. The PV controller increases the reference for the inverter output power by a small amount, and then detects the actual output power. If the output power is indeed increased, it will increase again until the output power starts to decrease, at which the controller decreases the reference to avoid collapse of the PV output due to the highly non-linear PV characteristic. Although the P 2) the PV system may always operate in an oscillating mode even with a steady-state sunshine condition, leading to fluctuating inverter output; and 3) the operation of the PV system may fail to track the maximum power point due to the sudden changes in sunshine. Open- and Short-circuit Method The open- and short-circuit current method for MPPT control is based on measured terminal voltage and current of PV arrays (T. Noguchi et al., 2002). By measuring the open-circuit voltage or short-circuit current in real-time, the maximum power point of the PV array can be estimated with the predefined PV current-voltage curves. This method features a relatively fast response, and do not cause oscillations in steady state. However, this method cannot always produce the maximum power available from PV arrays due to the use of the predefined PV curves that often cannot effectively reflect the real-time situation due to PV nonlinear characteristics and weather conditions. Also, the online measurement of open-circuit voltage or short-circuit current causes a reduction in output. Incremental Conductance Algorithm The main task of the incremental conductance algorithm is to find the derivative of PV output power with respect to its output voltage, that is dP/dV (C. Hua et al.,1998). The maximum PV output power can be achieved when its dP/dV approaches zero. The controller calculates dP/dV based on measured PV incremental output power and voltage. If dP/dV is not close zero, the controller will adjust the PV voltage step by step until dP/dV approaches zero, at which the PV array reaches its maximum output. The main advantage of this algorithm over the P and Mode 2 for perturb process. Mode 1 measures the power variation due to the previous voltage change and atmosphere change, and keeps the PV voltage constant for the next control period. Mode 2 measures the power variation and determines the new PV voltage based on the present and the previous power variations. Because the estimate process of Mode 1 stops tracking maximum power point by keeping the PV voltage constant, the tracking speed of MPCurrent(A)020801000 1 2 3 4time (s)Vpv4060ipvPower(kW)00.511.50 1 2 3 4time (s)P pvmaxP pvP pvmax -P pvFig. 8. The PV output of MPCurrent(A)020801000 1 2 3 4time (s)V pv4060ipvPower(kW)00.511.50 1 2 3 4time (s)Ppvmax -PpvP pvP pvmaxCanadian Solar Buildings Conference Montreal, August 20-24, 2004 Refereed Paper Fig. 9 shows the PV output power, voltage and current for the EPP control proposed in this paper. This figure shows the waveforms that are similar to those of Fig. 8. Therefore the proposed EPP control has the similar maximum power tracking performance as the PCurrent(A)020801000 1 2 3 4time (s)V pv4060ipvPower(kW)00.511.50 1 2 3 4time (s)P pvmax -P pvPpvP pvmaxFig. 11. The voltage, current and power of DC/DC converter under proposed EPP method(a) PV Voltage and reference (c) Power Voltage(V)8484.40 1 2 3 4time (ms)vpv *vpv84.885.20 1 2 3 4time (ms)13.51414.515Current(A)ipviL*iLPower(W)1203.80 1 2 3 4time (s)1204.41204.21204.0P pvPpvmax(b) Converter current, reference and PV current Fig. 10. The PV power of three MPPT methods under step changing irradiance Power (kW)00.40.81.21.60 0.1 0.2 0.3time (s)(i) ideal MPPT (ii) PCurrent(10A/div) V dcigVg