Network Coordinated Power Point Tracking for Grid-Connected Photovoltaic Systems

Abstract

Maximum power point tracking (MPPT) achieves maximum power output for a photovoltaic (PV) system under various environmental conditions. It significantly improves the energy efficiency of a specific PV system. However, when an increasing number of PV systems are connected to a distribution grid, MPPT poses several risks to the grid: 1) over-voltage problem, i.e., voltage in the distribution grid exceeds its rating; and 2) reverse power-flow problem, i.e., power that flows into the grid exceeds an allowed level. To solve these problems, power point tracking of all PV systems in the same distributed grid needs to be coordinated via a communication network. Thus, coordinated power point tracking (CPPT) is studied in this paper. First, an optimization problem is formulated to determine the power points of all PV systems, subject to the constraints of voltage, reverse power flow, and fairness. Conditions that obtain the optimal solution are then derived. Second, based on these conditions, a distributed and practical CPPT scheme is developed. It coordinates power points of all PV systems via a communication network, such that: 1) voltage and reverse power flow are maintained at a normal level; and 2) each PV system receives a fair share of surplus power. Third, a wireless mesh network (WMN) is designed to support proper operation of the distributed CPPT scheme. CPPT is evaluated through simulations that consider close interactions between WMN and CPPT. Performance results show that: 1) CPPT significantly outperforms MPPT by gracefully avoiding both overvoltage and reverse power-flow problems; 2) CPPT achieves fair sharing of surplus power among all PV systems; and 3) CPPT can be reliably conducted via a WMN.