A case study on monitoring Potential Induced Degradation (PID) recovery in multi-crystalline modules

Abstract

Potential Induced Degradation (PID) causes significant module degradation leading to decreased power output in photovoltaic (PV) power plants. Many PV power plants are constructed using transformerless inverters and may be susceptible to issues associated with the galvanic connection between the PV configuration and the power grid. This increases the likelihood of a leakage current between the PV system`s active circuit and the ground. The resulting electric field causes sodium (Na+) ions to drift to the cell and some at a certain concentration may diffuse into the PN junction creating shunting paths. The PID detection tools employed in this work are maximum power measurements, comparison of open circuit voltage (Voc) at low irradiance (200 W.m-2 ) and high irradiance (1000 W.m-2 ) and Electroluminescence (EL) imaging at 10% of Short-circuit current (Isc). These techniques are used to assess the degree of PID and to monitor the module recovery. This work explores two recovery methods for PID affected modules, forced recovery and natural recovery. Forced recovery involves reverse biasing the module terminals for a few hours while natural recovery, modules are left unbiased for several months. This yields a maximum power recovery of approximately 95% and 94% for forced and natural recovery respectively. These techniques are used to assess the degree of PID and to monitor recovery. This paper demonstrates that PID recovery on modules depends on two mechanisms, viz. drift or diffusion, or combined.