PID Prevention and Reversal Equipment
To understand the functioning of equipment used for reversal of PID (Potential Induced Degradation) in it is important to understand how and why PID occurs. PID is observed only in those Si crystalline solar panels which are subjected to large negative potentials with respect to ground. Tiny currents are induced with positive ions moving towards, and gradually accumulating in the semiconductor. Presence of these excess positive charges near the junction upsets the depletion layer EMF which is primarily responsible for pushing the solar current. Notice that this an effect like polarization in a capacitor (which opposes DC current through the capacitor). It is a cumulative effect, and susceptible panels may lose three fourths of their full power even in the first one to two years. Compare this with the normal expectation of a 25-year retention of 80% of power. PID Prevention PID can be prevented by: • Using certified PID resistant panels • Using strings with negative terminal grounded • Using isolation transformers between the strings and inverters. Anti-PID Equipment All these measures mean additional expense, and anti-PID equipment may become an economic necessity where design may forbid grounding of the negative side of the string. Two options are available. • Charge Equalizers - Equipment is now readily available on the market which will control and reverse this degradation. Charge equalizers, or simply PID boxes, take advantage of the polarization nature of the PID effect. The basic principle is very simple. A voltage bias is responsible for the polarization. Apply a reverse bias to remove it. A PID box becomes a series stage between the panel array and the inverter. It senses the PV output from the array. If the voltage is higher than a set threshold, as during a sunny day, the output is passed to the inverter. At night, when the voltage falls, the inverter is disconnected. A DC bias is applied to the array so that a controlled depolarization current flows from the semiconductor to the ARC, glass encapsulation, and the frame and mount. Overnight, the polarization charge is reversed and the PID effect removed. Charge equalizers can be used with new projects or old projects which have been diagnosed to be suffering from PID. • High Impedance Grounding- is another option where a transformer isolation can be avoided by grounding the negative terminal through a high resistance of the order of 22K Ohm. This option will prevent further PID but will not reverse it as effectively. Another disadvantage is that ground fault equipment must be provided additionally. Anti PID Equipment in the Market Multiple manufacturers are now supplying Anti PID equipment. Among the major suppliers are SMA Solar Technology who offer a PV Offset box which can be used for voltages up to 1000 V. PADCON offer their Float Controller CI 30 which handles up to 2.500kWp DC power for central inverter installations and plants with large string inverters. Edison Energy offer their Pidbull in two versions: one for string inverters and one for central inverters. Similarly, Ilumen also offer their PID box.