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String integrity in a solar photovoltaic system

This time, we'll explore string integrity in a solar photovoltaic system - what it means, when does a string fail, and how to detect it. In this context, a string is an array of photovoltaic panels interconnected by cables, which are then connected to a solar inverter input. One or more strings of panels can be connected to each inverter input, and typically, multiple strings are connected to a single inverter.

Installation of a solar array on a roof - the panels are typically interconnected in series as a string.
Figure 1. Installation of a solar array on a roof - the panels are typically interconnected in series as a string.
Photovoltaic solar system diagram
Figure 2. Photovoltaic solar system diagram with three strings per inverter.

Next, let's discuss the proper operation of strings in solar photovoltaic systems and how it relies on the integrity of fuses (or switches) and connectors (inter-connection cables). Strings are arrays of solar panels interconnected in a series. In central and mini-central solar PV systems, several strings connect in parallel to an inverter entry point. Each inverter entry point has an electronic device, the Maximum Power Point Tracking device, that stabilizes voltage and current to optimal levels.

In the above schematic photovoltaic system diagram, you can identify three solar inverters (INV), in which three strings are connected to each inverter. Each string is an array of five panels in the specific case, although in reality of course longer strings can be encountered - in commercial systems we will often encounter strings of fifteen panels or more. The common point of failure of the strings is in the connection point between a string and the input pointof an inverter, where there is a fuse or switch that may fail in the event of a voltage spike. A slightly less common point of failure is disconnection of a connector cable along the string between squental panels or a connector cable between the string and the inverter.

Since in most cases there is no current sensor for a string, when an event of fuse or switch induced dissconnection occurs, the string stops delivering current to the input point of an inverter, there is no possibility of a simple automated alert. In the case of a torn or disconnected cable - the situation is very similar, so the inverter stops receiving the current from the problematic string. The relative cost of a string disconnection event is fairly high - in the case shown, each string represents 11.1% of the total system power, which means that as long as the string remains disconnected, there is an 11.1% decrease in availability of the schematic system. In mini-central commercial photovoltaic systems of 50-200 kW scale, disconnection of a single string will typically reduce about 5% of system output.

As mentioned, in central and mini-central solar systems, string disconnection is a common problem and not trivial to detect. In order to identify an event of a string disconnection in mini-central systems (such as SMA, Fronius, Fimer) a comparative analysis of inverter current or power data is necessary, or alternatively a physical inspection of fuses/switches from time to time. These methods are not particularly effective because the first one is tricky for automation, requiring either a professional level of understanding inverter data or devoting additional time to calibrate automatic inverter comparison alerts in the monitoring software (and works only with systems equipped with two or more inverters). In the second case, the string fault identification comes very late, inducing quite a bit of costs in terms of lost output. More advanced methods for detecting disconnections in strings include adding additional current sensors at the string level (common among Chinese manufacturers such as Sungrow, Huawei and Growatt), or connecting additional sensors at the panel level (such as SolarEdge and Tigo). Notably, an automatic analysis of current data in solar inverters using Soltell's Sensorless performance analysis technology enables sufficient sensitivity to detect inverter power decline of 5% - easily detecting a string-level power anomaly for up to 20 strings on a single MPPT.

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