Solar PV Performance Analysis: 2.1. Inverter Availability

The performance of a solar PV site is based on (1) what % of equipment is available to produce energy (availability) and (2) how well that available equipment is producing energy (performance ratio). It's important to consider both in an evaluation of a power plant's performance. Today we’ll look at item 1 and specifically inverter availability.

Monitoring software typically generates alerts automatically based on pre-defined parameters, for example if an inverter is in fault status then an alert is created. Or if a string current is less than 80% of the maximum current on the inverter/combiner box for 2 consecutive hours, then this generates a string alert.

Looking first of all at inverter availability, three main issues we might see are:

(1) Inverter completely down

In this example we see the inverter stops producing at around 10:40.

Here we exclude issues such as a whole site trip (all inverters down) or comms issues. In this case this inverter has a critical fault and it has shut down. There can be many reasons for this fault (e.g. internal fan fault, DC insulation fault, technician forgot to restart the inverter after maintenance), but it will likely require a visit to site to assess, and/or assistance from the inverter manufacturer.

If the maintenance team assesses the fault and can’t immediately find the solution, a claim should be made with the inverter manufacturer. Whilst waiting for feedback from the inverter manufacturer, it would be helpful to switch out the inverter with a spare (in the case of string inverters) to minimise downtime.

(2) Inverter tripping on and off

In this case the inverter is heavily cycling on and off which will heavily damage inverter components. There can be several causes for this cycling: grid instability, AC supply instability, internal inverter fault, DC insulation fault etc.

For example you can check the AC supply to check if the inverter is tripping on under or overvoltage. An inverter with 220V AC supply may be set up to accept grid voltage in a 10% range of 220V, therefore 200-242V. If the grid voltage goes below 200V or above 242V then the inverter trips.

For the above example the issue linked back to grid overvoltage, the AC supply was frequently above 242V as we can see on the image above (at 10:25 the phase 1 and 3 voltage was >242v), which caused the inverter to trip. In this case it is possible to adjust the inverter overvoltage settings to make the inverters less sensitive to grid fluctuations. Prior to making any changes, the asset owner should obtain approval from the inverter manufacturer and the grid operator.

Generally, the inverter fault log should give more indications to what is causing any inverter tripping.

(3) Inverter curtailed to a certain capacity

In this case the inverter is technically available (which is why you cannot just rely on availability alone as a metric to evaluate performance) but the curtailment still reduces the overall site capacity available to produce energy.

This curtailment might be caused by low factory load (for zero export systems), grid export limitations, internal inverter issues such as fan faults, incorrect setting of inverter active power etc. In this case the inverter had an internal fan fault which limited the inverter's output to prevent overheating. Whatever the case, the root cause should be identified and actions put in place to mitigate.

Ensuring 100% inverter availability is critical. Here is the summary of what we’ve seen today:

Inverter Issue	Potential Cause 1	Potential Cause 2	Potential Cause 3	Potential Cause 4
Inverter Down	Inverter Fault	DC insulation fault	Technician forgot to restart after maintenance	-
Inverter Tripping On/Off	Grid instability	AC supply instability	Inverter fault	DC insulation fault
Inverter Curtailed	Low factory load	Grid export limitation	Inverter fault	Incorrect active power  setting

In the next post we’ll look at string availability checking!