"Is my PV plant generating maximum possible power?" – Part 1

Solar plant owners often wonder if their plant is generating the maximum possible power and giving them the best returns. They are referred to the plant’s PR number, but, the PR is a very top-level figure. There are many layers to the PR number. Like an onion, we need to peel off the various layers to get the answer the plant owner is seeking. This series of blogs attempts to do this.

Let us start by looking at how the power flows in a Solar plant and the major assets or equipment involved. Power is generated by the solar panels that are chained in series, called Strings. The DC power from the strings is fed to the inverters, which convert the DC power to low voltage AC form (LV). The AC power then flows through the transformer that converts the LV to suitable HV for transmission to nearest sub-station and coupling to grid. The power between assets is coupled through power transmission cables.
 
The first layer…
Starting from the grid side, the efficiency of the transformer can be easily determined using energy meters, located appropriately. A suitably sized and well-made transformer should perform at very high efficiency (95+) from day one. Simple steps like regular monitoring of transformer temperatures and preventive maintenance will ensure that the transformer maintains its performance throughout its life.
 
The second layer…
Inverters are now available with very high efficiencies (95+) and their reliability keeps improving. Watch out for spurious inverter trips, especially coinciding with peak generation hours. 
 
The third layer…
The power transmission cables, mentioned above, they are not so innocent! Every cable suffers from I 2R losses… Following good design practices and thumb rules, right at the design stage, can keep these losses minimal. The worst part is that the losses also maximize at the peak generation hours. So, any compromises in selecting the right cable size at the design stage (to save costs), can lead to continuous loss in generation and revenue throughout the plant life!
 
Megamic’s SolarNXT SCADA (Base Pack) has a ‘Loss Breakup’ report that separates out these layers, helping plant owners answer the question ‘Is my PV plant generating maximum possible power?’
 
The tricky layer…
The PV strings are the actual power generating elements. 
While the efficiency of the inverters and transformer can be determined by making direct electrical energy measurements using energy meters, estimating the PV strings efficiency is not that straight-forward. 
First, we need to measure the in-plane (or tilted plane) irradiance at the site using suitable measuring devices. By knowing the irradiance and the rated values of the PV panels, it is possible to estimate the power that the PV panels will generate ideally.

Figure 1 shows the curves of expected power (from irradiance) and actual measured power over a 12-hour period from 6 AM to 6 PM. The data for this curve is taken from a real SCB (string combiner box) to which 21 strings (24 panels each) are terminated.
 
Figure 1: Comparison of SMB-level Forecast Power and Actual Power for a 21-String Case
(Note the dips in power around mid-day, these correspond to the inverter trips mentioned earlier)
 
Clearly, there is a gap between the forecast power and the actual power. Multiple factors play a role in creating the gap between the forecast and actual powers. Identifying the correct factor for the shortfall in actual delivered power from the PV strings can be tricky.
 
In the next blog, we will discuss the dominant factors that determine the actual power generated by a PV string with real-life data.
…To be Continued…
 
To find the complete answer to the original question, watch out for the next part in this series. If you don’t want to miss the next part in the series, you can subscribe for the same using the ‘ Subscribe’ button below.