Since the use of trackers is so much wide spread, I am wondering whether there is a consensus on how you determine the accuracy of your MPPT. That is, to what extent are you -really- tracking the Maximum Power Point?

In this light, I would like to discuss these questions as well:
- What kind of figure of merit do you attach? Percentage of "theoretical" divided by "measured"?
- Would the definition change if there is a different tracker algorythm used? (self-oscillating MPPT or perturb-and-observe)
- When we are talking of tracking accuracies higher than "99%", how would you verify this? what kind of equipment do you use?
- Do you consider parasitic effects such as dynamic behaviour of both solar arrays and MPPT?

I am hoping we can address these topics in a precise an analytical manner. I would like to understand what exactly encompassess the definition "MPPT accuracy"

regards,
Bouke

Hello Bouke,

Basicically, they have yet to find a consensus on the measurement of random mppt efficiency. However, for grid connected mppts, there is a standard.

I do 2 things. I measure the converter efficiency (input power/output power), and i multiply that by the tracking efficiency.

You need an I/V curve sweeper for this, to find your actual Maximum power point, and then see where the MPPT actually tracks at. If your actual maximum power point is at 16.00 volts, and your tracker is regulating at 16.2volts +/-0.2 volts, then your tracking efficiency is 16/(16.2+0.2) = 97%.

Once again, your mppt efficiency is the converter efficiency multiplied by the tracking eff. If your MPPT drifts far from the actual maximum power point, then it is usually a problem with your algorithm rather than parasitics.

You will need accurate and expensive gear to measure near 1% accuracy.

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EFF(mppt) = EFF(converter) x EFF(tracker)

The most used factor for comparing grid-connected PV inverters is the so called European efficiency, which is a weighted conversion efficiency. It is a useful comparative tool for designers and consumers, as systems are installed in a wide range of solar resource regimes. It aims to approximate the integral of the conversion efficiency versus time over the entire day[1].

Please read the attached document from IEEE on grid mppt effecieny:
(Right click and save as)




Cheers,
Tony.

Hi Tony,

Thank you for that reading. It clearly shows that measurements on MPPT systems are quite difficult. Indeed, measuring the so called "tracker accuracy", or n_MPPT in the paper is one of the measurements actually not very well explained. I do assume also that for common, ground-based MPPT systems these trackers may be as accurate as 99.5%. But if this is a selling feature, how do you verify it?

In the past months I have discussed the topic with Yokogawa Europe and I they have recently provided a prototype WT3000 Power Analyzer to us with a dedicated functionality to measure both the RMS power and Peak power in an oscillating MPPT system (i.e. a Denzinger type MPPT) these are the ones we use most in our applications. We now intend to purchase one of these analyzers with dedicated MPPT Analysis functionality.

I think this setup now brings us one step closer to determining the true tracker accuracy, a parameter that can now be measured independently from the electrical efficieny of an MPPT system. However, with added measurement accuracy, there also comes the added difficulties of parasytics. The AC loading on the PV array produces phase shifting of current and voltage, and energy transfer between input filters and PV parasitics create assymmetric waveforms. On-the-fly measurements can thus be offset, and less accurate.

Your proposed method is therefore probably the more accurate: First find your true MPP in DC operation, then switch on the tracker and measure RMS power. The tricky bit here is: how do you exclude all other dynamics that may cause your measurement to offset itself?

More to come... stay tuned...
Bouke