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Voltage Drop and Bypass Diode Alternatives


Q:

Ch 3 Question: Will we be asked to calculate voltage drop on the exam?
 
Ch 4 Question: Have you used optimizer/chips instead of bypass diodes? Jinko modules use them and they appear to be much better than bypass diodes in terms of losses.
 
A:
 
To answer your question 3 about voltage drop first:
 
I do not think you will be asked to calculate voltage drop on the NABCEP PV Technical Sales Exam, but you should understand what voltage drop is. I was surprised when I took the NABCEP PV Installation Professional Exam, that there was not one question that includes a voltage drop calculation. If there is not a voltage drop calculation on that exam, it is even more unlikely that there would be a voltage drop calculation on the NABCEP PV Technical Sales Exam. I think this may be because voltage drop is not a safety issue for PV systems, it is just an efficiency issue. The problem with a test question on voltage drop would be, what is the operating voltage and current that we would use for the calculation, since the voltage and current is constantly changing all day long as the cold wind blows, the clouds pass and the sun beats down. There are no solid numbers that we can use for the calculations. To keep it simple, I use Vmp and Imp for voltage drop calculations, but others use different numbers based on location, temperature, airflow behind the array and the quality of the atmosphere in a location.
 
What I think you should know about voltage drop, is that it exists. Especially when you have to transmit power over longer distances or with lower voltages. You should know that it is a better idea to have your inverter by the house if you have a ground mount array that works at 420Vdc and the house is 240Vac. The higher the voltage, the more efficient it is over a long distance. Also, you might not have to know this for the exam, but 3-phase power has a benefit over single-phase power by a factor of 0.866 (that is the square root of 3 divided by 2).
Also, since this is a sales exam, you should know that the longer the distance or the lower the voltage, the more money you will end up spending on wire. If the PV is cheap, then you might want to get an extra module to make up for the wire loss. You can also weigh the extra module cost against the wire cost.
I always like to point out the tricky thing about voltage drop, which is what the industry will refer to as “voltage rise”. Voltage rise is in reality still voltage drop, but it looks like voltage rise when the ac side of the inverter will have the voltage rise as the current increases. Since Vdrop = IR (ohms law) and when current rises on the ac side of an interactive inverter, then the voltage at the inverter must also rise to make up for the lost voltage on the inverter output circuit. If the voltage at the inverter is not higher than the voltage at the interconnection, then there would be no way for the inverter to push power to the interconnection. This is also true with large power plants on the grid. The nuclear power plant down the block will make more voltage at the source to push the power to where it is going. (This doesn’t account for transformers, which change voltage). If you have an interactive inverter and the wire between the inverter and the interconnection is too small, then the voltage gets higher at the inverter and if the voltage gets to be 10% higher than grid voltage, then the inverter will go offline. That is good to know.
 
 
Jinko Optimizer Chip Question 4:
 
I have not used the Jinko optimizer chips. It sounds good and I will wait until it has been around for a while and has withstood the test of time. Another thing I like to do is wait and see if the large companies use technology in mass. If the banks and the big companies will use it, then it has probably been researched a bit. Have you used the Jinko optimizer chip? I remember at a PV show in Shanghai a few years ago ET solar had something like that and I have not seen it take off.
One of the best arguments that string inverter people use when they are defending themselves against module level MPPT (microinverters and power optimizers) is “Why put the module in the shade in the first place?” That being said, I got a look at the 2017 NEC and I think when the 2017 NEC is in effect in full force, that rooftops will all have module level control (optimizers and microinverters). The 2017 NEC says that by 1/1/2019 we will have to be able to bring the voltage down to 80V inside the array (pretty much 1 module) or the array needs to be listed as rapid shutdown or there are no exposed conductors. I am guessing that most people will just default to module level electronics.
 
Thanks,
Sean White


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