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Forum Index : Solar : Blocking diodes
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| windlight Guru  Joined: 03/03/2007 Location: AustraliaPosts: 331 |
Most new solar installs done up here (Daintree) seem to be done with grid connect solar panels, in strings of 3 or 4 into a MPPT with no blocking diodes. Now for many shading is an issue in the am. I don't know what is in the terminal box on said panels but it seems to me that this arrangement is shall we say unsatisfactory. "I like this place and willingly could waste my time in it" - (Act II, Scene IV). |
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
I checked the "grid connect" panels I use in an off-grid installation and found that there must be bypass diodes in the terminal box. Measure the short circuit current and progressively cover up one column at a time. Likewise to my recent surprise there are no blocking diodes in these panels. But, then again it depends on how the panels are shaded. I don't believe that there is any relationship between blocking diodes and shading. |
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zaphod Regular Member  Joined: 03/06/2018 Location: United KingdomPosts: 93 |
Are you talking about blocking or bypass diodes ?? Bypass diodes are normally fitted to the panel itself to bypass shaded sections, for example my 60 cell panels are in a single series chain with a bypass diode across each section of 20 cells. Blocking diodes would normally be connected externally to the panels when the panels are connected in parallel to prevent a panel with lower output shunting the other/s BUT they reduce efficiency due to there voltage drop. Normally in grid tied systems all panels are in a single series string, however some GTI's have multiple string inputs each with it's own MPPT controller, in those situations NO blocking diodes are required as the multiple series strings are not connected in parallel. I would say a GTI with 3 or 4 separate MPPT inputs is very effective indeed as it can account for up to four areas with completely different shading and insolation characteristics (e.g. East/West). Cheers Roger 1Kwp DIY PV + Woodburner + Rainwater scavanger :) |
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
zaphod, Let's take a simple case where one has one "grid-tie" panel (no blocking diode 33Voc) facing NE and another similar panel facing NW (Southern Hemisphere), both connected in parallel. keyword "virtual tracking" setup Starting in the morning the NE panel produces most of the power, ie current into the 27V battery. So, does the NW panel because it is producing little or no power then shunt power away from the NE panel? I could see that if the NW panel loaded voltage was less than 27Volts that this could be case. I have read comments to the effect that one should put separate MPPT controllers on each parallel connected panel. IE, for the above case one should have a MPPT controller on the NE panel and another one the NW panel. I also assume that PWM or ON/OFF controllers would not be a suitable choice, unless blocking diodes were used. According to these article here suggests that PWM controllers would be OK. I will measure what potential shunting current there is early in the morning and late afternoon, when there is unequal insolation on the NE and NW arrays. |
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| zaphod Regular Member Joined: 03/06/2018 Location: United KingdomPosts: 93 |
As with many things there are many if's but's & maybe's! Assuming you have two panels that are matched by the manufacturer then reverse current flow is unlikely provided your controller does not allow the panel voltage to exceed Vmpp BUT if the voltage is allowed to approach Voc then reverse current flow will occur to the extent the idle panel can absorb the entire output of the active panel simply as a result of ohms law and panel current times the internal resistance. The temperature coefficient acts to slightly improve the situation as the active panel will be hotter than the shaded one and hence have slightly lower Vmpp. In systems with mismatched panels (different manufacturers) the situation can be worse and in cases where the installed power of the active vs shaded panels are different sufficient reverse current can flow to cause a fire hazard. Blocking diodes prevent this situation at potentially a loss of <6W/panel. The nature of the controller be it PWM, MPPT or constant voltage is irrelevant other than its effect upon the operating voltage. Cheers Roger 1Kwp DIY PV + Woodburner + Rainwater scavanger :) |
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
Thanks for more information than supplied by several links I followed. I bought a 30A Schottky just for testing, however I don't think I have a problem that needs fixing! Cheers, Dave |
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| windlight Guru Joined: 03/03/2007 Location: AustraliaPosts: 331 |
I became interested in this when my immediate northern neighbor had significantly reduced output from his 5-6 year old new solar install. Three strings connected to one MPPT, individually they gave the expected voltage, but the amps were only about 40%. I checked what I could at ground level, my balance is not good enough to go onto roofs anymore, especially two story. He had gone with the cheapest quote instead of one of the company's that support this area. Apart from being cheep panels and no blocking diodes I believe the problem was caused by a failure of one connector, this together with shading caused significant damage to some panels. From my research this is just the scenario that can happen with no blocking diodes when connected to a common rail. Actually I am not convinced that even if blocking diodes had been fitted this meltdown could have been avoided, I just don't know. The supplier being Cairns based did not want to know about this problem, so after 3 years of this, the owner had a local installer replace all panels, the rest will probably end up in court. Local installer has now connected all panels as a nominal 324v string into a ABB Selectronics approved grid connect inverter, into the Selectronics pro. As I don't hear my neighbors generator running I assume this is now working. "I like this place and willingly could waste my time in it" - (Act II, Scene IV). |
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