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First of all, the test procedure DID imitate a "Global MPPT" algorithm. All data points where the Fronius inverter was off-MPPT (i.e. when it was causing the "Christmas light effect") were excluded from the results (even though it's arguable that these are valid data points). NREL noted this in "Inverter MPPT Error" section of the study, stating that there would be another 2% energy harvest gain for the microinverter system in the moderate shade scenario, if they had included these MPPT errors.
Regardless, that "Global MPPT" algorithms or dual-MPPT channels address shade mismatch are misleading for two reasons:
(1) The string-level MPPT will still "turn-off" any shaded sections of the array (using a bypass diode), even though there is a lot of diffuse light still reaching the shaded section. The result is that the string-level MPPT is ALWAYS exacerbating the impact of shade.
(2) The "Global MPPT" is still limited to the input voltage range of the inverter, so the term "Global" is really just marketing spin. And, this has a further consequence that the effectiveness of "Global MPPT" algorithms will be more limited in situations where the string is short or the weather is hot.
To summarize, if you rely on bypass diodes and "Global MPPT" algorithms to address mismatch effects, you will ALWAYS be at a disadvantage to micro inverters. It's only a question of how much.
Regardless, that "Global MPPT" algorithms or dual-MPPT channels address shade mismatch are misleading for two reasons:
(1) The string-level MPPT will still "turn-off" any shaded sections of the array (using a bypass diode), even though there is a lot of diffuse light still reaching the shaded section. The result is that the string-level MPPT is ALWAYS exacerbating the impact of shade.
(2) The "Global MPPT" is still limited to the input voltage range of the inverter, so the term "Global" is really just marketing spin. And, this has a further consequence that the effectiveness of "Global MPPT" algorithms will be more limited in situations where the string is short or the weather is hot.
To summarize, if you rely on bypass diodes and "Global MPPT" algorithms to address mismatch effects, you will ALWAYS be at a disadvantage to micro inverters. It's only a question of how much.
Areas with shading, small systems (~5kW) and complex roof geometries are perfect candidates for a micro inverter system. Commercial systems, regardless of how much they are pushed, make no sense if you take ROI and LCOE into consideration in my opinion. Many of the arguments fall apart under the slightest scrutiny- seeing a rooftop full of naked rails with micro inverters attached to them should scare anyone away when looking at all those potential points of failure on a rooftop with no shading.
An outlier with respect to his 100% failure rate could have merit if he wasn't the only one suffering from failures or environmental issues. I was in Las Vegas yesterday and a few installers talked to me about their failure rates in the hot areas of the SW United States. Many micro inverter manufacturers rate their product to 65C ambient, but one installer showed me a FLIR image of the area underneath a micro inverter rooftop installation that read 85C! His micro inverters had been installed for over a year and he had lost a total of 3 MONTHS of production because the inverters disconnect due to high temperatures. This happens more often than people realize in these hot areas.
An outlier with respect to his 100% failure rate could have merit if he wasn't the only one suffering from failures or environmental issues. I was in Las Vegas yesterday and a few installers talked to me about their failure rates in the hot areas of the SW United States. Many micro inverter manufacturers rate their product to 65C ambient, but one installer showed me a FLIR image of the area underneath a micro inverter rooftop installation that read 85C! His micro inverters had been installed for over a year and he had lost a total of 3 MONTHS of production because the inverters disconnect due to high temperatures. This happens more often than people realize in these hot areas.
Tags: inverter, Micro inverter
Each system is unique and its configuration depends of goals - in case when owner is connected to reliable grid and intended just to save some money and maybe sell some electricity, and has not more than 10 panels, then M215 is a good choice.
But if he has not reliable grid, he must have an energy (batteries, for example) storage, a completely different (and much cheaper) of MPPT battery chargers, I would recommend 150 or 300 V strings, and a few cheap synchronized transformerless converters.
And of course he must have some spare converters, batteries and chargers.
And if the goal is just an energy producing, we just throw out batteries and replace cheap chargers with cheap optimizers.
Here will be always lots of questions - for example, (by the way, how exactly) plug strings in parallel or use discrete converter for the each string, how to clean panels in reality, will broken converters just stop energy producing or turn into short circuit, which wires, connectors and insulators will survive climate, where to position converters and other stuff to safe it of lightning, mices, children, how to avoid unnecessary spends, etc. etc. - it is a kind of art.
But if he has not reliable grid, he must have an energy (batteries, for example) storage, a completely different (and much cheaper) of MPPT battery chargers, I would recommend 150 or 300 V strings, and a few cheap synchronized transformerless converters.
And of course he must have some spare converters, batteries and chargers.
And if the goal is just an energy producing, we just throw out batteries and replace cheap chargers with cheap optimizers.
Here will be always lots of questions - for example, (by the way, how exactly) plug strings in parallel or use discrete converter for the each string, how to clean panels in reality, will broken converters just stop energy producing or turn into short circuit, which wires, connectors and insulators will survive climate, where to position converters and other stuff to safe it of lightning, mices, children, how to avoid unnecessary spends, etc. etc. - it is a kind of art.
A failure of a roof-mounted micro inverter requires a much larger effort to replace, compared with a wall-mounted string inverter. I expect the cost differential in labor to replace a roof-mounted micro inverter vs. a wall-mounted inverter would be substantial. This may factor into the overall system availability if a homeowner opts to not replace a single failed micro inverter right away.
I have also heard the opposite case to hold true for certain (primarily government) installs where the budget to purchase the PV system is available, but the budget for O&M down the road is zero. In this case, it is expected that the system output degrades gradually over time with individual component failures, versus a complete system failure should a central inverter fail with no budget for repairs.
I do find this installer's claims of having failures on 100% of their installed microinverter systems to be hard to believe. This seems like an outlier to me, not in line with anecdotal evidence that I am hearing from other installers. However, I'm not an expert in reliability, so I'm not going to get into it.
I have also heard the opposite case to hold true for certain (primarily government) installs where the budget to purchase the PV system is available, but the budget for O&M down the road is zero. In this case, it is expected that the system output degrades gradually over time with individual component failures, versus a complete system failure should a central inverter fail with no budget for repairs.
I do find this installer's claims of having failures on 100% of their installed microinverter systems to be hard to believe. This seems like an outlier to me, not in line with anecdotal evidence that I am hearing from other installers. However, I'm not an expert in reliability, so I'm not going to get into it.
Addressing "point d", CEC efficiency testing is conducted by an independent test laboratory, according to a protocol established by the Sandia National Laboratory. Here is a link to a webpage where you can download the test standard.
Addressing "point a" (and part of "point c"), I believe the test protocol has specific requirements around power factor and harmonic distortion. Please refer to the link above for that info.
Addressing "point b", the test of power conversion efficiency does not include MPPT efficiency, though I agree that this is equally important. In fact, this is why Enphase publishes MPPT efficiency on its product datasheet.
Addressing "point c", the M215's efficiency remains very high at low power levels thanks to a patented technology called "burst-mode". This technology enables Enphase Micro inverters to cycle on and off and to interleave power stages at very high speeds (on the scale of micro-seconds), in order to optimize the power conversion efficiency all the way down to a fraction of a watt. And, your reaction to these efficiency numbers shows just how innovative and unique burst-mode technology is-- it's something that doesn't exist in any other inverter (or microinverter), and is one of the (many) things that makes Enphase the efficiency leader in micro inverters.
Addressing "point e", I'm not sure where your temperature information came from. We have operating temperature information published on the M215 product datasheet.
Tags: inverter, Micro inverter
a) What kind of a load was used, will these inverters have such a high efficiency with inductive (refrigerators, washing machines) or diode-capacitive one (TV sets, computers, monitors etc) load or they just mean very rare nowadays pure resistive load?
b) Does this efficiency include efficiency of its MPPT or it is just an inverter efficiency?
c) In some cases such a high efficiency is achievable by reducing waveform quality cause the output capacitor takes some energy to recharge 120 times per second and an output coil also has its resistance, what about waveforms quality in the whole range of loads? Such a plain curve in case of Enphase M215-60-2LL (95-96% efficiency in a load range 10-100%) makes me to suspect they have a reduced output filter, but Powercom SLK-1500 looks more (85-96% efficiency with load range 10-100%) realistic - all right, a good output filter reduces efficiency in case of small loads.
d) by the way - who made all these measurements? The problem is that most of contemporary digital voltmeters and ampermeters give wrong results with non-sine voltage and current measurements, so the better filter - the worst efficiency will be shown. In some cases (Tom Bearden's MEG) they ever show "over-unity!"
e) Why the Enphase M215-60-2LL results are actual just for 25-40 Centigrades? What if somebody installs them at the roof, will it have these 25-40 Centigrades?
b) Does this efficiency include efficiency of its MPPT or it is just an inverter efficiency?
c) In some cases such a high efficiency is achievable by reducing waveform quality cause the output capacitor takes some energy to recharge 120 times per second and an output coil also has its resistance, what about waveforms quality in the whole range of loads? Such a plain curve in case of Enphase M215-60-2LL (95-96% efficiency in a load range 10-100%) makes me to suspect they have a reduced output filter, but Powercom SLK-1500 looks more (85-96% efficiency with load range 10-100%) realistic - all right, a good output filter reduces efficiency in case of small loads.
d) by the way - who made all these measurements? The problem is that most of contemporary digital voltmeters and ampermeters give wrong results with non-sine voltage and current measurements, so the better filter - the worst efficiency will be shown. In some cases (Tom Bearden's MEG) they ever show "over-unity!"
e) Why the Enphase M215-60-2LL results are actual just for 25-40 Centigrades? What if somebody installs them at the roof, will it have these 25-40 Centigrades?
Tags: inverter, Micro inverter
Micro Inverters are no the be all end all solution but I feel they do make sense in some smaller applications. For example in small residential systems they do make sense. For a one or 2 module gird tied system micro inverters are likely the only solution. As system size increases above a few kW Micro inverters become too costly.
Another important consideration is the type of installation. In the residential space the roof is typically complex, has obstructions or may be exposed to shading from nearby objects. In these types of scenarios distributed MPPT solutions (micro inverters and DC power optimizers) do make sense. The ability to use different string lengths, different mounting orientations, different size modules, and shade tolerance are all valuable tools in rooftop PV system design. I must disagree with the conclusion that distributed solutions never add energy, and that shading rarely occurs. Shading is common in the residential space and there have been several independent studies that show that where shading occurs distributed technologies add substantial added energy. This makes perfect sense because with distributed MPPT solutions the current of the entire string is not reduced by shading one module in the string.
On the other end of the spectrum are ground mounted utility scale systems. In these types of systems many of the design advantages offered by distributed technologies are of less valuable. String length, module orientation, and module type are all simple inputs to the system design equation. In utility scale system shading is normally not an issue since nearby objects that would create shade are very uncommon. Without shading, the potential for increased energy yield typically comes from module mismatch. With a well matched array the lower efficiency of micros compared to larger inverters makes it difficult to produce additional energy. DC optimizers fare better here since their higher efficiency means that the lost energy recovered from mismatch can be larger than the losses incurred by inserting the optimizers into the system. Well matched is a key phrase since many systems installed during the many "PV booms" around the world are far from well matched.
Another important consideration is the type of installation. In the residential space the roof is typically complex, has obstructions or may be exposed to shading from nearby objects. In these types of scenarios distributed MPPT solutions (micro inverters and DC power optimizers) do make sense. The ability to use different string lengths, different mounting orientations, different size modules, and shade tolerance are all valuable tools in rooftop PV system design. I must disagree with the conclusion that distributed solutions never add energy, and that shading rarely occurs. Shading is common in the residential space and there have been several independent studies that show that where shading occurs distributed technologies add substantial added energy. This makes perfect sense because with distributed MPPT solutions the current of the entire string is not reduced by shading one module in the string.
On the other end of the spectrum are ground mounted utility scale systems. In these types of systems many of the design advantages offered by distributed technologies are of less valuable. String length, module orientation, and module type are all simple inputs to the system design equation. In utility scale system shading is normally not an issue since nearby objects that would create shade are very uncommon. Without shading, the potential for increased energy yield typically comes from module mismatch. With a well matched array the lower efficiency of micros compared to larger inverters makes it difficult to produce additional energy. DC optimizers fare better here since their higher efficiency means that the lost energy recovered from mismatch can be larger than the losses incurred by inserting the optimizers into the system. Well matched is a key phrase since many systems installed during the many "PV booms" around the world are far from well matched.
When you hear about the suitability of a particular inverter technology for a particular project, it comes from the perspective of this hard-earned experience. To the best of my knowledge, no peer reviewed paper has demonstrated a performance advantage for micro inverters in commercial applications -- let alone an LCOE advantage (taking into account the considerable difference in price points for string or central inverters compared to micro inverters). Perhaps you can show me otherwise?
I have seen recent work presented by NREL which demonstrates the performance advantage of the micro-inverter architecture for shaded conditions. This analysis appears to be well done and we appreciate the rigorous approach. Again, though, I wonder if the performance advantage (3.7% in the case of light shading) can overcome the price premium for the microinverter system.
So we tend to think that other factors are at work in the inverter market. The perceived ease of use of micro inverters shouldn't be discounted, for example. In the fast-growing North American market there are many new entrants so this factor is important. We are excited to be launching our SB 240 micro inverter system this year and look forward to serving the needs of these new entrants to the market and those companies who have built their businesses around the characteristics of microinverter technology. We have a few tricks up our sleeves and are optimistic about our chances to compete in this segment. After all, our track record is pretty good.
However our firm opinion is that over time even these new entrants might seek ways to improve returns and move towards more established technologies with proven gains in LCOE or ROI.
BTW, this might also explain your comment about "integrators turning to micro inverters for large-scale projects." I wonder if you can point to (for example) any of the Top 15 commercial systems integrators who are using microinverter technology for large-scale projects? If one supposes that these large, sophisticated integrators might be using global best practices, then it might be important to note that these companies rely on central inverters or decentralized string inverter architectures to deliver leading returns to PV investors.
I have seen recent work presented by NREL which demonstrates the performance advantage of the micro-inverter architecture for shaded conditions. This analysis appears to be well done and we appreciate the rigorous approach. Again, though, I wonder if the performance advantage (3.7% in the case of light shading) can overcome the price premium for the microinverter system.
So we tend to think that other factors are at work in the inverter market. The perceived ease of use of micro inverters shouldn't be discounted, for example. In the fast-growing North American market there are many new entrants so this factor is important. We are excited to be launching our SB 240 micro inverter system this year and look forward to serving the needs of these new entrants to the market and those companies who have built their businesses around the characteristics of microinverter technology. We have a few tricks up our sleeves and are optimistic about our chances to compete in this segment. After all, our track record is pretty good.
However our firm opinion is that over time even these new entrants might seek ways to improve returns and move towards more established technologies with proven gains in LCOE or ROI.
BTW, this might also explain your comment about "integrators turning to micro inverters for large-scale projects." I wonder if you can point to (for example) any of the Top 15 commercial systems integrators who are using microinverter technology for large-scale projects? If one supposes that these large, sophisticated integrators might be using global best practices, then it might be important to note that these companies rely on central inverters or decentralized string inverter architectures to deliver leading returns to PV investors.
Claiming that today's micro-inverters---which use mixed-signal ASIC technology to operate at 96% efficiency and deliver utility interactive and wireless networking capabilities---is anything like the microinverters of the 1980's is absurd.
The rate of evolution in microinverters is faster than string inverters in every dimension (from performance and reliability to cost and features), which is the whole point of why end-customers are so interested.
I would hope that the rate of micro evolution is progressing as they have a ways to go to catch up with string inverters. They aren't as reliable, they aren't as efficient and they aren't as cost effective as string inverters for larger installations. It baffles me to hear about 600kW to 2MW micro installs. I find it hard to believe that those LCOE/ROI works out in favor of the customers, and I have yet to be proven wrong. I would be happy to sell people 4000 SB240's, but I feel compelled to warn against it for multiple reasons, mainly, because it doesn't make sense. Micros just aren't there- yet.
Also, I think you need to reread the article as he never tried to compare micros from 30 years ago to the ones today. He only mentioned that they have been around a long time.
The rate of evolution in microinverters is faster than string inverters in every dimension (from performance and reliability to cost and features), which is the whole point of why end-customers are so interested.
I would hope that the rate of micro evolution is progressing as they have a ways to go to catch up with string inverters. They aren't as reliable, they aren't as efficient and they aren't as cost effective as string inverters for larger installations. It baffles me to hear about 600kW to 2MW micro installs. I find it hard to believe that those LCOE/ROI works out in favor of the customers, and I have yet to be proven wrong. I would be happy to sell people 4000 SB240's, but I feel compelled to warn against it for multiple reasons, mainly, because it doesn't make sense. Micros just aren't there- yet.
Also, I think you need to reread the article as he never tried to compare micros from 30 years ago to the ones today. He only mentioned that they have been around a long time.
Inverters main circuit mainly consists of three-phase or single-phase bridge rectifier, smoothing capacitor, filter capacitor, IPM inverter bridge, current limitation resistors, contactors and other components. Many common failures are caused by the electrolytic capacitors. The electrolytic capacitor life is determined by the DC voltage and the internal temperature on the capacitor both sides, the capacitor type is confirmed during the circuit design, so, internal temperature inside the electrolytic capacitor is critical important. Electrolytic capacitor will affect the inverter life directly, generally, temperature increase 10 ℃, inverter life reduce a half. Therefore, on one hand, considering proper ambient temperature in installing, on the other hand, reduce ripple current by taking some measures. Adopt power factor improved AC/DC reactors can reduce ripple current, thereby extend the electrolytic capacitor life.
During inverter maintenance, usually it's relative easy to measure the electrostatic capacity of to determine the capacitor deterioration, when the electrostatic capacity is less than rated 80%, insulation impedance is below 5 MΩ, it needs to replace the electrolytic capacitors.
During inverter maintenance, usually it's relative easy to measure the electrostatic capacity of to determine the capacitor deterioration, when the electrostatic capacity is less than rated 80%, insulation impedance is below 5 MΩ, it needs to replace the electrolytic capacitors.
Tags: inverter, Maintenace
PG is short for Pulse Generator, generally it is used for measuring rotational speed. The most common PG card is optical encoder.
PG card is a part of vector inverter, to convert the encoder different form signals to suitable for the controller, like: electrical level conversion, analog digital conversion, optical isolation, etc.
Vector control inverter is a high-performance inverter which can be comparable with DC converter.
In the vector control, it requires a motor speed feedback to the frequency inverter, this speed feedback is achieve by adding a rotary encoder (PG) to the motor, which means PG card feedback vector control inverter. In order to simplify the system, the feedback can be formed by operation of the inverter output signal, this control is called none PG card feedback vector control inverter, the performance has a slight gap than PG card feedback, but configuration is simple.
PG card is a part of vector inverter, to convert the encoder different form signals to suitable for the controller, like: electrical level conversion, analog digital conversion, optical isolation, etc.
Vector control inverter is a high-performance inverter which can be comparable with DC converter.
In the vector control, it requires a motor speed feedback to the frequency inverter, this speed feedback is achieve by adding a rotary encoder (PG) to the motor, which means PG card feedback vector control inverter. In order to simplify the system, the feedback can be formed by operation of the inverter output signal, this control is called none PG card feedback vector control inverter, the performance has a slight gap than PG card feedback, but configuration is simple.
Reconstruction scheme
Condenser pump is one of main auxiliary equipments in turbine system. It brings condensing water to deaerator through heating. Because of constant speed operation, flow only can be controlled by valve. Great throttle loss, high outlet pressure, damage on pipe and low efficiency lead to frequent leakages and energy waste. Furthermore, this kind of mechanic adjustment structure has bad quality, linearity and low switch rate. Frequent adjustments cause lots of faults, which increases maintenance cost and wastes resources.
2 100% capacity condenser pumps are equipped with 300MW generator. Model is 9LDTNA4. Rated flow is 841m3/h. Water lift is 280m. Speed is 1480rpm. Asynchronous motor power is 1000KW. During reconstruction, we add MV inverter to implement stepless adjustment. Power varies with load variety. It heightens efficiency and optimizes operation. Reconstruction complies tenet of "small change, more reliability, best economic benefit".
Advantages: make full use of existing equipment; low cost; keep original control mode same and ensure motor to run in direct on line when inverter is broken; easy maintenance.
Disadvantages: complicated connection; need to add MV breaker; need to add close circuit.
Normal operation: take an example of 51# pump in variable frequency state. Power goes to inverter through 50# switch, then output to 51# motor through 510#. At this time, direct on line switch of 52# is spare. Logics of each switch is following:
Conditions of switch on in direct on line:
Conditions of switch on of inverter
Operation process is following when a pump is broken.
When the pump controlled by inverter is broken, the other pump switches to direct on line automatically (same as original switch mode). When the broken pump is well repaired, it switches to variable frequency state. The spare pump runs only in direct on line, cannot run in variable frequency state.
Condenser pump is one of main auxiliary equipments in turbine system. It brings condensing water to deaerator through heating. Because of constant speed operation, flow only can be controlled by valve. Great throttle loss, high outlet pressure, damage on pipe and low efficiency lead to frequent leakages and energy waste. Furthermore, this kind of mechanic adjustment structure has bad quality, linearity and low switch rate. Frequent adjustments cause lots of faults, which increases maintenance cost and wastes resources.
2 100% capacity condenser pumps are equipped with 300MW generator. Model is 9LDTNA4. Rated flow is 841m3/h. Water lift is 280m. Speed is 1480rpm. Asynchronous motor power is 1000KW. During reconstruction, we add MV inverter to implement stepless adjustment. Power varies with load variety. It heightens efficiency and optimizes operation. Reconstruction complies tenet of "small change, more reliability, best economic benefit".
Advantages: make full use of existing equipment; low cost; keep original control mode same and ensure motor to run in direct on line when inverter is broken; easy maintenance.
Disadvantages: complicated connection; need to add MV breaker; need to add close circuit.
Normal operation: take an example of 51# pump in variable frequency state. Power goes to inverter through 50# switch, then output to 51# motor through 510#. At this time, direct on line switch of 52# is spare. Logics of each switch is following:
Conditions of switch on in direct on line:
Conditions of switch on of inverter
Operation process is following when a pump is broken.
When the pump controlled by inverter is broken, the other pump switches to direct on line automatically (same as original switch mode). When the broken pump is well repaired, it switches to variable frequency state. The spare pump runs only in direct on line, cannot run in variable frequency state.