Construction quality:
These inverters are built to a (very low) price. When you remove the lid and take a closer inspection of the circuit, you can see the evidence of this. Critical components such as power diodes and Mosfets are held against the case/heatsink with a single strip of metal that do not compensate for varying component size tolerances. The result of this is that some diodes/Mosfets are clamped tightly to the case whereas others sit loosely, not benefitting from effective heat transference. Several owners of the smaller, 300w inverters have reported Mosfets and diodes failing in these units. It is likely that these components have failed due to overheating caused by poor heatsinking and or load sharing. There is a mod for the Mosfet/diode mounting issue that I noticed on a popular video sharing site that looks like it might be effective, involving small adhesive rubber pads being placed between the Mosfets/diodes and the metal clamping strip. When I first activated my 600w inverter, I was getting an electric shock each time I touched the inverter case and earth. I place a multimeter between the inverter case and the household earth connection and got a reading of 160V AC! The problem turned out to be a small piece of aluminium swarf that was shorting one of the power Mosfet tabs to the inverter cassis. The unit worked fine in this situation but obviously the QA process in the factory doesn’t check for chassis isolation. The lack of any EMC filtering on the input and output is also a serious issue with these devices – see next paragraph! Before you hook this thing up to AC and if you feel comfortable opening the case, I suggest that you use a multimeter to confirm that all diodes and Mosfet metal tabs are electrically isolated from the case.
Electromagnetic Interference (Radio Noise)!
These inverters are about as RF dirty as you can get! If you are an Ham Radio operator or you live in the vicinity of one or even if you just enjoy listening to AM or Shortwave radio during the day, you will probably want to consider using some additional filtering on the DC and AC sides of the inverter. Hook these inverters up to the AC start generating and you will be generating wideband radio interference across a wide frequency spectrum for two blocks! Compounding the problem is that the solar panel wiring in particular acts like an efficient antenna, radiating the noise for all and sunder to hear. I am in the process of developing some ferrite/capacitor based low pass filters for the solar panel side, which should help. As for the AC side, I will try investing in a replacement EMI filtered, IEC style, AC socket to replace the cheap original socket. Hopefully this will tame the unit. Stay tuned for more info..
Rated Output:
It is probably safe(er) to assume that these Chinese grid tie inverters are very overrated on their published output power figures. The 600w Grid Tie inverter I refer to here may be able to provide up to 600w of peak output (downhill, with a tailwind!), but it cannot produce this kind of power for extended periods of time without overheating. Based on this assumption and the expeiences of others, it is probably best not to exceed 400w worth of solar panels on this device. With 400w worth of panels connected to the input, the AC output power should be around 320W at it’s peak during the summer months. In many countries, summertime ambient temperatures can reach 40+ degrees C and derating the unit in these conditions would be prudent course of action if you want to obtain a reasonable service life. Tests from other users indicate that efficiency drops considerably anyway once AC output exceeds 300w. If the smaller 300w inverters have similar characteristics, I would suggest not running any more than 200w worth of solar panels on those.
Input Operating Voltage (20-55v DC?)
Well no, in actual fact the input/solar operating voltage band of these inverters is actually quite narrow and requires that the user carefully select the configuration of his solar array. On bench tests, these inverters do not start generating meaningful power until the input DC/solar array voltage exceeds 28 volts! In practice, this requires that parallel strings of two series connected 12v or single 24v panels be used. Either arrangement should provide an open circuit voltage of around 40-42v DC and a maximum power point of the solar array (under load) of around 36V. A string of three 12V panels in series, while being advantageous as far as DC wiring losses are concerned, would produce an open circuit voltage of >60V and would therefore potentially exceed the rated input voltage of these inverters. The big electrolytic filter capacitors on the input circuit of the inverter are only rated at 63V, so you can see why the manufacturer has nominated 55V as the maximum input voltage.
Efficiency
Bench tested efficiency of these units appears to range from about 85-91%. Efficiency figures vary depending on input voltage and current and are to be expected with any grid tie inverter. It is likely that the specified maximum efficiency of 91% would only occur with solar array voltages ranging between 34-40V DC, which is where this inverters MPPT circuit seems to operate the best.
Power consumption
I know you are asking yourself ‘Power Consumption’, what is this guy talking about? Well, for some reason my 600w inverter does consume AC power when it is not generating i.e at night and under very poor light conditions. According to my inline AC power consumption meter, this inverter consumes around 9 watts of energy when not generating power. For the unit to actually start generating power, it must have at least 28v applied to it’s Solar input terminals. With only a small solar array connected and under very poor light conditions, it is possible for this unit to actually consume more power than it generates! These inverters will consume 108Wh’s of power every night (12hours). Assuming you have ~200watts worth of Solar panels connected up to the inverter, the first hour or so of low elevation morning sunlight will simply be replacing what the inverter itself has consumed the previous night. If you live in a region that does not see a lot of regular sunlight and you are only planning on putting up a small solar array, you might not end up with any net financial gain at all, particularly after the costs of the inverter, solar panels and associated hardware are taken into account.. Food for thought..?
Although another 600w inverter owner reckons that his unit does not consume any power during standby.. Go Figure..
Safety & Earthing
Interestingly, the earth pin on the inverters mains socket is not directly earthed to the inverters chassis or circuit board.
An earth wire does come off the socket and connects to the Printed Circuit Board, but from there, it simply connects to chassis via a series .001uF capacitor. It would seem as though the only function of this earth is to shunt (some of the) RF noise, generated in the inverter, to ground as it offers no personal protection against internal shorts etc.
The inverter chassis does have a dedicated earth terminal. In my installation, this is connected to the metal cabinet that encloses everything and then an earth cable is run to a nearby earth stake driven into the ground. I’m not sure if this improves the safety of the unit, but it may help down the track with interference filtering. There are issues here with balanced, un-balanced and grounding of neutral lines in AC circuits that I need to put some more thought into..
Earth Leakage questions:
This issue applies to any ‘plug-in’ type Grid Tie Inverter.
Many homes nowadays contain RCD’s (residual current devices) or ‘ground fault circuit interrupter’ sensors on their various mains circuits. These devices compare the current flowing in the Active and Neutral lines of the house wiring. If there is a disparity or imbalance of more than 5-30mA of current flowing in these lines, the RCD immediately shuts the power off. These devices likely prevent the electrocution of many people every year. However, if a circuit, normally protected by an RCD, has a grid tie inverter connected to it, will that RCD still activate if an earth leakage fault occurs? If a child for arguments sake, sticks a metal object into a mains socket, some of the current flowing through the body would be coming from the RCD protected mains circuit and some, I assume, would be flowing from the inverter. The RCD will only shut the power off once the residual current reaches 5-30mA, however, a similar amount of, if not more, current could be flowing from the inverter. Granted, the Grid Tie inverter will shut down (anti-islanding) once the RCD shuts down, but this may not occur, since not all the leakage current would be flowing through the RCD.
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