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The importance of GTI shutdown & suggestions / modifications

The PSWGT series of GTI’s all consume varying amounts of AC power from your grid when not generating. 

During standby (at night or very overcast conditions), the 300w GTI’s consume around 5-6 watts, the 600w unit consumes around 12 watts and the 1200w GTI, around 20 watts. I have not tested the larger 2000, 2500 or 3000w GTI’s but it stands to reason that the power draw of these units increases proportionately. This small power draw may not sound like a big deal, but it is a serious problem and can add up to a significant proportion of your daily generation capacity, particularly during the winter months as I will now try to demonstrate..

The following calculation is based on the 1200w GTI. You can transpose the figures for your own GTI if you have a smaller or larger unit. If you have multiple GTI’s in use, you will have to multiply the final figure by the number of GTI’s in use..The final figure may surprise you!.

GTI power draw in standby mode (at night and very overcast light conditions) = 20W

Winter months nighttime hours (no sun)         =   17-18hrs per day.

Winter power consumption  =  17hrs x 20w  =  340Wh per night!

Now ask yourself how long it would take for your system to produce that 340Wh’s of energy used every night by the GTI before you actually saw a net benefit?

But, it can get worse! The above example assumes good light levels throughout the day. With a small solar panel array and in very overcast conditions, the GTI may well consume more power than it generates during the day too! The GTI will not start generating power until the DC input is within the MPPT operating voltage band. This is generally 25-55v for the higher voltage GTI and probably between 12-20v for the lower voltage models. If you have a digital AC power meter inline with your GTI, (Killa-Watt style) you should not assume that those small watt figures being displayed are what you are generating. AC power meters cannot differentiate between power in and power out, so any reading less than ~20 watts is probably power draw rather than power generation.

So, now you can see the importance of reducing or eliminating the standby power consumption of these devices when they are not generating.

Option 1: (may be better for multiple GTI’s)

Use a commonly available AC timer switch on the AC side of the GTI/s. Set the switch to come on a little before the sun will be shining on your solar panel array and switch off a little after the panels begin to be shaded in the afternoon. This will eliminate all night GTI power consumption.

The disadvantage of this system is that the timer requires resetting at different times of year as the days become longer or shorter. The advantage is that it does not involve hardware modification, is cheap and one timer switch can be used for multiple GTI’s.

Option 2 (May be better for single/larger GTI’s):

Caution! This option requires internal GTI modification and should not be attempted by anyone who is not comfortable working on high voltage equipment! Make sure you disconnect the inverter from AC before attempting this one!!

Depending on the DC operating voltage of your GTI, you will require a n AC voltage rated power relay or a Solid State Relay (SSR)  for this mod.

If using a conventional relay, select one with a low coil operating current as this will consume less power from your array (higher coil impedance/resistance = less power consumption). Fit the SPST relay (N.O circuit) into one of the AC input lines inside the GTI. Run light duty DC wire from the DC/solar input to the relay coil (make sure you fit an appropriate diode across the relay coil to prevent back EMF from being generated when the relay closes). Despite the DC voltage reaching ~40V at times on my 28-55v GTI, a 24v relay seemed to handle the higher coil voltage without any problems. If you are concerned about this a resistor could be placed in series with one of the relay coil control lines.

As pointed out by 'XRing', using a Solid State Relay (SSR) may well have several advantages over a conventional relay in this situation as Solid State Relays have the following important characteristics:

Eliminates contact arcing (may protect the GTI FET's during AC switch-on).
Improved reliability (no contacts or coils to burn out)
No moving parts
Reduced current draw from the DC/Solar supply (SSR's only require a few mA on their control line)
More flexible adjustment of turn on voltage on the control lines (a simple resistor voltage divider circuit across the Solar input will permit the user to select the turn on or wake-up voltage for the SSR control lines).

SSR's have an inherent 'On Resitance' which is higher than mechanical relay contacts. This resistance produces some  degree of AC power loss, which results in the SSR generating heat at higher current levels. (hense the need for SSR's to be mounted to a heatsink). I have not actually tested the degree of power loss in this situation, so it may not be a significant factor, nevertheless it should be mentioned here. 

Conventional Relay Theory: With no DC voltage applied to the GTI input, the relay contacts are open and no AC voltage is applied to the inverter (no AC power is consumed). As the sun begins to shine on the solar array, the DC input voltage gradually rises until the relay energises and the AC contacts close (about 19v on my 24v relay). At this point, AC is applied to the inverter and normal GTI operation begins. As the relay only requires 20% of it’s rated coil voltage to maintain it’s closed state once energised, the relay contacts will stay closed if the DC voltage drops due to overcast conditions. At the end of the day, as the sun sets, the DC input voltage will slowly drop. When the DC input voltage drops to 5-10v, the relay contacts will open and AC is removed from the GTI, effectively putting it into sleep mode until the following day.

SSR theory: With a simple resistor voltage divider circuit (variable resistor on one side of the divider) across the DC/Solar input, the user can adjust the 'Wake-Up' voltage to any point between 3.5v - 30V. On the 24-55v GTI, I would suggest somewhere around 20V is a good point to turn the GTI on and off. Any higher than this and the GTI may cycle off everytime a cloud goes past the sun.