Need Advice on Grid Tie Setup

Melvin,

Your system is a "Grid Tied" system in function (that is how those inverters work). If the load of the facility always exceeds the power generated by the solar array + GT inverters, then it will never back feed the utility.

Of course, you cannot guarantee this unless you know the loads well enough (i.e., these are "base" load that are always one and do not cycle, turn off for holidays, and technically, if you have a power failure, the loads come back on before the 5 minute timeout of the GT inverters).

If you have a "typical" utility connection/local distribution system, the utility interface is "bi-directional"--You can back feed the utility. There are in some cases utility protection devices that measure the direction of power flow and turn off in the case of the back feed (safety as there are sometimes parallel current paths in the distribution system to share loads/backup in case there is a failure in one distribution leg, etc.... But this appears to be rare--at least in the US). In some cases, the reverse power flow detection may be installed if there is a large backup generator in the facility (I believe is it sometimes done--Not my area of expertise).

Another issue--In the "olden days", the mechanical utility meters used for logging power were bi-directional. The meter would run "forward" if you consume power, and "backwards" if you generate power to the grid. Newer meters may:

• Meter may not run backwards--So you may not get any credit for power "sold" to the utility.
• Meter may run forwards if back fed power--So you may get charged for power you sell to the utility.
• Meter may fag/report reverse power flow to the utility--Utility may choose to take action against the company for feeding power to the utility.
• Meter may attempt to figure out that you have GT Solar--In the US and other countries, the utility can (worst case) disconnect your power if you have any solar power GT connected to their utility power--Whether you back feed or not.
• Meter may have peak power/peak VA logging--For commercial billing in US, there are "reservation charges", basically peak 15 minute power usage in the last 1 or 12 months... Roughly equal to 1/2 the bill. For locations with large solar array, the solar back feed can be higher than the loads, and actually drive up the cost of utility connection (utility does not "care" which direction power/VA flows when calculating bill).

A standard GT inverter will always feed the maximum available solar array power to its power connection in the main panel/breaker box. A GT inverter cannot tell where the power goes (to local loads, out the utility connection, or shared by both). It will only turn off if the utility power frequency is ~1.0 Hz +/- over base line frequency or if the AC power is outside of allowed voltage (for 240 VAC US circuits; over 264 VAC or lower than 212 VAC).

-Bill