Solar Farm Interconnection Voltage Selection

[electroctopus]

Hey power engineers- probably a very amateur question- is it practical to interconnect a 85MWac solar farm to a 69kV line or substation (provided the line/substation has the capacity)? Or should we be looking at larger voltages like 115 - 138 kV?
(Also, any references on instructions, tips or thumb rules for selecting optimum interconnection voltage for power plants would be greatly appreciated)

Thanks in advance.

 

[MKFPE]

It can be done, Practical? Depends. You need to do a power flow study to determine if voltage drops and power losses are acceptable. It comes down to economics and that is different in every situation. I doubt a rule of thumb is of much value. You need to do the math, comparing options of installation costs vs. operating costs to get the project lifetime cost.

 

[JJ_Roy]

The biggest thing is not to exceed the bus (or equipment) current rating for the connection voltage for your interconnection. I see it all the time where an interconnection developer wants to put 3500A into one POI on a 3000A rated substation bus.

Second biggest is what MKFPE said, is the economics... I recommend finding a consulting company that familiar with the interconnection study process to do 'screening studies' for the potential POI's to see what your interconnection and upgrade costs may be for a potential project. ISO interconnection studies are going to get a lot more costly with the new FERC Order 2023! It's a real bummer to find out your project is not economically feasible due to upgrade costs after spending $250k in the ISO interconnection process. I know spending $$$ for good consultants is a tough nut to swallow for some developers but it often pays for itself over the long run.

 

[cranky108]

Assuming you are in North America, The threshold for NERC is 75 MVA, so all the lines between your farm and the grid will be considered BES. Most equipment above 100MVA typically uses distance relays which would make it easer to comply with PRC-023. That said many times 69kV will use distance protection, but will have so many load taps, that it may not be possible.

 

[bacon4life]

The optimum transmission voltage is often whatever is already close by. The US Department of Homeland Security publishes a transmission map at

https://hifld-geoplatform.opendata....s/explore?location=47.886006,-121.600023,7.92

Historically interconnection studies focused on what would be optimal for the next single generation plant being added. One ongoing change is that many interconnection studies are now done as pooled/batched studies, where the focus is what is the best way to accommodate everyone. Such a shift may result in building brand new higher voltage lines rather than incrementally increasing capacity of lower voltage transmission lines.

Lines at 69 kV and 138 kV tend to be single overhead conductors, so the easiest cap to check is the thermal limit on the conductors. Large conductors can carry more than 700 amps, but many 69 kV lines have small conductors that would be rated at less than 700 amps.

There are observed ranges regarding the relationship between Surge Impedance Load (SIL) and typical loads on a transmission line. That constraint likely means 69 kV might only be practical if it runs fairly short distance before interconnecting with higher voltages. These sites have more info on typical loading versus SIL:

https://voltage-disturbance.com/power-engineering/surge-impedance-loading-of-transmission-line/

https://web.engr.oregonstate.edu/~webbky/ESE470_files/Section 4 Transmission Lines.pdf

 

[wcaseyharman]

That’s over 700A at 69kV, seems like you’d need pretty big wire and no other generators could be on the system.
In general, at least in my utility, higher voltages are more reliable systems. I’m not sure what other utilities are doing, but ours is attempting to move away from sub transmission voltages like 55kV, so the equipment on those systems are old with limited spare parts.
Those higher currents on a potentially weak system will bring issues such as var absorption, voltage stability, and possible angular stability based on the system configuration.
We had a larger wind farm (150MW) on a weak 115kV system and we ended up running into voltage stability / controls stability issues and had to install a STATCOM to reduce voltage swings at high output power.
These problems are generally less pronounced at higher connection voltages because the system is stiffer. Ultimately as others have said systems studies will show the practicality of the interconnection. I’m

 

[crshears]

The utility controlling the circuits you want to connect to will almost certainly call the shots on this; Special Protection Schemes will very likely be required, since your facility would be affecting / influencing power flows to other customers / users, particularly if a 230/345/500 kV circuit is being used as a sales path; the additional power input could well cause congestion on that line, adversely limiting the ability of the Independent System Operator to authorize commercial transactions on that asset.

Cost somebody else money and you can expect to get into a sticky situation . . .

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]