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Utility interconnection requirements for large scale solar
3

Utility interconnection requirements for large scale solar

Utility interconnection requirements for large scale solar

(OP)
I was hoping to get some input from the utility guys on this one.

Basically I was wondering if there is a special set of requirments for interconnecting large scale PV solar systems into the utility system.  I know that most utilites have interconnection requirments and in looking at some of them online they seem to address the interconnection of small or large "generators" or generating stations onto the utility system.  I've seen the interconection requirements for different levels with small less than 10kW requring not much more than a disconnect up to large greater than 2MW systems requiring a study and certatin proteciton requirments for interconnecting.

I was curious though if the requirments are differnt for interconnecting an inverter based PV system as opposed to a generator system.  I'm not sure if there are different requiremnts necessary for the solid start interver as there may be the the rotating generator.

I appreciate any input or experiences.

Thanks

RE: Utility interconnection requirements for large scale solar

2
If the PV system is built to UL 1741, it generally complies with most utility interface reequirements for protection.  I'm not sure of the size limit of UL 1741, if any.  IEEE 1547 has general requirements for interconnection protection.  

The main electrical difference is that PV inverters are not really capable of sourcing a lot of fault current (compared to a generator), and their control systems are designed to shut down quickly on loss of voltage or excess current.  

Also, refer to Article 690 in the latest (2011) NEC.  

 

RE: Utility interconnection requirements for large scale solar

IEEE 1547 and UL 1741 are only intended for small-scale grid interconnections of distributed generation. Below is a quote from Section 1.3 of IEEE 1547:

"The criteria and requirements in this document are applicable to all distributed resource technologies with aggregate capacity of 10MVA or less."

I've done a couple of integration studies for connecting large(ish) PV farms (30 to 80MW) to the European grid, and the grid code has some essential requirements for large PV (or wind) farm integration that is not required by IEEE 1547, for example:

1) Reactive power capability - the PV farm has to be able to supply or absorb reactive power within certain pre-defined P-Q limits.

2) Low voltage ride through (LVRT) or fault ride through (FRT) - the PV farm must stay connected to the grid during a momentary fault or voltage dip at the PCC. Newer PV inverters will most likely be equipped with reactive power / voltage support during a fault, but that's typically not a grid code requirement (yet).

3) Voltage stability at the PCC

I'm not sure what the requirements are in the US, but I'm guessing that the transmission system operators are more or less wanting the same things. I read somewhere that the US has traditionally lagged behind Europe in the grid integration of wind and PV, and have poached all the lessons learned from past euro experiences.

For further reference, you can have a look at the UK National grid code (http://www.nationalgrid.com/uk/Electricity/Codes/gridcode/). It is a truly turgid document, but you'll find some tidbits about grid connection of non-synchronous generating plant.

RE: Utility interconnection requirements for large scale solar

(OP)
Thank you dpc and juleselec both for your valueable input.

I was curious how close UL1741 and IEEE 1547 are in content?  I have IEEE 1547 and am familiar with it, but do not have access to 1741.  I am wonding if it is basically the same information.

juleselec

I very interested to hear about your previous integreation studies since this is sort of what I am looking into.  I'm wonding what are the criteria for having a utility integration stduy performed?  Are they required for a certain size system, type of system, distribution/transmission POCC?  Natrually you would not perform an integration study for small systems such as a 100kw system so I imagine it the larger systems where this is looked at.

I'm also curious what aspects are looked at with the integration study.  I imagine fault contribution, load flow, harmonics etc... are looked at.  As you also mentioned above it sounds like reactive capability and voltage support are looked at as well.

RE: Utility interconnection requirements for large scale solar

UL 1741 and IEEE 1547 have supposedly been "harmonized" so they should be consistent.  UL 1741 contains more specific information on performance requirements, components, etc, but should not conflict with IEEE 1547 in any significant way.  It is more a concern to the manufacturers building the PV inverters and controls.  

RE: Utility interconnection requirements for large scale solar

Juleselec, I supose we have poached experences from Europe on grid interconnections. However, I haven't seen the same from the many wind and solar developers.

What I have seen is a more consistent product from the manufacturers to meet the somewhat common regulations. Which is good.

I believe the IEEE 1547 should be a minumim standard, and for larger farms, or fields, there should be additional requirements. These requirements should align to solve the problems expected with a larger instalation.

But if the additional requirements go to far, what would stop a developer from breaking the farm or field into several 10 MW instalations along your system.

The 10 MW limit, above which there maybe dispatch problems, and changes in system var requirements with generation, as well as other possible system issues.

RE: Utility interconnection requirements for large scale solar

(OP)
In looking through IEEE1547 it appears that it covers both utility interactive (line communicated) inverters and stand alone or islanded inverters (self communicated) inverters.  I had previously thought that this document only dealt with grid tied line communicated inverters?

It discusses inverters cablable of supplying reactive current and its effects on voltage drops/rises.  I did not think grid tied inverters are capable of supplying reactive current so I assume the self communicated inverters are the only ones cable of providing reactive current.

Also for fault consideration it disscuses self communicated inverters being cabple of supplying sustained fault currents for a longer period of time since they are not dependent on voltage synchronization being provided by the utility.  

I'm also curious what considerations there are for determining what voltage level to interconnect distributed genteration at?  Are there different factors to consider for interconnecting at low level distribution vs high level distribution or even at transmission levels?

RE: Utility interconnection requirements for large scale solar

Rockman,
The requirements will vary widely depending on the location and the utility.  You really can't make any predictions until a system impact study in complete.  I have seen requierments for low voltage ride through, direct transfer trip, and power factor control required by various utilities.
-JFPE

RE: Utility interconnection requirements for large scale solar

Rockman,
I agree with jfpe, the requirements for a study will largely depend on the transmission system operator (TSO). We don't choose the studies that we perform - rather, we are contracted out by the PV farm developer, who in turn gets their requirements from the TSO. The TSO will have their own rules and grid code, which may differ from other TSOs.  

RE: Utility interconnection requirements for large scale solar

Rockman:

It's really going to depend on the specifics of your project, and the utility (and/or TSO) you're hoping to interconnect with.

First you contact them, and tell them everything about your project.  They will probably still have questions.  Like do you also need transmission service to get your power from the interconnection point to your customer location?

Eventually they will tell you how much you pay for them to do the system impact studies.

Once you pay, they will do the studies, and then tell you everything else you have to pay for to mitigate your impacts to their system.

You will ultimately agree to this in your interconnection contract.

Meanwhile one of the 10 other developers attempting to do the same thing may get to the finish line first, in which case you need to check if you still have your financing.  And your customer.

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