PV Grid Tied Inverter System Not To Export, Can this be Done?ch nhveng ng 4

[ThePunisher]

Hi all,

I have a customer with 300 KW PV inverter system and they are planning to interconnect such to the grid. Their equiopment is not IEEE 1547 or UL 1741 tested for grid intertie.

I am thinking that if their inverter capacity or maximum output is less than the peak load or the inverter output can reduce current output during light loading to prevent export to utility, then it would eliminate complicated requirements for interconnection. However, is there an inverter that does that?

Alternatively, I was also thinking about splitting the PV-fed AC panelboard from normal utility panelboard and apply open transition tie contactors in the event PV is insufficent to power local loads. But I am looking for a seamless way of connecting inverter and utility without exporting so that the PV system is only for minimizing utility import only.

Regards

 

[waross]

Yes there are inverters that will do that.
Here is one unit. Beware this will not work properly on unbalanced lines. Not suitable for a 240 Volt inverter on a 120:240 Volt system.
There was a recent thread that explored this option.
thread248-430843
Look towards the end of the thread.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ThePunisher]

Thanks Bill,

I forgotto mention in my OP that the 300 kW “inverter” is actually 3 x 100 kW smaller inverters.

If I follow the case at the end of the thread, assuming the inverter group control system is able to load share, each inverter will have utility inputs and the outputs will be connected to a common bus. As I understand the thread, I am thinking as follows:

1. Each 100 kW PV system group connected to each inverter DC bus inverter
2. Each inverter will somehow be programmed to operate directly from PV
3. In the event of low PV output or evening, power will be taken from utility
4. Option to provide battery bank on each inverter with PV charging the battery bank and supplying power to DC bus for loads....in this case need larger PV capacity

Is this understanding of mine on the thread sound right?

On the other hand, what about having a smart meter to compare load vs PV generation and when PV generation > loads, will execute command to shed PV generation via PV solar cell disconnection or inverter output limitation?

Or use load bank during light loading?

Many stuff in mind, none proven to work perhaps today.

 

[ThePunisher]

And by the way, the above setup would likely be a converter-inverter setup or a UPS like

 

[waross]

If you can connect the inverters for 277 Volts or 347 Volts and use one on each phase of a 277:480 or 347:600 Volt service it may work well.
Have the inverters been purchased?
What voltage are you connecting at?
Are we discussing single phase inverters or three phase inverters.
If you are retrofitting an existing installation it becomes very challenging.
If you already have the inverters, what is the make and model?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ThePunisher]

Thanks again. No, we have not bought the inverters yet and this not a retrofit either. We have plenty of time to specify the right thing and get further assistance from manufacturer.

The service voltage for the loads is 600 V, the utility service is coming from 34.5-0.6 kV Delta-Wye grounded, step down transformer. The 600 V system will be 3 phase 3 wire, 60 Hz from solidly grounded system.

I am a bit confused of your statement:

“If you can connect the inverters for 277 Volts or 347 Volts and use one on each phase of a 277:480 or 347:600 Volt service it may work well.”

A sketch SLD may help me understand what you are getting at

Many thanks, star for you.

 

[bacon4life]

Why don't you want to purchase IEEE 1547 complaint inverters? Even if the panels do not export power back to the utility, the inverter would need to be listed if it will ever be in parallel with the grid.

 

[waross]

The inverters in the link use a current sensor monitoring one of the lines for current level. This information is used to control the output of the inverter. Monitoring only one line can be a problem if the line currents are unbalanced.

The service voltage for the loads is 600 V, the utility service is coming from 34.5-0.6 kV Delta-Wye grounded, step down transformer. The 600 V system will be 3 phase 3 wire, 60 Hz from solidly grounded system.

A neutral will be available if you chose to connect it.
Your line to neutral voltage on a 600 Volt system will be 347 volts. Three inverters could be connected from the line to neutral.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[FreddyNurk]

I have no knowledge of the US voltage variations, or the specifics of IEEE 1547, however; what you're requesting happens quite often in Australia, albeit to different required standards.

Most of the utilities here have set an arbitrary figure of 30kW, above this figure, standards compliance and / or additional protection is required. Quite a number of manufacturers make protection relays for this application, though whether they're suitable for use in terms of IEEE 1547 or UL 1741 I don't know.

At that sort of scale of inverter, it should be possible to arrange for some sort of power metering that is capable of interacting with the inverters in order to regulate the output so as to not export to the network. Ideally the inverter manufacturer already has a suitable product. At about 100kW per inverter size, in my experience they'd be 3 phase rather than single phase, along with all the considerations of a grounding system for a 3 phase connection (the last inverter I saw that size was ungrounded, and had IEEE 1547 compliance. It relied on DC bus ground fault detection, and needed an isolation transformer to connect it to the network).

I'd also imagine that the relevant utility and what they permit will have a great influence over what you can do. I can also advise that attempting to use equipment not compliant with standards often gets far more expensive, particularly when additional equipment is mandated by the utility as a result.

EDMS Australia

 

[jfpe]

What country are you working in? Any utility in the US will require an interconnection agreement an UL listed equipment even if the system will not export.

Also, what is the minimum load in you facility. I've seen systems where the inverter is connected through a shunt trip breaker and the installer places a reverse power flow relay at or near the service entrance. If the power flow goes negative, the relay trips the inverter breaker.


JFPE

 

[FreddyNurk]

I feel obliged to add, that whilst waross' link shows a good discussion on some of the inverter options, there's a good couple of orders of magnitude size different here. The thread referenced was originally discussing systems of up to about 1kVA, whereas the OP states 100kVA... 100kVA inverters 'shunt' connect to networks, whereas a lot of the smaller units similar to waross' thread link do a 'series' connect.

EDMS Australia

 

[waross]

Try this link:
Link
There is a 600 Volt model available.
Contact the manufacturer for information on running three units in no-export mode on the same system.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

I'm a little confused on your project. If you are interconnected to the utility (running in parallel), the import/export of power is mostly a commercial issue. The technical interconnection requirements for protection and safety don't really change much in my experience. Only difference to the system is the net kW crossing the meter. I'm sure inverter controllers can be configured to curtail output to have no net export. But I doubt that will have much impact on interconnection requirements.

 

[ThePunisher]

Thanks Freddy for your supplementary inputs. I have not come in contact with the PV system manufacturer but, like you, bacon4life and JFPE mentioned, these Grid-paralleled PV systems will be tested, listed and certified for such purpose.

As they are following Grid voltage for their output reference, they must not actively regulate voltage at the PCC.

Having metering that actively senses the local loads they serve and communicates with the inverters to somehow “shed” generation will be ideal and a reverse power relay that will trip the main breaker at PCC will work as well.

I do not know up to how much reactive or active power can be backfed without affecting the Distribution Utility, I do not have such specified in any of their standards.

 

[racookpe1978]

If you have (or are thinking of mounting) three different DC-AC inverters, how are you going to "parallel" them together? They will not automatically startup in phase.

You will need each of the three inverter AC outputs to be separately and permanently wired to their own part of the breaker panel with their output never to be connected.

 

[waross]

The old school electro-mechanical meters in some applications made assumptions. The standard residential meter in North America for use on 120:240 Volt service assumed that the voltages from line to neutral were equal. If, for instance, there was an unbalanced load and a greater voltage drop one one line than on the other, the metering was not as accurate.
eg: With voltage drops, one line has 120 Volts and the other line has only 114 Volts, The meter would assume that each lines had 114+120/2= 117 Volts.
The heavy load would be metered at 117/114 Volts or 102%.
The light load would be metered at 117/120 Volts or 97.5%
This was not a lot of error and was in the favour of the utility.
There was a scheme to meter unbalanced four wire three phase with a two element meter which made similar assumptions about the line to neutral voltages.
The old meters would run backwards when power was unintentionally exported.

The electronic revenue meters are able to accurately meter and record both positive and negative power on each line.
The default setting on many electronic meters is to sum the positive energy register with the negative energy register.
If you unintentionally export power you will be charged for the power that you export as well as for the power that you import.
racook; Many inverters have a provision to run a synchronizing signal so that they may be used in parallel.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[FreddyNurk]

racooke, parallel in this context isn't the same as trying to get synchronous machines in parallel.

Its entirely possible to run multiple inverter units connected to the one site, they will all 'self synchronise' with whatever is connected to their terminals, and in the absence of any control arrangement, will run as hard as they can based on whatever the connected solar capacity is on the DC side.

If there's no restriction on export, this is exactly what happens. If, however, there is a restriction on exporting, then some means of coordinating the output in order to not exceed the export limits at some point of coupling is required. Most of the time either hard limit reverse power protection is used, or if possible, some sort of coordinating device can be used to control the inverter outputs. The coordinating device doesn't replace any utility requirements for reverse power though.



EDMS Australia

 

[ThePunisher]

Thanks DPC for your comments. As your comments indicates that impacts of excess power from the PV system exported may be incidental to cause any adverse effects on the distribution utility system, there is no baseline limit or cut-off specified by the dist. utility.

If none is specified, we may either end up conducting a distribution impact assessment or ensure not to export power and ensure controls are in place to prevent the inverters to regulate the voltage at the PCC.

Depending on where these PV systems relative to the dist. system and whether there are there are line drop compensators and/or reclosersin the line circuit where these PV systems are connected to. Without a cut-off limit established by dist. utility, the impact of the grid parallelled PV systems cannot be determined without further studies.

Hence, having not to export will greatly help.

 

[ThePunisher]

HI Warross, DPC, JFPE and all,

This is also what I have in mind, as a follow up of the above post. The O/H line circuit where the 300 kW PV facility is interconnected to does not have an voltage regulating device or reclosers along the line. I am thinking that If the power flow does not reverse towards the source substation of the distribution utility and the flow goes towards the nearest load, I don not think its a big of a deal anyways for the distribution utility.

As long as the PV facility short circuit contribution and power flows (and voltages)are checked at the distribution utility to ensure that all equipment and performance characteristics (by simulations) are within specified limits...it should not be a problem. In such case, the distribution utility and facility owner should come to certain agreements.

Also, as long as the PV facility will have some form of sensing capability to detect the distribution utility line tripped or open-circuited (islanded) to allow the PV system to isolate itself from the distribution utility line, then it should not be a problem.

These are my thoughts after reading portions of the IEEE 1547-2003.

Comments?

 

[waross]

What matters is not what we say but rather what the utility says.

Bill
--------------------
"Why not the best?"
Jimmy Carter