PV backfeeding the grid 2

[stevenal]

Situation:
Utility customer meter is occasionally registering reverse power flow and PV panels are visible on the property. The customer has not gone through the net-metering approval process, so the utility has no record of the installation or knowledge if NRTL products were used, what standards were used, or if the AHJ has had a look at it.

The customer claims the PV system is isolated and has no desire to sign up for net-metering. The utility is reluctant to perform any checking on the customer side of the meter.

My first concern is regarding lineman safety, who might think lines were de-energized when not. Utility says the linemen test and ground without fail, so there is no concern here.

My next concern is for the customer. What happens if the inverter continues to operate in island mode during an outage and is out of sync when power is restored? While I’m aware that such occurrences are hard on synchronous machines, what happens to electronic inverters?

Any guidance is appreciated. Happy Independence Day.

 

[Heaviside1925]

OP,
I won't go down quite the rabbit hole I was considering but if it's a rural residential customer, with an "off-grid" type PV set up, they most likely have generator feeding an all-in-one MPPT, inverter, battery charger combination unit. The generator being auto started by the unit if the batteries get too low. There could be an accidental crosstie with the generator circuit and that's what is causing the back feed. I asked about a barn, or a machine shed because even though they have a PV system, they still have a grid service. I would suspect the PV system is supplying the house and the grid service is supplying the barn. The separation of the house and barn when installing the PV system could also be where an accidental crosstie could occur. Hopefully your meter foreman has some success in diagnosing what is going on, I just wanted to provide some ideas of what could be occurring based on past experiences with rural customers. Also, from some linemen I knew who worked for rural co-ops, when they would encounter a back feed line during an outage, they would stop and go find the farm or house with the lights still on and have a very direct conversation with the customer.

 

[cranky108]

There again, most home generators are too small to even excite the pole top transformer.

 

[wayne440]

There again, most home generators are too small to even excite the pole top transformer

I respectfully disagree. A typical 15kVA 7200 to 120/240 transformer will happily produce 7200V as long as there is no significant load on the line when a 2kW generator is connected to half of the secondary.

 

[JJ_Roy]

The company has a couple of live line demonstration trailers like this one (

https://www.youtube.com/watch?v=RNoSCkA8QlI)

that are run by a 5500W generator that back-feeds a 10kVA to feed another 10kVA to energize a secondary to light a streetlight, turn a meter by running a blender and a lightbulb, etc. It's also used to make arcs and blow fuses on the primary as well.

If you go to around 7:00 in the video, they energize the primary with one of those little "camping" generators. Just like wayne440 noted......

 

[cranky108]

So what are you calling a significant load? Typically a pole top transformer is connected to a whole distribution circuit.

 

[stevenal]

Typical is not the question. People pull out their generators and suicide cords when the situation is atypical. My service transformer serves my house and my neighbors. A third neighbor is on the same fused primary tap. If a squirrel gets into the wrong place, the tap fuse will blow causing loss of service to three houses. Any one of us might be tempted to connect a generator to keep some power on while waiting for the Serviceman. If done wrong, and the loading allows, the generator might backfeed the tap line without tripping.

 

[waross]

In rural areas, where I lived for over 10 years, it is common to have miles of single phase primary serving widely spaced farms or ranches.
In the event that a farmer taps a distribution pole with a 40 foot wide tillage machine and brings down a wire, there may be only one or two transformers left on the affected line.
A back-feed would energize the broken wire that the linemen would have to repair.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[iop95]

Most probably it's a grid-tie inverter with so called "reverse power limiting device" that send signal (RS485/Modbus) to inverter to reduce it's power to avoid reverse power when local consumption decrease and/or PV panel production increase.
Communication between device and inverter may be lost for some reasons and in this time frame, inverter power is not limited.

 

[Alistair_Heaton]

As a DIY solar installer and owner in Europe.

I suspect they have a AliExpress power transfer switch possibly automatic.

We have issues with the neutrals and these systems as the owner uses the same neutral as the grid where I am.

Two linesman got zapped, one with life changing injurys and now if you have power transfer for power outage the setup needs tested every 12 months.

It's relatively easy to get zero export inverters these days with a CT clamp just after the meter. But it's only on the live nothing on neutral.

I don't know much about the USA power setup.

My system I installed is 2 8.5kw 400V 3phase 50Hz inverters with battery on one, just for reference what I do know about. My system is grid tied without essential services output on the hybrid battery inverter. If the grid goes everything shuts down.

 

[3DDave]

Just to learn more about this: What is the downside for a customer to sign up for net metering?

 

[Alistair_Heaton]

Basically cost.

It's a common theme for DIY solar and grid tied round the world.

The USA paperwork side of things and sign offs by the electricians is collosal. Your over double the cost of the hardware.

My 28 panels and 8.5kw inverter latest installation cost under 5000$. If I had used a installer it would have been 11k in Estonia.

USA solar groups there are even more restrictions to do with inspections and fire code compliance etc.

The UL hardware is also restrictive and costs alot more than a simple Chinese string inverter.

 

[waross]

What is the downside for a customer to sign up for net metering?

It depends.
It really depends.
One factor is tariffs.
Example, Two adjacent Canadian Provinces.

A: Alberta;
Fixed costs; over $100 per month. Yes, if you use zero KWHrs your bill will still be over $100 per month.
Net metering; Exporting rate is 1/4 of importing rate. You must export four KWHr to recover the cost of importing one KWHr.

B: British Columbia;
Fixed costs; about $30 per month.
Net metering; one for one. Each KWHr exported pays for one KWHr imported.

That said, one person in Alberta has an interesting solution:
They DO NOT have a grid connection.
Their house runs on inverters and batteries.
They have solar panels to charge the batteries.
They do have a natural gas service.
They have a natural gas driven generator that starts automatically when needed to charge the battery bank.
They have a second natural gas driven generator as backup.

The home is partial off grid as it is serviced with natural gas but no electrical services. Electricity comes from roof mounted solar panels with a rated output of 6kw which feeds two large sets of Rolls battery banks with a combined storage capacity of 39kwh. The solar system is controlled by a Schneider XW 6848 invertor/charger 120/240 volt 6.8 kw and is supplemented with two AutoStart Generac N/G generators. One generator is 11 KW and the other is 16 KW. Only one generator is necessary to supplement when solar generation is low, the 2nd generator is simply meant to be a spare unit. TOTAL GAS BILL FOR LAST YEAR WAS $2750 which averages $229/month for utilities.

Link to listing


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[bacon4life]

Customer wiring in the USA must typically comply with the NEC and as well undergo an electrical inspection. Regardless of whether a customer adds new loads or solar, customers are legally required to use listed equipment and have the installation inspected. Thus I think it would be unfair to include these costs as a cost of solar net metering.

Signing up for solar net adds several complications:
1)As Waross described, production and consumption may have different prices.
2)Signing up for solar net metering requires interfacing with the utility provider. There is often a fee associated with the application, it takes time to do the application, and there can be delays waiting for the utility to approve the installation and/or replace the meter.
3)Utilities may have additional requirements (e.g. disconnect switches or production meters) for solar installations that exceed the minimums set the the NEC. Some of these requirements add hundreds of dollars to the project cost.
4)Signing up for a met meter tariff often obligates the utility to keep that tariff available to the customer for 10 years or more. Customers who signed up for early version of net metering tariffs get large subsidies whereas eventually net metered customers will connect under tariffs that are updated each year to reflect the current cost/benefits of solar.

 

[OperaHouse]

On solar forums, the discussion of grid tie inverters in the US set up for zero export and exporting power is often a topic. Theories about response time, exporting on one leg and drawing on the other, and setting export too close to zero are discussed. How these machines are set up apparently leaves a lot of room for error. The majority just don't have any technical understanding of what they are doing. Industry has made it pretty much plug and play. However, they can't quite grasp that 400A connections can't be made willy nilly.