DG - Inverter Protection

[EITtraining]

Hi all,

Not sure where to post this...

I have some trouble understanding equipment protection on a grid-tied PV system, specifically when there's a backup generator behind a transfer switch supplying the same load.

UL1741 specifies that the inverter should disconnect if it senses undervoltage, frequency variation and that it should have anti-islanding protection for safety reasons in case of an outage.

But what happens when the backup generator kicks in? I'm assuming if the inverter is grid-tied, it will see this power, interpret that the utility is back and connect to the system. My questions are:

1) Will the inverter actually connect after the standby generator powers up?

2) Assuming it will... if both generator (backup and solar) are about the same size, and their combined output power is higher than the load, what will prevent them from feeding each other (remember i can't return any excess to the utility because there's an outage). How does this "reverse power" protection actually works?

3) Is there any control on inverters to reduce their power output? Maybe disconnecting modules and sacrificing potential output for safety reasons. Let's say you have a 10kW inverter system supplying a 6kW load at noon on a sunny summer day. Are inverters smart enough to detect this and control their output accordingly? How?

Can anyone help or point me to reading resources for this specific information? Feel free to insult if the questions seem stupid -

 

[davidbeach]

If, if, if.

If the generator can maintain sufficiently stable voltage and frequency for at least 5 minutes the inverter will reconnect.

If the generator knows nothing of the inverter and remains in load following mode it will see the inverter as negative load.

If the inverter can never power the entire load on the island, the generator and inverter will continue to share the load.

See If #1.

The answer is probably not.

 

[waross]

If the inverter is not frequency agile, you had best inhibit it from running with the generator.
With droop control mode, the frequency on a 60 Hz set will vary from 60 Hz to 61.8 hz.
Aha, but what about isochronous mode for a true 60 Hz. With 100% block loading, many isochronous governors will drop to 58.2% and then slowly recover to 60 Hz. With a 100% load loss they will rise to 61.8 Hz and slowly recover to 60 Hz. Unless the governor is quite sophisticated (and expensive) a basic isochronous governor will respond to any load changes with classic PI (proportional plus integral) response.
The proportional band will be 2.91%. This is equal to 3% droop. (1.8Hz/61.8Hz = 2.91% proportional band. 1.8 Hz/60 Hz = 3% droop. The normal set-point for a 60Hz droop governor is 61.8Hz.)
If your inverter is not grid frequency agile JUST SAY NO to running with the generator.

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

 

[redfurry]

The anti-islanding features on a lot of PV inverters work by attempting to modify the frequency output on the assumption that on a typical distribution grid, the frequency of the grid is fixed and will 'push back' at attempts to change it.

I suspect that when tied to a relatively smaller generator with soft frequency control, the anti-islanding features would not work as planned and the inverter would have a tendency to drop off.

It sounds like you want the PV (and your local generator) to keep producing power in islanded mode, so maybe you should turn off the PV anti-islanding features and instead use a transfer-trip scheme tied to the utility substation to control the breaker which will island your isolated system.

 

[EITtraining]

Hey, thanks for the quick feedback...

What I can take out from these comments is that:

A single standby generator HAS to be run in isochronous mode when isolated from the grid.

If an inverter cannot be run in droop mode, trying to run both in parallel would create a real mess in system frequency with both systems fighting for control and the inverter connecting/disconnecting constantly.

So the smartest thing would be to make sure the inverter is isolated from the backup generator in case of an outage if it cannot be run in droop mode.

If the inverter is run in droop mode, it would also disconnect constantly if the frequency from the backup generator is not stable enough. Depending on the relative size of both generators, the disconnection of the inverter could be seen by the backup generator as a sudden load increase, the inverter connecting back would be seen by the backup generator as a sudden load decrease, which would compound the problem of system stability.

Sounds like a big mess, so unless it's specifically guaranteed by the inverter designer with a signed contract, it's best to just make sure the inverter is disconnected completely from any backup generator in case of an outage.

What would be the best way to do this? Is there any option besides doing it through the transfer switch, isolating the grid and PV system from the standby generator at the same time?

 

[waross]

Run in droop mode if possible. Most users are not aware that there is a slight frequency variation from the diesel generator. Droop control parallels well and is dependable.

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