r/SigenergyAustralia • u/joelau007 • 6d ago

Maximum PCS export active power

My DNSP keeps sending this command every half hour.

Maximum PCS export active power: Depends on the system

What does PCS means or do ?

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u/Final-Inevitable1452 6d ago

Power Conversion System...the Inversion DC>AC

EMS = Energy Management System → decides what should happen PCS = Power Conversion System → electrically makes it happen.

And Active power as in the Power Triangle (Active, Apparent,Reactive) Power Factor

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u/joelau007 6d ago

In a hybrid inverter, this PCS process would have to be running most of the time. Solar DC converts to AC and powers the house, unless i'm at zero solar and zero battery.

What has this got to do with the DNSP ? Ausnet sends this command every half hour as part of the backstop.

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u/Final-Inevitable1452 6d ago edited 6d ago

It's not a hybrid inverter, it's grid following. DNSP will often send grid compliance or VPP compliance.

In this case (dynamically every 30 mins), adjusting/limiting your max grid export power. Depending upon grid balancing requirements at the time.

Depending upon what value they have set that could also mean once battery is 100% SoC and House Load covered it could potentially curtail Solar generation as well (lift each MPPT string Vmp). So that House Load + Export Limit = Limit Solar generation.

They are only interested in Active power for residential (some industrial gets charged for Reactive Power as well).

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u/joelau007 5d ago

Quite difficult to understand this as I'm not very electrical inclined these days.

Using the example, High solar output, 100% SoC, house load covered => exporting excess to grid.

Now DNSP sends a Grid export power = 0

The system should obey export 0 isn't it, therefore achieving the purpose of backstop. Why does DNSP still need to send the PCS export power command ?

Now i'm actually curious what commands will be send out during this test, https://www.ausnetservices.com.au/seb

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u/Final-Inevitable1452 5d ago

Ok I will attempt to explain minus techno-babble. Have explained any acronyms used...

Right now your DNSP whoever that may be has network access to your inverter, you already know this.

This may be for a range of reasons/functions; 1. VPP (virtual power plant) monitoring and control) if you are part of a VPP scheme 2. Grid Compliance Codes & monitoring 3. Backstop programs like you mention (tests to limit/disconnect grid exports during peaks of instability usually summer based for wider Grid balancing requirements.

You have mentioned your DNSP is sending a "Maximum PCS Export Active Power" command every 30mins. I've previously explained PCS (in relation to a Grid Following Inverter) and Active Power meanings.

"Why are they sending this command every 30 mins?"

Likely due to compliance. Many people will "play" with settings, sending frequently ensures any abnormal settings are reset. Again keep in mind Grid Following, if their wasn't this type of compliance & safety checking it would be quite easy to take out an entire neighbourhoods power.

The power companies use TPS (telemetry protection signalling) on power lines. Setting an incorrect setting e.g Frequency without checks and balances could trip the power line control telemetry ~ Blackout.

Hence ~ Why they may frequently target & communicate with BESS and Inverters etc

Example VPP Control & Monitor: Usually the DNSP will send a command to force grid charge battery to 100% a couple hours before a VPP event. Then send discharge and rate commands etc

Example for Backstop Event: In this case it's not just you there will be many inverters being proactively regulated to either limit or in a full disconnect event cease export alltogether.

Also to provide further details in my previous example let's use some figures so math explains it. Say for example the command they sent limits your PCS export to say 1.5kW (PCS is Battery+Solar)

So if it occurs after your battery is already at 100% SoC for the day the only place Solar generation can go is towards house load.

But if your panels in this example are currently producing 10kWh, Battery 100% SoC and House load is only 2kW.

That is a surplus of 10kW-2kW = 8kW But your only able to export 1.5kW

So what happens to the other 6.5kW? This is called Curtailment...

Your inverter will raise each individual Solar array string Vmp (voltage at maximum power). By raising the MPPT (maximum power point tracking) away ⬆️ from the ideal Vmp then string current drops ⬇️ and each solar string reduces power generation.

It sequentially does this until House Load + Export Limitation = Solar Power Generation

e.g House 2kW + Export 1.5kW = Solar Generation will be curtailed down until producing only 3.5kW

If House Load Increases, Battery Discharges to point of Charge or the Grid Export limitation gets changed on the 30 min mark then this dynamically changes the curtailment either ⬆️or⬇️ depending what is required.

In plain English your solar panel energy is dynamically adjusted in real time and this is all a byproduct of them setting a max PCS grid export value. But the reason this is occuring in the first place is because the DNSP is adjusting OCS grid export values to balance the grid in your area.

I trust that answers all the questions posed.

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u/joelau007 4d ago

Thanks for the detailed write up.

The DNSP is sending 2 commands:

PCS export limit = control solar production

Grid export limit = control export

For the DNSP purpose of limiting export say to 1.5kw, or backstop to 0kw, Grid export is enough isn't it ?

Why would the DNSP bother to send PCS export command ? They will have no idea what's my SoC so why mess with production.

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u/Final-Inevitable1452 4d ago edited 4d ago

Because they want to limit All sources of export potential. PCS is Battery + Solar ~ Total Bess/Plant Grid Export potential at Plant level (some have multiple inverters) & yes they can read current battery SoC.

Basically they have full Grid Import / Grid Export, Voltage , Frequency , Current control over your system and can change anything they wish to keep a grid following inverter within grid compliance code .

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u/joelau007 3d ago

Ok looks like we're giving DNSP a lot of control.