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Solar Inverter Curtailment Scheme

Solar Inverter Curtailment Scheme

Solar Inverter Curtailment Scheme

We took on a small project to help curtail a solar array. The available kW (360) of the solar array exceeds the export limit set by the utility, (150kW). The plant load varies between 50-250 kW. There are no batteries to dump excess energy to. There is an intertie relay with CT's that are at the secondary main of the plant. There are 6 inverters to be controlled. A small PLC (SEL RTAC) was installed to handle the protocol conversion - the Inverters are Modbus TCP and the intertie relay is a SEL-that can speak a number of protocols over a number of channels. All devices are ethernet based communications on a managed switch within a VLAN. We have remote access to the PLC, but not the relay (that will be fixed).

What we are struggling with is being able to get the system to respond quick enough to avoid being dumped by the forward power intertie relay trip (2 sec). A call to system protection did not alter their thoughts on the time delay. There isn't any kind of irradiance sensor. Under lower output conditions, we are able to write to the inverters to curtail and how much. The minimum ramp time is 1 second. Currently, we start and stop based on a power level (relay status bit) to begin writing data and triggering start.

What complicates the situation is that the inverter breaker is a manually operated MCCB with a shunt trip. It's a 2.5 hr round trip to reset the breaker. The Inverter OEM's support staff don't seem inclined to provide guidance. I've read the manuals of similar inverters and 5-15 seconds is not unusual. From the research I've done, similar sized sights may have a battery system. Is this necessary when curtailing?

Solar is new in our area, so the design flaws our firm inherited are not as easy to fix as this project has a constrained budget.

Since Modbus doesn't time tag, I'm using SEL Virtual taqs in the RTAC to display what I hope is some kind of time reference to assist in analyzing via SOE. Until I have a serial connection to the relay, I'm limited in access.

I'm wondering if others have had to curtail Solar quickly and what were the control strategies?

Best Regards

RE: Solar Inverter Curtailment Scheme

Unfortunately as you've discovered, if the solar inverter system isn't fast enough (or its communications system isn't fast enough), there's little you can do.
I've had colleagues who have managed to get faster response by using native inverter communications protocols, but that is constrained on a per manufacturer basis. I also had a project where the proposed response time of each inverter was up to 60 seconds on a 25 inverter site.

There may be a possibility of retrofitting a suitable size contactor behind the MCCB, providing this is acceptable to the utility. This would allow you to close back on rather than dropping the site, although whether this is viable for you is another thing entirely.

You can't just peg the setpoints on the inverters to never be able to exceed 150kW aggregate?

EDMS Australia

RE: Solar Inverter Curtailment Scheme

Thank you for the response. We checked the prices for an electric reset accessory and it's 3x the cost of the MCCB. The contactor idea is something we will look into -perhaps on the refurbished market. The next time around, I'm referring the client to a former employer that can supply a Power Circuit Breaker, Electrically operated with a shunt trip and auxiliary contacts all in a single frame with the necessary CT/VT.

The Inverter OEM needed a reminder today that we discussed this problem 5 weeks ago, as they were playing dumb with us about response times. There is nothing like sending the same email chain back to them.

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