Your questions answered: Meeting isolation requirements for inverter-based standby power applications

On January 23, 2024, Consulting-Specifying Engineer hosted a webcast about “Meeting Isolation Requirements for Inverter-Based Standby Power Applications.” After the presentation, speakers Victor Bonachea and Robert Benavidez of ASCO answered questions from the audience. The following are answers to questions not answered during the live event.

Question: What NEC codes/sections apply to BESS? If a BESS is used for code required emergency power, how do codes differ from, for example, using a generator?

Answer: Article 705 and 706 discuss the use of microgrid and stored energy solutions for emergency power. Generator for most cases are single purpose and used explicitly used for standby power however these new energy landscape solutions often are used of economic reasons when not providing standby power. As a result they need additional controls, isolation needs and architectural considerations when being implemented.

Question: What are the challenges of using BESS as backup source?

Answer: The BESS and loads need proper isolation from other sources to serve as a backup power source. Additionally understanding that level of charge/runtime you will get from a BESS is critical considering that they cannot just be refueled to get added runtime.

Question: Are there any recommendations or requirements to separate BESS from building electrical service? Similar to emergency generator equipment separation.

Answer: Yes, Isolation is required. SourcePacT is the ideal solution since it is purpose designed and built for this task against the UL 3008 provisional standard. SourcePacT (Isolation Switch) serves to a BESS the same as a Transfer Switch to a generator.

Question: Are there any considerations for having UPS downstream of the BESS?

Answer: Architecturally, this is doable by having the inverter feed the upstream of the UPS just like a utility source. That said any specific interactions between the UPS and inverter would need to be reviewed by the UPS manufacturer based on the specs of the specific inverter.

Question: Would you need to be able to modulate the generation (PV) if the essential loads are less than the generation?

Answer: Yes proper control of a PV array is required and would need to be coordinated be done within the PV system (including its inverter)

Question: Is there a similar product that ASCO offers that can pair inverter-based generation and gas powered generation on the client side? for example, would you recommend this product for a client that has on-site cogen and solar (no BESS) that would like to isolate from the utility without turning off the solar inverters?

Answer: Depending on the desired sequence this can be supported in many ways by Schneider. Simpler smaller systems can be architected to work with a standard isolation switch like SourcePacT and a ASCO ATS but larger systems with more complex sequences can be served by solutions from the Schneider Microgrid team such as our Energy Control Center (ECC) solution our Ecostruxture Microgrid Flex solution as well.

Question: Is UL 3008 approved? Or is it still under investigation?

Answer: It is approved by UL and published as what they call an “Outline of Investigation” which in practicality is a provisional standard. It can be reviewed and used for listing like any other standard. As it matures and evolves and gets additional involvement it will then turn into traditional standard like UL 1008.

Question: I normally designed my system to switch the auto-xfer switch to a gen-set if normal power fails. What are the disadvantages of ATS switching to gen-set as compared with that of Isolation?

Answer: Transfer switches are used with generator because the sources most always be isolated from each other regardless of what source is connected to load. BESS need to operate in parallel with utility when operating grid tie mode but be isolated from it in grid forming mode. So while an ATS has really 2 key mode connected to source 1 OR connected to source 2 and Isolation switch allows them to be parallel in one state or isolated from each other in the other state.

Question: What are the fault ratings of the isolation switch?

Answer: Initial release of SourcePacT is 50kA for our 480v/600A design. However these will expand as we add more configurations.

Question: How would a transfer switch / generator work with the SourcePacT?

Answer: There are many ways these sequences can play out depending on how you architect that online. Please see the slide that shows some examples of those architectures. For a more detailed explanation let me know and perhaps we can setup some time to discuss.

Question: Is there a break during the transfer?

Answer: Yes this operates in a similar fashion to an open transition switch. A momentary break happens between the isolation event and reconnection events as the inverter switches modes.

Question: Do you have anything built in to the SourcePacT to prevent this from becoming a single point of failure?

Answer: SourcePacT and UL 3008 are designed to the same level of reliability and redundancy as a Transfer Switch solution.

Question: Can SourcePacT be built into a switchboard/switchgear as SE can provide with UL1008 solutions or contractor/PLC based throw over schemes?

Answer: Yes, our plan is to offer this open style in the future so that it can be integrated into other boards and gear.

Question: Is the device readable by metering apps like AlsoEnergy? can its status be displayed on their interface?

Answer: The initial release of this product does not support external comms however future FW upgrades will enable Modbus communication.

Question: Do the loads on the BESS experience an interruption when going from grid tied to grid forming mode and vice versa?

Answer: Yes this is true. This would be similar to how an open transition transfer occurs using ATS switching. In future versions this may change with additional feature sets.

Question: Inverters will typically turn off very fast when the utility goes down and remain off for 5 minutes waiting for a stable voltage. Does the SourcePacT send a signal to override that time to be utilized in under 10 seconds (similar to a generator) for emergency loads?

Answer: SourcePacT will send a signal to the inverter using a Form C dry contact to tell it if it needs to be in Grid Tie mode (looking for a utility source to sync to and run in parallel) or grind forming mode (product power independently of any other source and serve as a stand alone source). Of course you would need to the correct kind of inverter that offers these functions for this application.

Question: Can this system accommodate a generator for long periods of utility outage and the battery source runs out of capacity?

Answer: Yes, many architectures can be accomplished with SourcePacT including having a generator for use during an extended outage should you exhaust the batteries. Please refer to the slide with high level reference architectures for an example.

Question: Does it always disconnect the inverter on utility return or does it give synch command to inverter then close itself when the inverter is in synch with Utility?

Answer: Yes this operates in a similar fashion to an open transition switch. A momentary break happens between the isolation event and reconnection events as the inverter switches modes.

Question: Nirvana here is to use a BESS not only for backup power, but a fast-enough one to be equivalent to a UPS system to prevent computer loads from dropping offline. Will SourcePacT provide that capability so don’t have to “also” purchase/maintain a UPS, or is the switchover time still too long (e.g. over 30ms)? With an ATS and Genset, that is not possible because of generator startup time required, but with a BESS – might be possible.

Answer: SourcePacT is not designed for such a fast reaction time and neither are most BESS systems since the design of a BESS from a UPS are very different considering the difference in their intended reaction and run times.

Question: What language system does the SIS use to communicate to the BESS?

Answer: At this point the mode selection of inverter modes is accomplished using a hardwired Form C Dry Contact. Future versions will be considering adding digital comms protocols.

Question: Can you have a “generator first” architecture?

Answer: Yes that is possible and can be done with the existing components today but is a less common application due to the sustainability/green impact being counter to what most markets are demanding.

Question: Where will the inverter obtain a 60Hz reference signal? Currently the frequency reference is the utility line.

Answer: 60Hz reference is only need for Grid Tie mode and would be derived from the utility. In grid forming mode the inverter works independently from other sources and does not require a reference.

Question: What is the timeline for the signalling between loss of utility and restoration of power by the inverter based sources? Are there concerns about out of phase issues if you are switching quickly and have rotating machines that have not completely stopped?

Answer: Signaling would be dependent on customer time delays and inverter operation speed but SourcePacT can take its controls actions in approx. 1s assuming all timers are set to their fastest. Phases issues like you mention are possible and should be considered in a similar fashion to how someone would apply an open transition or delayed Transition ATS. Keep in mind large motor loads are more uncommon to be fed from BESS due to their high in rush needs and the in ability of inverters to support that in many cases.

Question: Do you see the source pack device operating and clearing in a sub-cycle time frame?

Answer: This more closely mirrors a open or delayed transition of an ATS meaning momentary outage will be expected during operation of SourcePacT. SourcePacT can operate in approximately 1s assuming all settings are set to their fastest, however this does not account for reaction time on the inverters side which would also need to be considered. These systems are not intended to replace UPS function since the designs between them differ greatly based on reaction time and run duration expectations.

Question: Does the SourcePacT have a programable delay for reconnection?

Answer: Yes. Similar to ATS the SourcePacT controller has user configurable time delays including one that will start upon the return of the utility source to delay the reclosure of the isolation device ensuring the utility source is stabilized and reliable. We usually see this sit around 20 mins but it can be customer set from 0 to hours long.

Question: Does this device become the new service disconnect for a facility and require the related N-G separation?

Answer: SourcePacT is not service rated because at this point UL 3008 restricts the isolation devices from serving as a service disconnect. This is something I expect to change as the standard evolves and that eventually SourcePacT will be service rated and comply to both the UL 3008 requirements as well as NEC 230.

Question: Is there an ISO communication protocol to communicate to the inverters? If not are there inverters that have been tested or recommended?

Answer: At this point the mode selection of inverter modes is accomplished using a hardwired Form C Dry Contact. Future versions will be considering adding digital comms protocols.

Question: Are you assuming all inverters are capable of “grid forming”?

Answer: Yes because that is the application this product is intended for. For inverters to serve as a backup source they must able to work in a grid forming mode. If the inverter does not support this then it cannot be used as a independent backup source and as such SourcePacT would not be applicable.

Question: Does SourcePacT carry a service entrance rating?

Answer: SourcePacT is not service rated because at this point UL 3008 restricts the isolation devices from serving as a service disconnect. This is something I expect to change as the standard evolves and that eventually SourcePacT will be service rated and comply to both the UL 3008 requirements as well as NEC 230.

Question: Is this only recommended if there is an inverter?

Answer: SourcePacT was designed thinking of solar/BESS inverters and how they require paralleling during normal operation but isolation from sources to work as a backup source. Other applications may be viable but would require a deeper review.

Question: Do you need external PTs on utility side to detected loss of source?

Answer: PTs are not required the controller in SourcePacT already has the necessary wiring to detect the line voltage directly.

Question: Are there any other product sizes beyond the 600A SourcePact switch?

Answer: Initial release is targeted to be 480v 600A 50kA. Additional voltages and ampacity options will be available soon.

Question: Does SourcePacT require a micro grid controller to determine when utility is out of voltage/frequency window and to isolate. Or is that Internal to sourcePacT?

Answer: That is internal to SourcePacT. The main concept behind SourcePacT is to have an all in one purpose built device that can do the necessary metering, logic and isolation actions without requiring any external controls or meters.

Question: Are there further complications if the generator ATS is designed as closed transition?

Answer: Depending on the overall architecture this could be something that requires closer evaluation between the BESS/Inverter and generator to ensure there are no issues if they were to momentarily parallel.

Question: Is SourcePacT available in ‘service entrance rated’ application? per NEC 230?

Answer: SourcePacT is not service rated because at this point UL 3008 restricts the isolation devices from serving as a service disconnect. This is something I expect to change as the standard evolves and that eventually SourcePacT will be service rated and comply to both the UL 3008 requirements as well as NEC 230.

Question: how much is your equipment for a 1000 amp service 480/277 volts? how much space does that take?

Answer: At this point SourcePacT is sized for 600A applications only sizing and pricing on larger sizes with be available in the future as those configs become available.

Question: With switchboard lead times exceeding 52 weeks what is the expected availability for SourcePacT?

Answer: SourcePacT as a standalone solution will be stocked in our warehouses at launch meaning any order should ship in less than a week.

Question: The National Renewable Energy Laboratory recommends to use PV+BESS system for peak shaving when utility is on. Can you comments?

Answer: Using BESS and PV in parallel with utility to reduce energy costs or reduced energy demand on the utility is a common application of those systems. SourcePacT would work well in these applications while in the close state allowing power to be used from both Utility and the DERs at the same time. The peak shaving controls however would be separate from SourcePacT.

Question: Any future plans to have this integrated into switchboard/switchgear construction to reduce footprint on larger capacities?

Answer: Yes, our plan is to offer this open style in the future so that it can be integrated into other board and gear.

Question: Are isolation switches only for ESSs or can they be used for other backup sources as well?

Answer: ESSs and article 700 systems as defined in NFPA are the most stringent in terms of requirements for backup. As a result in most other applications SourcePacT would also be acceptable as a result however the unique details of each project would need to be reviewed.

Question: Does the SourcePacT have the ability to delay a return to grid voltage in response to rolling brownouts so the inverter isn’t cycling between grid forming and grid following essentially seeing the brownouts in the system?

Answer: Yes. Similar to ATS the SourcePacT controller has user configurable time delays including one that will start upon the return of the utility source to delay the reclosure of the isolation device ensuring the utility source is stabilized and reliable. We usually see this sit around 20 mins but it can be customer set from 0 to hours long.

Question: Does SourcePact need to be rated for service entrance duty? If so, is it?

Answer: The UL 3008 provisional standard prevents the service entrance and isolation device from being the same device. This may change in the future but for now these need to be separate devices.

Question: What are the rated operating voltages of the SourcePact device and what is it’s fault current withstand capability?

Answer: Initial release is targeted to be 480v 600A 50kA. Additional voltages and ampacity options will be available soon.

Question: Why not use the main service disconnet to isolate?

Answer: In many cases a BESS is not rated to carry the full load of the facility, in this case use a service disconnect is not practical since it would overload the inverter. Additionally existing disconnects do not have the proper ratings, qualifications, or controls to do these functions.

Question: Can multiple units be linked to allow the system to serve multiple branches at the same time and giving priority. Similar to a hospital ATS system?

Answer: The isolation requirement is really only required to operate to isolate the inverter and essential branches from Utility so is only required at one point in the system since all the backed up feed would need to be down stream from that point. As a result what you are describing would need to function more like a Load Management solution to service those various loads.

Question: Is an isolation switch needed if your only function for the inverter is to power the life-safety lighting via UL924 relays?

Answer: An isolation switch is required to ensure the BESS does not backfeed utility regardless of loads. If the isolation is provided in some other compliant method then you are ok otherwise an isolation device is required.

Question: What examples have you heard from mis-applications of ATS equipment in the field for islanding inverter-based power systems?

Answer: Upon an outage and grid tied inverter must shutdown leaving no power available anywhere on the bus from either source. Most ATS solutions on the market today require at least one of the two sources to be energized to be used power a transfer between the source. In this case since no power is present the ATS would not be able to open or close to provide the required isolation and your inverter will be stuck connected to the utility and unable to turn on. Although their are workaround for this with ATS designs they tend to add unnecessary cost and complexity.

Question: So, for emergency mode, there is a “dead time” between inverter shut-off and isolation device signal that it is OK to resume? What good is that for a 24/7/365 application?

Answer: Various applications out there are able to accept such a delay. This is similar to the deadtime found while waiting for a generator to start or a transfer switch to transfer. Do not confuse a BESS with a UPS since these are designed very differently to accomplish different tasks. Exploring a way to accomplish this transition with a BESS with zero downtime may be something that is possible in certain applications with future versions of the product.

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