Maturity status |  |
Supported domains | HEMS |
Overview
The Time-Variable Import Limits feature allows customers to define a custom schedule to limit the grid power consumption. By setting specific import power limits for different times of the day or days of the week, the Energy Management System (EMS) uses available system flexibility (like batteries or controllable consumers like EVs or heat pumps) to "shave" power peaks. This aims to reduce demand/capacity-related electricity costs.
Defining Schedules
Limits are configured via the Tariff Timer API using the /systems/{systemID}/import-power-limit-schedule endpoint.
Granular Control: The user can define limits for different times and days of the week. For example 6kW during peak hours and 12kW during off-peak hours. It can also be differentiated by the day of the week, e.g. defining different limits for weekdays and weekends.
Local Time Alignment: Schedules automatically adjust for local time zones and daylight saving changes, ensuring your limits always apply exactly when intended. For this the correct
timezoneneeds to be specified, when creating the import power limit schedule.Automatic Fail-Safe: Import power limits are applied directly to the gridbox (via DER API constraints) independently from the cloud-based Time-of-Use (ToU) optimizations. Even if the ToU optimization fails, the local EMS will continue to respect the last synchronized import limit.
Optimal control: The defined limits are respected as an additional constraint in the ToU optimization, ensuring cost optimal behaviour while trying to respect the limits.
Sync Frequency: Import limit schedules are processed and synchronized every 30 minutes.
Example
Example
{
"timezone": "Europe/Stockholm",
"periods": [
{
"from": "00:00:00",
"to": "06:00:00",
"weekdays": [
"MO",
"TU",
"WE",
"TH",
"FR",
"SA",
"SU"
],
"maxImportTotal": 12000
},
{
"from": "06:00:00",
"to": "22:00:00",
"weekdays": [
"MO",
"TU",
"WE",
"TH",
"FR",
"SA",
"SU"
],
"maxImportTotal": 6000
},
{
"from": "22:00:00",
"to": "23:59:59",
"weekdays": [
"MO",
"TU",
"WE",
"TH",
"FR",
"SA",
"SU"
],
"maxImportTotal": 12000
}
]
}Interaction with Fuse Protection
It is important to understand how this feature interacts with the static fuse protection feature:
Conflict Resolution: If both a static Fuse Protection limit and a Time-Variable Import Limit are active, the system always applies the lower (more restrictive) value.
Scope: Physical Fuse Protection monitors power per-phase, whereas Time-Variable Import Limits apply to the total aggregate power at the Grid Connection Point (GCP).
Reserve State-of-Charge
To improve the compliance of the import power limits and let the end-user benefit from reduced peak-load costs, the ToU optimization tries to "reserve" some energy in the battery to account for forecasting errors (e.g. higher load and/or less PV power than expected). The cloud-based ToU control decisions will try to always keep the battery's state of charge above a defined limit. This minimum state of charge target is calculated by adding a reserveSoc (defined via the /systems/{systemID}/import-power-limit-schedule endpoint, which, if undefined, defaults to 0.15 or 15%) to the technical minimum state of charge defined in the battery driver.
Known Limitations & Technical Behavior
To manage user expectations, keep the following behaviors in mind:
Reactive Control: The EMS control logic is reactive. Brief power spikes beyond the limit may occur before the assets can adjust to compensate. This can also lead to small violations of the limit in the 15-min aggregated power values.
Flexibility Required: The system can only maintain the limit if there is "flexibility" available. If the battery is empty and the EV must charge to reach the target state of charge by the defined departure time, the limit may be exceeded to meet operational needs.
Asset Synchronization: If the GCP Meter asset is deleted and rescanned, the limits may not apply until the next 30-minute synchronization cycle.
Unlisted Periods: Any time window without a defined schedule is treated as unlimited grid import.
Visualization in XENON
The historical active import power limits (static and time-variable) are shown in the historical measurements view in the XENON Admin-Dashboard:
Administrators can also view the defined schedule (requires ImportPowerLimitScheduleRead permission):
The detailed view includes:
Active Limit: The kW limit currently being enforced.
Upcoming Limit: The next scheduled limit change.
Defined Schedules: The full weekly overview of programmed limits.
