Computed variables
Computed variables
Besides the raw variables read from the devices, Cube automatically generates some computed variables: derived values the system calculates from other signals. They are created when the plant is imported and behave like any other variable — you find them in Trend, dashboards, synoptics, and the realtime views.
For a photovoltaic plant, computed variables live on two levels: plant-level (on the CONTROL device) and per-inverter.
The CONTROL device
Every plant has a special device called CONTROL. It does not correspond to a real machine: it is a synthetic device created by Cube, one per plant, used to collect the aggregated values of the whole plant. It aggregates all the plant's real devices (excluding itself) and cannot be deleted.
On the CONTROL device Cube creates these values:
| Variable | What it represents |
|---|---|
| Irradiation | The plant irradiation, computed from the technical sensors. |
| Active Power | The sum of the active power of all inverters. |
| Producible Power | The sum of the producible power of all inverters. |
| Active Alarms | The number of active alarms on the plant. |
| Status counters | How many devices are in each state: Ready, Run, Manual Stop, Maintenance, Automatic Stop, Fault, Comm Error. |
How they are aggregated:
- Active Power and Producible Power are the sum — sample by sample, after a temporal alignment — of the same tag across all the plant's inverters.
- The status counters count how many devices are in each state (the Device Status abstraction); the Comm Error counter also includes devices whose status readings are invalid (no communication).
- Active Alarms counts the alarms that are active and not yet acknowledged (level ≥ 4).
Per-inverter computed variables
On each inverter Cube creates a few derived variables:
Irradiation
Every inverter has its own Irradiation variable: it is the reference irradiation for that inverter. The value is resolved as follows:
- if a specific irradiation sensor is assigned to the inverter, it uses that one;
- otherwise it falls back to the plant irradiation (the one on the CONTROL device).
Its purpose is to give each inverter the most relevant irradiation for its location — for example to compute its producible power or to compare it against the actual production. Unlike the plant irradiation, it is a simple pass-through of the chosen signal: it applies no statistical filter.
Producible Power
The producible power is the power the inverter could generate given the current irradiation conditions. Compared with the actual power, it highlights any underperformance. It is computed as:
Producible Power = Irradiation / 1000 × Nominal power (kW) × efficiency
where 1000 W/m² is the irradiation at standard conditions (STC) and efficiency is the inverter's efficiency (default 1). It uses the inverter's Irradiation (see above) and the nominal power defined by the model; if irradiation is missing or the nominal power is not configured, the value is not computed.
Status and Event
Each inverter also exposes INV Status and INV Event, i.e. the Device Status and Device Event abstractions. They are described in detail in Device Variables.