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Inverters, PV and batteries

MOI, MPS, MOB and COB: what an inverter, a string and a battery report, and the only control use case outside the grid pair.

Four use cases about the machines on the generation side of a building, and they nest the way the hardware does: an inverter with PV strings on one DC bus and a battery on another, presenting one AC connection to the house.

Layering, not repetition

MOI is explicit about it (§2.5.1): an inverter SHALL also support MPC, which carries the ordinary AC values — active power, current, voltage, frequency. MOI adds only what is specific to a machine converting DC into AC: apparent and reactive power, the power factor, the four yield counters, the nameplate, and a component temperature.

The same nesting appears in the entity tree. A PVString and a Battery are sub-entities of an Inverter, and an installation with two roofs has two PVString entities under one inverter — which is the point of MPS rather than MGCP. A shaded or failing string is invisible in a building's total and obvious in its own measurement.

One measurement layer, eleven use cases

MOI, MPS and MOB are built on the same usecases::monitoring machinery as MPC, MGCP, EVCEM, EVSOC, COB and the three HVAC thermometers. What they add is vocabulary, not mechanism:

use eebus::usecases::mob;
use eebus::usecases::monitoring::{Measurand, Quantity};

let mut battery = mob::monitored_unit(1)
    .with(Measurand::unphased(Quantity::DcPower))
    .with(Measurand::unphased(Quantity::StateOfCharge));

Measurand::unphased is the DC counterpart of total and on. A direct-current measurement has no phases, and the parameter description omits acMeasuredPhases rather than defaulting it to abc — a client waiting for phase information on a DC measurement would wait forever.

Three of a battery's values are easy to confuse and are separate quantities here for that reason:

• stateOfCharge is a percentage — how full.
• stateOfEnergy is watt-hours — how much is in there.
• useableCapacity is watt-hours — how much it could hold, after ageing.

A client that read stateOfEnergy as a percentage would see a 12 kWh battery as 12 000 per cent full.

COB — the other control use case

Everything above reads. COB writes, and it is the only control use case in the crate outside the LPC/LPP pair — with, unsurprisingly, the same safety shape: a heartbeat, a failsafe state, a Failsafe Duration Minimum between two and twenty-four hours, and a state machine that fails towards a known-safe number rather than towards the last thing it was told.

Its own idea is that a manager may control a battery in either of two frames:

use eebus::usecases::cob::{ControlMode, SetpointWrite};

// "Charge at 3 kW."
inverter.on_control_mode(ControlMode::Power, true, now);
inverter.on_setpoint(&SetpointWrite::active(3_000.0), true, now);

// Or: "hold the grid connection at zero, and work the rest out yourself."
inverter.on_control_mode(ControlMode::Pcc, true, now);
inverter.on_setpoint(&SetpointWrite::active(0.0), true, now);

pcc is the mode that self-consumption optimisation actually wants, and the reason the inverter rather than the manager does the arithmetic: an inverter reacts to the grid connection in milliseconds, and a manager on a network cannot.

Two details bite:

The sign convention is passive ([COB-001/1]): positive is consumption, negative is production. A setpoint of −3 000 W is discharging at three kilowatts, and a caller that gets the sign wrong charges when it meant to discharge.

Which setpoint power mode uses depends on the machine. A battery inverter has only an AC side, so [COB-011] sets AC power. A hybrid inverter also has PV strings on its DC bus, so [COB-012] sets DC power and the AC side is whatever the sun leaves over. Telling a hybrid inverter an AC number would not say what the battery should do, which is why InverterKind is a constructor argument rather than something to remember.

Deactivated is not zero

[COB-917] is the rule most easily got wrong. A deactivated setpoint hands the inverter to its default setpoint, which is a configured number — not nought, and not the last thing the manager said. EffectiveControl names which of the four is driving the machine:

EffectiveControl::Setpoint(3_000.0)   // the manager's number
EffectiveControl::Default(-500.0)     // deactivated or expired  [COB-917]
EffectiveControl::Failsafe(0.0)       // the manager went quiet  [COB-913]
EffectiveControl::Autonomous          // the inverter's own logic

In a household that is being billed for what the battery does, which of those four applied is the whole question — and it is not visible on the wire.