S2 example for PV
This page serves as a guide for implementing an S2 RM for a PV installation. It provides example S2 messages that help developers to understand how to interact with PV installations and what kind of flexibility they can offer.
[ add architecture picture ]
A PV installation produces electricity by converting solar energy into electricity. This electricity can be used for several purposes, such as self-consumption or earning money by delivering electricity to the grid.
Usually a PV installation is not flexible at all, as it only produces electricity when there is sun and depending on weather circumstances (e.g. such as cloud coverage) it will produce more or less, i.e. its production is intermittent. But many PV inverters also allow to control the maximum output power of the PV installation, also known as curtailment. This curtailment is for example needed when too much installations are connected to the same local grid and produce at high power, pushing the voltage of the network above its acceptable limits, or when prices for delivering electricity back to the grid become to low (or even negative) to be profitable.
S2 supports five different control types for flexibility (and one for no control possibility), each supporting a different way of modelling flexibility of a device. For PV installations the choice is somehow simple, as the capabilities of these installations are limited:
| Control capability of PV installation | Control type | Remark |
|---|---|---|
| No control possible | NOT_CONTROLABLE | As there is no way to control this PV installation, the NOT_CONTROLABLE ControlType should be selected. Although nothing can be controlled, devices can still send PowerMeasurements and PowerForecasts |
| Set maximum output power | POWER_ENVELOPE_BASED_CONTROL (PEBC) | Based on the constraints of the PV installation (e.g. the minimum and maximum power output of the inverter), the CEM can provide the Resource Manager of the PV installation a PowerEnvelope that describes the minimum (usually 0) and maximum (e.g. -2kW, negative for production) that the inverter of the PV installation should produce |
The table shows two ways to interact with a PV installation: just using the power measurements such that the CEM can optimize based on the production of the PV installation and using Power Envelope Based Control, which allows the CEM to instruct the PV installation to curtail.
We could also model the PV installation as a state machine using Operation Mode Based Control (OMBC). Different states (operation modes in S2 language) could be modeled that describe at which maximum power output the PV installation should adhere to, but this approach is more work to configure than PEBC, and does not increase the amount of flexiblity a PV installation has.
The Power Envelope Based Control is used for devices that can be influenced to use or produce a minimum and/or maximum amount of power over time. So the CEM cannot control the amount of power produced or consumed by the device directly, but it can dictate power limits, which can change over time. The Resource Manager informs the CEM with which power limits it can work using the PEBC.PowerConstraints message. Optionally, the Resource Manager can also instruct the CEM how much energy it needs to consume or produce in a certain timespan (by sending a PEBC.EnergyConstraints message), which the CEM shall take into consideration while setting power limits.
This example describes how a PV interter can expose its curtailment capabilities to the CEM and let the CEM control this inverter by sending it messages that will ask it to curtail itself.
The following sequence diagram is an example of what a message exchange between a CEM and RM could look like, but messages could also be sent in a different order (see also State of communication and the PEBC Message reference). ReceptionStatus messages are omitted for readability.
First an overview of the messages is presented graphically, next sections will describe the example messages in more detail.
For the RM to work correctly, some information is required to fill in the messages between the RM and the CEM.
The diagram below depicts this process and the data that is needed.
Image generated using the following PlantUML code:
@startuml
title Initialize communication between the RM and the PV installation
'participant CEM
participant RM
participant "PV Inverter" as PV
note over RM, PV: Initialize connection between RM and PV Inverter
RM <-> PV: connect using inverter protocol
PV -> RM: Send constraints, \ne.g. maximum production power
loop every x seconds
PV -> RM: Send PV measurements
PV -> RM: Send forecasts (if available)
end loop
@enduml
As shown, the RM needs to know what the maximum output power of the inverter is and how it can get measurements that can be send to the CEM.
[intro]
@startuml
title Example communication between CEM and RM of a PV Installation
participant CEM
participant RM
note over CEM, RM: WebSocket connected
CEM -> RM: Handshake
RM -> CEM: Handshake
CEM -> RM: HandshakeResponse
note over CEM, RM #lightblue: Initialized
|||
group Send RM configuration for this inverter and \nthe control type that this RM supports
RM -> CEM: ResourceManagerDetails
CEM -> RM: SelectControlType
note right: The CEM will select the \nPEBC control type for this RM
end group
note over CEM, RM #lightgreen: ControlType PEBC activated
|||
group #fff0b7 Send power or energy constraints of this PV Installation
RM -> CEM: PEBC.EnergyConstraint
RM -> CEM: PEBC.PowerConstraint
note right: The RM informs the CEM of the\n maximum power output of the PV inverter
end
|||
loop in no particular order
RM -> CEM: PowerMeasurement
RM --> CEM: PowerForecast [Optional]
CEM -> RM: PEBC.Instruction
note right: Here the CEM will ask the RM to curtail its production
RM -> CEM: InstructionStatusUpdate
note right: The RM will confirm (or not) this curtailment
end loop
|||
RM -> CEM: SessionRequest
note over CEM, RM: WebSocket disconnected
@enduml