SEP 2.0 DER Support¶
Smart Energy Profile 2.0 (SEP2, IEEE 2030.5) specifies a REST architecture built around the core HTTP verbs: GET, HEAD, PUT, POST and DELETE. A specification for the SEP2 protocol can be found here.
SEP2 EndDevices (clients) POST XML resources representing their state, and GET XML resources containing command and control information from the server. The server never reaches out to the client unless a “subscription” is registered and supported for a particular resource type. This implementation does not use SEP2 registered subscriptions.
The SEP2 specification requires HTTP headers, and it explicitly requires RESTful response codes, for example:
- 201 - “Created”
- 204 - “No Content”
- 301 - “Moved Permanently”
SEP2 message encoding may be either XML or EXI. Only XML is supported in this implementation.
SEP2 requires HTTPS/TLS version 1.2 along with support for the cipher suite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8. Production installation requires a certificate issued by a SEP2 CA. The encryption requirement can be met by using a web server such as Apache to proxy the HTTPs traffic.
SEP2 discovery, if supported, must be implemented by an xmDNS server. Avahi can be modified to perform this function.
SEP2 groups XML resources into “Function Sets.” Some of these function sets provide a core set of functionality used across higher-level function sets. This implementation implements resources from the following function sets:
- Device Information
- Device Capabilities
- End Device
- Function Set Assignments
- Power Status
- Distributed Energy Resources
Distributed Energy Resources¶
Distributed energy resources (DERs) are devices that generate energy, e.g., solar inverters, or store energy, e.g., battery storage systems, electric vehicle supply equipment (EVSEs). These devices are managed by a SEP2 DER server using DERPrograms which are described by the SEP2 specification as follows:
Servers host one or more DERPrograms, which in turn expose DERControl events to DER clients. DERControl instances contain attributes that allow DER clients to respond to events that are targeted to their device type. A DERControl instance also includes scheduling attributes that allow DER clients to store and process future events. These attributes include start time and duration, as well an indication of the need for randomization of the start and / or duration of the event. The SEP2 DER client model is based on the SunSpec Alliance Inverter Control Model [SunSpec] which is derived from IEC 61850-90-7  and [EPRI].
EndDevices post multiple SEP2 resources describing their status. The following is an example of a Power Status resource that might be posted by an EVSE (vehicle charging station):
<PowerStatus xmlns="http://zigbee.org/sep" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" href="/sep2/edev/96/ps"> <batteryStatus>4</batteryStatus> <changedTime>1487812095</changedTime> <currentPowerSource>1</currentPowerSource> <estimatedChargeRemaining>9300</estimatedChargeRemaining> <PEVInfo> <chargingPowerNow> <multiplier>3</multiplier> <value>-5</value> </chargingPowerNow> <energyRequestNow> <multiplier>3</multiplier> <value>22</value> </energyRequestNow> <maxForwardPower> <multiplier>3</multiplier> <value>7</value> </maxForwardPower> <minimumChargingDuration>11280</minimumChargingDuration> <targetStateOfCharge>10000</targetStateOfCharge> <timeChargeIsNeeded>9223372036854775807</timeChargeIsNeeded> <timeChargingStatusPEV>1487812095</timeChargingStatusPEV> </PEVInfo> </PowerStatus>
VOLTTRON’s SEP2 implementation includes a SEP2Agent and a SEP2 device driver, as described below.
SEP2Agent implements a SEP2 server that receives HTTP POST/PUT requests from SEP2 devices. The requests are routed to SEP2Agent over the VOLTTRON message bus by VOLTTRON’s MasterWebService. SEP2Agent returns an appropriate HTTP response. In some cases (e.g., DERControl requests), this response includes a data payload.
SEP2Agent maps SEP2 resource data to a VOLTTRON SEP2 data model based on SunSpec, using block numbers and point names as defined in the SunSpec Information Model, which in turn is harmonized with 61850. The data model is given in detail below.
Each device’s data is stored by SEP2Agent in an EndDevice memory structure. This structure is not persisted to a database. Each EndDevice retains only the most recently received value for each field.
SEP2Agent exposes RPC calls for getting and setting EndDevice data.
VOLTTRON SEP2 Device Driver¶
The SEP2 device driver is a new addition to VOLTTRON MasterDriverAgent’s family of standard device drivers. It exposes get_point/set_point calls for SEP2 EndDevice fields.
The SEP2 device driver periodically issues SEP2Agent RPC calls to refresh its cached representation of EndDevice data. It issues RPC calls to SEP2Agent as needed when responding to get_point, set_point and scrape_all calls.
These field IDs correspond to the ones in the SEP2 device driver’s configuration file, sep2.csv. They have been used in that file’s “Volttron Point Name” column and also in its “Point Name” column.
|Field ID||SEP2 Resource/Property||Description||Units||Type|
||Model (32 char lim).||string|
||Long-form device identifier (32 char lim).||string|
||Short-form device identifier (32 char lim).||string|
||Version (16 char lim).||string|
||AC power factor.||%||float|
||Usable capacity of the battery. Maximum charge minus minimum charge.||Ah||float|
||Maximum RMS AC current level capability of the inverter.||A||float|
||Maximum rate of energy transfer into the device.||W||float|
||Maximum rate of energy transfer out of the device.||W||float|
||Nominal energy rating of the storage device.||Wh||float|
||Continuous power output capability of the inverter.||W||float|
||Maximum power output. Default to WRtg.||W||float|
||AC lifetime active (real) energy output.||Wh||float|
||CONNECTED=0, AVAILABLE=1, OPERATING=2, TEST=3.||enum|
||Setpoint for maximum charge. This is the only field that is writable with a set_point call.||W||float|
||Power flow in or out of the inverter.||W||float|
||Output energy (absolute SOC). Calculated as (availabilityDuration / 3600) * WMax.||Wh||float|
||Control Mode: REMOTE=0, LOCAL=1.||enum|
||State of Charge %.||% WHRtg||float|
||DISCONNECTED=1, INITIALIZING=2, CONNECTED=3, STANDBY=4, SOC PROTECTION=5, FAULT=99.||enum|
Revising and Expanding the Field Definitions¶
The SEP2-to-SunSpec field mappings in this implementation are a relatively thin subset of all possible field definitions. Developers are encouraged to expand the definitions.
The procedure for expanding the field mappings requires you to make changes in two places:
- Update the driver’s point definitions in services/core/MasterDriverAgent/master_driver/sep2.csv
- Update the SEP2-to-SunSpec field mappings in services/core/SEP2Agent/sep2/end_device.py and __init__.py
When updating VOLTTRON’s SEP2 data model, please use field IDs that conform to the SunSpec block-number-and-field-name model outlined in the SunSpec Information Model Reference (see the link below).
For Further Information¶
- Information model specification: http://sunspec.org/wp-content/uploads/2015/06/SunSpec-Information-Models-12041.pdf
- Information model reference spreadsheet: http://sunspec.org/wp-content/uploads/2015/06/SunSpec-Information-Model-Reference.xlsx
- Inverter models: http://sunspec.org/wp-content/uploads/2015/06/SunSpec-Inverter-Models-12020.pdf
- Energy storage models: http://sunspec.org/wp-content/uploads/2015/06/SunSpec-Energy-Storage-Models-12032.pdf
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