Microgrid Concepts¤

A microgrid is a local electrical grid that connects a set of electrical components together. They are often built around a passive power consumer, to supplement the electricity consumed from the public grid with on-site power generation or storage systems.

Microgrids can also function in island-mode, without a grid connection, or without a local power consumer, but they have to have at least one of the two, to be meaningful.

Frequenz SDK Microgrid Model¤

The SDK aims to provide an abstract model of the microgrid that enables high-level interactions with microgrid components, without having to worry about (or even be aware of) location-specific details such as:

where the meters are placed,
how many batteries,
whether there's a grid connection or a passive consumer,
what models the inverters are, etc.
whether components are having downtimes, because metrics and limits get adjusted automatically when components are having downtimes.

Users of the SDK can develop applications around this interface once and deploy anywhere, and the SDK will take care of translating the requests and instructions to correspond to the specific microgrid configurations.

flowchart LR

subgraph Left[Measurements only]
direction LR
  grid["Grid Connection Point"]
  consumer["Consumer"]
  pv["PV Arrays"]
  chp["CHP"]
end

junction(( ))

subgraph Right[Measurements and control]
direction LR
  bat["Batteries"]
  ev["EV Chargers"]
end

grid --- junction
consumer --- junction
pv --- junction
chp --- junction

junction --- bat
junction --- ev

Grid¤

This refers to a microgrid's grid-connection-point. The power flowing through this connection can be streamed through grid_power.

In locations without a grid connection point, this method remains accessible, and streams zero values.

Consumer¤

This is the main power consumer at the site of a microgrid, and often the load the microgrid is built to support. The power drawn by the consumer is available through consumer_power

In locations without a consumer, this method streams zero values.

Producers: PV Arrays, CHP¤

The total CHP production in a site can be streamed through chp_power. PV Power is available through the PV pool described below. And total producer power is available through microgrid.producer().power.

As is the case with the other methods, if PV Arrays or CHPs are not available in a microgrid, the corresponding methods stream zero values.

PV Arrays¤

The total PV power production is available through pv_pool's power. The PV pool by default uses all PV inverters available at a location, but PV pool instances can be created for subsets of PV inverters if necessary, by specifying the inverter ids.

The pv_pool also provides available power bounds through the power_status method.

The pv_pool also provides a control method propose_power, which accepts values in the Passive Sign Convention and supports only production.

Batteries¤

The total Battery power is available through the battery_pool's power. The battery pool by default uses all batteries available at a location, but battery pool instances can be created for subsets of batteries if necessary, by specifying the battery ids.

The battery_pool also provides soc, capacity, temperature and available power bounds through the power_status method.

The battery_pool also provides control methods propose_power (which accepts values in the Passive Sign Convention and supports both charging and discharging), or through propose_charge, or propose_discharge.

EV Chargers¤

The ev_charger_pool offers a power method that streams the total power measured for all the EV Chargers at a site.

The ev_charger_pool also provides available power bounds through the power_status method.

The ev_charger_pool also provides a control method propose_power, which accepts values in the Passive Sign Convention and supports only charging.

Component pools¤

The SDK provides a unified interface for interacting with sets of Batteries, EV chargers and PV arrays, through their corresponding Pools.

All of them provide support for streaming aggregated data and for setting the power values of the components.

Streaming component data¤

All pools have a power property, which is a FormulaEngine that can

provide a stream of resampled power values, which correspond to the sum of the power measured from all the components in the pool together.
be composed with other power streams to for composite formulas.

In addition, the battery pool has some additional properties that can be used as streams for metrics specific to batteries: soc, capacity and temperature.

Setting power¤

All pools provide a propose_power method for setting power for the pool. This would then be distributed to the individual components in the pool, using an algorithm that's suitable for the category of the components. For example, when controlling batteries, power could be distributed based on the SoC of the individual batteries, to keep the batteries in balance.

How to work with other actors¤

If multiple actors are trying to control (by proposing power values) the same set of components, the power manager will aggregate their desired power values, while considering the priority of the actors and the bounds they set, to calculate the target power for the components.

The final target power can be accessed using the receiver returned from the power_status method available for all pools, which also streams the bounds that an actor should comply with, based on its priority.

Working with other actors to control Batteries¤

This section describes the details of the power manager's reconciliation algorithm for controlling batteries.

Adding the power proposals of individual actors¤

When an actor A calls the propose_power method with a power, the proposed power of the lower priority actor will get added to actor A's power. This works as follows:

the lower priority actor would see bounds shifted by the power proposed by actor A.
After lower priority actor B sets a power in its shifted bounds, it will get shifted back by the power set by actor A.

This has the effect of adding the powers set by actors A and B.

Example 1: Battery bounds available for use: -100kW to 100kW

Actor	Priority	Available Bounds	Requested Bounds	Requested Power	Adjusted Power	Aggregate Power
A	3	-100kW .. 100kW	None	20kW	20kW	20kW
B	2	-120kW .. 80kW	None	50kW	50kW	70kW
C	1	-170kW .. 30kW	None	50kW	30kW	100kW
					target power	100kW

Actor A sees the full system: Available Bounds = -100kW .. 100kW. It proposes a power of 20kW, but no bounds.
This becomes the operating point for actor B, so actor B sees bounds shifted by A's proposal = -120kW .. 80kW. It can operate only in this range.
Actor B proposes a power of 50kW on this shifted range, and if this is applied on to the original bounds (aka shift the bounds back to the original range), it would be 20kW + 50kW = 70kW.
So Actor C sees bounds shifted by 70kW from the original bounds, and sets 50kW on this shifted range, but it can't exceed 30kW, so its request gets limited to 30kW.
Shifting this back by 70kW, the target power is calculated to be 100kW.

Irrespective of what any actor sets, the final power won't exceed the available battery bounds.

Example 2:

Actor	Priority	Available Bounds	Requested Bounds	Requested Power	Adjusted Power	Aggregate Power
A	3	-100kW .. 100kW	None	20kW	20kW	20kW
B	2	-120kW .. 80kW	None	-20kW	-20kW	0kW
					target power	0kW

Actors with exactly opposite requests cancel each other out.

Limiting bounds for lower priority actors¤

When an actor A calls the propose_power method with bounds (either both lower and upper bounds or at least one of them), lower priority actors will see their (shifted) bounds restricted and can only propose power values within that range.

Example 1: Battery bounds available for use: -100kW to 100kW

Actor	Priority	Available Bounds	Requested Bounds	Requested Power	Adjusted Power	Aggregate Power
A	3	-100kW .. 100kW	-20kW .. 100kW	50kW	50kW	50kW
B	2	-70kW .. 50kW	-90kW .. 0kW	-10kW	-10kW	40kW
C	1	-60kW .. 10kW	None	-20kW	-20kW	20kW
					target power	20kW

Actor A with the highest priority has the entire battery bounds available to it. It sets limited bounds of -20kW .. 100kW, and proposes a power of 50kW.
Actor B's operating point is now 50kW. So, Actor B sees Actor A's limit of -20kW..100kW shifted by 50kW as -70kW..50kW, and can only operate within this range.
Actor B tries to limit the bounds of actor C to -90kW .. 0kW, but it doesn't have access to that full range and can only operate in the -70kW .. 50kW range, so its specified bounds get restricted to -70kW .. 0kW.
Actor C sees this as -60kW .. 10kW, because it gets shifted by Actor B's proposed power of -10kW.
Actor C proposes a power within its bounds and the proposals of all the actors are added to get the target power.

Example 2:

Actor	Priority	Available Bounds	Requested Bounds	Requested Power	Adjusted Power	Aggregate Power
A	3	-100kW .. 100kW	-20kW .. 100kW	50kW	50kW	50kW
B	2	-70kW .. 50kW	-90kW .. 0kW	-90kW	-70kW	-20kW
					target power	-20kW

When an actor requests a power that's outside its available bounds, the closest available power is used.

Comprehensive example¤

Battery bounds available for use: -100kW to 100kW

Priority	Available Bounds	Requested Bounds	Requested Power	Adjusted Power	Aggregate Power
7	-100 kW .. 100 kW	None	10 kW	10 kW	10 kW
6	-110 kW .. 90 kW	-110 kW .. 80 kW	10 kW	10 kW	20 kW
5	-120 kW .. 70 kW	-100 kW .. 80 kW	80 kW	70 kW	90 kW
4	-170 kW .. 0 kW	None	-120 kW	-120 kW	-30 kW
3	-50 kW .. 120 kW	None	60 kW	60 kW	30 kW
2	-110 kW .. 60 kW	-40 kW .. 30 kW	20 kW	20 kW	50 kW
1	-60 kW .. 10 kW	-50 kW .. 40 kW	25 kW	10 kW	60 kW
0	-60 kW .. 0 kW	None	12 kW	0 kW	60 kW
-1	-60 kW .. 0 kW	-40 kW .. -10 kW	-10 kW	-10 kW	50 kW
				Target Power	50 kW

Working with other actors to control PV inverters and EV chargers¤

The power manager reconciles power proposals for PV inverters and EV chargers similarly to batteries, but with one key difference:

There is no shifting of operating point between actors, and the powers are not added together.

Higher priority actors can strictly limit the bounds available to lower priority actors, but the power proposals by lower priority actors take precedence, as long as they are within the bounds set by higher priority actors.

This is because PV inverters can only produce power (negative power according to the PSC), and EV chargers can only consume power (positive power according to the PSC), and shifting bounds would make ranges available to actors that a PV inverter or EV charger can't operate in.

A PV pool Example

Actor	Priority	Available Bounds	Requested Bounds	Requested Power	Adjusted Power
A	4	-100kW .. 0W	-90kW .. 0kW	-20kW	-20kW
B	3	-90kW .. 0kW	-75kW .. -20kW	-50kW	-50kW
C	2	-75kW .. -20kW	None	-100kW	-75kW
D	1	-75kW .. -20kW	-60kW .. -60kW	-60kW	-60kW
E	0	-60kW .. -60kW	None	-20kW	-60kW
				Final Power	-60kW

Actor A with the highest priority has access to the entire range of the PV inverters. In this case -100kW .. 0W.
It wants to limit production to a maximum of -90kW, so it sets bounds of -90kW .. 0W. It also proposes a power of -20kW. If there are no lower priority actors, that power will get set to the inverters. But here, it gets overridden by lower priority actors.
Actor B limits the bounds further, and proposes its preferred power.
Actor C proposes -100kW, which is outside of what Actor B has allowed, so it gets clamped to -75kW, which is the closest Actor C can get to its requested power in its available range of -75kW .. -20kW.
Actor D wants exactly -60kW, so it clamps the bounds to -60kW .. -60kW, and sets -60kW, making sure Actor E or other lower priority actors can't change the power further.

Withdrawing power proposals¤

An actor can withdraw its power proposal by calling propose_power with None target_power and None bounds (which are the default anyway). As soon as an actor calls pool.propose_power(None), its proposal is dropped and the target power is recalculated and the component powers are updated.

When all the proposals for a pool are withdrawn, the components get reset to their default powers immediately. These are:

component category	default power (according to Passive Sign Convention)
Batteries	Zero
PV	Max production (Min power according to PSC)
EV Chargers	Max consumption (Max power according to PSC)