As one may have noticed, there are several types of control functions that can be applied to Storage and PVSystem elements in OpenDSS. Some of those operate on the same electrical measure, i.e., active, or reactive power. In these cases, depending on the nature of the control function, they may be allowed to operate simultaneously, or they might be mutually exclusive. In the first case, it is necessary to establish an order of precedence. The precedence rules applied in OpenDSS follows the summary of functional descriptions for smart inverters from [6] and in IEEE 1547-2018 [1] requirements, where applicable. They have been adapted to OpenDSS context and are summarized in Tables 1 and 2 for functions/settings that manage active and reactive power, respectively.

Table 1: Precedence of Watt Related Functions


Priority

Settings/

Functions

Description

1st: Fundamental Physical Limit


DER

Primary Source of Energy

A DER cannot produce/absorb active power that it does not have avail- able. These represent the ultimate limit of the DER. For PVSystem, it is represented by the available active power at a given time instant and for Storage it is represented by the energy capacity available (depending on the relation between kWhrated and kWhstored for charging state and between %stored and %reserve for discharging state).

2nd:

Nameplate and Device Limits Settings


Max Power Capability Setting

This is the configurable setting establishing the DER s maximum ac- tive power input/output: kV A, for PVSystem, and kV A and kWRated, for Storage. Higher or lower priority settings/functions may reduce the wattage output below this value, but nothing may increase it above this value.



3rd: Settings Actively Affecting Operating Boundaries


Limit DER

Power Function

These functions serve to reduce maximum allowed watt level to some percentage of the Max Power Capability Setting less than 100%. The cause varies: For Volt-Watt function, it’s the voltage at the DER ter- minal (autonomous function). For Limit DER Power Output Function, it’s a direct control command (for instance, sent by the operator), spec- ified through property %kWRated for Storage and %Pmpp for PVSys- tem. They may be simultaneously active with the same relative priority. The one requiring the greatest reduction in watts takes precedence. As functions intended to establish operating boundaries, these functions are higher priority than any of the dynamic or steady-state functions. Those functions may be active at the same time as these functions but must operate (even dynamically) within the boundaries established by these functions.



Volt-Watt Function

4th:

Dynamic Functions

None at

Current Version

These would be Dynamic Real-Power Support and Dynamic Volt-Watt functions. These are dynamic functions that would produce “additional” active power. See [6] for more information.


5th: Steady State Functions

Storage Direct

Ch./Dch.

Each of these functions/dispatch modes serve to manage the flow of real power into or out of the DER. These functions are mutually exclusive and cannot be active simultaneously. The first one refers to the direct dispatch command through properties kW , kvar, state, %charge, and

%discharge, whereas the second refers to any of the self-dispatch modes or any mode available through a StorageController CE.

Any Storage Dispatch

Function


Table 2: Precedence of Vars Related Functions


Priority

Settings/

Functions

Description

1st:

Fundamental Physical Limit


Self-Imposed Limits


None at current version

2nd: Nameplate and Device Limits

Settings


Max Power Capability Setting

This is the configurable setting establishing the DER s maximum reactive power input/output: kvarMax, kvarMaxAbs and the ascending linear vars limit specified through %PminNoV ars and %PminkvarMax for both PVSystem and Storage. Higher or lower priority settings may re- duce the vars output/input below these values, but nothing may increase it above these values.

3rd:

Settings Actively Affecting Operating Boundaries



None defined for Vars




None at current version

4th:

Dynamic Functions

DRC

Function

This is a dynamic function and produces “Additional” reactive current that adds-to or subtracts-from whatever the present reactive current level may be based on one1 of the 5th priority functions.


5th: Steady State Functions

Constant

PF Function

These functions instruct the DER as to the desired level of reactive power to produce at any time. For each function, the reactive power level is indirectly related to the watt output. These functions have equal priority but are never in conflict because they are mutually exclusive and only one may be effective at any time.

Constant

kvar Function

Volt-Var

Function



1At current version, DRC can only be applied on top of Volt-Var function.