Unbalanced Network Formulations

AbstractUnbalancedACPModel

Real-valued formulation from:

  • Formulation without shunts: Mahdad, B., Bouktir, T., & Srairi, K. (2006). A three-phase power flow modelization: a tool for optimal location and control of FACTS devices in unbalanced power systems. In IEEE Industrial Electronics IECON (pp. 2238–2243).

AbstractUnbalancedDCPModel

Applying all of the standard DC linearization tricks to the AbstractUnbalancedACPModel

SDPUBFModel

The BFM SDP relaxation as described in:

  • Gan, L., & Low, S. H. (2014). Convex relaxations and linear approximation for optimal power flow in multiphase radial networks. In PSSC (pp. 1–9). Wroclaw, Poland. doi:10.1109/PSCC.2014.7038399

Note that this formulation is complex-valued and additional steps are needed to implement this in JuMP.

SOCNLPUBFModel

The starting point is SDPUBFModel. The SDP constraint can be relaxed to a set of SOC constraints, starting from either the real or complex form of the matrix on which the PSD-ness constraint is applied.

  • Kim, S., Kojima, M., & Yamashita, M. (2003). Second order cone programming relaxation of a positive semidefinite constraint. Optimization Methods and Software, 18(5), 535–541. doi:10.1080/1055678031000148696
  • Andersen, M. S., Hansson, A., & Vandenberghe, L. (2014). Reduced-complexity semidefinite relaxations of optimal power flow problems. IEEE Trans. Power Syst., 29(4), 1855–1863.

SOCConicUBFModel

See SOCNLPUBFModel

LPUBFDiagModel

This formulation has originally been developed by Sankur et al.

  • Sankur, M. D., Dobbe, R., Stewart, E., Callaway, D. S., & Arnold, D. B. (2016). A linearized power flow model for optimization in unbalanced distribution systems. arXiv:1606.04492v2

and is here cast as only considering the diagonal elements of the linear formulation by Gan and Low:

  • Gan, L., & Low, S. H. (2014). Convex relaxations and linear approximation for optimal power flow in multiphase radial networks. In PSSC (pp. 1–9). Wroclaw, Poland. doi:10.1109/PSCC.2014.7038399

This furthermore leads to the imaginary part of the lifted node voltage variable W in Gan and Low being redundant, so it is substituted out in LPUBFDiagModel.

The full formulation from Gan and Low was supported as LPUBFFullModel in PowerModelsDistribution up to version 0.7.

FBSUBFPowerModel, FOTPUPowerModel, FOTRUPowerModel

The linear FBS and FOT formulations as described in:

  • Girigoudar, K., & Roald, L.A. (2021). Linearized Three-Phase Optimal Power Flow Models for Distribution Grids with Voltage Unbalance. 2021 IEEE Conference on Decision and Control (CDC).

Unbalanced Network Formulation Type Hierarchy

PowerModelsDistribution has a rich model type hierarchy similiar to PowerModels. At the top of the type hierarchy we can distinguish between conic, active power only, and branch flow models:

abstract type AbstractUnbalancedConicModel <: AbstractPowerModel end
abstract type AbstractUnbalancedActivePowerModel <: AbstractPowerModel end
abstract type AbstractUBFModel <: AbstractPowerModel end
abstract type AbstractUBFQPModel <: AbstractUBFModel end
abstract type AbstractUBFConicModel <: AbstractUBFModel end
const AbstractUnbalancedConicModels = Union{AbstractUnbalancedConicModel, AbstractUBFConicModel}

Several nonlinear (non-convex) models are available at the top level:

abstract type AbstractUnbalancedACPModel <: AbstractPowerModel end
abstract type AbstractUnbalancedACRModel <: AbstractPowerModel end
abstract type AbstractUnbalancedIVRModel <: AbstractUnbalancedACRModel end

The following relaxations are available under these hierarchies:

abstract type AbstractNLPUBFModel <: AbstractUBFQPModel end
abstract type AbstractConicUBFModel <: AbstractUBFConicModel end
const AbstractUBFModels = Union{AbstractNLPUBFModel, AbstractConicUBFModel}

abstract type SDPUBFModel <: AbstractConicUBFModel end
abstract type SDPUBFKCLMXModel <: SDPUBFModel end
abstract type SOCNLPUBFModel <: AbstractNLPUBFModel end
abstract type SOCConicUBFModel <: AbstractConicUBFModel end
const SOCUBFModels = Union{SOCNLPUBFModel, SOCConicUBFModel}

where UBF is an unbalanced variant of the Branch Flow models from PowerModels. Models which do not contain UBF in their name are Bus Injection Models e.g. AbstractUnbalancedACPModel. Finally, some linear unbalanced power flow models are available under the following hierarchy:

abstract type AbstractUnbalancedDCPModel <: AbstractUnbalancedActivePowerModel end
abstract type AbstractUnbalancedNFAModel <: AbstractUnbalancedDCPModel end
abstract type AbstractLPUBFModel <: AbstractNLPUBFModel end
abstract type LPUBFDiagModel <: AbstractLPUBFModel end
const LinDist3FlowModel = LPUBFDiagModel
abstract type FBSUBFModel <: AbstractLPUBFModel end

Unbalanced Power Models

Each of these Models can be used as the type parameter for an UnbalancedPowerModel:

mutable struct ACPUPowerModel <: AbstractUnbalancedACPModel @pmd_fields end
mutable struct ACRUPowerModel <: AbstractUnbalancedACRModel @pmd_fields end
mutable struct DCPUPowerModel <: AbstractUnbalancedDCPModel @pmd_fields end
mutable struct NFAUPowerModel <: AbstractUnbalancedNFAModel @pmd_fields end

mutable struct SDPUBFPowerModel <: SDPUBFModel @pmd_fields end
mutable struct SDPUBFKCLMXPowerModel <: SDPUBFKCLMXModel @pmd_fields end

mutable struct SOCNLPUBFPowerModel <: SOCNLPUBFModel @pmd_fields end
mutable struct SOCConicUBFPowerModel <: SOCConicUBFModel @pmd_fields end

mutable struct LPUBFDiagPowerModel <: LPUBFDiagModel @pmd_fields end
const LinDist3FlowPowerModel = LPUBFDiagPowerModel
mutable struct FBSUBFPowerModel <: FBSUBFModel @pmd_fields end
mutable struct FOTPUPowerModel <: AbstractUnbalancedACPModel @pmd_fields end
mutable struct FOTRUPowerModel <: AbstractUnbalancedACRModel @pmd_fields end

Optimization problem classes

  • NLP (nonconvex): ACPUPowerModel, ACRUPowerModel, IVRUPowerModel
  • SDP: SDPUBFPowerModel, SDPUBFKCLMXPowerModel
  • SOC(-representable): SOCNLPUBFPowerModel, SOCConicUBFPowerModel
  • Linear: LPUBFDiagPowerModel (LinDist3FlowPowerModel), FBSUBFPowerModel, FOTPUPowerModel, FOTRUPowerModel, DCPUPowerModel, NFAUPowerModel

Matrix equations versus scalar equations

JuMP supports vectorized syntax, but not for nonlinear constraints. Therefore, certain formulations must be implemented in a scalar fashion. Other formulations can be written as matrix (in)equalities. The current implementations are categorized as follows:

  • Scalar: ACPUPowerModel, ACRUPowerModel, IVRUPowerModel, DCPUPowerModel, NFAPowerModel, FBSUBFPowerModel, FOTPUPowerModel, FOTRUPowerModel
  • Matrix: SDPUBFPowerModel, SDPUBFKCLMXPowerModel, SOCNLPUBFPowerModel, SOCConicUBFPowerModel, LPUBFDiagPowerModel

Explicit Neutral Models

Explicit neutral (EN) models represent the neutral currents explicitly; unlike all the previous models, which implicitly assume they are grounded immediately where they originate.

There are two broad classes of formulations: IVR ones an ACR one.

IVR

AbstractExplicitNeutralIVRModel

This abstract type groups all EN IVR formulations, and is a subtype of AbstractUnbalancedIVRModel.

AbstractNLExplicitNeutralIVRModel

This abstract type groups non-linear EN IVR formulations, and is a subtype of AbstractExplicitNeutralIVRModel.

IVRENPowerModel

This concrete type is a non-linear EN IVR formulation.

IVRReducedENPowerModel

This concrete type is a non-linear EN IVR formulation, where the branches only have series current variables (instead of also total current variables).

AbstractQuadraticExplicitNeutralIVRModel

This abstract type groups quadratic EN IVR formulations, and is a subtype of AbstractExplicitNeutralIVRModel.

IVRQuadraticENPowerModel

This concrete type is a quadratic EN IVR formulation.

IVRReducedQuadraticENPowerModel

This concrete type is a quadratic EN IVR formulation, where the branches only have series current variables (instead of also total current variables).

ACR

AbstractExplicitNeutralACRModel

This abstract type is a subtype of AbstractUnbalancedACRModel, analogous to the IVR structure.

ACRENPowerModel

This is the only concrete ACR type amongst the EN formulations.

Overview of hierarchy

EN-IVR (AbstractExplicitNeutralIVRModel)
|
|-- NL (AbstractNLExplicitNeutralIVRModel)
|   |
|   |-- IVRENPowerModel
|
|-- Quadratic (AbstractQuadraticExplicitNeutralIVRModel)
    |
    |-- IVRENPowerModel
    |
    |-- IVRReducedENPowerModel

EN-ACR (AbstractExplicitNeutralIVRModel)
|
|-- ACRENPowerModel