REQUIREMENT Approval item:

# Application concept

The purpose of the neutral-earth voltage (NEV) capability improvement in GridLAB-D is to facilitate studying the effects of various technologies on the distribution power system. This will be focused on how unbalanced power conditions may affect smaller systems like microgrids, but also includes items such as protection schemes with increased renewable generation deployed. Tie-ins to the existing microgrids capabilities are expected.

To include NEV capabilities, the powerflow solver is being generalized to support *n*-phase connections. This will not only include the ability to model the neutral currents and voltages, but also the ability to model multiple independent, co-located circuits or multiple phase conductors.

# Use Case

The initial NEV capability will be used to model the IEEE test systems that support such information. The IEEE standard test feeder related to neutral earth voltages will be used for initial evaluation and testing of the implementation. Testing will eventually move to modelling proper fault currents on a system during an event with standardized test system.

# General Requirements

The neutral-earth voltage capability shall represent an *n*-phase, unbalanced power system and be able to solve any requisite powerflow equations. This will include explicit neutral currents and any neutral-earth-voltage impacts. The new solver method will work with existing objects, but will also provide flexibility to more accurately model neutral-related and generic *n*-phase contributions individual objects or operations may induce.

The final implementation of the NEV solver capability will meet these requirements:

## R1

The NEV solver method will retain all current capabilities of the existing Newton-Raphson powerflow solver method

### R1.1

The NEV solution approach will not be supported by the Forward-Backward Sweep method at this time.

### R1.2

The NEV solution approach will replicate or maintain all model validation checks within powerflow. e.g., phase checks, voltage checks, and properly specified parameters.

### R1.3

The NEV solution approach will retain the current level of functionality with the reliability and microgrids capabilities, until such time that they are supplanted by explicit NEV approaches.

### R1.4

The NEV solution implementation will support existing objects within GridLAB-D that interface with powerflow (i.e., generators objects, residential objects). When explicit NEV connections are not supported, the NEV implementation will fall back to an assumed "ABC" phasing structure.

## R2

The NEV solver shall support *n*-phase connections

### R2.1

The *n*-phase implementation will be built assuming an ideal, zero-impedance ground plane exists and is at zero potential.

### R2.2

The *n*-phase implementation will support the ability to explicitly model a user-defined number of cable connections.

#### R2.2.1

The *n*-phase implementation will support a user-defined number of explicit traditional "phase" conductors

#### R2.2.2

The *n*-phase implementation will support a user-defined number of explicit neutral conductors.

#### R2.2.3

The *n*-phase implementation will support a user-defined number of explicit ground conductors, understanding connections of "ground" will need to be defined in the context of R2.1.

### R2.3

The *n*-phase implementation will support overbuilt feeders -- e.g., a single utility pole may have two conductors that are "Phase A" of two different feeder circuits

### R2.4

The *n*-phase implementation will support generic transformer configurations

#### R2.4.1

Common transformer specifications in GridLAB-D (Wye-Wye, Delta-GWye, Delta-Delta, Single-phase-center-tapped) will be explicitly included

#### R2.4.2

A method for defining generic transformer connection specifications for types outside the "common" versions listed will be provided

#### R2.4.3

Regulators will support the generic transformer connection specifications.

##### R2.4.3.1

Regulators will remain a separate object within GridLAB-D, even if internally it links to a generic transformer object.

### R2.5

The *n*-phase implementation will support fault-current calculations.

### R2.6

The *n*-phase implementation will allow shunts between any of the *n*-phases and earth, or each other.

## R3

The NEV solver method will support reconfiguration capabilities.

### R3.1

The NEV solver method will allow line impedance recalculation when significant changes occur, such as parallel circuit changes or phase losses.

### R3.2

The NEV solver method will allow phases to be reconfigured. e.g., seasonal load balancing of a Phase B lateral is switched to Phase C for summer load considerations.

## R4

The NEV solver method will support multiple SWING/slack buses in the same GLM.

### R4.1

The NEV solver method will support multiple islands within the same GLM.

### R4.2

The NEV solver method will support multiple SWING/slack nodes on the same island.

## R5

The NEV solver method will expand support for childed powerflow objects.

### R5.1

The NEV solver method will support an infinite (within reason) number of childed powerflow object levels.

### R5.2

The NEV solver method will support multiple methods for parent-child connections in powerflow.

#### R5.2.1

The NEV solver method will support the standard GridLAB-D `parent` field for parent-child connections.

#### R5.2.2

The NEV solver method will support a new `connector` object to implement parent-child-like connections.

#### R5.2.3

The NEV solver method will support zero-length lines to implement parent-child-like connections.

#### R5.3

The NEV solver method will support ideal, zero impedance connections (`switch`, `fuse`, etc).