Line Shunt Conductance (ACLineSegment.gch)

Motivation

The Powerflow page's Y-bus construction models each line as a π-equivalent: a series impedance plus a shunt admittance split evenly across both ends. Almost every power-flow tool's documentation talks about that shunt term purely in terms of susceptance — line charging capacitance, the reactive effect of a long line's own capacitance to ground — because for the overwhelming majority of real lines the shunt's real part (conductance, representing corona loss or leakage) is negligible or simply zero.

CGMES's ACLineSegment schema doesn't assume that: alongside bch (susceptance) it defines gch (conductance) as a first-class, independent field, "of the entire line section" like every other ACLineSegment electrical attribute. Most real conformance fixtures leave it at zero, but not all — ENTSO-E's own MicroGrid-BE-MAS fixture has two lines (BE-Line_6, BE-Line_2) with non-negligible gch, together worth several MW of real power at nominal voltage.

The concepts

Every tool that models AC lines at all has shunt conductance somewhere in its branch admittance — this isn't a case of some supporting it and others not. What differs is how it's parameterized, and how visible a distinct "conductance" concept is in the data model. Three variants, all carrying the same physical information:

1. A direct conductance field, per line or per end

The most explicit form: a siemens value stored next to the susceptance. Two sub-variants matter for conversion:

• One value for the whole line — CGMES's own form, gch "of the entire line section". A converter must split it across the π-model's two ends, conventionally evenly: \(g_1 = g_2 = g_{ch}/2\), exactly the way \(b_{ch}\) is already split.
• One value per end (g1/g2) — strictly more general, since it can represent an asymmetric line whose two ends carry different shunts. A whole-line value maps into it trivially by halving; the reverse direction loses information unless the two ends happen to be equal.

2. One complex per-end shunt admittance

Instead of naming conductance separately, store a single complex number per end, \(h = g + jb\), and stamp it directly into the Y-bus diagonal: \(y_{11} = y_s + h_{or}\). The conductance is the real part — genuinely present and genuinely solved, just never given its own name. A grep for "conductance" in such a codebase finds nothing, which is a naming fact, not a capability one.

3. Derived from capacitance and a loss tangent

A physically-motivated alternative parameterization: store the shunt capacitance \(c_1\) and a dielectric loss tangent \(\tan\delta_1\), and derive the complex shunt from both:

\[ y_{shunt} = \omega c_1 \tan\delta_1 + j,\omega c_1 \]

so \(g = \omega c_1 \tan\delta_1\). Same information, expressed the way a cable datasheet expresses it. Converting into this form from a raw siemens pair means backing out the tangent as \(\tan\delta_1 = g_{ch}/b_{ch}\) rather than mapping the two fields across directly — and a converter whose source format has no loss-tangent equivalent has no way to produce a nonzero conductance at all.

Where this fits in gridoxide today

Before this fix, types::Line had a b_shunt field and nothing else — src/cgmes.rs's ACLineSegment conversion read bch but never gch, even though its own comment already documented gch as one of the fields "of the entire line section" (a stale comment that got ahead of the code, not the other way around). network::build_ybus had no way to stamp a real shunt term even if the conversion had wanted to.

The fix uses concept 1's whole-line form with the even split, since that's the same convention gridoxide's own bch handling already used and types::Line already has half-open-line self-loop folding logic that only needed a second field threaded through it:

#![allow(unused)]
fn main() {
pub struct Line {
    pub from: usize,
    pub to: usize,
    pub r: f64,
    pub x: f64,
    pub b_shunt: f64, // total line charging
    #[serde(default)]
    pub g_shunt: f64, // total shunt conductance (CGMES ACLineSegment.gch; usually 0)
}
}

#![allow(unused)]
fn main() {
// build_ybus: split shunt admittance (conductance + susceptance) equally to both ends of line
let y_shunt_half = Complex::new(ln.g_shunt / 2.0, ln.b_shunt / 2.0);
y.add(ln.from, ln.from, y_line + y_shunt_half);
y.add(ln.to, ln.to, y_line + y_shunt_half);
}

#[serde(default)] keeps every existing native-JSON and PGM-JSON fixture working unchanged. The PGM importer always sets g_shunt: 0.0: PGM's own line schema is concept 3, and PgmLine has no tan1 field to derive a conductance from — the same "no data, so no effect" stance the rest of that converter takes.

Why this mattered more than "a couple of MW out of a large network"

The missing MW didn't just make voltages a little off everywhere — it concentrated almost entirely into one bus's angle, and nowhere else. BE-Line_6/BE-Line_2 feed directly into the substation hosting MicroGrid-BE-MAS's StaticVarCompensator (see the StaticVarCompensator page), a voltage-magnitude-pinned bus. A pinned bus can absorb a reactive-power mismatch by adjusting its own Q injection, but it has no equivalent slack for an active-power one — so the several MW this fix restores had, before the fix, nowhere to go but that one bus's angle. Cross-validated against pypowsybl's own independent CGMES import (scripts/bench/cross_validate_cgmes_microgrid_be.py, with both tools pinned to the same reference bus so their angles are directly comparable): worst angle deviation across the whole fixture dropped from 0.34° to 0.07° once gch was included — a five-fold improvement concentrated almost entirely at that one substation, exactly where the missing real power was actually flowing in.

Tool reference