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// SPDX-License-Identifier: LicenseRef-PolyForm-Noncommercial-1.0.0
//! Impedance correction tables.
//!
//! Defines tap-position-dependent impedance scaling factors for transformers.
//! Large autotransformers have non-linear impedance-vs-tap characteristics;
//! correction tables allow the solver to adjust R and X at each tap position.
//! PSS/E RAW section: "IMPEDANCE CORRECTION DATA".
use serde::{Deserialize, Serialize};
/// An impedance correction table (PSS/E TAB field on transformers).
///
/// Each entry is a `(T, F)` pair where:
/// - `T` is the tap ratio (or phase angle in degrees for phase shifters)
/// - `F` is the impedance scaling factor (1.0 = nominal)
///
/// The transformer's R and X are multiplied by the interpolated F value
/// at the current tap position. Referenced by `Branch.tab` (the TAB1/TAB2
/// field in PSS/E transformer records).
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ImpedanceCorrectionTable {
/// Table number (I in PSS/E). Referenced by transformer `tab` field.
pub number: u32,
/// Up to 11 (tap_or_angle, scaling_factor) pairs.
/// Must be sorted by the first element (tap ratio or angle).
pub entries: Vec<(f64, f64)>,
}
impl ImpedanceCorrectionTable {
/// Interpolate the scaling factor for a given tap ratio or phase angle.
///
/// Uses linear interpolation between the two nearest entries.
/// Clamps to the boundary factor if `t` is outside the table range.
pub fn interpolate(&self, t: f64) -> f64 {
if self.entries.is_empty() {
return 1.0;
}
if self.entries.len() == 1 {
return self.entries[0].1;
}
// Below first entry
if t <= self.entries[0].0 {
return self.entries[0].1;
}
// Above last entry
let last = self.entries.len() - 1;
if t >= self.entries[last].0 {
return self.entries[last].1;
}
// Linear interpolation
for i in 0..last {
let (t0, f0) = self.entries[i];
let (t1, f1) = self.entries[i + 1];
if t >= t0 && t <= t1 {
let frac = (t - t0) / (t1 - t0);
return f0 + frac * (f1 - f0);
}
}
1.0
}
}
impl Default for ImpedanceCorrectionTable {
fn default() -> Self {
Self {
number: 1,
entries: Vec::new(),
}
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn interpolate_within_range() {
let table = ImpedanceCorrectionTable {
number: 1,
entries: vec![(0.9, 1.1), (1.0, 1.0), (1.1, 0.95)],
};
assert!((table.interpolate(0.95) - 1.05).abs() < 1e-10);
assert!((table.interpolate(1.0) - 1.0).abs() < 1e-10);
assert!((table.interpolate(1.05) - 0.975).abs() < 1e-10);
}
#[test]
fn interpolate_clamps_at_boundaries() {
let table = ImpedanceCorrectionTable {
number: 1,
entries: vec![(0.9, 1.1), (1.1, 0.9)],
};
assert!((table.interpolate(0.8) - 1.1).abs() < 1e-10);
assert!((table.interpolate(1.2) - 0.9).abs() < 1e-10);
}
#[test]
fn interpolate_empty_returns_one() {
let table = ImpedanceCorrectionTable {
number: 1,
entries: vec![],
};
assert!((table.interpolate(1.0) - 1.0).abs() < 1e-10);
}
}