StaticVarCompensator

Motivation

A Static Var Compensator (SVC) is a shunt-connected, power-electronically-controlled reactive power source: instead of a fixed shunt capacitor/reactor bank, it continuously adjusts its own reactive injection to hold the voltage at its connection point near a setpoint, within a capacitive/inductive rating. Electrically it behaves like a voltage-controlled bus for load-flow purposes — the same \(\vert V_k \vert\)-known, \(Q_k\)-unknown PV formulation the Powerflow page already describes for a generator, just without any active-power term.

CGMES represents one as its own StaticVarCompensator class (a RegulatingCondEq, the same base every SynchronousMachine and ExternalNetworkInjection also derive from), carrying:

• capacitiveRating / inductiveRating — the SVC's reactive range,
• slope — a droop coefficient for voltage-vs-reactive-power regulation,
• q (SSH) — a starting/fallback reactive power value,
• an optional RegulatingControl reference — the same voltage-mode-target mechanism SynchronousMachine uses.

The concepts

1. Ratings are reactances, not powers

capacitiveRating/inductiveRating read like power quantities ("...at maximum capacitive reactive power") but are documented, and universally treated by real tools, as reactance ratings in ohms. They have to be converted to a susceptance, and from there to a reactive-power rating, rather than used directly as a Mvar value:

\[ B = \frac{1}{X_{rating}}, \qquad Q \approx V^2 B \]

The deciding evidence is that powsybl-core's CGMES importer computes exactly 1 / rating to get the susceptance it stores. If capacitiveRating were already a Mvar quantity, taking its reciprocal to get a susceptance would make no dimensional sense. (See "A bug this caught" below for what happens if you believe the doc text instead.)

An absent or zero rating conventionally means unlimited rather than zero — mapped to \(\pm\infty\) (or ±Double.MAX_VALUE), the same "no rating means no limit" convention tap-changer xMin/xMax uses.

2. Three regulation behaviors, in increasing fidelity

Hard voltage pin. An SVC that is actively regulating in voltage mode is treated exactly like a PV bus: its controlled bus's \(\vert V \vert\) is fixed at the target and its \(Q\) is the free variable, clamped to \([B_{min}V^2,\ B_{max}V^2]\). This is the same mechanism as a generator's voltage control — the SVC contributes no \(P\) term, which is the only structural difference.

Droop (slope). A real SVC doesn't hold voltage exactly; it regulates along a droop characteristic, so that absorbing more reactive power comes with a slightly lower terminal voltage. The linearized form folds directly into the voltage equation as an extra term rather than being a post-hoc correction:

\[ V + \text{slope} \cdot Q_{SVC} = V_{target} \]

Standby / dead-band. An SVC may sit idle as a fixed susceptance while voltage stays inside a dead-band, only entering active regulation when voltage leaves it. Because that decision depends on the solved voltage, it can't be made before the solve — it needs an outer loop that toggles the bus between a fixed-susceptance PQ shunt and an active PV pin between passes, the same architectural pattern the Reactive Power Limits page describes for Q-limit switching, applied here to decide whether to regulate at all rather than how far a limit was exceeded.

A tool that implements only the hard pin still solves the common case correctly; droop and standby refine it.

3. The regulated bus need not be the SVC's own bus

RegulatingControl.Terminal is independent of the equipment's own terminal, so an SVC can regulate a remote bus. Two consequences for anything converting one: the bus whose voltage gets pinned is the control terminal's bus, while the per-unit base for converting the ohm ratings into a Q limit is the SVC's own physical bus's rated voltage. The two differ whenever regulation is remote.

4. Not regulating? Fall back to a fixed injection

An SVC with controlEnabled = false, a disabled RegulatingControl, or no RegulatingControl at all is not a voltage-controlled bus. The fallback is the SSH q value as a plain fixed reactive injection — an ordinary PQ contribution.

Where this fits in gridoxide today

src/cgmes.rs's StaticVarCompensator conversion (added alongside the ACLineSegment.gch fix — see the Shunt Conductance page — after this fixture's own SVC was found to be silently dropped entirely) mirrors SynchronousMachine's existing RegulatingControl-driven PV upgrade:

• If the SVC has an enabled, voltage-mode RegulatingControl, the controlled bus (concept 3 — possibly remote) is promoted from PQ to PV and pinned to the target voltage: concept 2's hard pin.
• Otherwise it falls back to concept 4's fixed Q injection from the SSH q value, using the same sign convention SynchronousMachine.q already established empirically (no negation — see that code's own comment for why the doc text alone isn't trustworthy here).

q_min/q_max follow concept 1 — a reactance rating converted to a per-unit reactive-power limit, \(Q \approx V^2 B \approx B_{pu}\) at \(V \approx 1\) pu (the same flat-voltage approximation SynchronousMachine's own min_q/max_q already make), anchored to the SVC's own physical bus's u_rated:

#![allow(unused)]
fn main() {
let z_base = own_bus.map(|b| buses[b].u_rated * buses[b].u_rated / s_base_va);
buses[controlled_bus].q_min = match (sc.inductive_rating, z_base) {
    (Some(x), Some(zb)) if x != 0.0 => zb / x,
    _ => -f64::INFINITY,
};
buses[controlled_bus].q_max = match (sc.capacitive_rating, z_base) {
    (Some(x), Some(zb)) if x != 0.0 => zb / x,
    _ => f64::INFINITY,
};
}

Two deliberate simplifications versus concept 2's fuller model, both consistent with gridoxide's existing scope elsewhere:

• No droop/slope. StaticVarCompensator.slope is not read at all — every regulating SVC is a hard voltage pin, the same simplification gridoxide already makes for SynchronousMachine.
• No standby/monitoring mode. A non-regulating SVC falls back to its fixed q injection permanently; there's no outer loop that would later switch it back into regulation if voltage left some dead-band, since gridoxide's plain newton_raphson doesn't run any outer loop for SVCs at all (only for PV→PQ switching, and only when newton_raphson_enforcing_q_limits is used instead of the default solver).

Tool reference

power-grid-model and lightsim2grid have no SVC concept at all: PGM's only shunt-connected component is Shunt, a fixed admittance, and lightsim2grid's ShuntContainer is a fixed injection stamped straight into the Y-bus diagonal. Both are consistent with their domains — SVCs are a transmission-level device.