A three-phase power-flow solver reported convergence for four transformer types it does not support, returning bus voltages near a million per unit with no warning. Ten other unsupported types were correctly rejected.

The system said: converged=True.
The evidence showed: 994,686 per unit on a 0.4 kV bus and a 10.9 MW residual on a 0.1 MW load. Four unsupported transformer types skip the gate that rejects the other ten.

§ 1

The record

FINDING · pandapower-unsupported-vector-groups-return-convergedverify · coverage illusion

A three-phase power-flow solver reported convergence for four transformer types it does not support, returning bus voltages near a million per unit with no warning. Ten other unsupported types were correctly rejected.

Claim: In pandapower develop at fd7346f, runpp_3ph on the network from closed issue #1041 returned converged=True with no warning for the vector groups Dy, Yy, Yd and Dd, with identical non-physical results (about 994,686 per unit on the 0.4 kV buses, 714% transformer loading, a 10.9 MW power-balance residual against a 0.1 MW load), because zero-sequence modelling is skipped for those four strings before the gate that rejects the other ten unsupported groups. All 18 cells matched their pre-registered prediction.

Status: OBSERVED Witnessed in one instance. No claim about how often.

Question: Passed is not correct

Subject: pandapower, an open-source power-system analysis library (third-party system); the classifier and the sanity bands are Omega's. third-party subject

Frame: The #1041 network verbatim; 18 two-winding vector-group strings from the source, the docs and the shipped standard types; shift_degree 0; each cell in its own process.

Method: Each string classified as documented-supported or unsupported before the run, with a predicted outcome per cell frozen in a pre-registration; the run compared against the prediction; the source tree diffed against the pinned archive after the run.

Oracle: Five authored sanity bands on converged results: finite values, low-voltage magnitudes in 0.9 to 1.1 per unit, external-grid power in band, power balance under 1e-3 MW, transformer loading under 100%.

Negative control: Present The supported Dyn group as a positive control and the known-silent Dy group as a negative control both reproduced earlier runs the same day to the digit; no environment drift.

Denominator: 18 of 18 cells matched their frozen prediction; 4 of 14 unsupported groups returned converged=True; 10 of 14 raised; 0 failed to converge.

Preregistration: PREREG-VECTOR-GROUP-BOUNDARY, hashed before execution and unchanged after; no deviations.

Limitation: One network, one load, zero phase shift, one commit. The four silent strings map to one model, so they are one observation, not four. No independent oracle was used for the supported groups, so plausible is the ceiling of what is said about them. Whether the zero-sequence skip is intended for another calculation path is upstream's call. The issue is open and unanswered; nothing has been fixed.

Source: artifact not public

Reproduce: not reproducible from a public clone

Independent reproduction: None known.

Upstream: REPORTED UPSTREAM Issue #3122, opened 6 September 2026; no maintainer response as of 20 September 2026. Link

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Where this sits

This finding answers Passed is not correct and supports the VERIFY stage of the operating method. It is an instance of the coverage illusion mechanism.

Evidence ledger Bring us a problem