Generator vmin_pu/vmax_pu setters don't persist values to OpenDSS C engine

[matheusduartedm]

Bug description

Setting dss.generators.vmin_pu and dss.generators.vmax_pu via the property API appears to succeed (the getter returns the new value), but the value is not persisted to the underlying OpenDSS C engine. This causes silent data corruption in power flow results.

The equivalent load properties (dss.loads.vmin_pu, dss.loads.vmax_pu) work correctly.

Reproduction

import py_dss_interface

dss = py_dss_interface.DSS()
dss.text("compile [master.dss]")

dss.generators.name = "sgen_0"

# Write via property API
dss.generators.vmin_pu = 0.0

# Read back via property — looks OK
print(dss.generators.vmin_pu)                    # → 0.0

# Read back via text command — value unchanged!
print(dss.text("? Generator.sgen_0.Vminpu"))     # → 0.90 (default)

Expected: Both should return 0.0.
Actual: The text command reveals the C engine still has the default value.

Root cause

The bug is in the OpenDSS C library (libOpenDSSC.so), not in py-dss-interface's Python code.

In GeneratorsF.py, the setters correctly call:

• GeneratorsF(11, value) for Vmaxpu
• GeneratorsF(13, value) for Vminpu

However, the C library's handler for these indices doesn't write the value to the active Generator object — it only echoes the parameter back. The equivalent DSSLoadsF handler (indices 29/35 for loads) works correctly.

Impact

When Vminpu/Vmaxpu are not actually set, OpenDSS uses the defaults (0.90/1.10) and may switch the generator model from constant power to constant impedance at unexpected voltage levels, producing different P/Q results without any warning.

Workaround

Use text commands instead of the property API:

dss.text(f"Generator.{name}.Vminpu=0.0")
dss.text(f"Generator.{name}.Vmaxpu=2.0")

Fix

PR #106 implements this workaround directly in GeneratorsF.py, replacing the broken C API calls with text commands while keeping the same public API.

Environment

• py-dss-interface version: 2.2.1 and master (2.3.0)
• Platform: Linux (tested)
• OpenDSS C library: bundled libOpenDSSC.so