Modeling and Control of OLTC-Based Volt/VAR Regulation in PV-Rich Distribution Feeders with Thermal Life Protection

High penetration of photovoltaic (PV) generation in medium-voltage distribution feeders causes midday over-voltage, rapid voltage fluctuations, and frequent On-Load Tap Changer (OLTC) operations. Conventional OLTC logic—typically a fixed deadband with short dwell times—responds poorly to fast PV variability, accelerating mechanical wear and exacerbating transformer hot-spot temperatures that reduce insulation life. This paper develops a comprehensive framework that (i) models the electrical and thermal behavior of a distribution transformer, feeder, PV inverters, and switched capacitors; (ii) introduces a supervisory Volt/VAR control that prioritizes inverter VAR droop → capacitor steps → OLTC; and (iii) embeds a thermal guard into OLTC decision-making to protect transformer life. The system is implemented in MATLAB/Simulink with multi-rate solvers. Case studies on a 33/11 kV, 10 MVA transformer feeding a 5 km radial feeder with 4 MW distributed PV show: (1) voltage compliance within ±5% under diverse conditions; (2) a ~55% reduction in tap operations compared with PV-rich conventional control; (3) peak hot-spot capped at ≈115 °C with ~35% lower daily loss-of-life; and (4) PV hosting capacity improved from ~30% to ~50% of feeder rating. The results demonstrate that asset-aware, hierarchical Volt/VAR control provides both power quality and asset longevity.