電圧調整

Voltage regulation is the percentage difference between no-load and full-load voltage at the point of connection. It is defined as (V_no-load − V_full-load) / V_full-load × 100. Per EN 50160, the PoC voltage must remain within ±10% of nominal.

A load tap changer (LTC) adjusts the transformer's turns ratio to compensate for voltage drop in the feeder. Each tap step (typically 1.25% or 1.5625% per IEC 60076-1) changes the secondary voltage by a fixed percentage. With ±8 taps (±12.5%), the transformer's effective secondary voltage can be raised or lowered by up to 12.5% to keep the PoC voltage within limits.

Feeder voltage drop is primarily caused by resistive losses (I²R) and inductive reactance (I·X) in the conductors. The formula V_drop = I·√(R²+X²)·cos(φ−atan(X/R)) captures both effects, where φ is the power factor angle of the load. Heavy loads with low power factor produce the largest voltage drops.

EN 50160 specifies that the voltage at the point of connection should not deviate by more than ±10% from the nominal supply voltage for 95% of the time measured over a one-week period. This is the standard that determines whether voltage regulation calculations pass or fail.

If voltage regulation exceeds ±10% even at the extreme tap positions, the feeder conductor should be upgraded (larger cross-section or parallel feeders). This reduces R_feed and X_feed, lowering V_drop and improving regulation. The tap changer range should be reserved for daily load variations, not for correcting an undersized feeder.

The voltage at the load point is V_PoC = V_tap − V_drop, where V_tap is the transformer secondary voltage at the selected tap position and V_drop is the impedance drop in the feeder. Regulation % is then (V_PoC − V_nominal) / V_nominal × 100. A positive regulation means the load sees a voltage below nominal.