Line Voltage Regulator Tap and Bandwidth

A step voltage regulator has three settings that interact -- set voltage, bandwidth, and time delay -- and getting them wrong produces either a feeder that sags at the end or a tap changer that wears itself out. An ANSI regulator ranges over 32 steps of five-eighths of one percent each, plus or minus ten percent, so on a 120 V base one step is 0.750 V and the full range is 12.0 V. THE BANDWIDTH IS A DEADBAND, not a target: the control does nothing while the sensed voltage stays inside it and moves one tap when it leaves. The first hard rule is that the bandwidth must exceed one tap step. A deadband narrower than the regulator's own correction means every operation overshoots out the far side and has to come straight back, which is hunting, and it spends a tap changer's rated operations in a fraction of its life. Ordinary practice is about one and a half to two steps, which on a 120 V base is a bandwidth somewhere near 1.5 to 2.0 V. Line drop compensation makes the regulator hold voltage at a point out on the feeder rather than at its own terminals, by subtracting a synthetic drop proportional to load current so the control sees what the regulation point sees. The R and X dials are in volts at rated CT secondary current and they encode the impedance out to that point, so the simulated drop reported here scales with the ratio of actual current to CT rating and with the load power factor. Set them too high and the regulator overcorrects at peak; set them to zero and the far end of the feeder sags exactly as far as the line drops. One single-phase regulator, steady state, ANSI 32-step ranging. It does not set the time delay, which is the third setting and the one that coordinates cascaded regulators and a regulator against a switched capacitor -- a downstream device must be slower than the upstream one or they fight each other. It does not derive the R and X settings from the impedance to the regulation point, which is where they should come from, and it does not produce a feeder voltage profile. Reverse power flow from distributed generation behind the regulator breaks the compensation logic entirely and needs a reverse-sensing mode this knows nothing about. ANSI C57.15, IEEE 1783, the regulator manufacturer's control manual, and the utility's voltage regulation practice govern.

volts per tap step = 0.00625 x base voltage (32 steps of 5/8 of one percent, plus or minus ten percent); bandwidth in steps = bandwidth / volts per step, which must exceed one; output at a tap = base x (1 + 0.00625 x tap); line drop compensation drop = (load current / CT rating) x (R x cos phi + X x sin phi).

ANSI 32-step regulator ranging and the line drop compensation relation, by name, with ANSI C57.15 named. One single-phase step regulator, steady state. The time delay is not set here and reverse power flow is out of scope. ANSI C57.15, IEEE 1783, the regulator manufacturer's control manual, and the utility's voltage regulation practice govern.

A multiplication by the ANSI step fraction and one trigonometric term; no manufacturer control table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: base_voltage_v, bandwidth_v, tap_position, load_current_a, power_factor, ldc_r_volts, ldc_x_volts, ct_rating_a, volts_per_step, bandwidth_steps, full_range_v, output_voltage_v, simulated_drop_v

• One single-phase regulator, steady state cascaded regulators need the time delay this does not setIEEE 1783
• The R and X settings are entered they should be derived from the impedance to the regulation pointthe utility's voltage regulation practice
• No reverse power flow generation behind the regulator breaks the compensation logicIEEE 1783