Method to reduce overvoltage on AC motor insulation from inverters with ultra-long cable

Stator insulation guidelines for allowable dv/dt and peak over-voltage such as NEMA MG-1, Part 31 and GAMBICA are well established. To meet these guidelines inverter output reactors are often used to reduce motor dv/dt and peak over-voltages for inverters with short output cables. However, an output reactor solution using ultra-long motor cables actually substantially increases motor peak over-voltage to damaging levels, depending on drive modulation strategy, carrier frequency and cable characteristics. Thus, an improved output [L-R] filter solution method is described using small 3%-5% output inductor with specially selected parallel damping resistors to reduce motor reflected wave overvoltage having cables up to and beyond 2,000 ft. AC drive modulation factors and motor cable characteristics contributing to the overvoltage transient are first described. Ineffectiveness of output reactor alone solutions is analyzed using theoretical model and simulation results. Effectiveness of output [L-R] filters to mitigate motor overvoltage is also analyzed, simulated, and demonstrated with experiment results. This paper provides guidelines for selecting reactor impedance rating and damping resistor power rating to keep motor PWM overvoltage transients to <; 1.3x dc bus, as transmission line cable model length increases to 2,000 ft. Finally, the paper defines the maximum allowable ultra-long cable length limit where even [L-R] filters become ineffective under a pulsed modulated power source, and which sinewave output filters are then required.