Large adjustable speed and soft-started drives with long cable lengths

The popularity of large voltage source inverters (VSIs) has increased for a wide range of applications. An overview is given on how cable length challenges associated with VSIs can be overcome. Load commutated inverters (LCIs) are however in many cases still more suitable for pure soft-start applications and for very large drives. Very limited literature does however exist describing how to overcome long cable-length problems in LCI applications. The associated overvoltage phenomenon due to resonance and voltage distortion is explained. Case studies and simulation results are presented based on actual configurations in large petrochemical plants with motor ratings ranging from 17 MW to 55 MW with cable distances up to 3 km. An evaluation is performed to determine when additional electrical filter and rotor position detection equipment are required. It is shown that with correctly designed systems the applicable motors can be started without the presence of unacceptable overvoltages, pulsating air-gap torques or shaft resonances. A general design approach is described to determine the requirements for VSIs and LCIs. Ultimately it is shown that effective, safe, reliable and low cost solutions exist to extend the capabilities and application of VSI and LCI systems.