New Harmonics Current Mitigation Technique in Induction Motor Driving Reciprocating Compressor

Reciprocating machines, and especially reciprocating compressors, produce large fluctuating torques. These cyclic torques induce periodic angular oscillations (i.e. torsional vibrations) to the driving shaft. When a reciprocating compressor is driven by an electric motor, also the rotor of the latter is subjected to these oscillations. Synchronizing torques, damping torques and pulsating primary currents are developed due to this vibratory motion of the rotor. Consequently the actual instantaneous line current is sum of instantaneous values of steady state mean load current and pulsating currents. The main standards associations (IEC, IEEE, NEMA, API) have issued standards and guidelines for the construction of electric motor driving compressors, designed for several application (chemical, petrochemical, natural gas, etc.). For example, API 618 establishes limits for electric motor current pulsation with respect to the full load current value, consistent with NEMA MG-1 and IEC 60034: for synchronous motors this limit is fixed to 66\%, while for induction to 40\%, and IEEE Standards 519-2014 has established recommendation for the design of electrical system that include both linear and non-linear loads which can be source of harmonics. In order to verify these limits, a dynamic study of the system composed by the compressor and the electric motor is required. This study was traditionally performed in a two-step procedure that separates the mechanical and the electrical systems. At first a Torsional Vibration Analysis (TVA) was carried-out, under the packager responsibility, modeling the mechanical system with a lumped parameter scheme without taking into account the electrical part; after that, the electric motor supplier performs a source current pulsation analysis on the electric motor system, based on the TVA results. On industrial TVA of compression units, only recently, the effect of the motor air-gap has been taken into account by adding a spring and a damper between motor and ground in the model. This model is more accurate than the traditional one, but is applicable only to the steady-state condition and still fails to consider the reciprocal effects between the two parts of the system. The new model links together the mechanical and the electrical part of the system and describes in a more accurate way the whole system behavior, getting at the same time more consistent results for the torsional behavior and the current pulsations. This has been possible by using a multi-physics simulation software capable of simultaneously solve the electrical and mechanical equations of the systems. Starting from this model, new mitigation techniques can be applied in reducing the harmonics effects in currents pulsations. In this paper, a new harmonics current mitigation technique, based on hybrid active filter, is presented for induction motor driving reciprocating compressors.