Practical Charts for Sub-Synchronous Resonance Assessment of Induction Motor Starts

Many electric distribution utilities employ series compensation to support system voltage when supplying large industrial loads located far away from the substation. Sub-synchronous resonance associated with induction motors is a well-documented phenomenon that still plague these systems and is typically handled by employing preventative measures that include sizing the series capacitor bank using rules-of-thumb. However, the maximum size of the series capacitor bank that allows the system to operate prior to creating a potential sub-synchronous resonance condition is highly dependent on the distribution system topology and induction motor parameters. As a result, these sizing practices do not prevent sub-synchronous resonance conditions in many real systems, resulting in increased number of switching operations of sub-synchronous resonance resistor insertion and of the by-pass breaker, which prevents the phenomenon but removes the series capacitor bank from the circuit when it is most needed. This paper proposes creating guidelines for sizing series capacitor banks by using practical charts derived from a large number of Electromagnetic Transient simulations. These charts can be easily adopted by utility engineers and only require basic knowledge of the distribution system and induction motor, such as Short-Circuit Ratio and motor nameplate.