Challenges to supplying large induction motor loads in a long radial transmission system

Load growth along a 349 km single radial 138 kV transmission system in BC, Canada comprises many induction motors in the order of 4 MW: over ten to be directly connected, and the others fed via Variable Frequency Drives (VFDs). Dynamic simulations shows a normally cleared fault in the transmission network that supplies the radial system causes the directly connected motors to stall, resulting in sustained low voltage, and small hydro generators located at the end of the radial system going out of step. Potential mitigating schemes including remedial action schemes, addition of STATCOM or synchronous condensers and transmission line reinforcement were simulated and evaluated from performance, costs and operating experiences perspective. A cost effective technical solution is identified that requires installing additional VFDs to achieve acceptable dynamic performance.