Impact of induction furnace interharmonics on distribution systems

Some modern electronically controlled induction furnaces are capable of producing a wide range of nonpower frequency currents. Connecting this kind of load to a typical North American distribution system can have quite undesirable consequences. The potential for disruption of other end user's sensitive loads is high. A typical distribution system will employ numerous switched capacitor banks and the system is always resonant at some frequency, frequently within the same range as that produced by an induction furnace. The power industry is accustomed to dealing with harmonic distortion and care is often taken to avoid resonances at a harmonic frequency, particularly, an odd harmonic. However, it is unprepared for loads that can inject nonharmonic frequency currents over a wide range of frequencies and can excite whatever resonance exists. The basic characteristic of the induction furnace load is described. Experience with an actual case study is also described. Several alternatives for addressing the problem are presented, including modifications to induction furnaces, reconfiguration of the power supply, and filtering options. The impact of utility deregulation on the identification and mitigation of such problems is also discussed.