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At Medium-frequency electric furnace In power-frequency power systems, reactive power compensation and odd-harmonic mitigation technologies are employed. A thyristor-based parallel resonant frequency converter is utilized, with power regulation achieved by adjusting the firing angle of the SCRs. In power-frequency power systems, odd harmonics are generated. The formula for calculating the characteristic harmonic spectrum of a frequency converter is: n = kP ± , where k is a positive integer (k = 1, 2, 3, ...). For a rectifier transformer supplying a single three-phase winding, the DC output exhibits six-pulse ripple, with characteristic odd harmonics at 5th, 7th, 11th, and 13th orders. For a rectifier transformer supplying a dual three-phase winding, the DC output exhibits twelve-pulse ripple, with characteristic odd harmonics at 11th, 13th, 23rd, and 25th orders.

In the triggering of a fully controlled thyristor bridge, due to factors such as asymmetric free-hanging leads, additional non-characteristic harmonics may arise. However, the primary issue is the pollution of the power grid by higher-order harmonics, which are predominantly characteristic harmonics. This leads to a decline in power quality, increased equipment damage, and reduced power output. In conventional low-voltage reactive power compensation devices, harmonic currents are amplified by the compensating capacitors, causing contactors to burn out, capacitors to swell, and capacitor banks to fail to operate properly. As a result, the load power factor deteriorates continuously, forcing enterprises to pay substantial reactive power adjustment fees. For medium- and large-sized intermediate-frequency furnaces with DC output pulses—whether 12-pulse or 24-pulse—capacitor banks cannot be put into operation normally, leading to a prolonged decline in the load power factor and high reactive power adjustment charges for enterprises. For intermediate-frequency furnaces below 1000 watts (excluding 1000 watts), using a variable-frequency device with a 6-pulse DC output and equipped with a low-voltage reactive power compensation system powered by industrial frequency, adopting measures such as changing the connection of compensating capacitors from delta to star configuration and switching current and voltage sampling to high-voltage-side CTs and PTs can ensure normal operation. The reactive power compensation and harmonic control equipment for intermediate-frequency furnaces should be installed on the same side as the power transformer supplying the variable-frequency power source. For industrial enterprises that have multiple transformers and various types of equipment or production lines, if the production process or installation space permits, all equipment generating harmonics—also known as active loads—should be concentrated on one or more specific transformers, while other equipment that does not generate harmonics should be grouped together on different transformers. In this way, harmonic mitigation efforts need only focus on transformers equipped with active loads, whereas other transformers require only standard low-voltage compensation capacitor banks. Consequently, the total investment will be significantly reduced. On the contrary, if all active loads are distributed across all transformers, although the harmonic current content per transformer may not be very high and the harmonic voltage levels slightly below national standards, after installing ordinary capacitor banks for reactive power compensation, the harmonic currents will be amplified to an intolerable level, and the busbar harmonic voltage will immediately exceed the allowable limits. Measurements taken at many user sites have shown that typical harmonic currents can be amplified by more than three to four times, resulting in total harmonic distortion of the main voltage increasing by more than double compared to the original level. In some individual users, harmonic currents have reached over 90%, and the total harmonic distortion of the voltage has climbed as high as 14%!

To address harmonic issues, it is necessary to install a passive fundamental-wave generator on each transformer, which would significantly increase the overall investment. The closer the medium-frequency furnace’s output power is to its full capacity, the higher its power factor; conversely, the lower the output power, the greater the ratio of higher-order harmonics to the fundamental power frequency. Therefore, during operation, the medium-frequency furnace should be designed and operated as efficiently as possible. Given the complexity of its composition and the fact that its characteristics vary with changes in melting materials and process conditions, this introduces considerable uncertainty into the parameter design of reactive power compensation and harmonic mitigation devices. To address these challenges, it is essential to conduct thorough and meticulous on-site investigations, perform comprehensive field tests, and carry out system design simulations. Moreover, equipment manufacturing must rely on high-quality, highly reliable components. To purify the power supply system and reduce operating costs for industrial users, a unified management approach has been adopted for reactive power compensation and harmonic mitigation devices, covering their design, installation, and operation alongside the medium-frequency furnaces themselves.