Frequency inverter in blown converter for energy saving

A Steel Refining Company has three sets of 15t oxygen upper blown converter (capacity is expanded to 20t now). In 2001, its yearly production is 710000t.

Because the converters discontinue to blow oxygen periodically, they require fan to run in variable speed in whole refining period in order to meet requirements of energy saving and environmental protection. Original speed adjustment mode is hydraulic coupling. High speed 2700r/min, low speed 800r/min. there are following problems due to limitation of hydraulic coupling.

• Adjustment range is 30%～90％, speed is unstable. Furthermore, low speed 800r/min is still high, which causes energy wasting. In high speed, there is slippage and smoke rising from converter mouth.
• After 10 years of operation, hydraulic coupling is serious aging. It needs to change bearing frequently, which leads to stop converter. That can not meet requirement of continuous production.
• Motor efficiency is low, particular in low speed.
• Low adjustment precision, inferior linearity, slow response.
• Big start current affects power grid stability.
• Motor can not switch to direct on line when hydraulic coupling is broken. Machine must stop to repair.
• Oil leakage pollutes environment seriously and corrupts floor heavily.

Thereby, it is urgent to reform hydraulic coupling.

At the end of 2001, the company began to expand the plant. In order to heighten fan efficiency and solve problems of hydraulic coupling, company decided to reform with frequency inverter. Requirements of frequency inverter is following:

• frequency inverter can run for long term without fault with high reliability.
• Motor can switch to direct on line when frequency inverter is broken.
• Adjustment range is wide and efficiency is high. frequency inverter should have logic control ability.
• frequency inverter can accelerate/decelerate automatically according to oxygen blowing period.
• frequency inverter has skip set for resonance points, which can avoid resonance points during operation and prevent fan from surging.

After investigation and comparison, the steel company purchased a 6KV/630KW frequency inverter at the end of 2001. The frequency inverter put into service on 14th March, 2002. Compared with original hydraulic coupling, frequency inverter has following advantages:

• Operation is stable, safe and reliable. For hydraulic coupling control mode, bearing has to be changed every 40 days. Changing bearing needs to stop for half a day, which brings much economic loss. For frequency inverter, it is free maintenance and only needs to change filter and no need to stop, which ensures production continuity.
• Energy saving effect is obvious and power consumption is lowered greatly.
• Soft start and soft stop function. Sine current supplied by frequency inverter reduces motor fault. frequency inverter sets skip resonance points to avoid operation on resonance points for long time. It reduces wear and tear of bearing, prolongs equipment lifetime and heightens utilization factor of fan.
• frequency inverter has completed protection. Compared with relay protection, frequency inverter protection is more sensitive and completed. During commissioning in site, frequency inverter protected and reported over current of motor. After inspection, we found one phase cable terminal was cracked. lack phase caused over current. we repaired in time and accident was avoided.
• Communication meets production need. Imbed PLC makes signal input flexible. Converter provides a pair of high speed nodes and a pair of low speed nodes. frequency inverter runs in high speed or low speed automatically according to nodes state. Gas flow and temperature signals which are in instrument cabinet are connected to frequency inverter. Instrument cabinet is dismantled and installation space is saved greatly. frequency inverter has speed meter, so original speed meter is canceled. frequency inverter can indicate motor speed in site.
• Adaptive ability for grid vibration is strong. Grid voltage reaches 6.9kV sometimes, but frequency inverter still works normally.

Because the frequency inverter has above advantages, the Steel Refining Company decided to purchase again for 2# and 3# converter. 2# and 3# frequency inverters put into service and passed the inspection on 30th July, 2002.