Wednesday, December 1, 2010
AGC sensors and Controls- changes in 1970-80
I like to share few personal AGC experiences encountered during 38 years in the field of rolling mill automation and controls. First one being Hot Strip Mills of Bokaro in India commissioned in 1975. I was one of the electrical commissioning engineer mainly involved with 132 /11 kV main step down substations and 11 kV supply system and motors. 11 kV synchronous motors of up to 110 MW was commissioned by me and my friends for Roughing Mill area. These stands were not with any AGC system, however 7 stand finishing mills were equipped with electro mechanical AGC system commissioned later on .. till then mill was running on manual thickness controls.The AGC system used DC screw down motors fed from circulating current type thyristor converters (SCR) and main motors were DC motors fed from Mercury Arc rectifiers fed from Zig Zag connected double star transformers.
Actual AGC control has been achieved through the use of Russian operational amplifier system called UBSR. This system allowed changes in amplification and time constants by changing capacitance and resistances using normal soldering of these passive devices. These changes were done based on time constants of mechanical systems or electrical motor circuits and we tried to achieve a step response within minimum possible time and allowed up to 4% overshoot in the first crest so that the feedback system is optimally damped. Sensors used were Selsyn transmitters, analogue tachos (they looked like DC motors those days) and X-Ray Thickness gauges. Inter stand minimum tension controls were achieved through the use of stall motor controlled loopers.
I can recall 1 incident which took place those days - Hot Strip Mill was commissioned during the time Emergency was declared in India. We had kept transformer no 2 of 132/11 kV main step down sub station no 2 ready for commissioning except for break down value of bushing oil which was less than 40 kV in the Russian pot we used to test the oil. While our commissioning department preferred internal heating by feeding the transformers at 11 kV and shorting of secondary terminals (calculation for short circuit current was made for these transformers having over 20% impedance), erection department went ahead with heating of bushings with flood lights covering then with Tarpaulins. One day while we returned after lunch at 1:60 PM (that's how we shall joke about being in time at 2.00 PM),due to lightening the Tarpaulin caught fire and the bushing cracked under heavy rains. Those days we use to develop affection with the equipments and I was in tears. Flame from the fire started reaching 132 lines of Transformer no 1 which was charged. Our efforts to convince the operator at MSDS 1 to switch off 132 kV power to this transformer was going in vain since he wanted permission from Sri BB Dutta, Chief Power Engineer and a difficult person. Then I remembers the differential relay for 132 kV air blast circuit breaker of MSDS1 located in our substation (commissioned by me earlier). I manually actuated that relay and saved major accident since the 132 kV line was about to snap due to the heat of the flame... In subsequent days my boss Mr JB Prasad got lot of praise for this action of mine...
Tomorrow I shall write about experience we had in Salem Steel Plant in India. I have included a photo of 2 Hi mill by MECON in that plant...in this post itself.