Wednesday, November 29, 2006

Insulation Value Crisis !

I was looking after the erection and commissioning of electrics at Bhilai Steel Plant (Site : Rajahara Mines) in Central India in the early sixties. Massive construction activity was on. The plant was being set up with the assistance of the erstwhile USSR.

A high-tension slip ring induction motor of 250 KW for driving a cone crusher was to be installed. The standard practice is that the insulation levels are checked and only when the IR (Insulation Resistance) value is more than several Mega ohms, the motor is energised.

On initial checking, the IR value was found to be much less than 1 Mega ohm. This was not considered unusual as the motor was shipped from the USSR long back and during several weeks of transportation and storage, moisture could have seeped in. As a standard remedial measure, the motor was put on heating, initially by external means and later by passing a current in the rotor of value higher than its rated load current. The motor was placed at its final location, which was about 25 meters below the ground level, in a pit. Since it was very cumbersome to get down into the pit every time to measure the IR value, the Meggar was kept at ground level, and a set of cables was sent down into the pit and connected to the motor.

The heating was started and, after every one-hour, the readings were taken. We were surprised to note that the IR value continued to remain low. After several rounds of heating, the result was the same much to the annoyance of the commissioning crew, and adding to the perplexity of the engineers.
Experts from the USSR were also at a loss.

The situation became very critical when it was known that the plant commissioning date was fixed just after 3 days to coincide with the visit of the then minister of Steel and Mines.

After many hours of lost sleep and anxiety of the team, it occurred to me to go down to the pit with the Megger and check the IR value connecting the Megger leads directly to the motor terminals. And lo, the value indicated was more than 100 Mega ohms, which was perfect! The readings were taken many times to ascertain that the values were consistent!

After analysis, it turned out that some one, while searching for a cable of suitable length, found an L.T. 3-core cable lying around, and 2 cores of this cable were used. Since the megger was a 10 kV HT Megger, it was measuring the insulation resistance between two cores of a low-tension cable. Even though the motor at the end of the cable had attained high insulation resistance after heating, the lead insulation itself was much lower and naturally the lower value was being indicated by the Megger. The Megger was measuring the inter-lead insulation and not the motor insulation value.

The team could then energize the electrics successfully and celebrated with a shot of vodka!

-------- Raghavendra Girimaji.