Grounds Stolen in Substation - Voltage Regulators

[jbhart]

Last week we had the grounds stolen off of 3 7200 volt step voltage regulators at one of our substations. We had some scada voltage alarms on these and a tech went to check it out. He immediately spotted the theft/problem and safely de-energized the units. They were sitting on a metal stands on a 24in thick concrete pad. Every point of contact on the pad had globs of molten glass and concrete particles blowing out from it. The wire from each regulators neutral bushing came down and was grounded to the grounding lug and then into the ground to attach to the ground grid. The wires were cut between the ground grid connection and the grounding lug on the case, leaving the neutral bonded to the case...The PT in one of the units is severely damaged and so is the concrete. We are trying to understand how much potential and current developed on the case of the regulator to do so much damage...and also how the theif managed to escape death/injury.

Any help or thoughts are appreciated.

 

[jghrist]

Any unbalanced load current would have to return through the ground and concrete to the tank ground and then to the SL bushing. If the ground connection between the single phase units was also stolen and if there were no metallic connection between tanks, then the current in the shunt winding (difference between source and load current) of each phase would have to flow through the concrete to complete each single phase circuit.

 

[waross]

Someone with a little electrical experience in his past and a pair of insulated gloves could cut 7200 volt grounds if he was careful. If these are autotransformer type voltage regulators, and they were at the center tap, niether boosting nor bucking there would be little current to ground. There would be less hazard at this time. As the load increased and the regulator tried to boost the line voltage, the voltage to ground across the missing ground connection would increase as the regulator acted as an open circuit CT. The regulator would probably saturate. As the taps went higher, the effective CT ratio would decrease and the current to ground would increase.
With the regulator at 1% boost look at the regulator as a 100:1 CT with X1 connected to the line and X2 connected to the steel frame sitting on the concrete.
With the boost on the 10% tap, the regulator would become a CT with a 10:1 ratio.
The regulator secondary would probably saturate under load.
The voltage developed to ground could be the saturation voltage of the regulator primary. The actual open circuit voltage would depend on both the load current and the tap position.

Bill
--------------------
"Why not the best?"
Jimmy Carter