The damage left 93 homes in the Scunthorpe area without power and Northern Powergrid said most were without power for some time.
The repair bill for the seven incidents is estimated to cost around £800,000.
Roderick Stuart, a spokesman for Northern Power Grid, said the thieves were lucky to be alive.
He said: “They cheated death. One wrong move and it would have killed them.
“We do believe this was an organised crime group and that is from the methods used to steal these transformers.
“They appear to be getting up the poles and attaching ropes or chains to the transformers and pulling them off the poles.
“These transformers were 11,000 volts and you would either have to be extremely lucky or know what you are doing to be able to remove them.
“They are getting very close to these 11,000 volt transformers. It is a miracle they are not electrocuting themselves.
“They disconnect cables and leave live cables hanging off.
“Anyone coming into contact with them, could be seriously injured or killed.
“In the year I have been working here, I have not come across this before.
“It seems to be quite a big operation.
“To get these transformers off the top of the poles, they would have needed the equipment to do that.”
Both Northern Powergrid and Humberside Police are appealing for residents to remain vigilant.
Thieves 'cheating death' when they steal transformers