I have a LG LRE30453SW Electric Range and the relay pcb keeps going bad. I get an F7 error when starting the oven. The first time I noticed the black line in, from the power terminal, to the relay pcb was blown. I replaced the relay pcb and it did start to heat the oven and we baked one thing and then it blew again. I replaced the relay pcb again and it blew again. What could be causing this to keep blowing? Do I need to replace the main pcb? The resistance on both bake and broil elements reads normal as does the oven sensor. Any help would be appreciated.

First, check the 220v line and breakers are properly grounded and that you are getting a constant 220v at the plug. Then you will need to check the transformer on the unit.To test a transformer you need to know what type of transformer it is first. There is a step up transformer which steps the voltage up and a step down transformer which steps the voltage down.
You will still test the transformers the same way for step up and step down transformers. If it is a step up transformer apply the incoming power to the transformer or plug it up. Now take your multimeter and place it on the AC volt scale and test the voltage if the voltage on the data plate says the transformer should step up to 220 volts and it doesn't then your transformer is bad.
Now repeat the same steps for a step down transformer. Plug up the incoming or primary power and test voltage on the outcoming or secondary side the voltage should step down to the proper value. If you don't read any voltage on the secondary or outgoing side then the transformer is bad.
To read the voltage place the two meter leads on the secondary side, or the side you will plug your equipment into. If the secondary side is an outlet then place the two meter leads into the top two slots in the plug and read voltage. You also want to make sure the incoming power is working as well. Otherwise you will be reading a bad reading every time.
If all that looks good, then you will need to replace the control board. Here is a link to the parts for your range.
Regards,Richard