My screen wont turn off even after pressing the power button which drains my battery and its show a notification as "USB connected "when i haven't connected any USB ...(samsung galaxy m30)
Welcome to Android Central! As belodion no doubt suspects, I'd consider a faulty USB port on the phone. Try blowing out the port with a can of compressed air, and clean its contacts with a Q-tip dipped in rubbing alcohol.
I can switch off the phone but the screen does not go off when i press the power button and even after cleaning the usb port it still shows usb connected when its not ..
If Galactic Zoo's suggestion doesn't work, then try wiping the cache partition: https://www.hardreset.info/devices/samsung/samsung-galaxy-m30/wipe-cache/. If that doesn't work, then the next step would be a factory reset. Beyond that, then it's either contacting Samsung for warranty repair (if it's still covered) or bringing it to a repair shop.