The MPC provides that zoning is not applicable to any existing or proposed building, or extension thereof, used, or to be used, by a public utility corporation, if upon petition of the corporation, the Pennsylvania Public Utility Commission shall, after a public hearing, decide that the present or proposed situation of the building in question is reasonably necessary for the convenience or welfare of the public. Where such authority is provided by state law, the PUC may issue a certificate of convenience and determine that zoning regulations shall not apply.