No public utility, and no subsidiary or affiliate of, or corporation holding a controlling interest in, a public utility, shall purchase or acquire, take or hold, any part of the capital stock of any other public utility, organized or existing under or by virtue of the laws of this state, without having been first authorized to do so by the commission; provided, however, that the commission may establish by order or rule categories of stock acquisitions which it determines will not be harmful to the public interest, and purchases within those categories are exempt from this section. Every assignment, transfer, contract, or agreement for assignment or transfer of any stock by or through any person or corporation to any corporation or otherwise in violation of any of the provisions of this article is void and of no effect, and no such transfer shall be made on the books of any public utility. Nothing in this section prevents the holding of stock previously lawfully acquired.