There are many BUILD UP news items referring to infringement, e.g.: http://www.buildup.eu/en/search/infringement
According to the EU treaties, the European Commission may take legal action – an infringement procedure – against an EU Member State that fails to implement EU law. The Commission may refer the issue to the European Court of Justice, which in certain cases can impose financial penalties.
Along these lines, the European Commission oversees that EU Member States fully and correctly apply EU energy law. When a Member State fails to do so, the Commission may take legal action by launching an infringement procedure and eventually referring the case to the Court of Justice of the European Union for a ruling.
Specific information regarding infringement procedures in relation to energy can be found at: https://ec.europa.eu/energy/en/topics/enforcement-laws