Power transmission network

A power transmission network comprising:

• an AC electrical network connected to a point of common coupling, the AC electrical network having a voltage, the point of common coupling being connectable to a further electrical device;
• and
• a processing circuit comprising a phase locked loop, the processing circuit configured to receive and process a voltage of the point of common coupling to determine a phase difference between the voltages of the AC electrical network and the point of common coupling during an exchange of power between the AC electrical network and the point of common coupling, the phase locked loop comprising:
• a phase detector,
• a loop filter, an output of the loop filter being configured to provide a phase difference signal corresponding to the phase difference;
• a first voltage controlled oscillator;
• and
• a second voltage controlled oscillator configured to lock the phases of the voltages of the AC electrical network and the point of common coupling prior to the exchange of power between the AC electrical network and the point of common coupling;
• wherein the phase locked loop is configured to switch between the first voltage controlled oscillator and second voltage controlled oscillator.