Initial designs for the ANGLE DC project; converting existing AC cable and overhead line into DC operation

ANGLE DC is an innovative project which will convert two existing AC distribution circuits from the island of Anglesey to the Welsh mainland to operate as DC circuits. At present, there is a double circuit 33kV AC connection between the Llanfair PG sub-station on Anglesey and the Bangor 33kV sub-station on the mainland. For the ANGLE DC project, the 3-phases of one AC circuit will be connected in parallel to form the positive conductor and the three phases of the other AC circuit will form the negative conductor. At each end of the circuit, an AC - DC converter will be installed. The use of Voltage Source Converter (VSC) technology for the converter inherently provides reactive power control capability on both AC networks, which will improve the voltage profile in the local region and hence reduce the operating losses in the networks. A key feature of the project is the retention of the existing AC cables and overhead line; hence attention was paid to the choice of DC voltage and DC current to avoid over-stressing the existing asset, parts of which are in excess of 50 years old. As discussed in the paper, the voltage was chosen to be ±27kV and the current was chosen to be 556A. This gave a scheme power rating of 30.5MW, which is a 23% increase in transmitted power capacity from the existing operational limit of 24.8MW. Initial voltage testing of the cable at 38kV indicated that operation under DC stress was possible and that no significant partial discharge was measured. To provide condition monitoring on the cable, an on-line partial discharge detection system will be installed when the circuits are converted for DC operation. The ANGLE DC project will demonstrate the viability of converting existing AC assets to DC operation and the up-lift in power transmission which can be achieved. For future applications, with less concern about the age of the assets, higher power up-lift may be anticipated.