A model of dual distribution voltage in a developed cosmopolitan city

11kV as distribution voltage for The Hongkong Electric Co., Ltd. (HEC) was first introduced in 1961, when the system peak demand was just 114 MW. Forty years later, the same distribution voltage has to support a system peak demand over 2500 MW with area load density over 100MVA/km² on average and 250MVA/km² in Central Business District on Hong Kong Island respectively. With such high area load density, a large number ofllkV cables are required to be laid side by side along the limited cable corridor under the narrow pavements of Hong Kong Island. The llkV cables cause mutual heating with each other and there is a limit to the number of cables that can be further added along the same route without overheating problem. On the other hand, the llkV system was originally designed as an open ring network, which will cause momentary supply interruption to customer when any cable is faulted or damaged. Despite the reduction of cable fault / damage rate in recent years, complete elimination of momentary supply interruption is impossible and thus there is still scope for further improvement in the System Average Interruption Frequency Index (SAIFI) with the adoption of a closed ring or dual feed distribution network. To address the inherent limitation of the existing llkV open ring network, HEC introduced a higher distribution voltage at 22kV with closed ring configuration in 2002. However, with the extensive development of the llkV cable system over the past 40 years, the concept of "dual distribution voltage" is adopted for the introduction of 22kV network in HEC. The paper will discuss the background leading to the introduction of 22kV; the basic idea of dual distribution voltage; the introduction sequence; equipment and interface considerations; operation and management aspects and the long term strategy.