The Intermountain Power Project (IPP) includes the Intermountain Generating Station, which consists of two, 750 MW generators and the transmission systems. The project was conceived in the early 1970s to fulfil the projected power and load diversification needs of a group of Utah and California utilities.
By mid-1986, the project was providing power to 36 utilities in Utah, Nevada, and California. IPP participants include 23 municipal utilities, six rural cooperatives, and one investor owned utility in Utah and six municipal utilities in southern California.
IPP is owned by the Intermountain Power Agency (IPA). IPA appointed the Los Angeles Department of Water and Power, as project manager in charge of design, construction, and operation of the project.
The IPP Southern Transmission System (STS) brings the major part of the power to Los Angeles by HVDC. When ABB received the HVDC converter station contract in 1982, the Intermountain Generating Station was planned for four units of 750 MW and the STS for two HVDC bipoles of 1600 MW each. In 1983 the generating plant was reduced to two units. After a period of study, in which also 500 kV AC transmission was studied, it was decided to build STS with one HVDC bipole of 1600 MW.
This meant a big change for the HVDC transmission compared to the two bipole solution. The STS takes the majority of the power generated in IPP and The Northern Transmission System (NTS) that supplies power to the Utah and Nevada participants has limited capability. When a pole (800 MW) of a single bipole has a forced outage, then the remaining pole has to instantly double its power to secure that the IPP generators remain synchronous with the Utah and Nevada system.
Each pole of the bipole was therefore designed for a temporary 100 % overload, which decreases at a rate of 60 MW/min to a continuous overload of 50 %. This resulted in major changes to the converter stations.
The IPP transmission raised the reliability criteria of HVDC to the high level where the technology is today. For the first time, a bipolar DC line was officially deemed to be equivalent to a double circuit AC line, which resulted in a substantial reduction of the spinning reserve in the Los Angeles area. The STS was up-rated from 1600 to 1920 MW some years ago.
Link to: The Intermountain HVDC transmission