HVdc Transmission and Integration into an AC Grid

Authors: Joe Mooney, POWER Engineers, Inc, and Brian K. Johnson, University of Idaho, USA

In the late 1990’s, power electronic technology that had been used for years in variable speed motor drives began being used in HVdc applications. Insulated-gate bipolar transistors (IGBT) were developed that had voltage ratings and current capabilities needed for HVdc applications.
The first of these was put into service in 1997. HVdc converters utilizing this technology are referred to as Voltage Source Converters (VSC). Two of the main improvements with VSC systems are the ability to control reactive power flow and the reduced harmonic filtering requirements.
Since 1997, VSC systems have been dominating the HVdc market for installations up to 1000 MW and they are becoming the standard for new installations. VSC HVdc systems raise the possibility of creating a HVdc grid, with several multi-terminal systems now in operation and consideration of a European HVdc supergrid.

LCC and VSC systems respond differently to disturbances and events on the AC and DC power system. Due to the high level of control that is available in a VSC system, the rectifier and inverter can respond as “ideal sources” in nearly any fashion desired by the controls engineer, and can often be viewed as current regulated voltage sources. 
That means the response of VSC systems to AC system faults and dynamic conditions is much different than conventional AC system sources, such as AC generators. An LCC system, on the other hand, is largely passive in its response and there is very little control action an LCC system can take for AC system faults. 
However, as with a VSC system, LCC converters can respond very quickly to other abnormal system conditions, such as power swings, and take corrective actions to help keep the power system stable and intact.

Benefits
HVdc transmission has many benefits that make it an attractive alternative to traditional AC transmission lines. HVdc is excellent for transmission of large blocks of power over great distances. The Pacific HVdc Intertie was constructed to make the inexpensive renewable hydro power from the Northwest available to the California market. In addition, in the winter time when loads were low in California, power could be shipped to the Northwest when loads were high. HVdc transmission is becoming the primary mode of transporting hydroelectricity and coal generation from western China to the load centers in the East.  By the end of 2016 there will be 34 LCC HVdc lines in China and many more are planned. In addition there are presently five VSC HVdc projects in China, with two more scheduled for completion by the end of 2018.  The longest transmission system in the world is the recently completed Rio Madeira HVdc system at 2375 km (1476 mi) with two ±600 kV, 3150 MW transmission lines based on LCC converters.

HVdc can also be used as an isolation point between AC systems or a point of interconnection between isolated ac systems, even systems operating at different frequencies. The US East and West grids have been interconnected via HVdc back-to-back systems for decades. The same is also true for ERCOT.  Although these connections allow the transfer of real power, the regions are still isolated from each other due to the nature of HVdc. There is a proposal for building an HVdc “Super Station” near Clovis, New Mexico that will be used to create a three-way tie between the Western and Eastern interconnects and ERCOT via an HVdc network with an ultimate design capacity of 30 GW. The Super Station will serve as a market hub for exchanging renewable energy between the three regions.
HVdc transmission lines often require smaller rights-of-way than their AC equivalent.  A HVdc bipole transmission line only requires two current carrying conductors. In comparison, an AC transmission system carrying the equivalent amount of power would require multiple lines with three conductors per tower. To illustrate this point, Figure 1 shows a comparison of right-of-way needs for 6000 MW transmission system utilizing ±800 kV DC and 800 kV AC. Using HVdc would require a single bipole transmission line and using AC would require two transmission lines resulting in about half the right-of-way requirements for the HVdc line. 

HVdc is ideal for underground and undersea cable because there is no distance limitation like there is with AC transmission. Cable applications on AC systems are typically limited to tens of miles due to the need to compensate for the cable charging current. With HVdc systems there is no charging current so the cable length has no physical distance limitation.  In addition, the cost of underground and undersea HVdc cables are less than equivalent voltage cables used on AC systems.

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