To avoid congestion and potential damage to individual grids in case of major power fluctuations, SGCC is planning to significantly strengthen the interconnection of its three regional grids in the east, central and south of the country (Figure 7). In 2009, cross-regional trade of electricity is 11.4% higher than the previous year and totalled at 158 TWh (Zhang, 2010). Depending on how quickly it can ramp and other conditions, a generating plant can be used as primary (frequency-response), secondary (spinning and nonspinning) or tertiary (replacement) reserves (Box 3) (IEA RETD, 2010).