Power grid main characteristics
The primary energy reserve in two western provinces, Yunnan and Guizhou, accounts for 90% of the total reserve in the area. In contrast, Guangdong electricity consumption accounts for 56%. It is a basic need to transfer electric power from west to east. Then there are two physical characteristics of CSG should be remembered:
Bulk power is transferred over 1000 km through HVAC/HVDC paralleling networks. It is nearly one-third of power consumption of Guangdong comes from western provinces, and CSG have a strong backbone composed of 8 HVAC and 7 HVDC lines including three +/- 800kV UHVDC. The capacity of the backbone reached 36GW. After a HVDC bipolar block, bulk power will be immediately transferred to parallel 500 KV AC lines, which may cause overload in AC lines and drastic voltage drop, even system blackout. Wide area speical protection systems were designed and used to prevent CSG from large disturbances such as HVDC block in past years.
Multiple HVDC links infeed Guangdong AC system. Nowadays there seven HVDC inverter stations located around Pearl River Delta, which is a relatively small area. Then faults in HVAC may lead to commutation failure of HVDC, malfunction of protection of HVDC, even block of HVDC. On the other hand, commutation failure or block of HVDC could has impact on protection and dynamic stability of HVAC. The interaction between HVAC and HVDC systems is the main concern during planning and operation of CSG.
Major incidents & preventive measures
Tropical Cyclone Yasi
For a bulk grid span over 1000 km like CSG, large disturbances may come from various reasons, such as multiple line faults, latent faults of AC relays and bipolar block of HVDC. Multiple line faults in CSG generally occurred during bad weathers. In CSG, latent faults had happened for a few times that protection latent faults result in big-area cascading faults.
Wide area SPS was developed and used successfully in past year the protect CSG from large disturbances. Recently CSG had commissioned a system named Integrated Defense System. The system screens all specified outages every 15 min. and evaluates the security of the whole system, as well as giving alarms and suggestions. The system played an important role during typhoons in 2013.
From January to February in 2008, Southern China experienced widespread and long-lasting ice storms, which is the biggest ice storm in southern China according to history records. CSG was the most serious victim of the storm. The maximum observed ice thickness was 118 mm. For 500 kV lines, 678 towers collapsed, 295 towers were damaged. During the storm, Guizhou grid, Guilin grid and north Guangdong grid were forced islanding. Guizhou grid maintained only 36% of its normal supply capacity. More than half of the counties, thousands of towns, over 18 million people in the province lost power for ten days.
Since 2008, CSG has carried out integrated schemes to secure the network from ice storm, including ice monitoring and de-iceing. Ice coating monitoring system collects ice coating information from 157 terminals in key zones, and monitors 123 lines at and above 110 kV. CSG installed 19 DC deicer to melt ice for 244 lines at 110 kV and above. Ice storms in 2010 and 2011 confirmed the effectiveness of the integrated schemes to secure the network.