An investigation into an incident that saw 600 turbines totalling 890MW suddenly disconnected from the grid in Jiuquan, northwest China, on February 24, has ruled that poor-quality products and management are to blame.
The result "of human errors rather than natural disaster", the incident sparked a voltage-fluctuation problem of unprecedented scale, say officials in the country. The impact of the incident quickly spread thoughout the network in Gansu Province and threatened the security of the larger Northwest China Grid, which covers five provinces.
"The grid voltage fluctuated by a wide margin in a short time," says Wang Ningbo, director of Gansu Grid's wind power technology centre. "This directly affected the power utilisation quality of a large number of users."
Located across ten wind farms in Guazhou County, the initial 298 turbines were suddenly cut off from the grid due to a short circuit involving a ground cable under a 35kV feeder line switch cabinet at the Jiuquan Qiaoxi wind farm, owned by China Power Investment Corporation. This, in turn, sparked a three-phase short circuit and the 330kV bus voltage in the Dunhuang transformer station instantly fell to 272kV, leading to the automatic disconnection of the 298 turbines.
With the sudden loss of output from those turbines, there was a voltage surge at the transformer station to 380kV. This forced another 300 turbines across six more Guazhou wind farms to be disconnected.
The investigation, led by the Northwest China Branch of the State Electricity Regulatory Commission (Serc), found that the turbines affected did not have the low-voltage ride-through (LVRT) safety features now stipulated in turbine supply contracts. LVRT enables turbines to continue operating in the event of a grid fault. This improves grid safety and stability and saves time when making adjustments on the grid.
No LVRT standards
The turbines for the Jiuquan Qiaoxi wind farm were bought through public tenders in 2008. At that time, China did not have explicit technical LVRT standards. The technical protocol for the turbine supply contracts was amended later to include the LVRT requirement, stipulating the need for upgrades in all installed turbines too.
According to Wang, the reality is that "almost all wind turbines from Chinese makers do not have LVRT capacity" yet. The only domestic machines to have passed LVRT tests so far are Goldwind's 1.5MW design and the 2.5 MW machine supplied by Guangxi Yinhe Avantis Wind Power's (GYAW). He notes though that the installed turbines supplied by foreign manufacturers in China are also lacking ride-through features, with LVRT functional modules left out to keep production costs down.
The Jiuquan incident investigation team has urged companies to accelerate LVRT upgrades. According to Wang, it costs CNY300,000-400,000 ($46,000-61,000) to upgrade one turbine. However, upgrades are vital, he insists.
Jiuquan is set to be China's first 10GW wind power complex. By mid-March, 13 wind farms totalling 5.5GW had been installed in the area, with 4.06GW of that fully operating and supplying power to the grid.
"If all the 5.5GW turbines are put into operation in the Jiuquan base, and they remain devoid of LVRT capacity, it will be very hard to control the operation of the grid when there is a grid fault or voltage dip," says Wang. "This might result in a terrible accident of voltage collapse in the power supply system."
There is no stated deadline for upgrades yet, however, and many firms are likely to wait for the new national standards currently being worked on by central government. "Compared with the current technical rules for connecting wind farms to the power system from the State Grid Corporation of China, the new national standard concerning the LVRT test will be more strict, adding requirements for response time and duration of reactive compensation under three-phase short-circuit of wind farms," explains Amanda Yang, an engineer with GYAW.
Meantime, there were also "obvious flaws" with the 35kV cable construction technologies and quality management of the Jiuquan Qiaoxi wind farm, notes the investigation team. "It has become urgent to improve technical norms among wind turbines, wind farm management, and the grid," says a spokesman for Serc's Northwest China Branch.
He Dexin, president of China Wind Energy Association (CWEA) agrees. "If we cannot properly harmonise programmes of wind farms and the grid, the eight 10GW-level wind power projects (planned for China) will be crippled."
China had just under 45GW of wind capacity installed by the end of 2010 and plans to increase that to 150GW by 2020. To accommodate the burgeoning volume of new capacity, State Grid has planned to speed up the construction of ultra-high-voltage (UHV) alternating-current synchronising transmission lines in the northern, central and eastern regions of the country over the next five years.
"With these UHV transmission lines, we will be able to send 80% of wind power from north-west China to the central and eastern parts," says Bai Jianhua, deputy chief engineer of the energy research institute under State Grid.