DC breaker testing in the Nan'ao Multi-terminal station

Testing of the DC circuit breaker that was recently installed in the Nan'ao multiterminal HVDC grid in China. The video shows the single pole manual short circuit test that was completed successfully, with the dc circuit breaker opening during the applied dc fault with both terminals maintaining  steady operation.

Brief Updates for 2017

Two brief updates on HVDC projects and related programs:

1.  Firstly, the construction of the Hubei-Chongqin back-to-back HVDC system based on VSC technology started earlier this year, more specifically on the 25th of May 2017. The project that is expected to be completed in late 2018 has a transmission rated power transfer of 50000MW, and a rated voltage of ±420KV, rated current at 1500A. Each station includes two sets of 1250 MW HVDC light  converters.
2. Secondly, the Jiangsu Zhongtian Technologies Co. Ltd announced the successful testing and production of a  ±525KV cross-linked polyethylene (XLPE) insulated cable for HVDC light applications.

The First ±800KV UHVDC project in Northeast of China come into service

On the 15^th of October, the Zhalute-Qingzhou ±800KV UHVDC project completed its pre-operation trial tests, with the three converter unit HVDC  (3*2500MW) came into service successfully, it is the fist UHVDC project connecting Northeast China to central China.

The project rated dc voltage is ±800KV, and its rated capacity at 10000 MW (a configuration that is becoming quite standard in the current generation of LCC-HVDC projects in China). The inverter station side of the UHVDC system is connected to two AC lines, a 500kV and an 1000kV AC system. The project is a link between the renewable energy generation centers in northeast of China and the main load centers, preventing wind curtainlment, an issue that has plagued quite a few of the wind plants, especially in the west of China. 

The project starts from the Zhalute converter station in Tongliao, Inner Mongolia Autonomous Region, crossing the provinces of Hebei, Tianjin, and Shandong. The total transmission distance is 1234km and the total cost of the project is 21.1 Billion RMB(~AU$4B).

The path of the Zhalute - Qingzhou UHVDC line 

Aerial photo of the converter station and the reactive compensators

Test of Synchronous Condenser for HVDC systems

On the 24^th November 2017, testing for the first 300MVAr large synchronous condenser for UHVDC projects, manufactured by the Harbin Electric Group, was completed. The testing report (see figures below) shows that the dynamic performance of the synchronous condenser meets the operational requirements of the State Grid Corporation of China (SGCC).

As investment in HVDC technologies continues and more and UHVDC projects are connected to the system, the need for reactive power support of the networks also becomes important. Cascading faults within the AC and the DC systems increase the threats of voltage instability and synchronism with the grid. Once an UHVDC station with large power capacity suffers an outage, the converter will receive a large amount of reactive power from AC system, which leads to major voltage drops and further cascading faults in the ac network. The large synchronous condenser will provide greater amounts of reactive power in the transient period to prevent voltage drop and help with the fast voltage recovery requirements in post-fault conditions.

The following picture show the HVDC performance after voltage drop with condenser and without condenser, the red one show the converter station with condenser.

At the time of writing, the synchronous condenser has been sent to both the Zhatelu and the Jiuquan 800kV UHVDC converter stations where it is being installed and commissioned for full testing. The installation of larger synchronous condensers with allow for new stabilization strategies in the grid when it comes into service.

Completion of the trial run in the Ximeng - Taizhou ±800kV UHVDC project

The Ximeng - Taizhou ±800kV UHVDC project completed its 1-week (168-hour) trial run on the 30th of September 2017. The project is one of the key components of the ‘Four UHVAC & Four HVDC plan’ as part of the broader ‘National Air Pollution Control Action Plan’. 

The ±800kV UHVDC Transmission line starts at the Ximeng converter station, which is  located in the Inner Mongolia Autonomous Region and terminates at the Taizhou converter station, crossing five provinces (Inner Mongolia, Hebei, Tianjin, Shandong, Jiangsu), and covering an overall distance at 1620km. The rated capacity of the UHVDC system is 10000 MW, with the inverter station connecting to two AC systems (a 500kV and a 1000kV AC systems).

The total cost of the project is 25,4 billion RMB (~AU$5.1B). The project is the second HVDC line to the Jiangsu province, which is a major load center and one of the biggest manufacturing centers in China. The first HVDC line project was the Jinbei-Jiangsu ±800kV UHVDC system which came into service on the 30^th of June, during the summer peak time. At that time the electricity demand at Jiangsu reached a peak of 100 GW, with the UHVDC connection providing a maximum output at 4500 MW, contributing to load supply and system stability.

Upon its completion, the Ximeng-Taizhou UHVDC Project will have the ability to transmit 55000 TWh annually, equally transport 25 million tons, and reducing CO₂ emissions by 49.5 million tons. The project will contribute to the control of air pollution around tein the Yangtze district.

The Ximeng - Taizhou UHVDC transmission line

Luxi hybrid BTB converter station

China Southern Power Grid announced the completion of the Luxi hybrid back-to-back HVDC project on the 26th of June 2017, according to Xinhua News. The project is  a vital part in the asynchronous interconnection between the grid of Yunnan province and the main China Southern Grid aiming to transfer power from the hydroelectric power plants in the south-west of China to the main load centers in the south (Estimated to more than 3000TWh per year).

The project is located in the Luoping County of the Qujing district in the province of Yunnan. It is a hybrid HVDC system forming a combination of HVDC classic (LCC) and HVDC light (VSC). The capacity of the VSCs in the system is 1000MW at a rated dc-link voltage of ±350 kV, while the capacity of bipole LCC reaches 1000MW for each pole.

The project was completed in two stages. The first stage construction included the VSCs of the monopole HVDC light unit and was completed in 2016, with the second stage of the project completed in June 2017. The converter station is connected to four 500KV transmission lines on one terminal and three more 500kV lines into the the side of the main grid.

The HVDC classic consists of a set of 12 pulse converter units, installed in each pole, with six 400MVA converter transformers, two sets of 160MVar reactive power compensators for the  rectifier station side, and three sets of 110MVar reactive power compensators in the inverter station side. 

Main valve hall of the Luxi station

Installation works at the valve hall

The project is a result of the many developments on hydropower along the Changjiang River in Yunnan, with the potential benefits driving the China Southern Grid to reach an agreement with the province of Yunnan for the connection of the new hydropower stations to the main grid. Analysis on the network showed that a synchronous connection between the Yunnan and the main grid increased the risk of cascading failures in both networks so an HVDC connection was the preferable choice. However, the weak AC system in the province of Yunnan did not allow for an HVDC classic solution to by built for the project. The final design solution was a hybrid HVDC project, taking advantage of the benefits provided by the HVDC light for fast response in real and reactive power despite the weak AC system, while the HVDC classic is able to transmit bulk power.