The Chuxiong UNHDC converter station is part of the China Southern Power Grid that supplies power to the Pearl River Delta region.
The largest and most important power center in terms of total energy consumption supplied by China Southern Power Grid is the Pearl River Delta region. The economics and industry in this region, still developing at an increasing rate, play an important role in China. The four main large cities in this region are Guangzhou, Shenzhen, Dongguan and Foshan, and the demand in each city exceeds 10,000 MW.
For the Pearl River Delta region, China Southern Power Grid (CSG) has constructed a long-distance ultrahigh-voltage (UHV) hybrid alternating-current/direct-current (AD/DC) power grid comprising eight 500-kVAC overhead lines, four high-voltage DC (HVDC) links (±500 kV, 3,000 MW) and one 1,418-km (881-mile) UHVDC link (±800 kV, 5,000 MW), which have, in total, a west-to-east load-transfer capacity of more than 25,000 MW. Therefore, dynamic reactive power demand and voltage stability are paramount to ensuring the supply of electrical energy to this region.
As the electric utility industry is developing rapidly to keep pace with China’s growing economy, there is an increasing demand for high-quality services. CSG’s response is to establish a strategy to provide a source of energy that is intelligent, efficient, reliable and generated by renewable energy sources, that is, green power.
System Control and Reliability Studies
A member of the flexible ac transmission system (FACTS) family of devices, the static synchronous compensator (STATCOM) can enhance transmission system control, reliability and operation, and improve system power quality. For CSG’s load center in the East, a STATCOM could significantly help with voltage recovery following faults in the eastern sector of the CSG service territory and also prevent failure of the HVDCs’ serial commutation.
Control and reliability studies were required to determine the optimum location for installing a STATCOM and what the capacity of the unit should be. The Electric Power Research Institute of CSG (EPRI-CSG) undertook in-depth, detailed studies that considered different operational modes, system development and hybrid AC/DC operation. The studies revealed the most severe system fault that could occur was a three-phase fault where a link circuit breaker failed to open.
The studies indicated that the best place for a STATCOM to be installed was either in the 500-kV Dongguan substation or in the 500-kV Hengli substation. When determining the required capacity of the STATCOM, CSG had to consider the conditions of each substation as well as the age of its equipment. After much consideration, CSG decided to install a ±200-MVAR STATCOM in its 500-kV Dongguan substation.
From 2007 to 2010, CSG, along with Rongxin Power Electronic Co. and Tsinghua University, established a consortium project to undertake the research and development of the 200-MVAR STATCOM.
STATCOM Main Circuit and Topology
The group recommended the STATCOM be connected to the 35-kV bus instead of the 10-kV bus to reduce the output current required and the losses. The study group also compared the preferred connecting modes, star and delta. In the star mode, the phase voltage was lower in steady-state while the three-phase voltages were different and probably much higher than rated value in unbalanced working condition. In contrast, in the delta mode, there were different current outputs in unbalanced conditions with no higher-phase voltages. Based on these findings, a STATCOM with delta connections was selected.
Considering the technology challenge and device’s limitation, the STATCOM is designed to be ±200 MVAR, with two ±100-MVAR voltage source converters (VSCs). VSCs are the kernel technology for the STATCOM; normally, there are two- or three-level converters and multilevel VSC applications.
Because of the limitation of the device and the expectation of achieving improved performance, the VSC is built up with H-bridge modules connected in series that use the injection-enhanced gate transistor devices. Each phase has 26 modules with two for redundancy to output 53 levels of voltage. With the high levels of voltage, the frequency of the pulse-width modulation (PWM) is only 250 Hz and, thus, provides very good performance regarding harmonic distortion and loss control.