#### Application

With the growing installation of power plants and the large penetration of variable renewable sources, the generated energy required has also increased in order to accommodate such emerging demand for electricity. However, transmission infrastructure investments have not increased proportionally over the last two decades. The transmission bottlenecks and stresses will finally lead to inevitable congestion on the transmission and sub-transmission system, which degrades system reliability and increases energy prices.

The approach for addressing the aforementioned challenge can be divided into two main categories: 1) to expand the transmission system by constructing new high voltage transmission lines (ac or dc), which may take years to be approved and completed due to the economic and environmental concerns. The price of new transmission lines is typically U.S. $0.3-1.3 million/km, but can exceed U.S.$ 6.3 M/km. Additionally, even if a newly built transmission line can relieve congestion to some extent, it is not sufficient to ensure the flexible controllability and high utilization efficiency of meshed transmission networks. 2) To fully utilize the existing transmission system, actively control the active power flow on the line. This approach can be a more efficient method to route the power flow in the large scale transmission networks, increase the power flow on underutilized lines, and also limit the power flow on congested lines.

#### Research

The prototype DCC providing adjustable dc current (0 â€“ 1000 A) for 115 kV/1500 A CVSR

The project focuses on the development of a power electronics based dc current controller (DCC) for continuously variable series reactors (CVSRs) to continuously vary the line reactance and control the power flow on the transmission and distribution system. Compared with FACTS series compensation devices, DCC only performs control function, is isolated from the high voltage ac line and can be of much lower voltage and power rating, ultimately reducing the system cost. But a regulated dc current from 0 to 1000 A must be supplied by the DCC under different ac load and dc flux bias conditions, which is a big challenge due to the non-linear characteristic relationship between induced back-emf and dc current. Furthermore, high reliability is critical for acceptance in utility applications. To minimize maintenance, a natural convection cooling system instead of forced air cooling is required and poses additional restrictions for the topology and device selection.

#### How WBG Can Help

To achieve the best current regulation, the DCC must dynamically overcome (V_{emf}), and then provide the necessary additional voltage for the voltage drop associated with resistance and inductance of CVSRâ€™s dc winding to obtain the desired dc current. Given that the induced back-emf mainly consists of even order harmonics with a minimum frequency of 120 Hz, this current regulation scenario would require the DCC output voltage to generate an ac voltage up to kilovolts range with high bandwidth, which would require high dc link voltage and fast switching frequency. The wide band gap (WBG) devices, especially high voltage SiC devices, enable the DCC to achieve this ideal current regulation, which contributes to the reduction of dc winding current harmonics and the improvement of the whole CVSR system performance.

• Sheng Zheng
• Jingxin Wang