Next-generation power electronics based power systems with significant renewable energy penetration

Next-generation power systems will consist of a significant penetration of large-scale inverter based renewable energy resources (IBRs) into transmission and subtransmission grids plus a wide integration of distributed energy resources (DERs) into distribution grids. Our research interests lie in the fields of electrical power systems, power electronics, and high voltage engineering, with a focus on modeling, control, protection, and security of power electronics based power systems in both transmission and distribution levels, to contribute to security, resiliency, efficiency, and sustainability of next-generation power systems.

Transmission Systems

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Modeling

Dynamic modeling and stability analysis
PMU-based online modeling of HVDC for real-time stability detection of hybrid AC/DC systems
  • We proposed an intelligent white-box modeling methodology to develop a new PMU based HVDC model. This model can be applied to online, fast, and accurately detect voltage stability of hybrid AC/DC systems by the Thevenin impedance matching. [J6, C6, R4]
  • Other applications of the proposed model include online frequency stability detection, fault location, and load shedding determination.
EMTP modeling and transient analysis
  • We proposed an electromagnetic model of the modular multilevel converter (MMC) based HVDC to analyze its energization transients. [C8, C9]
  • We designed a circuit breaker (CB) with a pre-insertion resistor (PIR), which can be applied to suppress inrush currents during energization of the MMC-HVDC. [C9]
  • We developed an innovative electromagnetic model of a complex CB based on Siemens PTI software and proposed a mathematical algorithm to estimate the amplitude of the direct-current (DC) component generated during energization of compensated lines. [C7]

The corresponding paper was selected by the technical program committee as the “Best Paper” (Top 5%) on power system modeling and simulation in the 2016 IEEE PES General Meeting (leading conference). The research work was highly praised by the world-class renowned researchers and experts. In addition, Dr. He was featured in a Siemens News article to acclaim the “Best Paper” and praise Dr. He’s contribution to the very innovative modeling of a complex circuit breaker.

Control and Stability

VSC-based novel excitation system
  • We invented paralleled multiple voltage/current source converter (VSC/CSC) based self-shunt excitation systems. The invented excitation systems can not only (1) achieve the function of the conventional excitation system, but are also able to (2) flexibly control the reactive power exchange of the ac side of the VSC/CSC with the generator terminal to provide additional damping. [P1, P2]
  • We developed an optimal coordinating damping control strategy of the VSC based self-shunt excitation system applying the LQR method to maximize the damping capability of the invented VSC based excitation system to suppress low frequency inter-area oscillations. [J4, J2]
Voltage and frequency control of VSC-HVDC connected offshore wind farms
  • We developed the droop-based coordinating control of the multi-terminal VSC-HVDC to integrate offshore wind generators to mainland transmission systems. [J5, R4]
  • We also developed frequency control strategies to enable offshore wind farms to provide primary and secondary frequency regulation services to mainland transmission systems. [R3]
Wide-area measurement and control
  • We proposed a direct measurement methodology to online identify the rotor angle of the synchronous generator. This methodology can be integrated into WAMSs to improve the reliability of rotor angle stability detection. [J3]
  • In the existing rotor angle stability detection schemes, the phase angle of the terminal voltage of a generator is usually assumed to be its rotor angle since the generator rotor angle is hard to be measured accurately. The caused error by this assumption can be eliminated by the proposed methodology effectively.

Protection

Impacts of IBRs on protection of power systems
  • We investigated the performance of doubly fed induction generators (DFIGs) during a short-circuit fault, and studied the impact of their low voltage ride through (LVRT) behavior on distance protection of AC grids. It is found that the fault on the terminal line of DFIG wind farms can induce mis-coordination of distance relays, leading to system security problems. [C5]
  • We explored the performance of the VSC-HVDC during a short-circuit fault on its AC side, and researched the corresponding impact on AC distance relays. It is verified that the control of the VSC-HVDC can result in the miscoordination of protective relays. [C6]
Protection coordination of hybrid AC/DC grids
  • We proposed an apparent impedance calculation algorithm, which utilizes the bus impedance matrix (Zbus) to calculate the impedances viewed by distance relays during a three-phase SC fault. [J5, R4]
  • The proposed protection coordination algorithm can be applied to fast identify the protective device settings on an AC grid that need to be adjusted due to HVDC control of offshore wind generators.
  • This algorithm can also determine the miscoordinated boundary and appropriate relay settings.

Equipment Operation and Maintenance

Detection of vegetation encroachment of overhead transmission lines
  • We proposed a deep-learning based detection framework with advanced stereovision that utilizes the images obtained from vision sensors mounted on power transmission towers.
  • The proposed framework provides an accurate, cost-effective, and practical vegetation detection methodology, which can provide early warning services for the vegetation management of utilities to detect vegetation encroachment of overhead power lines to avoid potential contacts. [C10, J8]

Distribution Systems

Based on the solid foundation of our research achievements in transmission grids, we are developing modeling, control, and protection strategies for future distribution systems with significant DER integration.

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Control

To be updated soon…

Protection

Impacts of DERs on protection of distribution systems
  • We performed a quantitative analysis of the short circuit fault characteristics of a distributed PV system and identified their impact on the traditional overcurrent protection of distribution systems. [C1]
Protection of distribution systems integrated with DERs
  • We proposed an intelligent current protection algorithm applying a machine learning algorithm, i.e., the radial bias function neural network (RBFNN). It can online detect the low fault currents that are fed from DERs and also difficult to detect by conventional relays in distribution grids. [C2]