Implementation of Electric Springs on DC Power Systems

Minghao Wang, Kwan-Tat Mok, Siew-Chong Tan, Shu-Yuen Ron Hui
The University of Hong Kong

With the booming of the electronic loads, renewable generations and electric vehicles, the DC grids are becoming a promising grid system for modern power delivery. Due to the integration of the renewable generations, nonlinear loads and occasional faults, there will be voltage instability issues of low-frequency bus voltage variation, harmonic noise and voltage flickers at the DC bus. In this project, the concept of electric springs is introduced to DC systems as a distributed means to improve the power quality of the emerging DC grids. Our research explores the operating principles and the applications of DC electric springs (DC-ES) in DC grids. The results show that the DC-ES can be an effective method to maintain the voltage stability for the DC grids.

We have conducted a study on both the series- and shunt-type DC-ES and have experimentally verified their applications in a 48-V DC-grid test bench. The outcomes of this project are included as follows.

1. We have investigated the operating modes of the DC-ES and built small signal models for both the series- and shunt-type DC-ES. The dynamic response of the series-type ES is dependent on the non-critical load, while the dynamic response of the shunt-type ES is independent of the non-critical load. The shunt-type ES has a faster dynamic response than that of the series-type ES.
2. The series-type ES can manipulate the power of non-critical load accordingly. Therefore, it can shave the load profile and can potentially achieve a smaller storage capacity than the shunt-type ES.
3. Both the series- and shunt-type ES can be used to compensate the low-frequency harmonic noise at the DC bus.
4. In the application of fault-ride-through, the shunt-type ES have a faster response and a better voltage regulation performance than that of the series-type ES. With the help of DC-ES, the bus voltage deviation during fault condition can be reduced.