Project: Alternative protection for the FREEDM System based on the Solid State Transformer

Project Objective: The primary objectives of this project are to develop an alternative protection strategy based on the FREEDM Green Hub system and the SST capabilities, implement the method in the digital real time simulation test bed, and verify the proposed protection method in hardware in loop testing with the digital test bed.

 

The FREEDM system contains a large number of power electronics based devices, including converters and solid state transformers, interfaced with distributed renewable energy resources to minimize environmental impact and to improve the reliability and security of the grid. As such, the traditional protection methods cannot be applied successfully because of various reasons including the synchronization time to acquire all the required measurements, the asymmetry existing between the phases, and the fault current level. Indeed, based on the loop configuration and the SST current limiting and under voltage capabilities, the direction of the currents through the terminals of each section along with the under voltage detection will determine the location of the fault as showed in Fig.1. Explicitly, once the normal current direction is defined for each section, only a faulted section will have the currents flowing from the FIDs to the cable and the other sections will have one current flowing from one FID to cable and the other from the cable to FID. The protection algorithm for each terminal and the corresponding complete section protection has been proposed. In addition, the backup protection is based on the under voltage and current limiting function so that an overload condition will not trigger the relay operation. Back up protection will be provided by the analysis of the current waveform (square wave paired with the under voltage detection) because a short circuit current limiter in the utility supply line, anywhere in the loop, produces a square-shape current wave form and there is also voltage drop in the system. The proposed protection strategy has been implemented on the digital test bed in RTDS as in Fig. 2. Simulation results from RTDS have verified the efficiency of the proposed protection strategy. In particular, two papers have been published/accepted by the prestigious intentional power system conferences.

References:

 Tatcho, P.; Zhou, Y.; Li, H., and Liu, L., “A real time digital test bed for a smart grid using RTDS” IEEE International Symposium on Power Electronics for Distributed Generation Systems (PEDG), pp: 658 – 661, 2010

 

CONTACT:                                     SPONSOR:

Dr .Hui Li, Associate Professor          NSF FREEDM

hli@caps.fsu.edu  (850) 644-8573