Design and Development of Protection Schemes
for FREEDM Smart Grid Systems
by
Pavanchandra Mandava
A Thesis Presented in Partial Fulfillment
of the Requirements for the Degree
Master of Science
Approved July 2014 by the
Graduate Supervisory Committee:
George Karady, Chair
Raja Ayyanar
Keith Holbert
ARIZONA STATE UNIVERSITY
August 2014
ABSTRACT
This research work describes the design and validation of protection schemes developed to
solve the problem of communication with an ability to detect and sectionalize the fault. Protection
schemes have been designed according to the requirements of Future Renewable Electric Energy
Delivery and Management (FREEDM) system. Due to the presence of distributed generation
(DG), power flow in the loop is bi-directional and conventional protection schemes may face the
problem of unwanted tripping. Hence customized protection schemes have been developed
specific to the FREEDM system. Former FREEDM students at ASU have developed ultrafast pilot
differential protection using fast analog communication (Ethercat communication) and modified
it in various ways to speed up the fault detecting capability of the algorithm. However, National
Science Foundation (NSF) complained the compatibility issue of communication, which cannot
be used for longer distances. FREEDM loop uses fault current limiter (FCL) to limit the fault
current and the sub-station solid state transformer (SST) reduces the system voltage to limit the
fault current to 2 per unit. This allows the protection scheme to detect fault current in 2-3 cycles.
However a much delayed fault detection is not encouraged as it will disrupt the power supply to
healthy parts of the system for a longer duration. Time inverse directional over current protection,
pilot directional protection and PMU based protection are developed in this thesis work addressing
the communication problem and at the same time with the ability to quickly detect the faults.
Validation of the protection scheme is done on Real Time Digital Simulator (RTDS) at Center for
Advanced Power Systems (CAPS) using SEL relays and simulation models are developed in
PSCAD.