Gerente

System Protection Schemes in Power Network
based on New Principles
Daniel Karlsson, ABB Automation Products AB
SE-721 59 Västerås, SWEDEN
daniel.h.karlsson@se.abb.com

Abstract
This report describes how a wide area protection
system against large disturbances can be designed
and implemented. Such a system can be used to
mitigate voltage collapse, loss of synchronism,
poweroscillations, etc. The protection system is
based on a number of protection terminals
connected via a system wide data communication
network and synchronized by GPS technology. The
system is very flexible and comprises a large
number of optional protection functions of various
complexity and capability.
Based on time synchronized measurements of
voltages and currents by phasor measuring units
(PMUs)at different locations in the network, realtime values of angular differences in the system
can be derived with a high accuracy and at a high
sample rate, e.g. half the power system frequency
(25/30 Hz). With this new type of real-time
measurements, efficient emergency actions, such
as PSS control, based on system wide data, load
shedding, etc., can be used to save the system
stability incase of evolving power oscillations.
Based on voltage instability predictors (VIPs), at
different locations of the power system, an
overview of the overall system condition can be
achieved and appropriate actions to mitigate a
voltage collapse can be taken. Both protective
actions, such as shunt capacitor switching or load
shedding, and emergency control, such as AVR
boosting and requestfor HVDC emergency power
support, can be implemented.
Introduction
The angle between the voltage phasors in different
parts of a power system has been known as a
critical parameter for AC systems, since the
introduction of large electric systems. It has,
however, not until recently, been possible to
measure and compare phasor angles from different
parts of the system with sufficientaccuracy for real
time power system applications.
The voltage stability phenomenon and appropriate
remedial actions, have been discussed during the
last decades. A lot of effort has been put into
studies of suitable indicators of evolving voltage
instability, from simple voltage measurements to
sophisticated indices based on the minimum
singular value of the Jacobian matrix and faster

thanreal time simulations. Load shedding and
other suitable actions to counteract the transition
into instability, in terms of “where, when and how
much” to shed, have also been investigated, mainly
on the academic level.
During the last ten years a lot of efforts have been
put into discussions, research, and theoretical work
in the area of system protection schemes (SPS)
also called wide areaprotection. Prototype testing
of transducers and algorithms, especially
developed for mitigation of system wide
disturbances, have also been performed during the
last years. It now seems reasonable to believe that
time has come to merge the accumulated
knowledge concerning power system behavior
during extreme contingencies, available actions for
system instability mitigation and, theavailable
measurement and communication technology into
commercially available products.
So far traditional relay manufacturers have been
focused on equipment protection, such as line
protection, transformer protection, etc. Equipment
protection is designed to isolate the faulted or
overloaded equipment and to maintain the power
supply to the healthy part of the system. System
protectionschemes, on the other hand, address
situations where no particular equipment is faulted
or overloaded, but the power system is severely
stressed and in transition towards instability,
resulting in a wide spread blackout, if no remedial
actions are taken.
The fast restructuring of the entire electricity
business puts a large pressure on the utilities to
utilize their investments to the maximum...