> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 Abstract— After deregulation of power systems, large problems with voltage stability incurred due to the large transits of power transmitted to large distances. Systems are very stressed and often work on their limits. Therefore, single contingency is sometimes enough to cause system collapsing. One of the most economic methods to prevent larger scale voltage collapse is undervoltage load shedding (UVLS). In this article, implementation of UVLS scheme is tested in New England Test System and then modeled in part of Croatian power system. Simulation of system behavior is presented as well. Index Terms— undervoltage load shedding, voltage, voltage stability, simulation I. INTRODUCTION ATELY, many problems concerning voltage stability incurred in power utilities. Systems often work on their stability limits and single contingency is sometimes enough to cause cascade blackouts of such stressed systems. Most significant examples of such incidents happened in 2003 in USA, Italy and London. With increased financial pressures and competition in the electric power industry, innovative and cost-effective voltage collapse countermeasures are necessary. Before the main disturbance that causes collapse, the system is weakened due to the planned or forced outages of main objects (lines and production facilities) and voltages on main buses sink. Larger scale blackouts have great impact on economies with significant financial losses. Therefore, methods and tools for prevention of total system collapse are needed in order to save costs. The most effective improvement of voltage stability limits is building of new transmission lines and generation facilities. However, it is very hard and expensive to get a new corridor for transmission lines or a new location for power plant. UVLS imposes itself to be such tool for voltage collapse prevention which is very economic and easy to implement. Manuscript submitted 02 March, 2007. M. Klaric is with the Dalekovod jsc, Zagreb, HR-10000, M. Cavica 4, CROATIA (corresponding author; e-mail: mario.klaric@dalekovod.hr). I. Kuzle is with the Faculty of Electrical Engineering and Computing, University of Zagreb, Zagreb, HR-10000, Unska 3, CROATIA (e-mail: igor.kuzle@fer.hr). S. Tesnjak is with the Faculty of Electrical Engineering and Computing, University of Zagreb, Zagreb, HR-10000, Unska 3, CROATIA (e-mail: sejid.tesnjak@fer.hr). II. UNDERVOLTAGE LOAD SHEDDING A. Basic principles As it was mentioned earlier, when the system is heavily loaded, voltages on most critical buses sink. This phenomenon is especially strong when there is a lack of reactive power support but it can also occur due to the poorly organized reactive power ancillary service. In figures 1 and 2, voltages on the most critical buses just before USA 2003 blackout are shown. Fig. 1. Voltages in main buses during 2003 blackout Fig. 2. Voltages and reactive power production in Eastlake 5 power plant before the voltage collapse in 2003 Example of Undervoltage Load Shedding Implementation Mario Klaric, Igor Kuzle, Senior Member, IEEE, Sejid Tesnjak, Senior Member, IEEE L 1-4244-0987-X/07/$25.00 ©2007 IEEE. 1-4244-0987-X/07/$25.00 ©2007 IEEE.