✔ adequate voltage controls and ability to dynamically Quadratic Approximation to Ferranti Rise Curve 1.250 1.200 Vo /Vc provide megavars (Mvars) to maintain voltages within limits ✔ the capability of being monitored at the system control center. the capacity of a restoration building block to affect the overall restoration plan can be measured in two ways: ✔ the total Mw capacity of the generating units will determine the amount of load that can be energized. ✔ the total Mvar absorption capability and the source impedance of the generating units will determine the length of the eHV transmission lines that can be energized. restoration building blocks have always been formed by using a single generating unit or power plant. the amount of transmission that can be energized is then limited by the Mvar absorption capacity of the black-start generating units as well as the minimum source impedance required to prevent transient over voltages. with phasor measurements and advanced governing control systems, it is feasible that a more extensive transmission corridor could be energized by simultaneously synchronizing generating units at multiple locations. the multiple generators would be able to combine their Mvar absorption capability so that a greater span of transmission circuits could be energized. 1.150 1.100 Voltage p.u. Squared 1.050 1.000 0 50 100 150 200 250 Line Length (mi) 300 350 figure 4. A Ferranti rise and a quadratic approximation. generating stations since the operators at these stations are more familiar with the operation of synch-check relays. a virtual synchroscope could be developed based on phasor measurements and power and angle calculations to remotely monitor any breaker in the system. the virtual synchroscope would allow synchronization operations to be performed manually or under program control from a remote location using any breaker in the system. Expansion of Electrical Islands Reintegrate with Neighboring Systems once one or more stable restoration building blocks are built, these can be expanded as electrical islands. the number of generation-load islands that can be built in parallel is usually matched to the number of control desks that are involved in the restoration. the expansion of each island requires the focused attention and supervision of one or two power system operators. the size of the islands can be determined in the planning stage, or they can evolve based upon the progress made by the crew at each operating console. current restoration methods use a manual approach to controlling the frequency of electrical islands. a single isochronous unit is designated for each electrical island. the unit operator monitors the unit frequency and manually controls the governor reference motor to maintain the island at the designated frequency. automatic island frequency-control programs can be developed to control multiple units in an island to ensure that frequency deviations following the addition of load blocks are minimized. prior to adding load blocks, the automatic island frequency-control program may direct the units to preposition frequency to a value greater than 60 Hz. this will increase the minimum frequency value that occurs after the load is added and thus limit damage to steam unit turbine blades. integration with neighboring utility systems offers the opportunity to significantly increase the amount of online generating capacity and system inertia. this may greatly increase the overall stability of the system and its ability to withstand major contingencies, such as the trip of a large generating unit. integration with neighboring systems requires coordination between different transmission operators and often requires review and approval by a regional transmission organization. the frequency difference between the systems also needs to be minimized in order to control the synchronizing angle and limit the flow between the systems after synchronization. Ferranti rise is a function of line length. Figure 4 shows the voltage at the open end divided by the voltage at the close end. a quadratic approximation provides a useful rule of thumb. a Ferranti rise of 5% occurs for lines of 150 mi. a Ferranti rise of around 20% occurs for lines of 300 mi. if a line is simultaneously energized from both ends, the Ferranti rise at the midpoint will be 25% of the Ferranti rise when energized from just one end. by arming breaker operations to occur simultaneously and by adding a synchroscope to measure the angle difference between buses at the ends of the line, it is possible to synchronize islands with very long lines while limiting the Ferranti rise at the midpoint. Combine Electrical Islands electrical islands are currently tied together using breakers with synch-check relays. the frequency difference between the islands needs to be minimized in order to control the synchronizing angle and limit the flow between the islands after synchronization. some utilities prefer to perform the synchronization at january/february 2014 Complete Restoration of Customer Loads load is typically restored based on the technical needs of the power system. initial load restoration is focused on providing load to online generators, dampening voltage transients, and consuming excess Mvars. the loads restored are ieee power & energy magazine 59