FRCC Standards Handbook. FRCC Automatic Underfrequency Load Shedding Program. Revision Date: July 2003

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1 F R C C FRCC Standards Handbook FRCC Automatic Underfrequency Load Shedding Program Revision Date: July 2003 FRCC Underfrequency Load Shedding Program Modification and Approval Process Requests to modify the FRCC UFLS program should be made to the chairperson of the FRCC Operating Committee (OC). The OC will direct the SWG and other appropriate groups to evaluate the proposed modifications. Final approval of FRCC UFLS program and modifications to the UFLS program rests with the FRCC OC. FRCC Underfrequency Load Shedding (UFLS) Requirements 1. All load serving members of the FRCC must install automatic underfrequency relays which will disconnect 56% of their customer demand in accordance with the following schedule. UFLS Step Frequency - (hertz) Time Delay 1 - (seconds) Amount of Load (% of member system) A B C D E F L M N Cumulative Amount of Load (%) 2. Deviations from the FRCC UFLS schedule must be assessed by the FRCC. Local area changes in the UFLS assignments must be compatible with the overall underfrequency protection for the FRCC. 1 Time Delay = Intentional delay + relay delay + breaker delay 2 FPL has 2/3 of Step A set at hertz as an aid to system stability. This high set Step A is concentrated in the Miami area.

2 3. All load serving members of the FRCC must annually provide the FRCC Relay Task Force with the amount of armed load connected to UFLS relays as a percentage of the entities projected summer peak load. The amount of UFLS in each of the four FRCC load shedding zones shall also be provided to the Relay Task Force. 4. The FRCC Stability Working Group ( SWG ) shall maintain a data base of the UFLS program. Equivalent representations of the UFLS program may be used in this data base provided they are consistent with the overall application of relays in each of the four load shedding zones. 5. The FRCC SWG shall assess the adequacy and effectiveness of the FRCC s UFLS program at least every five years or whenever there are significant changes in system conditions. The SWG shall review FRCC UFLS program requirements and documentation at least every five years. 6. All disturbance conditions involving operation of UFLS relays or frequencies below these relays setpoints shall be evaluated. The evaluation shall be assigned to the appropriate FRCC working group or ad-hoc working group as deemed appropriate. A report will be submitted to the FRCC Engineering and Operating Committees. 7. All load serving members of the FRCC must have an UFLS maintenance and testing program. Documentation of the program must be available to the FRCC on request. 8. FRCC members using frequency based automatic restoration of load must annually provide the Relay Task Force with the amount of automatically restored load as a percentage of the members projected summer peak load. 9. The system impact of frequency based automatic load restoration shall be assessed at least every five years or whenever there are significant changes in system conditions. FRCC Load Shedding Zones The FRCC load shedding zones are defined below in terms of the transmission circuits that form the zone boundary. The boundary terminal is underlined for those circuits that have tap load stations. FRCC Northeast Area Zone 1 Bradford-Deerhaven 138 Bunnell-Ormond 115 Bunnel-Volusia 230 Columbia-Suwanee 115 Duval-Hatch 500 Duval-Kingsland 230 Duval-Thalmann 500 Putnam-Volusia 230 Seminole-Silver Springs N. 230 #1 &2 Note: This zone definition represents a change from the currently defined UFLS zones. It moves Gainesville from zone 1 to zone 2 and Palatka/St. Augustine from zone 2 to zone1. The reason for making this change is the revised zone 1 boundaries coincide with the separation interfaces for the Poinsett North and Duval South 500 kv corridor outages. FRCC Central Area Zone 2 South Boundary is same as zone 3 North Boundary: Bradford-Deerhaven 138 Bunnel-Volusia 230 Duval-Poinsett 500

3 Ft.White N/S 230 Ft. White-High Springs 69 Poinsett-Rice 500 Putnam-Volusia 230 Seminole-Silver Springs North 230 #1&2 FRCC South Area - Zone 3 Brevard-Eau Galle 138 Brevard-Malabar 230 #1&2 Calusa-Ft. Myers 230 Cocoa Beach-Eau Galle 138 Charlotte-Ft.Myers 230 #1&2 Martin-Poinsett 500 Midway-Poinsett 500 FRCC Northwest Area Zone 4 Columbia-Suwanee 115 Ft.White N/S 230 Ft. White-High Springs 69 Jasper-Pinegrove 115 Jasper-Tarver 115 Pinegrove-Suwanee 230 Sub 20-South Bainbridge 230 Callaway-Port St. Joe 230 Scholz-Woodruff 115 Suwanee-Twinlakes 115 Background The FRCC UFLS program underwent a major revision in The addition of two 500 kv and one 230 kv interconnections with Southern Subregion Security Area raised FRCC import capability from zero to approximately 3600 MW. Although the potential for electrical separation was dramatically decreased by these interconnections, the potential consequences of a separation were increased because of the higher import capability. The required amount of automatic UFLS was raised from 36% to 41% with the setpoint of the first step raised from 59.0 to 59.7 hertz. Subsequent studies of the UFLS program s effectiveness have lead to an increase in the number of required UFLS steps. All load serving members of the FRCC must have 56% of their customer demand armed with automatic UFLS relays in accordance with the FRCC UFLS schedule. FRCC UFLS Coordination Issues Extreme contingencies such as loss of all lines on a 500kV right of way or all generators at a plant can cause a separation of the Florida transmission system if the outage occurs during conditions of high power import. While these types of events are rare, they have occurred and caused separations. Coordinated operation of controlled separation systems, UFLS, and reactive switching systems are essential for minimizing the impact on the bulk power system and customer service. Given the exposure to various extreme contingencies and the design of the controlled separation systems, the most likely separation point is in north Florida at Duval and Ft. White substations. This separation point disconnects most of the FRCC from the rest of the grid and necessitates a coordinated approach to load shedding and emergency controls throughout the FRCC. Unusual generator dispatch and/or line outage conditions can create a potential for separations within the FRCC region. The definition of the four FRCC load shedding zones is an attempt to assure uniform application of underfrequency relays. Non uniform application of underfrequency relays across a geographic area can adversely affect frequency recovery when an unforeseen separation occurs.

4 Loss of Generation The sudden trip of a Florida generator will result in the SOU-FRCC interconnection picking up about 90% of the lost generation. This increase in tie flow is due to the size of the Florida system relative to the rest of the Eastern Interconnection. The FRCC transmission systems are operated such that line loadings and voltages will not exceed rated values for the loss of the largest Florida generator. On line contingency analysis (the security analysis program) is used to assure secure operating conditions. The sudden loss of an entire power plant when the SOU to FRCC power transfer is at or close to its 3600 MW limit can cause low voltages and generator reactive overloads. A control center based, automatic load shedding scheme (see discussion of scheme LS1 in FRCC SPS Review and Coordination Procedure and Database documents) has been designed to recognize the low voltages, ties flow increases and generator reactive outputs associated with this condition and respond by tripping approximately 800MW of load in the South Florida area. A controlled separation of the transmission system will be initiated in the event the LS1 load shedding is insufficient to arrest voltage declines. This is accomplished by out-of-step relay systems installed at Duval and Ft. White substations. The loss of power plant followed by controlled separation is the most challenging disturbance scenario for underfrequency. The FRCC UFLS has been designed around the following generator protection and control practices: a) Steam turbine underfrequency protection set to respond to frequencies at or lower than 58.0 hertz for more than 12.0 seconds; b) Nuclear unit low reactor coolant flow protection can respond to frequencies below 57.0 hertz for more than 0.1 seconds; c) Digital turbine governor controls respond to 103% overspeed by closing intercept valves. These considerations establish frequency targets for the FRCC UFLS. The frequency should not fall below 57.0 hertz and should recover above 58.0 hertz in less than 12.0 seconds. Over shedding should not cause the frequency to exceed 103% overspeed (61.8 hertz). UFLS in an amount equal to the power deficit in the separated system will usually result in the frequency stabilizing at a level below 60 hertz. The FRCC UFLS schedule has been designed so that slightly more load is shed than the power deficit. This allows the frequency to recover to or slightly above 60 hertz and facilitates line reclosure via automatic synchronizing relays. All of the controlled separation schemes are coordinated with automatic synchronizing and synchronism check reclosing systems. The Florida system will generally experience temporary overvoltages following separation and UFLS. These temporary overvoltages generally get more severe with greater frequency declines and greater amounts of UFLS. Most transmission level shunt capacitor and reactor banks in South Florida have been equipped with underfrequency/overvoltage relays which result in faster switching than can be accomplished with the automatic voltage controllers normally used for automatic switching of reactive compensation. Loss of 500kV Right of Way A large portion of Florida imports flow on the 500kV lines on the east coast of Florida. From southeast Georgia to Midway substation (near Ft. Pierce, FL), there are two 500kV lines on separate towers sharing the same right of way. The simultaneous loss of both 500kV circuits on the same right of way can result in a controlled separation of the Florida transmission system if the outage occurs during a condition of high power imports. The exact location of the separation and the relay system involved depends on whether the right of way outage is north of Duval substation, south of Duval substation or north of Poinsett substation. All of the separation interfaces are in north Florida. Separations due to 500kV right of way outages are less severe than those caused by the loss of generation. The power deficit consists of the power that was flowing on the outaged 500kV lines and the lines opened by the controlled separation systems. The amount of power interrupted is always less than 3600MW (the maximum import).

5 Coordination with SERC The FRCC is interconnected only with the Southern Subregion of SERC. No contingencies, normal or extreme have been identified which would result in Florida and a sizeable portion of the Southern Subregion becoming separated from the Interconnected power system. Consequently, the FRCC s UFLS program is based on Florida specific concerns. Load Restoration No FRCC members currently use relays which automatically restore load disconnected by UFLS relays. The use of automatic load restoration systems is allowed under the policy stated below. Each utility shall be allowed to automatically restore no more than 15% of the amount of load that is shed by underfrequency relays after the frequency has recovered to hertz, under the following condition: 1. The relays shall be set so that 5% will be restored after 20 minutes of continuous operation at hertz, an additional 5% will be restored after 25 minutes of continuous operation at hertz, and the final 5% will be restored after 30 minutes of continuous operation at hertz. 2. Immediately if frequency is 60.3 hertz or above