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FRCC Standards Handbook
FRCC Generator Coordination Requirements
Revision Date:
November 2001
Introduction
Due to its peninsular nature, the FRCC Bulk Power Electric System has a greater exposure to
separation of transmission system and frequency deviations than other parts of the North
American Eastern Interconnection. Switching station outages or transmission right of way
outages can cause a transmission separation. Control and protection systems have been
developed and coordinated within the FRCC Region to minimize the disruption of service and
permit rapid restoration. The successful operation of these control and protection systems
relies upon generators instantaneously supporting grid voltage and frequency during these
events.
Generator Coordination Requirements
Generator protection and controls shall support the requirements of the interconnected
transmission systems to minimize the risk of generator tripping during temporary excursions in
voltage or frequency. All Generation equipment owners and/or operators operating within the
FRCC Region shall ensure that protection and control systems on all synchronous generators,
with a nameplate rating greater than 20.0 MVA, will not result in tripping of the generator under
the following conditions;
1) Frequency Excursion Ranges:
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No tripping results with frequency excursions between 60.5 Hz and 61.8 Hz from 0 to 10
seconds.
No tripping results with frequency excursions between 59.5 Hz and 60.5 Hz.
No tripping results with frequency excursions between 58.5 Hz and 59.5 Hz from 0 to 60
seconds.
No tripping results with frequency excursions between 58.0 Hz and 58.5 Hz from 0 to 10
seconds.
No tripping results with frequency excursions between 57.5 Hz and 58.0 Hz from 0 to 1
second.
2) Generator terminal voltages that are within 5 % of the rated nominal design voltage.
3) Generator terminal voltage deviations that exceed 5% but are within 10% of the rated
nominal design voltage and persist for less than 10.0 seconds.
4) Generator volts per hertz conditions that are less than 116% (of generator nominal
voltage and frequency) that last for less than 1.5 seconds.
5) Generator overexcited stator currents (or generator apparent impedance) less than
150% of nameplate rating persisting for less than 5.0 seconds.
A generator may be designed to trip under the above conditions if an equal or greater amount of
load is tripped simultaneously. This load must be located in the same FRCC load shedding
zone as the generator1 The owner of the generator must submit detailed control schemes to the
FRCC in order for the FRCC to examine the proposed scheme and to assess its effect for
potential disturbance conditions.
The FRCC must approve the scheme prior to its
implementation.
FRCC Control Areas and Transmission Providers have the obligation to ensure that all
generators connected to the FRCC Bulk Power Electric System meet the above conditions.
Documentation of the generator protection and controls that could respond to the conditions
above by tripping the generator shall be provided to the FRCC. In the event the generating
equipment owner cannot correct or mitigate these potential generator trip conditions, a request
for a waiver may be made to the FRCC. A waiver may be granted in circumstances where there
is no significant reliability impact to the FRCC Bulk Power Electric System.
Background
Common Generator Protection/Transmission System Interactions
The following types of generator protective relays may respond to emergency conditions in the
bulk power system.
Generator Underfrequency – Most generators are equipped with underfrequency relays that
disconnect the generator from the grid if the frequency is below its setpoint for greater than the
allowed time. These relays are used to protect against turbine blade metal fatigue. Allowed
level and duration curves from representative manufacturers are given in ANSI standard
C37.106. The allowed levels depend upon the individual turbine. Typical settings for Florida
generators are 58.0 hertz with a 12 second delay.
Generator Overspeed Control and Protection - The manufacturer generally supplies turbine
controls and protection to protect against overspeed. Overspeed conditions are normally
expected for a sudden opening of the generators breakers but can also be experienced during
system separation. The underfrequency load shedding program has been designed to shed
slightly more load than the generation imbalance. This is intended to create a slight
overfrequency condition that facilitates automatic resynchronization of transmission ties.
Typical overspeed trip settings for Florida generators are 66.0 hertz with no intentional delay.
Overexcitation (volts per hertz) – Generator magnetic flux densities are proportional to the
terminal voltage magnitude divided by the rotor speed. Overexcitation causes heating in the
generator stator as well as the step up transformer. Most Florida generators use a dual
level/time delay scheme to protect for this condition. A common low level trip setting is 110%
volts per hertz for greater than 45 seconds. A common high level setting is 118% volts per hertz
for more than two seconds. Temporary overvoltages will occur when transmission system
separations cause large amounts of underfrequency load shedding. It takes several seconds
for generator voltage regulators and reactive compensation controls to reduce overvoltages to
below emergency levels. Premature operation of volts per hertz protection increases the
overvoltage burden on the remaining generators and could lead to a cascading tripout of
generators.
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Under frequency load shedding zones are defined in the Policy 6 section of the FRCC Operating
Committee Handbook
Transmission Fault Backup/Overcurrent - Phase fault (distance) relays are often used to protect
the generator from faults on the transmission system. Various types of overcurrent relays are
sometimes used for this purpose. When backup distance relays are set to protect for long
transmission lines exiting the plant, the resulting impedance characteristic can restrict generator
loadability. The Florida system has a number of areas where low voltages and/or voltage
collapse can be occur following equipment outages. Generators near the problem area will
temporarily exceed their continuous reactive output capability as the field voltage goes to its
ceiling level. Maximum excitation limiters and/or protection circuits are generally installed to
protect for reactive overloads in accordance with ANSI Standard C50.13. Phase fault relays, if
set with the impedance reach too high, can respond to this system condition while the generator
is well within its short time capabilities.
Loss of Field (“LOF”) – Excitation system failures can cause a generator to become
underexcited to the point that the stator iron is overheated and the machine eventually losses
synchronism. Temporary transmission overvoltages following a separation/underfrequency load
shedding disturbance can cause the automatic voltage regulator (AVR) to adjust generator
reactive output such that it falls within the LOF characteristic. In most cases a underexcited
reactive ampere limiter (“URAL” also referred to as minimum excitation limiter) is provided with
the AVR to prevent this. Generating equipment owners should verify that URAL controls
coordinate properly with LOF protection.