Application - ABB Relion REC670 Applications Manual

1MRK 511 358-UEN A
9.4.2

Application

Generator prime movers are affected by abnormal frequency disturbances. Significant
frequency deviations from rated frequency occur in case of major disturbances in the system.
A rise of frequency occurs in case of generation surplus, while a lack of generation results in a
drop of frequency.
The turbine blade is designed with its natural frequency adequately far from the rated speed
or multiples of the rated speed of the turbine. This design avoids the mechanical resonant
condition, which can lead to an increased mechanical stress on turbine blade. If the ratio
between the turbine resonant frequencies to the system operating frequency is nearly equal to
1, mechanical stress on the blades is approximately 300 times the nonresonant operating
condition stress values. The stress magnification factor is shown in the typical resonance
curve in Figure 83.
300
250
200
150
100
50
IEC12000611 V2 EN-US
Figure 83: Typical stress magnification factor curve according ANSI/IEEE C37.106-2003
Each turbine manufactured for different design of blades has various time restriction limits
for various frequency bands. The time limits depend on the natural frequencies of the blades
inside the turbine, corrosion and erosion of the blade edges and additional loss of blade
lifetime during the abnormal operating conditions.
The frequency limitations and their time restrictions for different types of turbines are similar
in many aspects with steam turbine limitations. Certain differences in design and applications
may result in different protective requirements. Therefore, for different type of turbine
systems, different recommendations on the time restriction limits are specified by the
manufacturer.
Application manual
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1.0
Frequency or Resonant Frequency
Ratio
Standard
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IEC12000611-2-en.vsd
Section 9
Frequency protection
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