ABB Relion 670 Series Applications Manual page 285

1MRK 511 401-UEN Rev. K
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 132.
300
250
200
150
100
50
IEC12000611 V2 EN-US
Figure 132: 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.
However, the IEEE/ANSI C37.106-2003 standard "Guide for Abnormal Frequency Protection for
Power Generating Plants" provides some examples where the time accumulated within each
frequency range is as shown in Figure 133.
Bay control REC670
Application manual
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1.0
Frequency or Resonant Frequency
Ratio
Standard
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IEC12000611-2-en.vsd
Section 11
Frequency protection
279