ABB REC670 Applications Manual page 256

Section 10
Frequency protection
250
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 99.
300
250
200
150
100
50
-
Frequency or Resonant Frequency
IEC12000611 V2 EN-US
Figure 99: Typical stress magnification factor curve according ANSI/IEEE
C37.106-2003 Standard
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 100.
+
1.0
Ratio
IEC12000611-2-en.vsd
1MRK 511 401-UEN A
Bay control REC670 2.2 IEC
Application manual