As was mentioned previously, there is a significant change in the trend of the torque-speed signal over time. This confirms that the torque-speed curve behavior can be used as an obvious indicator with a view to perform a suitable CMS on wind generators during running. The fluctuation in the electric torque pulsations through the normal and abnormal conditions is due to the ripple and cogging torque. The value of b
can be
utilized to figure out the presence of the electrical faults in the wind generators, e.g. stator winding fault or rotor imbalance fault. Further, the proposed monitoring model is based on the parameters that control the operation condition, such as the generator reactance. The change of the generator reactance <., which is corresponding to the operation condition, is one of the most significant effects that lead up to electrical faults in the generator. Figure 4.13 shows the dramatic changes over time in the signal of the criterion b
during the
operation. When the generator suffers from an electric fault, such as stator inter-turn fault, the generator reactance decreases automatically, which increases the variable b
108 remarkably. Therefore, high values of the criterion b
represent a real impression of the
abnormal operation condition of the generator.
Figure 4. 13 The proposed indicator
trend.
Likewise, the excitation flux in the core of the generator and connected power transformers are directly proportional to the ratio of the voltage to the frequency on the terminals of the equipment. The losses that are due to the eddy currents and hysteresis rise the temperature and hence increase in proportion to the level of excitation. In the abnormal operation condition, the generator reactance <. might rapidly decrease, and the ratio of the change in the generator temperature increases with respect to the electrical torque and rotor rotational speed. Therefore, the ratio of the change in the generator temperature could be considered an indicator to detect the faults in the generators. Figure 4.14 illustrates the
109
behavior of the rate of change in the generator temperature with respect to the rotor rotational speed in the normal and abnormal conditions respectively.
Fig. 4. 14 The rate of change in the generator temperature with respect to the angular rotor speed.
Further, Fig 4.15 shows the rate of change in the generator temperature trend with respect to the electrical torque in the normal and abnormal conditions respectively. The increase in the generator temperature shows high values in the abnormal condition with respect to the generator output power as shown in Fig. 4.16, which illustrates the scatterplot of the generator temperature rise against the relative power output (%) in the normal and abnormal conditions. This figure clearly presents the contrast in the rise of the generator temperature with respect to the relative power output between these conditions.
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Fig. 4. 15 The rate of change in the generator temperature with respect to the electrical torque.
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The average generator temperature rise for each 50kW increment of the output power in the normal and abnormal conditions is illustrated in Fig. 4.17. The transition condition represents the transit from the normal to the abnormal situation.
Fig. 4. 17 The generator temperature rise trends against the relative output power.
The simulation results of the induced driving rotating permanent magnet torque show low values in the abnormal condition with respect to the rate of change in the rotating permanent magnet temperature as shown in Figs. 4.18, 4.19. The figures illustrate the trend of the of the permanent magnet torque of the wind generator in the normal and abnormal conditions with respect to the rate of change in the rotating permanent magnet temperature and the magnetization angle of the permanent magnet. The influence of the permanent magnet temperature on the driving permanent magnet torque and the magnetization angle is very obvious in the normal and abnormal conditions.
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Fig. 4. 18 The trend of the permanent magnet torque with respect to the rate of change in the magnet temperature when the magnetization angle is within (0 – 0.15 rad).
Fig. 4. 19 The trend of the permanent magnet torque with respect to the rate of change in the magnet temperature when the magnetization angle is within (0.15 – 0.475 rad).
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The high-driving permanent magnet torque with respect to the rate of change in the magnet temperature represents the normal operation condition when the magnetization angle is within the range (0 to 0.15 rad). In this situation, the wind generator produces proper torque in the normal condition. For instance, when the rate of change in the magnet temperature reaches 0.8, the generator produces approximately 3800 N. m and 3300 N. m in the normal and abnormal conditions respectively. The contrast in the induced torque between the normal and the abnormal is very obvious when the magnetization angle is within the range (0.15 to 0.475 rad). For instance, when the rate of change in the magnet temperature reaches 0.8, the generator produces approximately 2000 N.m and 650 N.m in the normal and abnormal conditions respectively. In the both cases, the high-rotating permanent magnet torque indicates that the wind generator does not suffer from any improper magnet temperature even though the magnetization angle is different.