According to the requirements of the "pre-regulation," the stator core test of a generator is conducted after reassembling or replacing parts, repairing the silicon steel sheets, or when necessary—such as when there are doubts about the temperature readings of the stator core measuring points, after the first overhaul, when the rotor has been removed, or after replacing the stator bars or wedges. The test aims to comprehensively assess whether the insulation between the laminations is good and to prevent overheating of the core, which could lead to accidents, by checking if the losses and temperature rise of each part exceed the specified limits.
**First, Test Wiring and Method**
1. **Test Wiring Diagram**
Figure 4-11 shows three different wiring configurations for the test. The instrument used should have an accuracy of at least 0.5 class. Prepare more than 20 alcohol thermometers, placing at least 10 in the circumferential and axial directions of the teeth, with 2 placed in the yoke area. Measure the initial temperature of the core and the ambient room temperature. If possible, use an infrared camera for temperature measurement and recording.
2. **Calculation of Excitation Coil and Measuring Coil**
The number of turns in the excitation coil is calculated using formula (4-5):
$$
N = \frac{U_1}{4.44 \cdot f \cdot B \cdot S}
$$
In this formula:
- $ B $: Magnetic flux density in the core, typically 1 T or 1.4 T depending on the cooling method (1 T for indirect cooling in turbo generators below 100 MW and small hydro-generators; 1.4 T for direct cooling in large turbo and hydro-generators).
- $ U_1 $: Voltage of the excitation coil, in volts.
- $ S $: Cross-sectional area of the stator core, in cm².
- $ L $: Axial length of the stator core, in cm.
- $ K $: Core filling factor, usually taken as 0.93.
- Other parameters like $ L_1, L_2, H, D_1, D_2, H_1 $ refer to specific dimensions of the core structure.
The excitation current is then calculated using formula (4-6), ensuring that the actual magnetic density $ B' $ matches the selected value. Adjustments may be required if there is a significant deviation.
**Second, Test Steps and Precautions**
1. Short-circuit the stator windings to ground using a 50 mm² cross-section cable. If a grounding point already exists, avoid grounding and just short-circuit.
2. Follow the wiring diagram (see Figure 4-n) and ensure the excitation and measuring coils are positioned perpendicular to each other.
3. Turn on the power, read the indicator value, and quickly turn it off. Calculate the actual magnetic density $ B' $ using formula (4-9):
$$
B' = \frac{U_2}{4.44fSW_2} = \frac{45U_2}{SW_2}
$$
Where $ U_2 $ is the induced voltage in the measuring coil, and $ W_2 $ is the number of turns.
4. Reconnect the power and check the temperature after 10 minutes using an infrared camera. If temperatures are excessively high, smoke, or redness is observed, stop the test immediately and record the data. Continue measuring every 10 minutes for 90 minutes (at 1 T) or 45 minutes (at 1.4 T), then turn off the power and conclude the test.
5. **Notes:**
- Ensure the excitation and measuring coils are well insulated and do not come into contact with the core. Avoid using metal-sheathed or armored cables.
- Prevent any foreign metal objects from falling into the generator.
- Minimize air flow that could affect temperature readings.
- If a local hot spot is detected but the temperature difference is not significant, increase the magnetic density to 1.2–1.4 T or adjust the test voltage accordingly.
**Third, Analysis and Judgment of Test Results**
1. At a magnetic density of 1 T, the highest temperature rise of the teeth should not exceed 25 K, and the maximum temperature difference between the teeth should not exceed 15 K.
2. The unit loss should not exceed 1.3 times the reference value, as defined for 1.4 T conditions.
**Additional Notes on Data Interpretation**
The test results should be analyzed in conjunction with the initial temperature measurements and the magnetic density applied during testing. Any abnormal temperature distribution or excessive loss indicates potential issues with the core’s insulation or structural integrity. It is essential to document all readings and compare them against standard thresholds to determine if further investigation or maintenance is required. Always follow safety protocols and ensure proper equipment calibration before conducting the test.
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