Static Tests

Tests are to be completed in accordance with the requirements of the Contract and scope of work, and in line with the following:

10.2.1 Heater Connections

Make sure that anti-condensation heaters are connected to a supply of the specified voltage in such a way that they are always switched on when the generator is standing idle.

10.2.2 Insulation Resistance

If the instruction in Section 5.3 regarding the connection of heaters has been carried out, there should be no problem due to low insulation resistance. However, the insulation resistance of the windings should be checked before the machine is started as follows:

10.2.2.1 Stator Windings

1. Neutral earthing arrangements should be disconnected and a 5000V Megger should be applied to the complete winding for 1 minute.

2. The insulation resistance depends on the winding temperature and should not be less than the following values:

Table 10-2 Insulation Resistance/Winding Temperature Relationship Winding Temperature (°C) 0 10 20 30 40 50 Insulation resistance (Mɏ) 128 64 32 16 8 4

3. If the reading is less than this value, first check that there is no moisture or dirt on the terminal insulators, check whether any other equipment such as lightning arrestors, voltage transformers etc, is connected to the stator terminals as this could affect the readings. If none of these is the cause, the stator winding should be dried out by one of the following methods:

a. Circulate hot dry air through the machine e.g. by means of fan heaters.

b. Short circuit the stator terminals through a suitable ammeter, drive the generator at its normal speed and switch the excitation to hand control. Adjust the

motorised hand control regulator to give 100% to 110% of full load current in the short-circuited stator.

Electrical Hazard

Even though the line terminals are shorted, an appreciable voltage may exist between the line and neutral terminals; the usual precautions must be taken to prevent

Notice

A check should be kept of the stator winding temperature, by means of the embedded temperature detectors provided, and these readings should not be allowed to exceed 110ºC.

10.2.2.2 Rotor Winding

1. The rotor winding should be Meggered at 500V by applying the test voltage between earth and one of the main field connections. Temporarily short diodes or diode bridge with a piece of fuse wire during this test. Where a rotor earth fault monitor is provided, it must be disconnected.

2. Rotor winding insulation resistance will normally be at least 2Mɏ if the machine is thoroughly dry. However, if it has been allowed to become damp almost any low reading may be obtained. Provided this is not less than about 10kɏ, the best way to dry the rotor out is to run the machine on load.

3. Running with the stator windings short-circuited, as for stator drying is an alternative method that may be used if desired. Several days of running may be required before the insulation resistance of a damp rotor gets up to 2Mɏ.

4. Note: If closed air circuit machines have to be dried out, a temporary air bleed system may be arranged to prevent re-circulation of moisture. Care should be taken not to introduce moisture or dust via the air bleed system. On water cooled machines, water will either condense out on the cooler fins to be removed by the drip tray and the leak detector, or it will be dissipated over a period of time (by the air being bled to seal the bearing shaft seals.)

10.2.3 High Voltage Testing of Windings on Site

High voltage testing is carried out in the works on all machines after running test in accordance with British, American or IEC Standards as specified.

Each of these Standards contain a recommendation that this test should not be repeated but allows reduced voltage tests to be carried out by agreement on machines installed on site.

10.2.3.1 DC HV Testing

British and American Standards allow d.c. to be used for high voltage testing by agreement. In this respect we would comment as follows:

1. We do not consider that high voltage testing on site is necessary or desirable on a machine that has already passed its tests in the factory in accordance with the above Standards. However, if the Purchaser decides to carry out such tests, we draw particular attention to the requirement that the machine shall be clean and thoroughly dried out.

2. We do not consider that dc high voltage testing serves a useful purpose since the potential distribution on stator end windings under dc testing is different from that which exists under ac for which the machine was designed. However, if the Purchaser wishes to carry out this test he should refer to the appropriate Standard. The value of the direct voltage shall be not greater than the RMS value of the alternating voltage specified for the acceptance test in the factory multiplied by 0.8 x 1.6 in the case of British Standards and 0.85 x 1.7 in the case of the American Standards. The maximum duration of the test is 1 minute.

3. After dc high voltage testing the windings should be earthed for a minimum period of 16 hours. WARNING: If the windings are earthed only momentarily, a voltage will build up following the removal of the earth connection.

4. If a failure should occur during a site test on a winding that has already passed the full voltage test in the factory, the responsibility for repair shall be on the Purchaser. The exception is where BRUSH is responsible for site erection and commissioning, and where the test is carried out by agreement and by our own personnel.

10.2.3.2 Routine HV Testing

If, after a major overhaul, it is felt necessary to prove the windings by HV testing the following procedure may be considered, but the decision to conduct these tests and the responsibility for all possible consequences remain with the Customer. In general, potentially destructive HV testing is not recommended. The simple insulation resistance measurement method is preferred (See Section 10.2.2).

1. Essential Pre-Conditions (Stator)

a. Remove neutral link and disconnect surge capacitors.

b. Measure the insulation resistance and polarisation index at 5000V on each phase in turn with the other two phases earthed.

c. The minimum Insulation Resistance (IR) for windings, measured for 1 minute, at various rated voltages and core temperatures are given below:

Table 10-3 Stator Insulation Resistance Values for Given Temperature and Voltage Rating Rated Voltage

d. For machines with a high number of slots it may be possible to reduce the above values.

e. The minimum polarisation index is 1.5.

10.2.3.3 Stator DC Leakage Current HV Test

1. Test Voltage = VL x —2 x 1.2. For example 13.8kV x —2 x 1.2 = 23.4kV dc 2. The test voltage, as defined above, should be applied in approximately 2kV

increments to each phase in turn to earth (with the other two phases earthed), holding for one minute at each stage.

3. Leakage current for each step should be recorded and plotted. A sound winding will provide a smooth steady increase in current with application of test voltage.

4. If for any reason the current becomes erratic, the test should be abandoned until the cause is identified. Upon completion, the current curves for the three phases should

10.2.3.4 Rotor Winding Routine Testing

5. Measure the insulation resistance to earth at 500V maximum (short out rotating rectifier diodes and, when fitted, disconnect the rotor earth fault detector).

6. The minimum acceptable value is 1Mɏ if the machine is dry. However, if it has been allowed to become damp almost any low value may be obtained. Provided the contaminant is known to be moisture then a value not less than 10kɏ is acceptable, the best way to dry the rotor out is to run the machine on load.

7. Note: If a winding is heavily contaminated, initial measurements to earth should be carried out using a low voltage ohmmeter, e.g. 'Fluke' or similar. If readings are low, then the 500V instrument must not be used.

8. If desired an ac impedance pole balance test may be performed as follows:

a. With the end caps still in situ apply a current, typically 5-10A RMS 50-60Hz, to the two conductors of the `D' lead. Measure the total voltage. Measure the voltage between each `D' lead conductor and the winding centre point; the sum of these measurements should equal the supply voltage.

b. On frame 6, 7 and 8 rotors (12 coil) the centre point is at the connection end between the two No. 6 coils at the bottom.

c. On frame 9 rotors (14 coil) the centre point is at the connection end between the No. 7 coils at the top.

d. For sound coils/poles, readings within ±5% are satisfactory.

9. Measure the winding resistance using a suitable low resistance bridge. Compare the value with the original test information contained in the Contract Quality Dossier.

Ensure the correct temperature correction factor is used.

10.2.3.5 Exciter Components

Measure the insulation resistance to earth at 500V maximum on the pilot exciter, main field, armature and rectifier assembly (short out rotating rectifier diodes and, when fitted, disconnect the rotor earth fault detector).

10.2.4 Auxiliary Equipment

All auxiliary equipment should be checked as far as possible by energising each item individually to make sure it operates correctly.