Visual Evaluation High Potential Electrical Testing Partial Discharge Analysis

Generator Testing Services

Visual Evaluation
Industrial Tests, Inc. generator specialists understand the signs of winding damage and failure. They also are familiar with winding support looseness and can evaluate the winding support system for looseness. Coil wedges are also inspected for proper tightness and signs of movement or failure. The entire HV stator coil system is also inspected for signs of corona discharge.

Electrical Testing:

Winding Copper Resistance
Industrial Tests, Inc. can measure the low resistance of the field and stator windings and test the pole balance of the generator field when accessible. Each phase of the stator winding is measured using a 10 amp or 25 amp micro ohmmeter.

Insulation Resistance
Initial megger testing is conducted at 2.5kv testing to determine the suitability of a generators winding for high voltage insulation stress testing.

Polarization Index
A Polarization Index (PI) of 3.0 is desired before proceeding with higher voltage tests. We typically utilize a special low energy test set, which is highly accurate and a good predictor of insulation condition.

Dielectric Discharge
In addition to the Polarization Index test we will also perform a Dielectric Discharge test. The Dielectric Discharge Test gives an indication of the amount of absorbed dirt and moisture.

Step Voltage
Another extremely useful test is an automated step voltage test. The step voltage test is based on the principle that an ideal insulator will produce identical resistance readings at all voltages. An insulator which is being stressed will show lower resistance values as voltages are increased.

The above tests, when taken together provide an excellent bench mark for the condition of a generator’s insulation system. When done annually they serve as an excellent predictor of future failure.

High Potential Testing
Industrial Tests, Inc. has numerous AC and DC Hypots and can perform these tests when request. Although these tests are extremely stressfull to the winding system, and must be conducted with care, they are a good indicator of winding failure.

Power Factor Measurement
The power factor tip-up test for stator coils has been used by electric machinery manufacturers and users since the original standard was issued in 1968. This recommended practice applies to stator coils or bars (half coils) of electric machinery operating at any voltage level. However, it is usually performed on machines with a voltage rating of 6 kV and higher. Individual stator coils outside a core (uninstalled), individual stator coils installed in a core, and completely wound stators can be field tested. The coil insulation under test is the major groundwall insulation that is external to the conductor structure.

Rotor Winding Short Turns
Using a computer analysis program developed by Generatortech, Inc we can produce data which will yield a highly accurate “snapshot” of the rotor winding insulation condition. The system can indicate if there are shorted turns in the rotor winding and pinpoints which coils are affected.

Partial Discharge Analysis
This test can be performed on-line or off-line. Our test equipment is portable and specifically designed to monitor Partial Discharge in motors and turbine generators equipped with Bus or Stator couplers. We can determine the magnitude, rate, and polarity of voltage pulses created by partial discharges. Additional parameters such as NQNs and peak pulse magnitudes are also calculated. Our equipment comes equipped with user selectable RS232, USB, or Ethernet (TCP/IP) communication modes. An external computer is used to acquire, display, and store Partial Discharge test results quickly and conveniently. The nature, region and severity of the Partial Discharge activity can be identified through analysis. Test data can indicate slot discharge, discharge from voids, delamination of the winding, loose windings, and end-winding discharge.

Corona Probe Measurements
Corona probe testing can pinpoint partial discharge location in each slot. It is useful in determining the precise location of partial discharge in a particular coil. The probe can also be used as a testing tool to measure levels of partial discharge.

Electromagnetic Core Imperfection Detection (EL CID) is used to check the integrity of inter-laminar insulation for stator cores of generators. A fault occurring within the laminated stator service results in a hot spot and cause a catastrophic failure of a generator in service. An El ID test can detect faults and defects in the stator core. It is particularly useful test before and after core repairs and should be done after a rewind to check for any damages created by the rewind.