How to test short in alternator rotor winding inside the slots?

[SA07]

Hi
What test can be done to test for short in an alternator rotor winding inside the slots? Alternator 45 MW 11kV 1500 rpm Thanks

 

[panter]

To begin with, you would have to measure resistance of the winding and compare it with the values from generator test sheet. As it is relatively small, you could determine the voltage drop with DC current of for example 100 A if it is less than the nominal values .

 

[LionelHutz]

You can apply AC voltage to the rotor and then measure the voltage on each coil/pole and compare. I don't know how much difference indicates a bad pole, but I know this is a test that sites with a lot of synchronous motors use to determine when a motor has rotor issues.

 

[Gr8blu]

For industrial equipment, the allowable variation is dependent on the type of test.

DC Drop Test (Apply DC to circuit, measure DC voltage across each pole). Allowable variation is 5% from average (most shops will raise concern at 3%).
AC Drop Test (Apply AC to circuit, measure AC voltage across each pole). Allowable variation is 10% from average (most shops will raise concern at 7-8%).

For a "wound in slot" design, the typical tests are:
1) insulation resistance (meggar) which looks for primarily surface ground paths or moisture absorption - performed at relatively low voltage
2) surge (looks for turn-turn fault) - performed at elevated voltage
3) dielectric (hipot) which looks for ground fault via coil insulation breakdown (pinhole) - dielectric test may be AC or DC, and is at significantly elevated voltage

Both 2) and 3) are considered "destructive" tests by some users, because of the el3evated voltage levels introducing added voltage stress to the insulation.

An 11KV alternator probably has a rotor winding rated somewhere between 120 and 500 Vdc. Test 1) would be performed at 500 Vdc. Test 2) at 1500 Vac. Test 3) at 2500 Vac OR 3400 Vdc.

 

[SA07]

Our insurance company is suggesting we do the following test:


The Recurrent Surge Oscillograph (RSO) test, also known as a surge comparison test, is a non-destructive method used to detect and locate inter-turn and ground faults in alternator rotors, especially for turbogenerators. It involves injecting a high-frequency, low-voltage waveform into the rotor winding and analyzing the reflected signals to identify any anomalies.

Here's a more detailed explanation:
What it tests for:

• Inter-turn shorts: Short circuits between turns of the rotor winding.
• Ground faults: Shorts between the winding and the rotor core (ground).
  

How it works:

• Time-domain reflectometry (TDR) principle:
  The RSO test is a form of TDR, which measures the time it takes for a signal to travel through a circuit and return when it encounters a fault.
  
  
• Pulse injection:
  A low-voltage, high-frequency pulse is applied to the rotor winding's slip rings.
  
  
• Signal analysis:
  The reflected signals are captured on an oscilloscope, and the differences in the returning signals are analyzed to detect faults.
  

Why it's important:

• Early fault detection:
  RSO testing can detect shorted turns that may not carry significant current during normal operation, preventing potential catastrophic failures.
  
  
• Condition assessment:
  It helps assess the condition of the rotor winding, especially before and after maintenance or repairs.
  
  
• Preventative maintenance:
  By identifying faults early, corrective actions can be taken, minimizing downtime and repair costs.
  

Key aspects of the test:

• Off-line test: The RSO test is typically performed when the generator is off-line and isolated.
  
  
• Custom test instrument: A specialized RSO reflectometer is used to perform the test.
  
  
• Interpreting results: The waveforms obtained from the test are analyzed to determine the location and severity of any faults.

Is there a special equipment which can do this test? Any reference, make

 

[esperantes]

SA07, You may use the Baker DX Analyser. Baker is now part of Megger.
Esperantes.

 

[Gr8blu]

SA07 - In addition to the Baker, there are two other fairly common testers in use. one is manufactured by Schleich, and the other is called a "PJ Tester".

PJ Electronics Inc. : Manufacturer of High Voltage Surge Testers - Worldwide

MTC3 | MULTI-PURPOSE WINDING TESTER | Schleich

The MTC3 testers offer almost unlimited options for combining and integrating different winding, safety, and functional test methods. Select the test method necessary for the test task out of a large pool of options.

www.motordiagnosticsystems.com

My personal preference is the PJ Tester, although it is (usually) the most expensive piece of equipment.
Either the MTC2 or MTC3 tester by Schleich will do the job.

By the way - the purpose of the Recurrent Surge Oscillograph (RSO aka "surge") test is NOT to find a ground fault. It is to find a turn-to-turn (or perhaps phase-to-phase) fault. It only discovers a ground fault if the original turn-turn (phase-phase) fault has destroyed enough of the coil to create a hard ground between the live conductor and the surrounding ground plane (in your case, rotor pole iron).

You do want to be sure you obtain a surge tester (which automatically drives multiple pulses in a defined period of time), and not an impulse tester (which only drives a single pulse every time you hit the "go" button).