Rotor winding problems

Bill Moore, PE, National Electric Coil

Moore’s presentation provided an overview of winding design and duties. This was followed by a comprehensive summary of field-winding failure modes—including root causes and what owner/operators can do to reduce the likelihood of occurrence of a particular problem. Each of the failure modes was described and illustrated with photographs showing the damage that a given type of failure can inflict on a winding.

Failure modes discussed included the following:

• Shorted coils—turn/coil distortion, foreign material.

• Shorted turns—cracked turn insulation, turn insulation extrusion and displacement, foreign material.

• Ventilation path restriction—turn insulation migration, turn distortion, block migration, coil migration.

• Restricted coil growth—improper blocking.

• Various conditions of winding open circuits—cracked turns, failed brazes, failed J-straps, failed turn-to-turn, failed coil-to-coil, failed pole-to-pole (photo).

• Grounds—conductor fracture, braze failure, insulation misassembly or failure.

Common tests were mentioned—including short turn detection via flux probe and by RSO test, and running impedance test. Access presentation

Electrical and mechanical tests

Izzy Kerszenbaum, PE, consultant @ IzzyTech

Generator rotor tests are covered extensively in IEEE and IEC publications and Kerszenbaum’s presentation identified seven such documents. He reviewed three overall approaches to assure generator reliability: monitoring, protective relaying, and testing and then identified 25 tests and their conditions of application—for example, online, offline, running, idle. Individually and collectively the documents noted provide extensive information of the purposes of, and procedures for, rotor testing.

The consultant provided expanded coverage of rotor testing, offering the following experience and lessons learned regarding electrical tests:

• Winding copper resistance.

  • Measurement accuracy requires significance to a minimum of four decimal places—typically with a digital low-resistance instrument.

  • Purpose is to assess for shorted turns, bad connections, wrong connections.

  • Rotor should be at room temperature when the test is performed.

  • Compare with original factory data and previous test results, if any.

• Winding insulation resistance to ground.

  • Insulation in good condition will be in the megohm range.

  • Acceptable minimum reading, in MW, by IEEE Standard 43 is the line voltage in kV 1.

  • The rotor winding must be completely dry before any testing is conducted.

  • Readings are sensitive to humidity, surface contamination, and temperature.

• Polarization Index (PI).

  • When the insulation system is clean and dry, the IR value tends to increase as the dielecric material in the insulation absorbs the charge.

  • When the insulation is dirty, wet, or a gross insulation problem exists, the charge does not hold and the IR value will not increase.

  • PI is the ratio of the resistance reading at 10 minutes to that at 1 minute.

  • The recommended PI value is 2.0.

• Other electrical tests:

  • Repetitive surge oscillograph (RSO).

  • Open circuit test.

  • Winding impedance test.

  • C-core test.

  • DC voltage-drop measurements.

  • Pole-drop (voltage drop) test.

Several rotor mechanical tests were discussed as well. Kerszenbaum divided these into surface and volumetric nondestructive examination techniques. Surface NDE, he said, can be visual, magnetic particle, liquid penetrant, and eddy current; volumetric NDE tests are radiography and ultrasonic.

Plus, mention was made of the following specific tests:

• Rotor-bore pressure test.

• Fretting-fatigue cracks in slot dovetails and wedges.

• Pitting of retaining rings, zone rings, fan hubs, and other shrunk-on members.

• Tooth-top cracking.

• Bearing-oil wipers-hydrogen seal running surfaces. Access presentation

Forging cracking issues

Neil Kilpatrick, Siemens Energy

The speaker, a career metallurgist, began with a focus on retaining rings, discussing numerous deterioration/failure modes, including these:

• Runaway over-speed expansion/rupture.

• Ring/rotor/end-plate fretting on some designs.

• Excessive damper winding and body currents caused by asynchronous or unbalanced stator phase currents, possibly resulting in one or more of the following:

  • Arc damage.

  • Metallurgical transformation/embrittlement.

  • Retaining ring/rotor welding.

• Corrosion and stress-corrosion cracking.

Next, Kilpatrick illustrated and discussed numerous causes of damage to, and possible failure of, the main field forging, including the following:

• Tooth/wedge fretting and galling.

• Potential for rotating/bending fatigue.

• Double-ground catastrophic current flow.

• Excessive damper current flow, possibly causing:

  • Arcing, melting.

  • Metallurgical transformation/embrittlement.

  • Wedge/rotor tooth cracking.

  • Retaining ring/rotor welding.

• Tooth-top cracking.

• Metallurgical discontinuities—including flaking, porosity, segregation.

Shaft extensions, the consultant said, are sites for a surprisingly large number of problems. He mentioned friction rubs, loss-of-oil events, bearing failure, journal wear, electrolysis and mechanical damage, rotating bending fatigue, electrical faults related to the collector and connections, coupling-bolt galling, and torsional fatigue, discussing a few in detail. Which issues, if any, are experienced, he said, depends in part on shaft configuration and operating duty. Access presentation

Excitation systems

John Demcko, PE, Arizona Public Service Co

Demcko, a 43-year industry veteran, began his presentation with the basics, first asking and then answering the question: What does an exciter do?

• Provides a source of magnetizing current for a synchronous generator’s rotor.

• Controls generator voltage.

• Supplies or absorbs MVARs from the power system.

• Enhances power-system transient stability.

Additionally, supplemental signals may be added for other purposes.

The APS senior consulting engineer went on to illustrate and discuss each of these topics and then continued his coverage of fundamentals by identifying the three types of excitation systems and their characteristics:

• Brushless. No collector rings and carbon brushes to wear out; vibration issues; electrically complicated and relatively slow to respond; new designs are low maintenance.

• Static. No rotating components, but collector rings and brushes are present; high performance; fast and mechanically simple.

• Rotating alternator/DC generator. Collector rings and brushes are present; vibration issues, complicated and obsolete.

Regardless of the type, excitation-system parameters include the following:

• Ratings of from a few hundred to many thousands of amps DC, depending on the machine.

• Must be capable of forcing 141% of the steady-state rating for several seconds.

• High reliability is a must.

• Redundant systems are common.

• Must enhance transient stability and not harm dynamic stability.

With the foregoing fundamentals in place, discussion turned to photographs illustrating system components and function, and requirements for reliable collector performance. An example of a modern static-exciter retrofit was presented, illustrated by photographs of the many components involved.

The presentation closed with a description of power-system stabilizers (PSS), a requirement of NERC and the Western Electricity Coordinating Council (WECC). The modern high-speed excitation system that solved the transient stability problem created another problem: dynamic instability. A properly tuned PSS will enhance the dynamic stability of the power system and increase local mode rotor damping of the machine it is controlling.

The PSS originated in the late 1960s as a supplemental excitation damper control (to prevent rotor forging damage/failure from torsional resonance). In effect, the system is a “cruise control” for turbine/generator rotor speed deviation from synchronous speed. The PSS is tuned by compensating for the measured phase lag measured between automatic voltage regulator’s summing junction and generator terminal voltage deviation. Access presentation