At annual meetings of the Generator Users Group (GUG), presentations by consultants and vendors, and participation by engineers from these companies in discussion sessions, are critical to the all-volunteer organization’s success. GUG activities are supported by the electric-power industry’s three major OEMs—GE/Alstom, Mitsubishi Hitachi Power Systems Americas (via MD&A), and Siemens Energy—and several third-party equipment and services providers. The latter group includes AGTServices, Doble Engineering, Environment One Corp, EthosEnergy Group, IRIS Power, National Electric Coil, Schweitzer Engineering, and Turbine Generator Maintenance (TGM).
Six consultant/vendor presentations from the 2016 meeting in San Antonio, August 22-25, are summarized below; four others were presented in the previous issue of CCJ ONsite. The links here enable quick access to the topics of greatest interest to you.
Endwinding vibration, Mladen Sasic, Iris Power.
Stator global VPI technology, Scott Robinson, Siemens Energy.
Generator testing and overhauls, Pawel Kwiatkowski and Darian Garcia, EthosEnergy Group.
Generator rotor thermal sensitivity, W Howard Moudy, National Electric Coil.
Emergency field rewind, Jamie Clark, AGTServices and Fred King, Duke Energy.
GE’s generator product/services offerings, various presenters.
Users wanting to dig deeper into these areas can access the presentations on the Power Users website. You must be registered to participate in the forum, a relatively simple process if you’re not already signed up.
Endwinding vibrational forces and duties have increased as generator ratings have increased and as more compact designs have evolved. Simultaneously, cost reductions for less robust designs have tended toward more vibration problems. Examples of dust generation from vibration are shown in Fig 1. Note that endwinding vibration has been found to have the highest total loss mitigation value on recent common generator failures. If not detected before failure, considerable collateral damage may occur (Fig 2).
Two common ways are used to assess vibrational issues:
Periodic visual inspection looking for evidence (dusting, fretting, greasing).
Periodic impact (bump) testing to identify natural frequencies and ensure they are not causing resonance.
Because each of these alternatives requires an outage, there has been a trend toward installing endwinding vibration detectors (fiberoptic non-metallic accelerometers) on units with suspected or known vibration issues. Sensors are installed in locations where high vibration is most likely. Displacement values are commonly read, but velocity/acceleration values may provide better analytical information. Displacement under 5 mils is usually considered safe, with 10 mils cause for concern and 20 mils considered dangerous.
The bottom line: Endwinding vibration monitoring systems can be a valuable resource on suspect windings to provide early warning and allow optimum scheduling and planning of needed repairs.
Stator global VPI technology
Siemens presented an overview of its generators using global vacuum pressure impregnation (GVPI)—including operating experience and repair options. The company introduced its GVPI system in 1988 and today has more than 1650 such stator windings in service with a total capability of 239,000 MVA. Thus far, these machines have combined for more than 25-million operating hours and 320,000 start/stop cycles—in round numbers. The units employ indirect-air, indirect-hydrogen, and water cooling. Voltage ratings extend to 22 kV, outputs to 870 MVA.
Design features of the GVPI insulation system for a stator bar are highlighted in Fig 1. The insulation ground-wall materials are applied over the copper in the following sequence: inner corona protection, ground-wall insulation, inner-outer corona protection, mica splitting layer interspersed with contact tape, and outer-outer corona protection layer.
The speaker said that, to date, there has been no report of a stator-bar failure attributed to this insulation system. GVPI windings, he continued, do not require re-wedging, re-tightening of enwinding structure, or re-tightening of the laminated core—for the entire lifecycle of the generator, with a reliability factor of greater than 99.9%.
Damage to GVPI stator windings is very unlikely, and when it occurs, the cause typically is impact by a foreign object. Should such damage occur, the OEM has repair procedures to suit both the situation and customer preferences.
Some operational issues that have been experienced include dusting of de-bonded structural components, which may occur at any bonded interface in the end winding (Fig 2). Also, stator endwinding natural frequencies may experience a shift over time and if they approach the driving frequency, loosening will be accelerated. This condition can be addressed by adding tangential blocking between the top-layer bar ends near the series connections.
Localized erosion of outer corona protection has been found on several units at the ends of the core (Fig 3). Methods have been developed for the repair of such findings.
Generator testing and overhauls
EthosEnergy Group’s (EEG) two-hour session was divided into these three topics:
Generator deterioration causes and corrective actions.
Overview of the company’s generator maintenance capabilities.
Illustration of EEG’s capabilities by review of 16 case studies.
The first slide illustrated the complexity of a powerplant and the “insignificance” of the generator. While comparatively small in size, the importance and complexity of the generator is hard to over-emphasize. In the figure, the generator is the tiny white object within the red ellipse.
The design life of generators is commonly considered to be about 30 years. Aging considerations include fatigue life of the forgings, stop/start cycles completed, equivalent operating hours remaining, the machine’s position on the “bathtub curve,” rate of increase of component failures, and the point at which plant’s economic feasibility becomes negative.
The four major stresses imposed on the generator were considered individually: electrical, mechanical, thermal, and environmental.
Electrical stresses listed were core back-iron overheating caused by over-excitation operation, overheating of core ends caused by under-excitation operation, core manufacturing or repair defects, partial-discharge activity, and surface contamination and moisture.
Mechanical stresses: core looseness, vibration and fretting, stator winding slot looseness and 60-Hz/120-Hz vibration, stator endwinding looseness and vibration, rotor component stresses caused by centrifugal forces, and abrasive material contamination.
Thermal stresses: core insulation damage, poor ventilation, continuous operation at high temperature or overload, differential expansion between components, and thermal cycling.
Environmental stresses: water absorption, oil contamination, acidic or alkaloid atmospheres, and carbon dust.
EthosEnergy Group has found that causes of in-service failures have been: 37% bearings, 33% stator windings, 11% unspecified, 6% shaft/coupling, 5% external devices, 5% rotor, and
3% brushes/slip-rings. By contrast, major problems found during inspection/test have been: 61% bearings, 10% unspecified, 8% stator windings, 8% shaft/coupling, 7% brushes/slip-rings, 4% external devices, and 2% rotor.
Test and inspection were discussed briefly. Tests commonly used on stators are: winding copper resistance, insulation resistance and polarization index, EL CID/loop test, partial discharge, insulation tan delta/power factor, and AC/DC hipot.
Tests commonly used on fields are winding copper resistance, insulation resistance and polarization index, repetitive surge oscillography (RSO), pole (and turn) drop, AC/DC HV hipot (on major repairs and rewinds).
Repairs (solutions) were discussed in detail. Topics covered included: stator core restacking, cleaning methods, repair of partial discharge indications, stator wedge testing and replacement, stress corrosion cracking of 18/5 retaining rings, exciter and collector issues, bearing and journal damage. Get the details by accessing the presentation via the Power Users website.
Remainder of the presentation was a detailed description of company capabilities for repairing generators, illustrated by a review of 16 case studies of work performed by EEG repair crews.
Generator rotor thermal sensitivity
Rotor thermal sensitivity normally can be attributed to one or a combination of the following factors: insufficient or unequal clearances, asymmetrical coil expansion, bound slot wedges, blocked ventilation passages, and shorted turns. Each of the above was discussed in this presentation.
Insufficient or unequal clearances can exist from one coil to another and/or from the coil ends to the steel end plate. This condition can cause forces to be applied that may result in a bending of the rotor forging and increased vibration.
Asymmetrical coil expansion can be caused by restriction of one or more coils and result in unequal coil expansion because of the lack of an adequate slip plane between coils and forgings. This may result in unequal expansion forces on the body forging causing it to bend and vibrate.
Bound slot wedges often result from deficient wedge design/incorrect installation, resulting in asymmetrical and restricted expansion which can place bending forces on the forging and cause vibration.
Blocked ventilation passages can occur throughout the ventilation circuits with one location mentioned in particular: radial discharge holes (photo).
Shorted turns can have a variety of causes: conductor movement, incorrect blocking issues, conductor restriction leading to ratcheting or distortion, connector issues, foreign material. The location and magnitude of the shorted turns has a significant influence on the level of thermal sensitivity; specifically, the closer the coil with shorted turns is to the pole head the greater the influence of the short.
The presentation also reviewed NEC’s solutions to the thermal sensitivity problems discussed.
Emergency field rewind
Numerous maintenance problems with generators in the Duke system were described with slides narrated by the utility’s Fred King and AGTServices Inc’s Jamie Clark (access the presentation for more excellent photography). Issues included broken J-straps (Fig 1). On another unit, a flux-probe test revealed shorted turns in a large coil. Inspection revealed the root cause as movement of turn insulation (Fig 2).
Failure of an exciter lead is shown in Fig 3 (left) with the upgraded connector to its right. Several cases of endwinding and connection-ring vibration have been experienced by Duke generators with indications as seen in Fig 4. Each of these was corrected by tie replacement and/or application of bonding resin (Fig 5).
Answers to several informal industry survey questions were provided by the presenters for everyone’s benefit. The percentages of “yes” responses follow the questions below:
Have you experienced J-strap failures? 50%
Do you require a pressure test on bore seals on hydrogen-cooled units? 89%
Have you found field slot-liner problems requiring field rewind? 53%
Have you operated a unit with one field ground? 47%
Do you require new copper for field rewinds? 5%
Do you require a high-speed balance after field rewind? 70%
Do you specify stator wedge materials for rewedge/rewind projects? 53%
GE’s generator product/services offerings
As a major generator manufacturer and supporter of the Generator Users Group, GE had most of the third day of the 2016 user-group conference for discussion of its product line. There were several presentations, several summarized briefly below. The editors suggest following up by reviewing the various GE powerpoints posted to the Power Users website.
GE has three general lines of generators: air-cooled, 30-340 MW; hydrogen-cooled, 90-590 MW, and water-cooled, 530-1800 MW. For cost and quality reasons, the company has adopted a modular design philosophy using long-time proven features and components.
The OEM’s recommendations are detailed in GEK103566, recently updated. GE is migrating towards removal of the first-year inspection requirement, and the latest GEK document focuses more directly on updated recommendations with less-intrusive inspections.
Generator uprate. This informative presentation discussed the important and not-well-understood generator kilowatt and kilovolt-ampere output issues, and the need for generator modification or replacement to safely support a plant uprate.
Generator fundamentals. Discussion included interesting sketches that illustrated how and why a generator can convert rotating torque energy from the turbine into electrical power for the grid. Various physical configurations of air- and hydrogen-cooled generators were described with numerous photos. Some of the major generator components were described in detail—including the stator core, stator wedging system, field winding, hydrogen seals, and excitation systems.
Excitation systems can be challenging components, as illustrated by a listing of 12 different excitations systems for GE and nine for Alstom, which was acquired recently by GE. Brushless and static excitation systems were discussed in detail, followed by coverage of generator protection systems.
GE Power Services. Worldwide, the OEM has more than 10,000 generators in service and about 1600 GW of installed capacity (both round numbers). With the recent purchase of Alstom, GE is now an amalgamation of 16 companies that existed 40 years ago. It was characterized as a growing and dynamic business.
Generator vibration and torsional dynamics. Every generator has some degree of thermal sensitivity and there are many possible causes; if the root cause is identified, corrective action can be taken. Motoring and negative-sequence events generally are well understood, and they occur occasionally. Depending on the severity of conditions, corrective actions may range from none need to scrapping of the rotor. Several additional important topics were addressed briefly: TIL 1292, “Generator Rotor Dovetail Inspection,” and turbine/generator torsional dynamics, generator vibration monitoring, generator bearing-metal temperature, and grid series compensation and SSR.
Global repair solutions. GE has power-generator repair facilities worldwide in 55 locations and staffed, in round numbers, by 4000 employees. Many of these sites are large, high-capability facilities. In the US, high-speed balance can be done only in Schenectady, NY, and Richmond, Va.
Generator monitoring. In support of industry trends toward condition-based maintenance (CBM), GE has increased focus on monitoring instrumentation. Some of the following devices were discussed:
Upgraded stator leak monitoring system (said to eliminate the need for hydraulic integrity testing during routine maintenance outages).
Shorted-turn flux probe.
Endwinding vibration detectors.
Collector health monitor.
Rotor shaft-voltage monitor.
Examples of instrumentation success stories also were presented.