There Is Not a Stator Winding That NEC Cannot Make!

Strictly speaking, the coils making up a stator winding have multiple turns per coil, and the stator bars in a winding have only one turn per bar. However, most power industry engineers use the terms interchangeably. Different manufacturers may have a decided preference for one term or the other. We at NEC use both terms, so no one is left thinking that we manufacture one but not the other. If it goes into a generator or high-voltage motor stator, we make it. We are experts and have been serving the power generation sector and industry for over 85 years.

High Voltage Stator Windings Manufactured at NEC's Brownsville Facility

Manufacturing starts with electrical grade copper. Individual copper strands are insulated with glass fiber filaments and coated with a "B" staged epoxy. Each insulated strand is stacked and transposed per design specifications. Strands are stacked and grouped into bundles, depending on the design specifications of the coil or bar being made.

Above – Left: Some or all conductors in inner water-cooled bars are hollow copper tubes. Example cross section is from a General Electric 200,535 kVA, 13.8 kV, 3600 RPM unit.
Right: Non-conductive hollow steel tubes are used with inner gas-cooled bars. Cross sections are from: (L) Westinghouse 493,280 kVA, 24 kV, 3600 RPM inner hydrogen-cooled unit, (C) Westinghouse 209,000 kVA, 13.8 kV, 3600 RPM unit inner hydrogen-cooled unit and (R) Westinghouse 159,000 kVA, 13.8 kV, 3600 RPM inner air-cooled unit.

The strand bundles are compressed and heated to achieve precise dimensions and cure the strand epoxy. The bundles are further shaped depending on the type of coil or bar being made. Custom-made dies are used for each design to shape the cell portion and the involute (end arm). In the case of multi-turn coils, strands are looped and then shaped in a series of dies, that both spread and bend the coil. In either case, consistency in coil or bar shape, especially in the involutes, is critical to good machine fit and avoiding conditions that may initiate corona after some time in operation. After shaping, testing assures the absence of strand shorts. Ground wall tape, is applied in a specified number of half-lapped layers. Some coil and bar designs use a mica flake-fiber glass tape with resin applied though a vacuum pressure impregnation (VPI) process for the ground wall insulation. Others use a B-staged ground- mica paper tape. Both processes require further curing in heated presses. (DOWNLOAD brochure detailing NEC's NeccoBondE insulation system.)

Above: Conventionally cooled stator coils and bars. (L) for a Westinghouse 21,400 kVA, 12 kV, 900 RPM unit. (C) Westinghouse 15,625 kVA, 13.8 kV, 3600 RPM unit. (R) Brush 9,490 kVA, 13.8 kV, 600 RPM unit.

Hydraulics are used to apply pressure and maintain an exact size for the duration of the cure. The temperature profile of the cure press is also very critical for a good end product. The cured coils are carefully checked for finished size and corona suppression treatment is applied to the coil surface. Coils/bars are subjected to extensive electrical testing before the shipment . READ MORE about NEC's coil manufacturing process.

 

NEC's Field Service

Services for Planned & Unplanned Outages

National Electric Coil's Field Service can be instrumental in the successful completion of a planned or unplanned outage. Whether an on-site installation or other generator field work, returning your unit/s to service as soon as possible is our ultimate goal.

On-Site Installations

Installation of our stator windings, stator cores and hydro field pole windings by NEC's Field Service or an Authorized Industry Partner (AIP) insures they will perform as designed. Care is taken to assure that the windings and laminations arrive complete and ready-to-install at your site. NEC's Field Service utilizes proprietary installation methods and techniques developed out of our long-term experience with all makes and sizes of machines and working under various site conditions. Our Field Service supervisors and senior staff bring years of experience to our crews. Many have worked for with various OEMs.

Stator Rewinds

Typical Field Service scope for a stator rewind installation will include:

1. Stator core preparation, including cleaning, repairs to interlaminar insulation, testing and re-painting. (Some scopes may include a core restack. In other cases, age of the unit or testing results may indicate problems requiring a core replacement.)
2. Installation of upgraded end winding support system (Certain fossil units).
3. Installation of new surge and bus rings ( may include provisions for a future Main-Neutral Interchange).
4. Installation of stator bars, RTDs and wedging and blocking system and testing of individual coils or groups of coils at specified points in the installation.
5. Brazing and circuit connections
6. Testing of the completed winding
7. Painting and miscellaneous finishing details

Stator Core Restacks

NEC's Field Service has the proven experience and extensive expertise to rig and perform core restacks for any type or size of hydro or fossil unit. This includes vertical core restacks for fossil units, which may provide a more budget-friendly option than some of the OEM approaches.

A viable stator core is important to the success of any rewind and to the long-term, reliable operation of the unit. In addition to its mechanical functions of supporting the stator windings and transferring torque to the frame, the stator iron provides a magnetic circuit, from pole to pole. It also helps transfer heat from the stator windings to the cooling medium. At the same time, it is the source of additional heat due to hysteresis losses.

During unit operation, an owner may see trending data or visual indicators during routine maintenance activities suggesting problems with the stator core. Or a unit may be scheduled for a rewind, and during the process of stripping the old winding and testing the core, problems may come to light.

National Electric Coil has extensive experience with stator core restacks for both hydro and fossil units. This experience ranges from repairs to individual laminations and partial restacks to complete stator core restacks. Although restacking a unit with its old iron is not unusual, there are some advantages to owners restacking with new laminations. A core restack with new iron may be found necessary if there are hot spots or other forms of degradation of the core iron. With earlier vintage units with core laminations of low-grade, high loss iron, new laminations made of the newer high-efficiency iron may be helpful in increasing a unit's power output. READ MORE

Hydro Rotor Services

NEC's Field Service plays an important role in services NEC provides for hydro rotors. Our skilled Field Service technicians make sure that the rotor is properly disassembled so field poles can be shipped to our Brownsville, Texas facility for refurbishment and repairs. If the rotor has been redesigned for for an uprate or frequency change, they make any necessary modifications to the spider and rim. When the refurbished poles are returned to the job site, they reinstall the poles, along with any modified keys or connectors. READ MORE

Integral Part of NEC's Project Management Team

Each project under taken by NEC is assigned a project manager whose job it is to coordinate the various internal resources that will be required for successful completion of the project. This includes resources of both people and equipment and spans the project from design and manufacturing to installation.

With the assistance of this project manager, NEC's Field Service personnel can draw upon the resources of NEC's engineering staff for advice or design clarifications or modifications, should an unusual condition or unforeseen problem arise during a rewind or core restack project. They also work closely with the engineering department when NEC provides maintenance or trouble-shooting services, assisting engineering with certain aspects of these projects, such as visual inspections and testing.

Where projects are lead by one of our AIPs, NEC supervisors supervise the installation, to insure that all procedures and techniques for installation are followed per our directives. Depending on the Partner's staffing plan for the project, NEC may also provide a full or partial winding crew.

Generator Field Work

 

The depth of experience NEC provides to a project is particularly valuable to NEC's other on-site services. Often an owner will see indications of a potential problem showing up in trending data or during a maintenance outage. Our experience shows that many units with problems may exhibit more than one symptom. For owners with limited manpower or lack of personnel with hands-on experience, the challenge can be making sense of these diverse indicators and homing in on the actual root cause.

Inspections, Testing & Cleaning

Some owners may not have the necessary training, certifications or experience to perform certain generator tests. Visual inspections by technicians with well-practiced eyes can important in going over the entire stator core and winding for evidence of any number of problems, from partial discharge or blocked vents, to loose or missing blocking and hardware or damaged laminations. NEC can provide Inspection, Testing and Cleaning services as part of a regular planned outage for maintenance.

Where a more trouble-shooting approach is needed, our team of qualified engineers and technicians will inspect your unit and perform various field testing. We also will analyze your maintenance logs and trending data, as well as our field testing data, to pinpoint your machine's problems.

NEC has lead the industry in identifying multiple generic design problems with Westac and Brush generator units, which have lead to high partial discharge, the subsequent break down of the winding's ground insulation and the deterioration of stator side packing. We have also been instrumental in identifying and correcting a number of field pole problems in certain older hydro units. NEC is also up-to-date in spotting and successfully repairing the machine-specific issues discussed in various industry TILs, OMMs and AIBs.

Stator Rewedges

Stator rewedges can be an important maintenance task. Loose wedges present a potential risk as a source for major unit damage. A typical rewedge starts with a wedge tightness survey. This survey is especially recommended if the unit has not been rewedged in the last ten years or if evidence of coil looseness is present. The survey reports on the location, condition and tightness of each wedge. It will also report on any slippage or movement of fillers. The actual rewedge picks up where the survey leaves off, replacing wedges and fillers where they have been found to be out of tolerance..