Richard Huber, P. Eng. 
Richard Huber Engineering, Ltd. 
North Vancouver, BC
Canada 
Technical Services Committee Member

Introduction
Recently I worked on new air-cooled machines rated at 13.8 kV that generated large quantities of ozone and had very high partial discharge levels. The basic problem with the windings in these machines was incorrect spacing of the end turns and the main leads. There was also a lack of space between the series and group connections in the windings.

Spacing requirements
The spacing suggested here is a guide only and original equipment manufacturers (OEMs) and service centers may have their own guide for such values. It should be noted, how­ever, that if spacing is much reduced from that suggested here, there is a real risk that partial discharge activity will develop in the winding.

For machines rated above 5 kV, ad­equate space must be provided between conductors to avoid partial discharge (see Figures 1 and 2). In addition, space between the main leads is required to avoid overheating of the conductors.   

For air-cooled machines, the space required to avoid partial discharge will vary according to the rated voltage of the machine, the thickness of the winding insulation and the dielectric constant of the insulation. In addition, surface defects and contamination on the winding components will affect the space requirements. Other items such as temperature, humidity and altitude will also complicate the issue.  

Fortunately, through testing and ex­perience, the many variables have been addressed by a nominal spacing of 12 mm (0.5”) for 13.8 kV rated machines. [If above 1000 m (3300 ft) altitude ad­ditional clearance may be required.] It is sufficient to account for surface defects, contaminants and the maximum oper­ating voltage of the machine, usually 1.05 times the nameplate value.

A more refined model is reported in [1] where specific irregularities were included in the test program for a 13.8 kV insulation system. The recom­mended minimum clearance between surface defects such as resin accumula­tions and adjacent bars or coils is 9 mm (0.35”).  For a 3 mm (0.12”) defect this means the spacing between conductors has to be 12 mm (0.5”). 

As shown in [1], under standard temperature and pressure, the partial discharge inception voltage (PDIV) between 5 kV and 20 kV can be approxi­mated as being linear with respect to electrode spacing. The spacing required for voltages within this range can then be calculated as a proportion of the spacing required for 13.8 kV. [9 mm (0.35”) plus an allowance for defects].

Obviously, for conductors in a winding where the voltage between them is less than the rated voltage, the space can be reduced. In practice, this does not usually happen as it is easier to set the spacing at one value for all conductors. An exception may be the spacing to ground.

Where the coils exit the slots, the spacing between front and back coils is much reduced. Since the coils are cov­ered with a stress grading material in this region, the spacing between coils can be reduced. However, the “drop” in the coil should be sufficient, that beyond the end of the grading mate­rial the suggested spacing is achieved.

For windings with multi-turn coils, it may be difficult to provide sufficient space between the series connections and the end of the adjacent coil. This is often an area where partial discharge occurs, when coils with large voltage differences are adjacent to each other. See Figure 3.

Problems created by inadequate spacing
In machines where spacing be­tween conductors is inadequate, partial discharge activity can be expected. There are risks associated with partial discharge activity. Under the right conditions, it can create ozone. This substance is a health hazard and can damage some of the materials inside the machine. If partial discharge ac­tivity is left unchecked, it can lead to surface tracking and possible flashover. If the partial discharge activity in­volves cables (usually the main leads) that contain synthetic materials they can be severely damaged. See Figure 4.

How to ensure adequate clearance
Achieving adequate end turn clear­ance in a new machine must start with a specification. The expected clearances must be stated to the manufacturer to give them the opportunity to provide a winding that meets the requirements. This inevitably results in economic trade-offs. To achieve the necessary spacing, in some cases will require a larger diameter and/or axially lon­ger coils. In some cases, the machine may not accommodate the new coils; alternatively, modifications may be required to the coil support structures. The additional cost has to be compared to the risks of severe partial discharge occurring in the winding and the ulti­mate reduction in winding life.

However, in some situations, it is a matter of the winding designer be­ing aware of the fundamental laws of physics as they apply to large rotating machines. For example, the main leads should never be tied together without space between them. Main leads should never pass directly through a steel bulk­head. The opening should be large and fitted with an insulating cover through which the leads would pass.

In a rewind situation, the size of the machine is established. So the best that can be done is to ensure care is taken during the rewind process to provide the necessary spacing wherever possible and to minimize the bulk of hand taped connections. In some cases, the winding configuration may be altered slightly to achieve the desired result.

For both the new machine and the rewind situation, it is always advisable for the owner/operator to inspect the work as it progresses. If anomalies do occur, they can be identified at an early stage and actions taken to overcome the problems.

An example of a machine where spacing between coils was a consideration is shown in Figure 5. The gap between phase groups in the end turn region has been increased slightly and the spacing be­tween the series connections at the beginning and end of adjacent phase groups has also been increased.

Remedial actions for existing problems
To overcome existing problems, the owner/operator has really only two options; increase the space between the offending conductors or fill the region with a material that will exclude air. An example of a change in main lead configuration that increased the space between leads is shown in Figure 6.  

Today one of the more frequently chosen materials to exclude air is a room temperature vulcanizing mate­rial (RTV). It is important that the application be done in a very neat manner with a minimum of working the material to reduce the possibility of voids or air pockets being developed in the material.  An example of one such application is shown in Figure 7.  

When filling gaps as shown in Figure 7, air flow is impeded. This can affect the cooling of the winding. Hence, the impact of altered airflow patterns should be considered prior to using this technique.

AVAILABLE IN SPANISH

Categories: AC motors, Stators/windings, Alternators/generators

Tags: Windings

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