scope:

Foreword

Many alternating-current, rotating electric machines are designed to have multiturn form-wound stator coils. In these cases, the winding has two separate but interrelated insulating barriers:

- One between the various turns (turn insulation), and

- One between the turns and ground (ground insulation).

Failure of either of these barriers will prematurely terminate the service life of the machine. A test level for the ground insulation of twice-rated voltage plus 1 kV has been in existence for many years. This guide suggests methods and test levels for the turn insulation.

Experience has shown that turn insulation failures can be precipitated by abnormal steep-front surges caused by factors such as lightning strokes, faulty breaker closures, or the malfunction of various types of switching devices. However, turn insulation failures can also be caused by surges during normal breaker operations when the circuit conditions are such that the rise time of the surge at the machine terminals is less than a few microseconds. A measure of protection from such surges may be provided by installation of devices such as surge capacitors at the machine terminals and surge arrestors, or by designing the coils with suitable turn insulation capability. When used for this purpose, capacitor ratings are usually chosen to extend the rise time of voltage surges to 5 µ s or longer.

The bibliography (Annex C) contains references that discuss the general surge environment and surge strength of electric machines.

Notice to users

Errata

Errata, if any, for this and all other standards can be accessed at the following URL: http:// standards.ieee.org/reading/ieee/updates/errata/index.html. Users are encouraged to check this URL for errata periodically.

Interpretations

Current interpretations can be accessed at the following URL: http://standards.ieee.org/reading/ieee/interp/ index.html.

Patents

Attention is called to the possibility that implementation of this standard may require use of subject matter covered by patent rights. By publication of this standard, no position is taken with respect to the existence or validity of any patent rights in connection therewith. The IEEE shall not be responsible for identifying patents or patent applications for which a license may be required to implement an IEEE standard or for conducting inquiries into the legal validity or scope of those patents that are brought to its attention.

Scope

This guide makes suggestions for testing the dielectric strength of the insulation separating the various turns from each other within multiturn form-wound coils to determine the acceptability of the coils. Typical ratings of machines employing such coils normally lie within the range of 200 kW to 100 mW (270 to 135 000 hp). Test voltage levels described herein do not evaluate the ability of the turn insulation to withstand abnormal voltage surges, as contrasted to surges associated with normal operation. The repetitive voltage surges (spikes) associated with adjustable frequency drives (AFD) are also not addressed here. This guide applies to

a) Individual stator coils after manufacture.

b) Coils in completely wound stators of original manufacture.

c) Coils and windings for rewinds of used machinery.

d) Windings of machines in service to determine their suitability for further service (preventivemaintenance testing).