IEC TS 60034-24:2009 pdf download – Rotating electrical machines – Part 24: Online detection and diagnosis of potential failures at the active parts of rotating electrical machines and of bearing currents – Application guide
The group of winding faults in item 3 marks the most severe deficiencies at the active parts. They all produce magnetic fields of fundamental frequency. Thus winding faults cannot be detected by a frequency analysis of the stator currents. The field waves, produced by winding faults, are of elliptic nature, which means the superposition of two reverse rotating waves, having the same number of poles and the same frequency, but different amplitudes. In principle such failures can be detected by exploring the negative sequence component of the current of fundamental frequency.
Especially in case of the most dangerous failure, an interturn fault of a high-voltage machine, when the high currents flow in only one of many turns per phase, this component is very small. A negative-sequence component of the current may also be caused by an unavoidable small asymmetry of the supply voltages (a negative sequence component of the voltage results in a negative sequence component of the currents, which is 6 to 1 0-times higher). Summing up, it is not recommendable to detect winding faults by means of a voltage/current analysis. Reliable detection of cage imperfection or defects (e.g. broken bars or end-rings) is possible by use of stator current analysis. Another disadvantage of the stator current analysis cannot be neglected. Statistics of insurance companies manifest that most of the winding faults occur during transient phenomena such as starting of motors, short-circuits at the terminals, etc., and cause high inrush currents. It is unfeasible to detect failures by current analysis during the interval of the transients.
5.3 Induced voltages of auxiliary turns embedded into the stator slots or other magnetic sensors sensing the air-gap flux An ideal diagnostic signal would be zero during operation of a healthy machine under steady- state and transient conditions, it would rise with the amount of the deficiency for all kinds of failures according to items 3 to 5 of Table 1 and would be able to distinguish between the failures. Solutions close to the optimum have been developed. These solutions are based on turns made by insulated wire, the diameter of which can be selected under solely mechanical aspects. Both coil-sides are incorporated in the stator slots of the main winding, usually during manufacturing of the machine between the upper layer of the winding and the slot wedge.
The assembly at a later stage is possible. The end- connections are led close to the end of the core. The same insight into the magnetic field at specific locations at the stator bore can eventually be achieved by other kinds of magnetic sensors instead of measuring turns. Usually several turns of the same pitch are series-connected and shifted against each other by a predetermined angle.
It is aimed to get finally a system of auxiliary measuring coils, for which the resulting winding factor is zero for all air-gap fields, which exist during normal undisturbed operation, and for which the winding factor is maximum for a field with that number of pole pairs, which is intended to be used as the reference field of the diagnosis. If a system of auxiliary coils can be found which fulfills the condition explained above for a reference field, which is amongst the fields generated by all failures of items 3 to 5, the coil system would be complete.
But there is one remaining difficulty: The fields produced by a winding fault according to item 3 of Table 1 , are of an elliptic nature. If one of them is chosen as reference field, the induced voltage of the coil system would vary with the location of the fault at the circumference. Such a situation is of course unacceptable. The problem can be eliminated by use of a second identical coil system, which is shifted against the first one by the angle π/(2ν), when ν is the number of pole pairs of the reference field.IEC TS 60034-24 pdf download.
