Technical Sciences / 5.Energetics

Rozum T.I., Polichshuk V.I.

Tomsk Polytechnic University

Protection of synchronous machine rotor windings from damage

Currently, generators with a capacity of 500-800 MW and above are used in large quantities, respectively, which requires timely troubleshooting, in turn, to turn short circuit in field winding due to the high operating temperature. First, a short circuit at two points belongs to a very dangerous damage to the field winding of synchronous machines [1]. Closures on the ground at one point winding for synchronous machines are not dangerous, as the current value in the scheme of arrangement is equal to zero. Lack of power in some corners is the asymmetry of the magnetic rotor, which is accompanied by vibration. Vibration can quickly lead to the destruction of the bearings and the shaft journal. A simple way to protect the excitation winding from turn-to-turn short circuit [2] of two magnetic sensors is suggested.

At a turn-to-turn short circuit current in the closed rotor coils of synchronous machine is missed, and a magneto motive force (MMF) of damaged poles and its magnetic field decreases in proportion to the number of turns shutting. Thereby, the protection device operation is based on a comparison of a magnetic field of synchronous machine coil-end leakage at two poles. To do this, it is necessary to use a pair of magnetic field sensors, MFS1 and MFS2, which are displaced along the air gap at a distance multiple of pole pitch . The example of those magnetic sensors disposition for the synchronous machine with at is shown in the Fig. 1. The pairs of excitation winding poles in this figure are marked as , ,.

Fig. 1 Versions of two magnetic sensors position for synchronous machine at

The protection is realized by the block scheme in Fig. 2 , where the magnetic sensors 1 and 2 are fastened on the end shield; the magnetic sensor is attached to the comparing element so that and in the inputs of comparing element for any n could be in the opposite phase, CB- comparison block, B - band-pass filter, TE - threshold element; LS – load-break switch.

The information sign selection is done by the graph-analytical analysis. In the magnetic sensor EMF is induced by stationary stator winding with a current frequency of Hz, and is induced by excitation winding rotating with an angular velocity at a constant current . EMF in the output of the filter F will be equal in value and its sum will be equal to zero. Therefore, when analyzing, the difference of EMF cannot be ignored in the future. At the same time, the value of EMF was determined by the magnetizing force of the rotor pole. Therefore, when the rotor is intact, these EMFs are equal to each other, and if the rotor is damaged, EMFs are different. The result is that the sum of the EMF of magnetic sensors 1 and 2 at the output of BS is determined by the amount of EMF from the excitation winding poles.

Fig.2 Block scheme of the device for implementation of the protection of the synchronous machine rotor with two magnetic sensors

As a result, at the synchronous rotation of the rotor with an intact winding EMF of the first converter is defined as:

, (1)

where – peak EMF from magnetizing force of the intact rotor pole; - angular line frequency; – time.

At turn-to-turn ground short circuit winding of one of the rotor poles, the magnetizing force decreases, as the current in closed windings is equated to zero. This will lead to a decrease the half-wave amplitude of EMF by induced in magnetic sensor 1 by that pole. Given the fact that all the poles of the rotor are transmitted nearby magnetic sensor 1 during the one complete circuit of the rotor relation in the circuit of some part of turns in one of the poles of the synchronous machine at . This relation expanded into Fourier's series and converted into account the filter F effect and at the number of analogous poles can be written as

, (2)

where ; – a number of turns in the pole winding.

EMF at the output of BS obtained by numerical expansion in Fourier’s series for the different location of magnetic sensors [3] is as follows:

, (3)

, (4)

. (5)

The analysis of the expressions obtained and Fig. 3 shows that to construct a rotor protection with from turn-to-turn ground short circuit, you can use all the components that are listed in (3), (4) and (5). However, the use of the second component of the series is more preferable, since it is present at any location of measuring transducers. The choice depends on the availability of the presence of the device protection signals and its power and in the fault-free excitation winding.

At closing of excitation winding to the ground at two points, any amount of turns can be closed and the data signal will be stronger.

In the proposed method of protection a signal of turn-to-turn short circuit in excitation winding can be as a harmonic as well a constant component of EMF sum at the output of magnetic sensor.

References

1. Федосеев А.М. Релейная защита электрических систем. – М.: Энергия. – 1976.– 559с.

2. Новожилов А.Н., Горюнов В.И., Полищук В.И., Воликова М.П., Новожилов Т.А. Способ защиты обмотки ротора синхронного генератора от витковых замыканий на двух индукционных преобразователях// Электричество. – 2010.- №8. – С.65–67.

3. Бессонов Л.А. Теоретические основы электротехники. – М.: Высшая школа, 1973. – 752 с.