EFFECT OF THE DAMPER ON HEATING OF THE END PARTS OF A TURBOGENERATOR ROTOR UNDER ABNORMAL OPERATING CONDITIONS

V. K. Ivanov, V. D. Klimovitskii, V. F. Chirikov

(Presented by the scientific seminar of the Departments of Electrical Machines and General Electrical Engineering)

Under abnormal operating conditions of a turbogenerator, the currents induced in the rotor body close at its end faces. Passing through the end wedges, the teeth, and the retaining ring at the place where it is fitted onto the rotor body, the currents cause heating of the end zone. With the growth of the unit power of turbogenerators, which is accompanied by an increase in linear loads, the intensity of losses in the end zone increases, which makes it necessary to reduce the permissible duration of turbogenerator operation under abnormal conditions. For example, for turbogenerators of the T2 series, with power up to 60 MW, the value \(I_2^2 t\), characterizing the losses released in the rotor under the action of the negative-sequence current \(I_2\) over time \(t\), is limited to 30; for TVF-100 turbogenerators, with power of 100 MW, this value is reduced to 15, and for turbogenerators of the TVV and TGV series with power of 150–300 MW—to 8. A significant improvement in the thermal state of the end zone can be achieved by installing on the rotor a special damper system that relieves the end zone of transverse currents. Similar systems at one time did not find wide application in turbogenerators, since their sufficient mechanical reliability was not ensured by the design of the retaining-ring assembly as a whole.

At SibNIIETI, a version of a damper system was developed for high-power turbogenerators, whose reliability is promoted by the direct fastening of the retaining ring only to the rotor body, and it was tested on a turbogenerator of type T2-0.75-2BP (power 750 kW, voltage 400 V, current 1355 A, speed 3000 rpm).

For the tests, the generator rotor was reconstructed. The remaining retaining rings made of magnetic steel were replaced by nonmagnetic ones with a cantilever fit on the rotor body (Fig. 1, a). Instead of slot wedges made up across the width, solid wedges were installed: in the middle part—of duralumin, and in the end parts—of nonmagnetic steel. To make it possible to assess the effectiveness of the damper system, the latter was placed only on the side of the slip rings. The damper was made of two layers of copper segments (Fig. 1, b) of comb-like shape, 1.5 mm thick, installed with overlap. The teeth of the segments were brought under the end slot wedges. By fitting the end wedges, good electrical contact with the teeth of the segment was ensured.

To record the heating of individual rotor elements, the latter was equipped with a system of copper-constantan thermocouples. The thermocouples were arranged

Fig. 1. Retaining-ring assembly with a damper.
a) general arrangement: 1 — field winding, 2 — retaining-ring insulation, 3 — damper segment, 4 — retaining ring, 5 — key, 6 — nonmagnetic-steel key, 7 — duralumin key;
b) damper segment

were installed on the surface of the rotor elements along three generatrices: on the large tooth, approximately 50 mm from its edge; on the small tooth, equidistant from the large rotor teeth; and on the wedges adjoining this small tooth. In this arrangement the thermocouples were divided into 5 zones: 13–15 mm from the end face (seating surfaces), 62–65 mm from the end face, and 300 mm from the end face (middle of the barrel). In addition, on both sides, thermocouples were installed at a distance of 14–20 mm from the end face in the “body” of the barrel (on the walls of the ventilation channel in the large tooth) and in the end wedge at a depth of 6–8 mm from the seating surfaces, as well as on the seating surfaces of the retaining rings (above the ventilation channels in the large tooth). The heads of all thermocouples had good thermal contact with the monitored elements by soldering or caulking, except for the thermocouples on the retaining ring, which were glued. The leads of all thermocouples were brought out through special radial drillings and the central bore to the current collector of the VNIIEM system.

The thermoe.m.f. was recorded in long-duration operating conditions by means of a potentiometer, and in transient conditions by using a loop oscillograph.

Abnormal conditions possible in operation, accompanied—

with significant heat release in the structural elements of the rotor, can be reduced, basically, to three types:

1) asynchronous;
2) prolonged unbalanced, caused by load unbalance, in which the magnitude of the stator negative-sequence currents is relatively small;
3) short-term unbalanced, caused by unbalanced short circuits or by malfunction of the switching equipment; in this case the negative-sequence currents may reach a considerable magnitude.

In accordance with this, the tests were carried out in three types of conditions: asynchronous, reverse-synchronous rotation, and two-phase short circuit. In the discussion that follows, the results of the conditions whose electrical parameters are given in Table 1 are used mainly.

Table 1

The level of losses released in the rotor \((P_s)\) in conditions 1 and 2 is close to the losses in the rotor winding at rated current, \(P_{\mathrm{rn}} = 8.6\) kW.

Tables 2–4 present the additional temperature rises (above the temperature rises in the ventilation thermal condition) at characteristic measurement points in all three conditions. Their comparison for the ends with a damper and without one shows quite clearly that, in the design investigated, the damper is effective in the presence of negative-sequence currents; in the asynchronous condition its influence is practically not manifested.

Table 2

Additional temperature rises of the seating surfaces in the asynchronous-running condition (°C)

Table 3

Additional temperature rises of the seating surfaces in the reverse-synchronous-rotation condition (°C)

Table 4.

Additional heating of seating surfaces in the nonstationary mode of a two-phase short circuit at \(I_2^2 t = 25\) (°C)

The results obtained make it possible to examine each mode in greater detail.

The distribution of heating in the asynchronous mode is sufficiently uniform (Fig. 2, a); no local concentration at the ends is observed. If the increase in heating in the middle part of the large tooth is attributed to poorer cooling of this part of the rotor, which is characteristic of a symmetrical radial ventilation system, then one may speak of practically uniform—

Fig. 2. Distribution of additional heating along the rotor length: 1 — large tooth, 2 — small tooth, 3 — wedge;
a) asynchronous-running mode, b) reverse-synchronous-rotation mode.

...uniform loss release along the length. At the same time, a noticeable difference is seen in the heating of the large tooth and the small tooth with the wedges adjacent to it.

The available number of thermocouples unfortunately permits only tentative discussion of the details of the distribution of heating along the rotor length in the reverse-synchronous rotation mode. A sharp concentration of heating is observed at the end on the side without the damper (Fig. 2, b), whereas on the opposite side the distribution of heating is uniform and its level is 2–2.5 times lower. The unloading effect of the damper on the wedge and the small tooth is appreciably greater than on the large tooth, since the latter has no direct electrical contact with the damper segments. The large increase in the temperature of the small tooth in the middle of the rotor should be attributed to the joint of the wedges at this point, which leads to redistribution of currents between the teeth and the wedges.

In the nonstationary mode of a two-phase short circuit, the effect of the damper on the various rotor elements is manifested still more differentially: the heating level is reduced for the large tooth and the retaining ring by more than a factor of 2, for the small tooth by approximately a factor of 4.5, and for the wedge by approximately a factor of 7 (Table 4). The character of the heating curves shows that by the time when \(I_2^2 \cdot t\) had reached 25, the heat exchange of the regions adjoining the thermocouples was small and, consequently, the heating reflected the ratio of losses in them. Therefore, the large difference observed in the experiment in the heating of the seating surface and the “body” of the large tooth (280 and 46°C, respectively) and of the end wedge (111 and 45°C, respectively) indicates a large concentration of losses at the seating surface, especially in the body of the large tooth.

The test in the mode of a two-phase short circuit at \(I_2 = 1.56\) was carried out up to the value \(I_2^2 t = 60\). Subsequent inspection of the rotor with the retaining rings removed revealed, on clean seating surfaces on the side where the damper was installed, a large number of dark-blue spots on the seating surfaces of the barrel and the retaining ring on the opposite side. Most of the spots were located on the large teeth. The color of the spots indicates local heating to temperatures on the order of 350°C. The maximum temperature recorded in the experiment, 330°C, was indicated by a thermocouple on the seating surface of a large tooth that was located at the center of one of the spots.

It should be noted that in all modes the heating of the small tooth as a whole is appreciably lower than that of the large tooth; moreover, the last mode shows that this difference in losses is still greater. It may be assumed that the shunting action of the slot wedges plays an important role in this. From this point of view, the rationality of the wide recent use of end wedges made of materials of high conductivity, for example bronze, is confirmed.

The results of the work performed show that the damper system studied effectively reduces the heating of the rotor end parts under abnormal conditions in the presence of negative-sequence currents. To increase the effectiveness of the damper system, it should be made so as to ensure direct electrical contact with the end part of the large tooth.