Cylindrical rotor inter-turn short-circuit detection

Transcription

1 Cylindrical rotor inter-turn short-circuit detection by Kobus Stols, Eskom A strayflux probe is commonly used in the industry to determine if any inter-turn short-circuits are present in the field winding of a synchronous generator with a cylindrical rotor. The voltage signal obtained from the probe is analysed and differences are typically expressed as a percentage error.it is the purpose of this paper to: Highlight the pitfalls when analysing a strayflux recording To provide a tripping philosophy. Flux in the generator Open-circuit flux Fig.1 illustrates the symmetrical flux pattern set up by the field winding under open-circuit conditions. The flux lines are not drawn on scale but serve merely to indicate that the main flux lines are more prominent than the stray-flux lines and that they are symmetrical around the direct axis (Xd). Fig. 1: Basic Open-circuit Fig. 2: Basic short-circuit Short-circuit flux Fig. 2 illustrates the symmetrical flux pattern set up by the field winding under short-circuit conditions.the main flux is Fig. 3: Basic On-load Fig.4: Tangential fluxprobe. Fig. 5: Radial fluxprobe. significantly reduced due to the armature reaction, and the stray-flux component is therefore more prominent. Analysis of the stray-flux can be done more accurately during the short-circuit condition. On-load flux The resultant flux will look different under loading conditions due to the angle between the flux of the rotor and stator windings, known as the load angle. Fig 3. clearly illustrates that there is no symmetry around the direct axis. Types of flux probes A flux probe is basically a small coil that is positioned in the air gap of a generator. The distance of the coil from the rotor body is not critical and can vary considerably. energize - March Page 20 The most common distances vary between 1 and 3 cm. The coils typically have between 700 and 800 turns of copper wire with a diameter of approximately 0,05 mm. The coil is normally obtained by winding the wire on a former with a diameter of less than 10 mm. The orientation of the coil with respect to the rotor body determines its designation. It is therefore possible to have a tangential probe (Fig. 4) as well as a radial probe. The radial type probe is the most common in industry and this paper will focus on this type. Identifying the physical properties from a stray flux recording The voltage signal shown in (Figs. 6 8) was obtained from a radial flux probe during a short-circuit test. The effect of the following physical properties is visible: The main flux that results in a 50 Hz fundamental waveform Pole faces A and B Damper windings Reduced number of turns in the coils closest to the poles The teeth width that is not uniform 50 Hz fundamental waveform The main flux causes a 50 Hz fundamental voltage waveform on which the strayflux signal is superimposed. The ultimate condition for measuring the rotor stray-flux for analysing purposes is when there is a short circuit applied to the terminals of the generator, due to the following reasons: A situation is created where the main flux from the rotor is opposed by the flux that results from the short circuit current in the stator. This means that only a small amount of flux is present in the air-gap which may influence the voltage peaks resulting from the strayflux Analysing the results is easy due to the fact that the absolute voltage spikes associated with the slots are expected to be of similar magnitude for all slots with the same number of turns. It is often not practical to apply a shortcircuit to the terminals of the generator, hence the short can applied to the HV side of the generator transformer. This means that the impedance of the

2 Fig. 6: Voltage signal recording from a stray-flux probe during a short-circuit test. Fig. 8: Ideal Strayflux without the 50 Hz fundamental waveform. Fig. 7: Stray flux superimposed on the 50 Hz fundamental waveform. Fig. 9: Effect from the damper winding slots on a stray flux recording. Number of slots 38 Number of slots with field 32 winding coils Number of turns in the slots adjacent to the polefaces 7 Number of turns in the other winded slots Teeth width 11 All not identical Table 1: Characteristics of the rotor. transformer will be present between the terminals of the generator and the applied short-circuit. The terminal voltage of the generator is an indication of the amount of flux in the core. The voltage across the impedance of the transformer will be visible on the terminals of the transformer, hence the levels of flux in the machine will be low but it will not be zero. This scenario will still provide recordings of the strayflux that will provide excellent results from an analysis point of view. Damper windings Fig. 9 shows the voltage effects associated with slots in the two pole faces which only contain damper windings. This is due to the behaviour of the flux lines when they enter or exit the magnetic core. Flux always enter or exits a magnetic core at angles of 90. The basic effect of this phenomenon with irregular shapes in the magnetic core is illustrated in (Figs ). The flux at the irregularities in the pole of the magnetic core contains both radial and tangential components. The same argument applies to the slots in the pole faces, hence the appearance of voltage peaks when these slots pass the flux probe. Reduced number of turns It is common to find that the coils closest to the poles have less turns than the rest. Manufacturers use this technique to reduce the harmonics generated by the machine. These reduced peaks should not be interpreted as shorted turns. Voltage peaks off sets, intervals and the width of the teeth. The main flux of a cylindrical rotor is shaped to produce a more sinusoidal voltage waveform. In addition to the reduced number of turns in the coils closest to the pole faces, it is also common to find that the slot spacing gets smaller near the quadrature axis. (Figs ). Analysing methodology Numbering/labelling The slots and the poles are labelled/ numbered as shown in Fig. 16 to illustrate the analysing methodology. The symmetrical point in a stray-flux recording depends on the type of fluxprobe used. The Xq axis will be the point of symmetry when a radial flux-probe is used whilst the Xd axis will be the point of symmetry if a tangential flux-probe is used. No field winding coils are present in the three slots on each of the pole faces. The damper windings, which are present in these slots, do not carry any current during the short-circuit test because there is no relevant movement between the rotating stator flux and the damper winding in the rotor because both rotate at 3000 rpm. energize - March Page 22 Fig. 10: The effect of irregular shapes on flux lines. The slots which contain damper windings only, i.e. pole A; slots 19, 20 and 21 and pole B; slots 38, 39 and 1, are therefore not shown in Fig. 16, nor will they be used during the analysis. Some of the characteristics of the rotor in the example are shown in Table.1 Example of an analysis for inter-turn problems The symmetry method The expected waveform for a healthy rotor is shown in Fig. 17. The waveform is symmetrical around the Xq axes due to the use of a radial flux-probe. The following example proves that the Xq axes are the point of symmetry: The voltage peaks associated with slots 3 and 18 are the same and they are also comparable with the voltage peaks of slots 22 and 37 The pattern of voltage peaks from slot 10 to slot 3 is the same as the pattern of the voltage peaks from slot 11 to slot 18 The pattern of voltage peaks from slot

3 Fig. 11: Effect from the damper winding slots on a stray flux recording. Fig. 16: Labelling and numbering of the field winding used during the analysis. Fig. 12: Effect narrow teeth/slot spacing. Fig. 17: Expected stray flux recording for a healthy rotor. Fig. 13: Equal slot spacing. Fig. 14: Condensed slot spacing near the Xq axis. Fig. 18: Stray flux recording of a faulty rotor. Fig. 15: The effect of condensed slot spacing on the peak-to-peak voltage. Fig. 19: Stray flux for slots 3 to to slot 22 is the same as the pattern of the voltage peaks from slot 30 to slot 37 Fig. 18 shows the actual waveform obtained from the radial flux-probe during the short circuit test. In order to illustrate the analysing methodology more clearly, the waveform in Fig. 18 is split into two separate recordings as shown in Fig. 19 and Fig. 20. The presence of a shorted turn will be evident in both the relevant slots for the specific coil as discussed earlier. It is for this reason that the stray flux recording of slots 3 to 18 will theoretical be identical to those of slots 22 to 37 if the polarity of the signal is ignored. It is already evident that possible problems exists in slots 3, 5, and 17 by merely looking at the symmetry of the waveform. The peaks shown in red in Fig. 21 indicate where the measured peak was expected to be. The energize - March Page 23 waveform in Fig. 22 also shows that possible problems exists in slots 23, 35 and 37. The peaks shown in red indicate where the measured peak was expected to be. The problem areas indicated in the recordings above are shown in a more familiar view in Fig. 23. By analysing the symmetry (or lack of it) of the recordings, it is suggested that some turns in certain slots are shorted with some or no resistance.

4 Fig. 20: Stray flux for slots 22 to 37. Fig. 21: Symmetry problems in slots 3,5 and 17. Fig. 22: Symmetry problems in slots 23, 35 and 37. Fig. 23: Coils with possible Interturn problems. Fig. 24: Measuring the one side of a voltage peak. Fig. 25: Measuring the other side of the same voltage peak. Slot Side 1 Side 2 Average Table 2: Stray flux peak voltages as measured. The possible problematic slots are: Slots 17 and 23 (Pole A Coil 2) Slots 5 and 35 (Pole B Coil 3) Slots 3 and 37 (Pole B Coil 1) Comparison of the respective coils method Another approach is to compare the absolute value of the voltage peaks for the opposing coils as in Fig. 24. The values of the two sides of each voltage peak were measured to calculate the average value for the peak as shown in the mentioned table. The voltage peaks for the various slots were measured and are indicated in Table 2. A shorted turn will have no current when compared to the corresponding healthy turns. This also implies that the ampereturns for the coil with the shorted turns will be less than the ampere-turns for the energize - March Page 24 Pole A Pole B Coil 1 Slot 22 + Slot 18 Slot 3 + Slot 37 Coil 2 Slot 23 + Slot 17 Slot 4 + Slot 36 Coil 3 Slot 24 + Slot 16 Slot 5 + Slot 35 Coil 4 Slot 25 + Slot 15 Slot 6 + Slot 34 Coil 5 Slot 26 + Slot 14 Slot 7 + Slot 33 Coil 6 Slot 27 + Slot 13 Slot 8 + Slot 32 Coil 7 Slot 28 + Slot 12 Slot 9 + Slot 31 Coil 8 Slot 29 + Slot 11 Slot 10 + Slot 30 Table 3: Specific slots is associated with specific coils. Pole A Pole B % error Coil V 2510 V 9,06% Coil V 3915 V 9,07% Coil V 3430 V 7,42% Coil V 3700 V 1,08% Coil V 3685 V 0,95% Coil V V 0,31% Coil V 2800 V 0,71% Coil V 2710 V 0,55% Table 4: Comparison of the relevant coils. healthy coil. It is for this reason that the coil with the highest value is assumed to be the healthy coil. The analysis also takes into account the number of turns per slot. The relevant slots for the rotor should be compared as indicated in Table 3 to determine the presence of interturn problems (Fig. 23). Accuracy The relation between the number of shorted turns and the percentage deviation are influenced by the following factors: Reduced number of turns in the coils closest to the poles The width of the teeth that is not uniform (see Fig. 15) The width of the teeth is such that it results in a offset that effects the strayflux signal (see Fig. 16) The flux in the air gap is not 100% sinusoidal. It is therefore not possible to get a 100% accurate relation between the % error and the number of shorted turns. It is especially the offset due to the width of the teeth that can greatly exaggerate the error when expressed as a percentage difference between the relevant peak-topeak voltages. Effects of Inter-turn short circuits in a field winding Inter-turn short circuits in the field winding (rotor winding) can cause high levels of vibration in a generator. Both thermal and magnetic unbalances are commonly associated with inter-turn shorts.

5 Thermal balance The heat of the rotor body is distributed in such a way that the rotor expands uniformly. The current in the field winding and the resistance of the winding results in the generation of heat (I 2 R), which is transferred to the rotor forging. A turn that is shortcircuited does not carry any current; hence temperature of such a turn will be significantly lower than its healthy counterpart. A shorted turn with no current will be cooler than its counterpart which is associated with the opposite pole. The pole with the shorted turn will therefore be slightly colder than the healthy, hence hotter pole. The difference in temperatures will result in the bowing of the rotor. More than one shorted turn near the Xq axis may have less of an impact on the delta T of the two poles than one shorted turn near one of the pole faces. Magnetic balance A shorted turn will also result in a different flux density in the one pole when compared to the other. This effect will also have a negative impact on the vibrations of the machine. The vibration effect will be higher the closer the shorted turn is to one of the pole faces. More than one shorted turn near the Xq axis may therefore affect the machine less than one shorted turn near the pole face. Excitation requirements A specific number of ampere-turns is required to set up the flux required for a specific quantity of reactive power. It is therefore necessary for the excitation system to compensate for a shorted turn by increasing the excitation current to get to the same ampere-turns value. Recommendations Tripping is required if the symptoms of Fig. 26: Excitation requirements. inter-turn short circuits exceed any one of the following limitations of the machine: Vibration levels Stability of the machine in the reactive power import region If the symptoms of inter-turn faults don't exceed the mentioned limits of the machine, it poses no direct risk to the machine and does not require automatic tripping. Contact Kobus Stols, Eskom, Tel , kobus.stols@eskom.co.za energize - March Page 25