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A1-208
Session 2004
© CIGRÉ
STATOR DEFORMATION OF LARGE HYDROGENERATORS AND ITS EFFECTS
ON THE MACHINES
C. AZUAJE*
A. MILLAN
C.V.G. ELECTRIFICACIÓN DEL CARONI, C.A. (EDELCA)
(Venezuela)
SUMMARY
This paper is about the evaluation of a stator roundness of hydrogenerators in some of the
EDELCA´s hydroelectric complexes in Venezuela.
Large hydrogenerators frequently have several parallel groups on stator windings. These
generators also have a neutral unbalance current and a phase unbalance current protection.
During the operation of these units, high values of current unbalance between neutrals/groups
in parallel per phase were found, reaching alarm and sometimes trip values causing the shut
down of operation of the generators. Complete vibration measurements of units were
performed and normal values were found; all vibration measurements taken on bearings and
static parts had acceptable values. In addition air-gap and displacement of stator bases were
measured during operating conditions and it was found that some of the sliding bases that
were suppose to slide in the radial direction during heating expansions and contractions were
stacked, not allowing the stator to keep its round shape for the operation.
Due to magnetic unbalance caused by a non uniform air-gap, the effect on the rotor structure
was investigated on a generator of 250 MVA at Macagua Power Plant using the Finite
Elements Method. As a conclusion of this study, limits for stator roundness were established
to avoid fatigue on rotor spider structure as a consequence of the cycling forces caused on the
rotor structure at the rotating speed frequency.
Keywords: Hydrogenerators - Stator Deformation - Synchronous Machine - Magnetic Force Air-gap.
cazuaje@edelca.com.ve
1.
INTRODUCTION
Air-gap uniformity in rotating machines is very important for their performance. Large
hydraulic machines always have non-uniformities in the air-gap that causes stator and rotor
vibrations, wear or damage on guide bearings, poles looseness, etc.
During operation, generator’s air-gap can be evaluated by direct or indirect methods; one of
the indirect methods utilizes unbalance current measurements. In EDELCA unbalance current
measurement systems have been installed in Power Houses 2 and 3 at Macagua. Direct
methods have been developed in the last 20 years. At this time direct measurement systems
have been installed in 12 generators of 250 MVA at Macagua II Project.
In a generator, a very uniform air-gap would produce very low unbalanced currents, while for
a non-uniform air-gap unbalanced currents would be higher. In conclusion, it is possible by
these measurements, to know generator’s air-gap quality.
Due to unbalanced phase current protection trips on generator 12 at Macagua Power Plant, an
investigation was initiated to determine the cause of high values of currents. The developed
study purpose was to know air-gap conditions, stator behavior and lack of roundness effect.
It is important to underline that unbalanced currents do not indicate quantitative uniformity of
the air-gap and only give qualitative information about it. A generator with a non-uniform airgap suffers of magnetic unbalance, vibrations and non-uniform temperature distribution on
stator.
2.
CHARACTERISTICS OF THE GENERATOR
The characteristics of the hydrogenerators at Power House 2 of Macagua Project, where this
study was conducted, are shown in the following table:
Table I: Characteristics of generators at Macagua Project.
MVA
Voltaje (KV)
# of phases
# of groups / phase
Winding connection
# of slots
# of slots/pole
# of poles
# of camping bars / pole
External stator radius (mm)
250
13.8
3
4
Υ
576
6,5454
88
4
7625
External rotor radius (mm)
7232
Air-gap (mm)
18
Current transformers were installed to measure unbalanced currents of parallel groups on “A”
and “C” phases of the generators. Stator windings have four groups in parallel per phase,
each one distributed in not more that 180° of the stator geometry, comparing the air-gap
between diametrically opposed winding groups as shown in Figure 1.
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Figure 1: Parallel groups comparison on phases “A” and “C”.
We can observe the axes of comparison of parallel groups of the same phase; one of the axes
compare two groups of phase “A” and the other, two groups of phase “C”. The two axes are at
75° between them as shown in Figure 1. With this arrangement it is possible to know the
orientation of the air-gap deformation or eccentricities between stator and rotor.
By this physical winding distribution, a relative eccentricity between stator and rotor is
followed by unbalanced currents between groups of the same phase. In this way the winding
group located on lower air-gap side would have higher induced voltage due to an increase of
flux density; the winding group of the same phase located on the side with higher air-gap
would have lower induced voltage. Unbalanced currents would flow from the group with
higher voltage to the group of the same phase with lower induced voltage. If eccentricity is
not aligned with any of the two axes, we would have unbalanced currents in all groups, but
with higher value on groups closer to the eccentricity axis.
3.
MEASUREMENTS
Due to the activation of the unbalance current protection on Unit 12 of Macagua Power Plant,
and with the purpose of evaluating the cause of the problem, it was decided to do the
following measurements on this generator: air-gap, phase and neutral unbalance currents,
stator bases displacement and vibrations for different operating conditions of the machine.
3.1. Air-gap
For air-gap evaluation, Mechanic Tolerances for Hidrogenerators criteria shown in Table II
[1] were applied. Measured values were compared with Table II to determine conditions of
stator and rotor roundness and air-gap non-uniformities.
Table II Guide of Tolerances of VibroSystm.
Generator Air Gap*
Erection
Air Gap Variation
13%
Stator Roundness
7%
Stator Concentricity
5%
Rotor Roundness
6%
Rotor Concentricity
1.2%
* Deviations in % of nominal air gap value
Accept
20%
12%
7.5%
8%
2.5%
Critical
30%
20%
10%
10%
4%
2
For the Generator 12, stator roundness was 3.05 mm, which is 16.94% of rated air-gap. A
polar graphic of air-gap from the monitor system shows a minimum air-gap at 46° and a
maximum at 303°. Minimum air-gap is aligned with the phase “A” axis.
Figure 2: Air-gap polar graphic.
A 23.4% of rated value of Air-gap variation was found, which is higher than the acceptable
value of 20% shown in Table II.
Generators of Macagua Project were designed to operate within 2.11 mm of eccentricity.
Eccentricity value found during measurements was acceptable.
3.2. Neutral and Phase Unbalance Currents.
Unbalance phase currents wave forms are not regular and it is possible to identify two
components, a component of constant amplitude indicated by the minimum peak to peak
value and a modulated component at rotation frequency.
The component of minimum amplitude indicate that there is a deformation or eccentricity in
generator’s stator, and the modulated component indicate a deformation in the rotor. It can be
observed in Figures 3 and 4.
Figure 3: Phase “A” unbalance current.
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Figure 4: Phase “C” unbalance current.
Comparing amplitude of phase “A” signal with phase “C”, it can be observed that the constant
component is higher in the phase “A” axis. This condition indicate an eccentricity in both
comparing axes, being higher in the phase “A” axis direction, such as it was observed in the
air-gap monitor system.
On Figure 5 is it possible to observe a neutral unbalance current which is a result of phase
unbalance conditions due to the air-gap deformations.
Figure 5: Neutral unbalance current
3.3. Stator Frame Bases Displacement.
The generator’s stator under study is supported over 12 soleplates which guide the frame
stator structure for thermo expansions and contractions in radial direction. Displacements of
bases were recorded during measurements under full load operating condition.
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Figure 6: Polar diagram of stator bases displacement.
On Figure 6 it is possible to observe that displacement of stator bases is not uniform, causing
a non-regular stator circular geometry. On the figure we see that bases # 2, 7 and 11 have
smaller displacement than others, the possible cause is higher friction on these bases. This
condition causes an excessive displacement of bases # 3 and 10 during the expansion period
due to a temperature raise. Base 3 is located near the position of maximum air-gap and base
11 is located on the area were minimum air-gap is.
3.4. Vibration Measurements.
Vibration measurements were also performed simultaneously with unbalance currents, air-gap
and bases displacement on the generator under study. Displacement sensors were installed at
guide bearings of turbine and generator. To evaluate vibration levels the standard ISO 7919-5
[2] was applied.
As can be observed in Figure 7, vibrations of the hydrogenerator under test were high at 60
MW, which is normal for partial load on a Francis turbine. For full load, vibrations are of
maximum amplitude of 150 microns peak to peak which is considered as acceptable for
continuous operation.
Figura # 1 Amplitudes Globales de Vibración Unidad 12
400
350
250
200
150
100
50
M
W
19
7
M
W
18
0
16
0
M
W
M
W
14
0
12
0
10
0
M
W
M
W
M
W
80
M
W
60
M
W
40
M
W
20
va
c_
ex
c
0
va
cí
o
Micrones (p-p)
300
Condición
Cojturb 0°
Cojturb 90°
Cojgen 0°
Cojgen 90°
Figure 7 Vibration amplitudes.
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It is important to observe that relative eccentricity between rotor and stator due mainly to
stator deformations, do not produce significant higher vibrations observables from static parts
of the machine.
4.
STATOR DEFORMATION AND CYCLIC FORCES ON ROTOR STRUCTURE.
Important stator deformations on hydrogenerators cause cyclic forces due to magnetic
unbalance over the rotor structure. In some cases, excessive deformations on the stator of a
generator have caused fatigue over parts of the rotor structure. Due to the importance of the
matter of stator deformation and the effect on rotor structure, a three dimensional Finite
Element Method (3D-FEM) analysis was performed on the generator’s rotor structure, to
determine the maximum forces due to magnetic unbalance, acceptables for continuous
operation. In this way it was possible to determine the limit of acceptable stator deformation
for continuous operation of generators at Macagua Project.
Figure 7: Maximum magnetic unbalance
5.
TOLERABLE STATOR DEFORMATION BY FEM ANALYSIS.
Based on rotor design characteristics of generators of Power House 2 at Macagua Project, an
investigation project was developed in cooperation with Simon Bolivar University. A 3DFEM analysis was applied over the complete rotor model; it was determined the maximum
acceptable force due to magnetic unbalance under rotating conditions at rated speed.
Once it was determined the maximum acceptable force for magnetic unbalance that for its
cyclic effect can produce fatigue on components of the rotor structure after long period of
operation, simulations with FLUX2D, a 2D-FEM application in electromagnetic fields, were
developed. The FEM analysis in electromagnetic was applied to determine the air-gap nonuniformity caused mainly by stator deformation or eccentricity between stator and rotor, that
would cause the magnetic forces that would fatigue the rotor structure. Simulation results
showed that an eccentricity or stator deformation higher than 10% of the 18 mm of nominal
air-gap of Power House 2 generators at Macagua Project, would produce fatigue caused by
magnetic unbalance after long periods of operation.
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VARIACIÓN DE ESFUERZO EN CARA POLAR
300
250
Fuerza (kN)
200
Valor critico:
(16.3 mm, 203
kN)
150
Valor nominal:
(18 mm, 170 kN)
100
50
0
13
14
15
16
17
18
19
20
Entrehierro (mm)
Figure 8: Magnetic force behavior vs. Air-gap per pole.
6.
7.
CONCLUSIONS.
•
To maintain the integrity of the generators at Macagua Power Plant and to increase
their expected operating life, it was determined that stator deformations have to be
under 10% of rated air-gap, in order to minimize the induced cyclic forces on the rotor
caused by magnetic unbalance.
•
Depending of design, for a large hydrogenerator it is important to determine the stator
acceptable deformation limit for a continuous operation without having any damage
on the rotor structure after long periods of operation. Magnetic unbalance produced by
a generator air-gap deformation, would cause cyclic forces on the rotor structure that
might cause fatigue over support parts.
•
Stator deformations not necessarily cause significant vibrations on stationary parts of
the machine. A complete evaluation is required utilizing other tools such as: air-gap
measurements during operation, phase unbalanced currents and displacement of stator
bases measurements are recommended. Application of all these tools can give the
required information to make the necesary analysis in order to make proper decisions
to correct the problem.
REFERENCES.
1.
VIBROSYSTM: Hydrogenerator Mechanical Tolerantes. www.vibrosystm.com
2.
INTERNATIONAL ORGANIZATION FOR STANDARDIZATION: Mechanical
Vibration of Non-Reciprocating Machines - Measurements On Rotating Shafts And
Evaluation Criteria - Part 5: Machine Sets In Hydraulic Power Generating And
Pumping Plants First Edition. ISO 7919-5. 1977/02/15.
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