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8/11/2019 Static Frequency Converters With Reduced Parasitic Effects
1/6
2004
35th Annual IEEE Power Elecrronirs Specialists Conference
Aar hm , G ermany , 2 4
Static frequency converters with reduced parasitic effects
Oliver Drubel Max Hobelsb erger
Drubel@,ieee.org
ALSTOM (Switzerland) Ltd.,
Turboge nerator Development, Zentralstr.
40
CH-5242 Birr, 0liver.drubel~alstom.nower.com
ALSTO M (Switzerland) Ltd.
Custom er Service
A b s l r a c l : Parasitic effects caused by static frequency
converters, like EMI, may endanger the reliable operation of
drives or turbine shaft arran gemen ts and diminish its lifetime.
Converter drive applications for large plants up to 500MVA
can cause shaft voltages up
to
SOOV,, if a non optim ised
converter design is chosen. This phenomenon is investigated.
Design options for the converter and the plant arrangement
are discussed. The influence of the most suitable solution and
their influence upon electromagnetic interference (EMI) and
the shaft voltages is shown and conformed by measurements
in real power plants.
K e y w o r d s :
Static frequency converter, EMI, shaft voltage,
power application
I
INTRODUCTION
Recent developmen ts in power generation equ ipment show an
increasing growth
in
the installed unit power of gas turbines.
Gas turbines with up to 260 MW are proven d esign with high
reliability. Whereas steam turbines are capable to run up
independently, gas turbines have to be started and driven until
they turn fast enough to allow ingnition and self-sustaining
operation. Ignition may not be possible until up to
80%
of
nominal speed.
For running up the turbine it is state of the art to use the
turbo-generator as motor. Electrical power is supplied
to
the
generator-motor by
a
variable-speed starting device like a
static frequency con verter (SFC). A typical start arrangemen t
is givenin fig. 1.
This SFC has to deliver considerable amounts of power, e.g.
5-15
MW for a typical large gas turbine. The SFC is
disconnected from the generator after running up the gas-
turbine to ignition and self-sustained operation. The
generator-motor is consequently used as generator. Usually
this start-up operation with active SFC takes only a cou ple of
minutes, e.g. 7 minutes w hich is only a short period of time.
Because of this short period some effects and phenomena
during start-up were not discovered until recently.
However due to modern use of gas-turbine plants to generate
peak-load turbines must be switched on and off several times
per day; and in between these times of full operation there are
also prolonged periods of active SFC-operation for
maintenance purposes. Therefore shaft voltages during start
static frequency con version have to be cared for as well.
A key component to reduce shaft voltages is the design of the
SFC [3]. Therefore the SFC is within the focus of this
investigation.
11 OBSERVED EFFECTS AT A TYPICAL POWER PLANT
Strong electromagnetic interferences occurred at sensors,
video screens and distributed control equipment (DCS) at a
typical
300
MVA gas-turbine plant during SFC-operation.
The powerful SFC and the close proximity of control
equipment
to
the SFC has been the main reason for these
disturbances. The grounding and shielding of systems was
then improved to reduce the interference to acceptable levels
which appea red to be qu ite costly.
Furthermore it was observed during maintenance works that
the fuse at the NDE shaft-grounding module ( RC-module
in
Fig. 1: Electrical power plant equipment for starting the gas
Fig. 2) had blown.
turbine
0-7803-8399-0/04/ 20.0082 4
EEE. 4365
mailto:Drubel@,ieee.orgmailto:Drubel@,ieee.org8/11/2019 Static Frequency Converters With Reduced Parasitic Effects
2/6
2004 35th Annu al IE EE Power Electronics Specialists Conference
The grounding arrangement is shown in Fig. 2 below. The
shaft has its main grounding at the drive-end (DE) of the
generator. At the non-driven end (NDE) the shaft is
additionally grounded by a mainly capacitive element.
Therefore no low-frequency current can flow. H ow we r high-
frequency voltage peaks (typically caused by the SCKs
thyristors oft he excitat ion system) are strongly dam ped [ I ] .
Tvbb.
Aacken. Germany,
2004
turbine. Every 6.66 ms a huge peak appeares, followed by a
much lower peak after 3.33 ms. It is evident that the rectifier
at the input-side of the SFC has to be considered as the
primary cause.
111 INVESTIGA TION O F THE SFC GENERATED SHA FT
VOLTAGE
Within electrical machines shaft voltages occur
on
account of
different effects [1-6,10]. With the application of power
electronic devices, new types of shaft voltage sources occur
[3-61. In general shaft voltages due to power electronics are
either capacitive or inductive.
After the introduction of power electronics for static
excitation the voltage peaks of the rectifier bridge caused
severe voltage peaks
on
the shaft [I]. These kind of peaks
occurred during normal operation. The voltages were coupled
by the capacitance between the rotor-field winding and the
shaft. Therefore the shaft current is different at the drive end
in comparison with the non-drive end. Reichert, Amman,
Posedel and Joho designed a method to suppress the shaft
voltages in 1988. In order to avoid a direct coupling of the
shaft at the non-drive end (NDE) they coupled the shaft
capacitively to ground, see
[ I ]
Some other publications related to experie nce with conve rter
fed asynchronous machines [4,6] an inductively coupled the
shaft voltage. Here the currents at drive end and non-drive
end must be the same.
Synchronized m easure men ts at the DE-side an d the NDE-side
showed that a positive p ulse at the NDE-side coincides with a
negative pulse at the DE-side and vice versa. So the picture of
a closed current loop arises with the current being induced by
changing mag netic fields in the stator core. This hypothesis is
supported by the observation that the pulses at NDE have
low- impedant sources. Current strengths of up to 70 A were
measured which could not be explained by capacitive
coupling.
These kinds of shaft voltages are clearly inductively caused
with the SFC as main root cause. Th e speed independent time
period of 3.33ms with stro ng pulses every 6.66 ms reveals the
strong relation to the SF C's rectifier. A typical electrical
design of a co nverte r fed plant drive is given in Fig.4. Fig. 4
reveals a strong asymm etry concerning the arrangement o f the
main choke between the rectifier and converter. According to
this model current measurement coils (Rogovsky coils) and
voltage senso rs have been installed at a multitude of
significant locations in the real equipment and corresponding
measurements were done.
.
SMI xmdm WGnumm WGxmdm
n o m
DEM
R C M
C
B m
CVBRUd
Fig
2:
Grounding scheme o f shaft train with KC-module
Measurements determined voltage peaks with abnormally
high amplitudes, fig.3, during active SFC-operation to be the
cause of th e blown fuse .
200
speed n i l 8 9 rpm
150
100
50
I
-
0
>
5 50
-100
-150
-200
c
v)
0.01 0.02 0.03
25OF; I
I
I
'
Time
s )
Fig. 3: Shaft voltage peaks at ND E during turbine run up
The shown voltage pattern is in its timely distribution of
peaks independent of the turning speed of the running-up
m
8/11/2019 Static Frequency Converters With Reduced Parasitic Effects
3/6
2004
35th
A n n u l IEEE Power
Electronics Specialists
Conference
Anchen.
Germany. 2W4
The static frequency converter consists of two thyristor
bridges. It is a current-type converter: Between both bridges a
D.C. circuit exists with one choke only in the positive branch.
The negative branch connects both bridges directly. The
bridge at the grid side is used to adjust the voltage level of the
D.C. circuit (and hence the DC current) and works asla
rectifier. During o ne period of 20m s each o f the 6 thyristors is
switched
on
and off. Three of them are connected to the
positive branch (link) with the choke, the other three are
directly connected to the output -rectifier bridge.
The bridge at the machine side works as a converter. A t lower
speeds it operates in block mode, afterwards it works as LCI
converter.
1.
It was determined that the start-up transformer could not be
the source of the pulses.
A
clear relationship between the shaft voltages and the
transformer currents should exist in case of the transformer as
voltage source. Figure 5 show s the summation of all three
transformer phase currents.
Shafivolaee
at non
d
end
I50
IO0
SO
0
-50
-100
-150
Fig. 5: Relationship between the summation current of all
transformer phases a nd the shaft voltage.
N o relationship between the shaft voltage and the summation
current can be found in Fig. 5
2.
Whereas the transformer as source of the new sha ft voltage
phenomenon could be excluded, the SFC seem s to be the real
source.
It is obvio us from Fig.
6
that a strong common-mode cu rrent
(summation of all phase currents)
flows
into the SFC and out
of it again without alteration. The conclusion is that the
current loop must be closed via the generator, ground, the
shielding of the SFC supply-cables and their shielding
capacitances. There are no other connections left. The
similarity between curve shap es of common mode current and
shaft voltage is also striking.
100
0
-100
Sh&vo lege
IMP 4456)
2001
50
t v
I I I I J I
0.0194
0.01 0.0198
Time
t ( s )
Fig. 6 : Current into the SFC and out of the SFC
3. In [ 6 ] it was shown how the shaft voltage is induced by a
circulating flux flow through the generator core. The
circulating flux flow can only occur
in
case of a difference
between the current in the positive and negative axial
direction. Therefore the current of the stator terminals and the
neutrals is measured, Fig. 7.
-200
a r r e n t at Etdmr neutral
w
4440)
-240
b -280
-300 (MP4436)
-3201
- 3 4 0 b , I , , , , I , , ,
~,
, , I , ,
, I ,
0.01976 0.01978 0.0198 0.01982 0.01984
Time
t s)
Fig. 7: Current into one generator terminal and out of the
generator neutral conn ection
Indeed figure 7 eveals a phase difference of about I0-15p
between the curren t at the phase-terminal and at the neutral
connection. The time constants of the shaft voltage-pulses and
the difference curren t between the stato r terminal-current and
neutral-current are fairly similar. It will cause a circular
magnetic flux in the stator core and hence induce the shaft-
voltage pulses.
In the following more em phasis is given
to
voltages measured
in or at the SFC during these switching processes. All these
voltage-surges and oscillations contribute to overall EM1 and
also to increased stress of components.
It can be seen in Fig. 8 that during valve-change-over of the
corresponding conducting thyristors of the rectifier-bridge
very strong current-oscillations appear in the two connected
cables a nd their shields.
4367
8/11/2019 Static Frequency Converters With Reduced Parasitic Effects
4/6
2 4
35th Annual IEEE
Power
Elecrrunics Specialisrs Conference
Aachen Germany, 2004
0.021 0.0212
o 021 Time
1
s)
Fig. 8: Current in the grounding-connections of the
transformer SFC cable a) Phase U, b) Phase V, c) Phase W
These currents flow in differential mode and do not influence
the shaft voltage. They produce however strong EM1 due to
the relatively high frequencies involved.
Fig. 9 and Fig. 10show the considerable common-mode
voltage pulses and steep voltage slopes to which the whole
SFC-system and also the generator winding is subjected. The
commo n mode voltage is given by the summ ation
o f
the three
terminal voltages,
F
0 5 10 I S
20 25
Tirnr rnr)
Fig. 10
Voltages to ground at the SFC input terminals
r j
2
. . . .
1 1
.
5
10 I5 20 25 ~~
Timc(rnr)
.~
Fig. 1 I Voltages at the links inside the SFC
The negative-link potential voltage (curve 3 is pushed down
very steeply
to
low values. At the same time the potentials at
the SFC-inputs are pushed up. However, when the positive-
link thyristors are switching almost no effect on the shaft-
voltage can be o bserved. This
is
due
to
the protecting effect
of the inductor in the positive link.
In
Fig. 1 1 it is clearly visible that
the
shaft-voltage pulses
coincide with every switching of the rectifier-thyristors
connected to the negative link. Fig. 12 gives
a
magnified view
of the processes.
2.5 s h # ' * a . m Y w h - t m
.nm*ndsma-
w
, W . b 8 r n W d r n
A.---
-.-. . ._
__.
wYp.*n..-.
-d.r.V*
,
> -0.5
0.W5
0.01 0.015
0.02
0.025 0.1
0.035
0.04 0045
0.05
Time
I) - 1 . 5
I_
Fig. 9: Commo n mode voltage at SFC-input terminals
-2.5
6.4 6.5 6.6
6.1
6.8
6.9 7.1 7
~
Time , lo
Fig. 12 Voltages of links inside the SFC
The potential of the negative link starts to slope steeply
downwards. The potential of the voltage between the choke
and generator follows and also the shaft voltage follows. This
is again a clear indication that switching of the rectifier
4368
8/11/2019 Static Frequency Converters With Reduced Parasitic Effects
5/6
2004
35th Annual IEEE Power Electronics Specialists Conference
thyristors connected to the negative link cause the pulses on
the shaft voltage.
IV IMPROVED SFC DESIGN
The influence of the SFC design is one of the dominant
factors for the parasitic effects, which can he seen at the shaft.
The direct connection of rectifier thyristors to the negative
link and with it to the generator connections has been
identified above as main cause for high frequency pulses.
This configuration can be used a s long as the shaft voltages
stay within appropriate limits [1,7-91. If these limits are
exceeded the SF C needs to be modified.
One very effective modification is
to
use a split choke within
the d.c. circuit of the SFC. A split or preferably symmetric
inductor will prevent steep gradients and HF-pulses from
travelling into the generator. The choke will significantly
reduce the voltage gradients to which the generator winding is
subjected and keep
it
at acceptable values. This design
ensures a very smooth operation. It is also an economically
acceptable solution because no large additional components
are needed in general: The main inductor is split into two
halfs, wh ich is often quite feasible. No big ch anges are caused
electrically to the SFC by this change: The internal DC-
current path and its control is still dominated by the same
inductances.
The beneficial influence of the choke on the high-frequency
generation ofth e SFC can he seenin Fig. 13.
The shaft voltage during a commutation at the positive link of
the SFC is only
a
third or even less than during a
commutation at the negative link. The main reason for this is
the strong HF-damping by the choke, see fig.
13
below. In
Fig. 13 it is clearly visible that the HF-content
of
the voltage
after the choke is almost negligible.
Aachen Germany 2004
ShaRvoltage(MP4456)
000
MOO
4000
d .c .
link
voltage
befors &e
choke,
tranrformrside MP 455)
d.c.
linkvoltage
a fe r & e choke,
MP4454)
- E -2oooE Y
.0176 0.0178
0.018
-4WE -30;j174
Time t
s)
Fig. 13Voltages before and after the choke, positive link
Beside the influence of the SFC (steepness of voltagelcurrent
changes), other parameters like the cable length between the
start up transformer and the SF C as well as between the SFC
end the generator have to be taken into account.
Generally, modifications to converters within plants of some
IOOMVA output have to be b ased on
a
deep understanding of
the phenomenon involved. The refore design modifications to
the static frequency converter (SFC) must take into account
all main com ponents which are involved. Mod ifications need
firstly
to
he simulated by using simplified mathematical
models
of the whole plant with all the main par& involved.
The effectiveness has
to
he shown in simulations. The
proposed design improvem ent is a result of these simulations.
VI SUCCESSFULLY IMPLEMENTED MODIFICATION
The influence of the modifications could be proven in the
meantime at several power plants which showed high shaft-
voltage pulses. All the plants had long cab les between start-
up transformer and SFC resulting in high coupling
capacitance to ground at the inputs of the SFC.
The main modification has been the implementation of a
symmetrical choke in both d.c. links of the SFC. The
influence of this change within the frequency converter
design is shown in fig. 15 .
4369
8/11/2019 Static Frequency Converters With Reduced Parasitic Effects
6/6
2004
35111
Annu al I EE E Power Elecrronics Specialists Conference
Aachen, German),, 2004
Time
t
@s)
Fig. 15: Measured shaft voltages with and without
symmetrical choke
The modification has been a complete success. The high-
freque ncy compon ents (-5OkHz) of the shaft-v oltage pulse
have almost completely disappeared. They are efkctiv ely
blocked by the inductors in both links. There is no low-
impedance grou nd-loop anymore for HF-bursts cau sed by the
SFC. Only a signal of considerable lower frequency -10
kHz) and much lower amplitude is left. The voltage peaks
have been reduced to a third of the original value. High
frequency signals causing EM1 have almost been eliminated
by this relatively simp le modification.
Furthe rmore it became clear that the source-impedance of the
pulses could be considerably increased by the additional
choke. That means that the capacitive grounding module at
the ND E-side of the generator can reduce the vo ltage pulses
even further without being subjected to increased
electricaVthermal stress due to high HF-c urrents .
It was possible to reduce the level of shaft voltages even
below the expe cted level.
V CONCLUSIONS
Measurements in a 300MVA power plant allowed to identify
the mechanism w hich causes the observed HF shaft-voltage
pulses at converter-fed large electrical machines. The root
causes for these pulses were a SFC which p roduces com mon-
mode HF-voltage pulses together with a mainly capacitively
conducting electrical loop which couples thc pulses
inductively onto the shaft .
Based on the improved understanding of the phenomenon,
mod ifications have been implemented in a power plant w hich
were able to reduce the shaft voltages by a factor of
3.6.
The key factor for the successful voltage reduction has been
a design-change within the converter. Instead of using a
single choke in the internal DC-loop in an asymmetric
configuration, two chokes are arranged in a symmetrical
way. The two chokes effectively block HF-pu lses and reduce
EM1 considerably.
VI REFERENCE
[ I ] Amann C ., Reichert K., Joho R., Possedel Z.: SHAFT
VOLTAGES IN GENERATORS WITH STATIC
EXCITATION SYSTEMS
-
PROBLEMS AND
SOLUTION, IEEE Trans. Energy Conversion, vol .
3,
no.
2,
j u n e
1988.
[2]
Torlay
I.
E., Corenwinder C., Audoli A., Herigault J.,
Foggia A.: Analysis of Shaft Voltages in Large
Synchronous Generators, International Electric
Machines and Drives Conference, 9-12 may 1999,
Seattle, Washington, US.
[3]
Link P. J.: Minimizing Electric Bearing Currents in
ASD Systems, IEEE Industry Application Magazine,
JulyiAugust 1999, pp. 55 - 66.
[4] Miitze A.: Bearing Cu rrents in Inverter-Fed AC-Motors,
Dissertation University of Darmstadt,
23.1.2004.
[5]
Cheng
S. ,
Lip0 T., Fitzgerald D.: Modeling of Motor
Bearing Currents in PWM Inverter Drives, IEEE
Trans. on Industry Applications, vol.
32,
no.
2,
march-
April 1993
[6]Hausberg V., Seinsch H. 0 : Kapazitive
Lagerspannungen und Strome bei umrichtergespeisten
Induktionsmaschinen, Electrical Engineering
(82).
pp,
153-162, 2000.
[7]
Amman C. U.:Wellenspannungen in grossen, statisch
erregten Turbogeneratoren, Dissertation ETH-Zurich,
1988.
[8] VDE
0141/5.76:
,,Bestimmungen f~ Erdungen in
Wechselstromanlagen
f
Nennspannung, VDE Verlag
[9] AIEE Co mm ittee Report: ,,Voltage gradients through
the ground under fault condition, Trans. AIEE Part 111,
(1958)
pp.
669-692.
[IO]
Hausberg V., Seinsch H. 0 : Kapazitive
Lagerspannungen und Strljme bei u mrichtergespeisten
Induktionsmaschinen, Electrical Engineering
(82).
pp.
153-162, 2000.
4370