Static Frequency Converters With Reduced Parasitic Effects

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.org


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


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.

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35th Annual IEEE

Power

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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

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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 .

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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.

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Static Frequency Converters With Reduced Parasitic Effects

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