Investigation of Bearing Currents in Dual Mode Operation of Synchronous Machine with Static Excitation System

Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 4, November 2013

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INVESTIGATION OF BEARING CURRENTS IN DUAL

MODE OPERATION OF SYNCHRONOUS MACHINE

WITH STATIC EXCITATION SYSTEM

Arun Kumar Datta1, Manisha Dubey

2 and Shailendra Jain

3

1Central Power Research Institute, Bhopal, India

2,3Maulana Azad National Intitute of Technology, Bhopal, India

ABSTRACT

Days are gone for rotating exciters. Most of the machines those require field excitation are fed from static

excitation system (SES). Static systems are prone to generate shaft voltage thereby causing the flow of

bearing currents. This paper has presented a thyristor based SES in 12-pulse configuration. This SES is

being applied on the field winding of a synchronous machine installed in a short circuit testing laboratory.

This machine is of large capacity and without prime mover. SES along with static frequency converter

makes this single machine to operate in dual mode (i.e. motor or generator). Functions of SES under

different operational modes have been looked into this paper. Causes of generation of shaft voltage and

bearing current in perspective with this machine under static excitation system are the main attraction of

this paper.

KEYWORDS

Synchronous machines, shaft voltage, bearing current, static excitation, parasitic capacitance, electric

discharge machining.

1.INTRODUCTION

Any synchronous machine, be a motor or generator requires dc power in the rotor winding to

magnetise the air gap. Four major options can be exercised to source this dc power:

1. Brushless excitation through rotating diodes and transformer.

2. Thyristor based controlled rectifier (static excitation system) feeding the rotor

winding through sliprings and brushes.

3. Separate dc generator feeding the synchronous machine rotor through sliprings and

brushes.

4. Rotating exciter coupled with the same shaft of the synchronous machine.

The first option i.e. brushless excitation system is very much useful for a machine where the rotor

field current requirement is low. But it has some design limitations for the large synchronous

machine with high field current. In this case the option of static excitation system (SES) with

rotor slipring and brushes are the best solution. But this method needs regular maintenance due to

wear & tear in the carbon brushes and slip rings. Prior to invention of solid state devices, most of

the machines were operated either with third or fourth option. But the later one was much

popular.


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SES came into picture in the beginning of sixties. In the year 1962 it was designed and

experimented on a steam turbine generator [1]. Very quickly it started replacing the old rotating

exciter of existing power generators. The reason was design flexibility and easy retrofit for both

small and large machines. From the steam turbine generator it was even extended up to the

captive power plant in paper and pulp industries [2].

The negative aspect of SES is observed as the flow of bearing current due to the presence of shaft

voltage. Though the shaft voltage is a very common phenomenon since the invention of electric

machines, but it is more predominant in the case of machines with static supplies [3]. Static

sources generate shaft voltage by means of capacitive coupling, inductive couplings and common

mode voltage (CMV). It is well known that one of the causes of bearing failure is the flow of

unwanted current through it. Many literature suggested different converter topologies [4-6] and

various filtering techniques [7-9] to minimise the CMV. In spite of several drawbacks in static devices no other systems are better than SES for not only this machine but for others too.

The bearing current studied in this paper has been noticed on a synchronous machine installed in

a short circuit test laboratory. Short circuit test is carried out on electrical power equipment to

check its practical performances in case of actual faults. These tests are performed as per the

various national and international standards. A high power three phase alternator is necessary to

conduct the short circuit tests. The alternator here is 1500MVA, 12.5kV, 69kA rating and without

prime mover. Static excitation system along with static frequency converter makes this machine

to start as a motor and thereafter converting to alternator to deliver power for the short circuit

tests.

This paper gives the detailed analysis of static excitation system and its behaviour in motor &

generator mode. Causes of generation of shaft voltage and flow of bearing current in this

synchronous machine are also looked into this paper.

2.BEARING CURRENT & ITS EFFECTS

Bearing current is the consequences of shaft voltage. It happens when the shaft voltage reaches up

to the break down voltage level of bearing lubricant. Bearing current will also flow when the

electrically charged shaft comes to a close proximity with the grounded bearing. Presence of shaft

voltage in electrical machines is unwanted but can not be avoided.

Figure 1. Bearing currents in a machine.

There are quite a few internal or external causes behind the shaft voltage. These are mainly

depends on the type of source voltage fed to the machine i.e., sinusoidal or non-sinusoidal.

Bearing currents caused by these shaft voltages are normally circulating or non-circulating types


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(fig.1). Both may also appear simultaneously depending on the supply source. Circulating current

makes a close loop around rotor, shaft, bearing, stator frame to the other end of the bearing and

shaft [10-12]. But the non-circulating current is a discharge mode current through ground.

Machine bearings are normally capable of withstanding a small amount of current. If the current

level is high and impulse in nature then bearing erosion takes place. It is commonly known as fluting or Electric Discharge Machining (EDM) [13]. As a result, bearing, shaft and nearby metal

surfaces become rough due to pittings on it (fig.2). EDM also creates electromagnetic

interferences [14] in the neighbouring electronic circuits.

Figure 2. Bearing & shaft erosion.

3.MACHINE & ASSOCIATED SYSTEM

The 200 ton weight synchronous machine is installed on a specially built concrete foundation. Its 3000 rpm rotor weighing approximately 40 ton is supported by two journal bearings on each end.

Figure 3. Synchronous machine with connected systems.


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Figure 3 depicts the synchronous machine and its associated system. The machine is connected

with two sources. Static excitation system (SES) feeds dc current in the rotor field winding. It is

done through carbon brushes & two sliprings from the exciter end. Stator gets it variable ac from

static frequency converter (SFC). As the machine doesn’t have any mechanically coupled

primemover, hence it is called motorless generator [15].

4.SES: DESCRIPTION

Synchronous machine can be operated either in motor or generator mode. DC supply in rotor

winding is mandatory for both modes. This dc current magnetises the air gap of the machine.

Among the various excitation methods the response of SES is very fast, which is the prime

requirement of the synchronous machine. SES converts input ac supply to pulsating dc through a

thyristor based controlled rectification. To reduce ripple and harmonic contents, the use of poly-

pulse converter [16] is in practice. It also improves the overall system power factor. Keeping this

in view a 12-pulse converter configuration is adopted here by combining two six pulse converter

(fig.4).

Figure 4. Schematic of SES feeding for rotor excitation.

SES has a special converter transformer with one primary and two secondary windings.

Secondaries are in star & delta configurations with a 30° phase shift between them. Each


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secondary output feed individual six pulse converter bridge. To improve the current rating two

thyristors are put in parallel in each leg of the bridge.

Rectified outputs of converter 1 & 2 are connected in series and feed the rotor winding. Input

voltage & current of the converters are measured by standard VT & CT but the output being a

pulsating dc, special Hall Effect transducers are used. One set of measured waveforms are shown in figure 5. Rotor current is controlled from the SES control cubicle through the thyristor gate

pulses. A dc circuit breaker does the switching operation between the SES and rotor circuit. For

de-energizing the rotor energy a de-excitation contactor with series resistance is put across the

rotor circuit.

a. Input voltages b. Input currents c. Output voltage & current

Figure 5. Input & output voltage & current in SES.

5.SES: OPERATION

SES is functional all the times with different output levels. The key operational modes are

discussed in the subsequent sub-sections.

5.1 During motoring

In the beginning SES injects current in the field winding and creates two magnetic poles in the

rotor. At the same time magnetic flux is induced in the stator winding after getting low frequency

current pulses from SFC. This flux makes to move the magnetized rotor. Stator input supply

frequency is increased by SFC up to 50 Hz to achieve the rated speed of 3000 rpm. SES controller

sets its output according to the rotor speed. It is understood from figure 6 that at low speed (150

rpm) field current drawn is more than the full speed (3000 rpm).

a. 150 rpm b. 3000 rpm

Figure 6. Field voltage & current at 150 & 3000 rpm.


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5.2 During generating

The basic purpose of the machine is to generate power for short circuit test. For this, the machine

has to be converted to generator from motor at is full speed. This is done in two steps. In the first

step stator input supply is blocked and in the second step the SES sets the field current

proportional to the required stator output voltage. As the rotor is spinning, machine behaves as an

alternator and is ready to supply required power in short circuit test. During the short circuit test

period, the generator output voltage & current are adjusted by SES with the feedback from VT &

CT (fig.4). Figure 7 shows the response plot at SES controller during the short circuit test on a

circuit breaker. Actual voltage and current waveforms for this test conducted with this generator

are shown in figure 8.

Figure 7. Response at the SES controller during test. Figure 8. Generator I & V during test.

6.PHENOMENON OF ELECTRIC DISCHARGE MACHINING (EDM)

It has already been mentioned that static supply is the main source of shaft voltage. SES output is

not pure dc rather it is pulsating in nature. It has also steep steps (high dv/dt) due to thyristors

commutation (fig.5.c). FFT analysis (fig.9) on the rotor current and voltage reveals that dc

waveforms contain multiple numbers of harmonics.

Figure 9. FFT on rotor voltage & current waveforms.


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In the presence of insulating material, air and lubricating oil as dielectric medium and copper

windings as conductor, parasitic capacitances are formed at many places in the machine. This is

the reason that every part of the machine including the source is coupled to each other through

distributed capacitances. These capacitances may form between stator to ground, stator to rotor,

stator to shaft, rotor to shaft, bearing to ground, cable to ground and also supply source to ground.

Capacitive coupling gets stronger with the presence of higher harmonics (as Xc=1/2πfC). Leakage current (Ic=C.dv/dt) flows all along the stator and rotor windings [14] due to this parasitic

capacitance C and high dv/dt. This leakage current generates high frequency flux. Harmonics and

other high frequency spikes in the stator and rotor power supply also generate high frequency

fluxes. These cumulative fluxes link with winding, core, frame, shaft, and other metallic parts. In

this way the inductive coupling is formed. Capacitive and inductive couplings together give an

additive effect for generation of shaft voltages for this long shaft machine. When these voltages

become high or the shaft comes in close proximity with the grounded bearing, discharge currents of small width but high magnitude takes place from the shaft to the bearing. This is called electric

discharge machining (EDM) and it is the main cause for eroding the bearing and shaft surfaces.

EDM has been observed in this machine (fig.10). This EDM shows the strong presence of shaft

voltage.

Figure 10. Electric discharge machining (EDM).

7.CONCLUSION

Bearing currents is very common phenomena in electric machines. It has been observed in many

forms. Machine with static sources are very much prone to generation of shaft voltages and the

subsequent flow of bearing currents. The machine discussed in this paper is 1500MVA, 12.5kV,

69kA, 50Hz, 3000rpm rating short circuit alternator. Output of the machine is utilised for short

circuit tests on electrical power equipments. Like other high power alternator this one is also

equipped with static excitation system. SES in this machine is active not only in motor and

generator but for braking and reversal operation also. In spite of many advantages the darker side

of SES is the contribution in generation of shaft voltages. Sparking on the contact areas of shaft

grounding brushes is the evidence of EDM due to the presence of shaft voltage. Shaft voltages

generated mostly due to capacitive and inductive couplings. Harmonics and other higher

frequencies in the SES output create these couplings. EDM has an effect mostly on the bearing &

shaft of the machine in the form of flutings or pittings on the surfaces. It also affects the chemical

composition of lubricants and nearby communication circuits. Efforts are on to minimise the effect of the bearing currents. Though the work has been carried out on a particular machine, but

it can also be implemented to other similar machines.


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ACKNOWLEDGEMENT Authors are thankful to the management of Central Power Research Institute, India for supporting

this work.

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AUTHORS

Arun Kumar Datta graduated in Electrical Engineering in 1992 from Govt. Engineering

College, Bilaspur in India. In 1990 he did his post graduation (M.Tech.) from MACT

(REC), Bhopal, India. In March 1993 he joined Central Power Research Institute (CPRI)

India and looking after the Operation & Maintenance of two 1500MVA short circuit

generator plant and 33kV substation at Bhopal unit. He is also the Deputy Quality

Assurance Co-ordinator of CPRI Bhopal unit. He had undergone training at the works of

M/s. Alsthom, France. He is a Certified Energy Auditor from Bureau of Energy

Efficiency (BEE) and also a member of Institution of Engineers (India). He has attended

many International & National Conferences and has many papers on his credential. Other

than his regular assignments he is also pursuing Ph.D. from Maulana Azad National

Institute of Technology (MANIT), Bhopal.

Manisha Dubey was born in Jabalpur in India on 15th December 1968. She received her

B.E (Electrical), M.Tech. (Power Systems) and Ph.D (Electrical Engg.) in 1990, 1997 and

2006 respectively. She is working as Professor at the Department of Electrical Engineering,

National Institute of Technology, Bhopal, India. Her research interests include power

systems, Genetic Algorithms, Fuzzy Logic systems and application of Soft Computing

Techniques in power system dynamics and control. She is the life member of IEEE, IE &

ISTE, etc.

Shailendra Jain received the B.E. degree in electrical engineering from Samrat Ashok

Technological Institute Vidisha (SATI), India, in 1990, the M.E. degree in power electronics

from Shri Govindram Seksaria Institute of Technology and Science, Indore, India, in 1994,

the Ph.D. degree from the Indian Institute of Technology, Roorkee, India, in 2003, and the

PDF from the University of Western Ontario, London, ON, Canada, in 2007. He is a

Professor in the Department of Electrical Engineering, National Institute of Technology,

Bhopal, India. His research interests include power electronics and electric drives, power

quality improvement, active power filters, high-power-factor converters, and fuel-cell-

based distributed generation. Dr. Jain was a recipient of the “Career Award for Young

Teachers” from the All India Council for Technical Education (AICTE), New Delhi,

India, for the year 2003–2004.

Investigation of Bearing Currents in Dual Mode Operation of Synchronous Machine with Static Excitation System

Documents

synchronous machine

large synchronous machine

field excitation

brushless excitation

single machine

static systems

functions of ses

rotor winding