Proceedings of the 6th WSEAS International Conference on Power Systems, Lisbon, Portugal, September 22-24, 2006
424
Harmonic Distortion Produced by Synchronous Generator in Thermal Power Plant
ELESCHOVA ZANETA, BELAN ANTON, MARTIN MUCHA
Department of Power Engineering
Slovak University of Technology
Ilkovicova 3, Bratislava
SLOVAK REPUBLIC
zaneta.eleschova@stuba.sk http://www.fei.stuba.sk
Abstract: - At the present time, the area of energy interference is highly relevant cause of these reasons: wide-range of
power electronics usage, using of new (renewable) sources of electric energy, massive usage of light sources with
electronic ballasts. Experimental results obtained with large synchronous generator in thermal-power plant are
presented in this paper, to provide experimental evidence that the synchronous machine does indeed have ability to
generate harmonic distortion in power system.
Key-Words: - Energy Interference, Harmonics, Synchronous Generator, Power Quality.
1 Introduction
Electromagnetic interference is any electromagnetic
effect, which can impair equipment functionality or
functionality of system. Electromagnetic interference is
any electromagnetic effect, which can impair equipment
functionality or functionality of system. From the
viewpoint of frequency spectrum and physical impact of
electromagnetic
interference,
we
can
divide
electromagnetic interference to low-frequency and radiofrequency. Low-frequency interference has bad impacts
on power system (energy interference) and to lowfrequency information transport systems (acoustics
interference).
Energy interference acts in frequency band from 0 Hz to
2 kHz and can cause harmonic distortion of voltage or
current wave. Energy interference impacts on controlling
and information systems, lightening equipments,
condensers, cables, protective device and electrical
protection. Energy interference sources are mainly
nonlinear loads which consume deformed non-sinusoidal
current from network (mainly power controlled
semiconductor changers), but also source of electric
power itself.
At the present time, the area of energy interference is
highly relevant cause of these reasons: wide-range of
power electronics usage, using of new (renewable)
sources of electric energy, massive usage of light sources
with electronic ballasts.
That is why it is important to define sources of energy
interference exactly and to analyze their impact on
power sys-tem equipment, mainly from point of view of
right and reliable function (electrical protections), losses
(transmission,
distribution
and
compensation
equipments) and possible damage of equipments (highvoltage insulating systems of rotary machines).
In the project we focus on possible sources of energy
interference:
renewable sources of electric power,
conventional sources of electric power,
selected light sources,
power equipments with semiconductor electronic
devices.
2 Harmonic Distortion in Power System
We assume that waveform of voltage and current in AC
circuits has sinus form with constant amplitude and
frequency, but more or less, each device produce
deformation of voltage and current waveform and
deviations from sinus waveform.
Currents of nonlinear equipments produce voltage drops
on net impedances. These voltage drops lead to voltage
wave-form deformation in comparison with the ideal
sinus wave-form.
In assumption of constant deformation of waveform,
volt-age or current waveform can be divided into more
sinus waveforms with different amplitude and phase,
while frequency of these components is multiples of
fundamental frequency. In Fourier transformation,
multiples of fundamental frequency are called
harmonics.
Proceedings of the 6th WSEAS International Conference on Power Systems, Lisbon, Portugal, September 22-24, 2006
Harmonics in power systems can cause:
network resonation for defined harmonic, that can
expose equipment of power system to higher strain
by overvoltage or overcurrent,
overheating and overloading of transmission,
distribution and compensation devices,
incorrect functionality of electric protection,
interference of telecommunication devices,
incorrect functionality of control circuits.
3 Harmonics Produced by Synchronous
Machines
3.1 Voltage
Harmonics
Synchronous Machines
Produced
by
If the magnetic flux of the field system is distributed
perfectly sinusoidal around the air gap, the e.m.f. (electro
motoric force) generated in each full-pitched armature
coil is
e.m.f. = 2f. sin t [V per turn]
(1)
Where f is the total flux per pole and f is frequency
related to speed and pole pairs.
However the flux is never exactly distributed in this
way, particularly in salient pole machines. A
non/sinusoidal field distribution can be expressed as a
harmonic series:
2 x
3.2x
F(x ) = F1 sin
+ F3 sin
5.2x
+ F5 sin
+ .....
(2)
If the oils are chorded to cover ( ) electrical radians,
the flux linked is reduced by cos( 2 ) and the e.m.f. is
reduced in proportion. The effective chording angle for
harmonics of order n is n. Hence the general coil/span
factor is
n
k sn = cos
2
E(t ) = E 1 sin t + E 3 sin 3t + E 5 sin 5t.....
(3)
The magnitudes of the harmonic e.m.f.s are determined
by the harmonic fluxes the effective electrical phase
spread of the winding, the coil span, and the method of
interphase connection.
For an integral slot winding with g slots per pole per
phase and an electrical angle between slots, the
distribution factor for the nth harmonic is
k dn
sin ng
2
n
g sin
2
( )
(4)
(5)
By suitable choice of kd and ks many troublesome e.m.f.
harmonics can be minimized or even eliminated. The
triplen harmonics in a three-phase machine are generally
eliminated by phase connection, and it is usual to select
the coil span to reduce 5th and 7th harmonic.
Slotting (the slots being on the stator) produce variation
of permeance. The fundamental rotor m.m.f.
(magnetomotive force) can be represented as a traveling
wave. The slot ripple component of flux density is of the
form
2x
2x
F1A 2 sin 2mg
t
cos
(6)
This can be resolved into two counter-rotating
components, which are slow-moving multi-pole
harmonics. Their wavelengths are
corresponding velocities are
and the
(2mg 1)
f
. As the number of
(2mg 1)
waves passing any point on the stator per second is
(speed wavelength) , obviously each component induces
an e.m.f of fundamental frequency in armature.
Relative to the rotor, however, these two waves have
different velocities. The rotor velocity being f, the
waves
The machine can be consider to have 2p fundamental
poles together with 6p, 10p, 2np harmonic poles, all
individually sinusoidal and all generating electromotoric
forces in an associate winding. The winding e.m.f. can
be expressed as a harmonic series:
425
travel
at
velocities
f and
f
2mg + 1
f with respect to rotor. In any closed rotor
f +
2mg 1
circuit each of these will generate currents of frequency
2mgf (by considering the ratio of speed to wavelength)
and these superimpose a time-varying m.m.f at
frequency 2mgf on the rotor fundamental m.m.f. This
can be resolved into two counter-rotating components
relative to the rotor, each traveling at high velocity
2mgf, and therefore at 2mgf f relative to the stator.
The resultant stator e.m.f.s have frequencies (2mg 1) f .
Slot harmonics can be minimised by skewing the stator
core, displacing the center line of damper bars in
successive pole faces, offsetting the pole shoes in
successive pairs of poles, shaping the pole shoes, and by
the use of composite steel bronze wedges for the slots
of turbogenerators.
Proceedings of the 6th WSEAS International Conference on Power Systems, Lisbon, Portugal, September 22-24, 2006
426
It can be shown that the distribution factor for slot
harmonics is the same as for the fundamental e.m.f.. It is
not reduced by spreading the winding. Fractional instead
of integral slotting should be used.
3.2 Synchronous Machines Source of Harmonic
Currents
Synchronous machines represent a source of harmonic
currents on two counts: the frequency conversion effect,
and the non-linear characteristic due to magnetic
saturation.
The frequency conversion effect: a synchronous
generator feeding an unbalanced, three-phase load may
experience the flow of a negative sequence current in the
rotor, which in turn may induce a third-order harmonic
current on the stator winding. In special cases when the
generator feeds static converter equipment the machine
can be important source of harmonic generation.
The saturation of the stators circuit represents another
harmonic source.
Fig. 1 Voltages of Generator rms values
4 Measurements on the Synchronous
Generator
Measurements were performed on a generator with a
round rotor in a thermal-power plant.
The goal of measurements was to detect harmonic
distortion produced by synchronous machines.
The measurements were performed in this case of
generator operation:
1. Measurement of voltage harmonics on non-load power
plant block: generator - transformer.
2. Measurement of voltage and current harmonics on
loaded power plant block: generator - transformer.
3. Measurement of voltage and current harmonics on
minimal-loaded power plant block: generator transformer.
4.1 Measurement of voltage
harmonics on non-load power
generator - transformer
Fig. 2 Voltage Harmonics Produced by Generator
4.2 Measurement of voltage
harmonics on loaded power
generator transformer
and current
plant block:
The power plant block was connected to the power system
and power output and a terminal voltage were regulated
from the power system dispatch.
Results from measurements are in fig. 3 6.
and current
plant block:
The generator was connected to a block transformer and
the generator was operated with nominal revolutions and
nominal voltage on its terminals during measurement.
The block transformer was disconnected from a power
system.
Results from measurements are in fig. 1 2.
Fig. 3 Voltages of Power Plant Block (on the 110 kV side
of transformer) rms values
Proceedings of the 6th WSEAS International Conference on Power Systems, Lisbon, Portugal, September 22-24, 2006
Fig. 4 Total Harmonic Distortion of Voltage on the
110 kV side of Transformer
427
Fig. 7 Voltages of Power Plant Block (on the 110 kV side
of transformer) rms values
Fig. 5 Currents of Power Plant Block (on the 110 kV side
of transformer) rms values
Fig. 8 Voltage Harmonics of Power Plant Block (on the
110 kV side of transformer)
Fig. 6 Total Harmonic Distortion of Current on the
110 kV side of Transformer
4.3 Measurement of voltage and current
harmonics on minimal-loaded power plant block:
generator transformer
The power plant block was connected to the power system
and power output was on minimal value.
Results from measurements are in fig. 7 12.
Fig. 9 Total Harmonic Distortion of Voltage on the
110 kV side of Block Transformer
Proceedings of the 6th WSEAS International Conference on Power Systems, Lisbon, Portugal, September 22-24, 2006
Fig. 10 Currents of Power Plant Block (on the 110 kV
side of transformer) rms values
Fig. 11 Current Harmonics of Power Plant Block (on the
110 kV side of transformer)
Fig. 12 Total Harmonic Distortion of Current on the
110 kV side of Transformer
5 Conclusion
Energy interference impacts on controlling and
information systems, lightening equipments, condensers,
cables, protective device and electrical protection.
Energy interference sources are mainly nonlinear loads
428
which consume deformed non-sinusoidal current from
network (mainly power controlled semiconductor
changers), but also source of electric power itself.
At the present time, the area of energy interference is
highly relevant cause of these reasons: wide-range of
power electronics usage, using of new (renewable)
sources of electric energy, massive usage of light sources
with electronic ballasts.
Experimental results obtained with large synchronous
generator in thermal-power plant are presented in this
paper, to provide experimental evidence that the
synchronous machine does indeed have ability to
generate harmonic distortion in power system.
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This work was supported by Science and Technology
Assistance Agency under the contract No. APVT-20002004
