Transcript aiktcDspace1824.2

eCourseware@AIKTC
INTRODUCTION


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Electrical energy transmitted through AC or DC
transmission is to be delivered at consumers terminals at
specified voltage level of constant magnitude without
deviation from ideal waveform.
Deviation from the perfect sinusoidal waveform is
generally expressed in terms of harmonic components.
Harmonics are defined as the content of electrical signals
whose frequency is an integral multiple of the
fundamental frequency produced by generators.
IMPORTANCE OF HARMONIC STUDY
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
HVDC transmission system generated harmonic currents
on AC side and harmonic voltages on DC side during
operation.
The harmonic currents generated at AC bus get
transmitted to AC system and cause following adverse
effects
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Heating of equipments connected
Instability of converter control
Generates telephone and radio interference in neighboring
communication line, thereby inducing harmonic noise.
Harmonics can lead to over voltages due to resonance when
filter circuits are employed.
GENERATION OF HARMONICS

Sources of harmonics are
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AC generator
Transformer
Converter along with its control devices
AC GENERATOR AS A SOURCE OF HARMONICS
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Although the waveform is usually good, an AC generator may
be regarded as a source of balanced harmonic voltages because
of non-uniform distribution of flux on the armature windings.
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TRANSFORMER AS A SOURCE OF HARMONICS:
Due to presence of DC component in transformer
secondary, there is magnetic distortion and magnetic
saturation which make transformers as sources of
harmonic voltages.
Magnitude of these harmonic depends on operating flux
density.
Converter transformers are usually operated at higher
flux densities than conventional three phase transformers,
therefore the possibility of generation of harmonics is
more in former than in latter.
HARMONICS DUE TO CONVERTERS
CHARACTERISTIC HARMONICS:
 Under balanced operating conditions, the converter is
supposed to produce fundamental and harmonics of the
orders 5,7,11,13,etc.
 Normal current harmonic produced on AC side are of
the order (np±1)
 Normal voltage harmonic produced on DC side are of
the order (np)
where ‘n’ is any positive integer and ‘p’ is pulse number.
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The above mentioned normal harmonics are called as
characteristic harmonics.
HARMONICS DUE TO CONVERTERS
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CHARACTERISTIC AC HARMONICS
HARMONICS DUE TO CONVERTERS

The current IA= IA1+IA2

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The line current of each converter bridge will be half of
the total current mentioned above ie



4 3 
1
1
1
1
Id cos t    cos11t    cos13t    cos 23t    cos 25t  .....
 
 11 
 13 
 23 
 25 



1
1
 1 
 1 
Id cos t    cos 11t    cos 13t    cos 23t    cos 25t  .....

 11 
 13 
 23 
 25 


2 3
The rms value of which is,



2 3 
1
1
 1 
 1 
Id cos t    cos11t    cos13t    cos 23t    cos 25t  .....
 2 
 11 
 13 
 23 
 25 



1
1
 1 
 1 
Id cos t    cos11t    cos13t    cos 23t    cos 25t  .....
 
 11 
 13 
 23 
 25 

6
HARMONICS DUE TO CONVERTERS

From the above the maximum value of fundamental and
harmonic currents will be,
 6
 Id
I 10  
  


 I 10 
Iho  

 h 

The second subscript “o” indicates that the overlap angle
“u” is assumed to be zero.
HARMONICS DUE TO CONVERTERS
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When “u” is non-zero, the expression for Ih is given by,


1
2
Iho A2  B 2  2 AB cos2α  u 
Ih 
cos α  cos δ
where,
u
u
sin h  1
sin h  1
2 ,B 
2 ,    u
A
h 1
h 1

The above expressions are valid only for u≤60
HARMONICS DUE TO CONVERTERS
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CHARACTERISTICS DC HARMONICS
From the Fourier analysis of DC voltage waveform, we
can obtain,


Vdo C 2  D 2  2CD cos2  u 
Vh 
2
where,
u
u
cosh  1
cosh  1
2 ,D 
2
C
h 1
h 1
1
2
HARMONICS DUE TO CONVERTERS
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NON CHARACTERISTIC HARMONICS:
The harmonics of the order other than characteristic
harmonics are termed as non-characteristic harmonics.
These are due to,
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Imbalance in the operation of two bridges forming the 12 pulse
converter
Firing angle errors
Unbalance and distortion in AC voltages.
Unequal transformer leakage impedances.
1
2
HARMONICS DUE TO CONVERTERS
The harmonics produced due to the 1st cause are termed
as residual harmonics. These are majorly due to difference
in the firing angles of the two bridges which lead to
unequal cancellation of harmonics of order 5,7,17,19etc
 The last three cause can lead to generation of triple or
even harmonics.
(1) EFFECT OF FIRING ANGLE ERRORS:
 The major source of firing angle errors in modern
converter stations is due to ripple in current f/b signal
and not due to equipments.
 The tolerance in the firing pulses should not exceed ±
0.2º

HARMONICS DUE TO CONVERTERS
(2) EFFECT OF UNBALANCED SYSTEM VOLTAGES:
 The presence of negative sequence component in AC
voltage shifts the zero crossing of the commutating
voltages.
 With IPC this introduces firing angle dissymmetry and
results in non-characteristic harmonics.
 With 5% negative sequence voltage, the 3rd harmonic
current generated can be as large 5% of the fundamental
component.
HARMONIC SUPRESSION- FILTERS
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The most easiest way to reduce harmonics is to use a
passive LC resonant filter that offers low or zero
impedance and bypasses the harmonic voltages to a level
specified in technical specification.
HARMONIC MODEL AND EQUIVALENT CIRCUIT:
HARMONIC MODEL AND EQUIVALENT
CIRCUIT:

The harmonic currents of order “h” generated are
divided into two components.

(a) harmonic currents in filter branch
Vh
I hc  Z ha
I hf   
Z hf Z ha  Z hf

(b) harmonic currents in AC network
I hc  Z hf
Vh
I ha  

Z ha Z hf  Z ha
and satisfies the condition,
I hc  I hf  I ha
HARMONIC ELIMINATION METHODS
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There are two means by which harmonics can be reduced
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(1) Increasing pulse number
(2) Use of filters
INCREASING PULSE NUMBER:
From converter analysis it is clear that the converter can
be operated at higher pulse number preferably 12 pulse
to reduce the harmonics.
The pulse number can be increased from 6-108 but it
demands a more expensive transformer.
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USE OF FILTERS
A filter is a network that responds more vigorously at
some frequencies than others and reduces the amplitude
of one or more fixed frequency currents or voltage.
They normally consist of one or more LC tuned circuits
with a fairly high Q(40), and can comprise a high pass
filter.
DESIGN OF AC FILTERS
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AC filters are designed to reduce telephonic interferance. This
can be measured by one of the following performing indices.
HARMONIC DISTORTION:
m
IhZh
D
100
h  2 V1

Where,
Ih-harmonic current
Zh-harmonic impedance
V1- fundamental component of line to neutral
voltage
m- highest harmonic considered
DESIGN OF AC FILTERS
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TELEPHONE INFLUENCE FACTOR (TIF):

I

TIF 
h

1
2 2
Z h Fh 
V1
where,
F1  5hf1 ph
ph- C message weighing (this reflects the frequency
dependent sensitivity of human ear and has a max value at
frequency of 1000hz)
DESIGN OF AC FILTERS
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TELEPHONE HARMONIC FORM FACTOR (THFF):
2
THFF 


50
1
 En

K n Pn  100

 E ph

IT PRODUCT:
IT 
 I
h

1
2 2
Fh 
FILTER CONFIGURATIONS
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Types of AC Filters
The different types of AC filters used inHVDC systems
were as follows
1. Band pass filter
2. Single tuned filter
3. Double tuned filter
4. High pass Filter
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i. Second order filter
ii. C type filter
TYPES OF AC FILTERS
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Single Tuned Filter
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Double tuned filter
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It was to filter out the characteristic harmonics of single
frequency. It was used to filter out 11th and 13th harmonics in
a converter system.
It was used to filter out two discrete frequencies. It was also
used to eliminate11th and 13th harmonics.
Advantages
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1. Only one inductor was subjected to full line impulse voltage
2. Reduced power loss at the fundamental frequency
TYPES OF AC FILTERS
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High Pass Filter
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Second order High pass filter
It was used to filter out the higherfrequencies. Tuning of these
filters was not critical.
C type High pass filter
It was used to reduce power losses at the fundamental
frequency. Capacitor was connected in series with inductor
to provide low impedance path to the fundamental component
of current. A third harmonic filter was used to filter out the
non-characteristic harmonic of third order. All the filter
branches were capacitive at fundamental frequency and supply
reactive power.
DC FILTERS
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The DC voltage on the converter side contains both
characteristic and non-characteristic and noncharacteristic harmonics.
These harmonics result in current harmonics in the DC
line and produce noise in telephone circuits.
Effectiveness of DC filter is judged by one of the
following,
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Maximum TIF value on DC high voltage bus
Maximum included noise voltage (INV) in millivolts/km in a
parallel test line situated one km away from HVDC line
Max permissible noise to ground in Db on telephone lines
close to HVDC lines.
DC FILTERS
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The DC filter used maybe of single tuned or double
tuned type to filter out 6th or 12th harmonic voltages and
high pass filters are used to filter out higher order
harmonics.
DC filters are designed only to eliminate DC harmonic
voltages and there is no need for filters to supply reactive
VARs as in case of AC filters.
POWER LINE COMMUNICATION AND RI
NOISE
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HVDC converters produce high levels of electrical noise
whose frequencies range in the carrier frequency band
from 22Khz to 500 khz
They also generate radio interference (RI) noise of
frequencies in mega hertz
Power line communication and radio interference (PLCRI) filters are used to minimize the interference with
power line carrier communication system.
POWER LINE COMMUNICATION AND RI
NOISE
ACTIVE FILTERS
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The active filter used in DC filtering is usually a hybrid
active filter i.e an active filter in series with shunt passive
filter.
Here, a double tuned filter (for12th and 24th) is
connected in series with VSC-based active filter.
IGBT devices are used in VSC with PWM.
The transformer provides galvanic separation between
VSC and HVDC line, it helps in raising voltage to desired
level.
The control strategy of active filter is to inject harmonic
voltage of appropriate magnitude and phase angle to
cancel harmonic currents flowing in the line.
OTHER METHODS OF HARMONIC
ELIMINATION

(1) ELEMINATION BY MAGNETIC FLUX
COMPENSATION
OTHER METHODS OF HARMONIC
ELIMINATION
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(2) HARMONIC INJECTION
OTHER METHODS OF HARMONIC
ELIMINATION
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(3)DC RIPPLE INJECTION:
In this a triple frequency current wave is generated such
that the magnitude is a function of the magnitude of DC
currents and wave shape.
This current is injected into neutral of the main
transformer secondary and flows through the conducting
transformer winding.
The modified currents in the transformer phases will then
contain only 12 pulse related current harmonics