RGPV EX7102 UNITIII

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Prof MD Dutt HOD Ex Department SRCT ThuakhedaBhopal MP India
READING MATERIAL FOR B.E. STUDENTS
OF RGPV AFFILIATED ENGINEERING COLLEGES
BRANCH VII SEM ELECTRICALAND ELECTRONICS
SUBJECT EHV AC AND DC TRANSMISSION
Professor MD Dutt
Addl GeneralManager(Retd)
BHARAT HEAVY ELECTRICALS LIMITED
Professor(Ex) in EX Department
Bansal Institute of Science and Technology
Kokta Anand Nagar BHOPAL
Presently Head of The Department ( EX)
Shri Ram College OfTechnology
Thuakheda BHOPAL
Sub Code EX 7102 Subject EHV AC AND DC TRANSMISSION
UNIT III

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EX 7102
RG PV Syllabus
UNIT III EHV AC AND DC TRANSMISSION
Components of EHV DC system, convertor circuits rectifier and inverter valves.
Reactive power requirement, harmonic generation. Adverse effect , classification,
remedial measures to suppress , filters and ground return. Convertor faults and
protection, harmonics mis operation. Commutation failure, Multi terminal DC lines.
INDEX
S No Topic UNIT III Page
1 Components of EHV DC system, convertor circuits rectifier
and inverter valves
3- 6
2 Reactive power requirement, harmonic generation. 7-9
3 Adverse effect , classification, remedial measures to suppress ,
filters and ground return
9-12
4 Convertor faults and protection, harmonics mis operation 12-20
5 Commutation failure, Multi terminal DC lines.). 20-23

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COMPONENTS OF EHV DC SYSTEM, CONVERTORCIRCUITS
RECTIFIER AND INVERTOR VALVES
The conversion from AC to DC and vice versa is done in HVDC convertor stations bu
using three phase bridge converters. A converter has two types of circuits.
1) Main circuit through which high power flows, the main circuit comprises of
converter transformers, thyristor valves, bus bars, series reactors etc.
2) Control and protection circuit for firing or blocking the valves in desired
sequence, monitoring etc.
The main circuit of converters in HVDC terminal stations are made up of series
connected thyristor valves. A thyristor can be turned ‘ON’ when the anode is positive
with respectto the cathode and a gate signal is provided. Once the thyristor is turned
ON, it can be turned OFF only when the current through it goes to zero and there is
minimum commutation margin when the voltage across the thyristor is reverse biased
i.e. the anode is negative with respect to cathode. Now the current is blocked until a
control pulse is applied to the gate. When not conducting, the thyristor should be
capable of withstanding the forward or reverse bias voltage appearing at its cathode
and anode.
The convertor valves are connected between convertor transformer and DC switchyard.
Each thyristor valve is made up several thyristor connected in series. Individual
thyristor rating is about 1.5Kv, 600 to 4000 A. The configuration of rectifier and
inverter is identical.. The triggering pulse to thyristor gates are delayed by angle ‘α’
called the delay angle, w.r.t the instant of natural commutation i.e the instant when the
phase voltage across subsequently conducting valve exceeds that across the proceeding
valve. The delay angle is varied by means of control circuit which gives the triggering
pulses to the gates of thyristor in each arm of the bridge in a definite sequence. With
delay angle less than 90˚, the convertor acts as rectifier mode and the delay angle ‘α’
between 90˚ and 180˚ the convertor works as inverter .
CONVERTERCIRCUITS
The basic HVDC converter is the three phase, full wave bridge circuit as shown in
figure below. This circuit is also known As “ GRAETZ BRIDGE”. This is a six pulse
converter and the 12 pulse converter is composedoftwo such bridges in series
supplied from two different three phase transformers with voltage differing in phase by
30˚. The pulse number of a converter is defined as the number of pulsation ( cycle of
ripple) of direct voltage per cycle of alternating voltage. The configuration for a given
pulse number is selected in such a way that both the valves and converter transformer

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utilization is maximum. The GRAETZ BRIDG has been universally used for HVDC
converters as it provides better utilization of converter transformer and a lower voltage
across the valve when not conducting. The voltage across the valve when it is not
conducting is referred to as the peak inverse voltage and is an important factor that
determines the rating of valves.
THREE PHASE , FULL WAVE BRIDGE CIRCUIT (GRAETZ CIRCUITS)
The converter transformer has on load taps on the AC side for voltage control. The AC
side windings of the transformer are usually star connected with grounded neutral; the
valve side winding are delta connected or star connected with ungrounded neutral. The
purposeof analysis, the following assumptions are made:-
1) The AC system including the converter transformer, may be represented by an
ideal sourceof constant voltage and frequency in series with lossless inductance
representing mainly the transformer leakage reactance.
2) The direct current Id is constant and ripple free;( becauseof the large smoothing
reactor)
3) The valves have zero resistance when conducting and infinite resistance when not
conducting.
The equivalent circuit shown below is representation of the bridge circuit.
EQUIVALENT CIRCUIT FOR THREE PHASE FULL WAVE BRIDGE
CONVERTER
Let the instantaneous phase voltages be
ea = Em sin (ωt +150˚)
eb = Em sin (ωt +30˚)

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ea = Em sin (ωt - 90˚)
Where Em is the peak value of line neutral voltage
Then the line voltage
eac = ea-ec =√3 Em sin (ωt +120˚)
Similarly
eba = √3 Em sin (ωt)
ecb = √3 Em sin (ωt -120˚)
To ease the understanding of the operation of bridge converter, let us assume negligible
sourceinductance Lc=0 and no ignition delay
NEGLECTING SOURCEINDUCTANCE
As shown the cathodes of valves 1,3 and 5 and anodes of valves 2,4 and 6 are
connected together. The valves are numbered in the order of firing. When the phase
voltage of phase a is more positive than the voltages of the other two phase, valve 1
conducts. Thepotential of the cathode of the three valves are now equal to that of the
anode of valve 1. As the cathodes of valve 3 and 5 are at a higher potential than their
anodes, these valves do not conduct. Similarly valve 2 conducts when phase c voltage is
more than the other two phases.
WAVE FORMS OF VOLTAGES AND CURRENTS OF GRAETZ BRIDGE
CIRCUIT
(a) SOURCE LINE TO NEUTRAL AND LINE-LINE VOLTAGES
(b) VALVE CURRENTS AND PERIODS OF CONDUCTION
(c) Phase current Ia

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From the wave form we see that valve 1 conducts when ωt is between -120˚ and 0˚,
since ea is greater than eb and ec . Valve 2 conducts when ωt is between -60˚ and 60˚.
As ec is more negative than ea and eb during this period, this shows the period of
conduction of each valve, and the magnitude and duration of current in it. Since the
direct current Id is assumed constant, the current in each valve is Id when conducting
and zero when not conducting.
Valve switching sequence without ignition delay and overlap
Just before the period ωt= o, valves 1 and 2a re conducting, just after ωt =0, eb becomes
more positive than ea and valve 3 ignites, valve 1 is extinguished because its cathode is
now at higher potential than its anode. For the next 60˚, valves 2 and 3 conduct. At
ωt=60˚, ea is more negative than ec, causing valve 4 to ignite and valve 2 to extinguish
and the sequence continues. The period and the conducting valves is given in the table
below:-
SNo Period Conducting Valves
1 -60˚ to 0˚ 1 and 2
2 0˚ to 60˚ 2 and 3
3 60˚ to 120˚ 3 and 4
4 120˚ to 180˚ 4 and 5
5 180˚ to 240˚ 5 and 6
6 240˚ to 300˚ 6 and 1
REACTIVE POWER REQUIRMENT, HARMONIC GENERATION

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Both the active power P and reactive power Q are equally important for satisfactory
operation of HVDC converters. The rectifier as well as the inverter absorb the reactive
power. The reactive power is required for
a) Mainly due to the delayed current conduction in the converter.
b) Partly due to the reactance of the converter transformer.
Supplying leading reactive power is called reactive power compensation. AC system
supplies active power P as well as some reactive power Q to the convertor; but this not
enough. Hence additional compensation is provide on AC side of the converter by
means of AC filter capacitors, AC shunt capacitors, synchronous condensers or static
VAr source. Reactive power compensation has a significant influence on the costof
HVDC substation.
As we know CosΦ = cosα
Where cos Φ is the displacement factor or vector power factor and angle ‘Φ’ is the
angle by which the fundamental line current lags the line to neutral source voltage.
When the delay angle ‘α’ is zero, the fundamental component of current is in phase
with the line to neutral sourcevoltage, As the delay angle is increased the displacement
angle between the fundamental component of the current and the line to neutral voltage
increase and current lags behind the voltage. Thus, the converter which may be a
rectifier or an inverter draws reactive power from the AC system.
The reactive power compensation/ consumption of converters vary mainly with :-
1) The active power
2) The delay angle ‘α’ of rectifier and extinction angle ‘γ’ of the inverter.
3) AC bus voltage and DC pole voltage.
4) The commutating reactance of converters
5) Mode of operation of HVDC system (monopolar , bi polar)
Variation of reactive power with active power
The reactive power demand of converter varies between 20% to 60% of the active
power flow. At full load, the reactive power absorbed bythe converter is about
60% of the active power flow for the normally used values of transformer reactance,

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delay angle and extinction angle . In practice reactive power is controlled in narrow
margin.
Generally AC filter capacitors are arranged in suitable switchable banks such that
the requirement of AC harmonics filters and reactive power compensation on AC
side are fulfilled by the AC filter banks under various conditions of power and
voltage of Ac and DC network condition.
The variation of reactive power Qd versus active power Pd is shown above in the
diagram.
In case higher compensation is required additional shunt capacitors are installed.
Synchronous condensers are use in special cases where AC bus needs
compensation of reactive power as well as additional short circuit level for
satisfactory converter operation and rotating inertia for improvement in dynamic
stability.
At the inverter terminal, the inverter end feeds power to AC load. So in addition to
reactive power required by inverter, the reactive power required by the AC load
should also be supplied. At the rectifier terminal the reactive power required by the
converter is supplied partly by filter capacitors and partly by AC network. Thus the
MVAr of filter capacitors at the rectifier end may be less than the MVAr of filter
capacitors at the inverter end.
The reactive power requirements of a converter can be reduced to zero or even
reversed if forced commutation is used. This also helps in avoiding commutation
failure sin inverters. Forced commutation is achieved by addition of a voltage
component to the normal commutation voltage to shift the zero crossing. This cam
be implemented easily by providing series capacitors. Forced commutation is
feasible if gate turn off ( GTO)or MOS controlled thyristors ( MCT) are usd in
converters. Or else , the auxiliary circuits required for forced commutation along
with increased rating of thyristor valves may be expensive as compared to the cost
of reactive power compensation devices required without forced commutation.
HARMONIC GENERATION
Any periodic non sinusoidal waveform can be resolved into a fundamental sine
wave of the same frequency and several other sinusoidal wave form of higher
frequency order known as Harmonics
Harmonic voltage and currents are integral multiple of fundamental frequency.
Therefore, any periodic non sinusoidal 50Hz current or voltage waveform is
considered to be the sum of basic 50Hz current or voltage and some combination of
harmonics.
Operation of thyristor converters in HVDC terminal substation generates several
current and voltage harmonic in AC and Dc system. These harmonics travel into AC
system and DC lines creating problems. Effect of each harmonic is analyzed
mathematically by applying the rules of circuit analysis of sinusoidal periodic
functions. The effect of original non sinusoidal waveform on the circuit is predicted

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by summing up the effect of componentsinusoidal waveform by using
superposition theorem.
Harmonics are eliminated by using AC shunt filters and DC shunt filters. AC
harmonic filters are provided on the bus bar of converter transformer. These AC
filters are R,L,C circuits connected between phase and earth. The value of L,C
parameters are so selected that the filter offers low impedance to harmonic
frequencies. Thereby harmonic currents are passed to earth and the harmonic
content in the AC network is minimized to acceptable level. The AC harmonic
filters provide reactive power required for satisfactory converter operation. The
requirement of AC filters and shunt compensation depends on the condition of AC
network. The AC harmonic filters constitutes a significant capital costof the
HVDC terminal substations. A large area in converter substation is covered by AC
filters.
DC filters are R,L,C shunt circuits connected between DC pole and neutral bus.
Smoothening reactor also helps in reducing DC current harmonics.
ADVERSE EFFECT S, CLASSIFICATION, REMIDIALMEASURES TO
SUPPRESS, FILTERS, GROUND RETURN
Harmonic caused by operation of HVDC converter travels into AC network and along
DC lines and have the following harmful effects:-
a) Harmonics cause heating of filter capacitors.
b) Harmonics cause additional heating of rotating machines such as synchronous
generators, synchronous motors, induction motors, power transformers etc, and
machines are to be derated .
c) Harmonics influence torque, speed characteristics of induction motors. The speed
as well as torque is reduced.
d) Harmonics of particular order may cause resonance in AC circuit and thereby
over voltage, flash over’s and insulation failures.
e) Harmonic causes noise in communication system.
f) Harmonic cause disturbance, in accuracy and instability in constant current
control system of HVDC converters.
g) Shunt capacitors and shunt reactors are heated due to harmonics.
h) DC harmonics cause telephone interference in adjacent telephone lines
i) DC harmonics cause disturbance in control system of HVDC converter.
The telephone interference TI is the most significant trouble caused by harmonics and
most difficult to overcome.
CLASSIFICATION
Operation of HVDC converter gives predominant current harmonics on AC side and
predominant voltage harmonics on DC side. AC current harmonics pass through AC
network impedance and producevoltage harmonics correspondingto the voltage drop

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in AC network impedance due to harmonic current. DC voltage harmonics travel along
HVDC line and generate corresponding DC current harmonics due to voltage
harmonics and DC impedance.
The harmonics are of various order from 2 to 60 , harmonics are classified as:-
1) Characteristics harmonics
2) Non Characteristics harmonics.
Characteristics Harmonics :- Harmonics which can be predicted by mathematical
analysis and are generally predominant are called Characteristics Harmonics.
Characteristics harmonics are always present even under ideal operation i.e balance Ac
voltages, symmetric three phase network and equidistant pulses.
By mathematical analysis of the original AC and DC waveforms of HVDC converters
the following characteristics harmonics are obtained.
nac = pq ± 1 i
ndc = pq ii
q= integers 0,1,2,3
p= pulse number of convertor ( 6 or 12)
Harmonics given by equation i are called characteristics harmonics on AC side
Harmonics given by equation ii are called characteristics harmonics on DC side
These are harmonics of definite order. Necessary filters are provided for diminishing
these harmonics.
The order of characteristics harmonics depends on the pulse number of the converter.
From nac = pq ± 1
Pulse no p=6 pulse no p=12
q 0 1 2 3 0 1 2 3
1 5 11 17 nac 1 11 23 35
7 13 19 13 25 37
The amplitude of harmonic waveform goes on reducing as the order of the harmonic
increases. Hence harmonics of 60th order and above are usually too small and are
neglected . for calculation for designing of filters, they are considered only for
designing filters with reference to permissible limits of telephone interference.
NON CHARACTERISTICS HARMONICS
Several other non characteristics harmonics may be present in small portion. Non
characteristics harmonics are harmonics of other order than characteristics harmonics
and are less predominant than the characteristics harmonics. The actual waveform
contain various characteristics and non characteristics harmonics. Non characteristics
harmonics are generated due to :
a) Imbalance in the operation of the bridges.

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b) Firing angle errors
c) Unbalance and distortion in AC voltages
d) Unequal transformer leakage impedances.
The harmonics produced due to imbalance in the operation of bridge is termed as
residual harmonics. These are produced mainly because of the difference in the firing
angles in the two bridges of a 12 pulse converter. The unequal leakage reactance’s of
the two converter transformer feeding the two bridges also lead to residual harmonics.
The last three cause can lead to the generation of triplen and even harmonics.
However such non characteristics harmonics are usually of smaller magnitude and
therefore separate tuned filters are generally not provide to filter them.
REMIDIAL MEASURES TO SUPPRESS
The operation of thyristor converters in HVDC system produces several characteristics
and non characteristics harmonics in AC and Dc wave form. Harmonics have harmful
effects on AC generators, Induction motors , capacitors, transformers , supply circuit
etc. They cause in the AC and DC waveform within specified limits.
Harmonics are minimized by
1) The use of converter bridges of higher pulse number.
2) The use of DC smoothening reactor.
3) The use of Ac and DC harmonic filters.
There are basically two type of filters that are used in HVDC system. They are:-
a) Passive filters
b) Active filters
The passive filters have been used in HVDC system right from the early days and are
still used. Passive filters are basically LC circuits whose inductance and capacitance are
so selected that the complete LC circuit resonates at a particular frequency. At this
resonate frequency, the filter offers low impedance to the harmonics of that frequency
and high impedance to other frequency harmonics. Thus the filters used to pass and stop
certain frequencies.
In HVDC systems, AC shunt filters and DC shunt filters are used for suppressing
characteristics and non characteristics harmonics. Series filters are used only for power
line carrier frequency range (80to 500Khz). Normally series filters are not used as main
harmonic filters.
FILTERS
AC FILTERS

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In HVDC converter station AC filters are installed at AC yard side. These filters
comprising of R,L,C banks connected in shunt i.e between line and ground at the AC
bus bar. Harmonics have to be filtered out sufficiently at the terminal so that the
harmonics entering the AC system are small and distortion caused by harmonic
currents is within limits. The penetration of harmonic into the AC system and resonance
conditions depends on the harmonic impedance of AC network. The harmonic
impedance is constantly varying as the circuit are being added or switched out and also
the system operating conditions are varying. In filter design elaborate studies are to be
undertaken considering various factors including the possible system resonance.
An LC circuit resonates at a frequency given by
f 0 = 1
2π√LC
At this frequency, the filter offers low impedance to harmonic of f 0 frequency and high
impedance to other frequencies. A typical AC filter has several parallel branches tuned
for specific frequencies such as 7th 9th 11th 13th etc. The respective tuned branches offer
low impedance to respective harmonic frequency currents order and conducts them to
earth. Hence, the harmonic contents in the network are reduced.
The impedance of AC network changes with load. The Ac filter requirements and the
AC shunt compensation requirement is affected by the equivalent impedance of the AC
network. Hence the Ac filter requirement changes with the power transfer through the
DC link.
AC filters are grouped in separately switchable groups, each having certain tuned
branches and a high pass branch.
To adjust the L and C parameters of a filter to some specific resonant frequency to
achieve desired harmonic suppressionis know as tuning of a filter. Tuning signifies the
sharpness of a filter. The sharpness of tuning is normally expressed in terms of Q factor
of an inductor or a capacitor or a circuit.
The Q factor = 2π[Maximum energy stored ]
Energy dissipated per cycle
In a complete cycle of sinusoidal AC wave, the maximum energy stored in an inductor
is ½ LIm².
Similarly average power of an inductor with resistance R is I²R. thus energy dissipated
per cycle,
= { Im }² R
√2
f
Where ‘f’ is the frequency, Thus the Q factor of an inductor

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= 2π ½ LIm² = 2πfL
( Im/√2 )²R R
f
The Q factor for an inductor = ωL/R
For a capacitor with capacitance ‘C’ and resistance ‘R’
The maximum energy stored
= ½ CVm²
= 1 Im²
2 ω²C
And the energy dissipated per cycle { Im }² R
√2
F
The Q factor of a capacitor = 1
ω0 CR
For a parallel RLC circuit resonance, the factor = ω0 CR
At the resonant frequency ω1 the current is maximum ω2 are the corresponding
frequencies when the value of current is 0.707Im. The power delivered to the circuit at
ω1and ω2 frequencies is( 0.707Im)² times i.e half of the maximum power Im²R.
The distance between ω1 and ω2 measured in Hz is called the band width of the filter.
The Q factor or quality factor can be expressed in terms of the bandwidth as
Qr = ω0 = f0
ω2 - ω1 f2 - f 1
The AC shunt filters thus
1) Divert harmonics to ground and reduce the harmonic contents in the main AC
network.
2) Provide reactive power compensation required on the AC side. Part of the
reactive power required by converter is supplied by AC shunt filters and the
remaining requirements is provided by shunt capacitor banks.
CHARACTERISTICSOFSERIES RLC FILTER
The design criteria for AC harmonic filters include

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1) To achieve the required reduction in the harmonic contents in the AC network
according to the specifications and statutory rules.
2) To reduce telephone interference.
3) To supply the required reactive power or a major part of it.
4) Minimum cost.
TYPES OF AC FILTERS
The following are the various type of AC filters that can be used:-
a) Single tuned filter
b) Double tuned filter
c) High pass filter
a) Single tuned filter, b) Double tuned filter c)Second orderhigh pass d) High pass
’C’ type
IMPEDANCE CHARACTERISTICS AS A FUNCTION OF FREQUENCY
a) Single tuned b) Double tuned c) High pass filter
DC FILTERS
The harmonics in the DC voltage contain both characteristics and non characteristics
harmonics. These harmonics result in the current harmonic in DC lines and cause
telephone interference. The harmonic current varies with the distance from the
converter station.

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DC filters are used to reduce DC harmonics to minimize the telephone interference. The
choice of DC filters also affect the overvoltage due to DC line resonance and line fault.
Therefore, in addition to the telephone interference criteria, the DC filters are designed
to avoid the resonance between DC filters and the DC line at lower order harmonics.
The design aspectof the DC filter is similar to the AC filter except that the reactive
power of DC filter is not significant. The rating of DC filter capacitors is determined by
maximum DC voltage, required capacitance and thermal rating due to harmonic current.
The size or MVAr rating of DC filter is much smaller than that of AC filter. This is
because the smoothening reactor smoothens the ripples in DC current or reduces the DC
harmonic current substantially. The various components in the configuration of Dc filter
branch include:-
1) Oil and air filled smoothening reactors connected in the pole bus.
2) Shunt tuned filter branches consisting of capacitors ,reactors and resistors.
3) High pass filter comprising of a capacitor and an inductor resistor combination
4) Surge capacitors connected in parallel between the pole bus and the neutral bus
and the earth.
GROUND RETURN
HVDC transmission lines use ground or sea water as the return conductoreither
continuously in the monopolar mode or for short time in case of emergency in bipolar
operation. These return paths are called ground return even when the sea water is used
as a return path ground electrodes are used at both ends of the HVDC link to transfer
current from the convener to the ground. There are three types of electrodes. Namely
1) Land electrodes
2) Shore electrodes located on a sea beach at a short distance from water line.
3) Sea electrodes located in the water at some distance from the coastline.
A monopolar line with ground return is more economical than a bipolar line due to the
elimination of the return conductor. Forthe same length of transmission, the resistance
offered by the ground in case of DC is much less as compared to CA transmission as
the DC spreads over a very large cross sectional area in both depth and width. In fact
the earth resistance in case of DC in independent of line lengths ( for long lines) and is
equal to the sum of the electrode resistance. As resistance in case DC is low, the power
losses are also reduced. Besides this, there are two more significant advantages of using
ground as the return, They are:-
a) A DC line can be built in two stages if the initial load requirement is low.
Initially the line will operate as a monopolar line with ground return and later on
it can be built as a bipolar line. Thus considerable part of the investment can be
deferred until the second stage.
b) The use of ground return improves the reliability / availability of the HVDC link.
In the event of an outage of one of the conductors ofa bipolar line, it can be
temporarily operated at almost half of its rated power by the use of the healthy

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line and the ground. Thus, the reliability of a bipolar line is equal to that of
double circuits 3 phase line.
CONVERTORFAULTS & PROTECTION, HARMONIC MISOPERATION
The Faults in a DC system are caused by
1) The malfunctioning of the equipment and controllers
2) The failure of insulation caused by external sources such as lightning pollution
In HVDC transmission system, the faults and abnormal conditions can occurin the
transmission line, converter, DC yard, AC yard, auxiliaries etc. The faults have to be
detected and the system has to be protected so that the disruption in the power
transmission is minimized.
For satisfactory operation of the converters, the valves of the converter bridge should
fire in a definite sequence and conductin a definite direction.
There are three basic types of faults that can occur in converters and given below:-
a) Faults due to malfunction of valves and controllers
1) Arc backs or back fire
2) Arc through or fire through
3) Misfire
4) Quenching or current extinction
b) Commutation failure in inverters.
c) Short Circuits
ARC BACK
The arc back is the failure of the valve to block in reverse direction and results in
temporary destruction of the rectifying property of the valve due to conductionin the
reverse direction. This is a major fault in mercury arc valves and is of random nature.
This is a non self clearing fault and results in severe stresses on transformer windings.
Arc back is more common during rectification than during inversion. Thyristor have the
advantage that they do not suffer from arc back.
ARC THROUGH
A malfunction in the gate pulse generator or the arrival of a spurious pulse can fire a
valve which is not supposed to conduct, butis forward biased. For example, in a bridge,
when valve 1 is successfully commuted its current to valve 3, the initial voltage across
it is negative for the duration of the extinction angle and then becomepositive. If valve
1 is fired at this time, the current will transfer back to valve 1 from valve 3. This is
known as ARC THROUGH. The arc through fault is likely to occurmainly at the
inverter end, as the valve voltages at the inverter is positive most of the time when they
are not in conducting mode. The effects of arc through are similar to that of
commutation failure- the voltage across the bridge falls as valve 4 is fired ( with valve 1
conducting) and the AC current goes to zero when valve 2 current goes to zero. The
firing of valve 5 is unsuccessfuland the bridge recovers to normal operation after valve
6 is fired and subsequentfirings are according to the normal sequence. Thus a single arc
through in thyristor valves can occurdue to malfunction in the control system, but

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probability is very small. The protection against persistent arc through is also through
the converter differential protection scheme.
MISFIRE
While an “arc through” is caused by the presence of unwanted gate pulse, misfire
occurs when the required gate pulse is missing and the incoming valve is unable to fire.
A misfire can occurin rectifier or inverter stations, but the effects are more severe at the
inverter station. This is due to the fact that in inverters, persistent misfire leads to the
average bridge voltage going to zero, while an AC voltage is injected into the link. This
results in large current and voltage oscillations in the DC link as it presents a lightly
damped oscillatory circuit viewed from the converter. The DC current may even
extinguish and result in large over voltage across the valves. The chances of misfire are
very small in modern converter stations because of duplicated converter controls,
monitoring and protective firing.
The effects of a single misfire are similar to those of commutation failure and arc
through. However, at the end of the cycle, the normal sequence of valve firings is
restored. Thus a single misfire is also self clearing.
CURRENT EXTINCTION
The extinction of current in a valve can occurif the current through it falls below the
holding current. This can arise at low values of the bridge currents when any transient
can lead to current extinction. The current extinction can result in over voltages across
the valve due to current chopping in the oscillatory circuit formed by the smoothing
reactor and the DC line capacitance.
The problem of current extinction is more severe in the case of short pulse firing
method. However, in modern converter stations, the return pulses coming from thyristor
levels to the valve group control, indicate the build up of voltage across the thyristor
and initiate fresh firing pulses. It may happen that a number of firing pulses may be
generated during a cycle when the link current is low.
PROTECTIONAGAINST OVERCURRENT
The over current protection in convertors is based on principle similar to those used in
AC systems. The factors considered in designing a protection system are:-
1) Selectivity
2) Sensitivity
3) Reliability
4) Back up
The main feature of converter protection is that it is possible to clear faults by fast
controller action (in less than 20ms) by blocking gate pulses or current regulation and
control. The selectivity is enhanced by high impedances of the smoothing reactor and
the converter transformer. The converters are divided into independent valve groups
such that the protection system must be able to switch off only the affected valve group.

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Consider a converter station of a 12 pulse converter ( 2 valve group per pole). The
protection system used per pole is shown in the figure below
OVERCURRENT PROTECTIONIN A POLE
The protection against converter faults discussed earlier is provided by valve group
differential protection, which compares the current on the valve side of converter
transformer to the DC current measured on the line side of the smoothing reactor. The
differential protection is employed because of its selectivity and fast detection. The over
current protection used as back up. The level of over current required to trip must be set
higher than that of the valve group differential protection to avoid tripping with faults
outside the stations.
The pole differential protection is used to detect ground faults which may not be
otherwise detected.
The fault clearing action of these protection circuits is to block the valves and at the
same time trip the AC circuit breaker of the affected group or pole. The fast tripping
sequence is used for internal faults where there is danger of valve damage. This
involves increasing of the delay angle of the rectifier to about 150˚ combined with the
signal to trip the Ac circuit breaker. The pulses are blocked after 29ms. This allow the
inverter action at the rectifier station to try to reduce the current before the converter is
blocked.
The faults are classified as:-
a) Internal faults which cause high over currents but are very infrequent. The
thyrister surge current ratings must be chosen to withstand these over currents.
b) Line faults which cause over currents in the range of 2 to 3 p.u These are limited
by current control.
c) Commutation failures at the inverters may be quite frequent, However, the over
currents are small and limited by current control.
OVER VOLTAGE IN A CONVERTER SYSTEM
The over voltage in a converter station are caused by :-

19
a) Disturbances on the AC side
b) Disturbances on the DC side
c) Internal faults in the converter
The type of over voltage can be classified into three categories.
1) The switching over voltage ( with wave front of more than 100ms)
2) Temporary over voltage ( lasting few seconds)
3) Steep front over voltages ( with wave front times in the range of 0.3 -3 msec)
DISTURBANCE OF THE AC SIDE
The lightning strokes in the AC network cause steep-fronted high over voltages. These
are, however, reduced in magnitude and steepness by AC filters. After they pass
through the converter transformer, they appear only as highly damped switching surges
across the converter.
The initiation and clearing of the faults in the AC system result in switching surges and
temporary over voltages. The energization of a converter transformer can cause high
over voltage ( upto 1.6p.u) due to the inrush magnetizing currents that last up to
100cycles. The voltage is also distorted due to even harmonics. This type of temporary
over voltage can cause severe stresses on the surge arrestors due to excessive energy
dissipation. Pre insertion resistors in the circuit breakers are very effective in
controlling these over voltages.
The temporary over voltages due to load rejection can be quite serous for converter
stations connected to weak AC systems. The load rejection may be caused by blocking
of the converters in responseto the action of the protection system.
DISTURBANCE OF THE DC SIDE
The steep wave front surges in DC overhead lines are produced bylightning strokes.
However, when they reach the converter through the smooting reactor they appear as
switching surges.
The switching surges at the converter are also caused by ground faults on a pole of
bipolar DC link. Because of the capacitive and inductive coupling between conductors,
the surges also occurat the healthy pole. The magnitude and the wave shape of these
surges arriving at the terminal are dependent on the type of termination; inductive
capacitive or resistive, The rate of decrease of voltage at the terminal is a variable and is
utilized in line fault detection.
The over voltage can also arise from the oscillation of current and voltage in the line
caused by suddenjumps in the converter voltage due to commutation failure and other
converter faults or injection of AC voltages of fundamental frequency and second
harmonic.
The switching of DC filter, parallel connection of poles can cause transient currents and
over voltages which will stress the neutral bus and filter reactors.
OVER VOLTAGE CAUSED BY CONVERTER DISTURBANCES

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The series connection on thyristors and the spread in delay times of thyristor turn on
result over voltages across the device during turn on. These over voltages are repetitive
and have to be taken into account in the valve design and the choice of snubbercircuit
parameters. The commutation over shootalso result in repetitive over voltages.
Transient over voltages of very steep front may result from internal converter faults,
such as a ground fault at the valve side of the smoothing reactor. The firing of bypass
pairs or closing of bypass switch across one converter generates over voltages across
the remaining converters.
The energization of the DC line from the rectifier side with the remote terminal blocked
can cause high over voltages at the inverter which is open ended. Such things should be
avoided by deblocking the inverter and limiting the rate of decrease of delay angle.
PROTECTIONAGAINST OVER VOLTAGES
The basic principles of over voltages protection are given below:-
1) The over voltage stresses in equipment must be limited by providing surge
arresters. The protection level of the surge arresters must be lower than the
breakdown voltages of the insulation.
2) Self restoring insulation such as air may be allowed to breakdown where there is
no danger to the safety of the personnel.
3) The operation of surge arresters must not be frequent. Hence the protective level
of arresters must be higher than the maximum operating voltages in the system.
4) There must be proper coordination of the insulation and over voltage protection
in different parts of the system.
The over voltages generated on the CA side should be limited by the arresters on the Ac
side. The over voltages generated on the DC side must be limited by Dc line. DC bus
and neutral bus arresters. The critical components suchas valves are directly protected
by arresters connected close to the components.
HARMONIC MIS OPERATION
Due to harmonic present in the system following operational problems are encountered
LOSSES AND HEATING DUE TO HARMONICS
Harmonic caused by HVDC converters travel in connected AC network and cause
additional losses and temperature rise in the synchronous generators, synchronous
motors, induction motors, power transformers etc. Harmonics have higher frequency
order and therefore cause additional hysteresis losses, eddy current losses.
Hysteresis loss is directly proportional to the frequency. Eddy current loss is
proportional to the square of frequency. The stray fluxes ( leakage fluxes) are also
increased with increased harmonics. The skin effect increases with increased frequency.
The total iron loss with harmonics is much higher than total iron loss for the
fundamental frequency. Higher iron losses cause more loss and more heating. With

21
higher harmonic content the motor , generator, transformer will have higher losses,
lesser efficiency and temperature rise with the same ampere load.
TORQUEAND SPEED OF INDUCTION MOTOR
Harmonic cause harmonic fluxes in air gap of induction machines. These rotate either
positive or negative sequence given by
n=6q± 1 ( as per expression of a 6 pulse converter)
where n is the order of harmonic and q is integers 1,2 etc
With +ve sign of 1, the flux is of +ve sequence i.e in the direction of rotating
fundamental magnetic field. With-ve sign of 1 the flux is of –ve sequence i.e opposite
to the rotating magnetic field of fundamental frequency.
Hence for harmonics 5,11,17 etc negative sequence magnetic flux is produced in air gap
resulting into reduced torque and speed.
COMMUTATION FAILURE
Failure to complete commutation before the commutating voltage reverses is known as
commutation failure.
The extinction angle is given by γ = 180 – α - µ
Because of the turn off time requirements of thyristor there is a need to maintain a
minimum value of extinction angle γmin. The overlap angle µ is a function of the
commutation voltage and the DC current. The reduction in the voltage or increase in the
current or both can result in an increase in the overlap angle which can result in γ<γmin.
This gives rise to commutation failure. The current in the incoming valve ( say valve 3)
will diminish to zero and the outgoing valve ( valve 1) will be left carrying the full link
current.
The firing of the next valve in sequence ( valve4) will result in the short circuit of the
bridge as bothvalves in the same arm will conduct. If commutation from valve 2 to
valve 4 is successful, only valves 1 and 4 are left conducting and this state continues
until the valve 6 is fired. The firing of valve 5 prior to the firing of valve 6 is
unsuccessfulas the valve 5 is reverse biased at the time of firing.
If the commutation from valve 4 to valve 6 is successful, the conduction pattern returns
to normal except that the bridge voltage is negative at the instant where valve 4 stops
conduction. If the causes which led to commutation failure in valve 1 in the first
instance have disappeared, the bridge operation returns to the normal state. Thus, single
commutation failure is self clearing.
The failure of two successivecommutations in the same cycle, is called ‘ double
commutation failure’ . If the commutation failure occurs when valve 4 is fired also, the
valves 1 and 2 are left in conducting state until the instant in the next cycle when valve
3 will be fired. The double commutation failure is more severe than the single
commutation failure. Double commutation failure is very rare but like the single
commutation failure, it is self clearing.
The following are the effects of a single commutation

22
1) The bridge voltage remains zero for a period exceeding 1/3rd of c cycle, during
which the Dc current tends to increase.
2) There is no Ac current for the period in which the two valves in an arm are left
conducting.
The recovery from a commutation failure depends on
a) The responseof the gamma controller at the inverter.
b) The current control in the link
c) The magnitude of the Ac voltage
On detection of a commutation failure, if the angle of advantage β is increased,
subsequent commutation failures may be averted. However, it depends upon the control
of DC current and the magnitude of AC voltage. The initial rate of rise of current in the
inverter is limited by the smoothing reactor and the current controller at the rectifier
helps to limit the current in the case of persistent commutation failure. It may be
necessary to reduce the current reference to limit the overlap angle in the case of low
voltages caused by faults in the Ac system.
Commutation failures are self clearing although in the caseof persistent commutation
failures, the converter differential protection removes the converter out of service.
During commutation failures when the two valves in an arm of bridge are left
conducting, the CA current goes to zero while the DC current continues to flow.
The commutation failure in a bridge can lead to consequential commutation failures in
the series connected bridge.
MULTITERMINAL DC LINES
A multiterminal Dc (MTDC) transmission system consists of three or more HVDC
terminal substations and interconnecting HVDC transmission line. MTDC systems are
more attractive and may fully exploit the economic and technical advantages of HVDC
technology.
The MTDC system interconnects three or more AC systems through HVDC lines.
Some terminal stations of the MTDC act in rectifier mode and transfer power from AC
system to the DC system where as after terminal operate in inverter mode and transfer
power from DC system to Ac system. The total power taken from rectifier substation
should be equal to the total power supplied by the inverter substation if the losses are
neglected.
MTDC systems with parallel connected converters are now preferred over series
connected converters. MTDC system provide solution to the controlproblems of
interconnection of three or more large AC systems. By MTDC system few HVDC
interconnection of high capacity can provide asynchronous tie ups between three or
more large AC systems.
MTDC NETWORK CONFIGURATION
There are two possible schemes for a MTDC system:-
1) Series scheme with constant current

23
2) Parallel scheme with constant voltage
The series connected MTDC scheme is normally run with constant current setting set
by the master converter station. The master station determines its voltage and other
converter station have to vary the Dc voltage. Flexibility of the power transfer requires
a wide range of transformer tapping at the series station. Thus , a large tap changing
ratio, large reactive power requirement, varying DC voltage are the main disadvantages
of a series connected MTDC system. Parallel MTDC schemes are therefore widely used
and they provide practical solution to the operational problems.
In the parallel connected MTDC scheme the converters are connected in parallel
and operate at a common voltage. The connections can be either radial or mesh. The
currents of each converter are adjusted to share the power allocation.
PARALLEL CONNECTED MTDC SCHEME WITH RADIAL NETWORK
In this system some substation poles acts in rectifier mode and others in the inverter
mode. Fora parallel connected MTDC system
Sum of rectifier outputs = sum of inverter out put + sum of losses
As seen from the direction of power in the diagram, station 1 and 4 acts as rectifier
whereas station 2 and 3 acts as inverter. The currents Id1, Id2,Id3 and Id4 are adjusted to
get the required P1,P2,P3 and P4.
Parallel connected MTDC system offer minimum operational and controlproblems. As
compared to series connected scheme, it results in fewer line losses, easier control and
offer more flexibility for future extension. The majority of proposed applications of
MTDC have considered parallel configuration with radial type connection as the mesh
connections requires greater length of DC lines.
Practical applications of parallel connected MTDC system include.
1) Parallel Tap off
2) Three terminal Dc system
3) MTDC system for bulk power transfer
4) MTDC system for interconnection
The MTDC system interconnect three or more network by asynchronous DC tie.
MTDC system have technical and economical advantage over equivalent two terminal
HVDC systems. They provide greater flexibility in dispatching the power between three
or more AC systems. In large interconnected systems, MTDC systems can provide

24
precise and faster control over dispatching of power. The frequency oscillations in the
interconnected systems can be quickly damped out by MTDC system control. The
inherent overload capability of MTDC systems can increase the transient stability
limits.

RGPV EX7102 UNITIII

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RGPV EX7102 UNITIII