RGPV EX7102 UNITIV

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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 IV

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EX 7102
RG PV Syllabus
UNIT IV EHV AC AND DC TRANSMISSION
Control of EHV DC system desired feature of control. Controlcharacteristics, Constant
current control, constant extinction angle control, Ignition angle control. Parallel
operation of HVAC and DC systems , Problems and Advantages.
INDEX
S No Topic UNIT IV Page
1 Control of EHV DC system desired feature of control 3- 5
2 Control characteristics, Constant current control 5-9
3 constant extinction angle control 9-12
4 Ignition angle control 12-20
5 Parallel operation of HVAC and DC systems 20-23
6 Problems and Advantages

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CONTROLOF EHV DC SYSTEM DESIRED FEATURE
The controlof power in an HVDC link can be achieved through the controlof current
or voltage. It is important to maintain constant voltage in the HVDC link and thus the
current is adjusted to meet the required power. This strategy is also helpful in optimal
utilization of the insulation.
SCHEMATIC DIAGRAM OF AN HVDC LINK
The direct current flowing from the rectifier to the inverter
Id = Vdor Cosα –Vdoicos γ
Rcr +Rl-Rci
The power ate the rectifier terminal
Pdr = VdiId
And the power at the inverter terminal
Pdi = VdiId = Pdr - Id²Rl
EQUIVALENT CIRCUIT
Thus the direct voltage at any point on the HVDC line and the current and therefore the
power can be controlled by controlling the internal voltages Vdr Cosα andVdicos γ. This
is achieved by the control of ignition angle or controlof the AC voltage through tap
changer of converter transformer. The tap changing control is slow and requires 5 to 6
sec/step and is therefore used as a complementary control. Gate controlaction is
initially used for rapid result, followed by the tap changing to restore the converter

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quantities i.e delay angle ‘ α ‘ for rectifier and restoration of extinction angle ‘ γ ‘ for
inverter to their normal range.
As the line resistance Rl and converter resistance Rcr and Rci are small, therefore a small
change in Vdor or Vdoi causes a large change in direct current Id. Forexample, a 25%
change in voltage at the rectifier or the inverter can cause the direct current to vary by
as much as 100% . This means that although the delay angle ‘ αr ‘ and extinction angle
‘ γi ‘ are kept constant., the direct current can vary over a wide range for small changes
in the magnitude of alternating voltage at either end. Such variations are not acceptable
for satisfactory performance of the power system. As the voltage changes can be
sudden, manual controlof converter angles is not feasible. Therefore, direct and fast
control of direct current by varying αr and or γi in responseto a feedback signal is
essential. The rapid control over the current is also desirable from the viewpoint of
limiting the over currents in thyristor valves which have limited short term over load
capacity.
For a given power transfer, the direct voltage profile along the line should be close to
the rated value. This minimizes the direct current and hence the line losses. As
mentioned earlier it is desirable to control the current and regulate the voltage
simultaneously in the DC link. Under normal conditions it is desirable to have current
control at the rectifier end because:-
1) The increase in power is achieved by reducing delay angle αr , which improves
the power factor, at the rectifier end for higher loadings and reduce the reactive
power requirement.
2) The inverter can now be operated at minimum extinction angel γ thus reducing
the reactive power requirement at the inverter end also.
3) The operation with minimum extinction angle at the inverter end and current
control at the rectifier end gives better voltage regulation.
4) The currents during line faults are automatically limited.
The current control from the inverter end worsens the power factor at higher loadings as
the extinction angle has to be increased. Increase in extinction angle γ means higher
losses in the snubber circuit.
Therefore, to achieve higher power factor, the delay angle of rectifier α and extinction
angle of inverter γ should be kept as low as possible. However, the rectifier has a
minimum delay angle limit of about 5˚ so as to ensure adequate voltage across the valve
before firing. The positive voltage appearing across each thyrister before firing is used
to charge the supply circuit providing the firing pulse energy to the thyrister. Hence the
rectifier normally operates with delay angle having values within the range of 15˚ to 20˚
so as to leave some spacefor increasing the rectifier voltage to controlthe DC power
flow.
For inverters it is necessary to maintain a certain minimum extinction angle to avoid
commutation failure. It is to be ensured that the commutation is completed with
sufficient margin to allow de ionization before commutating voltage reverses at α= 180˚
or γ= 0˚. The extinction angle γ is the difference between advance angle β and overlap
angle µ . the overlap angle µ depends on direct current Id and the commutation voltage.

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There is a possibility of change in direct current and alternating voltage even after
commutation has begun, therefore sufficient commutation margin above the minimum
extinction angle must be maintained. The typical value of extinction angle with
acceptable commutation margin is 15˚ for a 50Hz system.
Thus it is economical to operate the inverter at constant extinction angle ( CEA) which
is slightly above the minimum required. This results in reduced costof inverter stations,
reduced converter losses and reactive power consumption. The main drawback of CEA
control is the negative resistance characteristics of converter which makes it difficult to
operate stably when the Ac system is weak i.e having low short circuit ratio (SCR).
During normal operation, the rectifier operates at constant current control(CCC)and the
inverter with CEA control. However, under conditions of reduced AC voltage at the
rectifier, it is necessary to shift the current control to the inverter end to avoid rundown
of the HVDC link when the rectifier control hits the minimum limit. Therefore the
inverter must have current control in addition to the CEA control. A smooth transition
from CEA control to CC control takes place whenever required.
The power reversal in the HVDC link is obtained by the reversal of Dc voltage. This is
done by increasing the delay angle at the converter station operating earlier as the
rectifier, while reducing the delay angle at the converter station operating initially as the
inverter. Hence it is essential to provide both CEA and Cc controls at both the converter
stations.
CONTROLCHARACTERISTICSCONSTANTCURRENT CONTROL
The characteristics of each converter station consists of three segments, constant
ignition angle (CIA) corresponding to minimum delay angle αmin, constant current and
constant extinction angle (CEA). According to figure three parts of each converter
station characteristics are
Converter Station I Converter Station II Type
Segment ab Segment hg Minimum α (CEA)
Segment bc Segment gf Constant Current
Segment cd Segment fe Minimum γ (CEA)
The intersection of two stations characteristics determine the mode of operation. As
shown in figure the two characteristics intersect at point A . This point A lies on the
constant current segment of the characteristics of station I where as it is in the constant
extinction angle segment of the characteristics of station II .hence converter station I is
operating with constant current controland the station II is operating with constant
extinction angle control.

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CONVERTER CONTROLCHARACTERISTICS
For the same direction of power flow, there can be three modes of operation of the
HVDC links. The ceiling voltage of the rectifier determines the point of intersection of
the two characteristics and hence the modes of operation. The three modes are :-
1) Mode 1 CC at the rectifier and CEA at the inverter (operating point A) This is the
normal mode of operation.
2) Mode 2 With a slight dip in the AC voltage, the point of intersection drifts to point
B which means now the rectifier is operating with minimum α (CIA) and CEA at
the inverter.
3) Mode 3 With still lower AC Voltage, the intersection drifts to point C. This means
that the rectifier is now operating with minimum α (CIA) and CC control at the
inverter end.
Normally, the characteristics ab has more negative slope than the characteristics fe for
similar values of Rcr and RCi. This is due to fact that the slope ab is due to the
combined resistance ( Rcr + Rl) while the slope fe is due to resistance Rci . However
for low short circuit ratio (SCR) at the inverter, the slope fe can be more negative. In
practice, the constant current Characteristics may not be truly vertical. It depends on the
current regulator. With a proportional plus integral current regulator, the CC
characteristics is quite vertical.
As stated earlier the CEA characteristics of the inverter intersects the rectifier CC
characteristics at point ‘A’ for normal voltage. However, the CEA characteristics does
not intersect the rectifier characteristic at a reduced voltage as seen in figure. Therefore,
a big reduction in rectifier voltage would cause the current and power to be reduced to
zero after a short time depending on the DC reactors. This would cause the system to
run down.
To avoid the above problem, the inverter is also provided with a current controller. The
current controller at the inverter is set at a lower value than the current setting for the
rectifier. The difference between the rectifier current order and the inverter current
order is called the current margin, denoted Im. The current margin is usually 10 to 15%
Of the rated current so as to ensure that the two constant current characteristics do not
cross each other due to errors in measurement etc. Under normal operating conditions,
represented by point A, the rectifier controls the direct current and the inverter direct

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voltage. For a reduced rectifier voltage, may be due to a nearby fault, the operating
point is shifted to point ‘C’. The inverter takes over the current controland the rectifier
the voltage. During this operating mode he roles of the rectifier and invertors are
reversed. The change from one mode to another is known as mode shift.
When the current reference of converter station II is larger than that of station I, the
current margin is negative. The operating point now shifts to point ‘A’. The current Id
is the same as before but the polarity of direct voltage has changed. The power flow is
reversed and the characteristics is shown below.
CONTROLCHARACTERISTICS FOR NEGATIVE MARGIN
During power reversal, converter station I acts as an inverter and operates with
minimum CEA control whereas station II operates with CC control.
Thus the maintenance of propercurrent margin is important and it requires adequate
telecommunication channel. To prevent inadvertent power reversal in the HVDC link it
is essential to prevent the inverter from trasition to rectifier operation. This can be done
easily by providing minimum limits of the delay angle of the inverter ( of about 100° to
110° )
CONSTANT CURRENT CHARACTERISTICS
Usually DC transmission controlling and co-operation between rectifier and inverter has
been explained based on Ud/Id characteristics as shown in figure Traditionally rectifier
controls the current and inverter operates with constant commutation margin under
normal operation under steady state, typically rectifier would be act as constant current
sourcei.e constant current controland inverter will operate as counter voltage source i.e
constant extinction angle. The current order at the rectifier is determined by the
manipulation of power order and inverter DC voltage. To maintain stability at rectifier,
it is necessary to have less (Idref –Id) deviation in DC current and also
(γmeas –γref) deviations should be kept as low as possible for inverter stability. The
intersection of two modes give normal operation point .
ALPHA MINIMUM CHARATERISTICS AT RECTIFIER

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This charateristics is determined by the equation shown below,
Udc = Udro cosα – (dxN+drN)UdioN .Idc
IdcN
The above equation determines the dc voltage across the converter. If we assume
practical minimum alpha of5 degrees in order to have certain voltage across the valve
before firing and transformer reactance ((dxN+drN)UdioN / IdcN are also always constant.
Hence, increasing DC current reduces the DC voltage i.e negative slope determined by
the transformer reactance and DC current ( reduced voltage due to overlapping of valve
currents).
CONSTANT CURRENT CHARACTERISTICSAT RECTIFIER
The characteristics could also be explained by the same equation above , by assuming
current as constant and alpha as variable. It can be seen from figure that higher DC
voltage at minimum alpha and increasing of alpha decreases the DC voltage. The direct
current is determined based on current order, which could be selected between
minimum current capability and the rated current of valves. The maximum current
carrying capacity of valves would be determined for a transient time period to limit
valve stress.

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FIG 17 and FIG 18 from notes
CONSTANT EXTINCTION ANGLE CHARACTERISTICS
Inverter is normally operating as alpha max or constantcommutation margin mode in
order to have certain extinction angle to commutate the valves without fail. Under
normal operation, inverter operates at γ =17 at 50hz, it is not recommended to increase
or decrease to limit reactive power consumption and avoid commutation failure. At
steady state, inverter operates normally as constant DC voltage control mode. Assuming
gamma constant and Idc as variable gives negative slope characteristics. This slope
would be even more negative if the AC system is weaker.
Udc = Udio cosγ– (dxN - drN)UdioN .Idc
IdcN
ALPHA MINIMUM AT INVERTER
The power reversal could be obtained by increase the current order of the inverter
higher than rectifier. In case of DC line fault, it is recommended that both convertors
should operate as inverter to make the fault current in DC line to zero as fast as
possible. If there is no minimum alpha limit at inverter, it could also operate at rectifier
by reduced alpha cause feeding the DC fault. Therefore, always minimum alpha at the
inverter is limited to 110°. However, rectifier could be operating as inverter for reason
explained above. Also because of one more reason, inverter should have minimum
counter voltage to start current flow after the fault clearance .
Fig 19 from notes

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CONTROLOF MTDC SYSTEM
The basic control principle for MTDC system is generalization of the controlprinciple
for a two terminal (point to point) system. The controlcharacteristics for each converter
is composed ofsegments representing constant current control and constantfiring angle
control i.e CEA for inverter and CIA for rectifier. The converter characteristics along
with the DC network conditions establish the operating point of the system.
CONTROLOF SERIES CONNECTEDMTDC SYSTEM
CONVERTERCHARACTERISTICS OF SERIESMTDCSYSTEM
In a series connected MTDC system, current is controlled by one terminal station and
all other terminal station either operate at constant –angle ( α or γ) controlor regulate
voltage.fig above shows the control characteristics of a series connected MTDC system.

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The current control is assumed by the rectifier with lowest current order if he
sum of the rectifier voltage at the ordered current is greater than the sum of inverter
voltage. On the other hand, the inverter with higher current order assumes current
control when the sum of the inverter voltages is greater. Forseries systems, the voltage
reference must be balanced.
The operation of converters in series requires converter operation with high firing
angle. This can be minimized by tap changer controland backing off one bridge against
another.
CONTROLOF PARALLEL CONNECTED MTDC SYSTEM
In a parallel connected MTDC system, one of the terminal station establishes the
operating voltage of the HVDC system. All other terminal stations operate with
constant current control (CCC). Two or more rectifier and two or more inverters may be
connected in parallel. The V-I characteristics of a four terminal HVDC system in shown
in fig. It is assumed that two of the terminal stations are operating as rectifiers and the
other two terminals as inverters.
CONVERTER CONTROLCHARACTERISTICS OF PARALLEL CONNECTED
MTDCSYSTEM
In the parallel connected MTDC system, the sum of rectifier currents must exceed the
sum of inverter currents by value called current margin. All the converters will have
the current according to respective controlsettings. A selected terminal station known
as Voltage setting Terminal (VST) . or Master station establishes direct voltage profile
throughout the HVDC system. The voltage setting terminal is the one with smallest
ceiling voltage. This may be either a rectifier on CIA controlor an inverter on CEA
control. The DCvoltage of other terminal stations are equal to the voltage of VST plus
or minus the line voltage drops. It is assumed that rectifier1 is the VST on CIA mode.
To maintain stable controloperation, a positive current margin must be maintained.
When the VST is a rectifier, the operation is more stable. All the terminals operating as
inverters control current thereby avoiding the operation in the less stable CEA control
mode. The system is less dependent on communication and hence is more secure. On
the other hand, if an inverter is the VST, it is vulnerable to inadvertent overloading. It is

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unable to controlthe current at its terminals in the event of a system disturbance or load
change.
FIRING ANGLE CONTROL
The converter firing control system establishes the firing instants for the converter
valves so that the converter operates in the required mode of control: constantcurrent
CC and CEA controllers is closely linked with the method of generation of gate pulses
for the valves.
The two basic requirements for the firing pulse generation of HVDC valves are
1) The firing instant for all the valves are determined at ground potential and the
firing signals are sent to the individual thyristor by light signals through fiber
optic cable.
2) Although a single pulse is sufficient to turn on a thyrister, but the gate pulse
generator must send a pulse whenever required. If the particular valve is to be
kept in a conducting state. This is of particular importance when a thyrister is
operating at low DC currents and transients may reduce the current below the
holding current.
Two basic type of controls have been used for generation of converter firing pulses.
These schemes are as follows:-
a) Individual Phase Control (IPC)
b) Equidistant Pulse Control (EPC)
INDIVIDUAL PHASE CONTROL
There are two ways of achieving individual Phase control
a) Constant α control
b) Inverse cosine control
In the Constant α control, six timing commutation voltage are derived from the
converter AC bus through a voltage transformer. The six gate pulses are generated
at identical delay times subsequent to the respective voltage zero crossings. The
instant of zero crossing of a particular commutation voltage correspondsto α = 0 for
that valve. The time delays are produced byindependent delay circuits and are
controlled by a common controlvoltage. The control voltage ‘V’ is derived from
current controllers.
Figure below shoes a common arrangement for inverse cosine control. Six timing
voltage are obtained in the constant delay angle control. Each voltage is phase shifted
by 90° and is added separately to a common controlvoltage Vc. The zero crossingof
the sum of the two voltages initiate the firing pulse for a particular valve. The delay
angle α is proportional to the inverse cosine of the control voltage. The angle α is also
dependent on the amplitude and shape of the AC system voltage. Under steady – state
conditions, such a system controls each valve with constant commutation margin,
irrespective of the load, voltage variations and unbalance.

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INVERSE COSINE CONTROLARRANGEMENT
VOLTAGE WAVEFORM DURING FIRING
The individual phase control has the advantage of being able to achieve the highest
possible direct voltage under unsymmetrical or distorted supply conditions as the firing
instant of each valve is determined independently.
The major drawback of IPC system is the aggravation of harmonic stability problems,
particularly in power systems with low SCR. As the controlsignal is derived from the
alternating line voltage, any deviation from the ideal voltage wave forms will disturb
the symmetry of current wave forms. This in turn will cause additional wave forms
distortion and thereby introducing non characteristics harmonics. If AC system to
which the converter is connected is weak. The feedback effect may further distort the
alternating voltage and hence leads to harmonic instability.

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The problem of harmonic instability can be overcome by following measures:-
a) Altering the harmonic behavior of the AC network by using additional filters to
filter out non characteristics harmonic.
b) Use of filters in the control circuit to filter out non characteristics harmonics in
the commutation voltages. This method can be troublesome due to variation in
the supply frequency.
c) The use of firing angle controlindependent of the zero crossing of the AC
voltages.
EQUIDISTANCE PULSE CONTROL
The use of firing angle controlindependent of the zero crossings of the AC voltages is
the most attractive solution and leads to the equidistant pulse firing scheme . In this
scheme, valves are ignited at equal time intervals and the ignition angles of all valves
are retarded or advance equally so as to obtain the required control mode. There is only
indirect synchronization of the AC system voltage.
In the figure an equidistant pulse frequency control (EPC) based constant current
control system. This EPC firing scheme is based on pulse frequency control. The main
components of the system are a voltage – controlled oscillator (VCO) and a ring
counter. The VCO delivers pulses at a frequency directly proportional to the input
control voltage. The pulse train is fed to the ring counter which has six or twelve stage
depending on the pulse number of the converter. Only one stage of the ring counter in
cyclic manner.
BLOCK DIAGRAM OF EQUIDISTANT PULSE CONTROLBASED CONSTANT
CURRENT CONTROLSYSTEM.
As each stage is turned on, a short output pulse is produced once per cycle. Hence a
complete set of six or twelve output pulses is produced bythe counter at equal intervals
over a full cycle.

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Under steady state conditions, the output voltage of the control amplifier, (v2) is zero
and the voltage V1 is proportional to the 3 Phase AC line frequency ω. Thus pulses of
line frequency are generated and a constant firing delay angle ‘α’ is maintained. When
there is a change in current order I0, current margin Im or line frequency ω a change in
V3 occurs which in turn produces a change in the frequency of the firing pulses. A
change in firing delay angle results from the time integral of the differences between
line and firing pulse frequencies.
An alternative equidistant pulse controlfiring scheme is the pulse phase control in
which a steep change in control signal causes the spacing of only one pulse to change.
This results in a shift of phase only. These schemes provide equal pulse spacing in the
steady state.
The equidistant firing control gives a lower level of non-characteristics harmonics and
stable control performance when used with weak systems. Although EPC scheme has
replaced IPC scheme in all modern HVDC projects, it has its own limitations. The first
draw back is that under unbalanced voltage conditions, EPC results in less DC voltage
as compared to a system with IPC. Unbalance in the voltages results from single phase
to ground faults.EPC scheme also results in higher negative damping contributions to
torsional oscillations when HVDC is the major transmission link from a thermal station.
However, this problem is not so serious as the problem of non-characteristics harmonics
created in IPC.
FIRING SYSTEM
In modern converter, the valve firing and valve monitoring are achieved through an
optical interface. The basis valve firing scheme is shown in figure.
The valve controlgenerates firing signals. Light guides are used to carry the firing pulse
to each thyrister. Thus each thyrister level in independent, sharing only a duplicated
light sourceat the ground potential. At present, thyrister that that are triggered directly
by fibre optics, are being developed.
Each thyrister is provided with a special control unit that changes the light pulse to an
electrical pulse. The valve controlunit also includes many monitoring
And protective functions. Information about the condition of thyrister, required for
protection and monitoring of valves, is also transmitted by a light guide system for each
thyrister. The return pulse system coupled with short pulse firing scheme is used in
present day valve control units. A separate light guide is used to send a return pulse
whenever the voltage across the thyrister is sufficient and the power supply unit is
charged. During normal operation, only one set of light pulses are generated in a cycle
for each valve. However, during operation at low direct currents, many light pulses are
generated due to discontinues current.

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The implementation of the control can be achieved using analog or digital circuits. In
digital circuits, micro-processorbased controls can be used. With the availability of
high speed programmable controllers using bit slice architecture, converter control
using micro- processoris now feasible. Micro processorbased controllers are normally
more reliable than analog controllers and therefore superseded them.
VALVE FIRING SCHEME
PARALLEL OPERATIONOFAC ANDDC TRANSMISSIONLINE.
In a DC transmission line the power to be transmitted depends on the four parameters
Vr, Vi, α and β. These four parameters can be controlled nearly independently over the
desired range. By operating a DC transmission line in parallel with an AC transmission
system, we can achieve:-
1) Constant current flow
2) Constant power flow
3) Constant angle between AC bus voltages

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PARALLEL OPERATION OF AC AND DC TRANSMISSIONSYSTEM
Figure on earlier page shows parallel operation of AC and DC transmission line.
The figure below shows he power angle diagram for the AC line. The power
transmitted through an AC line is given by
P = Vs Vr sin δ
X
Where Vs and Vr are the voltages at the two ends of the line. X is the inductive
reactance of the line and ‘δ’ is the phase angle between Vs and Vr.
POWER ANGLE DIAGRAM FOR AC LINE
Normally AC transmission lines are operated at an angle δ of about 30°. The value of
30° allows a margin for additional power flow required to meet the transient

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fluctuations in the load or to meet suddenchanges in the system conditions arising
because of fault.
When an AC line is operated in parallel with a DC line, the AC line can be operated at a
much greater phase angle. The AC line can be operated with a phase angle value of
nearly 80°, thereby causing an increase in the power transmission capacity of nearly
95%. To increase the power transfer capacity the DC link should be controlled. The
control is achieved either by a signal proportional to the angle ‘δ’ orby measuring the
AC power flow. In either case the required signal should be proportional to the rate of
change of the controlling parameter as to obtain a stabilized power flow in the DC line.
During normal parallel operation of the AC and DC line, the power flow through the
DC line is kept small and delay angle ‘α’ of the rectifier is large. In an abnormal
condition such as a sudden increase in load or a fault, the power transfer through the
AC line reduces. During such abnormal situations, the power flow through the DC line
can be increased by quickly reducing the delay angle to a suitable value. Arrangement
for reversal of power in the DC line is also necessary to mitigate the sudden drop in the
sending end voltage, particularly when the voltage drop is due to faults in the CA
system.
PROBLEMS AND ADVANTAGES
EHC AC transmission line has the following inherent advantages:-
i) Voltage can be stepped up or stepped down in substation to have economical
transmission
ii) Parallel lines can be easily added.
iii) AC lines can be easily extended or tapped.
iv) Equipments are simple and reliable.
v) Operation of A system is simple and adopts naturally to the synchronously
operating AC system.
DISADVANTAGES of EHV AC transmission
Reactive losses While transferring power at a lagging power factor there will be drop
in voltage along the line. Where as if the reactive power is leading there is rise in
voltage. The reactive voltage drop or rise and the natural load do not put any restriction
on the distance over which the power may be transmitted. But to fix the voltage which
causes limitation in power transmission.
i) Stability Consideration:- The stable condition means the sending end and the
receiving end remains in synchronism with each other. If synchronism is lost the
system is called unstable. The stability limit is the maximum power flow without
losing synchronism.
P = Vs Vr sinδ

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X ( Vs = Sending end voltage, Vr= receiving end voltage, X= series
reactance, α =load angle)
ii) Current carrying capacity of conductors Thepermissible loading of an EHV AC
line is limited by transient stability limit and line reactance to almost one third of
thermal rating of conductors
iii) Ferranti Effect the rise of receiving end voltage for a lightly loaded line is
known as “Ferranti Effect”, Shunt reactors in the load end are generally used to
control this voltage rise.
iv) Number of lines: - A fault on any one phase of a 3phase AC trips all the
3phases. Hence an additional three phase line is always provided to maintain
continuity of power flow and transmission stability.
ADVANTAGES DC TRANSMISSIONLINE
Various advantages of HVDC transmission are:-
i) Cheaper in cost:- Bi polar DC transmission line requires two conductors
while AC system requires 3 wire.
ii) The potential stress is 1/√2 times less in case of DC system compare to AC
system of same operating voltage.
iii) The phase to phase and phase to ground clearance and tower size are smaller
in case of DC transmission.
iv) No skin effect:- There is no skin effect on DC transmission system.
v) Lower transmission losses:- As only two conductors are required in HVDC,
hence I²R losses are low for the same power transfer.
vi) Better voltage regulation, As there is no inductance hence voltage drop due to
inductance does not exists.
vii) Permissible loading on a EHV AC line is limited by transient stability. No
such limit exists in HVDC lines.
viii) Greater reliability A two conductorbipolar HVDC link is more reliable tha
3phase HVAC 3wire line.
ix) It is possible to generate power at one frequency and utilize it at some other
frequency.
x) Less dielectric power loss and higher current carrying capacity. Cable have
less dielectric loss with HVDC compare to HVAC
xi) Absence of charging current:- Due to absence of charging current in HVDC,
power can be transmitted to along distance by cables.
xii) Low short circuit current, In HVAC parallel lines results I larger short circuit
currents in the system.
xiii) Lesser corona loss:- the coronalosses are proportional to )f+25), f frequency.
The coronalosses are less in HVDC.
xiv) Lower switching surge level:- The level of switching surges due to DC is
lower compare to HVAC.
xv) Reactive power compensation :- HVDC does not require any reactive power
compensation, where as HVAC requires shunt or series compensation.

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DISADVANTAGES of HVDC
1. Costly Terminal equipments : The convertors required at both ends are more
expensive. The convertors have a very little over load capacity and absorb
considerable reactive power. The convertors producea lot of harmonics both on
Dc and AC side and may cause R.I. To remove ripples from the DC output,
filtering and smoothening equipments are to be provided. On AC side filters are
to be provided for absorbing the harmonics, and thus further increasing the cost
of convertor
2. HVDC circuit breakers comprises of circuit breaking capacitors, reactors etc,
which increase costseveral times than that of an AC circuit breaker.
3. More maintenance of insulators is required in HVDC system.
4. Circuit breaking in multi terminal DC system is difficult and costlier
5. Voltage transformation is not easier in case of DC system.

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