rgpv ex7102 unitiv

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READING MATERIAL FOR B.E. STUDENTS

OF RGPV AFFILIATED ENGINEERING COLLEGES

BRANCH VII SEM ELECTRICAL AND ELECTRONICS

SUBJECT EHV AC AND DC TRANSMISSION

Professor MD Dutt Addl General Manager (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 Of Technology

Thuakheda BHOPAL

Sub Code EX 7102 Subject EHV AC AND DC TRANSMISSION

UNIT IV

Prof MD Dutt HOD Ex Department SRCT Thuakheda Bhopal MP India

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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. Control characteristics, 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 Page1 Control of EHV DC system desired feature of control 3- 52 Control characteristics, Constant current control 5-93 constant extinction angle control 9-124 Ignition angle control 12-205 Parallel operation of HVAC and DC systems 20-236 Problems and Advantages


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CONTROL OF EHV DC SYSTEM DESIRED FEATURE The control of power in an HVDC link can be achieved through the control of 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 terminalPdi = 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 control of 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 control action is initially used for rapid result, followed by the tap changing to restore the converter Prof MD Dutt HOD Ex Department SRCT Thuakheda Bhopal MP India

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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. For example, 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 control of converter angles is not feasible. Therefore, direct and fast control of direct current by varying αr and or γi in response to 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 space for increasing the rectifier voltage to control the 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 cost of 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.

CONTROLCHARACTERISTICS CONSTANT CURRENT CONTROLThe 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 TypeSegment ab Segment hg Minimum α (CEA)Segment bc Segment gf Constant CurrentSegment 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 control and the station II is operating with constant extinction angle control.


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CONVERTER CONTROL CHARACTERISTICS

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 Prof MD Dutt HOD Ex Department SRCT Thuakheda Bhopal MP India

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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 control and 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.

CONTROL CHARACTERISTICS 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 proper current 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 source i.e constant current control and 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 RECTIFIERProf MD Dutt HOD Ex Department SRCT Thuakheda Bhopal MP India

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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 CHARACTERISTICS AT 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 CHARACTERISTICSInverter is normally operating as alpha max or constant commutation 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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CONTROL OF MTDC SYSTEM The basic control principle for MTDC system is generalization of the control principle for a two terminal (point to point) system. The control characteristics for each converter is composed of segments representing constant current control and constant firing 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.

CONTROL OF SERIES CONNECTED MTDC SYSTEM

CONVERTER CHARACTERISTICS OF SERIES MTDCSYSTEM

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) control or 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. For series 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 control and backing off one bridge against another.

CONTROL OF PARALLEL CONNECTED MTDC SYSTEMIn 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 CONTROL CHARACTERISTICS 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 control settings. 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 control or 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 control operation, 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 Prof MD Dutt HOD Ex Department SRCT Thuakheda Bhopal MP India

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unable to control the current at its terminals in the event of a system disturbance or load change.

FIRING ANGLE CONTROLThe converter firing control system establishes the firing instants for the converter valves so that the converter operates in the required mode of control: constant current 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 are1) 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 CONTROLThere are two ways of achieving individual Phase controla) Constant α controlb) 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 corresponds to α = 0 for that valve. The time delays are produced by independent delay circuits and are controlled by a common control voltage. 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 control voltage Vc. The zero crossing of 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 CONTROL ARRANGEMENT

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 control signal 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 control independent of the zero crossing of the AC voltages.

EQUIDISTANCE PULSE CONTROLThe use of firing angle control independent 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 CONTROL BASED CONSTANT CURRENT CONTROL SYSTEM.

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 by the 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 control firing 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 SYSTEMIn 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 control generates 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 source at 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 control unit also includes many monitoringAnd 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-processor based controls can be used. With the availability of high speed programmable controllers using bit slice architecture, converter control using micro- processor is now feasible. Micro processor based controllers are normally more reliable than analog controllers and therefore superseded them.

VALVE FIRING SCHEME

PARALLEL OPERATIONOF AC ANDDC TRANSMISSION LINE.

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 flow2) Constant power flow3) Constant angle between AC bus voltages


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PARALLEL OPERATION OF AC AND DC TRANSMISSION SYSTEM

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 sudden changes 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’ or by 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

transmissionii) 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 The permissible 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 TRANSMISSION LINEVarious 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 conductor bi polar 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 HVACxi) 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 corona losses are proportional to )f+25), f frequency.

The corona losses 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 produce a 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 cost several 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 costlier5. Voltage transformation is not easier in case of DC system.


rgpv ex7102 unitiv

Engineering