1

2Introduction1.1Transmission &Distribution

1.1 Development of Modern Power System - A Brief Historical Preview

The development of the modern day electrical energy system took a few centuries. Prior to 1800, scientists like William Gilbert, C. A. de Coulomb, Luigi Galvani, Benjamin Franklin, Alessandro Volta etc. worked on electric and magnetic field principles. However, none of them had any application in mind. They also probably did not realize that their work will lead to such an exciting engineering innovation. They were just motivated by the intellectual curiosity.

Between 1800 and 1810 commercial gas companies were formed - first in Europe and then in North America. Around the same time with the research efforts of scientists like Sir Humphrey Davy, Andre Ampere, George Ohm and Karl Gauss the exciting possibilities of the use of electrical energy started to dawn upon the scientific community.

In England, Michael Faraday worked on his induction principle between 1821 and 1831. The modern world owes a lot to this genius. Faraday subsequently used his induction principle to build a machine to generate voltage. Around the same time American engineer Joseph Henry also worked independently on the induction principle and applied his work on electromagnets and telegraphs.

For about three decades between 1840 and 1870 engineers like Charles Wheatstone, Alfred Varley, Siemens brothers Werner and Carl etc. built primitive generators using the induction principle. It was also observed around the same time that when current carrying carbon electrodes were drawn apart, brilliant electric arcs were formed. The commercialization of arc lighting took place in the decade of 1870s. The arc lamps were used in lighthouses and streets and rarely indoor due to high intensity of these lights. Gas was still used for domestic lighting. It was also used for street lighting in many cities.

From early 1800 it was noted that a current carrying conductor could be heated to the point of incandescent. Therefore the idea of using this principle was very tempting and attracted attention. However the incandescent materials burnt very quickly to be of any use. To prevent them from burning they were fitted inside either vacuum globes or globes filled with inert gas. In October 1879 Thomas Alva Edison lighted a glass bulb with a carbonized cotton thread filament in a vacuum enclosed space. This was the first electric bulb that glowed for 44 hours before burning out. Edison himself improved the design of the lamp later and also proposed a new generator design.

The Pearl Street power station in New York City was established in 1882 to sell electric energy for incandescent lighting. The system was direct current three-wire, 220/110 V and supplied Edison lamps for a total power requirement of 30 kW.

The only objective of the early power companies was illumination. However we can easily visualize that this would have resulted in the under utilization of resources. The lighting load peaks in the evening and by midnight it reduces drastically. It was then obvious to the power companies that an elaborate and expensive set up would lay idle for a major amount of time. This provided incentive enough to improve upon the design of electric motors to make them commercially viable. The motors became popular very quickly and were used in many applications. With this the electric energy era really and truly started.

However with the increase in load large voltage and unacceptable drops were experienced, especially at points that were located far away from the generating stations due to poor voltage regulation capabilities of the existing dc networks. One approach was to transmit power at higher voltages while consuming it at lower voltages. This led to the development of the alternating current.

In 1890s the newly formed Westinghouse Company experimented with the new form of electricity, the alternating current. This was called alternating current since the current changed direction in synchronism with the generator rotation. Westinghouse Company was lucky to have Serbian engineer Nicola Tesla with them. He not only invented polyphase induction motor but also conceived the entire polyphase electrical power system. He however had to face severe objection from Edison and his General Electric Company who were the proponents of dc. The ensuing battle between ac and dc was won by ac due to the following factors:

Transformers could boost ac voltage for transmission and could step it down for distribution.

The construction of ac generators was simpler.

The construction of ac motors was simpler. Moreover they were more robust and cheaper than the dc motors even though not very sophisticated.

With the advent of ac technology the electric power could reach more and more people. Also size of the generators started increasing and transmission level voltages started increasing. The modern day system contains hundreds of generators and thousands of buses and is a large interconnected network.

1.2 Introduction of Modern Power System

Modern electric power systems have three separate components - generation, transmission and distribution. Electric power is generated at the power generating stations by synchronous alternators that are usually driven either by steam or hydro turbines. Most of the power generation takes place at generating stations that may contain more than one such alternator-turbine combination. Depending upon the type of fuel used, the generating stations are categorized as thermal, hydro, nuclear etc. Many of these generating stations are remotely located. Hence the electric power generated at any such station has to be transmitted over a long distance to load centers that are usually cities or towns. This is called the power transmission. In fact power transmission towers and transmission lines are very common sights in rural areas.

Modern day power systems are complicated networks with hundreds of generating stations and load centers being interconnected through power transmission lines. Electric power is generated at a frequency of either 50 Hz or 60 Hz.

In an interconnected ac power system, the rated generation frequency of all units must be the same. In India the frequency is 50 Hz.

1.3 Need for EHV Transmission

To provide adequate grid system capacity, the electricity transmission lines need to operate at 400 kV or 765 kV. The existing transmission capacity is inadequate. So the developers are constructing to connect to EHV Transmission system.

EHV transmission provides more reliable and less constrained electricity network capacity.

(i)Increase in size of generating units

Volume Cost Volume

As voltage increases, Volume of conductor decreases and cost of the line decreases and the size of generating unit increases. Transmission of large amount of power over long distance is economically feasible for EHV transmission.

(ii)Increase in Transmission efficiency

As voltage increases, current flows through the line decreases and I2R loss reduces and transmission efficiency increases.

(ii)Pithead steam plants and remote hydro plants

Cost of transportation depends on cost of coal in thermal plants. Too avoid this, steam or thermal plants are situated near coal mines are called as Pithead steam plants.

Hydro plants are mostly situated at remoter places. In remoter places, Water availability is more, land and labour cost is cheap.EHV systems are needed to transmit large amounts of power over long distance as from pithead and remote hydro plants to load centers.

(iv)Number of circuits and land requirement

As voltage increases, number of circuits and land requirement for transmission decreases.

(v)Line Costs

As voltage increases, the line insulation cost/MW/Km decreases. The total line cost including the cost of losses/MW/Km decreases with increase in voltage.

(vi)Surge Impedance loading (SIL)

where ZC = surge impedance =

Surge Impedance loading (SIL) is proportional to the square of the operating voltage, so surge impedance loading increases as voltage increases.

Limitations of EHVAC Transmission

1. More insulation is required for the conductor and towers.

2. More clearance is required between the conductor and the ground.

3. More distance is required between the conductors. So the length of cross arms used increases.

4. The transformers, Switchgears and other terminal equipment should be designed to handle such high voltage.

5. Long bulk power transmission is not possible.

Advantages of EHVAC Transmission

Reduction in the cost of material for given power

Improvement of transmission efficiency

Reduction in percentage line drop

Improvement in voltage regulationEHVAC Systems in India

(i) 400 kV Line:

Dehar-panipat line

Obra-sultanpur line

sultanpur Lucknow line

Obra-Kanpur line

Kanpur Moradnagar line

Koradi-katwa line

Srinagar-Jammu-kashmir line

(ii) 765 kV Line:

Anpara-Unnao

Tehri-Meerut

Vindhyachal-Bina-NAgda

Kisanpur-Moga(operated at 400kV)

Monubulu-Sriperumbudur (operated at 400kV)

Agra-Gwalior

Pichor-Malanpur

1.4. History of HVDC transmission

An early method of high-voltage DC transmission was developed by the Swiss engineer Rene Thury. This system used series-connected motor-generator sets to increase voltage. Each set was insulated from ground and driven by insulated shafts from a prime mover. The line was operated in constant current mode, with up to 5000 volts on each machine, some machines having double commutators to reduce the voltage on each commutator. An early example of this system was installed in 1889 in Italy by the Society Acquedotto de Ferrari-Galliera. This system transmitted 630 kW at 14 kV DC over a distance of 120 km. Other Thury systems operating at up to 100 kV DC operated up until the 1930s, but the rotating machinery required high maintenance and had high energy loss. Various other electromechanical devices were tested during the first half of the 20th century with little commercial success

The grid controlled mercury arc valve became available for power transmission during the period 1920 to 1940. In 1941 a 60 MW, +/- 200 kV, 115 km buried cable link was designed for the city of Berlin using mercury arc valves (Elbe-Project), but owing to the collapse of the German government in 1945 the project was never completed[5]. The nominal justification for the project was that, during wartime, a buried cable would be less conspicuous as a bombing target. The equipment was moved to the Soviet Union and was put into service there.

Introduction of the fully-static mercury arc valve to commercial service in 1954 marked the beginning of the modern era of HVDC transmission. Mercury arc valves were common in systems designed up to 1975, but since then, HVDC systems use only solid-state devices.

Introducing Flexible AC Transmission Systems (FACTS) technology, high-speed controllable power electronics device will enable utilities to reduce transmission congestion and more fully utilize the existing transmission system without compromising the reliability and security of the system with the following potential benefits:

Increase the power transfer capability of existing transmission systems,

Directly control real and reactive power flow,

Provide fast dynamic reactive power support and voltage control,

Improve system stability and damp power system oscillations, and

Reduce financial costs and environmental impact by possible deferral of new transmission lines.

1.5 INDIA ADOPTS HVDC TRANSMISSION When the Talcher - Kolar, High Voltage Direct Current [HVDC] transmission was commissioned by Power Grid Corporation on Feb 14, the implication of the event may have been lost on many Indians.

For one, it signalled Indias arrival at the leading edge of world class, state of the art, electrical engineering technology. For another, it was a project completed nine months ahead of schedule by a public sector company. Finally,as an effective means of delivering generated power far away to places where it is needed, environmental gains are immense, as we will see.

In the second era of electricity revolution Alternating Current [AC] displaced Direct Current [DC] because its voltage can be stepped up and the lower current that results can be pumped through relatively thinner wires. At the receiving end the voltage can be stepped down to the 220 v we use. This claim for AC is still valid and used, but only over shorter distances typical in local networks.

HVDC comes into play if very high volumes of electricity need to be transmitted over distances above 800 km. In this very advanced technology AC is converted to DC and pumped into the lines. This may seem a convoluted, complicated way. It is indeed: very few countries can today master, install and manage HVDC systems. The advantages are lower line losses, slimmer hardware across the countryside, stable grid behavior, dispersed generation of power, and overall economy. Indias hydel riches are in the North East, coals in the East and consumers all over the land. Pristine locations can silently generate power and need not create polluting industries nearby as consumers. HVDC vacates massive quantum of power with ease to far away points.

India has been a pioneer developer of HVDC since 1990 when the 1000 mw Rihand - Dadri line was commissioned in UP. Since then many 500 mw lines have come up. The 2000 mw Talcher - Kolar link is the biggest so far and spans four states: Orissa, Andhra Pradesh, Tamil Nadu and Karnataka. The 5651 towers used are as high as the Kutb Minar. In all 100,00 metric tonnes of steel and 80,000 tonnes of cement were used. The project cost Rs.700 crores and was executed by Indians. As we already noted, we finished it ahead of time. These facts should give you a measure of the little-known developmental works of very high calibre that are going on in India right now. You should be justly proud of this achievement.

Greater plans are cooking. India is racing to a saturation point in electricity availability by 2012. 100,000 mw of power is planned to be added. HVDC technology will be waiting to ferry this power to all corners of India. And did you know that Mr. R V Shahi the Secretary in the Ministry of Power is not a bureaucrat but picked from the private sector?

Advantages of HVDC over AC transmission

In a number of applications the advantages of HVDC makes it the preferred option over AC transmission. Examples include:

Undersea cables, where high capacitance causes additional AC losses. (e.g. 250 km Baltic Cable between Sweden and Germany[7]).

Endpoint-to-endpoint long-haul bulk power transmission without intermediate 'taps', for example, in remote areas.

Increasing the capacity of an existing power grid in situations where additional wires are difficult or expensive to install.

Allowing power transmission between unsynchronized AC distribution systems.

Reducing the profile of wiring and pylons for a given power transmission capacity.

Connecting remote generating plant to the distribution grid, for example Nelson River Bipole.

Stabilizing a predominantly AC power-grid, without increasing maximum prospective short circuit current.

Reducing corona losses (due to higher voltage peaks) for HVAC transmission lines of similar power

Reducing line cost since HVDC transmission requires less conductor (i.e. 2 conductors one is positive another is negative)

Long undersea cables have a high capacitance. While this has minimal effect for DC transmission, the current required to charge and discharge the capacitance of the cable causes additional I2R power losses when the cable is carrying AC. In addition, AC power is lost to dielectric losses.

HVDC can carry more power per conductor, because for a given power rating the constant voltage in a DC line is lower than the peak voltage in an AC line. This voltage determines the insulation thickness and conductor spacing. This allows existing transmission line corridors to be used to carry more power into an area of high power consumption, which can lower costs.

Increased stability of power systems

Because HVDC allows power transmission between unsynchronized AC distribution systems, it can help increase system stability, by preventing cascading failures from propagating from one part of a wider power transmission grid to another, whilst still allowing power to be imported or exported in the event of smaller failures. This has caused many power system operators to contemplate wider use of HVDC technology for its stability benefits alone.

Possible health advantages of HVDC over AC transmission

A high-voltage DC transmission line would not produce the same sort of extremely low frequency (ELF) electromagnetic field as would an equivalent AC line. It is speculated by those who believe that ELF radiation is harmful that such a reduction in EM fields would be beneficial to health. The benefits would extend only to those near the transmission lines, as the electric and magnetic fields associated with high current AC transmission lines do not travel far beyond the actual lines themselves. These fields are, however, also associated with electrical equipment and household appliances. It should be noted that the current scientific consensusDisadvantages

The required static inverters are expensive and cannot be overloaded very much. At smaller transmission distances the losses in the static inverters may be bigger than in an AC power line, and the cost of the inverters may not be offset by reductions in line construction cost.

In contrast to AC systems, realizing multiterminal systems is complex, as is expanding existing schemes to multiterminal systems. Controlling power flow in a multiterminal DC system requires good communication between all the terminals; power flow must be actively regulated by the control system instead of by the inherent properties of the transmission line.

AC network interconnections

AC transmission lines can only interconnect synchronized AC networks that oscillate at the same frequency and in phase. Many areas that wish to share power have unsynchronized networks. The power grids of the UK, Northern Europe and continental Europe all operate at 50 Hz but are not synchronized. Japan has 50 Hz and 60 Hz networks. Continental North America, while operating at 60Hz throughout, is divided into regions which are unsynchronised: East, West, Texas and Quebec. Brazil and Paraguay, which share the massive Itaipu hydroelectric plant, operate on 60Hz and 50Hz respectively. However, HVDC systems make it possible to interconnect unsynchronized AC networks, and also add the possibility of controlling AC voltage and reactive power flow.

A generator connected to a long AC transmission line may become unstable and fall out of synchronization with a distant AC power system. An HVDC transmission link may make it economically feasible to use remote generation sites. Wind farms located off-shore may use HVDC systems to collect power from multiple unsynchronized generators for transmission to the shore by an underwater cable.

In general, however, an HVDC power line will interconnect two AC regions of the power-distribution grid. Machinery to convert between AC and DC power adds a considerable cost in power transmission. The conversion from AC to DC is known as rectification, and from DC to AC as inversion. Above a certain break-even distance (about 50 km for submarine cables, and perhaps 600-800 km for overhead cables), the lower cost of the HVDC electrical conductors outweighs the cost of the electronics.

The conversion electronics also present an opportunity to effectively manage the power grid by means of controlling the magnitude and direction of power flow. An additional advantage of the existence of HVDC links, therefore, is potential increased stability in the transmission grid.

1.6 RECTIFYING AND INVERTINGRectifying and inverting components

Early static systems used mercury arc rectifiers, which were unreliable. Nevertheless some HVDC systems using mercury arc rectifiers are still in service in 2005. The thyristor valve was first used in HVDC systems in the 1960s. The thyristor is a solid-state semiconductor device similar to the diode, but with an extra control terminal that is used to switch the device on at a particular instant during the AC cycle. The insulated-gate bipolar transistor (IGBT) is now also used and offers simpler control and reduced valve cost.

Because the voltages in HVDC systems, up to 800 kV in some cases, exceed the breakdown voltages of the semiconductor devices, HVDC converters are built using large numbers of semiconductors in series.

The low-voltage control circuits used to switch the thyristors on and off need to be isolated from the high voltages present on the transmission lines. This is usually done optically. In a hybrid control system, the low-voltage control electronics sends light pulses along optical fibres to the high-side control electronics. Another system, called direct light triggering, dispenses with the high-side electronics, instead using light pulses from the control electronics to switch light-triggered thyristors (LTTs).

A complete switching element is commonly referred to as a 'valve', irrespective of its construction.

Rectifying and inverting systems

Rectification and inversion use essentially the same machinery. Many substations are set up in such a way that they can act as both rectifiers and inverters. At the AC end a set of transformers, often three physically separate single-phase transformers, isolate the station from the AC supply, to provide a local earth, and to ensure the correct eventual DC voltage. The output of these transformers is then connected to a bridge rectifier formed by a number of valves. The basic configuration uses six valves, connecting each of the three phases to each of the DC rails. However, with a phase change only every sixty degrees, considerable harmonics remain on the DC rails.

An enhancement of this configuration uses 12 valves (often known as a twelve-pulse system). The AC is split into two separate three phase supplies before transformation. One of the sets of supplies is then configured to have a star (wye) secondary, the other a delta secondary, establishing a thirty degree phase difference between each of the sets of three phases. With twelve valves connecting each of the two sets of three phases to the two DC rails, there is a phase change every 30 degrees, and harmonics are considerably reduced.

In addition to the conversion transformers and valve-sets, various passive resistive and reactive components help filter harmonics out of the DC rails.

1.7 HVDC LINK - CONFIGURATIONSMonopole and earth return

In a common configuration, called monopole, one of the terminals of the rectifier is connected to earth ground. The other terminal, at a potential high above, or below, ground, is connected to a transmission line. The earthed terminal may or may not be connected to the corresponding connection at the inverting station by means of a second conductor.

If no metallic conductor is installed, current flows in the earth between the earth electrodes at the two stations. Therefore it is a type of Single wire earth return. The issues surrounding earth-return current include

Electrochemical corrosion of long buried metal objects such as pipelines

Underwater earth-return electrodes in seawater may produce chlorine or otherwise affect water chemistry.

An unbalanced current path may result in a net magnetic field, which can affect magnetic navigational compasses for ships passing over an underwater cable.

These effects can be eliminated with installation of a metallic return conductor between the two ends of the monopolar transmission line. Since one terminal of the converters is connected to earth, the return conductor need not be insulated for the full transmission voltage which makes it less costly than the high-voltage conductor. Use of a metallic return conductor is decided based on economic, technical and environmental factors. Modern monopolar systems for pure overhead lines carry typically 1500 MW. If underground or sea cables are used the typical value is 600 MW.

Most monopolar systems are designed for future bipolar expansion. If overhead power transmission lines are used, the used electricity pylons are often designed to carry two conductors and in many cases they do also. The second conductor is either unused, used as electrode line or permanently parallelized with the other (as in case of Baltic-Cable).

Bipolar

In bipolar transmission a pair of conductors is used, each at a high potential with respect to ground, in opposite polarity. Since these conductors must be insulated for the full voltage, transmission line cost is higher than a monopole with a return conductor. However, there are a number of advantages to bipolar transmission which can make it the attractive option.

Under normal load, negligible earth-current flows, as in the case of monopolar transmission with a metallic earth-return; minimising earth return loss and environmental effects.

When a fault develops in a line, with earth return electrodes installed at each end of the line, current can continue flow using the earth as a return path, operating in monopolar mode.

Since for a given power rating bipolar lines carry only half the current of monopolar lines, the cost of the second conductor is reduced compared to a monopolar line of the same rating.

In very adverse terrain, the second conductor may be carried on an independent set of transmission towers, so that some power may continue to be transmitted even if one line is damaged.

A bipolar system may also be installed with a metallic earth return conductor.

Bipolar systems may carry as much as 3000 MW at voltages of +/-533 kV. Submarine cable installations initially commissioned as a monopole may be upgraded with additional cables and operated as a bipole.

Back to back

A back-to-back station is a plant in which both static inverters are in the same area, usually even in the same building and the length of the direct current line is only a few meters. HVDC back-to-back stations are used for

coupling of electricity mains of different frequency (as in Japan)

coupling two networks of the same nominal frequency but no fixed phase relationship

different frequency and phase number (for example, as a replacement for traction current converter plants)

Different modes of operation (as until 1995/96 in Etzenricht, Drnrohr and Vienna).

The DC voltage in the intermediate circuit can be selected freely at HVDC back-to-back stations because of the short conductor length. The DC voltage is as low as possible, in order to build a small valve hall and to avoid parallel switching of valves. For this reason at HVDC back-to-back stations valves with the highest available current rating are used.

APPLICATIONS OF HVDC CONVERTERS

The first application for HVDC converters was to provide point to point electrical power interconnections between asynchronous a.c. power networks. There are other applications which can be met by HVDC converter transmission which include:

1. Interconnections between asynchronous systems. Some continental electric power systems consist of asynchronous networks such as the East, West, Texas and Quebec networks in North America and island loads such as the Island of Gotland in the Baltic Sea make good use of HVDC interconnections.

2. Deliver energy from remote energy sources where generation has been developed at remote sites of available energy, HVDC transmission has been an economical means to bring the electricity to load centers. Gas fired thermal generation can be located close to load centers and may delay development of isolated energy sources in the near term.

3. Import electric energy into congested load areas. In areas where new generation is impossible to bring into service to meet load growth or replace inefficient or decommissioned plant, underground d.c. cable transmission is a viable means to import electricity.

4. Increasing the capacity of existing a.c. transmission by conversion to d.c. transmission. New transmission rights-of-way may be impossible to obtain. Existing overhead a.c. transmission lines if upgraded to or overbuilt with d.c. transmission can substantially increase the power transfer capability on the existing right-of-way.

5. Power flow control. A.C. networks do not easily accommodate desired power flow control. Power marketers and system operators may require the power flow control

capability provided by HVDC transmission.

6. Stabilization of electric power networks. Some wide spread a.c. power system

networks operate at stability limits well below the thermal capacity of their transmission conductors. HVDC transmission is an option to consider to increase utilization of network conductors along with the various power electronic controllers which can be applied on a.c. transmission.First HVDC link in the world Swedish main Load

Voltage : 100 kV

Power : 20 MW, This is the world first D.C.Link.

HVDC lines in India.

1. Rihand-Delhi HVDC transmission system

Voltage : 400 kV Power : 1000 MW

Type : Bipolar HVDC transmission system

2. Talcher-Kolar HVDC transmission system

Voltage : 500 kV

Power : 2000 MW

Type : Bipolar HVDC transmission system

3. Chandrapur-padghe HVDC transmission system (Western Region) Voltage : 500 kV

Power : 1500 MW

Type : Bipolar HVDC transmission system

4. Hirma -Jaipur HVDC transmission system (Northern Region) Voltage : 600 kV

Power : 3000 MW

Type : Bipolar HVDC transmission system

Other HVDC Links1. Korba STPS-Karamsad HVDC transmission system

Voltage : 765 kV

Power : 4000 MW

Type : Bipolar HVDC transmission system

2. Talcher-Bangalore HVDC transmission system (Southern Region)

Voltage : 500 kV

Power : 2000 MW

Type : Bipolar HVDC transmission system 1.8 COMPARISION BETWEEN HVDC AND EHVAC TRANSMISSION1.8.1 Economics of Power Transmission

Sl.No.HVDCEHVAC

1.For lines designed for same insulation level, It can carry more power with two conductors (Positive and negative)It can carry as much power with three conductors of the same size

2.For a given power level, It requires less right of way, cheaper towers, and reduced conductor costs.For a given power level, cost of towers, conductors are high.

3.Cable insulation required is less.Cable insulation required is more.

4.Power losses are reduced because of two conductors (operational cost reduces)Power losses are increased because of three conductors(operational cost increases)

5.Do not require compensationCost for compensation devices are high

6.Terminal equipment cost is high due to the presence of converters and filtersTerminal equipment cost is low

7.Absence of skin effect reduces power losses, thereby operational cost reducesDue to skin effect power losses increases thereby operational cost increases

8.Dielectric losses in the power cables is less.Dielectric losses is high.

9.Corona loss and radio interference is less compared to that of A.C. This leads to the choice of economic size of conductors for D.C.Corona loss and radio interference is high thereby operational cost increases

10.Maintenance cost is highMaintenance cost is low

Cost

Fig. Variation of costs with line length

Fig shows the Variation of costs for D.C and A.C transmission system with distance. A.C transmission is more economical upto the breakeven distance and D.C transmission is more economical after breakeven distance. Breakeven distances may vary from 55km to 600km depending on p.u line cost.1.8.2 Technical Performance

(i) Full Control over power transmitted

In HVDC, power carrying capacity of D.C is unaffected by the distance of transmission as shown in fig. (Long distance bulk power transmission).In EHVAC power transfer in A.C depends on Power transfer =

Where =Angle between sending end and receiving end voltages. For a given power level, increases with distance. Power carrying capacity of an A.C.line decreases after some distance as shown in fig. Power transfer is limited by the consideration of transient and steady state stability

HVDCEHVAC

(i)Voltage Control:

D.C converter station requires reactive power related to the line loadings, the line itself does not require reactive power control.It is complicated due to line charging and inductive voltage drops.

Voltage profile is flat due to surge impedance loading.

Requires reactive power control to maintain constant voltages at the ends from inductive to capacitive as the line loading is increased. Reactive power requirements increases with the increase in line length.

(ii)Short circuit current:

The contribution of D.C line to short circuit current is only upto rated current of D.C lineInterconnection of two A.C systems by an A.C line increases the short circuit current in the system.

(iii)Line Compensation:

Does not require line compensationRequires shunt and series compensation in long transmission lines to overcome line charging and stability limitations.

Static VAR system are used to increase power transfer and voltage control

(iv)Problems of A.C interconnection:

Two systems are interconnected which have different frequencies.

For synchronous D.C tie lines, no need of coordinated control for interconnection.Two systems are not interconnected which have different frequencies.

Two power systems are interconnected when they have coordinated using tie line power and frequency. The problems arise due to the presence of large power oscillations which can lead to frequency tripping, increase in fault level, transmission of disturbances from one to the other.

(v)Ground Impedance:

Ground im,pedance is negligible, so D.C link can operate using one conduutor with ground return.Ground current cannot be permitted in steady-state due to high magnitudes of ground impedance which will result telephone interference.

(vi)Economical use of underground cables or submarine cables are possible Not Possible

(vii)Fast control to limit fault currents in D.C: D.C. breakers in two terminal D.C links are avoided. But cost of D.C. breakers are highCannot avoid breakers.

(viii)Inability to use transformer to change voltage levels Transformers are used to change voltage level

(ix)Cost of conversion equipment is highNo conversion equipment cost

1.8.3 Reliability

It is the probability that an item or a collection of items will perform satisfactory, under specified condition during a given period. This is called reliability.

Reliability of D.C is good compared to that of A.C. The performance of thyristor valves is much reliable than mercury are valves, and control and protection is to improve the reliability level. The development of direct light triggered thyristors (LTT) has been used to improve the reliability. There are two measures of overall system reliability

PART-B

1. With a neat sketch explain the Structure of a general transmission and distribution system Apr/May 2005Basic Structure of a Power System

The basic structure of a power system is shown in Fig. 1.1.

Fig. 1.1 A typical power system.General structure of a power system

1. Power generating plants, where energy resources, such as natural gas and coal are converted to electricity

2. Giant transmission lines move electricity intrastate from power plants at very high voltage.

3. At sub stations, voltage is stepped down or reduced for use at large industrial complexes

4. At additional sub stations farther down the line, power is stepped down again ,so it can be used in homes for lights and appliances.

It contains a generating plant, a transmission system, a subtransmission system and a distribution system. These subsystems are interconnected through transformers T1 , T2 and T3 . Let us consider some typical voltage levels to understand the functioning of the power system. The electric power is generated at a thermal plant with a typical voltage of 22 kV (voltage levels are usually specified line-to-line). This is boosted up to levels like 400 kV through transformer T1 for power transmission. Transformer T2 steps this voltage down to 66 kV to supply power through the sub transmission line to industrial loads that require bulk power at a higher voltage. Most of the major industrial customers have their own transformers to step down the 66 kV supply to their desired levels. The motivation for these voltage changes is to minimize transmission line cost for a given power level. Distribution systems are designed to operate for much lower power levels and are supplied with medium level voltages.

The power distribution network starts with transformer T3, which steps down the voltage from 66 kV to 11 kV. The distribution system contains loads that are either commercial type (like office buildings, huge apartment complexes, hotels etc) or residential (domestic) type. Usually the commercial customers are supplied power at a voltage level of 11 kV whereas the domestic consumers get power supply at 400-440 V. Note that the above figures are given for line-to-line voltages. Since domestic customers get single-phase supplies, they usually receive 230-250 V at their inlet points. While a domestic customer with a low power consumption gets a single-phase supply, both industrial and commercial consumers get three-phase supplies not only because their consumption is high but also because many of them use three-phase motors. For example, the use of induction motor is very common amongst industrial customers who run pumps, compressors, rolling mills etc.

The main components of a power system are generators, transformers and transmission lines.

In this module we shall discuss the models of these components that will be used subsequently in power system studies.

Power generation

A power generation plant is a facility designed to produce electric energy from another form of energy, such as:

Figure 1. Power Generation Plant to transmission line

Heat (thermal) energy generated from:

fossil fuels; coal ,petroleum, natural gas

solar thermal energy

geothermal energy

nuclear energy

Potential energy from falling water in a hydroelectric facility

Wind energy

Solar electric from solar (photovoltaic) cells

Chemical energy from: fuel cells,batteries

There are many different types of electric power generating plants. The major types generating electric power today are shown below.

Transmission Lines

Transmission lines carry electric energy from one point to another in an electric power system. They can carry alternating current or direct current or a system can be a combination of both. Also, electric current can be carried by either overhead or underground lines. The main characteristics that distinguish transmission lines from distribution lines are that they are operated at relatively high voltages, they transmit large quantities of power and they transmit the power over large distances.

Types of transmission lines

Overhead Transmission Lines

Sub transmission Lines

Underground Transmission Lines

Overhead AC transmission lines

Overhead AC transmission lines share one characteristic; they carry 3-phase current. The voltages vary according to the particular grid system they belong to. Transmission voltages vary from 69 kv up to 765 kv. The following are examples of different overhead transmission line structures in use today. The DC voltage transmission tower has lines in pairs rather than in threes (for 3-phase current) as in AC voltage lines. One line is the positive current line and the other is the negative current line.

Sub transmission lines

Sub transmission lines carry voltages reduced from the major transmission line system. Typically, 34.5 kv to 69 kv, this power is sent to regional distribution substations. Sometimes the subtransmission voltage is tapped along the way for use in industrial or large commercial operations. Some utilities categorize these as transmission linesUnderground transmission lines

Underground transmission lines are more common in populated areas. They may be buried with no protection, or placed in conduit, trenches, or tunnels.

Substations

A substation is a high-voltage electric system facility. It is used to switch generators, equipment, and circuits or lines in and out of a system. It also is used to change AC voltages from one level to another, and/or change alternating current to direct current or direct current to alternating current. Some substations are small with little more than a transformer and associated switches. Others are very large with several transformers and dozens of switches and other equipment. There are three aspects to substations:

Depending on the purpose, the substations may be classified into five categories.

Generating Substations (or) Step up Substations

Grid Substations

Secondary Substations

Distribution Substations

Special Purpose Substations.

Depending on the physical features, the substations may also be of four types.

Outdoor Type

Indoor Type

Pole mounded (or) Open (or) Kiosk Type

Underground TypeSubstations functions:

Substations are designed to accomplish the following functions, although not all substations have all these functions:

Change voltage from one level to another

Regulate voltage to compensate for system voltage changes

Switch transmission and distribution circuits into and out of the grid system

Measure electric power qualities flowing in the circuits

Connect communication signals to the circuits

Eliminate lightning and other electrical surges from the system

Connect electric generation plants to the system

Make interconnections between the electric systems of more than one utility

Control reactive kilovolt-amperes supplied to and the flow of reactive kilovolt-amperes in the circuits

Distribution system A distribution system originates at a distribution substation and includes the lines, poles, transformers and other equipment needed to deliver electric power to the customer at the required voltages. Customers are classed as:

Industrial Customer

Commercial Customer

Residential Customer

Transportation Customer

The electric energy is supplied to consumers through a distribution system. A distribution system can be subdivided into three major parts.

Feeders,

Distributors,

Service mains. Feeders Feeders are designed based on the current density.

Distributors Distributors are designed based on the Voltage drop

Service Mains Service Mains are conductors between a distributor and the metering point of the consumers premises. Sub Mains

The diameter of conductors of sub mains is more than the service mains.

2. Make a comparison between HVDC and EHVAC System Apr/May 2005Sl.No.HVDCEHVAC

1.For lines designed for same insulation level, It can carry more power with two conductors (Positive and negative)It can carry as much power with three conductors of the same size

2.For a given power level, It requires less right of way, cheaper towers, and reduced conductor costs.For a given power level, cost of towers, conductors are high.

3.Cable insulation required is less.Cable insulation required is more.

4.Power losses are reduced because of two conductors (operational cost reduces)Power losses are increased because of three conductors(operational cost increases)

5.Do not require compensationCost for compensation devices are high

6.Terminal equipment cost is high due to the presence of converters and filtersTerminal equipment cost is low

7.Absence of skin effect reduces power losses, thereby operational cost reducesDue to skin effect power losses increases thereby operational cost increases

8.Dielectric losses in the power cables is less.Dielectric losses is high.

9.Corona loss and radio interference is less compared to that of A.C. This leads to the choice of economic size of conductors for D.C.Corona loss and radio interference is high thereby operational cost increases

10.Maintenance cost is highMaintenance cost is low

3. What are FACTS devices?

FACTS means Flexible Alternating Current Transmission Systems Flexible alternating current transmission systems (FACTS) devices are used for the dynamic control of voltage, impedance and phase angle of high voltage AC lines.Advantages

Improved steady state system performance

Increase system security

Reduces power system oscillations.

Improved in system transient or dynamic stability.

Reduced financial costs and environmental impacts

Below the different main types of FACTS devices are described:

(i) Static Var Compensators (SVCs)

Static Var Compensators (SVC) is a shunt connected static VAR generator or absorber whose output is adjusted to control capacitive or inductive current so as to maintain or control specific parameters of the electrical power system.

Static Var Compensators (SVCs) the most important FACTS devices, have been used for a number of years to improve transmission line economics by resolving dynamic voltage problems. The accuracy, availability and fast response enable SVCs to provide high performance steady state and transient voltage control compared with classical shunt compensation. SVCs are also used to dampen power swings, improve transient stability, and reduce system losses by optimized reactive power control.

(ii) Thyristor controlled series compensators (TCSCs)

TCSCs are an extension of conventional series capacitors through adding a thyristor-controlled reactor. Placing a controlled reactor in parallel FACTS For cost effective and reliable transmission of electrical energy with a series capacitor enables a continuous and rapidly variable series compensation system. The main benefits of TCSCs are increased energy transfer, dampening of power oscillations, dampening of sub synchronous resonances, and control of line power flow.

(iii) STATCOM (Static Synchronous Compensator)

STATCOM is a static synchronous generator operated as a shunt connected static VAR compensator (SVC) whose inductive and capacitive output current can be controlled independelly of the A.C.supply system.. STATCOMs are GTO (gate turn-off type thyristor) based SVCs. Compared with conventional SVCs (see above) they dont require large inductive and capacitive components to provide inductive or capacitive reactive power to high voltage transmission systems. This results in smaller land requirements. An additional advantage is the higher reactive output at low system voltages where a STATCOM can be considered as a current source independent from the system voltage. STATCOMs have been in operation for approximately 5 years.

(iv) Unified Power Flow Controller (UPFC).

Unified Power Flow Controller (UPFC) is a combination of static synchronous compensator (STATCOM) and a static synchronous series compensator (SSSC) which are coupled through a D.C.link, to allow bi-directional flow of real power between serious output of SSSC and shunt output of STATCOM

Unified Power Flow Controller (UPFC). Connecting a STATCOM, which is a shunt connected device, with a series branch in the transmission line via its DC circuit results in a UPFC. This device is comparable to a phase shifting transformer but can apply a series voltage of the required phase angle instead of a voltage with a fixed phase angle. The UPFC combines the benefits of a STATCOM and a TCSC.

4. Compare STATCOM and SVC

Sl.No.STATCOMSVC

1.It uses Gate Turn -off ThyristorsIt uses conventional Thyristors

2.It has short time overload capabilityIt cannot have short time over load capability

3.It reduces System HarmonicsIt generates System Harmonics

4.Even with very week A.C.system, It maintains stable voltageEven with very week A.C.system, Its operation is difficult.

5.Better performance during transientsSlow performance during transients

6.It operates both inductive and capacitive regions.It operates mostly in capacitive regions.

7.It is based on voltage source converterIt is a voltage regulator and variable susceptance controller.

8.It can serve as a real power exchanger if it has an energy source at D.C.bus.

Two Marks1. Define Electric supply system.

The system which enables the supply of electric power from, a power generating station to consumers premises is known as electric supply system.

2. What are the main components of a power system? ( Nov - 04)

The main components of a power system are generation units (generators, transformers, etc.) ,Transmission Lines and distribution network.

3. Define electric power system

Electric Power Systems, components that transform other types of energy into electrical energy and transmit this energy to a consumer. The production and transmission of electricity is relatively efficient and inexpensive, although unlike other forms of energy, electricity is not easily stored and thus must generally be used as it is being produced.

4. Define Power distribution grid.

Electrical power is a little bit like the air you breathe: You don't really think about it until it is missing. Power is just "there," meeting your every need, constantly. It is only during a power failure, when you walk into a dark room and instinctively hit the useless light switch that you realize how important power is in your daily life. You use it for heating, cooling, cooking, refrigeration, light, sound, computation, entertainment... Without it, life can get somewhat cumbersome.

Power travels from the power plant to your house through an amazing system called the power distribution grid.

5. Why all transmission and distribution systems are 3 phase systems?

A 3 phase a.c circuit using the same size conductors as the single phase circuit can carry three times the power which can be carried by a 1 phase circuit and uses 3 conductors for the 2 phases and one conductor for the neutral. Thus a 3 phase circuit is more economical than a 1 phase circuit in terms of initial cost as well as the losses. Therefore all transmission and distribution systems are 3 phase systems.

6. Why the transmission systems are mostly overhead systems?

Because of the cost consideration, the transmission systems are mostly overhead systems.

7. Why all overhead lines use ACSR conductors?

ACSR conductors comprises of hard drawn aluminium wires stranded around a core of single or multiple strand galvanized steel wire. They provides the necessary conductivity while the steel provides the necessary mechanical strength. Has less corona loss. The breaking load is high and has less weight.

8. Why transmission lines are 3 phase 3 wire circuits while distribution lines are 3 phase 4 wire circuits?

A Balanced 3 phase circuit does not require the neutral conductor, as the instantaneous sum of the 3 line currents are zero. Therefore the transmission lines and feeders are 3 phase 3 wire circuits. The distributors are 3 phase 4 wire circuits because a neutral wire is necessary to supply the 1 phase loads of domestic and commercial consumers.

8. Why overhead line conductors are invariably stranded?

They are stranded to make them flexible during erection and while in service.

9. State the advantages of interconnected systems.

Any area fed from one generating station during overload hours can be fed from another power station and thus reserved capacity required is reduced, reliability of supply is increased and efficiency is increased.

10. What are the problems of interconnection?

1.it increase the amount of current which flows when a short circuit occurs on a system and thereby requires the installation of breakers which are able to interrupt a larger current`

2. Synchronism must be maintained between of all the interconnected systems.

11.Define one line diagram.

A simplified diagram by omitting the completed circuit through the neutral and by indicating the components of the power system by standard symbols rather than by their equivalent circuits.

Now represent a practical power system where a lot of interconnections between several generating stations involving a large number of transformers using three lines corresponding to R, Y and B phase will become unnecessary clumsy and complicated. To avoid this, a single line along with some symbolical representations for generator, transformers substation buses are used to represent a power system rather neatly. For example, the system shown in fig with three lines will be simplified to figure 2.9 using single line.

Fig.1 single line representation of a power system

12.Draw the general structure of a power system Nov/Dec-2004

1. Power generating plants, where energy resources, such as natural gas and coal are converted to electricity

2. Giant transmission lines move electricity intrastate from power plants at very high voltage.

3. At sub stations, voltage is stepped down or reduced for use at large industrial complexes

4. At additional sub stations farther down the line, power is stepped down again ,so it can be used in homes for lights and appliances.

13. Mention the disadvantages of a 3 wire system

In 3 wire system a third wire is required. The safety is partially reduced. A balancer is required and therefore cost is increased.

14. What are the advantages of a 3 wire dc distribution system over a 2 wire dc distribution system?

If 3 wire system is used to transmit the same amount of power over the same distance with same efficiency with same consumer voltage we require 0.3125 times copper as required in 2 wire system.

15. Mention the differences between 3 wire and 3 phase 4 wire distribution system?

3 phase 3 wire is employed for balanced loads, but 3 phase 4 wire is employed for unbalanced loads.

3 phase 3 wire is used for transmission but 3 phase 4 wire is used for distribution of power to consumers.

16. Why overhead line conductors are invariably stranded?

They are stranded to make them flexible during erection and while in service.

17. Mention the demerits of HVDC transmission. Electric power cannot be generated at high dc voltages.

The dc voltages cannot be stepped up for transmission of power at high voltages.

The dc switches and circuit breakers have their own limitations.

18. List two merits of HVDC transmission system Nov/Dec-2004

For lines designed for same insulation level, It can carry more power with two conductors (Positive and negative)

For a given power level, It requires less right of way, cheaper towers, and reduced conductor costs.

Absence of skin effect reduces power losses, thereby operational cost reduces

Dielectric losses in the power cables is less.

Corona loss and radio interference is less compared to that of A.C.

Cable insulation required is less.

Power losses are reduced because of two conductors (operational cost reduces)

Do not require compensation

19. What are the advantages of high voltage ac transmission.

The power can be generated at high voltages.

The maintenance of ac substation is easy and cheaper.

20. Mention the disadvantages of high voltage ac transmission.

An ac line requires more copper than a dc line.

The construction of an ac line is more complicated than a dc transmission line.

Due to skin effect in the ac system the effective resistance of the line is increased.

21. Mention the limitations of using very high transmission voltage.

a) The increased cost for the insulation of the conductor.

b) The increased cost for the transformers, switch gears and other terminal apparatus.

22. Mention the terminal equipments necessary in HVDC system.

Converters, mercury arc valves and thyristors.

23. Why HVDC line do not require any reactive power compensation?

Due to absence of charging currents.

24. Mention the equipments that supply reactive power in HVDC converter stations?

AC filters

Static shunt capacitors

Synchronous condensers

StaticVAR compensators.

25. Why dc transmission is economical and preferable over ac transmission for longer distances?

Because with longer distances, the saving in cost of dc overhead lines become greater than the additional expenditure on terminal equipment.

26. Why is voltage regulation better in case of dc transmission ?

Because of absence of inductance in dc systems .

27. What are the advantages of adopting EHV/UHV for transmission of ac electrical

power ?

Reduced line losses

High transmission efficiency

Improved voltage regulation

Reduced conductor material requirement

Flexibility for future system growth

increase in transmission capacity of the line

Increase of SIL.

28. Mention the problems associated with an EHV transmission?

The problems associated with EHV transmission are corona loss and radio interference, requirements of heavy supporting structures erection difficulties and insulation requirements.

29. What for series and shunt compensation provided in EHV lines?

Series compensation is provided to reduce the series reactance of the line so as to improve stability, voltage regulation and transmission efficiency.

Shunt compensation is provided to reduce the line susceptance so as to improve the voltage regulation under light load condition.

30.What is the voltage that has been selected for HVDC transmission ?

400 KV, 500 KV, 600 KV , 800 KV, 1000 KV etc.

31. What are service mains? Apr/May 2005

Service mains are conductors which connect the consumers terminals to the distributor.

32. What is the usable voltage for secondary distribution?

415 /240 V (415 volts for 3-phase loads and 240 volts for 1-phase loads)

33. Why galvanized steel wire is not suitable for EHT lines for the purpose of transmitting large amounts of power over long distance?

--because of Poor conductivity

--High internal reactance & eddy current & hysteresis

34. Mention the transmission voltages that are used in India?

33/66/110/132/220/400/765 KV.

765KV is EHV.

35. On what basis the conductor size is determined for transmission lines up to 220 KV?

It is determined on the basis of its current carrying capacity.

36. On what basis the conductor size of EHV line is determined?

It is decided on the basis of corona.

37. Mention the sources of audible noise generation in EHV transmission systems?

Corona

Humming of transformers

Cooking Systems

Mechanical and Electrical auxilliaries

38. Mention the material universally employed for overhead transmission line?

ASCR (Aluminium Conductor Steel Reinforced)39. What is the economical value of span for 400kv transmission line?

350 - 400mt.

40. Give four advantages of EHVAC Transmission. Apr/May 2005

Reduction of current and losses Reduction of volume of conductor material.

High transmission efficiency

Improved voltage regulation

Reduced conductor material requirement

Flexibility for future system growth

increase in transmission capacity of the line

Increase of SIL.

41. What is FACTS

FACTS means Flexible Alternating Current Transmission Systems Flexible alternating current transmission systems (FACTS) devices are used for the dynamic control of voltage, impedance and phase angle of high voltage AC lines.Advantages

Improved steady state system performance

Increase system security

Reduces power system oscillations.

Improved in system transient or dynamic stability.

Reduced financial costs and environmental impacts

42. What is SVC

Static Var Compensators (SVC) is a shunt connected static VAR generator or absorber whose output is adjusted to control capacitive or inductive current so as to maintain or control specific parameters of the electrical power system.

Static Var Compensators (SVCs) are the most important FACTS devices, have been used for a number of years to improve transmission line economics by resolving dynamic voltage problems. The accuracy, availability and fast response enable SVCs to provide high performance steady state and transient voltage control compared with classical shunt compensation. SVCs are also used to dampen power swings, improve transient stability, and reduce system losses by optimized reactive power control.

43. What is UPFC?

Unified Power Flow Controller (UPFC) is a combination of static synchronous compensator (STATCOM) and a static synchronous series compensator (SSSC) which are coupled through a D.C.link, to allow bi-directional flow of real power between serious output of SSSC and shunt output of STATCOM Unified Power Flow Controller (UPFC). Connecting a STATCOM, which is a shunt connected device, with a series branch in the transmission line via its DC circuit results in a UPFC. This device is comparable to a phase shifting transformer but can apply a series voltage of the required phase angle instead of a voltage with a fixed phase angle. The UPFC combines the benefits of a STATCOM and a TCSC

44. What is STATCOM?

STATCOM is a static synchronous generator operated as a shunt connected static VAR compensator (SVC) whose inductive and capacitive output current can be controlled indwependelly of the A.C.supply system.. STATCOMs are GTO (gate turn-off type thyristor) based SVCs. Compared with conventional SVCs (see above) they dont require large inductive and capacitive components to provide inductive or capacitive reactive power to high voltage transmission systems. This results in smaller land requirements. An additional advantage is the higher reactive output at low system voltages where a STATCOM can be considered as a current source independent from the system voltage. STATCOMs have been in operation for approximately 5 years.

45. What are the various types of HVDC?

Monopolar HVDC transmission system

Bipolar HVDC transmission system

Homopolar HVDC transmission system

46. Give any three HVDC lines in India.

1. Rihand-Delhi HVDC transmission system

Voltage : 400 kV

Power : 1000 MW

Type : Bipolar HVDC transmission system

2. Talcher-Kolar HVDC transmission system

Voltage : 500 kV

Power : 2000 MW

Type : Bipolar HVDC transmission system

3. Chandrapur-padghe HVDC transmission system (Western Region) Voltage : 500 kV

Power : 1500 MW

Type : Bipolar HVDC transmission system

4. Hirma -Jaipur HVDC transmission system (Northern Region) Voltage : 600 kV

Power : 3000 MW

Type : Bipolar HVDC transmission system

Other HVDC Links

1. Korba STPS-Karamsad HVDC transmission system

Voltage : 765 kV

Power : 4000 MW

Type : Bipolar HVDC transmission system

2. Talcher-Bangalore HVDC transmission system (Southern Region)

Voltage : 500 kV

Power : 2000 MW

Type : Bipolar HVDC transmission system

47. Which is the first HVDC link in the world?

Swedish main Load

Voltage : 100 kV

Power : 20 MW, This is the world first D.C.Link.

48. What is the highest A.C. transmission voltage we have in India?

765KV.

49. What are the limitations of EHV A.C.Transmission?1. More insulation is required for the conductor and towers.

2. More clearance is required between the conductor and the ground.

3. More distance is required between the conductors. So the length of cross arms used increases.

4. The transformers, Switchgears and other terminal equipment should be designed to handle such high voltage.5. Long bulk power transmission is not possible.

50. Mention the need of going for EHV A.C. transmission.

EHV A.C. transmission. Provide more reliable and less constrained electricity network capacity.

As the size of the generating unit increases due to increase in voltage ,the cost of the line decreases.

Transmission efficiency increases.

Cost of the line decreases.

Surge impedance loading increases.

51. List out the practical transmission and distribution voltage level s commonly used. Primary transmission : 110kV/132kV/220kV/400kV/765kV

Secondary transmission : 66 kV/33 kV

Primary Distribution : 11 kV/6.6 kV

Secondary Distribution : 400 V for 3 phase; 230 V for single phase

52. What are the advantages of DC Transmission system Crossing of large bodies of water. Ac Cables have too much capacitance. Capacitance does not affect dc. Also, no ionic motion in cables. No induced currents in sheath. Dc Lines do not require compensation. Ac Lines must be compensated with series/shunt capacitance to reduce the total reactance:

Stability Considerations: Phase angles between ac systems interconnected through dc link can be arbitrary. This is unlike steady-state and transient stability limits for ac transmission systems. Thus even back-back, i.e., Sakuma Freq. Changer (Japan), Itaipu (Brazil) where 50 and 60 Hz systems are connected together.

Interconnection between systems of different frequencies is possible

Supply of power to highly populated urban areas via underground cable (Kingsnorth System).

Earth return operation possible because of low earth impedance to dc. Effective voltage on the dc line is the actual dc voltage. For ac lines, it is1/2 times the peak ac voltage. This results in more transmitted dc power for the same insulation level.

No skin effect. Hence better use of conductor cross section.

53. What are the disadvantages of DC Transmission system Harmonic interference with communication circuits

Navigation/compass errors and Corrosion when earth or sea return is used

High cost of conversion equipment

Transformation (step up/down) is not possible

Tapping of dc is difficult High Reactive power Requirements

Lack of skill sets for engineers, maintenance staff and operators54. Compare HVDC and FACTS.

1. FACTS Controllers can be retrofitted into the existing line, but not in HVDC.

2. Installation cost is less for FACTS Compared to that of HVDC.

3. FACTS devices provide VAR Compensation.

4. FACTS device control the line impedance or inject phase shift.

5. FACTS device increases the stability margin.6. FACTS device uses special dampers which are used to improve dynamic stabilityReference1.http://www.answers.com/topic/high-voltage-direct-current2.http://filpower.umr.edu/papers.htm

UNIT-I INTRODUCTION

Distance

Power

D.C

A.C

Break even point

Distance

D.C

A.C

Break even point

2A.S.S.Murugan SL/EEE,KLNCE A.S.S.Murugan SL/EEE,KLNCE

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