EHV AC AND DC TRANSMISSION SYSTEMUnit-I Constitution of EHV a.c. and d.c. links, Kind of d.c. links, Limitations and Advantages of a.c. and d.c. transmission, Principal application of a.c. and d.c. transmission, Trends in EHV a.c. and d.c. transmission, Power handling capacity. Converter analysis garetz circuit, Firing angle control, Overlapping ROLE OF EHV AC TRANSMISSION Industry requires vast amount of energy such as hydro, thermal, oil for transportation and industry, natural gas for domestic of which electrical energy forms a major fraction. It is only 120 years since the installation of the first central station by Edison using dc. But the world has already consumed major portion of its natural resources in this short period and is looking for sources of energy. Hydro-electric and coal or oil-fired stations are located very far from load centers for various reasons which requires the transmission of the generated electric power over very long distances. This requires very high voltages for transmission. The very rapid development of dc transmission since 1950 is playing a major role in extra-long-distance transmission, complementing or supplementing EHV ac transmission. BRIEF DESCRIPTION OF ENERGY SOURCES AND THEIR DEVELOPMENT Two broad categories: (1) Transportable (2) Locally Usable. 1. Transportable type is obviously hydro-electric and conventional thermal power. But locally generated and usable power is by far more numerous and exotic. These are also called 'Alternative Sources of Power'. Twelve such sources of electric power are listed here. 2. Locally Usable Power Conventional thermal power in urban load centers Micro- hydel power stations Nuclear Thermal: Fission and Fusion BRIEF DESCRIPTION OF ENERGY SOURCES AND THEIR DEVELOPMENT Wind Energy Ocean Energy: (a) Tidal Power. . (b) Wave Power. and (c) Ocean thermal gradient power Solar thermal Solar cells or photo-voltaic power Geo-thermal Magneto hydro-dynamic or fluid dynamic Coal gasification and liquefaction Hydrogen power Biomass Energy: (a) Forests (b) Vegetation (c) Animal refuse. Radio Interference. Use of bundled conductors. Corona Energy Loss. High electrostatic field under the line.PROBLEMS/LIMITATIONS POSED IN USING HIGH VOLTAGE Increased Current Density because of increase in line loading by using series capacitors. Increased Short-Circuit currents and possibility of Ferro resonance conditions. Switching Surge Over voltages which cause more havoc to air-gap insulation than lightning or power frequency voltages. High surface voltage gradient on conductors. Corona problems: Audible Noise. . Carrier Interference. and TV Interference. Single-pole reclosing to improve stability. resulting in possible sub-synchronous resonance conditions and high short circuit currents. but causing problems with arcing . Insulation coordination based upon switching impulse levels. for both lightning and switching-surge duty. Shunt reactor compensation and use of series capacitors.PROBLEMS/LIMITATIONS POSED IN USING HIGH VOLTAGE Use of gapless metal-oxide arresters replacing the conventional gap-type Silicon Carbide arresters. 1.2 ADVANTAGES OF HVDC No reactive power loss No Stability Problem No Charging Current No Skin & Ferranti Effect Power control is possible Requires less space compared to ac for same voltage rating and size Ground can be used as return conductor Less corona loss and Radio interference . ADVANTAGES OF HVDC Cheaper for long distance transmission Asynchronous operation possible No switching transient No transmission of short circuit power No compensation problem Low short circuit current Fast fault clearing time . 1.3 DISADVANTAGES OF HVDC Cost of terminal equipment is high Introduction of harmonics Blocking of reactive power Point to point transmission Limited overload capacity Huge reactive power requirement at the converter terminals Cooling of HVDC sub-station HVDC system control . 1.4 COMPARISION OF AC & DC TRANSMISSION • The relative merits of the two modes of transmission (AC and DC) which need to be considered by a system planner are based on the following factors: • Economics of Transmission • Technical performance • Reliability . 4. .1. Cont.1 Economics of power transmission Cost of a transmission line includes • Investment includes: Right of Way (ROW) Transmission towers Conductors Insulators Terminal equipment • Operational costs includes: Cost of losses.. if its assumed that the insulator characteristics are similar for AC and DC and depend on the peak level of voltage applied with respect to ground. then it can be shown that for lines designed with the same insulation level.• The characteristics of insulators vary with the type of voltage applied. . a DC line can carry as much power with two conductors (with positive and negative polarities with respect to ground) as an AC line with 3 conductors of the same size. For simplicity. The other factors that influence the line costs are the costs of compensation and terminal equipment .The corona effects on DC conductors tend to be less significant than for AC and this also leads to the choice of economic size of conductors with DC transmission. . . • No skin effect with DC is also beneficial in reducing power loss marginally.Comparison of ROW • This Implies that for a given power level. • The dielectric losses in case of power cables is also very less for DC transmission. • The power losses are also reduced with DC as only two conductors are used. cheaper towers reduced conductors and insulator costs. . DC lines requires less ROW with Simpler. • DC lines do not require compensation but the terminal equipment costs are increased due to the presence of converters and filters. . • The other factors that influence the line cost are the cost of compensation and terminal equipment.Comparison of ROW • The corona effects tends to less significant on DC conductors than for AC and this leads to choice of economic size of conductors with DC transmission. The breakeven distances can vary from 500Km to 800Km in overhead lines.Variation of cost with line length AC tends to be more economical than DC for distances less than Break even distance and costlier for longer distances. . The ability to enhance transient and small signal stability in associated AC networks 3. Fast control to limit fault currents in DC lines.2 Technical performance The DC transmission has some positive features which are lacking in AC transmission. Full control over power transmitted 2. These are mainly due to the fast controllability of power in DC lines through converter control. This makes it feasible to avoid DC breakers in two terminal DC links. The advantages are: 1.1. .4. For a given power level. The power carrying capability of an AC line is a function of distance but in DC lines it is unaffected by the distance of transmission. .STABILITY LIMITS The power transfer in AC lines is dependent on the angle difference between voltage phasors at the two ends. The maximum power transfer is limited by the considerations of steady state and transient stability. this angle increases with distance. For constant voltage at the line terminal. The Voltage profile varies with the line loading.VOLTAGE CONTROL The voltage control in AC lines is complicated by line charging and inductive voltage drops. The voltage profile in a AC line is relatively flat only for fixed level of power transfer corresponding to surge impedance loading (SIL) or normal loading. . the midpoint voltage is reduced for line loading higher than SIL and increased for loadings less than SIL. Series capacitors and shunt inductors are used for this purpose. it is necessary to provide shunt compensation at regular intervals. In AC cable transmission. mainly to overcome of the line charging and stability limitations.LINE COMPENSATION AC lines require shunt and series compensation in long distance transmission. . The increase in power transfer and voltage control is possible through the Static Var Systems (SVS). the operation of AC ties can be problematic due to The presence of large power oscillations which can lead to frequent tripping.the automatic generation control of both systems have to be coordinated using tie line power and frequency signals. Increase in fault level Transmission of disturbances from one system to the other .PROBLEMS OF AC INTERCONNECTION • When two power systems are connected through AC ties (Synchronous interconnection). • Even with coordinated control of interconnected systems. GROUND IMPEDANCE • In AC transmission. The ground return is objectionable only when buried metallic structures (such as pipes) are present and are subject to corrosion with DC current flow. but also result in telephone interference. • But ground impedance negligible for DC currents and a DC link can operate one conductor with ground return (Mono polar operation). . the existence of ground (Zero sequence) current cannot be permitted in steady-state due to high magnitudes of ground impedance which will not only affect efficient power transfer. 3 RELIABILITY • The reliability of DC transmission is quite good and comparable to that of AC systems.1. • It must be remembered that the performance of Thyristor valves is much more reliable than mercury arc valves and further developments in devices. protection is likely to improve the reliability level.4. control. . • An exhaustive record of existing HVDC links in the world is available from which the reliability statistics cab be computed. Of recordable AC faults ..Cont. There are two measures of overall system reliability • Energy availability • Transient reliability Energy availability=100 (1–{Equivalent outage time})% Total Time Transient reliability = 100*No. Of times HVDC systems performed as designed No. Recordable AC system faults are those faults which cause one or more AC bus phase voltages to drop below 90% of the voltage prior to the fault.Cont. It is assumed that the short circuit level after the fault is not below the minimum specified for satisfactory converter operation. Both energy availability and transient reliability of existing DC systems with thyristors valves is 95% or more.. . This is the factor specifying the performance of HVDC systems during recordable faults on the associated AC systems. Kind of D.C. Links The DC links are classified into three types: Monopolar 2) Bipolar 3) Homopolar • Monopolar link: . The ground return is objectionable only when buried metallic structures (Such as pipes) are present and are subject to corrosion with DC current flow. Having one conductor (-ive Polarity preferred in order to reduce the Corona effect) and ground is used as return path. The major drawback in this system is power flow is interrupted due to either converter failure or DC link.Monopolar link cont.. . During fault in one pole it will operate as Monopolar link. .Bipolar link There are two conductors. one is operated at positive and other is negative. This is very popular link in HVDC. . Ground is always used as return path. two or more conductors have same polarity. During fault in one pole it works as Monopolar.Homopolar link • In this link. Normally negative polarity is used (less corona loss and radio interference). AC transmission • Voltage can be stepped up or stepped down in transformer substation to have economical transmission voltage. • Control of power flow in the network is simple and natural. . • Line can be tapped easily. • System incorporation. extended easily.1. • Back born transmission network.6 Application of EHV. • Parallel line can be added. • Power flow in a particular line cannot be controlled easily and quickly. Asynchronous connection of AC system with different frequencies.DC transmission • The main areas of application based on the economics and technical performances. are Long distance bulk power transmission.1. The underground of submarine cables.7 Application of EHV. Control and stabilize the power system with power flow control. . .8. three types of HVDC is possible. (or) HVDC transmission where bulk power is transmitted from one point to another point over long distance. For this DC system is the best option. Bulk Power transmission Back to back connection Modulation of AC system 1.8 Based on the interconnection.1 Bulk power transmission: The transfer the power from one end to another end without tapping power in between.1. emergency support as per our requirement.(Or) Back to Back link where rectification and inversion is carried out in the same converter station with very small or no DC lines .8.1.2 Power flow control (Back to Back HVDC) • If two regions are very nearby. we can monitor the power flow from one region to another to control. 8.3 Modulation of AC system This is basically used to control the power and stabilize the system. It is also used to connect two different frequencies system. .1. (Modulation of AC) AC system is connected parallel with DC system. It is mainly used to modulate the power of AC lines.(or)Parallel connection of AC and DC links. Where both AC and DC run parallel. 1.9 Principle parts of HVDC Transmission . Schematic diagram of a typical HVDC converter station . 0 Various Parts of HVDC transmission • • • • • • • Converters Converter transformers Smoothing reactors Harmonic filters Overhead lines Reactive power source Earth electrode .2. • Each HVDC lines have at least two converters.0. one at each end. Rectifier can be used as inverter or vice versa.2.1 CONVERTERS Converters are the main part of HVDC system. • Sending end converter works as Rectifier (It converts AC power to DC power). • Several thyristors are connector in series and/or in parallel to form a valve to achieve higher voltage / current ratings. . So generally it is call it as CONVERTERS. • In case for reversal of operation. However converter as receiving end works as Inverter (It converts DC power to AC power). Required voltage rating • Valves in series • Bridges in series • Combination of above. .How to achieve required current & voltage rating Required current rating • Valves in parallel • Thyristors in parallel • Bridges in parallel • Combination of above. . • To allow reasonably short commutation angle during inverter operation. • To withstand high peak inverse voltage during non conducting phase.Main requirements of the Valves Bridge converters are normally used in HVDC systems • To allow current flow with low voltage drop across it during the conduction phase and to offer high resistance for non conducting phase. . etc. SVC.Cont. • Smooth control of conducting and non conducting phases.. • Two versions of switching converters are feasible depending on whether DC storage device utilized is an inductor-Current source converter or Capacitor-Voltage source converter. • CSC is preferable for HVDC system • VSC is preferable for FACTS like STATCOM. . Comparison of CSC and VSC Inductor is used in DC side Capacitor is used in DC side Constant current Constant voltage Higher losses More efficient Fast accurate control Slow control Larger and more expensive Smaller and less expensive More fault tolerant and more reliable Less fault tolerant and less reliable Simpler control Complex control Not easily expandable for in series Easily expanded in parallel for increased rating . 2 CONVERTER TRANSFORMERS • For six pulse converter. • Six single -phase two windings • Three single. • However for 12 pulse configuration.phase three windings • Two three.phase two windings .0.2. a conventional three phase or three single phase transformer is used. following transformer are used. • In converter transformer it is not possible to use winding close to yoke since potential of its winding connection is determined by conducting valves hence entire winding are completely insulated. . it produces greater eddy current loss and hot spot in the transformer tank. • As leakage flux of a converter transformer contains very high harmonic contents. the leakage impedance of converter transformer is higher than the conventional transformer.• In case of 12-Pulse configuration. . one will have starstar connection. • On-line tap changing is used to control the voltage and reactive power demand. • Since fault current due to fault across valve is predominantly controlled by transformer impedance. and another will have star delta connection to give phase shift of 30 . if two three phase transformers are used. 2. . • Normally Partial or total air cored magnetically shielded reactor are used.3 SMOOTHING REACTORS • These reactors are used for smoothing the direct current output in the DC line. • It also limits the rate of rise of the fault current in the case of DC line short circuit. • The saturation inductance should not be too low.0. • Disc coil type windings are used and braced to withstand the short circuit current. 2.0.4 HARMONIC FILTERS: • Harmonics generated by converters are of the order of np 1in AC side and np is the DC side. Where p is number of pulses and n is integer. • Filters are used to provide low impedance path to the ground for the harmonics current. • They are connected to the converter terminals so that harmonics should not enter to AC system. Cont.. • However, it is not possible to protect all harmonics from entering into AC system. • Magnitudes of some harmonics are high and filters are used for them only. • These filters are used to provide some reactive power compensation at the terminals. 2.0.5 OVERHEAD LINES • As Monopolar transmission scheme is most economical and the first consideration is to use ground as return path for DC current. • But use of ground as conductor is not permitted for longer use and a bipolar arrangement is used with equal and opposite current in both poles. • In case of failure in any poles, ground is used as a return path temporarily. • The basic principle of design of DC overhead lines is almost same as AC lines design such as configurations.. . • The choice of conductors depends mainly on corona and field effect considerations. • The number of insulators and clearances are determined based on DC voltage. towers and insulators etc.Cont. 2.0.6 REACTIVE POWER SOURCE • Converter does not consume reactive power but due to phase displacement of current drawn by converter and the voltage in AC system, reactive power requirement at the converter station is about 50-60% of real power transfer, which is supplied by filters, capacitors and synchronous condensers. • Synchronous condensers are not only supplying reactive power but also provide AC voltages for natural commutation of the inverter. • Due to harmonics transient special designed machines are used. 2.0.7 EARTH ELECTRODES • The earth resistivity of at upper layer is higher (~4000 ohm-m) and electrodes cannot be kept directly on the earth surface. • The electrodes are buried into the earth where the resistivity is around (3-10 ohm-m) to reduce transient over voltages during line faults and gives low DC electric potential and potential gradient at the surface of the earth. Cont…. • The location of earth electrode is also important due to Possible interference of DC current ripple to power lines, communication systems of telephone and railway signals etc. Metallic corrosion of pipes, cable sheaths etc. Public safety. The electrode must have low resistance (Less than 0.1 ohm) and buried up to 500 meters into the earth. the current flowing is P = E sinδ / √3 LX ……………………. at the load P.. (4) .f.Power Handling Capacity and Line loss • The Power Handling Capacity of a single circuit is P = E2 sin δ / LX ………………………… (1) • At unity p.. (2) • The total power loss in the three phase will amount to P= 3 I2 r ……………………… (3) • Therefore the percentage power loss is % P = 100 r sin δ / X ………………….. The power handling capacity of line at a given voltage level decreases with line length. 2.Cont… • The following important and useful conclusion can be drawn for preliminary understanding of trends relating to power handling capacity of A.C. being inversely proportional to length L. 1. .One 750kv line can normally carry as much power as four 400kv circuits for equal distance of transmission. transmission lines and line losses. The required get power is made available at the potential individual thyristor for electrically triggered thyristor valves. the light signal can be used to directly fire for individual capacitor. for light triggered thyristor valves.Firing Angle Control Two basic requirements for the firing pulse generation of HVDC valves The firing instant for all the valves are determined at ground potential and the firing signals sent to individual thyristors by light signals through fiber optics cables. Howe ever. . . the gate pulse generator must send a pulse whenever required. .Cont. • Equidistant pulse control (EPC). There are two basic firing schemes • Individual phase control (IPC). While the signal pulse is adequate to turn on a thyristor. Garetz Circuit: . Assumptions made for analysis : Without overlap . . . . . . . . . . . . With overlap . Analysis of Mode-1 . . . . . . . . . Analysis of mode-2 . . . . . . RGPV QUESTIONS . RGPV QUESTIONS .