HVDC TransmissionElectrical Engineering 8th Semester Subject Code : EE801A Multi-Terminal HVDC Systems 08 HVDC Transmission 2 .Lesson Plan Module Content Hours 1. Control of HVDC Converters and Systems 08 4. Introduction 04 2. Harmonics and Filters 10 5. Analysis of HVDC Converters 06 3. Faults and Protection Schemes in HVDC Systems 04 6. Colin Adamson and N. . Arrillaga.Text & Reference Books Text Books: 1. 2. E. Padiyar. Kamaraju. Kimbark. HVDC Transmission 3 . G. New Age International. S. Direct Current Transmission (Volume I). Hingorani. Kamakshaiah & V. W. London. HVDC Transmission. R.Reference Books: 1. 3. Tata McGraw Hill Education. Garraway Limited. J. High Voltage Direct Current Transmission. HVDC Power Transmission System. Peter Pregrinu. K. 2. High Voltage Direct Current Power Transmission. Wiley-Interscience. speed and size. Edison and George Westinghouse) • Limitations of Early Low Voltage DC Systems High losses and voltage drop Transformation of voltage required (Higher voltage means lower voltage drop) Commutators of DC Machines impose limitations on the voltage.Introduction Introduction to DC Power Transmission Technology • War of Currents (Thomas A. (Voltage per bar < 21 V) high voltage per commutator → high number of bars → large diameter and low speed for less centrifugal force → heavier machines and more associated maintenance costs HVDC Transmission 4 . cheap and efficient) Advancement of Synchronous Generators • The Thury System (1880 .1911) HVDC Transmission 5 .Introduction Introduction to DC Power Transmission Technology • Rise of AC Power System Advent of Transformers for stepping up and stepping down the voltage level Development of Polyphase Induction Motors (simple. rugged. Introduction Introduction to DC Power Transmission Technology • Major Problems in HVAC Systems Insulation Stability Reactive Power and Line Loading Ferranti Effect and Skin Effect Power Control Interconnection HVDC Transmission 6 . • Basis for comparison Current Limit & Voltage Limit Economy & Terminal Equipment Reactive Power & Surge Impedance Loading Ground Return Stability Circuit Breaking Control of Power Short-Circuit Current Amount of Power Transfer Generating Units HVDC Transmission 7 .Introduction Comparison of AC and DC Transmission • Technical Limitations & Economic Limitations Cheapest method by which a certain amount of power at a certain load factor can be transmitted reliably over a certain distance. limiting factor is direct voltage > peak value normal working voltage. number of • For same power. therefore losses are less. Current Flow • For Underground (UG) Cables. • For same power. (To avoid permanently increased sag) conductors required is more. Thermal loading due exceed permissible maximum to Ohmic losses temperature. Comparison of AC and DC Transmission Basis Remarks HVAC Transmission HVDC Transmission for comparison Current Limit • For Overhead (OH) Lines. maximum working • Switching Surges ≈ 2-3 times • Switching Surges ≈ 1. of AC voltage 8 . conductors required is less. resistance (To avoid insulation damage) Voltage Limit • For OH Lines. number of depends on . increased in foul weather. • Ambient Temperature of insulation in contact • Charging current in UG cables Temperature must not exceed permissible limits the maximum possible • Conductor maximum temperature. • Radio interference is decreased in foul weather. AC resistance.7 voltage and minimum conductor normal peak voltage times normal voltage Switching and size are limited by loss and radio • Radio interference is Lightning Surges interference due to corona. load current. • Duration of therefore losses are more. • AC resistance is slightly • DC resistance is lower than Conductor temperature must not higher value due to skin effect. • Insulation must withstand - • For UG Cables. the line charging employed to counteract the line length. than steady state stability. reactance. limit Steady state stability (Less than the half of thermal and transient stability • Critical power (or torque) angle limit) • Maximum transferable • Maximum transferable power power is only limited by is dependent upon line thermal loading. excessive voltage power and line charging. • Ferranti Effect Stability • Loss of synchronism and hunting • Transient stability is lower • Very high transient stability due to sudden loading / un-loading. Loading (SIL) capacitive line charging. 9 . current (proportional to line length) reactive power requirements. Comparison of AC and DC Transmission Basis Remarks HVAC Transmission HVDC Transmission for comparison Reactive Power & • Voltage profile along line depends • For long and heavily loaded • No problem of reactive Surge Impedance on inductive voltage drop and OH lines. heavily depend upon • For UG cables. pose a serious problem. sides of the line require Reactive power fixed. if sending drop (or rise) occurs at the however converters at both and receiving end voltages are kept middle portion of the line. huge amount of reactive requirements of line • Line compensation must be power. cheaper and require •In UG cables. Amount of Power •Power transfer capability should be • Power transfer capability is • For same amount of power Transfer independent of line length. than that of the AC cables. reliable power less right-of-way. permissible reactive power and right-of working stresses are higher way requirements. • In DC cables. is easy and rapid. permissible working stresses are lower. direction. • Elimination of faults are complicated. simpler. • Fast control to limit fault currents. • In AC cables. dependent upon power factor transfer. transfer is dependent on permissible • Power transfer is complicated working stress.e. Control of power reliability of the system. line losses). DC lines are of operation (i. by line configuration. 10 . Comparison of AC and DC Transmission Basis Remarks HVAC Transmission HVDC Transmission for comparison Control of Power • Power control and power • Power control is not possible • Bi-directional control over modulation is necessary for better without additional power transmitted in either performance and increased components. Ground Return • Use of ground as return is not • Ground can be used as possible due to very low return by using specially penetration. Ideally suited for bulk power transmission. • Transmission loss is low. amount of power at a certain load factor can be transmitted reliably • Right-of-way requirements • For same power. Comparison of AC and DC Transmission Basis Remarks HVAC Transmission HVDC Transmission for comparison Economy • Total Cost = Fixed Cost + Running • Initial cost is low but • Initial cost is high. tower commissioning is high. primarily and Terminal Cost transmission cost increases due to very expensive Equipments tremendously with the converter station • Cheapest method by which a certain increase in line length. erection and line commissioning is low. and cost due to conductor. equipments. constructed ground electrodes. way requirements and cost tower erection and line due to conductor. Right-of- over a certain distance. 11 . possible due to presence of complicated due to absence natural current zero. possible. Generating Units • Asynchronous operation is • Asynchronous operation is and Tie-line Power not possible. complicated • Tie-line power control is easy and rapid. of natural current zero in DC line current. 12 . • Fault level increases with • Fault level remain addition of new lines. Short-circuit • Limited short-circuit fault • Limited overload capability Currents level. of the converter valves. Control • Ideally suited for off-shore • Tie-line power control is power generation. Comparison of AC and DC Transmission Basis Remarks HVAC Transmission HVDC Transmission for comparison Circuit Breaking • Circuit breaking is important for • Normal circuit breaking is • Circuit breaking is rapid fault clearing. unchanged with addition of new lines. HVDC Transmission 13 . especially in foul weathers.Introduction Advantages of HVDC Transmission • Greater power per conductor and higher efficiency of transmission. • Line power factor is always unity and line does not require reactive compensation. • Ground return can be used. • No problem of charging current and cables can be worked at higher voltage gradient. • Synchronous operation is not required and hence distance is not limited by stability. • May interconnect AC systems of different frequencies. • Low short-circuit current on DC line and does not contribute to short circuit current of AC system • Tie line power control is easy and rapid. • Less corona loss and radio interference. • Simpler line construction and commissioning. This reactive power needs to be supplied from AC side at both ends. • Converters have limited overload capacity. • Reliable multi-terminal DC systems are yet to be established because of lack of HVDC circuit breakers. • Complexity of control. • Converter stations require huge amount of reactive power. • Converter stations generate harmonics on both AC and DC sides. which are to be eliminated using huge filters. HVDC Transmission 14 .Introduction Limitations of HVDC Transmission • Converters and valves are expensive. • For interconnection of AC systems having different frequencies or for asynchronous operation. especially for sea water crossing (such as for feeding power to a far-off island using submarine cables).Introduction Applications of HVDC Transmission • For bulk power transmission over long distances by overhead lines. • For supplying power to special types of load centers (such as congested urban areas) where it is difficult to acquire right of way for overhead lines and where the lengths involved make the AC cables impractical. • For interconnection of systems using long cables. HVDC Transmission 15 . Mean Time To Failure (MTTF) 4. Mean Time To Repair (MTTR) • Monopolar operation is possible in the event of outage of one pole. • Reliability is rapidly increasing with the development of high quality valves and switches. • Main parameters for reliability – 1.Introduction Reliability of HVDC Transmission • Reliability of HVDC transmission system is comparable to that of EHVAC transmission system. 2. 3. HVDC Transmission 16 .