High Voltage Direct Current Transmission – Proven Technology for Power Exchange 2 Contents Chapter Theme Page Contents 3 1Why High Voltage Direct Current? 4 2 Main Types of HVDC Schemes 6 3 Converter Theory 8 4Principle Arrangement of an 11 HVDC Transmission Project 5 Main Components 14 5.1 Thyristor Valves 15 5.2 Converter Transformer 18 5.3 Smoothing Reactor 21 5.4 Harmonic Filters22 5.4.1 AC Harmonic Filter 23 5.4.2 DC Harmonic Filter 25 5.4.3 Active Harmonic Filter 26 5.5 Surge Arrester 28 5.6 DC Transmission Circuit 5.6.1 DC Transmission Line 31 5.6.2 DC Cable 33 5.6.3 High Speed DC Switches 34 5.6.4 Earth Electrode 36 5.7 Control & Protection 38 6System Studies, Digital Models, 45 Design Specifications 7Project Management 46 3 1 Why High Voltage Direct Current ? 1.1 Highlights from the High Line-Commutated Current Sourced Self-Commutated Voltage Sourced Voltage Direct Current (HVDC) History Converters Converters The transmission and distribution of The invention of mercury arc rectifiers in Voltage sourced converters require electrical energy started with direct the nineteen-thirties made the design of semiconductor devices with turn-off current. In 1882, a 50-km-long 2-kV DC line-commutated current sourced capability. The development of Insulated transmission line was built between converters possible. Gate Bipolar Transistors (IGBT) with high Miesbach and Munich in Germany. voltage ratings have accelerated the At that time, conversion between In 1941, the first contract for a commer- development of voltage sourced reasonable consumer voltages and cial HVDC system was signed in converters for HVDC applications in the higher DC transmission voltages could Germany: 60 MW were to be supplied lower power range. only be realized by means of rotating to the city of Berlin via an underground DC machines. cable of 115 km length. The system The main characteristics of the voltage with ±200 kV and 150 A was ready for sourced converters are a compact design, In an AC system, voltage conversion is energizing in 1945. It was never put four-quadrant operation capability and simple. An AC transformer allows high into operation. high losses. power levels and high insulation levels within one unit, and has low losses. It is Since then, several large HVDC systems Siemens is offering voltage sourced a relatively simple device, which requires have been realized with mercury arc converters for HVDC applications with valves. ratings up to 250 MW under the trade little maintenance. Further, a three-phase plus synchronous generator is superior to a name HVDC Power Link Universal The replacement of mercury arc valves Systems. DC generator in every respect. For these by thyristor valves was the next major reasons, AC technology was introduced development. The first thyristor valves This paper focuses upon HVDC trans- at a very early stage in the development were put into operation in the late mission systems with high ratings, i.e. of electrical power systems. It was soon nineteen-seventies. with line-commutated current sourced accepted as the only feasible technology converters. for generation, transmission and distri- The outdoor valves for Cahora Bassa bution of electrical energy. were designed with oil-immersed thyristors with parallel/series connection However, high-voltage AC transmission of thyristors and an electromagnetic firing links have disadvantages, which may system. compel a change to DC technology: Further development went via air- • Inductive and capacitive elements of insulated air-cooled valves to the air- overhead lines and cables put limits insulated water-cooled design, which is to the transmission capacity and the still state of the art in HVDC valve design. transmission distance of AC trans- mission links. The development of thyristors with higher •This limitation is of particular signi- current and voltage ratings has eliminated ficance for cables. Depending on the the need for parallel connection and required transmission capacity, the reduced the number of series-connected system frequency and the loss eva- thyristors per valve. The development of luation, the achievable transmission light-triggered thyristors has further distance for an AC cable will be in the reduced the overall number of range of 40 to 100 km. It will mainly components and thus contributed to be limited by the charging current. increased reliability. • Direct connection between two AC Innovations in almost every other area systems with different frequencies is of HVDC have been constantly adding not possible. to the reliability of this technology with • Direct connection between two AC economic benefits for users throughout systems with the same frequency or the world. a new connection within a meshed grid may be impossible because of system instability, too high short-circuit levels or undesirable power flow scenarios. HVDC = high voltage direct current DC = direct current Engineers were therefore engaged over AC = alternating current generations in the development of a IGBT = insulated gate bipolar technology for DC transmissions as a transistor supplement to the AC transmissions. 4 1.2 Te chnical Merits of HVDC 1.3 Economic Considerations 1.4 Environmental Issues The advantages of a DC link over an AC For a given transmission task, feasibility An HVDC transmission system is basi- link are: studies are carried out before the final cally environment-friendly because decision on implementation of an HVAC improved energy transmission possi- •A DC link allows power transmission or HVDC system can be taken. Fig.1-1 bilities contribute to a more efficient between AC networks with different shows a typical cost comparison curve utilization of existing power plants. frequencies or networks, which can between AC and DC transmission not be synchronized, for other reasons. considering: The land coverage and the associated • Inductive and capacitive parameters right-of-way cost for an HVDC overhead do not limit the transmission capacity •AC vs. DC station terminal costs transmission line is not as high as that or the maximum length of a DC •AC vs. DC line costs of an AC line. This reduces the visual overhead line or cable. The conductor •AC vs. DC capitalised value of losses impact and saves land compensation for cross section is fully utilized because new projects. It is also possible to in- there is no skin effect. The DC curve is not as steep as the AC crease the power transmission capacity curve because of considerably lower line for existing rights of way. A comparison For a long cable connection, e.g. beyond costs per kilometre. For long AC lines between a DC and an AC overhead line 40 km, HVDC will in most cases offer the cost of intermediate reactive power is shown in Fig. 1-2. the only technical solution because of compensation has to be taken into the high charging current of an AC cable. account. This is of particular interest for trans- mission across open sea or into large The break-even distance is in the range cities where a DC cable may provide the of 500 to 800 km depending on a number only possible solution. of other factors, like country-specific cost elements, interest rates for project •A digital control system provides financing, loss evaluation, cost of right accurate and fast control of the active of way etc. power flow. •Fast modulation of DC transmission AC- DC- power can be used to damp power tower tower oscillations in an AC grid and thus improve the system stability. Fig. 1-2: Typical transmission line structures for approx. 1000 MW There are, however, some environmental issues which must be considered for the Costs Total AC Cost converter stations. The most important ones are: •Audible noise Total •Visual impact DC Cost • Electromagnetic compatibility • Use of ground or sea return path in monopolar operation AC Losses DC Losses In general, it can be said that an HVDC system is highly compatible with any environment and can be integrated into it without the need to compromise on DC Line any environmentally important issues of today. AC Line DC Terminals AC Terminals Break-Even Transmission Fig. 1-1: Distance Distance Total cost/distance 5 2 Main Types of HVDC Schemes 2.1 DC Circuit 2.2 Back-to-Back Converters 2.3 Monopolar Long-Distance Tr ansmissions The main types of HVDC converters are The expression Back-to-back indicates distinguished by their DC circuit arrange- that the rectifier and inverter are located For very long distances and in particular ments. The following equivalent circuit in the same station. for very long sea cable transmissions, a is a simplified representation of the return path with ground/sea electrodes DC circuit of an HVDC pole. Back-to-back converters are mainly used will be the most feasible solution. for power transmission between adjacent AC grids which can not be synchronized. They can also be used within a meshed grid in order to achieve a defined power HVDC ±±Id flow. Cable/OHL Ud1 Ud2 System 1 System Electrodes 2 System AC AC HVDC Fig. 2-1: Equivalent DC circuit Fig. 2-4: Monopole with ground return path The current, and thus the power flow, is System 2 System controlled by means of the difference 1 System In many cases, existing infrastructure or AC between the controlled voltages. The AC environmental constraints prevent the current direction is fixed and the power use of electrodes. In such cases, a direction is controlled by means of the metallic return path is used in spite of voltage polarity. The converter is de- Fig. 2-2: Back-to-back converter increased cost and losses. scribed in the next section. HVDC Cable/OHL System 1 System LVDC 2 System AC AC Fig. 2-5: Monopole with metallic return path HVDC = high voltage direct current DC = direct current AC = alternating current Ud = DC voltage 12-pulse Fig.