High Voltage Direct Current(HVDC) By Pavel Gabriel

Introduction Electric power transmission was originally developed with direct current. The availability of transformers and the development and improvement of induction motors at the beginning of the 20th Century, led to greater appeal and use of ac transmission. Power transmission using HVDC lines is more cost effective than with AC lines, but requires more complicated and expensive interconnections (converters) at each end of the line. The application of HVDC power transmission is therefore limited to transmission of large energies over long distances. Moreover, HVDC is the only practical option when transporting electric energy through cables over long distances. Another special application of HVDC is as a link between AC networks of different frequencies, or between AC networks that become unstable when tied togethter directly. The HVDC link is here short and the term "back-to-back" is used to characterize such a configuration. Dc transmission became practical when long distances were to be covered or where cables were required. Originally, mercury arc valves were used in the converters. Thyristors were applied in the late 1960’s and solid state valves became a reality.

The HVDC Technology The fundamental process that occurs in an HVDC system is the conversion of electrical current from AC to DC (rectifier) at the transmitting end, and from DC to AC (inverter) at the receiving end. There are three ways of achieving conversion: Natural Commutated Converters. Natural commutated converters are most used in the HVDC systems as of today. The component that enables this conversion process is the thyristor, which is a controllable semiconductor that can carry very high currents (4000 A) and is able to block very high voltages (up to 10 kV). Capacitor Commutated Converters (CCC). An improvement in the thyristor-based commutation, the CCC concept is characterised by the use of commutation capacitors inserted in series between the converter transformers and the thyristor valves. The commutation capacitors improve the commutation failure performance of the converters when connected to weak networks.

The components of an HVDC transmission system Forced Commutated Converters. This type of converters introduces a spectrum of advantages, e.g. feed of passive networks (without generation), independent control of active and reactive power, power quality. The valves of these converters are built up with semiconductors with the ability not only to turn-on but also to turn-off. They are known as VSC (Voltage Source Converters). The components of an HVDC transmission system To assist the designers of transmission systems, the components that comprise the HVDC system, and the options available in these components, are presented and discussed. The three main elements of an HVDC system are: the converter station at the transmission and receiving ends, the transmission medium, and the electrodes.

AC Filters and Capacitor Banks The converter station: The converter stations at each end are replica’s of each other and therefore consists of all the needed equipment for going from AC to DC or vice versa. The main component of a converter station are: Thyristor valves VSC valves Transformers DC filters AC Filters and Capacitor Banks

Transmission medium For bulk power transmission over land, the most frequent transmission medium used is the overhead line.This overhead line is normally bipolar, i.e. two conductors with different polarity. HVDC cables are normally used for submarine transmission. The most common types of cables are the solid and the oil-filled ones. The solid type is in many cases the most economic one. Its insulation consists of paper tapes impregnated with a high viscosity oil. No length limitation exists for this type and designs are today available for depths of about 1000 m. The self –contained oil-filled cable is completely filled with a low viscosity oil and always works under pressure. The maximum length for this cable type seems to be around 60 km. The development of new power cable technologies has accelerated in recent years and today a new HVDC cable is available for HVDC underground or submarine power transmissions. This new HVDC cable is made of extruded polyethylene, and is used in VSC based HVDC systems.

Thyristors Load voltage regulated by thyristor phase control. Red trace: load voltage Blue trace: trigger signal. Thyristors can be used as the control elements for phase angle triggered controllers, also known as phase fired controllers. Phase control (PFC), also called phase cutting, is a method of pulse width modulation (PWM) for power limiting, applied to AC voltages. It works by modulating a thyristor, SCR, triac, thyratron, or other such gated diode-like devices into and out of conduction at a predetermined phase of the applied waveform.

Advantages of HVDC over AC Despite alternating-current being the dominant mode for electric power transmission, in a number of applications HVDC is often the preferred option. Undersea cables (Ex: 250 km Baltic Cable between Germany and Sweden) Increasing the capacity of an existing power-grid in situation where additional wire are difficult or expensive to instal Allowing power transmission between unsynchronised AC distribution systems. Reducing the profile of wiring and pylons for a given power transmission capacity Stabilising a predominantly AC power-grid Long undersea cables have a high capacitance.This causes AC power to be lost extremely quickly in reactive and dielectric losses, even on cables of a modest length. HVDC can carry more power per conductor,because for a given power rating the constant voltage in a DC line is lover 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 ued to carry more power into an are of high power consumption,which can lower the costs!

Disadvantages of HVDC over AC The disadvanteges of HVDC are in conversion,switching ,control ,availability of link capacity and maintenance. HVDC is less reliable and has lower availability than AC systems, mainly due to the extra conversion equipment. Single pole systems have availability of about 98.5%, with about a third of the downtime unscheduled due to faults. Fault redundant bipole systems provide high availability for 50% of the link capacity, but availability of the full capacity is about 97% to 98% The required static inverters are expensive and have limited overload capacity.At smaller transmission distances the losses in static inverters may be bigger than in an AC transmission line! In contrast to AC systems ,realizing multiterminal systems is complex,as is expanding existinf schemes to multiterminal systems. High voltage DC circuit breakers are difficult to build because some mechanism must be included in the circuit breaker to force to zero! Operating a HVDC scheme requires many spare parts to be kept,often exclusively for one system as HVDC systems are less standardized than AC systems and technology chandes faster!

Cost of HVDC Transmission Costs vary widely depending on the specifics of the project such as power rating, circuit length, overhead vs. underwater route, land costs, and AC network improvements required at either terminal. A detailed evaluation of DC vs. AC cost may be required where there is no clear technical advantage to DC alone and only economics drives the selection. For an 8 GW 40 km link laid under sea, the following are approximate primary equipment costs for a 2000 MW 500 kV bipolar conventional HVDC link (exclude way-leaving, on-shore reinforcement works, consenting, engineering, insurance, etc.) Converter stations = 110 milion of euro Subsea cable+installation= 1 milion / km So for an 8 GW capacity ,40 km lenght the cost will be of almost 750 milion of euro! But there are requiered another 300 milion for the other works!

Iceland-UK(1100MW) Uk-Ireland Norway-UK(1200MW) Germany-Norway(600MW) Norway-Denmark Finland to Sweden Vyborg Estonia to Finland Norway to Netherlands Denmark to Norway Denmark to Sweden(600MW) Sweden to Sweden Germany-Germany Denmark to Denmark None Sweden to Poland Germany to Sweden Denmark to Germany Ireland to Uk UK to Netherlands(1000MW) France to Uk Italy to France Italy to Italy Spain to Spain Till 28 is EuroMed