Transmission and Distribution Networks: AC versus DC

Transcription

1 Transmission an Distribution Networks: AC versus DC D.M. Larruskain 1, I. Zamora 1, A.J. Mazón 1, O. Abarrategui 1, J. Monasterio 2 1 Department of Electrical Engineering University of the Basque Country - Bilbao (Spain) phone: , fax: , 2 Avnet Iberia S.A.U. ieplaesm@lg.ehu.es, iepzabei@bi.ehu.es, iepmasaj@bi.ehu.es, gauoihan@yahoo.es, jorge.monasterio@avnet.com Abstract The fast evelopment of power electronics base on new an powerful semiconuctor evices has le to innovative technologies, such as HVDC, which can be applie to transmission an istribution systems. The istribution voltage level is smaller than the transmission one, thus the power electronics are less expensive in istribution. The technical an economical benefits of this technology represent an alternative to the application in AC systems. Some aspects, such as eregulation in the power inustry, opening of the market for elivery of cheaper energy to customers an increasing the capacity of transmission an istribution of the existing lines are creating aitional requirements for the operation of power systems. HVDC offer major avantages in meeting these requirements. Key wors: irect current, alternating current, power transmission, istribution. 1. Introuction The transmission an istribution of electrical energy starte with irect current (DC) in the late 19th century, but it was inefficient ue to the power loss in conuctors. Alternating current (AC) offere much better efficiency, since it coul easily be transforme to higher voltages, with far less loss of power. AC technology was soon accepte as the only feasible technology for generation, transmission an istribution of electrical energy. However, high-voltage AC transmission links have isavantages an engineers were engage in the evelopment of a technology for DC transmissions as a supplement to the AC transmissions. The invention of mercury arc rectifiers an the thyristor valves, mae the esign an evelopment of line-commutate current source converters possible. High Voltage Direct Current (HVDC) transmission finally prove to be technically feasible. The worl's first commercial HVDC transmission link, was built in 1954 between the Sweish mainlan an the islan of Gotlan, with a rating of 20 MW, 200 A an 100 kv. HVDC transmission base on current source converters has been in use for 50 years. The Insulate Gate Bipolar Transistors (IGBT) with high voltage ratings have accelerate the evelopment of voltage source converters for HVDC applications in the lower power range. Voltage Source Converter (VSC) HVDC transmission has come into use in the last years. This paper presents a comparison between the AC an DC transmission system technology. Economical, technical an environmental consierations of the AC an DC power flow are stuie. It reviews the unerlying technology an iscusses the HVDC systems from a esign, construction, operation an maintenance points of view. The paper also presents a numerical analysis of the power increase that can be achieve on an existing istribution network when a 3- phase link is substitute by DC link. 2. The HVDC technology The HVDC transmission systems are point-to-point configurations where a large amount of energy is transmitte between two regions. The traitional HVDC system is built with line commutate current source converters, base on thyristor valves. The operation of this converter requires a voltage source like synchronous generators or synchronous conensers in the AC network at both ens. The current commutate converters can not supply power to an AC system which has no local generation. The control of this system requires fast communication channels between the two stations. A. Components of HVDC transmission system The most relevant components that comprise a HVDC system, are the following: - The Thyristor or IGBT valves make the conversion from AC to DC an thus are the main component of any HVDC converter. Each single valve consists of a certain amount of series connecte thyristors (or IGBTs) with their auxiliary circuits.

2 AC bus Transformer Converter Smoothing reactor Transmisión cable AC Filter DC Filter Control System Fig. 1 HVDC system components - The Converter Transformers transform the voltage level of the AC busbar to the require entry voltage level of the converter. - The Smoothing reactor, which main functions are: Prevention of the intermittent current Limitation of the DC fault currents Prevention of resonance in the DC circuits - The Harmonic Filters, on the AC sie of a HVDC converter station, which have two main uties: To absorb harmonic currents generate by the HVDC converter To supply reactive power Also DC filter circuits have to be use. Besies Active Harmonic filters can be a supplement to passive filters ue to their better performance. - Surge arrester, which main task is to protect the equipment of over-voltages. - DC Transmission circuit, which inclue DC Transmission line, cable, high spee DC switches an earth electroe. - Control an Protection. A HVDC station requires consierable lan because the transformers, filters an phase correction capacitors are place outoors. However, the valves an control equipment are place in a close air-conitione/heate builing, this istribution is ue to the fact that the completely enclose system requires a large builing an is too expensive. B. Feasibility of HVDC transmission A HVDC system can be monopolar or bipolar. The monopolar system uses one high voltage conuctor an groun return. This is avantageous from an economic point of view, but is prohibite in some countries because the groun current causes corrosion of pipe lines an other burie metal objects. However, in Europe, monopolar systems are in operation. Most of them are use for submarine crossings. The bipolar system uses two conuctors, one with plus an one with minus polarity. The mi point is groune. In normal operation, the current circulates through the two high voltage conuctors without groun current. However, in case of conuctor failure, the system can transmit half of the power in monopolar moe. Besies, this operation can be maintaine for a limite time only. Recently, ABB an Siemens starte to buil HVDC systems using semiconuctor switches (IGBT or MOSFET) an pulse with moulation (PWM). The capacity of a HVDC system with VSCs is aroun MW. Operating experience is limite but many new systems are being built worlwie. The PWM controlle inverters an rectifiers, with IGBT or MOSFET switches, operate close to unity power factor an o not generate significant current harmonics in the AC supply. Also the PWM rive can be controlle very accurately. Typical losses claime by ABB for two converters is 5%. 3. HVDC Operation an Maintenance In general, basic parameters such as power to be transmitte, istance of transmission, voltage levels, temporary an continuous overloa, status of the network on the receiving en, environmental requirements etc. are require to initiate a esign of a HVDC system. For tenering purposes a conceptual esign is one following a technical specification or in close collaboration between the manufacturer an the customer. The final esign an specifications are in fact the result of the tenering an negotiations with the manufactures/suppliers. It is recommene that a turnkey approach be chosen to contract execution, which is the practice even in evelope countries. In terms of construction, it can take from three years for thyristor-base large HVDC systems, to just one year,

3 epening on the ifferent technologies, to go from contract ate to commissioning. To the extent that the term operation enotes the continual activities that are aime at keeping the system availability at esigne levels, moern HVDC links can be operate remotely, in view of the semiconuctor an microprocessor base control systems inclue. There are some existing installations in operation completely unmanne. Moreover, moern HVDC systems are esigne to operate unmanne. This feature is particularly important in situations or countries where skille people are few, an these few people can operate several HVDC links from one central location. Maintenance of HVDC systems is comparable to those of high voltage AC systems. The high voltage equipment in converter stations is comparable to the corresponing equipment in AC substations, an maintenance can be execute in the same way. Maintenance will focus on AC an DC filters, smoothing reactors, wall bushings, valvecooling equipment, semiconuctor valves. In all the above, aequate training an support is provie by the supplier uring the installation, commissioning an initial operation perio. Normal routine maintenance is recommene to be one week per year. The newer systems can even go for two years before requiring maintenance. In fact in a bipolar system, one pole at a time is stoppe uring the time require for the maintenance, an the other pole can normally continue to operate. Depening on the in-built overloa capacity it can take a part of the loa of the pole uner maintenance. In aition, preventive maintenance shall be pursue so that the equipment will achieve optimally balance availability with regar to the costs of maintenance, operating isturbances an planne outages. As a guieline value, the aim shall be to achieve an availability of 98 % accoring to Cigrè protocol While HVDC systems may only nee a few skille staff for operation an maintenance, several factors influence the number of staff neee at a station. These factors are: local routines an regulations, working conitions, union requirements, safety regulations, an other local rules can separately or together affect the total number of personnel require for the type of installe equipment. 4. HVDC system costs The cost of a HVDC transmission system epens on many factors, such as power capacity to be transmitte, type of transmission meium, environmental conitions an other safety, regulatory requirements etc. Even when these are available, the options available for optimal esign (ifferent commutation techniques, variety of filters, transformers etc.) rener it is ifficult to give a cost figure for a HVDC system. Nevertheless, a typical cost structure for the converter stations can be as shown in Fig % 14% Cost Structure 8% 5% 10% 20% 10% Fig. 2 Cost Structure 16% Valves Converter transformers AC Filters Control Other equipment Civil works, builings Engineering Erection, commissioning Freight, insurance 7% The cost of the traitional HVDC system is high because of the nee for filters, capacitors an other auxiliary equipment. The traitional HVDC system is esigne for the transmission of large amounts of energy measure in hunre of megawatts. This system is not economical less for than 20 MW loas. The price must be base on few ata, as rate power, transmission istance, type of transmission an voltage level in the AC networks where the converters are going to be connecte. When the voltage is lower the price goes own, so in istribution networks the total cost is lower than in the transmission ones. Following, an example of approximate cost is shown. These values can be use only to compare ifferent systems: HVDC system 50 MW, 100kV, Thyristor converter. Approximate per unit value is: 500 EUR/kW HVDC Light 50 MW, +/-84kV, IGBT converter pair. Approximate per unit value is: 150 EUR/kW Transformer 50MVA, 69kV/138kV. Approximate per unit value is: 7,5 EUR/kVA For a bipolar line HVDC transmission, a price of 190 keur/km is assume, converter stations are estimate to 190 MEUR. Besies, for a ouble circuit AC transmission a price of 190 keur/km (each) is assume, AC substations an series compensation (above 600 km) are estimate to 60 MEUR. It shoul be pointe out that the relationship between the cost an capacity (MW) is not linear, because the cost of the control system, communication system, auxiliary

4 electrical supply are more or less inepenent of the size of the converter. Similarly the size of the place an builing also has a lower limit. The cost figures inicate can be reasonably use above 10 MW. 5. DC versus AC. The vast majority of electric power transmissions use threephase alternating current. The reasons behin a choice of HVDC instea of AC to transmit power in a specific case are often numerous an complex. Each iniviual transmission project will isplay its own set of reasons justifying the choice. A. General characteristics The most common arguments favouring HVDC are: 1) Investment cost. A HVDC transmission line costs less than an AC line for the same transmission capacity. However, the terminal stations are more expensive in the HVDC case ue to the fact that they must perform the conversion from AC to DC an vice versa. On the other han, the costs of transmission meium (overhea lines an cables), lan acquisition/right-of-way costs are lower in the HVDC case. Moreover, the operation an maintenance costs are lower in the HVDC case. Initial loss levels are higher in the HVDC system, but they o not vary with istance. In contrast, loss levels increase with istance in a high voltage AC system meium, ifferent local aspects (permits, cost of local labour etc.) an an analysis must be mae for each iniviual case (Fig. 3). 2) Long istance water crossing. In a long AC cable transmission, the reactive power flow ue to the large cable capacitance will limit the maximum transmission istance. With HVDC there is no such limitation, why, for long cable links, HVDC is the only viable technical alternative. 3) Lower losses. An optimize HVDC transmission line has lower losses than AC lines for the same power capacity. The losses in the converter stations have of course to be ae, but since they are only about 0.6 % of the transmitte power in each station, the total HVDC transmission losses come out lower than the AC losses in practically all cases. HVDC cables also have lower losses than AC cables. 4) Asynchronous connection. It is sometimes ifficult or impossible to connect two AC networks ue to stability reasons. In such cases HVDC is the only way to make an exchange of power between the two networks possible. There are also HVDC links between networks with ifferent nominal frequencies (50 an 60 Hz) in Japan an South America. 5) Controllability. One of the funamental avantages with HVDC is that it is very easy to control the active power in the link Above a certain istance, the so calle "break-even istance", the HVDC alternative will always give the lowest cost. The break-even-istance is much smaller for submarine cables (typically about 50 km) than for an overhea line transmission. The istance epens on several factors, as transmission Cost Losses DC line cost DC terminal cost Total AC cost Losses DC line cost Total DC cost DC terminal cost Distance (km) 6) Limit short circuit currents. A HVDC transmission oes not contribute to the short circuit current of the interconnecte AC system. 7) Environment. Improve energy transmission possibilities contribute to a more efficient utilization of existing power plants. The lan coverage an the associate right-of-way cost for a HVDC overhea transmission line is not as high as for an AC line. This reuces the visual impact. It is also possible to increase the power transmission capacity for existing rights of way. There are, however, some environmental issues which must be consiere for the converter stations, such as: auible noise, visual impact, electromagnetic compatibility an use of groun or sea return path in monopolar operation. In general, it can be sai that a HVDC system is highly compatible with any environment an can be integrate into it without the nee to compromise on any environmentally important issues of toay. Fig. 3 HVAC-HVDC cost

5 B. Power carrying capability of AC an DC lines It is ifficult to compare transmission capacity of AC lines an DC lines. For AC the actual transmission capacity is a function of reactive power requirements an security of operation (stability). For DC it epens mainly on the thermal constraints of the line. If for a given insulation length, the ratio of continuousworking withstan voltage is as inicate in equation (1). k DC withs tan voltage = (1) AC withs tan voltage( rms) Various experiments on outoor DC overhea-line insulators have emonstrate that ue to unfavourable effects there is some precipitation of pollution on one en of the insulators an a safe factor uner such conitions is k=1. However if an overhea line is passing through a reasonably clean area, k may be as high as 2, corresponing to the peak value of rms alternating voltage. For cables however k equals at last 2. A line has to be insulate for overvoltages expecte uring faults, switching operations, etc. AC transmission lines are normally insulate against overvoltages of more than 4 times the normal rms voltage; this insulation requirement can be met by insulation corresponing to an AC voltage of 2.5 to 3 times the normal rate voltage. AC insulation level k 1 = 2.5 rate AC voltage( ) = (2) On the other han with suitable conversor control the corresponing HVDC transmission ratio is shown in equation (3). k 2 E p DC insulation level = =1.7 (3) rate DC voltage( ) Thus for a DC pole to earth voltage V an AC phase to earth voltage E p the relations (4) exist. insulation length require for each AC phase Insulation ratio = (4) insulation length require for each DC pole an substituting (1), (2) an (3) equations, we obtain equation (5) for the insulation ratio. V p k E Insulation ratio = k 1 k 2 V p (5) DC transmission capacity of an existing three-phase ouble circuit AC line. The AC line can be converte to three DC circuits, each having two conuctors at ± V to earth respectively. Power transmitte by AC: Power transmitte by DC: P = 6 E I (6) a p L P = 6 V I (7) On the basis of equal current an insulation I L = I (8) k V = 1 k E p k 2 The following relation shows the power ratio. P P V k k k = 1 a E p 2 (9) (10) For the same values of k, k 1 an k 2 as above, the power transmitte by overhea lines can be increase to 147%, with the percentage line losses reuce to 68% an corresponing figures for cables are 294 % an 34% respectively. Besies, if the AC line is converte, a more substantial power upgraing is possible. There are several conversions of AC lines to DC lines proposals [2], these conversions are carrie out as a simple reconstruction. The most feasible of them is Double Circuit AC Conversion to Bipolar DC, it implies tower moifications that maintain all the conuctors at a height above groun of 1 to 2 meters below the original position of the lowest conuctor uring the whole construction phase. Two new crossarms are inserte at the level of the ol intermeiate crossarm. No change is mae to the conuctors, the total rate current remains the same, which means that the transmitte power increases proportionally to the aopte new DC line-togroun voltage. The conversion of lines where an increase of phase to groun voltage can be higher than 3, is possible when all the conuctors of one AC circuit are concentrate in one DC pole. The line to line (LL) AC voltage is ouble for use with DC, thus the transmitte power will increase by 3.5 times. U ( DC) = 2Urms( AC) (11)

6 Table I: Transmitte power an losses for the original ouble circuit AC lines an the converte DC lines AC 0,7 A/mm2 1.0 A/mm2 1.4 A/mm2 Voltage (kv) Joule % of P Joule % of P Joule % of P (per 10Km) (per 10Km) (per 10Km) DC 0,7 A/mm2 1.0 A/mm2 1.4 A/mm2 Voltage (kv) Joule % of P Joule % of P Joule % of P (per 10Km) (per 10Km) (per 10Km) Table I shows the transmitte power an relative losses as a function of current ensity for istribution networks with a 25 mm iameter conuctor. Only Joule losses are taken into account, because they are the most important in istribution voltage levels. The numerical values are function of the efine factors, an are proximate values that show the magnitue orer of the power increases that can be achieve. 6. Conclusions The construction of new overhea electric lines is increasing ifficulty, thus there is a nee to look at alternatives that increases the power transfer capability of the existing right of ways. It is technically feasible to achieve a substantial power upgraing of existing AC lines through their conversion for use with DC, by using the same conuctors, tower boies an founations, but with changes in tower hea an insulation assemblies. When using existing AC lines to transmit DC power, the lines are alreay built, so that cost can be save. The istribution networks cost is lower than the transmission ones, because of the lower voltage level applie to the semiconuctor cost. It is also remarkable the fast evelopment of multiterminal DC systems. DC transmission has many more avantages, such as stability, controlle emergency support an no contribution to short circuit level. References [1] Jos Arrillaga, High Voltage Direct Current Transmission, E. Peter Peregrinus, [2] Alessanro Clerici, Luigi Paris, Per Danfors, HVDC conversion of HVAC lines to provie substantial power upgraing, IEEE Transactions on Power Delivery, Vol. 6, No.1 January [3] Roberto Ruervall, Jan Johansson, Interconexión e sistemas eléctricos con HVDC. Seminario internacional e interconexiones regionales CIGRE, Santiago e Chile, Noviembre [4] Mesut E. Baran, Nikhil R. Mahajan, DC Distribution for Inustrial Systems: Opportunities an Challenges, IEEE Transactions on Inustry Applications, vol. 39, No. 6, November/December [5] Ambra Sannino, Giovanna Postiglione, an Math H. J. Bollen, Feasibility of a DC Network for Commercial Facilities, IEEE Transactions on Inustry Applications, vol. 39, No. 5, September/October 2003.