HVDC Transmission Opportunities and Challenges

Transcription

1 HVDC Transmission Opportunities and Challenges Dr B.R. Andersen Andersen Power Electronic Solutions Ltd, UK Keywords: HVDC, Power Transmission. Abstract HVDC transmission has a number of technical advantages compared with HVAC transmission, which have been highlighted in many papers. However, HVDC transmission has remained a niche technology, being used only for long distance overhead transmission, submarine cable and frequency conversion. Nevertheless, HVDC projects often provide strategically important enhancements and costeffective additions to ac networks. Recent developments in energy policies and stronger environmental lobbies could make HVDC transmission attractive for many more applications. This paper reviews technical issues faced by users of HVDC transmission and discusses how HVDC could be made more generally acceptable as a transmission solution. 1. Introduction HVDC transmission has been in use for more than 50 years. It has proved to be a reliable and valuable transmission media for electrical energy and has a number of technical advantages compared with HVAC transmission. Nonetheless, a comprehensive HVDC/HVAC system planning approach is not commonly found within utilities, and therefore full advantage is not being taken of the HVDC technology. Recent developments in energy policies and stronger environmental lobbies have a significant impact on the design and construction of electrical power transmission networks, and could provide a number of opportunities for HVDC transmission. However, HVDC transmission is perceived to be expensive, difficult to integrate in an ac network, to require highly skilled personnel to operate and maintain, and to have high power losses. This paper will briefly describe the present HVDC technology, will discuss the developments in energy policies and transmission networks, and will outline opportunities that may arise for HVDC transmission. inally, the paper will review the challenges presently preventing greater acceptance of HVDC as the solution to transmission needs, and will discuss how these challenges could be overcome. 2. Characteristics of HVDC Transmission The characteristics of an HVDC scheme have been described in numerous papers and documents[1][2]. Therefore, only a brief overview of the main characteristics of the technology and its present implementation will be provided here. HVDC transmission is now available in two different technologies, i.e. line-commutated current-sourced converter (LCC HVDC) and self-commutated voltage sourced converters (VSC Transmission). Both technologies convert ac to dc and vice versa, and use direct current for transmission between terminals. This means that power transmission can be performed between asynchronous networks. There is no reactive power flow on the dc line, therefore, there is no technical limit to the transmission distance. The limit to distance is economic, since the power loss in the transmission line may eventually become unacceptably high, when practical conductor diameters are used. The practical transmission distance increases with the voltage. 2.1 LCC HVDC igure 1 shows a simplified diagram of a mono-polar LCC HVDC scheme, which has one converter at each end and provides a single transmission block. It is generally considered equivalent to a single-circuit ac transmission link. Convertor Transformer Rectifier Harmonic ilters + I dc Inverter Convertor Transformer igure 1. Simplified Monopolar HVDC scheme The rectifier takes power from its ac network and the inverter injects power into its ac network. Control systems control the two converters such that the desired active power is transmitted between the two. One terminal controls the dc voltage, and the other the direct current. The active power between the converters is fully controlled and does not depend on the magnitude, phase angle or frequency of the ac voltage at either end of the HVDC scheme. The ability to rapidly control the active power can be very beneficial, e.g. for the damping of power swings in one of the network. The line-commutated converter depends on the ac system voltage for its satisfactory operation. Thyristor switch the converter ac terminals between the two dc terminals. The thyristor can be turned on by a gate signal when the voltage Harmonic ilters

2 across it is forward biased. The thyristor can conduct current in one direction only, and it turns off when the current through it attempts to reverse. Two series connected six pulse bridges are typically used, giving 12-pulse operation. The line commutated converter operates at a lagging power factor, partly because of the inductance of the converter transformer, but primarily because the firing of the thyristors in the rectifier has to be delayed relative to the voltage crossing to control the dc voltage. Similarly, the commutation at the inverter has to be completed at least 10º electrical before the voltage crossing, to enable the thyristor to build up its withstand voltage before the voltage becomes forward biased. The commutation process also results in the generation of substantial amounts of harmonic current, primarily at harmonics of the order 12n±1 on the ac side. AC harmonic filters are used to reduce the harmonic distortion at the ac terminals. The filters are capacitive at fundamental frequency, and are subdivided into banks, which are switched in and out as required to limit the harmonic distortion and to provide reactive power compensation/ac voltage control. The ac harmonic filters and reactive power banks together with their ac switchgear occupy a very large land area. Therefore, an HVDC converter station is many times (>10 times) larger than an equivalently rated ac substation. Because of their capacitance the ac harmonic filters/reactive power banks can result in large ac over-voltages during load rejection and dynamic conditions, e.g. during fault recovery. Typically an LCC HVDC scheme needs to be connected to a point in the ac network where the short circuit power is at least 2.5 times the rating of the HVDC scheme, in order to achieve stable and satisfactory operation. The total rating of LCC HVDC schemes in service today (2006) is approximately 60GW, the largest scheme having a rating of 6,300MW. The highest dc voltage used for a LCC HVDC scheme is ±600kVdc. Typically, an LCC HVDC scheme will achieve an availability of 98-99%, and will have an efficiency at full load of >98.3%, including the loss in both terminals, but excluding the loss in the transmission line. 2.2 VSC Transmission igure 2 shows a simplified diagram of a VSC Transmission scheme. The scheme has one converter at each end and is a single transmission block, and is generally considered equivalent to a single-circuit ac transmission link. The Voltage Sourced Converter (VSC) creates an ac voltage by switching the ac terminals between the dc terminals. The switches use Insulated Gate Bipolar Transistors (IGBTs), which can be switched on and off by a gate signal, even if there is current flowing through the switch at the time it is instructed to switch. The IGBT can withstand voltage and conduct current in one direction only, and use a diode connected in anti parallel, to enable the converter to conduct direct current in both directions. In some converters, e.g. the 3-level neutral point clamped converter, one or more intermediate dc voltage levels can be created, such that the waveshape more closely resembles a sinusoid. The converter switches can be switched on and off several times during each power frequency cycle, if required. Typically the converter switches are operated at a mean frequency of about 1kHz, and is switched in such a way that certain lower order harmonics are eliminated. As a result, filters are required only for higher frequency harmonics, and can be much lower rating than those used for LCC HVDC schemes. Sending End VSC A ~ = + - igure 2. Simplified VSC Transmission scheme Seen from the ac network, the VSC Transmission terminal is equivalent to a voltage source with an amplitude and phase angle determined by the control system. igure 3 shows the operating capability of a VSC terminal, with the three circles indicating the capability for different ac voltages. DC cable Thermal limit Inductive Rectifier Mode Inverter Mode R dc I dc P conv igure 3:VSC Transmission Scheme Operating Capability The active power exchange with the ac network is controlled primarily by the phase angle of the voltage created by the VSC. The active power balance on the dc side needs to be preserved, and therefore one terminal is allocated to control the direct voltage, whilst the other controls the active power exchanged at its ac network connection.. The reactive power exchange with the ac network is controlled primarily by the magnitude of the voltage created by the VSC. The reactive power exchange can be controlled independently at the two converters, and independently of the active power transmission. The ability to control the reactive power at the ac terminals is one of the most significant differences between a VSC Transmission scheme and a LCC HVDC scheme. Typically, because of the controllability of the reactive power, the ac harmonic filters are not need subdivided or switchable, and the space occupied by a VSC Transmission substation is less than 35% of that of a LCC HVDC terminal. + - Receiving End VSC B = ~ Desired Reactive Power Desired Active Power Q conv Capacitive U ac = Max U ac = Nom U ac = Min

3 Another significant difference is that the VSC Transmission scheme generates its own ac voltage from the dc capacitor, which means that it can operate as a power supply to a passive ac network. In this operating mode the scheme is operated similarly to a motor drive, which in fact is the technology from which VSC Transmission has been derived. VSC Transmission was introduced in 1997 through a 3MW, ±10kVdc technology demonstrator. There are now 7 schemes in service world wide, with a further scheme under construction, which will give a total rating of >1200MW, the largest being a ±150kVdc, 350MW scheme. All schemes in service have used cables as the transmission media. Availability records for VSC Transmission have not been published. Typically, a VSC Transmission scheme using the latest technology will have an efficiency at full load of >96.5%, excluding the power loss in the transmission line. 3. Developments in Energy Policies The first effects of the unbundling of the electricity supply industry, has typically been a move towards generation providing a quick return on investment. In the UK, and in many other places, this resulted in the so-called dash for gas and consequently to the closure of older less efficient thermal power stations. In the unbundled and competitive environment little investment was made in more risky and longer term generation plant, such as Nuclear or Hydro power stations. Nevertheless, the use of more efficient generating plant was beneficial in the short term for the environment, resulting in a reduction in CO 2 gas. The siting of new generation plant was determined largely by the access to the gas, and in spite of the connection charging structure encouraging siting of new generation to suit network loading, ac networks still came under increasing stress. With growing concerns over Global Climate Change, many governments decided to encourage the development of renewable energy sources through subsidies and preferential price levels within the energy market. At this juncture wind generation is the most efficient and cheapest source of renewable power, and the political support has resulted in a dramatic growth in this source of energy. The best wind resource is often located at considerable distance from major load centres. or example, in the UK excellent wind resources can be found in the northern parts of Scotland, where a generation factor of >45% can be found in many land based locations. Transport of remote wind energy puts additional stress on the transmission network and enhancement of the transmission infrastructure is required. Recent events, such as the energy dispute in early January 2005 between the Russian Confederation and Ukraine, has highlighted the need for a balanced energy portfolio. Whilst wave and tidal current generation, bio-mass, bio-crops and other renewable energy sources could play a significant role in obtaining the desired diversity, other sources such as Nuclear Power and clean generation from coal with CO 2 recovery are also being seriously considered. However, the new generation resources are unlikely to be located at the optimum point from the perspective of network loads and power flows. Interconnections between national networks are being recognised as being essential for mutual support and to provide economic access to diverse energy sources. Some of the links now being considered involve considerable transmission distance over land or as submarine links. 4. Developments in Transmission Networks In Europe the public resistance to overhead lines has grown steadily during the last couple of decades. The objections are caused by fear of detrimental health effects from magnetic fields. urthermore, objections are raised on environmental grounds, including visual impact, audible noise, impact on birds and other wildlife. With electricity being considered to be cheap, the public seems prepared to pay the extra cost of mitigation of the environmental issues. Approval for a new transmission line requires a lengthy and time-consuming public enquiry, even when an existing line is to be replaced by a higher voltage line. The full exploitation of wind resources in North Scotland is dependent on the strengthening of the transmission infrastructure, to enable the energy to be exported to load centres in England. However, permission to build new or upgrade existing ac lines, if granted, could be delayed by at 5 and possibly 10 years because of the objections and necessary public enquiries. Because of the difficulty of constructing new lines, the network operators use a number of measures to increase the capacity of the existing lines. Such measures include uprating of the line s current capability, e.g. by re-conductoring or permitting higher operating temperature. Achieving higher controllability of the power flow in the ac network is also an important part of increasing the overall transmission capacity. This is achieved by the installation of series compensation (controlled or fixed), phase angle control (e.g. quadrature boosters) and shunt reactive power control (fixed, breaker switched or dynamic). Increasing the transmission capability of an ac network without additional lines or increasing the ac voltage will of course increase the power loss in the transmission network. In some cases the addition of an underground HVDC cable solution may prove to be more economic, than an ac overhead line when taking all factors into account. The issues to be considered includes: Capital Cost of stations (HVDC converters or HVAC substations, including installation and cost of land) Capital Cost of land for line ( overhead line requires much more than that required for a cable route) Cost of Consent Process (EIA, Consultation, Legal, etc) Cost of Delay, including loss of opportunities Power losses, both for scheme itself and its impact on the existing ac network Reliability & Availability, including cost of potential black outs caused by delay in network strengthening.

4 Maintenance & Operation Cost Auxiliary service benefits, e.g. impact on overall network transmission capability In the past cable links have been considered to cost up to 10 times as much as an overhead line. However, when taking into account developments in cable and converter technology the HVDC cable option may be the most economic option! In Sweden such a comparison is presently being carried out for the Snitt 4 grid re-inforcement project [4]. The comparison is between a 400km long 400kV ac overhead line and a ±300kVdc option. Network owners and operators will be forced to use cables for more and more stretches of transmission, particularly in urban areas, because of growing opposition to overhead lines. or long distances, say >100km, dc cable transmission could be the most economic option. 5. Challenges and Opportunities 5.1 Wind Power and Energy Diversity One of the challenges of wind power is its intermittency. In order to ensure continuation of power supply during periods of calm, other sources of power generation must be provided to take over power supply during such periods. Today long term energy storage is available only as pumped storage or reservoir storage for hydro power. Hydrogen energy storage needs substantial R&D to reduce cost. Strong interconnections from strategic points in the ac network to geographically remote points in other networks could provide both energy diversity and alleviate the intermittency of wind power. By overlaying the existing networks with Super Interconnectors, bottlenecks could be avoided and power losses reduced. Super Interconnectors would enable mutual support between networks, and would thus allow more intermittent sources to be applied overall. In principle, Super Interconnectors could be merchant links and used for energy trading during normal conditions, but would need to provide support to the underlying ac networks, when required. However, public/ governmental ownership would seem more appropriate, showing the commitment to renewable power generation and a means of increasing energy diversity and system security. Super Interconnectors would be long distance and with high rating of individual power blocks, and would be terminated at strong points in the ac network. Very high voltage HVDC links, say 800kVdc or even 1000kVdc, could provide an economic solution to Super Interconnectors. At this juncture the largest power HVDC scheme is 2 x 3150MW, ±600kVdc at Itaipu, Brazil. However, the need for projects with higher rating and dc voltage has been identified in China, India, Africa and Brazil, and some manufacturers have started R&D activities linked to such projects. The challenge of the higher voltage has to be met for the HVDC converter and for overhead lines and cables. or very high power land based applications, it might be interesting to develop Gas Insulated dc Lines as an alternative to cables. 5.2 AC Network Enhancement The capacity of an ac network could be increased by addition of HVDC overhead lines or dc cables, or by the conversion of ac lines to dc operation. Where the main objection to the construction of overhead lines is the fear of its impact on health, there could be an argument for the use of dc transmission instead of ac transmission. This is because the magnetic field from a bipolar dc line is static and generally less than the earth s magnetic field at any location accessible to the public. However, to categorically prove that a dc line has no negative health impact would be a challenge. Converting an ac line to HVDC operation could provide an increase in transmission capacity of up to 100% of the existing capacity [3]. Such an increase in capacity could be very worthwhile. During the B4 session at the 2004 CIGRE meeting a new method for the conversion of ac lines to HVDC operation was outlined [5]. The idea is to use all three conductors of the ac line, with three converters, one being capable of bi-directional current operation, and to cycle the current duty between the three converters as shown in igure 4. Pole 1 Pole 2 Pole 3 igure 4 Operation of 3 line, 3 pole HVDC scheme The idea has the advantage of utilisation of the full current capability of the three existing conductors, and does not require a metallic earth return. urther development work would be necessary to make it a realistic option. 6. HVDC System Challenges The technology required to imbed HVDC schemes in an ac network is available today. However, there are a number of challenges to overcome in order to enable HVDC transmission to be the technology of choice for more of the opportunities that will become available. The following subsections will review these issues. 6.1 Cost and value of HVDC rms Thermal Limit The market for HVDC has traditionally been relatively small, and there are only very few manufacturers capable of providing such systems. With few projects, almost all of which require bespoke engineering, the benefit of mass production are not available, and costs become relatively high. Naturally, cost levels could never become similar to

5 those for an ordinary ac substation of similar rating, because the advantageous technical performance of a HVDC system necessitates the use of many more components than an ac substation. The challenge for proponents of HVDC is to ensure that the value of the technical characteristics of an HVDC system are fully recognised: ull and fast control of the power flow Enhancement of ac networks (power oscillation damping capability, increased transmission capacity of parallel lines, etc) No contribution to system short circuit level AC voltage control (smooth control with VSC Transmission) Additionally, the HVDC manufacturers must continue their R&D to improve performance and continue to drive down prices, as has been seen during the last decade or so, such that all opportunities can be targeted and the market can grow. inally, increases in volume would result in a reduction of price levels. 6.2 Power Loss The power loss in a HVDC converter station is higher than that in an ac substation, because of the conversion between ac and dc and the harmonics produced by this process. However, the power loss in a HVDC transmission line can be 50 to 70% of that in an equivalent HVAC transmission line. Thus for large distances, an HVDC solution may have lower loss. Nevertheless, it would be desirable to reduce the power loss in the converter stations. Thyristors are highly efficient conversion devices, and the efficiency of each LCC HVDC converter station is typically about 99.3%. The efficiency of the converter stations in a VSC Transmission scheme is today typically around 98.2%, a value which is significantly higher than that for the first generation of the technology. R&D for VSC Transmission has power loss reduction as a high priority. However, whilst a further significant reduction is likely, the power loss is unlikely to become as low as that of a LCC HVDC scheme, because of the use of transistors, rather than thyristors. A significant reduction in the power loss of a HVDC scheme might result from new generations of semi-conductors, e.g. the use of Silicon Carbide, diamond or other materials. Meanwhile, proponents of HVDC must elaborate on the overall power loss comparison. This should take into account the loss in converters, dc lines and any power loss reduction in the ac network, e.g. elimination of loop power flows and balancing of power flow in ac lines. 6.3 Complexity of HVDC schemes An HVDC system is relatively complex, but is in fact easy to operate, since the control system executes the necessary detailed sequences and control commands to achieve high level objectives given by the operator, e.g. power order and reactive power exchange with the ac network. Specially trained personnel are required for maintenance and fault finding of HVDC scheme equipment. These activities occur infrequently, but time has to be allocated to enable the personnel to keep their skills up to date, and this cost needs to be taken into account by the Owner. This situation is not that different from the maintenance and fault finding of the complex SCADA systems in ac substations. In fact, Vendors of the equipment or specialist companies often provide this service. By providing long term service contracts as an extension to the HVDC scheme contract, the costs become known, and the network operator can take these costs into account in his comparisons. The maintenance and fault finding requirements could be reduced by further development of the monitoring system. Self diagnosis of problems is performed by some HVDC solutions, providing the personnel with step by step instruction for its rectification. Such systems remove the need for day to day involvement of specialists. However, specialists are likely to still be required to solve the more rare problems Dispatch and Control of HVDC Scheme Network operators wish to dispatch an HVDC scheme as if it were a generator or a large controllable load. One of the great benefits of any type of HVDC scheme is that its active power can be controlled irrespective of the ac voltage phase angle or angle at its terminals. Grid codes typically stipulate that a generator has to be able to operate with a controllable power factor, and that the reactive power capability has to be available throughout most of its operating range. Typically, ac voltage controllability is also required. The ability of a VSC Transmission scheme to control the reactive power at its two terminals independently of each other and independently of the active power transmission is a valuable technical benefit in this respect. A LCC HVDC scheme can change its power factor by the switching of ac harmonic filters and shunt capacitors/reactors. The resulting control of reactive power/ac voltage is in steps, which is generally acceptable to the ac network, particularly if the ac network is relatively strong. Smooth control of the reactive power by a LCC HVDC scheme could be achieved by the addition of a SVC at the ac terminals. In principle, the reactive power could also be controlled by the insertion of a TCSC in series with the converter transformer impedance. The reactive power could also be controlled by the converter firing angle, and the steady state impact at the other terminal could be eliminated through converter transformer tapchanger action. urther developments in this area could improve the performance and acceptability of LCC HVDC. 6.5 Integration of HVDC scheme in AC network Integration of a HVDC terminal into an ac system requires some specialist engineering. The large ac harmonic filters, particularly for LCC HVDC, can cause significant overvoltages during fault recovery, if the ac network strength is relatively weak. Development in HVDC control has resulted in improved performance during and after faults in the ac network, and the performance can be optimised to suit particular network requirements. Nonetheless, the

6 performance is different from that of an ac connection, and network planners have a natural tendency to use the more familiar ac options, even though the system performance could, in some cases, be improved with an HVDC scheme. The dynamic and transient performance of an HVDC scheme can be improved by the incorporation of dynamic reactive power control capability. This capability is already available with VSC Transmission, and could be added to LCC HVDC, either through new circuit topologies and control algorithms, or by the addition of new components, such as shunt or series reactive power compensation. 6.6 Harmonics All power electronic converters produce harmonics as a byproduct of the conversion process. In order to prevent these harmonics spreading into the ac network, where they could cause problems, ac harmonic filters are used at the ac terminals of the HVDC scheme. As the number of converters connected to an ac network increases, the harmonic pollution in the network increases, as filtering is not perfect. Therefore, the harmonic pollution that a new scheme is permitted to contribute is reduced, making ac harmonic filtering increasingly difficult, and therefore expensive. Since LCC HVDC produces harmonics at relatively low frequencies (primarily 550Hz and above), the problem is worse for this type of HVDC than it is for VSC Transmission (usually >1kHz). Another issue is that the ac harmonic filters and any shunt capacitor banks used for reactive power compensation can actually cause magnification of the distortion caused by other remote harmonic sources. HVDC manufacturers need to consider new converter topologies and the commercialisation of low-cost active ac harmonic filters, which would provide adaptable filtering of harmonics over a broad range. 6.7 Operation of HVDC Scheme with Ground return The cost of an HVDC system can be significantly if it is permissible to operate with a single/hv metallic conductor. urthermore, the power loss in the transmission line during earth return operation is almost half of that applicable to operation with a LV metallic return conductor. Early HVDC schemes routinely used earth or sea electrodes for the neutral return current, when operating in mono-polar mode. Naturally, care must be taken in the design and location of electrodes, since the direct current flowing between them could result in corrosion of metallic structures. During the last decade environmentalists have increasingly expressed concerns for the wellbeing of organisms and creatures in the vicinity of the electrodes. No detrimental effects have been proven, but planning permission for electrodes has become difficult to obtain. Deep earth electrodes were tested with mixed success on the Baltic cable scheme, and more R&D would be necessary to achieve a satisfactory solution. CIGRE Working Group B4-44 Planning Guidelines Dealing with HVDC Environmental Issues is looking at the issues of earth electrodes, as well as a number of other environmental factors, e.g. audible noise, magnetic fields, etc. 6.8 Stability of Network with multi-infeed of HVDC If HVDC were used for many more applications in a network, then the issue of interaction between multiple HVDC schemes would become increasingly important. Commutation failures, which are typically caused by large voltage dips or sudden ac voltage phase angle changes, could be caused by disturbances on another HVDC scheme, and interaction between schemes could potentially cause instability, unless appropriate steps were taken. The problems are not insurmountable as witnessed by several examples where HVDC converters terminate electrically close to each other, and where good performance has been experienced. It should be noticed that VSC Transmission does not suffer from commutation failures, and is therefore not likely to suffer from instability, even if several HVDC terminate in close proximity to each other. 7 Conclusions Many technical papers have explained the technical advantages resulting from the use of HVDC transmission., However, HVDC is not suitable for all transmission applications. Rather than writing yet another paper focusing on all the beneficial features of HVDC, the author decided to use the opportunity of this keynote paper to discuss some of the technical challenges which have to be faced when applying an HVDC scheme. The author strongly believes that the growth in environmental opposition and the need for energy diversity will result in a dramatic growth in the application of HVDC schemes, as a solution to future power transmission challenges. To enable the full potential for HVDC schemes to be exploited, it is necessary to take into account the issues which have been highlighted in this paper. Some aspects requires education of the public, some training of planners and the advisors of investors, and some requires R & D, primarily by the HVDC manufacturers. References [1] E. W. Kimbark, Direct Current Transmission, John Whiley & Sons Inc, 1971 [2] CIGRE WG B4.37, VSC Transmission, CIGRE Brochure 269, April [3] A. Orzechowski, Analysis of Possible Enhancement of Transmission Capacity while Converting 220kv Alternating Current Overhead Lines into Direct Current Lines, Paper B4-105, CIGRE Session 2004 [4] Nordel, Prioritised Cross-Sections status report, June 2005 [5] Lionel O. Barthold, Hartmung Huang, "Conversion of AC Transmission Lines to HVDC Using Current Modulation," IEEE PES Inaugural 2005 Conference and Exposition, Durban, South Africa, July 11-15, 2005