Starting with a brief history of the evolution of HVDC light technology , the paper gives the definition of “HVDC LIGHT”, and compares it with Conventional HVDC. This paper focuses on the HVDC light converter technology and about the light cable. The advantages of HVDC light cables over AC under ground cables and over DC under ground cables are discussed. The active and reactive power control by HVDC light are seen and Emergency Power and Black Start Capability of HVDC Light, the new technology prospects and the applications, by considering the possible economical and environmental considerations are discussed.
Introduction:-
As the size of a concentrated load in cities increases due to the on-going urbanization, metropolitan power networks have to be continuously upgraded to meet the demand. Environmental issues are also becoming more and more of a concern all over the world. Strong forces are pushing for replacing old local generation with power transmission from cleaner sources. Land space being scarce and expensive, substantial difficulties arise whenever new right-of-way is to be secured for the feeding of additional power with traditional transmission lines. With increasing power levels, the risk of exceeding the short-circuit capability of existing switchgear equipment and other network components becomes another real threat to further expansion. Increasing demands on the power quality in urban areas is also a factor to consider for the power system engineer. The HVDC Light system is a solution to these problems.
This technology is designed to transmit large quantities of power using underground cables and at the same time adds stability and power quality to the connected networks. The cables are easily installed underground using existing right of ways, existing cable ducts, roads, subways, railways or channels. The HVDC Light converter stations are compact and by virtue of their control, they do not contribute to the short-circuit levels. As its name implies, HVDC Light is a high voltage, direct current transmission technology and is well suited to meet the demands of competitive power market for transmission up to 1100 MW.
Evolution of "HVDC Light" Technology:
The reforms that have or are being introduced in the electricity sector in many countries have radically changed the business environment of the electricity supply industry, featuring, among other, the opening of the electricity markets, competition and electricity trade across regions and national borders. The utility company has now a new dimension in its competitive strategy. It does no longer need to restrict its generating assets within fixed geographic bounds, but can to a greater extent take advantage of remote energy resources, both small and large.
Recent development efforts in transmission technology have focussed on compact, small weight and cost-effective, so-called voltage source converters (VSC), using novel high power semiconductors that can be switched at high frequencies. In parallel, a scientific and engineering breakthrough in extruded DC cable technology, makes it now possible to manufacture lightweight, high-power DC cables that are easily installed, using conventional ploughing techniques.
By combining the advances made in VSC and DC cables, a new breed of electricity transmission and distribution technology emerges: The "HVDC Light" technology. The new technology extends the economical power range for High Voltage Direct Current transmission (HVDC) downwards to just a few MW. Transmission of electricity over long distances using underground DC cables is both economical and technically advantageous. HVDC Light is thus an alternative to conventional AC transmission or local generation in many situations. Possible application fields include the feeding of distant loads and the connection of distant generation plants. By feeding a remote load from the main grid, it is feasible to shut down small, expensive and possibly polluting generation plants, as well as eliminate the associated fuel transport. This makes the new technology very attractive from both an economical and environmental point of view.
What is HVDC Light?
HVDC Light is the successful and environmentally-friendly way to design a power transmission system for a submarine cable, an underground cable or network interconnection. HVDC Light is HVDC technology based on voltage source converters (VSCs). The new transmission technology is called "HVDC Light", thus emphasizing the lightweight and compactness features intrinsic to it as well as its competitiveness in applications in the low end of the power scale.
HVDC Light cables have extruded polymer insulation. Their strength and flexibility make the HVDC Light cables well suited for severe installation conditions both underground as a land cable and as a submarinecable.The converter station design is based on voltage source converters (VSCs) employing state of the art turn-on/turn-off IGBT power semiconductors that operate with high frequency pulse width modulation. HVDC Light has the capability to rapidly control both active and reactive power independently of each other, to keep the voltage and frequency stable. This gives total flexibility regarding the location of the converters in the AC system since the requirements of short-circuit capacity of connected AC network is low (SCR down to zero).
The cable system is complete with cables, accessories and installation services. The cables are operated in bipolar mode, one cable with positive polarity and one cable with negative polarity. The cables have polymeric insulating material, which is very strong and robust. This strength and flexibility make the HVDC Light cables perfect for severe installation conditions.
- The submarine cables can be laid in deeper waters and on rough bottoms.
- The land cables can be installed less costly with ploughing technique.
- HVDC cables can now also go overhead with aerial cables
HVDC Light Converter Technology:
Conventional HVDC converter technology is based on the use of line-commutated or phase-commutated converters (PCC). With the appearance of high switching frequency components, such as IGBT’s (Insulated Gate Bipolar Transistor) it becomes advantageous to build VSC (Voltage Source Converters) using PWM (Pulse Width Modulation) Technology.
HVDC Light uses Pulse Width Modulation to generate the fundamental voltage. It controls the magnitude and phase of the voltage freely and almost instantaneously and allows independent and very fast control of active and reactive power flows. PWM voltage source converter does not contribute to the short-circuit power, as the AC current can be controlled by the converter valve
The key part of the HVDC Light converter consists of an IGBT valve bridge. No special converter transformers are necessary between the valve bridge and the AC-grid. A converter reactor can separate the fundamental frequency from the raw PWM waveform. If the desired DC voltage does not match the AC system voltage, a normal AC transformer may be used in addition to the reactor. A small shunt AC-filter is placed on the AC-side of the reactor. On the DC-side there is a DC capacitor that serves as a DC filter.
New Development in Transmission Technology:
In parallel with the development of the IGBT based VSC technology, the development of extruded DC cables exhibiting an outstanding voltage withstand capability, low weight per unit length as well as very high service reliability tipped the balance in favour of HVDC technology in transmission and distribution. The HVDC Light cable is a new design triple extruded, polymeric insulated DC-cable, which has been successfully type-tested to 150kV DC. It is a new lightweight cable similar in appearance and characteristics to a standard AC, XLPE cable except that the problem associated with space charges which breakdown the insulation when using AC, XLPE cables on DC has been over come with this new design. The cables are operated in bipolar mode, one cable with positive polarity and one cable with negative polarity. The cables have polymeric insulating material, which is very strong and robust. DC cables do not exhibit the reactive power problem encountered in AC cables and can therefore be used for transmission over long distances. This strength and flexibility make the HVDC Light cables perfect for severe installation conditions:
- The land cables can be installed less costly with plowing technique.
- The submarine cables can be laid in deeper waters and on rough seabeds.
- HVDC cables can also be installed as overhead cables.
Active and Reactive Power Control:
The fundamental frequency voltage across the converter reactor defines the power flow between the AC and DC sides. Changing the phase angle between the fundamental frequency voltage generated by the converter and the voltage on the AC bus controls the active power flow between the converter and the network. The reactive power flow is controlled by the width of the pulses from the converter bridge.
In an HVDC Light system the active and reactive power can be controlled at the same time like in a synchronous converter, but the control is much faster, in the millisecond range. This fast control makes it possible to create any phase angle or amplitude, which can be done almost instantaneously providing dependent control of both active and reactive power. From a system point of view it acts as a motor or a generator without mass.
Emergency Power and Block Start Capability:
A VSC transmission system will be a very valuable asset during a grid restoration. It will be available almost instantly after the blackout and does not need any short circuit capacity in order to become connected to the grid. The benefits will differ if one or both ends are exposed to the blackout. The following list highlights some aspects:
No need for short circuit power for commutation. Black start capability if equipped with a small diesel generator feeding auxiliary power (or power from another grid).
Fast voltage control is available in both ends virtually instantly after the auxiliary power is back.
Can energize a few transmission lines at a lower voltage level avoiding severe Ferranti over voltage and allow remote end connection of transformers/reactors at a safer voltage level.
When active power is available in the remote end the VSC connection can feed auxiliary power to local plants making sure that they have a stable frequency to on.
When the local plants are synchronized to the grid they can ramp up power production at a constant and safe speed and do not initially have to participate in frequency control.
DC underground cables provide significant advantages, compared with overhead power lines.These include:
- Reduced environmental impact, an underground cable has no visual impact.
- Faster and easier issue of permits using DC underground cables.
- Virtually no magnetic radiation associated with the bi-polar DC cable.
Compared with AC underground cables the HVDC Light cable also has some significant advantages to be considered:
- DC cables require only two cables between each converter station.
- DC-cables have no technical limit to distance.
- DC cables can carry up to 50% more power than the equivalent AC cable.
Being considerably more compact and lightweight than classic HVDC, HVDC Light enables transmission of electrical power to, from, and between offshore installations where distances prohibit AC transmission
Applications of New DC Technology: HVDC-Light
HVDC light is expected to become the preferred alternative in many electricity supply applications such as:
- Connection of small-dispersed electricity generators to a grid:
With the independent control of reactive and active power afforded by the VSC scheme, the varying operating conditions of the wind power units can be allowed without negative impact on the power quality level of the grid. The underground cable also helps in minimizing the impact of environmental factors on the reliability and availability of the transmission while keeping the visual impact on the environment down to a minimum. Furthermore, the VSC technology allows a variable frequency to be used in the wind generator, thus making the plant operate at the speed that gives maximum power. The variable speed operation scheme can boost the energy delivery of the wind power plant by 5-25%, thus improving the economy of the installation. Obviously, the HVDC-light technology is very suited for the collection, transmission and distribution of electricity from small, run-of-the-river, hydro power plants.
- Delivery of electricity to islands:
Electricity from a remote power system, supplied via an HVDC-Light link, can successfully replace diesel-generating sets that are commonplace in such cases. Transporting electricity with the new HVDC Light scheme effectively cuts down the relatively expensive operating costs associated with the transportation of diesel fuel to the islands as well as with the relatively low energy conversion efficiency of smaller diesel-generator units. Other possible applications are the supply of electricity to offshore oil and gas platforms, where space and equipment weight are at a premium, or the use of excess gas from oil platforms to generate and transmit electric power to mainland locations. At present, designs in the range 1-60 MW with cables operating at 100 kV are being considered. In future both power and voltages are expected to increase, yielding much higher transmission capacities.
- Feeding electric power to large and rapidly growing cities:
As the size of a concentrated load increases due to on-going urbanization, the metropolitan power network has to be continuously upgraded to meet the demand. Land space being scarce and expensive, substantial difficulties arise whenever new right-of-way is to be secured for the feeding of additional power. Furthermore, with increasing power levels, the risk of exceeding the short-circuit capability of switchgear equipment and other network components becomes a real threat to further expansion. Consequently new power in feed solutions is required. The HVDC-light technology meets both demands: The cables are easily installed underground, the converter stations are compact and by virtue of their control, they do not contribute to short-circuit levels.
- Feeding of electric power to remotely located loads:
Small cities, mining districts, villages and other places that are located far from any electrical network, can now be economically fed from larger networks via an HVDC-light link. In this way, the advantages afforded by large electricity networks are brought to basically any place on land or even offshore. In the past, for loads in the range below 100 MW, local generation was necessary if the distance between the existing electric grid and the load was beyond what is possible to achieve economically using traditional AC technology. The new DC technology makes it possible to cost effectively bridge across large distances with a minimum of losses.
Environmental Considerations:
- Magnetic fields are eliminated since HVDC Light cables are laid in pairs with DC currents in opposite directions.
- It offers no overhead lines, neutral electromagnetic fields, oil-free cables and compact converter stations.
- The cable insulation is power electronic based are not dangerous.
Conclusion:
The technical development that has recently taken place in the field of electrical transmission, coupled to a changing business environment of the electricity supply industry and the deregulation of energy sector at large, lead to a growing attractiveness of electrical transmission.
The hallmarks of the new technology are: short lead times, cost effectiveness, compactness, environmental friendliness, and ease of application. It is anticipated that this technology will quickly become the preferred alternative for transportation of energy, in many application cases where electricity transmission was not considered previously.
References:
- B. Normark, D. Ravemark, “Underground transmission with HVDC Light”.
- “Power System Stability benefits with VSC DC-Transmission Systems”, CIGRE.
- K. Eriksson, “HVDC Light – An excellent tool for City Center Infeed”.
Technical Paper on HVDC Light Technology
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