Multi-terminal DC transmission and DC grid technology

With the shortage of traditional energy sources and the worsening of environmental degradation, countries around the world have recognized the need for energy use and development. Fund projects: National Natural Science Foundation International (regional) cooperation and exchange projects from traditional energy sources to green renewable energy sources, etc. Clean energy transition. As of June 2012, China's grid-connected wind power capacity has reached 52.58x103GW, becoming the world's largest wind power country; at the same time China's photovoltaic power generation capacity will reach 4GW. But limited by the power system capacity, most of the renewable energy is not available. Effective use, even power. With the large-scale renewable energy access to the power grid, traditional power equipment, power grid structure and operational technology are becoming more and more inadequate in accepting ultra-large-scale renewable energy. For this, new technologies, new equipment and new power grid structures must be adopted. To meet the profound changes in the future energy landscape. Multi-terminal DC transmission systems based on conventional DC and flexible DC and DC grid technology are one of the effective technical means to solve this problem.

At present, foreign research on multi-terminal DC transmission and DC grid technology is getting deeper and deeper. The International Power Grid Conference established six working groups to conduct research work on DC grid feasibility, planning, DC converter models, topology, power flow control, control protection, and reliability. In addition, Europe has proposed SuperGrid in 2008 to fully utilize renewable energy while achieving full use of electricity trading and renewable energy between countries; and in April 2010, a technology was established. TWENTIES, a cooperative organization for R&D and demonstration projects, uses innovative tools and integrated energy solutions to achieve large-scale low-voltage ride-through power generation for wind power generation and other renewable resources, designed to meet the needs of large-scale wind power entering the European power system. Eliminate obstacles and help Europe achieve its 20/20/20 goal, which is to achieve in Europe by 2020: 20% reduction in carbon dioxide emissions; 20% increase in energy efficiency; 20% of electricity consumption comes from renewable sources. In 2011, based on the background of the aging of power transmission equipment, the emergence of power transmission bottlenecks and the frequent blackouts, the United States proposed the 2030 Grid 2030, that is, the future US power grid will be established from the east coast to the west coast, north to Canada. South to Mexico, mainly using superconducting technology, power storage technology and more advanced DC transmission technology backbone grid.

This paper starts with the background analysis of the development of DC transmission, and outlines the conventional DC and flexible DC transmission technologies at both ends. Based on this, the basic concepts of multi-terminal DC transmission and DC grid are analyzed in depth, and the differences and connections between multi-terminal DC and DC grids are combined. Their respective technical characteristics discuss the technical bottlenecks needed to be built in the future DC grid, and provide suggestions and suggestions for the future development of China's power grid technology.

1 Development of transmission technology 1.1 AC and DC transmission technology People's understanding and research on transmission technology began with DC.

In 1882, the first long-distance DC transmission in history.

With the vigorous development of high-voltage AC transmission technology and the formation of large-area AC power grids, AC transmission also encounters its inherent system synchronization, transmission stability, transmission efficiency and lower DC system and other technical bottlenecks; The problem of safe operation has also become increasingly prominent. According to incomplete statistics, from 1965 to the present, large-scale blackouts (load loss of 28 million kW) occurred in the world up to 25 times, only on July 30 and 31, 2012, the world's major AC voltage levels and the first time in the northern part of India. Operating time Tab.1MaorACvoltagelevelsin serial voltage level / kV commissioning time / annual commissioning country US former Soviet Union US former Soviet Union The former Soviet Union China has two consecutive major blackouts, causing about 600 million people to be affected. Therefore, the question of whether the AC transmission system is the only technical solution for future power transmission is again raised.

1.2 HVDC transmission technology While the AC transmission technology is becoming more and more mature, HVDC technology has also developed with the development of high-power power electronic devices and high-voltage commutation technology, overcoming the early DC technology bottleneck problem. In the 1950s, high-voltage and large-capacity The successful development of the controllable mercury arc rectifier marks the return of HVDC technology to the historical stage. HVDC technology refers to the direct current transmission technology that converts the AC power of the sending end into DC power by the rectifier station, delivers the DC power to the inverter station through the DC line, and then converts the DC power into the AC power through the inverter station. . Mainly experienced three important stages of development.

In 1928, the mercury arc valve with gate control capability was successfully developed. In 1954, the world's first mercury arc valve DC transmission project was put into operation in Sweden. However, due to the complicated manufacturing technology, high price, high failure rate, low reliability and inconvenient maintenance, the mercury arc valve is gradually replaced by thyristor commutation technology.

In 1956, American Bell, compared with AC transmission, HVDC technology has the advantages of no stability problems, high transmission efficiency, fast and reliable regulation, and saving transmission corridors. Due to the high cost of the converter station equipment, it usually has technical and economic advantages when the transmission distance is greater than 800km. At present, due to the lack of high-voltage DC circuit breakers and DC/DC transformers, the development of multi-terminal DC transmission and DC grid technology is limited.

UHV grids (1000kV AC and ±800kV conventional DC) are one of the best ways to solve long-distance and large-capacity power transmission in China, both economically and technically.

Comparison project AC transmission technology HVDC technology Reactive power existence does not exist Stability is low High synchronization problem exists There is no transmission distance limitation Limited theoretically Unlimited distributed power supply acceptance is not suitable for system impedance magnitude Fault current rise rate is small Large large storage It is easy to be difficult to cut off the fault current. It is difficult to cut off the fault current. However, in the large-scale new energy grid connection, due to the intermittent nature of new energy power generation, the AC grid cannot directly complete the acceptance of new energy. Therefore, the UHV backbone grid of “strong and strong” combined with the regional DC grid will form the basic form of China's future grid architecture.

2 DC transmission technology at both ends 2.1 Conventional DC transmission technology The research on HVDC reached a climax, the key technologies gradually matured, and the voltage level of HVDC projects applied in engineering practice continued to increase.

Since the thyristor only has a trigger turn-on function, the active power transmitted by the LCC system is controlled by adjusting the firing angle. A large amount of reactive power is consumed at the rectifier of the power transmitting end and the inverter of the power receiving end. This requires configuring filters and capacitors on the AC side to compensate for reactive power. Especially under transient conditions, the range of reactive power varies greatly. When the current is reversed, the polarity of the HVDC system needs to be reversed. If the line uses a cable, the charge and discharge problems of the cable capacitor under polarity reversal conditions will not be ignored.

The maximum capacity of the Jinping-Sunan DC project is 7.2x103MW (800kV/4.5kA), and a higher voltage class 1100kVHVDC system is under development.

2.2 Flexible DC transmission technology With the advancement of power semiconductor device technology, the emergence of high-power insulated gate bipolar transistors (IGBT) and the development of pulse width modulation (PWM) and multi-level control technology, self-replacement The phase of the voltage source converter (VSC) technology of HVDC has been rapidly developed in the past decade.

There are advantages such as reactive power and active power, independent control, no filtering and reactive power compensation equipment, power supply to passive load, and voltage polarity change when power flow is reversed, as shown in Table 3. Therefore, VSC is more suitable for building multi-terminal DC transmission and DC grid. At present, the loss of VSC-HVDC-terminal is about 1.6%, and the loss of the converter valve accounts for 70% of the loss. The maximum capacity of the VSC-HVDC project that has been put into operation at this stage is in the construction stage. At present, the main constraint to increase the transmission capacity of this technology is the voltage level limitation of XLPE cables. Due to the bidirectional conductivity of the VSC converter valve, when the DC side fails, the short-circuit current AC part can be cut off by the AC breaker; however, the short-circuit current discharged by the DC line and the DC side support capacitor will be difficult to be blocked. For this purpose, VSC-HVDC systems at both ends usually use cable connections.

Table 3 Comparison of LCC technology and VSC technology Project LCC technology basic components thyristor harmonic component strong lower harmonic component weaker harmonic component reactive/active power consumption large amount of reactive power completely independent control loss /% Maximum capacity and AC grid connection mode Converter transformer series reactor and transformer current inversion voltage polarity reversal current polarity reversal DC side fault control by adjusting the firing angle control out of control DC side inductance size DC side capacitance is small (using cable Larger) The short-circuit current rise rate is small, and the controllable large-network demand has entered a rapid development stage. Various new topologies and modulation methods are emerging; the development goal is to make the total loss of the converter station less than 1% and the transmission capacity greater than 1500MW. , has the ability to limit and cut off the DC side fault current.

2.3 Hybrid DC transmission technology Hybrid DC transmission technology (hybrid HVDC) is a combination of conventional direct current transmission and flexible direct current transmission, that is, one end of the transmission line is LCC and the other end is VSC. This technology can not only retain most of the advantages of flexible direct current transmission technology. And can optimize the project cost.

Hybrid HVDC transmission has great advantages for the connection of offshore power grids. The compact voltage source converter is suitable for offshore platforms and can be connected to electrical islands. The current source converter side can be placed on land that does not require a large volume of the converter station, and can be connected to a land-based power grid. Since the voltage polarity of the voltage source type converter is fixed, the current flow of the current source type inverter is fixed, so the power flow cannot be directly reversed. The system needs to be shut down when the power flow is reversed, and the voltage polarity at one end needs to be changed. The topology of some voltage source converters can now directly change the voltage polarity to achieve power flow reversal. In order to avoid the reversal of the power flow, the hybrid line can only consider the unidirectional power flow when planning.

The initial stage of the development of 3 multi-terminal DC transmission technology network is a transmission system that is connected by series, parallel or hybrid mode by more than three converter stations, which can realize multi-power supply and multi-drop power reception. (a) - (d) are topological structures of series, hybrid, radial parallel, and ring-connected parallel.

Schematic diagram of the connection mode of multi-terminal DC transmission The parallel converter stations operate with the same level of DC voltage. The power distribution is realized by changing the current of each converter station; the series converter stations are operated with the same level of DC current. Power distribution is achieved by varying the DC voltage; both parallel and series hybrids add the flexibility of multi-terminal DC wiring. Compared with the series type, the parallel type has smaller line loss, larger adjustment range, easier implementation of insulation coordination, more flexible expansion mode and outstanding economy. Therefore, the multi-terminal DC transmission engineering that has been operated at present is adopted. Parallel wiring.

The basic principle of multi-terminal DC transmission was proposed in the mid-1960s, but so far there are only five true multi-terminal conventional DC transmission projects, as shown in Table 4. The first three projects have been operated in a multi-terminal DC mode; the Nelson River in Canada and the Pacific Line HVDC transmission project in the United States also have the characteristics of a 4-terminal DC transmission system.

Due to the complexity of the control and protection technology of the multi-terminal DC system, the difficulty in manufacturing the high-voltage DC circuit breaker, and the need to change the polarity of the voltage, the current DC power transmission project currently in operation is listed in Table 4. Multi-terminal DC transmission of the multi-terminal DC transmission project Project commissioning time / annual number of operating voltage / kV rated power / MW Italy one Corsica - Sardinia Canada Quebec New England æ›° New Xinkang Canada Nelson River US Pacific line most of the two ends of the DC transmission system .

Since the VSC-HVDC technology has the characteristics of not changing the polarity of the voltage when the power is turned over, it is more suitable for forming a multi-terminal DC system. With the continuous improvement of the level of turn-off devices and DC cable manufacturing, VSC-HVDC will become the most important transmission mode in multi-terminal DC transmission and DC grid in high-voltage and large-capacity power transmission. Table 5 shows an overview of the multi-terminal flexible DC transmission project under construction.

Table 5 Overview of multi-terminal flexible DC transmission projects under construction Tab. Serial number multi-terminal DC transmission engineering commissioning time / annual end number operating voltage / kV rated power / MW (Sweden - Norway) South Australia wind farm (China) Zhoushan (China) In addition, the United States is Plan to build a multi-terminal hybrid DC transmission project GBX multi-terminal DC project, as shown.

The voltage level is 600kV; the LCC converter station at both ends, the middle drop point is the 345kVVSC converter station; the project aims to transfer the renewable energy of the Southwestern Power Alliance to the power market of the Midwest Regional Power Market and PM Company.

4 DC grid technology and its challenges 4.1 DC grid concept The possible topology of multi-terminal DC system development in the future is shown in (a), which is the simplest implementation form of multi-terminal HVDC transmission system, which leads to multiple converter stations from the AC system. Through multiple sets of point-to-point DC connections to different AC systems, multi-terminal DC has no mesh and no redundancy; since it cannot provide redundancy, it is difficult to be called a network. When any converter station or line in the topology fails, the entire line and the converter stations connected to both sides of the line will all be out of operation, and the reliability is low.

The topology diagram of multi-terminal DC transmission can form a true DC grid if the DC transmission lines are connected to each other on the DC side to form a 'one-to-many point' and ''multi-point-to-point' form, such as (b) It is shown that each AC system is connected to the DC grid through a converter station, and multiple DC lines are connected between the converter stations through the DC circuit breaker. When a fault occurs, the circuit breaker can be used to selectively cut off the line or the converter station. . The true DC grid has the following characteristics: 1) The number of converter stations can be greatly reduced, and only one location is required at each connection point with the AC grid, which not only can significantly reduce the construction cost, but also can reduce the overall transmission loss; Each of the converter stations can transmit (transmit or receive) power separately, and can change its transmission status from transmission/reception to reception/send without affecting the transmission status of other converter stations; 3) owning more More redundancy, even if a line is out of service, you can still use other lines to ensure reliable power transmission.

The DC grid will be an intelligent and stable AC/DC hybrid wide-area transmission network with advanced energy management systems. Different clients, existing transmission networks, microgrids and different power sources can be effectively used in this network. Manage, optimize, monitor, control and respond to any power problem in a timely manner. It integrates multiple power supplies and transmits and distributes power over thousands of kilometers with minimal loss and maximum efficiency.

4.2 Three stages of DC grid development The DC grid is a multi-terminal DC transmission system with point-to-point DC transmission and multi-terminal DC transmission. It has no grid structure and redundancy, and is not a true “grid” because of this stage. There is no redundancy in the topology. This topology is usually used as a backup for communication or for connecting two asynchronous communication systems.

The topology of the second stage, as shown in (b), has initially assumed the prototype of the DC transmission network, in which all the busbars are AC busbars, and the traditional transmission lines are replaced by DC lines connected between the two converter stations. . In this topology, all DC lines are fully controllable. It may contain two transmission modes, VSC and LCC. Different DC lines may work at different voltage levels, and more complicated power flow control is needed to maintain frequency stability. The main problem at this stage is the need for a large number of converter stations. In a normal large power grid, the number of branches is generally 1.5 times the number of nodes, which requires the number of converter stations to be 2x1.5x DC nodes. If the third topology is used, the number of converter stations is the same as the number of DC nodes. This is important because the converter station is the most expensive, sensitive, and most lossy component in the DC grid.

The stage 3 topology is shown in (c). The topology at this time is an independent network. Compared with phase 2, not every DC line has a converter station at both ends, but the DC station is only through the converter station. The network is integrated with the AC grid. In a separate DC grid, the individual DC lines can be freely connected and can be used interchangeably as a connection, not just as a connection to an asynchronous AC grid. In addition, Phase 3 can greatly reduce the number of converter stations, which is of great economic importance. So as a true DC grid, the topology of (c) is the future development trend.

4.3 DC grid technology challenges Although the point-to-point DC transmission technology and engineering are relatively mature, there are many challenges in building a future DC grid. This paper believes that it is necessary to make breakthroughs in the following aspects.

The simulation of the DC grid also includes two kinds of offline simulation technology and real-time simulation technology. The off-line simulation technology is to establish a mathematical model for the DC grid on a computer and solve it by mathematical method to carry out simulation research. For offline simulation of DC grid, the mathematical model of grid should be established first. Because there is a fundamental difference between DC grid and AC grid in topology and operation principle, the mathematical model used for DC grid simulation must be re-established. In addition, there are fewer inertia links in the DC grid. Therefore, the response time constant of the DC grid is at least two orders of magnitude smaller than that of the AC grid. The system simulation is mainly for electromagnetic transient simulation. The simulation step size is small and the resource requirements are high. The current off-line simulation system cannot meet the needs of DC grid simulation.

Full digital real-time simulation is the development trend of international simulation research. However, due to the relatively complex topology of DC grid, its power flow distribution and coordinated control are more complicated. System simulation of DC grid, especially DC commutation characteristics and control protection system. Accurate simulation requires high requirements for the node of the simulation technology; therefore, for the simulation of the fast electromagnetic transient process of the DC grid including high-power power electronic devices such as IGBTs and thyristors, the accuracy of the current digital simulation cannot meet the DC grid system. Simulation requirements. For this reason, whether it is offline simulation or real-time simulation of DC grid, it is necessary to re-examine the simulation modeling method applicable to DC grid on the basis of improving the resources of simulation platform.

Just as the grid frequency is an important indicator of the active power balance in an AC system, the power balance indicator in a DC network is the DC voltage. When the DC network power is excessive, the DC voltage will rise; otherwise, the DC voltage will drop. As shown, for a DC grid, C and T represent capacitors and switching devices in the VSC, and L is the load in the grid, which is the inverter voltage. The corpse 1, the heart is the input and output power of the inverter, and if the two are not equal (a) AC control principle (b) DC control principle AC and DC grid control characteristics comparison diagram In the DC grid, control the DC voltage of the entire network to maintain Stability is a prerequisite for the normal operation of the system. With the change of the DC grid control object, its operation control method is essentially different from the traditional AC system; meanwhile, as mentioned above, since the response time constant of the DC grid is at least two orders of magnitude smaller than the AC grid, this pair of DC The network's control system will be an extremely harsh challenge.

Since the DC grid has high requirements on the response time of the protection system, traditional AC system protection, such as overcurrent protection, distance protection and differential protection, are not suitable for direct application to the DC grid. For example, overcurrent protection is a measure of protection against a corresponding protection action (such as tripping a circuit breaker) when the current exceeds a certain threshold. It is simple and non-selective. Compared with AC systems, complex impedance measurements in DC grids have fundamentally different characteristics, especially fault resistance; therefore, conventional distance protection is no longer suitable for fault protection of DC grids. For differential protection, if a fault occurs near the DC bus, the fault on the other side of the line will not be measured after a certain delay (may take several ms on a longer line), far from meeting the DC grid. The need for protection. Therefore, it is necessary to study the new protection principle and protection method applicable to the DC grid according to the operating characteristics of the DC grid.

In addition, in the DC grid protection strategy, the grid size, converter topology and circuit breaker configuration will affect the coordinated protection strategy. The converter and DC bus in the DC grid can be used as shown. Selective protection.

―The type of grid topology that can be selectively protected. The number represents the number of the converter station to which the circuit breaker circuit belongs. The second number represents the position of the circuit breaker, 1 represents the adjacent DC bus 1, and 3 represents the adjacent DC bus 2, 2 represents Located between the two.

When DC bus 1 and DC bus 2 fail, the circuit breakers with the 2nd corners 1 and 3 can be selectively disconnected; if the DC cable fails, the 2nd corner is broken. When the converter station fails, the second corner of the converter that is connected to the converter station is 1 and 2 and the breaker is disconnected at the same time. In this topology, the working efficiency of the converter station and the busbar can be fully improved, and at the same time, when a fault occurs in a certain link of the power grid, the normal operation of other non-faulty links of the power grid is not affected.

Wide-area measurement and fault detection technology for DC grids.

The wide area measurement system (WAMS) applied to the AC grid is composed of a global positioning system, which can dynamically measure and calculate the operating state phasor and generator power angle of the power system, and is also widely used in the power system. Many areas of steady state and dynamic analysis and control. Similarly, DC grids require a wide-area measurement technique for DC systems for a wide range of unified coordinated control and protection, state estimation, voltage stability analysis, fault detection, and processing. However, since the voltage and current in the DC grid do not have zero crossings of the rising edge and zero crossings of the falling edge, the PMU of the AC grid and its algorithm cannot be applied to the DC grid.

Also due to the response time of the DC grid, the DC grid fault detection technology needs to reduce the detection time and improve the response speed in the traditional detection technology. The fault detection of some key equipments needs to change the traditional technology "fault information collection, failure information reporting, fault information processing, fault processing command, fault processing", using local fault information processing and fault handling functions. All need to be based on fast and accurate fault detection technology.

DC grid safety and reliability assessment technology.

In order to prevent economic losses caused by large-scale power outages caused by major accidents in the DC grid, the DC grid and the AC system are also facing safety and reliability assessment problems in the three stages of planning, design and operation. At present, the research on reliability evaluation of AC transmission and transmission system and LCC-HVDC itself is relatively mature, but the reliability evaluation technology for VSC-HVDC and DC grid including different transmission methods and key equipment of DC grid is still in its infancy.

There are many VSC-HVDC components, the control system is complex, and the fault tolerance is poor. When evaluating the reliability, it is necessary to re-establish the evaluation model and evaluation method, and the reliability index should be adjusted at the same time. On the other hand, the key equipment of the current DC grid, such as DC circuit breakers, DC/DC transformers, etc., has no industrial products, and the evaluation models, evaluation methods and reliability indicators of the equipment need to be re-established.

Like the AC system, the operation of the DC grid also requires a large number of standards; at the same time, once the network is formed, the operating standards of the DC grid are quite different from the traditional point-to-point DC transmission standards, as shown in Table 6.

Table 6 Standard difference between DC grid and point-to-point DC transmission Tab.6DifferencesbetweentheDC No. Category Point-to-point DC grid power required voltage does not need current required AC grid short-circuit capacity requires power control does not need DC overhead line/cable fault protection time does not need DC The circuit breaker action time does not need to require DC breaker circuit breaking current. It does not need communication protocol and signal. The most urgent need is to set the voltage level standard. As with AC systems, multiple standard voltage levels need to be defined. Once a voltage level is selected, the entire system is set to this voltage.

Secondly, it is able to standardize the equipment that can be connected to the DC system, including DC circuit breakers, DC/DC transformers, etc., and also includes the standardization of the converters, because there are many manufacturers of converters and different manufacturers. The flow devices must be able to connect and operate reliably, while the independent control functions of the various converters of the various manufacturers cannot negatively affect each other, and should even operate in a positive manner.

If the control speeds of different inverters are significantly different, the operating characteristics of the converter in the event of system failure may have a serious impact on the operation of the system. Therefore, the independent control level transmission protocol and communication protocol of the inverter must comply with a certain Some standards.

Development of key equipment for DC power grid.

It mainly includes high-voltage DC circuit breakers, large-capacity DC/DC transformers and high-voltage DC cables.

Unlike the DC transfer switch, which can only turn off the normal operating current, the DC circuit breaker has the ability to cut off the fault current. At present, the commonly used high voltage DC circuit breaker has three kinds of current breaking modes, namely a mechanical circuit breaker based on a conventional switch, a solid state circuit breaker based on pure power electronic devices, and a hybrid circuit breaker based on the combination of the two. The current mechanical high-voltage DC circuit breaker can cut off the short-circuit current within tens of milliseconds. The cutting speed of this fault current cannot meet the requirements of the DC grid. Solid-state circuit breakers can easily overcome the breaking speed limit, but generate a lot of losses during steady-state operation. The hybrid circuit breaker combines the good static characteristics of the mechanical circuit breaker and the dynamic characteristics of the solid-state circuit breaker without arc and rapid breaking. It has the advantages of low running loss, short breaking time, long service life, high reliability and good stability, but Fast switch manufacturing is very demanding.

In addition to the above method of directly breaking the short-circuit current, it is also conceivable to add a current limiter to the circuit breaker switching current, because for a mechanical switch that needs to be extinguished, the larger the current, the more difficult it is to extinguish the arc; In power electronics, turning off large currents can cause dynamic overvoltage in the device. The larger the current amplitude, the higher the overvoltage.

Therefore, in the loop, the link that limits the peak value of the short-circuit current is maintained, and the low-resistance state is maintained during normal operation. When the fault occurs, the resistance is increased, the short-circuit current is limited to a certain lower value, and then the lower current is interrupted. The manufacturing difficulty of the breaking current portion is greatly reduced, and the breaking capacity can be improved at the same time.

In addition, during the development process, the circuit breaker or its independent components must undergo functional testing. For high-voltage DC circuit breakers, direct testing is unrealistic and a synthetic test method must be used. On the other hand, compared with AC circuit breakers, DC circuit breakers have a strong interaction with the system, and the test stress of the circuit breaker must be able to truly reflect the actual power level. The test method and test circuit of the traditional switch are not suitable for the whole type test of the DC circuit breaker. Therefore, the equivalent test method for the high-voltage DC circuit breaker and the new synthetic test circuit are also one of the research directions of the DC circuit breaker.

At present, large multinational companies (such as ABB and Alstom) have carried out relevant research and are expected to complete 320kV/2.6kA/16kA respectively at the end of 2012 (voltage level/normal operating current/maximum breaking current, the same below) Development of a 120kV/1.5kA/7.5kA high voltage DC circuit breaker prototype.

Since there is no uniform voltage standard in the current DC grid, there are many DC lines of various voltage levels. If these DC circuits of different voltage levels are connected to form a network and fully improve the operational flexibility of the DC grid, the DC/DC converter is Essential equipment. DC/DC transformers have not been used in DC transmission so far, but there will be many applications in the future DC grid. Ideally, DC/DC transformers need to achieve the following functions: higher, controllable, step-by-step ratio for connecting DC systems of different voltage levels; different types of converters can be connected; DC system poles Power balance; trend direction bidirectional controllable; low loss, low cost, small size; has a certain fault current withstand capability.

At present, the research of large-capacity DC/DC transformers is in the stage of circuit topology, simulation calculation and principle prototype. There is no report on industrial prototypes; there are mainly two kinds of transformer technology used, namely high-frequency transformers and power electronic components transformers. ABB has developed a prototype of DC/DC transformer based on thyristor and IGBT, respectively. The maximum parameters are 4/80kV-5MW (input voltage/output voltage-capacity level, the same below) and 2/40kV-3MW. An important transmission medium in DC transmission is another bottleneck that limits the increase in the transmission capacity of HVDC transmission. The electric field distribution in the insulating layer of the AC cable is inversely proportional to the dielectric constant, and the dielectric constant is less affected by temperature, and no space charge is generated in the insulation; however, for DC cables, the electric field distribution is proportional to the resistivity of the material. Distribution, and the insulation resistivity generally varies exponentially with temperature, which will form a space charge in the insulation of the cable, thereby affecting the electric field distribution. The polymer insulation has a large number of local states, and the space charge effect is serious. Therefore, it can be considered to reduce and eliminate. The space charge in the insulating material is the key to the development of DC cables. In addition, for conventional direct current transmission, changing the direction of the current requires changing the polarity of the voltage. At this time, the polarity superimposition will cause the voltage on the DC cable to be as high as 2.5 times the delivery voltage, which is easy to break through the cable.

At present, the highest voltage level for industrial use is 320kV; high-voltage cables of 500kV are undergoing relevant tests. It is foreseeable that in the next 5 years, the voltage and capacity levels of the wrapped and extruded DC cables will be increased to 600kV/2.4GW and 600kV/2GW respectively; and the voltage and capacity of the DC cable will be in the next 10 years or so. The grade will reach 750kV/3GW. The market demand for DC grid will be the driving force behind the development of DC cable technology.

5 Conclusion UHV AC-DC transmission technology is an effective means to solve the problem of long-distance large-capacity power transmission in China. However, multi-terminal HVDC and DC grid technologies will be effective technical means to solve the problem of grid-connected and consumption of regional new energy in China. .

Multi-terminal DC transmission is a stage of DC power grid development, which can achieve multi-power supply and multi-drop power reception. The DC transmission lines are connected to each other on the DC side to form a true DC grid.

The utility model has the advantages that the number of converter stations is greatly reduced, the converter station can transmit power separately, the transmission state can be flexibly switched, and the reliability is high.

VSC-HVDC has the technical advantage that the voltage polarity does not change when the power flow is reversed. Therefore, with the continuous improvement of the manufacturing level of the switchable device and the DC cable, VSC-HVDC will become the most important in the DC power grid in the high-voltage and large-capacity power transmission. Transmission method.

Key technical issues that need to be addressed in building a future DC grid include system simulation, control and protection techniques, fast fault detection techniques, safety and reliability assessment methods, standardization, and development of critical equipment including high-voltage DC circuit breakers and DC/DC transformers. .

The key technologies of the DC grid and the corresponding technologies of the AC grid have certain similarities, but there are essential differences between the two.

This is mainly due to the fact that there are fewer inertia links in the DC grid, and the response time constant is at least two orders of magnitude smaller than the AC grid. These key technologies cannot refer to and follow the relevant technologies of the AC grid and need to be re-examined.

In the next 10 years or so, it will be the stage of rapid development of DC grid technology and construction. Ultimately, the interconnected power grid will become the basic form of China's power grid architecture.

Handwheels

Aluminum Handwheels,Aluminum Spoke Handwheel,Cnc Machine Tool Handwheel,Aluminum Alloy Spoke Handwheel

Hebei Baisite Machinery Accessories Co.,Ltd , https://www.jiaomujian.com