Study on the topology of a system for delivery of electricity from deep sea wind through voltage source converter

It is a difficult problem to maintain high reliability while designing the topology of the voltage source converter (VSC) for deep-sea wind power in a compact manner. In this paper, the topology of the converter, the main wiring type and grounding method, and the DC system wiring scheme are studied. The advantages and disadvantages of each scheme are compared in offshore wind power application scenarios. Finally, the half-bridge converter topology and the true bipolar with metal return line topology are recommended for the topology design of the VSC system for deep and remote offshore wind power. The power operating range is also presented. The scheme proposed in this paper paves the way for promoting the lightweight and highly reliable system for the delivery of electricity from deep sea wind through VSC.


Introduction
The system topology is the basis for the design of an offshore voltage source converter (VSC) system, which is an important content from planning and design to engineering application.It is the key to equipment layout, site optimization, and economic cost, which can provide theoretical guidance for subsequent engineering design.The area of the converter station, the cost of investment, and especially the load-bearing capacity of the offshore platform have become the main constraints in the design of a system for the delivery of electricity from large-scale new energy through VSC.Offshore converter stations in the engineering design are hoping to minimize the total weight and total volume of the converter station, to achieve the purpose of compact design [1][2].
Offshore wind power platforms are difficult to construct and the construction conditions are complex.Construction of the key equipment required (such as offshore wind turbine foundation piling and wind turbine lifting) specializing in the availability of large ship equipment.Ship class cost is high, thus the construction cost is relatively high.This makes the offshore wind power platform in the design and use process pursue economy.The overall space of the platform is relatively small, and the installation of related equipment needs to fully consider the limited space.In addition, offshore wind farms are generally farther away from the shore, coupled with unfavorable sea conditions such as large waves caused by typhoons, storm surges, and other weather, and the accessibility is poor.Operation and maintenance are more difficult, maintenance costs are high, and the reliability of the system needs to be improved as much as possible [3].VSC system needs to ensure reliability while taking into account the economy, which puts forward higher requirements for the design of the main wiring of the VSC system.Foreign research on flexible DC transmission technology and its engineering applications in the field of offshore wind power started earlier, and a relatively complete research framework has been formed in the field of main circuit optimization design.Hussennether and Rittiger briefly introduced the main wiring, and converter topology, and used grounding modes of 800 MW and 576 MW offshore wind power projects as examples respectively [4].In [5], several HVDC system-level topologies applicable to offshore wind power transmission were proposed.In [6], a system-level topology optimization algorithm was proposed, which can meet the engineering planning and design requirements.Ahmad et al. proposed a structural design scheme for the offshore wind farm delivery system without using an offshore AC platform [7].Domestic engineering research and application of offshore wind power flexible DC technology began in 2010 with the Shanghai Nanhui flexible DC transmission project.Subsequently, relevant research institutions and universities gradually carried out systematic research in the field of topology construction.Li et al. proposed the main connection scheme of the Zhoushan multi-terminal flexible DC transmission project converter station [8].Mao et al. took the Zhoushan multi-terminal flexible DC project as an example, analyzed and compared the four grounding methods from the perspectives of AC-side insulation effect, overvoltage and shortcircuit current level in the converter station, and grid-side voltage drop, and recommended the best grounding scheme for Zhoushan project [9].Liu put forward the general design principles of the main wiring of the multi-terminal flexible DC transmission system and pointed out the problems that may be faced in the engineering application and the corresponding solution measures [10].
In summary, at present, domestic and foreign research on the topology construction of the VSC system has formed a system, but lack of research on the characteristics of the deep offshore wind power transmission project.This paper carries out the adaptability analysis and research on converter topology, main wiring type and grounding method, and DC system wiring scheme.The advantages and disadvantages of each scheme in offshore wind power application scenarios are compared to determine the final main circuit design scheme.

Converter topology study
The bridge arms of the Modular Multilevel Converter (MMC) are cascaded by using a Sub-Module (SM).Each bridge arm of MMC consists of N sub-modules and a series reactor (bridge arm reactor), and the upper and lower bridge arms of the same phase form a phase unit.
The converter topology is mainly studied for two topologies: half-bridge and full-bridge.Halfbridge structure converter loss is minimized, the least number of devices used, and is currently the most widely used topology.In the normal state, its external performance voltage varies between VC and 0. The half-bridge submodule has no fault-clearing capability in case of fault.The voltage variation range of the full-bridge submodule under normal operating conditions is consistent with the half-bridge case.However, the full-bridge structure converter can isolate the fault current.When fault blocking occurs on the DC side, the fault current can be isolated through the shutdown operation of the IGBT.However, due to more converter devices, the converter loss is greater than the half-bridge structure converter.
Compared with half-bridge wiring, full-bridge wiring equipment cost, loss, and footprint will be significantly increased, and the probability of equipment damage and operation and maintenance costs have also increased.For the delivery engineering of electricity from deep sea wind through VSC, full consideration should be given to the restricted offshore platform, unattended offshore station, and lightweight offshore equipment.If there is no specific demand, a half-bridge converter topology can be preferred.

Main wiring type selection
The system for delivery of electricity from deep sea wind through VSC is generally a two-terminal DC transmission system with DC transmission lines.The DC transmission system electrical main wiring basic type is mainly divided into two, as shown in Figures 1 and 2. Figure 1 shows that each station consists of a converter unit symmetrical unipolar structure type (pseudo-bipolar structure).Figure 2 shows that each station converter constitutes a true bipolar structure type.
The symmetrical unipolar wiring scheme utilizes a 6-pulsation bridge structure or a 1-basic converter unit (i.e., modular multilevel converter) structure.The neutral potential is clamped by a suitable grounding device on the AC or DC side.The potentials of the DC poles are symmetrical positive and negative.The true bipolar wiring scheme uses two basic converter units to form the positive and negative poles, respectively.The two poles can be operated independently, and a metal return line is used in the middle to form a return current path.
According to the different grounding methods adopted by the VSC, the DC line scheme mainly includes two types of metal return line schemes and earth loop schemes.For the bipolar structure of the system, a metal return line or earth (or seawater) return line (grounding pole line and grounding pole) can be used as the two basic wiring methods.For unipolar systems, only the grounding electrode method may be used.However, different DC line scheme for adaptability of the system for delivery of electricity from deep sea wind through VSC is different, and should also be combined with the grounding method comprehensive analysis.

Grounding method study
One of the key problems in the design of VSC is to select a reasonable grounding method.The grounding device can provide the reference potential for the whole converter station system, and its specific arrangement determines the main wiring of the system and the form of the converter station site layout.At the same time, it also provides basic data for the subsequent over-voltage insulation calculation and verification and lightning arrester protection design configuration.The grounding method of the single-pole structure of VSC can be DC-side grounding or AC-side grounding.DC side grounding is generally used in DC pole line parallel clamping resistance grounding.AC side can choose transformer Y winding neutral grounding or transformer valve side by star reactor grounding methods.At present, the single-pole structure of the domestic operation of VSC is used on the AC side of the grounding.If conditions allow, we generally set the grounding pole at both ends of the converter station.However, for the VSC offshore wind power engineering, taking into account the special characteristics of the offshore station, only the onshore single side of the converter station grounding method can be used.

3.2.1.
Ground through the DC pole wire in parallel with the clamping resistor.In this way, regardless of the wiring of the AC side coupled transformer, two clamping resistors of equal resistance value are connected in parallel to the DC pole line to the ground.The advantage of this grounding method is simple, direct, effective, and low cost.The disadvantage is that after grounding by DC resistance, the resistor is a long-term load with large power loss during normal operation on the DC line side.Resistor deviation after long-term operation will lead to DC pole line voltage deviation.This grounding method is generally used in projects with lower voltage levels on the DC side.

The Y winding is grounded via a grounding resistor through the coupling variable.
The valve side of the coupling transformer adopts a Y winding, and the valve side is grounded through the neutral point of the Y winding by a grounding resistor.The grounding method has been applied in the Yu-E project.Its advantage is the direct use of transformer Y winding neutral point grounding, grounding equipment less.The disadvantage is that it completely depends on the transformer Y winding to withstand the fault DC voltage and fault current, which puts forward high requirements on the transformer.This grounding method requires the use of an ungrounded winding type or the third winding with a triangular connection on the network side to isolate the AC and DC.Therefore, it is suitable for projects with other neutral grounding points on the network side, such as joint construction projects with substations.

3.2.3.
The neutral point is formed through a reactor and grounded via a resistor.This method is mainly used when the valve-side winding is a △-type grounding method, which cannot be grounded through the neutral point of the transformer Y winding.Therefore, it is necessary to artificially form a neutral point through the reactor and then ground through the grounding resistor.The advantage of this method is that the reactor shares the fault current, and the pressure on the coupling transformer is lower.The reactor can also play a role in limiting the short-circuit current.This method is widely used in foreign projects, domestic Zhoushan, and other projects, and the technical program is mature.Early projects in the reactor are limited by the manufacturing level, inductance value can not be made larger.The reactive power absorbed by itself is large, which has a large impact on the system.This year, with the continuous improvement of the manufacturing level, the power operation of the system has a very small impact on the limitations of the circle diagram.Comparing the above two schemes, the third grounding method is recommended for the characteristics of offshore wind power transmission engineering.

The grounding method of the bipolar structure of VSC is generally DC-side grounding.
The typical grounding method of a double large earth return line is shown in Figure 3.If the neutral point grounding method of double large earth is adopted at both ends, the system can be equated to two independent unipolar earth circuits.With higher safety and symmetrical operation in the earth circuit, there is no running current through.In case of fault, the system can be converted to unipolar operation with high reliability.The bipolar system can also be grounded at one neutral point.The reliability and flexibility of this method are poor, and it is seldom used in the project.The typical grounding method of the bipolar system using the metal center wire wiring method is shown in Figure 4. Generally, the metal center wire is selected to be grounded on one side, and the wiring method is more flexible.However, for the use of three conductors to form a transmission line, the line structure is complex and high cost.

Main wiring type
Typical wiring patterns include half-bridge single-pole wiring, full-bridge single-pole wiring, halfbridge double-pole large-earth return wiring, full-bridge double-pole large-earth return wiring, halfbridge double-pole metal return wiring, and full-bridge double-pole metal return wiring.
The symmetrical unipolar wiring scheme is simple in construction.During normal operation, the normal AC voltage is applied to the valve side of the coupling transformer.The transformer can be similar to the normal AC transformer structure, and the equipment manufacturing is easy.A unipolar symmetrical wiring scheme is currently more common in the VSC wiring scheme and has been put into operation in Shanghai Nanhui, Zhoushan multi-terminal, Nan'ao multi-terminal, and other projects that are used in this wiring scheme.This a more suitable main wiring scheme for offshore wind power VSC engineering from the perspective of investment and lightweight.However, the pseudo-bipolar has a higher level of overvoltage.After a pole ground fault, the other pole doubles the voltage to ground, which is a more stringent test of equipment insulation.
The bipolar wiring scheme is characterized by higher reliability.one pole fails, the other pole can continue to operate without causing power interruption.However, the AC side connection area of each pole is subjected to an AC voltage with a DC bias during normal operation.The magnitude of the DC bias voltage is half of the DC pole line voltage.The requirements of this condition make it difficult to manufacture the transformer and associated equipment for the coupling zone.The equipment and footprint of the bipolar solution are larger, increasing the investment in the offshore platform.The use of metal return lines increases the investment in DC cables.If the grounding pole forms will increase the impact on the marine environment, the construction will be more difficult.If the platform footprint and load-bearing capacity of the offshore VSC are considered, the half-bridge pseudo-bipolar structure form with less footprint, smaller investment, less difficulty in equipment manufacturing, and less impact on the environment is considered.We adopt transformer valve side star reactor via resistance grounding method.
When the capacity reaches 2000 MW and above, if the symmetrical unipolar topology is still used, the equipment insulation level is high.The net distance is large, the individual equipment voltage and current stress are large, and the design and manufacture are more difficult.However, once a key equipment failure occurs, such as converter failure, transformer failure, or DC line failure, it will result in an overall platform shutdown or even a long-term power outage of the wind farm.Not only the power loss is huge and the availability is low, but also the incidental large equipment maintenance costs will be caused.If a true bipolar topology with a metal return line is used, the other pole can be maintained in operation in case one pole is out of service.The power output of the turbine is guaranteed with high reliability.The energy dissipation device is configured on the DC side of the receiving end land station.The bridge arm reactor is configured on the DC side of the converter valve, eliminating the offshore DC reactor.The onshore DC reactor is retained, and the coupling transformers of the onshore and offshore stations are of double-winding type.In addition, it should be noted that the output power of the turbine is not fully generated in most cases, so unipolar shutdown does not even cause power loss in most cases.This greatly reduces the maintenance surcharge of the system.The above analysis shows that the true bipolar topology is more advantageous in offshore wind power transmission projects.
The symmetrical bipolar with metal return line topology is more flexible due to the metal return line.The availability rate is higher, and the normal operation of the other pole is not affected when one pole fails.The disadvantage is that as the transmission distance increases, the cost of the metallic return line increases accordingly.However, in the bipolar without metallic return line topology, if one pole fails, the whole system is shut down.At the same time, due to the long restart time after turbine shutdown, the system power interruption time is longer, which is not suitable for offshore wind power through VSC.Therefore, the true bipolar with metal return line topology is recommended to be used in the system for delivery of electricity from deep sea wind through VSC, as shown in Figure 5.

Study of the power operating range of a converter station
The PQ circle diagram is the ability of the converter station to inject active and reactive power into the AC system at a specified AC voltage with continuously adjustable coupling transformer taps.The power here refers to the amount of power injected into the AC system from the grid side of the coupling transformer.The PQ diagram is mainly related to the parameters of the converter transformer and the bridge arm reactors.Therefore, the PQ circle diagram calculation can be carried out on the half-bridge symmetrical unipolar structure.During the calculation, the power direction is defined as positive for power absorption from the AC system (rectification, inductive reactive power) and negative for injection into the AC system (inverter, capacitive reactive power).The constraints are as follows.a. the coupled variable capacity is not exceeded; b. modulation ratio within specified limits; c.DC power limitations; d.RMS limitations on bridge arm current; e. AC bus voltage within the steady-state operating range; f.DC port voltage within the steady-state operating range.
For the operation of two sets of coupling transformers at sea, it is assumed that the rated capacity of the equivalent coupling transformer after the two sets of coupling transformers at sea which are connected in parallel is 1400 MVA, and the short-circuit impedance is 15%.The horizontal coordinate of the diagram is the active power (MW), the vertical coordinate is the reactive power (Mvar), and the operating range is shown in Figures 6 and 7.

Conclusions
It is a difficult problem to maintain high reliability while designing the topology of the soft direct feeder system for deep-sea wind power in a compact manner.In this paper, the topology of the converter, the main wiring type and grounding method, and the DC system wiring scheme are studied.Comparison results show that the converter adopts a half-bridge converter topology, and the DC system adopts a true bipolar with metal return line topology with high reliability and good economy.The power operating range is also presented.The scheme proposed in this paper provides a boost to the development of deep offshore wind power.

Figure 3 .
Figure 3.Typical grounding method of double large earth return line.

Figure 4 .
Figure 4. Bipolar system using a metal center line wiring mode.

Figure 6 .
Figure 6.PQ operating range of offshore station during the operation of two sets of offshore coupling transformers.

Figure 7 .
Figure 7. PQ operation interval of the land station during the operation of two sets of coupling transformers at sea.

Figure 8 .
Figure 8. PQ operation interval of the offshore station during long-term operation of the offshore single-group coupling transformer.

Figure 9 .
Figure 9. PQ operation interval of the onshore station during long-term operation of the offshore single-group coupling transformer.