Research on relay protection technology of high temperature superconducting cable system

High-temperature superconducting cables have many advantages such as low line loss, large transmission capacity, small space occupation, and environmental friendliness, which provide a highly efficient, compact, reliable, and green way of power transmission for power grids, and have great potential for application in large-capacity power transmission and urban power grid expansion and reconstruction. However, high-temperature superconducting cables have a complex structure, which is quite different from traditional cable lines in terms of operating conditions and fault mechanisms, and the requirements for the secondary system are also more stringent. Relying on the project of China Power Grid, “Research and Demonstration Application of Superconducting Transmission Technology in Urban Distribution Networks”, the thesis carries out an in-depth research on relay protection technology adapted to the access of high-temperature superconducting cables.


Introduction
High-temperature superconducting cable access to the power grid, not only affects the short-circuit current distribution characteristics of the power grid near the access point, but also puts forward higher requirements for the configuration scheme and coordination strategy of adjacent component protection.Therefore, according to the structure and operating characteristics of high-temperature superconducting cables, combined with the actual situation of access to the power grid, in-depth research on relay protection technology adapted to the access of high-temperature superconducting cables has important theoretical and practical significance to ensure the operational safety of the cable, improve the reliability of power supply of the grid and the stability of operation, and it is also a high-temperature superconducting cables need to focus on the key technical issues to be resolved in the process of the practical implementation of high-temperature superconducting cables.

Status of foreign applications
In 2001, Denmark's Nordic superconductor company and the Nordic cable company, together with the research and development of a 30m/36kV/2kA three-phase AC thermally insulated high-temperature superconducting cables, and put into operation in the year the power grid [1] .2011 September, by the German RWE, France Nexans and the German Karlsruhe Institute of Technology (KIT), the Ampa City cable, which is the most powerful superconducting cable in the world, has been put into operation in the world.In September 2011, the Ampa City project undertaken by RWE (Germany), Nexans (France) and Karlsruhe Institute of Technology (KIT) (Germany) was officially launched to develop a high-temperature superconducting cable with a length of 1,000 m, a voltage level of 10 kV and a capacity of 40 MVA for the connection of the Dellbrügge substation to the Herkules substation, which has a superconducting current limiter in series to limit the level of short-circuit currents.The high-temperature superconducting cable is manufactured in two segments, which are connected via intermediate joints.The Ampa City project has not only produced the longest superconducting transmission system in the world today, but also for the first time a superconducting cable has been used in conjunction with a superconducting current limiter in a real power grid.The superconducting transmission system completed testing in 2013, was connected to the grid on April 30, 2014, and has been in continuous operation for two years, except for a few minutes of blackout during the June 9, 2014 Ela storm [2] .The Superconductivity Project in St. Petersburg, Russia, plans to fabricate a DC high-temperature superconducting cable with a rated current of 2.5kA, a rated voltage of 20kV, and a rated capacity of 50MW, and put it into actual operation on the St. Petersburg city grid.The superconducting cable is 2.5km long and is manufactured in five segments connected by four intermediate joints.The cable is made of a generation of superconducting strips manufactured by Sumitomo Corporation, with 22 strips of de-current conductors, two layers, each with a critical current of 160 A, and 19 strips of return conductors, one layer, each with a critical current of 180 A [3] .In 2015, the U.S. Navy entered into an exclusive supply contract with AMSC on the development of superconducting cables for use in power supply for ships.During the course of this project, AMSC, in conjunction with the Naval Metal Processing Center of Excellence, not only optimized the production cost of the HTS degaussing cable, but also streamlined the cable manufacturing process to meet the delivery and cost targets for the production of the entire ship.In October 2014, Chubu University in Japan, Sumitomo Electric Industries, and Sakura Internet Corporation, among others, collaborated in the development of a superconducting direct-current cable with a total length of 1,000m for multiple connection segments.The cable consists of three segments of 468.8m, 372.5m and 125m in length connected by two intermediate joints, with a rated current of 2.5kA, a rated voltage of ±10kV [4] .

Domestic application status
April 19, 2004, in Kunming City, Yunnan Province, Puji substation, the use of domestic new materials, new technologies developed successfully 30m/35kV/2kA thermally insulated three-phase AC high-temperature superconducting cables formally connected to the network, marking the United States, Denmark, China has become the third realization of high-temperature superconducting cables in the world after the country's parallel operation of the network.2004, the Chinese Academy of Sciences, Institute of Electrical Engineering, Gansu Province, with the support of the national "863" major projects.Ltd. in the national "863" major projects under the support of the research and development of 75m/10.5kV/1.5kAthermally insulated three-phase AC high-temperature superconducting cables successfully passed the system testing and debugging, and in February 2011 to the grid test operation.2012, Henan Zhongfu Industry Co. Ltd [5] .put a 360m/10kA DC superconducting cable into grid operation in Gongyi City, Henan Province, to supply power to its electrolytic aluminum production plant, achieving energy savings of more than 65% compared with conventional cables of the same transmission capacity.The cable is the world's first DC high-temperature superconducting cable put into industrial operation.In 2017, Futong Group Superconducting Technology Application Co., Ltd.successfully wound a set of cold insulated three-phase AC superconducting cables of 100m/35kV/1.0kAby using second-generation high-temperature superconducting strips, and put the cables into the test operation of the power grid of the Binhai Hi-Tech Development Zone of Tianjin in the same year, after completing the assembling and all kinds of experimental tests.

Relay protection programme for high-temperature superconducting cables
3.1 Short-circuit protection configuration 3.1.1Fibre optic current differential protection configuration scheme Considering that the 10kV system accessed by the high temperature superconducting cable is a neutral point grounded system through small resistance, when asymmetric grounding short circuit occurs, zero sequence current will be generated, so zero sequence current differential protection can be added on the basis of split-phase current differential protection.Normal operation of the power system three-phase load current symmetry will not produce zero sequence current, so the zero sequence current differential protection is not affected by the load current, and withstand the transition resistance ability is stronger, can better make up for the shortcomings of split-phase current differential protection.Therefore, when the high-temperature superconducting cable adopts the optical fibre current differential protection as the main protection, it should be equipped with split-phase current differential protection and zero sequence current differential protection at the same time.In addition, due to the high hazard of internal short circuit in superconducting cable, when the main protection refuses to operate, if the fault is removed by the backup protection of the higher-level components, the duration is long and may cause serious consequences [6] .In order to improve the reliability of the main protection, the fibre-optic current differential protection adopts a dualised configuration.
The above optical fibre current differential protection principle is simple, high sensitivity, strong resistance to transition resistance, not subject to power system oscillation, overload, PT disconnection and other complex factors, and fast action speed, can reliably respond to various types of short-circuit faults on the line of high-temperature superconducting cables, and effectively ensure the safety of superconducting cables and the operation of the power grid.

Staged directional current protection configuration scheme
High-temperature superconducting cable access to the local power grid after the operation of two kinds of operation: open-loop operation and closed-loop operation, in the closed-loop operation, in order to avoid the current protection error, should be added to the directional components, constituting a directional current protection [7] .In order to ensure the safety of high temperature superconducting cable operation, while taking into account the selectivity of the protection action, the directional current protection adopts a three-stage configuration, which is divided into the directional current flow protection, directional time-limited current flow protection and directional time-limited overcurrent protection.The current break protection and time-limited current break protection are phase-to-phase current protection installed according to the needs of high-temperature superconducting cable current-bearing capacity, which can be used as the "backup" protection of high-temperature superconducting cable loss of over-protection [8] .Current break protection according to the superconducting cable "limit current" set, no delay action; time-limited current break protection according to the superconducting cable's current carrying capacity and neighbouring components of the protection of the action time and so on set, its purpose is to ensure that when the neighbouring components fail, under the premise of the high temperature superconducting cable operation safety, the priority of the neighbouring components by the protection of their own excision Fault; fixed-time overcurrent protection as a superconducting cable phase-to-phase short-circuit near backup protection and feeder phase-to-phase short-circuit remote backup protection.

Layered loss of overrun protection
Hierarchical loss-of-overrun protection can be divided into three levels according to the magnitude of superconducting cable currents, as shown in Figure 1.Level 1 protection is mainly used to react to loss-of-overrun faults triggered by severe short circuits in superconducting cables or in the near zone.When a severe short-circuit occurs, the superconducting cable is severely lost in excess due to the large short-circuit current, the temperature rises rapidly, and the current withstand time is very short, so it needs to be removed as soon as possible.Therefore, when the short-circuit current exceeds the large fault current i2 shown in Figure 1, the protection operates without delay.The protection of level I can be realised by the current-failure protection.
Level 2 protection is mainly used to react to the loss of super faults caused by "non-severe" short circuits such as short circuits at the far end of the neighbouring wires of superconducting cables.When a "non-severe" short-circuit occurs, the short-circuit current flowing through the superconducting cable is between i1 and i2 in Figure 2-4, and the superconducting cable can continue to operate for a certain period of time due to its current resistance, thus creating conditions for selective removal of external faults.The protection of level 2 is mainly composed of inverse time limit overheating protection, loss of super fault phase action protection and fixed time limit current protection [9] .
Inverse time limit overheating protection: The heat generation of the superconducting cable after loss of super is proportional to the square of the RMS value of the short-circuit current.Inverse time limit overheating protection through the online calculation of the short-circuit current size, and according to the superconducting cable in the current under the allowable current carrying time, to determine the protection action time.The larger the short-circuit current is, the shorter the action time of the inverse time overheating protection is, which can effectively ensure the safety of cable operation while giving full play to the current-carrying capacity of the cable.
Loss-of-overrun fault phase-sequential action protection: When a short-circuit fault occurs outside the cable and causes an overall loss-of-overrun of the cable, the protection identifies the external fault tripping situation through the amount of current and voltage changes.Once the external fault is removed, the superconducting cable is accelerated to jump off through the successive action, in order to shorten the superconducting cable's running time under the loss-of-overrun state and reduce the heat generation, thus facilitating the superconducting cable to be put into operation again after recovering the superconducting state and improving the reliability of power supply to the grid.
Fixed-time current protection: The basic principle is the same as that of traditional fixed-time current protection, and its main function is to standby with short-circuit protection to further improve the reliability of backup protection [10] .
Level 3 protection is mainly used to react to loss-of-overrun faults (mainly localised loss-of-overrun faults) caused, for example, by abnormalities in the cable body or faults in the cooling system.Such faults develop and change relatively slowly and are detected mainly by changes in non-electrical quantity signals such as temperature, flow and pressure to improve reliability and accuracy.The protection of level 3 mainly consists of temperature and temperature rise trans-domain protection, flow deviation protection, differential pressure trans-domain protection, fibre-optic temperature measurement protection based on the characteristics of temperature distribution along the line, and low-temperature cooling system fault tripping.
Temperature and temperature rise out-of-range protection: When a high-temperature superconducting cable is out of range, its resistance increases, which generates and accumulates heat, leading to an increase in the temperature of the superconducting cable.Temperature and temperature rise out-of-range protection monitors the temperature and temperature rise of the superconducting cable in real time, and when the temperature or temperature rise is out of range, the superconducting cable will be jumped off to avoid overheating and damage to the cable.
Low flow protection: When the circulating pump stops, the liquid nitrogen channel is blocked or the liquid nitrogen leaks, the cold quantity provided by the cryogenic cooling system cannot maintain the low temperature conditions required for the normal operation of the superconducting cable, which in turn causes the loss of super fault.The above cryogenic system failure can be indirectly reflected by the liquid nitrogen flow information, therefore, when the flow is low, the superconducting cable should be removed in time to ensure its operational safety.
Differential Pressure Protection: Liquid nitrogen channel blockage and liquid nitrogen leakage and other cryogenic system failures can also lead to superconducting cable import and export differential pressure beyond the domain, therefore, when the import and export differential pressure beyond the domain, the superconducting cable should be removed in a timely manner to ensure the safety of its operation.
Fibre optic temperature measurement and protection based on the temperature distribution characteristics along the cable: after the superconducting cable is out of overrun, a large amount of Joule heat will be generated and accumulated in the overrun area, resulting in changes in the size and distribution characteristics of the temperature along the cable.Fiber optic temperature measurement protection through the distributed fiber optic temperature measurement technology for online measurement of the temperature along the cable, and according to the size of the temperature along the line and changes in the identification of faults, can effectively solve the problem of detecting local faults, to better ensure the safe operation of superconducting cables.
Fault trip of cryogenic cooling system: Due to the complex structure of cryogenic cooling system, which involves many components such as refrigerator, liquid storage tank, liquid nitrogen pipeline and circulating pump, the chance of failure is relatively high.In order to ensure the reliability and rapidity of cryogenic system fault detection, the cryogenic control system can send a fault trip signal to the overrun protection device through communication and hard contact.

Self-recovering control of superconducting cables
After the superconducting cable is connected to the network, the main reasons for its withdrawal from operation are: superconducting cable system failure and external power grid failure, in which the superconducting cable system failure includes cable body failure and cryogenic refrigeration system failure.When the cable is withdrawn from operation due to superconducting cable system failure, it needs to be shut down for maintenance, and it can be put into operation again (manually) only after removing the fault; when the superconducting cable is withdrawn from operation due to loss of super caused by external power grid failure, since the superconducting cable system itself does not fail, it should be put into operation as soon as possible after the superconducting cable body recovers the superconducting state to achieve the superconducting cable self-recovery control function under the failure of the external power grid.function, in order to improve the reliability of grid power supply.The self-recovery control of superconducting cable consists of failure detection, heat accumulation calculation, exit detection, recovery time calculation and low-temperature system operation state detection, etc.After the superconducting cable is exited from the operation due to an external fault, it will be detected whether it meets the conditions of self-recovery input, and if it meets the conditions, it will send out a closing allow signal, and the superconducting cable will be put back into the operation again.

Simulation models
In order to verify the effectiveness of the proposed protection method, the three identical axial superconducting cables are taken as an example, and the multi-physics field simulation software COMSOL is used to build its heat transfer model.The cooling method of the superconducting cable system is single-end cooling (liquid nitrogen inlet and outlet are at the same end), the inlet temperature of liquid nitrogen is 70K, the maximum permissible temperature rise is 6K, the flow rate of liquid nitrogen is 0.3kg/s.The main geometric parameters of the cable components are shown in Note: T is the real-time temperature of the material.

Simulation verification
In order to comprehensively evaluate the operational performance of the proposed localized loss-of-overrun detection and protection method, simulation analyses were carried out for the cases of no localized loss-of-overrun, slight localized loss-of-overrun and severe localized loss-of-overrun, respectively.In the simulation analysis, the number of measurement points is set as 7/m, the noise amplitude is selected as 10% of the maximum temperature rise of superconducting cable during normal operation, the threshold value Th=0.6, the threshold values of r1 and r2 of the percentage of lost overrun area are selected as 3% and 8%, respectively, and the threshold value of low temperature TL=76*1.05=79.8K,and the threshold value of high temperature TH=150/1.2=125K. Due to the limitation of the length of the paper, only some of the simulation results are given in the following.some of the simulation results are given below due to space limitation.

No localized loss of super
The simulation results show that when the superconducting cable is free of localized loss of super faults, there will be no wave characteristics although the temperatures along the conductor layer of the cable are different at different load levels.Figure 2 and Figure 3 show the temperature along the conductor layer of the cable with rated load (current of 2.5 kA, cable equivalent resistance of 8.4 × 10-5 Ω) and the results of the MMG transformation MT, that is, the simulation results when the cable is rated load without localized loss-of-overrun.Figure 4 shows the effect of the corresponding finite element model solution results.From Figure 2, it can be seen that when the superconducting cable has no localized loss-of-overrun faults, the temperature waveform along the conductor layer does not have wave crest characteristics, and its MMG transformation result MT will not be larger than the threshold value, so the protection is reliable and not inadvertently activated.

Minor localized loss of superlatives
Figure 5 and Figure 6 show the temperature change curve along the superconducting cable and the MMG transform result MT when a slight loss of super fault occurs somewhere in the superconducting cable, and it is easy to see that the Gaussian filtering algorithm given in the paper not only has a good performance of noise cancellation, but also better maintains the characteristics of the peak edges in the original waveforms, which is conducive to improving the accuracy of the waveform feature extraction.

Severe localized loss of super
Assuming that the superconducting cable has two simultaneous loss-of-overrun faults, the simulation is used to obtain the temperature change curve along the conductor layer and the MMG transformation results MT; according to the simulation results, it can be seen that, in the two loss-of-overrun regions, the multi-resolution morphology gradient value MT exceeds the threshold, and the loss-of-overrun faults can be accurately identified.At the same time, through the coordinates of the starting point and ending point of the wave crest, the ratio of the total width of the loss of superheating area to the length of the cable can be calculated as [(160.80-120.15)+(329.70-301.65)]/500=13.74%,which fulfills the tripping criterion, and the protection will send out the tripping command to remove the superconducting cable.The above simulation results show that the proposed protection method can correctly detect the location and width of each localized superconducting heat generating area and reliably operate under strong background noise, no matter whether a localized slight loss of superconducting fault or multiple severe loss of superconducting faults occur.

Selection of handling equipment
At present, during the construction of high-temperature superconducting cable system, it is often necessary to move various types of small and medium-sized equipment from the ground to the crate, or from the crate to the ground, which is carried by manpower.Due to the large weight of some of the equipment, handling generally requires more people to work together, which is a waste of manpower, affecting the efficiency of the work, but also prone to cause heavy fall pressure injuries or personnel sprains and other safety risks.Thus, the development of an electric loading and unloading operation plate car, the boom can be rotated, you can easily circulating pumps, superconducting cables, etc. from the ground to the plate car or from the plate car lifted to the ground, speed up the handling speed, reduce the pressure injury, sprain safety risks, enhance the effectiveness of the verification.Functions are as follows: 1, with electric lifting, boom steering function; 2, the use of outreach support mechanism to stabilise the bottom of the work crate, assisting the body to maintain a smooth lifting mechanical arm action; 3, electric loading and unloading work crate within the overall size of 70 * 110cm, lifting a maximum weight of 125kg.The parameters of the device are shown in Table 3 below: .Conclusion High-temperature superconducting cables are not only a technological innovation of the primary power transmission system, but also put forward new and higher requirements for the secondary system.In order to ensure the operation safety of high-temperature superconducting cables and power systems, it is necessary to carry out in-depth research on relay protection technology adapted to the access of high-temperature superconducting cables.Based on the project of "Research and demonstration application of superconducting transmission technology in urban distribution network" of China Power Grid, the paper focuses on the construction scheme of high-temperature superconducting cable protection system, the principle of realization, and the configuration scheme and coordination strategy of grid protection adapted to the access of superconducting cables to carry out a systematic research.The main work and research results of the paper are as follows: (1) The overall construction program of superconducting cable protection system is proposed, which lays a solid foundation for the development of the protection system.According to the structure and operation characteristics of high-temperature superconducting cables, the fault types and generation mechanism of high-temperature superconducting cables are analyzed.The overall construction scheme of superconducting cable protection system based on short-circuit protection and loss of super protection is proposed.(2) A method for detecting and protecting the local loss of superconducting cable faults based on distributed fiber-optic temperature measurement technology is proposed, and its good performance is verified by digital simulation.In view of the problem of local failure detection, the paper proposes a local failure detection and protection method for superconducting cables based on distributed fiber optic temperature measurement technology, according to the characteristics of temperature peaks along the conductor layer at the point of failure after the occurrence of local failure of superconducting cables.Simulation results show that the method has good noise resistance, can not be acted when there is no localized loss of super fault, and is not affected by load changes; in the event of localized loss of super faults can be correctly detected in the location and width of each loss of

Figure 1
Figure 1 Schematic diagram of hierarchical lost overrun protection.

Figure 2
Figure 2 Temperature along the cable conductor layer.

Figure 3
Figure 3 Results of MMG transformation of TS.

Figure 4
Figure 4 Effect of finite element model solution results.

Figure 6
shows the temperature TS along the line, the MMG transformation results MT, and the results of the calculation of the coordinates of the starting point, end point and peak point of the wave crest in the TS waveform (in the figure, Ti(x,y): x represents the distance from the first end of the cable, and y represents the temperature; T1 represents the starting point of the wave crest, T2 represents the peak point of the wave crest, and T3 represents the end point of the wave crest).From Figure6), it is easy to see that the result of MMG transformation of temperature TS along the line MT will exceed the threshold value at the point of loss of super, which can accurately determine the occurrence of local loss of super fault.At the same time, through the determined coordinates of the starting point and end point of the wave crest, it can be calculated that the ratio of the width of the lost overheating area to the length of the cable is: (220.35-201.90)/500=3.69%>3%,which meets the alarm criteria, and the protection reliably sends out an alarm signal.

Figure 5
Figure 5 Temperature along the cable conductor layer.

Figure 6
Figure 6 TS, MT and localized loss of super detection results.

Table 1
the heat capacity and thermal conductivity of the materials are shown in Table2.

Table 1
Geometric parameters of the components of the superconducting cable.

Table 2
Heat capacity and thermal conductivity of superconducting cable materials.

Table 3
List of equipment parameters.