Overview of development,design,testing and application of compact gas-insulated DC systems up to ±550 kV

1 Introduction

For ensuring higher power ratings over very long distances,the high-voltage direct current (HVDC) technology is technically superior to the conventional highvoltage alternating current (HVAC) technology.One of the drivers of HVDC technology is the growing demand for the integration of renewable electric energy resources,resulting in a change of the existing electric power transmission system.Based on the increasing demand for space-saving and reliable HVDC solutions,compact gas-insulated systems for HVDC applications are under development worldwide.While gas-insulated systems for AC applications are common standard since decades [1],the experience in the development and testing of gas-insulated DC systems is limited today.The insulation requirements and testing procedures for gas-insulated AC systems have been incorporated in standards such as the IEC 62271 series since decades.However,for DC GIS products,specific standards are currently not available,though recommendations for the testing requirements are being prepared in CIGRÉ JWG D1/B3.57,along with the physical background and insulation requirements [2] [22].

While in gas-insulated AC systems the electric field distribution remains unchanged after energization,in DC systems it changes from electrostatic field to electric flow field.Intensive research on this field during the last years shows a growing understanding of the basic characteristics of DC insulation,including charge-carrier based processes in the gaseous and solid insulating media and at the gassolid interface [3]-[9].

These phenomena must be considered during the design and development of gas-insulated DC systems to ensure high reliability and for determining the appropriate testing strategies.

2 Special aspects of DC insulation for development

The key to evaluating the performance of an insulating system under DC voltage stress is to have a comprehensive understanding of the behaviour of the charge carriers in the insulating media and,in particular,at the gas-solid interface.Shortly after energizing with DC,the electrostatic field,which is determined by the permittivities,is transformed into the electric flow field,which is determined by the conductivities.In a typical gas-insulated system,this capacitive-resistive transition can last from a few hours to months and is accompanied by the accumulation of surface and space charges,which also affect the electric field strength [2],[5],[8].

Conventional models have been used to describe the behavior of the insulating system with RC elements while using distinct conductivity values.Taking practical field strength into account,previous studies [2],[4],[8],[10] have shown that besides natural ionization,additional charge carriers from voltage dependent sources are present.Therefore,conduction has to be understood as the movement of different charge carrier species within the insulating material [8],[10].The determining complex physical processes are influenced by several parameters,such as non-linear material properties dependent on temperature,humidity and additional charges.Besides,the duration of the field transition is depending on the conductivity.Results confirm that a higher conductivity is reducing the time for the field transition [8],[12],[13].

Since the operating currents cause an inhomogeneous temperature distribution between conductor and enclosure,the solid insulator experiences the full temperature gradient.The conductivity of the material is varying by orders of magnitude in the relevant temperature range [11].Hence,the conductivity is increasing in the vicinity of the heated conductor,resulting in an enhanced electric field strength in the colder regions of the insulation (Fig.1) [8],[13],[14].

Under AC condition (Fig.1,top) the electric field distribution is determined by the permittivities.Further,it neither exhibits transient behavior after energization with a constant voltage nor dependency on the temperature.The highest field strength is occurring at the inner conductor in the insulating gas.This condition,which also occurs shortly after energization under DC conditions,has to be tested through a typical dielectric voltage withstand test under AC and DC conditions.

Under DC condition,the electric field distribution is determined by the charge carrier-based conduction processes in the gas and in the solid.Without a temperature gradient (Fig.1,middle),under cold conditions,the field strength is the highest close to the conductor within the solid insulator.Hence,it is recommended that long-term tests be performed to investigate the possible aging processes,since the current experience is very limited.

Fig.1 Electric field distribution of a conical insulator under AC voltage (top),under DC voltage without a temperature gradient (middle) and under DC voltage with a temperature gradient between conductor and enclosure (bottom)

With a current,flowing in the conductor,the temperature in the system is increasing and a temperature gradient is arising (Fig.1,bottom).Since the conductivity of the solid material increases with the temperature,the electric field is shifted to colder regions.Hence,the field strength is no longer the highest at the center of the insulator but at its outer area close to the grounded enclosure.In the shown example,the stress at the gas-solid interface increases,while the field strength close to the conductor decreases.This mode represents the typical operation conditions and should be revised in thermoelectric tests.

An important criterion is the insulator’s resistivity.The higher the resistivity of the insulating material,the longer the charging time will be.Further,the lower the resistivity,the higher the power losses resulting from the flow of electric charge through the insulator will be.The optimum is reached when losses are at an acceptably low level,and the amount of charge accumulated is limited to a reasonable range.

3 Modular design of DC GIS

The development of DC GIS is based on the technology used for AC GIS with,in principle,the same components and a newly designed DC insulator to meet the requirements of the electric field distribution under DC voltage stress.The elementary functions of gas-insulated switchgear assemblies without a circuit-breaker under DC operation are:

·Set up a safe isolating distance with earthing on both sides

·Measurement of current and voltage

·Protection against overvoltage

·Connection to other transmission media (DC cable,DC overhead line,DC GIL)

The modular structure of AC GIS has been proven in many applications worldwide with high reliability and has been optimized over the last 50 years since the first AC GIS has been installed in Germany in 1968 [1][19].

The gas-insulated system is independent from external ambient conditions,like humidity,dust,ice,wind,corrosion,etc.,because of its gas and watertight encapsulation.The grounded enclosure can be located inside a building.Underground installation is possible as well.

Respecting all relevant design requirements including geometry and material characteristics,development has led to the modules shown in Fig.2,which can be arranged flexibly according to the project’s needs,e.g.in a bay layout for a DC switchyard (Fig.3).

Fig.2 Modules of DC gas-insulated switchgear assemblies

Fig.3 Principle structure of a compact DC substation - converter pole feeder as DC switchyard in/out bay

Disconnector and earthing switches

At the core of the switchgear assemblies is the disconnector.Together with the earthing switches on either side of the insulating gap,the disconnector ensures the safe insulation and earthing of de-energized circuits.The makeproof earthing switch also enables the safe discharge of potential resulting from residual DC charges.

Surge arrester

Encapsulated surge arresters ensure the protection from overvoltage.Their active parts consist of metal oxide varistors with a strongly nonlinear current-voltage characteristic.Depending on the overvoltage surge requirements,more than one surge arrester may be used.

Voltage and current measurement

Gas-insulated RC voltage dividers map high-voltage linearly over a frequency range from DC up to 30 kHz.They are designed for an optimal transient behaviour up to 2 MHz [17].Current detection relies on the zero-flux measurement principle [16] for rated currents up to 5000 A.The modules within the GIS carry up to three encapsulated measuring heads.

Interface modules

The interface modules enable the transition from the gas-insulated switchgear assemblies to other equipment.Gas-to-air bushings are available for the transition to an overhead line.The cable terminations follow IEC 62271-209.The conductor link within the module can be removed to separate cable and GIS for on-site cable testing.Another interface module is available for the transition to a gasinsulated line.

Passive Modules

Several passive modules (e.g.T-shaped) are available for flexible configuration.Compensation modules provide options to deal with heat dilatation and make access to single modules within the arrangement for service and repairs.

4 Testing of gas-insulated DC systems

The testing requirements for gas-insulated HVDC systems are currently not standardized.Recommendations are under development in CIGRE JWG D1/B3.57.According to these,the electrical and mechanical requirements of IEC 62271 series,that are independent of the type of operating voltage,should be fulfilled [15],[22].Additional electric and thermoelectric tests must be performed so that the special features of the DC voltage conditions in terms of the electric field distribution of the insulators,which are affected by the accumulation of electrical charge carriers,and the operationrelated inhomogeneous temperature distribution,are also considered.

Table1 lists the tests performed on ±550 kV DC GIS.All the tests were successful.Hence,the ±550 kV DC GIS is ready for application in HVDC converter and transition stations.Technical data is summarized in Table 2.

Table1 Overview of performed type tests ±550 kV DC GIS

Performed Test Result Standard Dielectric test Passed IEC 62271-1 IEC 62271-102 IEC 62271-203 CIGRE JWG D1/B3.57 Temperature-rise test Passed Mechanical endurance test /Low and high temperature test Passed Short-time withstand current and peak withstand current test Passed Internal arc test (already done during ±320 kV development) Passed Insulator tests Passed Insulation system test Passed CIGRE JWG D1/B3.57

Table2 Technical data of DC GIS

Technical data 8DQ1 DC Undc nominal DC voltage ±500 kV Urdc rated continuous DC voltage (max.continuous operating voltage) ±550 kV ULI rated lightning impulse withstand voltage (1.2 / 50 μs) ±1550 kV LI ● superimposed with DC ±550 kV DC ＆ ±1550 kV LI ● across the open isolating distance ±550 kV DC ＆ images/BZ_97_2082_811_2099_829.png1550 kV LI USI rated switching impulse withstand voltage (250 / 2500 μs) ±1175 kV SI superimposed with DC ±550 kV DC ＆ ±1175 kV SI across the open isolating distance ±550 kV DC ＆ images/BZ_97_2083_1012_2100_1030.png1175 kV SI Irdc rated nominal current 5000 A DC Ik rated short-time withstand current 50 kA AC (1 s)Ambient temperature range (-30 … +50) °C

4.1 Type testing

Dielectric tests

For the dielectric test,all relevant DC GIS components were assembled [19].In addition to the conventional highvoltage tests performed using an AC voltage,lightning impulse (LI) voltage and switching impulse (SI) voltage,further tests considering the DC-specific requirements were performed.

Further,in addition to the DC withstand voltage tests,composite voltage tests using lightning and switching impulse voltages,which were superimposed on a DC voltage,were conducted after DC prestressing for 2 h.To ensure proper testing,an optimized test circuit,including a sphere gap for superpositioning was employed [20].DC pre-stress longer than two hours was not required,as the main objective of the tests is the gaseous insulation only.The isolating area between the two types of disconnector switches was stressed during the combined voltage tests,wherein a DC voltage was applied at one terminal and a lightning or switching impulse voltage at the other.Table 3 summarizes the performed tests.

Table3 Dielectric tests for ±550 kV DC GIS

Test Level AC withstand voltage test with PD measurement 700 kV AC 467 kV AC (PD)DC withstand voltage test (10 min) ±825 kV DC LI withstand voltage test (15 impulses) ±1550 kV LI SI withstand voltage test (15 impulses) ±1175 kV SI Composite voltage test* DC + LI with 2 h DC pre-stress ±550 kV (15 impulses)±550 kV DC ＆±1550 kV LI Composite voltage test* DC + SI with 2 h DC pre-stress ±550 kV (15 impulses)±550 kV DC ＆±1175 kV SI

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* all polarity combinations
** bipolar polarity

Test Level Combined voltage test**,DC + LI across the isolating distance (15 impulses)±550 kV DC ＆1550 kV LI Combined voltage test**,DC + SI across the isolating distance (15 impulses)images/BZ_98_972_553_990_571.png±550 kV DC ＆1175 kV SI images/BZ_98_973_658_990_676.png

Temperature-rise tests

For AC GIS,temperature-rise tests are required to determine whether the system temperatures are within the acceptable limits.This is required for DC GIS also.However,the temperature-rise tests cover an additional DC-specific aspect:the temperature distribution and the temperature gradient between the high-voltage conductor and the enclosure determine the electric field distribution at the insulators.Further,as electro-thermal tests are usually performed with AC current,the relation between the AC and DC current at a given temperature and temperature gradient is of interest.As in the dielectric tests,the test object consisted of all major DC GIS components.The tests were conducted using AC and DC currents,whose magnitudes were increased in steps,as shown in Table 4.

Table4 Temperature-rise tests for ±550 kV DC GIS

Test Level Temperature-rise test AC 50 Hz 2500 A | 3150 A | 4000 A | 5000 A Temperature-rise test DC 2500 A | 3150 A | 4000 A | 5000 A

All tests were passed according to IEC 62271-203 requirements,also for a current of 5000 A DC.The temperature distribution in the test object was measured using more than 100 thermo-couples,to provide a basis for the preparation of electro-thermal tests.

Electro-thermal tests

For AC GIS,these tests are not required,as the electric field distribution does not depend on the temperature distribution within the GIS.On the other hand,electrothermal tests are essential,given the temperature dependence of the field transition.The special requirements for the tests,according to the recommendations of CIGRE JWG D1/B3.57,are considered in the so-called insulation system test as an electro-thermal test.

The test setup (Fig.4) was designed to be a closed loop,including a current transformer to induce an AC current,to enable a temperature distribution within the GIS,comparable to the maximal operating current.Comparing the outcomes of the AC and DC temperature-rise tests,4700 A AC resulted in the same temperature gradient (Δϑ = 30 K) as caused by 5000 A DC.In the thermally stationary state (high load condition with a temperature gradient),the DC GIS was charged with DC voltage for more than 120 hours [7],after this process at least 90% of the DC voltage charge has been reached at all points in the insulation system (the time period is labeled t90).Subsequently,the insulation system was stressed with composite voltages of DC and superimposed with lightning and switching impulses.

The tests performed are listed in Table 4.All the tests were successful.

Table5 Insulation system test ±550 kV DC GIS

* all polarity combinations

Pre-test (zero load - without temperature gradient)Test Level AC withstand voltage test (1 min) 700 kV AC DC withstand voltage test (10 min) ±825 kV DC LI withstand voltage test (15 impulses) ±1550 kV LI SI withstand voltage test (15 impulses) ±1175 kV SI Insulation system test (high load - with temperature gradient)Test Level Composite voltage test* DC and LI voltage with ＞120 h of DC pre-stress ±550 kV (3 impulses)±550 kV DC ＆±1550 kV LI Composite voltage test* DC and SI voltage with ＞120 h of DC pre-stress ±550 kV (3 impulses)±550 kV DC ＆±1175 kV SI

Fig.4 Test setup of insulation system test

Short-time withstand and peak withstand current test

It must be ensured that the GIS exhibits safe thermal and mechanical performances under short-circuit conditions.Even though the short-circuit current requirements of DC systems are typically lower than those of AC systems,the according tests were performed as required for AC in agreement with IEC 62271 series for a short-circuit withstand current of 50 kA,a peak factor of 2.7 p.u.and a short-circuit duration of 1 s.All tests were passed successfully.

Internal arc test

Internal arc faults are extremely rare in gas-insulated systems; however,they must be considered during the design of the enclosures and overpressure devices used as rupture discs,in accordance with the IEC 62271 series.A disconnector module was tested under conditions of arcing due to an internal fault with a peak current of 63 kA,a short-circuit current of 47.5 kA and a duration of 300 ms,as shown in Fig.5.The test was performed with direct current.Neither fragmentation nor the burn through of the enclosure occurred during the test,and the rupture disk worked properly.Hence,it was concluded that the system had passed the test.

Fig.5 Oscillogram of internal arc test current

Long-term ageing tests

Data on the long-term performance of DC GIS are limited.The aging of the bulk epoxy material must be considered in the case of AC systems.For DC applications,aging is expected to be of lower importance because of reduced aging effects at DC voltage [21].Nevertheless,tests are recommended due to the limited experience.To investigate the aging behavior,DC long-term tests were performed on real-sized GIS insulators under conditions of accelerated aging.For up to 40.000 hours,more than 130 insulators have been tested at high DC voltage of both polarities,as given in Table 6.The test results indicate a better long-term performance of epoxy insulators under DC voltage compared to AC voltage.This confirmed the suitability of the DC GIS insulator design [22].

Table6 DC long-term ageing test with ±500 kV

Insulator type pcs electric stress*Time at ±500 kV Result(small epoxy volume) 32 2.8 41.600 h (4.7 a)No puncture Cylindrical type Cylindrical type(medium epoxy volume) 51 2.8 41.600 h (4.7 a)No puncture Cylindrical type(large epoxy volume) 13 1.0 29.800 h (3.4 a)No puncture Cylindrical type(small,extra high stress) 30 5.4 9.700 h (1.1 a)No puncture

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* normalized to DC GIS service stress

Insulator type pcs electric stress*Time at ±500 kV Result No puncture DC GIS insulator type 8 1.0 30.400 h (3.5 a)

4.2 Routine testing

The purpose of routine tests is to evaluate the integrity of the manufactured assembly units.These tests should reveal any faults that may be present in the material and assembly.Based on IEC 62271-1,the routine testing process consists of the following steps:

a) dielectric test on the main circuit

b) tests on auxiliary and control circuits

c) measurement of the resistance of main circuit

d) tightness test

e) design and visual checks

Since the tests (b) to (e) are not depending on the type of voltage,used they can be performed without any constraints.

DC voltage has not been used widely for the routine testing of the dielectric of gas-insulated systems.Therefore,the recommendations of CIGRÉ JWG D1/B3.57 suggest performing dielectric tests on the main circuit using an AC voltage,in keeping with IEC 62271-203.Accompanying partial discharge (PD) measurements should detect possible material and manufacturing defects existing in the transport units.In particular,the ability to evaluate the partial discharges under AC voltage stress using patterns would be highly valuable.

The JWG recommends for AC pre-stress (1 min) before PD measurement

and for the PD measurement (＞1 min)

Where Urdc is the rated DC voltage,that is,the maximum continuous operating voltage.[22]

4.3 On-site testing

The purpose of onsite dielectric testing is to check the dielectric integrity of the equipment after transport and erection,in order to eliminate factors that may give rise to internal faults in the future.The most reliable technique for detecting and identifying these factors is AC voltage testing along with PD measurements.Because there have been few studies on PD measurements under DC voltage conditions and given that there is a lack of reliable measurement techniques,onsite tests should be performed using an AC voltage,in keeping with the recommendations of CIGRÉ JWG D1/B3.57.

The tests should be performed at a nominal gas-filling pressure.Further,the test voltage should be supplied by a resonant test system.Finally,the test should be accompanied by PD measurements,which should be performed using ultrahigh-frequency (UHF) PD sensors.

As DC GIS are assembled under clean factory conditions,the gas compartments are generally free of free-moving particles; this can be confirmed through high-voltage routine tests and the accompanying PD measurements.Nevertheless,the post-transport integrity and cleanliness of the system as well as its complete assembly onsite along with the functioning of the gas works should be confirmed through high-voltage tests and PD measurements performed onsite after erection.For DC GIS,CIGRÉ JWG D1/B3.57 recommends the use of the same procedures as those for AC GIS,namely,those given in IEC 62271-203.The AC test voltage values to be used for the PD measurements and high-voltage tests should be calculated in the same manner as those for the DC GIS routine tests; these are described in the previous section.The voltage is typically applied in steps and includes a potential conditioning phase.Further,the main high-voltage test is followed by a partial discharge test [22].

5 Standardization

As mentioned in Section 3,there are no standards currently available for testing gas-insulated HVDC systems,such as DC GIS and DC GIL.However,recommendations have been under development at CIGRÉ since 2014 (JWG D1/B3.57 “Dielectric testing of gas-insulated HVDC systems”).

In 2017,efforts at international standardization were started by IEC Technical Committee (TC) 17 and its subcommittees.Subsequently,a task force investigated the necessity of standardization for DC GIS and DC GIL.To begin with,the business and market needs were analyzed.It was determined that there is a growing market for DC GIS and DC GIL,owing to an increase in the number of offshore installations in the North Sea in Europe as well as the number of HVDC converter stations to be installed onshore in Japan.A new DC GIS installation with a rated voltage of ±250 kV was scheduled to start operating in Japan in 2019 as a part of the Hokuto converter station.Moreover,the first offshore installation of a DC GIS with a rating of ±320 kV has already been ordered [18].

Given these scheduled installations,in 2018,the task force concluded that now is the ideal time to start developing the desired standards.

Furthermore,the task force compared the existing AC IEC standards for GIS and GIL with the requirements determined based on DC applications.They found major differences of the requirements and tests for dielectrics corresponding to AC and DC stresses.For instance,some sections of IEC 62271-203 (AC GIS) and IEC 62271-204 (AC GIL) can be used after corrections while those related to dielectric testing will need to be rewritten completely based on the results of CIGRÉ JWG D1/B3.57.Hence,new standards are necessary.[22] A decision was taken in 2018 with respect to IEC SC17C for DC high-voltage switchgear assemblies and transmission lines to develop several new standards,including the following two:

·IEC 62271-5Technical Specification on general requirements for DC switchgear devices ＆ DC switchgear assemblies

·IEC TS 62271-316DC Gas Insulated Switchgear Assemblies (DC GIS).

6 Application of DC GIS

In the instances that require space saving,independence from external conditions,the prevention of transient interference caused by lightning strikes,and aesthetic station planning with high visual attractiveness,gas-insulated HVDC systems are more suitable for use in converter and transition stations than air-insulated ones.[19]

6.1 Converter station

High-voltage DC transmission requires that converter stations be established at the grid connection points to the AC power grid.In particular,offshore converter platforms require space-saving installations as well as independence from external environmental conditions.Furthermore,owing to their poor accessibility,they must exhibit high reliability and low maintenance requirements [19].

The DC switchyard with a disconnector and earthing switches along with measuring devices and surge arresters is installed between the converter reactors and the DC voltage cable.With respect to the air clearances,the DC switchyard occupies relatively more space within offshore platforms.A typical configuration with air insulation is shown in Fig.6 with top and front view.

When using DC GIS with the same functional properties,the DC switchyard can be placed close to the grounded wall between the converter and reactor room (Fig.10).Further,it has been reported that,compared to air-insulated configurations (DC AIS),the gas-insulated design (DC GIS) can save 70-95% more of the space required by the switchyard,thus ensuring that the air clearance requirements are met.This allows the size and weight and hence the costs of the entire platform to be reduced significantly (Fig.7).[19]

Fig.6 DC GIS within reactor room in offshore platform

Fig.7 DC GIS may reduce platform size by 10%

For onshore converter stations,limited space is typically not as critical as for offshore converter platforms.In any case,DC GIS can reduce the volume of the DC switchyard by up to 95% (Fig.8).However,depending on the HVDC transmission system topology used,reconfigurations between two systems can help ensure that redundancies are available in the event of single-pole converter failure.The necessary connections in the converter stations result in increased space requirement,which can be reduced by using gas-insulated HVDC components.[19]

Fig.8 DC GIS reduce switchyard space by 95%

6.2 Transition station

Transition stations ensure connectivity between the different transmission media,such as overhead lines,cables,and gas-insulated lines.They enable the separation and safe earthing of the line sections and can include the necessary measurement systems for measuring the current and voltage as well as analysis systems for online line-fault location.They also allow for the possibility of accessing the conductor for high-voltage testing during commissioning as well as for repairs and the offline determination of line faults.All the equipment necessary for the above-mentioned functions while ensuring a compact and aesthetic station design is available in the gas-insulated modules for DC GIS installations (Fig.9).[19]

Fig.9 Example of transition station layout with DC GIS for connecting different transmission media

In addition to the transition stations located between different transmission media,very long cable lines require cable-cable transition stations.These stations integrate the measuring systems that can locate line faults by identifying transient voltages,which are related to high-voltage cable failures.In this case,elements for the application of online and offline location systems are foreseen.

In Germany,the government has decided to privilege underground installations for the north-south DC transmission lines with a length of approximately 700 km each.This has been done to connect renewable energy sources such as those related to wind power in the north with the load centers in the south.These long and underground DC transmission lines need to be separated in length,if necessary,to allow for sections for repair as well as high-voltage testing.DC GIS allow for space saving while also integrating all the necessary functionalities and are independent of the different transmission media that will be connected.Further,owing to their modular design,even complex arrangements and layouts can be implemented with little interfacial engineering.DC GIS can also be installed underground to keep the visual impact of transition stations to the absolute minimum and reliably prevent unauthorized access.

At the same time,these transition stations represent a clear separation between the cable sections,which may be important if the cables are provided by different cable manufacturers.For the purposes of maintenance and repair,it is essential to ensure that the sections are disconnected and earthed.The layout of the cable-cable transition stations includes cable-sealing ends,a voltage divider,and a decoupling option for transients (e.g.,a capacitor) as well as a disconnector and earthing switches (Fig.10).

Fig.10 DC cable to cable transition in air-insulated technology (DC AIS)

In order to prevent the transient interference caused by lightning strikes,air-insulated systems might be enclosed within a structure.As an example,for connecting the DC cables of two systems with Un = ±320 kV,a hall for the primary equipment with dimensions of 50 m × 50 m and a height of approximately 20 m would be required,while keeping the relevant air clearances in mind (Fig.11).The footprint of this structure would be 2500 m2.

Fig.11 Example of a cable-cable transition station for two systems of Un=±320kV in air-insulated technology

These transition stations can be built much more compact in gas-insulated design using DC GIS.The necessary components of the same functionality are available up to a voltage of ±550 kV.[19]

A gas-insulated RC divider with a bandwidth ranging from that corresponding to DC voltages to approximately 1 MHz and high-amplitude accuracy can be used for decoupling the transient signals,that is,for online cablefault location and even partial discharge measurements.After the gas works,the measurement systems can be connected to the high-voltage conductor via T-shaped passive modules for offline fault location.

Furthermore,the effort involved in civil works and system erection and commissioning can be reduced by prefabricating the DC GIS for connecting the cable sections in containers and delivering them to the construction site pretested (Fig.12).This will increase the installation speed onsite and reduce the risk of failure.Underground installation is also possible which would prevent lightning from striking the high-voltage conductor.Each container has a standard length of 13 m and height of 3 m and can contain a DC GIS installation with a voltage rating of up to ±550 kV.

Two systems with a voltage rating of ±320 kV are shown in Fig.16.Containers for DC GIS must have dimensions of 13 m × 18 m = 234 m2.They can thus reduce the footprint from 2500 m2 in the case of air-insulated systems(see Fig.11) by approximately 90%.[19]

Fig.12 Example of a cable-cable transition station for two systems of up to ±550 kV with DC GIS

7 Conclusion

DC GIS provide a compact technical solution with high functional density and are optimized for projects with limited space,such as offshore HVDC converter platforms,onshore HVDC converter stations,and the transition stations placed between different transmission media.Compared to technically equivalent air-insulated switchgear,DC GIS require up to 95% less space.Hence,the size of the HVDC offshore converter platform can be reduced by up to 10% while the footprint of the transition stations,such as those for long DC cable lines,can be decreased by up to 90%.

DC GIS can also be readily delivered prefabricated and pretested in modular containers for civil works and commissioning.This also helps in keeping the visual impact of transition stations to a minimum while also reliably preventing unauthorized access.

The development of high-voltage DC GIS with voltage ratings of up to ±550 kV can thus be considered complete.Further,the developed systems have passed the tests prescribed based on the recommendations of CIGRÉ JWG D1/B3.57 and the relevant parts of the IEC 62271 series.

Acknowledgements

This work was supported by the Federal Ministry for Economic Affairs and Energy,Germany(FKZ:03ET7511C) and the Bavarian Ministry of Economic Affairs,Regional Development and Energy (FKZ:IET-1208-0018).