Gas-insulated lines for HVAC transmission for long distance bulk-power transmission (HVAC)

Thèse de Bachelor , 2018 , 89 Pages , Note: 76

Ingénierie - Technique énergétique

Various converters, generators, and other high power facilities are situated several kilometres away from metropolitan areas. Thus, numerous renewable energy sources with an interconnection between the generation and transmission technology require an efficient and high capacity system which entails the implementation of gas-insulated lines.

Gas-insulated lines (GIL) are a resourceful High Voltage (HV) transmission technology, which offers an alternative to traditional cables or overhead lines (OHL). Conventional cables are unable to provide adequate long-distance bulk power transmission as the ratings for voltage or useful power at the load aren’t sufficient in proportion to the line length. OHL with a lattice tower is a typical transmission method on shore with submarine Cross-linked Polyethylene (XLPE) cables or GIL utilised underground in challenging areas due to their flexibility and functional characteristics.

A 420/550 kV GIL representation comprising of a multiphase model and constant RLC values presents an adequate analysis of the system. Fundamentally, GIL offers encouraging results given the high-pressured insulation medium, skin effect and returning current. Furthermore, by utilising a common platform for the simulations, a fair comparison with various HV transmission alternatives is accomplished. Additionally, this investigation incorporates an overview of the overvoltage and recovery characteristics of the simulated GIL with an adequate power supply concerning the installation of a synchronous generator and wind turbine.

This research examines the economic factors with a distinctive focus on substantial investments made into offshore wind farms and GIL functioning in High Voltage Alternating Current (HVAC). The cost for this particular technical solution may provide an alternative to High Voltage Direct Current (HVDC) networks with Voltage Source Converters (HVDC-VSC) and submarine XLPE cables. The price per generating hour accumulates over time which reflects the total costs. Ultimately, the cost per unit is related to the demand.

Finally, the literature review includes technical data, background information and a verification process for the simulation results.

Contents

Abstract

List of Abbreviations

1. Introduction
1.1. Renewable energy: Integration with GIL
1.2. Power transmission techniques: Roles and problems
1.2.1. Conventional transmission systems
1.2.2. GIL applications
1.3. Project goals and incentives
1.3.1. Aims and objectives
1.3.2. Project incentives

2. Literature review
2.1. Advantages of GIL
2.1.1. High-reliability standards
2.1.2. Low transmission losses
2.1.3. Low magnetic field ratings: GIL review
2.1.4. No ageing
2.2. Technical data and design of coaxial GIL: Background research
2.2.1. Case study: Second generation GIL at Palexpo exhibition hall
2.2.2. Design of a 420/550 kV GIL

3. Transmission characteristics of GIL
3.1. GIL: PSCAD model and simulations
3.1.1. EMTDCS/PSCAD: GIL simulation model
3.1.2. Simulation results: 0.7 and 0.8 power factor operation
3.1.3. 300km GIL results
3.1.4. Verification of the results
3.1.5. Loading conditions effect on GIL
3.1.6. Leading power factor effect on the GIL: PSCAD limitation
3.1.7. Lagging power factor effect on the GIL School of Electrical and Electronic Engineering

4. Transmission technologies
4.1. Transmission characteristics of OHL
4.1.1. OHL: Design and implementation
4.2. Underground XLPE cables analysis
4.2.1. Comparison of transmission technologies
4.3. Wind turbine analysis and energisation: Overvoltage and transient recovery voltage (TRV) of GIL integrated with OHL
4.3.1. Design and specifications of GIL with a renewable wind source
4.4. HVDC technology: HVDC systems and economic analysis
4.4.1. Alternative voltage control strategy
4.4.2. Financial expenditures and Return on Investment (ROI) of transmission technologies

5. Conclusion
5.1. Review and recommendation
5.1.1. Content review: Key elements of the investigation
5.1.2. Transmission technology proposal
5.1.3. Future work

References

Appendices
Appendix 1 - Progress report
Appendix 2 - Tables concerning the overvoltage and recovery analysis
Appendix 3 - Automated arc welder: Prototype
Appendix 4 - MATLAB load configuration and numerical analysis School of Electrical and Electronic Engineering

Abstract

Various converters, generators, and other high power facilities are situated several kilometres away from metropolitan areas. Thus, numerous renewable energy sources with an interconnection between the generation and transmission technology require an efficient and high capacity system which entails the implementation of gas-insulated lines.

Gas-insulated lines (GIL) are a resourceful High Voltage (HV) transmission technology, which offers an alternative to traditional cables or overhead lines (OHL). Conventional cables are unable to provide adequate long-distance bulk power transmission as the ratings for voltage or useful power at the load aren’t sufficient in proportion to the line length. OHL with a lattice tower is a typical transmission method on shore with submarine Cross-linked Polyethylene (XLPE) cables or GIL utilised underground in challenging areas due to their flexibility and functional characteristics.

A 420/550 kV GIL representation comprising of a multiphase model and constant RLC values presents an adequate analysis of the system. Fundamentally, GIL offers encouraging results given the high-pressured insulation medium, skin effect and returning current. Furthermore, by utilising a common platform for the simulations, a fair comparison with various HV transmission alternatives is accomplished. Additionally, this investigation incorporates an overview of the overvoltage and recovery characteristics of the simulated GIL with an adequate power supply concerning the installation of a synchronous generator and wind turbine.

This research examines the economic factors with a distinctive focus on substantial investments made into offshore wind farms and GIL functioning in High Voltage Alternating Current (HVAC). The cost for this particular technical solution may provide an alternative to High Voltage Direct Current (HVDC) networks with Voltage Source Converters (HVDC-VSC) and submarine XLPE cables. The price per generating hour accumulates over time which reflects the total costs. Ultimately, the cost per unit is related to the demand.

Finally, the literature review includes technical data, background information and a verification process for the simulation results.

School of Electrical and Electronic Engineering

List of Abbreviations

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1. Introduction

1.1. Renewable energy: Integration with GIL

It is well established to date that global warming is having a significant long-term effect on the World. Harmful chemicals released into the atmosphere are a consequence of burning fossil fuels and utilising carbon-intensive sources to supply energy into the National Grid and abroad. The UK government have invested their time and resources into further offshore wind developments as ‘the current installed offshore wind capacity of 5.7 GW is due to increase to 10 GW by 2020, and then to increase by an estimated 10 GW by 2030’ ¹. Equally, the United Kingdom is a prime location along with other states to install offshore wind farms. In response to the issues concerning the integration of renewable energy sources into bulk power networks and systems, many companies across the World are developing creative ideas concerning the generation and transport of power with the intention of reducing greenhouse emissions and acquire useful output power rating.

Offshore wind-generated power is potentially an infinite source of energy. In Great Britain, the average rated capacity of wind turbines in 2016 was 4.8 MW which is over 15 % greater than the previous year’s ratings ². Correspondingly, grid- connected offshore wind farms contributed to the net additional consumption of 1,558 MW within the same year ². The distance to shore for various wind farms ranges just over 200 km with the outmost wind farm projects accounting for the most significant cumulative capacity concerning power generation ².

The European Wind Energy Association (EWEA) have recorded all relevant data concerning the planning and development of grid-connected offshore wind farms for the past 36 years ². The trends and statistics timeline report released by EWEA in February 2016 identified a total of 63.5 GW projects in the planning phase which is a substantial amount regarding the average power consumed across Europe every day ³. A significant portion of land is needed to configure large wind farms to avoid densely populated areas such as large cities in case of possible interferences. The demand for clean, renewable energy is growing following the recommendations of the American electric power organisation and the EWEA to avoid an over- dependence on a few fossil fuels ⁴.

Smart-grid is a reliable power storage system required for the management of new renewable resources concerning power flow control as the generated power fluctuates over time. Additionally, offshore wind farms need an efficient and specific long-range transmission technology as a result of the isolated areas, strategically selected for constant high wind rates. GIL is a potential candidate for this issue as it features all the necessary characteristics such as low transmission losses and longterm reliability. A particular GIL system could potentially expand the use of renewable energy in locations where OHL aren’t feasible. Consequently, the potential for renewable energy could grow concerning time as GIL is operational above ground, vertically, directly buried and in tunnels alongside other GIL.

During 2017, the manufacturer Statoil successfully trialled the world’s first floating wind farms across the north-east coast of Scotland which is revolutionary in the energy and transport industry ⁵. The software working with each wind turbine allows more flexibility and achieves aerodynamic stabilisation by dampening the motion of the upright tower using the blades for various manoeuvres. Many similar cases across the World provide the possibility of a compact solution such as GIL.

1.2. Power transmission techniques: Roles and problems

1.2.1. Conventional transmission systems

An efficient and reliable transmission system is essential for sufficient renewable energy sources. OHL integrated with HVAC networks in rural areas, or parliamentary constituencies comprises of Aluminium Conductor Steel Reinforced (ACSR) cables. Conversely, offshore transmission employs underground cables with XLPE insulation. This underground setup is installed underground in many cases in the 21st century as the environmental and visual influences are minimal.

After a specific transmission distance, reactive compensation is necessary to sustain the useful power. XLPE cable design with optimal Direct Current (DC) ratings is required to optimise the particular power system in focus. HVDC-VSC situated at both ends of the power system ensures the consumers an adequate HVAC supply as the transmission setup regulates the reactive compensation at the load and provides additional active power. Subsequently, the HV conversion platforms restrict the practicality of XLPE cables concerning the potential transmission distance and expenditures.

1.2.2. GIL applications

The first generation GIL application consists of pure sulphur hexafluoride (SF6) for insulation in contrast to the second generation, developed to reduce costs and improve efficiency. The second generation of GIL entails a coaxial, aluminium conductor for electrical transmission and an insulating-gas mixture of 20 % SF6, and 80 % nitrogen (N2) ⁶. The gas insulation is under a substantial amount of pressure, specified in more detail in chapter 2 to maintain a compact and efficient transmission. The setup operates with a large outer aluminium coated enclosure for the gas compounds and a protection layer.

Previously, the insulation of traditional cables comprises of an XLPE sheath or paper with oil to reduce the losses and achieve a higher useful power rating at the load. SF6 is an insulation gas for substations and GIL due to its dielectric strength. The main issue concerning SF6 as an insulator is that it’s very harmful. Its global warming potential (GWP) value is 23,900 times that of carbon dioxide (CO2) over a 100-year time horizon ⁷. Alternative gas insulation compounds are researched to compromise for hindrances concerning the high GWP.

The general public negatively views OHL due to its visual impact. Lattice transmission towers operate with two electric circuits and protection cables for reliability purposes. Several economic factors such as tourism and house prices are affected by these design features. A long-term and feasible alternative is underground GIL.

GIL has relatively low losses in comparison to other transmission procedures. The resistance, inductance, and capacitance per phase are relational to the space between the cross-sectional area of the conductor and the enclosure of the pipe. A relatively small capacitance is also a feature for the GIL, as no reactive compensation is necessary for transmission lengths of 100 km ⁸. The simulations conducted reflect these particular characteristics. The most extensive operational GIL is 12.5 km, constructed by Siemens to deliver HV transmission from a vast hydropower plant in Xiloudu, Southwest China ⁹. The GIL technology installation is enclosed within the dam which entails a vertical design ⁹. Evidently, the generated power flows directly through the GIL for reliability and future development purposes. The hydropower plant deals with real-time peaks in demand for energy, and gas- insulated transmission technology is an exceptional candidate for this studied case.

A 550 kV GIL with a nominal current rating of 4,000 A was preferred to distribute power from a 10,200 MW capacity hydroelectric dam project in Sichuan province, China. This example is one of the highest recorded ratings for GIL. Larger power generation facilities involve Ultra High Voltage (UHV) power with 1,000 kV Alternating Current (AC) and ±800 kV DC systems ¹². The thickness of the GIL conductor and enclosure is increased to supply higher voltage ratings, in contrast to traditional double-circuit alteration for tailored HV networks.

1.3. Project goals and incentives

1.3.1. Aims and objectives

This project aims to evaluate the practicality of GIL by conducting an investigation and performing simulations. The simulation-based report is undertaken, with a particular interest in various compilers or rule-based solvers. In this investigation, the study analyses the fundamentals and technicality of the GIL design. Moreover, a direct comparison with cables functioning underground and overhead transmission technology is appropriate for this research and further studies. The objectives of this project are as follow:

- Simulate a nominal pi ()-section; representation of the GIL with sufficient dimension characteristics for leading and lagging load conditions.  Determine the maximum transmission distance where the transmission technologies are feasible regarding various regulations.
- Determine the optimisation setting of the GIL with adequate analysis and calculations.
- Describe HVDC technologies and the characteristics of the HVDC-VSC design.
- Conduct an economic comparison between HVAC GIL and other technologies.
- Simulate a wind turbine model with a switching or transformation component for the GIL and OHL.

1.3.2. Project incentives

The installation of overhead pylons is a requirement to achieve reliable and safe transmission of power. The National Grid is responsible for this particular necessity in the United Kingdom as they’re committed to delivering secure energy.

Overhead transmission technologies have been a subject of debate due to its capabilities and visual impact in the Isle of Anglesey, North Wales. Over the next few years, the Isle of Anglesey (Ynys Môn) is expected to host a series of significant projects in the energy and transport industry, which are likely to create numerous positive benefits, in addition to adverse impacts ¹⁴. The energy island programme has proposed significant energy developments by 2025 with an updated nuclear power station, 299 MW biomass power infrastructures, tidal and solar power schemes ¹⁵. Additional lattice constructed pylons and a substantial amount of land is required to fulfil the proposal.

Subsequently, campaigners have been opposing particular aspects of the proposed plan for several years with regular protests through various media outlets. Figure 1 illustrates a 400 kV HV transmission line, held by a lattice tower from Wylfa nuclear power plant, crossing the Menai Strait, Anglesey to mainland Wales¹⁶.

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Figure 1: Pylons from Wylfa crossing a stretch of tidal water¹⁶

The majority of the islands assembly members, as well as 30 county councillors, are opposing the current operations and future development plans for an additional line of pylons from Wylfa, all calling for a compromise which entails underground cable transmission technologies ¹⁷. Hence, a significant increase in demand is evident for submarine, XLPE cables carrying power from the island of Anglesey to mainland Wales. A potential, long-term and feasible solution for this subject involves underground GIL. This compact solution is more efficient than conventional cables with technical characteristics providing a different set of advantages and disadvantages. With a reliable conductor and gas insulation, a more appealing pylon design is also a possibility. Ultimately, as Wylfa nuclear power station is sea-bound, underground cables from the north of the island to England is another possibility. Renewable energy sources and efficient transmission alternatives originated from extensive research and appropriate funding. Solar city and space-based solar power are instances of exceptional proposals made in the technological industry. Besides, many creative ideas and recommendations concerning the future of energy generation and transmission involve GIL. The GIL application entails an outer housing and protection layer. The insulation medium surrounding the aluminium conductor is under a substantial amount of pressure to reduce voltage losses and keep full functionality of the technical design. There have been no significant reported issues with GIL, nor its epoxy resin spacers and ground encapsulation to date. Figure 2 shows a primary sample of a GIL ¹⁸. The installation of the GIL entails a per-section instalment which benefits local occupants living on the Isle of Anglesey regarding the occupied land per kilometre.

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Figure 2: GIL, maximum power and minimum impact¹⁸

2. Literature review

2.1. Advantages of GIL

2.1.1. High-reliability standards

There are many advantages concerning the integration of GIL into the National Grid as it’s established as the utmost reliable transmission technology in the World ²⁰. The typical setup comprises of a protection layer, which ensures a successful operation. Ultimately, the system is operational for many years as the outer layer is coated with polyethylene to avoid long-term corrosion. As the technology is fundamentally laid underground based on its design and characteristics; lightning overvoltage is an improbable concern. Equally, automated arch welding systems or additional layering technologies are implemented for reliability commitments concerning health and safety obligations.

2.1.2. Low transmission losses

The GIL features a low resistance per kilometre and low power loss of 51 W per phase meter providing a 420/550 kV design as the definition of the power loss is the voltage multiplied by the current squared. The typical GIL resistances range between 6 mΩ and 8 mΩ /per km, ‘depending on the outer diameter (500 mm or 600 mm) and the wall thickness of the enclosure and conductor pipe (6 mm to 15 mm)’²¹.

2.1.3. Low magnetic field ratings: GIL review

The Electromagnetic Field (EMF) level of exposure has strict regulations due to the possible harm it may cause; hence, it’s imperative to follow the health and safety guidelines diligently. Magnetic field spectrum analyser with a probe set is utilised for electric and magnetic field response tests based on the current rating of the GIL. To ensure full functionality of electronic devices within proximity and ensure the protection of the public, guidelines are set to limit the magnetic field ratings per meter. For a 3000 A GIL, the magnetic field rating is 1 μT within a 10-metre distance which is the requirement in numerous countries or states ²². The EMF is deficient for the GIL design as a result of the operational frequency of 50 Hz or 60 Hz, a 180° phase shift with the coaxial conductor, and the grounding system connected to the outer layer. If the installation of GIL is one metre underground, the magnetic field strength is less significant than 0.5 μT. Additionally, the maximum exposure levels recommendations of the electric and magnetic field by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) are specified in Table 1 ²³.

Table 1: ICNIRP Guidelines and restrictions for electrical current induced fields²³

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Reference level at 50 Hz Electric field 5 kV/m 10 kV/m

Magnetic field 100 μT 500 μT

The GIL is simulated on Quickfield software to visualise the field distribution. Typical voltage ratings are directly relational to the electrical field which corresponds to the ICNIRP regulations. Equation 1 expresses the electric field regarding the voltage in vector form and denotes the force per unit change from the aluminium conductor.

Where: ‘V’ is the voltage, ‘x’, ‘y’ and ‘z’ are the coordinated dynamics regarding the electric field spectrum instigated away from the design.

Equation 1 reflects the characteristics of the descending field about the conductor. Correspondingly, Figure 3 illustrates three 420 kV GIL in a vertical orientation within an arch-shaped tunnel. The insulation medium maintains full compact functionality of the voltage as the outer shell is rated at 0 V to return the current.

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Figure 3: single-circuit GIL, Electric field simulation on Quickfield scaled by 10⁵

The electric field simulation illustrates the exceeded field limit of 10 kV/m recorded within negligible proximity concerning the GIL. Therefore, the GIL is safe for human contact, and one of the significant incentives is for GIL to function underground as a precaution. Figure 4 demonstrates an experiment with multiple transmission technologies, focusing on the EMF ratings at different distances away from each conductor ²⁴. The OHL entails a double-circuit per phase operation with two expected peak readings concerning the experiment.

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Figure 4: OHL, Cable and GIL electric field evaluation experiment²⁴

The research concluded that the EMF range is between 15 to 20 times smaller within the vicinity of the GIL compared to the other transmission technologies²⁴. Consequently, GIL is safely operational at a higher rate of cumulative areas. Finally, the Siemens GIL technology satisfies the utmost stringent magnetic flux regulation in Switzerland which comprises of 1 μT²⁵.

2.1.4. No ageing

The thermal and electric characteristics of the GIL aren’t affected by age; hence consistent load ratings are expected. The aluminium conductor and insulating gas are re-usable as it’s a compact transmission solution. Equally, it’s operational for over 50 years depending on the project conditions²⁶. As the EMF and temperature readings are low for underground cables, there are practically no thermal, mechanical or electrical ageing limitations. Non-technical advantages concerning the GIL include its functionality in high temperatures of 70 °C in a tunnel and 50 °C directly-buried ²⁷. Switching impulse and lightning voltage tests with difficult loading conditions provide very little stress for the GIL design. Consequently, the GIL is a very encouraging long-term transmission solution.

2.2. Technical data and design of coaxial GIL: Background research

2.2.1. Case study: Second generation GIL at Palexpo exhibition hall

The directional adjustment of underground GIL applications ranges between 4° to 90° to function in delicate areas such as under the Palexpo exhibition hall in Geneva ²⁹. The characteristics of a long-distance GIL are nominal concerning the technical data of the second generation GIL under the exhibition hall specified in table 2 ²⁹.

Table 2: Technical data of 2nd generation GIL, Palexpo project²⁹

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GIL was chosen to replace a 220 kV OHL in this particular case as its design ensures low magnetic field readings, equally shown in Figure 3²⁹. Additionally, a new layering processing technology was adopted for the GIL setup to minimise its visual effect, concerning the exhibition hall ²⁹. The current rating comprising of 3150 A with a 420 kVrms source is utilised for the simulations conducted on PSCAD as it’s an adequate thermal current rating for subjective cooling concerning the design. The simulation characteristics of the GIL are specified in chapter 3 with the technical data in table 2 applied to the electrical design. The GIL offers a significant alternative to OHL and XLPE cables.

2.2.2. Design of a 420/550 kV GIL

The construction of GIL entails the use of mechanical CNC machines and software such as Solidworks or CAD software packages. The manufacturing facilities provide tailored dimensions for the voltage ratings. The coaxial structure of GIL ensures quality assurance with precise dimensions and a particle trap to protect the functional transmission system from contamination concerning the gas insulation. Nominal pressure for gas systems is 480 kPa with valves and temperature monitoring devices resulting in 0.5 % leakage per year by volume. Figure 5 illustrates an engineering- based design about the coaxial dimensions of a 420/550 kV GIL including the necessary labels and angles ³⁰.

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Figure 5: HVAC GIL engineering drawing sample design³⁰

Table 3 illustrates the description and geometrical characteristics of the 420/550 kV GIL design in Figure 5³¹. The cross-sectional areas are based on the insulating gas medium, free outer space, the inner-conductor, and outer-conductor. Equally, the radiuses for each conducting phase are specified under length (mm) with the grounded outer-shell denoted by 'R4'.

Table 3: Coaxial geometrical dimensions of the GIL³¹

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Additionally, the design encompasses a 5 mm polyethylene layer for corrosion protection.

The inner conductor is isolated by cast resin insulators and insulating gas towards the outer-enclosure. The outer-enclosure is grounded and has no HV potential with the scale of the design based on the standardised HV impulse testing and dielectric dimensioning for power transmission. Expression 2 applies to the GIL regarding the electric field limit for the insulation medium.

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Where: ‘Ub’ is the voltage stress across the conductor, ‘Ex’ is the breakdown electric field stress at point ‘x,’ ‘R4’ is the outer enclosure radius and ‘R2’ is the phase outer- radius. Considering equation 2; the geometry characteristics of the GIL design entails a unity value of ln (R4/R2) given an optimal gap distance and a suitable field value. The ratio between ‘R4’ and ‘R2’ for an optimal gap distance is 1:2.718, meaning a similar proportion between the two measurements ensures efficiency and reliability. For the 420 kV, geometrical characteristics of the GIL specification in Table 3; the ratio is 1:2.854. Hence, given a lightning impulse withstand voltage of 1550 kV peak value and the design characteristics based on the Compressed Gas Insulated Transmission Bus Systems (CGIT) testing ³¹ ; the maximum electric field strength is founded to be 16.607 kV/mm.

3. Transmission characteristics of GIL

3.1. GIL: PSCAD model and simulations

3.1.1. EMTDCS/PSCAD: GIL simulation model

Many solvers and compilers developed for several disciplines employ rule-based functionality including reversed solving capabilities to simulate researched energy solutions. Electromagnetic Transients including DC (EMTDC)/PSCAD is utilised to simulate an OHL, XLPE cable, and GIL. A simulation of a GIL entails the use of nominal- sections on EMTDC/PSCAD’s educational software to represent three single-cores GIL. Equally, the power factor concerning the various loads ranges between 0.7 and 1.0. The loads for different lengths of the GIL are obtained by implementing numerical analysis techniques with a plotting function. An iterative method comprising of a Dirac delta function or standard deviation is a justified approach concerning the accuracy of the results. Figure 6 illustrates a 200 km GIL model constructed on PSCAD with an ideal (R = 0 Ω) sending-end voltage (VS) of 420∠0° kVrms. Voltmeters to ground, ammeters, and several multimeters provide the per phase voltage, current, active and reactive power readings. It is imperative the zero, and positive sequence characteristics are equal since the design comprises of inner and outer solid conductors with no faults.

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Figure 6: 200 km GIL model in PSCAD, 50 km length for individual PI sections

Measurement components illustrate the rating of the GIL over time by implementing overlay graphs with signal and data labels to observe the plotted voltage, current, active and reactive power. The apparent base power (SB) is set to 1 MVA for the multimeters to ensure an appropriate scale for the plotted data. The load configurations are set accordingly with the variable RL output to achieve the desired power factor and continuous thermal current rating. The thermal current rating is 3150 A with simulations performed for verification purposes comprising of a similar setting at the load.

The initial simulated GIL length is 100 km to observe the minimal reactive compensation. A standard deviation technique on MATLAB is implemented to obtain each load configurations specified in Table 4.

Table 4: RL load connected at the end of the 50 Hz GIL

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The current is lagging behind the voltage over time to avoid irregular current flow readings concerning ‘I1’ and ‘E1’for example specified in Figure 6. Intermittent readings are expected for a unity power factor load configuration as there’s no reactive compensation. Hence, overlapping plotted results is expected for the GIL.

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Figure 7 shows multiple iterations concerning the output results of a 200-kilometer

GIL operating at 0.7 power factor, 50 Hz. The output resistive component is the x- axis, and the y-axis denotes the resulting output current. The desired load current is about the turning point of 4.45 kA. The FORTRAN solver operates continuously as the necessary readings are based on the various transmission lengths.

Figure 7: Standard deviation graph on MATLAB for a 200 km, 0.7 power factor GIL

The thermal current is multiplied with√2 to acquire the nominal load current. The inductive and capacitive reactance per phase characteristics is acquired with the designated expressions: 3 and 4.

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Where: ‘f’ is the operational frequency, ‘L’ is the induction, ‘C’ is the capacitance and ‘/m’ represents per metre.

The relationship between the inner and outer conductor are relative to the characteristics of the GIL design. The voltage limit of the GIL is proportional to the dimensions requirements. Consequently, the AC resistance and capacitance per phase decrease as the GIL dimensions increases. Table 5 illustrates the characteristics of the 420/550 kV GIL simulated on PSCAD which offers a lower inductance per metre than any other design ³².

Table 5: Characteristics of the applied GIL data at 50 Hz³²

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Capacitance per phase: C/km 0.0542 μF/km XC: 58.72876129 [Mohm*m]

CGIT delivers economical GIL solutions in association with AZZ and ABB. The nominal gas insulation for the 420/550 kV system employs SF6, followed by N2 which are distributed from separate compressed gas cylinders to establish the appropriate pressure and gas mixture for the technical transmission system. The weight of the SF6 gas, in this case, is 47.31 kg per phase meter ³³. There’s no fire hazard for the setup as its pressure and protection layer ensures consistency. Conversely, Insulation comprising of oil and paper may induce fire risks and other health safety issues. The electrical transmission of GIL ranges between 100 km and 500 km concerning the simulations conducted. Providing the EMTDC/PSCAD model with an established solver; an accurate analysis of the GIL performance is reviewed which corresponds to its fundamental characteristics.

3.1.2. Simulation results: 0.7 and 0.8 power factor operation

The simulations demonstrate the effectiveness of GIL providing the power factor is lagging. Hence, inductive loads are implemented. Plotted peak voltage and current profiles, specified in (a) and (b), corresponds to the peak active and reactive power characteristics shown in (c) and (d).The data indicates the reactive compensation required for different line lengths with the flow of current regulated at varying rates concerning the distances covered and the operational power factor. The simulation results are shown in Figure 8 based on a 0.7 power factor operation.

(a) Voltage Drop Profiles (b) Peak Current Profiles

(c) Active power Profiles (d) Reactive power Profiles

Figure 8: 0.7 power factor, EMTDC/PSCAD simulation results for GIL plotted in sections (a), (b), (c) and (d) with 50 km per phase readings

The reactive power is minimal for a 100 km GIL which corresponds to the statements by H.Koch in⁸ regarding the compensation for the technical system. The second set of simulations performed utilises a 0.8 power factor load with constant thermal power and current ratings.

The frequency remains at 50 Hz to compare the various load configurations based on the power factor. Figure 9 denotes the simulation results when implementing the 0.8 power factor load settings for different GIL lengths specified in Table 1.

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(a) Voltage Drop Profiles

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(b) Peak Current Profiles

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(c) Active power Profiles

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(d) Reactive power Profiles

Figure 9: 0.8 power factor load, EMTDC/PSCAD simulation results for GIL plotted in sections (a), (b), (c) and (d) with 50 km per phase readings The GIL design is a solidly based conductor with gas insulation, and its characteristics are similar to a solid conductor of a conventional cable. The current accumulates as the distance covered increases with the useful power and voltages declining as a consequence of the resistance per kilometre. The current flow is lagging behind and is inducing the voltages higher to improve the system by employing the constant inductive load. The reactive compensation enhances the efficiency and influences the magnetic field waves within the gas mixture. Besides, limitations of the GIL include its visual impact, the temperature of the enclosure, relative gas-tightness and time constraints for installation. It is imperative the reactive power is supplied to maintain the voltage standards, as it’s a by-product of an HVAC system.

The current is accumulating in proportion to the distances, phase, and reactive compensation. Typically, the useful power or energy dissipates in the form of heat or general losses, with the voltage losses denoted by the phase or responsive compensation sent and received. Subsequently, a lower power factor results in more significant reactive power compensation between the sent and received HV buses. Equally, the voltages situated at the load are far more significant based on a 0.8 power factor inductive load in comparison to a 0.7 power factor operation based on Figure 9, Figure 8, and the power system fundamental characteristics. The current rise for the particular GIL operating at a 0.8 power factor is 0.325 kA for a 100 km GIL and 1.326 kA for a 500 km procedure.

3.1.3. 300km GIL results

Security and Quality of Supply Standards (SQSS) is a standard license condition for the national electricity transmission system and safety ¹³. The voltage limit regulation set by the SQSS license concerning a transmission technology is -10 % and +5 % with several proposals suggesting a voltage limit alteration to -6 % and + 6 % ¹³. The regulation determines the maximum transmission distance of the GIL without external reactive compensation from local generators. A 300 km GIL simulation with the RL output components specified in Table 4 was conducted for a variety of loads comprising of 0.7, 0.8, 0.9, and 1.0 power factor settings. The transmission distance covered accumulates with recorded readings every 50 km. The transmission range for the SQSS voltage limit is an essential requirement for a power system design and analysis. Expression 5 provides the theoretical three-phase voltage conversion, and the error denotes the accuracy of the results in expression 6.

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Where: ‘V’ is the voltage applied to the power system, ‘(RMS)’ is the root mean square, ‘THV’ is the theoretical voltages, ‘SV’ is the simulated voltages.

Consequently, the theoretical voltage supply is 342.93, the SQSS regulation is 308.64 kV concerning the descending voltages, and the error for the conducted simulations is 0.035 %.

If the voltage flowing in an alternating form is too low, the HV systems automatically disconnect with circuit breakers or manually with similar actions for manual tap changers regarding the transformers operational requirements. Figure 10 illustrates the voltage drop profiles of a 300 km GIL and the SQSS regulation of -10 %. The three voltage curves represent the effect of various power factor loads and the maximum operational distance threshold for the GIL.

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Figure 10: Different power factor configured loads analysis of a 300 km GIL

As the simulated load configurations concerning the power factor increase, closer to unity rating; the voltage transmission is more efficient and encouraging. The simulations illustrate a 79.59 km difference between a 0.7 and 0.9 power factor load concerning an SQSS regulation of -10 % set by Ofgem for the voltage thresholds. An apparent thermal power limit ‘S’ is shown in expression 7 justifies the nameplate data for the power systems. The thermal power apparent limit is 2292 MVA with a 420 kVrms source and corresponding 3150 A thermal current. Correspondingly, the reactive power demand increases to compensate for the falling active power for long- distance transmission with the phase and magnitude values analysed in real-time. Besides, figure10 may provide an approximation of other power factor settings at the load of the GIL concerning the SQSS regulation and voltage readings. Power flow equations for active and reactive power received are shown in expression 8 and expression 9 where the current is lagging the voltage. Equally, the apparent thermal potential is denoted by expression 7.

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[...]

• Université: University of Manchester
• Cours: Electrical and electronic engineering
• Note: 76
• Auteur: Llion Stephen (Auteur)
• Année de publication: 2018
• Pages: 89
• N° de catalogue: V437194
• ISBN (ebook): 9783668775046
• ISBN (Livre): 9783668775053
• Langue: anglais