Comparative study of HVAC and HVDC transmission systems
Introduction
Energy is essential for economic growth and modern civilization without which poverty endures. Natural energy resources often occur in remote places far away from big cities. Coal, oil, gas, geothermal energy and hydropower sources usually occur in barren lands, deserts and mountains whereas users live in far off populated cities. Chinese coal and hydropower resources occur in the north and the south, but industry and population clusters occur in east. Solid, liquid, gas and electric energies may be transported from remote places to the big cities and industries by trains, trucks, ships, pipelines and power transmission lines. Comparative energy, transportation studies show that the vehicles consume a significant amount of energy, pipelines cause gas leakages and power lines have resistive losses [1]. Pipelines have gas leak losses and delayed delivery issues. It takes long time to fill the pipeline to reach desired pressure and flow rates. When the distances between energy sources and consumers are in the range of thousands of miles the high voltage transmission lines have been found to be superior energy transport method [2]. Oil, liquid natural gas (LNG), compressed natural gas and hydrogen fuels have different considerations due to long hauled international sea routes. High voltage transmission lines (HVTL) are capable of fast and efficient energy transfer over long distances [3]. HVTL have the ability to integrate widespread renewable energy sources [4]. Electric power transmission systems come as high voltage alternating current (HVAC), extra or ultra high voltage AC (EHVAC or UHVAC), high phase order (HPO) HVAC, 50, 60 and fractional frequency transmission systems, high voltage direct current (HVDC), monopole HVDC, bipole HVDC, tripole HVDC and composite AC/DC transmission lines [5], [6].
National utilities, international manufacturers (ABB, Siemens, GEC etc.), R&D institutes (i.e. EPRI etc) and universities have been carrying out comparative studies of HVAC versus HVDC lines for power transfer capabilities with minimum line losses and environmental impacts [7], [8], voltage and rotor stability of generators during power system faults, line (LCC) and capacitor commutated converter (CCC) reliabilities [9], steady and transient stability [10], [11], state space (SPWM) and state vector pulse width modulation (SVPWM) techniques [12], power quality, reliability, dynamic performance of current source converter (CSC), voltage source converter (VSC) and modular multilevel converter (MMC) VSC based HVDC in various configurations [13], gate firing schemes [14], [15], sequential and simultaneous AC/DC power flow studies [16], HVAC and HVDC links enabling integration of offshore wind farms and remote solar parks as without integrating distributed renewable energy sources the energy transition remains incomplete [17], [18]. Superconducting transmission lines have been proposed subject to public acceptance [19]. Power electronics causes 100 Hz to 5 kHz harmonics which interfere with environment to enhance ozone generation. Phase shifted inverters stabilize output voltage under varying frequency conditions, yet these produce ozone far more than simple PWM inverters [20]. Power transmission losses decrease with an increase in the voltage level, but continued racing with increasing AC/DC voltage levels might create environmental concerns due to induced 1–4 kV voltages in gas pipelines [21], [22] and plumes of ozone generation [23], [24]. If we keep in mind externalities, then coal transport by train might prove cheaper [25]. Ambient ozone concentration is 50 ppb (0.05 ppm) though some industrial processes require 600 ppm concentrations. Moderate high voltages allow high power transfer [26], [27] with low zone generation compared to UHVAC lines. High ozone concentrations cause asthma and lung disease [28].
Utility deregulation, distributed power generation, wind farms, solar parks and smart grid visions are constantly changing the façade of modern power system. War of currents after several decades has transformed into a newfangled war of voltages between fast growing economies. After the advent of DC transformers [29], possibility of DC circuit breakers, power line communication abilities on HVDC links [30] and renewable energy integration flexibilities the utilities are racing on HVDC and UHVAC lines to decrease transmission line losses to benefit distributed energy sources [31], [32]. HVDC systems are being modeled to allow quick renewable energy integration despite low reactive power capacities [33], [34]. HVDC and UHVAC transmission lines are being undertaken to increase power transfer capability and minimize line losses reducing greenhouse gas emissions. Fossil fuel based power plants emit carbon dioxide and HVAC lines generate ozone. Ozone generation rate of HVDC lines is half of HVAC lines [23]. Ozone nearby HVDC and HVAC lines is not any big environmental concern [35], yet still high voltage race requires a techno-economic debate.
The aim and objective of this study is to provide a comprehensive background to research students, utility managers and climate change experts on the current state of the art technology in the field of high voltage transmission lines. To achieve the objective, this paper introduces historical events related to transitions from DC to AC and again back to DC by comparing power transfer capabilities, AC/DC power system configurations, techno-economic constraints, technology and future directions. The scope of this work is limited to description of HVAC, HVDC, UHVAC and composite transmission systems topologies, comparative power transfer capabilities of high voltage transmission lines and high voltage limitations in the context of animal safety and environmental concerns. This work compares the performance of existing high voltage AC/DC power systems and points out future opportunities.
Section snippets
War of currents and voltages
War of currents between Thomas Edison and Nicolas Tesla has changed into a war of voltages today. Technical debates on high voltage AC and DC systems have been reported in literature [36], [37], [38]. Thomas Edison (1847–1931) and Nicolas Tesla (1856–1943) were pioneers of direct (DC) and alternating current (AC) power systems. Edison (self educated at Cooper Union) architected the century [39] by lighting the world and giving a bright future [40]. Tesla (a graduate of Graz University of
High voltage transmission lines
Several researchers have reviewed HVAC and HVDC system׳s potential benefits, design complexities, conversion problems, power transfer constraints, economic constraints in light of climate change challenges the world faces today [47], [48]. Power engineers converted the DC lines into AC lines in early twentieth century and started converting the AC lines back into HVDC or HPO HVAC lines by the end of the century. The earlier decision was based on transformer׳s ability to raise the voltage level,
Techno-economic analysis
Beyond break-even-distance HVDC is more economic than HVAC. Break-even distances for the submarine or underground cables and overhead transmission lines are 50 and 600 km. HVAC cables are common, but HVDC cables are rare. New Zealand, Spain, Switzerland and Canada, have laid down 220, 380, 400 and 500 kV, 8–37 km long, DC cables. UHVDC outweighs UHVAC lines, despite extra expenses on expensive DC circuit breakers and converter electronics, when power is to be delivered over 1000–3000 km distances
HVDC circuit breakers
HVDC lines have the higher power transfer capability as compared to equivalent HVAC lines, but they suffer serious limitations of HVDC circuit breakers. LCC HVDC tripping times are several times higher than HVAC circuit breakers. ABB, Siemens and others are trying to develop HVDC circuit breakers, yet all of them are miles away. As of 2011, ABB and Siemens had invested $200 and €110 millions of R&D activities [111]. HVDC circuit breakers market potential is estimated to be $10 billion in next
Grid, step and touch voltages
Grid potential rise (GPR) occurs when large currents flow through earth grid impedance. The term GPR refers to the voltage difference between local and remote earth. Grid potential rise may be given bywhere
where R1 is the earth resistance of grid conductor, R2 is grid resistance of earthing electrode and Rm is the mutual earth resistance of grid and electrodes.
Step voltage is the potential gradient between the feet of a walking person on soil near a live
Climate change and energy transition
Global greenhouse gas emissions were 53 billion tonnes of CO2 in 2014 out of which 39.50 billion tonnes were CO2 emissions. The average concentration of greenhouse gases is 430 ppm out of which 402 ppm is CO2 concentration which is a level never seen in last 800,000 years. The average temperature rise has been recorded to be 1 °C from 1880 to 2015 which is halfway down the IPCC target of 2 °C by 2100. Energy sector contributes 35% of total GHG emissions, which more than agriculture and
Conclusions and future trends
Utility deregulation, distributed power generation, wind farms, solar parks and smart grid visions are constantly changing the façade of modern power system. War of currents after several decades has transformed into a newfangled war of voltages. China, Brazil and India are constructing UHV and HVDC lines to decrease transmission line losses. Power transfer capability of HVAC lines is limited by reactance but HVDC lines can be loaded up to the conductor thermal limit. A bipolar HVDC line can
Acknowledgments
This research study was carried out by partial funding of Higher Education Commission Pakistan (Project ID No. 299). Authors are thankful to PEPCO for discussing future transmission lines augmentation plans.
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2022, Electric Power Systems ResearchCitation Excerpt :Thus, they are sited in remote areas and require long-distance transmission networks to transmit the generated power to load centres. Under this paradigm-shift, the high-voltage direct current (HVDC) technology became a lucrative solution for long-distance power transmission, since the HVDC technology is more economical and efficient than the ac transmission grid under such circumstances [2,3]. Whereas the large-scale adoption of HVDC networks also raises stability and control challenges, since the dynamic characteristics of the ac power network are significantly influenced by the HVDC network.