Abstract
Alternating current (AC) had been the viable and typical choice of media for electrical power transmission to homes and businesses for a century. However, high voltage alternating current (HVAC) transmission proved to have its constraints such as its inability to directly connect two AC power networks operating at different frequencies. The limitations of HVAC transmission alongside the need for renewable power integration and bulk power transfer to load centres stimulated the implementation of a reliable and supplementary transmission technology known as point-to-point high-voltage direct-current (HVDC) links. HVDC transmission thrived as a groundbreaking technology as it is appropriate for economical long-distance power transmission with lower electrical losses. In this project, a Voltage source converter (VSC) based HVDC system is modelled, simulated and controlled using PSCAD, a software for simulating power systems in the time domain.
Section 1: Introduction
1.1 Introduction
Renewable energy sources such as wind energy are known to be dependable and unwavering alternatives to the conventional burning of fossil fuels as they are usually cost effective, surplus, widely distributable and emit no greenhouse gases during operation [2]. However, harnessing these resources require the deployment of power transmission technologies which are based on power electronics as they are typically very distant from central areas. To facilitate the grid integration of renewable power sources offshore to main onshore and to transfer bulk power, point to point HVDC links are utilized.
The efficiency and flexibility of HVDC systems make them suitable for electric power transmission over long distances. HVDC systems consist of overhead or underground transmission lines and converter stations capable of converting gigawatts worth of power from HVAC to HVDC or vice versa. HVDC converters are at the core of the converter stations as they carry out the conversion process. HVDC converters are mostly bi-directional; they could either rectify electrical power from an AC source to DC or invert DC power back to AC before transmitting to an AC network or grid.
However, technological advancements of semiconductor devices led to the arrival of another variety of HVDC systems known as the Voltage source converter (VSC) based HVDC system which uses gate turn off thyristors or Insulated gate bipolar transistors (IGBT) as the fundamental unit of construction of its converter stations. They operate at higher than line switching frequencies between 1-2 KHz. The technology is said to extend the economical power range of HVDC transmission to a few tens of megawatts and reaches ratings ±640 kV and 3,000 MW – enough electricity to power several millions of households as it ensures power transmission over 2 KM [7].
1.2 Historical background of HVDC Systems
The world mostly used HVAC for above the ground cross country transmission. However, an extensive amount of research indicated that DC had an edge over AC in certain situations. Active power flow through an AC system depended on the parameters of the network while active power flow through a DC transmission line was controllable. DC connections at high voltages ensured economical bulk power transmission over large distances [3] whereas AC required shunt compensations, had higher losses over longer distances and caused stability problems [4].
Early converter technology was concentrated on switching devices capable of withstanding high voltages and mercury-arc rectification was regarded as the most appropriate for handling large currents [5]. The world’s first fully commercial HVDC transmission project was constructed by ASEA, a Swedish electrical conglomerate was completed in 1955 between Västervik on the mainland and Ygne on the island of Gotland and Sweden. The operations on the system began with a transmission capacity of 20MW and a rated voltage of 100kV with converter stations built on mercury-arc valves and adopted a 12-pulse converter topology. A vital step towards the improvement of HVDC systems was the replacement of the inefficient mercury arc valves with thyristors in the 1970’s. The discovered potential of high-power solid-state thyristors rendered mercury arc valves obsolete. The main reason for this was the unavailability of mercury arc valves and the incessant need for maintenance such as valve overhauls. The Eel River Converter station in Eel River Crossing, New Brunswick, Canada was the first operative thyristor based HVDC station in the world and was commissioned by General Electric (GE) in 1972.
Thyristors restrained the functionality of HVDC systems as they entailed that HVDC systems should dependably contain synchronous machines for commutating voltages. Fortunately, the constant progression of fully controlled electronic devices marked another epoch in the history of HVDC systems. In the 1990s, the Insulated gate bipolar transistor (IGBT) became an alternative to the thyristors as the main building block of the valves. The major distinction between the thyristor and the IGBTs was the turn-off capability of the latter which made it suitable for the construction of self-commutated converters also known as voltage-source converters (VSC) and VSC based HVDC systems. VSC-HVDC systems are quickly maturing as a technology and are currently the typical choice for most of the commissioned, small to medium power scale HVDC projects. The first VSC-HVDC systems was constructed in 1997 at Hellsjön, Sweden [15]. VSC-HVDC systems are expected to be the key technology for power transmission over long distances for the foreseeable future.
Section 2: Literature Survey
2.1 The Classic HVDC System
Classic HVDC transmission employs line commutated current source converter (CSC) which are constructed by utilizing thyristor valves to perform commutation. The fundamental unit of the converter station is the six-pulse bridge topology which comprises of three pairs of thyristors starting and stopping conduction in a synchronized sequence. Classical HVDC configurations usually implement 12-pulse converters to eliminate fifth and seventh harmonics in the system [3]. A 12-pulse converter bridge is the combination of two 6 pulse bridges in parallel or series connection. Transformers of a 12-pulse bridge topology are of star-star-delta three-winding configurations or a hybrid of star-star and star-delta configurations [3]. In the classic HVDC system, DC current is constantly unidirectional and flows through a large inductance [10]. Converters of the classic HVDC system operate as current sources – they inject harmonic currents into AC networks. Classical HVDC systems are heavily reliant on AC voltage for commutation and only function with lagging current. As a result, they depend on synchronous AC grids for power conversion [10]. Operation with lagging current entails that reactive power is also required to operate the system. The required reactive power of the system directly proportional to the power transmitted. Filters are installed at the AC side of the system to filter current harmonics and provide reactive power
Figure 1; HVDC system based on CSC technology [2]
Figure 2:12-pulse converter topology with star-star-delta three winding transformer configuration [3]
2.2 Components of Classical HVDC System
The classic HVDC system consists of: AC and DC filters, compensation equipment, DC reactors, DC cables, Converters and Transformers. Each component of the classical HVDC system performs fundamental operations to enhance overall performance of the system [11].
AC Filters
Switchable filters are capable of diminishing current harmonics and generating reactive power. Tuned series resonance filters with very low impedance enable attenuation of 11th and 13th harmonics. Higher order harmonics with lower current levels are mitigated by damped broadband filters [11].
DC Filters
HVDC converters produce voltage ripples in the frequency band during AC/DC conversion. which result in interference to telephone in close proximity to an HVDC overhead line. DC filters are used to reduce these voltage ripples. Active filters are known to be more flexible than passive filters and also more economical for more complex tasks [6].
Transformers
Transformers are located behind AC harmonic filters [3]. The primary function of transformers is to regulate AC voltage level to a DC level suitable for transmission. The most common design of transformers is the single-three-phase-winding design and three identical transformers are needed for each converter [11]. The windings on one converter side are connected in star and the windings on the other converter are connected in delta.
Converters
HVDC converters are fundamental parts of all HVDC systems as they perform AC/DC conversion. Converters at the receiving end of an AC network are the rectifiers which convert AC power to DC and converters at the sending end of AC networks are inverters which invert DC power back to AC before it its transmitted to the grid. Converters of the classic HVDC system are current source converters with DC current kept constant [6]. 12 pulse converter bridges are connected to AC systems with either star-star winding structure or star-delta winding structure to ensure that 5th and 7th harmonics through the transformers are in opposite phase and consequently reduce distortion In AC systems due to HVDC converters [6].
2.3 Applications of Classic HVDC Systems
Provision of point to point interconnection between asynchronous AC networks.
Limitless distance of transmission for both submarine or underground cables and overhead line.
Energy transmission from distant energy sources: Remote sites have been refined to enable efficient energy generation. HVDC transmission has been a cost-effective approach to transmit electricity to load centers and congested load areas and has been a fitting replacement for inefficient power plant [6].
Stability enhancement of Electric Power Networks: HVDC transmission ensures rapid and accurate control of magnitude and direction to augment stability of the power system.
2.4 VSC based HVDC Systems
VSC-HVDC systems are also known as self-commutating HVDC systems for both their turn-on and turn-off capability. The VSC-HVDC converter valves are built on fully controlled semiconductor devices such as gate turn off (GTO) thyristors and Insulated gate bipolar transistors (IGBT). The simple VSC-HVDC topology is the conventional two-level-three-phase bridge. Usually, many series connected IGBTs are connected in series at each converter station operating as switching devices to deliver high blocking voltage capability and increase DC bus voltage level of the system [12]. IGBTs used with pulse width modulation (PWM) is the typical control method for VSC-HVDC systems. PWM is the switching procedure used to control magnitude and phase of voltages. The controllability of the IGBT has many benefits such as: High dynamic performance; multi-terminal possibility; and flexible control of passive and reactive power [15]. A benefit of Self-commutated systems over Line-commutated HVDC systems include a smaller contribution to short circuit current and no risk for sub-synchronous interaction. Voltage stability has also been significantly improved because of reactive power control. VSC-HVDC systems are commercially known as HVDC light or HVDC plus [11].
Figure 3: Two-level VSC-HVDC Configuration. [15]
2.5 Operation Of VSC-HVDC systems
Figure 4: VSC based HVDC topology [6]
Fig.7 shows the topology of a VSC-HVDC system. The primary aim of the VSC-HVDC is to convert AC power to DC and transmit DC power from the rectifier to the inverter. A VSC-HVDC consists of Converters, phase reactors, transformers, passive high-pass filters and DC cables or transmission lines [6]. The components of the VSC-HVDC system and their functions are stated below. DC cables are usually installed in bipolar pairs to mitigate magnetic fields [11].
DC Capacitors are installed between DC cable and converter. DC Capacitors are implemented for provision of low inductive path for turned off current, reduction of voltage ripples and to prevent abrupt power variations during transient periods. Phase reactors operate as AC filters to attenuate high frequency harmonics of AC currents which are a result of switching operation of voltage source converters and also as a limiter to limit short circuit current. Phase reactors are mainly used for control of active and reactive power flow. Transformers are installed right before AC filters.
Transformers are mainly used to rectify voltage of an AC system to a suitable voltage level for the converter. It is possible to use simple connections for transformers such as two winding instead of conventional three to eight winding for transformers. Normal transformers could be used with tap changers to enhance reactive power. The voltage source converters are constructed by placing series-connected IGBTs in order of either of the two basic configurations: the two level six-pulse bridge and three level twelve–pulse bridge. The two level six-pulse bridge configuration is the most widely used three-phase configuration. Voltages at AC output of each phase are switched between two discrete levels analogous to electric potentials of the positive and negative DC terminals, ± 0.5Vdc. If the upper of the two connected valves is switched on, the AC output terminal is connected to the positive DC terminal resulting in an output voltage of +0.5Vdc.
Contrarily, If the lower valve of a phase is switched on, the AC output terminal is connected to the negative DC terminal resulting in an output voltage of -0.5Vdc. Both valves of each phase should never be switched on simultaneously. If both valves of each phase are switched on, there would be an uncontrolled discharge through the DC capacitor and the converter equipment would be at the risk of destruction.
The two-level six-pulse bridge configuration consists of six valves. Each valve is a combination of an IGBT and an anti-parallel diode. Anti-parallel diodes are connected to each IGBT to establish 4-quadrant operation of the converter [15]. DC smoothing reactors are substituted for DC smoothing capacitors. The DC capacitors improve commutation and act as filters for DC harmonics [15]. To achieve higher power rating of the HVDC system, voltage-source- converter configurations are connected to control devices, water-cooled heatsinks, diodes etc [11].
2.6 Advantages and Applications of VSC-HVDC Systems
The major distinction between the VSC-HVDC systems and the classic HVDC system is the controllability of the former [6]. The unrestrained controllability of the VSC-HVDC systems prompts advantages which lead to more distinct applications. The consensus is that VSC-HVDC systems have very higher technical potential compared to the contemporary LCC-HVDC system and few economic limitations. Listed below are some of the advantages and applications of VSC-HVDC systems.
Improved power quality: Rapid response as a result of increased switching frequency ensures efficient, rapid and independent control of active and reactive power [11]. Furthermore, larger switching frequency reduces power quality disturbances such as voltage dips, flicks and harmonics. Power quality is of crucial importance to grid operators and consumers.
Absence of commutation failures: The classic HVDC system is prone to commutation failures caused by disturbances in AC systems. However, the converters of VSC-HVDC systems employ self-commutating semiconductor devices and the presence of a certain amount of AC voltage is no longer needed [11]. This drastically reduces the possibility for commutation failures in VSC-HVDC systems [6].
Telecommunication is inessential: inverter and rectifier converter stations operate independently of each other and telecommunication is no longer required [6]. This improves the efficiency and rapidity of controllers.
Appropriate for Multi-terminal DC grids: VSC-HVDC systems are very fitting for multi terminal DC grids due to the independent operation of inverter and rectifier stations.
Supplying industrial installations: VSC converters have the capability of producing AC voltages to any preset frequency value in the absence of rotating machines. VSC-HVDC systems are practical for feeding passive AC networks and industrial plants [6].
Section 3: Overview of the Project
3.1 Project Objectives
The primary objectives of the project are to model, simulate and control a VSC-HVDC link. Simulation and schematic design of the VSC based HVDC system is performed on PSCAD, a rapid, accurate and easy to use power system simulation software for analysis, optimization, verification and design of power systems [8]. Milestones of the project include: understanding the theory behind HVDC systems; Learning to perform simulations and schematically construct simple circuits on PSCAD; construction of two 2-level 3- phase converter configurations on PSCAD which are the rectifier and inverter both with identical layouts but exactly opposite functions; understanding the control process of a converter on PSCAD; Simulation of a control scheme consisting of converters, transformers, human machine interfacing (HMI), Proportional integral (PI) controllers and comparators; and simulation of a functional VSC-HVDC power system using PSCAD.
The essence of the control scheme is to aid control of magnitude of alternating voltage, current, active and reactive power transfer of converter configurations with the use of PWM. PI controllers would be used to control values of current in the system. Simulation results are analysed with sinusoidal waveforms displayed on PSCAD. Converters are the most important components of any HVDC system. Therefore, it is imperative that both the inverter and rectifier networks are functioning efficiently. The basic control process is observed and learnt on PSCAD while extensive research is carried out by engaging with requisite literature and meeting with my supervisors for further explanation. All components of the system must be properly cascaded as they determine the reliability of the system. Lastly, case studies on VSC-HVDC topologies would be conducted with the aid of accessible data on HVDC systems and a final report on the entire project would be written.
3.2 Progress
Over the last 11 weeks, key milestones along the initial project timeline have been reached. They include the following:
Understanding background theory behind HVDC systems: Research papers, dissertations, journals, websites, textbooks on power electronics and HVDC systems have been identified and studied. Succeeding modules from previous years at university have been revisited and relevant content relating to the project have been examined, they include the following: Power engineering and electrical materials (EN1085); Machine and power electronics (EN2708); Power system analysis (EN2709). A four-hour guest lecture on HVDC was attended in week 6. These were the measures taken to gain understanding on the theory of HVDC systems.
PSCAD: Rudimentary knowledge on use of the simulation platform, PSCAD to model basic schematic diagrams was acquired between weeks 3 and 6. Supervisors supplemented the learning process with further explanation on the utilization of the software.
Construction of a Voltage-source converter with a controller: The acquisition of basic knowledge on PSCAD to create schematics was essential for the construction and design of a functioning converter for the HVDC system. A controller with NOT gates, comparators is designed and implemented for the control of the phase and magnitude of 3 voltages, Ea, Eb, Ec and the turn-off of each IGBT. Fig 8 and 9 are the Constructed 2-level 3-phase voltage-source converter and the Designed controller for the voltage-source converter on PSCAD.
Figure 5: Constructed 2-level 3-phase voltage-source converter on PSCAD
Figure 6: Designed controller for the voltage-source converter
Simulation Result
Figure 7: Sinusoidal Pulse width modulation for voltages, Ea, Eb & Ec.
The simulation result is obtained by running the program for the converter and controller. In Fig.9, the 2 inputs of all three comparators are a triangular waveform which is the carrier with a frequency of 1000Hz and a sinusoidal waveform. Sinusoidal inputs of the comparators are 120° out of phase with frequencies of 50Hz. Both inputs of a comparator are compared to produce a SPWM signal. A NOT gate is connected to each output of a comparator to ensure that only one valve per pair is switched on at during a duty cycle. Fig.10 depicts the sinusoidal pulse width modulation (SPWM) of 3 voltages. Voltages Ea, Eb and Ec are shown with blue, green and red respectively. The essence of employing SPWM instead of a conventional high amplitude square wave is to diminish the harmonic distortions of the system.
3.3 Conclusion
The project is proceeding fairly compared to its original plan but should perform optimally with its current plan. The limited availability of the CIREGS lab and the lack of familiarity with power electronics concepts are the major constraints on the progress of project. The construction of the converter’s controllers and a simple power system seems challenging but is definitely within the bounds of possibility. The simulation model for a VSC-HVDC link should be available in due time.
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