Power Electronics Application: HVDC
- 1. Course: Distribution Generation and Smart Grid Prof. (Dr.) Pravat Kumar Rout Sunita S. Biswal Department of EEE, ITER, Siksha ‘O’Anusandhan (Deemed to be University), Bhubaneswar, Odisha, India Class-12: Power Electronics Application: HVDC
- 2. HVDCTransmission HVDC POWER transmission systems and technologies associated with the flexible ac transmission system (FACTS) continue to advance as they make their way to commercial applications. Both HVDC and FACTS systems underwent research and development for many years, and they were based initially on thyristor technology and more recently on fully controlled semiconductors and voltage-source converter (VSC) topologies. The ever increasing penetration of the power electronics technologies into the power systems is mainly due to the continuous progress of the high-voltage high power fully controlled semiconductors.
- 3. Continue... The fully controlled semiconductor devices available today for high-voltage high-power converters can be based on either thyristor or transistor technology. These devices can be used for a VSC with pulse-width modulation (PWM) operating at frequencies higher than the line frequency. These devices are all self-commuted via a gate pulse.
- 4. WhyVSC? Typically, it is desirable that a VSC application generates PWM waveforms of higher frequency when compared to the thyristor-based systems. However, the operating frequency of these devices is also determined by the switching losses and the design of the heat sink, both of which are related to the power through the component. Switching losses, which are directly linked to high-frequency PWM operation, are one of the most serious and challenging issues that need to be dealt with inVSC-based high-power applications. Other significant disadvantages that occur by operating a VSC at high frequency are the electromagnetic compatibility/electromagnetic interference (EMC/EMI), transformer insulation stresses, and high frequency oscillations, which require additional filters.
- 6. Two distinct technology Type-1: Line-commutated current-source converters (CSCs) that use thyristors (CSC-HVdc): This technology is well established for high power, typically around 1000 MW, with the largest project being the Itaipu system in Brazil at 6300 MW power level. The longest power transmission in the world will transmit 6400 MW power from the Xiangjiaba hydropower plant to Shanghai. The 2071 km line will use 800 kV HVdc and 1000 kV ultrahigh-voltage ac transmission technology.
- 7. • Line commutated current source converters (CSCs) that use thyristors. •This technology is well established for high power around 1000 MW • A matured technology today and they have been used extensively in present power system. •The CSCs have the natural ability to withstand short circuits as the dc inductors can assist the limiting of the currents during faulty operating conditions.
- 8. Continue... Second: Forced-commutated VSCs that use gate turn-off thyristors (GTOs) or in most industrial cases insulated gate bipolar transistors (IGBTs). (VSC-HVdc): It is well-established technology for medium power levels, thus far, with recent projects ranging around 300–400 MW power level.
- 9. • UseVSCs with pulse width modulation (PWM) operating at frequencies higher than the line frequencies •These devices are all self-commutated via a gate pulse • Operating frequency is higher in-comparison to thyristor-based systems •This technology around 300-400 MW (till 2009) • Recently developed •The world’s firstVSC-based PWM-controlled HVdc system using IGBTs was installed in March 1997 (Hellsj¨on project, Sweden, 3 MW, 10 km distance, ±10 kV).
- 10. Continue... • The VSCs are more vulnerable to line faults, and therefore, cables are more attractive forVSC-HVdc applications. • Faults on the dc side of VSC-HVdc systems can also be addressed through the use of dc circuit breakers (CBs) . • In the event of the loss of a VSC in a multi-terminal HVdc, the excess of power can be restricted by the advanced dc voltage controller.
- 11. How does HVDC transmission system work? In generating substation, AC power is generated which can be converted into DC by using a rectifier. In HVDC substation or converter substation rectifiers and inverters are placed at both the ends of a line. The rectifier terminal changes the AC to DC, while the inverter terminal converts DC to AC. The DC is flowing with the overhead lines and at the user end again DC is converted into AC by using inverters, which are placed in converter substation. The power remains the same at the sending and receiving ends of the line. DC is transmitted over long distances because it decreases the losses and improves the efficiency.
- 12. A system having more than two converter stations and one transmission line is called a‘two terminal DC system’ or a‘point-to-point system’. Similarly, if substation has more than two converter stations and interconnecting DC terminal lines, it is called multi-terminal DC substation.
- 13. Core HVDCTechnologies Two basic converter technologies are used in modern HVDC transmission systems.These are: 1. Conventional line-commutated current source converters (CSCs) 2. Self-commutated voltage source converters (VSCs).
- 14. Advantages of VSC as opposed to a line- communicated CSC Avoidance of commutation failures due to disturbances in the AC network. Independent control of the reactive and active power consumed or generated by the converter Possibility to connect the VSC-HVDC system to a weak ac network or even to one where no generation sources is available, naturally, the short circuit level is very low. Faster dynamic response due to higher PWM than the fundamental switching frequency (phase-controlled) operation, which further results in reduced need of filtering, and hence small filter size. No need of transformers to assist the commutation process of the converters fully controlled semiconductors
- 15. Continue... Advanced power flow control capability, which allows a rapid switch of power flow direction by reverse the current direction but not the voltage polarity; Capability of multi-terminal interconnection; Capability to independently control active and reactive flows at each terminal by the converters; Possibility to connect the VSC-HVDC system to a “weak” ac network; Capability of paralleled operation of DC network on regional AC grid; and Reduced construction and commissioning time of a HVDC system.
- 16. Summary of fully controlled high power semiconductors Acronym Type Full Name IGBT Transistor Insulated Gate BipolarTransistor IEGT Transistor Injection Enhanced GateTransistor GTO Thyristor GateTurn-offThyristor IGCT Thyristor Integrated Gate CommutatedThyristor GCT Thyristor Gate CommutatedTurn-offThyristor
- 17. Comparison between AC and DC Transmission Investment Cost: DC transmission requires fewer conductors than AC transmission - 2 conductors per DC circuit whereas three conductors per 3 phase AC circuit. HVDC allows line supporting towers to be smaller and, hence, requires lesser right-of-way. Thus, clearly, HVDC transmission line would cost lesser than an HVAC line. However, the terminal converter stations in HVDC are much more expensive which are not required for HVAC transmission. Over a specific distance, called as break-even distance, HVDC line becomes cheaper than HVAC. The break-even distance for overhead lines is around 600 km and for submarine lines it is around 50 km.
- 18. Continue... Losses: Skin effect s absent in DC. Also, corona losses are significantly lower in the case of DC. An HVDC line has considerably lower losses compared to HVAC over longer distances Controllability: Due to the absence of inductance in DC, an HVDC line offers better voltage regulation. Also, HVDC offers greater controllability compared to HVAC.
- 19. Continue... Asynchronous interconnection: AC power grids are standardized for 50 Hz in some countries and 60 Hz in other. It is impossible to interconnect two power grids working at different frequencies with the help of anAC interconnection.An HVDC link makes this possible. Interference with nearby communication lines: Interference with nearby communication lines is lesser in the case of HVDC overhead line than that for an HVAC line. Short circuit current: In longer distance HVAC transmission, short circuit current level in the receiving system is high.An HVDC system does not contribute to the short circuit current of the interconnectedAC system.
- 20. CSC-HVDC System Configurations Depending upon the function and location of the converter stations, various configurations of HVdc systems can be identified. The ones presented in this section involve CSC-HVdc configurations but similar types of configurations exist forVSCHVdc with or without transformers. Back-to-Back CSC-HVDC System Monopolar CSC-HVDC System Bipolar CSC-HVDC System Multiterminal CSC-HVDC System
- 21. Back-to-Back CSC-HVDC System In this case, the two converter stations are located at the same site and there is no transmission of power with a dc link over a long distance. A block diagram of a back-to-back CSCHVdc system with 12-pulse converters is shown in Fig. The two ac systems interconnected may have the same or different frequency (asynchronous interconnection).
- 22. Monopolar CSC-HVDC System In this configuration, two converters are used that are separated by a single pole line, and a positive or a negative dc voltage is used. Many of the cable transmissions with submarine connections use a monopolar system. The ground is used to return current. Fig. shows a block diagram of a monopolar CSC-HVdc system with 12- pulse converters.
- 24. Continue... This is the most commonly used configuration of a CSCHVdc system in applications where overhead lines are used to transmit power. The bipolar system is two monopolar systems.The advantage of such system is that one pole can continue to transmit power in case the other one is out of service for whatever reason. In other words, each system can operate on its own as an independent system with the earth return. Since one is positive and one is negative, in case that both poles have equal currents, the ground current is zero theoretically, or, in practice, within a difference of 1%. The 12-pulse-based bipolar CSC-HVdc system is depicted in Fig.
- 26. Continue... In this configuration, there are more than two sets of converters. A multi-terminal CSC-HVdc system with 12-pulse converters per pole is shown in Fig. In this case, converters 1 and 3 can operate as rectifiers while converter 2 operates as an inverter. Working in the other order, converter 2 can operate as a rectifier and converters 1 and 3 as inverters. By mechanically switching the connections of a given converter, other combinations can be achieved.
- 27. VSC-HVDC FUNDAMENTAL CONCEPTS A basicVSC-HVdc system comprises of two converter stations built withVSC topologies. The simplestVSC topology is the conventional two-level three-phase bridge shown in Fig.
- 28. Continue... Typically, many series-connected IGBTs are used for each semiconductor shown in Fig. in order to deliver a higher blocking voltage capability for the converter, and therefore in-crease the dc bus voltage level of the HVdc system. It should be noted that an anti-parallel diode is also needed in order to ensure the four-quadrant operation of the converter. The dc bus capacitor provides the required storage of the energy so that the power flow can be controlled and offers filtering for the dc harmonics. TheVSC-HVdc system can also be built with otherVSC topologies.
- 29. Continue... The converter is typically controlled through sinusoidal PWM (SPWM), and the harmonics are directly associated with the switching frequency of each converter leg. Fig. presents the basic waveforms associated with SPWM and the line-to-neutral voltage waveform of the two- level converter. Each phase leg of the converter is connected through a reactor to the ac system. Two-level sinusoidal PWM method: reference (sinusoidal) and carrier (triangular) signals and line-to-neutral voltage waveform.
- 30. VSC-HVDC multilevel topologies Multilevel converters extend the well-known advantages of low- and medium-power PWM converter technology into the high-power applications suitable for high-voltage high-power adjustable-speed drives and large converters for power systems through VSC-based FACTS and HVdc power transmission. There are numerous multilevel solid-state converter topologies reported in the technical literature. However, there are two distinct topologies, namely, the diode-clamped neutral-point-clamped (NPC) converter and the flying capacitor (FC) converter.
- 31. Five-level FCVSC phase leg Five-level PWM line-to-neutral voltage waveform
- 32. Neutral-point-clamped (NPC) phase leg Three-level PWM line-to-neutral voltage waveform
- 33. Perspective Applications Submarine and Underground CableTransmission Long-Distance Bulk-Transmission Asynchronous Interconnection OffshoreTransmission of Renewable Energy Infeed Large UrbanAreas DC Segmented Grid WeakAC Network Connections
- 34. EmergingApplications VSC-HVDC can be effectively used in a number of key areas as follows: small, isolated remote loads power supply to islands infeed to city centers remote small scale generation off-shore generation and deep sea crossings multi-terminal systems
- 35. Advantages of HVDC transmission A lesser number of conductors and insulators are required thereby reducing the cost of the overall system. It requires less phase to phase and ground to ground clearance. Their towers are less costly and cheaper. Lesser corona loss is less as compared to HVAC transmission lines of similar power. Power loss is reduced with DC because fewer numbers of lines are required for power transmission.
- 36. Continue... The HVDC system uses earth return. If any fault occurs in one pole, the other pole with ‘earth returns’ behaves like an independent circuit.This results in a more flexible system. The HVDC has the asynchronous connection between two AC stations connected through an HVDC link; i.e., the transmission of power is independent of sending frequencies to receiving end frequencies. Hence, it interconnects two substations with different frequencies. Due to the absence of frequency in the HVDC line, losses like skin effect and proximity effect does not occur in the system. It does not generate or absorb any reactive power. So, there is no need for reactive power compensation. The very accurate and lossless power flows through DC link.
- 37. Disadvantages of HVDC transmission Converter substations are placed at both the sending and the receiving end of the transmission lines, which result in increasing the cost. Inverter and rectifier terminals generate harmonics which can be reduced by using active filters which are also very expensive. If a fault occurs in the AC substation, it may result in a power failure for the HVDC substation placed near to it Inverter used in converter substations have limited overload capacity. Circuit breakers are used in HVDC for circuit breaking, which is also very expensive. It does not have transformers for changing the voltage levels. Heat loss occurs in converter substation, which has to be reduced by using the active cooling system. HVDC link itself is also very complicated.
- 38. Types of Converters There are basically two configuration types of three phase converters possible for this conversion process. 1. Current source converter (CSC) 2. Voltage source converter (VSC)
- 39. Comparison: on AC side CSC VSC Acts as a constant voltage source Requires a capacitor as its energy storing device Requires large ac filters for harmonic elimination Requires reactive power supply for power factor correction Acts as a constant current source Requires an inductor as its energy storing device Requires a small ac filters for higher harmonic elimination Reactive power supply is not required as converter can operate in any quadrant.
- 40. Comparison: on DC side CSC VSC Acts as a constant current source Requires an inductor as its energy storing device Requires DC filters Provides inherent fault current limiting features Acts as a constant voltage source Requires a capacitor as its energy storing device Energy storage capacitors provide DC filtering capability at no extra cost Problematic for DC line side faults since the charged capacitor will discharge into the fault
- 41. Comparison: Switches CSC VSC Line commutated or forced commutated with a series capacitor. Switching occurs at line frequency i.e only single pulsing per cycle. Lower switching losses. Self commutated Switching occurs at high frequency i.e. Multiple pulsing within one cycle Higher switching losses
- 42. Comparison: Rating range CSC VSC 0-550 MW per converter up to 600 KV 0-1000 MW up to 450KV
- 43. References Flourentzou, N., Agelidis, V. G., & Demetriades, G. D. (2009). VSC-based HVDC power transmission systems: An overview. IEEE Transactions on power electronics, 24(3), 592-602. Jacobson, B., Karlsson, P., Asplund, G., Harnefors, L., & Jonsson, T. (2010, August). VSC-HVDC transmission with cascaded two- level converters. In Cigré session (pp. B4-B110). Jacobson, B., Karlsson, P., Asplund, G., Harnefors, L., & Jonsson, T. (2010, August). VSC-HVDC transmission with cascaded two- level converters. In Cigré session (pp. B4-B110).
- 44. Questions Differentiate between CSC andVSC based converter. What are the major CSC-HVDC based System configurations? What are the advantages and disadvantages of HVDC system? Give the comparison between AC and DC high voltage transmission.