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Active Damping

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Abstract—The use of superconductors for power transmissionhas been studied for decades. The lossless nature of the super-conducting cables makes the system less stable operationallysince the damping normally provided by resistive losses iseliminated. Breaker actions during routine system operationsor in response to faults can trigger high frequency oscillationsbetween the inductances and capacitances in the system. Thesecapacitances are either power factor correction capacitors orparasitic phase-to-phase or phase-to-ground capacitances inthe lines or cables. The resulting transients are referred to aselectromagnetic transients and can often see 100% or greaterover-voltages. In a system using conventional conductors, theseries resistance will damp these oscillations within a numberof 60 Hz cycles. In a superconducting system, these oscillationswill persist, with only the light damping from the frequencydependent resistance of the superconductors, creating a long-lasting distortion on voltage and current. Many of the tradi-tional methods to damp transient oscillations can be used butthey won’t cover every situation effectively. Power electronicconverters could also be used to damp these oscillations.Either a shunt- or a series-connected converter could be used.However, the series-connected converter would need to carrythe full line current at all times, but a shunt-connectedconverter need be used to damp oscillations only when theyoccur. Key issues are rating the converter and reducingenergy losses.I. INTRODUCTIONThe discovery of high temperature superconductivity [1]sparked renewed interest in its application to the powerarea. One such application, low voltage power transmission,could have a significant impact on the layout of power gridsin the future. A superconducting power system would oper-ate at optimum generator voltages, resulting in a singlevoltage level from generation to distribution subsystems.There would no longer be a need to step up to high voltagelevels for long distance transmission because I2R losseswould have been eliminated from the lines. Low voltageoperation eliminates the need for high voltage insulationand large transformers [2,3] A superconducting powersystem could haveeither ac or dc transmission and distribu-tion. Both have their advantages and disadvantages.Implementing a system using direct current transmis-sion eliminates losses resulting from eddy currents in themetal matrix surrounding the superconductor [4]. There willbe significant added costs up front for converters and afurther penalty for converter losses. However, this is offsetby the enhanced ability to control system operation. Such asystem is referred to as a low voltage direct current (LVdc)transmission system. Manuscript Received September 15, 1998.This work was supported by the University of Idaho EngineeringExperiment Station and the UI Power Applications Research Group.The lossless nature of the superconducting cablescreates new problems because eliminating losses alsoeliminates the damping normally provided by resistive ele-ments. Breaker actions during routine system operations orin response to faults can trigger high frequency oscillationsbetween the line and transformer inductances and capaci-tances from power factor correction capacitors or parasiticphase-to-phase or phase-to-ground capacitances in thecables. The resulting transients are referred to as electro-magnetic transients and can often see 100% or greater over-voltages. In a system using conventional conductors, theseries resistance will damp these oscillations within a few60 Hz cycles. In a superconducting system, these oscilla-tions will persist, with only the light damping from the fre-quency dependent resistance of the superconductors, creat-ing a long-lasting distortion on voltage and current. Loadswill also provide some damping to these oscillations butoscillations on the transmission system will persist. Manyof the traditional methods to damp transient oscillations canbe used but they won’t cover every situation effectively.Power electronic converters could also be used to dampthese oscillations. This paper will explore these options forac and dc systems.II. TRANSMISSION SYSTEMSA. AC Power SystemsAC power systems tend to be connected in a meshed net-work, having parallel current paths between most systembuses. Power is usually generated at 12kV to 24kV by syn-chronous generators. The voltage is than stepped up tohigher levels by transformers for long distance transmis-sion, typically in the 132kV to 500kV range. The voltage isstepped down again for distribution to local loads in a dis-tribution system branching out from the transformer sub-station. Distribution voltages often range from 13.2kV to69kV.Early superconducting systems will probably operate atthese same levels. Superconducting cables will probablyinitially replace existing lines to increase the current capa-bility of a given corridor. Eventually, the step up to highertransmission voltage levels may be eliminated.B. DC Transmission OptionsDirect current transmission sees limited use at present.It is mainly used to connect asynchronous systems or forhigh capacity point-to-point transmission over long dis-tances in the form of HVdc transmission systems. Thesepoint-to-point lines are typically 300 or more miles long.Most existing systems are two-terminal point-to-point sys-tems due to control limitations that are partly tied to the useB.K. Johnson and H.L. HessUniversity of Idaho, Moscow ID 83844-1023Active Damping for Electromagnetic Transients in Superconducting Systemsof line-commutated current source converters. However,voltage source inverter (VSI) based HVdc systems are be-coming available, these systems have fewer limitations insystem control. Advantages of dc transmission must be bal-anced against an additional cost for power conversion andthe potential complexity of multiterminal systems. Super-conducting dc systems also have the same advantage assuperconducting ac systems in that neither needs to havethe current stepped down to low levels (and the voltageconsequently stepped up to high levels) to reduce the I2Rlosses. However, converter performance may be better withhigher voltage and lower current levels [4].Initial application of superconducting low voltage dc(LVdc) systems will probably begin with point-to-point dcsystems, such as a high-capacity link into a transmission-limited urban area. The low voltage level allows for


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