Design and Control of a Dynamic Voltage Restorer

Research output: ResearchPh.D. thesis

Abstract

This thesis is the main results from a Ph.D. project under the research programme "Local Compensation" at Aalborg University. The main topics in the thesis are the design and control of a Dynamic Voltage Restorer (DVR). The DVR is a series connected device, which primarily can protect sensitive electric consumers against voltage dips and surges in the medium and low voltage distribution grid.

The thesis first gives an introduction to relevant power quality issues for a DVR and power electronic controllers for voltage dip mitigation. Thereafter the operation and the elements in a DVR are described. The advantages and disadvantages are treated by inserting the DVR in either the medium voltage distribution system or in the low voltage distribution system. Different topologies for a DVR are investigated on a converter and system level and the protection issues are treated.

The design of a DVR is treated and two prototype DVRs are designed and specified. The first DVR is a low voltage DVR (LV-DVR) rated for 10 kVA for insertion in a 400 V low voltage grid and the second DVR is a high voltage DVR (HV-DVR) rated for 200 kVA for insertion in a 10 kV medium voltage distribution system.

The control issues are treated and voltage controllers are discussed and designed. The focus is on compensation methods where the phases of the load voltages are unchanged and under normal voltage conditions the DVR is inactive and performs no switchings to reduce losses. If a voltage dip is detected the DVR injects the missing voltage until the energy storage is completely drained or the voltages have returned to normal voltage levels. The control of the HV-DVR is a combined feedforward and feedback control to have a fast response time and load independent voltages. The control is implemented in a rotating dq-reference frame, which gives a very good compensation of the positive sequence component and a damping of the negative sequence to less than 1/4 of the negative sequence component in the supply voltages. Zero sequence components are not detected or compensated with the chosen control method.

System models are build in order to aid the design process and perform simulations on a DVR. The simulations have been compared with measurements in order to be able to test the validity of the models. A good agreement have been found between simulations and measurements, which indicates that the system is sufficiently modelled for the investigations.

The control of the LV-DVR is described and test results with the LV-DVR are  presented. Thereafter the control of HV-DVR is described and the tests of the HVDVR are divided in high voltage/low power tests at Aalborg University and high voltage/low power tests at DEFUs test facilities at Kyndby, Sealand. The stationary tests indicate that the DVR can compensate different kind of loads, however can non-linear load lead to oscillations in the line-filter of the DVR and thereby an increased harmonic distortion of the load voltages. The dynamic tests with the LVand HV-DVR indicate a good compensation of symmetrical and non-symmetrical voltage dips. In most cases the DVR is capable of restoring the load voltages within 2 ms. During the transition phases load voltage oscillations can be generated and during the return of the supply voltages short time over-voltages can be generated by the DVR. Both of the described events can be a potential problem for sensitiv loads.

During the thesis knowledge is gathered about power quality and power electronics for voltage quality improvements. In addition knowledge about the design and control of a DVR either connected at the low voltage or the medium voltage level.

During the Ph.D. an extensive amount of measurements are performed to test the operation of a DVR and the potential of a DVR. The testing at a high voltage/high power level at a realistic test location gave insight in placing series compensation equipment at this level. The level is characterized by having a high short circuit level and the system is inductor grounded. The DVR is installed to protect a medium voltage load consisting of a 10/0.4 kV distribution transformer and different low voltage loads. The voltage dips are generated by performing controlled short circuits in the grid and thereby the equipment is well verified with relative realistic voltage dips.

The conclusion is that the DVR is an effective apparatus to protect sensitive loads from short duration voltage dips. The DVR can be inserted both at the low voltage level and at medium voltage level. The series connection with the existing supply voltages makes it effective at locations where voltage dips are the primary problem. However, the series connection makes the protection equipment more complex as well as the continuous conduction losses and voltage drop.

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Details

This thesis is the main results from a Ph.D. project under the research programme "Local Compensation" at Aalborg University. The main topics in the thesis are the design and control of a Dynamic Voltage Restorer (DVR). The DVR is a series connected device, which primarily can protect sensitive electric consumers against voltage dips and surges in the medium and low voltage distribution grid.

The thesis first gives an introduction to relevant power quality issues for a DVR and power electronic controllers for voltage dip mitigation. Thereafter the operation and the elements in a DVR are described. The advantages and disadvantages are treated by inserting the DVR in either the medium voltage distribution system or in the low voltage distribution system. Different topologies for a DVR are investigated on a converter and system level and the protection issues are treated.

The design of a DVR is treated and two prototype DVRs are designed and specified. The first DVR is a low voltage DVR (LV-DVR) rated for 10 kVA for insertion in a 400 V low voltage grid and the second DVR is a high voltage DVR (HV-DVR) rated for 200 kVA for insertion in a 10 kV medium voltage distribution system.

The control issues are treated and voltage controllers are discussed and designed. The focus is on compensation methods where the phases of the load voltages are unchanged and under normal voltage conditions the DVR is inactive and performs no switchings to reduce losses. If a voltage dip is detected the DVR injects the missing voltage until the energy storage is completely drained or the voltages have returned to normal voltage levels. The control of the HV-DVR is a combined feedforward and feedback control to have a fast response time and load independent voltages. The control is implemented in a rotating dq-reference frame, which gives a very good compensation of the positive sequence component and a damping of the negative sequence to less than 1/4 of the negative sequence component in the supply voltages. Zero sequence components are not detected or compensated with the chosen control method.

System models are build in order to aid the design process and perform simulations on a DVR. The simulations have been compared with measurements in order to be able to test the validity of the models. A good agreement have been found between simulations and measurements, which indicates that the system is sufficiently modelled for the investigations.

The control of the LV-DVR is described and test results with the LV-DVR are  presented. Thereafter the control of HV-DVR is described and the tests of the HVDVR are divided in high voltage/low power tests at Aalborg University and high voltage/low power tests at DEFUs test facilities at Kyndby, Sealand. The stationary tests indicate that the DVR can compensate different kind of loads, however can non-linear load lead to oscillations in the line-filter of the DVR and thereby an increased harmonic distortion of the load voltages. The dynamic tests with the LVand HV-DVR indicate a good compensation of symmetrical and non-symmetrical voltage dips. In most cases the DVR is capable of restoring the load voltages within 2 ms. During the transition phases load voltage oscillations can be generated and during the return of the supply voltages short time over-voltages can be generated by the DVR. Both of the described events can be a potential problem for sensitiv loads.

During the thesis knowledge is gathered about power quality and power electronics for voltage quality improvements. In addition knowledge about the design and control of a DVR either connected at the low voltage or the medium voltage level.

During the Ph.D. an extensive amount of measurements are performed to test the operation of a DVR and the potential of a DVR. The testing at a high voltage/high power level at a realistic test location gave insight in placing series compensation equipment at this level. The level is characterized by having a high short circuit level and the system is inductor grounded. The DVR is installed to protect a medium voltage load consisting of a 10/0.4 kV distribution transformer and different low voltage loads. The voltage dips are generated by performing controlled short circuits in the grid and thereby the equipment is well verified with relative realistic voltage dips.

The conclusion is that the DVR is an effective apparatus to protect sensitive loads from short duration voltage dips. The DVR can be inserted both at the low voltage level and at medium voltage level. The series connection with the existing supply voltages makes it effective at locations where voltage dips are the primary problem. However, the series connection makes the protection equipment more complex as well as the continuous conduction losses and voltage drop.

Original languageEnglish
Place of PublicationAalborg
PublisherInstitut for Energiteknik, Aalborg Universitet
ISBN (Print)87-89179-42-0
StatePublished - 2002
Publication categoryResearch

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