WO2019009706A1 - Power grid and flexible current transmission system forming part thereof - Google Patents

Power grid and flexible current transmission system forming part thereof Download PDF

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Publication number
WO2019009706A1
WO2019009706A1 PCT/NL2018/050425 NL2018050425W WO2019009706A1 WO 2019009706 A1 WO2019009706 A1 WO 2019009706A1 NL 2018050425 W NL2018050425 W NL 2018050425W WO 2019009706 A1 WO2019009706 A1 WO 2019009706A1
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WO
WIPO (PCT)
Prior art keywords
power
variable frequency
frequency converter
power grid
powerline
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Application number
PCT/NL2018/050425
Other languages
French (fr)
Inventor
Jan Abraham Ferreira
Original Assignee
Technische Universiteit Delft
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Publication of WO2019009706A1 publication Critical patent/WO2019009706A1/en

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Classifications

    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/01—Arrangements for reducing harmonics or ripples
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/18—Arrangements for adjusting, eliminating or compensating reactive power in networks
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/30—Reactive power compensation
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/40—Arrangements for reducing harmonics

Definitions

  • the invention relates to a power grid comprising at least one powerline suspended with intermediate isolators from a high-voltage support structure and connected through apparatus connecting cables with a flexible current transmission system to provide or to retain power from the grid, so as to condition the power transmitted through the power grid, wherein said power flows through the grid at a fundamental frequen ⁇ cy of for instance 0, 50 or 60 Hz.
  • the power and current may therefore be either AC or DC.
  • the invention also relates to such a flexible current transmission s ystem .
  • FACTS Flexible alternating current transmission systems
  • STATCOM static synchronous compensator
  • active filters which can act as either a source or sink of reactive AC power to an electricity network[l] [2]. If connected to a source it can also provide active AC power.
  • STATCOMs are highly complex devices and belong to the most expensive components in a power grid system because of their high power rating. Even though STATCOMs are modular they cannot be moved around in a power system to deal with changes in instabilities associated with the evolution of the network [5] [6] . Grid owners are reluctant to use traditional STATCOMs in AC networks or to couple DC/DC converters in DC networks because they are dimensioned to match to the system rating making them expensive and bulky. These "fat" converters are conforming to the old school of thought of power system de ⁇ sign; building systems from large components that have predefined functionality.
  • a power grid and a flexible current transmission system are proposed in accordance with one or more of the appended claims.
  • the invention applies both to alternating current systems and to direct current systems .
  • the flexible current transmission system is a variable frequency actuator system that is arranged to provide or to retain a variable frequency power signal to the power grid, wherein said variable frequency actuator system operates at a frequency and phase orthogonal to the fundamental frequency of the power grid. Accordingly the variable frequency actuator system is capable to decouple and eliminate disturbing non- fundamental frequencies from the fundamental grid-frequency.
  • the variable frequency actuator system using non- fundamental frequencies of the grid frequency of 0, 50 or 60 Hz, i.e. sub-, super- and inter-harmonics, it is possible with relatively low power equipment to condition disturbances in the power flow in the power grid.
  • variable frequency actuator system comprises a floating variable frequency converter with a restricted energy storage which is connected to a medium frequency converter that drives the floating variable frequency converter to condition the power that the floating variable frequency converter provides or retains from the power grid.
  • the medium frequency converter is connectable or connected to an external energy storage, power source or sink so as to enable it to provide power or to retain power from the power grid.
  • the floating variable frequency converter comprises both a positive polarity variable frequency converter module and a negative polarity varia- ble frequency converter module which are placed in series with each other, and wherein each of said modules connects to a powerline dedicated to the concerning variable frequency converter module, and wherein the medium frequency converter is electrically connected to the said positive and negative po- larity variable frequency converters in counter phase.
  • the power associated with the medium frequency converter does not directly penetrate into the power grid because the opposing polarity voltages provided to the positive polarity variable frequency converter module and to the negative polarity varia- ble frequency converter module sum to zero. Accordingly this is a relatively simple and effective tool to condition the power that is transported through the power grid.
  • variable frequency actuator system connects a first powerline to a second power- line; further the first powerline connects to a multiple frequency and phase current sensor which in turn connects to a current source controller that drives the variable frequency actuator system so as to condition power on the first power- line and provide clean power to the second powerline.
  • a current source controller drives the variable frequency actuator system using a small signal network analyz ⁇ er for analysis of the power equivalent circuit network and employing a frequency selective damping organ and phase com ⁇ pensation organ in order to condition said power from the first powerline before it transmits as cleaned up power to the second powerline.
  • the mentioned network analyser is an instrument that provides information to construct a small signal circuit model. Because it usually has a very limited power capability it can only measure small signal impedances.
  • -figure 1 shows a section of a power grid with a flexible current transmission system according to the invention
  • -figure 2 shows a control and conversion block diagram of a variable frequency actuator system employed in the invention
  • -figure 4 shows a circuit embodiment in which a ca- pacitive voltage divider network provides a common mode voltage separation between parts of the variable frequency actuator system of the invention.
  • FIG. 5 shows a circuit embodiment in which a transformer galvanic isolation provides the common mode voltage separation between parts of the variable frequency actuator system of the invention.
  • figure 1 shows part of a power grid which is known per se, comprising at least one powerline 3 suspended with several isolators 2 from a high- voltage support structure 1 and connected through apparatus connecting cables 5 with a flexible alternating or direct cur- rent transmission system 6 to provide or to retain power from the grid so as to condition the power transmitted through the power grid.
  • the flexible current transmission system is a variable frequency actuator system 6 that is arranged to provide or to retain a variable frequency power signal to the power grid.
  • the variable frequency actuator system 6 comprises a floating variable frequency converter 7 with an energy storage which can have restricted capacity, and which is connected to a medium frequency converter 10 to condition the power that the floating variable frequency converter 7 provides or re ⁇ tains from the power grid.
  • the energy storage can be restricted because it needs only be dimensioned to absorb the energy fluctuations generated by the pulsating power resulting from power conversion between the said medium frequency and the variable frequencies in the power grid.
  • the medium frequency converter 10 is connectable or connected to an external energy storage, power source or sink generally denoted with reference 9 so as to enable the medium frequency converter 10 to provide power or to retain power from the power grid by appropriately driving the floating variable frequency converter 7.
  • Figure 3 shows that the floating variable frequency converter 7 comprises both a positive polarity variable frequency converter module 11 and a negative polarity variable frequency converter module 12 which are placed in series with each other, and wherein each of said modules 11, 12 connects to a powerline 3 which is dedicated to the concerning module 11, 12, and wherein the medium frequency converter 10 is electrically connected to the said positive and negative polarity variable frequency converters 11, 12 in counter phase.
  • the result of this is shown in the top of the figure, wherein the powerline connected to the positive polarity variable frequency converter module 11 is spoiled with a third harmonic voltage distortion of 10%.
  • the medium frequency converter 10 drives the positive polarity variable frequency converter module 11 in counter phase with the negative polarity variable frequency converter module 12 with a signal that is shown in the top of the figure in the middle. This driving signal is a ⁇
  • This medium frequency would typically vary between 200Hz and 2kHz, depending on the required bandwidth of the variable fre- quency actuator system.
  • the switching frequency of the semiconductor devices, and the inductor and capacitor values of modules 11, 12 are dimensioned accordingly.
  • the powerline connected to the negative polarity variable frequency converter model 12 exhibits a cleaned up power, sourced by a 1kHz medium frequency converter, in which the original voltage distortion has been reduced with 80% in this example.
  • Figure 3 shows the presence of a 7th harmonic voltage distortion on the first powerline 3' which is substantially mitigated on the second powerline 3' ' by means of the 1kHz voltage that is present on a connection 3' ' ' between the positive and negative polarity variable frequency converters 11, 12. Therefore the invention proposes a three port power apparatus comprising these positive and negative polarity variable frequency converters 11, 12 as well as the medium frequency converter 10, wherein two ports of converters 11, 12 are connected to the power grid system, and the third port between said positive and negative polarity variable frequency converters 11, 12, is connected to the medium frequency converter 10, that provides an auxiliary power source and absorbs or transmits power at a different than the fundamental frequency of the power grid.
  • both the medium frequency and the variable frequencies exclude the fundamental power frequency of the power grid system, so that, based on the known principle that power flows at different frequencies are orthogonal (decoupled) , the invention makes possible to mitigate such orthogonal frequencies with a compact device that is not affected by the power flow at the fundamental frequency in the main power grid system.
  • Figure 3 further schematically shows that there is a high voltage isolation separation 8a between the positive and negative polarity variable frequency converters 11, 12 on the one part, and the medium frequency converter 10 on the other part .
  • Both figure 4 and figure 5 show the circuitry of figure 3 in more detail and show two possible embodiments wherein the high voltage isolation separation is applied.
  • the voltage separation requirements could vary from a few kilovolts to hundreds of kilovolts, depending on the specific application.
  • the high voltage and associated high electric field is either handled by the dielectrics of capacitors and/or by the insulation between a primary and secondary winding of a medium frequency transformer.
  • Figure 4 shows that the medium frequency converter 10 connects to the positive and negative polarity variable frequency converters 11, 12 through capacitors 13, 14, 15.
  • Figure 5 shows another embodiment in which the medium frequency converter 10 connects to the positive and negative polarity vari- able frequency converters 11, 12 through a transformer 16.
  • the medium frequency converter 10 thus modulates voltage waveforms composing anti-phase medium frequency voltage waveforms that are superimposed on in-phase variable frequency voltage waveforms .
  • Figure 4 and figure 5 show further that the medium frequency converter 10 drives the positive and negative polarity variable frequency converters 11, 12 in counter phase, which means that in principle driving the positive and negative polarity variable frequency converters 11, 12 does not introduce any serial voltage to the power line resulting in effective decoupling with the 0, 50 or 60 Hz power flow. Accordingly the invention provides that a minimal amount of power is needed to mitigate an instability in the power grid or to steer the power flow in the right direction associated with non-DC resp. non-50Hz-current and voltages -or non-60 Hz in case of a 60 Hz AC power system.
  • the non-fundamental component of power that causes the instabilities and resonances in the power grid is very small.
  • variable frequency actuator only needs to be rated for a small fraction of the pow- er handled by the grid, but because of the complexity of the interactions of the various frequencies, a sophisticated control system as discussed hereinafter with reference to figure 2 is preferably applied.
  • this provides a "more brains” and “less brawn” approach.
  • the invention can thus be applied to compensate and mitigate the non-fundamental power in the grid, and is also applicable in DC systems.
  • variable frequency actuator system 6 connects a first powerline 3 ' to a second powerline 3'', wherein the first powerline 3' is provided with a multiple frequency and phase current sensor 21 which connects to a current source controller 17 that drives the variable frequency actuator system 6 so as to condition power on the first powerline 3' and provide it as cleaned up power to the second powerline 3' ' .
  • the current source controller 17 preferably drives the variable frequency actuator system 6 using a small signal network analyzer 18 for analysis of the power on the first powerline 3 r and employing a frequency selective damping organ 19 and phase compensation organ 20 to condition said power on the first powerline 3' and provide it to the second powerline 3'' as cleaned up power.
  • PEDG Distributed Generation

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  • Power Engineering (AREA)
  • Inverter Devices (AREA)

Abstract

Power grid comprising at least one powerline (3, 3', 3' ' ) suspended with intermediate isolators (2) from a high-voltage support structure (1) and connected through apparatus connecting cables (5) with a flexible current transmission system to provide or to retain power from the grid, so as to condition the power transmitted through the power grid, wherein said power flows through the grid at a fundamental frequency of for instance 0, 50 or 60 Hz, and wherein the flexible current transmission system is a variable frequency actuator system (6) that is arranged to provide or to retain a variable frequency power signal to the power grid, wherein said variable frequency actuator system (6) operates at a frequency and phase orthogonal to the fundamental frequency of the power grid. The variable frequency actuator system (6) comprises a floating variable frequency converter (7) having both a positive polarity variable frequency converter module (11) and a negative polarity variable frequency converter module (12) which are placed in series with each other, and wherein each of said modules (11, 12) connects to a powerline (3', 3' ' ) dedicated to the concerning variable frequency converter module, and wherein the medium frequency converter (10) is electrically connected to the said positive and negative polarity variable frequency converters (11, 12) in counter phase.

Description

Power grid and flexible current transmission system forming part thereof
The invention relates to a power grid comprising at least one powerline suspended with intermediate isolators from a high-voltage support structure and connected through apparatus connecting cables with a flexible current transmission system to provide or to retain power from the grid, so as to condition the power transmitted through the power grid, wherein said power flows through the grid at a fundamental frequen¬ cy of for instance 0, 50 or 60 Hz. The power and current may therefore be either AC or DC.
The invention also relates to such a flexible current transmission s ystem .
Flexible alternating current transmission systems (FACTS) are known and based on power electronics equipment and have become an established technology to enhance controllability and increase power transfer capability of AC power grids. A member of the FACTS family of devices is the so-called static synchronous compensator (STATCOM) and active filters which can act as either a source or sink of reactive AC power to an electricity network[l] [2]. If connected to a source it can also provide active AC power.
Increasing dynamics in power systems connected to the known power grid is putting the grid under unprecedented stress levels. Currently utilities worldwide are experiencing problems integrating renewable energy sources into the grid. Various resonance phenomena [3] [4] occur between natural frequencies of the system and non-50Hz frequencies generated by wind power systems and PV systems. The general approach followed is to add active filters in the form of STATCOMs to the system, or to modify the controllers of the power electronics interfaces, for example those of the wind driven generators.
STATCOMs are highly complex devices and belong to the most expensive components in a power grid system because of their high power rating. Even though STATCOMs are modular they cannot be moved around in a power system to deal with changes in instabilities associated with the evolution of the network [5] [6] . Grid owners are reluctant to use traditional STATCOMs in AC networks or to couple DC/DC converters in DC networks because they are dimensioned to match to the system rating making them expensive and bulky. These "fat" converters are conforming to the old school of thought of power system de¬ sign; building systems from large components that have predefined functionality.
It is an object of the invention to break new ground and to provide more flexibility in the design of power grid systems.
It is also an object of the invention to provide more flexibility in conditioning the power that is provided and transported through the power grid system.
It is still a further object of the invention to pro- vide less bulky and less costly equipment to condition the power that is transported through the power system grid.
According to the invention a power grid and a flexible current transmission system are proposed in accordance with one or more of the appended claims. The invention applies both to alternating current systems and to direct current systems .
In a first aspect of the invention it is proposed that the flexible current transmission system is a variable frequency actuator system that is arranged to provide or to retain a variable frequency power signal to the power grid, wherein said variable frequency actuator system operates at a frequency and phase orthogonal to the fundamental frequency of the power grid. Accordingly the variable frequency actuator system is capable to decouple and eliminate disturbing non- fundamental frequencies from the fundamental grid-frequency. By the variable frequency actuator system using non- fundamental frequencies of the grid frequency of 0, 50 or 60 Hz, i.e. sub-, super- and inter-harmonics, it is possible with relatively low power equipment to condition disturbances in the power flow in the power grid.
Advantageously the variable frequency actuator system comprises a floating variable frequency converter with a restricted energy storage which is connected to a medium frequency converter that drives the floating variable frequency converter to condition the power that the floating variable frequency converter provides or retains from the power grid.
It is preferred that the medium frequency converter is connectable or connected to an external energy storage, power source or sink so as to enable it to provide power or to retain power from the power grid.
It is further preferred that the floating variable frequency converter comprises both a positive polarity variable frequency converter module and a negative polarity varia- ble frequency converter module which are placed in series with each other, and wherein each of said modules connects to a powerline dedicated to the concerning variable frequency converter module, and wherein the medium frequency converter is electrically connected to the said positive and negative po- larity variable frequency converters in counter phase. The power associated with the medium frequency converter does not directly penetrate into the power grid because the opposing polarity voltages provided to the positive polarity variable frequency converter module and to the negative polarity varia- ble frequency converter module sum to zero. Accordingly this is a relatively simple and effective tool to condition the power that is transported through the power grid.
Suitably and for functional reasons there is a common mode separation between the positive and negative polarity variable frequency converters on the one part, and the medium frequency converter on the other part. An effective separation is necessary because the voltage amplitude of the medium voltage converter is typically ten to hundred time smaller than the power system voltage. This is best achieved by arranging that the medium frequency converter connects to the positive and negative polarity variable frequency converters through one of a capacitor or capacitors, and a transformer, or both.
In a preferred embodiment the variable frequency actuator system connects a first powerline to a second power- line; further the first powerline connects to a multiple frequency and phase current sensor which in turn connects to a current source controller that drives the variable frequency actuator system so as to condition power on the first power- line and provide clean power to the second powerline. One thing and another can be suitably realized by ar¬ ranging that the current source controller drives the variable frequency actuator system using a small signal network analyz¬ er for analysis of the power equivalent circuit network and employing a frequency selective damping organ and phase com¬ pensation organ in order to condition said power from the first powerline before it transmits as cleaned up power to the second powerline. The mentioned network analyser is an instrument that provides information to construct a small signal circuit model. Because it usually has a very limited power capability it can only measure small signal impedances.
The invention will hereinafter be further elucidated with reference to the drawing of an exemplary embodiment of an apparatus according to the invention that is not limiting as to the appended claims.
In the drawing:
-figure 1 shows a section of a power grid with a flexible current transmission system according to the invention;
-figure 2 shows a control and conversion block diagram of a variable frequency actuator system employed in the invention;
-figure 3 shows a functional diagram of the variable frequency actuator system according to the invention;
-figure 4 shows a circuit embodiment in which a ca- pacitive voltage divider network provides a common mode voltage separation between parts of the variable frequency actuator system of the invention; and
- figure 5 shows a circuit embodiment in which a transformer galvanic isolation provides the common mode voltage separation between parts of the variable frequency actuator system of the invention.
Whenever in the figures the same reference numerals are applied, these numerals refer to the same parts.
Making reference first to figure 1 it shows part of a power grid which is known per se, comprising at least one powerline 3 suspended with several isolators 2 from a high- voltage support structure 1 and connected through apparatus connecting cables 5 with a flexible alternating or direct cur- rent transmission system 6 to provide or to retain power from the grid so as to condition the power transmitted through the power grid.
The flexible current transmission system is a variable frequency actuator system 6 that is arranged to provide or to retain a variable frequency power signal to the power grid.
The variable frequency actuator system 6 comprises a floating variable frequency converter 7 with an energy storage which can have restricted capacity, and which is connected to a medium frequency converter 10 to condition the power that the floating variable frequency converter 7 provides or re¬ tains from the power grid. The energy storage can be restricted because it needs only be dimensioned to absorb the energy fluctuations generated by the pulsating power resulting from power conversion between the said medium frequency and the variable frequencies in the power grid.
The medium frequency converter 10 is connectable or connected to an external energy storage, power source or sink generally denoted with reference 9 so as to enable the medium frequency converter 10 to provide power or to retain power from the power grid by appropriately driving the floating variable frequency converter 7.
Figure 3 shows that the floating variable frequency converter 7 comprises both a positive polarity variable frequency converter module 11 and a negative polarity variable frequency converter module 12 which are placed in series with each other, and wherein each of said modules 11, 12 connects to a powerline 3 which is dedicated to the concerning module 11, 12, and wherein the medium frequency converter 10 is electrically connected to the said positive and negative polarity variable frequency converters 11, 12 in counter phase. The result of this is shown in the top of the figure, wherein the powerline connected to the positive polarity variable frequency converter module 11 is spoiled with a third harmonic voltage distortion of 10%. The medium frequency converter 10 drives the positive polarity variable frequency converter module 11 in counter phase with the negative polarity variable frequency converter module 12 with a signal that is shown in the top of the figure in the middle. This driving signal is a β
medium frequency modulation of the variable frequency convert¬ er modules 11, 12, albeit in counter phase with each other. This medium frequency would typically vary between 200Hz and 2kHz, depending on the required bandwidth of the variable fre- quency actuator system. The switching frequency of the semiconductor devices, and the inductor and capacitor values of modules 11, 12 are dimensioned accordingly. As a result the powerline connected to the negative polarity variable frequency converter model 12 exhibits a cleaned up power, sourced by a 1kHz medium frequency converter, in which the original voltage distortion has been reduced with 80% in this example.
Figure 3 shows the presence of a 7th harmonic voltage distortion on the first powerline 3' which is substantially mitigated on the second powerline 3' ' by means of the 1kHz voltage that is present on a connection 3' ' ' between the positive and negative polarity variable frequency converters 11, 12. Therefore the invention proposes a three port power apparatus comprising these positive and negative polarity variable frequency converters 11, 12 as well as the medium frequency converter 10, wherein two ports of converters 11, 12 are connected to the power grid system, and the third port between said positive and negative polarity variable frequency converters 11, 12, is connected to the medium frequency converter 10, that provides an auxiliary power source and absorbs or transmits power at a different than the fundamental frequency of the power grid. According to this invention both the medium frequency and the variable frequencies exclude the fundamental power frequency of the power grid system, so that, based on the known principle that power flows at different frequencies are orthogonal (decoupled) , the invention makes possible to mitigate such orthogonal frequencies with a compact device that is not affected by the power flow at the fundamental frequency in the main power grid system.
Figure 3 further schematically shows that there is a high voltage isolation separation 8a between the positive and negative polarity variable frequency converters 11, 12 on the one part, and the medium frequency converter 10 on the other part . Both figure 4 and figure 5 show the circuitry of figure 3 in more detail and show two possible embodiments wherein the high voltage isolation separation is applied. The voltage separation requirements could vary from a few kilovolts to hundreds of kilovolts, depending on the specific application. The high voltage and associated high electric field is either handled by the dielectrics of capacitors and/or by the insulation between a primary and secondary winding of a medium frequency transformer.
Figure 4 shows that the medium frequency converter 10 connects to the positive and negative polarity variable frequency converters 11, 12 through capacitors 13, 14, 15. Figure 5 shows another embodiment in which the medium frequency converter 10 connects to the positive and negative polarity vari- able frequency converters 11, 12 through a transformer 16. The medium frequency converter 10 thus modulates voltage waveforms composing anti-phase medium frequency voltage waveforms that are superimposed on in-phase variable frequency voltage waveforms .
Figure 4 and figure 5 show further that the medium frequency converter 10 drives the positive and negative polarity variable frequency converters 11, 12 in counter phase, which means that in principle driving the positive and negative polarity variable frequency converters 11, 12 does not introduce any serial voltage to the power line resulting in effective decoupling with the 0, 50 or 60 Hz power flow. Accordingly the invention provides that a minimal amount of power is needed to mitigate an instability in the power grid or to steer the power flow in the right direction associated with non-DC resp. non-50Hz-current and voltages -or non-60 Hz in case of a 60 Hz AC power system. The non-fundamental component of power that causes the instabilities and resonances in the power grid is very small. Therefore the variable frequency actuator only needs to be rated for a small fraction of the pow- er handled by the grid, but because of the complexity of the interactions of the various frequencies, a sophisticated control system as discussed hereinafter with reference to figure 2 is preferably applied. Hence, as opposed to the prior art this provides a "more brains" and "less brawn" approach. The invention can thus be applied to compensate and mitigate the non-fundamental power in the grid, and is also applicable in DC systems.
Turning now to figure 2 it is shown that the variable frequency actuator system 6 connects a first powerline 3 ' to a second powerline 3'', wherein the first powerline 3' is provided with a multiple frequency and phase current sensor 21 which connects to a current source controller 17 that drives the variable frequency actuator system 6 so as to condition power on the first powerline 3' and provide it as cleaned up power to the second powerline 3' ' .
The current source controller 17 preferably drives the variable frequency actuator system 6 using a small signal network analyzer 18 for analysis of the power on the first powerline 3r and employing a frequency selective damping organ 19 and phase compensation organ 20 to condition said power on the first powerline 3' and provide it to the second powerline 3'' as cleaned up power.
Although the invention has been discussed in the foregoing with reference to exemplary embodiments of the power grid and flexible current transmission system of the invention, the invention is not restricted to these particular embodiments which can be varied in many ways without departing from the invention. The discussed exemplary embodiments shall therefore not be used to construe the appended claims strictly in accordance therewith. On the contrary the embodiments are merely intended to explain the wording of the appended claims without intent to limit the claims to these exemplary embodiments. The scope of protection of the invention shall there- fore be construed in accordance with the appended claims only, wherein a possible ambiguity in the wording of the claims shall be resolved using these exemplary embodiments.
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[17] Wiik, J. A.; Wijaya, F.D.; Shimada, R. ; Characteristics of the Magnetic Energy Recovery Switch (MERS) as a Series FACTS Controller, IEEE Transactions on Power Delivery, Year: 2009, Volume: 24, Issue: 2, p : 828 - 836.
[18] Ferreira, J. A.; Dorland, P.; De Beer, F.G; An Active Inline Notch Filter for Reducing Acoustic Noise in Drives, IEEE Transactions on Industry Applications, 2007, Volume: 43, Issue: 3, pp 798 - 804
[19] Sun, J.; Impedance-Based Stability Criterion for Grid- Connected Inverters; IEEE Transactions on Power Electronics; Year: 2011, Volume: 26, Issue: 11, Pages: 3075 - 3078
[20] Paz, F. and Ordonez, M. ; An Embedded Impedance Measurement for DC Microgrids Based on a Lock-In Amplifier; IEEE Power Electronics in Distributed Generation { PEDG) conference, 2016, pp 1-6
[21] Xu, L; Fan, L.; Miao, Z; "DC impedance-model-based resonance analysis of a VSC-HVDC system," IEEE Trans. Power Del, vol. 30, no.3, pp. 1221-1230, June 2015.

Claims

1. Power grid comprising at least one powerline (3, 3', 3'') suspended with intermediate isolators (2) from a high-voltage support structure (1) and connected through apparatus connecting cables (5) with a flexible current transmis¬ sion system to provide or to retain power from the grid, so as to condition the power transmitted through the power grid, wherein said power flows through the grid at a fundamental frequency of for instance 0, 50 or 60 Hz, characterized in that the flexible current transmission system is a variable frequency actuator system (6) that is arranged to provide or to retain a variable frequency power signal to the power grid, wherein said variable frequency actuator system (6) operates at a frequency and phase orthogonal to the fundamental frequency of the power grid.
2. Power grid according to claim 1, characterized in that the variable frequency actuator system (6) comprises a floating variable frequency converter (7} with a restricted energy storage, which converter is connected to a medium frequency converter (10) that drives the floating variable frequency converter (7) to condition the power that the floating variable frequency converter (7) provides or retains from the power grid.
3. Power grid according to claim 2, characterized in that the medium frequency converter (10) is connectable or connected to an external energy storage, power source or sink (9} so as to enable it to provide power or to retain power from the power grid.
4. Power grid according to claim 2 or 3, characterized in that the floating variable frequency converter (7) comprises both a positive polarity variable frequency converter module (11) and a negative polarity variable frequency converter module (12) which are placed in series with each other, and wherein each of said modules (11, 12) connects to a power- line (3' , 3'') dedicated to the concerning variable frequency converter module, and wherein the medium frequency converter (10) is electrically connected to the said positive and nega- tive polarity variable frequency converters (11, 12) in coun¬ ter phase.
5. Power grid according to claim 4, characterized in that there is a common mode separation {8a, 13-16) between the positive and negative polarity variable frequency converters (11, 12) on the one part, and the medium frequency converter (10) on the other part.
6. Power grid according to claim 4 or 5, characterized in that the medium frequency converter (10) connects to the positive and negative polarity variable frequency converters (11, 12) through one of a capacitor or capacitors {13, 14, 15), and a transformer {16), or both.
7. Power grid according to any one of claims 1 - 6, characterized in that the variable frequency actuator system (6) connects a first powerline (3' ) to a second powerline (3''), wherein the first powerline (3') is provided with a multiple frequency and phase current sensor (21) which connects to a current source controller (17) that drives the variable frequency actuator system (6) so as to condition power from the first powerline (3') and provide it as clean power to the second powerline {3' ' ) .
8. Power grid according to claim 7, characterized in that the current source controller (17) drives the variable frequency actuator system (6) using a signal network analyzer (18) for analysis of the power on the first powerline (3' ) and employing a frequency selective damping organ (19) and phase compensation organ (20) to transmit cleaned up power to the second powerline {3' ' ) .
9. Flexible current transmission system, character- ized in that the system is a variable frequency actuator system (6) that is arranged to provide or to retain a variable frequency power signal to an external power grid that operates on a fundamental frequency, wherein said variable frequency actuator system (6) operates at a frequency and phase orthogo- nal to the fundamental frequency.
10. System according to claim 9, characterized in that the variable frequency actuator system (6) comprises a floating variable frequency converter (7) with a restricted energy storage which is connected to a medium frequency con- verter (10) to condition the power that the floating variable frequency converter (7) provides or retains from the external power grid.
11. System according to claim 10, characterized in that the medium frequency converter (10) is connectable or connected to an external energy storage, power source or sink
(9) so as to enable it to provide power or to retain power from the external power grid.
12. System according to claim 10 or 11, characterized in that the floating variable frequency converter (7) comprises both a positive polarity variable frequency converter module (11} and a negative polarity variable frequency converter module (12) which are placed in series with each other, and wherein each of said modules (11, 12) is connectable to a pow- erline, and wherein the medium frequency converter (10) is electrically connected to the said positive and negative polarity variable frequency converters (11, 12) in counter phase .
13. System according to claim 12, characterized in that there is a common mode separation (8a, 13-16) between the positive and negative polarity variable frequency converters (11, 12) on the one part, and the medium frequency converter
(10) on the other part.
14. System according to claim 12 or 13, characterized in that the medium frequency converter (10) connects to the positive and negative polarity variable frequency converters (11, 12) through one of a capacitor or capacitors (13, 14, 15), and a transformer (16), or both.
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CN109659960A (en) * 2019-01-16 2019-04-19 四川大学 A kind of joint frequency modulation control strategy improving wind power plant alternating current-direct current grid-connected system frequency
CN109659960B (en) * 2019-01-16 2022-06-28 四川大学 Combined frequency modulation control strategy for improving frequency of wind power plant alternating current-direct current grid-connected system
WO2021015769A1 (en) * 2019-07-24 2021-01-28 Motoyama Dean Hatsuo Device and process for detecting and mitigating reverse power-flow
CN112436520A (en) * 2020-11-27 2021-03-02 上海电力大学 Alternating current power spring feedback linearization decoupling control method
CN112436520B (en) * 2020-11-27 2023-11-17 上海电力大学 Feedback linearization decoupling control method for alternating-current power spring
CN112327637A (en) * 2020-12-01 2021-02-05 上海电力大学 Power spring feedback linearization control method based on robust disturbance observation
CN112327637B (en) * 2020-12-01 2022-05-27 上海电力大学 Power spring feedback linearization control method based on robust disturbance observation

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