WO2019129729A1 - Compensation de déséquilibre par redistribution optimale du courant - Google Patents

Compensation de déséquilibre par redistribution optimale du courant Download PDF

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Publication number
WO2019129729A1
WO2019129729A1 PCT/EP2018/086691 EP2018086691W WO2019129729A1 WO 2019129729 A1 WO2019129729 A1 WO 2019129729A1 EP 2018086691 W EP2018086691 W EP 2018086691W WO 2019129729 A1 WO2019129729 A1 WO 2019129729A1
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WO
WIPO (PCT)
Prior art keywords
current
phase
converter
sequence
grid
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PCT/EP2018/086691
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English (en)
Inventor
Jeroen STUYTS
Jeroen TANT
Fjo De Ridder
Sven De Breucker
Original Assignee
Vito Nv
Katholieke Universiteit Leuven
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Application filed by Vito Nv, Katholieke Universiteit Leuven filed Critical Vito Nv
Publication of WO2019129729A1 publication Critical patent/WO2019129729A1/fr

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Classifications

    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/12—Arrangements for reducing harmonics from ac input or output
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R29/00—Arrangements for measuring or indicating electric quantities not covered by groups G01R19/00 - G01R27/00
    • G01R29/16—Measuring asymmetry of polyphase networks
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R29/00—Arrangements for measuring or indicating electric quantities not covered by groups G01R19/00 - G01R27/00
    • G01R29/18—Indicating phase sequence; Indicating synchronism
    • G—PHYSICS
    • G05—CONTROLLING; REGULATING
    • G05F—SYSTEMS FOR REGULATING ELECTRIC OR MAGNETIC VARIABLES
    • G05F1/00—Automatic systems in which deviations of an electric quantity from one or more predetermined values are detected at the output of the system and fed back to a device within the system to restore the detected quantity to its predetermined value or values, i.e. retroactive systems
    • G05F1/70—Regulating power factor; Regulating reactive current or power
    • 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
    • H02J3/26—Arrangements for eliminating or reducing asymmetry in polyphase 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
    • H02J3/01—Arrangements for reducing harmonics or ripples
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/0003—Details of control, feedback or regulation circuits
    • H02M1/0016—Control circuits providing compensation of output voltage deviations using feedforward of disturbance parameters
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/0003—Details of control, feedback or regulation circuits
    • H02M1/0016—Control circuits providing compensation of output voltage deviations using feedforward of disturbance parameters
    • H02M1/0019—Control circuits providing compensation of output voltage deviations using feedforward of disturbance parameters the disturbance parameters being load current fluctuations
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/0003—Details of control, feedback or regulation circuits
    • H02M1/0016—Control circuits providing compensation of output voltage deviations using feedforward of disturbance parameters
    • H02M1/0022—Control circuits providing compensation of output voltage deviations using feedforward of disturbance parameters the disturbance parameters being input voltage fluctuations
    • 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
    • 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
    • Y02E40/50—Arrangements for eliminating or reducing asymmetry in polyphase networks

Definitions

  • the present invention relates to converters and method of operating them and to provide Unbalance Compensation by Optimally Redistributing Currents.
  • More and more power electronic (PE) converters are connected to the low-voltage grid (LV grid). They are used to interface distributed generation (DG), like solar and small- scale wind and they allow you to connect a battery or to charge a plug-in electric vehicle (PEV). These converters have in common that they convert the grid’s AC to DC for the consumer or the other way around. More applications exist for these AC-DC converters on the LV grid, like controlling motors (e.g. compressors and heat pumps) and generators (e.g. small wind turbines and micro water turbines) for all sorts of applications. In addition, future applications, like DC microgrids, will have an AC-DC converter as a crucial component.
  • DG distributed generation
  • PEV plug-in electric vehicle
  • the topology is composed of a number of half bridges (two switches in series), which together can compose a full bridge (two half bridge in parallel, i.e. a single-phase converter). Three half bridges are required for a three phase converter without neutral connection (i.e. one half bridge per phase). Adding a fourth connection is required for a neutral connection (e.g. a fourth leg as in Figure 1). Furthermore, each half bridge, except the one for the neutral connection, uses an LCL-filter. This topology is the most common for low voltage converters, the actual technology behind the components can vary, but the overlaying architecture is the same.
  • the typical European residential LV grid has three phases at 400V line-to-line and a neutral (230V phase voltage).
  • 230V phase voltage For example in parts of Belgium 43% of the connections are single phase grid connections (typically 40 A) to this three-phase four- wire grid. 23% has a three- phase grid connection (typically 25 A).
  • the other 34% is connected to the old 3x230V topology (no neutral, 230V line-to-line) [1]
  • a converter can be used to respond to an unbalanced grid in many different ways. It can inject balanced currents, it can inject balanced powers (e.g. active or reactive), it can inject only the positive sequence ... [4] . The injection of currents independent of unbalance has been achieved in [5]. A converter could also actively compensate unbalance [6], which can e.g. decrease grid losses [7], [8] . Also different unbalance compensation techniques are available [9], but some are dependent on the available hardware [10] (i.e. a neutral connection or not). The connection to the neutral is required to be able to compensate not only the negative sequence, but also the zero sequence [10] .
  • [7] proposes a centralized solution and a local solution.
  • the local solution uses a fixed droop relationship for the active power output, based on the voltage in the phase(s).
  • the solution is however voltage based and considers active power only.
  • [9] is capable of compensating negative or zero sequence, but not both at the same time.
  • a more refined form of the previous solution is the central control of a group of single phase inverters where the combined output of the controlled inverters is adjusted until e.g. current balance is improved beyond a certain target [17] .
  • this directly affects the positive sequence active and reactive power of the controlled inverters. For example, if the controlled group are PV inverters, adjustment of the combined output causes unwanted curtailment of the solar energy.
  • Load balancing can also be achieved by using a UPS in parallel to the load and power source.
  • the phase imbalance of the load is compensated by the UPS, although it is not stated how the UPS manages to compensate the imbalance and is able to transfer power between phases [18].
  • a three phase inverter without neutral connection is used in [18], so the transfer capacity between phases is limited, but this is not addressed in the description.
  • the current harmonics present in the dc-bus can flow unimpeded to the neutral conductor, increasing the already present harmonic current in this conductor.
  • the neutral current is drawn from the middle of the dc-bus, the balance of the dc-bus is disturbed and depending of the neutral current direction, the voltage of the lower part of the dc-bus will increase or decrease compared to the upper part.
  • Space Vector Modulation is no longer applicable, forcing the dc-bus voltage to increase.
  • the solution proposed in [20] recognizes that the injection of positive and negative sequence components should be limited to prevent excessive currents in the converter.
  • the proposed solution in [20] only takes the positive and negative sequence into account, as it assumes a three-phase converter without neutral connection, and neglects the zero sequence current.
  • the solution proposed in [20] can give priority to the positive sequence current.
  • the positive and negative sequence currents have a sinusoidal representation in [20], which means that the maximum negative sequence current needs to be calculated at every phase angle as the rotation of the positive and negative sequence current is opposite, resulting in a different maximum allowable negative sequence current at each phase angle for a given maximum phase current and positive sequence current.
  • the limit value of the negative sequence current thus changes constantly, even within a single grid-period, and forces the negative phase current controller to track this limit value. This produces an erratic negative sequence current.
  • a control architecture for the control of the positive and negative sequence components is described in [21].
  • the zero sequence component is omitted, as the neutral wire is not connected, and hence, no optimization of the set points for the negative and zero sequence components is performed.
  • a further control architecture for multi-level inverters is known from [23] .
  • [23] proposes a control method for minimizing the common mode voltage with the aim of reducing the zero sequence current component of the current between the LCL-filter and the neutral point of the dc-bus (neutral point divides the dc-bus in 2 halves). [23] focuses on the control and related minimization of the zero sequence current,.
  • a method and system is disclosed according to embodiments of the present invention to determine unbalance compensating currents to be injected or consumed by a three-phase converter by current redistribution.
  • First load-current is decomposed to outputs of at least negative- and zero-sequence components (I neg and I zer ), followed by determining a positive-sequence component l pos,i nv ,i ) as a function of active power (P), reactive power ( Q ) and positive-sequence voltage (U), and determining a phase current, to be injected per phase and a optionally a neutral current.
  • An advantage of embodiments of the present invention is the ability to compensate negative and zero sequence, both at the same time.
  • An advantage of embodiments of the present invention is the ability take into account how much current can be injected in the phases and to work with a neutral connection.
  • Embodiments of the present invention are flexible, not expensive and can be used at low and medium voltage at any power level.
  • An advantage of embodiments of the present invention is that a transformer is not required to inject the currents calculated by method or system embodiments of the present invention.
  • An advantage of embodiments of the present invention is that they consider currents and also reactive power.
  • An advantage of embodiments of the present invention is that unbalance compensation can be optimised continuously and is not based on a fixed amount (or per voltage).
  • An advantage of embodiments of the present invention is the ability to compensate for the negative sequence and the zero sequence.
  • An advantage of embodiments of the present invention is the ability to operate on local measurements alone and also to compensate the zero sequence.
  • An advantage of embodiments of the present invention is to provide a good or to improve or to maximize the unbalance-compensating current injected by an inverter under all grid circumstances at all times.
  • the unbalance-compensating current consists of or comprises a negative and a zero sequence current which is injected by the inverter with the purpose to reduce the negative and zero sequence current present in the grid or the property it is installed in.
  • a method to determine unbalance compensating currents to be injected or consumed by a three-phase converter by current redistribution comprising the steps:
  • a transformer is not required to inject the currents.
  • the method can comprise evaluating:
  • phase current determining the phase current, to be injected for phase i according to:
  • I Z er,i the zero sequence current of phase i in complex notation
  • I ⁇ , ⁇ hn the current through the converter leg connected to phase i
  • Io, ⁇ hn the current through the converter leg connected to the neutral
  • I ⁇ Dmax the maximal phase current through the converter
  • Io,max the maximal neutral current through the converter
  • Ipos,inv,i the positive sequence current of phase i
  • Ip positive sequence component flowing through a grid, injected or consumed by the inverter or a combination of these
  • Iz the zero sequence to be compensated.
  • optimization aspects of the present invention can be formulated as a second order cone problem.
  • the three-phase converter can be a series or parallel converter.
  • the load current can be measured or estimated by calculation.
  • At least negative- and zero- sequence components can be provided in vector format.
  • a computer based system to determine unbalance compensating currents to be injected or consumed by a three-phase converter by current redistribution comprising:
  • the system can also comprise means for evaluating:
  • phase current means for determining a phase current, to be injected for phase i according to: i,ihn pos,mv,i n l neg,i z l zer,i
  • I Z er,i the zero sequence current of phase i in complex notation
  • Li nv I the current through the converter leg connected to phase i
  • Io, ⁇ hn the current through the converter leg connected to the neutral
  • Ic Dmax the maximal phase current through the converter
  • Io,max the maximal neutral current through the converter
  • Ipos,inv,i the positive sequence current of phase i
  • Ip positive sequence component flowing through a grid, injected or consumed by the inverter or a combination of these
  • Iz the zero sequence to be compensated.
  • the means for evaluating can be subject to
  • the three-phase converter can be a series or parallel converter.
  • Means for measuring the load current or means for estimating the load current by calculation can be provided.
  • the at least negative- and zero- sequence components can be provided in vector format.
  • a converter controller system comprising:
  • a grid estimator which takes the 3 phase voltages as an input and outputs positive, negative and zero sequence voltages, grid frequency, and grid-angle, and a
  • a current controller receiving output from the computer based for controlling switching elements of the three-phase converter.
  • grid estimating which takes the 3 phase voltages as an input and outputs positive, negative and zero sequence voltages, grid frequency, and grid-angle
  • Embodiments of methods of the present invention can be run on an inverter with a parallel connection to a grid and to a property it is installed in, but can also be used in series connected converters, i.e. converters that are connected to the grid at one side and connected to a device or building at the other side.
  • series connected converters i.e. converters that are connected to the grid at one side and connected to a device or building at the other side.
  • Embodiments of the present invention will be described with reference to the parallel case, but the present invention is not limited to this case but can be implemented successfully in the case of a series connected converter.
  • embodiments of the present invention are suited for both parallel and series connected converters.
  • a determination of a current unbalance can be based both on direct measurement of the current in the grid or property and/or on the derivation of the grid/property current based on other parameters.
  • the direct measurement of the current and its unbalance can, for example either be a current measurement directly connected to the current compensating converter of embodiments of the present invention or a measurement performed in a e.g. in a smart grid where the current present in the grid is transmitted to the current compensating converters of that smart grid.
  • Embodiments of the present invention can assume that the current unbalance is measured or estimated and uses either measurement or estimation as an input.
  • Embodiments of the present invention comprise injection of negative and zero sequence currents, the underlying inverter technology is presumed to have a neutral connection next to the phase connections.
  • Embodiments of the present invention can function with different neutral point connections, e.g. a split-cap implementation or a fourth half-bridge in a conventional three-legged three-phase inverter. It is anticipated that embodiments of the present invention are not limited by the chosen technology as long as the technology allows the injection of negative and zero sequence currents.
  • a first optional limitation of some embodiments of the present invention is that the primary function of the inverter preferably remains unaffected, being the injection or absorption of positive sequence current, which consists of or comprises the active and reactive current.
  • the set-points of the active and reactive current can depend on the underlying application and grid requirements, such as active power of a PV inverter, (dis)charge power of a battery-inverter, mechanical drive power and reactive power compensation of a motor drive, and should not be impeded by the unbalance-compensating current settings.
  • a second optional limitation of some embodiments of the present invention is that the hardware limits of the inverter should preferably be respected.
  • an inverter is configured where all phases have the same current rating, and allows the neutral connection to have a current rating which differs from the phases.
  • the current rating of the neutral connection depends on the functional requirements of the inverter. For example if the emphasis lies on active/reactive power injection and unbalance compensation is only allowed at one third of the power rating, then the current rating of the neutral connection might be as low as 1/4 ⁇ of the current rating of the phases.
  • Embodiments of the present invention can take the above limitations into account and can for example determine the optimal set-point of the negative and zero sequence current. In case none of the limitations are met, the inverter will fully compensate the negative and zero sequence currents of the targeted grid or the property it is installed in. If the limitations do occur, embodiments of the present invention can make an optimization between the injected negative and zero sequence currents while the positive sequence active and reactive currents remain unaltered. Embodiments of the present invention are able to recalculate the negative and zero sequence currents settings very quickly (ms range), such that any alteration in the positive sequence current can immediately be taken into account. In any case, the positive sequence remains unaltered at all times and will always be injected/consumed or passed through.
  • a converter is preferably fully programmable. If this is not the case or if the controller for the converter is not suitable for upgrading to the present invention then a new controller can be added.
  • Embodiments of the present invention can provide the converter with the possibility to compensate unbalance.
  • Embodiments of the present invention can be used to calculate the currents that need to be injected by a PE (power electronic) converter to compensate unbalance.
  • PE power electronic
  • the present invention avoids unwanted curtailment of generation units or undesired load shaving as the positive sequence active and reactive power remains unaffected and current unbalance issues are solved by the injection of negative and zero sequence currents.
  • the present invention ensures that the positive sequence current required by the load or generation is not adversely affected by the negative and zero sequence currents and optimizes the applied negative and zero sequence currents within the limits of the converter hardware.
  • Embodiments of the present invention can give priority to the positive sequence current, for example.
  • Embodiments of the present invention do not need to have positive and negative sequence currents having a sinusoidal representation. This avoids that the maximum negative sequence current needs to be calculated at every phase angle as the rotation of the positive and negative sequence current is opposite, resulting in a different maximum allowable negative sequence current at each phase angle for a given maximum phase current and positive sequence current.
  • Embodiments of the present invention avoid that the limit value of the negative sequence current thus changes constantly, even within a single grid-period, and forces the negative phase current controller to track this limit value.
  • Embodiments of the present invention avoid that an erratic negative sequence current is produced. Also the zero sequence current is not omitted in embodiments of the present invention.
  • the positive, negative and zero sequence current have a vector representation, which means that the amplitude and phase angle are available separately.
  • the available current limit for the negative and zero sequence current can thus be calculated directly by the algebraic difference between the current limit of the converter-phases and the positive sequence current. This fixes the available negative and zero sequence current and avoids the erratic behaviour of [20] .
  • the ratio between the injection of the negative and zero sequence current is optimized such that the current unbalance is minimized, this optimization is not performed in [20] as only the negative sequence current is taken into account.
  • the present invention preferably can be used with a controller for the positive, negative and zero sequence components of the current.
  • Embodiments of the present invention provide a determination of the optimal set points of the negative and zero sequence components such that the current unbalance (e.g. negative and zero sequence current components) present in the grid or the installation of the property are either minimized or eliminated.
  • Embodiments of the present invention provide a determination of the optimal set-points for the negative and zero sequence currents with the aim to minimize the current unbalance in the grid or property.
  • Figure 1 shows some of the connections of a converter with which embodiments of the present invention can be used such that it becomes a compensator.
  • Figure la) shows a series connected compensator.
  • Figure lb) shows a parallel connected compensator
  • Figure lc) shows a parallel connected compensator in a separate grid (e.g. that of a consumer) Note that the connection to the neutral line is optional but preferred.
  • FIG. 2 shows an embodiment of the present invention.
  • Figure 3 shows a decomposer which can be used in accordance with embodiments of the present invention.
  • Figure 4 shows a solver, running asynchronously, communicating over a buffer with the real-time synchronous converter or inverter controller according to an embodiment of the present invention.
  • FIG. 5 shows further embodiment of the present invention.
  • Figure 6 shows a prior art converter without current redistribution.
  • Figure 7 shows a converter in accordance with an embodiment of the present invention with current redistribution.
  • Figure 8 shows results from application of embodiments of the present invention being complete compensation, no limits, enabled at 0.1 s
  • Figure 9 shows results from application of embodiments of the present invention with limits of phase and neutral currents to lApp, enabled at 0.1 s.
  • Figure 10 shows results from application of embodiments of the present invention with limit of currents to 2A, while injecting 500VAr with the inverter, enabled at O.ls.
  • Figure 11 shows of an inverter that can be used with embodiments of the present invention.
  • Figure 12 shows an embodiment of the present invention implemented on an engineering PC communicating with a control computer over a TCP/IP LAN.
  • PE Power Electronic
  • PEV Plug-in Electric Vehicle
  • DG Distributed Generation
  • UPS Uninterrup table Power Supply
  • SOCP Second-Order Cone Program
  • Ii.inv I the current through the converter leg connected to phase i
  • Io, ⁇ hn the current through the converter leg connected to the neutral
  • I ⁇ max the maximal phase current through the converter
  • Ipos,inv,i the positive sequence current of phase i
  • I p positive sequence component flowing through a grid, injected or consumed by the inverter or a combination of these
  • I z the zero sequence to be compensated.
  • an“inverter” is interchangeable with“converter”, e.g. in the description and claims.
  • the name‘inverter’ is typically used for applications that transfer power from the DC side to the AC side (e.g. photovoltaic panels connected to the grid).
  • Converters are applications that transfer power between the DC side and the AC side, independent of the direction of power. So an inverter is also a converter. Applications such as fast-charging of electric vehicles, controlling AC drives ... all use converters.
  • Embodiments of the present invention provide controlling of a converter, hence provide a‘converter controller”.
  • Converter controller is to be understood as covering an“inverter controller”. Description of the preferred embodiments
  • the present invention is suited for both parallel and series connected converters and is only limited by its use in three-phase power systems. It can be applied to virtually any converter, as long as it is connected to at least three-phases of the power system. An additional connection to the neutral conductor is preferred but not required.
  • embodiments of the present invention can be used with a 3- phase grid (lines 1, 2, 3 and neutral) and a converter controller 2 providing current redistribution, the converter controller 2 running on a processing unit 3 of a computer and controlling an inverter 4.
  • the inverter 4 is controlled in such a way that the unbalanced currents flowing through the grid (here to the left) become balanced with the additional currents from the inverter 4 (balanced currents go to the right).
  • the grid can be any grid, e.g. a distribution grid or a local grid from a residence or company.
  • the converter can be connected to any of a low-voltage grid, medium-voltage grid, high- voltage grid or any other voltage level grid. It has been tested in a low- voltage grid purely as an example.
  • Embodiments of the present invention provide a converter, which can be used, for example, for solar panels, fast-charging of cars or only compensating unbalance.
  • the grid is capable of being used for distributed generation, so that bidirectional power flows are not a problem (e.g. installing the correct fuses. If a neutral connection is not yet established and the converter supports this, it would be advisable to establish this connection.
  • Adjustments to the converter to which the present invention can be applied are for example: o Firstly, firmware of the converter can be altered/upgraded/updated, so that it includes embodiments of the present invention. In cases where the controller of the converter is not good enough a new controller can be installed.
  • an optional neutral connection can be available or would preferably be added to the converter, if it’s not already present. Adding this to existing converters can be quite cumbersome, so that it can be better to change this in the design phase of the converter and add an extra switching leg.
  • Embodiments of the present invention support less than perfect balancing but which is still an optimal balancing taking into account the limitations of the system. This can be caused by limitations of the system i.e. are actually due to external causes. Embodiments of the present invention are capable of injecting the best possible unbalance compensation under all situations, i.e. limited by the (re)active power injection or limited by the hardware which will determine how much current can be injected or consumed. Removal of external limitations, results in complete balancing when embodiments of the present invention are used.
  • the converter maybe only capable of injecting 2App per phase and at the same time, that converter is also injecting 500VAr.
  • the present invention can still compensate unbalance optimally for that converter, taking into account these limits. If the converter would be twice as big (4App), an overall better result could be achieved, yet again for that converter the optimum for that converter will be achieved.
  • Embodiments of the present invention use a grid/load current.
  • the grid/load current can be calculated or measured by any suitable means.
  • a current measurement device can be used, or a device able to estimate or measure e.g. the current based on a voltage measurement.
  • embodiments of the present invention can calculate the correct pulse widths for switches like IGBTs. Driving the witches with Pulse Width Modulation (PWM), for example, the switches inject, on average, the required current. But other techniques like Space Vector Modulation (SVM) can also be used.
  • PWM Pulse Width Modulation
  • SVM Space Vector Modulation
  • Embodiments of the present invention generate current setpoints. When a converter applies these setpoints, current will flow from and to the converter in the different phases and optionally the neutral conductor. These compensating currents will at least partially eliminate the unbalanced currents (e.g. in the grid or from a load). This can result in phase shifted currents and voltages, as well as changes in amplitude of the currents and voltages.
  • Embodiments of the present invention can be used with multi-phase inverters that are connected to two or more, e.g. three phases and a neutral, but the present invention can be used with other inverters and embodiments described below which are targeted towards three-phase inverters with a neutral connection, for example.
  • Different neutral point connections can be used with embodiments of the present invention, e.g. a split-cap implementation or by using a fourth half-bridge such that a four-legged three-phase inverter is obtained.
  • the multi-phase inverter can be used in a parallel or serial set-up, and the implementation can be performed on a parallel connected inverter.
  • Embodiments of the present invention can improve or optimise the set-points of the unbalance compensating current for a given unbalance situation, i.e. the improved or optimal negative and zero sequence currents are determined such that the remaining unbalance in the given grid-node can be reduced or minimized.
  • This reduction or optimisation process can take several restrictions or limitations into account.
  • embodiments of the present invention can ensure optionally that the positive sequence current has priority and is not affected by injection of negative and zero sequence current.
  • Positive sequence current encompasses, for example, at least the fundamental frequency active and reactive current.
  • Positive sequence current can, for example, be affected by e.g. a voltage or frequency droop control, such that the injected active and reactive power can still be adjusted depending on the grid situation.
  • a second restriction or limitation can be for example that the hardware limitations of the inverter are respected.
  • a first hardware limitation can be the maximal phase current, which is identical in all phases and should not be exceeded.
  • a second hardware limitation can be that the maximal neutral current is respected, which can be different from the maximal phase current.
  • a third restriction or limitation can be that the injected negative and zero sequence currents do not amplify or overcompensate the given current unbalance, i.e. the given current unbalance will disappear in the ideal scenario.
  • the obtained solution of embodiments of the present invention will preferably improve or maximize the injected negative and zero sequence currents within these restrictions. If the unbalance compensating inverter has ample ampacity in its phases and neutral, the current unbalance can be made to disappear entirely.
  • Embodiments of the present invention can use a vector representation of the positive, negative and zero sequence current.
  • the limit values can be calculated economically as the amplitude and phase angle are separately available.
  • a known limit value of the converter currents and a fixed positive sequence current, the injected negative and zero sequence current remain stable.
  • Embodiments of the present invention can be compatible with any vector representation of the grid.
  • Embodiments of the present invention can take the restrictions of the hardware and fundamental (re)active current pro-actively into account, such that additional limiters at the output of the current controllers of the negative and zero sequence current are not required.
  • Embodiments of the present invention can be augmented for example by including the harmonic compensation currents.
  • the amplitude of the injected harmonic currents are simply added to the amplitude of the positive sequence currents and the present invention remains otherwise unaltered o If the harmonic compensation currents are unbalanced, the amplitudes of the injected harmonic phase and neutral currents are added to their respective boundary conditions (i.e. the first four equations of the boundary conditions, representing Ll, L2, L3 & N) and the present invention remains otherwise unaltered.
  • Pan [24] calculated, based on the theory of Steinmetz, the properties of an ideal compensator that can be realized with inductors and capacitors. For a three-phase four- wire network, two compensators are required. One connected in Delta (D) and one in Star (Y). Concluding that:
  • Embodiments of the present invention can compensate unbalance by means of a power electronic converter with, for example, full control over the positive, negative and zero sequence. However, it is enough to only compensate the negative and zero sequence of the load to fully compensate unbalance. When this requires more current than is available, special actions are preferably taken.
  • the result is that only the negative sequence component can be or has to be compensated, as no zero sequence can be injected due to the absence of a neutral connection.
  • any three-phase current can be decomposed in positive, negative and zero sequence components.
  • the positive sequence can come from the grid, as this is a balanced current, which transports the active power. Either one of the two other components will lead to an unbalanced current. Therefore, the ideal current response from a 4-leg converter is the sum of the negative and zero sequence current. A 3-leg converter cannot deliver the zero sequence as it has no neutral connection. To maximally limit ( unbalanced ) current in the grid, the ideal current response of this converter is therefore the negative sequence current. As a consequence, the grid delivers the sum of the positive and zero sequence.”
  • Achieving perfect unbalance compensation is therefore relying on measuring or calculating the currents, decomposing them in negative and zero sequence and injecting the opposite currents.
  • An important aspect is the determination of the current(s).
  • hardware constraints will preferably need to be taken into account.
  • An important feature of a converter is that it can inject or consume balanced power e.g. active and/or reactive power. It is important that a converter which is used for current redistribution is still able to do its primary job: inject or consume power. However, in many instances the converter will not be injecting or consuming the designed or maximal amount of power (e.g. solar in the night, morning or at noon at which time power generation is high but consumption is lower). Furthermore, the unbalance that needs to be compensated will be very situation and time dependent as well. The amount of unbalance to be compensated as well as the balanced power that needs to be consumed or injected is thus time and case dependent.
  • balanced power e.g. active and/or reactive power. It is important that a converter which is used for current redistribution is still able to do its primary job: inject or consume power. However, in many instances the converter will not be injecting or consuming the designed or maximal amount of power (e.g. solar in the night, morning or at noon at which time power generation is high but consumption is lower
  • an inverter preferably should be capable of compensating as much unbalance as possible ; given its constraints. This triggers the use of an optimization, for example.
  • unbalance compensation comprises injecting negative and zero sequence components. Injecting or consuming balanced power is injecting or consuming positive sequence currents. Any actively controlled three-phase converter is therefore theoretically capable of compensating some sort of unbalance, if controlled correctly. Injecting negative and zero sequence currents does not necessarily conflict with injecting positive sequence. This can be enabled by a hardware constraint optimization.
  • An objective of embodiments of the present invention is to reduce or minimize (preferably well or optimally) unbalanced currents in the grid which cause unwanted losses in the grid.
  • An optimization according to embodiments of the present invention is to minimize or maximize an objective function.
  • a suitable objective function to calculate how much unbalance can be compensated is then the losses in the grid, caused by the currents. This also means that the amount of unbalanced current is decreased or is decreased to a minimum.
  • the objective function as:
  • I t are the grid currents of the 1-3 respective phases, which are squared to calculate a representation of the losses. This assumes an equal resistance of the phases.
  • the formulation of the objective function can be expanded, as to clearly include the compensation terms and the positive sequence:
  • I p is the positive sequence component which is flowing through a grid, injected or consumed by the inverter or a combination of these
  • I n is the negative sequence that needs to be compensated i.e. (either the grid, a (sum of) load(s) or both)
  • I z is the zero sequence that needs to be compensated (i.e. either the grid, a (sum of) load(s) or both). This is split up per phase (e.g. 1, 2 and 3).
  • a n and a z are now the variables for optimization, as these indicate how much negative and zero sequence is compensated. It is clear that when both are 1, the compensation is perfect, as only the positive sequence remains, and that when it is 0, there is no compensation.
  • i neg is the negative sequence current of phase 1 in complex notation
  • i zer is the zero sequence current (of phase 1) in complex notation
  • operation is to determine the absolute value
  • / inv is the current through the i th converter leg
  • hiac is the maximal phase current through the converter and / 0 max is the maximal neutral current through the converter. If thac can be equal to / 0 mai , but this is not required. If hiac can be different for each phase, but this is not a requirement.
  • Two additional boundary conditions can be formulated that limit the solution space.
  • the parameters a n and a z should preferably, for example never get any number not within a limited range, e.g. not in the range [0,1] thus including the boundaries. Any value below 0 would result in an additional injection of negative and zero sequence, as would any value over 2.
  • the range [1 2] would mean a sign was wrong in the definition of the equation. Adding these boundaries considerably limits the solution space, which should speed up an algorithm computing the solution.
  • the current through phase i can be calculated as:
  • I pos ,inv,i is the positive-sequence current of phase i that needs to be injected by the converter rather than the one of the load or grid.
  • This type of problem can be formulated, for example as a second-order cone program (SOCP).
  • SOCP second-order cone program
  • the norm of vectors is used, which is ideal for SOCP.
  • SOCP is convex, which guarantees that there is one global minimum [26], [27]
  • the previously presented problem has two factors as a solution: a n and a z , that determine how much negative- and zero-sequence current should be injected.
  • the solution proposed in embodiments of the present invention is a very good way to compensate unbalance.
  • the currents that are required as data input can be the result of a measurement or an estimation e.g. determining the positive, negative and zero- sequence decomposition.
  • the solution determines how much current should be injected in each inverter leg (i.e. each connection), using the formula’s for the current through a phase and the neutral.
  • phase current to be injected by the converter, for phase i can be calculated as: i,ihn roe,ihn,i n l neg,i z l zer,i 1 where all components are now known.
  • the neutral current is calculated as: 2
  • FIG. 3 shows a block diagram of an implementation of an online decomposition unit that can be used with any of the embodiments of the present invention.
  • the input is the three-phase current measurement 1 of the current to be compensated and the detected grid frequency w.
  • the outputs are the positive-, negative- and zero-sequence components of the current l pos , l neg and I zer .
  • the three-phase input current / is sent through an adaptive band-pass filter at frequency w, which has the same structure as a SOGI (see P. Rodriguez, A. Y. Timbus, R. Teodorescu, M. Liserre, and F. Blaabjerg,“Flexible Active Power Control of Distributed Power Generation Systems During Grid Faults,” IEEE Trans. Ind. Electron., vol. 54, no. 5, pp. 2583-2592, Oct. 2007).
  • the outputs of the filters stage are the filtered in-phase component / (per phase) and quadrature component I q (per phase) of the three-phase input. The calculation is as follows:
  • the quadratic sum gives the filtered amplitude 1/1 of the current (per phase), i.e.
  • the input to the online decomposition unit is the current measurement i.e. of the current / to be compensated and determined grid frequency w and the outputs are the sequence components I pos (e.g. optionally not used), I neg and l zer .
  • the three-phase input current / is sent through an adaptive band-pass filter at frequency w, which is the same structure as a SOGI.
  • the output of this filter stage is an in-phase component / and quadrature component I q .
  • the positive-sequence of the inverter l pos ,inv,i needs to be determined which is a function of active power P, reactive power Q and positive-sequence voltage U.
  • the inverter will have a setting for active power P and reactive power Q.
  • the positive-sequence voltage U the calculation becomes:
  • the required grid frequency w for the current decomposition as well as the positive- sequence voltage U can be derived with a Frequency or Phase Locked Loop, to the likes of e.g. [5], [6]
  • the path 19 of the software synchronous converter or inverter controller process 10 executed on a processing engine is shown.
  • the relevant data 11 for inputting to the asynchronous optimisation process 20 are measured and/or calculated.
  • This data is written in a step 12 to a buffer which is part of the synchronous process 10.
  • the data is read in step 13 from the buffer.
  • the asynchronous problem is constructed in step 14 and solved in step 15.
  • step 16 the solution is written to a buffer.
  • the solution is read into a buffer which is part of the synchronous process 10.
  • the asynchronous solution is adapted to the state of the synchronous process.
  • Figure 5 shows an implementation as an embodiment of the present invention.
  • the block‘Decompose’ 5 is as described with respect to Figure 3.
  • the block‘Construct positive sequence’ 6 calculates Equation 3, e.g. using a digital processing engine.
  • the gathered data is sent via a buffer to the optimization, which sends the results back to construct the solution.
  • the blocks ‘Gathering data’ 7 and Optimization’ 8 depend on the actual solver being used and an example has been described with respect to Figure 4 e.g. using a digital processing engine.
  • the construction of the solution is achieved according to Equations 1 and 2 in the“Construct Solution block” 9 e.g. using a digital processing engine.
  • the implementation of Figure 6 shows a conventional converter control scheme without current redistribution: the PLL (Phase Locked Loop) uses the voltage measurement t/ 123 to determine the grid-angle Q and passes this information, together with the voltage measurement, on to a current controller. Voltage measurement devices are shown as circled V’s.
  • the current controller gets its settings from a power setpoint (P set and Q set ) ⁇
  • the current controller determines the voltage U 123 sp that needs to be applied by the converter, applying limits if the setpoint cannot be reached.
  • the voltage setpoint is applied by the converter, using for example PWM (Pulse Width Modulation), thereby requiring the DC-bus voltage U dc .
  • PWM Pulse Width Modulation
  • FIG 7 shows an implementation of a current redistributing system according to embodiments of the present invention for compensating unbalance with a regular converter.
  • This embodiment can be included in a converter controller having a processing engine.
  • Voltage measurement devices are shown as circled V’s.
  • the measurements are provided to a grid state-estimator 22 (e.g. an expanded version of a Frequency or Phase Locked Loop), whereby the positive, negative and zero-sequence of the voltages U pnz are provided, as well as the frequency w.
  • a grid state-estimator 22 e.g. an expanded version of a Frequency or Phase Locked Loop
  • the current / 1230 resulting from the current redistribution provided by the current redistributor 24 according to embodiments of the present invention as described with reference to Figure 5 for example, is then sent to a conventional current controller, where the limiting action is no longer required. But now also the current from the neutral conductor needs to be controlled. From there, PWM signals are generated to control the switching elements in the converter.
  • Figure 7 shows a parallel implementation, but other configurations are included within the scope of the present invention, for instance those in Figure 1. Note that the neutral connection is optional but preferred otherwise one cannot act on that conductor and as such, additional neutral measurements have to be included.
  • a measurement system provides the required information to the optimizer.
  • the presented optimization is reformulated to an SOCP and is solved online with ECOS [28] as explained above.
  • the generated set points are implemented and injected using a current controller. This control is implemented on the setup described in [9] .
  • Grid current [A] The grid current is the current flowing in the (distribution) grid or after the point of common coupling. It is the sum of the load current and the inverter current.
  • the grid current is made up of three phase currents (Ll, L2 & L3) and the neutral current (N)
  • the load current is the current of the load that is to be compensated. This can be any current, made up of any combination of loads and/or distributed generation and represents thus also the current flowing in a grid before any compensation.
  • the load current is made up of three phase currents (Ll, L2 & L3) and the neutral current (N)
  • the inverter current is the current that compensates the load according to embodiments of the method of the present invention. It adheres to the boundary conditions applied and optimally compensates the load current.
  • the inverter current is made up of three phase currents (Ll, L2 & L3) and the neutral current (N)
  • o Q the amount of reactive power that is present in the grid (ideally as three- phase balanced positive sequence).
  • the unbalance compensation is always enabled at 0.05s, i.e. the first vertical line.
  • limits are imposed on the inverter and thus its ability to compensate the unbalance of the load completely.
  • the results are visualized in Figure 9 limit phase and neutral currents to lApp, enabled at 0.1 s.
  • the limits on phase and neutral current are lApp and 0.7Arms.
  • the grid current is reduced from a single-phase unbalanced load to a more balanced three-phase load, although the result is still unbalanced, due to the imposed limits.
  • the neutral current is reduced by one amp, the maximum allowed by the limits and the current in phase 2 has decreased, while that in phase 1 and 3 has increased.
  • the decrease of Du clearly shows that unbalance is compensated. P and Q remain unaltered and S also decreases.
  • the converter or inverter is clearly injecting the maximum of 1A per phase and in the neutral. Furthermore, as P and Q remain unaltered, it is also clear that unbalance compensation is achieved without injecting or consuming any (re)active power. This embodiment indicates that the imposing of limits works. The optimizer finishes in 11. lms.
  • limits are imposed on the inverter and thus its ability to compensate unbalance, while the inverter is injecting balanced current itself.
  • the results are visualized in Figure 10 enabled at O.ls.
  • the limits on phase and neutral current are 2App, l.4Arms and the inverter is injecting 500VAr.
  • the grid current is reduced from an unbalanced load to a more balanced load, although the result is unbalanced, due to the imposed limits.
  • the neutral current is reduced and the current in phase 2 has decreased, while that in phase 1 and 3 has increased.
  • the decrease of Du clearly shows that unbalance is compensated. P and Q remain unaltered and S also decreases.
  • the inverter is clearly injecting the maximum of 2A per phase and neutral. Furthermore, as P and Q remain unaltered, it is also clear that unbalance compensation is achieved without injecting or consuming any additional (re)active power, while still being able to inject the desired amount of reactive power. This embodiment indicates that imposing of limits function correctly, while being able to inject power.
  • the optimizer finished in 10.8ms.
  • the current redistribution computer based method has been implemented on an actual 3ph+N inverter, including
  • FIG 11 is a drawing of an inverter to which any of the embodiments of the present invention can be applied: on the left is the DC connection, which links to three identical half-bridges. Each halfbridge connected to an LCL-filter stage and then to the AC side.
  • An embodiment of the present invention has been implemented on a processing engine comprising a dual core 2.26 GHz PC with 4 GB of RAM.
  • the principle is shown in the Figure 12
  • This embodiment of the present invention can be implemented on a wide variety of microprocessors, CPU or GPU.
  • the engineering PC is adapted to communicate with the control computer (having processing engine and memory) over a TCP/IP local area network.
  • This control computer controls the inverter by calculating the setpoints and processing the measurements.
  • the communication between the control computer and the inverter happens over a real-time fieldbus.
  • the converter or inverter is the one shown in Figure 7.
  • This inverter- system was tested on the grid of an office building with realistic grid voltages and current unbalances and is able to inject current into this grid.
  • the current redistribution on an operational inverter-system has been tested in a relevant environment, e.g. testing of a prototype in a realistic simulated operational environment.
  • Embodiments of the present invention can be implemented in all technical sectors which utilize three-phase inverters. These sectors include, for example
  • Controlled current unbalance in own grid The current unbalance in the own grid, introduced by large single-phase load/generation, is compensated by the grid-tied inverter, but the maximum amplitude of the negative and zero sequence current can be higher than the positive sequence current.
  • a single-phase 32 A EV charger is connected to the 20 A three-phase grid of a property; If the unbalance- compensating inverter can inject an additional 20 A in the EV phase and draw 10 A from the 2 other phases, the vehicle can still be charged.

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Abstract

L'invention concerne un procédé et un système permettant de déterminer des courants de compensation de déséquilibre destinés à être injectés ou consommés par un convertisseur triphasé par le biais d'une redistribution de courant. Un premier courant de charge est décomposé de manière à faire sortir au moins des composantes de séquence négative et nulle (I neg et I ze r), puis une composante de séquence positive (l pos,inv,i ) est déterminée en fonction de la puissance active (P), de la puissance réactive (Q) et d'une tension de séquence positive (U), et un courant de phase destiné à être injecté par phase et un courant neutre sont déterminés.
PCT/EP2018/086691 2017-12-31 2018-12-21 Compensation de déséquilibre par redistribution optimale du courant WO2019129729A1 (fr)

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