SE1851656A1

SE1851656A1 - Power supporting arrangement for an ac network

Info

Publication number: SE1851656A1
Application number: SE1851656A
Authority: SE
Inventors: Haofeng Bai; Jan Svensson; Nicklas Johansson
Original assignee: Abb Schweiz Ag
Priority date: 2018-12-21
Filing date: 2018-12-21
Publication date: 2020-06-22
Also published as: SE543496C2

Abstract

A power supporting arrangement comprises a first voltage source converter (10) and a first energy storage system (16), where the first voltage source converter has a first alternating current, ac, side for connection to a first ac grid (20) and a first direct current, dc, side for connection to a first end of a dc link (14). The dc link has a second end for connection to a second dc side of a second voltage source converter (12), which second voltage source converter (12) in turn has a second ac side connectable to a second ac grid (22). The first energy storage system (16) is connected to the first ac side of the first voltage source converter (10) and is jointly configured with the first voltage source converter (10) to support one of the ac grids.

Description

lO POWER SUPPORTING ARRANGEMENT FOR AN AC NETWORK FIELD OF INVENTION The present invention relates to a power supporting arrangement for an alternating current (ac) network.

BACKGROUND An increase of converter-connected renewable energy sources (solar,wind), so called non-synchronous generation (NSG) is seen globally. Insome electrical grids, the penetration of NSG can be very high during partsof the year. High levels of NSG in a grid bring challenges e.g. when itcomes to frequency stability, voltage stability, short-circuit power levelsand harmonic stability. To meet the challenges, grid codes are successivelymodified in many countries. In Europe the transmission system operator(TSO) in the United Kingdom (UK) National Grid has been a driver inhighlighting the challenges which they see in their system studies whichinclude high NSG scenarios. European Network of Transmission SystemOperators (ENTSO-E) has also issued new Grid code implementationguideline documents (IGDs) focused on grid code developments needed to meet the challenges seen.

In order to meet these challenges it may be necessary to use energy storage systems for supporting a grid.

One way to connect an energy storage system for such support is throughplacing an energy storage system, inside a direct current (dc) system,which placing may be a placing of the energy storage elements of such anenergy storage system in the submodules of a modular multilevel converter(MMC). The thus modified converter may then be used for supporting anac system. This is for instance described in the article “A New Hybrid MMC with Integrated Energy Storage”, by Ping Wang, Tao Zhang and Rui lO Li, 2017 IEEE Energy Conversion Congress and Exposition (ECCE), 1-5Oct. 2017. However, when the energy storage system is placed in the dcsystem in this way the rating of the converter has to increase. It is also possible that the complexity of the converter is increased.

There is therefore a need for a simpler realization allowing the converter rating to be limited.

SUMMARY OF THE INVENTION The present invention is directed towards providing a power supportingarrangement comprising a first voltage source converter achievingimproved supporting functionality without an increase in rating of the first VOltage SOUTCC COIIVCITCT.

This object is according to a first aspect achieved through a powersupporting arrangement comprising a first voltage source converter and afirst energy storage system. The first voltage source converter has a firstalternating current, ac, side for connection to a first ac network and a firstdirect current, dc, side for connection to a first end of a dc link. The dc linkhas a second end for connection to a second dc side of a second voltagesource converter and the second voltage source converter in turn has asecond ac side connectable to a second ac network. The first energy storagesystem is connected to the first ac side of the first voltage source converterand is jointly configured with the first voltage source converter to support one of the ac networks.

The power supporting arrangement may comprise a first control unitconfigured to control the first energy storage system and the first voltagesource converter to perform the concerted support of said one of the ac networks. lO According to a variation of the aspect, the ac network supported by thefirst voltage source converter and first energy storage system is the first acnetwork and the power supporting arrangement further comprises thesecond voltage source converter and a second energy storage systemconnected to the second ac side of the second voltage source converter. Inthis case the second energy storage system is jointly configured together with the second voltage source converter to support the second ac network.

The power supporting arrangement may in this case also comprise asecond control unit configured to control the second energy storage systemand the second voltage source converter to perform the concerted support of the second ac network.

According to another variation, the first energy storage system, when beingjointly configured together with the first voltage source converter tosupport one of the ac networks, is configured to provide power to thesupported ac network corresponding to inertia of a virtual synchronous machine.

In this respect, the first voltage source converter, when being jointlyconfigured together with the first energy storage system to support one ofthe ac networks, may be configured to perform power synchronizationcontrol. The first energy storage system may in turn be configured toemulate the synchronous machine either alone or together with the firstvoltage source converter. In the latter case the first voltage sourceconverter provides the electrical dynamics of the virtual synchronous machine.

The first control unit may in the different situation outlined above beconfigured to control the first energy storage system and the first voltagesource converter to perform the concerted support of said one of the acnetworks through controlling the first energy storage system to provide power to the supported ac network corresponding to inertia of a virtual lO synchronous machine, to control the first voltage source converter toperform power synchronization control, to control the first energy storagesystem to emulate the synchronous machine either alone or together withthe first voltage source converter and to control the first voltage sourceconverter to provide the electrical dynamics of the virtual synchronous machine.

The second energy storage system, when being jointly configured togetherwith the second voltage source converter to support the second ac network,may be configured to provide power to the supported second ac network corresponding to inertia of a virtual synchronous machine.

In this respect, the second voltage source converter, when being jointlyconfigured together with the second energy storage system to support thesecond ac network, may be configured to perform power synchronizationcontrol. The second energy storage system may in turn be configured toemulate the synchronous machine either alone or together with the secondvoltage source converter. In the latter case the second voltage sourceconverter provides the electrical dynamics of the virtual synchronous machine.

The second control unit may in the different situation outlined above beconfigured to control the second energy storage system and the secondvoltage source converter to perform the concerted support of the second acnetwork through controlling the second energy storage system to providepower to the supported ac network corresponding to inertia of a virtualsynchronous machine, to control the first voltage source converter toperform power synchronization control, to control the first energy storagesystem to emulate the synchronous machine either alone or together withthe first voltage source converter and to control the first voltage sourceconverter to provide the electrical dynamics of the virtual synchronous machine. lO It is additionally possible that the first energy storage system and the first voltage source converter when being jointly configured to support one of the ac networks are configured to stabilize the frequency of the ac network.

In this case the first control unit may control the first energy storagesystem and the first voltage source converter to stabilize the frequency of the ac network.

It is similarly possible that that the second energy storage system and thesecond voltage source converter when being jointly configured to supportthe second ac network are configured to stabilize the frequency of thesecond ac network. In this case the second control unit may control thesecond energy storage system and the second voltage source converter to stabilize the frequency of the ac network.

It is additionally possible that the first energy storage system and the firstvoltage source converter when being jointly configured to support one ofthe ac networks are configured to, if there is a fault in the ac network,jointly provide a fault current to the ac network. The first energy storagesystem may furthermore be configured to supply the negative phasesequence current and the first voltage source converter may be configuredto supply the positive phase sequence current of the fault current or viceversa. In this case the first control unit may be configured to control thefirst energy storage system and the first voltage source converter to jointlyprovide the fault current to the ac network, which control may involvecontrolling the first energy storage system to supply the negative phasesequence current and controlling the first voltage source converter tosupply the positive phase sequence current of the fault current or vice VGYSEI.

It is in a similar way possible that the second energy storage system andthe second voltage source converter when being jointly configured tosupport the second ac network are configured to, if there is a fault in the second ac network, jointly provide a fault current to the second ac lO network. The second energy storage system may furthermore beconfigured to supply the negative phase sequence current and the secondvoltage source converter may be configured to supply the positive phasesequence current of the fault current or vice versa. In this case the secondcontrol unit may be configured to control the second energy storagesystem and the second voltage source converter to jointly provide the faultcurrent to the second ac network, which control may involve controllingthe second energy storage system to supply the negative phase sequencecurrent and controlling the second voltage source converter to supply the positive phase sequence current of the fault current or vice versa.

According to another variation the first energy storage system and the firstvoltage source converter when being jointly configured to support one ofthe ac networks are configured to jointly supply reactive power to the acnetwork. This may involve the first control unit controlling the first energystorage system and the first voltage source converter to jointly supply reactive power to the supported ac network.

It is in a similar way possible that the second energy storage system andthe second voltage source converter when being jointly configured tosupport the second ac network are configured to jointly supply reactivepower to the second ac network. This may involve the second control unitcontrolling the second energy storage system and the second voltagesource converter to jointly supply reactive power to the supported ac network According to a further variation, the first voltage source converter isconfigured to provide a voltage waveform for the first ac network and thefirst energy storage system is configured to operate an active filterremoving harmonics in the waveform. This may involve the first controlunit controlling the first voltage source converter to provide the voltagewaveform for the first ac network and the first energy storage system to operate an active filter removing harmonics in the waveform. lO It is in a similar way possible that the second voltage source converter isconfigured to provide a voltage waveform for the second ac network andthe second energy storage system is configured to operate an active filterremoving harmonics in the waveform. This may involve the second controlunit controlling the second voltage source converter to provide the voltagewaveform for the second ac network and the second energy storage system to operate an active filter removing harmonics in the waveform.

According to yet another variation, the first energy storage system isconfigured to act as a power source during black start of the first acnetwork and the first voltage source converter. It may also be configured toact as a power source for the black start of the second voltage sourceconverter. In this case the first control unit may be configured to controlthe first energy storage system to act as a power source during black startof the first ac network and the first voltage source converter as well aspossibly to act as a power source for the black start of the second voltage SOUTCC COIIVCITCI' According to another variation there is a first transformer having aprimary side connected to the first ac side of the first voltage sourceconverter and a secondary side for connection to the first ac network,where the first transformer is set to adapt the voltage level of the first acside of the first voltage source converter to the voltage level of the first acnetwork. The first energy storage system may additionally be connected inshunt with the first voltage source converter via a set of auxiliary windingsof the first transformer. As an alternative it may be shunt-connected at apoint between the primary side of the first transformer and the first ac sideof the first voltage source converter. As yet another alternative it may beshunt-connected at a point between the secondary side of the first transformer and the first ac network. lO In a similar fashion there may be a second transformer having a primaryside connected to the second ac side of the second voltage source converterand a secondary side for connection to the second ac network, where thesecond transformer is set to adapt the voltage level of the second ac side ofthe second voltage source converter to the voltage level of the second acnetwork. The second energy storage system may additionally be connectedin shunt with the second voltage source converter via a set of auxiliarywindings of the second transformer. As an alternative it may be shunt-connected at a point between the primary side of the second transformerand the second ac side of the second voltage source converter. As yetanother alternative it may be shunt-connected at a point between the secondary side of the second transformer and the second ac network.

It is also possible that if the first ac network delivers overload powerintended for the second ac network, the first energy storage system isconfigured to draw the overload power from the first ac side of the firstvoltage source converter and the second energy storage system isconfigured to supply the overload power to the second ac side of thesecond voltage source converter. This may involve the first control unitcontrolling the first energy storage system to draw the overload powerfrom the first ac side of the first voltage source converter and the secondcontrol unit controlling the second energy storage system to supply theoverload power to the second ac side of the second voltage source COIIVGITCT.

The first ac network may be a part of a first ac grid and the second acnetwork may be a part of a second ac grid. Alternatively the first acnetwork and the second ac network may be two parts of one grid. They may thus both belong to one and the same ac grid.

The present invention has a number of advantages. This coordinatedoperation of the first energy storage system and the first voltage source converter may be beneficial to optimize the control actions and minimize lO the converter station cost. The converter rating may also be allowed to belowered. The dynamic performance of the dc system may additionally be improved.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will in the following be described with reference being made to the accompanying drawings, where fig. 1 schematically shows a first embodiment of a power supportingarrangement comprising a first voltage source converter, a second voltagesource converter, a DC link and a first and a second energy storage system,where the energy storage systems are connected in a first way to thevoltage source converters, fig. 2 schematically shows a second way of connecting the first energystorage system to the first voltage source converter, fig. 3 shows a third way of connecting the first energy storage system to thefirst voltage source converter, fig. 4 schematically shows a frequency dip in the frequency of an ac gridand ways to handle this frequency dip, fig. 5 schematically shows the required active power for providing virtualinertial power for different inertial constants as a function of ROCOF atthe PCC, fig. 6 schematically shows required energy storage for proving virtualinertial power for different inertial constants as a function of maximumfrequency deviation, fig 7a shows an electrical model of a synchronous machine, and fig. 7b shows an electrical model of a voltage source converter.

DETAILED DESCRIPTION OF THE INVENTION In the following, a detailed description of preferred embodiments of the invention will be given. lO Fig. 1 shows a first voltage source converter (VSC) 10 connected to asecond voltage source converter (VSC) 12 via a direct current (dc) link 14comprising a first pole P1 and a ground connection GR. The first VSC 10has a first dc side connected to a first end of the dc link 14, while thesecond VSC 12 has a second dc side connected to a second end of the dclink 14. The first VSC 10, the second VSC 12 and the dc link 14 may beincluded in a direct current (dc) system, which may be a high voltage directcurrent (HVDC) system. The first VSC 10 has a first alternating current(ac) side which is connected to a first ac network 20 via a first transformerT1 and the second VSC 12 has a second ac side connected to a secondalternating current (ac) network 22 via a second transformer T2. The firsttransformer T1 has a primary side with a number of primary windings eachconnected to a corresponding phase of a phase leg of the first VSC 10 and asecondary side with a corresponding number of secondary windings eachconnected to a corresponding phase of the first ac network 20. Thereby theprimary side of the first transformer T1 is connected to the first ac side ofthe first VSC 10. In a similar manner the second transformer T2 has aprimary side with a number of primary windings each connected to acorresponding phase of a phase leg of the second VSC 12 and a secondaryside with a corresponding number of secondary windings each connectedto a corresponding phase of the second ac network 22. The firsttransformer T1 is set to adapt a voltage level of the first ac side of the firstVSC 10 to a voltage level of the first ac network 20, while the secondtransformer T2 is set to adapt a voltage level of the second ac side of the second VSC 12 to a voltage level of the second ac network 22.

There is also a first energy storage system 16 comprising a number ofenergy storage elements and a first further VSC. The energy storageelements may as an example be batteries that are connected in strings,which strings may be connected in parallel with a dc side of the firstfurther VSC. The first energy storage system 16 may additionally comprise switches for selectively switching strings and individual energy storage lO ll elements of such strings across the dc side of the first further VSC. Thefirst energy storage system 16 is connected to the first ac side of the firstVSC 10. There is here also a second energy storage system 18 comprisingenergy storage elements connected to a second further VSC in the sameway as in the first energy storage system 16. The second energy storagesystem 18 is connected to the second ac side of the second VSC 12. As canbe seen in fig. 1 the first energy storage system 16 is connected to a numberof (three) auxiliary windings of the first transformer T1, while the secondenergy storage system 18 is connected to a number of (three) auxiliary windings of the second transformer T2.

The first VSC 10, the first transformer T1 and the first energy storagesystem 16 may here be provided in a first converter station 28, while thesecond VSC 12, the second transformer T2 and the second energy storagesystem 19 may be provided in a second converter station 30, where the dc link 14 interconnects the converter stations 28 and 30.

The dc link 14 may comprise at least one dc pole P1 and optionally also aground connection GR. In yet some further variations it may additionally comprise a second pole (not shown).

The first VSC 10 may be realized as a modular multilevel converter (MMC)or a two-level converter. The second VSC 12 may likewise be a VSC in theform of an MMC or a two-level converter. However, other types of VSCs are also contemplated.

In the example shown in fig. 1, both the first and second VSC 10 and 12 isan MMC. The MMC is in this case made up of a number of parallel phaselegs, where each phase leg comprises a number of cascaded submodules. Asubmodule may in turn have a bipolar voltage contribution capability, forinstance realized as a full-bridge submodule, or have unipolar voltagecontribution capability, for instance realized as a half-bridge submodule. A submodule therefore comprises at least one energy storage element and a lO 12 number of switches configured to insert the energy storage element withone out of a maximum of two different polarities in a phase leg or tobypass the energy storage element. The energy storage element may withadvantage be a capacitor. Thereby each submodule also has a submodulevoltage. This submodule voltage is inserted into the phase leg or bypassed in order to form a waveshape.

In one version of the first and second converters 10 and 12, each phase legis made up of submodules with bipolar voltage contribution capability. Inanother variation each phase leg is made up of submodules with unipolar voltage contribution capability. In other variations each phase leg is madeup of a mixture of submodules with unipolar and bipolar voltage contribution capabilities.

The phase legs are also divided into an upper phase arm and a lower phasearm, where the upper phase arm comprises a number of submodules SMUconnected to a phase leg midpoint via an upper arm reactor Lu, while thelower phase arm comprises a number of submodules SML connected to the phase leg midpoint via a lower arm reactor Ll.

In the first and second converters 10 and 12, the phase legs are thusconnected in parallel between the first pole P1 and ground GR.Furthermore, the midpoint of a phase leg forms an ac terminal forconnection to a corresponding ac phase of the first and second ac networks and 22, which are three-phase ac networks.

Although fig. 1 shows two energy storage systems it is possible to omit one,such as the second energy storage system 18. Furthermore, the first VSC 1oand the first energy storage system 16 may be included in a powersupporting arrangement, which may be a power supporting arrangementfor one or both of the AC networks 20 and 22. In case the second energystorage system 18 is present, also the second energy storage system 18 and the second VSC 12 may be included in the power supporting arrangement. lO 13 In the latter case it is also possible that the dc link 14 is a part of the powersupporting arrangement. The first and second transformer T1 and T2 mayalso be included, where the second transformer may be omitted in casethere is no second energy storage system in the power supportingarrangement. In case there is no second energy storage system, it isadditionally possible that the first energy storage system 16 and the secondVSC 12 form the power supporting arrangement. In the example shown infig. 1, the dc link 14, the first and second VSCs 10 and 12 and the first andsecond energy storage systems 16 and 18 may therefore together form thepower supporting arrangement. Also the first transformer T1 and thesecond transformer T2 may be included in the power supporting arrangement.

The power supporting arrangement provides power support of one or bothof the networks 20 and 22. In this support the first energy storage system16 is jointly configured with the first voltage source converter 10 to supportan ac network, which in the example in fig. 1 is the first ac network. In thesame example of fig. 1, the second energy storage system 18 is jointlyconfigured together with the second VSC 12 to support the second ac network 22.

In order to perform the support there is a first control unit 24 controllingthe first VSC 10 and the first energy storage system 16 and optionally also asecond control unit 26 controlling the second VSC 12 and the secondenergy storage system 18. The control of an energy storage system istypically carried out through controlling the included further VSC. If a switching arrangement is present, this may also be controlled.

As can be seen above, there are a number of variations that may be madeof the power supporting arrangement. There are also several ways in which an energy storage system may be connected to a corresponding VSC. lO 14 As can be seen in fig. 1 each energy storage system is connected to a set ofauxiliary windings of a transformer used to adapt a voltage of thecorresponding dc system to the voltage of the connected ac network. Thesetransformers are the first and second transformers T1 and T2. Throughthis type of connection, the energy storage system is connected in shuntwith the ac side of the corresponding dc system VSC, i.e. the VSC of the dcsystem 14. The first energy storage system 16 is thus connected in shuntwith the first VSC 10 via a set of auxiliary windings of the first transformerT1 and the second energy storage system 18 is connected in shunt with the second VSC 12 via a set of auxiliary windings of the second transformer T2.

Fig. 2 shows another way in which the first energy storage system 16 maybe connected to the first converter 1o. In this case the first energy storagesystem 16 is connected to an internal AC bus via a first supportingtransformer ST1, where the internal ac bus stretches between the first ACside of the first VSC 10 and the primary windings of the first transformerT1. The first energy storage system 16 is thus connected to a secondary sideof the first support transformer ST1, the primary side of which isconnected to the internal ac bus. Put differently, the first energy storagesystem 16 is shunt-connected at a point between the primary side of the first transformer T1 and the first ac side of the first VSC 1o.

Fig. 3 shows a third way in which the first energy storage system 16 may beconnected to the first VSC 1o. In this case it is again connected to thesecondary side of the first support transformer ST1. However, the primaryside of the first support transformer ST1 is in this case connected to thesecondary side of the first transformer T1. The support transformer ST1 isthus connected to a bus running from the secondary side of the firsttransformer T1 to the first AC network 20. Put differently, the first energystorage system 16 is shunt-connected at a point between the secondary side of the first transformer T1 and the first ac network 20. lO It can in this way be seen that the first energy storage system 16 isconnected in shunt with the AC side of the first VSC 10 in all the different ways of connecting it.

When there is provided a second energy storage system 18, it is withadvantage connected in the same way in relation to the second VSC 12, ashas been described above in relation to the first energy storage system 16and the first VSC 1o.

The power supporting arrangement is provided for supporting a first acgrid connected to the dc system, where the first ac grid may be a grid madeup of the first ac network 20. However, the first ac grid may also comprisethe second ac network 22. The first and second ac networks 20 and 22 maythereby be two parts of the same synchronous ac grid. When this is thecase the dc system may be a so-called embedded system. Anotherpossibility is that the second ac network 22 is part of a second ac grid. Inthis case the power supporting arrangement may also be provided to support the second ac grid.

In such an ac grid there may be a high degree of non-synchronous generators (NSGs), such as wind farms and solar panels.

The support may be performed using coordinated control of the first VSC10 and the first energy storage system 16, which coordinated control is carried out by the first control unit 24.

In such support, it is possible that the first VSC 10 operates in a powersynchronization control (PSC) mode, while the first energy storage system16 is controlled as a virtual synchronous machine in order to provideinertial support. This means that the power delivered by the first VSC 10 toor from the first ac grid is coordinated with the power delivered by the firstac grid from or to the dc system, while the first energy storage system 16 provides inertial support through being operated as a virtual synchronous lO 16 machine. The first energy storage system 16 is thereby configured toemulate the synchronous machine alone. Thereby the first VSC 10performs PSC control, while the first energy storage system 16 performsvoltage control. It can furthermore be seen that the first energy storagesystem 16 when being jointly configured together with the first voltagesource converter 10 to support an ac grid is configured to provide power tothe supported ac grid corresponding to inertia of a virtual synchronousmachine. It can in this case also be seen that the first voltage sourceconverter 10 in this joint control is configured to perform power synchronization control.

As an alternative it is possible that the coordinated control is such that thefirst energy storage system 16 and the first VSC 10 are jointly controlled asa virtual synchronous machine in order to support the first ac grid. Thefirst energy storage system 16 and first voltage source converter 10 are thereby together configured to emulate the synchronous machine.

When supporting an ac grid, the first energy storage system 16 and the firstVSC 10 may be jointly configured to stabilize the frequency of this ac grid.

A discussion regarding support for such increasing of frequency stability ofan ac grid will now be given with reference being made to fig. 4 - 6, wherefig. 4 shows a frequency dip in the system frequency of an ac grid and waysto handle this frequency dip as an illustration of frequency stabilitychallenges in a grid with a high penetration of non-synchronousgenerators. Fig. 5 shows required active power change (in per cent ofstation rating Sb) to supply virtual inertia with an inertia constant as asynchronous generator with the same rating as a function of rate of changeof frequency (ROCOF) at the point of common coupling (PCC), where thepoint of common coupling is the node in the ac grid to which thearrangement is power supporting arrangement is connected. Fig. 5 alsoshows a grid goal. This goal refers to the system frequency (and is the ROCOF below which a grid being run by a grid operator should operate. lO 17 The system frequency refers to the mass-weighted average of the nodefrequencies in the grid. Mass refers here to the rotating inertia of thesynchronous machines connected at the different nodes. Fig. 6 showsrequired energy storage (in seconds of supply at P=Sb) to supply virtualinertia with an inertia constant as a synchronous generator with the same rating as a function of maximum deviation of the PCC frequency.

One key requirement for such support may be the requirement forprovision of virtual inertia to improve frequency stability in a grid withhigh NSG penetration. High penetration of wind/ solar and lesssynchronous generators results in less grid inertia and larger grid frequency variations during power imbalances as is illustrated in fig. 4.

In a grid with low inertia, the ROCOF after a major loss of generation maybe high. If turbine governors of synchronous generators in the grid areunable to respond quickly enough, the nadir frequency will be too low andload shedding schemes may be set in operation causing blackouts in someparts of the grid. To improve the situation, grid-connected converters suchas wind and solar converters as well as HVDC links, may in the future berequired to provide synthetic inertia. Furthermore, also requirements onconverters to mimic synchronous machines in their control, so calledvirtual synchronous machine (VSM) control, may become required toresolve many of the issues associated with grid with a high penetration ofNSG. This requirement may also be combined with a requirement ofemulating a certain machine inertia. Such requirements may lead to a needfor energy storages to be connected to the converters. In the special case ofan HVDC link connected between two asynchronous ac systems, the powerrequired for provision of synthetic inertia (with or without VSM controlrequirement) may be sourced from the other grid provided that frequencyevents do not occur simultaneously in both interconnected grids. However,in the case of an embedded HVDC link which connects different parts of asingle synchronous ac grid, additional energy storage will also likely be required in the HVDC case. lO 18 A synchronous machine outputs additional active power when thefrequency drops since rotational energy of the machine and turbine is lowered and the surplus is converted to electrical energy.

The electro-mechanical equation of a single synchronous machine (SM) can be written Pm - P, = wmJ dwm (1)d: Here, J is the moment of inertia [kg*m2], (om is the rotational speed of themachine [rad/s], Pm is the supplied mechanical power from the turbine[W] and Pe is the output electrical power [W]. The inertia constant H [s] is furthermore defined as _ E, _ JwmozSB ZSB H (2) Here, Ef is the rotational energy in the machine [J ], (om is the nominalrotational speed of the machine [rad/s], and SB is the [MVA] rating of themachine. Common values of H for different types of synchronousgenerators (including turbines) are 2-8 seconds (lower for hydro and higher for steam turbines).

Based on Eqs. (1) and (2), the machine response to a change in gridfrequency can be calculated to better understand the requirements of aconverter mimicking a synchronous machine either by virtual inertiaemulation using regular dq-current control or by VSM control. In Figure 5,the active power output change of a synchronous machine during afrequency change is plotted as a function of the machine ROCOF. Notethat this very basic calculation does not take into account differences in thelocal machine ROCOF and the system ROCOF which will always be there lO 19 in a multi-machine system. Depending on power oscillations in the grid,the frequency at different nodes in the grid will oscillate around theaverage frequency. Thus also the ROCOF will depend on the location in thegrid and oscillate around the system ROCOF.

From Figure 5, it is seen that the active power provision as calculated fromEqs. (1) and (2) is proportional to H and the ROCOF. The results may alsoprovide an indication of the required additional rating of a converterproviding virtual inertia. Assume for example that the converter isoperating at 1 p.u. active power provision. If it is assumed that the localROCOF in the grid always stays below 0.5 Hz/ s, and the virtual inertiaconstant of a converter emulating a synchronous machine is set to 5seconds, this would require an overrating of 10 % of the converter since thepower output would have to increase with 10 % in the event of a rapid frequency decay to provide inertial support.

The required energy for the inertial response is calculated based on thesame equations and plotted in Figure 6. It can be seen that the requiredenergy for the inertial response is proportional to H and the maximumfrequency deviation in the grid (which defines the lowest speed of themachine and thus the energy deviation from the rotational energy atnominal speed). This may also provide an indication of the requiredadditional energy storage requirements of a converter providing virtualinertia. Assume that the maximum frequency deviation at the node wherethe converter is connected is in all cases less than -1 Hz and the virtualinertia constant of the converter is set to 5 seconds. From the figure it canbe concluded that the required energy storage is E/ SB=o.2 s. For a 500MVA converter this would equal to 500 MVA*o.2=1oo MJ. This amount ofenergy needs to be delivered to the grid before the frequency nadir which may occur a few seconds after the loss of generation. lO Note that this inertial energy output would have to be supplied to the ACgrid in addition to the normal energy output from the converter which istransmitted through the HVDC link during the event.

Today there is no general grid code requirement for inertial support forHVDC. However, the recent grid code proposal from national gridoperators such as National Grid of the United Kingdom and recentImplementation Guideline Documents (IGDs) from European Network ofTransmission System Operators (ENTSO-E) make it plausible that such arequirement will exist in the future. As is well-known a grid code is atechnical specification defining the parameters a facility connected to apublic electric system has to meet in order to ensure safe, secure andeconomic proper functioning of the electric system. Furthermore, there arealso indications that significant overload capability of grid-connectedconverters will be required in the future. As HVDC converters are largeand have a large impact on the grid, it is likely that the new requirementswill come earlier for HVDC converters than for other smaller converters.Today, many HVDC links provide frequency control functionality (such asAPzk*Af) , but few (if any) provide inertial support (such as APzk*dAf/ dt).In order to affect the system frequency in a grid, active power must beprovided from an external source such as another grid or an energy storage unit.

If the VSC is the sole source of such support, then a marginal inertial support for small frequency deviations can be provided.

With the expected requirements on inertial support for HVDC convertersin embedded connections additional energy storage units may therefore berequired to be connected to the stations. This problem can be addressed by connecting an energy storage system to a converter.

With an energy storage system in place connected at the HVDC converter station, several interesting applications exist. The primary application may lO 21 be the above-described virtual inertia support from the energy storage system.

The operation of such virtual inertia support will in the following be described in some more detail.

In case the first ac network 20 needs support, for instance because thefrequency drops too fast or drops below a minimum frequency deviationthreshold, then the first control unit 24 may coordinate the control of thefirst energy storage system 18 and the first VSC 10. It may moreparticularly control the first energy storage system 16 to provide virtualinertial support, which may be done through controlling the first energystorage system 18 to emulate a synchronous machine through injectingpower into the ac connection between the first VSC and the first acnetwork corresponding to the needed virtual inertia while at the same time controlling the first VSC 10 to operate in PSC control mode.

This coordination may be beneficial to optimize the control actions and minimize the station cost.

To provide virtual inertia through controlling the first energy storagesystem 16 as a separate unit to provide virtual inertia may be one optionbut coordinative PQ control between the first VSC 10 and the first energystorage system 16 may enable a better control result and an even more cost-efficient solution.

In this case the combination of first energy storage system 16 and first VSC10 may be controlled to emulate a synchronous generator. Thereby theyare together configured to emulate the synchronous machine, where thefirst energy storage system 16 emulates the mass-mechanical dynamics ofsuch a synchronous machine providing virtual inertial while the first VSC emulates the electrical dynamics of the synchronous machine. lO 22 The first control unit 24 may additionally do this using a mapping of anelectrical model of a synchronous machine onto an electrical model of thefirst VSC 10 and through applying the network voltage E and obtains acontrol signal through applying the network current and the networkvoltage in this mapped model. This emulation may be the emulation described in WO 2010/ 022766, which is herein incorporated by reference.

The emulation in the first VSC 10 may more particularly involve the following operation.

In the synchronous dq reference frame, the electrical dynamics of a synchronous machine are given by: Lqledis/dt = GM - VS - (RR +jc01LG)i5 + (RR -jwlLMﬁM (3) diM/dï = (RR/LM)*(is - iM) (4) where is is a stator current vector, eM a back emf (electromotive force)vector, i.e. a magnetizing voltage, vs a stator voltage vector, iM amagnetizing current vector, L., the total (stator and rotor) leakageinductance, LM the magnetizing inductance and m1 the angular line frequency.

An equivalent electric circuit corresponding to equation (3) above is shownin fig. 7a, which shows from right to left a rotor branch including a currentdependent voltage source jwlLMiM and the rotor resistance RR. This rotorbranch is in turn connected in parallel with a magnetizing branchincluding the magnetizing inductance LM through which the magnetizingcurrent iM is running. A voltage source providing the back emf eM is at oneend connected to a first connection point interconnecting the parallelmagnetizing and rotor branches and at another end connected to a first end of the total leakage inductance LG, through which the stator current is lO 23 is running. The stator voltage vs is finally provided between a second endof the magnetizing inductance LM and a second connection point interconnecting the parallel magnetizing and rotor branches.

In case the VSC is provided with a pure inductive filter facing the networkand having inductance L, the dynamics of the converter current are givenby: Ldi/dt = V - E -jcolLi (5) where i is a converter current vector, v is a converter voltage vector, E is anetwork voltage vector, L the filter inductance and m1 the angular line frequency.

An equivalent electric circuit corresponding to equation (5) above is shownin fig. 7b, which shows from right to left a voltage source providing avoltage v and at a first end being connected to a first end of the filterinductance L, through which the converter current i is running. Thenetwork voltage E is here provided between a second end of the filter inductance L and a second end of the voltage source v.

Through comparing the two circuits in fig. 7a and 7b it can be seen that ifthe stator current is is mapped onto the converter current i, i.e. that theconverter current i is set equal to the stator current is, and the statorvoltage vs is mapped onto the network voltage E, while the leakageinductance LG is set equal to the filter inductance L, the converter voltage vin fig. 7b corresponds to the back emf eM plus the voltage across themagnetizing branch (including the magnetizing inductance LM) and therotor branch (including the rotor resistance RR and the current dependentvoltage source jcolLMiM) in fig. 7a. This opens up the possibility of emulating the electrical dynamics of the synchronous machine with the lO 24 VSC by selecting the converter voltage reference, i.e. the control signal for the voltage converter, as: VREF = GM -RRi + (RR -jcolLMﬁM (6)and withdiM/dt = (RR/LM)*(i - iM) (7) Here VREF, eM, i and iM are variable vectors, while RR, m1 and LM areconstant. RR and LM may furthermore be chosen freely. Provided that v =VREF, which can be assumed if the control device has a negligible time delayand operates in its linear region, i.e. that overmodulation is avoided, the electrical dynamics of the synchronous machine are thereby emulated.

Through equation (7) the differential equation (4), which sets out therelationship between the stator current is and the magnetizing current iMthrough the magnetizing inductance LM, is applied on the convertercurrent i. Through equation (6) the control signal VREF is furthermoreobtained through combining three terms, which combining here includessumming up these three terms. The three terms are here a first term (-RRi) that is dependent on the converter current i and is here amultiplication of this current with a first factor -RR, a second term [(RR -jw1LM)iM] being dependent on the magnetizing current iM and here amultiplication of the magnetizing current iM with a second factor (RR -jwlLM) as well as a third term eM representing the variable back emf, i.e.representing the back electromotive force of the synchronous machine model.

These two equations (6) and (7) may in one embodiment of the present invention be all that is used for controlling a VSC. lO The emf voltage term may be provided as a variable that is dependent onthe network voltage E and more particularly as a variable that isdependent on a difference between a network voltage reference and thenetwork voltage. This difference may then be provided as the differencebetween the network voltage reference and the absolute value of thenetwork voltage. This difference may furthermore be amplified with a gain as well as possibly high-pass filtered.

An exciter that provides such an emf term may then provide it according to: GM = [KE/(1+STE)l*(EREF-| El) (8) where KE is the exciter gain, TE is the exciter time constant and EEEE is anetwork voltage reference vector. Both the gain and time constant mayhere be set freely. The gain may for instance be set to a gain of above one,involving amplification, of one, involving no amplification or between oneand zero, involving an attenuation. If the gain KE is selected as a realnumber, the dq frame reference will be approximately aligned with thenetwork voltage. Here a high exciter gain may with advantage be chosen,in the region of ten times the normalized network voltage, while the time constant may have a value of a few hundred ms.

In this way the electrical dynamics of a synchronous machine are emulatedand used for controlling a VSC, where as was discussed above the firstcontrol unit 24 controls the first energy storage system 16 to emulate themass mechanical dynamics based on equations (1) and (2) discussedabove. The control using the emulation of the electrical dynamics of asynchronous machine performed by the first VSC 10 may here be donetogether with regular PQ control. The voltage VREE may thus be added to a voltage reference used for regular PSC control. lO 26 The first VSC 10 and the first energy storage system 16 are thus jointlycontrolled to emulate a synchronous machine supporting the first acnetwork 20, where the inertia required for supporting the first ac network may then be provided by the first energy storage system 16.

The control may thus be a control in order to the support the first acnetwork 20, in which case the first VSC and first energy storage systemmay be controlled to perform the support. In case the second networkneeds support, it is instead possible that the second VSC 12 and the secondenergy storage system 18 are controlled in a similar way by the secondcontrol unit 26. It can in both the above-discussed cases be seen that thesupport can be achieved without using the dc link, which allows the converter rating to be lowered.

Moreover, in case the second ac network 22 needs support and there is nosecond energy storage system 18, then the first VSC 10 and the first energystorage system 16 may be used to provide this support. As an alternative,when the power supporting arrangement lacks the second energy storagesystem 18, it is possible that the first energy storage system 16 and thesecond VSC 12 are jointly controlled to provide the support of the second ac network 22.

As stated earlier, the primary application may be the virtual inertiasupport from the energy storage system. However, several otherapplications of coordinated VSC and energy storage system operation exist. Some possible applications are listed below.

The support of a network may also or additionally generally involveproviding coordinated reactive power support to a connected ac network.If this is done using the first energy storage system 16, the ratingrequirements on the first VSC 10 may be lowered. Also in this case the firstenergy storage system 16 and first VSC 10 may be jointly controlled to perform the reactive power support through jointly supplying reactive lO 27 power to the ac network 20, where again the first 16 energy storage systemmay or may not provide virtual inertia of a virtual synchronous machine and possibly both be controlled to emulate the synchronous machine.

In case there is a fault in the first ac network 20 it may additionally be ofinterest to support the fault current. When being jointly configured tosupport an ac network and if there is a fault in this ac network, the firstenergy storage system 16 and the first VSC 10 may thus be jointlyconfigured to provide a fault current to the ac network. In this support it ispossible that the first energy storage system 16 and first VSC 10 arecontrolled to share the current that must be injected into the network tobehave like a VSM. This control may involve controlling the first energystorage system 16 to provide the negative phase-sequence current and thefirst VSC 10 to provide the positive phase-sequence current. The firstenergy storage system 16 may thereby be configured to supply the negativephase sequence current and the first VSC 10 may be configured to supplythe positive phase sequence current of the fault current. Thereby thepositive-phase sequence voltage may be boosted and the negative-phasesequence voltage may be dampened. Here it may be mentioned that alsothe opposite operation is possible, i.e. that the first energy storage system16 is configured to supply the positive phase sequence current and the firstVSC 10 is configured to supply the negative phase sequence current of the fault current.

As is stated above, it seems likely that grid codes for large grid-connectedconverters will evolve with requirements for virtual inertia provision orvirtual synchronous machine emulation. Also requirements for short termoverload capability are currently being proposed. Assuming requirementson virtual inertia provision/VSM operation, embedded HVDC links willlikely need to install energy storage connected to the station(s). If this isdone, coordinated control of the energy storage system and the VSC of thedc system will likely be beneficial for improving the dynamic performance and for enabling a lower total system cost. lO 28 When two energy storage systems 16 and 18 are provided on opposite sidesof the dc link 14 of an embedded dc system, then these can be used for providing the virtual overload capability.

This type of control provides an overload capacity through the dc link 14without actually attempting to increase the rating of the first and secondVSCs 10 and 12.

If for instance excess power is to be transmitted from the first ac network20 to the second ac network 22, the first control unit 24 may control thefirst energy storage system 16 to receive the excess power and the secondcontrol unit 24 may control the second energy storage system 18 to deliverthe excess power to the second ac network 22, thereby a virtual power path is created between the two energy storage systems 16 and 18.

It can thereby be seen that if the first ac network 20 delivers overloadpower intended for the second ac network 22, the first energy storagesystem 16 may be configured to draw the overload power from the first acside of the first voltage source converter 10 and the second energy storagesystem 18 may be configured to supply the overload power to the second ac side of the second voltage source converter 12.

To enable a virtual power link for enhancing the overload capability of theHVDC link (e. g. 0.3 p.u. additional active power, for instance during 20 s)one of the energy storage systems may thus charge and the other onedischarge energy. From the grid point of view this will be seen as anincreased power ﬂow through the HVDC link. This naturally would onlywork as long as the charge state of the energy storage systems is neitherfull nor empty. Depending on the maximum energy content and state ofcharge of the energy storage systems chosen the time duration available for this functionality will vary. It may for instance be possible to make in 20 s. lO 29 As was discussed above, this type of ﬁinctionality may for example be usedfor transient stability improvement in an Embedded dc link application i.e.when the first and second ac networks are a part of the same ac grid. Thisis an interesting application for dynamic control of the dc link to enablehigher pre-fault power ﬂow across the power corridor and improves the transient angle stability.

Coordinated control for provision of short term overload power may alsoreduce the rating requirements for the VSC phase arms. System costsavings and performance improvements may thus be achieved if coordinated control as outlined herein is performed.

Another possible use of the power supporting arrangement is for blackstart. If for instance the first ac network 20 comprises wind turbines, it isadditionally possible that the first energy storage system 16 is used atpower up of the first AC network 20 in which wind turbines starts toproduce energy. The first energy storage system 16 can thereafter be usedto energize the first VSC 10 and possibly also the second VSC 12. It can beseen that it is in this way possible to use the first energy storage system 16for black start of both the AC networks and the DC system. The first energystorage system 16 may thereby be configured to act as a power sourceduring black start of the first ac network 20 and the first VSC 10 andoptionally also during black start of the second VSC 12. Naturally apossibly present second energy storage system 18 may also be provided for black start of the second VSC 12 and/ or the second ac network 22.

Since the first energy storage system 16 is connected in shunt with the ACside of the first VSC 10, it is additionally possible to control the first energystorage system 16 and the first VSC 10 in such a way that the performancecan be increased. The first VSC 10 may be controlled to provide awaveform and the first energy storage system 16 may be controlled to filterthis waveform. This may involve controlling the converter to output a two- level waveform or a quasi-two-level waveform and controlling the energy lO storage system to reduce the harmonics in this two-level waveform. Thisprinciple is of course not limited to a two-level waveform but can also beapplied on more advanced waveforms, such as those generated by MMCs.This has the advantage of allowing a lowering the required capacitor sizesused in the first VSC 10 to be made.

As can be seen above, the invention has a number of advantages. Thecoordinated control of the energy storage system and the first VSC isbeneficial for improving the dynamic performance of the dc system and forenabling a lower total system cost. Reduced valve rating possible of the dcsystem converters is also possible. Moreover, using the energy storagesystem will help the dc system to handle future grid code for fault current injection, e.g., negative sequence current and overload requirements.

Each control unit may be realized in the form of discrete components, suchas one or more Field Programmable Gate Arrays (FPGAs), ApplicationSpecific Integrated Circuits (ASICs) or Digital Signal Processors (DSPs).However, each control unit may also be implemented in the form of aprocessor with accompanying program memory comprising computerprogram code that performs the desired power supporting control functionality when being run on the processor.

From the foregoing discussion it is evident that the present invention canbe varied in a multitude of ways. It shall consequently be realized that the present invention is only to be limited by the following claims.

Claims

1. A power supporting arrangement comprising a first voltagesource converter (10) and a first energy storage system (16), said firstvoltage source converter having a first alternating current, ac, side forconnection to a first ac network (20) and a first direct current, dc, side forconnection to a first end of a dc link (14), said dc link having a second endfor connection to a second dc side of a second voltage source converter(12), the second voltage source converter (12) in turn having a second acside connectable to a second ac network (22), wherein the first energystorage system (16) is connected to the first ac side of the first voltagesource converter (10) and being jointly configured with the first voltage source converter (10) to support one of the ac networks.

2. The power supporting arrangement according to claim 1,wherein the first energy storage system (16) when being jointly configuredtogether with the first voltage source converter (10) to support one of theac networks is configured to provide power to the supported ac network corresponding to inertia of a virtual synchronous machine.

3. The power supporting arrangement according to claim 2,wherein the first voltage source converter (10) when being jointlyconfigured together with the first energy storage system (16) to supportone of the ac networks is configured to perform power synchronization control.

4. The power supporting arrangement according to claim 2 or 3,wherein the first energy storage system (16) when being jointly configuredtogether with the first voltage source converter (10) to support one of the ac networks is configured to emulate the synchronous machine alone.

5. The power supporting arrangement according to claim 2 or 3, wherein the first energy storage system (16) and first voltage source lO 32 converter (10), when supporting an ac network, are together configured toemulate the synchronous machine, where the first voltage source converter provides the electrical dynamics of the virtual synchronous machine.

6. The power supporting arrangement according to any previousclaim, wherein the first energy storage system (16) and the first voltagesource converter (10) when being jointly configured to support one of the ac networks are configured to stabilize the frequency of the ac network.

7. The power supporting arrangement according to any previousclaim, wherein the first energy storage system (16) and the first voltagesource converter (10) when being jointly configured to support one of theac networks are configured to, if there is a fault in said ac network, jointly provide a fault current to the ac network.

8. The power supporting arrangement according to claim 7,wherein the first energy storage system (16) is configured to supply thenegative phase sequence current and the first voltage source converter (10)is configured to supply the positive phase sequence current of the fault CUITCIIÉ OI' VlCC VCYSEI.

9. The power supporting arrangement according to any previousclaim, wherein the first energy storage system (16) and the first voltagesource converter (10) when being jointly configured to support one of theac networks are configured to jointly supply reactive power to said ac network.

10. The power supporting arrangement according to any previousclaim, wherein the first voltage source converter (10) is configured toprovide a voltage waveform for the first ac network (20) and the firstenergy storage system (16) is configured to operate as an active filter removing harmonics in the waveform. lO 33

11. The power supporting arrangement according to any previousclaim, wherein the first energy storage system (16) is configured to act as apower source during black start of the first ac network (20) and the first voltage source converter (10).

12. The power supporting arrangement according to any previousclaim, further comprising a first transformer (T1) having a primary sideconnected to the first ac side of the first voltage source converter (10) and asecondary side for connection to the first ac network (20), said firsttransformer (T1) being set to adapt the voltage level of the first ac side ofthe first voltage source converter (10) to the voltage level of the first ac network (20).

13. The power supporting arrangement according to claim 12,wherein the first energy storage system (16) is connected in shunt with thefirst voltage source converter (10) via a set of auxiliary windings of the first transformer (T1).

14. The power supporting arrangement according to claim 12,wherein the first energy storage system (16) is shunt-connected at a pointbetween the primary side of the first transformer (T1) and the first ac side of the first voltage source converter (10).

15. The power supporting arrangement according to claim 12,wherein the first energy storage system (16) is shunt-connected at a pointbetween the secondary side of the first transformer (T1) and the first ac network (20).

16. The power supporting arrangement according to any previousclaim, wherein the ac network supported by the first voltage sourceconverter (10) and first energy storage system (16) is the first ac network(20) and the power supporting arrangement further comprising the second voltage source converter (12) and a second energy storage system (18) lO 34 connected to the second ac side of the second voltage source converter(18), said second energy storage system (18) being jointly configuredtogether with the second voltage source converter (12) to support the second ac network (22).

17. The power supporting arrangement according to claim 16,wherein if the first ac network (20) delivers overload power intended forthe second ac network (22), the first energy storage system (16) isconfigured to draw the overload power from the first ac side of the firstvoltage source converter (10) and the second energy storage system (18) isconfigured to supply the overload power to the second ac side of the second voltage source converter (12).

18. The power supporting arrangement according to any previousclaim, wherein the first ac network (20) is a part of a first ac grid and the second ac network (22) is a part of a second ac grid.

19. The power supporting arrangement according to any of claims 1- 17, wherein the first ac network (20) and the second ac network (22) are two parts of one ac grid.