WO2018215071A1

WO2018215071A1 - Energy storage system

Info

Publication number: WO2018215071A1
Application number: PCT/EP2017/062699
Authority: WO
Inventors: Nicklas Johansson; Tomas Tengner
Original assignee: ABB Schweiz AG
Current assignee: ABB Schweiz AG
Priority date: 2017-05-25
Filing date: 2017-05-25
Publication date: 2018-11-29
Anticipated expiration: 2019-11-25

Abstract

The present invention discloses a method for providing an energy storage to an electrical power grid. The method comprises connecting an energy storage system (ESS), in series with a transmission line of an electrical power grid, or in series with a shunt unit connected to the transmission line, the energy storage comprising an energy source connected to the transmission line or the shunt unit via an H-bridge connection, measuring an AC grid frequency in the electrical power grid, discharging the ESS when the measured AC grid frequency drops a predetermined magnitude from a normal condition, and charging the ESS when the measured grid frequency is back to normal condition. An energy storage and a series energy storage are also presented.

Description

ENERGY STORAGE SYSTEM

TECHNICAL FIELD

The invention relates to a method for providing an energy storage for an electrical power grid, an energy storage system, and a series energy storage system thereof.

BACKGROUND

In electrical power grids of today, a dramatic increase in the amount of renewable energy sources (RES) is seen. This leads to increased challenges for utilities, transmission system operators (TSOs) and distribution system operators (DSOs). One reason for this is the non-dispatchable nature of the RES. Another reason is their impact on the grid voltage and frequency stability. RESs do generally not automatically provide the grid with inertia like power sources connected through synchronous generators do. A sufficient amount of inertia in the grid is important since more inertia lowers the rate of change of the AC frequency (ROCOF) in the grid in the event of a power imbalance and thus limits the maximum deviation in the grid frequency before the operators participating in the frequency control of the grid have had time to act. This in turn reduces the risk for harmful effects like load shedding schemes going into operation. In Ireland where a very high penetration of RES is seen, a market mechanism for inertial response/fast frequency control has been proposed.

In some grid systems, wind turbines are required to be equipped with an inertial response scheme which acts if the frequency ventures outside of an allowed range. Such schemes use the rotational energy of the wind turbine to temporarily output more or less power (depending on the direction of the frequency change) to improve the frequency stability of the grid. It should be noticed that the rotational speed of the wind turbines will need to be restored after such an event. This would lead to a power output variation in the other direction after a number of seconds. Another way for the TSO to ensure a sufficient amount of inertia in an electrical power grid is to install synchronous condensers which provide inertia in the same way as regular synchronous machines do. However synchronous condensers are bulky and have relatively high idling losses. Alternatively, energy storage units like battery energy storage units can be deployed to provide synthetic inertia/ fast frequency control and more long- term AC frequency control. This is a small but growing application.

WO2013170899 describes a battery energy storage arranged to be connected to a capacitor link, which is connected in parallel to a power converter. SUMMARY

An object of the present invention is to enable improved inertia/fast frequency control and long-term frequency control for an electrical power grid.

According to a first aspect presented herein, a method for providing an energy storage to an electrical power grid is presented. The method comprises connecting an energy storage system, ESS, in series with a transmission line of an electrical power grid, or in series with a shunt unit connected to the transmission line, the energy storage comprising an energy source connected to the transmission line or the shunt unit via an H-bridge connection, measuring an AC grid frequency in the electrical power grid, discharging the ESS when the measured AC grid frequency drops a predetermined magnitude from a normal condition, and charging the ESS when the measured grid frequency is back to normal condition.

The total energy stored of the presented ESS can be used in the electrical power grid, since it is connected in series. Also, no transformer or DC/DC converter is required to connect the ESS to the electrical power grid. Further, thyristors can be used instead of e.g. IGBTs in the energy storages for the electrical power grids.

The method may further comprise bypassing the ESS when neither discharging nor charging is required. Each leg of the H-bridge connection may comprise a power conductor switch with anti-parallel diodes, to charge/discharge the ESS. Each power conductor switch may be a thyristor, an IGBT or a GTO.

The discharging may be maximized by closing tandem leg switches of the power conductor switches shortly after a current zero crossing of a current in the transmission line.

The discharging may be slowed by closing tandem leg switches of the power conductor switches delayed after a current zero crossing in the transmission line. The charging may be maximized by opening all leg switches of the power conductor switches.

The energy source may comprise one or more batteries, one or more supercapacitor, a combination of batteries and supercapacitors, or a combination of batteries, supercapacitors and normal capacitors. The ESS may comprise an energy storage connected in series, via an H-bridge connection, in each transmission line of a multi phase electrical power grid.

The method may further comprise connecting one or more further ESS in series with the ESS.

According to a second aspect, a series ESS is presented. The series ESS comprises an energy source having a positive connection point and a negative connection point, wherein the energy source has an energy time constant T of at least 200 ms, preferably at least 500 ms, and a power conductor switch connected to the positive or negative connection point of the energy source, wherein the power conductor is configured to vary the DC voltage level of the energy source.

The time constant T may be the energy stored in the energy storage divided with the maximum power rating of the ESS. The series ESS may further comprise a first power conductor switch connected between a first connection point of the series ESS and the negative connection point, second power conductor switch connected between the positive connection point and the first connection point, a third power conductor switch connected between a second connection point of the series ESS and the negative connection point, and a fourth power conductor switch connected between the positive connection point and the second connection point, wherein the series ESS thereby connects the energy source between the first and second connection points via an H-bridge, to provide active power between the first and second connection points.

The energy storage may be configured to vary the DC voltage level from 0.65 to 1.0 p.u., preferably from about o to about 1 p.u.

Each power conductor switch may comprise a thyristor and an antiparallel diode, an IGBT and an antiparallel diode, or a GTO and an antiparallel diode. The energy source may be a battery, a supercapacitor, or a combination of energy storage and normal capacitor.

The series ESS may comprise one or more further ESS in series with the ESS.

According to a third aspect, a series ESS for providing an energy storage to an electrical power grid is presented. The ESS comprises an energy source arranged to be connected, via an H-bridge, to a transmission line of an electrical power grid, at least four power conductor switches, with anti- parallel diodes, wherein each power conductor switch is connected to the energy source and the transmission line to form the H-bridge of the ESS, the ESS being configured to measure an AC grid frequency in the electrical power grid and to discharge the ESS when the measured grid frequency drops a predetermined magnitude, and to charge the ESS when the measured grid frequency increases a predetermined magnitude. Charging of the ESS may be used to mitigate a non-desired effect when the grid frequency goes higher than a nominal value, higher then by a

predetermined magnitude or outside a deadband.

The ESS may further comprise a controllable generator, SVC, STATCOM, tap-changer of a transformer, or high-voltage, direct current (HVDC) system, wherein the ESS and the generator, SVC, STATCOM, tap-changer of a transformer, or HVDC system are controlled coordinated.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described, by way of example, with reference to the accompanying drawings, in which:

Fig. l schematically illustrates a series energy storage system (ESS) according to an embodiment; Fig. 2 schematically illustrates SSSC operation vs one embodiment of series ESS operation;

Fig. 3 shows a diagram schematically illustrating required ESS mean power;

Fig. 4 shows two diagrams schematically illustrating active power output;

Fig. 5 schematically illustrates a test ESS; Figs. 6 and 7 show diagrams schematically illustrating discharge of ESS for case A; Figs. 8 and 9 show diagrams schematically illustrating discharge of ESS for case B;

Figs. 10 and 11 show diagrams schematically illustrating discharge of ESS for case C; Figs. 12 and 13 show diagrams schematically illustrating discharge of ESS for case D; and

Figs. 14-18 schematically illustrate embodiments of implementations of series ESS (SESS).

DETAILED DESCRIPTION

The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout the description.

Products for grid connected storages on the market are today generally shunt connected devices. An energy storage system (ESS) connected in series with a transmission line of an electrical power grid is presented herein.

The presented series ESS is in Fig. 1 shown for one phase of a transmission system (all phases have the same configuration of components). Fig. 1 illustrates one phase of an H-bridge converter circuit connecting an energy storage 5 in series with an AC transmission line 6. Each leg of the H-bridge has a power conductor switch and an antiparallel diode, 1-4, to connect the energy storage 5 to the transmission line 6.

The energy storage unit 5 is in the Fig. 1 shown as a supercapacitor but this may alternatively be a battery such as an electrochemical battery. The energy storage unit may further include a normal (non-storage) high voltage capacitor for filtering purposes. The illustrated configuration of a series ESS has some similarity in structure to a Static Synchronous Series Compensator (SSSC), which may be constructed with a cascaded H-bridge topology, but the basic operation is different, which is illustrated in Fig. 2. The use of thyristors as power conductor switches, instead of the IGBTs used in normal chain-link SSSC configuration, is also presented. During discharge of the series ESS, the thyristors may fundamentally be switched shortly after a line current zero crossing and this mode of operation is herein called basic operation.

Depending on the turn-on time (time from forward blocking mode to forward conducting mode) of the thyristors in the ESS, which may be controlled, the ESS may be used to interchange active and reactive power with the grid. This is done by injecting a square-wave voltage in series with the grid AC voltage by proper switching of the thyristors when the thyristors are blocked, and the ESS will be fully charged thorough the diode in the H-bridge configuration. The controllability may also be used to e.g. shift the phase of the AXJ vector.

The basic operation of the presented ESS is different from the SSSC as illustrated in Fig. 2. While the SSSC can only supply reactive power with the injected voltage AXJ in quadrature to the line current, the ESS will in basic operation mainly supply and consume active power with the injected voltage vector in parallel or anti-parallel to the line current. The injected power of the ESS will in basic operation thus be P¾3*AU_phase*Iiine with Auphase being the voltage of the supercapacitor/battery and lime the line RMS (root mean square) current. This voltage will vary during discharge and charge of the ESS. The power that can be drawn from or charged into the ESS is thus proportional to the line current and to the voltage of the energy storage unit. If the turn-on time of the thyristors is delayed, the discharge active power can still be controlled, but then the reactive power injection of the ESS also varies. The total energy stored in the energy store of the ESS can be used, since they can be completely discharged by the line current. In shunt connected solutions, normally only a part of the total energy stored can be used, since the energy storage can only be partly discharged via a DC/DC converter. For a voltage range between 50-100 % of the rating in a shunt example, the utilized energy is 75%. The ESS energy rating would be around 25 % less than the shunt storage energy rating.

A time constant T for the ESS is the energy stored (in Joule) in the energy store divided with the maximum power rating (in Watt) of the ESS. To be able to act as an energy store, and not only as a reactive compensator, the time constant T may be at least 200 ms, preferably at least 500 ms. The voltage level of the ESS may for a supercapacitor vary from about o to about 1 power unit (p.u.). The voltage variation for a battery may be less, about 0.65 to about 1.0 p.u.

The voltage magnitude across the energy store is restricted from the grid requirements for voltage variations and will thus be low compared to the line- line voltage. The current rating of the ESS however needs to be substantial to handle the full line current. Due to high current rating, the power rating of the device will still be substantial despite a low voltage rating.

No transformer is required to connect the ESS to the transmission line, even though topologies with transformers may be constructed if beneficial for other reasons.

A DC/DC converter is often required for shunt connected energy storages, which however is avoided with the presented ESS topology connected in series.

To reduce the cost and complexity of the ESS, thyristors may be used instead of e.g. IGBTs, which may be beneficial for losses, overload capability and price etc.

Some characteristics of the ESS are: 1. The discharge and charge speed of the ESS and thus the energy exchanged with the grid is dependent on the line current on the line where it is installed. Thus, the ESS may advantageously be installed on lines where the load is high and predictable when it is required to be used in order to have a large output power and thus fast discharge speed. For the frequency support

(inertial response) application, this may for example be on lines which have a high loading when renewable energy sources (RES) generation is high and the grid system has low inertia.

2. The discharge and charge of the ESS will be dependent on the charge-state of the energy storage. A high charge state/high DC voltage of the energy storage indicates a high possible charge/discharge speed (high active power input/output).

3. The voltage injected by the ESS will locally increase the line voltage during discharge and decrease the line voltage during charging. The possible voltage rating and power rating of the ESS is thus dependent on the acceptable voltage variation on the line where it is installed. This voltage magnitude impact of the ESS may be lowered by varying the turn-on time of the thyristors, thus interchanging reactive power with the grid.

Voltages in an electrical power grid vary significantly for different loading conditions. Each transmission system operator (TSO) has their own voltage tolerance band for which they call normal operation.

An acceptable voltage injection will thus be dependent on the grid and the location of the ESS within it. Voltage disturbance of the ESS in the grid may be balanced by the action of voltage regulators of generators and voltage- regulated transformers in the grid. Since the ESS will vary the voltage at its location it may also change the power which close-by voltage-dependent loads draw. This may counteract the energy input to the grid injected by the ESS. To avoid this, the ESS may be localized such that its influence of voltage-dependent loads is minimized, e.g. far away from such loads or close to generators which stabilize the voltage magnitudes at the load centres. Inertia application

To understand requirements for an ESS to be used to supply inertia/fast frequency control in an electrical power grid, it is useful to study a synchronous machine and its active power contribution when a power grid experiences a large frequency dip.

A simplified calculation may be made based on the rotational energy stored in a synchronous ιοοο MVA generator, with an inertia constant H=6 s, as shown in Fig. 3. The plot of required mean ESS power after fault until nadir, to replace inertial energy of a 1000 MVA synchronous machine, is shown. The required energy is shown for the two cases, 120 MJ and 240 MJ, respectively. The rotational energy of the machine, which is delivered to the grid when two different frequency dips occur, is illustrated. A dip of Af_max of -0.5 Hz, which is a common lower limit in many power grids, results in an rotational energy loss/energy output from the synchronous machine of 120 MJ. A dip of -1.0 Hz, which is a very large deviation, sometime observed in countries with inertia issues like Ireland, results in an energy output of 240 MJ from the machine.

To get the requirements on an ESS used to replace the inertial response of a machine providing an energy output of 240 MJ, the time it takes for the operators to respond in the grid is also needed, since this is the time the ESS has to act before the power balance is restored in the grid and the frequency reaches the minimum value, the nadir. This time varies depending on the type and settings of the operators as well as on the magnitude of the lost power input (power output of the tripped generation). In NORDEL grid this time is commonly 6-12 s for a large generator trip, but both larger and smaller values than this can be found. Fig.3 shows the required mean ESS power output from the time of the fault until the nadir to replace the inertial energy of the assumed machine. In other words, Fig. 2 gives an indication of the required power rating of an ESS to improve the frequency stability in the same way that a 1000 MVA machine with inertial constant H=6 s does. In times of decommissioning of synchronous generators the results can also be interpreted as an indication of the required ESS power rating needed to replace a large generator with regards to its inertial response. The ESS energy ratings are in this example 120 and 240 MJ for the two assumed maximum frequency deviations, respectively. The example indicates that a comparably small power rating is required for the ESS to compensate for the inertial response of a synchronous machine of a large power rating. However, the presented ESS will have a declining power output and thus the maximum power rating of the presented ESS be substantially higher than the mean power output during an inertial response event. To further understand the contribution of a synchronous machine to frequency stability, a simulation using the Nordic 32 grid is illustrated in Fig. 4. Fig. 4 shows the AC frequency and power output of a 1000 MVA, H=6 s synchronous generator after a large generator trip at t=2 s in the grid. A peak in the power output coinciding with maximum frequency derivative at about t=3 s is seen in Fig. 4. The magnitude of the peak (compared to the pre-fault value) is in the range of 50 MW. After t=3 s the output power declines until the operator response has caught up and after the nadir at about t= 5 s, the increase in output power can be fully attributed to the turbine governor response (increased power thrust from the synchronous generator). The inertia of the machine results in an energy demand to supply the increasing rotational energy during this time after the nadir until the frequency is stable again.

High load is in electrical power grids often associated with lowered voltages. The ESS should be placed on transmission lines where the load is significant to secure sufficient power/energy output during a frequency drop. This may e.g. be on transmission lines carrying large amounts of RES power at times when the penetration of RES is high and the inertia in the grid is thus low.

The discharge of a supercapacitor, from the above ESS presented in an inertia application, will be somewhat similar to the inertial response of a generator. In the ESS case, the output power would however peak initially and then decay as the capacitor is discharged. The comparison with a generator has focused on a drop in frequency, since it is the most common problem, e.g. due to a major generator trip. The ESS may however also be used to mitigate frequency variations during frequency peaks, for example due to major load trips. Verification - inertia application

The inertia boosting application presented herein has been simulated to verify the functionality of the ESS. The simulated test system is shown in Fig. 5·

The test system data is as follows. U=40okV (line-line). The ampere capacity of the 400 kV line is assumed to be 1.5 kA and the impedance of the transmission line is corresponding to an overhead line of about 50 km

(distance from generator to load). The ESS has C (per phase) = 1 F, with a supercapacitor voltage rating 11.5 kV (5% of the system line-phase voltage).

The line current (la, lb, Ic) is Iii_ne=i kA, and an inductive load with P =720 MW, and Q=30 MVAr.

The energy stored in the Supercapacitors is 200 MJ at rated voltage. When this total energy is released, it is equivalent of the inertial energy release of a 1000 MVA, H=6 s synchronous machine, when it experiences a frequency drop of about -0.85 Hz. For the response of an ESS to aid in reducing the maximum frequency drop in the grid, energy from the ESS has to be supplied to the grid before the turbine governors of power plants in the grid has obtained power balance in the grid (which happens at the nadir).

Variable names are shown in Fig. 5, except for the active and reactive power input of the ESS, which are AP=Pr-Ps and A=Qr-Qs. Four cases are simulated and verified, cases A, B, C and D.

Case A: Full discharge of the ESS - the thyristors are turned on (closing of power conductor switch), connecting the energy source to the transmission line, without delay at current zero crossings. The thyristors la, b, c/4a, b, c and 2a, b, c/3a, b, c operate in tandem to ensure that the inserted voltage vector is in phase with the line current vector. The results are illustrated in Figs. 6 and 7 for a 1 kA line current. The presented results are obtained assuming ideal thyristor devices. In practice, a certain amount of delay (in the order of about 1 ms) of the turn-on time of the thyristor device after current zero crossings will be required. This will yield a certain amount of reactance power interchange with the grid.

Case B: Full charge of the ESS - all thyristors are blocked (opening of switch) and the supercapacitors are charged through the remaining diode bridge. The results are illustrated in Figs. 8 and 9 for a 1 kA line current.

Case C: Bypass of the ESS - the thyristors la, 3a, lb, 3b, lc, 3c are

continuously turned on without delay at all current zero crossings. The results are illustrated Figs. 10 and 11 for a 1 kA line current. Case D: Slower discharge of the ESS - the thyristors are turned on with a 2 ms delay at the current zero crossings. The thyristors la, b, c/4a, b, c and 2a, b, c/3a, b, c operate in tandem. The results are illustrated in Figs. 12 and 13 for a 1 kA line current.

Case A - Fig. 6:

The active and reactive powers flowing into and out of the ESS are Ps, Qs, and Pr, Qr, respectively. The difference between these is the power supplied by the ESS, ΔΡ and AQ. In this case the receiving end active power is larger since the ESS is discharging. It can be seen that the output power of the ESS is decreasing with time as it is discharged. Qs and Qr are close to equal showing there is no significant reactive power contribution from the ESS. The output power of the ESS peaks in the range of 30 MW and then declines.

The RMS voltages at the sending and receiving end of the ESS are UsRMS and UrRMS. It can be seen that the ESS increases the line voltage by initially about 5 % which then declines as the ESS discharges. In a normal grid with generators at both ends, the voltage at the receiving end will be well controlled by the generator's automatic voltage controllers (AVR). The loads at the receiving end will thus likely not see a significant voltage variation when the ESS is triggered (apart from a possible initial transient). However, the voltage variation caused by the device may impact loads close by when the grid is weak. If the loads are voltage sensitive, this will impact their power demand thus diminishing the effect of the power injection by the ESS.

The energy storage voltages, i.e. supercapacitor cell voltages, are Ucella, b, c. These can be seen to start at full charge, n.skV and then decline to about 2 kV after 10 seconds. At this stage about 97 % of the capacitor energy has been supplied to the grid. Case A - Fig. 7:

Line currents la, lb and Ic are symmetric.

Capacitor currents Icella, b, c, which can be seen to be rectified to discharge the capacitors for both current directions.

Gate turn-on pulses Gia, G2a, G3a, G4a is illustrated for all thyristors in phase a. It can be seen that G1/G4 and G2/G3 operate in tandem.

The sending and receiving end voltages in phase a, Usa, and Ura, are shown in two different scales, which illustrate the voltage increase injected by the ESS.

The square voltage injected by the ESS in phase a, is USCa. Case B - Fig. 8:

The same plots are shown as in case A, but with the ESS configured for charging instead of for discharging. It can be seen that all curves are reversed in time compared to case A, with the active power input to the ESS being the highest at the last stage of the charging. The voltage injection from the ESS is now negative, depressing the receiving end voltage. As before, this voltage variation will be counteracted by generator AVRs in the receiving end in a operating system. Case B - Fig. 9:

The capacitor currents can be seen to be rectified in the other direction compared to case A in this case. The thyristors can be seen to be blocking in this mode of operation. The receiving end voltage can be seen to be lowered compared to the sending end voltage. The injected square voltage wave is phase shifted by 180 degrees compared to case A.

Case C - Figs. 10 and 11:

In case C the capacitor cells are bypassed from the system and inject no series voltage. No charging or discharging of the capacitors occurs in this state. Case D - Figs. 12 and 13:

The discharge of the capacitors is slowed down and the active power output of the ESS is lowered compared to case A. This is achieved by introducing a 2 ms delay from the current zero crossing when turning on thyristors. This yields a slower discharge rate but also introduces a reactive power exchange between the ESS and the grid. It can be seen that the RMS voltage injection by the ESS is reduced compared to case A in this case. Rating of

supercapacitor

The energy storage technology for the ESS may be based on traditional high voltage capacitors, but due to the high energy storage capacity requirement, electrolytic dual layer capacitors (ELDC), also called supercapacitors or ultracapacitors, are preferred. Supercapacitors uses high surface area activated carbon electrodes and an organic electrolyte to provide energy densities as high as 10 Wh/kg. As a consequence the maximum capacitor cell voltage is limited to 2.5 - 3.0 V. This will mean that many supercapacitors will need to be connected in series for this application. Li-ion batteries could potentially also be used. They provide 10 - 25 times higher energy density, but suffer from lower specific power and limited cycle life, and they must not be discharged below a certain voltage. For an ESS aimed for providing inertia into an electrical power grid, the ESS should release most of its energy in less than 10 seconds, and for such operation, the supercapacitors are ideal.

A dimensioning of a supercapacitor stack for inertia application is provided below. The requirements are: 11.5 kV maximum voltage, 1.5 kA, 1 F per phase, which equals 200 MJ of energy storage.

The table provides typical data of the Maxwell BCAP3000 3.0V/3000F and the Skeleton SCE4500 2.85V/4500F supercapacitor cells. It can be noticed that the Skeleton SCE4500 has both better specific power and energy density. Hence, the values of Skeleton SCE4500 cell are used below.

To get the 11.5 kV rating, 11500/2.8 = 4036 cells are needed. The energy storage capacity per cell is 0.01828MJ, and to reach 200MJ/3,

200/(3*0.01828) = 3618 cells are needed, which is less than 4036. Hence, only one string of series connected supercapacitor cells will be needed per phase. One supercapacitor cell may weigh about Vi kg. The total number of cells for all three phases will then be 12108, and the total weight of these cells will be in the range of 6000 kg.

Other applications

One other application, with an electrochemical battery used as the energy source in the ESS configuration, is also presented. An installation with a battery ESS may be use to assist e.g. a power plant with frequency control services. In this way, the power plant can be kept at a stable level of active power output which may be beneficial, e. g. from an efficiency and/or reliability point of view, while the ESS handles he required fast response during frequency events. Connection of an ESS on a generator output, i.e. behind a meter, would potentially ensure a stable and high line current seen by the ESS, thus ensuring a sufficient output power of the ESS to comply with grid requirements for frequency control. The voltage on the grid side of the ESS may also be used as a feedback voltage to the AVR of the generator, thus ensuring a stable point of common coupling (PCC) voltage and grid code compliance for voltage. This would avoid a voltage disturbance when the ESS is in operation.

The output power of the ESS will be directly proportional to the line current and the ESS voltage. For a high power output and low discharge time the ESS should thus be located at a transmission line where a stable high current is flowing when ESS charge/discharge is required. The output power of the ESS will not be stable but decline with the energy source voltage during discharge. For inertia application it can be noted the inertial response of a synchronous machine is also not stable but instead dependent on the rate of frequency change which normally is varying during a frequency event. What matters most for the ability to limit the frequency deviation during a frequency drop is to supply the grid with energy faster than the turbine governors can supply it. This is a local control action on dedicated plants, wherein no human operator needs to be involved. A varying power output should be acceptable for this application.

The use of the ESS in a grid will introduce losses in the grid. Use of the bypass mode will reduce such losses. However, also in bypass mode the ESS will introduce losses due to semiconductor on-state losses. The bypass losses will be significantly lowered if a mechanical switching device is connected in parallel to the ESS, which can be used during bypass mode. Adding of a mechanical bypass switch may be done as long as the increased turn-on time of the ESS due to the switch operation is acceptable. Such a mechanical l8 bypass switch will anyhow likely be used for bypass during faults in the grid and during maintenance.

The ESS may also be combined with fixed shunt compensation, Static Var Compensators (SVC)/StatComs and HVDC applications. An ESS in series with a capacitor for shunt compensation is illustrated in Fig. 14 (one phase illustrated only).

An ESS in series with an inductor for shunt compensation is illustrated in Fig. 15 (one phase illustrated only).

An ESS system implemented for a SVC/STATCOM is in Fig. 16 illustrated with a first ESS in series with a SVC and a second ESS in series with a

STATCOM (one phase illustrated only).

An ESS in series with a HVDC converter is illustrated in Fig. 17 (one phase illustrated only).

An ESS system implemented in a STATCOM chain-link configuration is illustrated in Fig. 18, with an ESS in each phase of a three phase STATCOM.

A method for providing an energy storage to an electrical power grid is presented according to an embodiment. The method comprises connecting an energy storage system, ESS, in series with a transmission line of an electrical power grid, or in series with a shunt unit connected to the transmission line, the energy storage comprising an energy source connected to the transmission line or the shunt unit via an H-bridge connection, measuring an AC grid frequency in the electrical power grid, discharging the ESS when the measured AC grid frequency drops a predetermined magnitude from a normal condition, and charging the ESS when the measured grid frequency is back to normal condition.

The shunt unit may e.g. be a reactor, capacitor, STATCOM, or SVC. The AC grid frequency may e.g. be measured in the electrical power grid locally at the ESS or at a remote location in the grid.

The predetermined magnitude may e.g. be +- 0.1-0.3 Hz

The discharge of the ESS may be fast, and the charge may be slow over time back to a normal charge level.

The method may further comprise bypassing the ESS when neither discharging nor charging is required.

Each leg of the H-bridge connection may comprise a power conductor switch with anti-parallel diodes, to charge/discharge the ESS. Each power conductor switch may be a thyristor, an insulated-gate bipolar transistor (IGBT), or a gate turn-off thyristor (GTO).

The discharging may be maximized by closing tandem leg switches of the power conductor switches shortly after a current zero crossing of a current in the transmission line. The discharging may be slowed by closing tandem leg switches of the power conductor switches delayed after a current zero crossing in the transmission line.

The charging may be maximized by opening all leg switches of the power conductor switches. The energy source may comprise one or more batteries, one or more supercapacitor, a combination of batteries and supercapacitors, or a combination of batteries, supercapacitors and normal capacitors.

The ESS may comprises an energy storage connected in series, via an H- bridge connection, in each transmission line of a multi phase electrical power grid.

The method may further comprise connecting one or more further ESS in series with the ESS. A series ESS is according to an embodiment presented with reference to Fig. l. The series ESS comprises an energy source 5 having a positive connection point and a negative connection point, wherein the energy source has an energy time constant T of at least 200 ms, preferably at least 500 ms, and a power conductor switch connected to the positive or negative connection point of the energy source, wherein the power conductor is configured to vary the DC voltage level of the energy source.

The time constant T may be the energy stored in the energy storage divided with the maximum power rating of the ESS. The series ESS may further comprise a first power conductor switch 1 connected between a first connection point of the series ESS and the negative connection point, a second power conductor switch 2 connected between the positive connection point and the first connection point, a third power conductor switch 3 connected between a second connection point of the series ESS and the negative connection point, and a fourth power conductor switch 4 connected between the positive connection point and the second connection point, wherein the series ESS thereby connects the energy source between the first and second connection points via an H -bridge, to provide active power between the first and second connection points. The energy storage may be configured to vary the DC voltage level from 0.65 to 1.0 p.u., preferably from about o to about 1 p.u.

Each power conductor switch may comprise a thyristor and an antiparallel diode, an IGBT and an antiparallel diode, or a GTO and an antiparallel diode.

The energy source may be a battery, a supercapacitor, or a combination of energy storage and normal capacitor.

The series ESS may comprise one or more further ESS in series with the ESS.

A series ESS for providing an energy storage to an electrical power grid is presented. The ESS comprises an energy source arranged to be connected, via an H-bridge, to a transmission line of an electrical power grid, at least four power conductor switches, with anti-parallel diodes, wherein each power conductor switch is connected to the energy source and the transmission line to form the H-bridge of the ESS, the ESS being configured to measure an AC grid frequency in the electrical power grid and to discharge the ESS when the measured grid frequency drops a predetermined magnitude, and to charge the ESS when the measured grid frequency increases a predetermined magnitude.

The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.

Claims

1. A method for providing an energy storage to an electrical power grid, the method comprising: connecting an energy storage system, ESS, in series with a transmission line of an electrical power grid, or in series with a shunt unit connected to the transmission line, the energy storage comprising an energy source connected to the transmission line or the shunt unit via an H-bridge connection; measuring an AC grid frequency in the electrical power grid; discharging the ESS when the measured AC grid frequency drops a predetermined magnitude from a normal condition; and charging the ESS when the measured grid frequency is back to normal condition.

2. The method according to claim l, further comprising bypassing the ESS when neither discharging nor charging is required.

3. The method according to claim 1 or 2, wherein each leg of the H-bridge connection comprises a power conductor switch with anti-parallel diodes, to charge/discharge the ESS.

4. The method according to claim 3, wherein each power conductor switch is a thyristor.

5. The method according to claim 3, wherein each power conductor switch is an IGBT.

6. The method according to claim 3, wherein each power conductor switch is a GTO.

7. The method according to any one of claims 3 to 6, wherein discharging is maximized by closing tandem leg switches of the power conductor switches shortly after a current zero crossing of a current in the transmission line.

8. The method according to any one of claims 3 to 7, wherein discharging is slowed by closing tandem leg switches of the power conductor switches delayed after a current zero crossing in the transmission line.

9. The method according to any one of claims 3 to 8, wherein charging is maximized by opening all leg switches of the power conductor switches.

10. The method according to any one of claims 1 to 9, wherein the energy source comprises one or more batteries, one or more supercapacitor, a combination of batteries and super capacitors, or a combination of batteries, supercapacitors and normal capacitors.

11. The method according to any one of claims 1 to 10, wherein the ESS comprises an energy storage connected in series, via an H-bridge connection, in each transmission line of a multi phase electrical power grid.

12. The method according to any one of claims 1 to 11, further comprising connecting one or more further ESS in series with the ESS.

13. A series energy storage system, ESS, comprising: an energy source (5) having a positive connection point and a negative connection point, wherein the energy source has an energy time constant T of at least 200 ms, preferably at least 500 ms; and a power conductor switch connected to the positive or negative connection point of the energy source, wherein the power conductor is configured to vary the DC voltage level of the energy source.

14. The series ESS according to claim 13, wherein the time constant T is the energy stored in the energy storage divided with the maximum power rating of the ESS.

15. The series ESS according to claim 13 or 14, comprising a first power conductor switch (1) connected between a first connection point of the series ESS and the negative connection point; a second power conductor switch (2) connected between the positive connection point and the first connection point; a third power conductor switch (3) connected between a second connection point of the series ESS and the negative connection point; and a fourth power conductor switch (4) connected between the positive connection point and the second connection point; wherein the series ESS thereby connects the energy source between the first and second connection points via an H-bridge, to provide active power between the first and second connection points.

16. The series ESS according to any one of claims 13 to 15, wherein the energy storage is configured to vary the DC voltage level from 0.65 to 1.0 p.u., preferably from about o to about 1 p.u.

17. The series ESS according to any one of claims 13 to 16, wherein each power conductor switch comprise a thyristor and an antiparallel diode, an IGBT and an antiparallel diode, or a GTO and an antiparallel diode.

18. The series EES according to any one of claims 13 to 17, wherein the energy source is a battery, a supercapacitor, or a combination of energy storage and normal capacitor.

19. The series ESS according to any one of claims 13 to 18, comprising one or more further ESS in series with the ESS.

20. A series energy storage system, ESS, for providing an energy storage to an electrical power grid, the ESS comprising: an energy source arranged to be connected, via an H-bridge, to a

transmission line of an electrical power grid; at least four power conductor switches, with anti-parallel diodes, wherein each power conductor switch is connected to the energy source and the transmission line to form the H-bridge of the ESS; the ESS being configured to measure an AC grid frequency in the electrical power grid and to discharge the ESS when the measured grid frequency drops a predetermined magnitude, and to charge the ESS when the measured grid frequency increases a predetermined magnitude.

21. The ESS according to claim 20, further comprising a controllable generator, SVC, STATCOM, tap-changer of a transformer, or high-voltage, direct current (HVDC) system, wherein the ESS and the generator, SVC, STATCOM, tap-changer of a transformer, or HVDC system are controlled coordinated.