Battery Energy Source Arrangement And Voltage Source Converter System

Abstract

The invention relates to power networks, and in particular to a battery energy source arrangement and voltage source converter system in such network. The battery energy source arrangement includes battery energy storage, having one or more parallel-connected battery strings and a mechanism for connecting a voltage of the battery strings to a load. The battery energy source arrangement further includes battery string voltage adapter devices connected in series with respective ones of the one or more battery strings wherein the battery string voltage adapter devices are designed to handle only a fraction of the voltage handled by the battery strings.

Description

FIELD OF THE INVENTION

The invention relates generally to the field of electric power transmission networks, and in particular to active power support in power transmission networks.

BACKGROUND OF THE INVENTION

It is sometimes advantageous to be able to control not only the reactive power, but also to inject or absorb active power, also denoted real power. The STATCOM can therefore be provided with an energy source on its DC side in order to provide some active power, in addition to the reactive power generated to the network. For example, the real power can be utilized either as a source of reserve power when an energy deficit occurs within the network, or as a control power for managing transients and electromechanical oscillations in the network. The energy source may for example comprise conventional capacitors, super capacitors or electrochemical batteries.

FIG. 1 illustrates a basic STATCOM arrangement, or voltage source converter system, having an active power source on its DC side, wherein the energy source in the illustrated example comprises a number of parallel-connected battery strings 2. Each battery string 2 comprises a number of battery modules and each battery module in turn contains a number of battery cells. The STATCOM 1 is also connected to a power network 3 in a conventional manner via a transformer (not shown) to adapt the converter voltage to the network voltage and to provide galvanic isolation.

Often, when electrochemical batteries are used as the energy source, a high number of battery modules must be connected in series to match the DC voltage of the STATCOM. Moreover, in order to obtain the desired active power and duration of the battery energy storage, a number of battery strings often has to be connected in parallel, as is illustrated in the figure. The STATCOM DC voltage is controlled and all the parallel-connected battery strings are connected to this controlled voltage.

During operation of the battery energy storage, the cells age differently. Further, some of the cells may have failed and in order to continue operation, the broken cell(s) of the battery modules of the battery string will be bypassed. Moreover, some aged battery modules will be replaced by new modules. For batteries with a low internal resistance, a small voltage offset will result in charge redistribution. The performance of the whole battery energy storage will be reduced, as it will be dominated by the battery cell with the lowest performance.

To bypass the failing battery cell(s) is thus done at the expense of reduced charging voltage of the battery string. Another solution is to bypass the whole battery string, but then an even greater capacity redundancy in the form of oversized batteries and additional battery strings would be required to meet capacity requirements at all times. Today, parallel-connected battery strings are thus not run in an optimal fashion.

In view of the above, it would be desirable to provide an improved way of handling failing battery cell(s) and/or battery strings.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an active power source, and in particular a battery energy source arrangement, for a load, in particular a voltage source converter, wherein the above disadvantages are overcome or at least alleviated.

It is a particular object of the invention to provide a battery energy source arrangement for a voltage source converter wherein the maximum effect is obtained from the active power source, providing an optimized operation thereof.

It is another object of the invention to provide a battery energy source arrangement for a voltage source converter, wherein voltage differences due to performance variations of individual cells of a battery are prevented.

It is still another object of the invention to provide a battery energy source arrangement for a voltage source converter, wherein the service life of the battery energy source is maximized.

It is yet another object of the invention to provide a cost-efficient battery energy source arrangement.

These objects, among others, are achieved by a battery energy source arrangement and a voltage source converter as claimed in the appended independent claims.

In accordance with the invention an improved battery energy source arrangement is provided. The battery energy source arrangement comprises a battery energy storage, which in turn comprises one or more parallel-connected battery strings. The battery energy source arrangement further comprises connection means for connecting a voltage of the battery strings to a load, such as a voltage source converter. The battery energy source arrangement is characterized by battery string voltage adapter devices connected in series with a respective one of the one or more battery strings. The battery string voltage adapter devices are designed to handle only a fraction of the voltage handled by the battery strings. By means of the invention active power can be provided in an optimized manner. Maximum effect of the battery energy storage is enabled as well as fully loadable battery energy storage. Voltage differences due to performance variations of individual cells of the battery energy storage are prevented, thereby prolonging the service life of the battery energy storage. Further, by means of the invention, it is possible to disconnect parallel-connected battery strings faster compared to the state of the art. Further still, by means of the invention, the service life of circuit breakers used to disconnect battery strings can be prolonged, as the current in each battery string can be controlled to zero before the circuit breakers of each battery string are opened. As the battery string voltage adapter device is designed so as to be able to handle only a fraction of the total load voltage, the battery string voltage adapter device can be designed and dimensioned in a most cost-efficient manner.

In accordance with an embodiment of the invention, the battery string voltage adapter devices are arranged to add a certain voltage to their respective battery strings so that the voltage level of the battery strings are optimized. The optimization comprises in an embodiment to provide an equal voltage over each of the one or more battery strings including the string voltage adapter device. The service life of the battery energy storage is thereby maximized. In particular, the battery string voltage adapter devices may comprise means for adding a voltage equal to the difference of a controlled load voltage and an actual battery string voltage.

In accordance with an embodiment of the invention, the battery string voltage adapter device comprises a conventional H-bridge, which provides a cost efficient solution.

In accordance with another embodiment of the invention, the battery string voltage adapter device comprises further an overvoltage/short-circuit current protection device. The circuitry of the battery string voltage adapter device is thereby protected against overvoltages and short-circuit currents of the battery strings in a convenient manner.

Further embodiments are defined in the dependent claims.

The invention also relates to a voltage source converter system comprising a voltage source converter and a battery energy source arrangement, whereby advantages similar to the above are achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art arrangement.

FIG. 2 illustrates an embodiment of a battery energy source arrangement for a voltage source converter in accordance with the present invention.

FIG. 3 illustrates an embodiment of a battery string voltage adapter device in accordance with the invention.

FIG. 4 illustrates another embodiment of a battery string voltage adapter device in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 illustrates an embodiment of the invention. The same reference numerals are used throughout the figures for same or corresponding parts.

A battery energy source arrangement 4 in accordance with the invention comprises connection means for being connected in parallel with a voltage source converter 1 and further comprises a battery energy storage 2, preferably comprising lithium ion batteries, and battery string voltage adapter devices 7_i, in the following denoted adapter devices 7_i.

The battery energy storage 2 comprises one or more battery strings 5₁, . . . , 5_i, . . . , 5_n connected electrically in parallel across common busbars 10a, 10b. Each battery string 5₁, . . . , 5_i, . . . , 5_n comprises series connected battery modules, wherein each battery module comprises battery cells 6, having any suitable nominal individual voltage, for example 3.4 V. Further, any suitable number of battery cells 6 can be connected electrically in series giving a suitable nominal voltage, e.g. 624 V, for a battery module. Several series-connected battery modules make up a battery string 5_i to provide for example 10 kV up to 80 kV DC or more depending on for example power levels of the load to which the battery energy source arrangement 4 is connected and the desired duration of the battery energy source arrangement 4. Several battery strings 5₁, . . . 5_i, . . . , 5_n may be connected in parallel to provide necessary power and energy.

In the present application, the battery strings 5 are high-voltage battery strings and an increased need of active power can be met by adding a suitable number of parallel-connected battery strings. Each battery string 5₁, . . . , 5_i, . . . , 5_n is connected in parallel with the voltage source converter 1.

As mentioned, the battery energy source arrangement 4 further comprises adapter devices 7₁, . . . , 7_i, . . . , 7_n, one for each battery string 5₁, . . . , 5_i, . . . , 5_n. The adapter devices 7₁, . . . , 7_i, . . . , 7_n are connected in series with each respective battery string 5₁, . . . , 5_i, . . . , 5_n. In particular, the adapter device 7_i is connected to a battery string 5_i preferably via a DC breaker 8_n^a. The adapter device 7_i is also connected to a local DC bus 9a, 9b. The adapter devices 7₁, . . . , 7_i, . . . , 7_n provide an optimized operation of the battery energy storage 2, as will be described in the following.

The adapter device 7_i is arranged to add, upon need, a voltage Δu_i to the battery string 5_i to which it is connected. Each battery string 5₁, . . . 5_i, . . . , 5_n has its own adapter device 7₁, . . . , 7_i, . . . , 7_n. If one or more battery cells 6 of the battery string 5_i fail they may be bypassed by any known bypass arrangement (not shown). The adapter device 7_i for the battery string 5_i comprising the failing and bypassed battery cell(s) 6 then adds a suitable voltage Δu_i to the battery string 5_i. The voltage Δu_i is chosen so as to render the battery string voltage u_{string, i} optimal, for example optimal in the sense of providing maximum effect and enabling the battery string 5_i to be fully loaded.

All the battery strings 5₁, . . . , 5_i, . . . , 5_n preferably have the same value of the battery current. Thereby all the batteries will become fully charged and also discharged at the same time. However, due to different reasons, e.g. failing cells, the battery strings 5₁, . . . , 5_i, . . . , 5_n have different voltage levels for the same current level. The adapter device 7_i is therefore connected to each battery string 5_i in order to adapt the voltage level of the respective battery string 7_i.

In order to provide a cost efficient design, the voltage potential between the adapter devices 7₁, . . . , 7_i, . . . , 7_n is preferably about the same. All the adapter devices 7₁, . . . , 7_i, . . . , 7_n are connected together with a local DC bus 9a, 9b. All the adapter devices 7₁, . . . , 7_i, . . . , 7_n are further connected either to the positive 10a or the negative 10b busbar of a bus 10a, 10b connecting the battery strings to load, such as the voltage source converter 1. The local energy storage, e.g. a capacitor, of each of the adapter devices 7₁, . . . , 7_i, . . . , 7_n is limited and the local DC-voltage bus 9a, 9b is in an embodiment of the invention used for transferring energy between the adapter devices 7₁, . . . , 7_i, . . . , 7_n.

More in detail, in order to keep the voltage level of the local DC bus arrangement 9a, 9b constant, the total power of all adapter devices 7₁, . . . , 7_i, . . . , 7_n should be zero. Assuming that the current of the battery strings 5₁, . . . , 5_i, . . . , 5_n are equal gives

I_{string 1}=i_{string 2}= . . . =i_{string n}

The average voltage of all the battery strings 5₁, . . . , 5_i, . . . , 5_n should be equal to the controlled DC voltage U_dc from the voltage source converter 1:

$\frac{1}{n}\left(U_{\mathrm{string}1}+U_{\mathrm{string}2}+\dots +U_{\mathrm{stringn}}\right)=U_{\mathrm{dc}}$

The added voltages Au, should therefore be set according to:

Δu₁=U_dc−U_string1

Δu₂=U_dc−U_string2

Δu_n=U_dc−U_stringn

The voltages Δu_i are controlled by a control system, schematically indicated at reference numeral 12. The control system 12 retrieves data from different parts of the system, for example voltages and currents of the battery strings and voltage source converter voltages.

The control system 12 comprises software and algorithms for performing the voltage adaptation of the individual battery strings 5₁, . . . , 5_i, . . . , 5_n. In particular, the control system 12 comprises means for determining the individual battery string voltages and means for controlling the voltages of the individual battery strings so as to be optimal, for example by software implementing algorithms utilizing the above equations. The algorithms may be programmed for optimizing the voltage of the battery strings 5₁, . . . , 5_i, . . . , 5_n so that an equal voltage over each of the battery strings 5₁, . . . , 5_i, . . . , 5_n and their corresponding adapter device is accomplished: U₁+Δu₁= . . . =U_{string i}Δu_i= . . . =U_n+Δu_n. For example, so that the added voltages Δu₁, . . . , Δu_i, . . . Δu_n are equal to the difference of the controlled load voltage U_dc and the actual battery string voltage.

FIG. 3 illustrates an embodiment of the adapter device 7_i in accordance with the invention. The adapter device 7_i need only handle a fraction of the voltage that the battery string 5_i handles and the components can be chosen accordingly. The illustrated adapter device 7_i comprises an H bridge, having four switching elements in a conventional manner. A standard H-bridge design may be used. In particular, the H-bridge consists of two phase-legs, each having two valves in series. Each valve consists of a controllable power semiconductor, for instance a MOSFET (metal-oxide-semiconductor field-effect transistor), an IGCT (Integrated Gate Commutated Thyristor) or an IGBT (Insulated-Gate Bipolar Transistor) and an anti-parallel diode. In parallel with the two phase-legs a capacitor is connected in such a way as to form a loop with low inductance. To achieve the function in accordance with the invention, the mid-point in between the two valves of one of the two phase-legs of the H-bridge is connected to the positive busbar 10a, whereas the mid-point of the other phase-leg in the H-bridge is connected to the positive terminal of the battery string through a reactor. Furthermore, to achieve the function in accordance with the invention, the connection described above is repeated for each of the parallel battery strings. The described embodiment thus comprises as many H-bridges as the number of battery strings connected in parallel. Several methods can be adopted and implemented in the control system 12 for controlling the valves. However, the total size of all the capacitors of the H-bridges should be chosen such that the resulting voltage variations on the local DC bus 9a, 9b are acceptable.

The adapter device 7_i is preferably self-supporting, i.e. needs no external power source. In the above-described embodiment of the adapter device 7_i may be blocked, by means of signals from the control system 12, so as to load the capacitor of the adapter device. Thereby no external voltage source is required.

FIG. 4 illustrates another embodiment of an adapter device for use in the invention. In particular, in order to protect the adapter devices against over-voltages and short-circuits in the battery strings, a protection circuit 11 is preferably used for handling overvoltage and short-circuit currents. Any suitable protection circuit 11 design may be used.

In FIG. 2 conventional DC breakers 8_i^a,8_i^b are illustrated connected to the plus and minus pole of the battery strings 5_i and to the positive and negative DC busbar 10a, 10b. The DC breakers 8_i^a,8_i^b, two for each battery string 5_i, are arranged to connect and disconnect the battery strings 5_i. The adapter device 7_i in accordance with the invention provides an additional advantage in that the current in each battery string 5_i can be controlled to zero before the DC breakers 8_i^a,8_i^b of each respective battery string are opened. The service life of the DC breakers 8_i^a,8_i^b can thereby be prolonged. In particular, the adapter device 7_i of a particular battery string 5_i may be blocked and when the current through the particular battery string is zero, the battery string can be readily disconnected.

Alternatively, the time for disconnecting the battery strings 5_i can be minimized. In the prior art, the battery strings are disconnected by adapting the converter 3 DC-voltage to a first battery string 5_i and then the DC breakers 8_i^a,8_i^b are opened. This procedure is then repeated for each battery string. By means of the innovative battery energy source arrangement 4, great timesavings can be accomplished by controlling the battery string currents and then opening the DC breakers 8_i^a,8_i^b simultaneously.

The present invention provides another advantage in that the battery strings may be connected into an operative state more rapidly than prior art solutions. More particularly, when the battery strings are disconnected from its load, for example a voltage source converter, they may be connected to each other. Thereby the voltages of the battery strings and their respective adapter devices may be redistributed and all battery strings and their respective adapter devices may be loaded to the same level. Having the battery strings equally loaded in turn enables a fast connection to the operative state, i.e. connection to the voltage source converter.

The present invention provides yet another advantage in that short-circuits of the battery strings can be easily detected. In particular, if the voltage over the local DC bus comprises sudden increases this can be taken as an indication of one or more failing battery strings. Appropriate action can then be taken quickly. It is further possible to determine which one of the battery strings is failing. This can be done by measuring the current in each battery string, as a short-circuited battery string has an increased current.

It is noted that the adapter device 7_i of the invention only need to be able to handle a fraction of the total load voltage, for example a few percent. As an example, if the battery string 5_i is dimensioned for a voltage of about 80 kV, then the adapter device 7_i may be dimensioned to handle a voltage of, for example, 1-3 kV. The adapter device can thus be accordingly dimensioned in a most cost-efficient manner.

It is further noted that it is possible to utilize different currents in the different battery strings. In particular, the battery strings may have different capacity or be of different types, and battery sources of different loading can thus still be used, providing a cost-efficient solution wherein there is no need to unnecessarily discard battery strings.

A voltage source converter system in accordance with the invention comprises a voltage source converter 1 and a battery energy source arrangement 4 as described above.

Although not shown in the figures, additional DC power source(s) may be connected in parallel with the voltage source converter 1 of such voltage source converter system. One such additional DC power source conventionally used is capacitor devices.

Claims

1. A battery energy source arrangement comprising a battery energy storage having one or more parallel-connected battery strings, and a mechanism for connecting a voltage of said battery strings to a load, characterized by battery string voltage adapter devices connected in series with respective ones of said one or more battery strings wherein said battery string voltage adapter devices are designed to handle only a fraction of said voltage handled by said battery strings.

2. The battery energy source arrangement as claimed in claim 1, wherein said battery string voltage adapter devices are arranged to add a voltage to a respective battery strings so as to optimize the voltage level of said battery strings.

3. The battery energy source arrangement as claimed in claim 2, wherein said optimization comprises providing an equal voltage over each of said one or more battery strings and their respective battery string voltage adapter devices.

4. The battery energy source arrangement as claimed in claim 1, wherein said battery string voltage adapter devices comprises means for adding a voltage equal to the difference of a controlled load voltage and an actual battery string voltage.

5. The battery energy source arrangement as claimed in claim 1, wherein said battery string voltage adapter devices are arranged to be controlled by a control system.

6. The battery energy source arrangement as claimed in claim 5, wherein said control system comprises an algorithm for optimizing the voltage level of each of said battery strings.

7. The battery energy source arrangement as claimed in claim 1, wherein said battery string voltage adapter device comprises an H-bridge.

8. The battery energy source arrangement as claimed in claim 7, wherein said battery string voltage adapter device is self-supporting.

9. The battery energy source arrangement as claimed in claim 7, wherein said battery string voltage adapter device further comprises an overvoltage and short-circuit current protection device.

10. The battery energy source arrangement as claimed in claim 1, wherein said battery energy source arrangement is arranged for use in a high-voltage application.

11. The battery energy source arrangement as claimed in claim 1, arranged to be connected to a voltage source converter.

12. A voltage source converter system comprising a voltage source converter and a battery energy source arrangement as claimed in claim 1, wherein said battery energy source arrangement is arranged parallel-connected to the voltage source converter.

13. The voltage source converter as claimed in claim 12, wherein said battery energy source arrangement and said voltage source converter are connected across a common busbar.

14. The voltage source converter as claimed in claim 12, wherein said voltage source converter is connectable to a high voltage power network.

15. The voltage source converter as claimed in claim 12, wherein said battery string voltage adapter devices are arranged to be connected to a local busbar, which in turn is arranged to be connected to said common busbar and to said battery string of said battery energy source arrangement.