WO2015124911A1

WO2015124911A1 - Battery control system

Info

Publication number: WO2015124911A1
Application number: PCT/GB2015/050443
Authority: WO
Inventors: Marco DEGANO; Saul LOPEZ AREVALO; Emil Ernest; Bradford Palmer; Krzysztof PACIURA
Original assignee: Cummins Generator Technologies Limited
Priority date: 2014-02-20
Filing date: 2015-02-17
Publication date: 2015-08-27
Also published as: GB201403013D0

Abstract

A battery control system is disclosed for controlling a set of batteries which are arranged to be charged from and to supply power to a multi-phase AC power line. The multi-phase AC power line comprises a neutral connection (24), and the set of batteries comprises a first battery (16) and a second battery (18) connected in series with a mid-point of the batteries connected to the neutral connection. The system comprises conversion means (20) for converting an AC input on the AC power line to DC for charging the set of batteries in a first mode of operation, and for converting a DC input from the set of batteries to AC for supply to the AC power line in a second mode of operation, and control means (26) for controlling operation of the conversion means. The control means is arranged to control the conversion means such that, during the first mode of operation, the conversion means balances the state of charge of the first battery and the second battery.

Description

BATTERY CONTROL SYSTEM

The present invention relates to a battery control system, and in particular to a battery control system for controlling batteries which are arranged to be charged from and to supply power to a multi-phase AC power line. The present invention has particular (but not exclusive) application in a back-up battery system for an electrical generating set, or for a generator powered by a renewable energy source such as a wind or wave turbine. Electrical generating sets or "gensets" typically comprise a prime mover such as a diesel engine coupled to a generator for generating electricity. Generating sets have many applications, including use in situations where there is no connection to a power grid or as an emergency power-supply if the grid fails. Generating sets may also be used for more complex applications such as peak-lopping, grid support and export to the power grid.

A battery back-up system may be provided for use in conjunction with the generating set. The battery back-up system typically supports the generating set during transients, as well as providing other advantages such as fuel savings and help with overload situations. Usually the battery back-up system is charged by the generating set when it is not supplying its full rated power, and is used to support the generating set when it is at or near to full power.

Typically a single battery, or a set of batteries connected in series or parallel with two output terminals, is used as the back-up power supply. However such an arrangement may not be suitable for use in multi-phase systems, in particular where an unbalanced load may be present. In such situations it may be desirable to provide two series connected batteries with a neutral point between them, in order to assist in supplying different loads on different phases.

A problem which may arise when using two series connected batteries is that the batteries may end up with a different charge. This may arise, for example, due to variations in the characteristics of the batteries, or due to the control mechanism employed when supplying power from the batteries. Over time this may lead to an unbalanced output when the batteries are supplying power. According to one aspect of the present invention there is provided a battery control system for controlling a set of batteries which are arranged to be charged from and to supply power to a multi-phase AC power line, the multi-phase AC power line comprising a neutral connection, and the set of batteries comprising a first battery and a second battery connected in series with a mid-point of the batteries connected to the neutral connection, the system comprising:

conversion means for converting an AC input on the AC power line to DC for charging the set of batteries in a first mode of operation, and for converting a DC input from the set of batteries to AC for supply to the AC power line in a second mode of operation; and

control means for controlling operation of the conversion means, wherein the control means is arranged to control the conversion means such that, during the first mode of operation, the conversion means balances the state of charge of the first battery and the second battery.

The present invention may provide the advantage that, by providing a conversion means which balances the state of charge in the batteries during a mode of operation in which the batteries are being charged, a simple mechanism for balancing the state of charge can be achieved which can at least partially make use of existing components. In particular, it may be possible to incorporate charge balancing as part of the charging process without requiring separate circuits for charge balancing, or separate charging processes for each battery. Preferably the first mode of operation is a charging mode in which the set of batteries is charged from the AC power line, and the second mode of operation is a discharge mode in which the set of batteries supply power to the AC power line.

Preferably the conversion means is arranged to balance the state of charge in the first battery and the second battery as part of the process of converting the AC voltage on the AC power line to DC for charging the batteries. This may allow the state of charge of the batteries to be balanced without the need for separate charge balancing circuitry. The conversion means may be a converter, and is preferably a bidirectional AC/DC converter. For example, the conversion means may comprise an inverter/rectifier circuit. The inverter/rectifier circuit may act as a controllable rectifier in the first mode of operation, and an inverter in the second mode of operation. This can allow the same circuitry to be used for conversion from AC to DC and DC to AC. However, if desired, separate circuitry could be provided.

The conversion means may be a two-level converter. This can provide a simple arrangement in which, in the second mode of operation, the conversion means has an output which switches between two different levels in order to supply power to the AC power line. The conversion means may have two terminals for connection to the set of batteries, and the conversion means is preferably arranged to be connected across the series connection of the first battery and the second battery.

The multi-phase AC power line may have any number of phases greater than one. For example, the multi-phase AC power line may have 2, 3, 4 or 5 phases, or any other number. In a preferred embodiment, the AC power line is a three phase power line. Preferably the AC power line comprises a conductor for each phase, and for the neutral connection.

Preferably a filter is provided between the AC power line and the conversion means. The filter may acts as an input filter in the first mode of operation, and an output filter in the second mode of operation. This may allow the system to change easily from one mode of operation to another, and avoid the need for separate filters.

The control means may comprise a current control loop for controlling the current supplied to the first and second batteries in the first mode of operation. This can allow the rate at which the batteries are charged to be controlled, thereby ensuring optimum charging of the batteries. In this case, the current control loop may control the current supplied to the first and second batteries so as to balance their state of charge. The system may further comprise means (such as one or more charge sensors) for measuring the state of charge of the first and second batteries, and the control means may be arranged to control the conversion means in dependence on a measured state of charge. For example, the control means may be arranged to control the conversion means in dependence on a difference in a measured state of charge of the first battery and a measured state of charge of the second battery. A measure of the state of charge of the batteries may be obtained, for example, by measuring the battery voltage, or by measuring both voltage and current, or by coulomb counting, or any other method.

Preferably the neutral connection provides a return path for current flow (i.e. a return path from the mid-point of the batteries to the neutral connection of the AC power line). This can allow a different current to flow in each battery, and thus facilitate balancing the state of charge of the batteries.

The control means may be arranged to adjust a current flowing in the neutral connection. This arrangement may facilitate balancing of the state of charge in the batteries as part of the conversion process. In a multiphase AC power line, a current flowing in a neutral connection may be referred to as a common mode current. In order to assist with the control process, the system may further comprise means for obtaining a measure of a common mode current. In this case, the control means may be arranged to control the conversion means in dependence on the measure of the common mode current. This arrangement may allow the current flowing in the neutral connection to be adjusted.

If desired, a measure of the common mode current could be obtained by means of a current sensor arranged to sense the current on the neutral connection.

However, it may be possible to derive the common mode current from the phase currents on the AC power line. Since the phase currents may be sensed for other purposes (such as for operation of the conversion means as an inverter), this may allow the common mode current to be obtained without the need for a dedicated current sensor on the neutral connection. Thus, the system may further comprise means for sensing the current of each phase of the AC power line, and the control means may be arranged to control the conversion means in dependence on the sensed currents. Preferably a current sensor is provided for each phase of the AC power line. The control means may be arranged to derive a measure of the common mode current from the sensed currents.

The control means may comprise means for converting the sensed currents into a rotating frame of reference. This may simplify the control mechanism. For example, the control means may be arranged to apply the direct-quadrature-zero (c/qO) transformation. Thus the sensed currents may be converted into measures of direct current, quadrature current, and common mode current.

Preferably the control means comprises a control loop for controlling the common mode current so as to balance the state of charge in the first battery and the second battery. In order to achieve this, the control means may be arranged to compare the measure of the common mode current to a reference value to yield a difference signal, and to control the conversion means in dependence on the difference signal.

The control means may be arranged to derive the reference value from a measure of the state of charge of the first and second batteries. For example, the reference value may be proportional to a difference between a measure of the state of charge of the first battery and a measure of the state of charge of the second battery.

The present invention may also provide a battery system comprising a first battery and a second battery connected in series, and a battery control system in any of the forms described above. The conversion means may be connected across the series connection of the first battery and the second battery.

The battery system may comprise a battery management system, and the battery management system may be arranged to provide a measure of the state of charge of the first battery and the second battery and/or a measure of battery voltage. The AC power line may be connected to a main source of power, such as an electrical generating set. In this case the battery system may be arranged to supply auxiliary power to the AC power line in the second mode of operation. Preferable the AC power line is arranged to be connected to a load.

In one embodiment, the battery system is a back-up battery system for an electrical generating set. Thus, the invention may also provide a generating apparatus comprising an electrical generating set and a battery system as described above. However, the invention may also be used with other sources of electrical power, such as generators powered by renewable energy sources, or local grids with a mixture of energy sources.

According to another aspect of the present invention there is provided a battery control system which controls a set of batteries which are arranged to be charged from and to supply power to a multi-phase AC power line, the multi-phase AC power line comprising a neutral connection, and the set of batteries comprising a first battery and a second battery connected in series with a mid-point of the batteries connected to the neutral connection, the system comprising:

a converter which converts an AC input on the AC power line to DC to charge the set of batteries in a first mode of operation, and converts a DC input from the set of batteries to AC to supply the AC power line in a second mode of operation; and

a control unit which controls operation of the converter,

wherein the control unit is arranged to control the converter such that, during the first mode of operation, the converter balances the state of charge of the first battery and the second battery.

According to another aspect of the present invention there is provided a method of controlling a set of batteries which are charged from and supply power to a multi-phase AC power line, the multi-phase AC power line comprising a neutral connection, and the set of batteries comprising a first battery and a second battery connected in series, with a mid-point of the batteries connected to the neutral connection, the method comprising: in a first mode of operation, converting an AC input on the AC power line to DC for charging the batteries;

in a second mode of operation, converting a DC input from the batteries to AC for supply to the AC power line; and

controlling the first mode of operation and the second mode of operation, wherein, in the first mode of operation, the step of converting an AC input on the AC power line to DC comprises balancing the state of charge of the first battery and the second battery. Features of one aspect of the invention may be provided with any other aspect. Apparatus features may be provided with method aspects and vice versa.

Preferred features of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:

Figure 1 shows an overview of a power supply system in accordance with an embodiment of the invention;

Figure 2 shows parts of the battery back-up system in more detail;

Figures 3 and 4 show control topologies used in a charging mode of operation;

Figure 5 shows a control topology used in a paralleling mode of operation; Figure 6 shows a control topology used in a standalone mode of operation;

Figures 7(a), 7(b) and 8 show typical voltage, current and power waveforms in charging and paralleling modes of operation; and

Figure 9 shows parts of a bidirectional AC/DC converter in more detail.

System overview

Figure 1 shows an overview of a power supply system in accordance with an embodiment of the invention. The power supply system comprises a local grid 10 to which is connected a main source of electrical power, such as a generator set and/or a generator powered by an alternative energy source such as a wind turbine. The local grid 10 is connected via a contactor 12 to an AC power line 15, to which is connected a load 14. During normal operation, the local grid 10 supplies AC electrical power to the load 14. A back-up system is provided to support the local grid during transients such as when connecting or disconnecting a load. The back-up system comprises two series connected batteries 16, 18 which are connected to the AC power line 15 via bidirectional AC/DC converter 20 and filter 22. The batteries 16, 18 are charged by the local grid 10 when it is not supplying full load.

In the arrangement of Figure 1 , the local grid 10 supplies three phase AC power to the load 14. In some circumstances the load may be unbalanced, that is, the load on one phase may be different from the load on another phase. Such a situation may arise, for example, where each of the three phases is used to supply a separate single phase load. In order to accommodate this, a neutral line 24 is connected between the local grid 10, the load 14, the filter 22, and the midpoint of the two series connected batteries 16, 18. The neutral line can provide a return path when the load is unbalanced. Thus the neutral line can facilitate the supply of an unbalanced load.

Figure 2 shows parts of the battery back-up system in more detail. Referring to Figure 2, the bidirectional AC/DC converter 20 is controlled by a control unit 26. Current sensors 28 sense the currents on each of the three phases at the input to the bidirectional AC/DC, and the sensed values are fed to the control unit 26. Charge sensors 30, 32 sense the state of charge of each of the batteries 16, 18. The sensed state of charge values are fed to the control unit 26.

The charge sensors 30, 32 may be provided as part of a commercially available battery management system. Alternatively, the state of charge may be obtained by measuring the battery voltage, or by measuring both voltage and current, or by coulomb counting, or any other method which enables an indication of the state of charge of the batteries to be obtained. The control unit 26 controls charging of the batteries 16, 18 from the three phase AC power line 15, and supply of power from the batteries to the three phase AC power line 15, based on the various sensed values. The control unit 26 may include a processor which is programmed to carry out the control functions, which are described below. Alternatively hard-wired electrical components, or a mixture of electrical components and a processor, could be used to carry out the control functions.

The battery back-up system of Figures 1 and 2 is operable in three different modes, namely, charging, paralleling and stand alone. In each mode a control method is implemented which is based on the direct-quadrature-zero (dqO) frame.

Referring to Figures 1 and 2, the bidirectional AC/DC converter 20 is connected to the AC power line 15 through a filter 22. By using this configuration, the system can be easily changed from one operation mode to another. In the charging mode the filter 22 is used as the input filter, while in the paralleling and standalone modes the filter 22 is used as the output filter.

When the system is operating in charging mode, the bidirectional AC/DC converter 20 acts as a rectifier and DC current is supplied to the batteries 16, 18 in order to charge them. In paralleling mode, both grid 10 and batteries 16, 18 (through the bidirectional AC/DC converter 20) are connected to the load 14. The bidirectional AC/DC converter 20 acts as an inverter, and its output is filtered by filter 22 to produce a sinusoidal voltage which is applied to the load 14. In standalone mode, the grid 10 is disconnected from the load by contactor 12, and only the batteries 16, 18 supply power to the load. In this mode the bidirectional AC/DC converter 20 acts as an inverter, as in the paralleling mode.

Each of the three modes is described in more detail below.

Charging mode

In the charging mode, the batteries 16, 18 are charged using the three phase AC power supplied by the generating set. This mode of operation is split into two sub-modes: current mode and voltage mode. The system starts operating in current mode.

The control topology used in the current mode is shown in Figure 3. The three phase inputs A, B, C on the AC power line 15 are connected via the filter 22 to the AC/DC converter 20. The filter 22 comprises an inductor and a capacitor for each of the three phases, with each capacitor connected between phase and neutral.

The currents i_a, i_b, i_c at the inputs to the AC/DC converter 20 are sensed by the current sensors 28. The three sensed values are fed to converter 46, which converts the three values in the stationary abc frame using the direct-quadrature- zero transformation (c/qO) to obtain a set of variables in the rotating dqO frame. The angle Θ used in the dqO transformation is calculated by using the current measured at the input of the AC/DC converter. The outputs of the converter 46 are the direct axis current i_d, the quadrature axis current i_q, and the common mode current i₀. The common mode current i₀ is a measure of the current in the neutral line 24, and is obtained from the currents i_a, i_b, i_c at the input to the AC/DC converter using the direct-quadrature-zero transformation, without the need to measure the current in the neutral line directly. In the case of a balanced load the common mode current i₀ is zero.

In this mode of operation a current loop is used to control the charging of batteries. Each of the direct axis current i_d, the quadrature axis current i_q, and the common mode current i₀ is controlled using its own control loop, as explained below.

The control loop for the direct axis current comprises comparator 34 and PI (proportional integral) controller 40. The direct axis current i_d is compared to a reference value i_d ^* in comparator 34. The reference value i_d ^* is given by:

Where i_d ^* _c is the required battery current (normally the optimum current for charging the batteries from a low state of charge), V_DC is the voltage across both batteries, and V_s is the input voltage to the AC/DC converter (phase-to-neutral RMS). For this expression, the power balance is considered, i.e. input power is equal to output power. The value V_DC can be obtained using a voltage sensor connected across the two batteries 16, 18, or by adding together the battery voltages supplied by a battery management system. The value V_s can be obtained by sensing the phase-to-neutral voltages of the inputs A, B, C using known voltage sensors and calculating the RMS (root mean square) value.

The output of the comparator 34 is an error signal which is fed to proportional- integral (PI) controller 40. The PI controller 40 produces a direct axis control signal V_d, for control of the direct axis current.

The control loop for the quadrature axis current comprises comparator 36 and PI controller 42. The quadrature axis current i_q is compared to a reference value t^* in comparator 36. In this arrangement the reference value is set to zero. The output of the comparator 36 is fed to PI controller 42, which produces a quadrature axis control signal V_q.

The control loop for the common mode current comprises comparator 38 and PI controller 44. The common mode current i₀ is compared to a reference value to in comparator 38. The reference value i₀ ^* is set by regulator circuit 50. The regulator circuit 50 receives the state of charge of each of the batteries 16, 18, as measured by the charge sensors 30, 32, and sets the value i₀ ^* to be proportional to the difference in the state of charge. Thus the reference value i₀ ^* is given by: i₀ ^* = K_p(SOC₂ - SOC₁)

Where SOd is the state of charge of the first battery, SOC₂ is the state of charge of the second battery, and K_p is a constant. If desired, the value of K_p can be adjusted in order to set the rate at which the state of charge of the batteries is equalized. The output of the comparator 38 is fed to PI controller 44, which produces a common mode control signal V₀.

The direct axis control signal V_d, quadrature axis control signal V_q, and common mode control signal V₀ are fed to converter 48. The converter 48 applies the zero-direct-quadrature (Odq) transformation, to convert the signals from the rotating dqO frame back to the stationary abc frame. Pulse width modulation (PWM) circuits 49 are then used to control each of the phases in the AC/DC converter 20. The various control loops described above allow the AC to DC conversion process to be controlled in such a way that the appropriate currents are supplied to the batteries 16, 18 in order for them to be charged at an appropriate rate, and for the states of charge of the batteries to be balanced.

Once the nominal voltage in the batteries has been reached, the system switches to operation in the voltage charging mode. Figure 4 illustrates the control topology employed in the voltage charging mode. In this mode, the current flowing to the batteries is minimal; current flow is only required to keep the voltage of the batteries at its nominal level.

In the arrangement of Figure 4, a PI controller 52 is used to give the reference value i_d ^* . The input to the controller 52 is the total battery charge. This is obtained by adding together the state of charge of each of the batteries 16, 18 in the adder 54. The reference value t^* is set to zero, and reference value i_Q ^* is proportional to the difference in the state of charge of the batteries, as in the topology of Figure 3. Other parts of the system operate in the same way as described above with reference to Figure 3. The control strategies described above can be used to control the output of the AC/DC converter in such a way that the two batteries 16, 18 can receive different amounts of charging current. In a normal three wire inverter the common mode or zero sequence current (l₀ = (l_a + l_b + l_c)/3) has to be zero. However in the topology described above the addition of the neutral wire connected to the mid- point of the batteries allows the common mode current to be regulated. In this case the path is closed as a neutral current (l_n = 3 x l₀) in addition to the active and reactive current. The common mode current can be chosen arbitrarily, and will flow in both batteries with the opposite sign (l₊ = -I. = l_n/2). The common mode current is superimposed onto the active current regulated by the l_d current regulator.

During charging, the state of charge across each battery is equalised by setting the l₀ current reference i_Q ^* to be proportional to the difference in the state of charge of the batteries in such a way that, at the end of the charging process, both batteries will be in the same state of charge. This is achieved by setting the constant of proportionality, K_p, to the appropriate value.

Paralleling mode

In the paralleling mode, both the generator set and the batteries are connected to the load. The bidirectional AC/DC converter 20 operates as an inverter and produces a three phase AC voltage, which is applied to the load 14. In this mode of operation, the batteries 16, 18 transfer energy to the load 14 together with the generator set 10, so that both the generator set and the batteries supply power to the load. The paralleling mode has both the g and 0 current references equal to zero, and the d current reference is given a negative value. In this mode the state of charge of the batteries is not considered in the current control loop. PI controllers are employed for each of the d, q and 0 components, in a similar way to in the charging mode. The angle Θ used in the dqO transformation is calculated by using the current measured at the output of the inverter. The control topology for the paralleling mode of operation is shown in Figure 5.

Standalone mode

In the standalone mode, the generator set 10 is disconnected from the load 14 by the contactor 12 shown in Figure 1 . In this mode, only the batteries 16, 18 supply power to the load. This mode of operation is similar to the paralleling mode, except that the full load is supplied entirely by the battery inverter system. The bidirectional AC/DC converter 20 operates as an inverter, and the filter 22 is used as output filter. The control topology used in this operation mode is shown in Figure 6. This topology has two control loops. The inner loop is the current loop and the outer loop is the voltage loop. Both the current and voltage controller are PI controllers. The angle Θ is generated internally by the control unit. Controlled voltages are measured at the load using voltage sensors 51 . The reference values are constants. An interface may be proved to modify the reference values if required.

System operation

Figures 7(a) and 7(b) are graphs showing typical operation in the charging and paralleling modes. The voltage waveforms of both batteries are shown in Figure 7(a), and the current waveforms are shown in Figure 7(b). The charging mode takes place from 0 to 0.625s. During the charging mode, the batteries start charging in current mode (0.0 to approximately 0.29s) and then, when the nominal voltage is reached, they change to the voltage charging mode. At this instant, as it can be seen in Figure 7(b), the current falls to almost zero and stays in this condition until the system changes mode of operation.

At around 0.625s the system changes to the paralleling mode of operation. In this mode power flows from the batteries to the load. As a consequence, current flows out of the batteries, and the voltage of each battery reduces with time.

In the example shown in Figures 7(a) and 7(b), battery 1 initially has a higher state of charge than battery 2. As a consequence, the output of battery 1 has a higher voltage than that of battery 2. During the initial charging period (0.0 to approximately 0.075s) a higher current flows into battery 2 than battery 1 , in order to balance the state of charge. In this example, a current initially flows from battery 1 and into battery 2, and thus the voltage of battery 1 drops initially. Once the state of charge of the two batteries is equalized, essentially the same current flows into both batteries, and the output voltages are equalized. During the paralleling mode, variations in the characteristics of the batteries may cause the state of charge of the batteries to vary with time. This is shown in Figure 7(a) as a slight difference in the battery voltage during the paralleling mode. The power consumed and delivered by the system is shown in Figure 8. Positive power represents power flowing into the AC/DC converter and transferring to the batteries. Negative power represents the system operating in the paralleling or standalone mode. In this case, power flows from the batteries to the load. Figure 9 shows parts of the bidirectional AC/DC converter in more detail. In this embodiment, the bidirectional AC/DC converter is a two-level inverter/rectifier module comprising comprises six insulated gate bipolar transistors (IGBTs) T1 -T6 distributed in three legs. An anti-parallel diode D1 -D6 is connected across each of the transistors T1 -T6. Depending on the operation mode, the module acts as rectifier or as an inverter. Alternatively, separate inverter and rectifier circuits could be used instead. The bidirectional AC/DC converter is connected to the filter 22 on one side, and to two battery packs 56, 58 on the other side.

When the system is operating in the charging mode, the inverter/rectifier module acts as an active rectifier and its output is used to charge the two battery packs 56, 58. In this mode the controller 60 is used to control the amount of current supplied to the batteries, in the way described above with reference to Figures 3 and 4. When the system is operating in paralleling or standalone mode, the inverter/rectifier module acts as an inverter and supplies power from the battery packs to the load. In this mode, the controller 60 acts as an inverter controller to control the AC voltage supplied to the load, in the way described above with reference to Figures 5 and 6.

In this embodiment each battery pack 56, 58 consists of a battery and a battery management system. Each battery management system measures the state of charge of its battery, and supplies the measured state of charge to the controller 60.

In both modes of operation, a reference signal generator 62 is used to provide the appropriate reference values to the controller 60. The reference signal generator receives inputs from various sensors, such as current sensors 28, charge sensors 30, 32, and voltage sensors on either side of the bidirectional AC/DC converter. If desired, a user input could be provided to adjust the reference values. This can allow, for example, the rate of battery charging and the rate of charge balancing to be controlled through adjustment of the parameters i_d ^* _c and K_p.

Although embodiments of the invention have been described with reference to a back-up power supply system for a generator set, the present invention is applicable to any situation in which two series connected batteries are charged from and supply power to a multiphase supply. For example, the back-up battery system described used with a local grid, which may include a generator set and/or other source of electrical power. The present system may be particularly suitable for use with renewable energy sources such as wind power generators, wave power generators, etc. While embodiments of the invention have been described with reference to particular examples, it will be appreciated that the invention is not limited to these embodiments, and variations in detail may be made within the scope of the appended claims.

Claims

1 . A battery control system for controlling a set of batteries which are arranged to be charged from and to supply power to a multi-phase AC power line, the multi-phase AC power line comprising a neutral connection, and the set of batteries comprising a first battery and a second battery connected in series with a mid-point of the batteries connected to the neutral connection, the system comprising:

2. A system according to claim 1 , wherein the first mode of operation is a charging mode in which the set of batteries is charged from the AC power line, and the second mode of operation is a discharge mode in which the set of batteries supply power to the AC power line.

3. A system according to claim 1 or 2, wherein the conversion means is arranged to balance the state of charge in the first battery and the second battery as part of the process of converting the AC voltage on the AC power line to DC for charging the batteries.

4. A system according to any of the preceding claims, wherein the

conversion means is a bidirectional AC/DC converter.

5. A system according to any of the preceding claims, wherein the

conversion means comprises an inverter/rectifier circuit.

6. A system according to any of the preceding claims, wherein the

conversion means is a two-level converter.

7. A system according to any of the preceding claims, wherein the conversion means is arranged to be connected across the series connection of the first battery and the second battery.

8. A system according to any of the preceding claims, wherein a filter is provided between the AC power line and the conversion means.

9. A system according to claim 8, wherein the filter acts as an input filter in the first mode of operation, and an output filter in the second mode of operation.

10. A system according to any of the preceding claims, wherein the control means comprises a current control loop for controlling the current supplied to the first and second batteries in the first mode of operation.

1 1 . A system according to claim 10, wherein the current control loop controls the current supplied to the first and second batteries so as to balance their state of charge.

12. A system according to any of the preceding claims, further comprising means for measuring the state of charge of the first and second batteries, wherein the control means is arranged to control the conversion means in dependence on a measured state of charge.

13. A system according to claim 12, wherein the control means is arranged to control the conversion means in dependence on a difference in a measured state of charge of the first battery and a measured state of charge of the second battery.

14. A system according to any of the preceding claims, wherein the neutral connection provides a return path for current flow.

15. A system according to any of the preceding claims, wherein the control means is arranged to adjust a current flowing in the neutral connection.

16. A system according to any of the preceding claims, further comprising means for obtaining a measure of a common mode current.

17. A system according to claim 16, wherein the control means is arranged to control the conversion means in dependence on the measure of the common mode current.

18. A system according to any of the preceding claims, further comprising means for sensing the current of each phase of the AC power line, wherein the control means is arranged to control the conversion means in dependence on the sensed currents.

19. A system according to claim 18, wherein the control means is arranged to derive a measure of the common mode current from the sensed currents.

20. A system according to claim 18 or 19, further comprising means for converting the sensed currents into a rotating frame of reference.

21 . A system according to any of 18 to 20, wherein the sensed currents are converted into measures of direct current, quadrature current, and common mode current.

22. A system according to any of claims 16, 17, 19 or 21 , wherein the control means comprises a control loop for controlling the common mode current so as to balance the state of charge in the first battery and the second battery.

23. A system according to any of claims 16, 17, 19, 20 or 22, wherein the control means is arranged to compare the measure of the common mode current to a reference value to yield a difference signal, and to control the conversion means in dependence on the difference signal.

24. A system according to claim 23, wherein the control means is arranged to derive the reference value from a measure of the state of charge of the first and second batteries.

25. A system according to claim 23 or 24, wherein the reference value is proportional to a difference between a measure of the state of charge of the first battery and a measure of the state of charge of the second battery.

26. A battery system comprising a first battery and a second battery connected in series, and a battery control system according to any of the preceding claims.

27. A battery system according to claim 26, further comprising a battery management system, wherein the battery management system is arranged to provide a measure of the state of charge of the first battery and the second battery.

28. A battery system according to claim 26 or 27, wherein the AC power line is arranged to be connected to a main source of power, and the battery system is arranged to supply auxiliary power to the AC power line in the second mode of operation.

29. A battery system according to any of claims 26 to 28, wherein the battery system is a back-up battery system for an electrical generating set.

30. A generating apparatus comprising an electrical generating set and a battery system according to any of claims 26 to 29.

31 . A method of controlling a set of batteries which are charged from and supply power to a multi-phase AC power line, the multi-phase AC power line comprising a neutral connection, and the set of batteries comprising a first battery and a second battery connected in series, with a mid-point of the batteries connected to the neutral connection, the method comprising:

in a first mode of operation, converting an AC input on the AC power line to DC for charging the batteries;

controlling the first mode of operation and the second mode of operation, wherein, in the first mode of operation, the step of converting an AC input on the AC power line to DC comprises balancing the state of charge of the first battery and the second battery.