Patent Publication Number: US-2023159177-A1

Title: Electrical power system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This specification is based upon and claims the benefit of priority from United Kingdom Patent Application No. 2116919.8, filed on 24 Nov. 2021, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     Technical Field 
     This disclosure relates to an electrical power system for provision of a stabilised DC power supply from an electrical storage unit. 
     Description of the Related Art 
     The use of an Electrical (or Energy) Storage System (ESS) is becoming an important part of advanced electrical power systems for aerospace, marine and automotive applications. An ESS typically employs a bulk energy storage medium such as a high-density battery. In some applications the ESS is generally used intermittently to provide high power for short periods of time such as for engine starting, rotating generator load-levelling (e.g. supplying load peak demands only) or during emergency conditions such as loss of a rotating generator. In other applications, including hybrid-electric and purely electric applications, the ESS may be used continuously or at least for more sustained periods. In the case of electric Vertical Take-off and Landing (eVTOL) applications, the ESS may provide a high power for relatively short periods during take-off and landing and a somewhat lower power during flight. 
     It is possible to connect an electrical storage battery directly to a DC network with the battery terminal voltage determining the operating voltage of the whole DC network. Such an approach is illustrated in  FIG.  4   , which illustrates a DC electrical power system  400  with a direct battery connection. The system  400 , which could be used in practice on an aircraft or ship, comprises a directly connected battery  401  and two DC power sources derived from rotating generators  402 ,  403  connected to respective HP and LP spools  404 ,  405  of a gas turbine, with an AC:DC converter  406 ,  407  connected to each respective generator  402 ,  403  to provide a DC voltage VDC at a voltage supply bus  408 . In such a system, the DC network voltage changes depending on the current drawn from the battery  401 , due to natural source resistance regulation, and the state of charge of the battery  401 , i.e. its internal back EMF. Thus, the direct voltage magnitude is controlled by the battery  401 , which varies with load current and the SOC (state of charge) of the battery  401  itself. Typically, the direct voltage VDC could change by up to around 20%, which results in a higher current draw to deliver the same rated power at a lower voltage and means that all loads must be designed to operate with a variable supply voltage. Furthermore, it means the average DC network voltage could change by up to 20% with other power sources, for example rotating generators coupled to the DC network through associated power electronic converters, having to follow the DC voltage set by the battery  401 . 
     As a result, it is normal practice to connect a battery ESS to a DC network via a fully rated DC:DC power electronic converter, as illustrated schematically in  FIG.  5   . The electrical power system  500  comprises a DC:DC converter  501  connected between a battery  502  and DC bus  503  to provide a regulation function and to allow the voltage across the battery  502  to change as it discharges its stored energy whilst maintaining a near-constant direct voltage at the electrical network it is supplying via the DC bus  503 . The DC:DC converter  501 , which may be bi-directional to allow for charging and discharging of the battery  502 , must be rated to pass all the electrical power provided from the battery  502  to the attached DC network. For example if the ESS  502  supplies 100 kW, the DC:DC converter  501  must also be rated for at least 100 kW. Given the intermittent nature of the power provided by the ESS  502  in some applications, the fully rated DC:DC converter  501  can remain idle for much of the time, meaning it can be considered a wasteful overhead for the electrical system  500  when not in use. 
     A problem with the above approach, particularly when using a battery ESS in aircraft propulsion applications, is that a fully rated converter will add substantial weight. It would be advantageous to be able to reduce the overall weight of an electrical supply system incorporating a battery ESS while not compromising on the available rated power. 
     SUMMARY 
     According to a first aspect there is provided an electrical power system comprising: 
     a DC power bus having first and second DC power bus terminals; 
     an electrical storage unit having first and second terminals, the second terminal connected to the second DC power bus terminal; 
     a DC:DC converter having first and second DC:AC converters and a transformer connected between the first and second DC:AC converters, the first DC:AC converter a connected between the first terminal of the electrical storage unit and the first DC power bus terminal; and 
     a controller connected to control a switching operation of one or both of the first and second DC:AC converters. 
     The DC:DC converter may be bidirectional and the controller connected to control a switching operation of each of the first and second DC:AC converters. In alternative examples the DC:DC converter may be unidirectional and the controller connected to control a switching operation of the second DC:AC converter, the first DC:AC converter comprising a passive rectifier. 
     The second DC:AC converter may be connected across a DC power source. The DC power source may be the electrical storage unit or the DC power bus. Where the DC power bus is a first DC power bus, the electrical power system may comprise a second DC power bus, the DC power source being the second DC power bus. Where the electrical storage unit is a first electrical storage unit, the electrical power system may comprise a second electrical storage unit, the DC power source being the second electrical storage unit. 
     The first and second DC:AC converters may be configured to operate at up to around 20% of a power rating of the electrical storage unit. The power rating of the electrical storage unit may be 50 kW or more, for example between around 50 kW and 150 kW. In a specific example the power rating may be around 100 kW. 
     Each of the first and second DC:AC converters may comprise a plurality of semiconductor devices, the controller configured to control a switching operation of each of the plurality of semiconductor devices. The controller may be configured to operate the first DC:AC converter a to provide a bypass connection between the electrical storage unit and the DC power bus and disable the second DC:AC converter when a voltage across the electrical storage unit is above a threshold level. 
     The first DC:AC converter may be configured to provide a voltage across the DC power bus that is greater than a voltage across the electrical storage unit. The first DC:AC converter may alternatively be configured to provide a voltage across the DC power bus that is less than a voltage across the electrical storage unit. 
     The DC:DC converter may be a dual active bridge, DAB, converter. 
     The electrical power system may comprise a capacitor connected between the first terminal of the electrical storage unit and the first DC power bus terminal and a half-bridge or full-bridge switching circuit connected across the capacitor. 
     According to a second aspect there is provided an aircraft power and propulsion system comprising the electrical power system of the first aspect, the propulsion system comprising a DC:AC converter connected to the DC power bus, an electric machine and a fan, the electric machine connected to the DC:AC converter to receive power from the DC power bus to drive the fan. 
     According to a third aspect there is provided an aircraft comprising the electrical power and propulsion system of the second aspect. 
     The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore except where mutually exclusive any feature described herein may be applied to any aspect and/or combined with any other feature described herein. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described by way of example only with reference to the accompanying drawings, which are purely schematic and not to scale, and in which: 
         FIG.  1    shows a general arrangement of a turbofan engine for an aircraft; 
         FIG.  2 A  is a schematic illustration of a hybrid electric aircraft propulsion system; 
         FIG.  2 B  illustrates an electrically powered propulsor such as may be used in a hybrid electric propulsion system; 
         FIG.  3    is a schematic illustration of a purely electric aircraft propulsion system; 
         FIG.  4    is a schematic illustration of an example DC electrical power system with a direct battery connection; 
         FIG.  5    is a schematic illustration of an example DC:DC converter between a battery and a DC power supply bus; and 
         FIGS.  6  to  13    are schematic illustrations of various alternative example DC electrical power systems. 
     
    
    
     DETAILED DESCRIPTION 
     A general arrangement of an engine  101  for an aircraft is shown in  FIG.  1   . The engine  101  is of a turbofan configuration, and thus comprises a ducted fan  102  that receives intake air A and generates two pressurised airflows: a bypass flow B which passes axially through a bypass duct  103  and a core flow C which enters a core gas turbine. 
     The core gas turbine comprises, in axial flow series, a low-pressure compressor  104 , a high-pressure compressor  105 , a combustor  106 , a high-pressure turbine  107 , and a low-pressure turbine  108 . 
     In operation, the core flow C is compressed by the low-pressure compressor  104  and is then directed into the high-pressure compressor  105  where further compression takes place. The compressed air exhausted from the high-pressure compressor  105  is directed into the combustor  106  where it is mixed with fuel and the mixture is combusted. The resultant hot combustion products then expand through, and thereby drive, the high-pressure turbine  107  and in turn the low-pressure turbine  108  before being exhausted to provide a small proportion of the overall thrust. 
     The high-pressure turbine  107  drives the high-pressure compressor  105  via an interconnecting shaft. The low-pressure turbine  108  drives the low-pressure compressor  104  via another interconnecting shaft. Together, the high-pressure compressor  105 , high-pressure turbine  107 , and associated interconnecting shaft form part of a high-pressure spool of the engine  101 . Similarly, the low-pressure compressor  104 , low-pressure turbine  108 , and associated interconnecting shaft form part of a low-pressure spool of the engine  101 . Such nomenclature will be familiar to those skilled in the art. Those skilled in the art will also appreciate that whilst the illustrated engine has two spools, other gas turbine engines have a different number of spools, for example three spools. 
     The fan  102  is driven by the low-pressure turbine  108  via a reduction gearbox in the form of a planetary-configuration epicyclic gearbox  109 . Thus in this configuration, the low-pressure turbine  108  is connected with a sun gear of the gearbox  109 . The sun gear is meshed with a plurality of planet gears located in a rotating carrier, which planet gears are in turn meshed with a static ring gear. The rotating carrier drives the fan  102  via a fan shaft  110 . It will be appreciated that in alternative embodiments a star-configuration epicyclic gearbox (in which the planet carrier is static and the ring gear rotates and provides the output) may be used instead, and indeed that the gearbox  109  may be omitted entirely so that the fan  102  is driven directly by the low-pressure turbine  108 . 
     It is increasingly desirable to facilitate a greater degree of electrical functionality on the airframe and on the engine. To this end, the engine  101  of the present embodiment comprises one or more rotary electric machines, generally capable of operating both as a motor and as a generator. The number and arrangement of the rotary electric machines will depend to some extent on the desired functionality. Some embodiments of the engine  101  include a single rotary electric machine  111  driven by the high-pressure spool, for example by a core-mounted accessory drive  112  of conventional configuration. Such a configuration facilitates the generation of electrical power for the engine and the aircraft and the driving of the high-pressure spool to facilitate starting of the engine in place of an air turbine starter. Other embodiments, including the one shown in  FIG.  1   , comprise both a first rotary electric machine  111  coupled with the high pressure spool and a second rotary electric machine  113  coupled with the low pressure spool. In addition to generating electrical power and the starting the engine  101 , having both first and second rotary machines  111 ,  113 , connected by power electronics, can facilitate the transfer of mechanical power between the high and low pressure spools to improve operability, fuel consumption etc. 
     As mentioned above, in  FIG.  1    the first rotary electric machine  111  is driven by the high-pressure spool by a core-mounted accessory drive  112  of conventional configuration. In alternative embodiments, the first electric machine  111  may be mounted coaxially with the turbomachinery in the engine  101 . For example, the first electric machine  111  may be mounted axially in line with the duct between the low- and high-pressure compressors  104  and  105 . In  FIG.  1   , the second electric machine  113  is mounted in the tail cone  114  of the engine  101  coaxially with the turbomachinery and is coupled to the low-pressure turbine  108 . In alternative embodiments, the second rotary electric machine  113  may be located axially in line with low-pressure compressor  104 , which may adopt a bladed disc or bladed drum configuration to provide space for the second rotary electric machine  113 . It will of course be appreciated by those skilled in the art that any other suitable location for the first and (if present) second electric machines may be adopted. 
     The first and second electric machines  111 ,  113  are connected with power electronics. Extraction of power from or application of power to the electric machines is performed by a power electronics module (PEM)  115 . In the present embodiment, the PEM  115  is mounted on the fan case  116  of the engine  101 , but it will be appreciated that it may be mounted elsewhere such as on the core of the gas turbine, or in the vehicle to which the engine  101  is attached, for example. 
     Control of the PEM  115  and of the first and second electric machines  111  and  113  is in the present example performed by an engine electronic controller (EEC)  117 . In the present embodiment the EEC  117  is a full-authority digital engine controller (FADEC), the configuration of which will be known and understood by those skilled in the art. It therefore controls all aspects of the engine  101 , i.e. both of the core gas turbine and the first and second electric machines  111  and  113 . In this way, the EEC  117  may holistically respond to both thrust demand and electrical power demand. 
     The one or more rotary electric machines  111 ,  113  and the power electronics  115  may be configured to output to or receive electric power from one, two or more dc busses. The dc busses allow for the distribution of electrical power to other engine electrical loads and to electrical loads on the airframe. The dc busses may further receive electrical power from, or deliver electrical power to, an energy storage system such as one or more battery modules or packs. 
     Those skilled in the art will appreciate that the gas turbine engine  101  described above may be regarded as a ‘more electric’ gas turbine engine because of the increased role of the electric machines  111 ,  113  compared with those of conventional gas turbines. 
       FIG.  2 A  illustrates an exemplary propulsion system  200  of a hybrid electric aircraft. The propulsion system  200  includes a generator set  202  comprising an engine  210  and electrical generator  211 , and a battery pack  230 . Both the generator set  202  and the battery pack  230  are used as energy sources to power an electric motor-driven propulsor  204 , an example of which is shown in  FIG.  2 B . 
     The illustrated propulsion system  200  further comprises an AC:DC converter  212 , a dc distribution bus  213 , a DC:AC converter  214  and a DC:DC converter  217 . It will be appreciated that whilst one generator set  202  and one propulsor  204  are illustrated in this example, a propulsion system  200  may include more than one generator set  202  and/or one or more propulsor  204 . 
     A shaft or spool of the engine  210  is coupled to and drives the rotation of a shaft of the generator  211  which thereby produces alternating current. The AC:DC converter  212 , which faces the generator  211 , converts the alternating current into direct current, which is fed to various electrical systems and loads via the dc distribution bus  213 . These electrical systems include non-propulsive loads (not shown in  FIG.  2 A ) and the motor  215  which drives the propulsor  216  via the DC:AC converter  214 . 
     The battery pack  230 , which may be made up of a number of battery modules connected in series and/or parallel, is connected to the DC distribution bus  213  via the DC:DC converter  217 . The DC:DC converter  217  converts between a voltage of the battery pack  230  and a voltage of the DC distribution bus  213 . In this way, the battery pack  230  can replace or supplement the power provided by the generator set  202  (by discharging and thereby feeding the DC distribution bus  212 ) or can be charged using the power provided by the generator set  202  (by being fed by the DC distribution bus  213 ). 
     Referring to  FIG.  2 B , in this example the propulsor  204  takes the form of a ducted fan. The fan  216  is enclosed within a fan duct  219  defined within a nacelle  221 , and is mounted to a core nacelle  222 . The fan  216  is driven by the electric machine  215  via a drive shaft  224 , both of which may also be thought of as components of the propulsor  204 . A gearbox  220  may be provided between the electric machine  215  and the drive shaft  224  to allow the fan  216  to rotate at a different (typically lower) rotational speed to the electric machine  215 . 
     The electric machine  215  is supplied with electric power from a power source, for example the generator set  202  and/or the battery  230  via the DC bus  213 . The electric machine  215  of the propulsor  204 , and indeed the electric machine  211  of the generator set  202 , may generally be of any suitable type, for example of the permanent magnet synchronous type. 
     Those skilled in the art will recognize the propulsion system  200  of  FIGS.  2 A-B  to be of the series hybrid type. Other hybrid electric propulsion systems are of the parallel type, while still others are of the turboelectric type or have features of more than one type. The configuration of the more electric engine  101  of  FIG.  1    may be considered similar to a parallel hybrid system, with the main distinction being the roles of the electric machines. For example, the electric machines of a more electric engine are generally only used in motor mode to start the engine and to improve engine operability, whereas the electric machines of a parallel hybrid propulsion system are used to motor the spools to meaningfully add to the amount of propulsive thrust produced by the turbomachinery. 
     Those skilled in the art will also appreciate that the hybrid architecture illustrated in  FIG.  2 A  is only one example, and that other architectures, including architectures with ac distribution busses, are known and will occur to those skilled in the art. 
       FIG.  3    illustrates an exemplary electric propulsion system  300  of a purely electric aircraft. Alternative electric propulsion system arrangements are known and will occur to those skilled in the art. 
     The propulsion system  300  includes a battery pack  330  that feeds a high voltage (HV) DC distribution bus  313 , possibly via a DC:DC converter (not shown), which delivers power to one or more synchronous motors  315  via a DC:AC converter  314 . The one or more motors  315  drive the one or more propellers  316  that propel the aircraft. 
     The DC electrical power systems  400 ,  500  illustrated in  FIGS.  4  and  5    have been described above in relation to the background. 
       FIG.  6    illustrates an example DC electrical power system  600  in simplified form, the electrical power system  600  having a DC power bus  601 , an electrical storage unit  602  (for example a battery) and a DC voltage source  603 . The DC voltage source  603  is connected in series between the electrical storage unit  602  and the DC power bus  601  and is arranged to compensate for variations in the voltage across the electrical storage unit  602 . A voltage across the voltage source  603  may for example add to or subtract from a voltage across the electrical storage unit  602  to maintain a required stable voltage across the DC power bus  601 . For example, as the voltage across the electrical storage unit  602  decreases during discharging, the voltage across the voltage source  603  may be increased to compensate. 
       FIG.  6    illustrates the voltage source  603  connected between a positive terminal of the electrical storage unit  602  and a positive terminal  605   a  of the DC power supply bus  601 . The voltage source  603  may alternatively be provided between a negative terminal  605   b  of the DC power supply bus and the negative terminal of the electrical storage unit  602 . A voltage source may alternatively be connected to both positive and negative terminals  605   a ,  605   b.    
     In the example in  FIG.  6   , first and second capacitors  604   a ,  604   b  are connected in series between first and second DC power bus terminals  605   a ,  605   b . A ground or common connection  606  is provided between the first and second capacitors  604   a ,  604   b , thereby providing a differential output. In alternative single-sided examples, the second DC power bus terminal  605   b  may be connected to a common connection. 
     Because the DC voltage source  603  is connected in series, it may be rated at a lower power rating than the electrical storage unit  602 . If, for example, the voltage across the electrical storage unit  602  varies by up to 20%, the DC voltage source  603  may only be required to have a 20% power rating compared to the power rating of the electrical storage unit  602 . 
     When the voltage source  603  conducts direct current, this either sinks or sources power. When sourcing power, i.e. when the electrical storage unit  602  is discharging, the voltage source  603  draws power from a DC power source, which may be the electrical storage unit  602  or another DC power source such as another electrical storage unit, the DC power supply bus  601  or another DC power supply bus. When sinking power, i.e. when the electrical storage unit  602  is charging, the voltage source sends power to the electrical storage unit  602 , another electrical storage unit, the DC power supply bus  601  or another DC power supply bus. The voltage source is therefore required to be bidirectional to allow for the electrical storage unit  602  to be charged and discharged while maintaining a stable voltage across the DC power supply bus  601 . 
     In an example implementation, the electrical storage unit  602  may be a battery capable of providing a nominal 540 V, resulting in a differential +/−270 V across the DC power supply bus  601 . The voltage source  603  may be configured to supply up to 20% of the battery nominal voltage, i.e. up to around 108 V to compensate for up to a 20% loss in voltage across the battery  602  as the battery  602  discharges. If a voltage source is connected on each side of the DC voltage supply  601 , each source may have a reduced rating, for example each being rated at up to 10% of the total power rating of the battery  602 . 
     Because there needs to be voltage isolation between the series connected voltage source  603  and the power source or sink to which it is connected, the voltage source  603  is provided in the form of a DC:AC:DC dual active bridge (DAB) converter. Examples of electrical power systems having such a converter are illustrated in  FIGS.  7  to  13   , as described below. 
       FIG.  7    illustrates schematically an example electrical power system  700  comprising a DC power bus  701 , an electrical storage unit  702  and a DC voltage source  703  in the form of a bidirectional DC:DC converter. The DC power bus  701  has first and second DC power bus terminals  705   a ,  705   b . The electrical storage unit  702  has first and second terminals  706   a ,  706   b , the second terminal  706   b  connected to the second DC power bus terminal  705   b . The second terminals  705   b ,  706   b  may be negative or ground terminals and the first terminals  705   a ,  706   a  positive terminals. The bidirectional DC:DC converter  703  comprises first and second DC:AC converter circuits  707   a ,  707   b  and a transformer  708  connected between the first and second DC:AC converters  707   a ,  707   b . The first DC:AC converter  707   a  is connected between the first terminal  706   a  of the electrical storage unit  702  and the first DC power bus terminal  705   a.    
     Each of the DC:AC converter circuits  707   a ,  707   b  is in the form of a switched mode power converter comprising a plurality of semiconductor devices, the switching operation of each being controlled by a controller  709 . The controller  709  may be part of the PEM  115  as described above in relation to the engine  101  shown in  FIG.  1   . Switching sequences to achieve a desired DC:AC conversion will be well known to the skilled person. Each semiconductor device comprises a power transistor in parallel with a diode. Switching of the power transistors by signals received from the controller  709  (connections to which are not shown in  FIG.  7    for clarity) allows a proportion of power flowing to or from the electrical storage unit  702  to be transferred between the converter circuits  707   a ,  707   b  via the transformer  708 . 
     The transformer  708  serves to isolate the converter circuits  707   a ,  707   b  and to convert an AC current from one converter circuit to an AC current in the other converter circuit. 
     As shown in  FIG.  7   , during discharge of the battery  702  power flows from the battery  702  to the DC power bus  701  through the first and second converters  707   a ,  707   b . If the battery has discharged to an extent that the voltage available has reduced by 20%, the battery will only provide 80% of the power P (i.e. 0.8 P) directly to the DC power bus  701 . The remaining power (0.2 P) flows from the battery  702  through the second converter circuit  707   b  and is transformed by the transformer  708  and added to the power (0.8 P) flowing from the battery  702 . The converter circuits  707   a ,  707   b  are controlled by the controller  709  with switching sequences to convert 20% of the power so that this adds to the 80% provided directly, with the result that the DC power bus  701  receives the full power at the required nominal voltage. 
     Instead of being connected across the battery  702 , the second DC:AC converter  707   b  of the DC voltage source  703  may be connected across the DC power bus, as shown in the electrical power system  800  in  FIG.  8   , or a second DC power bus as shown in the electrical power system  900  in  FIG.  9   . The second DC power bus  909  may be connected across a second electrical storage unit (not shown) similar to the electrical storage unit  702 . Other components of the systems  800 ,  900  are similar to those in the example illustrated in  FIG.  7   . The example in  FIG.  9    enables power to be transferred between two DC power buses while maintaining galvanic isolation, which can be useful in cases where one DC bus is experiencing heavy loading requiring ESS support while another DC bus is more lightly loaded. In some examples, the second DC:AC converter  707   b  may be switchably connectable between two or more of the electrical storage unit  702 , the DC power bus  701 , a second DC power bus  909  and a second electrical storage unit (not shown). 
     A partially rated dual active bridge approach, as shown in  FIGS.  6  to  9   , may enable improvements in cost, size and weight compared to use of a fully rated DC:DC converter. In particular, the use of lower voltage semiconductors in the LV side [for example at a 20% voltage rating] of the DAB means the associated conduction losses will be lower than would be incurred with semiconductors rated for the full battery voltage. There is also the potential to improve the efficiency of the converters by applying soft switching [i.e. switching at around zero voltage and/or zero current] of the transistors to minimise switching losses, which is widely known for DAB converters. Also, with advanced Silicon Carbide [SiC] MOSFETs, there is the possibility to apply “synchronous rectification” methods, in which MOSFETs are controlled to carry current in a reverse direction to support or replace diode conduction with the benefit of reduced conduction losses. The use of soft switching and synchronous rectification may further reduce power losses during voltage stabilisation operation and hence reduce the burden on the thermal management and cooling system for the electronic power system. 
     In cases where the battery  702  is charged to its nominal capacity such that the voltage across the battery  702  is equal to the required DC bus voltage, the converter  703  may be bypassed by disabling the second DC:AC converter  707   b  and switching the first DC:AC converter  707   a  to a bypass mode, as illustrated in  FIG.  10   . In this mode, the connection between the first terminal  706   a  of the battery  702  and the first terminal  705   a  of the DC power bus is shorted by turning on all the semiconductor devices to provide parallel current paths, shown schematically by paths  1001 ,  1002  in  FIG.  10   . Current from the battery  702  to the DC power bus  701  may then be carried by the MOSFETs and diodes of the semiconductor devices, allowing conduction losses presented by the converter  707   a  to be minimised and with no switching losses. In a general aspect therefore, the controller  709  may be configured to operate the first DC:AC converter  707   a  to provide a bypass connection between the battery  702  and the DC power bus  701  and to disable the second DC:AC converter when a voltage across the battery  702  is above a threshold level. The voltage across the battery  702  may for example be measured by a voltage sensor, the output of which is provided to the controller  709 . 
     The examples described above relate to “boost” series injection methods where the battery discharges to a voltage below that of the DC network. The same approach may also be applied to “buck” series injection methods where the battery is designed to have a voltage higher than the DC network. This may be accommodated by connecting the series injection voltage source in the reverse direction, i.e. with the polarities of the semiconductor devices in the first DC:AC converter reversed. In a general aspect therefore, the first DC:AC converter  707   a  may be configured to provide a voltage across the DC power bus  701  that is either greater than (a boost converter) or less than (a buck converter) a voltage across the electrical storage unit  702 . 
     In addition to the dual active bridge isolated DC:DC converter, a further possibility, illustrated in the electrical power system  1100  in  FIG.  11   , is to utilise a bidirectional “flyback” converter  1103 , which also provides the necessary isolation between the two sides of the converter. As with the other examples, the converter  1103  comprises a first DC:AC converter  1107   a , a second DC:AC converter  1107   b  and a transformer  708 . Operation of the semiconductor devices in each converter  1107   a ,  1107   b  is controlled by a controller  709  as with the other examples, with the difference being in the switching signals provided. A flyback converter differs from a DAB converter in that power flow through the converter is pulsed, i.e. it stores energy in one transformer winding during the first part of the switching cycle and then releases it during the second part of the switching cycle. Other possible DC to DC converter topologies with galvanic isolation may also be used. 
     Battery energy storage is generally characterised by having a low internal series resistance, which allows the battery to provide high levels of current to the DC network. This is observed when initially charging the battery from the DC network and normally requires the temporary introduction of a soft-start resistor which can be bypassed by a mechanical contactor once the battery and DC network voltages are equalised. Other disturbances and faults on the DC network can lead to large differences between the battery and DC network voltages which can also lead to temporarily high levels of current flowing. A directly connected battery may be susceptible to these potentially high current transients. A fast acting circuit breaker may therefore be used to connect the battery to the DC network, which can respond to interrupt the current before it gets too high to cause damage. Solid state circuit breakers can operate in microseconds and would therefore be a preferable choice for dealing with such problems. 
     The series voltage source can in some instances be used to compensate for the difference between the battery and DC network voltage in order to reduce or eliminate high current surges flowing between the two. By varying the value of the series injection voltage, a voltage is developed across the equivalent battery resistance, which in turn controls the current flowing in or out of the battery and may be used to vary the power exchanged with the ESS. This method of current control is dependent on the voltage capability and polarity of the series voltage source, whether in the form of a boost or buck converter, together with the series resistance of the battery. Given that the series voltage source is variable and can be changed at a rate set by the controller of the DAB or flyback converter, the series compensation method provides a degree of current control which may be superior to a directly connected battery. In a general aspect therefore, the controller may be configured to operate the DC:DC converter to limit current flow through the electrical storage unit during charging or discharging of the electrical storage unit. 
     The converter connected in parallel with the electrical storage unit can be used to cancel or minimise the impact of a voltage disturbance on the DC network which might otherwise lead to cause large dynamic currents to flow into the battery terminals. For example, in the case of an over-voltage surge on the DC power bus, a compensating current may be drawn by the second converter  707   b  ( FIG.  7   ) to counteract the voltage surge by raising the series voltage across the first converter  707   a . Current pushed towards the battery  702  can thereby be diverted away by the parallel converter  707   b  to protect the battery  702  from excess current. 
     In the example electrical power systems described herein, series voltage compensation is achieved using a fixed capacitor  710 , which is connected in series between the first battery terminal  706   a  and the first DC power bus terminal  705   a . An extension to this may be used in which the capacitor  710  is switched in series with the battery  702  and/or may have its polarity reversed so the voltage injection may be positive or negative giving both boost and buck functionality. Example electrical power systems  1200 ,  1300  are illustrated in  FIGS.  12  and  13    respectively in which a half-bridge switching circuit ( FIG.  12   ) and a full bridge switching circuit ( FIG.  13   ) is connected across the capacitor  710  connected between the first terminal  706   a  of the battery  702  and the first terminal  705   a  of the DC power bus  701 . Switching of the capacitor  710  may be achieved using mechanical contactors, although it may be preferable to use semiconductor switches. It is important to note that the series voltage source  703  can be bypassed or inserted more rapidly when using semiconductor switches, enabling the series voltage to be changed between 0.2 per-unit and 0 without the delay introduced by charging and discharging the series converter capacitor  710 . 
     In the electrical power system  1200  illustrated in  FIG.  12   , a half-bridge switching circuit  1201  enables the series compensation voltage to be connected or bypassed. In the electrical power system  1300  illustrated in  FIG.  13   , a full-bridge switching circuit  1301  enables the series compensation voltage to be connected in the positive or negative direction or be bypassed. As such it enables situations where the battery voltage is lower than the DC network or higher than the network to be addressed. The bypass mode may be used when the battery voltage is approximately equal to the DC network in a similar manner to that described above in relation to the system illustrated in  FIG.  10    but without the need to discharge the DC capacitor providing the injection voltage. 
     The examples illustrated in  FIGS.  12  and  13    are shown in connection with a dual active bridge DC:AC:DC arrangement and with the second converter  707   b  connected across the electrical storage unit  702 , although other topologies including the alternative flyback arrangement illustrated in  FIG.  11    may also be used. Similarly the single phase dual active bridge could be implemented with more than one phase, for example using a 3-phase AC link and isolation transformer. 
     An advantage of the examples illustrated herein is that a stabilised DC voltage may be provided from a battery-based ESS using only a partially rated power electronic converter, which is a significant improvement over a directly connected battery. The use of a partially rated converter means the electrical plant will tend to be cheaper, lighter, smaller and more efficient than the traditional method of voltage stabilisation using a fully rated DC:DC converter. In some implementations the series converter may be operated in an electronic bypass mode when the loading on the battery is low and/or the state of charge [SOC] is high. Here the battery voltage defines the operating voltage of the DC network to within acceptable limits [e.g. 5% of nominal voltage]. In a general aspect therefore, the controller may be configured to operate the first DC:AC converter to provide a bypass connection between the electrical storage unit and the DC power bus and disable the second DC:AC converter when a voltage across the electrical storage unit is within a threshold range, e.g. within 5%, of a voltage across the DC power bus. 
     A further advantage is that the electrical power system can provide bidirectional power transfer capability which is required during battery discharge and re-charge modes of operation. In some examples, however, unidirectional power transfer may be sufficient such as for battery-powered applications where charging of the battery is not carried out during travel. 
     Some aircraft may for example be powered battery powered during flight and re-charged when grounded. In such cases, the first converter  707   a  (see e.g.  FIG.  7   ) may comprise a passive rectifier since a switching capability in the first converter is not required. 
     The electrical power system can offer some degree of current control which is better than a directly connected battery and may remove the need for the normal soft-start resistor-contactor solution when initially charging the battery from the DC network. 
     Boost and Buck solutions are possible to suit the specific application requirements. 
     The series converter can be designed to exhibit very low losses by using zero voltage and/or zero current switching, using synchronous rectification, using low voltage semiconductors in the 20% series converter part [inherent low conduction losses] and by employing electronic bypass where possible. 
     The real power required for the series voltage source can be exchanged with the battery, the DC network or another bespoke power supply operating at an optimised voltage. Exchanging for example a 20% power requirement with another DC power channel provides a means of transferring power between to two channels with galvanic isolation. 
     It may be possible to combine the boost and buck arrangements to produce a bidirectional buck-boost arrangement capable of injecting for example between 0-20% voltage in series with the battery in either the positive or negative directions. This would require the transistor-diode switches in the converter to be replaced with semiconductor-based switches with bidirectional voltage and bidirectional current capability which are well known in the power electronics industry and used in AC to AC “matrix” converters. This buck-boost variant is another method of achieving the performance of switched series voltage compensation shown in the example illustrated in of  FIG.  13   . 
     Various examples have been described, each of which feature various combinations of features. It will be appreciated by those skilled in the art that, except where clearly mutually exclusive, any of the features may be employed separately or in combination with any other features and the embodiments extend to and include all combinations and sub-combinations of one or more features described herein. 
     It will also be appreciated that whilst the embodiments have been described with reference to aircraft and aircraft propulsion systems, the electric machine drive techniques described herein could be used for many other applications. These include, but are not limited to, automotive, marine and land-based applications.