WO2011135293A2

WO2011135293A2 - Power supply for telecom base station

Info

Publication number: WO2011135293A2
Application number: PCT/GB2011/000644
Authority: WO
Inventors: Jawad Al-Tayie; Robert Seliga; Sitaram Ganeshan; Simon David Lount; Richard Mark Hardy; Emil Ernest; Rauf Abdul Sattar
Original assignee: Cummins Generator Technologies Limited
Priority date: 2010-04-30
Filing date: 2011-04-26
Publication date: 2011-11-03
Also published as: GB2492690A; GB201007343D0; GB201217958D0; WO2011135293A3; GB2492690B

Abstract

Power supply apparatus for a telecommunications base station disclosed. The power supply apparatus comprises means for receiving an AC mains power supply (10), conversion means (18; 60; 70; 72) for converting the AC mains power supply to a DC voltage, a DC bus (20) for supplying DC power to at least one DC load (22, 24) in the base station, and a standby generator set (34). Means (14) are provided for detecting an irregularity in the mains power supply, and power for the base station is supplied from the standby generator set on detection of an irregularity in the mains power supply. The standby generator set (34) is a variable speed generator set, and the conversion means is operable to convert an output of the variable speed generator set to a substantially constant DC voltage to supply the DC bus when the base station is powered by the variable speed generator set.

Description

POWER SUPPLY FOR TELECOM BASE STATION

The present invention relates to a power supply for a telecommunications (telecom) base station. In particular, the present invention relates to a power supply which can provide a back-up source of power in the event of a failure of the main power supply to the base station.

Telecommunications base stations usually have their power supplied from the electrical grid mains. In some cases back up power is supplied from a stand-by generator set. Some telecommunication sites rely entirely on a generator set as no grid mains is available. Where a generator set is employed, depending on the size of the installation, the rating is generally between 7.5kVA and ^"lOOkVA.

Typically telecommunications base station electrical loads comprise air-conditioning, switch mode power supply (SMPS), battery banks, base transceiver (BTS) units, transmitter equipment (Tx) and lighting. Telecommunications generator sets are usually over-sized to cope with the transient demand of air-conditioning systems and to provide short and rapid charging for battery banks. Duty cycles of the generator set vary depending on the equipment installed and whether a mains supply is present and how reliable it is.

Most modern telecommunications installations employ a synchronous field generator set consisting of a wound field generator driven by a reciprocating internal combustion engine. The generator is equipped with a controllable excitation system that provides substantially constant terminal voltage over a wide range of leading or lagging power factors. The frequency is stabilised by the speed governor of the engine. The engine is usually a diesel engine and the speed is usually fixed at 1500 or 1800 rpm to provide a 50 or 60 Hz output frequency. Existing generator sets for telecommunications base stations tend to be inefficient due to their fixed speed operation and the need to be over-sized to cope with transient loads.

According to one aspect of the present invention there is provided power supply apparatus for a telecommunications base station, the power supply apparatus comprising:

means for receiving an AC mains power supply; conversion means for converting the AC mains power supply to a DC voltage; a DC bus for supplying DC power to at least one DC load in the base station; a standby generator set;

means for detecting an irregularity in the mains power supply; and

means for supplying power for the base station from the standby generator set on detection of an irregularity in the mains power supply;

wherein the standby generator set is a variable speed generator set, and the conversion means is operable to convert an output of the variable speed generator set to a substantially constant DC voltage to supply the DC bus when the base station is powered by the variable speed generator set.

The present invention may provide the advantage that the speed of the engine can be varied to match variations in the load demand, while supplying a DC load in the base station. This can allow the engine to be operated at optimal speed for the demanded load. This can lead to increases in efficiency and fuel saving and reduce the overall size and weight of the generator set. Furthermore, the same converter which is used to convert the mains power supply to a DC voltage can also be used to convert the variable output of the generator set to a substantially constant DC voltage. This can allow a single converter to be used, reducing the size and cost of the system.

An irregularity in the mains power supply may be loss of the mains power supply, or any other abnormality such as voltage drop. Preferably the conversion means is operable to convert a variable voltage, variable frequency output of the variable speed generator set to a substantially constant DC voltage to supply the DC bus.

In order to convert a variable output of the generator set to a substantially constant DC voltage, the power supply apparatus may further comprise control means for controlling operation of the conversion means in dependence on the power demand of the base station when the base station is powered by the variable speed generator set. In one embodiment of the invention the conversion means comprises a plurality of switched mode power supplies. At least some of the switched mode power supplies may be existing switched mode power supplies in the base station, although additional switched mode power supplies may be provided to account for variations in the output voltage of the generator set.

The switched mode power supplies may be connected in parallel, and at least one of the switched mode power supplies may be connected and disconnected as the power demand of the base station changes. For example, at low power fewer switched mode power supplies may be connected than at high power. A distributor may be provided for distributing power from the mains power supply or the variable speed generator set to the plurality of switched mode power supplies. For example, the distributor may be arranged to control the distribution of power to the switched mode power supplies in dependence on the power demand of the base station.

Preferably the switched mode power supplies include a voltage regulator. This may assist in converting the variable output of the generator set to a substantially constant DC voltage.

In another embodiment of the invention the conversion means comprises an AC/DC converter comprising a rectifier and a buck or boost circuit. The converter may replace the switched mode power supplies which would otherwise be present in the power supply apparatus. Preferably the converter further comprises a voltage regulator for regulating the output of the buck or boost circuit. This may assist in converting the variable output of the generator set to a substantially constant DC voltage. Some of the electrical loads in the base station may require an AC supply. Thus the power supply apparatus may further comprise an inverter for converting the DC bus voltage to an AC output voltage. In this case the inverter is connected to the DC bus. This arrangement can allow the variable speed generator set to provide an AC output of substantially constant voltage and/or frequency, via the DC bus and inverter.

In other embodiments of the invention, the power supply apparatus comprises an AC/AC converter operable to convert the variable voltage output of the variable speed generator set directly to a substantially constant AC output voltage. The AC/AC converter may be a cycloconverter, such as that described in United Kingdom patent application number GB 2 450 891. Alternatively, the AC/AC converter may be an IGBT (insulated gate bipolar transistor) based PW (pulse width modulation) converter, or any other converter capable of converting from one AC voltage to another. Preferably the AC/AC converter is also capable of converting from one frequency to another. Preferably the AC/AC converter is operable to convert a variable voltage, variable frequency output of the variable speed generator set to a substantially constant AC voltage and/or frequency, or to an AC voltage and/or frequency which are variable in dependence on the requirements of the AC load.

The AC/AC converter may be connected in parallel with an AC/DC converter, and may be supplied with either the mains power or the output of the generator set (depending on which is supplying the power to the base station). In this arrangement, the AC/DC converter may supply the DC bus, while the AC/AC converter may convert the variable output of the variable speed generator set directly to the voltage and/or frequency required by the AC components in the base station. Such an arrangement may simplify the connections which need to be made by the consumer, making the apparatus easier to install.

In another example, the AC/AC converter may be operable to convert the variable voltage and frequency AC output of the generator set to the same voltage and frequency as the AC mains. This may allow the output of the generator set and AC/AC converter simply to replace the mains power when the mains power has failed. In this case, the converter may consist of switched mode power supplies usually present in the unit. In this arrangement the power supply apparatus may further comprise means for synchronising the output of the AC/AC converter with the mains power supply, in order to allow parallel operation, or to facilitate the handover from one to the other.

In any of the arrangements described above, the power supply apparatus may further comprise one or more batteries connectable to the DC bus. The batteries may be arranged to supply power to the DC bus when an irregularity in the mains supply is first detected. If the mains fault is intermittent, this can allow the batteries to supply the DC bus for a short period without the need for the generator set, or can allow time for the generator set to start and come up to speed. Furthermore, the batteries may provide a backup source of power when the variable speed generator set needs to increase its speed to meet an increase in load demand. This can help to maintain the DC bus at a substantially constant voltage. Preferably the batteries are chargeable from the DC bus. A charge/discharge unit may be provided which charges the batteries from the DC bus when there is sufficient power from the main power supply (mains or generator set), and manages the supply of power from the batteries to the DC bus when there is insufficient power from the main power supply.

The power supply apparatus may further comprise means for connecting a renewable energy source directly to the DC bus. Preferably the renewable energy source has its own converter which is connected directly to the DC bus. In this way the energy supplied by the renewable source can supplement the main power supply regardless of whether it comes from the mains or the generator set. In any of the arrangements described above, the power supply apparatus may further comprise a speed controller operable to control the speed of the variable speed generator set in dependence on the power demand of the base station. In this way the speed of the generator set may be controlled such that it operates at or close to the most efficient speed for the demanded power. In order to achieve this, the power supply apparatus may further comprise means for sensing the power demand and feeding the sensed power demand to the speed controller.

The speed controller may be integrated with control means for controlling operation of the conversion means. This can allow, for example, the amount of boost to be adjusted in accordance with adjustments to the speed of the generator set, which may improve the overall control mechanism. This arrangement may also allow algorithms such as load anticipation to be used.

The power supply apparatus may further comprise means for switching power for the base station from the mains power supply to the standby generator set on detection of an irregularity in the mains power supply. Alternatively, in some circumstances, a combination of the AC mains power supply and the generator set may be used to power the base station. The standby generator set preferably comprises a generator and a prime mover. The generator may be a permanent magnet generator, or a synchronous field generator. In a preferred embodiment, the generator is an axial flux permanent magnet alternator, although other types of generator may be used instead. The mains supply and/or the generator set may be single phase, three phase, or any other appropriate number of phases. The prime mover may be an internal combustion engine such as a diesel engine. Preferably, torque and fuel consumption maps of the engine are programmed into an electronic speed governor for load adaptive speed control.

In another aspect of the invention, there is provided a telecommunications base station comprising a power supply apparatus in any of the forms described above. The telecommunications base station preferably comprises at least one DC load connected to the DC bus.

According to another aspect of the present invention there is provided power supply apparatus which supplies power to telecommunications base station, the power supply apparatus comprising:

an input which receives an AC mains power supply;

a converter which converts the AC mains power supply to a DC voltage; a DC bus which supplies DC power to at least one DC load in the base station;

a standby generator set;

a detection unit which detects an irregularity in the mains power supply; and a switching unit which switches power for the base station from the mains power supply to the standby generator set on detection of an irregularity in the mains power supply;

wherein the standby generator set is a variable speed generator set, and the converter is operable to convert an output of the variable speed generator set to a substantially constant DC voltage to supply the DC bus when the base station is powered by the variable speed generator set.

Corresponding methods are also provided, and thus, according to another aspect of the present invention, there is provided a method of supplying power to a

telecommunications base station, the method comprising:

receiving an AC mains power supply;

converting the AC mains power supply to a DC voltage;

supplying DC power to at least one DC load in the base station;

detecting an irregularity in the mains power supply;

supplying power for the base station from a standby generator set on detection of an irregularity in the mains power supply;

operating the standby generator set as a variable speed generator set; and converting an output of the variable speed generator set to a substantially constant DC voltage for use by the base station at least in part using the same conversion means as was used for converting the AC mains power supply to a DC voltage.

At least part of the conversion means may be an existing converter in the base station.

Features of one embodiment may be provided with any of the other embodiments as appropriate. Any of the apparatus features may be provided with the method aspect and vice versa.

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

Figure 1 shows a first embodiment of a power supply apparatus for a telecommunications base station;

Figure 2 shows parts of a switched mode power supply;

Figure 3 shows a second embodiment of a power supply apparatus;

Figure 4 shows a third embodiment of a power supply apparatus;

Figure 5 shows parts of an AC/DC converter;

Figure 6 shows a fourth embodiment of a power supply apparatus;

Figure 7 shows a fifth embodiment of a power supply apparatus; and

Figure 8 shows a sixth embodiment of a power supply apparatus.

Figure 1 shows a first embodiment of a power supply apparatus for a

telecommunications base station. Referring to Figure 1 , the apparatus comprises terminals 12 for receiving power from a mains power supply 10, a mains failure detection and switch-over unit 14, a distribution board 16, and a plurality of switched mode power supplies 18 for supplying a DC bus 20. The DC bus 20 is used to supply DC power to parts of the telecommunications base station, such as transmitter equipment 22, base transceiver unit 24, and battery banks 26. The DC bus 20 also supplies power to an inverter 28, which converts the DC bus voltage to an AC voltage for supply to air conditioning 30 and lighting 32. In the present embodiment the DC bus voltage is 48V. However it will be appreciated that any suitable bus voltage could be used. Furthermore, while Figure 1 shows an inverter with a rating of 3kW, in practice the inverter will be rated in accordance with the expected power demand of the equipment it is to supply, and any suitable rating could be used. In an alternative arrangement (not shown), an interface is used in the distribution board to direct the mains power to the AC loads, and the air conditioning and lighting are supplied directly from the mains when mains power is being used.

The power supply apparatus of Figure 1 also comprises a variable speed generator set 34 comprising an engine 36, a permanent magnet alternator 38 and a speed controller 40. The variable speed generator set may any suitable generator set, such as that described in International patent application number WO 01/56133, the contents of which are incorporated herein by reference. A master controller 42 is provided to control the operation of the switched mode power supplies 18 and the distribution board 16. Although not shown in Figure 1 , the master controller 42 and the speed controller 40 may be linked, or may be part of the same control unit. In normal operation, power for the telecommunications base station is supplied from the mains power supply 10. The distribution board 16 distributes the mains power to the switched mode power supplies 18. The switched mode power supplies 18 are connected in parallel, and convert the AC mains voltage to a regulated DC voltage for supply to the DC bus 20. Five switched mode power supplies are shown by way of illustration in Figure 1 , but in practice any number could be used, depending on the power ratings of the switched mode power supplies and the power requirements of the telecommunications base station.

If a failure or abnormality occurs in the mains power supply, this is detected by the mains failure detection and switchover unit 14. In this case, power for the base station is switched from the mains supply 10 to the generator set 34. This may involve starting the engine 36 and bringing it up to the speed necessary to generate the required power. While this is happening, power may be supplied to the DC bus 20 by the battery banks 26. At other times, the battery banks are charged from the DC bus.

When power is being supplied from the generator set 34, the speed of the engine 36 is varied so that it operates at optimum efficiency for the demanded power. The power drawn from the DC bus 20 is sensed by power sensor 44 (consisting of a voltage sensor and current sensor) and fed back to the speed controller 40. The speed controller produces an analogue or digital speed signal which is in turn used by the engine electronic governor to actuate the speed of the engine 36. As the power demand changes (for example as units such as air conditioning 30 and lighting 32 are switched on or off), the speed of the engine 36 is varied accordingly.

Alternatively or in addition the power output of the generator 38 could be sensed, as shown by the dotted line.

In contrast to the mains power supply 10, the output of the variable speed generator set 34 is a variable voltage, variable frequency output. When power is being supplied by the generator set 34, the switched mode power supplies 18 are operated to convert the variable frequency, variable voltage output of the generator set to a regulated DC voltage for the DC bus 20. The switched mode power supplies are monitored by the master controller 42, and switched on and off as power demand changes. The power at the DC bus is monitored in order to determine how many SMPS's are required. The distribution board is used to distribute power to those SMPS's which are switched on.

By using a variable speed generator set, and integrating the mains converter with that of the generator set, the arrangement described above can provide fuel savings, reduced size, lower weight, a smaller package and compact generator set foot print. Furthermore the use of multiple SMPS's provides redundancy and can ensure that the power required by the telecommunications equipment is still supplied even if one or more SMPS unit should fail.

If desired, the speed controller 40 and master controller 42 could be linked, for example to provide load anticipation capability.

The inverter 28 can allow AC power to be made available for the lighting and air conditioning loads 30, 32. If DC lighting is used and no air conditioning is required or has its own control electronics, the inverter may not be needed. Figure 2 shows parts of a switched mode power supply 18 for use in the arrangement of Figure 1. Referring to Figure 2, the switched mode power supply comprises a rectifier 46, chopper circuit 48, chopper control circuit 50, and output filter 52. The rectifier 46 receives the AC power supply from either the mains 10 or the generator set 34, and converts it to an unregulated DC voltage. The rectifier 46 also includes a power factor correction (PFC) circuit. The unregulated DC output of the rectifier 46 is converted to a regulated DC voltage by the chopper circuit 48 and chopper control circuit 50. The chopper circuit may be a boost circuit or buck circuit, as appropriate. The regulated DC voltage is filtered by filter 52, and output for supply to the DC bus 20 in Figure 1.

Figure 3 shows a second embodiment of a power supply apparatus for a

telecommunications base station. The arrangement is similar to that described above with reference to Figure 1. Parts in common with Figure 1 are given the same reference numeral and are not described further.

The arrangement of Figure 3 uses a renewable energy source 54, such as wind, solar and hydro power, in addition to the mains power supply 10 or the generator set 34. The output of the renewable energy source is integrated directly on to the common DC bus 20 as shown in Figure 3. The renewable energy source has its own converter which allows it to connect directly to the DC bus. In this way the energy supplied by the renewable energy source can supplement the main power supply regardless of whether it comes from the mains or the generator set.

Depending on the availability of the natural resources (wind, sun, water, etc.), the renewable source 54 can be used continuously to supply power to the common DC bus. Thus the renewable source may complement both mains and generator set power. This scheme will deliver greater fuel savings and contribute to reducing greenhouse gases. A renewable energy source as shown in Figure 3 can also be used with any of the other embodiments described herein.

Figure 4 shows a third embodiment of a power supply apparatus for a

telecommunications base station. In the arrangement of Figure 4, a wide-range AC/DC converter 60 is used in place of the switched mode power supplies of the previous embodiments. The input to the AC/DC converter 60 can be delivered by the mains supply 10 or the generator set 34. The AC/DC converter needs to be capable of accepting the variable voltage, variable frequency output of the generator set, and converting it to a substantially constant DC voltage for supply to the DC bus 20. For example, the converter 60 may have an input voltage range of 80 to 270 Vrms (single or three phase) and an input frequency range of 50 - 400 Hz.

In the arrangement of Figure 4, the engine speed controller measures the power on the common DC bus using a single current sensor. The current sensor can be a Hall Effect device, a simple shunt resistor or any other current sensing device. In some circumstances it may also be desirable to sense the DC bus voltage, in order to compensate for any voltage drop. However in many situations there will be a large battery bank which will minimise voltage variations, allowing a single current sensor to be used. Figure 5 shows parts of the AC/DC converter 60 in the arrangement of Figure 4.

Referring to Figure 5, the AC/DC converter comprises a rectifier 62, buck converter circuit 64, and buck converter controller 66. The rectifier 62 receives the AC power supply from either the mains supply 10 or the generator set 34, and converts it to an unregulated DC voltage. The unregulated DC output of the rectifier is stepped down and regulated by the buck converter circuit 64 and controller 66, to produce a regulated DC voltage for supply to the DC bus. The buck converter circuit 64 may comprise an inductor and a switch, and the controller 66 may vary the duty cycle of the switch to maintain the DC output voltage at a substantially constant value (for example, 48V).

Figure 6 shows a fourth embodiment of a power supply apparatus for a

telecommunications base station. In the arrangement of Figure 6, a wide range AC/DC converter 60 and an AC/AC converter 62 are incorporated within the same enclosure, to provide a single power converter unit 64. The AC/ AC converter 62 is operated in parallel with the AC/DC converter 60, with the AC/DC converter 60 supplying the DC bus 20, and the AC/AC converter 62 supplying the air conditioning unit 30 and lighting 32. This arrangement simplifies the connections that need to be made by the customer. The AC/DC converter 60 may be as described above with reference to Figures 4 and 5. The AC/AC converter 62 may be an IGBT (insulated gate bipolar transistor) based PW (pulse width modulation) converter. Alternatively, the AC/AC converter may be a cycloconverter, as described in UK patent application number GB 2 450 891 , the contents of which are incorporated herein by reference. The AC/AC converter input can either be provided by the mains supply 10 or the generator set 34.

Alternatively, an inverter may be incorporated within the enclosure 64 instead of the AC/AC converter. The inverter may be supplied from the output of the AC/DC converter 60. In the arrangement of Figure 6, power is measured directly from the terminals of the generator 38, or from within the power converter 64, and is sent to the speed controller 40. This simplifies the connection and makes the system more attractive from a consumer's point of view.

Figure 7 shows a fifth embodiment of a power supply apparatus for a

telecommunications base station. In this arrangement an AC/AC converter 66 is incorporated with the engine 36 and generator 38. The AC/AC converter 66 modifies the variable amplitude and frequency of the generator 38 to a fixed output voltage, which may be a single phase or a three phase. Power is measured from the AC/AC converter and sent to the speed controller 40 to control the speed of the engine 36. The AC/AC converter 66 may be a cycloconverter, as described in GB 2 450 891.

The arrangement of Figure 7 allows a back up generator unit 68 with a fixed AC output voltage to be provided. This simplifies the integration of the back up generator unit into the telecommunications power supply system, while taking full advantage of variable speed operation. An advantage of this scheme is that the system may be retrofitted to an existing installation and the original SMPS devices 70 can be left in position without any need for modification.

In the arrangement of Figure 7, it is possible to eliminate the automatic mains failure detection and switch over device 14, as shown by the dotted line. In this case, a loss of mains detection function and a reconnection function can be implemented in a microcontroller which controls the AC/AC converter. A mains voltage

synchronization function can be implemented as part of the reconnection function. Figure 8 shows a sixth embodiment of a power supply apparatus for a

telecommunications base station. In this arrangement an AC/DC converter 72 is incorporated with the engine 36 and generator 38. The variable voltage and frequency of the generator 38 is modified by the AC/DC converter 72 to a fixed DC voltage. In this way a variable speed generator set 74 is provided which produces a DC voltage which can be fed directly to the telecommunications equipment.

In the arrangement of Figure 8, the mains supply can be fed to the AC/DC converter 72. In this case an interface 76 is provided between the mains supply 10 and the input of the AC/DC converter, to control the supply of power.

In the various embodiments described above, a variable speed generator set is used as the back-up power supply for a telecommunications power supply apparatus. The mains supply and the generator set may be single phase or three phase. An axial flux permanent magnet alternator (PMA) is used instead of a synchronous field generator. The speed of the engine is varied to match the variations in the load demand. The torque and the fuel consumption maps of the engine are programmed into the electronic speed governor for load adaptive speed control.

Operating the engine at optimal speed leads to increases in efficiency and fuel saving as well as reducing the overall size and weight of the generator set. Also the package of the generator set can be optimized to give the best envelop to suit the available space.

In the embodiments described above, parts which are in common with other embodiments have been given the same reference numeral, and the description of those parts has not been repeated. It will be appreciated that the embodiments described above are given by way of example, and variations in detail are possible within the scope of the claims.

Claims

1. Power supply apparatus for a telecommunications base station, the power supply apparatus comprising:

means for receiving an AC mains power supply;

conversion means for converting the AC mains power supply to a DC voltage; a DC bus for supplying DC power to at least one DC load in the base station; a standby generator set;

means for detecting an irregularity in the mains power supply; and

2. Power supply apparatus according to claim 1 , wherein the conversion means is operable to convert a variable voltage, variable frequency output of the variable speed generator set to a substantially constant DC voltage to supply the DC bus.

3. Power supply apparatus according to claim 1 or 2, further comprising control means for controlling operation of the conversion means in dependence on the power demand of the base station when the base station is powered by the variable speed generator set.

4. Power supply apparatus according to any of the preceding claims, wherein the conversion means comprises a plurality of switched mode power supplies.

5. Power supply apparatus according to claim 4, wherein at least some of the switched mode power supplies are existing switched mode power supplies in the base station.

6. Power supply apparatus according to claim 4 or 5, wherein the switched mode power supplies are connected in parallel, and at least one of the switched mode power supplies is connected and disconnected as the power demand of the base station changes.

7. Power supply apparatus according to any of claims 4 to 6, further comprising a distributor for distributing power from the mains power supply or the variable speed generator set to the plurality of switched mode power supplies.

8. Power supply apparatus according to claim 7, wherein the distributor is arranged to control the distribution of power to the switched mode power supplies in dependence on the power demand of the base station.

9. Power supply apparatus according to any of claims 4 to 8, wherein the switched mode power supplies include a voltage regulator.

10. Power supply apparatus according to any of the preceding claims, wherein the conversion means comprises an AC/DC converter comprising a rectifier and a buck or boost circuit.

1 1 . Power supply apparatus according to claim 10, wherein the conversion means further comprises a voltage regulator for regulating the output of the buck or boost circuit.

12. Power supply apparatus according to any of the preceding claims, further comprising an inverter for converting the DC bus voltage to an AC output voltage.

13. Power supply apparatus according to of the preceding claims, further comprising an AC/AC converter operable to convert a variable voltage output of the variable speed generator set to a substantially constant AC output voltage.

14. Power supply apparatus according to claim 13, wherein the AC/AC converter is connected in parallel with an AC/DC converter.

15. Power supply apparatus according to claim 13, wherein the AC/AC converter is operable to convert the variable voltage and frequency AC output of the generator set to the same voltage and frequency as the AC mains.

16. Power supply apparatus according to claim 15, further comprising means for synchronising the output of the AC/AC converter with the mains power supply.

17. Power supply apparatus according to any of the preceding claims, further comprising one or more batteries for supplying power to the DC bus.

18. Power supply apparatus according to any of the preceding claims, further comprising means for connecting a renewable energy source directly to the DC bus.

19. Power supply apparatus according to claim 18, wherein the renewable energy source has its own converter which is connected directly to the DC bus.

20. Power supply apparatus according to any of the preceding claims, further comprising a speed controller operable to control the speed of the variable speed generator set in dependence on the power demand of the base station.

21. Power supply apparatus according to claim 20, further comprising means for sensing the power demand and feeding the sensed power demand to the speed controller.

22. Power supply apparatus according to claim 20 or 21 , wherein the speed controller is integrated with control means for controlling operation of the conversion means.

23. Power supply apparatus according to any of the preceding claims, further comprising means for switching power for the base station from the mains power supply to the standby generator set on detection of an irregularity in the mains power supply.

24. A telecommunications base station comprising a power supply apparatus according to any of the preceding claims.

25. A telecommunications base station according to claim 24, comprising at least one DC load connected to the DC bus.

26. A method of supplying power to a telecommunications base station, the method comprising:

receiving an AC mains power supply;

converting the AC mains power supply to a DC voltage;

supplying DC power to at least one DC load in the base station; detecting an irregularity in the mains power supply;

27. A method according to claim 26, wherein at least part of the conversion means is an existing converter in the base station.