WO2003028187A1

WO2003028187A1 - Power supply apparatus

Info

Publication number: WO2003028187A1
Application number: PCT/ZA2002/000149
Authority: WO
Inventors: Manuel Dos Santos Da Ponte; Lech Grzesiak
Original assignee: Manuel Dos Santos Da Ponte; Lech Grzesiak
Priority date: 2001-09-26
Filing date: 2002-09-26
Publication date: 2003-04-03

Abstract

This invention provides a power supply apparatus (10) including a controllable power supply system (12) and a converter system (14). The system (12) comprises: a power source (16) first decoupling converter means (18) for generating an intermediate DC output from an electrical output of the source (16), which intermediate DC output is substantially independent of variations in the electrical output of the source (16); and first control means (26, 36) controlling the operation of the first decoupling converter (18), to dynamically vary the power output of the first decoupling converter (18). The converter system (14) comprises: second decoupling converter means (22) for generating a regulated DC output from the first intermediate DC output to supply a load (24) with appropriate power; and second control means (40) controlling the operation of the second converter means (22), to regulate the power output of the second converter means (22) according to the power requirements of the load (24).

Description

POWER SUPPLY APPARATUS

FIELD OF THE INVENTION

This invention relates to a power supply apparatus.

BACKGROUND OF THE INVENTION

In many conventional electric power supplies, a power source comprises a mains power supply grid and/or various electricity generating systems. One common (popular) example is an engine-driven electric generator set which typically operates at constant speed to supply electric power to a load. In practice, the load may vary substantially in time due to gradually changing load demands or cyclic loads (of "short" duration) such as thermostatically controlled heaters and refrigeration and air conditioning systems, including machinery for numerous other energy conversion processes. When loads include induction motors, very high peak power demands are imposed on the power supply during start-up and acceleration. This peak power demand can be several times the rated power. The source is usually sized to meet the peak load requirement of the load and in many cases "oversized" to maintain high quality power supply under other adverse load conditions such as non-linear loads caused by DC equipment powered by rectified AC converters such as computers, radios, and the like.

In applications where the load is powered by a strong supply grid, this practice of sizing according to peak load demands or over sizing to maintain high quality of power supply produces good results. However, in applications where the grid is weak or the load is powered by an on-site generating system, the peak load demands cause problems such as voltage fluctuations. In the case of engine driven generator sets, frequency variations are also problematic. In order to overcome the problems caused by peak load demands, generator sets must be sized according to peak load demand, which can be very wasteful due to the fact that the average load may typically be only 20% to 30% of the peak load or even less.

A very common problem associated with weak supply grids is power interruptions when protection devices kick in as a result of voltage fluctuations or loss of any one phase. In applications where backup reserve power systems and/or generating systems are used, backup power systems start up automatically and run until power is restored when the supply voltage is within the specified limits. This often means that the backup generator sets or reserve power units operate for long periods even though there is power available, albeit outside the normal limits. Often, the reserve capacity of the back-up power systems is unnecessarily depleted.

Another common problem associated with loads powered by engine-driven generator sets is prolonged engine operation under light load conditions. This situation is very common in loads powered by on-site generator sets and is the cause of numerous premature engine failures along with excessively high operating and maintenance costs. This is also true for loads connected to weak supply grids and which are equipped with backup generator sets. In this instance, disruptions in grid supply are caused by voltage variations which are interpreted by the system control as an abnormal supply condition or power failure.

A controllable power supply apparatus, which addresses the problems associated with light load operation, has been proposed in the applicant's own earlier PCT patent application no. PCT/EP97/07273 published under international publication no. WO 98/28832. This earlier apparatus responds to variations in load demand by varying its output power accordingly, and thus operates efficiently while providing a quality output power independently of variations in the load even during low-load demands. This earlier apparatus can also utilize batteries, or other energy storage devices to supplement the power source during peak-load demands. Furthermore, the apparatus can serve as a power hub between a plurality of power sources, including renewable energy systems, and diverse loads with onerous and varying load demands.

Even though this earlier apparatus operates effectively and efficiently, it does have certain operational limitations. For example, the natural operation of the earlier apparatus produces a non-constant intermediate DC output voltage in order to provide a voltage window having at least two threshold voltages as described in PCT patent application no. PCT/EP97/07273. This necessitates complicated control of the apparatus to maintain stability.

When a supplementary energy source is employed, the above problem is compounded by the on-off operation of the parallel converter controlling the power flow from the supplementary energy storage device to the intermediate DC link. When an inverter is used as an output means, the problem is further compounded by the limitation in the input voltage of the inverter.

Furthermore, when a plurality of power sources is used, it is very difficult to control the apparatus so as to maintain stability.

It is an object of the invention to provide an improved power supply apparatus which ameliorates at least some of the above problems.

SUMMARY OF THE INVENTION

According to the invention there is provided a power supply apparatus which includes a controllable power supply system and a converter system, wherein the controllable power supply system comprises: at least one power source having an electrical output; first decoupling converter means for generating an intermediate DC output from the electrical output of the at least one power source, which intermediate DC output is substantially independent of variations in the electrical output of the at least one power source; first sensor means for monitoring the voltage and/or current of the at least one power source and/or for monitoring the voltage and/or current of the intermediate DC output, and for generating first output signals corresponding thereto; and first control means responsive to the first output signals to control the operation of the first decoupling converter, to regulate the input and/or output voltage and/or current of the first decoupling converter; and wherein the converter system comprises: second decoupling converter means for generating a regulated DC output from the first intermediate DC output to supply a time varying load with appropriate power; second sensor means for monitoring the voltage and/or current of the second converter DC output and for generating second output signals corresponding thereto; and second control means responsive to the second output signals of the second sensor means, said second control means controlling the operation of the second converter means, to regulate the input and/or output voltage and/or current of the second converter according to the power requirements of the time varying load.

The first control means typically only regulates the input current or the output voltage of the first converter. The second control means typically only regulates the output voltage of the second converter.

The at least one power source may include an uncontrollable power source such as any one or any combination of the group including: at least one electric grid; at least one uncontrollable solar power source; at least one uncontrollable hydro-electric generator; at least one uncontrollable gas turbine/generator; and at least one uncontrollable wind turbine. The at least one power source may include a controllable power source such any one or any combination of the group including: at least one controllable solar power source; at least one controllable hydro-electric generator; at least one controllable gas turbine/generator; at least one controllable wind turbine; at least one controllable fuel cell; and at least one engine-driven generator, or other internal/external combustion engine.

Where the at least one power source includes a controllable power source, the first control means may, in response to the first output signals, also control operation of the at least one controllable source.

It is to be appreciated that when the at least one power source includes a controllable power source, the controllable power supply system operates substantially in the same manner as the apparatus described in PCT patent application no. PCT/EP97/07273, except that the intermediate DC output of this invention can be allowed to vary more substantially than that of the earlier apparatus due to the addition of the converter system having a second decoupling converter.

The controllable power supply system may include at least first energy storage means arranged to be charged from the intermediate DC output and to discharge energy into the intermediate DC output when the voltage of the intermediate DC output falls below a nominal value. The at least first energy storage means typically includes at least one capacitor.

The converter system may include at least second energy storage means arranged to be charged from the regulated DC output and to discharge energy into the regulated DC output when the voltage of the regulated DC output falls below a nominal value. The at least second energy storage means typically includes at least one battery and/or at least one capacitor. The at least one battery may include at least one electrochemical and/or at least one electromechanical battery.

The apparatus may include an additional energy storage system comprising a third energy storage device, wherein the additional energy storage system runs in parallel with the power supply system and is fed into the converter system to form the intermediate DC output and wherein the additional energy storage system is configured to supplement the provision of power to the converter system load when an onerous load is to be supplied. The third energy storage device typically comprises at least one battery and/or at least one capacitor. The at least one battery may include at least one electrochemical and/or at least one electromechanical battery. The additional energy storage system may be configured to discharge the third energy storage device when the voltage measured at the intermediate DC output falls below the terminal voltage of the third energy storage device.

The apparatus may include two or more power supply systems as described above running in parallel with each other and being fed into the converter system to form the intermediate DC output.

The control means of each power supply system may be configured so that each power supply system is prioritised in order that the higher priority systems are controlled to feed power into the converter system before the lower priority systems when the load draws ever increasing power. The control means of each power supply system may be configured according to a time varying prioritising of the power supply systems.

The control means of at least one controllable power supply system may be configured to control the power output of the source of that controllable power supply system according to the available energy reserves of the source. The available energy reserves of the source may be sensed by an energy reserve sensor of the apparatus. The power output of that source may be controlled to not exceed a predetermined value where the available energy reserves of that source are below a certain value. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a highly simplified block diagram of a power supply apparatus, according to one embodiment of the invention;

Figure 2 shows the speed, intermediate DC output voltage, and the DC input currents of the first and second DC to DC converters of Figure 1 , when a step up load is to be supplied;

Figure 3 shows another embodiment of the invention where an additional energy storage system is added to the apparatus of Figurel ;

Figures 4 and 5 show examples of the possible topologies of typical converters for the apparatuses of Figures 1 and 3, respectively;

Figures 6 and 7 show various differing topologies for the converters; Figures 8a and 8b show a schematic diagram of multi source version of the invention and a graph of the different reference voltages used, respectively; and

Figures 9a and 9b show a schematic diagram of a first power supply system 12.1 including a controllable power source 16.1 that is used to supplement a second power supply system 12.2 which includes a source in the form of an uncontrollable electric grid 16.2. and a graph of the different reference voltages used, respectively.

DETAILED DESCRIPTION OF THE DRAWINGS

With particular reference to the drawings, reference numeral 10 generally indicates a power supply apparatus according to the invention. The apparatus 10 includes a controllable power supply system 12 and a converter system 14.

The controllable power supply system 12 includes a power source having an electrical output. In this embodiment of the invention, the power source is in the form of a controllable electrical source 16 such as an engine/generator, a fuel cell or another source of electrical energy which can be controlled to vary its output.

The source 16 is connected to a decoupling converter means 18 comprising, in this embodiment of the invention, a DC to DC converter (for example, a combination of a step-down and a step-up chopper or a DC-DC converter with HF-transformer if galvanic isolation is needed). The output of converter 18 is referred to herein as an intermediate DC output, having a variable DC-voltage.

The controllable power supply system 12 also includes at least first energy storage means 20, for example a capacitor bank, arranged to be charged from the intermediate DC output and to discharge energy into the intermediate DC output when the voltage of the intermediate DC output falls below a nominal value. The storage means 20 operates like big dynamic energy buffer.

The storage means 20 supplies the converter system 14 and, in particular, a second decoupling converter means 22, in this embodiment a DC-DC converter, for generating a regulated DC output from the first intermediate DC output to supply a load 24 with appropriate power. In some embodiments, the load 24 is an output converter means such as an inverter which converts the voltage of the regulated DC output to an AC (n-phase) waveform for supplying an external load. However, in other embodiments, the load 24 may be an external DC-load.

The function of converter 22 is to keep the regulated DC output thereof independent from variations in the intermediate DC output and from variations in a load 24 connected thereto. The converter 18 serves the important function of decoupling the intermediate DC output from the source 16. For example, where the source 16 is an engine/generator set, the decoupling effect of the converter 18 permits operation of the engine/generator over a wide speed range (variation of voltage), a wide range of torque (alternator current), and various torque characteristics (alternator current characteristics) thereby optimising the operation of the power source independently from the voltage level of the intermediate DC output. Therefore, a wide range of intermediate DC output voltages (from 0 up to maximum designed voltage) is permitted.

The system 12 further includes a first control means which includes a control circuit 26 comprising, for example, an analogue and/or digital circuit which can readily be implemented utilizing a microcomputer running under the control of appropriate software. However, for purposes of the following description, reference is made to a control "circuit". A voltage sensor 28 monitors the value of the input of the converter 18, generating a voltage signal V_1i which is fed to a function generator 30. The function generator 30 produces a reference current signal I_ref1 which is fed to the control circuit 26 for controlling the converter 18. A voltage sensor 32 monitors the value of the intermediate DC output voltage, generating a voltage signal V_1 which is fed to the control circuit 26.

Additionally, a current sensor 34 measures the current output of the source 16 to the converter 18 and generates a current signal l_1 which is applied to the control circuit 26. The control circuit 26 is also supplied with voltage reference signal V_ref1. The output V_1 of the voltage sensor 32 is also applied to a controller 36 which has a reference voltage V_ref applied thereto, and which generates an electrical output signal which is applied to a control system 38 of the source 16. It is to be appreciated that the controller 36 forms part of the first control means and that the control system 38 forms part of the source 16.

Depending on the nature of the source 16, the control system 38 might be, for example, a fuel injection controller of an internal combustion engine or a controller which controls gas (for example, hydrogen and oxygen) flow in a fuel cell. The second decoupling converter 22 serves the important function of decoupling the intermediate DC output from the regulated DC output. The decoupling effect of the converter 22 permits operation of the first storage means 20 over a wide voltage range. Accordingly, the apparatus 10 permits a wide range of first intermediate DC output voltage variation (from 0 to max designed voltage). However, the control system 40 of the converter 22 permits operation with a small voltage variation in the regulated DC output (theoretically nearly zero).

The system 14 includes a control circuit 40 for the second converter 22. The circuit 40 can comprise an analogue and/or a digital circuit, and can readily be implemented utilizing a microcomputer running under the control of appropriate software. However, for purposes of the following description, reference is made to a control "circuit". A voltage sensor 42 measures the voltage of the regulated DC output and generates a voltage signal V_3 which is applied to the control circuit 40. Additionally, a current sensor 44 measures the input current of the converter 22 and generates a current signal l_3 which is applied to the control circuit 40. The control circuit 40 is also supplied with voltage and current reference signals V_ref3 and I_ref3.

The converter system 14 also includes a second energy storage means 46, for example a capacitor bank, arranged to be charged from the regulated DC output and to discharge energy into the regulated DC output when the voltage of the regulated DC output falls below a nominal value. The storage means 20 operates like big dynamic energy buffer. The second energy storage device 46 is typically much smaller than the first energy storage device 20.

The apparatus 10 shown in Figure 1 effectively operates with a voltage control loop, which is arranged so that the converter 18 effectively regulates the voltage of the intermediate DC output with a maximum voltage determined by the reference voltage V_ref1. Accordingly, the value of VDC_1 is adjusted by the control loop according to Vjref 1 , irrespective of a varying input voltage from the source 16. At the same time, in this mode of operation, the converter 18 controls the current which it passes according to the current reference I_ref1 so that the source 16 is loaded optimally.

For example, in the case of an engine/generator set, when the engine/generator is operating within its variable speed range, the engine is loaded according to a desired curve which approximates an optimum power/speed characteristic of the engine. Figure 2 shows the speed, intermediate DC output voltage, and the DC input currents of the first and second DC to DC converters by step up load. When the load power demand increases suddenly, the converter 22 increases its input current to provide a constant voltage to the regulated_. DC output. Simultaneously, the converter 18 regulates the current drawn from the source 16. This effectively means that the first intermediate DC output is starved of power, causing the load 24 to draw energy across the converter 22 from the energy storage device 20 which augments the intermediate DC output. As energy is drawn from the storages device 20, the output of the device 20, and hence the voltage VDC_1 of the intermediate DC output, will fall.

When the value of VDC_1 , as detected by the first voltage sensor 32, falls below a voltage threshold Vjref, the input signal V_1 applied to the controller 36 of the control system 38 controls the source 16 to increase its power output. In the case of engine/generator, the control system 38 will increase the speed of the engine, resulting in a corresponding increase in the output voltage of the generator. This increase in voltage increases the power supplied to the converter 18 and hence enables the converter 18 to supply higher power to the intermediate DC output without exceeding the source current set by the reference I_ref1. The source 16 thus satisfies the load 24 and recharges the energy storage device 20. The voltage VDC_1 of the intermediate DC output rises until it is restored to the voltage threshold determined by the voltage reference signal V_ref, and the source 16 will stabilize at the new, higher level of output power.

When the load power demand decreases, the balance between the source 16 and the load 24 will again be disturbed. In this case, the voltage VDC_1 will rise, and the voltage/speed controller 36 will operate to reduce the output power of the source 16 (for example, by reducing the engine speed in the case of an engine/generator set), reducing the voltage of the source 16 and accordingly allowing the converter 18 to reduce its output voltage back to its nominal value.

In the above described mode of operation, a form of current control is used in conjunction with the main voltage control scheme. By controlling or limiting the current drown from the source 16, the voltage VDC_1 of the intermediate DC output is caused to vary (without limiting the voltage drop) with variations in load demand.

The control of converter 18 can be described as a dominated current control with output voltage limit (maximum value of intermediate DC output voltage). In contrast, the control of converter 22 can be described as a dominated voltage control with current limit. Theoretically, it can be said that converter 16 is provided to keep its input current at reference value l_ref 1 (for

, example, constant) until output voltage is lower than limit V__ref1. In contrast, converter 22 is provided to keep its output voltage at reference value V_ref3 (for example, constant) until the input current is lower that limit I_ref3.

Figure 3 shows an embodiment of the invention where an additional energy storage system 48 is added to the apparatus 10 of Figurel . Accordingly, where applicable like reference numerals have been used to indicate like or similar features.

In this arrangement, the energy storage devices 20 and 46 typically comprise one or more capacitors or ultra-capacitors. The energy storage system 48 includes a third energy storage device 50 which typically comprises at least one battery and/or at least one ultra-capacitor. When in the form of a battery, the energy storage device 50 is connected via an interface circuit 52 to the intermediate DC output. In a simple embodiment, the interface circuit 52 comprises only a diode. A cathode should be connected to the intermediate DC output (input of converter 22) and an anode connected to a positive pole of the battery.

In this arrangement, the battery 50 can be arranged to be charged from an internal or an external energy source. Energy could be derived from the controllable energy source 16 or from, for example, an auxiliary mains source, a solar panel or another energy source. The output 54 of such source is fed to a third decoupling converter means 56, the characteristics of which are determined by the nature of the output 54. The output of the converter 56 is fed via a current sensor 58 to the battery 50 and its operation is controlled by a control circuit 60 which is responsive to the output of the current sensor 58, a voltage sensor 62, and current and voltage reference signals I_ref4 and V_ref4.

In Figure 3, the first energy storage device 20 discharges into the load 24 via converter 22 when the value of VDC_1 drops due to increased load demand. When the value of VDC_1 drops further, below the voltage threshold V_ref, the power controller 36 is activated as described above with reference to Figure 1. The battery 50 discharges when the voltage V_1 measured at the intermediate DC output falls below the terminal voltage of the battery 50. For example, the rated intermediate output voltage can be 400V and the voltage of the battery 50 can be 120 VDC. In this voltage window (from 400V DC to 120V DC) the battery 50 is not used.

It is to be appreciated that, even though the block representing the third energy storage device 50 is shown connected in series between the current sensor 58 and the interface circuit 52, the third energy storage device 50 is actually connected in parallel and fed into the interface circuit 52 so as to discharge into the converter system 14 when required.

In the arrangement of Figure 3, the battery 50 is typically used to supply the load 24 relatively infrequently compared with the banks of capacitors 20 and 46. This is desirable, since the duty cycle of the battery 50 is then dramatically reduced, while capacitors 20, 46 can survive a much greater number of charge/discharge cycles than the battery. Therefore, this arrangement provides the apparatus 10 with substantial flexibility and reserve energy capacity, while enhancing the reliability and longevity of the apparatus 10.

It is to be appreciated that, in Figures 1 and 3 and the descriptions related thereto, the controllable power source can be replaced by a non-controllable power source. In such an application, similar benefits can be obtained to those for a controllable power source. Obviously, the first power control means will then not control the source but only the first converter 18.

This embodiment of the invention finds typical application in the case of an electrical grid source where the first converter functions as a regulator to control the power drawn from the grid source thereby protecting the grid source from exceeding its power supply limits which in turn prevents activation of overload protection devices of the grid source.

It is also to be appreciated that, where the grid source is intermittently weak the first storage means is able to discharge energy into the converter system for a limited period of time allowing the converter system to provide the required power to the load for such limited period of time. This in turn allows the grid source to recover from its overload situation

Furthermore, should the voltage output of the grid source vary substantially over time then the converter system is able to compensate for such variation and provide a constant voltage output.

The above embodiment for a grid source finds similar application in wind turbine sources.

Figures 4 and 5 show examples of the possible topologies of typical converters for the apparatuses of Figures 1 and 3, respectively. Figures 4 and 5 include each include an inverter circuit for an embodiment of the invention where the load is to be supplied with an AC current. A simple single-transistor buck-boost converter has been used for both decoupling converters 18, 22. In this embodiment, operation with constant alternator current (constant engine torque) requires a reference current calculation of the first decoupling converter 18. The current through an inductor of the converter 18 should be proportional to the power if the output voltage of the converter 18 is constant. If it is not the case, a calculation of the power is required.

Reference formulae to calculate the average current in the inductor 70 are: P = T-Ω

and τ^« ₌ — 5 T^ref -Q,

^U out 'off

where is the Ω is the angular speed of the engine, T is the engine torque, T^ref is the reference torque, U_out is the output voltage of the converter, tof_t is the transistor switching off time, and 7} is the switching time period.

In Figure 6 two different topologies for the converters 18 and 22 are shown. In the Figure 6a, a simple configuration with a diode rectifier, a LC filter, and two DC-DC converters is presented. Converter 18 has buck type topology and converter 22 makes use of a boost converter.

A similar modification of the converters 18, 22 is shown in Figure 6b. To achieve lower current harmonics in the generator, a controlled sine wave rectifier comprising a diode rectifier and a DC to DC buck converter is used.

A further possible topology for the converters 18, 22 is shown in Figure 7. In this embodiment, an AC to DC current link converter 18 is used to convert AC variable frequency voltage supplied by the source 16 to a controlled DC voltage. On the AC side, there is provided an input filter consisting of capacitors while in the DC side there is provided a series inductance. The input filter smoothens and shapes the input current waveforms, and also reduces voltage spikes caused by commutation of the switches. In order to control the output current of the AC to DC converter 18, two switches of the same branch must be brought into conduction at the same time. These states (short circuit states) must take place in precisely determined sequence and their time duration determines the average value of the output voltage of the converter 18. The output voltage may also be phase-controlled such that the phase-shift between the input current and voltage waveforms may be positive or negative, due to the possibility of switching on and off at arbitrary time instances.

In the variable speed region, a control system of the AC to DC converter 18 adjusts and regulates input current. A reference current for the control circuit of the converter 18, similar to the reference current I_ref1 in Figure 1 , is set at a predetermined value, for example, as a function of speed which approximates the torque-speed curve of the engine. At a low load condition, the control system operates as a voltage stabilizer and keeps the DC voltage on a predetermined reference level (maximum value of intermediate DC output voltage level).

During transient conditions, the voltage on capacitor C1 can vary freely and can even theoretically fall to nearly zero. The second DC-DC boost type converter 22 operates with a voltage feedback and a stabilised output voltage on the given reference level which is independent from any variation in the load.

Figure 8a shows schematic diagram of multi source version of the invention. In this embodiment, three power supply systems 12.1, 12.2, 12.3, runnning in parallel with each other, are fed into the converter system 14 to form the intermediate DC output. Each power supply system includes a different energy source ie. a controllable energy source 16.1, such as that depicted in Figure 1 ; an uncontrollable wind turbine 16.2; and an uncontrollable solar source 16.3, respectively. The control of the converters 18.1 , 18.2, 18.3 and the controllable source 16.1 is totally independent of each other. Accordingly, no connection between the control means of the converters 18.1, 18.2, 18.3 and the source 16.1 is required.

It is to be appreciated that the different sources 16.1 , 16.2, 16.3 are allocated different priorities due to, for example, the cost of power provided by each source 16.1, 16.2, 16.3. The prioritising of the sources 16.1 , 16.2, 16.3 is achieved by the selection of appropriate reference voltages for each control circuit 26.1 , 26.2, 26.3 which controls the energy flow between sources 16.1 , 16.2, 16.3 and the intermediate DC output. The higher the priority of the source the higher the reference voltage applied.

Figure 8a shows an example of possible combination of priorities. In this example, a solar source 16.3 is allocated the highest priority and accordingly energy is always drawn from this source 16.3. A wind turbine defines the second source 16.2 and energy from the wind turbine 16.2 is discharged into the converter system 14 when the intermediate DC output voltage falls below a level depicted as V_ref 1_1.

For a larger load, when energy from the solar source 16.3 and the wind turbine 16.2 are insufficient, the intermediate DC output voltage falls below the level depicted as V_ref1 which causes the controllable energy source 16.1 to supply energy together with solar source 16.3 and wind turbine

16.2 to the converter system 14.

When the intermediate DC output voltage falls below the level depicted as V_ref the control circuit 36.1 increases the power output of the controllable power source 16.1. Accordingly, the controllable power supply system 12.1 comprising the controllable power source 16.1 adapts its power output according to the load power demand and always follows the load by controlling the current drawn from the generator (minimum speed operating region) or controlling the speed (variable speed region).

It is to be appreciated that the control circuit 26.3 is set with reference voltage V_ref1_2 to limit the power output of the source 16.3 should it start generating too much power.

Figure 9 shows schematic diagram of a first power supply system 12.1 including a controllable power source 16.1 that is used to supplement a second power supply system 12.2 which includes a source in the form of an uncontrollable electric grid 16.2. A grid AC to DC converter 18.2 (or a combination of AC/DC rectifier and DC/DC converter) is used to control the current drawn from the grid 16.2. It is possible to adjust the maximum level of current and in this way limit the power drawn from grid 16.2. When a large load is present, the first power supply system 12.1 supplements the supply from the grid 16.2. In this embodiment, no connection between the control means of the first and second power supply systems 12.1 , 12.2 is required.

It is to be appreciated that in other embodiments similar to those described in Figures 8 and 9, the control means of each power supply system may be configured according to a time varying prioritising of the power supply systems.

It is also to be appreciated that, in another embodiment of the invention similar to that described in relation to Figures 8 and 9, a control means of at least one controllable power supply system is configured to control the power output of the source of that controllable power supply system according to the available energy reserves of the source. The available energy reserves of the source are typically sensed by an energy reserve sensor of the apparatus. The power output of that source may be controlled to not exceed a predetermined value where the available energy reserves of that source are below a certain value or may be controlled in a time varying manner related to a variation in the reserves of the source. An example of such a source, is a hydro-electric source where the water reserves thereof may vary substantially necessitating different control strategies for the use of such reserves.

In the above description and related drawings, it is to be appreciated that, where the power source is in the form of a controllable power source, the controllable power supply system operates substantially in the same manner as the apparatus described in PCT patent application no. PCT/EP97/07273. However, it is important to understand that the intermediate DC output of this invention can be allowed to vary more substantially than that of the earlier apparatus due to the addition of the converter system having a second decoupling converter.

In the earlier apparatus an energy storage (capacitor bank or/and battery) is connected in parallel to the output of the converter. An important function of this operation is the control of charging and discharging of this energy storage system, witch is crucial for transient compensation caused by a step load. The control strategy of the earlier apparatus was based on multilevel predetermined reference voltage. Two of these levels are responsible to enable or disable (start/stop) operation of the converter during step-up or step-down mode.

To simplify the control algorithm and robustness of the earlier apparatus the converter system described above has been added and a new control strategy has been applied in this invention. Accordingly, it is no longer necessary to define different voltage levels for the control of energy flow from the source (when in the form of a generator) and the energy storage. The first converter operates in the variable speed region as a current controller and permits regulation of the load torque of the engine. At minimum speed (low load) the first converter operates as a voltage controller to stabilise the output voltage on a defined level higher than the voltage level for main voltage control (speed adjustment). In this way number of reference voltage levels is reduced and better performance of the apparatus is achieved. Furthermore, the number of nonlinear controllers is limited. Also, control of the second converter is totally independent (decoupled) from the control of the engine and from the control of the first converter.

It is also to be appreciated that, even where the above description and related figures (in particular Figure 1 ) describe a power source in the form of a controllable power source, this invention is not limited to such controllable power source. This invention also includes in its scope a power source in the form of an uncontrollable power source such any one or any combination of the group including: at least one electric grid; at least one uncontrollable solar power source; at least one uncontrollable hydro-electric generator; at least one uncontrollable gas turbine/generator; and at least one uncontrollable wind turbine.

When the power source is in the form of an uncontrollable power source, the first control means does not control operation of the controllable source to dynamically vary the power output of the source. Accordingly, the first control means only controls the operation of the first decoupling converter, to dynamically vary the power output of the first decoupling converter. This finds ready application where the power source is an electric grid which provides a weak power output which needs to be supplemented as in Figure 9; where the grid is used to supplement another power source; and/or where the manner in which electricity drawn from the grid is to be controlled due to, for example, varying tariffs of grid electricity at different times.

This invention is not limited to the precise constructional details described above. It is to be appreciated that various combinations of aspects of the invention described above are possible and that all such combinations form part of the scope of this invention.

Claims

1. A power supply apparatus which includes a controllable power supply system and a converter system, wherein the controllable power supply system comprises: at least one power source having an electrical output; first decoupling converter means for generating an intermediate DC output from the electrical output of the at least one power source, which intermediate DC output is substantially independent of variations in the electrical output of the at least one power source; first sensor means for monitoring the voltage and/or current of the at least one power source and/or for monitoring the voltage and/or current of the intermediate DC output, and for generating first output signals corresponding thereto; and first control means responsive to the first output signals to control the operation of the first decoupling converter, to regulate the input and/or output voltage and/or current of the first decoupling converter; and wherein the converter system comprises: second decoupling converter means for generating a regulated DC output from the first intermediate DC output to supply a time varying load with appropriate power; second sensor means for monitoring the voltage and/or current of the second converter DC output and for generating second output signals corresponding thereto; and second control means responsive to the second output signals of the second sensor means, said second control means controlling the operation of the second converter means, to regulate the input and/or output voltage and/or current of the second converter according to the power requirements of the time varying load.

2. An apparatus as claimed in claim 1 , wherein the at least one power source includes at least one uncontrollable power source.

3. An apparatus as claimed in either one of claims 1 or 2, wherein the at least one power source includes at least one controllable power source.

4. An apparatus as claimed in claim 3, wherein the first control means, in response to the first output signals, also controls operation of the at least one controllable source.

5. An apparatus as claimed in any one of claims 1 to 4, wherein the controllable power supply system includes at least first energy storage means arranged to be charged from the intermediate DC output and to discharge energy into the intermediate DC output when the voltage of the intermediate DC output falls below a nominal value.

6. An apparatus as claimed in any one of claims 1 to 5, wherein the converter system includes at least second energy storage means arranged to be charged from the regulated DC output and to discharge energy into the regulated DC output when the voltage of the regulated DC output falls below a nominal value.

7. An apparatus as claimed in any one of claims 1 to 6, which includes an additional energy storage system comprising a third energy storage device, wherein the additional energy storage system runs in parallel with the power supply system and is fed into the converter system to form the intermediate DC output and wherein the additional energy storage system is configured to supplement the provision of power to the converter system load when an onerous load is to be supplied.

8. An apparatus as claimed in claim 7, wherein the additional energy storage system is configured to discharge the third energy storage device when the voltage measured at the intermediate DC output falls below the terminal voltage of the third energy storage device.

9. An apparatus as claimed in any one of claims 1 to 8, which includes two or more power supply systems running in parallel with each other and being fed into the converter system to form the intermediate DC output.

10. An apparatus as claimed in claim 9, wherein the control means of each power supply system is configured so that each power supply system is prioritised in order that the higher priority systems are controlled to feed power into the converter system before the lower priority systems when the load draws ever increasing power.

11. An apparatus as claimed in claim 10, wherein the control means of each power supply system is configured according to a time varying prioritising of the power supply systems.

12. An apparatus as claimed in claim 9, wherein the control means of at least one controllable power supply system is configured to control the power output of the source of that controllable power supply system according to the available energy reserves of the source.

13. An apparatus as claimed in claim 12, wherein the available energy reserves of the source of that controllable power supply system are sensed by an energy reserve sensor of the apparatus.

14. An apparatus as claimed in either one of claims 12 or 13, wherein the power output of the source of that controllable power supply system is controlled to not exceed a predetermined value where the available energy reserves of the source of that controllable power supply system are below a certain value.

15. An apparatus as hereinbefore generally described.

16. An apparatus as specifically described with reference to or as illustrated in the accompanying drawings.