Description
Power supply system and method
The present invention relates to power supply systems, particularly to standalone electricity supply systems supplying power to grid-independent electrical loads.
Electrical power supply systems of the above kind consist of energy generation and storage systems placed at or near a power load center. Unlike large electric power plants, such standalone systems small in terms of capacity and are installed close to the consumer. Such power supply systems include, for example, fuel cells, microturbines, photovoltaic systems, Stirling Engines, wind turbine systems, as the electrical energy source.
Electrical loads on any supply system vary continuously as commanded by various consumers. In standalone electricity supply systems, prime mover generator set or electrical energy source could have slow acting voltage & frequency regulation control loop. This leads to voltage-& frequency variation of the electrical supply system. These variations can adversely affect operation & performance of sensitive electrical loads. This type of behavior needs to be controlled to maintain a good quality of electrical supply of system.
The object of the present invention is to provide a power supply system and method that provides improved power quality supplied to sensitive electrical loads.
The above object is achieved by the system according to claim 1 and the method according to claim 10.
The underlying idea of the present invention is to segregate the electrical load for the power supply system into a first and a second electrical load, namely, a sensitive electrical

load and non-sensitive electrical load respectively. Both of these loads have independent power converters and distribution lines, deriving power from a common DC link. This allows power output of the power converter of the non-sensitive (second) load to be varied in such a manner that the power demand of the sensitive (first) load is always maintained during transient periods.
In an exemplary embodiment, said power generation means comprises an electrical power source and an input-side power conversion device adapted for converting an electrical output of said electrical power source into direct current electrical power, said direct current electrical power output from said input-side power conversion device being supplied to said direct current link.
In one embodiment, said controller is in communication a sensing and feedback module, said sensing and feedback module adapted to sense electrical and/or mechanical parameters of power generation means and power conversion devices, said .controller adapted for the controlling of the.electrical power outputs of said first and second power conversion devices based upon said sensed parameters.
In a further embodiment, the proposed system further comprises an energy storage device capable of drawing electrical power from said direct current link during charging of said energy storage device and supplying supplementary electrical power to said direct current link during discharging of said energy storage device, said energy storage device being electrically coupled to said direct current link via a bi-directional power conversion device, wherein said controller is further adapted to operatively control said bi-directional power conversion device for controlling the rate of charging and/or discharging of said energy storage device. The energy storage device is used for augmenting the power supply during increased electrical loads. By controlling the rate of charging/discharging of the

energy storage device, it is possible to avoid rapid charging/discharging of the energy storage device, thus improving the life of the energy storage device.
In a still further embodiment, to allow greater control of the power supplied to the loads, the proposed system further comprises an energy sink adapted to consume electrical power from said direct current link unutilized by said electrical loads, wherein said controller is further adapted to control the consumption of energy by said energy sink.
In a still further embodiment, the proposed system comprises a capacitor connected across said direct current link adapted to filter the direct current output of said power generation means.
The electrical power source may comprise a Stirling engine coupled to a generator, or a fuel cell, or a wind turbine system, or photovoltaic cell.
. The power conversion devices may comprise insulated gate bipolar transistors (IGBT), metal oxide semiconductor field-effect transistors (MOSFET), gate turnoff thyristors, or combinations thereof.
The present invention is further described hereinafter with reference to illustrated embodiments shown in the accompanying drawing, in which:
FIG is a schematic diagram of a power supply system according one embodiment of the present invention.
As described in detail hereinafter, embodiments of the present invention solves the aforementioned problems of the prior art solve by providing an appropriate selection of power converter topology, intelligent segregation of electrical loads into sensitive electric loads & non sensitive electrical loads, and an appropriate control

strategy. Referring to the FIG is illustrated a power supply system 10, which is typically a standalone power supply system delivering power to grid-independent electrical loads. The power supply system 10 comprises power generation means 12 operable to generate DC (direct current) electrical power. The power generation means 12 includes an electrical power source 34 and an input-side power conversion device 36. The electrical power source 34 may include, for example, a prime mover that converts heat energy into mechanical energy and an electrical generator rotationally coupled to the prime mover that converts this mechanical energy into electrical power. In the illustrated embodiment, the electrical power source 34 comprises Stirling engine 34 as the prime mover which is coupled to a generator, which may be single phase or three phase alternating current (AC) generator, or even a direct current (DC) generator. The prime mover of the electrical power source 34 may however include any other type of internal or external combustion engines, such as microturbines. Alternately, the electrical power source may include a fuel cell (producing a DC power output), or a wind - turbine system, or a photovoltaic cell. The present invention is particularly advantageous for electrical power sources ' having slow acting voltage and frequency regulation control loop.
The electrical output 64 of the electrical power source 34, which may be AC or DC power depending on the type of electrical power source or generator used, is fed to the input-side power conversion device 36 which converts it into DC power that is supplied to a DC link 14. The input-side power conversion device 36 may thus comprise an AC to DC converter (referred to as AC-DC converter) including a controlled or uncontrolled rectifier, or alternately a DC to DC converter (referred to as DC-DC converter). The input-side power conversion device may accordingly employ high power semiconductor switches, for example insulated gate bipolar transistors (IGBT), or alternately metal oxide semiconductor field-effect transistors (MOSFET) or gate turnoff thyristors.

The DC power from the input-side power conversion device 36 is filtered by a capacitor 40 connected across the DC link 14. The capacitor 40 functions as a temporary energy storage means and reduces the ripple in the rectified DC output from the power conversion device 36.
In accordance with the present invention, the electrical load on the system 10 is segregated into two distinct loads, namely a sensitive electrical load 30 and a non-sensitive electrical load 32, each being supplied power from the DC link 14 via separate load-side power conversion devices 16 and 18. The power conversion device 16 is a DC-AC converter, for example, of the type described earlier, adapted for converting the power in the DC link 14 into a first AC power output 56 that is supplied to the sensitive electrical load 30. The sensitive electrical load 30 typically includes electrical load that is sensitive to fluctuations in power supply, for example, appliances such as television, computers, among others. The power conversion device 18 is also a DC-AC converter, for example, of the type described earlier,, adapted for converting the power in the DC link 14 into a second AC power output 58 that is supplied to the non-sensitive electrical load 32, including, for example, motors. The system 10 includes a controller 20 adapted for providing control signals 22 and 24 respectively to the two load-side power conversion devices 16 and 18 to control the power output 56 and 58 supplied to the tow loads 30 and 32. In particular, the present invention attempts to improve the quality of power supply the sensitive electrical load 30 by adapting the controller to vary the output 58 of the power conversion device 18 being supplied to the non-sensitive electrical load 32, such that the voltage and frequency of the output 56 of said power conversion device 16 being supplied to the sensitive electrical load 30 is maintained relatively constant in comparison to that of output 58 of the power conversion device 18. This ensures that appropriate voltage and frequency levels are maintained for the sensitive load 30 during transient periods. Power output of the power

conversion device 18 is restored once governor of electrical power source 34 increases or reduces its output. Although not explicitly shown in the FIG, appropriate interfacing & driver circuitry is provided that conditions the outputs 22, 24 from controller 20 so that semiconductor switches of the .converters 16 and 18 can be controlled. This typically consists of voltage level shifters, buffers & driver circuits for power devices.
The unutilized power from the DC link (that is not utilized by the loads 30 and 32) is consumed by an energy sink 38, including, for example, a braking resistor, coupled to the DC link. In order to control he power supply to the electrical loads 30 and 32, the energy consumption by the energy sink 38 is controllable by control signals 26 from the controller 20, which is also useful for regulating the voltage across the capacitor 40 under special conditions.
The controller 20 implements a closed loop control of the system 10 by communicating with a sensing and feedback module 28. As shown in the. FIG., the sensing and. feedback module is in communication with the input-side power conversion device 36 of the power generation means 12 via communication link 48, and to the load-side power conversion devices 16 and 18 by communication links 50 and 52 respectively. The sensing and feedback module 28 consists of sensors adapted to sense electrical, thermal, mechanical parameters such as voltages, currents, temperatures at the input-side and load-side power converters and feed it to controller 20 in a suitable form. The controller 20 then generates the control signals 22, 24, 26 based these sensed parameters received from the sensing and feedback module 28. The controllers 20 could comprise, for example a digital signal processor (DSP), a field programmable gate array (FPGA), or a microcontroller, among others.
An energy storage device 42, such as a battery is coupled to the DC link 14. Under normal operations, the battery 42 is

charged by drawing power from the DC link 14. However, under certain conditions, for example, during a sudden increase in power demand from the electrical loads, the battery 42 is discharged to provide supplemental electrical power to the DC link 14, to meet the increase in the load. In one embodiment, the battery 42 may be discharged to provide power to the DC link 14 during starting of the electrical power source 34, i.e., Stirling engine in this example. The generator of the electrical power source 34 now acts as a motor to turn the Stirling engine. When the Stirling Engine torque exceeds the motoring torque, electrical power from the generator is fed back into the DC link 14. For this purpose, bi-directional power converters (DC-DC converter or AC-DC converter as the case may be) may be used in the input-side power conversion device 36. Instead of a battery, the energy storage device 42 may alternately include a fly-wheel, or an ultracapacitor.
The energy storage device 42 is electrically coupled to the DC link 14 via a bi-directional DC-DC power conversion device 44, including, for example, high power semiconductor . switches, such as IGBT or MOSFET, among others. The bidirectional DC-DC power conversion device 44 is controlled by control signals 46 from the controller 20 for controlling the charging and/or discharging of said energy storage device 42 as well as the rate of charging/discharging of the energy storage device 42. A closed loop feedback control of the energy storage device 42 is established by the communication of the bi-directional DC-DC power conversion device 44 with the sensing and feedback module 28 via communication link 62. Accordingly, the sensing and feedback module is further adapted to sense electrical and/or mechanical parameters of the bi-directional DC-DC power conversion device 44 and provide a feedback to the controller 20. The controller 20 is adapted to implement a control strategy that ensures that the battery 42 is operated to maximize it lifetime by avoiding rapid charging or discharging of the battery 42.

The present invention thus advantageously provides an improvement in quality of power supplied to sensitive loads in standalone electrical power supply systems. A further advantage of the present invention is that rapid charging & discharging of the energy storage device can be avoided, prolonging their life. This reduces life cycle cost for the energy storage device.
Summarizing, the present invention relates to a system and method for power supply. The proposed system comprises power generation means operable to supply electrical power to a DC link. The system a first power conversion device and a second power conversion device, each operable to convert the electrical power in said direct current link respectively into a first electrical power output and a second electrical power output. The proposed system further includes a controller adapted for controlling the power outputs of said first and second power conversion devices by varying said second electrical power output of said second power conversion device such that the first electrical power output of said, first power conversion device is maintained relatively constant in comparison to said second electrical power output of said second power conversion device.
Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternate embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that such modifications can be made without departing from the spirit or scope of the present invention as defined.

We claim,
1. A power supply system (10), comprising:
- power generation means (12) operable to supply electrical
5 power to a direct current link (14),
- a first power conversion device (16) and a second power
conversion device (18), each operable to convert the
electrical power in said direct current link (14)
respectively into a first electrical power output (56) and a
10 second electrical power output (58), and
- a controller (20) adapted for controlling the power outputs
(56, 58) of said first and second power conversion devices
(16, 18) by varying said second electrical power output (58)
of said second power conversion device (18) such that the
15 first electrical power output (56) of said first power
conversion device (16) is maintained relatively constant in comparison to said second electrical power output (58) of said second power conversion device (18).
20 2. The system (10) according to claim 1, wherein said first . . . electrical power output (56) of said first power conversion device (16) and said second electrical power output (58) of said second power conversion device (18) are supplied respectively to a first electrical load (30) and a second
25 electrical load (32), said first electrical load (30) being more sensitive to fluctuation in power supply than said second electrical load (32).
3. The system (10) according to any of the preceding claims, 30 wherein said power generation means (12) comprises an electrical power source (34) and an input-side power conversion device (36) adapted for converting an electrical output (64) of said electrical power source (32) into direct current electrical power, said direct current electrical 35 power output from said input-side power conversion device (36) being supplied to said direct current link (14).

4. The system (10) according to any of the preceding claims,
wherein said controller (20) is in communication a sensing
and feedback module (28), said sensing and feedback module
(28) adapted to sense electrical and/or mechanical parameters
5 of power generation means (12) and power conversion devices (16, 18, 36), said controller (20) adapted for the controlling of the electrical power outputs (56, 58) of said first and second power conversion devices (16, 18) based upon said sensed parameters. 10
5. The system (10) according to any of the preceding claims,
further comprising an energy storage device (42) capable of
drawing electrical power from said direct current link (14)
during charging of said energy storage device (42) and
15 supplying supplementary electrical power to said direct
current link (14) during discharging of said energy storage device (42), said energy storage device (42) being electrically coupled to said direct current link (14) via a bi-directional power conversion device (44), wherein said
20 controller (20) is further adapted to operatively control said bi-directional power conversion device (44). for controlling the rate of charging and/or discharging of said energy storage device (42).
25 6. The system (10) according to any of claims 2 to 5, further comprising an energy sink (38) adapted to consume electrical power from said direct current link (14) unutilized by said electrical loads (30, 32), wherein said controller (20) is further adapted to control the consumption of energy by said
30 energy sink (38) .
7. The system (10) according to any of the preceding claims,
further comprising a capacitor (40) connected across said
direct current link (14) adapted to filter the direct current
35 output of said power generation means (12).
8. The system (10) according to any of the preceding claims,
wherein said electrical power source (34) comprises a

Stirling engine coupled to a generator, or a fuel cell, or a wind turbine system, or photovoltaic cell.
9. The system (10) according to any of the preceding claims, 5 wherein said power conversion devices (16, 18, 36, 44)
comprise insulated gate bipolar transistors (IGBT), metal oxide semiconductor field-effect transistors (MOSFET), gate turnoff thyristors, or combinations thereof.
10 10. A method for power supply, comprising:
- supplying electrical power to a direct current link (14),
- converting the electrical power of said direct current link (14) into a first electrical power output (56) and a second electrical power output (58), and
15 - controlling said first and second electrical power outputs (56, 58) by varying said second electrical power output (58) such that the first electrical power output (56) is maintained relatively constant in comparison to said second electrical power output (58).
20

The present invention relates to a system and method for power supply. The proposed system (10) comprises power generation means operable to supply electrical power to a DC link (14). The system (10) a first power conversion device (16) and a second power conversion device (18), each operable to convert the electrical power in said direct current link (14) respectively into a first electrical power output (56) and a second electrical power output (58). The proposed system (10) further includes a controller (20) adapted for controlling the power outputs (56, 58) of said first and second power conversion devices (16, 18) by varying said second electrical power output (58) of said second power conversion device (18) such that the first electrical power output (56) of said first power conversion device (16) is maintained relatively constant in comparison to said second electrical power output (58) of said second power conversion device (18).