WO2012084315A2

WO2012084315A2 - Electrical interface

Info

Publication number: WO2012084315A2
Application number: PCT/EP2011/069094
Authority: WO
Inventors: Robin Gupta; Mahesh Pr
Original assignee: Siemens Aktiengesellschaft
Priority date: 2010-12-22
Filing date: 2011-10-31
Publication date: 2012-06-28
Also published as: WO2012084315A3

Abstract

An electrical interface (25) for a first induction machine (20) coupled to a first prime mover (15), comprising a first inverter (40) comprising a first DC port and a first AC port, the DC port electrically coupled to a DC bus (60), the DC bus (60) being adapted to be electrically coupled to a DC storage device (30), an LC filter (57) connected to the AC port, the LC filter (57) adapted to be electrically coupled to the first induction machine (20), a DC link (50) connected to the DC bus (60), and an electronic load control unit (55) connected to the DC bus (60), the electrical load control unit (60) adapted to be electrically coupled to a controllable load device (35), wherein the first inverter (40) is controllable to provide a first input electrical power to the first induction machine (20) for cranking of the first prime mover (15), controllable to receive an amount of a first electrical power generated by the first induction machine (20) that is not consumed by at least one of an electrical load (32) electrically coupled to the first induction machine (20) and the DC storage device (30), and controllable to supply a first reactive power required by the first induction machine (20) and the electrical load (32).

Description

Electrical interface The present invention relates to an electrical interface for an induction machine.

Induction machines coupled with low speed prime movers are used in electrical power generation systems operated as a distributed generation unit. Such systems are used for providing electrical power to a load. The system can operate as a distributed power generation unit in an off-grid mode, grid connected mode or grid interactive mode. In such

systems, voltage and frequency of the induction machine are required to be controlled to achieve a desired output. This voltage and frequency control is achieved locally at the system.

It is an object of the embodiments of the invention to provide an electrical interface for an induction machine with reduced electrical components and reduced losses.

The above object is achieved by an electrical interface for a first induction machine coupled to a first prime mover according to claim 1.

This achieves in reduction in electrical components required for cranking the first prime mover, controlling the first induction machine and controlling the amount of electrical power not consumed by at least one of the electrical load and the DC storage device. The amount of electrical power not consumed by at least one of the electrical load and the Dc storage device can be provided to the controllable load device, and thus, stabilization of an electrical power generation system is achieved.

According to an embodiment, the electrical interface further comprises a controller configured to provide a first control signal to the first inverter and a second control signal to the electronic load control unit, wherein the controller is configured to generate the first control signal responsive to an output voltage of the first induction machine, a current flowing through the LC filter and a predetermined reference voltage, and configured to generate the second control signal responsive to a voltage across the DC link. Controlling the controller responsive to the output voltage of the induction generator, the induction current and the predetermined reference voltage enables controlling the inverter using a multiple loop feedback control technique. This achieves in improved stability and provides fast dynamic compensation.

According to yet another embodiment, the controller comprises a first comparator adapted to receive the output voltage of the first induction machine and receive the predetermined reference voltage, the first comparator configured to output a first reference signal responsive to a difference between the output voltage of the first induction machine and the predetermined reference voltage, a voltage controller

adapted to receive the first reference signal and operable to output a second reference signal, a second comparator adapted to receive the second reference signal and the current and configured to output a third reference signal responsive to a difference between the second reference signal and the current, a current controller adapted to receive the third reference signal and operable to output a fourth reference signal, and a modulation unit adapted to receive the fourth reference signal and output the first control signal. The voltage controller of the outer feedback loop control enables in controlling the first output voltage with respect to the predetermined reference voltage with minimum steady-state error. The current controller of the inner feedback control enables in achieving improved stability and provides fast dynamic compensation.

According to yet another embodiment, the voltage controller is a proportional + resonant controller and the current controller is a proportional controller. The proportional + resonant controller provides efficient tracking of the output voltage of the first induction machine with respect to the predetermined reference voltage. The proportional controller provides the advantage of reduced computational complexity. According to yet another embodiment, the controller is configured to generate the second control signal responsive to the voltage across the DC link being greater than a first threshold voltage. This enables in electrically connecting the controllable load device to the DC bus.

According to yet another embodiment, the electrical interface further comprises a charging module comprising an input port electrically connectable to the first induction machine, and an output port electrically connectable to the DC storage device. The charging module charges the DC storage device by using the electrical power produced by the first induction machine .

According to yet another embodiment, the electrical interface further comprises a bi-directional DC-DC converter having a first port and a second port, the first port coupled to the DC bus and the second port electrically connectable to the DC storage device, the bi-directional DC-DC converter being controllable responsive to the voltage across the DC link. This enables in charging the DC storage device using the electrical power from the DC bus and in providing electrical power from the DC storage device to the load.

According to yet another embodiment, the electrical interface further comprises a second inverter having a second DC port and a second AC port, the second DC port electrically coupled to the DC bus and the second AC port adapted to be

electrically coupled to a second induction machine coupled to a second prime mover. Thus, two induction machines can be connected to the electrical interface. For example, the second induction machine can be operated during peak hours when the electrical power consumption by the load is higher. According to yet another embodiment, the second inverter is controllable to provide a second input electrical power to the second induction machine for cranking of the second prime mover .

According to yet another embodiment, the second inverter is controllable to receive a second electrical power generated by the second induction machine and controllable to supply a second reactive power required by the second induction machine.

According to yet another embodiment, the electrical interface further comprises a DC-DC converter having a first port and second port, the first port being electrically connected to the DC bus and the second port being adapted to be

electrically coupled to a DC power generating device. This achieves in providing a hybrid electrical power generation system. Additionally, the first inverter controlling the first induction machine can be used for providing the

electrical power generated by the DC power generating device to the electrical load. This achieves in reduction of

electrical components of the electrical interface required for controlling the electrical power generation system. Thus, reduction in system looses is achieved.

According to yet another embodiment, the electrical interface further comprises a power conversion device having a first terminal port and a second terminal port, the first terminal port being electrically connected to the DC bus and the second terminal port adapted to be electrically coupled to an electrical power grid.

According to yet another embodiment, the power conversion device is controllable responsive to the voltage across the DC link. Thus, the electrical power from the DC bus can be provided to the electrical power grid and electrical power from the electrical power grid can be provided to the DC bus. According to yet another embodiment, the controllable load device is at least one from the group consisting of an electrolyzer, a desalination device, a resistance, an electrical light, and an electrical motor.

According to yet another embodiment, wherein the first prime mover and the second prime mover are low speed prime movers. Low speed prime movers are used in distributed power

generation units.

Embodiments of the present invention are further described hereinafter with reference to illustrated embodiments shown in the accompanying drawings, in which:

FIG 1 illustrates an electrical power generation system for generating electrical power according to an embodiment herein,

FIG 2 illustrates a block diagram of a system for

generating a first control signal according to an embodiment herein,

FIG 3 illustrates a graphical representation of power

verses time characteristics of a typical low speed prime mover,

FIG 4 illustrates an energy generation system according to a second embodiment herein,

FIG 5 illustrates an electrical power generation system according to a third embodiment herein,

FIG 6 illustrates an electrical power generation system according to a fourth embodiment herein, and

FIG 7 illustrates an electrical power generation system according to a fifth embodiment herein. Various embodiments are described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purpose of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments. It may be evident that such embodiments may be practiced without these specific details.

Referring to FIG. 1, an electrical power generation system 10 for generating electrical power is illustrated according to an embodiment herein. In the shown example of FIG 1, the electrical power generation system 10 is illustrated as a distributed power generation unit, wherein the power

generated is consumed by a local load. The electrical power generation system 10 comprises a first prime mover 15, an induction machine 20, an electrical interface 25, a DC storage device 30 and a controllable load device 35. The first prime mover 15 is mechanically coupled to the induction machine 20. An electrical load 32 can be electrically

connected to a terminal 37 of the first induction machine 20. According to an aspect, the first prime mover 15 is a low speed prime mover having a maximum speed of about 3000 rpm. For example, the first prime mover can be a Stirling engine. In aspects, where the electrical power generation system 10 is used to generate electrical power using renewable energy sources, the first prime mover 15 can be operated using renewable energy sources, such as, wind energy, solar energy, geothermal energy, and hydro energy to generate mechanical energy. The induction machine 20 coupled to the first prime mover 15 is used to convert the mechanical energy into electrical power.

The electrical interface 25 comprises a first inverter 40, a controller 45, a DC link 50 and an electronic load controller 55. The first inverter 40 comprises a first AC port and a first DC port. For example, the first inverter can be a three phase four leg inverter, a three phase three leg inverter with a delta-start transformer, and the like. The first AC port of the first inverter 40 is electrically connected to an LC filter 57 which in turn is electrically coupled to the terminal 35 of the induction machine 20. The DC port of the inverter 40 is electrically coupled to a DC bus 60. In the shown example of FIG 1, the DC link 50 is connected in parallel to the DC bus 60. In the shown example of Fig 1, the DC link 50 is a capacitor. However, other decoupling devices can also be used as the Dc link 50 instead of the capacitor. The electronic load controller 55 is connected in parallel to the DC bus 60 and can be electrically coupled to a

controllable load device 35. The DC bus 60 is also

electrically connected to a DC storage device 30. For

example, the DC storage device 30 can be a battery.

According to an aspect, a first output voltage 75 measured across the terminal 37 of the induction machine 20 and a current 80 flowing through the passive devices of the LC filter 55 are provided to the controller 45 as inputs. The passive devices of the LC filter 55 include the inductors and the capacitors of the LC filter 55. The current 80 can either be the current flowing though the capacitors or the inductors of the LC filter 55. The controller 45 is configured to generate a first control signal 85 responsive to the first output voltage 75, the current 80 inductor and a first predetermined reference voltage. The first predetermined reference voltage can be provided as an input to the

controller 45 or can be stored at a memory internal or external to the controller. The first control signal 85 generated by the controller 45 is provided to the inverter 40 and the inverter 40 is controllable responsive to the first control signal 85. The inverter 40 responsive to the first control signal 85 is controllable to provide a first reactive power required by the first induction machine 20 and the electrical load 32. The generation of the first control signal 85 will be described in more detail in paragraphs below.

According to another aspect, the electronic load controller 55 is controllable responsive to a voltage 90 measured across the DC link 50. As, illustrated in the example of FIG 1, the voltage 90 measured across the DC link 50 is provided to the controller 45 as an input and the controller 45 is configured to generate a second control signal 95 responsive to the voltage 90 across the DC link 50 being greater than a first threshold voltage. The second control signal 95 is provided to the electronic load controller 55. Typically, the voltage 90 across the DC link 50 increases if the electrical power generated by the induction machine 20 is greater than the electrical power consumed by the electrical load 32 and the DC storage device 30 and decreases if the electrical power generated by the induction machine 20 is less than the electrical power demand of the electrical load 32 and the electrical power required for charging the DC storage device 30. The first threshold voltage can advantageously be determined such that the voltage 90 across the DC link 50 being greater than the first threshold voltage indicates that the electrical power being generated by the induction machine 20 is greater than the electrical power demand of the

electrical load 32 and the electrical power required for charging the DC storage device 30. For example, the first threshold can be selected based on the electrical ratings of the electrical load 32 and the DC storage device 30. The first threshold voltage can be stored on a memory internal or external to the controller 45. The electronic load controller 55 connected to the controllable load device 35 is configured to electrically connect and disconnect the controllable load device 35 to the DC bus 60 responsive to the second control signal provided by the controller 45. For example, the electronic load controller 55 can be a switch.

Referring still to FIG 1, according to an aspect, the

electrical interface 25 further comprises a charging module 72, having an input port and an output port. The input port is electrically coupled to the terminal 37 of the induction machine 20 and the output port is electrically coupled to the DC storage device 30. The charging module 72 is adapted to charge the DC storage device 30 using the electrical power generated by the induction machine 20. In the present

example, where a charging module 72 is used to charge the DC storage device 30 such that the electrical power for charging the DC storage device 30 does not flow through the inverter 40, the electrical power consumed by the electrical load 32 and the electrical power consumed for charging the DC storage device 30 is the electrical power consumed by a load

electrically connected to the first induction machine 20.

FIG 2 with reference to FIG 1, illustrates a block diagram of a system 100 for generating the first control signal

according to an embodiment herein. According to an embodiment herein, the first control signal 85 is generated using multiple loop feedback control technique. The system 100 comprises a first comparator 105, a voltage controller 110, a second comparator 115, a current controller 120 and a

modulation unit 125. The first comparator 105 is configured to receive the first output voltage 75 measured across the terminal 37 of the induction machine 20 and to receive the first predetermined reference voltage, designated as 130. The first comparator 105 is configured to output a difference between the first output voltage 75 and the predetermined reference voltage 130 as a first reference signal 135. The voltage controller is operably coupled to the first

comparator 105 to receive the first reference signal 135. In an aspect, the voltage controller 110 is configured to control the first output voltage 75 with respect to the predetermined reference voltage 130 with zero steady-state error and output a second reference signal 140 responsive to the first reference signal 135. According to an embodiment, the voltage controller 110 can be a proportional + resonant controller. However, other controllers, such as, a

proportional-integral controller and the like can also be used. Using a proportional + resonant controller provides efficient tracking of the output voltage of the first

induction machine with respect to the predetermined reference voltage.

The second comparator 115 is configured to receive the second reference signal 140 outputted by the voltage controller 110 and a current 80 flowing through an inductor or a capacitor of the LC filter 57 and is configured to output a third reference signal 150 responsive to a difference between the second reference signal 140 and the current 80. The third reference signal 150 outputted by the second comparator 115 is provided to the current controller 120 and the current controller 120 is configured to compensate the third

reference signal 150 with respect to system disturbances and output a fourth reference signal 155. This achieves in improved stability and provides fast dynamic compensation. The modulation unit 125 is adapted to receive the fourth reference signal 155 and output the first control signal 85 as a modulated output responsive to the fourth reference signal 155. In an aspect, the first control signal 85 is a pulse-width-modulated signal, wherein the modulation is performed responsive to the fourth reference signal 155.

The first comparator 105, voltage controller 110, second comparator 115, current controller 120 and the modulation unit 125 forms an outer loop of feedback with the first output voltage 75 as the outer loop feedback variable. The second comparator 115, current controller 120 and the modulation unit 125 forms an inner loop of feedback with the current 145 as the inner loop feedback variable. Thus, as illustrated in the example of FIG 2, the inner feedback look is commanded by the second reference signal 140 provided to the second comparator 115. Advantageously, the current controller 120 can be a proportional controller as any steady-state error will not affect the outer loop feedback accuracy. However, other controllers can also be used as the current controller 120. Using a proportional controller provides the advantage of reduced computational complexity.

Referring now to FIG 1 and FIG 2, initially for the induction machine 20 to generate electrical power, the first prime mover 15 is required to be cranked. According to an

embodiment herein, the first inverter 40 can be controlled responsive to the first control signal 85 to provide a first input electrical power to the induction machine 20 and the induction machine 20 operates as an induction motor for providing the initial mechanical energy for cranking of the first prime mover 15. FIG 3 illustrates a graphical

representation of power verses time characteristics of a typical low speed prime mover. In the example of FIG 3, power is denoted by the y-axis and time is denoted by the x-axis. From the plot 170, the increase of power from zero towards negative indicates that power is being consumed by the prime mover 15. The power is provided to the prime mover 15 till the point 172 of the plot 170 at which the plot 170 starts moving towards zero. The time period indicated by the dashed line 175 indicates the cranking mode of the prime mover 15. Thereafter, the plot 170 moving towards zero and then in the positive direction from zero indicates an intermediate mode as the prime mover 15 has not yet started producing a steady state power. The intermediate mode is indicated as the time period between the line 175 and the line 180. The plot 170 being constant after the intermediate mode indicates that a steady state mechanical power is being generated by the prime mover 15.

Referring now to FIG 1, FIG 2 and FIG 3, the first input electrical power is provided to the induction machine 20 for cranking of the prime mover 15 for the time period indicated by the line 175. The electrical power to be provided to the induction machine 20 is received from the DC storage device 30. Advantageously, the predetermined reference voltage provided to the first comparator 105 can start with reduced voltage and reduced frequency and thereafter can be increased linearly to the desired output voltage and frequency at the terminal 37 of the induction machine 20. This enables in starting the operation of the inverter 40 with reduced voltage and reduced frequency and thus, enables in reducing the size of the DC storage device 30. In the intermediate mode, the prime mover 15 is driven using any renewable or non-renewable energy sources and the induction machine 20 operates as a generator for generating electrical power. The electrical load 32 can be electrically connected to the terminal 37 of the induction machine 20 when the prime mover 15 generates steady state mechanical power. In an aspect, the electrical load 32 can be electrically connected to the terminal 37 of the induction machine 20 after a predetermined time period after the initial start of the prime mover 15. The predetermined time period is the time required for the prime mover 15 to generate steady state mechanical from the initial start position.

Referring now to FIG 1 and FIG 2, the inverter 40 is

controllable to provide the first reactive power required by the induction machine 20 and the electrical load 32. The inverter 40 provides the reactive power to the induction machine 20 and the electrical load 32 responsive to the first control signal 85 provide by the controller 45. The first control signal 85 being generated responsive to the

predetermined reference voltage 130, first output voltage 75 and the current 80 flowing through the inductor or capacitor of the LC filter 57 enables in efficient voltage and

frequency control of the output of the terminal 37 of the induction machine 20.

Referring still to FIG 1 and FIG 2, the charging module 72 is configured to charge the DC storage device 30 using the electrical power generated by the induction machine 20 when the prime mover 15 starts generating steady state mechanical power. According to an aspect, when the electrical power generated by the induction machine 20 is greater than the electrical power demand of the electrical load 32 and the electrical power required for charging the DC storage device 30, an amount of electrical power greater than the electrical power consumed by the electrical load 32 and the DC storage device 30can be provided to the controllable load device 35 via the DC bus 60. The electronic load controller 55 is configured to electrically connect the controllable load device 35 to the DC bus 60 responsive to the second control signal 95. The controllable load device 35 electrically connected to the DC bus 60 can consume the amount of the generated electrical power not consumed by the electrical load 32 and the Dc storage device 30. The first amount of electrical power not being consumed by the load is provided to the DC bus 60 by the first inverter 40. The first inverter

40 receives the amount of electrical power at the AC port and provides the converted DC power to the DC bus 60 via the DC port. Providing the first amount of electrical power not being consumed by the electrical load 32 and the Dc storage device 30 to the controllable load device 35 enables the electrical power generation system 10 to be stable.

For an example, the controllable load device 35 can include, but, not limited to, an electrolyzer, a desalination device, a resistance, an electrical light, an electrical motor and the like. For example, in an aspect, if the controllable load device 35 is an electric motor, a flywheel can be

mechanically coupled to the electric motor and, thus, the additional electric power generated can be stored using the flywheel. According to another aspect, if the controllable load device 35 is a resistance, heat can be generated to be used for heating applications. Referring still to FIG 1 and FIG 2, in an aspect, when the electrical power generated by the induction machine 20 is not sufficient to meet the electrical power requirement of the electrical load 32, electrical power from the DC storage device 30 can be used to meet the electrical power demand of the electrical load 32. The electrical power stored at the DC storage device 30 can be provided to the electrical load 32 via the DC bus 60 and the first inverter 40. In aspects, where the electrical power generated by the induction machine 20 is not sufficient to meet the electrical power requirement of the electrical load 32, there is a decrease in the voltage 90 across the DC link 50. Due to this decrease, electrical power stored at the DC storage device 30 can be provided to the DC bus 60. On electrical power being available at the DC bus 60, the first inverter 40 can provide electrical power from the DC bus 60 to the electrical load 32. This enables in meeting the electrical power demand of the electrical load 32. FIG 4 illustrates the energy generation system 10 according to a second embodiment herein. In the shown example of FIG 2, the DC storage device 30 is electrically connected to the DC bus 60 using a bi-directional DC-DC converter 185. In the present example, as the DC storage device 30 is electrically connected to the DC bus 60 using the DC-DC converter 185, the electrical power for charging of the DC storage device 30 is provided by the inverter 40 via the DC bus 60. Thus, in the present example, the electrical power consumed by the

electrical load 32 is considered as the total power

consumption of the load as the DC storage device 30 receives the electrical power from the DC bus 60. According to an aspect, the controller 45 is configured to generate a third control signal 190 responsive to the voltage 90 across the DC link 50. The third control signal 190 generated by the controller 45 is provided to the DC-DC converter 185 and the DC-DC converter 185 is controllable responsive to the third control signal 190. In an aspect, electrical power from the DC storage device 30 can be supplied to the DC bus 60 in case the voltage 90 across the DC link 50 is below a second threshold voltage. The decrease in the voltage 90 across the DC link 50 below the second threshold voltage indicates that the electrical power generated by the induction machine 20 is less than the electrical power demand of the electrical load 32.

Referring still to FIG 4, advantageously, the second

threshold voltage can be selected such that the voltage 90 being less than the second threshold voltage indicates that the electrical power generated by the induction machine 20 is less than the electrical power requirement of the electrical load 32. Thus, the second threshold voltage selected is less than the first threshold voltage and can be stored on a memory internal or external to the controller 45. The

controller 45 on determining the voltage 90 across the DC link 50 being less than the second threshold voltage

generates the third control signal such that the DC-DC controller 185 is configured to provide electrical power from the DC storage device 30 to the DC bus 60 responsive to the third control signal. The first inverter 40 provides the electrical power from the DC bus 60 the electrical load 32. This enables in meeting the electrical power demand of the electrical load 32.

According to an another aspect, when the voltage 90 across the DC link 50 is greater than the second threshold voltage but less than or equal to the first threshold voltage, the controller 45 can be configured to generate the third control signal such that the DC-DC converter 185 is configured to charge the DC storage device 30 by using the electrical power from the DC bus 60 responsive to the third control signal. The voltage 90 across the DC link 50 being greater than the second threshold voltage but less than or equal to the first threshold voltage indicates that the electrical power

generated by the induction machine 20 can meet the electrical power requirement of the electrical load 32 and can be used charge the DC storage device 30. Thus, at circumstances when the electrical power being generated by the induction machine 40 is greater than the electrical power demand of the

electrical load 32, the additional electrical power can be used to charge the DC storage device 30.

According to a further aspect, the controller 45 is

configured to generate the second control signal 95 when the voltage 90 across the DC link 50 is greater than the first threshold voltage. The electronic load controller 55

responsive to the second control signal 95 electrically connects the controllable load device 35 to the DC bus 60. The voltage 90 being greater than the first threshold voltage indicates that the electrical power generated by the

induction machine 20 is greater than the electrical power required by the electrical load 32 and for charging the DC storage device 30. Thus, the controllable load device 35 consumes the electrical power not being consumed by the electrical load 32 and the DC storage device 30 and achieves in stabilizing the electrical power generation system 10. FIG 5 illustrates the electrical power generation system 10 according to a third embodiment herein. The electrical interface 25 of FIG 1 further comprises a second inverter 195 having a second DC port and a second AC port, the second DC port being electrically connected to the DC bus 60. According to an aspect, the second AC port of the second inverter 195 can be electrically connected to a second induction machine 200 operably coupled to a second prime mover 205. According to an aspect, the second prime mover 205 is a low speed prime mover having a maximum speed of about 3000 rpm. For example, the second prime mover can be a Stirling engine. In the present example, as the electrical load 32 is not directly connected to the second induction machine 200 the second inverter 195 is controlled using an open loop control

approach, i.e., a control mechanism without a feedback. Thus, according to an aspect, the controller 45 is configured to generate a fourth control signal 210 responsive to a second predetermined reference voltage. The second predetermined reference voltage can be provided as an input to the

controller 45 or can be stored at a memory internal or external to the controller 45. In an aspect, the first predetermined reference voltage can be used as the second predetermined referenced voltage. For example, the first predetermined reference voltage of FIG 1 can be used as the second predetermined reference voltage in case the electrical ratings of the first induction machine 20 and the second induction machine 200 are same. The fourth control signal 210 generated by the controller 45 is a modulated signal, wherein the controller 45 performs the modulation responsive to the second predetermined reference voltage. In an aspect, the fourth control signal 210 can be a pulse-width modulated signal .

The fourth control signal 210 generated by the controller 45 is provided to the second inverter 195 and the second

inverter 195 is controllable responsive to the fourth control signal 210. The second inverter 195 responsive to the fourth control signal 210 controls the output voltage and the frequency at the second terminal 212 of the second induction machine 200. The second inverter 195 responsive to the fourth control signal is controllable to provide a second reactive power required by the second induction machine 200. The second induction machine 200, similar to the first induction machine 20, operates as an induction motor for cranking the second prime mover 205 and operates as an induction generator to produce electrical power when the second prime mover 205 generates steady state mechanical power. According to an aspect, the second inverter 195 can be controlled responsive to the fourth control signal 210 to provide a second input electrical power to the second induction machine 200 and the second induction machine 200 operates as an induction motor for providing the initial mechanical energy for cranking of the second prime mover 205. Once the second prime mover 205 starts generating steady state mechanical power, the

induction machine 200 can start operating as an induction generator and generate electrical power. The second prime mover 205 can be driven using renewable or non-renewable energy sources for generating steady state mechanical power. Advantageously, the second predetermined reference voltage provided to the controller 45 can start with reduced voltage and reduced frequency and thereafter can be increased

linearly to the desired output voltage and frequency at output of the induction machine 200. This enables in starting the operation of the second inverter 195 with reduced voltage and reduced frequency, and thus, enables in reducing the size of the DC storage device 30. The second input electrical power provided to the second induction machine 200 can be provided from the DC storage device 30 or from the electrical power generated by the first induction machine 20.

Referring still to FIG 5, the electrical power generated by the second induction machine 200, can be provided to the electrical load 32 via the second inverter 195 and the first inverter 40. The second inverter 195 converts the electrical power generated by the second induction machine 200 to DC and the first inverter 40 converts the converted DC power to AC. The first inverter 40 can provide the electrical power provided to the DC bus 60 by the second induction machine 200 to the electrical load 32. In aspects, where the electrical power requirement of the electrical load 32 is met by the electrical power produced by the first induction machine 20, the electrical power produced by the second induction machine 200 can be used to charge the DC storage device 30. In aspects, where the electrical power produced by the second induction machine 200 is not consumed by the electrical load 32 or the DC storage device 30, electrical power can be provided to the controllable load device 35 via the

electronic load controller 55. Additionally, if the

electrical load 32 or the DC storage device 30 consumes only a portion of the electrical power produced by the second induction machine 200, the additional amount of electrical power not being consumed can be provided to the controllable load device 35. Moreover, in aspects, if the electrical power produced by the first induction machine 200 and/or the second induction machine 200 is not sufficient to meet the

electrical power demand of the electrical load 32, the electrical power stored at the DC storage device 30 can be provided to the electrical load 32 via the DC-DC converter

185 and the first inverter 40. This enables in operating two inductions machines 20, 200 in parallel.

Referring still to FIG 5, advantageously, the DC-DC converter 185 and the electronic load controller 55 can be controlled responsive to the voltage across the DC link 50 as described in the example of FIG 4. The controller 55 can be configured to generate the second control signal and the third control signal responsive to the voltage 90 across the DC link 50. The electronic load controller 55 and the DC-DC converter 185 are controllable responsive to the second control signal and the third control signal respectively. The second inverter 195 can be a three phase three leg inverter. The second inverter 195 can also be a three phase four leg inverter. However, as the electrical load 32 is not directly connected to the second induction machine 200, it is not required that the second inverter 195 be a three phase four leg inverter. FIG 6 illustrates the electrical power generation system 10 according to a fourth embodiment herein. In the shown example of FIG 6, the electrical interface 25 of FIG 1 further comprises a DC-DC converter 215 having a first port and a second port. The first port is electrically coupled to the DC bus 60 and the second port is electrically coupled to a DC power generating device 220. Advantageously, the DC power generating device 220 can be a device generating electrical power using renewable energy, such as, a solar power

generating system, or a fuel cell. The combination of the first prime mover 15 and the first induction machine 20 and the DC power generating device 220 can be used as a hybrid system as the electrical power generation system 10 will be producing electrical energy using two different types of energy sources. In the shown example of FIG 6, the electronic load controller 55 and the bi-directional DC-DC converter 185 are

controllable responsive to the voltage 90 measured across the DC link 50. The electrical power generated by the DC power generating device 220 is provided to the DC bus 60. In aspects, where the electrical power requirement of the electrical load 32 is met by the electrical power generated by the first induction machine 20, the electrical power generated by the DC power generating device 220 can be used to charge the DC storage device 30. In case the DC storage device 30 is completely charged, the electrical power

produced by the DC power generating device 220 can be

provided to the controllable load device 35 via the

electronic load controller 55. In aspects, where the

electrical power requirement of the electrical load 32 is greater than the electrical power generated by the first induction machine 30, the electrical power generated by the DC power generating device 220 can be provided to the

electrical load 32 by the first inverter 40 from the DC bus 60. The DC electrical power produced by the DC power

generating device 220 is converted to AC electrical power by the first inverter 40. Additionally, in aspects, if the electrical power requirement of the electrical load 32 is not met by the electrical power produced by the first induction machine 20 and the electrical power produced by the DC power generating device 220, electrical power from the DC storage device 30 can be supplied to the electrical load 32 to meet the electrical power requirement. Additionally, in aspects where the electrical power produced by the first induction machine 20 is greater than the

electrical power demand of the electrical load 32, the amount of electrical power not consumed by the electrical load 32 can be used for charging the DC storage device 30 via the bi- directional DC-DC converter 185. If the electrical power produced by the first induction machine is still greater than the electrical power consumed by the electrical load 32 and the DC storage device 30, the additional electrical power can be provided to the controllable load device 35 via the electronic load controller 55.

Referring still to FIG 6, the bi-directional DC-DC converter 185 and the electronic load controller 55 can be controlled by the controller 55 responsive to the voltage 90 across the DC link 50 as explain in the example of FIG 5. The controller 55 can be configured to generate the second control signal and the third control signal responsive to the voltage 90 across the DC link 50. The electronic load controller 55 and the DC-DC converter 185 are controllable responsive to the second control signal and the third control signal

respectively .

Referring now to FIG 7, the electrical power generation system 10 according to a fifth embodiment herein. In the shown example of FIG 7, the electrical interface 25 of FIG 6 further comprises a power conversion device 225 having a first terminal port and a second terminal port. The first terminal port is electrically coupled to the DC bus 60 and the second terminal port is electrically coupled to an electrical power grid 230. The electrical power grid 230 can be an AC power grid or a DC power grid. Thus, accordingly, the power conversion device 225 can be selected. For example, if the electrical power grid is an AC power grid, the power conversion device 225 can be a front end rectifier. The first terminal port of the power conversion device 225 can be an AC port and the second port can be a DC port. If the electrical power grid is a DC power grid, the power conversion device 225 can be a bi-directional DC-DC converter. The first terminal port of the power conversion device 225 can be a first DC port and the second terminal port can be a second DC port of the bi-directional DC-DC converter.

In the shown example of FIG 7, the power conversion device 225 can be turned on and turned off responsive to a voltage 235 measured across the electrical power grid 230. The controller 45 is operably coupled to the electrical power grid 230 and is configured to generate a fifth control signal 240 responsive to the voltage measured across the electrical power grid. The power conversion device 225 is turned on if the electrical power grid 230 is available and is turned off if the electrical power grid 230 fails. When the power conversion device 225 is in the on state, the electronic load controller 55 is turned off. The second control signal provided to the electronic load controller 55 can be

generated by the controller 45 such that the electronic load controller 55 is turned off when the power conversion device 225 is in the on state. For example, the second control signal to turn off the electronic load controller 55 can be generated by the controller 45 responsive to the voltage measured across the electrical power grid 230. The electrical power generated by the DC power generating device 220 is provided to the DC bus 60. The bi-direction DC-DC converter 185 is configured to charge the DC storage device 30 when the electrical power grid 230 is available.

Referring still to FIG 7, the power conversion device 225 in the on state is controllable responsive to the voltage 90 across the DC link 50 and can provide electrical power from the DC bus 60 to the electrical power grid 230 or can consume electrical power from the electrical power grid 230 and provide the same to the DC bus 60. For example, if the electrical power generated by the first induction machine 20 is not sufficient to meet the electrical power demand of the electrical load 32, the electrical power provided to the DC bus 60 by the DC power generating device 220 can be provided to the electrical load 32 via the first inverter 40. The first inverter 40 converts the DC power received from the DC bus 60 to AC power and provides the same to the electrical load 32. As mentioned above, the bi-direction DC-DC converter 185 is configured to charge the DC storage device 30 when the electrical power grid 230 is available. The DC storage device 30 can be charged using the electrical power generated by the first induction machine 20 or the DC power generating device 220 if the electrical power is not consumed by the electrical load 32 completely. In aspects, where the electrical power produced by the first induction machine 20 and the DC power generating device 225 is consumed by the electrical load 32 completely, electrical power from the electrical power grid 230 can be used for charging the DC storage device 30. The electrical power from the grid can be provided to the DC bus 60 via the power conversion device 225. This is achieved as the power conversion device 225 is controllable responsive to the voltage 90 across the DC link 50. The bi-directional DC- DC converter 185 can be controlled by the controller 55 responsive to the voltage 90 across the DC link 50 as explain in the example of FIG 6. The controller 55 can be configured to generate the third control signal responsive to the voltage 90 across the DC link 50.

In another aspect, if the electrical power generated by the first induction machine 20 and the DC power generating device 225 is greater than the electrical power demand of the electrical load 32 and the electrical power required to charge the DC storage device 30, the additional electrical power not used by the electrical load 32 and the DC storage device 30 can be provided to the electrical power grid 230 by the power conversion device 225.

Referring still to FIG 7, according to an aspect, the

controller 45 can be configured to control the power

conversion device 225 responsive to the voltage 90 across the DC link 50. The controller 45 is configured to determine if the voltage 90 across the DC link 50 is less than or equal to the first threshold voltage or greater than the first

threshold voltage and generate a sixth control signal 245 and a seventh control signal 250 respectively. The power

conversion device 225 responsive to the sixth control signal 245 is configured to provide electrical power from the electrical power grid 230 to the DC bus 60. The power

conversion device 225 responsive to the seventh control signal 250 is configured to provide electrical power from the DC bus 60 to the electrical power grid 230. The voltage 90 across the DC link 50 being less than the first threshold voltage indicates that the electrical power generated by the first induction machine 20 and the DC power generating device 220 is not sufficient to meet the electrical power demand of the electrical load 32 and the electrical power required to charge the DC storage device 30. The voltage 90 across the DC link 50 being greater than the first threshold voltage indicates that the electrical power generated by the first induction machine 20 and the DC power generating device 220 is greater than the electrical power demand of the electrical load 32 and the electrical power required for charging the DC storage device 30.

Referring still to FIG 7, in aspects, when the electrical power grid 230 has failed or is not available, the power conversion device 225 is turned off and the electronic load controller 55 is turned on. At the failure of the electrical power grid 230, the controller 45 generates the fifth control signal responsive to the voltage measured across the

electrical power grid 230 and the power conversion device 230 is turned off responsive to the fifth control signal. The controller 45 also generates the second control signal responsive to the voltage measured across the electrical power grid 230 and the electronic load controller 45 is turned on responsive to second control signal. On the

electronic load controller 45 being turned on, the electrical interface 25 operates similar to as described in FIG 6 as the electrical interface 25 is operably coupled to the DC power generating device 220 and the DC storage device 30. Referring still to FIG 7, advantageously, in an aspect, if the system 10 is used to supply electrical power to a

plurality of electrical loads, all the critical and non- critical electrical loads can be connected to the systems towards the first induction machine 20 during the

availability of the electrical power grid 230. During the failure of the electrical power grid 230, the non-critical electrical loads can be electrically disconnected. This ensures that uninterrupted electrical power supply is

maintained to the critical electrical loads. In another aspect, the critical electrical loads can be electrically connected towards the first induction machine 20 side and the non-critical loads can be electrically connected towards the electrical power grid 230 side. During the failure of the electrical power grid 230, if desired, the non-critical loads can be electrically connected to the first induction machine side 20. Critical electrical loads are the electrical loads which require uninterrupted electrical power supply. Non- critical electrical loads are the electrical loads which do not require uninterrupted electrical power supply.

Referring now to FIGS 1 through 7, the electrical power generation system 10 may comprise the embodiments described in different combinations. For example, in a combination, the electrical power generation system may comprise the first induction machine, the second induction machine and the DC power generating device. In another combination, the

electrical power generation system may comprise the first induction machine, the second induction machine and can be connected to the electrical power grid.

The embodiments described herein enable efficient control of an induction machine and load balancing of an electrical power generation system with reduced electrical components. As the electrical power required for cranking of the prime mover is provided by the inverter controlling the voltage and frequency of the induction motor, only one inverter is used for the cranking and controlling the parameters of the induction motor. Thus, the reduction of components achieves in reduction of losses of the system. The DC storage device can be used for providing the electrical power required for the initial cranking in a distributed power generation unit. The electronic load controller can connect the controllable load device to the DC bus so that the extra electrical power in the DC bus can be consumed, thus, achieving stabilization of the electrical power generation system. While this invention has been described in detail with reference to certain preferred embodiments, it should be appreciated that the present invention is not limited to those precise embodiments. Rather, in view of the present disclosure which describes the current best mode for

practicing the invention, many modifications and variations would present themselves, to those of skill in the art without departing from the scope and spirit of this

invention. The scope of the invention is, therefore,

indicated by the following claims rather than by the

foregoing description. All changes, modifications, and variations coming within the meaning and range of equivalency of the claims are to be considered within their scope.

Claims

Patent Claims:

1. An electrical interface (25) for a first induction machine (20) coupled to a first prime mover (15), comprising:

- a first inverter (40) comprising a first DC port and a first AC port, the DC port electrically coupled to a DC bus (60), the DC bus (60) being adapted to be electrically coupled to a DC storage device (30),

- an LC filter (57) connected to the AC port, the LC filter (57) adapted to be electrically coupled to the first

induction machine (20),

- a DC link (50) connected to the DC bus (60), and

- an electronic load control unit (55) connected to the DC bus (60), the electrical load control unit (60) adapted to be electrically coupled to a controllable load device (35), wherein the first inverter (40) is controllable to provide a first input electrical power to the first induction machine (20) for cranking of the first prime mover (15), controllable to receive an amount of a first electrical power generated by the first induction machine (20) that is not consumed by at least one of an electrical load (32) electrically coupled to the first induction machine (20) and the DC storage device (30), and controllable to supply a first reactive power required by the first induction machine (20) and the

electrical load (32) .

2. The electrical interface (25) according to claim 1, further comprising a controller (45) configured to provide a first control signal (85) to the first inverter (40) and a second control signal (95) to the electronic load control unit (55), wherein the controller (45) is configured to generate the first control signal (85) responsive to an output voltage (75) of the first induction machine (20), a current (80) flowing through the LC filter (57) and a

predetermined reference voltage, and configured to generate the second control signal (95) responsive to a voltage (90) across the DC link (50) .

3. The electrical interface (25) according to claims 1 or 2, wherein the controller (45) comprises:

- a first comparator (105) adapted to receive the output voltage (75) of the first induction machine (20) and receive the predetermined reference voltage, the first comparator (105) configured to output a first reference signal (135) responsive to a difference between the output voltage (75) of the first induction machine (20) and the predetermined reference voltage,

- a voltage controller (110) adapted to receive the first reference signal (135) and operable to output a second reference signal (140),

- a second comparator (115) adapted to receive the second reference signal (140) and the current (80) and configured to output a third reference signal (150) responsive to a

difference between the second reference signal (140) and the current (80),

- a current controller (120) adapted to receive the third reference signal (150) and operable to output a fourth reference signal (155), and

- a modulation unit (125) adapted to receive the fourth reference signal (155) and output the first control signal (85) .

4. The electrical interface (25) according to claim 3, wherein the voltage controller (110) is a proportional + resonant controller and the current controller (120) is a proportional controller.

5. The electrical interface (25) according to claims 3 or 4, wherein the controller (45) is configured to generate the second control signal (95) responsive to the voltage (90) across the DC link (50) being greater than a first threshold voltage .

6. The electrical interface (25) according to any one of the claims 1 to 5, further comprising a charging module (72) comprising an input port electrically connectable to the first induction machine (20), and an output port electrically connectable to the DC storage device (30) .

7. The electrical interface (25) according to anyone of the claims 1 to 5, further comprising a bi-directional DC-DC converter (185) having a first port and a second port, the first port coupled to the DC bus (60) and the second port electrically connectable to the DC storage device (30), the bi-directional DC-DC converter (185) being controllable responsive to the voltage (90) across the DC link (50) .

8. The electrical interface (25) according to anyone of the claims 1 to 7, further comprising a second inverter (195) having a second DC port and a second AC port, the second DC port electrically coupled to the DC bus (60) and the second AC port adapted to be electrically coupled to a second induction machine (200) coupled to a second prime mover

(205) .

9. The electrical interface (25) according to claim 8, wherein the second inverter (195) is controllable to provide a second input electrical power to the second induction machine (200) for cranking of the second prime mover (205) .

10. The electrical interface (25) according to claims 8 or 9, wherein the second inverter (195) is controllable to receive a second electrical power generated by the second induction machine (200) and controllable to supply a second reactive power required by the second induction machine (200) .

11. The electrical interface (25) according to anyone of the claims 1 to 7, further comprising a DC-DC converter (215) having a first port and second port, the first port being electrically connected to the DC bus (60) and the second port being adapted to be electrically coupled to a DC power generating device (225) .

12. The electrical interface (25) according to anyone of the claims 1 to 11, further comprising a power conversion device (225) having a first terminal port and a second terminal port, the first terminal port being electrically connected to the DC bus (60) and the second terminal port adapted to be electrically coupled to an electrical power grid (230) .

13. The electrical interface (25) according to claim 12, wherein the power conversion device (225) is controllable responsive to the voltage (90) across the DC link (50) .

14. The electrical interface (25) according to anyone of the claims 1 to 13, wherein the controllable load device (35) is at least one from the group consisting of an electrolyzer, a desalination device, a resistance, an electrical light, and an electrical motor.

15. The electrical interface (25) according to claims 1 or 8, wherein the first prime mover (15) and the second prime mover (205) are low speed prime movers.