WO2017163126A1

WO2017163126A1 - A power generation system and a cell site incorporating the same

Info

Publication number: WO2017163126A1
Application number: PCT/IB2017/000369
Authority: WO
Inventors: Arvind Kumar Tiwari; Somakumar Ramachandrapanicker; Yashomani Y KOLHATKAR; Govardhan Ganireddy; Sharath Sridhar ARAMANEKOPPA; Rahul Radhakrishna PILLAI
Original assignee: General Electric Company
Priority date: 2016-03-22
Filing date: 2017-03-21
Publication date: 2017-09-28
Also published as: PH12018501925A1; BR112018069319A2; ZA201806266B

Abstract

A power generation system (101) is disclosed. The power generation system (101) includes a variable speed engine (106) and a doubly- fed induction generator (DFIG, 108) coupled thereto. The DFIG (108) includes a generator (112) to generate a first electrical power, a rotor side converter (114) and a line side converter (116) electrically coupled to the generator (112). The power generation system (101) further includes a PV power source (110) and/or an energy storage device (122) electrically coupled to a DC-link (118) between the rotor side converter (114) and the line side converter (116). The PV power source (110) is configured to generate a second electrical power and the energy storage device (122) is configured to supply a third electrical power. The DC-link (118) is configured to be coupled to at least one DC local (104) to enable supply of a DC power to the DC local (104).

Description

A POWER GENERATION SYSTEM AND A CELL SITE INCORPORATING THE SAME

BACKGROUND

[0001] The present application relates generally to generation of electrical power and more particularly relates to a power generation system employing a variable speed engine and a photovoltaic (PV) power source.

[0002] Typically, power generation systems such as generators use fuels such as diesel, petrol, and the like to generate an electrical power that can be supplied to local electrical loads. Reducing consumption of the fuels is an ongoing effort in achieving low cost and environment friendly power generation systems. To that end, various hybrid power generation systems are available that use a generator operated by a constant speed engine as primary source of electricity and some form of renewable energy source such as a wind turbine as a secondary source of electricity.

[0003] In such hybrid power generation systems, as an amount of power generated by the renewable energy source increases, the power generated by the generators operated by the constant speed engine needs to be reduced. In order to do so, the constant speed engine needs to be operated at low loads. Typically, the constant speed engine has low efficiencies at loads lower than certain threshold limit (e.g., 25%). Moreover, the operation of the constant speed engine at such low loads adversely impacts health of the constant speed engine and overall maintenance cycle.

[0004] Furthermore, such power generation systems may be coupled to various types of loads, for example, electrical devices that are operable using an alternating current (AC) power and electrical devices that are operable using direct current (AC) power at a point of common coupling. Typically, power generation systems supply the AC power at the point of common coupling. Therefore, to supply appropriate DC power to the electrical devices that are operable using the DC power, additional power converters are required. Use of such additional power converters further increases overall cost of the power generation systems.

BRIEF DESCRIPTION

[0005] In accordance with an embodiment of the invention, a power generation system is disclosed. The power generation system includes a variable speed engine. The power generation system further includes a doubly-fed induction generator (DFIG) mechanically coupled to the variable speed engine. The DFIG includes a generator having a rotor winding disposed on a rotor and a stator winding disposed on a stator, wherein the generator is configured to generate a first electrical power. The DFIG further includes a rotor side converter electrically coupled to the rotor winding. Furthermore, the DFIG includes a line side converter electrically coupled to the stator winding at a point of common coupling (PCC), wherein the rotor side converter and the line side converter are electrically coupled to each other via a Direct Current (DC) link, and wherein the PCC is configured to be coupled to an electric grid. Moreover, the power generation system also includes at least one of a photo voltaic (PV) power source and an energy storage device electrically coupled to the DC-link, wherein the PV power source is configured to generate a second electrical power and the energy storage device is configured to supply a third electrical power. The DC-link is configured to be coupled to at least one DC local load to enable supply of a DC power to the DC local load, wherein the DC power at the DC-link is based on at least one of the first electrical power, the second electrical power, the third electrical power, and a grid power.

[0006] In accordance with an embodiment of the invention, a cell site is disclosed. The cell site includes a base station operable using a DC power. The cellular tower station further includes a power generation system electrically coupled to the base station. The power generation system includes a variable speed engine. The power generation system further includes a DFIG mechanically coupled to the variable speed engine. The DFIG includes a generator includes a rotor winding disposed on a rotor and a stator winding disposed on a stator, wherein the generator is configured to generate a first electrical power. The DFIG further includes a rotor side converter electrically coupled to the rotor winding. Furthermore, the DFIG includes a line side converter electrically coupled to the stator winding at a PCC, wherein the rotor side converter and the line side converter are electrically coupled to each other via a DC- link, and wherein the PCC is configured to be coupled to an electric grid. Moreover, the power generation system also includes at least one of a PV power source and an energy storage device electrically coupled to the DC-link, wherein the PV power source is configured to generate a second electrical power and the energy storage device is configured to supply a third electrical power. The DC-link is configured to be coupled to at least one DC local load to enable supply of a DC power to the DC local load. [0007] In accordance with an embodiment of the invention, a method for operating a power generation system employing a DFIG, wherein the DFIG includes a generator electrically coupled to a rotor side converter and a point of common coupling (PCC), the PCC is electrically coupled to a line side converter and an electric grid. The method includes determining a desired operating speed of a variable speed engine based on an amount of one or more of a second electrical power supplied by a PV power source and a third electrical power supplied by an energy storage device to a DC-link between the rotor side converter and the line side converter, wherein the variable speed engine is mechanically coupled to the generator. The method further includes operating the variable speed engine at the determined operating speed to generate a first electrical power by the generator. Furthermore, the method includes supplying a DC power to a DC local load from the DC-link, wherein the DC power at the DC-link is based on at least one of the first electrical power, the second electrical power, the third electrical power, and a grid power.

DRAWINGS

[0008] These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

[0009] FIG. 1 is a block diagram of an electrical power distribution system, in accordance with aspects of the present specification;

[0010] FIG. 2 is a flowchart of an example method for operating a power generation system, in accordance with aspects of the present specification; and

[0011] FIG. 3 is a block diagram of cellular tower system coupled to the electrical power distribution system of FIG. 1, in accordance with aspects of the present specification.

DETAILED DESCRIPTION

[0012] The specification may be best understood with reference to the detailed figures and description set forth herein. Various embodiments are described hereinafter with reference to the figures. However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these figures is for explanatory purposes as the method and the system may extend beyond the described embodiments.

[0013] In the following specification, the singular forms "a", "an" and "the" include plural referents unless the context clearly dictates otherwise. As used herein, the term "or" is not meant to be exclusive and refers to at least one of the referenced components being present and includes instances in which a combination of the referenced components may be present, unless the context clearly dictates otherwise.

[0014] As used herein, the terms "may" and "may be" indicate a possibility of an occurrence within a set of circumstances; a possession of a specified property, characteristic or function; and/or qualify another verb by expressing one or more of an ability, capability, or possibility associated with the qualified verb. Accordingly, usage of "may" and "may be" indicates that a modified term is apparently appropriate, capable, or suitable for an indicated capacity, function, or usage, while taking into account that in some circumstances, the modified term may sometimes not be appropriate, capable, or suitable.

[0015] In accordance with some aspects of the present specification, a power generation system is disclosed. The power generation system includes a variable speed engine. The power generation system further includes a doubly-fed induction generator (DFIG) mechanically coupled to the variable speed engine. The DFIG includes a generator includes a rotor winding disposed on a rotor and a stator winding disposed on a stator, where the generator is configured to generate a first electrical power. The DFIG further includes a rotor side converter electrically coupled to the rotor winding. Furthermore, the DFIG includes a line side converter electrically coupled to the stator winding at a point of common coupling (PCC), where the rotor side converter and the line side converter are electrically coupled to each other via a Direct Current (DC) link, and where the PCC is configured to be coupled to an electric grid. Moreover, the power generation system also includes at least one of a photo voltaic (PV) power source and an energy storage device electrically coupled to the DC-link, where the PV power source is configured to generate a second electrical power and the energy storage device is configured to supply a third electrical power. The DC-link is configured to be coupled to at least one DC local load to enable supply of a DC power based on at least one of the first electrical power, the second electrical power, the third electrical power, and a grid power to the DC local load. A cellular tower station employing the power generation system and a method for operating the power generation system are also presented. [0016] FIG. 1 is a block diagram of an electrical power distribution system 100, in accordance with aspects of the present specification. The electrical power distribution system 100 may include a power generation system 101. In some embodiments, the power generation system 101 may be configured to be coupled to a direct current (DC) local load 104 to enable supply of a DC power to the DC local load 104. In some embodiments, the power generation system lOlmay be coupled to an electric grid 102 at a point of common coupling (PCC) 103. In some embodiments, the power generation system lOlmay be coupled to both the electric grid 102 and the DC local load 104. In certain embodiments, the power generation system 101 may be coupled to the PCC 103 via a transformer (not shown in FIG. 1). In some embodiments, the PCC 103 may also be configured to be coupled to an alternating current (AC) local load 105 to enable supply of an AC power to the AC local load 105.

[0017] The electric grid 102 may be representative of an interconnected network for delivering electricity from one or more power generation stations (different from the power generation system 101) to consumers (e.g., the AC local load 105) through high/medium voltage transmission lines. The AC local load 105 may include electrical devices that are operable using the AC power either from the electric grid 102 or from the power generation system 101.

[0018] In some embodiments, the power generation system 101 may include one or more variable speed engines such as a variable speed engine 106, a doubly-fed induction generator (DFIG) 108, and at least one of a photo-voltaic (PV) power source 110 and an energy storage device 122. The DC local load 104 coupled to the power generation system 101 may include electrical devices that are operable using the DC power supplied via the power generation system 101. In a non-limiting example, the power generation system 101 may be disposed at cell site (not shown in FIG. 1, see FIG. 3) where the DC local load 104 may include a base station disposed at the cell site. In some embodiments, the power generation system 101 may include a central controller 124. In some embodiments, central controller 124 may be coupled to the variable speed engine 106 and the DFIG 108 via respective control buses (shown using solid connectors). The central controller 124 may be configured to control the operations of the variable speed engine 106 and the DFIG 108.

[0019] The variable speed engine 106 may refer to any system that may aid in imparting a controlled rotational motion to rotary element(s). For example, the variable speed engine 106 may be an internal combustion engine, an operating speed of which may be varied under the control of the central controller 124. For example, the variable speed engine 106 may be a variable speed reciprocating engine where the reciprocating motion of a piston is translated into a rotational speed of a crank shaft connected thereto. The variable speed engine 106 may be operated by combustion of various fuels including, but not limited to, diesel, natural gas, petrol, LPG, biogas, producer gas, and the like. The variable speed engine 106 may also be operated using waste heat cycle. It is to be noted that the scope of the present specification is not limited with respect to the types of fuel and the variable speed engine 106 employed in the power generation system 101.

[0020] The DFIG 108 may include a generator 1 12. In a non-limiting example, the generator 1 12 may be a wound rotor induction generator. The generator 1 12 may include a stator 126 and a rotor 128. The generator 1 12 may further include a stator winding 130 disposed on the stator 126. The generator 1 12 may also include a rotor winding 132 disposed on the rotor 128. The generator 1 12 may be electrically coupled to the PCC 103 to provide a first electrical power (voltage and current) at the PCC 103. The stator winding 130 may be coupled (directly or indirectly) to the PCC 103.

[0021] The DFIG 108 may be mechanically coupled to the variable speed engine 106. In some embodiments, the rotor 128 of the generator 1 12 may be mechanically coupled to the crank shaft of the variable speed engine 106, such that rotations of the crank shaft may cause a rotary motion of the rotor 128 of the generator 1 12. In some embodiments, the crank shaft of the variable speed engine 106 may be coupled to the rotor of the generator 1 12 through one or more gears. As will be appreciated, due to such coupling of the variable speed engine 106 with the generator 1 12, a rotational speed of the rotor of the generator 1 12 may be also be varied depending on the operating speed of the variable speed engine 106. In some embodiments, the generator 1 12 may generate the first electrical power at the stator winding 130 depending on at least one of the operating speed of the variable speed engine 106 and an electrical excitation provided to the rotor winding 132.

[0022] Typically, a slip of the generator 1 12 may be defined as represented by Equation (1):

S = ... Equation (1) where, N_r represents operating speed of the rotor 128 in revolution per minute (rpm) and N_s represents a synchronous speed of the generator 1 12. Further, N_s is represented by Equation (2): Equation (2) where, / represents frequency of current flowing through the stator winding 130, and p represents number of stator poles.

[0023] The generator 1 12 may operate in different modes depending on the operating speed (rpm) of the rotor 128. For example, the generator 1 12 may operate in sub-synchronous mode if N_r is lower than N_s. The generator 1 12 may operate in synchronous mode if N_r is same as N_s. The generator 1 12 may operate in super-synchronous mode if N_r is greater than N_s. In some embodiments, when the generator 1 12 operates in super-synchronous mode, the generator 1 12 may be configured to generate additional electrical power (hereinafter referred to as a fourth electrical power) at the rotor winding 132.

[0024] In some embodiments, the DFIG 108 may further include a rotor side converter 1 14 and a line side converter 1 16. Each of the rotor side converter 1 14 and the line side converter 1 16 may act as either an AC -DC converter or a DC- AC converter under the control of the central controller 124. The rotor side converter 1 14 may be electrically coupled to the rotor winding 132. Further, the line side converter 1 16 may be electrically coupled to the stator winding 130 at the PCC 103. The line side converter 1 16 may further be coupled to the PCC 103, directly or via a transformer (not shown in FIG. 1). In one embodiment, the rotor side converter 1 14 and line side converter 1 16 are also coupled to each other. For example, the rotor side converter 1 14 and the line side converter 1 16 are electrically coupled to each other via a DC-link 1 18.

[0025] Further, the power generation system 101 may include at least one of the PV power source 1 10 and the energy storage device 122 electrically coupled to the DFIG 108 at the DC- link 1 18. The PV power source 1 10 may include one or more PV arrays (not shown in FIG. 1), where each PV array may include at least one PV module (not shown in FIG. 1). A PV module may include a suitable arrangement of a plurality of PV cells (diodes and/or transistors). The PV power source 1 10 may generate a DC voltage constituting a second electrical power depending on solar insolation, weather conditions, and/or time of day. Accordingly, the PV power source 1 10 may be configured to supply the second electrical power to the DC-link 1 18. A maximum amount of the second electrical power producible by the PV power source 1 10 may be referred to as "PV rating." [0026] The energy storage device 122 may include arrangements of one or more batteries, capacitors, and the like. In some embodiments, the energy storage device 122 may be electrically coupled to the DFIG 108 at the DC-link 118 to supply to supply a third electrical power to the DC-link 118. A maximum amount of the third electrical power the can be supplied by the energy storage device 122 may be referred to as "energy storage device rating."

[0027] Furthermore, as the PV power source 110 and/or the energy storage device 122 may be electrically coupled to the DC-link 118 to supply the second electrical power, it may be desirable to appropriately select power ratings of the rotor side converter 114 and the line side converter 116. In some embodiments, appropriate selection of the power ratings of the rotor side converter 114 and the line side converter 116, may aid in operating the rotor side converter 114 and the line side converter 116 at their respective maximum efficiencies under the control of the central controller 124. The power ratings of the rotor side converter 114 and the line side converter 116 may be referred to as a maximum amount of power that may be handled by each of the rotor side converter 114 and the line side converter 116.

[0028] In one embodiment, when only the PV power source 110 is coupled to the DC-link 118, the power ratings of the rotor side converter 114 and the line side converter 116 may be selected based on the PV rating. In some embodiments, the value of the power rating of each of the rotor side converter 114 and the line side converter 116 may be selected equal to half of the PV rating. In some embodiments, the power rating of each of the rotor side converter 114 and the line side converter 116 may be equal to the PV rating.

[0029] In certain embodiments, when the energy storage device 122 is also coupled to the DC-link 118, the power ratings of the rotor side converter 114 and the line side converter 116 may be selected based on the PV rating and the energy storage device rating. In some embodiments, the power ratings of the rotor side converter 114 and the line side converter 116 may be may be equal to half of a sum of PV rating and the energy storage device rating. In some embodiments, the power rating of each of the rotor side converter 114 and the line side converter 116 may be equal to the sum of PV rating and the energy storage device rating.

[0030] In some embodiments, the power generation system 101 may optionally include one or more DC-DC converters. In some embodiments, the power generation system 101 may include a first DC-DC converter 134. The first DC-DC converter 134 may be electrically coupled between the PV power source 110 and the DC-link 118. The second electrical power may be supplied from the PV power source 110 to the DC-link 118 via the first DC-DC converter 134. The first DC-DC converter 134 may be operated as a buck converter, a boost converter, or buck-boost converter under the control of the central controller 124.

[0031] In some embodiments, the power generation system 101 may include a second DC- DC converter 136. The second DC-DC converter 136 may be electrically coupled between the energy storage device 122 and the DC-link 118. The third electrical power may be supplied from the energy storage device 122 to the DC-link 118 via the first DC-DC converter 134. In some embodiments, the energy storage device 122 may receive a charging current via the second DC-DC converter 136. The second DC-DC converter 136 may be operated as a buck converter, a boost converter, or buck-boost converter under the control of the central controller 124.

[0032] In some embodiments, the power generation system 101 may include a third DC-DC converter 138. The third DC-DC converter 138 may be electrically coupled between the energy storage device 122 and the PV power source 110. In some embodiments, the third DC-DC converter 138 may be configured to charge the energy storage device 122 via the PV power source 110. For example, in some embodiments, the energy storage device 122 may receive a charging current via the third DC-DC converter 138 from the PV power source 110. The third DC-DC converter 138 may be operated as a buck converter, a boost converter, or buck-boost converter under the control of the central controller 124.

[0033] Moreover, in some embodiments, the DC-link 118 may be configured to be coupled to at least one DC local load, such as, the DC local load 104. The DC-link 118 may be configured to be coupled to the DC local load 104 to enable supply of the DC power from the DC-link 118 to the DC local load 104. In some embodiments, the DC-link 118 may be configured to be directly coupled to the DC local load 104. While in some other embodiments, the DC-link 118 may be configured to be coupled to the DC local load 104 via a fourth DC-DC converter 140. The fourth DC-DC converter 140 may be electrically coupled between the DC local load 104 and the DC-link 118. The fourth DC-DC converter 140 may be operated as a buck converter, a boost converter, or buck-boost converter under the control of the central controller 124. While in some embodiments, the fourth DC-DC converter 140 may form a part of the power generation system 101, in some other embodiments, the fourth DC-DC converter 140 may be disposed outside the power generation system 101. [0034] In some embodiments, the DC power at the DC-link 118 may be based on at least one of the first electrical power, the second electrical power, the third electrical power, the fourth electrical power, and the grid power. For example, during operation of the power generation system 101, under the control of the central controller 124, the DC-link 118 may carry the DC power based on sources including, but not limited to, the first electrical power (via the line side converter 116), the fourth electrical power (via the rotor side converter 114 in super- synchronous mode), the second electrical power (directly or via the first DC-DC converter 134) and the third electrical power (directly or via the second DC-DC converter 136), and the grid power (via the line side converter 116 if the grid power is available at the PCC 103).

[0035] The power generation system 101 may include some or all of the first DC-DC converter 134, the second DC-DC converter 136, the third DC-DC converter 138, or the fourth DC-DC converter 140. Moreover, in some embodiments, some or all of the first DC-DC converter 134, the second DC-DC converter 136, the third DC-DC converter 138, or the fourth DC-DC converter 140 may include a DC- AC converter (not shown in FIG. 1) coupled to an AC- DC converter (not shown in FIG. 1). In some embodiments, the DC-AC converter is electrically isolated from the AC-DC converter. In a non-limiting example, the DC-AC converter may be coupled to the AC -DC converter via a high frequency AC transformer.

[0036] Furthermore, in some embodiments, the power generation system 101 may optionally include a common mode filter 142 coupled between the DC-link 118 and at least one of the PV power source 110, the energy storage device 122, and the DC local load 104. For ease of illustration, in FIG. 1, the common mode filter 142 is shown as coupled on a common DC line that couples the PV power source 110, the energy storage device 122, and the DC local load 104 with the DC-link 118. In some embodiments, the common mode filter 142 may be configured to minimize a flow of common mode current between the DC-link 118 and the at least one of the PV power source 110, the energy storage device 122, and the DC local load 104. For example, the common mode current may be constituted by a current flowing in a same direction on both positive bus (not shown in FIG. 1) and negative bus (not shown in FIG. 1) of the DC-link 118. In some embodiments, the common mode filter 142 may minimize the flow of the common mode current by grounding the currents flowing in the same direction on both the positive bus and the negative bus of the DC-link 118.

[0037] Additionally, in some embodiments, under the control of the central controller 124, the AC power available at the PCC 103 may be based on sources including, but not limited to, the grid power (via the line side converter 116 if the grid power is available at the PCC 103), the first electrical power from the stator winding 130, the DC power from the DC-link 118 (via the line side converter 116). As previously noted, the AC local load 105 may be electrically coupled at the PCC 103. Accordingly, in some embodiments, the PCC 103 may be configured to enable supply of the AC power to the AC local load 105.

[0038] In some embodiments, in addition to being operatively coupled to the variable speed engine 106, the generator 112, the rotor side converter 114, and the line side converter 116, the central controller 124 may be operatively coupled to at least one of the first DC-DC converter 134, the second DC-DC converter 136, the third DC-DC converter 138, and the fourth DC-DC converter 140 to control their respective operations. Furthermore, in some embodiments, the central controller 124 may be operatively coupled the DC local load 104 and the AC local load 105 to selectively connect and disconnect the respective electrical device to manage load.

[0039] The power generation system 101 may be operated in various operating conditions under the control of the central controller 124. The operating conditions may include grid connected mode of operation, an islanded mode of operation, and a transition mode of operation. The grid connected mode of operation may be defined as a situation when a grid power is supplied / available at the PCC 103 from the electric grid 102. The islanded mode of operation may be defined as a situation when the power generation system 101 is not connected to the electric grid 102. The transition mode of operation may be defined as a mode of operation when the power generation system 101 is to be transitioned from the grid connected mode of operation to the islanded mode of operation. Such situation may arise when the grid power cuts-off and the power generation system 101 needs to be controlled to generate sufficient electrical power to meet a load requirement of the DC and/or AC local loads 104, 105.

[0040] In one embodiment, the central controller 124 may include a specially programmed general purpose computer, a microprocessor, a digital signal processor, and/or a microcontroller. The central controller 124 may also include input/output ports, and a storage medium, such as, an electronic memory. Various examples of the microprocessor include, but are not limited to, a reduced instruction set computing (RISC) architecture type microprocessor or a complex instruction set computing (CISC) architecture type microprocessor. Further, the microprocessor may be of a single-core type or multi-core type. Alternatively, the central controller 124 may be implemented as hardware elements such as circuit boards with processors or as software running on a processor such as a commercial, off-the-shelf personal computer (PC), or a microcontroller. In certain embodiments, the variable speed engine 106, the rotor side converter 114, the line side converter 116, the first DC-DC converter 134, the second DC-DC converter 136, the third DC- DC converter 138, and/or the fourth DC-DC converter 140 may include controllers / control units / electronics to control their respective operations under a supervisory control of the central controller 124. The central controller 124 may be capable of executing program instructions for controlling operations of the power generation system, the electrical devices constituting the DC local load 104, and/or the electrical devices constituting the AC local load 105. In some embodiments, the central controller 124 may aid in executing a method for operating a power generation system (see FIG. 2) by controlling operations of the variable speed engine 106, the DFIG 108, the first DC-DC converter 134, the second DC-DC converter 136, the third DC-DC converter 138, and/or the fourth DC-DC converter 140, the electrical devices constituting the DC local load 104, and/or the electrical devices constituting the AC local load 105.

[0041] FIG. 2 is a flowchart of an example method for operating the power generation system 101, in accordance with aspects of the present specification.

[0042] At step 202, a desired operating speed of the variable speed engine 106 may be determined based on an amount of one or more of the second electrical power supplied by the PV power source 110 and the third electrical power supplied by the energy storage device 122 (if the energy storage device 122 is present) to the DC-link 118 between the rotor side converter 114 and the line side converter 116. In some embodiments, the central controller 124 may be configured to determine the desired operating speed of the variable speed engine 106. In certain embodiments, the central controller 124 may be configured to determine the desired operating speed of the variable speed engine 106 additionally on at least one of a total load requirement (i.e., load requirement of one or both of the DC local load 104 and the AC local load 105), an availability of the grid power, the power ratings of the rotor side converter 114 and the line side converter 116, an efficiency of the variable speed engine 106, and efficiencies of the rotor side converter 114 and the line side converter 116. The efficiency of the variable speed engine 106 may be defined as a percentage of a chemical energy (e.g., an energy generated due to burning of fuels) that is translated in to mechanical power output of the variable speed engine 106. Similarly, efficiencies of the rotor side converter 114 and the line side converter 116 may refer to a ratio of a respective output power and an input power. Once the desired operating speed of the variable speed engine 106 is determined, the variable speed engine 106 may be operated at the determined desired operating speed to generate the first electrical power by the generator 112, at step 204.

[0043] As previously noted, in some embodiments, under the control of the central controller 124, the DC-link 118 may carry the DC power based on sources including, but not limited to, the first electrical power (via the line side converter 116), the second electrical power (directly or via the first DC-DC converter 134) and the third electrical power (directly or via the second DC- DC converter 136), the fourth electrical power (via the rotor side converter 114 in super- synchronous mode), and the grid power (via the line side converter 116 if the grid power is available at the PCC 103). Accordingly, in some embodiments, at step 206, the DC power may be supplied to the DC local load 104 from the DC-link 118.

[0044] Additionally, in some embodiments, at step 208, the AC power may be supplied to the AC local load 105 via the PCC 103. As previously noted, in some embodiments, under the control of the central controller 124, the AC power available at the PCC 103 may be based on sources including, but not limited to, the grid power (via the line side converter 116 if the grid power is available at the PCC 103), the first electrical power from the stator winding 130, the DC power from the DC-link 118 (via the line side converter 116).

[0045] FIG. 3 is a block diagram illustrating a cell site 300 coupled to the electrical power distribution system 100 of FIG. 1, in accordance with aspects of the present specification. It is to be noted that description of the electrical power distribution system 100 is not repeated herein for the sake of brevity.

[0046] The cell site 300 may represent a cellular telephone site that may host infrastructure and equipment facilitating cellular communication. For example, cell site 300 may include a tower 302, sometimes also referred to as a radio mast, and a shelter 304. Further, the cell site 300 may also include one or more antennas such as the antenna 306 disposed on the tower 302. The antenna 306 may aid in transmission and reception of cellular communication. Furthermore, the cell site 300 may also include one or more base stations, such as, a base station 308 and one or more devices housed in the shelter 304. The devices may include, but are not limited to, an air-conditioner 312, a fan 314, lighting system 316, etc. It is to be noted that certain equipment of the cell site 300 may be operable using a DC power. In a non-limiting example, the base station 308 may be operable using the DC power. The equipment such as the air-conditioner 312, the fan 314, and the lighting systems 316 may be operated using AC power. [0047] In some embodiments, the cell site 300 may be powered, partially or fully, via the power generation system 101. The cell site 300 may be electrically coupled to the power generation system 101, as depicted in FIG. 3. In some embodiments, all equipment of the cell site 300 which are operable using DC power may be electrically coupled to the DC-link 118. For example, the base station 308 may be electrically coupled to the DC-link 118 to receive the DC power from the DC-link 118. In some embodiments, the base station 308 may be electrically coupled to the DC-link 118 via a DC-DC converter 318 (similar to the fourth DC-DC converter 140). While in some embodiments, the DC-DC converter 318 may form a part of the power generation system 101, in some other embodiments, the DC-DC converter 318 may be disposed outside the power generation system 101. As previously noted, the DC power at the DC-link 118 may be based on at least one of the first electrical power, the second electrical power, the third electrical power, the fourth electrical power, and the grid power.

[0048] In some embodiments, the equipment of the cell site 300 which are operable using AC power may be electrically coupled to the PCC 103 to receive the AC power from the PCC 103. As previously noted, the AC power at the PCC 103 is based on at least on one or more of the first electrical power, the grid power, and the DC power at the DC-link 118.

[0049] Any of the foregoing steps and/or system elements may be suitably replaced, reordered, or removed, and additional steps and/or system elements may be inserted, depending on the needs of a particular application, and that the systems of the foregoing embodiments may be implemented using a wide variety of suitable processes and system elements and are not limited to any particular computer hardware, software, middleware, firmware, microcode, etc.

[0050] Furthermore, the foregoing examples, demonstrations, and method steps such as those that may be performed by the central controller 124 may be implemented by suitable code on a processor-based system, such as a general-purpose or special-purpose computer. Different implementations of the systems and methods may perform some or all of the steps described herein in different orders, parallel, or substantially concurrently. Furthermore, the functions may be implemented in a variety of programming languages, including but not limited to C++ or Java. Such code may be stored or adapted for storage on one or more tangible or non-transitory computer readable media, such as on data repository chips, local or remote hard disks, optical disks (that is, CDs or DVDs), memory or other media, which may be accessed by a processor- based system to execute the stored code. Note that the tangible media may comprise paper or another suitable medium upon which the instructions are printed. For instance, the instructions may be electronically captured via optical scanning of the paper or other medium, then compiled, interpreted or otherwise processed in a suitable manner if necessary, and then stored in the data repository or memory.

[0051] In accordance with some embodiments of the invention, the power generation system may be operated at higher efficiencies by ensuring that converters (the rotor side converter and the line side converter) and the variable speed engine are operated at the best efficiency for a given load requirement. Moreover, wear and tear of the variable speed engine may also be reduced, since lower speed of operation increases the life of internal mechanical components of the variable speed engine. Moreover, the PV power source may be utilized as primary power source leading to more environmental friendly and cost effective power generation system. Also, overall fuel consumption by the variable speed engine may be reduced as the PV power source may be utilized as primary power source. Additionally, as the DC local load is coupled at the DC-link, use of additional converters may be greatly avoided or minimized, resulting in additional cost savings.

[0052] The present invention has been described in terms of some specific embodiments. They are intended for illustration only, and should not be construed as being limiting in any way. Thus, it should be understood that modifications can be made thereto, which are within the scope of the invention and the appended claims.

[0053] It will be appreciated that variants of the above disclosed and other features and functions, or alternatives thereof, may be combined to create many other different systems or applications. Various unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art and are also intended to be encompassed by the following claims.

Claims

1. A power generation system, comprising:

a variable speed engine;

a doubly-fed induction generator (DFIG) mechanically coupled to the variable speed engine, wherein the DFIG comprises:

a generator comprising a rotor winding disposed on a rotor and a stator winding disposed on a stator, wherein the generator is configured to generate a first electrical power;

a rotor side converter electrically coupled to the rotor winding; and a line side converter electrically coupled to the stator winding at a point of common coupling (PCC), wherein the rotor side converter and the line side converter are electrically coupled to each other via a Direct Current (DC) link, and wherein the PCC is configured to be coupled to an electric grid; and

at least one of a photo voltaic (PV) power source and an energy storage device electrically coupled to the DC-link, wherein the PV power source is configured to generate a second electrical power and the energy storage device is configured to supply a third electrical power, wherein the DC-link is configured to be coupled to at least one DC local load to enable supply of a DC power to the DC local load, and wherein the DC power at the DC-link is based on at least one of the first electrical power, the second electrical power, the third electrical power, and a grid power.

2. The power generation system of claim 1, wherein the at least one DC local load comprises a base station disposed at a cellular tower.

3. The power generation system of claim 1, wherein the PCC is further configured to be coupled to an alternating current (AC) local load to enable supply of an AC power to the AC local load, wherein the AC power at the PCC is based on at least on one or more of the first electrical power, the grid power, and the DC power at the DC-link.

4. The power generation system of claim 1, wherein the generator is further configured to generate a fourth electrical power at the rotor winding when the generator is operated in a super-synchronous mode, wherein the DC power at the DC-link is based on at least one of the first electrical power, the second electrical power, the third electrical power, the fourth electrical power, and the grid power.

5. The power generation system of claim 1, further comprising a first DC-DC converter coupled between the PV power source and the DC-link.

6. The power generation system of claim 1, further comprising a second DC-DC converter coupled between the energy storage device and the DC-link.

7. The power generation system of claim 1, further comprising a third DC-DC converter coupled between the energy storage device and the PV power source, wherein the third DC-DC converter is configured to charge the energy storage device via the PV power source.

8. The power generation system of claim 1, wherein the DC-link is configured to be coupled to the DC local load via a fourth DC-DC converter.

9. The power generation system of claim 1, further comprising a common mode filter coupled between the DC-link and at least one of the PV power source, the energy storage device, and the DC local load, wherein the common mode filter is configured to minimize a flow of common mode current between the DC-link and the at least one of the PV power source, the energy storage device, and the DC local load.

10. The power generation system of claim 1, further comprises a controller configured to determine a desired operating speed of the variable speed engine based on an amount of one or more of the second electrical power and the third electrical power to the DC- link and at least one of a total load requirement, an availability of the grid power, power ratings of the rotor side converter and the line side converter, an efficiency of the variable speed engine, and efficiencies of the rotor side converter and the line side converter.

11. The power generation system of claim 10, wherein the power ratings of the rotor side converter and the line side converter are selected based on at least one of a PV rating and an energy storage device rating, wherein the PV rating is a maximum amount of the second electrical power producible by the PV power source and the energy storage device rating is a maximum amount of the third electrical power supplied by the energy storage device.

12. The power generation system of claim 11, wherein the power rating of each of the rotor side converter and the line side converter is equal to half of the PV rating or half of a sum of the PV rating and the energy storage device rating.

13. The power generation system of claim 11, wherein the power rating of each of the rotor side converter and the line side converter is equal to a maximum of the PV rating or a sum of the PV rating and the energy storage device rating.

14. A cell site, comprising:

a base station operable using a direct current (DC) power; and

a power generation system electrically coupled to the base station and comprising:

a variable speed engine;

a rotor side converter electrically coupled to the rotor winding; and a line side converter electrically coupled to the stator winding at a point of common coupling (PCC), wherein the rotor side converter and the line side converter are electrically coupled to each other via a Direct Current (DC) link, and wherein the PCC is configured to be coupled to an electric grid; and at least one of a photo voltaic (PV) power source and an energy storage device electrically coupled to the DC-link, wherein the PV power source is configured to generate a second electrical power and the energy storage device is configured to supply a third electrical power,

wherein the base station is electrically coupled to the DC-link to receive a DC power from the DC-link.

15. The cell site of claim 14, further comprising one or more devices operable using an alternating current (AC) power, wherin the one or more devices are coupled to the PCC for receiving the AC power.

16. The cell site of claim 14, wherein the base station is electrically coupled to the DC-link via a DC-DC converter.

17. A method for operating a power generation system employing a doubly-fed induction generator (DFIG), wherein the DFIG comprises a generator electrically coupled to a rotor side converter and a point of common coupling (PCC), the PCC being electrically coupled to a line side converter and an electric grid, the method comprising:

determining a desired operating speed of a variable speed engine based on an amount of one or more of a second electrical power supplied by a photo voltaic (PV) power source and a third electrical power supplied by an energy storage device to a Direct Current (DC) link between the rotor side converter and the line side converter, wherein the variable speed engine is mechanically coupled to the generator;

operating the variable speed engine at the determined operating speed to generate a first electrical power by the generator; and

supplying a DC power to a DC local load from the DC-link, wherein the DC power at the DC-link is based on at least one of the first electrical power, the second electrical power, the third electrical power, and a grid power.

18. The method of claim 17, further comprising supplying an AC power to an AC local load via the PCC based on at least one of the first electrical power, the second electrical power, the third electrical power, and the grid power to the DC local load.

19. The method of claim 17, wherein the desired operating speed of the variable speed engine is determined further based on at least one of a total load requirement, an availability of the grid power, power ratings of the rotor side converter and the line side converter, an efficiency of the variable speed engine, and efficiencies of the rotor side converter and the line side converter.