SOLAR POWER PLANT, METHOD OF CONTROLLING A SOLAR POWER PLANT AND A
DC/DC CONVERSION SYSTEM
The present disclosure relates to a DC/DC conversion system, a method for controlling a solar power plant and a solar power plant.
In a typical solar power plant, solar power are collected by a plurality of
photovoltaic devices transforming the solar power into DC (direct current) electric power, which power is subsequently converted into AC (alternating current) electric power of a power transmission grid.
US 2012/0212066 (El) describes a photovoltaic power system, wherein a number of photovoltaic (or solar) panels (lOla-d in figure 2) are connected in a string to an AC/DC inverter (204). Each photovoltaic panel (lOla-d) is connected to the string via a dedicated DC/DC converter (205a-d). Having one DC/DC converter for controlling each photovoltaic panel can provide high efficiency for the photovoltaic panels, but is rather costly for larger scale systems. US 2011/0273917 (E2) illustrates a photovoltaic system wherein DC power from an array of photovoltaic (PV) panels is converted into AC power and fed into a power transmission grid. The system includes (see §13, §14) a utility grid adapted to provide AC power, a PV array adapted to provide DC power and a photovoltaic interface adapted to provide a direct interface with the PV array and the utility grid. To reduce costs of such systems, the system of E2 integrates a PV system or array (11) with a transmission or utility grid (12) using a photovoltaic interface (IGSI 13) that replaces both a conventional inverter and distribution transformer. The PV interface (13) provides a single DC/DC converter unit (33 in figure 9) that handles an array of PV modules, instead of a single PV module. This system may be used for conversion at low DC voltage levels and low AC voltage levels, for example 120/240
volts AC/DC (see §27), but E2 also suggests a solution for conversion of medium high voltage levels, at about 20 kV (see §38). The interface (13) between an array of PV modules and the grid, also provides active filtering and reactive power compensation. However, E2 suggests a system or unit that includes many components and has many intermediate steps resulting in a complex system, increasing losses and costs (see figures 9 and 10).
It is an objective of the present disclosure to at least alleviate a problem in the prior art in respect of energy production. For this purpose, a conversion system for converting low voltage to high voltage is provided.
A high voltage output provides a suitable interface for power transmission with low losses. In accordance with the invention, a DC power conversion system for converting low voltage DC power from a plurality of DC power sources, such as solar panels, to a high voltage DC power transmission link is provided. The DC power conversion system comprises a set of first DC/DC converters, a medium voltage DC grid and a second DC/DC converter that comprises a high voltage DC output. Each DC/DC converter of the set of first DC/DC converters comprises a low voltage DC input, each low voltage DC input being adapted for connection to an array of low voltage DC power sources. The DC grid of a medium voltage level, connects the output terminals of the set of first DC/DC converters to input terminals of the second DC/DC converter, and provides an all DC connection of the set of first DC/DC converters and the second DC/DC converter. Thus, this DC power conversion system provides a DC connection of the low voltage DC power sources to the high voltage DC output, without any intermediate steps of AC power collection grid.
The system avoids costs related to conversion to an intermediate AC power grid. Without conversion to AC power, and conversion up to high voltage DC power, the
system provides an economic alternative to prior systems for further transfer of power generated at, especially, large solar power plants. When arranging a solar power plant at large distances from a power grid, the system provides an HVDC output suitable for connection to a high power and high voltage DC link. The system can also be used for storing and supplying energy by means of for example batteries as DC power sources.
In an embodiment, each of the first DC/DC converters is an isolated DC/DC converter. This has the benefit of providing electrical isolation between the DC power sources and the medium voltage grid. Preferably also, the second DC/DC converter is an isolated DC/DC converter providing isolation between the medium voltage grid and a high voltage DC transmission system, and thus protects the first DC/DC converters from high voltage.
In an embodiment, the set of first DC/DC converters are cascaded and serially connected between input terminals of the second DC/DC converter.
In an alternative embodiment, each output of the first DC/DC converters is connected, in parallel, to the input terminals of the second DC/DC converter by means of a bus bar.
In an embodiment, each DC/DC converter is adapted to perform a maximum power point tracking control of a respective array of the low voltage DC power sources. Using maximum power point control for an array of power sources reduces costs in comparison to using maximum power point control of each DC power source. The efficiency of each individual DC power source may be reduced when controlling an array of DC power sources making the embodiment suitable especially for large systems.
This disclosure also provides a solar power plant comprising a plurality of photovoltaic modules as DC power sources and the DC power conversion system, as described above. In this solar power plant, the photovoltaic modules are arranged in a plurality of arrays, each array are arranged to feed DC power to a respective input of the DC power conversion system.
This disclosure also provides a method of controlling a solar power plant. The method includes controlling a power transfer from a plurality of arrays of interconnected photovoltaic modules, to a high voltage DC connection. Especially, the method comprises a first and a second step of conversion. The first step of comprises converting DC power supplied from each array into DC power at medium voltage DC level, which conversion preferably includes performing a maximum power point control of each array. The second step of conversion comprises converting the medium voltage DC power into DC power at a high voltage DC level at the high voltage DC output. The second step of converting includes adapting the high voltage DC level to the voltage level of a high voltage DC transmission link.
In an embodiment, the first step of converting is performed by means of one dedicated isolated DC/DC converter for each array.
In an embodiment, the second step of converting is performed by means of a single medium to high voltage DC/DC converter, which medium to high voltage DC/DC converter is arranged to receive the medium voltage DC power from each of the isolated DC/DC converters.
In an embodiment, the method of controlling a solar power plant includes collecting the medium voltage DC power from the first step of conversion by means of a serial connection of the isolated DC/DC converters, the serial connection providing an output being directly connected to an input of the medium to high voltage DC/DC converter.
In an alternative embodiment, the method of controlling a solar power plant, includes collecting the medium voltage DC power from the first step of conversion at a DC bus bar, the bus bar being directly connected to respective outputs of the isolated DC/DC converters and directly connected to an input of the medium to high voltage DC/DC converter.
Brief Description of the Drawings
The invention is now described, by way of example, with reference to the accompanying drawings, in which:
Fig la is a schematic illustration of a first embodiment a solar power plant with HVDC connection to a transmission system.
Fig 2 is a schematic illustration of a second embodiment of a solar power plant with HVDC connection to a transmission system.
Fig 3 is a schematic illustration of a method of controlling a solar power plant according to an embodiment of the invention.
The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.
In some countries low voltage power may be defined as up to lkV, medium voltage as 3kV to 30 kV, and high voltage as 60 kV. The system of the invention may suitably be adapted in accordance with such voltage levels.
This disclosure refers to voltage levels of three different levels; low voltage, medium voltage and high voltage. The low voltage range is used for DC voltages up to 1.5 kV. The medium voltage range refers to DC voltages between 2 kV and 50 kV. High voltage refers to a DC voltage level from 60 kV and above.
Figure 1 illustrates a solar power plant 1 comprising a plurality of PV (photovoltaic) arrays 2a, 2b,...,2n, wherein each PV array 2a-n comprises a plurality of
interconnected PV panels PV1, PV2, PVj. Each PV panel consists of one unit comprising interconnected photovoltaic cells. Each PV panel is arranged to receive energy from sun light and transform the energy into electric DC energy. Each PV array should typically include many PV panels, for example more than one thousand panels to produce DC power of about 0.5-3 MW. For clarity purposes, the figure only illustrates six PV panels in each PV array 2a-n, but the number of PV panels in each PV array 2a-n may, for example, be between 3000 to 10000 panels, for example 8000 PV panels in 320 parallel lines with 25 PV panels serially connected in each line. The PV panels of each PV array 2a-n are arranged in series and in parallel to produce the electric DC power at an output (3a-n) of about 1000 V, such as up to 1.5 kV. In the example each PV panel produce 250 W and 8000 PV panels produce 2 MW. In this example, 320 parallel lines PV panels producing DC electric power at 3.125 V yields a total of 1 kV at the common output (3a-n) of the PV panels of each PV array 2a-n.
The choice of 1 kV is suitable in respect of international standards for operating voltage levels of PV power plants. The proposed topology is, however, suitable for voltage levels of about 1.5 kV. In literature, often AC voltage levels up to 1 kV are referred to as low voltage AC, and DC levels up to 1.5 kV are referred to as low voltage DC.
The solar power plant 1 also comprises a plurality of DC/DC converters 4a, 4b, 4n adapted for transforming input DC power from a lower voltage into DC power at a
higher voltage level. Each DC/DC converter 4a-n has a respective input 3a-n connected to the output of a respective PV array 2a-n. The DC/DC converters are serially connected in a cascaded topology to provide a medium voltage DC collection grid 5. The medium voltage DC grid 5 is connected to the input of a medium to high voltage DC/DC converter 6, the output 8 of which provides DC power of high voltage for subsequent transmission on a HVDC transmission link (not illustrated). Each DC/DC converter 4a-n is controlled by a respective controller 9a-n. Alternatively, a common controller may be used for a plurality or all DC/DC converters 4a-n. Each controller 9a-n in the embodiment of figure 1, is a single dedicated controller 9a-n incorporated in each DC/DC converter 4a-n to control the DC/DC converter 4a-n. Each controller 9a-n is adapted to control the DC power from the respective PV array 2a-n by controlling the total voltage of the PV array 2a-n at the respective input 3a-3n of the respective DC/DC converter 4a-n. The controller is also adapted to control the voltage of the DC power fed to the collection grid 5 by controlling the output voltage of the respective DC/DC converter 4a-n. The control of each of the PV arrays 4a-n may suitably be based on maximum power point (MPP) tracking of the total array 2a-n of PV modules. For these control purposes, each controller 9a-n is adapted to monitor the DC voltage and DC current from each array 2a-n so that the DC voltage can be controlled in relation to the DC current, or power, fed from the respective array 2a-n. This is in contrast to systems having a separate control, such as MPP tracking, of each PV module.
Thus, the solar power plant collects solar energy and connects (at 3a-n) the produced electrical DC power from the PV arrays 2a-n at low voltage DC power to a high voltage DC power at the output 8 of the medium to high voltage DC/DC converter 6 by means of a DC connection. This connection comprises a set of first DC/DC converters 4a-n with a low voltage DC input, a medium voltage DC collection grid 5 and a second DC/DC converter 6, i.e. the medium to high voltage DC/DC converter 6. The medium to high voltage DC/DC converter 6 includes a controller 10 adapted for controlling the output power to the high voltage DC connection 8. In
this way, the solar power plant of figure 1, is controlled in two main control steps. A first step wherein each controller 9a-n of the DC/DC converters 4a-n performs a power and voltage control, such as maximum power point control, of each array 2a- n of PV panels, and a second step wherein the controller 10 of the medium to high voltage converter 6 controls the output voltage of the high voltage DC connection 8.
Each DC/DC converter 4a-n of the cascaded DC/DC converters 4a-n is an isolated DC/DC converter 4a-n. In this way a galvanic isolation is provided between the PV arrays and the output 8 at the HVDC transmission system by means of the isolated DC/DC converters 4a-n. The output of each DC/DC converter 4a-n includes a bypass switch 7a-n, by means of which a faulty DC/DC converter 4a-n or faulty PV array 2a- n can be bypassed.
Figure 2 illustrates a second embodiment of a solar power plant 11 comprising a plurality of PV arrays 2a-n connected by means of a DC system to a high voltage DC output 8 for feeding the collected and transformed DC power to a HVDC
transmission system. The PV arrays 2a-n are of the same type as in figure 1, and each PV array 2a-n provides DC power at an input 3a-n of a respective first DC/DC converter 4a-n. A medium voltage DC collection grid 15 of the DC interconnection system connects the outputs of every DC/DC converter 4a-n to the input of a second DC/DC converter 6, which is a medium to high voltage DC/DC converter 6 having an output 8 for the HVDC transmission system or link. The medium voltage DC bus bar collection grid 15 comprises a first and a second bus bar conductor 15a, 15b. Each DC/DC converters 4a-n is connected to the bus bar conductors 15a, 15b of the medium voltage DC collection grid 15. Thus, the bus bar conductors 15a, 15b are connected to the input of the high voltage DC/DC converter 6.
Thus, the main difference between the embodiments of figures 1 and 2 are the medium voltage DC collection grid; being cascaded DC/DC converters in a serial connection to the input terminals of the medium to high voltage DC/DC converter,
and parallel connections of the DC/DC converters to a bus bar connected to the input terminals of the medium to high voltage DC/DC converter, respectively.
Galvanic isolation between the PV panels and the output 8 at the HVDC
transmission link is provided by means of the set of first DC/DC converters 4a-n, where each of the first DC/DC converters is an isolated DC/DC converter 4a-n.
Figure 3 illustrates an embodiment of a method for converting power from a solar power plant.
The method starts with collecting, step 31, low voltage DC power by means of a plurality of arrays of interconnected PV modules.
Subsequent to the step of collecting, the method includes a first step 32 of conversion, wherein the collected low voltage DC power is converted into a medium voltage DC power.
The collected low voltage DC power provides an input for the first step of conversion. The collection and first step of conversion is controlled by means of a dedicated DC/DC converter 4a-n for each array 2a-n of PV modules, which DC/DC converters 4a-n preferably perform a maximum power point tracking for the respective array 2a-n.
The medium voltage DC power, which is output from each DC/DC converter, is collected 33 in a medium voltage DC grid 5, 15.
The last step 34 comprises converting the power from medium voltage DC into high voltage DC, by means of the medium to high voltage DC/DC converter 6. The second step of converting includes adapting the high voltage DC level to the voltage level of a high voltage DC transmission link, at output 8.
The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.