Electrical transmission or interconnection systems – Switching systems – Condition responsive
Reexamination Certificate
2002-06-10
2004-10-05
Riley, Shawn (Department: 2838)
Electrical transmission or interconnection systems
Switching systems
Condition responsive
C307S080000
Reexamination Certificate
active
06800964
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The invention relates to a system for feeding direct current into the alternating current network. It serves for optimising the energy input and is preferably used for converting direct current from photovoltaic systems having a plurality of rows of series-connected photovoltaic elements. The proposed system can, in particular, be advantageously operated in the case of relatively large network feeding systems.
2. Description of the Prior Art
Known photovoltaic systems usually consist of a plurality of inverters, which are connected at their input side in each case to one or more rows of series-connected photovoltaic elements. The inverters and photovoltaic elements are hard wired, so that no connection variants during operation are possible.
In arrangements of this kind, it is disadvantageous that operation of the system with optimum input power is only possible temporarily. This condition occurs more or less randomly when the current generators connected to an inverter just happen, by virtue of the prevailing insolation, to supply the output powers that are necessary for optimum operation of the inverters.
Since, in our latitudes, insolation is subject to severe fluctuations, both within the course of a day and dependent on weather and season, insolation with the intensity suitable for optimum operation is relatively rarely available. At other times, the inverters operate in partial load. In the course of a year, these times add up to by far the greater portion of time.
In partial load mode, the efficiencies of the inverters decrease significantly and, in particular in the lower partial load mode, only poor efficiencies are present. Photovoltaic systems according to the prior art therefore operate with unsatisfactory efficiency for by far the greatest portion of a year. The poor efficiency leads to losses in the feeding of the generated energy into the alternating current network, which reduce the profitability of the system.
SUMMARY OF THE INVENTION
Against this background, the object of the invention is to provide a system that feeds current for alternating current generators into the alternating current network, and whose efficiency is significantly higher than that of known systems.
Starting from a system with at least two active inverters, which are in each case characterised by optimum input performance and in each case are connected at their input side to at least one direct current generator of fluctuating output power and a measurement device, this object is solved in that
switching elements for carrying out a switchover operation are provided,
and the switchover operation effects a separation of at least one current generator from a first inverter and a connection of this/these current generator(s) to a second inverter,
a control device for initiating the switchover operation is provided,
and the initiation of the switchover operation is dependent on the fulfilling of a condition, which can be stored in the control device.
The proposed arrangement generally has a plurality of inverters, which are connected at the input side in each case to one or more direct current generators of fluctuating output power. If the fluctuation of the output power is manifested in a decrease of the output power, there is inevitably a fall in the input power of the inverters corresponding thereto and thereby in their efficiency. To counteract this effect, according to the gist of the invention, the number of current generators connected to an active inverter is kept variable. In practice, this object is achieved by means of corresponding switching elements. These elements permit one or more of the current generators assigned to an inverter to be separated from the latter and connected to a different inverter.
Here, the initiation of the switchover process is dependent on the satisfaction of a condition, which is specifiable by the control device. An essential assertion of the condition—which will be discussed in greater detail below—is that a connection of current generators occurs whenever the input power to the inverter under consideration has fallen off to the extent that it can no longer operate in the optimum efficiency range. In the case of inverters without connected current generators, the control device usually carries out deactivation.
According to a feature of the invention, the switchover operation is carried out for all inverters and current generators whose input and output powers satisfy the condition. This procedure leads to the situation that the number of active inverters within the system is reduced; the number of the current generators connected to the active inverters within the system is increased. For the respective active inverters, this means that in each case they can operate at or close to the optimum operating point. The result of this, for the entire system for feeding electrical energy from direct current generators into the alternating current network, is operation in the optimum efficiency range. Losses in feeding are avoided in the case of systems according to the present invention; the profitably of the system is increased compared with systems according to the prior art. With the embodiment of photovoltaic systems according to the present invention, they can be operated with optimum efficiency throughout the entire year.
In a further development of the invention, preferred conditions are disclosed, on the meeting of which the initiation of the switchover operation is dependent, and which can be stored in the control device.
According to the present invention, a possible condition is satisfied when the measurement device ascertains that the actual input power applied to a second inverter has fallen to a value which is below the optimum input power by an amount which corresponds to the total of the output powers of n current generators connected to a first inverter, n being a natural number whose smallest value is equal to 1 and whose maximum value is equal to the number of all the current generators connected to the first inverter, and n being specifiable by the control device.
From this further development, two variants are of particular interest, which, according to a feature of the invention, are provided where n is equal to 1 or n is equal to the number of all current generators connected to the first inverter. Both variants are described in greater detail below. Variant one comprises the further development in which n is equal to the number of all current generators connected to a first inverter. Variant two is the further development where n is equal to 1. As regards the content, these conditions mean that in variant one all current generators connected to an inverter are connected to another inverter, in variant two, by contrast, to only one of the current generators.
The content formulated in the two variant can be illustrated most simply by starting from an embodiment of a system in which, according to another feature of the invention, each inverter is connected at its input side to a current generator and the inverters and the current generators are in each case identical to one another.
For the starting state of this system, it is assumed that all current generators have an output power which corresponds to the optimum input power of the inverters. With falling output power, the output powers to all current generators are reduced and the corresponding input powers to the inverters are constant. When the input power to all inverters has fallen to 50% of the optimum input power, the aforementioned condition for switchover is satisfied.
The conditions defined by variant one and variant two are identical in the present case, since the number n of current generators connected to an inverter is equal to 1 and 1 also corresponds to the maximum number of connected current generators. The condition formulated in variant one (or two) is then satisfied as follows:
The actual input power at a second inverter has fallen to 50% of the optimum input power
The output power at a level of 50% of t
Riley Shawn
Schindler Edwin D.
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