Patent Application
20130155738
The invention relates to a system and method for controlling reactive power in a power conversion system.
With the rising cost and scarcity of conventional energy sources and concerns about the environment, there is a significant interest in alternative energy sources such as solar power and wind power. Solar power generation uses photovoltaic sources to generate electricity from the sun. Multiple photovoltaic sources are electrically coupled to one another in such systems to generate electricity. The electricity is supplied to utilities via a power distribution network including a power grid.
In response to utility requirements, power conversion systems regulate the output voltage provided to the utilities. A reactive power command is typically calculated based on the difference between the actual output voltage and the required output voltage. However, in a three-phase power conversion system, there is often an unbalance in the output voltages at each phase as a result of different loads being connected at each phase.
Existing power conversion control systems typically compute the required output voltage based on a balanced power conversion system and ignore the unbalanced voltage condition. Voltage unbalances may lead to higher maintenance costs and under-capacity utilization of the three-phase equipment and components of the power conversion system.
Hence, there is a need for an improved system to address the aforementioned issues.
In one embodiment, a power conversion system is provided. The power conversion system includes a three-phase power converter electrically couplable to a photovoltaic power source for converting DC power to three-phase AC power. The power conversion system also includes sensors for measuring voltage levels of the AC power at each phase. The power conversion system further includes a controller for generating and transmitting independent reactive power commands for each phase of the three-phase power converter based at least in part on the voltage levels and an existing voltage imbalance.
In another embodiment, a method for controlling reactive power in a power network is provided. The method includes converting DC power to three-phase AC power. The method further includes measuring voltage levels of the AC power at each phase. The method also includes generating independent reactive power commands for each phase of the three-phase power converter based at least in part on the voltage levels and an existing voltage imbalance. The method further includes transmitting the independent reactive power commands for each phase to the power converter for controlling the reactive power.
In yet another embodiment, a non-transitory computer-readable medium comprising computer-readable instructions of a computer program that, when executed by a processor, cause the processor to perform a method is provided. The method includes converting DC power to three-phase AC power. The method further includes measuring voltage levels of the AC power at each phase. The method also includes generating independent reactive power commands for each phase of the three-phase power converter based at least in part on the voltage levels and an existing voltage imbalance. The method further includes transmitting the independent reactive power commands for each phase to the power converter for controlling the reactive power.
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:
Embodiments of the present invention include a system and method for controlling reactive power in a power network. The system includes a three-phase power converter electrically couplable to a photovoltaic power source for converting DC power to three-phase AC power. The three-phase AC power is transmitted to a power grid. The power conversion system includes a sensor electrically coupled between the power grid and the power converter that measures voltage levels of the AC power at each phase. The measured voltage levels are provided to a controller that generates independent reactive power commands for each phase of the three-phase power converter based at least in part on the voltage levels and an existing voltage imbalance between the three phases. The controller transmits the independent reactive power command to the three-phase power converter to maintain or reduce the voltage imbalance. As described herein, the term “voltage imbalance” is referred to as a magnitude imbalance and should not be considered otherwise.
In one embodiment, the controller 22 further includes a condition election module 24 that may comprise a programmable module, an operator module or a combination of both. The condition election module 24 enables the controller 22 to operate at a selected one of a plurality of reactive power compensation modes.
Examples of reactive power compensation modes include a mode to maintain the existing voltage imbalance, a mode to at least partially reduce the existing voltage imbalance, and a mode to maintain or reduce the existing voltage imbalance only upon predetermined conditions. In a more specific embodiment, the predetermined conditions include night time, cloud cover, and a power network emergency conditions. In some embodiments, the night time and cloud cover conditions may more specifically include a condition in which the power converter 14 has excess operating capacity which in one embodiment is defined as a difference between a rated power and an actual operating power of the power converter. The power network emergency condition may include a condition of the existing voltage imbalance rising above a threshold voltage imbalance.
The controller 22 receives the measured voltage levels of each of the phases and computes the voltage imbalance in the power network based on the selected operating mode. In one embodiment, the controller may select the operating mode automatically or selects a mode based on an operator command. The controller 22 generates the independent reactive power commands for each of the phases and transmits the independent reactive power commands to the three-phase power converter 14. The independent reactive power commands allow the power converter 14 to generate reactive power to mitigate the voltage imbalance according to the selected operating mode. Several non-limiting examples of converters for which such control is useful include center-tapped power converters, a three-phase four leg power converters, and neutral point clamped power converters.
The various embodiments of the system for controlling reactive power in a power network described above include sensors electrically coupled between a power converter and a power grid that measure a voltage level of each phase of the power conversion system. The sensors are electrically coupled to a controller that generates independent reactive power commands that control reactive power generation in the power conversion system to either maintain or reduce the existing voltage imbalance in the power network. The controller generates the independent reactive power commands for each of the phases of the three-phase power converter based on the existing voltage imbalance between each of the phases. This results in better efficiency and lower maintenance costs of the grid equipment and the utility.
It is to be understood that a skilled artisan will recognize the interchangeability of various features from different embodiments and that the various features described, as well as other known equivalents for each feature, may be mixed and matched by one of ordinary skill in this art to construct additional systems and techniques in accordance with principles of this disclosure. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
