Patent Application
20130320770
The invention relates to a system and method for reactive power compensation in power networks.
Electric power networks are used for transmitting and distributing electricity for various purposes. Electric networks include multiple devices interconnected with each other to generate, transmit, and distribute electricity.
Electrical power networks experience voltage variations during operation that are caused by the variation in generation of the active and the reactive power by different power generating devices and variable consumption of the active and reactive power at different loads in the electrical power network.
Electric power networks to which large amounts of renewable power generation are connected can have large and rapid voltage variations at and around the points of interconnection that lead to excessive operation of voltage regulating devices such as on-load tap changing transformers and capacitors. Due to limited operating speeds of the voltage regulating devices, a constant voltage cannot always be maintained at all the network buses in the power network. Excessive operation of mechanically-switched transformer taps and capacitors leads to increased maintenance and diminished operating life of the switched devices.
One approach for mitigating the voltage variation mentioned above is to provide a closed loop controller, with or without voltage droop characteristics. The controller adjusts the reactive power supply to compensate the voltage variation using mechanically switched reactors and capacitors as well as dynamic devices such as static VAR compensators (SVCs) and static synchronous compensators (STATCOMs). More specifically, in some renewable power generation systems the closed loop controller adjusts the operating power factor of the power converter to adjust the reactive power for mitigating the voltage variation. The closed loop controller, however, may undesirably interact with other voltage controllers in the power network during this process. Furthermore, the closed loop controller tends to compensate for the reactive power demand of the network and connected loads, which leads to increased losses in the reactive power source and sub-optimal utilization of its dynamic capabilities.
An alternative approach for mitigating voltage variations in the power network is to individually compensate the self-induced voltage variation for each of the power generating devices. The amount of reactive power required for compensating a self-induced voltage variation is computed based on an approximate voltage drop equation which results in a constant power factor operation. However, this method tends to be inaccurate under high power conditions and may lead to overcompensation in the electric power network resulting in undesired voltage variations and increased losses.
Another approach is to compute the amount of reactive power based on the exact voltage drop equation which results in a variable power factor operation. However, this method is computationally complex and requires additional data.
Hence, there is a need for an improved system to address the aforementioned issues.
In one embodiment, a reactive power control system is provided. The reactive power control system computes a required value for a reactive power based on a state observer method for at least one electrical element in an electrical system. The reactive power control system also generates a reactive power command based on the required value of the reactive power. The reactive power control system further transmits the reactive power command to the electrical element in the electrical system for generating the required value of reactive power to compensate for a voltage change induced by the respective electrical element in the electrical system.
In another embodiment, a solar power generation system is provided. The system includes at least one photovoltaic module for generating DC power. The system also includes at least one power converter for converting DC power to AC power. The system further includes a reactive power control system. The reactive power control system computes a required value for a reactive power based on a state observer method for at least one power converter in the solar power generation system. The reactive power control system also generates a reactive power command based on the required value of the reactive power. The reactive power control system further transmits the reactive power command to the respective power converter in the solar power generation system for generating the required value of reactive power to compensate for a voltage change induced by the respective power converter in the solar power generation system.
In another embodiment, a method including the steps of, computing a required value of a reactive power based on a state observer method for at least one electrical element in an electrical system, generating a reactive power command based on the required value of the reactive power and transmitting the reactive power command to the respective electrical element for generating the required reactive power to compensate for a voltage change induced by the respective electrical element in the electrical system is provided.
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 reactive power control system coupled to an electrical element in an electrical system. The respective electrical element induces a voltage change in the electrical system during operation. The change induced by the respective electrical element is compensated by the reactive power control system coupled to the respective electrical element. The reactive power control system computes a required value for a reactive power based on a state observer method for the respective electrical element in the electrical system. The reactive power control system further generates a reactive power command based on the required value of the reactive power. The reactive power command is transmitted by the reactive power control system to the respective electrical element for generating the required value of the reactive power to compensate for the voltage change induced by the respective electrical element in the electrical system.
The total voltage change at node (i) is the sum of the variation caused by the active power output Pi and the reactive power output Qi provided by the electrical element 16 coupled at node (i) represented by ΔVii, and voltage change induced by the remaining electrical elements (18,
For understanding of the invention, one example for reactive power compensation for change in voltage induced by the electrical element 16 would be discussed below.
The number and nature of the sensitivity coefficients (si) depend on the model implemented for the observation module. One example for possible sensitivity coefficients (si) is the voltage sensitivity coefficient with respect to active power (δVi/δPi) and the voltage sensitivity coefficient with respect to reactive power (δVi/δQi) at node (i).
The sensitivity coefficients (si) adopted by the reactive power control system 12 needs to be initialized at the start of the control operations. The sensitivity coefficients (si) can be initialized by different approaches. One exemplary approach for initializing the voltage sensitivity coefficients is to induce and measure a change in voltage (ΔVi) at node (i). A change in voltage at node (i) caused by the electrical element 16 can be induced by a change in active power output (ΔPi) of the electrical element 16 at node (i) and by a change in reactive power (ΔQi) the electrical element 16 at node (i). The initial values for the sensitivity coefficients (δVi/δPi) and (δVi/δQi) are obtained in two steps in an example embodiment.
In the first step, the active power output (Pi) of the electrical element 16 at node (i) is kept unchanged for a predefined interval of time resulting in (ΔPi=0) and reactive power output (Qi) of the electrical element 16 at node (i) is actively changed by (ΔQi). The change in voltage (ΔVi) at node (i) due to the change in reactive power output (ΔQi) is then measured. From the measurement, a first estimate for δVi/δQi can be obtained as δVi/δQi≈ΔVi/ΔQi.
In the second step, the reactive power output (Qi) of the electrical element 16 at node (i) is kept unchanged for a predefined interval of time resulting in (ΔQi=0) and the active power output (Pi) of the electrical element 16 at node (i) is actively changed by (ΔPi). The change in voltage (ΔVi) at node (i) due to the change in active power output (ΔPi) is then measured. From the measurement, a first estimate for δVi/δPi can be obtained as δVi/δPi≈ΔVi/ΔPi. The reactive power control system 12 uses the initial values of δVi/δPi and δVi/δQi to initialize the control operations for the electrical element 16.
After initialization, the sensitivity coefficients si are continuously estimated by the state observer module 44 which in one embodiment comprises an extended Kalman filter. At first, the system module 38 provides a new set of expected sensitivity coefficients {tilde over (s)}{tilde over (sι)} based on a system model and the last set of sensitivity coefficients si-1. In a second step, {tilde over (s)}{tilde over (sι)} and the actual value of the active power output Pi 30 is used in the observation module 40 to create an expected value of the voltage {tilde over (V)}ι, which is compared to the measured value of the voltage Vi 26. The difference is then used by the observation module to update the sensitivity coefficients si. The updated sensitivity coefficients si are then used by the processing module 42 to calculate the value of reactive power output Qi, which is required to compensate for a voltage change induced by the active power output Pi 30 of the electrical element 16.
In one embodiment, the operation of the reactive power control system 12 is continuous. The sensitivity coefficients si-1 at time instance ti-1 are determined as discussed above and based on the last estimate of the sensitivity coefficients si-1, the system module 38 predicts a new set of sensitivity coefficients {tilde over (s)}{tilde over (sι)} at actual time ti. Using this prediction, the actual active power Pi and the actual reactive power Qi, the observation module 40 updates the sensitivity coefficients si. Once updated, the processing module 42 calculates the value of the reactive power Qi which is required to cancel out the voltage change induced by the active power output Pi.
The estimated sensitivity coefficients (si). are transmitted to the processing module 42 that computes the required value of reactive power for compensating the voltage change induced by the active power output Pi at time ti. The processing module 42 further generates a reactive power command (34,
The above mentioned operation is repeated continuously during operation of the electrical system. Although the example was provided for direct reactive power for purposes of example, similar techniques can be applied to other reactive parameters such as reactive current and power factor.
The solar power generation system 50 includes photovoltaic modules 64 that generate DC power. Each of the power converters 52, 54 is coupled to some of the photovoltaic modules 64 and converts DC power generated from them to AC power and transmits the AC power to a power grid 66. Each of the power converters 52, 54 induces a variation in voltage at the respective point of interconnection 60, 62 to the electric power grid 66. Each of the reactive power control systems 56, 58 is coupled to the respective power converters 52, 54 for compensating the voltage variation induced by the power output of the respective power converters 52, 54.
The reactive power control system 56, 58 of each of the respective power converters 52, 54 measures a voltage of the AC power at the respective point of interconnections 60, 62. Each of the reactive power control system 56, 58 generates a reactive power command 68, 70 based on the above mentioned state observer method for each of the respective power converters 52, 54 for compensating the individual voltage variations induced by each of the power converters 52, 54. In one embodiment, the reactive power command 68, 70 may include a command to generate the required value of reactive power or reactive current or adjust the power factor of the power converters 52, 54 during operation.
The various embodiments of the reactive parameter compensation system described above provide a more efficient and reliable electrical system. The system described above reduces voltage variations and increases an overall efficiency of the electrical system.
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.
