The invention relates to determining leakage inductances of a double fed induction generator.
Wound rotor induction machines are electrical rotating machines in which the rotor comprises windings. One example of such machines is a double fed induction machine commonly used as a generator; a double fed induction generator (DFIG). Double fed induction generators may be used in a configuration as shown in the example of
Estimation errors in machine parameter values are a common source for errors in the control of electrical machines. As regards electrical machines used as generators, such as double fed induction generators, these estimation errors may lead to inaccurate control, resulting in output inaccuracy for reactive and active power, for example. Poor following of reference set point may be compensated for e.g. by using a higher level control loop that acts on measured and requested error values and compensates the reference according to the actual output. However, higher level controllers need to have slow dynamics in order not to interfere with higher dynamics controllers. During a LVRT (Low Voltage RideTrough) situation, the slower controllers will have a negative effect on the performance due to their natural slow dynamic behavior, which is inherent in them because they are not based on generator modelling.
For example, in case of double fed induction generators, leakage inductance values given by the machine manufacturer or, if not available, values estimated by the user of the machine have been used in the control of the generator. If these leakage inductance values for some reason do not accurately match the real values, these incorrect values may lead to poor control performance of the double fed induction generator.
The object of the invention is thus to provide a method and an apparatus for implementing the method so as to solve or at least alleviate the above problem. The object of the invention is achieved with a method, an arrangement, an inverter, and a frequency converter that are characterized by what is stated in the independent claims. Preferred embodiments of the invention are described in the dependent claims.
The invention is based on the idea of obtaining estimates of the machine leakage inductances on the basis of measurements performed on the double fed induction generator while it is operating.
An advantage of the invention is that a better estimation of actual machine leakage inductances improves the control performance and reduces the need for slow dynamics higher level controllers thus improving both the operation during normal power generation and a response time during transients in the network. The invention enables identification of the leakage inductance values automatically so that user given generator data can be verified and tuned according to actual values. The invention is also cost-efficient and easy to implement even in existing devices.
In the following, the invention will be described in more detail in connection with preferred embodiments with reference to the accompanying drawings, in which
The application of the embodiments herein is not restricted to any specific double fed induction generator, DFIG, system, but it may be applied to various DFIG systems. In addition, the use of the invention is not restricted to any specific basic frequency or to any specific voltage level, for example.
Us=stator voltage of the DFIG
Is=stator current of the DFIG
Rs=stator resistance of the DFIG
Lls=stator leakage inductance of the DFIG
Lm=magnetizing inductance of the DFIG
Ur′=voltage over the magnetizing inductance Lm
Ur′=rotor voltage of the DFIG referred to the stator
Ir′=rotor current of the DFIG referred to the stator
Rr′=rotor resistance of the DFIG referred to the stator
Llr′=rotor leakage inductance of the DFIG referred to the stator,
where the rotor quantities of the DFIG referred to the stator equal to actual rotor side quantities as follows:
Ur′=Neff Ur (where Ur is the actual rotor voltage)
Ir′=1/Neff Ir (where Ir is the actual rotor current)
Rr′=Neff2 Rr (where Rr is the actual rotor resistance)
Llr′=Neff2 Llr (where Llr is the actual rotor inductance),
where
Neff=effective turns ratio of the DFIG.
In other words, quantities referred to the stator side merely represent another manner of representation of the quantities in question but are readily indicative of the actual quantities. In a similar manner, quantities of the stator side could be referred to the rotor side. In practice, rotor side quantities referred to the stator side are often readily available and used in practical applications.
Based on the equivalent circuit of
Thus, at a given frequency f, the rotor leakage inductance of the DFIG can be calculated on the basis of the stator voltage of the DFIG, the rotor voltage of the DFIG, the rotor current of the DFIG and the rotor resistance of the DFIG.
Moreover, considering that the rotor resistance may be negligible in comparison to the leakage inductance, or the value of the rotor resistance may be unknown, term Rr′
Thus, at a given frequency f, the rotor leakage inductance of the DFIG can be calculated on the basis of at least the stator voltage, the rotor voltage and the rotor current.
In a similar manner, based on the equivalent circuit of
Thus, at a given frequency f, the stator leakage inductance of the DFIG can be calculated on the basis of the stator voltage of the DFIG, the rotor voltage of the DFIG, the stator current of the DFIG and the stator resistance of the DFIG.
Moreover, considering that the stator resistance may be negligible in comparison to the leakage inductance, or the value of the stator resistance may be unknown, term Rs
Thus, at a given frequency f, the stator leakage inductance of the DFIG can be calculated on the basis of at least the stator voltage, the rotor voltage and the stator current.
According to an embodiment, one or more leakage inductances of a double fed induction generator can be determined during operation of the DFIG by performing at least one of a) and b) as follows:
a) Controlling a rotor voltage of the DFIG such that a stator current of the DFIG approaches zero; in response to the stator current of the DFIG being equal or below a first predetermined threshold, determining values of a stator voltage of the DFIG, a rotor voltage of the DFIG and a rotor current of the DFIG, or quantities indicative thereof; and calculating a rotor leakage inductance of the DFIG on the basis of at least the determined values of the stator voltage of the DFIG, the rotor voltage of the DFIG and the rotor current of the DFIG, or the quantities indicative thereof. According to an embodiment, the rotor leakage inductance of the DFIG is calculated on the basis of the determined values of the stator voltage of the DFIG, the rotor voltage of the DFIG and the rotor current of the DFIG, or the quantities indicative thereof, and a rotor resistance of the DFIG or a quantity indicative thereof.
b) Controlling a rotor voltage of the DFIG such that the rotor current of the DFIG approaches zero; in response to the rotor current of the DFIG being equal or below a second predetermined threshold, determining values of the stator voltage of the DFIG, the rotor voltage of the DFIG and the stator current of the DFIG, or quantities indicative thereof; and calculating a stator leakage inductance of the DFIG on the basis of at least the determined values of the stator voltage of the DFIG, the rotor voltage of the DFIG and the stator current of the DFIG, or the quantities indicative thereof. According to an embodiment, the stator leakage inductance of the DFIG is calculated on the basis of the determined values of the stator voltage of the DFIG, the rotor voltage of the DFIG and the stator current of the DFIG, or the quantities indicative thereof, and a stator resistance of the DFIG or a quantity indicative thereof.
Thus, either one of the stator and rotor leakage inductances or both of them can be determined. According to an embodiment, the operation of the DFIG refers to a state where the DFIG 10 is in operation, i.e. connected to an AC network 70 and its stator side is synchronized with the AC network 70 connected to the stator 12 of the DFIG. However, during such operation of the DFIG the rotor speed is not necessarily at the synchronous speed but a slip may be present, i.e. the rotor speed may be below or above the synchronous speed. Preferably during the operation of the DFIG 10 the stator voltage of the DFIG 10 is within a nominal voltage range of the stator voltage of the DFIG whereby it can be assumed that the leakage inductances of the DFIG are not saturated. The DFIG main inductance saturation generally does not impact the result of the determination of the leakage inductances. It is further noted that the determination becomes more accurate when the slip of the DFIG is not too small. This is due to the fact that at an exact synchronous speed the rotor voltage is small because the rotor voltage is proportional to the slip. The amount of slip providing optimal determination results depends on the characteristics of the DFIG in question and thus no specific values for the slip are given here.
An apparatus implementing the control functions according to any one of the embodiments herein, or a combination thereof, may be implemented as one unit or as two or more separate units that are configured to implement the functionality of the various embodiments. Here the term ‘unit’ refers generally to a physical or logical entity, such as a physical device or a part thereof or a software routine. One or more of these units, such as the control unit 50, may reside in an electric drive or a component thereof such as the inverter 30, for example.
An apparatus, such as the control unit 50, according to any one of the embodiments herein may be implemented at least partly by means of one or more computers or corresponding digital signal processing (DSP) equipment provided with suitable software, for example. Such a computer or digital signal processing equipment preferably comprises at least a working memory (RAM) providing storage area for arithmetical operations and a central processing unit (CPU), such as a general-purpose digital signal processor. The CPU may comprise a set of registers, an arithmetic logic unit, and a CPU control unit. The CPU control unit is controlled by a sequence of program instructions transferred to the CPU from the RAM. The CPU control unit may contain a number of microinstructions for basic operations. The implementation of microinstructions may vary depending on the CPU design. The program instructions may be coded by a programming language, which may be a high-level programming language, such as C, Java, etc., or a low-level programming language, such as a machine language, or an assembler. The computer may also have an operating system which may provide system services to a computer program written with the program instructions. The computer or other apparatus implementing the invention, or a part thereof, may further comprise suitable input means for receiving e.g. measurement and/or control data, and output means for outputting e.g. control data. It is also possible to use a specific integrated circuit or circuits, or discrete electric components and devices for implementing the functionality according to any one of the embodiments.
The invention according to any one of the embodiments, or any combination thereof, can be implemented in existing system elements, such as electric drives or components thereof, such as inverters or frequency converters, or similar devices, or by using separate dedicated elements or devices in a centralized or distributed manner. Present devices for electric drives, such as inverters and frequency converters, typically comprise processors and memory that can be utilized in the functions according to embodiments of the invention. Thus, all modifications and configurations required for implementing an embodiment of the invention e.g. in existing devices may be performed as software routines, which may be implemented as added or updated software routines. If the functionality of the invention is implemented by software, such software can be provided as a computer program product comprising computer program code which, when run on a computer, causes the computer or corresponding arrangement to perform the functionality according to the invention as described above. Such a computer program code may be stored or generally embodied on a computer readable medium, such as suitable memory, e.g. a flash memory or a disc memory from which it is loadable to the unit or units executing the program code. In addition, such a computer program code implementing the invention may be loaded to the unit or units executing the computer program code via a suitable data network, for example, and it may replace or update a possibly existing program code.
It is obvious to a person skilled in the art that as technology advances, the basic idea of the invention can be implemented in a variety of ways. Consequently, the invention and its embodiments are not restricted to the above examples, but can vary within the scope of the claims.
Number | Date | Country | Kind |
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14187738.1 | Oct 2014 | EP | regional |