The traction of railway electrification is more and more widely used in modern railway transportation because of its advantages of idle speed, heavy load, saving of primary energy, and reduction of environmental pollution. However, the electric traction load is a fluctuating single-phase high-power rectification load, and it is one of the most severe harmonic sources in the current power system. In order to reduce the adverse effect of the traction load on the power system, it is necessary to install a filter at the outlet of the pulling arm of the electric traction station to absorb the harmonic current generated by it.
The traditional passive filter cannot meet the filtering requirements of the electric traction load because of its own defects. The active power filter (APF) developed rapidly in recent years has received more and more attention and application due to its many advantages. APF is an active compensation device that calculates the harmonic current of the load through the detection link, and then controls the output of the main circuit to provide a compensation current that is in antiphase with the harmonic current of the load to eliminate harmonics. Among them, the real-time detection of harmonic currents is undoubtedly an important link to ensure the APF compensation characteristics.
For harmonic current detection, most of the current applications are detection methods based on instantaneous reactive power theory and generalized instantaneous reactive power detection methods based on dqO systems. Recently, adaptive harmonic detection based on artificial neural network has been proposed. Method etc. They mainly have two problems: First, because of the time delay of sampling and calculation, real real-time detection cannot be realized; second, the traction load changes rapidly and is a strong time-varying load. The first two detection methods all contain a low-pass filter. The latter needs to meet the actual comprehensive training sample. This has a significant time lag in the current change transient process and cannot respond to rapid changes in harmonic currents.
For APF to be compensated in real time, it is necessary to know the harmonic current at the next switching time in advance, but the next moment has not yet arrived and the current cannot be detected in practice. Based on this, this paper proposes a predictive harmonic current detection method, which can accurately obtain the harmonic current at the future time through the prediction link, and can track the rapid changes of the load, and realize real-time control when the next moment arrives. Beatless control.
2 Harmonic current detection principle of single-phase circuit Considering the method of harmonic current detection of three-phase circuit based on instantaneous reactive power theory, it is found that the detected three-phase signals are always first converted into two-phase signals in mutually perpendicular coordinate systems. Then calculate further. For a single-phase circuit, the above method is simplified, and only a one-phase current and an actual current lag TV4 (here, a power frequency cycle) are constructed, and a hypothetical two-phase coordinate system signal can be directly formed.
Set the current value of the single-phase circuit to more than one, and sina>r and costwi are the standard sinusoidal signals obtained by the zero-crossing synchronization and phase-locked loop of the voltage signal. The "-" above the variable
Indicates the DC component, "" indicates the AC component, and ip and q respectively represent the active and reactive DC components of the current. The fundamental wave current can be obtained by inverse transform corresponding to iP and L. with…
f. Subtract the fundamental component from the full current to obtain the harmonic current component ish=K-Ki (7) The principle of harmonic current detection of a single-phase circuit based on the above method is shown. Among them, the plant "is lagging behind / 4 links, C22 and C3 as shown in equation (8) 3 predictive harmonic current detection algorithm This paper proposes a predictive harmonic current detection algorithm to solve the aforementioned time delay problem, in order to achieve rapid Tracking, as shown in its implementation principle, where ia is the load current detected by the sensor, PM is a prediction link.
Predictive Single-phase Circuit Harmonic Current Detection Principle 3.1 Load current grading situation The k-period sampling point sequence +1 point prediction value GU+1) consists of two parts: hU fixed component, i2U+l) is the disturbance component.
For a fixed component, when the load current is gradually changing, taking into account its periodic characteristics, the linear extrapolation algorithm is used in this paper, and there are disturbance components. The change is closely related to several previous sampling points. To improve the calculation accuracy, second-order extrapolation is used. law.
3.2 Severe change of load current According to the locomotive running status and the number of locomotives on the power supply arm, the magnitude of the traction load often has some drastic changes. At this time, the harmonic current component also changes drastically, and some corrections must be made to its prediction algorithm. Calculations show that when the total electric current of the electric locomotive changes, the fundamental wave component also has similar changes. When the electric locomotive passes through the zoned pavilion, it can instantly increase one locomotive or reduce one for a certain power supply arm of the traction power supply system. For locomotives, the load current of the traction arm exhibits a step change characteristic. In this case, the following steps can be used to handle the above-mentioned steps.
After this treatment, the predicted harmonic currents can quickly track the load changes and solve the hysteresis effect of the general low-pass filter.
In the actual calculation, firstly, they all contain 3, 5, 7, 9 and 11 equal harmonic components, and b and b represent the harmonic current results detected by conventional algorithms and prediction algorithms under different variations. It can be seen that when the load current gradually changes, the harmonic current obtained by the prediction algorithm is basically consistent with the theoretical harmonic current, while the conventional algorithm has a large change of the load current model with a dramatic change in the load current model with drastic changes ( The current is increased by a factor of 1 in 1/4 cycle. It is shown that when the load current changes dramatically, the harmonic current obtained by the conventional algorithm has a one-cycle time lag, while the prediction algorithm has almost no time lag.
5 Conclusion This article presents a predictive harmonic current detection method. The condition can still give accurate prediction values, and overcome the time lag problem of other methods, which lays the foundation for filtering the harmonic current by the active filtering system. This harmonic detection method can be used for harmonic current detection such as general rectification load and electric traction load.
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