Research on improved Harmonic current detection method based on SOGI-PLL instantaneous reactive Power

In view of the traditional instantaneous reactive power detection has low anti-harmonic interference ability, poor real-time performance, and insufficient accuracy when the power grid frequency fluctuates, this paper improved the detection link. The working principle of single-phase harmonic detection is analyzed, and the improved second-order generalized integrator is added to make the phase-locked loop detect the voltage phase accurately and efficiently. Based on the comparison and compensation simulation results of the active compensation device, the quality of grid-connected current of the inverter is improved effectively, and the precision of phase locking is increased.


INTRODUCTION
With the rapid development of electrified railway, a large number of electric locomotives are integrated into the power grid, and the power quality problems such as harmonic distortion and voltage fluctuation become more and more obvious [1] .In the process of power management, firstly, the harmonic current of nonlinear load is taken as the reference instruction current by the detection link.Through the control of the active compensation device, the traction network outputs an equal and opposite compensation current [2][3] .In order to obtain the input signal of traction power supply system timely and accurately, the single-phase circuit harmonic current detection method is selected.This method is simple, real-time and robust, and has been widely used in the field of detecting harmonic current in power grid.
At present, it is necessary to detect phase locked loop based on instantaneous reactive power detection method.It is used to track the phase of positive sequence voltage of power grid.Its performance directly affects the level of detection performance.Traditional PLL cannot accurately and quickly lock voltage signal [4] when voltage distortion occurs.Synchronous coordinate system phase locked loop (SRF-PLL), the negative sequence component of grid voltage will be mapped to the double power frequency component under synchronous rotation coordinate.Because the basic principle of soft phase-locked loop based on d q transformation is to realize the voltage component of q axis to be zero, the double frequency fluctuation will inevitably cause disturbance to the phase-locked loop, thus reducing the steady-state of the phase-locked loop [5][6] .In view of the above situation, the literature [7] applies the phase-locked loop (DSOGI-PLL) based on the second-order generalized integrator to the i pi q harmonic detection algorithm, which can lock the grid voltage well in the case of asymmetry or distortion of the grid voltage, but fails to deal with the case that the grid contains DC component.Based on the second-order generalized integrator phase-locked loop can rapidly accurate extraction of ac signal in the power grid voltage distortion and have the ability to filter out low harmonic [8][9] .Aiming at the shortcomings of the traditional instantaneous reactive power theory i p -i q method, this paper adopts the improved single-phase harmonic current detection method based on the improved phase-locked loop, and verifies the current detection effect through simulation.First, take two arm currents: By phase-locked loop traction arm alpha side voltage and phase, and pass by is cosine circuit with believe Numbers sin  and cos ,figure 2 modified second-order generalized integrator structures respectively with I  and I  multiplied: In Formula (2), each quantity can be expressed as: Filter out its pulsating component, and the average active power of the two supply arms can be obtained: 1 ( ) 2 Final fantasy(4) income with sin  and cos multiplication, the product of the results for the two side This method USES the change smaller voltage phase information, less to construct three-phase coordinate transformation, the basic idea for power supply arm through a side to detect the corresponding voltage phase t  ,the single phase current I  and I  through mathematical method to calculate the active reactive power reference value, which is mainly composed of straight section two, and through the LPF filter out the ripple component part DC, left the active power, reactive power and harmonic current compensated b the system are obtained.

Improved second order generalized integrator principle
The structure of the improved second-order generalized integrator is shown in Figure 2. Diagram v as input signal;  is the error signal; k is control coefficient.When resonance frequency 0  error with the input signal is 0, v ahead ' qv 90  phase and amplitude, same v and ' v have the same phase and amplitude signal  coefficient with k after " qv subtraction does not contain the DC component of ' qv and at the same time the introduction of high order harmonic [10,11] , therefore is adopted in the channel low-pass filter to filter out higher harmonic, its transfer function is: To induce ' qv oscillators at 50HZ,the LPF link is set to 50HZ.For the selection of k value, considering the filtering effect and reponse speed of SOGI, k is set as 2 here.

Single phase locked loop structure
Phase-locked loop structure as shown in figure 3, i v for power grid voltage, 0  as the center of the voltage-controlled oscillator frequency,  estimates for power grid frequency, phase and its estimate for the grid voltage respectively  and  .The phase discriminator is realized by a simple multiplier, the phase difference between the voltage into the relevant voltage signal, through a PI controller to achieve loop filtering, filter out the high frequency AC component of the front stage, the function of the voltage control oscillator by a linear integrator and sine function to achieve, generate AC signal synchronous with the input signal, the voltage into phase.
Figure 3 Based on the improved SOGI phase-locked loop structure SOGI does not product a double frequency term in the phase locking process, so it can have a faster response than traditional PLL.it has certain performance of harmonic suppression and high precision of phase locking when the fundamental frequency voltage is tracked under non-ideal conditions such as asymmetry and distortion of grid voltage.

Simulation analysis
In order to verify the effectiveness of the improved phase-locked loop harmonic detection proposed in this paper, a simulation model is built with Matlab/Simulink to simulate the asymmetry and distortion of power grid voltage.As can be seen from Figure 4(a) and (b), when the traditional phase-locked loop is used for voltage phase tracking, the dynamic response of the system is slow and the frequency fluctuates around 0.8HZ.When the improved phase-locked loop detection is adopted, the system not only has good performance when the grid voltage fluctuates, but also can accurately track the phase and quickly enter the stable state with better anti-interference ability.It can be seen from the simulation figure6 that the content of the 5th, 7th and 11th harmonics is relatively large when the load changes.In order to verify the effectiveness of the single-phase harmonic detection method, after the load changes of 0.02s, the active compensation device controlled by the improved single-phase harmonic current detection method is added when t=0.1s, and the current distortion rate of the grid is 1.35%.5, 7, 11 harmonic suppression effect is ideal, after about 0.03s system to reach the balance; It can be seen from the simulation figure7 of current tracking that the system can almost track the change of the reference instruction current in real time and has good steady-state accuracy, which verifies the correctness of the theoretical analysis.

conclusion
Due to the voltage frequency fluctuation of the power grid, a phase-locked loop detection method based on an improved second-order generalized integrator is used to track the voltage phase effectively, which improves the accuracy and response speed problems caused by the nonlinearity and volatility of the load.Considering that the load is single-phase load, the harmonic detection method based on instantaneous reactive power theory is adopted to effectively suppress the harmonics of the system.

FIG. 1
FIG.1Detection method of harmonic current in single-phase system based on voltage phase of traction armαandβside power frequency positive sequence current a I  and a I  ,then with the current power supply arm I  and I  respectively minus a I  and a I  ,the result is the active compensation device to be compensation current c I  and c I  .

FIG. 2
FIG. 2 Structure of improved second-order generalized integrator (a) Output phase waveform of traditional PLL and improved PLL (b) Voltage fluctuation phase locked loop frequency waveform Figure4 Voltage fluctuation phase-locked loop frequency When t=0.02s, the load changes abruptly.As shown in Figure 5, the current distortion rate THD is 23.98%.In the case of current disturbance in the grid, the tracking frequency of the traditional phaselocked loop takes about 0.1s and fluctuates greatly, with certain overshot.However, the adjustment time of the improved phase-locked loop is about 0.04s, which can still lock the grid voltage accurately and quickly, and the ripple ripple is very small.(a) Load mutation frequency waveform Fundamental (50Hz) = 77.59, THD= 23.Load current harmonic distortion rate THD Figure5 Simulation waveform of injected harmonic phase

Fundamental
Figure7 Compensation current through single-phase harmonic detection link