Novel Control Strategy for Multi-level Active Power Filter without Phase-locked-loop

Active power filter (APF) using novel virtual line-flux-linkage oriented control strategy can not only realizes no phase-locked-loop (PLL) control, but also achieves a good inhibitory effect to interfere. However, there are some problems in the conventional method, such as the error of amplitude, the shift of phase angle and the non-determinacy of initial oriented angle. In this paper, two one-order low-pass filters are adopted instead of the pure integrator in the virtual line-flux-linkage observer, which can steady the phase and amplitude. Furthermore, an original scheme of harmonics detection under the rotating coordinate is advanced based on the simplified space vector pulse width modulation (SVPWM) strategy. Meanwhile, by using the new SVPWM algorithm, the voltage space vector diagram of the three-level inverter can be simplified and applied into that of two-level inverter, and this makes the control for Neutral Point potential easier.


Introduction
The use of nonlinear loads such as power electronic devices leads to serious harmonics pollution and lager voltage fluctuation.Moreover, lager unbalance voltage and current distortions in the power system are harmful to the electrical equipment and power systems.Shunt active power filter (SAPF) can well compensate the harmonic whose frequency and amplitude both changes, and it is also recognized as an effective way to manage the grid harmonic, to reactive power pollution and to improve the quality of the power.Active power filter (APF) is a new power electronics device for dynamic harmonics restriction and reactive compensation without influenced by system inductance [1,2], besides it has such functions: adaptive ability for the parametric variation of system and load, automatic tracing and compensation for the varying harmonics [3,4].There are some remarkable advantages in nature of three-level APF, such as lower distortion of output waveform, lower endure voltage and less switch loss, high efficiency and low electromagnetic interference (EMI), so it helps to raise the installed capacity and improves the harmonics compensation effect as well as system reliability.
On the one hand, the traditional harmonic current detection which is based on instantaneous reactive power adopts phase-lock-loop (PLL) to acquire the voltage vector angle [5][6][7], when the grid voltage fluctuation is more serious, PLL will be in unlocked condition because of the larger frequency offsets which can not accurately track the phase position.To solve the problem, this paper uses the virtual line-flux-linkage orientation to observe the vector angle, which converts the observation of vector angle to the flux.By using the voltage integral on the AC side of the active filter to estimate the grid flux, PLL can be omitted; meanwhile, the power grid interference can be inhibited well.On the other hand, the traditional harmonic current detection requires that the integrated vector combined by the sine and cosine function should be synchronous and phase coincidence to the integrated vector of the three-phase positive sequence fundamental voltage, otherwise, the detection accuracy of the fundamental positive sequence reactive component will be affected by phase difference [8].Therefore, the harmonic detection principle based on the rotating coordinates is proposed in this paper.The application of virtual flux in active filter system also includes controlling generation of compensation current, so it has a very good control to harmonic current detection and compensation current ge-neration, and the voltage space vector modulation strategy SVPWM (Space Vector Pulse Width Modulation) can be easily applied to active filter control.
In this paper, the simplified three-level SVPWM with neutral point potential adaptive control is applied to control multi-level active filter, based on this, a novel multilevel voltage active filter phase reconstruction algorithm is proposed.

Mathematical Model for Three-Level APF and the Rotating Coordinate Based Harmonics Detection Diagram
Figure 1 is the main circuit of NPC three-level APF, while vector diagram of the virtual line-flux-linkage oriented system is shown in Figure 2. When the virtual line-flux-linkage is at axis, then mathematical model of three-level APF is in reference frame [9,10]: where:   In ( 2) to (6), : AC side inductance of APF; d : capacitance of DC side; m : peak voltage of power system; : the D-axis and Q-axis AC current of APF;  q : the D-axis and Q-axis components of switching function.
According the virtual line-flux-linkage oriented strategy showed in Figure 2, the reference compensation current can be obtained: where: ,

The diagram of Novel Virtual Line-Flux-Linkage Observer
The essence of virtual line-flux-linkage oriented method is to gain accurate space angle ( ) estimation of the oriented vector   , as shown in Figure 2, the angle can be obtained: Then the estimation of  is converted to the evaluation of  and  components of the virtual line-flux- linkage.(10) In (10), v  and v  are the  ,  voltage quantity at AC side of three-phase APF, respectively.Integrated both sides of (10), It can be seen from ( 11) that the flux linkage can be evaluated by the voltage integral at AC side of APF, while it brings a problem of the integral initial value which cause an error of the flux linkage, furthermore, the method of pure integrator doesn't retrain the DC component of the input signal, even a little DC component can make the integrator saturation.In this manner, the flux linkage in  axes can be a circular trajectory with DC offset corresponding to the centre of the circle, meanwhile, it cause the inaccuracy of virtual line-fluxlinkage oriented angle and impact both the authenticity of current feedback and the veracity of voltage space   vector supply, a A in the process of starting, and even can't start.Therefore, in order to achieve an accurate flux estimate, it's necessary to make some measures to eliminate influence of the integral initial value.
In the traditional virtual flux estimation, in order to solve initial value pro 1 ) in flux estimation.It can be seen s from the formula of the first order that the first or-filter der inertia is similar to the pure integrator when the frequency  of the inusoidal input is far greater than Howev r, it is also required that the first order i a e nerti should have a certain decay to the direct flow, that means f  should be a certain value, and not too small, otherwise the ecay will be very slow .So the first order low d pa mea ss filter has contradictions between approximating pure points and decaying of the DC component, that ns f  must maintain a certain value to decay, while it also should be far less than  to approximate the integral.Know from the above analysis.
Low-pass filter is adopted by conventional virtual line-flu nkage estimate to substitute for the purely integrator to least the DC drift, howev , it would lead to errors of amplitude and phase angle.As desc x-li er ribed by the ), the novel virtual lineflux-linkage observer and the comparison of three observers are showed in Figure 3 and Figure 4 respectively.Acc , it is quite easy to found ording to Figure 4 that the proposed algorithm respond faster than that of conventional way.It can be seen from the bode figure

Novel Voltage Estimation for Three-Level APF
In this paper, the simplified three-level SVPWM algohm is adapted, as shown in Figure 6.The voltage ace vec re space vectors, and all the hexes are centered by vertexes of inner one.Hence, two-level SVPWM algorithm can be applied to calculate the duration-time and the switch sequence of voltage vector [11].
Because of inherent problem in the topology of the diode-clamping three-level converter, the various switch states have different impacts on the neutral-point potential.The middle vectors can cau viation because of asymmetric parameter in practice.Small vectors will cause the fluctuation of neutral-point potential.There is a striking contrast effect between the two different middle vectors corresponding to different switch status vector [11].
There exist the regions that are overlapped by adjacent small hexagons as shown in Figure 6.So if the reference voltage vector stays at those regions, S can have any values that are possible.1_ is the corrected reference voltage vector when the index S has the value of 1, and is the corrected reference voltage vector when the index S has the value of 2. If the two-level plane of S = 1 is selected switching sequence decided by reference voltage vector is given as follows: (0 ) (0 0 1) (1 0 1) (0 1 1) , the , the dwelling time of negative short vectors is shorter than the positive ones, neutral-point potential will rise.So the neutral-point potential can be controlled by changing th As can be seen from (11), on the basis of the proposed algorithm, the voltage at the terminals of three-level rec-e corresponding the value of index S. tifier is estimated, which is shown in Figure 7.
When the reference voltage is in the first sector, esti-  mated three-phase voltages are given by: where, are the estimated voltage of DC bus voltage of threeare the duration-time of three-phase for two-level algorithm, respectively.

Design and Experimental Results Analysis
To verify the viability of proposed non PLL control scheme for multi-level APF and evaluate performance of this method, the test platform is established and operated, and its control sketch is showed in Figure 9.As shown in Figure 10, the structure of full-digital controller is composed of DSP (TMS320F2812 of TI) and FPGA.The system can implement DC bus voltage contro

The Main Circuit
l, har-  monics detection, close osis and etc.

The Principles of the Main Circuit Parameter Calculation
1) The design of inlet inductance on AC side The design of AC filter inductor has two principles: for one thing, it is the power of active power filter to track and control compensation current, and make sure it can still produce a corresponding compensation current while the load current has larger current rate of change; for another, it is to satisfy the requirements of trackin the size of compensation current ripple.
It can be known from [12] that: The maximum inductance value can be acquired by the above equation, while the minimum inductor value is determined by the size of allowable ripple current, the size of the ripple current should be limited within the prescribed range while choosing the inductor value.capacitance value is not unlimited, so the design principles of DC bus capacitor is to work as a minimum capacitance while APF can be under normal operative condition.
It can be known from [12] that: here, c S is the compensating capacity of APF, T is the control cycle of DC bus voltage, is the maximum allowable value of the voltage wave.
3) The controllable range of DC bus voltage DC bus voltage selection should be considered as follows: it is not only necessary to function, but also should not lead to too fierce current ch ase APF should be greater than the peak value of AC power grid line voltage, that is

The Experimental Results Analysis
The gri gh-power applications system, th not be ignored if pacity or in high pronsformer leakage reactance unlocked condition thus it can not accurately track the phase position which ca d the whole system shock.Thus, in the event that the orientation angle is not exactly, APF can compensate the harmonic of the nonlinear loads, on the co trary, it will cause system shock.
The virtual flux observer proposed in this paper uses two first-order low-pass filters instead of pure integrator.As the integral part has characteristics of a low-pass filter, the amplitude of n times harmonic attenuate by n times of fundamental when it comes through pure integrator, which has some effect on filtering high harmonic, that is to say, the virt nted has a good inhibition on the grid voltage distortion.It can be seen from Figure 12, while using active filter control based on novel virtual fl line-flux kage switch smoothly before and after net-side pow e virtual flux angle can be able to accurately track the phase position without the effect of grid voltage distorn.The traditional virtual line-flux-linkage uses ter instead of pure integrator which causes a certain error of amplitude and phase a gle, the new virtual line-fl ter instead of pure integral can be made no amplitude error and phase shift.As can be seen in Figure 1 o vector lead to higher current vector burr, and al makes the current vector stabilize rapidly.Figure 14 shows the harmonic and fundamental wave detected by the load current rotational coordinates detection method virtual line-flux-linkage orientation based.The method realizes harmonic current detection without PLL, and compare with the traditional i p q i  by freque detection method PLL based, it is not restrained ncy shift and it can omit the PLL circuit avoiding the effect that out of control of PLL brings to system of the active filter.Moreover, not only the fundamental component of the detecting current becomes more flexible but also the detection accuracy of harmonic current is higher.
Figure 15 is the final compensated line current.It is easy to found that the curre nearly close to sinusoidal nd the third, fifth, seventh, ninth, eleventh and thirteenth harmonics are filtered off mostly.But there exist high harmonics by selecting the switching frequency of 2 kHz, and it is easy to be solved by the method of a high-pass filter (HPF) in parallel with the power system.
Figure 16 is the system response in the condition of abrupt change of 50% load more.The system has an excellent dynamic performance that it can be stable after 2-3 periods.According to Figure 16(a), the APF based on virtual flux oriented vector control system uses a voltage outer loop and the DC bus voltage coincide with the given voltage when the system comes to be stable.The voltage amplitude fluctuation is minimal which means that there's energy exchange just between the DC side of active filter and load while the system arrive at a steady state condition.As can be seen in Figure 16(  hese results proved that the novel control algorithm with virtual line-flux-linkage-oriented based control system possess strong robustness, and also demonstrated that the simplified three-level SVPWM strategy with neutral point potential self-adaptation is valid.

Conclusions
The experimental results show that the scheme of adopting the proposed virtual line-flux-linkage-oriented observer can realize the non-PLL control for APF, and displayed that the load current detection under rotatin ordinate oriented by virtual line-flux-linkage can n only detects harmonics cu t rapidly but also makes e compensating current tracked reference fast, and indicated that APF system with the method of discusse flux oriented and rotating coordinate based harmonics detection has the characteristics of decreasing line total harmonic distortion (THD) significantly and superior dynamic property, thus it is obvious that leading the simplified three-level SVPWM strategy with neutral point potential self-adaptation in control can improve the system's overall performance.
upper and down DC voltage relevant to neutral point;

:
The D-axis and Q-axis fundamental current of nonlinear load through low pass filter from , The D-axis and Q-axis component harmonics current of nonlinear load.

Figure 1 .
Figure 1.Topology of a diode NPC three-level shunt APF.

Figure 2 .
Figure 2. Vector diagram of the virtual line-flux-linkage oriented system.

1 TV
respectively voltage space vectors of the corresponding switching sequence (0-1-1) and (0 0-1), both negative short vectors;(the corresponding voltage vectors.The voltage vector 1NV and 1P V have the same output voltage 1V , and dwelling tim f the 1N V , are equal.If the above switching se nc p the dwelling time of negative short vectors is longer than the positive ones; neutral-point potential will decline.If the two-level plane of S = 2 is selected, the ching sequence is given as follows:

Figure 7 .
Figure 7.The novel three-level voltage estimation algorithm.

Figure 8 .Figure 9 .
Figure 8. Experiments waveform of the novel three-level voltage estimation algorithm.
allowable value that compensation current deviate from the reference current; f is the current frequency of the fundamental wave; * a I is the effective reference current.osses cause current in the APF mpensation current and the energy pulse caused by the apacitance is difficult to maintain a constant.The larger the capacitor is, the smaller such fluctuations would be, but the 2) The design of the DC bus capacitor Owing to both the energy pulse and switching l d by harmonic and reactive co filter inductor energy storage on AC side, the c Copyright © 2010 SciRes.EPE achieve a certain current anges.The DC voltage threshold of three-ph can not be tracked.Take grid fluctuations and linear cone a er be the current trol rang nd other factors into consid ation, the DC voltage m E is the peak value of the phase voltage on AC side.According to the above design principles, the main cuit parameters of the APF test platf U cir orm are as follow: ansformer: 380 V d-side transformer is used to buck-boost in the hi e effect caused by the leakage reactance of transformer can the non-linear load is of a large ca portion harmonic.As can be seen in Figure 11, the voltage distortion caused by tra makes the phase-lock-loop in observer with two first-order low-pass fil no 3, the rientation angle and amplitude errors of the virtual flux the novel gorithm responses faster than traditional methods which a) Power line voltage: 380 V/50 Hz, tr /120 V, and system impedance is neglected; b) Three-phase uncontrolled rectifier bridge is adopted as the nonlinear load, its R = 8  inductance L = 1.5 mH; c) Incoming inductance of APF: L = 1.7 mH, DC bus voltage: d U = 320 V, capacitance: C = 2300 uF.

Figure 11 .Figure 12 .
Figure 11.Control of the active filter based on phase-lockphase.

Figure 13 .
Figure 13.The vectorgraph of power current in  axes, (a) Using low-pass filter instead of pure integrator; (b) Novel virtual flux observer.

Figure 14 .
Figure 14.The harmonics current and the fundamental wave.

Figure 15 .Figure 16 .
Figure 15.The compensated source current and FFT analysis.