A simple digital control algorithm for three phase shunt active filter: simulation and experimentation

Abstract

A novel and simple i_freal control algorithm using digital signal processor (DSP) has been proposed and realized for a three phase shunt active filter (SAF). The simulation and prototype construction of SAF is conducted to compensate the reactive power and harmonics in a distribution system. The major feature of the proposed i_freal algorithm is that it does not require unit vector templates and any transformations for the reference current generation of SAF. This reduces the computational complexity and makes the control flexible and faster. The simulation is conducted in MATLAB/SIMULINK while DSP TMS320LF2407 is employed in the digital implementation of hysteresis current control (HCC) for experimentation. The hardware results correlate with the simulation results in reducing the total harmonic distortion (THD) of the source current and achieving unity power factor.

Keywords

shunt active filter (SAF) / power quality / voltage source inverter (VSI) / digital signal processor (DSP) / total harmonic distortion (THD) / power factor improvement

Introduction

The widespread application of nonlinear loads such as switched mode power supplies, voltage controllers, adjustable speed drives in day to day life introduces power pollution in the distribution network. This leads to under utilization of the available system in view of the low power factor and increased harmonic content of the source current. The research in the area of power quality aims at reducing the effect of these nonlinear loads on source current. The passive filters find their way useful in achieving this objective only for simple configurations. Due to their bulkiness and possible resonance, the passive filters could not be employed as and when required. By further research in the area of power quality, the active power filters (APFs) based on voltage source inverter (VSI) topology became the remedial solution [1]. The main purpose of APFs installed by individual consumers for a specific load is to compensate the current harmonics or current imbalance of their own harmonic producing loads [2]. The APFs can be connected with the system either in shunt, series or together in combination. The APF connected at the load side in shunt with the system, known as shunt active filter (SAF), prevents the harmonic component from entering into the system and also compensates the reactive power supplied by the source. SAF is operated as a controlled current source to inject a current which is equal and opposite to the harmonic and reactive components of the load current. Consequently, the line current becomes sinusoidal with unity power factor consisting of only the active component of the non linear load current [3].

The source current harmonic suppression using the icosϕ algorithm has been discussed in Ref. [4] using analog circuits. In addition to the use of individual sensors for source voltage, load current and injected current, biquad filter is used to extract the fundamental component of load current. The extensive use of electronic devices makes the circuit much complex and cumbersome. The instantaneous reactive power and synchronous detection algorithm has been compared with an algorithm based on icosϕ in Ref. [5], and it is shown that there is a better reduction in source current harmonics with the icosϕ algorithm. The extraction of fundamental component of the load current by the integration of power signal over a complete cycle and sampling the integrator output at the end of the cycle is described in Ref. [6]. Also, the algorithm proposed in Ref. [7] uses the synchronous reference frame theory (d-q-0) to determine the suitable current reference. The simulation results of shunt active filter with an adaptive hysteresis band controller are presented in Refs. [6] and [7]. The use of transformations and extraction of power signal and sampling the integrator output can be very well appreciated in simulation. However, sampling each and every instant of power signal in practice requires much computational effort. A new control algorithm has been proposed in Ref. [8], based on the instantaneous active current component (i_d) method, to generate current reference. The i_d method requires ‘dq’ transformation of the harmonic load current for further processing. The implementation of active power filter based on instantaneous power theory is discussed in Ref. [9]. This control algorithm demands two transformations, known as Park and Concordia matrix transformation, and digital filters in real time. The use of all these transformations involves matrix operations with sine and cosine functions. The performance comparison of SAF with synchronous detection method (SDM) and nonlinear autoregressive moving average (NARMA) is conducted in Ref. [10]. Complicated equations of SDM are avoided using NARMA-L2 controller but the realization of the NARMA-L2 controller is not conducted in real time.

The results of simulation and prototype implementation using DSP of a three phase active power filter with proportional integral (PI) and artificial neural network (ANN) controllers are given in Ref. [11]. DSP is used to generate the reference source current using the unit vector templates. The shunt active filter employing fast Fourier transform in Ref. [12] depicts the extraction of fundamental source current from the distorted load current for calculating the reference compensation current. This method requires complex transformations for its control. The operation of shunt active filter employing fuzzy based control is discussed in Ref. [13]. However, the implementation of fuzzy controller in real time will be complex. The neutral clamped capacitor topology requires a separate control to share the DC voltage between the two split capacitors at the DC bus. In a three leg bridge inverter, when used to compensate the four wire system, this control causes extra ripple in the APF current [14]. A new control scheme to eliminate harmonics and to compensate the reactive power has been proposed in Ref. [15]. The reference currents are generated in Ref. [15] using two PI controllers and a pulse-width modulation (PWM) controller is used for controlling the SAF. A DSP based optimal algorithm proposed in Ref. [16] uses three independent hysteresis current controllers for controlling the SAF current. A sample and hold circuit to generate the reference current for the SAF, which eliminates the need of complicated transformations for reference currents with additional electronic circuitry, is proposed in Ref. [17]. Two separate controllers, namely sliding mode controller (SLMC) and PI controller, are used in Ref. [18] to control the DC capacitor voltage and line current respectively. While implementing in real time, the design and tuning of the controller is a tedious process. If the output signal of SLMC is not limited, the system stability may be lost and improper PI tuning will result in unbalance three phase current even though the system is in balanced condition. To overcome the drawbacks discussed above, a simplified control scheme, called i_freal algorithm, is proposed in this paper. In the proposed algorithm, considering the design of SAF for a fixed load, the magnitude of the real component of fundamental load current (i_freal) is obtained from the active power of the load to be compensated. The value of i_freal thus obtained is also confirmed with the result of fast Fourier transform on load current waveform in simulation and using Fluke Power Quality analyzer in real time. A voltage equivalent to this current, i_freal, is obtained through the suitable design of output resistance of the Hall effect voltage sensor that senses the supply voltage. This equivalent voltage, which is in phase with the supply voltage, is used as the desired unity power factor source current for the control of SAF. Thus, the use of phase locked loop (PLL) and other complex transformations are avoided to generate the reference current.

The proposed scheme is flexible and can also work effectively for a change in real power demand of the load while the reactive power demand is within the capability of the SAF, by suitably changing the output resistance of the voltage sensor giving the voltage equivalent to the real component of the fundamental load current. Also the DSP will consume less time in computation since the present control scheme does not involve any transformations. The satisfactory working of this control scheme for a single phase SAF has also been tested in Ref. [19].

System description

The general block diagram of a distribution system connected with SAF is shown in Fig. 1. The three phase supply is connected to a resistive and inductive (RL) load fed by a diode bridge rectifier drawing a current ‘i_L(t)’. The load current ‘i_L(t)’ supplied by the source (i_s(t)) consists of the harmonic component ‘i_h(t)’, in addition to the fundamental real ‘i_freal(t)’, and reactive ‘i_freac(t)’ components. The SAF is connected in shunt with the system by a transformer for isolation. The SAF is to be designed to supply a current ‘i_SAF(t)’ in such a way to compensate the reactive and harmonic components of the load current so that the source supplies only the active component of the load current, which will be in phase with the source voltage yielding unity power factor.

i_s\left(t\right)=i_{\mathrm{L}}\left(t\right)=i_{\mathrm{f}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{l}}\left(t\right)+i_{\mathrm{f}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{c}}\left(t\right)+i_{\mathrm{h}}\left(t\right).

If SAF current

i_{\mathrm{S}\mathrm{A}\mathrm{F}}\left(t\right)=-\left(i_{\mathrm{f}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{c}}\left(t\right)+i_{\mathrm{h}}\left(t\right)\right),

then

i_{\mathrm{s}}(t){=i}_{\mathrm{L}}(t){+i}_{\mathrm{SAF}}(t){=i}_{\mathrm{freal}}(t).

The motivation of the proposed i_freal algorithm relies in the estimation of the reference current of SAF without the use of unit vector templates of the source voltage. This further enhances the chance of making the source current in phase with the source voltage.

Proposed control algorithm and operation

The block diagram of the proposed control algorithm for a three phase (R, Y, B) system is demonstrated in Fig. 2. Suitably designed voltage sensors are used for providing a voltage equivalent to the real component of the fundamental load current (i_freal) from the source voltage. This method is quite simple and straight forward as i_freal is directly estimated from the active power of the load to be compensated and verified by Fourier analysis in simulation and by using a power quality analyzer in real time.

The load current and injected current from the SAF are sensed using current sensors. The reference current for the SAF ‘i_ref’ is calculated by subtracting the ‘i_freal’ from the load current ‘i_L’. Later the actual SAF current ‘i_SAF’ is compared with the reference current and gating pulses are generated by the hysteresis current control technique.

Hysteresis current control

The hysteresis current control (HCC) technique used for the control of shunt active filter is depicted in Fig. 3. The actual current is compared with the reference current and its associated hysteresis band. This technique decides the switching pattern of active filter in such a way to maintain the actual injected current of the filter to remain within a desired hysteresis band (HB) as indicated in Fig. 3. The real time digital implementation of HCC using DSP controller is illustrated in Section 6. This digital implementation uses three hysteresis current control loops and three phase pulses are generated independently.

The switching logic is formulated as follows:

If i_SAF<(i_ref - HB), the upper switch is turned ON and lower switch turned OFF,

If i_SAF>(i_ref + HB), the upper switch is turned OFF and lower switch turned ON.

The switching frequency of the hysteresis current control technique depends on the speed in which the actual current changes from the upper limit to the lower limit of the hysteresis band, or vice versa. Therefore, the switching frequency does not remain constant throughout the switching operation, but varies along with the current waveform. The variation of switching frequency is permitted as the controller is faster and the IGBTs employed in VSI are able to operate even at a switching frequency of 20 kHz. Furthermore, the filter inductance can be used to smoothen out the active filter current.

Simulation of three phase shunt active filter

The simulation block diagram of a three phase distribution system connected to a nonlinear load with a three phase SAF is shown in Fig. 4. The nonlinear load consists of a three phase diode bridge rectifier with RL load. The resistance and inductance (R_L and L_L) are assumed for representing the line drops. The simulation parameters are furnished in Appendix 1. The simulation is conducted in MATLAB 7.0 and the results are discussed in Section 7.

Laboratory implementation

The efficacy of the proposed algorithm is verified with a hardware set up in the laboratory. The DSP based prototype of the three phase SAF shown in Fig. 4 has been developed in the laboratory. The system parameters are the same as that of the simulation as given in Appendix 1. A photograph of the experimental set up is presented in Fig. 5.

The power circuit consists of a three phase distribution system feeding a diode bridge rectifier fed RL load. A SEMIKRON built VSI constructed with IGBTs and necessary gate driving circuits, which acts as shunt active filter, is connected in shunt with the system through filter parameters R_f and L_f. The real component of the fundamental load current (i_freal) is calculated in open loop from the active power of the load and verified using the fluke power quality analyzer EN50160. The LEM LV-25P voltage sensor circuit is designed, as given in Appendix 2, to produce an output voltage equivalent to the value of i_freal. The three phase load current and SAF current are sensed by the LEM LA-55P Hall-effect current sensors. Nine out of the 16 channels present in the analog to digital converter (ADC) available in the digital signal processor TMS320LF2407A, are utilized to acquire the sensed signals. The reference current for the three phases of SAF are generated using the i_freal algorithm and further the switching pulses are generated using the hysteresis current control technique implemented in DSP as given in Fig. 6.

The necessary gating pulses for the VSI are derived through three out of four channels of the digital to analog converter (DAC 7625U) available in the TMS320LF2407 EVM board. The generated three phase gate pulses are used to switch the VSI suitably to inject a current ‘i_SAF(t)’ in such a way to cancel the harmonic and reactive parts of the load current.

Simulation and experimental results

The simulation and experimental results of the proposed three phase shunt active filter in steady-state operation are shown in Figs. 7 to 15. The proposed system is connected to a 400 V, 50 Hz three phase network feeding a diode bridge rectifier with a RL load. A scaled down three phase balanced supply voltage of 35 V (root mean square (RMS) phase to neutral) is applied to the system. Both simulation and hardware results are presented aside for the sake of clarity. The results of uncompensated distribution system are given in Figs. 7 to 10 and those of compensated are grouped in Figs. 11 to 15 for better understanding.

Figures 7 and 8 depict the conditions of the source voltage and source current in the three phases. Figure 9 shows the source current waveform superimposed with the supply voltage. It clearly indicates that the source current is lagging the source voltage with a 10.4% THD in real time, as shown in Fig. 10. The results of SAF compensated system are shown in Figs. 11 to 15. The lower and upper hysteresis bands of the reference current are shown in Fig. 11. It can be noticed from the superimposed waveforms of Fig. 12 that the compensating current of SAF is leading the source voltage. The improvement in power factor toward unity can be observed from the superimposed waveforms of source voltage and compensated source current in Fig. 13. The source current THD is improved to 1.48% in the simulation and 2.3% in real time due to the parasitic effects of actual system components, when 66 V is maintained at the DC link of VSI. The compensated source current in the three phases and the harmonic spectrum are depicted in Figs. 14 and 15 respectively.

Conclusions

A simple and easy to implement i_freal algorithm for three phase shunt active filter is proposed. This algorithm provides a unique method for generating the reference currents for SAF. This approach is quite straight forward provided that the input supply is sinusoidal and the SAF is designed for a specific load. The difficulty in tuning of hysteresis band during the analog implementation of HCC is avoided by digital implementation using TMS320LF2407A, where the flexibility for changing the hysteresis band is quite high. The simulation and experimental results show good performance of SAF in harmonic reduction and input power factor improvement.

References

[1] MillerT J E. Power Electronic Control in Electrical Systems. 1st ed. Newnes Power Engineering Series, 2002

[2] AkagiH. New trends in active filters for power conditioning. IEEE Transactions on Industry Applications, 1996, 32(6): 1312–1322

[3] LinB R, YangT Y. Implementation of active power filter with asymmetrical inverter legs for harmonic and reactive power compensation. Electric Power Systems Research, 2005, 73(2): 227–237

[4] BhuvaneswariG, NairM G. Design, simulation and analog circuit implementation of a three phase shunt active filter using the i cos ф algorithm. IEEE Transactions on Power Delivery, 2008, 23(2): 1222–1235

[5] JouH L. Performance comparison of the three phase active power filter algorithms. IEE Proceedings. Generation, Transmission and Distribution, 1995, 142(6): 646–652

[6] JogA N, ApteN G. An adaptive hysteresis band current controlled shunt active power filter. In: Proceedings of IEEE Conference on Compatiability in Power Electronics. Gdansk, Poland, 2007, 1–5

[7] KaleM, OzdemirE. An adaptive hysteresis band current controller for shunt active power filter. Electric Power Systems Research, 2005, 73(2): 113–119

[8] RahmaniS, Al-HaddadK, FnaiechF. A general algorithm applied to three phase shunt active power filter to compensate for source and load perturbations simultaneously. In: Proceedings of IEEE International Symposium on Industrial Electronics. Ajaccio, France, 2004, 1(2): 777–782

[9] PatrascuC, PopescuD, IacobA. Power quality improvement using a DSP controlled active power filter. In: Proceedings of the International Conference on Electrical Power Quality and Utilization. Barcelona, Spain, 2007, 1–4

[10] GeorgeM, BasuK P. Three phase shunt active power filter. American Journal of Applied Sciences, 2008, 5(8): 909–916

[11] EL-KholyE E, EL-SabbeA, El-HefnawyA, MharousH M. Three phase active power filter based on current controlled voltage source inverter. International Journal of Electrical Power & Energy Systems, 2006, 28(8): 537–547

[12] VodyakhoO, KimT, KwakS, EdringtonC S. Comparison of the space vector current controls for shunt active power filters. IET Power Electronics, 2009, 2(6): 653–664

[13] ElmitwallyA, AbdelkaderS, ElkatebM. Performance evaluation of fuzzy controlled three and four wire shunt active power conditioners. In: Proceedings of IEEE Power Engineering Society Winter Meeting, Singapore, 2000, 1650–1655

[14] BalcellsJ, LamichM, CapellaG. LC coupled shunt active power filter (APF): new topology and control method. In: Proceedings of IEEE International Conference on Electrical Power Quality and Utilization. Lodz, Poland, 2007, 1–6

[15] ChandraA, SinghB, SinghB N, Al-HaddadK. An imporved control algorithm of shunt active filter for voltage regulation, harmonic elimination, power factor correction and balancing of nonlinear loads. IEEE Transactions on Power Electronics, 2000, 15(3): 495–507

[16] GeorgeS, AgarwalV. A DSP based optimal algorithm for shunt active filter under nonsinusoidal supply and unbalanced load conditions. IEEE Transactions on Power Electronics, 2007, 22(2): 593–601

[17] DixonJ W, GarciaJ J, MoranL. Control system for three phase active power filter which simultaneously compensates power factor and unbalanced loads. IEEE Transactions on Industrial Electronics, 1995, 42(6): 636–641

[18] AkagiH, FujitaH. A new power line conditioner for harmonic compensation in power systems. IEEE Transactions on Power Delivery, 1995, 10(3): 1570–1575

[19] SrinathS, PrabhakaranS, MohanK, SelvanM P. Implementation of single phase shunt active filter for low voltage distribution system. In: Proceedings of the 16th National Power Sytems Conference (NPSC). Hyderabad, India, 2010, 295–300

RIGHTS & PERMISSIONS

Higher Education Press and Springer-Verlag Berlin Heidelberg