Comparison of Control Algorithms for Shunt Active Filter for Harmonic Mitigation

Comparison of Control Algorithms for Shunt Active Filter for Harmonic Mitigation

A.Giri Prasad¹ ,K.Dheeraj² ,A.Naveen Kumar³, Electrical and Electronics Engineering Department ST.Peters Engineering college, Kompally, Hyderabad

Abstract:- Shunt Active Filter generates the reference current, that must be provided by the power filter to compensate harmonic currents demanded by the load. This paper presents different types of SRF methods for real time regeneration of compensating current for harmonic mitigation. The three techniques analyzed are the Synchronous Reference Frame Theory (SRF), SRF theory without synchronizing circuit like phase lock loop (PLL) also called instantaneous current component theory and finally modified SRF theory. The performance of Shunt Active Power Filter in terms of THD (Total Harmonic distortion) of voltage and current is achieved with in the IEEE 519 Standard. The comparison of all methods is based on the theoretical analysis and simulation results obtained with MATLAB/SIMULINK

Index termsSynchronous Reference Frame, instantaneous current component theory, Modified SRF, Active Filter, Harmonics.

1. INTRODUCTION

   The increasing use of power electronic based loads (adjustable speed drives, switch modern power supplies, etc) to improve system efficiency and controllability is increasing concern for harmonic distortion levels in end use facilities and on overall power system. The Active Power Filter uses power electronic switching to generate harmonic currents that cancel harmonic content from non linear loads. Over the recent years, power quality has been given attention due to the intensively use of power electronic Controlled applications in all branches of industry, such as controlling or converting AC power to feed electrical loads.

   The non-linear loads have led to the concerns over the allowable amounts of harmonic distortion injected into the supply system. Standards such as IEEE-519 have emerged to set and impose limits and recommended practices so that the harmonic distortion levels are kept in check, thereby promoting better practices in the design and operation of power system and electric equipment.

   Based on observations from various references, a practical limit of less than 5% of the total harmonic distortion(THD) must be employed by any system designers and/or end-users to ensure compliance with the established standards. Many efforts have been expended to develop active power filters and conditioner that can soften the power quality problems.

   One of the cornerstones of the active filter is its control strategy that is implemented in the active filter controller. The performance of an active filter depends mainly on the selected reference generation scheme.

   The control strategy for a shunt active power filter generates the reference current, that must be provided by the power filter to compensate reactive power and harmonic currents demanded by the load. This involves a set of currents in the phase domain, which will be tracked generating the switching signals applied to the electronic converter by means of the appropriate closed-loop switching control technique such as hysteresis or deadbeat control.

   Several methods including instantaneous real and reactive power theory have been proposed for extracting the harmonic content. This paper presents a different modification based on the same principle and compares its performances with sinusoidal source and balanced load condition. The Modified SRF method called, in this paper, Filtered Modified Reference Frame Method (FMRF), because it uses filters and is based on the modified reference frame method .

   Fig. 1: Basic principal of shunt current compensation in active

2. SRF METHODS

Among the several methods presented in the literature, the Synchronous Reference Frame method (SRF) is one of the most common and probably it is widely used method. This section is organized as to describe succinctly the SRF methods. The three methods presented in this section with some results

obtained with the above mentioned methods. The nonlinear load considered is a three-phase diode bridge rectifier.

1. Synchronous Reference Theory (SRF)

   In the SRF, the load current signals are transformed into the conventional rotating frame d-q. If is the transformation angle, The transformation is defined by:

   source current in d-q coordinates. Fig 2 illustrates a configuration of the SRF method. There is no need to supply voltage waveform for a SRF based controller. However the phase position angle must be determined using voltage information. The SRF harmonic detection method can be reasonably summarized as a block diagram as shown in Fig.3.

   Where x denotes voltages or currents.

   (1)

   Fig.3: SRF harmonic detection

2. Instantaneous Current Component (id-iq) Theory:

   Fig.1 shows the schematic block diagram of the shunt active filter with controller. The block diagram consists of variable sensing system, Reference Estimation System, PWM signal generator and system controller.

   The variable sensing block senses the system variables like supply current, load current and compensating current, DC link voltage or current. PWM signal generator and system controller generate switching signals for converter switches based on the error produced by reference signal and actual system variables.

   Fig. 2: Basic Synchronous Reference Frame Configuration

   In the SRF is a time varying angle that represents the angular position of the reference frame which is rotating at constant speed in synchronism with the three phase ac voltages. To implement the SRF method some kind of synchronizing system should be used. In phase-locked loop (PLL) is used for the implementation of this method. In this case the speed of the reference frame is practically constant, that is, the method behaves as if the reference frames moment of inertia is infinite. The fundamental currents of the d-q components are now dc values. The harmonics appear like ripple. Harmonic isolation of the d-q transformed signal is achieved by removing the dc offset. This is accomplished using high pass filters (HPF). In spite of a high pass filter, a low pass filter is used to obtain the reference

   Fig.4: AF control system based on the instantaneous active and reactive current component Id – Iq method.

   In this method the currents Ici are obtained from the instantaneous active and reactive current components Id and Iq of the nonlinear load. In the same way, the mains voltages vi and the polluted currents Ii in as in the previous method by 2 and 3. However, the load current components are derived from a synchronous reference frame based on the Park transformation, where represents the instantaneous voltage vector angle 4

   (2)

   (3)

   (4)

   With transformation the direct voltage component is udq=u=

   and the quadrature voltage component is always null, Uq=0 , so due to geometric relations 4 becomes

   .(5) Instantaneous active and reactive load currents Id and

   Iq can also be decomposed into oscillatory and average terms Id=I*d+Id , and Iq=I*q+Iq . The first harmonic current of positive sequence is transformed to dc quantities, i dq1h i.e., this constitutes the average current components. All higher order current harmonics including the first harmonic current of

   negative sequence, , are transformed to non-dc quantities and undergo a frequency shift in the spectra, and so, constitute the oscillatory current components. These assumptions are valid under balanced and siusoidal mains voltage conditions. Eliminating the average current components by HPFs the currents that should be compensated are

   obtained, and .

   (6)

3. Modified (id-iq) Theory

   The method suggested in this section is based on the modified (id-iq) method (FMRF). The principle is the same. However there are two differences in the determination of the instantaneous position of the rotating reference frame. In spite of using the voltages to calculate the transformation angle, low pass filters (LPF) are used to reduce harmonics of the network signals, and consequently use on the control process approximate sinusoidal waveforms, fig.6.

   Fig. 5: Principal of modified (id-iq) method

   The second modification consists in separating the coefficient and to use a filtered coefficient. This new modification is important because the system will presents better results to inverse sequence components. These concepts are presented in fig. 5 using block diagrams. The modified synchronous reference frame method has excellent results in balanced sinusoidal and unbalanced ac mains.

   In all cases studied in this paper, the load is a three phase diode bridge with an inductive circuit on its dc side. The LPF, LPF (cutoff frequency fc) and LPF (cutoff frequency fd) have different functions. Filters LPF, LPF are set to filter the ac mains and to avoid the influence of voltage harmonics presented on the network point of common coupling. The LPF is set to avoid the oscillation of the parameter that is due to the inverse sequence component. The low pass filter used for LPF, LPF, and LPF, the type of LPF are of 4th order Butterworth and 1st order chebyshev type with appropriate cutoff frequencies. In this method the cutoff frequency of the filter was set at 8 Hz and the cutoff frequency of the alpha and beta filters were also set to 60 Hz in Butterworth filter and passing band frequency of 20 Hz is used in chebyshev type of LPF.

   III RESULTS AND ANALYSIS

   In order to evaluate the performance of all the methods simulation studies are carried out. In FMRF method it is observed that the supply current is close to sinusoidal and it remains in phase with the supply voltage, therefore, unity power factor is maintained at the output of supply system. From the figures 6 to 17 are results corresponding the three SRF Theories and Comparison is shown in the Table 1

   Fig 6: Performance of SRF theory: (1) Load current

   (2) Source current (3) Compensating current

   4000

   3000

   2000

   1000

   0

   -1000

   12

   10

   8

   Mag

   6

   4

   2

   0

   Selected signal: 10 cycles

   0 0.05 0.1 0.15 0.2

   Time (s)

   Fundamental (50Hz) = 469.6 , THD= 3.84%

   0 20 40 60 80 100

   Harmonic order

   4000

   3000

   2000

   1000

   0

   -1000

   15

   Mag

   10

   5

   0

   Selected signal: 10 cycles

   0 0.05 0.1 0.15 0.2

   Time (s)

   Fundamental (50Hz) = 469.6 , THD= 1.01%

   0 5 10 15 20

   Harmonic order

   Fig. 9: SRF Theory, Chebyshev type filter: (1) Source current for 10 cycles (2) FFT analysis

   Fig. 7: SRF Theory, Butterworth type filter: (1) Source current for 10 cycles (2) FFT analysis

   Fig. 8: Performance of SRF theory: (1) Load current

   (2) Source current (3) compensating current

   Fig. 10: Performance of id-iq theory: (1) Load current

   (2) Source current (3) Compensating current

   Selected signal: 10 cycles. FFT window (in red): 7 cycles

   150

   100

   50

   0

   -50

   0

   0.05

   0.1

   Time (s)

   0.15

   0.2

   Fundamental (50Hz) = 10.06 , THD= 2.02%

   0.15

   0.1

   0.05

   0

   0

   200

   400

   600

   800

   1000

   Frequency (Hz)

   Mag

   Fig. 11: id-iq Theory Butterworth type filter: (1) Source current for 5 cycles (2) FFT analysis

   4000

   3000

   2000

   1000

   0

   -1000

   15

   Mag

   10

   Selected signal: 10 cycles

   0 0.05 0.1 0.15 0.2

   Time (s)

   Fundamental (50Hz) = 469.6 , THD= 1.01%

   Fig. 12: Performance of id-iq theory: (1) Load current

   (2) Source current (3) Compensating current

   Selected signal: 10 cycles. FFT window (in red): 6 cycles

   5

   0

   0 5 10 15 20

   Harmonic order

   Fig.15: Modified id-iq Theory Butterworth type filter:

   1. Source current for 10 cycles (2) FFT analysis

      150

      100

      50

      0

      0

      0.05

      0.1

      Time (s)

      0.15

      0.2

      Fundamental (50Hz) = 10.29 , THD= 2.00%

      0.1

      0.08

      0.06

      0.04

      0.02

      0

      0

      200 400 600 800 1000

      Frequency (Hz)

      Mag

      Fig. 13: id-iq Theory Chebyshev type filter: (1) Source current for 6 cycles (2) FFT analysis

      Fig. 16: Performance of Modified id-iq theory: (1) Load current

   2. Source current (3) compensating current

4000

3000

2000

1000

0

-1000

12

10

8

Mag

6

4

2

0

Selected signal: 10 cycles

0 0.05 0.1 0.15 0.2

Time (s)

Fundamental (50Hz) = 453.9 , THD= 2.95%

0 5 10 15 20

Harmonic order

Fig. 14: Performance of Modified id-iq theory: (1) Load current

(2) Source current (3) compensating current

Fig. 17: Modified id-iq Theory Chebyshev type filter: (1) Source current for 10 cycles (2) FFT analysis

In real filtering, a Butterworth type filter is normally chosen, but chebyshev filter is also equally compatible for preparing experimental prototype. This particular filter type was chosen, in order to obtain magnitude and phase characteristics as close as possible to an ideal filter since its magnitude response is maximally flat in the passband and is monotonic in both passband and stopbands. To minimize the influence of the HPFs phase responses, an alternative HPF (AHPF) can also be used by mean of a low-pass filter (LPF) of the same order and cutoff frequency, simply by the difference between the input signal and the filtered one, which is equivalent in performance.

TABLE 1

Comparison of the Different SRF Methods

BW = Butterworth, CH = Cheybshev (Type of Filter Used)

IV. CONCLUSION

This paper presents the compensation performance of all the different SRF techniques under sinusoidl voltage source condition as shown in table-1. Results are similar with gained source THD under IEEE 519, but under various filter type the chebyshev type filter is having superior performance compare to Butterworth filter for all methods. The Synchronous Reference

Frame method is one of the most common and performing methods for detection of harmonics in active filters. An Improved Synchronous Reference Frame Method for the control of active power filters was presented. It is called Filtered Modified Reference Frame Method (FMRF) and is based on the same principle as the Synchronous Reference Frame method. However, this new method explores the fact that the performance of the active filter to isolate harmonics depends on the speed of the system that determines the rotating reference frame, but doesnt depend on its position. So, the delay introduced by the ac voltage filters, used for the detection of the reference frame, has no influence on the detection capability of the method. Compared with other methods, this new method presents some advantages due to its simplicity and its rudeness to perturbations on the ac network.

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