A COMPARISON OF DIFFERENT COMPENSATION CURRENT GENERATION SCHEMES FOR SUPPRESSING CURRENT HARMONICS GENERATED BY COMPACT FLUORESCENT LAMPS.
Active Power filters (APF) are an effective method of compensating for harmonic distortion. APF see wide application in industry where Adjustable Speed Drives (ASD) and Thyristior bridge rectifiers are used.
Different current estimation techniques are employed in separating harmonics from the fundamental current. Most of such techniques have been developed for large rectifier fed industrial loads. However, when these schemes are used with a smaller load having greater harmonic distortion such as compact fluorescent lamps (CFL), the results are unsatisfactory.
This work focuses on analyzing the performance of current estimation techniques when used with a load composed of compact fluorescent lamps (CFL). Percentage current harmonic distortion (%IHD) of resulting waveform after harmonic compensation has been taken as a figure of merit.
Key Words: Power Quality; Active Power Filter; Self Tuning Filter, Compact fluorescent Lamp.
Harmonics are sinusoidal waveforms having frequencies that are integral multiples of power line frequency. . Non- linear devices draw current in the form of pulses. Such waveforms are rich in higher order harmonics that create problems in the power system. Some of these problems are over heating of neutral conductors, overloading and early failure of transformers, mal-operation of protective devices. Harmonics in voltage result from current harmonics. These may get amplified due to interaction with capacitors installed for power factor correction. 
Harmonic mitigation techniques are therefore, a much studied area of interest. There are largely two approaches to solving the harmonic suppression problem: active and passive. The passive approach involves harmonic cancellation, damping, or shifting them to other frequencies [2, 3, 4,5] The active method uses active power filters to compensate for harmonic and reactive currents. This method tends to be more effective when working with loads such as AC drives that generate random harmonics . The shunt active power filter (SAPF), introduced by Gyugyi and Strycula , is a popular configuration for implementing an active power filter. It has good harmonic suppression characteristics. In addition, it can compensate for reactive power.
Harmonic current estimation methods are essential to Active power filter schemes. These methods calculate the amount of harmonics present in a current waveform. Most of these techniques are used when compensating for large industrial loads as adjustable speed drives (ASD) and Silicon Controlled Rectifier (SCR) bridges. However, when used with smaller loads having individually more Current Harmonic Distortion (IHD) performance of these current estimation techniques varies. This paper analyses the performance of instantaneous reactive power theory (IRPT, p-q theory)  and d-q theory when working with Compact Fluorescent Lamps (CFL) installed in large numbers.
To this end, Simulink models have been developed for the p-q and the d-q theories. CFL equivalent model has been used to simulate a per phase balanced non-linear load. The results of how effectively these two schemes work in this manner have been summarized
2. HARMONIC CURRENT ESTIMATION METHODS
This section discusses two major harmonic current estimation techniques
2.1 Instantaneous Reactive Power Theory (IRPT, p-q Theory)
IRPT theory was proposed by Akagi et al . In this method three phase currents and voltages a-b-c are transformed into two-phase co-ordinates a-AY-0 using Clarke transformation as in equation (1). Where 0 represents the equivalent zero sequence components. Instantaneous active and reactive powers are then calculated as in equations (3-5). Each of the Active Power (P) and reactive power (Q) are composed of continuous and alternating terms. The continuous terms reflect fundamental components of current and voltage. Whereas, alternating terms reflect harmonic part of the waveforms. A low pass filter (LPF) or a high pass filter (HPF) is used to separate fundamental and harmonic parts. Compensation currents in a-AY axis are computed according to equation (6). Inverse-Clarke transformation is then applied to obtain reference compensation currents Ia- Ib- Icas in equation (2). 
Where p' and q' are the harmonic part of real and reactive power respectively Under distorted grid voltage conditions, IRP-theory doesn't produce accurate current references for APF. Additional LPF have to be used to filter out voltage harmonics [8,9].
2 Stationary Reference Frame Method (d-q method)
The d-q method is based on Park transformation. Load current a-b-c are transformed into two co-ordinate system d- q-0, where 0 represents the zero sequence component as in equations (7). Inverse park transform is given in equation (8). Park transform requires angular position of the rotating reference frame. To this end, a phase locked loop (PLL) is required. Once the currents (Ia,Ib,Ic) are transformed into (Id, Iq, I0) a high pass filter (HPF) or low pass filter (LPF) in feed forward arrangement is used to separate harmonic components from fundamental ones. Inverse Park transform is used to obtain compensation current reference. 
Where represents the zero sequence components. Performance of d-q method depends on the angular reference generated by a phase locked loop (PLL). Grid voltage reference is filtered by LPF to obtain pure sinusoidal waveforms for PLL input [8, 9]
Six different scenarios have been implemented to assess the performance of different harmonic current estimation techniques under varied circumstances. The IRP-theory has been used with Low Pass filters (LPF); with Self Tuning Filters (STF); and both. Current Harmonic Distortion (IHD) w.r.t fundamental is taken as a measure of effectiveness of the method being used. In similar way d-q theory has been used with LPF, STF and both.
Finally the effect of zero sequence currents has been highlighted by adding and removing them from equations (7) and (8) of the Park transform
4. SIMULINK MODEL
A Simulink model has been developed for testing the effectiveness of various current estimation techniques when handling highly distorted current waveforms: such as those produced by CFLs. This includes a three phase programmable source module, Voltage- Current measurement blocks, CFL model and a harmonic current estimation block. Sim power systems block set has been utilised
4.1 Compensation Current Calculation
Compensation current is calculated from the non-linear load current (Ia Ib Ic) and source voltages (Va, Vb,Vc). This section calculates two parameters: firstly the compensation current that needs to be injected into the system and secondly the current waveform resulting from mathematical addition of the compensation and load current. Figure-2 shows the inputs and output of current estimation block Figure-3 shows the usage of an STF block in a-AY reference frame to filter out voltage harmonics. The pure sinusoidal reference of the grid voltage thus obtained is input to a phase locked loop (PLL) and angular position of the rotating angular d-q frame can be obtained.
Figure-4 shows the park transformation of load current using angular reference obtained as in Figure-3. LPF are used in feed forward configuration to obtain harmonic compensation currents.
Table 1 details Low Pass filter (LPF) parameters
###Filter###Cutt-off frequecy###Angular Cutt-
4.2 Compact Fluorescent Lamp (CFL) Modelling
Compact florescent lamps employ a small inverter circuit to create high frequency arcs that cause fluorescence in inert gas filled tubes. The authors in  have proposed a method to model Compact fluorescent lamps wherein the said inverter circuit and tube can be modelled as an equivalent resistance. This simplifies the overall modelling where many CFLs are to be simulated at one time. An equivalent model for 23 watt CFL lamp has been used as shown in Figure 5 whereas details of parameters are given in Table 2.
Table 2: parameters for CFL used in simulation
###Input resistance###3.59 ohms
4.3 Self-Tuning Filter (STF) Model
Self-tuning filter refers to the arrangement shown in figure 6
It has characteristics similar to a Band-Pass filter. There is no phase delay between input terms and corresponding output terms Equations (8) and (9) govern an STF
Where K is a constant such that total gain of STF is zero.
Self-Tuning Filter (STF) can simplify the control scheme used in active power filters . An STF is added after Clarke transformation block. It takes distorted a-AY Voltages as input and gives undistorted a-AY voltages as output as in equations (10). The STF method can be used to separate harmonic components from fundamental ones by using the STF in feed forward configuration as in equations (11). Harmonic current can be calculated when applied after the Clarke transformation of distorted load currents. The STF has been used with the p-q and d-q theories  Simulink model for STF has been shown in figure-7 whereas table 3 gives details of its parameters.
Table 3: Self Tuning Filter parameters
###Fundamental frequency (f)###60
###Fundamental frequency angular (W)###2f
5. RESULTS: HARMONICS LEVELS AFTER FILTERING
This section presents the results of filtering distorted current waveforms through active power filter using different current harmonic detection methods
5.1 Frequency Domain Analysis
FFT tool (Sim Power systems) in Simulink (R) has been used to analyze three current waveforms. i.e: a Thyristor bridge rectifier fed DC motor drive; a bank of twenty CFLs per phase, 23 watt each; CFLs under distorted mains voltage. The current harmonic distortion (IHD) with respect to the fundamental is compared
Table 4: Comparison of current harmonic distortion
###Thyristor bridge fed DC###29.29###20 A
###23 watt###126.40###10 A
###23 watt###217.18###15 A
###CFL bank with distorted
Referring to table 4 and figures 8-10 it is evident that the current harmonic distortion of CFL waveform is far greater than Thyristor fed DC motor. Under distorted mains voltage IHD increases further
5.2 Instantaneous Reactive Power Theory. (P-Q theory) Results.
Figure 11 shows resultant waveform achieved through p-q theory based harmonic compensation. The IHD is still at 62% of fundamental.
5.3 Instantaneous Reactive Power Theory. (P-Q theory) Under Distorted Voltage Conditions Figure 12 shows resultant waveform achieved through p-q theory based harmonic compensation under distorted grid voltage. The IHD increases to 142% of fundamental.
Compensation current calculation involves grid voltage in two dimensional co-ordinates Va and VAY as in equation (6). Distorted grid voltage implies that these quantities are distorted as well. So in- accurate harmonic current references are produced.
5.4 Stationary Frame Method (d-q method)- Using Low Pass Filters in Feed Forward Configuration
Figure 13 shows results of using d-q theory where LPF have been used in feed-forward configuration to filter out harmonic current. The current waveform after harmonic compensation is a nearly perfect sinusoid with IHD at 0.01% Instantaneous reactive power theory (P-Q theory) using Figure 14 shows results of p-q theory based compensation where Self tuning filters tuned to the fundamental frequency are used to filter out harmonic part of active and reactive power. The results are anomalous with THD further increasing to 261.9%
5.5 Stationary Frame Method (d-q Method)- Using STF in Feed Forward Configuration
Figure 15 shows results of using d-q theory with STF to filter out current harmonics from Id and Iq. The IHD in this case is 31.3%. A zero order hold has been introduced in both d and q legs of the STF.
5.6 Stationary Frame Method (d-q Method)- Using STF And Low Pass Filter in Feed Forward Configuration
Figure 16 shows the result of using STF to filter out grid voltages before phase locked loop (PLL) block. This produces a more accurate reference of angular position for the d-q frame. LPF has been used in feed forward configuration to separate harmonic currents in d-q.
This method gives 0.0% IHD in the resultant waveform
5.7 Effect of Zero Sequence Current
Most current estimation techniques have been developed to work with three phase three wire systems where zero sequence current is non-existent. Leaving out zero sequence current in the scenarios being analysed result in poor performance.
Table 5: Comparison of results: different current estimation techniques used
Current###Filtering###Filtering###% IHD of
Method###used in###used in###Current
p-q theory###none###Low Pass###91.8
###filter in feed
p-q theory###none###Low Pass###142.1
(distorted###filter in feed
d-q theory###Low pass###Low Pass###0.01
d-q theory###STF###Low pass###0.00
Table-5 summarizes the results presented for harmonic compensation. Waveforms being analyzed are a mathematical sum of harmonic compensation current and the initial distorted waveforms. This is an ideal experiment where the inverter switching and carrier frequency effects are not accounted for. Thus %IHD here represents the degree of effectiveness of each current estimation method employed.
The STF block filters out voltage harmonic distortion effectively. However, in case of current waveform where IHD is much greater it doesn't produce appreciable results. Incorporating a Zero order hold in both d-q legs of STF improves overall performance of the current estimation scheme
The IHD of current waveform is greater for current drawn by CFLs compared to that of Thyristor bridge rectifiers. IHD further increases for a distorted mains voltage.
For a given IHD of distorted current, different current estimation schemes produce different results depending on what method is used to separate harmonic component from the fundamental. In all circumstances, pre-processing of mains voltage to extract fundamental voltage reference improves harmonic current estimation. Excluding zero sequence currents from the current estimation process causes the scheme to perform poorly and IHD in resulting waveform is appreciably large Comparing all six scenarios discussed STF based filtering for distorted voltages and LPF filtering for distorted currents give the best results in combination.
Authors acknowledge the support by Philips Pakistan Pvt.Ltd. in this research project.
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|Date:||Apr 30, 2015|
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