IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 29, NO. 5, OCTOBER 2014 2055
A Proportional-Resonant Current Controllerfor Selective Harmonic Compensation in a
Hybrid Active Power FilterLeopold Herman, Student Member, IEEE, Igor Papic, Senior Member, IEEE, and Bostjan Blazic, Member, IEEE
Abstract—This paper deals with reactive power compensationand harmonics elimination in medium-voltage industrial networksusing a hybrid active power filter. It proposes a hybrid filter as acombination of a three-phase, two-level, voltage-source converterconnected in parallel with the inductor of a shunt, single-tuned,passive filter. This topological structure greatly decreases thevoltage and current stress over the elements of the active filter.Since the topology is composed of a single-tuned branch, thecontrol algorithm also has to ensure sufficient filtering at otherharmonic frequencies. We propose using a proportional-resonant,multiloop controller. Since the controller is implemented in asynchronous-reference frame, it allows us to use half the numberof resonators, compared with the solution using proportional-in-tegral controllers in the harmonic-reference frame. Theoreticalanalyses and simulation results obtained from an actual industrialnetwork model in PSCAD verify the viability and effectivenessof the proposed hybrid filter. In addition, the simulation resultsare validated by a comparison with the results obtained from areal-time digital simulator.
Index Terms—Harmonic distortion, hybrid filters, proportional-resonant controller, reactive power.
I. INTRODUCTION
N ONLINEAR loads, which, these days, form a large por-tion of the overall electrical load, are known to be a major
source of current harmonics in the electrical system. In addition,most of these loads impose varying reactive-power demandsthat have to be compensated in order to improve the powerfactor (PF) and efficiently deliver the active power to the loads.This results in harmonic distortion-related problems, reducingthe quality of the electrical power and the performance of thepower system. The operation of these devices may, therefore,prove to be very problematic [1].Traditional solutions to reduce the harmonic current flows
into the supply system and to improve the power factor at thecustomer-utility point of common coupling (PCC) involve theplacement of resonant-tuned passive filters at the PCC of the
Manuscript received August 23, 2012; revised February 19, 2014; acceptedJuly 03, 2014. Date of publication August 19, 2014; date of current versionSeptember 19, 2014. This work was supported by the Slovenian ResearchAgency under the Electrical Power Systems Research Program no. P2-356.Paper no. TPWRD-00887-2012.The authors are with the Faculty of Electrical Engineering, University of
load. These filters represent a well-established technology; how-ever, in addition to their fundamental task of providing reactive-power compensation and harmonics filtering, they may causeunwanted resonance conditions. Their other limitation is an in-ability to adapt to the changing conditions in the network andtheir size [2]–[4].With the development of power electronics, active filters are
becoming increasingly important, because they do not cause res-onance with the system, and they also enable rapid dynamic re-sponses to changing conditions. The central part of these devicesis the power converter, which is controlled in such away in orderto improve selected power-quality (PQ) parameters. The maindisadvantage of active filters is their high investment and oper-ating costs [5]–[7].To overcome the aforementioned disadvantages, passive and
active filters can be combined into a single device. This enablesus to minimize the required power ratings of the active filter(reducing the overall price) and to dampen the harmonic reso-nances caused by the passive part. These devices are referred toas hybrid active power filters (HAPFs) [8]. Various hybrid-filtertopologies have been proposed for reactive-power compensa-tion and harmonic-current filtering. The most common variantsare when the active filter is added in series to the passive filter(series HAPF) [9]–[12] and a parallel configuration of the pas-sive and active parts [13]–[15]. In the first case, the requiredvoltage rating of the voltage-source converter (VSC) is small,since most of the voltage drop occurs on the capacitor; however,the active filter may conduct a fully rated fundamental current ifno coupling transformer is used. In the second case, the VSC hasa lower current rating, but it has to be designed for the nominalvoltage. An extensive overview of other topological structurescan be found in [16]–[19].The hybrid filter investigated in this paper is composed of
a three-phase, two-level, VSC connected in parallel with a pas-sive filter inductor (Fig. 1). This topology has not received muchattention since it was first reported in [20], where the authors re-ported poor dynamical behavior of the active part of the filter.This paper proposes an improved version of the topologicalstructure and using a proportional resonant current controllerimplemented in the SRF. The main advantage of this structure isthat the voltage drop on the capacitor reduces the VSC voltageratings, while the inductor conducts the fundamental reactivecurrent. In this paper, the rating requirements of this topologyare analyzed and compared with the series HAPF topology andthe pure active filter. As will be shown, the required power
2056 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 29, NO. 5, OCTOBER 2014
Fig. 1. Scheme of the proposed shunt-connected HAPF.
rating of the active part is very low, typically only 1%–2% ofthe load rating.Since the topology investigated in this paper is composed of
a single-tuned branch, the control structure also has to ensuresufficient filtering of the current harmonics at other harmonicfrequencies. Therefore, designing the controller is an importantand challenging task, due to its impact on the performance andstability of the overall system. The control principles previouslypresented for different HAPF structures are mainly wideband,where the proportional (P) control law is the most common andis usually implemented in the SRF [6], [9], [14]. Due to thehigh value of the proportional constant required for sufficientfiltering, the proportional control structure does not performwell with the topology investigated herein. Namely, it showspoor transient performance [22]. The poor dynamic behavior ofthis topology in the case of a fluctuating load has also been re-ported in [20], where a proportional-integral (PI) regulator im-plemented in the HRF was used to control the active part of theHAPF.To enhance the transient performance of the HAPF, this
paper proposes a proportional-resonant current controllerimplemented in the SRF. Resonant controllers have taken onsignificant importance in recent years due to their high selec-tivity and good performance [23]–[29]. They are equivalent tothe conventional PI controllers implemented in the HRF for thepositive- and negative-sequence reference frames. Thus, theycan achieve similar steady-state performance to PI controllers,with the following advantages:• only one regulator is needed for compensating both har-monics at ;
• if implemented in the HRF, it allows half the number of res-onators, compared with the solution using PI in the HRF;
Fig. 2. Simplified equivalent circuit of the HAPF.
This paper is organized as follows. The system configurationand rating analysis are presented in Section II. In Section III, thecontrol system is developed. The simulated network is describedin Section IV and the proposed HAPF performance is evaluatedin Section V. Simulation results are validated in Section VI.Finally, conclusions are drawn in Section VII.
II. SYSTEM CONFIGURATION
Fig. 1 shows the proposed circuit configuration. In thissystem, a nonlinear load is supplied by a balanced voltagesource and compensated by the proposed HAPF. A ripplefilter is used to reduce the high-frequency harmonic currentsinjected into the network.In Fig. 2, a simplified, equivalent circuit of the proposed topo-
logical structure is shown. The symbols used are as follows:supply voltage, and are the supply-system resistance andinductance (short-circuit impedance and transformer impedanceconnected in series), and are the load resistance and in-ductance, is the passive filter capacitance, and arethe passive filter resistance and inductance, and , and
are the ripple filter inductances and capacitance. The ac-tive part of the HAPF is presented with an ideal voltage source,while the nonlinear load is considered to be a current source .
A. Rating Analysis
The active filter has to provide a small component of funda-mental voltage at the PCC in order to divert the fundamentalreactive current to flow in the inductor. This voltage is equal tothe voltage drop on the inductor for a pure passive filter and itdepends on the tuned frequency of the passive part. It can beexpressed as
(1)
Here, is the fundamental frequency (i.e., 50 Hz) and is thetuned frequency of the passive filter.The harmonic voltage across the active part of the hybrid
filter consists of two components: the component due to the dis-torted supply voltage and the component due to the har-monic load current flowing in the passive impedance. Thisvoltage is given by
(2)
To obtain the worst case, no load impedance was consideredwhen deriving (2), and the coupling impedance of the active
HERMAN et al.: PROPORTIONAL-RESONANT CURRENT CONTROLLER FOR SELECTIVE HARMONIC COMPENSATION 2057
TABLE IEXAMPLE CASE PARAMETERS
Fig. 3. Active filter rated power versus tuned frequency of the passive part.
filter was neglected and the harmonic current flowing in thesystem was considered to be zero (ideal filtering).The current flowing in the active part also consists of two
parts: the harmonic component due to the distorted supplyvoltage and the distorted load current. It is given by
(3)
The power-rating requirement of the active part is finally givenby
(4)
To better illustrate the power-rating requirements of the in-vestigated HAPF’s topological structure, let us consider the (hy-pothetical) example case in Table I. For this case, the requiredpower of the active part is given by
(5)
As is clear from Fig. 3, the power rating strongly depends onthe tuned frequency of the passive filter. The hybrid active filterof the proposed topology is thus the most suitable for tuned filterbranches, where the tuned frequency is as close as possible tothe filtered harmonic.For the same load conditions, the rating of the active part for
the series HAPF is equal to 0.0251 p.u., while the required ratingof the pure active filter for this case is 0.25 p.u. (which doesnot include the reactive power compensation). An extensiveoverview of the power-rating requirements for most commoncurrent-sink HAPF topologies can be found in [30].
III. CONTROL SYSTEM DESIGN
Proportional-resonant (PR) controllers are equivalent to con-ventional PI controllers implemented in the -reference frame,separately for the positive and negative sequences. Therefore,the PR controller is capable of simultaneously tracking the ref-erence for the positive and negative sequence with zero steady-state error. For example, a sixth harmonic PR compensator iseffective for the fifth and seventh harmonics of both sequences;hence, four harmonics are filtered with one PR filter imple-mented in the SRF [23].
A. PR Controller Transfer Function
The relationship between the -components and the-components is given by an anticlockwise rotating Park’s
vector
(6)
(7)
Thus, the influence of the Park transformation can be expressedas the frequency shift of all the frequencies in the frequencydomain. The equivalent transfer function of the PR controller isin the SRF. can be derived from a PI controller imple-mented in positive- and negative-sequence HRFs, taking intoaccount (6) and (7)
(8)
(9)
For the nonideal integrators of , the PRcontroller transfer function takes the form
(10)
where is the cutoff frequency, representing the limits ofthe integrator. In this paper, several HPR controllers are addedin a cascade to control several harmonics simultaneously. Thecurrent controller takes its final form
(11)
Equation (9) describes an ideal PR controller with infinite gainat the tuned frequency and no phase shift and gain at the otherfrequencies. The disadvantage of such a controller in practicalapplications is the possible stability problem associated with in-finite gain, which can be avoided by using nonideal integrators:(10). Another feature is the very narrow bandwidth of the idealPR controllers, whichmakes them highly sensitive toward slightfrequency variation in a typical power system [23].
B. Control Algorithm
Fig. 4 shows the proposed control scheme, which includesharmonic detection (Fig. 5), the PR-current regulation (11), dc
2058 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 29, NO. 5, OCTOBER 2014
Fig. 4. Control block diagram of the HAPF.
Fig. 5. Block diagram of the harmonic current detection.
Fig. 6. DC bus voltage control.
voltage control (Fig. 6), and the fundamental current diversioncontroller (Fig. 7). The PR controller is closed with the mea-sured and filtered values from the actual system. Since the objec-tive is to eliminate the harmonics from the supply current, thiscurrent represents the PR controller input. At first, the funda-mental-frequency component has to be extracted from the mea-sured current. This is done by using first-order, high-pass filtersin the fundamental frequency and synchronous reference framewith a cutoff frequency of 20 Hz. Fundamental angular fre-quency is obtained using a conventional qPLL system [31]. Dueto the fact that the supply current may also contain, in the case ofunsymmetrical line conditions, some fundamental negative-se-quence component, this component is suppressed by the secondpart of the harmonic detection unit, where Park’s transformationis performed with a clockwise synchronous rotation. As a result,the output of the harmonic detection unit is the supply-currentharmonics which, in the next step, are compared to the referencevalues. The difference is then applied to a PR controller tunedon the resonance frequencies of , , and . Theoutput of the controller is the variable , which is addedto the voltage reference .
Fig. 7. Feedforward control for the fundamental current diversion.
C. DC Bus Voltage Control
Proper control of the dc bus voltage is essential for the op-eration of this HAPF. The principle of controlling the dc busvoltage is based on active power control, that is, charging thedc capacitor with active power will increase the voltage whilereleasing a certain amount of active power, will decrease it. Ac-cording to -theory, a dc component in the -coordinates cor-responds to the active power and, thus, dc bus voltage control isimplemented in the SRF.As can be seen from Fig. 6, the difference between the ref-
erence value and the measured and filtered actual value is ap-plied to a PI controller, which adjusts the direct axis current (thequadrature axis is set to zero). A low-pass filter (LPF) with acutoff frequency of 15 Hz eliminates the harmonics from themeasured dc bus voltage. The resulting control signal is addedto the voltage reference .
D. Fundamental Current Diversion
In order to achieve the minimum current rating of the ac-tive part, the fundamental-frequency filter current needs to bediverted into a parallel inductance. This is done with a simplefeedforward controller, represented in Fig. 7. It calculates thevoltage appearing across the passive filter inductor , whichwould occur in the absence of the active filter, using (1). As aresult, only a small fundamental frequency current is flowingthrough the active elements, which is required for charging thedc capacitor.
E. Current Control Transfer Function
Fig. 8 shows the main current control block diagram of thehybrid filter. The harmonic content of the system current is fil-
HERMAN et al.: PROPORTIONAL-RESONANT CURRENT CONTROLLER FOR SELECTIVE HARMONIC COMPENSATION 2059
Fig. 8. Main current-control block diagram.
tered from the measured system current. The transfer functionfor the harmonic-detecting circuit can be expressed as
(12)
where is the cutoff angular frequency of the HPFs that ex-tract the dc component in the –coordinates, and is the fun-damental angular frequency. The detected harmonic currentis compared with the current reference , and the differencerepresents the input to the PR controller. This results in the
production of the reference voltage to be generated bythe inverter.In the real control circuit [implemented on a digital signal
processor (DSP)], the output signal is inherently delayed withrespect to the input signal. The time delay is represented as
(13)
where 100 s is the sampling period. The plant transferfunction is defined as (see Fig. 2)
(14)
Finally, the open-loop transfer function of the hybrid filter con-troller is given by
(15)
F. Filtering Characteristic
The filtering characteristic of the HAPF depends primarilyon the control algorithm; however, all of the algorithms havesomething in common. It is well known that power-electronicconverters in harmonic filtering applications may be controlledto behave in a similar fashion to a passive element [21], [32]. Aswill be shown, the PR-controlledHAPF proposed hereinmimicsseveral parallel resonant circuits added in parallel and tuned tothe characteristic harmonic frequencies.The filtering characteristic can be obtained by calculating the
equivalent model of the network (Fig. 2). Applying Kirchhoff’scircuit laws yields
(16)
Fig. 9. Equivalent circuit for the filtering characteristic analysis.
Taking into account (11) and (14) and neglecting the resis-tance of the passive filter inductor, we obtain
(17)
Letting:
(18)
we obtain
(19)Equation (19) defines the filtering characteristic of the HAPF,which depends on the passive filter inductor and capacitorequivalent impedances and , the system impedance, and the active power filter transfer function given by (11).
As can be seen from Fig. 9, representing a single-phase equiv-alent circuit of the system with a connected HAPF, the activefilter behaves as a pure resistor , with several parallel
circuits added in series.
IV. NETWORK UNDER STUDY
To illustrate some practical implications of the proposedPR-controlled HAPF and to evaluate its filtering performance,the operation is demonstrated on a real industrial networkmodel. A simplified scheme of the modelled system is shown inFig. 10. This system was chosen because it represents a typicalexample of an industrial network with a poorly designed pas-sive compensator, producing unwanted resonant amplificationsof the current harmonics. Since the proposed HAPF topologyenables retrofitting applications, the existing reactive powerfilter is upgraded with the active part. As will be shown, theactive part damps the resonances and ensures sufficient filteringof the characteristic harmonics.
2060 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 29, NO. 5, OCTOBER 2014
Fig. 10. Simplified single-line diagram of a real industrial network.
A. System Data
The network parameters are given in Table II. A 35-kV dis-tribution network is powered through a 110-kV transmissionsystem, which is represented by a stiff voltage source with ashort-circuit power rating of 3750MVA. The passive filter ratedat 5.4 MVAr is connected on the secondary side of the trans-former TR I (35-kV voltage level). Two large, adjustable-speed,thyristor-controlled, motor drives—DCM I and DCM II (ratedat 2.5 MW and 2.15 MW)—are also connected to the network.These two motor drives are the main source of the harmonicdistortion in the network. The linear load is modeled with theimpedance .
B. Active Filter Parameters
The parameters of the active part are given in Table III. Therated power of the active filter is 75 kVA, which is approxi-mately 1.4% of the passive part’s rating. The ripple filter blockin Fig. 1 is used to reduce the high-frequency harmonic currents(generated by the PWM inverter) injected into the network. Itsresonant frequency is approximately 2600 Hz.The dc side of the VSC is only built with a capacitor. To keep
the voltage ripple as low as possible, the capacitance is set to1200 F. The dc bus voltage is controlled to a value of 4 kV bya PI controller with 2 and 0.5 that adjusts thereference active current. The firing pulses for driving the semi-conductor switches [insulated-gate bipolar transistors (IGBTs)]are generated using PWM, with a 10-kHz carrier signal.
C. Tuning of the Main Current Control Loop
The parameters of the PR controller are most often tuned bymeans of Bode diagrams [27]–[29]. Fig. 11 shows the Bodediagrams of the open-loop transfer function (15) for positiveand negative frequencies on a linear scale. The diagrams areobtained with controllers tuned at harmonics
, with 0, 100, 250, and 450
TABLE IIINDUSTRIAL NETWORK PARAMETERS
TABLE IIIACTIVE FILTER PARAMETERS
and 0.5, 2, and 15, respectively. Since the controller isimplemented in the SRF, only one regulator is needed for com-pensating both harmonics at 1, 2, 3. To observethe effect of the harmonic detection transfer function, the har-monic detection transfer function is included in the calculationof the open-loop transfer function.As can be seen, the proportional constant defines the
crossover frequency at which the magnitude is 0 dB. Inother words, it defines the bandwidth of the filter. To ensurethe stability, has to be high enough so that all of the fil-tered harmonics are lower than the .When the resonant termsare added, the overall frequency response is modified only inthe vicinity of each tuned harmonic frequency and, thus, theirimpact on the stability can be neglected. The Bode character-istic depicted in black, is obtained with 15 and450, which ensures stable operation of the overall system witha phase margin of 76 and provides an adequate tradeoff be-tween the bandwidth, the transient performance, the selectivity,and the stability.
HERMAN et al.: PROPORTIONAL-RESONANT CURRENT CONTROLLER FOR SELECTIVE HARMONIC COMPENSATION 2061
Fig. 11. Open-loop Bode diagram of the main current control loop.
Fig. 12. Frequency characteristics of harmonic propagation/damping.
V. HYBRID FILTER PERFORMANCE EVALUATION
The HAPF filtering characteristic is analyzed using (18). Thesteady- and transient-state performances are demonstrated inPSCAD software.
A. Filtering Performance Analysis
Fig. 12 shows the relationship between the and for dif-ferent values of and . When 0, the HAPFbehaves as a pure passive filter tuned to 314 Hz. It creates aparallel resonance point very close to the 5th harmonic com-ponent with the ratio reaching high values of more than5 dB. This may result in overheating and a shorter lifespan forcertain equipment (transformers, cables, filters), the occurrenceof noise and vibrations (motors, generators), the incorrect op-eration of certain devices (computers, printers), and equipmentoutages or destruction. In the past few years, several cases werereported by this particular customer, related to the problem ofharmonic resonance.To overcome this problem, the active part of the HAPF needs
to dampen the parallel resonance. As can be seen from Fig. 12
Fig. 13. Simulated waveforms in the steady state—passive filter.
and follows from (19), increasing the proportional control partcauses the active filter to act as an additional fictitious re-
sistance added in series to the system impedance thatincreases the damping performance of thefilter. The parallel res-onance gets completely damped for values of . It shouldalso be noted that is null at the fundamental frequency and,thus, no additional losses occur at this frequency due to the op-eration of the active filter.On the other hand, by increasing the integral control part, high equivalent system impedances at the selected har-
monic frequencies are created. If is high enough, the systemimpedance is much higher than the filter impedance, which di-verts almost all of the harmonic currents injected by the non-linear load into the filter branch. This can be seen in Fig. 12 as alow (negative) gain of the ratio at , 1, 2,3. Thus, it can be assumed that the HAPF will show very goodfiltering performance for these harmonic currents produced bythe load. The filtering performance will be further evaluated.
B. Steady-State Performance Evaluation
In this subsection, the results of computer simulations usingthe PSCAD software package are shown. The HAPF results arecompared to the results obtained with a pure passive filter. Dueto the transparency, only the waveforms for one phase (L1) areshown.Fig. 13 shows simulated waveforms of the passive filter
(voltage at the filter PCC , supply current and loadcurrent ) in the steady state. The harmonic content of the
and in terms of the percentage of the fundamentalcomponent is given in Table IV. It is clear from the waveformsthat the 5th harmonic is particularly problematic. It reachesa value of more than 15% of the filter’s fundamental current,while the 7th harmonic reaches only 0.79%. Consequently, thevoltage at the filter PCC is also highly distorted withthe 5th harmonic. It reaches a value of 4.5%. These results wereexpected, as the frequency-response characteristics showed thatthere is a parallel resonance point close to the 5th harmonic.Fig. 14 shows the simulated waveforms of the HAPF under
the same conditions as Fig. 13. After starting the active filter,the voltage and current distortions decrease significantly. Bothwaveforms are nearly sinusoidal. The supply current has theTHD reduced to 1.47%. The 5th and 7th harmonic componentsare very small, that is, 1.39% and 0.35%. The PCC voltage THD
2062 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 29, NO. 5, OCTOBER 2014
TABLE IVHARMONIC CONTENT OF VOLTAGE AT THE PCC AND SUPPLY CURRENT IN
PERCENT OF THE FUNDAMENTAL COMPONENT
Fig. 14. Simulated waveforms in the steady-state—HAPF.
is now only 0.52%. It is also important to note that there is avery low fundamental frequency component in the active filtercurrent in the steady state.
C. Transient State Performance Evaluation
Fig. 15 shows how the hybrid filter behaves during startup. Atfirst, only the passive filter is operating, and after 50 ms of sim-ulation, the active filter starts. The system needs approximately150 ms to reach the steady state. After that, the supply currentand the PCC voltage waveforms become almost sinusoidal.Fig. 16 shows the HAPF active and reactive power outputs. It
is clear that the active filter does not affect the generation of thereactive power. It even slightly increases after putting the activepart in the operation.Fig. 17 shows simulated waveforms of the hybrid filter for a
step load decrease/increase of 50%. The supply current is dis-torted for approximately half a cycle after the occurrence of theload change and becomes almost a sinusoidal waveform witha THD of 1.47%. The load change does not produce any otherunwanted effect (e.g., unstable operation) and, thus, it can be
Fig. 15. Simulated waveforms in the transient state—HAPF startup.
Fig. 16. Simulated waveforms in the transient state—active filter and reactivepower output during HAPF startup.
Fig. 17. Simulated waveforms in the transient state–50% step load decrease/increase.
concluded that the current control loop as well as the dc-sidecontrol loop work properly and stably during load variations.
VI. RTDS RESULTS
In this section, the PSCAD simulation results are validatedby comparing them with the results obtained from the real-time
HERMAN et al.: PROPORTIONAL-RESONANT CURRENT CONTROLLER FOR SELECTIVE HARMONIC COMPENSATION 2063
Fig. 18. Schematic overview of the testing setup.
Fig. 19. RTDS waveforms in the steady state—HAPF.
digital simulator (RTDS), which is one of the simulators thatmakes real-time calculation of power-system electromagneticphenomena possible. The achieved calculation time stepsare about 50 s for the modelling of power-electronics ele-ments such as IGBT converters, even as low as 1.5 s [33].Special hardware also makes it possible for the importing andexporting of signals to external devices, which is a basis forthe closed-loop testing of external equipment (e.g., DSP) with apower system model. In this way, the RTDS user has the possi-bility to analyze the external device itself as well as its impacton the rest of the modelled system. Therefore, we can considerthe model within the RTDS simulator to be a replacement of areal system [34].A schematic overview of the testing setup is presented in
Fig. 18. Within the RTDS simulator, a power system modelwith the proposed HAPF (Fig. 1) has been created, wherethe output signals correspond to the following: three-phasevoltages at the filter PCC , three single-phase systemcurrents , and the voltage at the dc side of the inverter
TABLE VHARMONIC CONTENT OF VOLTAGE AT THE PCC AND SUPPLY CURRENT IN
PERCENT OF THE FUNDAMENTAL COMPONENT
Fig. 20. RTDS waveforms in the transient state—50% step load increase.
. By determining the analog output card (GTAO) ratio, theoutput from the RTDS simulator is in the form of seven voltagesignals . These signals are fed into theTexas Instruments TMS320F28335 hardware platform (32-bfloating-point, 150 MHz) as the control system that converts[analog-to-digital converter (ADC)] and amplifies these signalsand produces voltages and currents that correspond to thosein the model. The proposed control algorithm (Fig. 4) is im-plemented in the C language, using the TMS320C2000 CodeCompose Studio as a development environment and a pre-warped bilinear (Tustin) transform as a digitization technique.The hardware produces six firing pulses that are led back to theRTDS simulator via the digital input card (GTDI).
A. Steady-State Performance Evaluation
Fig. 19 shows the RTDS results of the HAPF operation underthe same conditions as Fig. 14. As can be seen, the system cur-rent and the PCC voltage waveforms are nearly sinusoidal. Thesupply current has the THD reduced to 2.01%, while the PCCvoltage THD is only 1%. The harmonic content of the and
2064 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 29, NO. 5, OCTOBER 2014
in terms of the percentage of the fundamental component isgiven in Table V.
B. Transient State Performance Evaluation
Fig. 20 shows simulated waveforms of the hybrid filter for astep load change from 50% to 100%. The supply current isdistorted for less than three fundamental cycles and after that,the benefits of the HAPF are clearly seen. The load change doesnot produce any other unwanted effect.A comparison of the results in Figs. 14–17 and Figs. 19 and
20 shows very good matching, which validates the simulationresults obtained with PSCAD software.
VII. CONCLUSION
In this paper, a hybrid active power filter for reactive powercompensation and harmonics filtering has been presented. It iscomposed of a small-rating VSC connected in parallel with theinductor of a shunt single-tuned passive filter. Since the ratedpower of the active filter is relatively low, the HAPF representsa viable solution for reactive power compensation and harmonicfiltering.A PR current control scheme for selective harmonics com-
pensation with the HAPF has been proposed. As shown, eachcontroller acts as a resonant filter tuned to a certain harmonicfrequency. The proper selection of the parameters ensures highselectivity and improves the transient performance of theHAPF.Another key feature is that each pair of harmonics ,
is filtered by one controller and, thus, importantsavings in terms of computational burden are achieved.Theoretical analysis, along with the simulation results, ob-
tained from a real industrial network model, verifies the effec-tiveness of the proposed hybrid filter, which represents an ex-cellent solution for reactive power compensation and harmonicfiltering.
REFERENCES[1] R. C. Dougan and H. W. Beaty, Electrical Power Systems Quality.
New York, USA: McGraw-Hill, 2002.[2] J. C. Das, “Passive filters—potentialities and limitations,” IEEE Trans.
Ind. Appl., vol. 40, no. 1, pp. 232–241, Jan./Feb. 2004.[3] M. H. Shwehdi and M. R. Sultan, “Power factor correction Capaci-
tors; essentials and cautions,” in Proc. IEEE Power Eng. Soc. SummerMeeting, Jul. 2000, vol. 3, pp. 1317–1322.
[4] D. F. Pires, C. H. Antunes, and A. G. Martins, “Passive and activeanti-resonance capacitor systems for power factor correction,” in Proc.Int. Power Electron. Motion Control Conf., 2006, pp. 1460–1465.
[5] H. Akagi, “Active harmonic filters,” Proc. IEEE, vol. 93, no. 12, pp.2128–2141, Dec. 2005.
[6] H. Akagi, “Trends in active power line conditioners,” IEEE Trans.Power Electron., vol. 9, no. 3, pp. 263–268, May 1994.
[7] B. Blazic and I. Papic, “Improved D-StatCom control for operationwith unbalanced currents and voltages,” IEEE Trans. Power Del., vol.21, no. 1, pp. 225–233, Jan. 2006.
[8] H. Akagi, S. Srianthumrong, and Y. Tamai, “Comparisons in circuitconfiguration and filtering performance between hybrid and pure shuntactive filters,” in Proc. IEEE 38th Ind. Appl. Conf., Oct. 12–16, 2003,vol. 3, pp. 1195–1202.
[9] H. Fujita and H. Akagi, “A practical approach to harmonic compensa-tion in power systems series connection of passive and active filters,”IEEE Trans. Ind. Appl., vol. 27, no. 6, pp. 1020–1025, Nov./Dec. 1991.
[10] S. Bhattacharya, P.-T. Cheng, and D. M. Divan, “Hybrid solutionsfor improving passive filter performance in high power applications,”IEEE Trans. Ind. Appl., vol. 33, no. 3, pp. 732–747, May/Jun. 1997.
[11] B. Singh and V. Verma, “An indirect current control of hybrid powerfilter for varying loads,” IEEE Trans. Power Del., vol. 21, no. 1, pp.178–184, Jan. 2006.
[12] R. Inzunza and H. Akagi, “A 6.6-kV transformerless shunt hybrid ac-tive filter for installation on a power distribution system,” IEEE Trans.Power Electron., vol. 20, no. 4, pp. 893–900, Jul. 2005.
[13] F. Z. Peng, H. Akagi, and A. Nabae, “A new approach to harmoniccompensation in power systems-A combined system of shunt passiveand series active filters,” IEEE Trans. Ind. Appl., vol. 26, no. 6, pp.983–990, Nov./Dec. 1990.
[14] Z. Chen, F. Blaabjerg, and J. K. Pedersen, “Hybrid compensation ar-rangement in dispersed generation systems,” IEEE Trans. Power Del.,vol. 20, no. 2, pt. 2, pp. 1719–1727, Apr. 2005.
[15] V. F. Corasaniti, M. B. Barbieri, P. L. Arnera, and M. I. Valla, “Hybridactive filter for reactive and harmonics compensation in a distributionnetwork,” IEEE Trans. Ind. Electron., vol. 56, no. 3, pp. 670–677, Mar.2009.
[16] L. Chen, Y. Xie, and Z. Zhang, “Comparison of hybrid active powerfilter topologies and principles,” in Proc. Int. Conf. Elect. Mach. Syst.,Oct. 17–20, 2008, pp. 2030–2035.
[17] Z. Shuai, A. Luo, W. Zhu, R. Fan, and K. Zhou, “Study on a novelhybrid active power filter applied to a high-voltage grid,” IEEE Trans.Power Del., vol. 24, no. 4, pp. 2344–2352, Oct. 2009.
[18] A. Luo, C. Tang, Z. K. Shuai, W. Zhao, F. Rong, and K. Zhou, “A novelthree-phase hybrid active power filter with a series resonance circuittuned at the fundamental frequency,” IEEE Trans. Ind. Electron., vol.56, no. 7, pp. 2431–2440, Jul. 2009.
[19] P. Jintakosonwit, S. Srianthumrong, and P. Jintagosonwit, “Implemen-tation and performance of an anti-resonance hybrid delta-connectedcapacitor bank for power factor correction,” IEEE Trans. Power Elec-tron., vol. 22, no. 6, pp. 2543–2551, Nov. 2007.
[20] J. Hafner, M. Aredes, and K. Heumann, “A shunt active power filterapplied to high voltage distribution lines,” IEEE Trans. Power Del.,vol. 12, no. 1, pp. 266–272, Jan. 1997.
[21] S. Senini and P. J. Wolfs, “Hybrid active filter for harmonically un-balanced three phase three wire railway traction loads,” IEEE Trans.Power Electron., vol. 15, no. 4, pp. 702–710, Jul. 2000.
[22] J. H. Sung, S. Park, and K. Nam, “New hybrid parallel active filterconfiguration minimizing active filter size,” in Proc. Inst. Elect. Eng.,Elect. Power Appl., Mar. 2006, p. 93.
[23] R. Teodorescu, F. Blaabjerg, M. Liserre, and P. C. Loh, “Proportional-resonant controllers and filters for grid-connected voltage-source con-verters,” Proc. Inst. Elect. Eng., Elect. Power Appl., vol. 153, no. 5,Sep. 2006.
[24] C. Lascu, L. Asiminoaei, I. Boldea, and F. Blaabjerg, “High perfor-mance current controller for selective harmonic compensation in ac-tive power filters,” IEEE Trans. Power Electron., vol. 22, no. 5, pp.1826–1835, Sep. 2007.
[25] R. I. Bojoi, G. Griva, V. Bostan, M. Guerriero, F. Farina, and F.Profumo, “Current control strategy for power conditioners usingsinusoidal signal integrators in synchronous reference frame,” IEEETrans. Power Electron., vol. 20, no. 6, pp. 1402–1412, Nov. 2005.
[26] M. Liserre, R. Teodorescu, and F. Blaabjerg, “Multiple harmonics con-trol for three-phase grid converter systems with the use of PI-RES cur-rent controller in a rotating frame,” IEEE Trans. Power Electron., vol.21, no. 3, pp. 836–841, May 2006.
[27] R. Bojoi, L. Limongi, D. Roiu, and A. Tenconi, “Frequency-domainanalysis of resonant current controllers for active power conditioners,”in Proc. IEEE 34th Annu. IECON, Nov. 2008, pp. 3141–3148.
[28] A. G. Yepes, F. D. Freijedo, J. Doval-Gandoy, O. Lopez, J. Malvar,and P. Fernandez-Comesaña, “Effects of discretization methods on theperformance of resonant controllers,” IEEE Trans. Power Electron.,vol. 25, no. 7, pp. 1692–1712, Jul. 2010.
[29] J. Miret, M. Castilla, J. Matas, J. Guerrero, and J. Vasquez, “Selectiveharmonic-compensation control for single-phase active power filterwith high harmonic rejection,” IEEE Trans. Ind. Electron., vol. 56, no.8, pp. 3117–3127, Aug. 2009.
[30] S. Senini and P. J. Wolfs, “Analysis and comparison of new and ex-isting hybrid filter topologies for current harmonic removal,” in Proc.Australasian Univ. Power Eng. Conf., Sep. 27–30, 1999, pp. 227–232.
[31] L. Feola, R. Langella, and A. Testa, “On the behavior of three-phaseinverters in the new smart grid context,” in Proc. 2nd IEEE EnergyconConf. Exhibit., 2012, pp. 521–526.
[32] H. Funato, A. Kawamura, and K. Kamiyama, “Realisation of negativeinductance using variable active-passive reactance (VAPAR),” IEEETrans. Power Electron., vol. 12, no. 4, pp. 589–596, Jul. 1997.
HERMAN et al.: PROPORTIONAL-RESONANT CURRENT CONTROLLER FOR SELECTIVE HARMONIC COMPENSATION 2065
[33] “Real-Time Digital Simulation for the Power Industry—Manual Set,”ver. RSCAD 2.024.2, RTDS Technologies, Winnipeg, MB, Canada.
[34] U. Rudez, P. Osredkar, and R. Mihalic, “Overcurrent protection relaytesting with Real Time Digital Simulator hardware,” Electrotech. Rev.,vol. 79, no. 1, 2012.
Leopold Herman (S’06) was born in Trbovlje, Slovenia, on April 16, 1984. Hegraduated from the University of Ljubljana, Faculty of Electrical Engineeringin 2008.Currently, he is a Researcher at the Faculty of Electrical Engineering, Uni-
versity of Ljubljana, Ljubljana, Slovenia. His research interests include power-quality and power system simulations.
Igor Papic (S’97–M’00–SM’06) received the B.Sc., M.Sc., and Ph.D. de-grees in electrical engineering from the Faculty of Electrical Engineering ofthe University of Ljubljana, Ljubljana, Slovenia, in 1992, 1995, and 1998,respectively.Currently, he is a Professor at the University of Ljubljana. From 1994 to
1996, he was with Siemens Power Transmission and Distribution Group,Erlangen, Germany. In 2001, he was a Visiting Professor at the Universityof Manitoba, Winnipeg, MB, Canada. His research interests include powerconditioners, flexible ac transmission systems devices, power quality, andactive distribution networks.
Bostjan Blazic (S’02–M’06) received the B.Sc, M.Sc., and Ph.D. degrees inelectrical engineering, from the University of Ljubljana, Slovenia, in 2000, 2003and 2005, respectively.Currently, he is an Assistant Professor with the Faculty of Electrical Engi-
neering, University of Ljubljana. His research interests include power quality,smart grids, mathematical analysis, and the control of power converters.
