CC BY
30Journal of Siberian Federal University. Engineering & Technologies, 2018, 11(5), 550-559
y^K 621.300
Harmonics and Neutral Line Current Compensation in Three-Phase Four-Wire Power Systems
Maxim O. Chernyshov, Valery P. Dovgun, Irina G. Vazhenina, Sergei A. Temerbaev and Victor V. Novikov*
Siberian Federal University 79 Svobodny, Krasnoyarsk, 660041, Russia
Received 20.09.2017, received in revised form 07.04.2018, accepted 06.05.2018
Load unbalance and modern office equipment result in a significant neutral current in three-phase four-wire low-voltage power systems. This paper considers a new configuration of hybrid power filter for power quality management in three-phase four-wire low voltage power systems. The proposed hybrid filter is composed of a single-phase power converter and a three-phase passive filter connected in series. The hybrid filter can be operated in both passive and hybrid mode. The compensating performance of the filter is confirmed with computer simulation using MATLAB software. Analysis and simulation proved that the hybrid filter is an effective solution for neutral current mitigation.
Keywords: three-phase four-wire power system, hybrid power filter, neutral line current.
Citation: Chernyshov M.O., Dovgun V.P., Vazhenina I.G., Temerbaev S.A., Novikov V.V. Harmonics and neutral line current compensation in three-phase four-wire power systems, J. Sib. Fed. Univ. Eng. technol., 2018, 11(5), 550-559. DOI: 10.17516/1999-494X-0053.
Компенсация высших гармоник и токов нейтральных проводников в трехфазных четырехпроводных сетях
М.О. Чернышов, В.П. Довгун, И.Г. Важенина, С.А. Темербаев, В.В. Новиков
Сибирский федеральный университет Россия, 660041, Красноярск, пр. Свободный, 79
Современное офисное оборудование и несимметрия нагрузок являются основной причиной увеличения токов нейтральных проводников в трехфазных четырехпроводных сетях низкого
© Siberian Federal University. All rights reserved
* Corresponding author E-mail address: chernyshov.m.o@gmail.com, irina-vazhenina@mail.ru
напряжения. В статье рассмотрена новая конфигурация силового гибридного фильтра для управления качеством электроэнергии в трехфазных четырехпроводных сетях. Предложенный гибридный фильтр образован последовательным соединением трехфазного пассивного фильтра, настроенного на частоту доминирующей третьей гармоники, и однофазного инвертора. Фильтр может работать как в пассивном, так и в гибридном режиме. Компенсационные характеристики предложенного фильтра проанализированы с помощью моделирования в MATLAB. Анализ показал, что предложенный гибридный фильтр является эффективным средством ослабления гармоник и токов нейтральных проводников.
Ключевые слова: трехфазная четырехпроводная сеть, гибридный силовой фильтр, ток нейтрального провода.
Introduction
Three-phase four-wire distribution power systems are widely used in office buildings and commercial complexes to supply single-phase and three-phase loads. These loads include office equipment, computer systems, fluorescent and diode lighting systems, adjustable-speed drivers and have nonlinear characteristics. Nonlinear loads create problems of high current harmonics and excessive neutral current since triplen harmonics are summed in the neutral conductor. Under the worst case, the neutral current could be 1.73 times exceed the phase current [1, 2]. High level of neutral harmonic current results in transformer and conductor overheating, voltage distortions [3]. Singlephase loads may be distributed unequally, which results in serious load unbalance and excessive neutral-line fundamental current. Therefore excessive neutral currents are becoming a significant problem in modern low voltage power distribution systems.
Different neutral current compensation techniques have been proposed [4-8]. A passive neutral current suppressor in the form of zig-zag transformer connected in parallel to the load was considered in [4]. However, the effectiveness of passive neutral-current suppressors depends on the ratio between the impedance of the distribution system and the passive filter. Furthermore the zig-zag transformer provides a low impedance path for zero-sequence components of the utility voltage.
A hybrid neutral-current suppressor consisting a zig-zag transformer connected in parallel with the load and a single-phase active filter which in its turn is connected in series with the neutral conductor was proposed in [5]. However, a converter inserted at the neutral line may cause fluctuations of the voltage between neutral points of the load and the distribution system [8]. The neutral voltage variations may cause improper operation of the sensitive electronic equipment.
Several neutral current compensation systems in the form of three-phase four-wire active power filter were proposed in [5, 6]. But this configurations has the disadvantage of complexity of control and a big number of semiconductor switches. The capacity and cost of three-phase four-wire converters are very high as compared with other types of power filters. This limits wide application of active power filters. Hybrid power filters (HPF) may be more attractive solution for neutral current mitigation.
In this paper the new configuration of the hybrid filter for neutral-line current and voltage mitigation in tree-phase four-wire distribution power systems is considered.
System configuration of hybrid power filter
The configuration of the proposed power filter is shown in Fig. 1. It comprises a three phase passive filter tuned for third harmonic and a single-phase power converter connected in series. Power
N
VSA
Vsb
[M]
Lr
adc - cb
2)
Vdc
Fig. 1. The system configuration of the hybrid filter for neutral current harmonics mitigation
converter acts as an active filter. The voltage rating of the power converter can be significantly reduced because major part of the phase to neutral voltage drops on the passive filter. The hybrid filter can operate in either passive or hybrid mode. In Fig. 1 ADC - analog to digital converter, CB - control block.
Computation of the output voltage of the power converter is performed by the digital two-band frequency-dividing filter which realizes bandpass and band-stop magnitude characteristics. The output voltage of the active filter is proportional to harmonic components of the neutral-line current IN:
,, _ D V^'+P j
AF AFIN T AFh N
(h)
(1)
where /',n and (',■' are fundamental and harmonic components of the neutral current, respectively; RAF1 and R i,./, are transfer resistances of the active filter on the fundamental and harmonics frequencies. The output signals of the two-band filter are amplified ¡and sentto the PWM circuit as the modulation signal.
The current flowing through the neutral conductor is only the zero sequence component. The zero-sequence equivalent circuit of the three-phase four-wire distribution power system with the proposed hybrid filter is shown in Fig. 2, where V° is a zero-sequence voltage source modeling the
Fig. 2. The zero-sequence equivalent circuit of the distribution power system
unbalanced utility voltage or nonsymmetrical linear load, IL is the zero- sequence current source caused by the nonlinear load. Zs and ZN are the phase-line impedance and neutral-line impedance of the distribution system, respectively. ZPF is the impedance of the passive filter. Active power filter is considered as a current controlled voltage source with output voltage according to the formula (1).
According to Fig.2 the neutral-line current IN canbederived as
U 3V0 37 J0
p _ unN __^v__|__j apf1 l__(2)
n 7 n 7s + 7pf+ 3 (( + Raf) 7s+7pf + 3(7 n + Raf )
Voltage in the point of common coupling (PCC)
(z pf +3RAFyS , 7pf(7s + 3Raf)1.
V _ y-pf ' -"-^af r s__+ ¿-pfy-s ' af 1 l (3)
FCc zs + Zpf + q(v +RfS) zs +zPF + f(zN +raf)
Eqaarions (2,3 V mayN e apnffe d fo rthe analysss of c omp)nsatingchNractevistics of the hybrid Filterfot zero-sequence componenFs. Ac ¡seen in (2, 3), operation of the active power filter is equivalent to inserting a resistor RAF in series with neutral-line impedance of the distribution system. Hence, harmonics of the neutral-line current and the voltage VPCC can be suppressed more effectively.
CaSFelatkin Of The Compensating Sagnal
Conttfl sirategies to generate compenssting; senate foe acCive filteri ore bost] on the frequency-fonmin r^iri^tv^nrdqa:vv^n teehme[uey S9, lni. ^ntrol strategy in tqe frequent0 domain is based on the Fourier anafyass ofrheCi))ortodeuitvct secoitage. Tht tech-o rf^orl^^i^tiio^ic oomnonents are separated from Ciileried signals and combined Vo Fsrm coapgnsctCfeg commocC^ tldi ehe discrete Fourier transformfDFTf lotvt aceueacy iir nevtOatkmirr eituatkms. CommoiJy vetd cakulation methods in thr rime demCnaretfeinitratontcvc j^f^a^t^^^eiar-tOai^O^e^i^ip^avf^noac^]^, veural network theory, notch filtvr apptoacF, adeptive ^^^nal processing. Most of ttosF slgorithmn hovi n much better dynamic rfs°onre -Fen tne DPT Sis tlets rearssts samprntdtiaei ea ins somarhtdrsoh rignal is performed by the twoeraed indnffeimculsa revnaneo (SIR)filtariFig. 3g. It reaiizse twn dzueioecomplementary transfer foaatlons:
/((:) = |g4llM--C]. (4)
where A(z) is a second-order all-pass transfer function. Fj(z) is a notch-type transfer function and F2(z) isa bandpass-type transferfunctiontuned to the fundamental frequency ra0.
A ladder-form realization of all-pass IIR digital filter is shown in Fig. 4. In Fig. 4 x(n) and y(n) are i nputand output signals, respectively.Transfer functionof tedrler an-pass fiher is the following:
A(A=Y(z) = z-2+kl(l + k2)z-1+k2 W X(z) k2z-2+kl(\ + k2)z-1 +1'
- 553 -
Fig. 3. Two-band IIRfilter
Thepoly nomialsof nomiootorand denominator afequaeion(6) havemicror sy mmetry.L adder IIR fiiteo (Fig. 4C realczos all-pans transfoenocctcon moduf of which is equal to 1 inthe all frequency raoge. According to (4, 5 ), transfer functions of till twod-ond IlR filOei, shown sn Ftg. 3, aet tlo following:
F (z ) = 1 (z -2 + 2k(z-1+s\s + k2) sW 2 k, z~2 +k,(S + L Vs+S
s(( + kr2 2)
F2 (z )=2 2s" )r
2W 2 k2z-2 ukj (u k2)"r u1
(7)
(8)
where k1 is the adaptive coefficient, which should converge to - cosœ0 for reject a sinusoid with frequency œ0. Suppressionfrequencyofthenotchfiltercanbemodified by k1 and stopband width by k2.
The all-pass IIR filter in Fig. 4 is adapted using adaptive algorithms related to the lattice finite impulse response (FIR) filters. The lattice structure of second-order FIR filter is shown in Fig. 5. Ad aptation algorithm of the FIR lattice was considered in [11].
MATLAB-based modclin- of the pos-os od Cller
frafillonslln, nonlineao loodt aire repreienled aicufcerf sio^]r^€is. However itiiiicC^-jti^iisediode rectlfierr fd moot anse s have capacitive smoothing filters. This type of nonlinear load is equivalent to a
Fig. 4. Ladder structure of second-order all-pass IIR filter
X(n) _ r„(n-l) n(n)_ t'j(li-l)
e,(n)
e2(n)
Fig. 5. Second-order FIR lattice filter
voltage source with small impedance [2, 12]. Common shunt passive and active filters have been shown in [12] to have different compensation characteristics for current-source nonlinear loads and voltage-source nonlinear loads. In this section we consider compensation characteristics of the proposed filter for different rectifier loads.
Case A: diode rectifier has the RLC load. The MATLAB model of the distribution power system, hybrid filter and the load is shown in Fig. 6. The nonlinear load consists of three single-phase diode rectifiers with a LC smoothing filters. Each rectifier is connected between line and neutral point. The linear load is modeled by the series RL network in each phase. Parameters used in the computer simulation are shown in Tabl el.
Fig. 7 shows the simulation results for the neutral current in the case of applying the passive filter and the hybrid filter. The passive filter is connected in 0,2s. The filter works in the passive mode until 0,4s when the power convertor starts. As seen in Fig. 7, the neutral current is attenuated from 30 A to less than5.3A (RMS).
The harmonic content of the neutral current is shown in Fig. 8. The total harmonic distortion (THD)ofthe PCC voltagelsless than 4%.
Fig. 9, 10 show the simulation results for the unbalanced RLC load. In this case the neutral current contains both triplen and fundamental harmonics. Active power filter does not influence over neutralline fundamental harmonic.
Table 1. Model parameters used in the simulation (RLC load)
Paeameter Value
rs, ls 0,1 Q, 318 ^H
rn 0,1 Q
Rl, Ll, Cl 40 Q, 1.2 mH, 530 ^F
Rll, Lll 15 Q, 47.7 mH
Vs 220 V, 50 Hz
Fig. 6. TheMATLABmodelofthehybridfilter andthedistributionpower system(RLC load)
Fig. 7. Waveform of theneutral 1 ine current IN (balanced RLC load)
Fig. 8. Spectrum of the neutral line current IN (balaneed RLC load)
0,3 0,35
time (ms)
Fig. 9. Waveform of theneutral line; current/«(unbalancedRLC load)
Fig. 10. Spectrum of the neutral line current IN (unbalanced RLC load)
Case B: diode rectifiers have the RC load. The MATLAB model of the load, hybrid filter and the distribution power system is shown in Fig. 11. The nonlinear load consists of three single-phase diode rectifiers with a load of capacitor and resistor connected in parallel. The parameters used in the computer simulation are shown in Table 2.
The simulation results for the neutral current in the case of the balanced RC load are shown in Fig. 12. The neutral current is attenuated from 25 A to less than 3 A (RMS). The harmonic content of the neutral current is reported in Fig. 13. In this case the THD of the voltage in the PCC is less than 2.5 %.
Fig. 11.TheMATLABmodelofthe hybrid filterandthe distributionpowersystem(RCload)
Table 2. Model parametersused in the simulation (RC load)
Param eter Value
RS,LS 0,1 W, 318 ^H
Rn 0,1 Q
Rl, Ll, Cl 40 Q, 937.5 ^F
Rll, Lll 15 Q, 47.7 mH
Vs 220 V, 50 Hz
Fig. 12. Waveformoftheneutrallinecurrent IN (balancedRCload)
The simulation results for the neutral current for the unbalanced RC load are shown in Fig. 14. The harmonic content of the neutral current is reported in Fig. 15.
Simulation results verify that the proposed hybrid filter is able to compensate for harmonic neutral currents in cases when nonlinear load has characteristics of current source or voltage source.
Conclusion
The overload of the neutral conductor is becoming a serious problem in modern three-phase four wire distribution low voltage power systems. In this paper, a hybrid filter is proposed to compensate
30 25 20 15 10 5 0
,
. 1 1
123456789 10 11 12 13 14 15 16 17 18 19
" Without HF ■ Passive mode u Hybrid mode
Fig. 13. Spectrum of the neutral linecurrent /)(balanc edRC load)
80 40
i" 0 -40 -80
Without filter
0,15
Passive mode
0,2
0,25
0,3 0,35
time (ms)
0.4
Fig. 14. Waveform of the neutral line current/jvliinbalanced RC load)
Hybrid mode
0.45
0,5
Fig. 15. Spectrum of the neutral line current IN (unbalanced RC load)
harmonics and neutral current in three-phase four-wire distribution power systems. The proposed filter is composed of a three-phase passive filter and a single-phase power converter connected in series. The output voltage of the active filter is proportional to harmonic components of the neutral-line current.
The simulation results verify that the proposed filter demonstrates good compensation characteristics for different nonlinear loads. It can provide a better neutral current mitigation than the passive filter.
References
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