Power quality improvement in off-grid renewable energy systems Текст научной статьи по специальности «Электротехника, электронная техника, информационные технологии»

Journal of Siberian Federal University. Engineering & Technologies 7 (2014 7) 821-831

УДК 621.311

Power Quality Improvement in Off-Grid Renewable Energy Systems

Sergey A. Temerbaev, Valery P. Dovgun* and Elena S. Shevchenko

Siberian Federal University 79 Svobodny, Krasnoyarsk, 660041, Russia

Received 15.06.2014, received in revised form 21.08.2014, accepted 04.10.2014

In this paper a grid-interfacing converter system with voltage and current harmonics compensation capability is considered. The idea is to integrate the power generating functions of the converter with the power filter harmonic mitigation capabilities. The analysis of compensation characteristics of different hybrid powerfilter configurations is carried out. The control method based on adaptive digital system processing is proposed. The current detecting scheme independently controls fundamental current and harmonic distribution system currents. Computation of the harmonic compensation current is performed by the adaptive notch infinite impulse response (IIR) digital filter. The filter's parameters are made adaptive with respect to the grid frequency fluctuations. MATLAB/SIMULINK power system toolbox is used to simulate the proposed system. The presented simulation results confirm the effectiveness of the proposed method.

Keywords: renewable energy system, harmonics, hybrid power filter, power quality.

Улучшение качества электроэнергии в автономных системах электроснабжения, использующих возобновляемые источники энергии

С.А. Темербаев, В.П. Довгун, Е.С. Шевченко

Сибирский федеральный университет Россия, 660041, Красноярск, пр. Свободный, 79

В статье рассмотрены конверторы, обладающие способностью компенсировать высшие гармоники токов и напряжений в автономных системах электроснабжения. Идея заключается в объединении функций конвертора и силового фильтра гармоник в одном устройстве. Проведен анализ основных конфигураций гибридных фильтрокомпенсирующих устройств. Предложен метод управления характеристиками фильтра, основанный на использовании алгоритмов адаптивной цифровой обработки сигналов. Схема выделения составляющих

© Siberian Federal University. All rights reserved Corresponding author E-mail address: Vdovgun@sfu-kras.ru

*

тока обеспечивает независимый контроль гармоники основной частоты и высших гармоник. Вычисление компенсирующего сигнала осуществляется с помощью цифрового режекторного фильтра с бесконечной импульсной характеристикой. Для моделирования предложенной системы использована среда MATLAB/SIMULINK power system toolbox. Результаты моделирования подтверждают эффективность предложенного подхода.

Ключевые слова: возобновляемые источники энергии, гармоники, гибридные силовые фильтры, качество электроэнергии.

Introduction

In classical centralized power systems, large power generation plants produce most of the power, which is then transferred over long distance transmission lines. However, necessity of electricity access in remote area has led to expanding decentralized power systems mainly based on renewable energy sources (RES). An off-grid renewable energy system is a stand-alone power system located in a remote area. The last decades a large number of renewable generation units have been installed in the low voltage distribution systems. Photovoltaic (PV) and wind power systems are typical examples of renewable energy sources. RES require a power electronic converter to interface with the utility grid, because the generated power is dc or has an ac frequency that is either nonconstant or higher than the utility frequency [1, 2]. Renewable generation units may operate in utility grid-connected, connected in the hybrid generator systems (in parallel with diesel generators or miniturbines), or a stand-alone mode.

A good power quality is an important factor for the reliable operation of electrical loads. High penetration of nonlinear loads such as compact fluorescent lamps, light-emitting diode lamps, switching mode power suppliers give rise to serious challenges in power quality, especially for off-grid power systems. Harmonic distortion produced by nonlinear loads causes several problems such as increased power losses in customer equipment, power transformers, flicker, shorter life of organic insulation. The installation of RES to the distribution system may have a significant impact on power quality. In some cases inverter-based renewable generation units can increase the current and voltage distortions in the system to which they are connected.

Between the different technical options available to power quality improving the active power filters (APF) have proved to be an important alternative to compensate for current and voltage disturbances in power distribution systems. In recent years active power filters have been widely investigated for the compensation of harmonic currents.

However, in many cases installing of pure active filters is not cost effective solution because their ratings are close to load. An attractive solution is to integrate primary power generation function of the converter with the APF capabilities for current and voltage disturbance mitigation. In other words, power converter should be able to maintain power transfer between the renewable energy source and the local grid, and be able to improve the power quality at the point of common coupling.

Another cost-effective solution may be hybrid compensating device consisting of an active filter and shunt passive filter. One of advantages of this hybrid power filter is the decrease in the required power rating of the converter.

The shunt active filter for current harmonic compensation in a renewable energy system is considered in [3, 4]. In this works feedforward-type active filter, which detects load current harmonics and generates an opposite current to cancel out distortions, is used.

Calculation of compensating signals is the important part of AHF control and affects their transient as well as steady-state performance. Different control methods have been proposed, ranging from the use of fast Fourier transform (FFT) to the instantaneous P-Q theory, artificial neural networks and adaptive notch filters.

The current harmonic compensating approach, considered in [3, 4], uses an adaptive line enhancer (ALE) based on finite impulse response (FIR) digital filter. The serious limitation of ALE is a relatively slow rate of convergence. Application an infinite impulse response (IIR) notch filter is attractive since it requires much smaller filter length and provides better convergence properties compared to the ALE with FIR filter.

This article focuses on power quality improvement in the off-grid renewable energy systems with the help of hybrid power filters (HPF). Active part of HPF may be integrated with the power electronic converter. Compensating characteristics of different HPF configurations, suitable for integration with renewable energy units, are considered. The load harmonic compensation is performed through using the adaptive two-band infinite impulse response (IIR) filter. Simulation results confirm the effectiveness of the proposed method.

Renewable Energy System Configuration

Renewable energy sources are interfaced with the grid through power converters. Depending on their operation in off-grid system, power converters may have the following modes of operation: voltage-source, current-source, and active power filter mode [2, 6]. In voltage-source mode converter can operate in off-grid system as a grid-forming unit. In this mode of operation the ac voltage and frequency are controlled to meet the power quality requirements.

According to [1, 2] most of the power converters in RES systems operate in current- source mode. PV inverters and wind power systems are typically renewable generation units, operating in current-source mode with dc to ac energy conversion. In this mode control of RG unit is performed by injecting of current with the same frequency and phase of grid voltage. Amplitude of current depends on the power available from a renewable source.

In active power filter mode AC current or voltage harmonics are generated to mitigate waveform distortions.

Hybrid Power Filter Analysis

There are two main categories of APF exist: shunt filters and series filters. The shunt APF should operate as a current source and inject the compensation current into power system to cancel the harmonic current, produced by current type nonlinear load [5]. The series active filter operates as a voltage source and generates a voltage proportional to the source current harmonics. The series APF can effectively suppress harmonics generated by voltage-type nonlinear loads.

In this section the following topologies of hybrid power filters are considered:

1. HPF with shunt passive filter and shunt active power controlled by the PCC harmonic voltage VCPh.

2. HPF with passive filter and active filter connected in series and controlled by the grid harmonic current IGh.

3. Combined HPF with shunt passive filter and series active filter controlled by the grid harmonic current IGh.

A. Shunt hybrid power filter controlled by the harmonic voltage VCPh.

This topology is shown in Fig. 1. It is suited with the DPG unit connected in parallel to the grid. The grid is presented by a sinusoidal voltage generator in series with grid impedance ZG = Rg + jraLG. The nonlinear load is presented as harmonic current source. ILh is the harmonic of distorted load current, and/Gh is the harmonic of the grid current. The passive filter has two branches tuned to 5-th and 7-th harmonics. The shunt AF is assumed to be an ideal current source that injects a compensating current, proportional to the harmonic components of the coupling point voltage VCPh:

^AF = Gaf ' VCPh-

With reference to Fig. 1, the grid current and PCC voltage can be deduced as a function of the load current In

I OH =

Y0

Yt+YPf+Gf

1LH ; VCPh

1

d+m+Gf

-i,

(i)

According to (0) active powrr filter attenuates propagotion af voltage and curreno harmonics generate d by the nonlinear lhad.

Fig. 2 shows a set of curves representing a frequency characteristics of distributing factor IGhHLh and arans-resistance VGh/ Vfh.

Equations (1) yield the following:

0,I

OifGaf>>\Yg+Ypf

(2)

The line current and CP voltage are almost sinusoidal if (2) is satisfied for each harmonic of order h.

B. Serres hybrid power filter controlled by the grid harmonic current IGh.

This configuration is shown in Fig. 3. The AF is equivalent to controlled voltage source:

ThF/i = Ra/ ' .(Oh-

The grid current and PCC voltage

Z.

IgH =

'if

Vp =

Z gZPf

Zp/+Zf+ Rf

The AF is controlled in such a way fo act as a resiatoc in series with grid impedance. Frequency cearncteristics of distributing factor ihh/OLc and trano-tesistance VGh/ULh are precented in Fig. 2.

The larotrRatiu chosen, the lowec the hafmonic current IGO find Vcuh will fee. In this structure the rating of APF is very small. It is prefeered tor power filter mode.

Gh

T

VCPh 1Lh

'-AF I I

*

Lh

Z

Fig. 1. Topology of shunt HPF

1.5-

50 150 250 350 450 550 650 750 850 950

Frequency, Hz

Frequency, Hz

-Gaf= 1; • ** Gaf= 5; Gaf= 10

Fig. 2. Frequency characteristics of shunt HPF

ZG

L Gh VCPh *Lh

1 " ZPP JL

Fig. 3. Topology of series HPF

A. Combined hybrid power filter controlle d by the load harmonic current IGh. This configuration is shown in Fig. 55.

(Compensating voltage of AF is proportional to the harmonic components of the grid current:

VAFh = Kaf ' ICh•

The grid curnent and PCC volfage

J Z* J V Z"Z + Kaf) J

Gh Z + Z + K Lh; VCPh Z + Z + K Lh ■ Zpf + Zg + Kaf Zpf + Zg + Kaf

A set of curves representing a frequency characteristics of distributing factor IGhHLh and transresistance VCPhIVLh is shown in Fig. 4.

The comb ined topology is suited with the RES units in the voltage-source mode.

Frequency, Hz

Frequency, Hz

-Raf= 1; ••• Raf= 5; — If 1(0

Fig. 4.Foequency characteristics of series HI5!7

lOh

Zp

. L h

Fig. 5. Topology of combined HPF

Calculation of the Compensating Signal

The control strategies to generate compensating signals aire based on the frequency-domain or time-domain tnchniques [1, 2, 5]. Control strategy in the frequency domain is fased on the Fourier aoplysis of the distorted current or voltage. The high-order harmonic components are separated from distorted signnlr and combieed to form compensating cpmmands. But the discrete Fourier transform (DFT) lrres accuracy in nonstaZionary siruatione.

Commonly used calculation methods in the time domain are the instantaneous active and reactive (P-Q) thnory approacli, nenral network theory, notch filter approach, adaptive signal processing. Most of these algorithms have a much better dnnamic reepense than the DFT. In nhis paper computation of the load harmonic compensation current is performed by the adaptive notch IIR filter.

The magnitude characteristic of the ideal notch filter is defined as:

fi m ^ m0

H(em) = J 0 (3)

[1 m = m0

50 150 250 350 450 550 650 750 850 950

Frequency, Hz

5

2.5

50 150 250 350 450 550 650 750 850 950

Frequency, Hz

-Ka/=\; ••• Kaf=5; Kap 10

Fig. 6. Frequency characteristicsof combined HPF

In (3) ct0 is the notch frequency. Notch filter extracts fundamental sinusoid from distorted current waveform without harmfully phas e shifting of the high-o rder harmonics. The ideal notc h filter has zero bandwidth. However, zero bandwidth cannot be realize d in practice.

The simpOest type of adaptive notch digioal filtee is the adaptive line enhancee (ALE) proposed by 13. Widoow [en The adantation of the finiee impulse response (FIR) filter is realized by using the least mean sfeare (LMS) algorithm. Shortcomings of this ALE are relatively low convergence speed and potential instability.

An infinite impulse response (IIR) filtes proviOes r sharper mngnitude response than the FIR adaptive line enhancor. Also, it requires mucf smrller filter length, than the ALE based on FIR filher.

Consider the two-rand infinite impulse response (iIR) filter (Fig. 7). It realizes Swo double-compremenrary transfer functions:

whereA(z) is a second-otder all-pass transfer function. F^z) is a notch-aype transfer function and F2(e) is a .andpass-type tonnsfer function. Tlie tuning. frequency is -qual to co0.

A latfice-form rea(ization of all-pass IIR. digital filten is shown in Fig. 8. Input and output signals in Fig. 8 are x(n) and y(n). Transfer function of the lattice all-pass filter is the following:

^(z^^^ZL-p+AcZ)],

1 ( X(Z) 2 [

(4)

(5)

Az)

Y(z) _ z~2 +jfc1(l + jfc2)z"1 +k2 X{z) ~ h2z-2 +kl{l + k1)z-1 +1

(6)

l'Ile polynomials of numerator and denominator of Equation (6) have mirror symmetry. Accordingly, lattice IlR-filter realizes all-pass transfer function with module equal 1 in the all frequency eange. Using equation (4), the transter function of the notch fiiter, shown in Fig. 6, is presented as:

where krt is tht adaptive coefficient which should converge tea -coscod foe reject a sinusoid with frequency cr0.

Adaptive CIR filter in Fig. 8 is adapted by unrng adaptive algorithms related fo the lattice FIR filters. Thn lattice siructure of second-order FIR filtec is shown in Fig. 9. In this article, gradient lattice algorirhm [8] is used for adaptation purposes. It ha s been chosen because o. its low complexity and high-speed convergence. Update of this coefficients k and kt2 by using gradient algorithm is given as foUows:

JlltO

1 (z~2 nrfcjz_l nlflln^f r fz~r n^lln^fz-1 nl '

(7)

k. (n n lf = kt lnf (e, ln)r_i (n - lf n e,._i (nfr, lnf),

D»

where ^ is an adaptation step. Parameter D,(r) is defined as:

Dt ln) = DDi{n) n e r_l (n) n rhln - lf

where f ic a forgetting factor: 0< ft <1.

X ( z )

+> A (z)

Y ( z )

Fig. 7. Two-band IIR filter

1 -k.

z

V"

x(n)

Fig. 8. Lattice structure of second-order all-pass IIR filter

Fig. 9. Lattice structure of second-order FIR filter

Simulation Results

The system was simulated using MathLab/Simulink to verify the proposed algorithm. Fig. 10 shows the model and parameters are presented in Table I.

Schematic diagram of proposed controlled shunt HPF is shown in Fig. 7. The linear load is defined as resistance R, non-linear load includes a rectifier with RL load on the dc side. Passive filter includes two LC circuits, tuned to 5th and 7th harmonics. Simulation process is divided into steps: connection of PF and connection of AF.

The grid supplies a non-linear current load, the Till), of grid current is 21,4 %. At the first simulation step the passive filter is connected in 0,2s and a waveform of grid current is more sinusoidal {THD,=18,8%). Simulation results are performed in Fig. 11.

Rfl lg

* VA ■ * 'W » b

*VJ V-22D riYi

i

r

r

R

J I

nJ

RUI

Fig. 10. MATHLAB scheme ofshunt HPF system

Table 1. Parametersof the Model

Grid Load Passive filter

AC=220 "V R1=3.7 Q C5=263.9 ^F

f=50 Hz L1=10 mH L5=1.57 mH

Rg=0,1 Q R2=15 Q C7=143.25 ^F

Lg=0,2 mH L7=1.47 mH

Time

Fig. 11. Grid current on the first simulation step.

Time

Fig. 12. Grid current on the second simulation step

The second step is APF connection in 0,5 s (Fig. 12). The resulting THD¡ of grid current is equal to 3,12 %.

Conclusion

In this paper a grid-interfacing converter system with voltage and current harmonic compensation capability is considered. Compensating characteristics of different hybrid power filter topologies are considered. A novel adaptive method for grid current harmonic compensation is proposed. The load harmonic compensation is performed by using the lattice-form adaptive notch IIR filter. It is shown that adaptive notch filter can be successfully employed in active harmonic filter for the sake of harmonic mitigation. The proposed approach does not need any training of the notch filter. The performance of the proposed control system is verified by computer simulation in MATLAB/SIMULINK. The simulation results are presented showing the effectiveness of the proposed method.

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