Evaluation of Different Positive Sequence Detection

Evaluation of Different Positive Sequence Detection Structures Applied to Grid-Connected Systems Mohammad Jamarani

Mohammad Pichan

Electrical Converters & Power Systems Dept., IRIEE, ACECR Tehran, Iran [email protected]

Electrical Converters & Power Systems Dept., IRIEE, ACECR Tehran, Iran [email protected]

Adib Abrishamifar Department of Electrical Engineering, Iran University of Science and Technology Tehran, Iran [email protected] Abstract— Effects of three different positive sequence detector structures including qPLL, EPLL, and SOGI-PLL on performance of a 10KVA shunt active filter are investigated in this paper. The evaluation is performed through simulation of practical test conditions including balanced and unbalanced grid voltage, frequency variation, and harmonic polluted grid voltage. These studies lead to recognizing the SOGI-PLL as a proper method for grid-connected power applications, especially for the ones with digital control systems. Keywords- grid-connection, positive sequence detector, digital control.

I.

INTRODUCTION

A possible solution in order to eliminate harmonics in grid current _generated by non-linear loads such as: switching power supplies, electric furnaces, high-voltage DC systems, adjustable speed drives, fluorescent lamps, and AC/DC converters/inverters is using active power filters. Active power filters have been known as the best tool for harmonic mitigation as well as reactive power compensation, load balancing, voltage regulation, and voltage flicker compensation. The most popular type of active power filters is the shunt active filter (SAF) as depicted in “Fig. 1” The shunt active power filter senses the load current and injects a current into the system to compensate harmonics and reactive currents of load [1]. A common point among all of the control methods applied to active power filters is that detection of fundamental component of grid voltage, which is used for synchronization between grid voltage and output current of the active filter, is the key element to generate proper reference signals for SAF and also to provide the unity power factor.

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Mehdi Fazeli Electrical Converters & Power Systems Dept., IRIEE, ACECR Tehran, Iran [email protected]

The information about the phase angle, amplitude, and frequency of grid voltages is supposed to be provided by a phase locked loop (PLL).The main idea of phase-locking is the ability to generate a sinusoidal signal whose phase is coherently following that of the main component of the input signal [2]. Figure 2 shows the general structure of a PLL, which is composed by a Phase detector (PD) required for determining the phase difference between the input and output signals of PLL, a loop filter (LF) which is a low-pass, and a voltage controlled oscillator (VCO) that generates the sinusoidal waveform with respect to the phase angle calculated in PD and available through LF [2-4].

Figure 1. Ccompensation scheme for non-linear load

Figure 2. Block diagram of a PLL

Should such distortions as harmonic pollutions, variations of fundamental frequency, and unbalances exist in the network voltage, a simple PD or a zero-crossing detector cannot be sufficient to extract correct information about phase, frequency, and amplitude, required to generate proper reference signals for SAF. In this case, a more advanced system than a simple PLL is required. Such system is called positive sequence detector (PSD) and is in charge of extracting correct information about the network voltage with fundamental frequency. Designing a proper control system for grid-connected power applications where disturbances are usual to happen necessitates including a PSD structure. Successful digital implementation of this control system depends on some parameters such as: the amount of physical memory that is required in the hardware, amount of mathematical calculations to be done in the digital processor, and other parameters related to digital programming. In this work, a comparison between different PSD structures is performed in order to evaluate the performance of an active power filter when the reference signals are generated using these PSD systems. This paper is organized as follows: Section II is an overview of the system used to evaluate the PSD structures, including descriptions of network, SAF, and non-linear load. Section III is about different PSD structures. Section IV presents the simulation results the PSD systems to evaluate and compare their performances. Section V presents a comparative study based on simulations in section 4, and finally, conclusions are presented in section VI. II.

SYSTEM DESCRIPTION

As previously depicted in Figure 1, the system simulated in this work includes three main parts which are briefly explained in the following: A. Grid In order to prevent the non-linear load from affecting the voltage characteristic of the network, a voltage source with zero impedance is used. B. SAF The shunt active filter in this work is an IGBT-based voltage source converter (VSC) and the instantaneous Power theory with hysteresis current controller (HCC) is used to control and generate the switching pattern of SAF [5]. C. Load The non-linear load is selected such that the highly unfavorable situation _drawing about 30% current harmonics of 5th, 7th, 11th, 13th, 17th, and 19th orders_ is simulated. This load is composed by a three phase resistance, Rac, in parallel with a diode rectifier which feeds an RL series load.

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III.

PDS STRUCTURES

Being capable of detecting phase angle of utility voltage as quick as possible, while adequately filtering the effect of distortions in sampled signal from utility, are the characteristics which make a PSD structure acceptable for practical applications. In this section, as presented in “Fig. s 3, 4, and 5”, three different PSD structures are investigated. These systems are qPLL-based PSD, EPLL-based PSD, and SOGI-based PSD structures, respectively [3], [4], [6-13]. Using three single-phase PLLs to create a three phase algorithm leads to great complexity. Additionally, this three phase system is very difficult to implement. The solution is to transform calculations to stationary reference frame. As illustrated in “Fig. s 3, 4, and 5”, the outputs of such systems are the network voltage with the fundamental frequency in αβ coordinates [6]. A. QPLL-based PSD Using the active/reactive power theory (PQ theory) to form a PLL, this PSD Structure is resulted and is depicted in Fig. 3. Through calculating imaginary component of power, q', which is done internally, the Data required to calculate phase angle of the network voltage, θ, becomes available. Finally, the data required to calculate the positive sequence component of voltage is acquired through real component of power p' and a low-pass filter. B. EPLL-based PSD EPLL-based PSD replacing the PD in PLL system with an adaptive notch filter (ANF), a new enhanced PLL (EPLL) structure is achieved as in Fig. 4. In comparison with the regular q-PLL system, this new configuration is capable of presenting high noise immunity along with better ability of rejecting harmonic distortions. C. SOGI-based PSD In the general structure of a PLL system, as previously depicted in Fig. 5, the PD block is intended to detect the phase error between reference an measured signals, which are the network and PSD’s output signals, respectively. Another way other than using a PD to extract information about phase, frequency, and amplitude of a signal is using two in-quadrature signals which can be provided through SOGIs. IV.

SIMULATION RESULTS

To compare performance of the shunt active filter when using the different PSD structures mentioned in section 3, some practical test conditions are simulated in MATLAB/SIMULINK and the results are presented in this section. The system response is studied in the following cases: step change in network frequency, balanced network voltage

with no harmonics, unbalanced network with voltage harmonics, and finally, the general effectiveness of SAF in reducing current harmonics.

A. Frequency fluctuation As depicted in Fig. 6, for the three PSD structures, response to a step change of 1Hz in grid frequency, as well as transient time in detecting the frequency of fundamental component of grid voltage is compared. Although there is not a considerable difference among the structures when the step fluctuation of frequency occurs, this is the transient behavior of the structures that distinguishes them. It is the SOGI-PLL structure, which has the relatively shortest transient time with less amplitude of over and undershoots. B. Voltage unbalancee

Figure 3: qPLL-based PSD

Figure 7 presents the results of the test condition simulated according to IEC related standard for grid voltage unbalance equal to 13% [7] in Fig. 7-a, and the three phase signals generated as reference based on calculations made by PSDs in Fig.s 7-b to 7-d. It is clear that among all, SOGI-based PLL structure has the relatively highest ability in identifying voltage unbalance and therefore generating balanced reference signals. C. Voltage harmonics and unbalancee According to IEC standard in [7], 13%of voltage unbalances same as previous section, as well as 5% of 5th order and 2% of 7th order harmonics are integrated with the grid voltage in the simulated system. Figure 8 depicts the results of fast Fourier transform (FFT) applied to the three phase output reference voltages of PSD structures. As it can be seen, filtering in q-PL is weak and lets the harmonics to the output. In EPLL, filtering for 5th and 7th order harmonics is strong but calculations generate 3rd harmonic. For SOGI-based PLL, filtering is proper and harmonic content of the generated signal is considerably reduced and also no mis-calculated harmonics are added.

Figure 4. EPLL-based PSD

D. General effectiveness of SAF In normal conditions of grid voltage, when no unbalance or voltage harmonics exist in the network, performance of the SAF when using the previously discussed PSD structures is evaluated. The non-linear load used for this purpose generates almost 30% of current harmonics. In Figure 9, the ability of SAF in eliminating current harmonics and variations in this characteristic due to different PSDs used is presented. It is clear that SOGI-PLL can effectively reduce the current harmonic content of grid current which includes harmonics from 5th, 7th, 11th, 13th, 17th, and 19th order. On the other hand, performance with qPLL is not desirable and EPLL is in between.

Figure 5. SOGI-based PSD

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6-a 6-b 6-c Figure 6. Transient time of the three PSD structures in detecting fundamental frequency and step change (6-a): q-PLL, (6-b): EPLL, (6-c): SOGI-PLL

7-a

7-b 7-c Figure 7. Unbalance of grid voltage and reference signals generated by PSDs in such conditions (7-a): grid voltage, (7-b): q-PLL, (7-c): EPLL, (7-d): SOGI-PLL

7-d

8-a 8-b 8-c 8-d Figure 8. Fourier analysis of the grid voltage and references generated by PSD structures in presence of harmonics and unbalance. (6-a): grid voltage, (6-b): q-PLL, (6-c): EPLL, (6-d): SOGI-PLL

9-a

9-b

9-c

Figure 9. Performance of SAF while allpying the PSD structures, baseed on Fouries analysis of grid current (9-a): load current, (9-b): q-PLL, (9-c): EPLL, (9-d): SOGI-PLL

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9-d

V.

COMPARATIVE STUDY

Based on the simulation results presented in the previous section, as well as theoretical ideas, table 1forms a comparison among the PSD structures considered in this work. The PSD structures considered in this work can be divided to PLL-methods that can be implemented to be frequency-adaptive and methods that are not able to be implemented frequency-adaptive. Complexity of digital implementation, difficulty of tuning the controller parameters,

ability of rejecting voltage harmonics, transient time, and finally, overall performance in cooperation with SAF are the aspects of comparison. It should be noted that complexity of digital implementation for these systems in DSPs or FPGAs, is mostly affected by using trigonometric functions in the structure for which a large space of memory or amount of memory would be required.

Table I. Comparative study of considered PSD structures PSD structure

Frequency-adaptive implementation

Complexity of Digital implementation

Complexity of tuning

Rejection of harmonics

Transient behavior

qPLL

Yes

High

Low

Low

Medium

EPLL

No

High

High

Medium

Low

SOGI-PLL

Yes

Low

Medium

High

good

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[4]

[5]

A. Emadi, A. Nasiri, S. B. Bekiarov, “Uninterruptable Power Supplies and Active Filters” CRC Press, ISBN: 0-8493-3035-1, October 2004. M. Karimi-Ghartemani, M.R. Iravani, “A New Phase-Locked Loop (PLL) System,” 44th IEEE MWSCAS, vol. 2, pp. 421-424, 2001. N. Hoffmann, R. Lohde, M. Fischer and F. W. Fuchs, L. Asiminoaei, P.B. Thøgersen, “A Review On Fundamental Grid-Voltage Detection Methods under Highly Distorted Conditions in Distributed Power Generation Networks,” Energy Conversion Congress and Exposition (ECCE), 2011 IEEE, pp. 3045 – 3052. Xiao-Qiang GUO, Wei-Yang WU, He-Rong GU, “Phase Looked Loop and Grid Synchronization Methods for Grid Interface Converters: A Review,” Electrical Review, ISSN 0033-2097, R. 87 NR 4/2011. A. Shahrdad, M. Pichan, A. Abrishamifar, M. Fazeli, “A 10 KVA FPGA-Based Active Power Filter for Battery Charger Applications,”

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