Multipurpose Compensation Scheme for Voltage Sag / Swell and Selective Harmonics Elimination in Distribution Systems

Voltage harmonics, sag, and swell are the most harmful disturbances in distribution systems. This paper introduces a novel effective controller method for simultaneous compensation of both voltage sag/swell and voltage harmonics by using multifunctional dynamic voltage restorer. In proposed controller method called FFT with integrated ISRF, ISRF detects the magnitudes of voltage sag/swell quickly and precisely, and FFT extracts the selective components of voltage harmonics very effectively. The proposed method integrates the superior properties of ISRF and FFT methods. FFT integrated ISRF is applied for the first time to provide the compensation of both sag/swell and selective harmonics together. The proposed system has ability to compensate symmetrical/asymmetrical sag/swell and symmetrical/asymmetrical selective harmonics which are 5th, 7th, 11th, and 13th. The controlled system is modelled in PSCAD/EMDTC and compared with conventional methods. The performance results verify that the proposed method compensates voltage disturbances effectively in the system.


Introduction
Voltage, current, frequency deviations, and waveform distortions that lead to equipment failure, monetary loss, and different negative consequences are known as power quality problems in distribution systems.Sag, swell, and voltage harmonics are the most crucial power quality problems.Voltage sag is a short term drop in the amplitude of grid voltage.Short circuit faults and starting up of large loads cause voltage sag problems in distribution systems [1] and [2].Swell is an increase in the amplitude of grid voltage.Voltage swell is not as widespread as voltage sag, but it could be more harmful and destructive [3].Voltage harmonics distortion defined as a distortion of the fundamental sinusoidal voltage waveform alternating at 50/60 Hz and repeats in every cycle [4].While sag/swell causes the damage of electronic equipment and failure of systems, voltage harmonics induce overheating and losses in cables, transformers, and motors.There are various custom power devices to cope with these problems in distribution systems.Among these devices, Dynamic Voltage Restorer (DVR) is the most effective device to compensate these power quality problems.DVR is an inverter based structure which is located between sensitive load and grid in the system.The main components in a conventional DVR are inverter, dc-link capacitor, filter, and injection transformer [5], [6] and [7].DVR injects controlled voltage in series to mitigate the influence of voltage disturbances on sensitive loads.
The main functionality of a conventional DVR is to compensate only sag/swell problems in the system [8].In the literature, DVRs have been recently applied to compensate both sag/swell and voltage harmonics, which are also named as multifunctional DVRs.Table 1 shows the compensation capabilities of multifunctional DVRs in [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19] and [20].Besides, these studies employ several reference generation methods to compensate multiple voltage disturbances at the same time.Among these studies, [11] and [15] examine the mitigation of only symmetrical voltage harmonics with sag/swell compensation.In addition, Instantaneous power theory and Perceptron based control algorithm are used for alleviation of symmetrical/asymmetrical sag/swell and only symmetrical voltage harmonics.In [12], the most common topology called as SRF theory which transforms 3-ϕ voltages to d-q components is used for symmetrical sag/swell and voltage harmonics.The asymmetrical problems of sag/swell and voltage harmonics are compensated in [20] by using ISRF.This paper presents a novel reference generation method based on Fast Fourier Transform (FFT) with integrated Improved Synchronous Reference Frame (ISRF) in order to compensate sag/swell with selective voltage harmonics via multifunctional DVR.The ISRF method applied in this study is more accurate and fast among sag/swell detection techniques.In addition, FFT is very effective approach to extract the components of voltage harmonics.This method shows superior properties of ISRF and FFT.The proposed FFT with integrated ISRF method is applied for the first time to compensate both sag/swell and asymmetrical selective voltage harmonics, simultaneously.[14], [16] and [19] Symmetrical/asymmetrical voltage sag and symmetrical voltage harmonics [13], [17] and [18] Symmetrical voltage sag/swell and symmetrical voltage harmonics [12] Symmetrical/asymmetrical voltage sag/swell and only symmetrical harmonics [11] and [15] Symmetrical/asymmetrical voltage sag/swell and all components of symmetrical/asymmetrical voltage harmonics [20] Symmetrical/asymmetrical voltage sag/swell and symmetrical/asymmetrical selective voltage harmonics proposed study In the proposed study: • The main contribution of this study is the elimination of symmetrical/asymmetrical sag/swell and symmetrical/asymmetrical voltage harmonics, simultaneously.
• ISRF is selected for compensation of sag/swell due to the fast speed detection property.• Harmonic compensation is achieved by using FFT method instead of ISRF.

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• FFT achieves compensation of both symmetrical and asymmetrical voltage harmonics.
• The system has ability to compensate up to 30 % sag/swell with the attenuation of selective voltage harmonics which are 5 th , 7 th , 11 th , and 13 th .

FFT Integrated ISRF
DVRs, which are connected between grid and sensitive load, are implemented to inject controlled voltage in series to prevent adverse influence of voltage issues on sensitive loads [20], [21], [22] and [23].Figure 1 shows the proposed reference generation method for multifunctional DVR.
The control strategy is a fairly critical issue in DVR.The primary purpose of the control system is to compensate sag/swell problems and to attenuate the effects of voltage harmonics in distorted grid voltages.Voltage detection is one of the most important subjects in a control system.Many voltage detection methods are presented with different control algorithms in literature.Sag/swell and harmonics must be compensated rapidly and accurately.
The flow chart of FFT with integrated ISRF is presented in Fig. 2. According to the proposed controller, grid voltages are firstly measured and converted to per unit (pu) values.In the next step, the per unit signals are used to generate the magnitudes of fundamental component and selective harmonics signals by using ISRF and FFT methods, respectively.Then, the mathematical processes are performed to produce reference signals for sag/swell and harmonics.Finally, reference signal is obtained and compared with carrier signal in Pulse Width Modulation (PWM), and switching signals for IGBTs in inverter are generated to inject controlled voltage in series for compensation.

Sag/Swell Detection: ISRF
Conventional SRF theory or dq-transformation cannot achieve detection of voltage disturbances under asymmetrical condition.In order to eliminate the drawback of conventional SRF, different approaches have been developed.Conventional dq-transformation is not feasible for a single phase voltage measurement because of 3-ϕ information that is used at the same time.Therefore, this transform process causes inaccurate voltage detection under asymmetrical (unbalanced) conditions.In this study, ISRF method is used to detect sag/swell signals in asymmetrical conditions, as shown in Fig. 3.In this study, ISRF eliminates the requirement of other this figure in publication version.
Best regards.
Dr. Mustafa İNCİ, et al. phases to generate magnitude information for singlephase.In proposed method, dq transform is realized for each phases, separately.To apply dq transform for each phase voltage, three symmetric virtual signals can be produced by a single voltage (for example phase-a) [20], which is transformed into dq transform given in Eq. ( 1), Eq. ( 2) and Eq. ( 3).Virtual signals for second input (In 2 ) and third input (In 3 ) in dq transform are generated applying mathematical equations on actual input (In 1 ).These processes are separately executed for all phases.
In conventional dq transformation, the input voltages must be equal to 2π  3 rad phase difference to detect voltage signals accurately.There is a relationship be-

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Fig. 3: ISRF method for a single phase.
tween interconnection of phases in a 3-ϕ system.Relationship between symmetrical 3-ϕ voltages are expressed by Eq. ( 1), Eq. ( 2) and Eq. ( 3): In ISRF method, virtual signals are generated by using only single phase information [20], as shown in Fig. 3.This operation is performed separately for all phases in multiple frame.According to Eq. ( 1), Eq. ( 2) and Eq. ( 3), the application of these signals in dq transform gives slow response for detection of voltage sag/swell.In order to achieve faster detection, virtual signals for In 2 and In 3 are reproduced by using a single voltage.These signals are expressed in Eq. ( 4) and Eq. ( 5).π is equal to −1 in phasor form and 4π 3 is obtained by the multiplication of π 3 and −1.In this way, In 2 is generated virtually by delay of π 3 .In a balanced system, the sum of phasor voltages is zero (In 1 + In 2 + In 3 = 0).As a result, In 3 is defined as the negative of the sum of In 1 and In 2 .
In ISRF, In 1 and its virtual signals (In 2 and In 3 ) are firstly transformed into α and β components for each phase in multiple frames.
In the next step, α-β components are converted to dq components according Eq. (7).
In dq reference frame, d-and q-components are orthogonal signals and used to define the magnitude of a single phase voltage via proposed controller.The square root of sum of squares of d-and q-components in Eq. ( 8) gives the magnitude for a single-phase voltage.
In sag/swell detection, magnitude of voltage sag/swell in pu (M ag ss ) is extracted from ISRF.Measured magnitude signal (M ag ss ) is extracted from 1 pu to calculate the depth of sag/swell (SS depth ).

Detection of Selective Voltage Harmonics: FFT
Several harmonic detection methods have been applied in power quality applications in literature.In this study, FFT based harmonic detection technique in [24] is used to obtain the harmonics of the grid voltage.The FFT, which consists of small Discrete Fourier Transform (DFT) components has a rapid response due to less complex calculations.Among FFT techniques, Cooley-Tukey is the most commonly applied algorithm [25] and [26], as illustrated in Fig. 4. The DFT form of the grid voltage is defined as where k is the harmonic frequency index, N is the number of sampling points, W k = e −j2πk/N .The Eq. ( 10) can be written in polar form as where V m and θ indicate the magnitude and phase angle of k-th harmonic, respectively.
In Cooley-Tukey algorithm, the voltage samples are categorized as odd samples (2n + 1) and even samples (2n) [26].Thus, Eq. ( 10) can be rewritten as By substituting W nk N/2 by W 2nk N , the Eq. ( 12) becomes By using symmetric W properties, Eq. ( 13) becomes The N/2-point DFTs can be reduced by N/4-point DFTs in order to reduce the computational cost.If the sample number is selected as the power of 2 (N = 2r), then it can be degraded until 2-point DFTs which is known as radix-2 FFT algorithm.Therefore, the calculations are decreased, and FFT becomes faster.In this study, 1024 points are exploited in 1-period.As a result, radix-2 FFT algorithm is applied as illustrated in Fig. 5.  employed to calculate the depth of sag/swell, FFT extracts the 5 th , 7 th , 11 th , and 13 th harmonic components.

512-point DFT
Reference signal of sag/swell (Ref ss ) is in phase with grid-side voltage.In order to determine Ref ss , the depth value of sag/swell is multiplied by a sine function in-phase with grid-side voltage.In this way, reference signal of sag/swell is determined as where θ n is instantaneous phase angle.
In order to compensate voltage harmonics, the inverse voltage is supplied to the grid.As a result, reference signal is the summation of Ref ss and negative Ref har : The final reference signal is compared with carrier signals in Sinusoidal PWM to generate controlled voltages for compensation process.

Case Studies
In this section, performance results of the proposed method based on ISRF and FFT are presented for application in multifunctional DVR system.The system includes a sensitive load which has a capacity of 1 MVA.It is fed from 690 Vrms (peak value of phase voltage is 560 V) 3-ϕ supply.The system parameters are given in Tab. 2. The proposed DVR is designed to compensate up to 30 % 3-ϕ sag and selective voltage harmonics (5 th , 7 th , 11 th , and 13 th ) at different gridside THD values.The system and proposed controller model is implemented in PSCAD/EMTDC to compensate sag/swell and selective voltage harmonics at the grid side.Different case studies are analysed using PSCAD/EMDTC to verify the controller method.These case studies are: • Case I: Performance comparison of SRF and ISRF for asymmetrical sag detection.
• Case II: Simultaneous compensation of: symmetrical selective voltage harmonics and symmetrical sag, symmetrical selective voltage harmonics and asymmetrical swell.
• Case III: Simultaneous compensation of: asymmetrical selective voltage harmonics and asymmetrical sag, asymmetrical selective voltage harmonics and asymmetrical swell.
Firstly, the performance results of ISRF and conventional SRF are compared for asymmetrical sag cases.As shown in Fig. 7, ISRF is applicable for asymmetrical sag conditions while conventional SRF cannot achieve accurate detection (Case I).ISRF detects sags fast and accurately within 1.3 ms, 2.4 ms, and 0.3 ms for phasea, phase-b, and phase-c, respectively.Table 3 and Tab. 4 shows the THD values before/after compensation for Case II and Case III, respectively.Harmonic compensation capabilities of ISRF and FFT was compared.In Case II, the system was used for compensation of symmetrical voltage harmonics and symmetrical sag/asymmetrical single phase swell at same time.In Case II-A, 3-ϕ sag occurs at 0.3 s for 5 periods, and the peak value of 3-ϕ voltages is reduced to 392 (0.7 pu) from 560 V (1 pu) in addition to voltage harmonics compensation.DVR compensates 0.3 pu sag, and THD values of load voltages are diminished to 2.14 %, 2.21 %, and 2.03 % from 11.38 % for phase-a, phase-b, and phase-c, respectively.In Fig. 9, individual harmonic spectrum before/after compensation is presented for Case II.
Case III-A and Case III-B present asymmetrical voltage harmonics (THDA = 9.81 %, THDB = 7.98 %, and THDC = 10.8 %) which distort voltage waveforms.Two phase asymmetrical sag and asymmetrical voltage harmonics condition (Case III-A) at grid-side is presented in Fig. 10(a).In single phase fault, voltage value of phase-a decreases to 0.7 pu from 1 pu during the period of 0.9-1.0s.Besides, different THD values appear at grid-side: 9.81 % at phase-a, 7.98 % at phase B, and 10.8 % at phase-c, respectively.In this case, both asymmetrical selective voltage harmonics and asymmetrical single phase sag is compensated at the same time.THD value at load-side is reduced to 1.49 % at phase-a, 1.62 % at phase-b, and 1.78 % at phase-c, respectively.In Case III-B, 3-ϕ asymmetrical voltage swell occurs from 1.3 s to 1.4 s, as shown in Fig. 10(b  (1 pu) at phase-a, phase-b, and phase-c, respectively.In Fig. 11, individual harmonic spectrum before/after compensation is presented for Case III.

Conclusion
In this study, a novel reference generation method based on ISRF and FFT is introduced for detection of sag/swell and extraction of voltage harmonics.Symmetrical/asymmetrical sag/swell and symmetrical/asymmetrical selective voltage harmonics are compensated by using the proposed method.Performance results of 30 % sag and 25 % voltage swell  compensation are analyzed in addition to asymmetrical selective voltage harmonics (5 th , 7 th , 11 th , and 13 th ).THD level in system is reduced nearly to 2 % from 12.36 % with the proposed controller.The case studies verify that the proposed method performs very effectively and compensates voltage disturbances in the system.

Fig. 1 :
Fig. 1: Proposed reference signal generation based on FFT with integrated ISRF method and Dynamic Voltage Restorer.

Figure 6
Figure6shows reference signal generation method using ISRF and FFT, simultaneously.While ISRF is

Fig. 6 :
Fig. 6: Proposed reference signal generation for voltage sag/swell and selective voltage harmonics.

Figure 8 (
a) shows voltage waveforms of grid-side, injected, and load-side voltages.In Case II-B, single phase voltage swell condition occurs during five periods.The peak value of grid side voltage increases to 672 V (1.2 pu) from 560 V (1 pu) at phase-c.The compensation of 20 % single phase swell and symmetrical selective voltage harmonics are presented in Fig. 8(b).
Tab. 1: Compensation capabilities of multifunctional DVRs in literature studies and the proposed study.
Tab. 3: THD and harmonic values of case studies before/after compensation-symmetrical condition.