Power Quality Improvement by Using Discrete Wavelet Transform Based DSTATCOM

Abstract

The discrete wavelet transform-based control technique for the distribution static compensator (DSTATCOM) is utilized in this paper to improve power quality at common points of interconnection (CPI). The discrete wavelet transform, the time frequency analysis technique, is used here to split distorted load current of each phase in order to recover the line frequency of harmonic components for estimating the respective active power components. The difference between the estimated reference active component and the detected load currents is utilized to create reference currents for DSTATCOM\'s voltage source converter (VSC) management. The performance of DSTATCOM is presented using MATLAB software under varied linear and non-linear load circumstances. Under various load situations, total harmonic distortion (THD) of the source current is less than 5% with a power factor of unity.

Introduction

I. INTRODUCTION

This In the present days, power quality of supply has become the major concern for electrical customers and utilities. Low power quality causes instabilities, shortens life of equipment, and causes power service equipment to malfunction, among other things. Voltage sag, harmonic distortion, voltage swell, and flicker are the four types of power supply problems. To improve the quality of power supply by properly detecting source of disruptions [1]. Power quality issues in distribution systems include harmonics in currents, lag power factor, excessive reactive power, and so on. These issues are produced by the use of non-linear and unbalanced loads in the distribution system. Using power electronic equipment such as rectifiers in the distribution system may cause voltage distortion and increase neutral currents in the power supply [2]. A low level of power quality indicates high level of disturbances, while a low level of disturbances indicates high level of power quality, with consumers and providers agreeing solely on the acceptable degree of disturbances. The power quality restrictions are maintained in accordance with international standards. If these restrictions are exceeded, the device will not operate for an extended period of time or will malfunction. Installing custom power devices (DSTATCOM) at the CPI to improve power quality and provide consumers with a stable power supply [3].

Power monitoring is essential to record performance of power supply and identify power quality issues at CPI. Wavelet analysis was created to address the shortcomings of Fourier analysis and brief time Fourier analysis. Time constraint Fourier analysis offers uniform time and the  frequency resolution over whole time frequency domain. The wavelets, which are functions of the time and the scale, aid in looking at the signal at several scales, also known as multi-scale analysis. Wavelets are often employed to detect signal discontinuities such as spikes, jumps, and non-smooth characteristics [4]. Wavelet, which is widely used for recognizing various power quality disturbances, was used to create the control algorithm for these devices in order to mitigate the current-related power quality disturbances. This study proposes a control technique for DSTATCOM based on Discrete Wavelet Transform (DWT) to mitigate power quality disruptions. Here, fundamental component of the load current is derived from deconstructed level and employed in the control method to determine the reference active current component. It also aids in simultaneous assessment of the PQ disturbances such as unbalancing and the THD based on load current decomposition levels [5].  

A. Power Quality Problems Are

1. Harmonics Distortion
2. Voltage Swell, Sag/Dips, Voltage unbalance, Flicker and Transients
3. Voltage Magnitude and Frequency, Voltage Fluctuation
4. Hot Grounding Loops
5. Ground Potential Rise Measurement and Management of quasi-dynamic, quasi-static and Transient type phenomena [6].[10]

B. Solution To Improve Power Quality Problems

To mitigate the above mentioned power quality problems various generalized methods have been suggested in the literature. Various method have been proposed to detect and localize the power quality events such as Discrete Fourier Transform (DFT), Short Time Fourier Transform (STFT), and Wavelet Transform. One of the important methods to reduce noise in various signals is Wavelet transform. Wavelet Transforms have emerged a fast and effective tools for automated detection and effective characterization of PQ disturbances .Discrete Wavelet Transform (DWT) based multi resolution analysis (MRA), which is used as multi resolution decomposition in PQ literature, analyze the signal at different frequencies with different resolutions [8]. To improve the power quality there are different ways to mitigate voltage dips, swell in transmission and distribution systems. At present, a wide range of very flexible controllers are emerging in power applications .Among these, the distribution static compensator is most effective devices, which is based on VSC principle. A new PWM control scheme has been implemented to control the electronic valves in two level VSC used in D-STATCOM. The term power quality, broadly refers to maintain sinusoidal bus voltage at rated magnitude and frequency in an uninterrupted manner from the reliability point of view. For a well-designed generating plant generates voltages almost perfectly sinusoidal at rated magnitude and frequency. Power quality problems start with transmission system and stay valid until end user in distribution system [7].

II. PROPOSED CONCEPT

Figure 1 depicts a schematic diagram of the three phases of DSTATCOM. CPI connects the nonlinear and linear loads. The interface inductor, voltage source converter, DC bus capacitor, and loads are all part of the DSTATCOM. The gate signals control the three-phase VSC. The gating signals are created by the hysteresis current controller in accordance with the control algorithm.

To cancel the compensating currents of the high frequency switching component, interface inductors are inserted between the midpoints of each leg of VSC and the main supply. The placement of VSC generates high frequency switching noise, which is minimized by putting a ripple filter comprising capacitor and resistor at CPI. The three phase bridge rectifier and non-linear R-L load are connected to the system's DC side.

1. Control Strategy

In this strategy, the estimated and sensed phase voltages of CPI are  , supply currents ( , currents of load ( ), and the DC bus voltage of VSC  ( )  are the control algorithm's feedback signals, these detected waves are processed in following order to the generate VSC gate pulses, are exposed in fig.2.

The fundamental load current retrieved with the co-efficients of each phase is in the quadrature to detected the load current of that phase with no delay. As a result, components ( ) of the predicted quadrature fundamental load currents are utilized to calculate the corresponding active power components ( ) respectively. The absolute (abs) magnitude of corresponding estimated quadrature components at each zero crossing of the related to each phase templates is used to extract these active power components. This extraction process's mathematical formulation for phase 'b' load current is as follows:

???????B. With Dstatcom Case

1. The System Performance for Balanced Linear and Non-linear Loads

The supply voltages of the system are shown in Figure.10. Figure.11. represents the three-phase source currents under balanced load conditions. Figure.12. shows the load current as a result of non-linear load. Small magnitude is the difference between source and load currents of balanced non-linear load situations because of the source impedance. The supply’s unity power factor controls the reactive power under balanced non-linear load while also efficiently reducing current harmonics on the grid side. Figure.13. represents the R-L load currents are connected across the CPI. To maintain the unity power factor operation with respect to the CPI, VSC feeds the reactive power to the distribution system locally.  DSTATCOM‘s compensating currents are depicted in Figure.14. The compensatory current primarily generates the harmonics to load current. Figure.15. Shows the DC voltage, it is maintained constant under balanced linear and non-linear load conditions.

The TABLE shows the performance comparison for %THD of a source voltage, source current and load current due to in the without and with DSTATCOM condition respectively. The without DSTATCOM condition THDs of source voltage and current of source and loads are more. The performance of the proposed control system with DSTATCOM is better as the distortion on the source current is less with this technique. DWT-based DSTATCOM is also having the additional ability to provide additional information like harmonic estimation in the group. So, the operation of DSTATCOM with this control system is within acceptable limits.

2)  System Performance under Unnalanced Linear and Non-linear loads with dstatcom

The performance of the distribution system with the DSTATCOM is examined; the output results are shown below. Figure.17. depicts the CPI voltages when the system is linked to the DSTATCOM under unbalanced conditions. Figure.18. shows the source current under unbalanced with DSTATCOM condition. Figure.19. and figure.20 are examples of formal represents the load currents due to non-linear and linear load-balanced conditions.  The source currents and load currents are sinusoidal in the system up to t = 0.04s. Following, an unbalance is introduced into the system by removing the load of the ‘b’ phase at t = 0.04s. The VSC has begun to generate reactive compensating currents in order to balance and sinusoidalize the supply currents uniformly under unbalanced load conditions. The unbalance in load is controlled at t = 0.06s, the system resumes its previous performance. The three-phase diode bridge rectifier is linked across CPI to provide the nonlinear load. Figure.21. Represents the compensating currents with unbalanced nonlinear load, the compensating currents generated by VSC are added with harmonics polluted load currents to make supply currents balanced on the grid side. Figure.22. shows the DC link voltage of the system with DSTATCOM under unbalanced linear and nonlinear load conditions. It is maintained throughout the constant voltage.

???????

Conclusion

This paper proposed the DWT-based control algorithm was applied in real-time for DSTATCOM to the improve the power quality in the distribution system. This DWT control approach uses the MRA method, which includes FIR filters and IDWT to extract basic component of the load current. Extracted the basic load component is then utilized to estimate the reference balanced source currents. in this paper, the proposed algorithm controls the THD of the source and load currents, unbalancing to balance conditions of the ‘R’, ‘Y’ and ‘B’ phase currents and the reactive power compensation in distribution system. According to specifications, THD of the supply current is less than 5%, indicating that the DSTATCOM with suggested control system is performing satisfactorily. The distribution system\'s power quality and performance have been upgraded. Appendix Line impedance: 1) Load: 50kw( RL with 0.8 pf lag) 2) Three-phase diode rectifier load : 3) Ripple filter: Capacitance of DC bus: 2250µF Voltage of DC bus: 700V Voltage of AC line: 415V, 50Hz

References

[1] Bhavna Jain, Shailendra Jain and R.K. Nena, “Investigations on Power Quality Disturbances Using Discrete Wavelet Transform,” International Journal of Electrical, Electronics and Computer Engineering, 2(2): pp.47-53, 05-August, 2014. [2] A.Rohini, M.Joorabian, “Modeling and control of DSTATCOM using adaptive hysteresis band current controller in 3-phase 4-wire distribution systems,” IEEE conference on Power Electronics, Drive Systems, pp.291-297, Feb 2014. [3] Raj Kumar, Bhim Singh and D.T.Shahani, “Control of DSTATCOM Using S-Transform,” IEEE Innovative Smart Grid Technologies, pp.226-231, 2018. [4] Y.H.Gu and M.H.J Bollen, “Time-Frequency and Time Scale Domain Analysis of Voltage Disturbance,” IEEE Trans. Power Del., vol.15, no.4, pp.1279-1284, Oct.2020. [5] Raj Kumar, Bhim Singh, D.T.Shahani and Chinmay Jain,” Dual-Tree Complex Wavelet Transform Based Control Algorithm for Power Quality Improvement in a Distribution System”, IEEE Transactions on Industrial Electronics, vol.64, no.1, pp.764-772, 2017. [6] L.Angrisani, P.Daponte, M.D Apuzzo and A.Tesla, “A Measurement Method Based on the Wavelet Transform for Power Quality Analysis,” IEEE Trans. Power Delivery, vol.13, pp.990-998, Oct. 1998. [7] M.Steinbuch and M.J.G van de Molengraft, “Wavelet Theory and Applications

Copyright

Copyright © 2023 Mr. Voggu Haricharan , S Radha Krishna Reddy . This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.