Abstract

A novel time–current-rate-based inverse characteristic curve for relays in a DC microgrid is proposed in this paper. Line current rise rate is used as actuating quantity, ensuring quick line fault clearing (relay operating time is in order of a few . The advantage of using line current rise rate as actuating quantity is that for a line short-circuit fault, it does not vary significantly for grid-connected and islanded modes of operation and varying network topologies of DC microgrid. Consequently, using the proposed characteristic, a single set of optimal relay settings is obtained using an optimization solver in MATLAB. The obtained settings ensure reliable and selective coordination between primary and backup relays for various pole-to-ground and pole-to-pole line faults under different operating conditions with multiple sources in two different 4-bus 400V low voltage DC microgrids. The maximum relay operating time for a high resistance fault with the proposed curve is for the first considered low voltage DC microgrid. Simulations in the Real Time Digital Simulator and comparisons with previous schemes (one comparison on the second considered DC microgrid), conventional standard inverse, and extremely inverse curves for DC microgrid protection indicate the effectiveness of the proposed scheme.

Introduction

The ease of integration of solar Photo-Voltaic (PV) and Battery Energy Storage Systems (BESSs) based renewable energy resources in a DC microgrid, compared to an AC microgrid, has led to the increased expansion and research interest of DC microgrids over AC microgrids [1], [2]. Also, DC microgrid can be a great way to power remote towns, shipboards, spaceships, and grids with sensitive loads, where the quality of power is crucial [3], [4]. However, the rate of rise of current is very high in a DC microgrid during a line fault due to the rapid discharge of the DC link capacitor and the associated low line impedance [5]. Hence, in general, the DC microgrid protection schemes follow a three-time frame based protection methodology in which the protection of the line, feeder, and the source should be done within a few milliseconds, and seconds, respectively [6]. Several DC Circuit Breakers (CBs) have been developed to support the quick isolation of lines during a fault in a DC microgrid, among which the solid-state CBs have a response time in the range of a few

Conclusion

This paper proposes a novel inverse time relay characteristic for the protection of DC microgrids against line faults using line current rise rate as an actuating quantity. It can coordinate relays in a DC microgrid with non-unit fault detection schemes or act as a backup scheme for relays with communication-based primary method. The relay coordination using the proposed characteristic is tested on a V LVDC microgrid. A single set of optimal relay settings ( and ) and a common set of values of and in (8) are obtained, considering all possible modes of operation, network configurations, and worst-case PG, PP LRFs and PG HRFs. Consequently, the proposed approach does not require an online update of and with a change in operating condition of DC microgrid. The efficacy of the proposed curve is validated with the obtained optimal unique relay settings for various PG, PP LRFs, PG HRFs for GC, islanded modes of operation, ring, radial network configurations, and intermittency of generation from renewable energy sources. Zero relay miscoordinations with a maximum primary relay operating time with the proposed curve are and for LRF and HRF faults, respectively, proving the effectiveness of the proposed technique in LVDC microgrid.

Further, the issues of using SI and EI curves to coordinate relays in a LVDC microgrid using line current and line current rise rate as actuating quantity and issues with using line current as actuating quantity in proposed scheme are discussed. It is observed that the conventional EI curve with line current rise rate may be used for relay coordination in DC microgrid. However, the high relay operating time (maximum primary relay operating time being ) may lead to prolonged line fault isolation. Additionally, the proposed scheme is fast and reliable, compared to EI curves, since the sum of primary relay operating times, considering all fault scenarios, is with the proposed curve, compared to the EI curve, which gives . The comparison with [16], [17] reveals the efficacy of the proposed scheme with the relay operating times in the proposed scheme being in the range of a few for line faults in LVDC microgrid.

Highlights

Abstract

Keywords

Abbreviations

1. Introduction

2. Microgrid model

3. Description of the proposed multi-functional algorithm

4. Simulation results

5. Comparison with other existing techniques

6. Conclusion

CRediT authorship contribution statement

Declaration of Competing Interest

References

Abstract

A large part of the current microgrids (MGs) operate in grid-connected mode and act as slaves following the voltage and frequency dictated by the main grid. Therefore, these MGs are not expected to be operated in parallel as islands. Hence, according to the IEEE 1457-2018 standard, the power generation units in these grid-connected MGs (GCMGs) have to be disconnected in less than 2 s if islanding occurs. On the contrary, if the islanding operation (IO) does not occur, these MGs exchange power with the electrical grid. This paper explores the feasibility of a multi-functional algorithm (MA) for these GCMGs in an attempt to address three tasks simultaneously; (i) power sharing, (ii) voltage support and, (iii) islanding detection (ID). The MG object of study comprises an electronically interfaced photovoltaic (PV) unit supplemented by a battery energy storage system (BESS) equipped in combination with a bidirectional charger. The model of the GCMG has been implemented in MATLAB, where an extensive set of simulations has been performed. The results demonstrate the effectiveness and benefits of the proposed MA, which fulfils several functions all at once.

1. Introduction

Conceptually, MGs are self-sufficient entities formed by an aggregation of generation units, energy storage systems and a set of controllable loads [1]. These MGs can be either connected to the grid (i.e., the abovementioned GCMGs) or operating in stand-alone mode [2]. Even though these GCMGs can be powered by different technologies (Wind, PV, mini-hydro, etc.), rooftop PV units are becoming very popular in residential and industrial applications due to their multiple benefits. Even though these GCMGs require the main grid, if both generation and storage devices are properly coordinated and sized, the amount of energy exchanged with the grid can be significantly reduced. Therefore, these GCMGs are becoming less grid-dependents in terms of exchanged energy.

In this regard, multi-functional algorithms are gaining momentum in MGs. For example, a fully distributed MA control has been proposed in [3], while references [4], [5], [6] used MAs for optimal MG management. If the peak power is shaved and the energy supplied by the grid is curtailed, this will imply a reduction in the electricity cost [7], [8]. Undoubtedly, this aspect is crucial for these MGs; for instance, see the techno-economic analysis carried out in [9].

Albeit these MGs offer several advantages for the users and the DNs (e.g., losses reduction and voltage support) [10], [11], some issues may occur due to their inherent intermittency. In particular, voltage regulation is among the most concerning issues for the distributor operators (DSOs). Although these PV-based distributed generations (DGs) are commonly operated at unity power factor, some grid codes require reactive power support by the voltage source converters (VSCs). Therefore, optimal Volt/VAR strategies have been investigated for such purposes, see [12]. Presently, the electronically-interfaced grid-connected generation units which provide grid support are also known as grid-following VSCs, see [13].

Abstract

Document Sections

I. Introduction

II. Circulating Current

III. Proposed Method to Eliminate Circulating Current

IV. Simulation and Experimental Results

V. Discussion

VI. Conclusion

Abstract:

Parallel pulsewidth modulation rectifiers are a good solution for high-power ac–dc applications, which enhance quality and reliability. However, the main and inevitable shortcoming of parallel structures is a generation of circulating current. Various methods are proposed to reduce or eliminate circulating current, most of which tend to change or limit the switching pattern of parallel converters. This article presents an interfacing circuit with low rating elements, which eliminates the circulating current without any limitation for the switching pattern of parallel converters. Therefore, voltage and current waveforms are in high quality. Moreover, unlike previous methods that had complex control and high mathematical computations, the presented method has simple control. Therefore, multiple modules can be implemented in a modular fashion with a fast dynamic response. Simulation and Laboratory results are presented to validate the analysis.

INTRODUCTION

he use of parallel PWM rectifiers (PPWMR) is essential in a wide range of applications in electrical industry such as renewable energy systems and motor drives [1, 2]. High power conversion, higher reliability, enhanced power quality, improved current and voltage waveforms are among the benefits of parallel structure. The main problem with the application of such structures is the inevitable creation of circulating current, which will damage semiconductor devices, increase losses, cause distortion in current and voltage waveforms, reduce efficiency and reliability of the system [3, 4]. Circulating currents are categorized as high and low frequency components [1, 5] which are due to different switching states of parallel rectifiers and different control system of converters respectively. Also, different design components and unbalanced voltages are of concern in low frequency circulating currents.

Low frequency circulating currents are normally reduced using a droop control strategy [6] and also PI controllers which are dependent on circuit performance in different states. Among these strategies are PI controller with feed forward controller [7], deadbeat control method [8], using a PIR with Manuscript received November 12, 2020; revised April 18, 2021 and June 16, 2021; accepted July 14, 2021. H. A Porkia and J. Adabi are with the Faculty of Electrical and Computer Engineering, Babol Noshirvani University of Technology, Babol 47148- 71167, Iran (e-mail: j.adabi@nit.ac.ir). F.Zare is a Professor with School of Information Technology and Electrical Engineering, University of Queensland, Brisbane, Australia, (e-mail: f.zare@uq.edu.au). feed forward term which requires information of switching vectors in each period [9], improved D-∑ digital method [10], finite-time controller and zero-vector feed forward control method [5]. [11-14] introduces a strategy based on an average model. [15] investigated elimination of circulating current due to third harmonic order.

Main solutions for high frequency circulating current reductions are mainly classified in three main categories: Filters, PWM strategies and topology reconfiguration. Each of which has its advantages and disadvantages. Coupled inductor (CI) [16-18], common mode inductor (CMI)[19], inter-phase inductor [20] and modified LCL (MLCL) [6] are the main structures in the category of filter-based techniques. Mainly, they have issues regarding size, cost and losses. However, according to [3, 6], MLCL leads to internal common mode resonance current and external zero-sequence resonance circulating current.

This article presents a novel fractional order proportional–integral–derivative (FOPID) controller with fractional filter for an automatic voltage regulator (AVR) system. The proposed controller has seven independent parameters to be set. These parameters are optimally tuned by using sine–cosine algorithm (SCA) capable of balancing exploration and exploitation phases to find the optimal solution. The optimally tuned controller named as SCA-FOPIDFF is employed to enhance the performance of the AVR system. In order to show the effectiveness of the proposed controller, a FOPID controller with integer filter called as SCA-FOPIDF, a traditional FOPID controller without filter called as SCA-FOPID, a PID controller with fractional filter called as SCA-PIDFF, and a PID controller with integer filter called as SCA-PIDF are also designed for the AVR system using the SCA with the same objective function. In addition, the superiority of the proposed controller is also validated by the comparison with published studies proposing optimally tuned PID and FOPID controllers for the AVR system. Frequency response characteristics of the proposed controller obtained from bode analysis are presented. Finally, the robustness of the designed controllers are examined separately against both parameter uncertainties in the AVR system and external disturbances injected into the AVR system. Considering the overall results presented, it is clear that the fractional filter in the proposed SCA-FOPIDFF controller has significantly improve the AVR system performance and that the proposed controller can be successfully applied to the AVR system.

1. Introduction

Real power losses in power generating units is a general issue needing to be solved. Researchers study on automatic voltage regulator (AVR) systems to minimize these power losses. The main aim of these AVR systems is to keep generator voltage at the desired level with high accuracy. Ensuring this accuracy increases the durability of equipment designed with the rated voltage in a power system network. In addition, the stability and robustness of the AVR system remarkably affect the safety of the power system [1]. Therefore, increasing the efficiency of the AVR system by developing new control techniques is still a current and important issue. In particular, improvement of the transient response of the AVR system within the stability limits is heavily studied.