Smart Protection System For Future Power System Distribution Networks With Increased Distributed Energy Resources

ABSTRACT

This thesis investigates the impact of increased penetration of distributed energy resources (DERs) on the power system distribution network protection system which has been designed on the premise of passive radial network with unidirectional power flow. The investigation involved developing a multistage morphological fault detection and diagnostic tool called the decomposed open-close alternating sequence algorithm using a signal processing technique called mathematical morphology. This investigation culminated in proposing new strategies for; adaptive overcurrent protection in AC radial distribution network with increased DER penetration and high impedance arc-fault detection in AC and DC power distribution networks.

TABLE OF CONTENTS

COVER PAGE

TITLE PAGE

APPROVAL PAGE

DEDICATION

ACKNOWELDGEMENT

ABSTRACT

LIST OF ABBREVIATIONS

CHAPTER ONE

• INTRODUCTION
• BACKGROUND OF THE PROJECT
• PROBLEM STATEMENT
• RESEARCH AIM
• RESEARCH QUESTIONS
• RESEARCH CONTRIBUTIONS
• SIGNIFICANCE OF THE RESEARCH
• RESEARCH METHODOLOGY
• STRUCTURE OF THE STUDY

CHAPTER TWO

LITERATURE REVIEW

• INTRODUCTION
• OVERVIEW OF POWER SYSTEM PROTECTION
• BASIC COMPONENTS OF PROTECTION SCHEME AND THEIR FUNCTIONS
• CONVENTIONAL DISTRIBUTION NETWORK
• CHALLENGES IN MODERN DISTRIBUTION NETWORKS
• INTEGRATION OF RE BASED DERS
• CHALLENGES IN DISTRIBUTION NETWORK OC PROTECTION SYSTEM DESIGN DUE TO INCREASED PENETRATION OF RE BASED DERS
• FAULTS ON AC SYSTEMS

CHAPTER THREE

3.0       METHODOLOGY

• DESIGNING THE MULTISTAGE MM ARC FAULT DETECTION ALGORITHM
• RESEARCH METHODOLOGY UTILIZING THE MMTECHNIQUE
• BACKGROUND OF MM BASEDTECHNIQUES
• DESIGN OF THE MORPHOLOGICAL ALGORITHM FOR POWER SYSTEM FAULT DETECTION
• ATTRIBUTES OF THE DOCAS ALGORITHM
• DETECTING POWER SYSTEM DISTURBANCES

CHAPTER FOUR

• INTRODUCTION
• THEVENIN EQUIVALENT PARAMETER ESTIMATION
• EFFECT OF PV SYSTEM PENETRATION ON FEEDER SUBSTATION FAULT CURRENT LEVEL
• PV SYSTEMS FAULT CURRENT CONTRIBUTION
• EFFECT OF FAULT LOCATION ON FEEDER SUBSTATION FAULT CURRENTLEVEL
• APPLICATION OF       MFD   OUTPUT        IN        ADAPTIVE    RADIAL DISTRIBUTION FEEDER OC PROTECTION
• ADAPTIVE OVERCURRENT THRESHOLD VALUE
• SIMULATIONS ANDDISCUSSION

CHAPTER FIVE

• CONCLUSION
• SUMMARY
• FUTURE DIRECTIONS
• REFERENCES

LIST OF ABBREVIATIONS

CHAPTER ONE

1.0                                                         INTRODUCTION

This chapter introduces the topic of the thesis and provides background discussion related to the topic including general overview of distribution network protection, its components and their functional attributes. The technical challenges introduced due to the increased penetration of RE based DERs in distribution network feeder protection are discussed. The discussions further highlight the coexistence of AC and DC systems within the distribution network made possible by the advances in power electronic converters and interfacing technology, and the need to provide effective protection in both systems.

The current trend in the design and utilisation of modern distribution networks includes diverse generating sources including renewable energy (RE) sources directly connected to the distribution feeders as distributed energy resources (DERs). The inclusion of RE based DERs changes the passive unidirectional power delivery nature of the radial distribution networks to active networks with bidirectional current flow. The conventional OC protection system in radial distribution feeder relies on current magnitude as a threshold metric, thus it is imperative that sufficient fault current magnitude above the predefined threshold limit during fault must exist for this scheme to be effective. The direct interconnection of RE based DERs at the distribution feeder contravenes this fundamental requirement as the DERs supply power to distributed loads along the feeder length thereby reducing the current supply emanating from the feeder substation. This can affect the coordination of the protective devices in the feeder OC protection scheme hence compromising its effectiveness in responding to fault conditions reliably.

Furthermore, other fault conditions such as high impedance faults (HIFs) resulting from fallen conductors as well as energised conductors making unwanted contacts with tree trunks and branches are quite common in medium (MV) and low voltage (LV) distribution networks. Unlike OC faults, this category of faults generates a low current magnitude rendering the feeder OC protection scheme ineffective in detecting such faults. This inherent difficulty can be exacerbated by increased levels of RE based DERs in the distribution feeders.

In recent times, the advances in power electronic converters and interfacing technology has enabled the creation of DC subsystems within the AC distribution networks to directly supply DC power to DC loads from DC sources such as photovoltaic(PV) based RE sources or through AC-DC inverters from traditional AC sources. This adds further challenges in designing protection systems for DC systems as the protection mechanism in AC systems cannot be applied in DC systems.

These technical challenges introduced due to the increased penetration of RE based DERs define the scope of the research problem. The aims and objectives of the research, the research questions and the research contributions are also defined in this chapter.

1.1                                             BACKGROUND OF THE SUDY

Existing distribution feeders and their integrated protection systems are not designed for high penetration of renewable energy (RE) based distributed energy resources (DERs). The overcurrent protection systems are designed considering the passive, unidirectional current flow. However, integration of the RE based DERs such as PV systems through power electronic inverter interfaces fundamentally changes the distribution network from passive to active network with bidirectional current flow. The increased use of inverter interfaced RE based DERs and loads will result in increased harmonic injection affecting power quality. Moreover, increased penetration of RE based DERs will reduce the level of fault current magnitude from the feeder substation source. This will adversely affect the feeder protection system to provide effective protection as the fault current could fall below the overcurrent threshold.

Faults in power systems (both in AC and DC system) are inevitable and will occur at one time or another. Certain fault types, such as high impedance faults (HIF) in AC systems generate low fault current magnitude as opposed to high fault current magnitude from common short circuit faults which renders the feeder overcurrent (OC) protection mechanism ineffective in HIF detection. This type of faults must be detected and isolated as they can cause fire hazards and increase the risk of electrocution. The inherent difficulty in HIF detection using OC protection scheme in medium- (MV) to low voltage (LV) where HIFs are a common occurrence can be aggravated by penetration of RE based DERs. HIF detection and classification based on feature extraction rather than simply using current magnitude as a metric for HIF detection will fail. This is due to low fault current magnitude from HIFs and moreover, increased penetration of RE based DERs that reduces the fault current magnitude.

Short circuit faults on the other hand result in large fault current having potential to cause severe damage to power system apparatus and switchgear as well as causing instability to the unaffected portion of the power system, thus must be speedily detected and isolated. Short-circuit fault conditions generate transients in fault current with an exponentially decaying DC-offset. The DC-offset distorts the fault signal waveform and may compromise the integrity of the relay algorithms such as those based on fast Fourier transform (FFT) and wavelet transform (WT) thereby resulting in computational delays in the detection of the fault condition. As the accuracy and speed of convergence of conventional FFT and WT relies on the periodicity of the fault current and voltage, their effectiveness under DC-offset and HIFs are limited. Moreover, most DC-offset suppression techniques utilise parameter estimation, and can add additional computational delay.

Fault protection systems in DC distribution are at their infancy as compared to the fault protection systems in AC distribution. Faults in DC systems including DC side of PV system exhibit characteristics quite different from AC system generally because of different voltage (V) and current (I) characteristics in DC systems. DC systems generally suffer from short circuit as well as open circuit faults resulting from mechanical separation of conductors, and in most cases resulting in sustained arcing. An overcurrent protection strategy using current magnitude as a threshold metric is applied for all types of faults in the DC power systems including PV systems. However, not all fault conditions on the DC system can be adequately protected using such a strategy. One such fault condition is the DC arc-fault occurring on the DC systems including the PV system. DC arc-fault can either be a parallel fault (a short-circuit fault) or a series fault (an open-circuit fault). In PV systems, the detection mechanism relies on backfed current to detect theses faults. The nature of the faults, especially the series fault contravenes the logic in its detection using current as the threshold metric. The difficulty in DC arc-fault detection is compounded in PV systems, particularly at low irradiance which also includes night to day transition and partial shading. The fast action of the maximum power point tracking (MPPT) algorithm to put the system at different MPP operation also imposes additional difficulties in the task of developing accurate reliable DC arc-fault detection techniques.

In this work, a fault detection and diagnostic tool call the decomposed open- closed alternating sequence (DOCAS) morphological fault detector (MFD) has been proposed for application in fault detection in both AC and DC systems. The DOCAS algorithm is a multistage morphological filter constructed from two nonlinear Mathematical Morphological (MM) filters called the Morphological Median Filter (MMF) and the Alternating Sequential Filters (ASF). The MM based technique analyses the topography of the input signal waveforms by means of a probing signal called the structuring element (SE) in complete time domain. MM has the ability to detect seemingly insignificant changes in the topography of the signal waveform being investigated. The DOCAS algorithm uses a decomposed weighted SE to enhance its performance in fault detection. The designed structure of DOCAS algorithm allows it to be seamlessly applied in fault detection in both AC and DC systems without any structural change. The characteristics of the MM technique make the DOCAS algorithm convenient for the detection and classification of HIFs as well as DC Arc-Faults in PV systems.

The performance of the DOCAS algorithm has been tested in radial  distribution feeder with PV based RE sources connected as DERs for short circuit faults studies. From these studies, a strategy for adaptive radial distribution feeder OC protection with built-in DC offset suppression capability is proposed. The DOCAS algorithm’s capabilities in HIF detection and classification based on feature extraction has been tested on various contact surfaces using the IEEE 13 bus test system. The test results showed that DOCAS is capable of extracting successfully the two target HIF features including randomness and arc extinction and re-ignition characteristics. A strategy for HIF detection based on the extraction of the two target features is proposed. The DOCAS algorithm was tested in a radial distribution feeder with PV based RE sources as DERs for DC arc-fault detection on the PV side. The performance of the algorithm has been remarkable with all cases of DC arc-fault detected under all simulated conditions including low irradiance and changing maximum power point (MPP).

1.2                                                                              PROBLEM STATEMENT

The challenges and issues in power system protection considering the changing landscape in distribution network due to the integration of RE based DERs are summarised here, and they constitute the problems that motivated this research.

The conventional distribution network feeder OC protection has been designed for passive radial network with unidirectional current flow. The penetration of the RE based DERs at the feeder affects the conventional feeder OC protection as well as introduce challenges in the redesign and implementation of feeder OC in the following way;

1. Distribution network feeder is no longer passive; it is dynamic with bidirectional current
2. Network topology changes intermittently due to RE based DERs switching in and

• Increased fault current contribution by the DERs affecting the feeder OC relay threshold (pick up) parameter setting thus compromising the relaying (switching)

1. Prevalence of DC-offset and its effect on fault current magnitude estimation for OC protection. This is an existing challenge that must be considered.

Challenges in High Impedance Fault Detection in Distribution Network

High impedance faults are an existing phenomenon, whether with or without DER penetration will always be difficult to detect. While research has spanned decades, a universal HIF detection and classification algorithm is yet to be developed. This is because of the challenges imposed by the characteristics of the HIFs. These include;

1. Low fault current magnitude typically, between 10-50 A which is much lower than the OC protection threshold (pick up) parameter setting. This makes it difficult or impossible for the OC protection mechanism to respond to HIFs. Thus, requiring HIF detection strategy that does not rely on OC
2. The HIF current waveform is erratic and has asymmetrical positive and negative half cycles with shoulder
3. The HIF current has high frequency harmonic components from 2 to 10 kHz
4. HIF current build-up
5. Non-stationary frequency spectrum
6. Highly random, with non-linear voltage-current (V-I) characteristics. No two HIF will exhibit same characteristics
7. HIF characteristics are dictated by the contact surface, the network condition, the environment and the weather

Challenges in DC Arc-Fault Detection in DC Bus and PV Strings under Low Irradiance

DC loads and DC sources are connected through a DC bus. In a PV system, particularly on the DC side, fault can occur at the input to the inverter or at the DC bus (bus formed by connecting the PV strings) or at the PV strings. The challenges in DC Arc-Fault detection that forms the basis for the motivation in the DC arc-fault detection proposed in this research include;

1. The existing (conventional) DC OC protection scheme is incapable of detecting DC arc-fault.
2. There is no natural zero-crossing on DC system faults, including DC Arc fault making it difficult in DC Arc-fault

• DC Arc-faults are difficult to detect in PV systems, under low irradiance, partial shading and day to night transition

1.3                                                        RESEARCH AIMS

The aim of this research is to develop a Fault Detection and Diagnostic Tool that can be used seamlessly in AC and DC distribution systems. The attributes of this tool are then used to propose strategies for; Adaptive OC protection in distribution networks with increased DER penetration, HIF detection and DC Arc-Fault detection in PV systems which are defined herein.

Strategy for an Adaptive Feeder OC Protection Scheme

The feeder OC protection would have the following attributes;

1. Adaptive to the change in landscape of power system structure at the distribution
2. Be versatile in the presence of bidirectional current flow and increased fault current injection from the DERs resulting in lower current magnitude at the feeder

• Having an OC threshold that is adaptive to the changing current

1. Fast computation of fault current magnitude including suppression of the exponentially decreasing DC-offset.
2. Must be able to detect power system fault and issue trip signal within 1 cycle of the fundamental

Strategy for HIF Detection and Classification

The attributes of the HIF detection and classification strategy would include;

1. Operate in tandem with OC protection system
2. Detect and classify HIF based on the HIF characteristics

• Must be able to differentiate between a HIF and non HIF related disturbances

1. Detect HIFs within a reasonable time delay

Strategy for DC Arc-Fault Detection in PV Systems

The attributes of the DC Arc-Fault detection in PV systems would include;

1. DC Arc-fault detection based on DC Arc phenomena that does not require a threshold parameter. In other words, make use of the chaotic behaviour of the DC arc phenomena to detect DC arc-fault.
2. Detect Arc fault under all conditions including, low irradiance, partial shading, night to day

• Must be able to identify the faulted PV string

1.4                                                                              RESEARCH QUESTIONS

To meet the research aims, the following research questions are defined;

1. What is the effective strategy for fault detection and diagnosis that can be applied in both AC and DC power networks with DC-offset suppression capability in AC power system fault detection as well as convenience of application in adaptive overcurrent protection in radial distribution feeders with increased RE based DER penetration?
2. What are the analytical and computational methods for developing a fault detection and diagnostic tool that can be seamlessly utilized in both AC and DC power systems?

• Are the analytical and computational tools capable of performing feature extraction for high impedance faults detection?

1. Are the analytical and computational analytical tools capable of performing feature extraction for DC Arc-faults detection?

1.5                                                                              RESEARCH CONTRIBUTIONS

The following are the contribution from this research:

-Tool for Fault Detection and Diagnosis: A fault detection and diagnostic tool based on Mathematical Morphology for time-domain analysis of the fault signal called the decomposed open close alternating sequence (DOCAS) morphological fault detector algorithm is proposed. This tool is a multistage filter based on two nonlinear morphological filters namely; the morphological median filter (MMF) and the alternating sequential filter (ASF). The MMF filter is comprised of two cascaded stages where the output of the first stage becomes the input to the second stage while the ASF has two layers; the open-close and the close-open alternating sequential filters each with four stages. The two layers of the ASF operate simultaneously, and each stage of the ASF is cascaded in a hierarchical manner where the output of the previous stage is cascaded to the next stage. This operational sequence is achieved through the decomposition of the filtering signal call the structuring element (SE) into two SEs used in the two different filters. The underlying nature and computation technique in the DOCAS algorithm makes it possible for its application in both the AC and DC power system fault detection and diagnosis. The complete process in developing the DOCAS algorithm is presented in section 3.4.

-Methods for analysing impact of RE based DERs on fault current magnitude and Adaptive Inverse Time Overcurrent Relaying for Radial

Distribution Feeders with RE based DERs: An analytical method by means of Thevenin parameter estimation and two distance factors, distance to the RE based DER and distance to fault, to analysing the impact of the level of DER penetration on feeder current magnitude is proposed. Then a technique for adaptive pickup setting in overcurrent relays in distribution feeder overcurrent protection schemes using the DOCAS MFD output is proposed. Moreover, a technique for determining the relay trip time using inverse time overcurrent relay based on the adaptive pickup parameter is proposed. The techniques are presented in Chapter 4.

-Method for the Detection and Classification of HIFs: A technique for the detection and classification of HIFs based on feature extraction using the DOCAS algorithm was developed. The HIF features extracted from the DOCAS MFD output include the HIF randomness and the arc extinction and re-ignition feature. The feature extraction and HIF detection and classification are presented in Chapter 5.

-Method for DC arc-fault Detection: A technique for DC arc-fault detection, with applications in PV systems at any level of irradiance using the DOCAS algorithm was developed. The technique uses the DOCAS algorithm to generate MFD spikes to the chaotic behaviour of the sustained DC arc when ignited. The DOCAS algorithm generates MFD spikes in response to the rate of change of the arc, and the spikes sustained if the DC arc-fault exist.

1.6                                                                              SIGNIFICANCE OF THE RESEARCH

Power system infrastructure, including switchgear and powerlines at all subsystems of the power delivery system are constantly exposed to elements such as changing weather conditions and ageing and are prone to damage of fault conditions. In many circumstances the faults occurring on or involving powerlines have been blamed as sources of catastrophic bushfires and wildfires resulting in substantial destruction of properties and sometimes tragic loss of lives. For instance, on the 7^th  of February 2009, major bushfire, known as the Black Saturday bushfires in the state of Victoria in Australia destroyed lots of properties, livestock as well tragic loss of 173 lives. The Victorian Bush Fire Royal Commission identified electricity distribution infrastructure as the cause of these fires [24]. The Royal Commission based on its investigations made 67 recommendations of which 8 are directly to the electricity distribution infrastructure. The Victorian government in response to the recommendations of The Royal Commission established a Powerline Bushfire Safety Task force to investigate the recommendations directly related to the electricity distribution infrastructure. The Task force furnished a report on the 30^th of  September 2011 to the Victorian government, and out of several recommendations, one was for further research into improving safety by identifying and introducing new technology and methods for reducing risks and preventing bushfires ignited by the electricity distribution system.

Powerlines span several hundreds of kilo meters and are exposed to weather and often come into contact with trees and vegetation. Fallen powerlines due to support structure failure and/or powerlines coming into contact with vegetation almost always result in arcing. Unlike short circuit faults, these conditions are classified as high impedance faults (HIFs) and generate low fault current magnitudes resulting in not being detected by the overcurrent protection system. The longer the arcing fault conditions persist, the higher the risk of igniting fire and electrocution of people. These conditions must be detected and eliminated before they escalate into any catastrophic events like bushfire and wildfires.

Bushfires and wildfires are disastrous events that are bound to happen if the arcing sources such as HIFs are not detected and removed. HIFs generally result in low fault current magnitude thus making it extremely difficult for their detection using the conventional overcurrent (OC) protection scheme. Thus, specialise algorithm specifically for arc-fault detection including HIFs have to be developed that can work in tandem with the conventional OC protection system. Thus, the DOCAS algorithm satisfies this requirement. Moreover, the DOCAS algorithm can be used as part of an online intelligent power line condition monitoring system, for the detection of arcing events.

Fire in DC systems including PV based DERs is major cause for concern. The DOCAS algorithm can be seamlessly utilised in DC systems to detect the occurrences of DC arc-fault as fire prevention mechanism in DC systems including PV systems.

1.7                                              RESEARCH METHODOLOGY

In the course of carrying this study, numerous sources were used which most of them are by visiting libraries, consulting journal and news papers and online research which Google was the major source that was used.

1.9                                               STRUCTURE OF THE STUDY

This thesis consists of seven chapters including Chapter 1 which covers the Introduction where the problem statement, the research objectives, the research questions and research contributions were defined. The overviews of the rest of the chapters of the thesis are presented herein: two presents the literature review of the related works,  chapter three describes methods used, chapter four discusses the result analysis, chapter five is on summary of findings, conclusion and recommendation.