ABSTRACT
Over the last decade, the power system has seen a significant rise in the installation of Distributed Generation(DG). Although implementation of DG has several advantages by supporting the system, its steady growth requires special consideration to issues relating to high DG penetration. The challenges faced by integration of DG cover many aspects, extending from technical to regulatory issues. In terms of its implementation, one of the main problems associated is its effects on the protection system. Faults in a power system can occur at any time. Having the protection devices affected is not desirable as it could be dangerous for both the customers and the power network. Therefore, simulations have been performed by forming case studies that relate to the real-world power system designs to deeply understand all the issues faced by the protection system with the connection of DG. The result indicates problems faced by protection devices in terms of device coordination, sensitivity and overall changes in fault contributions with respect to different fault and DG locations. Furthermore, a guideline has been developed to understand the impact of DG on fault protection system that can provide recommendations for DG connections.
Abbreviations and Symbols
EPS – Electrical Power System
DG – Distributed Generation
PS – Protection System
FC – Fault Contributions
CB – Circuit Breaker
Chapter 1. INTRODUCTION
1.1 BACKGROUND
This report is devoted to the protection side of the electrical power system (EPS), thus, adding to its design. Faults in a power network can be dangerous and if neglected, can be extremely damaging for the system including damage to components as well as threatening the safety of customers. The purpose of the protection system(PS) is to isolate faults by removing the faulty circuits from the network in order to ensure the customers receive a safe, reliable and continuous supply of power which is extremely important [1]. Hence, it is important to identify these problems thoroughly. Existing traditional protection schemes have been followed for a long time and have been used to select protection devices and their settings.
However, with the presence of Distributed Generation(DG), present schemes become affected and may no longer be able to function to their maximum ability. DG is usually a much smaller power source compared to the main source which is normally connected at the distribution level or the load end of the feeder [2]. The implementation of DG brings several benefits to a power network them being reduced power flow, decreasing over losses, aiding voltage profile and overall improving the network stability [3] as well as providing the option for expansion of the local network [2] therefore making it very desirable.
1.2 PROBLEM STATEMENT
One of the main issues faced by the integration of DG is its implementation in a radial power network. A radial power system has a main power source that feeds power to the loads after stepping down the voltage from the generation/ transmission side. This type of system has a unidirectional flow of current [1].The traditional PS has been designed with respect to the same. Adding DG on the low voltage side disrupts the unidirectional flow of current hence, making it bidirectional [4-6] and contributing to all faults making it difficult for the PS to function to their maximum ability and causing problems such as increased or decreased fault current and miss-coordination of devices[5-7]. IEEE Standard 1547 states that in case a fault occurs in a system in which DG is connected, the DG must disconnect in order for the PS to work normally for a radial system[8]. As DG helps the system, disconnection of DG is not desirable for the load. But, at the same time, DG connection makes faults difficult to be detected. Therefore, it is important to improve PS settings in order to work with DG without any issues for safe implementation.
1.3 OBJECTIVES
Therefore, the objectives of this project are to analyse the PS performance with different types of DG at different locations in the feeder with the impact of their intermittency and uncertainty as well as identify the issues where DG can interfere with the PS selectivity and grading of operation between two or more protective devices and how protection function be affected per their existing setting. Finally, the outcome of these objectives is to develop a guideline to evaluate the impact of DG on the EPS that can provide recommendations for its connections.
1.4 Report Layout
The report has been divided into four main sections. The table below discusses the basic overview of the chapters.
Table 1: Report Layout
Chapter 2 – Literature review
In the literature review, all research related the project has been defined. Understanding existing problem and comparison between existing schemes
Chapter 3 – Theoretical Model
The theoretical model chapter explains the problem using theory. It also explains how the shape of the research design has been chosen.
Chapter 4 – Design & Analysis
This sections of the report include the two main experiments performed in order to understand the effects of DG and hence develop recommendations.
Chapter 5 – Conclusion
The conclusion briefly describes the problem, the analysis of the experiments performed and future goals of this project.
Chapter 2. LITERATURE REVIEW
This literature review intends to explain what impacts can the implementation of Distributed Generation(DG) have on the electrical power system (EPS). Current literature illustrates that while there are several advantages that DG has to offer, there are many disadvantages that come along with its implementation, one of them being its effects on the protection system(PS). This literature review aims to discuss the various effects and see what contribute to those effects and what can be done to achieve normal operation of the PS.
2.1 Distributed Generation(DG)
By the early 21st century, DG had started to take an immense rise in the market due to its benefits although its effects were equally interesting to study. The authors of [9] analysed DG thoroughly and presented a basic understanding of what it is, its purpose and explaining different types of DG varying from renewable energy resources to different types of fuel cells. This paper also briefly discussed its negative effects on the distribution network including problems faced with the PS, voltage levels and stability. [2] also discussed the benefits of DG regarding the liberalisation of the electricity market and how DG can act as an alternative to expand the local EPS network. While these papers gave a good overview of what DG is, negative effects were not vastly discussed.
2.2 Effects of DG
2.2.1 Device Malfunction
This was although talked in more depth by the authors of [10] and [4]. In [10], problems associated with overcurrent protection and islanding as well as other issues such as voltage flicker and regulation for a radial power system were discussed. DG can add to the overall fault contributions (FC) of the ES which can lead to miss-coordination of protective devices. Also, different types of DG can have a different effect and that mainly single DG unit may have a lesser impact compared to multiple DG units connected to a PS. [4] talked about different DG characteristics such as size and location having different fault contributions.
Protection coordination was also discussed by the authors of [5] and[6] who explained that different fault current may be seen by different protection devices and hence, affecting performance ability. DG can also cut into the reach of protections devices causing them to either falsely trip or cause a fault to go undetected. S. Javadian [6] also examined the fault locations. This presented an insight on what all factors can affect the PS which can be used in understanding the effects more broadly although they type of DG has not been discussed and no solution for this problem was has been proposed.
2.2.2 Size, Location, and Type of DG/Fault
Fault and DG location can play a big role in affecting the ability of the PS. The authors of [11] studied this fact by performing circuit analysis of a feeder by placing DG and the fault at different locations. For this analysis, they analysed the fault contributions of the circuit initially without DG and then performed three main circuit analysis with DG and fault at different locations. Placing DG into the system showed an overall decrease in the source fault contributions. From the first case (DG before and fault after a recloser), it was found that the recloser would read both DG and the source fault contributions hence, an increase in the overall fault contributions without DG which could cause a miss-coordination between any downstream devices as it could cause faster tripping of the device. For the second case (both DG and fault after the recloser), the analysis showed that the recloser would only see the fault contributions of the source hence decreasing the overall fault current seen by the fault causing the recloser to become less sensitive and take longer to trip. The third case (fault before while DG after recloser) showed a fault current flowing through the recloser even though for the fault being placed before it. This presented the fact that if the contributions of the DG exceeded the trip value of the recloser, it would break which would be unacceptable in this case. These case studies provided an understanding of how different location of DG and fault could affect the PS differently although did not take into consideration the size and type of DG which can also be a factor in determining fault contribution. Although this method can be applied by analysing the fault contributions in multiple bus locations.
After learning that size and location of DG/fault can have different effects, it is important to understand how differently they affect the ESP. The authors of [12] talked about the location of fault and also introduced effects of type of DG. It was acknowledged that as faults move further away from the source, the fault contributions decrease. In terms of DG type, Inverter type sources inject less fault current compared to self or separately excited DG generators. This fact was further analysed by the authors of [13]. Simulations were performed on different types of DG sources of different sizes for different locations on fault. It was shown by simulation that a fault closer to the source had higher fault contribution compared to a fault further away from the source. The experiment also proved that increase in DG size will increase the overall fault contributions and that inverter type DG i.e. Solar PV will have lesser FC than generator type such as Wind Turbine Generators. (include ending statement (critique))
2.3 Existing Schemes
2.3.1 DG Disconnection
Due to all the effects imposed by the implementation of DG, solution for reducing them is critical. The authors of [10] mention IEEE Standard 1547 which requires DG to be disconnected if a fault occurs. This causes the PS to turn back in to a fully radial system, and in turn can ensure proper functionality of protective devices according to the traditional settings. Ramesh Bansal [14] also discusses this solution. Although this supports in minimising the effects of DG and does not require new setting to be made, it is not completely desirable as DG aids the EPS by providing power. Hence, it is essential to solve the issue without disconnecting DG from the EPS.
2.3.2 Other Schemes
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Text that leads in the purpose of the paper you will discuss. Link this in with a sentence that brings in the paper e.g. This [fact] was studied by [Authors (year) Harvard Style or Authors [X] IEEE Style] who [did what]. Talk about the design of the experiment and findings. Critique the experimental design and findings including gaps or strengths and link this to the story you are trying to tell about your research.
Chapter 3. THEORETICAL MODEL
This chapter covers the basic theory involved with fault detection and explains the relationship between theory and the problems associated with the implementation of distributed generation(DG). Moreover, how this relationship will be used to design the experiment has been covered too.
3.1 Fault Current Calculations
When a fault occurs in an electrical power system(EPS), fault current flows through the system based on the circuit design, i.e. impedance of networks, sources and fault as well as type of fault[1]. This current can then be calculated for different locations of the fault.
3.1.1 Relationship and Expectations
Varying the position of the fault would vary the impedance of the network through which, the current flows. Hence, faults upstream would have higher fault current compared to faults downstream due to increase in impedance. Therefore, making a fault less detectable as we move further away from the source.
3.2 Protection System(PS)
Protection of the EPS is essential. The main objective of the PS is to minimise the duration of the fault in order to ensure safe and reliable power is delivered to the customers. Many devices are used for the necessary protection mainly circuit breakers, relays and instrument transformers.
3.2.1 Circuit Breakers (CB)
CB is one of the major components used for EPS. These devices are capable of automatically breaking a circuit under any conditions[15]. This is achieved with the help of instrument transformers and relays.
3.2.2 Relay
Relays are devices that are used to control CBs [16]. These devices respond to variations in circuit which are detected with help of instrument transformers, therefore, commanding the CBs to trip. Most relays used in a radial EPS are overcurrent [1], i.e. measure the magnitude of current flowing through the system and hence, respond accordingly with a time delay. Relay settings are set by a given equation [17]
… Equation 1
where t refers to the tripping time of the device, TD is the time dial setting and denotes multiples of pickup current while P, A and B are device constants.
With multiple CBs in an EPS, correct coordination is required which is achieved by time discrimination. Time discrimination is calculated from equation 1 for different relays hence, for upstream relays, time discrimination increases.
3.2.2 Relationship and Expectations
With the implementation of DG, overall fault current flowing through the circuit may vary depending on their locations while relay setting remains unchanged. Therefore, an increase or decrease in the current would affect the relay sensitivity by measuring a different magnitude compared to conditions without DG.
Moreover, relay selectivity may be affected. Selectivity refers to the ability of the relay to distinguish between different types of fault conditions that require no actions [15]. For cases where DG is placed after the fault, relays will detect the fault contributions(FC) of the DG and may trip the CB.
3.3 Experiment
Situations can then be created in order to test the above-mentioned matters by creating a system with various loads and line impedances and placing DG and fault at different locations for fixed device settings and observing the changes in the circuit with the implementation of DG. The aim of the experiment shall be to observe if the aforementioned theory corresponds to simulations performed and ultimately gain a better understanding of the effects of DG.
Chapter 4. DESIGN & ANALYSIS
This part of the report aims to experimentally analyse the effects of DG on an EPS. Two major case studies have been conducted with and without the presence of DG and have been compared. Table 2 shows the type of case study and analysis performed.
Table 2: Case Studies
Type
Analysis
Case Study 1: Fault Detection and Sensitivity
⎝ Change in Fault Current
⎝ Worst Fault and DG Location
Case Study 1: Nuisance Tripping for Fault on Adjacent Feeder
⎝ Device Coordination
⎝ Worst Fault and DG Location
⎝ Change in Fault Current
The MATLAB model for both case studies and the system specifications have been provided in appendix c.
4.1 Case Study 1: Fault Detection and Sensitivity
The single line diagram(SLD) for case study 1 is given in figure 1. It includes a main power source with four main busses each having a load connected to them and each line consisting of a circuit breaker(CB) which has been used to detect and isolate faults. The system also consists of a DG source.
Figure 1: Bus configuration for Case Study 1
4.1.1 Scenario
The CB in each line detects and isolates faults. While there is no DG present in the system, the main power source acts as the only contributor of fault current and the same current is seen by the fault as well as the respective line CB. Although the inclusion of DG can change the fault current seen by the CBs and can hence, affect their performance ability as DG implementation decreases source fault contributions(FC) [18]and thus fault current seen by the CB. This reduction depends on various DG parameters, i.e. size and location as well as fault location [11, 13, 19]. In this case study, only the location of fault and DG have been varied and its effects have been observed. Figure 2 shows the flow of fault current in the circuit and how CB reads different current value in comparison to the fault.
Figure 2: Flow of current for Case Study 1
4.1.2 Results
The impacts of DG and fault location have been inspected. Each of them has been moved along the feeder. DG has been connected parallel to each load individually and the fault has been moved around to observe effects. Fault has been considered only on the lines and not on the terminals of the load or the DG. The change in fault current has been stated in terms of current multiples of the fault current seen by the CB.
Figure 3: Reduction in fault current for different fault locations
The change in fault current observed by the respective CBs for different locations of fault and DG with respect to the most upstream fault is shown in Figure 3. It refers to the reduction of current for the CB that trips the fault. As the fault moves away from the main source, the reductions in fault current tend to increase and the closer the DG is to the main source, the reductions are greater compared to DG further away from the source. Hence, it can be concluded that the worst-case scenario for fault is the very end whereas the worst-case scenario for DG is the very beginning for the reduction in fault current.
Although a decreasing current pattern is seen for fault moving away from the feeder, an overall increase in current was spotted after the installation of DG.
Figure 4: Rise of initial fault current
Figure 4 displays the amount of increase in current magnitude seen by the CBs that trip the fault after installation of DG. It can be observed that after the implementation of DG, the magnitude of current seen by different devices changes. It was found that the closer the DG to the fault, the higher magnitude of current is seen by the fault. Lastly, the increase pattern shows that as DG moves further away from the source, FC from DG and fault current seen by the CB increase for a fault just before the DG.
For a fault at line 1 and DG at bus 3, CB 1 should clear out the fault although CB in line 2 trip before CB in line 1 causing the devices miss-coordinate amongst themselves. This was due to the FC of the DG that caused the CB to trip.
4.2 Case Study 2: Nuisance Tripping for Fault on Adjacent Feeder
The single line diagram(SLD) for this study is given in figure 5. It includes a main power source supplying two parallel feeders with the main feeder having four main busses and the parallel feeder consisting of 3 busses each having a load connected to them. Each line consists of circuit breakers(CB) which have been used to detect and isolate faults.
Figure 5: Bus configuration for Case Study 2
4.2.1 Scenario
In the case of two parallel feeders, a fault in one feeder will not cause a CB in the other feeder to trip. Although, integration of DG means it will contribute to the fault current. For a DG connected in second feeder and the fault in first feeder, the FC will not only be seen by CBs in feeder 1 but also by CBs in feeder 2 (DG FC) as shown in figure 6. Therefore, it can cause CBs in the healthy feeder to trip which is not a suitable situation. This varies depending on the DG parameters (size and location) as well as fault location. In case study 2, only the location of fault and DG have been varied and its effects have been observed.
Figure 6: Flow of current for Case Study 2
4.2.2 Results
The impacts of DG and fault location have been analysed. The fault has been moved along feeder 1 while DG has been positioned at different locations in feeder 2. DG has been connected parallel to each load individually. Similar to Case Study 1, fault has been considered only on the lines and not on the terminals of the load or the DG. The change in fault current has been stated in terms of current multiples of the fault current seen by the CB.
Figure 7: Reduction in fault current for different fault locations
The change in fault current observed by the respective CBs for different locations of fault and DG with respect to the most upstream fault is shown in Figure 7. It refers to the reduction of current for the CB that trips the fault. Similar to the observations in case study 1, it is depicted from the figure that as the fault moves further away from the main source and DG, fault current decreases and closer the DG is to the main source, the reductions are greater compared to DG further away from the source. Hence, it can be established once again that the worst-case scenario for fault is the very end whereas the worst-case scenario for DG is the very beginning.
As DG has been placed in the healthy feeder, DG FC shall increase the fault current observed by the CB for both feeders.
Figure 8: Fault current seen by CBs of Healthy Feeder
Figure 8 shows the increase detected by the CBs in the healthy feeder. When a fault occurs in feeder 1, the DG in the healthy feeder contributes heavily adding to the FC causing CB to detect this. It can also be seen that as DG moves away from the fault, less current is contributed by the DG.
Performing this experiment also triggered CBs in the healthy feeder for two cases. CB 4 and 5 tripped for a fault in line 1 for DG at bus five and six respectively although did not trip for a fault further downstream feeder 1. This was the cause of DG FC higher than the pickup current of the relay which causes it to trip hence affecting relay selectivity and device coordination. Thus, it can be concluded that for DG closer to the fault and main source, the FC caused miss-coordination of protection devices causing a healthy feeder to trip unnecessarily.
Chapter 5. CONCLUSIONS
The integration of distributed generation(DG) brings along many benefits although introduces several matters that interfere with the existing protection method which need to be correctly examined to maintain acceptable functionality of protection devices. In this thesis, problems associated with DG implementation have been recognised and comprehensively evaluated by simulations. Case studies were developed with regard to typical electrical power system(EPS) settings and design. Device coordination has been identified as one of the main issues with DG implementation followed by sensitivity of the relays.
5.1 Recommendations
Through thorough analysis of case studies, a brief procedure has been suggested for assessing the effect of DG implementation on existing protection system(PS). The procedure has been established in view of significance of the PS.
Step 1: Check for device coordination – Worst location for DG is after the fault but not too far away.
Step 2: Check for parallel feeder device setting for no miss communication – worst fault and DG locations are closest to DG.
Step 3: Check for reduction in Fault current – worst location for DG is the beginning of the feeder while worst location of fault is the end of feeder.
In addition, worst DG location is at the end of the feeder and worst fault location is closer to the DG.
Calculating maximum permissible DG penetration requires to have knowledge regarding satisfactory level of change of PS permitted as well as more comprehensive information regarding PS setting and system configuration.
5.2 Future Work
In ECTE 451, major concerns and effects regarding implementation of DG have been analysed and recommendations have been suggested. The recommendations allow DG users to have a basic understanding of the effects DG will bring along with its installation and what issues to check for. In ECTE 458, a method for islanding detection and anti-islanding protection system for DG shall be developed while also introducing a new protection strategy for islanding protection.
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APPENDIX A
APPENDIX B
LIST OF FIGURES
Figure 1: Bus configuration for Case Study 1 10
Figure 2: Flow of current for Case Study 1 11
Figure 3: Reduction in fault current for different fault locations 12
Figure 4: Rise of initial fault current 13
Figure 5: Bus configuration for Case Study 2 14
Figure 6: Flow of current for Case Study 2 15
Figure 7: Reduction in fault current for different fault locations 16
Figure 8: Fault current seen by CBs of Healthy Feeder 17
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