臺灣博碩士論文加值系統

微電網是一種新結構的配電系統，主要是由分散式電源組成，像是可再生能源、分散式發電、儲能系統、負載、能源管理系統和保護裝置等。微電網主要的優點為：(1)提高可再生能源的普及率；(2)改善電力系統的可靠度；(3)具有高效率並對環境有較低的影響。然而，微電網具有以下幾項挑戰：(1)故障保護與協調；(2)分散式電源的電力潮流控制；(3)市電併聯與孤島運轉模式之間的無縫切換；(4)電力品質問題。本文著重於開發交流與直流微電網的故障保護系統。

為了保護多重接地的交流微電網，本論文提出一套快速且高適應性的故障保護系統，此保護系統可解決微電網中多項因素所造成的故障保護問題，包括：因逆變器式的分散式電源與旋轉式的分散式電源之混用所造成的問題、微電網在孤島運轉模式下故障電流因逆變器之限流所造成的問題、雙向的故障電流/電力潮流所造成的問題。換言之，新型的故障保護系統在交流微電網中應該具有自行適應的能力，以期處理隨插即用及非主從之微電網特性。根據卡方分佈統計法、一項簡化的故障電流分析方法以及藉由通訊支援的系統，於本論文提出之快速且高適應性的故障保護系統中，可根據交流微電網的不同電源及負載組合(例如：微電網中電源和負載分路之併網及斷開)來自動調整其相對應的電驛跳脫位準。除此之外，此快速且高適應性的交流微電網保護系統可以準確的偵測、判別，並於1.5週波內定位故障區段，以期未來配合靜態開關防止微電網中逆變器式的分散式電源於微電網故障期間之快速跳脫。於台灣核能研究所中的380V交流微電網試驗平台所做的模擬與試驗結果，可用來驗證本文所提出的快速且高適應性的交流微電網故障保護系統。

本論文亦提出新的直流微電網故障保護系統，使用快速熔絲結合電力電子開關與數位電驛來保護低電壓的直流微電網。尤其是直流微電網的數位電驛包含不同的故障偵測模組，像是：差動電流保護模組、方向性過流/過電流保護模組、低電壓/過電壓保護模組，以及根據直流電流與電壓的時間導數之保護模組來保護於直流微電網的極對極與極對地故障。本文對於直流微電網數位電驛的不同故障保護模組之間、數位電驛之間以及熔絲與數位電驛之間的保護協調策略亦進行分析，以確認此保護系統具有高選擇性。此新的直流微電網保護協調系統之宗旨為縮短故障排除時間且保護系統有好的成本效益同時具有高選擇性，因此，直流電源像是太陽光電、蓄電池和燃料電池系統、電源轉換器等，都應該由快速熔絲來保護以求得其成本效益。直流微電網之數位電驛則用來保護電源/負載分路、幹線和一般的直流匯流排，目的在於讓保護系統具有高選擇性。本文提出之漏電流保護方法是對於高阻抗接地/非接地直流微電網架構(例如：對稱的直流單極微電網或直流雙極微電網)進行保護。依據低電壓直流微電網試驗平台之模擬與試驗結果，本文評估所提出的直流微電網保護系統及其保護協調策略之有效性。

Microgrid (MG) is an emerging configuration of power distribution systems, which mainly consists of distributed energy resources (DERs) such as renewable energy sources and distributed generators (DGs), energy storage systems (ESS), loads, energy management system (EMS), and protective devices. Main advantages of the microgrid are: (i) facilitating high penetration of renewable energy sources; (ii) improving reliability of the power system, and (iii) having high efficiency and low environment impacts. The microgrid, however, meets with certain challenges in (i) fault protection and coordination, (ii) power-flow control of distributed energy resources, (iii) seamless operation transitions of the microgrid between grid-connected and islanded operation modes, and (iv) power quality issues. This dissertation focuses on the development of fault protection systems for both the AC and DC microgrids.

A fast and adaptable (FA) fault protection system is developed to protect multi-grounded AC microgrids. The proposed fault protection system can solve protection problems caused by the mixed combination of inverter-based DGs and rotating-based DGs in the microgrid, limitation of fault currents by DGs’ inverters under an islanded operation mode of the microgrid, and bi-directional fault currents/power flows. In other words, the novel fault protection system should be self-adaptive, aiming to deal with plug-and-play and peer-to-peer characteristics of the AC microgrid. Based on a Chi-square distribution statistic method, a simplified fault current analysis approach, and the support of communication system, tripping thresholds in the FA fault protection system can be automatically adjusted corresponding with the variational AC-microgrid DG and load combinations (e.g., connection and disconnection of source and load branches to and from the AC-microgrid). Additionally, the fast and adaptable AC-microgrid protection system can accurately detect, identify, and locate the faults within one and half cycles with the future combined with use of solid-state switches, aiming to prevent the fast tripping of inverter-based DGs during the fault period of the AC-microgrid. Simulation and experiment results of a multi-grounded 380V AC-MG test-bed at Institute of Nuclear Energy Research - Taiwan are available to validate the fast and adaptable AC-MG fault protection system proposed.

As regards a novel DC-microgrid fault protection system, fast-acting fuses (FAFs) are combined with power electronic switches and digital relays to protect low-voltage DC microgrids. In particular, a DC-microgrid digital relay contains various fault protection modules such as differential current protection module, directional overcurrent/overcurrent protection modules, under-/over-voltage protection modules, and protection modules based on time derivatives of DC current and voltage to protect DC microgrids against pole-to-pole and pole-to-ground faults. Protection coordination strategies among different fault protection modules in a DC-microgrid digital relay, among the digital relays, and between the fuses and the digital relays are analysed to ensure high selectivity of the protection system. Aims of the novel DC-microgrid protection coordination system are to shorten critical fault clearing time and get cost-effectiveness while still ensuring high selectivity for the protection system. As a result, it is proposed that DC power sources such as PV arrays, battery and fuel-cell systems, and power converters should be protected by fast-acting fuses to get the cost-effectiveness of the protection system. DC-microgrid digital relays are required to protect source/load feeders, trunk lines, and common DC buses to get the high selectivity of the protection system. Leakage-current protection solutions are proposed for high-impedance grounded/ungrounded DC-microgrid configurations (e.g., symmetric DC mono-polar microgrids or DC bi-polar microgrids). Simulation and experiment results from a low-voltage DC microgrid test-bed are available with the effectiveness of the proposed DC-microgrid fault protection system as well as the protection coordination strategies being also evaluated.

List of Contents

摘要 i
Abstract iii
Acknowledgement v
List of Contents vi
List of Figures xvii
List of Tables xxviii
List of Abbreviations xxxi
List of Publications xxxiii

Chapter 1 - Introduction
1.1 Overview of AC and DC Microgrids 1
1.2 Protection Challenges of AC and DC Microgrids 3
1.2.1 Protection challenges of both AC and DC microgrids 3
1.2.2 Additional protection challenges of AC microgrids 5
1.2.3 Additional protection challenges of DC microgrids 6
a) Inadequacy of traditional AC circuit breakers for the DC-microgrid protection 6
b) Challenges in protection coordination of overcurrent relays 6
1.3 Literature Review 7
1.3.1 Protection of AC microgrids 7
a) AC microgrid protection at the grid-connected operation mode 7
b) AC microgrid protection at the islanded operation mode 7
c) Other AC microgrid protection solutions 8
1.3.2 Protection of DC microgrids 9
a) Non-unit DC-fault protection solutions 9
b) Unit DC-fault protection solutions 10
c) DC-fault protection devices 10
1.4 Development of AC Microgrid and DC Microgrid Protection Systems 11
1.4.1 Development of a fast and adaptable AC-microgrid fault protection system 11
a) Summarised limitations of the existing AC-microgrid fault protection solutions
11
b) Necessity to develop a fast and adaptable (FA) AC-microgrid fault protection system 13
c) Approaches 14
1.4.2 Development of a novel DC-microgrid fault protection system 16
a) Summarised limitations of the existing DC-microgrid fault protection solutions
16
b) Necessity to develop a novel DC-microgrid fault protection system 17
c) Approaches 19
1.5 Contributions 19
1.5.1 AC-microgrid fault protection 19
1.5.2 DC-microgrid fault protection 21
1.6 Outline of the Dissertation 24

Chapter 2 - Generalised Grounding Model Structures of AC and DC Microgrids
2.1 A Generalised Grounding Model Structure of AC Microgrids 26
2.1.1 PV systems in a multi-grounded AC microgrid 30
2.1.2 Energy storage systems in a multi-grounded AC microgrid 31
2.1.3 Non-renewable DG units in a multi-grounded AC microgrid 32
2.1.4 Grounding of a multi-grounded LVAC microgrid 32
a) Impact of the grounded isolation transformers on the microgrid protection 32
b) Impact of the grounding of power generators at DG branches on the MG protection 34
2.1.5 Main differences in fault behaviours between multi-grounded and uni-grounded
LVAC microgrids 36
2.2 Demonstrate by Simulating a Multi-Grounded 380V INER Microgrid 38
2.2.1 Structure of a multi-grounded 380V AC microgrid test-bed 38
2.2.2 Simulation model of Zone 1 of the INER microgrid 39
a) Modelling of a 65kW micro gas-turbine generation system 39
i) Inner current control loop of a power rectifier 43
ii) Outer voltage control loop of a power rectifier 43
i) Inner current control loop of a power inverter (referring to Fig. 2.9) 44
ii) The outer loop design of a power inverter 44
b) Modelling of a PV generation system 47
i) Modelling of a PV array 49
ii) DC/DC chopper and DC/AC inverter 50
iii) Model of a LCL Filter 50
c) Modelling of a 100kWh battery power conditioning system 51
d) PSCAD simulation model of Zone 1 and Zone 2 52
2.3 A Generalised Grounding Model Structure of DC Microgrids 54
2.3.1 Characteristics of an asymmetric monopolar DC configuration 55
2.3.2 Characteristics of a symmetric monopolar DC configuration 56
2.3.3 Fault protection of asymmetric &; symmetric monopolar DC configurations 57
2.3.4 A generalised grounding model structure of LVDC microgrids 58
2.3.5 The number and location of grounded points in the DC microgrid 60
2.4 Demonstrate by Simulating a 48V DC-Microgrid Test-Bed 61
2.4.1 Structure of a community-sized low-voltage DC microgrid 61
2.4.2 Structure of a 48V DC microgrid test-bed 64
2.4.3 Simulation models of asymmetric and symmetric monopolar DC microgrids 66
a) Simulation models of two-wire and three-wire symmetric monopolar DC microgrids 66
b) Simulation model of an asymmetric monopolar 48V DC MG 68
2.5 Summary 69

Chapter 3 - Theoretical Background for Development of AC and DC Microgrids Fault Protection Systems
3.1 A Simplified Fault Current Estimation Approach for Grid-Connected AC MGs 72
3.1.1 Calculating the grid fault current component Ifault_grid 74
3.1.2 Calculating fault current coefficients of DG units 76
3.1.3 Calculating the total fault current seen by each protective relay 78
3.1.4 Evaluation of the simplified fault current estimation approach 79
a) During the grid-connected operation mode 79
b) During the autonomous operation mode 86
3.2 A Chi-Square Distribution Statistic Method Applied for Islanded MG Protection 86
3.2.1 Chi-Square distribution 86
3.2.2 Confidence intervals for variances and standard deviations 87
3.3 Overview of DC Fault Current Calculations 89
3.4 Operating Principle of Fault Protection Modules in a DC Digital Relay 93
3.4.1 Protection modules based on time derivatives of DC current and voltage 94
a) The time derivative of DC current 95
b) The time derivative of DC voltage 95
c) The system impedance under fault conditions 96
d) Evaluation of protection modules based on time derivatives of DC current &; voltage 96
3.4.2 Differential current protection module 97
3.4.3 Overcurrent protection modules 98
a) Forward-direction overcurrent protection 99
b) Backward-direction overcurrent protection 100
c) Distinguish faulted sections by the I2t response 101
d) Distinguish and detect faulted sections by the rate of current change 102
3.4.4 Under/over-voltage protection modules 105
a) Over-voltage (OV) protection 107
b) Under-voltage (UV) protection 108
3.4.5 Combined application of fault protection modules in a DC digital relay 109
3.5 Selection of Fast-Acting Fuses for Protecting the DC Microgrid 112
3.6 Theoretical Study of DC Leakage-Current Protection Solutions 113
3.6.1 Evaluate AC voltages/currents of the rectifier with a grounded AC power source
113
3.6.2 Detecting a voltage unbalance between positive and negative poles 115
3.6.3 Measurement of a ground potential rise at neutral-to-ground points 116
3.6.4 Measurement of the common-path currents in the DC system 117
3.7 Summary 117

Chapter 4 - A Fast and Adaptable Fault Protection System for Multi-Grounded AC Microgrids
4.1 General Description 120
4.2 AC-Microgrid Fault Protection during the Grid-Connected Mode 124
4.2.1 Operating principle of the fast and adaptable AC-MG protection system 124
4.2.2 Detecting the directional change of phase currents 132
4.2.3 Protection modules in a MG digital relay selected to protect load branches 135
4.2.4 Protection modules selected to protect DG/energy-storage source branches 135
4.2.5 Protection modules in a MDR selected to protect trunk lines/common buses 138
4.3 AC-Microgrid Fault Protection during the Islanded Operation Mode 139
4.3.1 Phase I - Fault detection and identification 142
a) Calculating the parameters and 146
b) Setting the parameter based on the Chi-square distribution 147
4.3.2 Phase II – Part a – Detecting the directional change of phase currents 151
a) Step 3-2(a) – Determine the faulted phases to detect the directional change of currents on the faulted phases selected 154
b) Step 3-2(b) – Calculating parameters Ipost, Vpost, Ipre, and Vpre 155
c) Step 3-2(c) – Determine whether the direction of currents on the faulted phases is changed or not 162
4.3.3 Phase II – Part b - Fault location 163
a) Case 1 - Fault location on a trunk line/common bus 163
b) Case 2 - Fault location at DG/energy storage source branches or load branches
164
4.4 Summary 165

 
Chapter 5 - A Novel Fault Protection System for Low-Voltage DC Microgrids
5.1 General Description 168
5.2 Design Requirements of a Novel DC-Microgrid Fault Protection System 171
5.2.1 Possible fault types in the DC microgrid 172
5.2.2 Common fault characteristics in three grounding configurations of DC MGs 173
5.2.3 Fault locations in the DC microgrid 174
5.3 Protection of DC Power Sources and Power Converters with High Fault Current (P1~P2, P5~P6) 176
5.3.1 Setting parameters of fast-acting fuses (P1 ~ P2) 177
5.3.2 Setting tripping thresholds of protection modules in a DC-MG digital relay 179
5.4 Protection of DG/Energy-Storage Source Branches with High Fault Current (P3, P7, and P13) 181
5.5 Protection of Load Branches with High Fault Current (P14~P19) 182
5.6 Protection of Trunk Lines and Common DC Buses with High Fault Current (P9~P10) 183
5.7 Coordination Strategies in the Novel DC-Microgrid Protection System with High Fault Current 184
5.7.1 Distinguish between the faulted and overloaded conditions 184
5.7.2 Distinguish among different faulted locations 186
5.7.3 Protection coordination among fault protection modules in a DC-MG digital relay
187
5.7.4 Protection coordination among DC-microgrid digital relays 188
5.7.5 Protection coordination between DC-microgrid digital relays and fuses 191
5.8 DC Leakage Current Protection Solutions 192
5.9 Summary 193


 
Chapter 6 - Validation of a Fast and Adaptable AC-Microgrid Fault Protection System
6.1 Overview of the Validation 197
6.2 General Description on Simulation Tests 198
6.3 Fault simulation results of INER microgrid at the grid-connected mode 202
6.3.1 Fault protection of a load branch (faults at F3) 209
6.3.2 Fault protection of a common AC bus (faults at F6) 214
6.3.3 Fault protection of the trunk lines (faults at F1 and F5) 220
a) Three-phase to ground and phase-to-phase faults at the trunk line-F1 220
b) Three-phase to ground and phase-to-phase faults at the trunk line-F5 225
6.3.4 Fault protection of PV and MT source branches (faults F4 and F2) 230
a) A three-phase to ground fault at the location F2 231
b) A three-phase to ground fault at the location F4 231
6.4 Simulated Sensitivity Tests on a Fast and Adaptable AC-MG Fault Protection System under the Islanded Operation Mode 233
6.4.1 Simulation results for faults on a trunk line (F1) 236
a) Single-phase to ground fault at Zone 1 (phase-a is the faulted phase) 236
b) Three-phase to ground fault at Zone 1 237
c) Phase-to-phase fault at Zone 1 238
6.4.2 Simulation results for faults on a load branch (F3) and a PV branch (F4) 239
6.5 Simulated Tests on Back-up Protection Solutions of the Fast and Adaptable AC-MG protection system 241
6.5.1 Low-impedance single-phase to ground fault on a trunk line F1 of Zone 1 241
6.5.2 High-impedance single-phase to ground fault on a trunk line-F1 of Zone 1 243
6.5.3 High-impedance single-phase to ground fault on a load branch-F3 of Zone 1 244
6.6 Simulated Dependability Tests on a Fast and Adaptable AC-MG Protection System under the Islanded Operation Mode 245
6.7 Staged Fault Tests on a Fast and Adaptable AC-MG Protection System under the Islanded Operation Mode 247
6.7.1 SPGF at a trunk line (F1) of Zone 1 under the islanded operation mode 249
6.7.2 TPGF at a trunk line (F1) of Zone 1 under the islanded operation mode 254
6.7.3 SPGF at a MT source branch (F2) of Zone 1 under the islanded operation mode 256
6.7.4 TPGF at a MT source branch (F2) of Zone 1 under the islanded operation mode 257
6.7.5 SPGF &; TPGF at a trunk line (F7) under the islanded operation of Zones 1 &; 2 259
6.7.6 SPGF &; TPGF at a source feeder (F8) under the islanded operation of Zones 1 &; 2
263
6.7.7 Observation on fault behaviours of two IBDG types in the INER microgrid 267
a) Observations 267
b) Evaluation of Type-I/II – IBDG fault behaviours on the novel FA protection module 269
6.8 Staged Dependability Tests of a Novel FA Fault Protection Module 273
6.9 Summary 275


Chapter 7 - Validation of a Novel DC-Microgrid Fault Protection System
7.1 General Operation Principles of the Novel DC-MG Protection System 278
7.1.1 Fault protection modules embedded in a DC-MG digital relay 278
7.1.2 Coordination strategies in the novel DC-microgrid fault protection system 279
7.1.3 DC leakage-current protection solutions 279
7.1.4 Main characteristics of the novel DC-microgrid fault protection system 280
7.2 Fault Simulation Cases of Low-Voltage DC Microgrids 281
7.3 Staged Fault Tests at a LVDC Microgrid Test-Bed 287
7.4 Evaluation on the Novel DC-Microgrid Fault Protection System by Staged Fault Test Results 289
7.4.1 Fault protection of DC power sources/converters by fast-acting fuses 289
a) Without protection of fast-acting fuses 289
b) With protection of fast-acting fuses 290
c) Comparing fault clearing time of fast-acting fuses (FAFs) &; no-fuse breakers (NFBs) 292
7.4.2 Evaluated operation of fault protection modules embedded in a DC-MG digital relay
294
7.4.3 Evaluation on detecting DC-leakage currents 297
7.5 Evaluation on the Novel DC-Microgrid Fault Protection System by Simulation Results 297
7.5.1 Fault protection of DC power sources 299
a) Operation of the digital relay – P1 for faults at F1 300
b) Operation of the digital relay – P5 for faults at F4 302
c) Protection coordination between digital relays-P1 and -P2 for faults at F2 304
7.5.2 Fault protection of power converters 305
a) Operation of the digital relay – P2 for faults at F2 305
b) Protection coordination between digital relays – P2 and -P3 for faults at F3 307
7.5.3 Fault protection of DG-source/ESS branches 309
a) Operation of the digital relay –P3 for faults at F2 309
b) Protection coordination between P2 and P3, and between P6 and P7 for faults at F3
311
7.5.4 Fault protection of trunk lines/common buses 312
a) Operation of digital relays –P3 –P7, and –P9 for faults at F3 312
b) Operation of digital relays –P9 and –P10 for faults at F5 316
7.5.5 Fault protection of load branches 319
a) Differentiate between faults at F6 and F7 by a digital relay–P13 319
b) Differentiate between faults at F8 and F9 by the digital relay–P15 321
c) Differentiate among faults at F8, F10 and F11 by the digital relay–P15 323
d) Protection coordination between P14 and P13 for faults at F7 324
e) Differentiate between faulted and motor-starting cases by a digital relay–P14 327
7.6 Simulated Evaluation on DC Leakage-Current Protection Solutions 328
7.6.1 Measurement of ground-return currents 329
7.6.2 Measurement of a ground potential rise at neutral-to-ground connection points 329
7.6.3 Detecting a voltage unbalance between positive and negative poles to ground 330
7.6.4 Evaluation of AC voltages/currents at the AC side of power rectifiers 331
7.7 Summary 334


Chapter 8 - Conclusions and Future Works
8.1 Conclusions 336
8.1.1 AC and DC microgrid protection challenges and countermeasures 336
a) A simplified fault current estimation approach for grid-connected AC microgrids
336
b) A Chi-square distribution statistic method applied to the AC-MG protection system under the islanded operation mode 337
c) Fast-acting fuses combined with DC-microgrid digital relays and power electronic switches to protect the LVDC microgrids 337
8.1.2 Designing the generalised model structures for system grounding simulation of AC and DC microgrids 338
8.1.3 The fast and adaptable AC-microgrid fault protection system developed 338
8.1.4 The novel DC-microgrid fault protection system developed 339
8.1.5 The fast and adaptable AC-microgrid fault protection system validated through the simulation and experiment results 340
8.1.6 The novel DC-microgrid fault protection system validated through the simulation and experiment results 340
8.2 Future Works 342


References 343
Appendix A - Fault characteristics of common AC power sources in AC microgrid 351
Appendix B - Fault current behaviours of DC/AC power inverters 357
Appendix C - Fault characteristics of common DC power sources 360
Appendix D - Fault behaviours of AC/DC rectifiers in DC-microgrid 362
Appendix E - Fault behaviour of DC/DC converters in DC-microgrid 379
Appendix F - Calculate parameter ‘Ipost’ at the faulted phases by a novel FA fault protection module 388
Appendix G - Calculate parameters ‘Ipre’ and ‘Vpre’ at the faulted phases by a novel FA fault protection module 392
Appendix H - Calculate parameter ‘Vpost’ at the faulted phase in case of three-phase or single-phase to ground faults 394
Appendix K - Calculate parameter ‘Vpost’ at two faulted phases in case of double-phase or double-phase to ground faults 396
Appendix L - Calculate parameters ‘Vpost’ and ‘Vpre’ corresponding with different power factors 398
Appendix M - Specification of a 48V DC microgrid test-bed 400



List of Figures


Figure 1.1 Typical configurations of LVDC and LVAC microgrids [9] 2
Figure 1.2 A general AC-DC hybrid MG configuration 3


Figure 2.1 General configuration of a multiple-grounded AC microgrid 28
Figure 2.2 Two common topologies of PV systems are used to connect to the AC microgrid
30
Figure 2.3 Connection diagrams of typical energy storage devices in the AC microgrid 31
Figure 2.4 Schematic representation of the CERTS microgrid [14] 36
Figure 2.5 Schematic representation of a part of the 380V AC INER microgrid 37
Figure 2.6 A multi-grounded 380V AC-microgrid test-bed [79] (continued) 39
Figure 2.7 Modelling of a 65kW micro-turbine generation system (continued) 41
Figure 2.8 A control block of the AC/DC power rectifier (continued) 46
Figure 2.9 Control diagrams of a DC/AC power inverter in PSCAD software 47
Figure 2.10 The data of a HCPV generation system installed in a 380V AC INER MG [79]
48
Figure 2.11 PSCAD simulation model of a HCPV generation system in the 380V AC MG (continued) 49
Figure 2.12 PSCAD simulation model of a battery power conditioning system at Zone 1 52
Figure 2.13 A PSCAD simulation model of Zone 1 and Zone 2 of the INER microgrid 53
Figure 2.14 An asymmetric monopolar DC-microgrid configuration 55
Figure 2.15 Two symmetric monopolar DC-microgrid configurations 56
Figure 2.16 A generalised grounding model structure of DC microgrids is changed to form a high-impedance grounded symmetric monopolar DC configuration 58
Figure 2.17 Geographic location and some real pictures of the LVDC Dongkeng microgrid
62
Figure 2.18 A single-line diagram of the battery station-1 in the LVDC Dongkeng microgrid
63
Figure 2.19 A single-line diagram of the battery station-2 in the LVDC Dongkeng microgrid
63
Figure 2.20 A single-line diagram of the battery station-3 in the LVDC Dongkeng microgrid
64
Figure 2.21 A real picture of the experimental 48V DC microgrid 65
Figure 2.22 A single-line diagram of the experimental 48V DC microgrid at the CSIST 65
Figure 2.23 PSCAD simulation model of the community-sized LVDC microgrid (a two-wire symmetric monopolar DC-microgrid configuration) 67
Figure 2.24 PSCAD simulation model of a three-wire symmetric monopolar DC-microgrid configuration with a high-impedance grounding system 68
Figure 2.25 PSCAD simulation model of the asymmetric monopolar DC-microgrid configuration with a low-impedance grounding system 69


Figure 3.1 Fault current contribution from the ith DG to the rth relay 77
Figure 3.2 Downstream and upstream fault current contributions on a trunk line of the MG
80
Figure 3.3 Downstream and upstream fault current directions on a trunk line of the looped MG configuration 82
Figure 3.4 Chi-square distributions along with the corresponding degrees of freedom 87
Figure 3.5 The values for _right^2 and _left^2 corresponding with a α significance level 88
Figure 3.6 The value _(n-1,1-α)^2 corresponding with a 1 - α confidence level 88
Figure 3.7 Typical curves of short-circuit currents for various power sources [89] 89
Figure 3.8 Standard approximation function of the short-circuit current [89] 90
Figure 3.9 The battery fault current waveform, with Ibatt – the fault current, Vs_batt – the fault voltage, and Pbatt – the battery power 91
Figure 3.10 Fault current characteristic of the PV power source 92
Figure 3.11 Fault current characteristic of the capacitor, Icap – the capacitor current 93
Figure 3.12 DC fault current and voltage waveforms under the over-damping condition, with I0 – the DC fault current and Edc – the DC fault voltage in the LVDC microgrid 97
Figure 3.13 Setting parameters Imax+, Ids, T+ for a forward overcurrent protection module [91]
99
Figure 3.14 Setting parameters Imax-, Ids, and T– for a backward-direction overcurrent protection module [91] 101
Figure 3.15 Current (top) and the i_t^2 response (bottom) for 1-mΩ (left) and 500-mΩ (right) faults as an example of differentiating faulted locations by the i_t^2 response 102
Figure 3.16 Fault and overload current curves distinguished by the current change rate 103
Figure 3.17 Distinguishing different faulted sections in the DC MG by the current change rate
105
Figure 3.18 Curves represent the operating principle of a positive OV protection module
107
Figure 3.19 Curves represent the operating principle of a negative OV protection module
107
Figure 3.20 Curves represent the operating principle of a positive UV protection module
108
Figure 3.21 Curves represent the operating principle of a negative UV protection module
108
Figure 3.22 The combined operation of fault protection modules in a DC-MG digital relay
111
Figure 3.23 The operating principle of the DC fuse 112
Figure 3.24 An AC/DC power rectifier with a grounded AC power source and a HI P-G fault
114
Figure 3.25 A typical symmetric monopolar DC MG with different faulted locations 116


Figure 4.1 Three operation zones of a multi-grounded 380V AC INER microgrid 123
Figure 4.2 Three important protection zones of Zone 1 in the 380V AC INER microgrid
124
Figure 4.3 The operating principle of a fast and adaptable AC-MG fault protection system under the grid-connected operation mode (Notes 1~4 have been explained in the above paragraph) 126
Figure 4.4 A single-line diagram of Zone 1 of the multiple-grounded 380V AC INER MG [79] 127
Figure 4.5 Downstream fault currents are observed at MDR6 and its tripping signal 128
Figure 4.6 Upstream fault currents are observed at MDR7 and its tripping signal 129
Figure 4.7 HI single-phase to ground fault currents and an ‘alarm’ signal of MDR3 130
Figure 4.8 Criteria for selecting the overcurrent relay pick-up currents [86] 131
Figure 4.9 Detection of the directional change in phase currents by comparing the pre-fault and fault phase-angles 134
Figure 4.10 Over-current and under-voltage protection modules and protection modules 46 and 47 are used for fault protection of load branches 135
Figure 4.11 Essential fault protection modules are installed in an AC-microgrid digital relay for the protection of DG/energy-storage source branches 137
Figure 4.12 Fault protection modules are selected for protecting the trunk line/common bus
138
Figure 4.13 Typical configuration of a communication-assisted MG digital relay is used for the multi-grounded AC-microgrid protection under the islanded operation mode
142
Figure 4.14 The operating principle of a novel FA fault protection module under the islanded mode of a multi-grounded AC-microgrid (to be continued) 145
Figure 4.15 The Chi-square distribution of a random variable Id,t 149
Figure 4.16 Distortion of a fault current waveform when Fourier transforms are not used
152
Figure 4.17 Fault behaviour of a PV source branch (i.e., a Type I – IBDG source branch) recorded in the multi-grounded 380V AC INER microgrid 153
Figure 4.18 Step 3-2(a) - Determining the faulted phases and detecting the directional change of currents on the faulted phases selected (to be continued) 154
Figure 4.19 Calculate the parameter Vpost through the reference phase voltages for a three-phase fault and single-phase to ground faults 156
Figure 4.20 Calculate the parameter Vpost through the fault phase voltages right after the fault inception time with respect to DPFs and DPGFs 157
Figure 4.21 Step 3-2(b) - Calculating four parameters Ipost, Vpost, Ipre, and Vpre on the faulted phase determined in case of three-phase or single-phase to ground faults 158
Figure 4.22 Step 3-2(b) - Calculating four parameters Ipost, Vpost, Ipre, and Vpre at two faulted phases determined in case of double-phase or double-phase to ground faults 159
Figure 4.23 MG digital relays are installed in a multi-grounded 380V AC INER microgrid
164


Figure 5.1 Five main DC-fault protection zones in a typical DC-microgrid configuration (an ungrounded symmetric monopolar DC-microgrid configuration) 169
Figure 5.2 Five main DC-fault protection zones of a typical DC-microgrid configuration (a high-impedance grounded symmetric monopolar DC-microgrid configuration)
170
Figure 5.3 Five main DC-fault protection zones of a typical DC-microgrid configuration (a grounded asymmetric monopolar DC-microgrid configuration) 171
Figure 5.4 A protection system design framework for load branches of the DC-microgrid
182
Figure 5.5 Curves represent the operating principle of the UV protection module [91] 185
Figure 5.6 Curves represent the operating principle of the overcurrent protection module [91]
185
Figure 5.7 Fault and overload conditions are distinguished by the current change rate [91]
186
Figure 5.8 The time-grading based coordination strategy of non-unit and unit protection modules in a DC-MG digital relay to protect the trunk lines (or the common buses)
187
Figure 5.9 Protection coordination between Relay P3 and Relay P2 with respect to the bus fault F3 by using the time-derivative of the DC voltage 188
Figure 5.10 Protection coordination: (a) between Relay P13 and Relay P14 with the fault F7; (b) between Relay P13 and Relay P15 with respect to the fault F8 189
Figure 5.11 Protection coordination between the relay P13 and the relay P14 by using the I2t response with respect to the fault F7 190
Figure 5.12 The protection coordination strategy among DC-microgrid digital relays 191


Figure 6.1 A single-line diagram of the multi-grounded 380V AC INER microgrid (including Zone 1 and Zone 2) 201
Figure 6.2 PSCAD simulation model of Zone 1 in the multi-grounded 380V AC INER MG
202
Figure 6.3 A time-chart shows the operation simulation of the multi-grounded 380V AC INER MG (only Zone 1) at the grid-connected mode 209
Figure 6.4 SPGF currents and voltages at F3 and the tripping time of the OC protection module in the MDR3 211
Figure 6.5 DPGF currents and voltages at F3 and the tripping time of the OC protection module in the MDR3 (followed) 212
Figure 6.6 TPF currents and voltages at F3 and the tripping time of the OC protection module in the MDR3 (followed) 213
Figure 6.7 PPF currents and voltages at F3 and the tripping time of the OC protection module in the MDR3 213
Figure 6.8 Three-phase fault (TPF) currents and voltages measured at MDR7, MDR4, and
MDR5 and their fault tripping signals 217
Figure 6.9 Phase-to-phase fault currents and voltages, the sequence components of current
(ip, in, iz) and voltage (vp, vn, vz) measured at MDR7, MDR4 &; MDR5 and their tripping signals 218
Figure 6.10 Total harmonic distortion percentages of phase currents and voltages seen at MDR7, MDR4 &; MDR5 and their tripping signals with respect to the HISPGF
219
Figure 6.11 Downstream fault currents observed at MDR6 and its tripping signal with regard to a three-phase to ground fault at the trunk line-F1 222
Figure 6.12 Upstream fault currents observed at MDR7 and its tripping signal (BRKMDR7) with regard to a three-phase to ground fault at the trunk line-F1 223
Figure 6.13 Downstream fault currents and fault voltages observed at MDR6 and its tripping signal (BRKMDR6) with regard to a phase-to-phase fault at the trunk line-F1 224
Figure 6.14 Upstream fault currents and fault voltages observed at MDR7 and its tripping signal (BRKMDR7) with regard to a phase-to-phase fault at the trunk line-F1 225
Figure 6.15 Downstream fault currents and fault voltages observed at MDR15 and its tripping signal (BRKMDR15) with regard to a three-phase to ground fault at the trunk line-F5 227
Figure 6.16 Upstream fault currents and fault voltages observed at MDR1 and MDR6 and their tripping signals (BRKMDR1 and BRKMDR6) for a three-phase to ground fault at a trunk line- F5 228
Figure 6.17 Downstream fault currents and fault voltages observed at MDR15 and its tripping signal (BRKMDR15) with regard to a phase-to-phase fault at the trunk line-F5
229
Figure 6.18 Upstream fault currents and fault voltages observed at MDR1 and MDR6 and their tripping signals (BRKMDR1 and BRKMDR6) for a phase-to-phase fault at F5
230
Figure 6.19 Downstream fault currents (Ia, Ib, and Ic) and fault voltages (Va, Vb, and Vc) observed at MDR1 and its tripping signal (BRKMDR1) for a three-phase to ground fault at F2 232
Figure 6.20 Downstream fault currents (Ia, Ib, and Ic) and fault voltages (Va, Vb, and Vc) observed at MDR4 and its tripping signal (BRKMDR4) for a three-phase to ground fault at F4 233
Figure 6.21 A time-chart shows the operation simulation of Zone 1 of the INER microgrid
234
Figure 6.22 Simulation results of fault voltages and currents at MDR7 (Vrelay7, Irelay7) and MDR6 (Vrelay6, Irelay6) with respect to the SPGF at a trunk line-F1 and the 18kW load branch 236
Figure 6.23 PSCAD simulation results for the TPGF at the faulted location-F1 during the islanded operation mode of the 380V AC INER microgrid 237
Figure 6.24 Simulation results of the PPF in the multiple-grounded 380V AC microgrid (a faulted location on the trunk line - F1) during the islanded operation mode 238
Figure 6.25 Simulation results of the TPGF and LISPGF at a load branch of the INER microgrid (a faulted location-F3 as seen in Fig. 6.1) during the islanded operation mode 240
Figure 6.26 Simulation results of the TPGF and LISPGF at a PV-source branch in the 380V AC microgrid (a faulted location-F4 as seen in Fig. 6.1) during the islanded operation mode 241
Figure 6.27 Simulation results for the LISPGF in the multiply grounded 380V AC microgrid (a faulted location-F1) at the islanded operation mode 242
Figure 6.28 PSCAD simulation results for the HISPGF in the multiple-grounded 380V AC microgrid (a faulted location on the trunk line - F1) during the islanded operation mode 243
Figure 6.29 Simulation results of the HISPGF at a load branch of the multi-grounded AC MG (i.e., a faulted location-F3 as seen in Fig. 6.1) under the islanded operation mode
244
Figure 6.30 Current and voltage waveforms at Case 1 – an operation transition test of Zone 1 from the grid-connected mode into the islanded mode 246
Figure 6.31 Current and voltage waveforms at Case 2 – an operation transition test of Zone 1 from the islanded mode into the grid-connected mode 246
Figure 6.32 SPGF currents (Ia, Ib, &; Ic) and voltages (Va, Vb, &; Vc) seen at MDR6 249
Figure 6.33 SPGF currents (Ia, Ib, &; Ic) and voltages (Va, Vb, &; Vc) seen at MDR5 249
Figure 6.34 SPGF currents (Ia, Ib, &; Ic) and voltages (Va, Vb, &; Vc) seen at MDR4 249
Figure 6.35 Fault voltage and current waveforms and the output voltage signals of the novel FA protection modules in MDR6 and MDR7 with respect to the SPGF-F1 252
Figure 6.36 TPGF currents (Ia, Ib, &; Ic) and voltages (Va, Vb, &; Vc) seen at MDR6 254
Figure 6.37 TPGF currents (Ia, Ib, &; Ic) and voltages (Va, Vb, &; Vc) seen at MDR4 254
Figure 6.38 TPGF currents (Ia, Ib, &; Ic) and voltages (Va, Vb, &; Vc) seen at MDR5 254
Figure 6.39 Fault voltage and current waveforms and an output voltage signal of the novel FA protection module in MDR1 with respect to a single-phase to ground fault-F2
257

Figure 6.40 Fault voltage and current waveforms and an output voltage signal of the novel FA protection module in MDR1 with respect to a three-phase to ground fault-F2
258
Figure 6.41 Single-phase to ground fault currents (Ia, Ib, &; Ic) and voltages (Va, Vb, &; Vc) are seen at MDR13 (left) and MDR14 (right) 260
Figure 6.42 Three-phase to ground fault currents (Ia, Ib, &; Ic) and voltages (Va, Vb, &; Vc) are seen at MDR13 (left) and MDR14 (right) 260
Figure 6.43 TPGF currents (Ia, Ib, &; Ic) and voltages (Va, Vb, &; Vc) are measured at MDR8
264
Figure 6.44 SPGF currents (Ia, Ib, &; Ic) and voltages (Va, Vb, &; Vc) are measured at MDR8
264
Figure 6.45 Fault currents and voltages at the Type-I IBDG branch and its fault tripping time
268
Figure 6.46 Fault currents and voltages at the Type-II IBDG branch and its fault tripping time
268
Figure 6.47 Fault currents and voltages at the Type-I IBDG branch and its fault tripping time
268
Figure 6.48 Fault currents and voltages at the Type-II IBDG branch and its fault tripping time
268
Figure 6.49 The operation of the novel FA fault protection module in MDR4 is depicted through the waveforms with respect to the SPGF at F1 (phase-a is the faulted phase) 270
Figure 6.50 The operation of the novel FA fault protection module in MDR4 is depicted through the waveforms with respect to the TPGF at F1 271
Figure 6.51 Current &; voltage waveforms measured at MDR4 (left) and MDR5 (right) during the operation transition of Zone-1 from the grid-connected mode into the islanded mode 273
Figure 6.52 Current &; voltage waveforms measured at MDR4 (left) and MDR5 (right) during the operation transition of Zone-1 from an islanded mode into a grid-connected mode 274


 
Figure 7.1 PSCAD simulation model of an asymmetric monopolar DC-microgrid multiply grounded with the low-impedance (continued) 283
Figure 7.2 PSCAD simulation model of a symmetric monopolar DC microgrid multiply grounded with the high impedance (continued) 285
Figure 7.3 Five fault protection zones and different faulted sections (F1~F12) in a generalised grounding model structure of LVDC microgrids 287
Figure 7.4 A faulted section and current/voltage measurement points in a LVDC-MG test-bed
288
Figure 7.5 The 48V DC microgrid test-bed with installation of FAFs and NFBs 290
Figure 7.6 Currents and voltages are investigated during the pole-to-pole fault at T1 without installation of fast-acting fuses (a time-interval between two consecutive samples is 0.2ms) 291
Figure 7.7 Currents and voltages are investigated during the pole-to-pole fault at T1 with installation of fast-acting fuses (a time-interval between two consecutive samples is 0.2ms) 292
Figure 7.8 Time-current characteristics of a fast-acting fuse – 50LET and a no-fuse breaker - NFB16-NFB350CN-2P350A-250Vdc with respect to the P-P fault at T1 293
Figure 7.9 Time-current characteristics of a fast-acting fuse – 32LET and a no-fuse breaker - NFB5-C60N-2P3563A-150Vdc with respect to the P-P fault at T1 294
Figure 7.10 The battery and MPPT output currents and voltages are investigated during the pole-to-pole fault at T1 (a time-interval between two consecutive samples is 0.2ms)
296
Figure 7.11 Positive-pole to ground (VP-G) and negative-pole to ground (VN-G) voltages are measured at the 48V DC microgrid test-bed during a negative-pole to ground fault at T1 297
Figure 7.12 Current and voltage observed at the digital relay – P1 and its tripping signal with respect to the P-G (top) and P-P (bottom) faults at F1 301
Figure 7.13 Current and voltage observed at the digital relay – P5 and its tripping signal with respect to the P-G (top) and P-P (bottom) faults at F4 303
Figure 7.14 Currents and voltages are observed at P1 and P2, and a tripping signal (BRKP2) with respect to the P-G (top) and P-P (bottom) faults at F2 (continued) 305
Figure 7.15 Current (I2) and voltage (V2) observed at the digital relay – P2 and its tripping signal with respect to the P-G (top) and P-P (bottom) faults at F2 306
Figure 7.16 Voltages observed at the digital relays – P2 and –P3 with respect to the P-G (top) and P-P (bottom) faults at F3 (the time interval -t between two consecutive samples is 1ms) 308
Figure 7.17 The current and voltage are observed at the digital relay – P3 and its tripping signal with respect to the P-G (top) and P-P (bottom) faults at F2 310
Figure 7.18 Voltages observed at the digital relays – P6 and –P7 with respect to the P-G (top) and P-P (bottom) faults at F3 (the time interval-t between two consecutive samples is 1ms) 311
Figure 7.19 Currents are observed at digital relays – P3, -P7, and –P9, and their tripping signals with respect to a P-G fault at F3 (the time interval-t between two consecutive samples is 1ms) 314
Figure 7.20 Currents (I3, I7, and I9) are observed at the digital relays – P3, -P7, and –P9, and their tripping signals with respect to a P-P fault at F3 315
Figure 7.21 Tripping signals of a main protection module (i.e., a differential-current protection module) and back-up protection modules in a digital relay-P3 for a P-P fault at F3 316
Figure 7.22 Currents observed at the digital relays – P9 and –P10, and their tripping signals with respect to a P-G fault at F5 317
Figure 7.23 Currents observed at the digital relays – P9 and –P10, and their tripping signals with respect to a P-P fault at F5 318
Figure 7.24 Tripping signals of a main protection module (i.e., a differential-current protection module) and back-up protection modules in a digital relay-P10 for a P-P fault - F5 318
Figure 7.25 Currents and voltages are observed at the digital relay – P13 to differentiate between P-P (or P-G) faults at F6 and F7 320
Figure 7.26 Currents (I15) and voltages (V15) are observed at the digital relay – P15 to differentiate between P-P (or P-G) faults at F8 and F9 322
Figure 7.27 Currents (I15) and voltages (V15) are observed at the digital relay – P15 to differentiate among P-P (or P-G) faults at F8, F10, and F11 324
Figure 7.28 Currents and voltages measured at the relays P14 and P13 with respect to P-P and P-G faults at F7 when the relay P10 is closed 325
Figure 7.29 Currents (I13 and I14) and voltages (V13 and V14) measured at the relays P14 and P13 with respect to the P-P and P-G faults at F7 when the relay P10 is opened
326
Figure 7.30 Currents (I14) and voltages (V14) are measured at the relay P14 with respect to P-G and P-P faults at F7 and a motor starting case at the load branch 328
Figure 7.31 Ground-return path currents (Ig02 and Ig03) are measured at the relays P2 and P3, respectively, with respect to a positive pole-to-ground fault at F2 329
Figure 7.32 The ground potential (Vg02 and Vg03) is measured at the relays P2 and P3, respectively, with respect to a positive pole-to-ground fault at F2 330
Figure 7.33 Negative-pole to ground voltages (V2neg and V3neg) are measured at the relays P2 and P3 respectively, with respect to a positive pole-to-ground fault at F2 330
Figure 7.34 Phase and sequence current components are observed at the AC side of power rectifier on the WT source branch; and the DC-fault current is measured at the relay-P13 for a positive pole to ground fault at F6 332
Figure 7.35 Phase and sequence current components are observed at the AC side of power rectifier on the WT source branch; and the DC-fault current is measured at the relay-P13 for a negative-pole to ground fault at F6 333


List of Tables


Table 1.1 Main contributions for the AC-microgrid fault protection 20
Table 1.2 Main contributions for the DC-microgrid fault protection 22


Table 2.1 Adaptability of the generalised grounding model structure of AC microgrids to different grounding types for the microgrid operation simulation (to be continued) 28
Table 2.2 Advantages and disadvantages of the generator grounding methods 35
Table 2.3 Technical parameters of a 65kW micro-turbine generation system [79] 40
Table 2.4 Simulation situations of the 380V AC INER microgrid operation 54
Table 2.5 Adaptability of the generalised grounding model structure of DC microgrids to form DC-microgrid configurations with different grounding types 59
Table 2.6 Specifications of NFBs in a 48V DC microgrid test-bed 64


Table 3.1 System parameters of the radial AC microgrid configuration 80
Table 3.2 Fault current contribution of DG-i, DG-j, and the utility grid to the relays-i &;-j 81
Table 3.3 System parameters of the looped AC microgrid configuration 83
Table 3.4 Setting parameters of directional OC protection modules 99
Table 3.5 Setting parameters of under-voltage/over-voltage protection modules and their applications to the DC-microgrid protection 106
Table 3.6 Main and back-up protection modules selected to protect different operation zones
110


Table 4.1 Main functions of fault protection modules embedded in a MG digital relay 121
Table 4.2 Definitions of standard DOC/OC relay characteristics [98] 132
Table 4.3 The operation principle of a novel FA fault protection module 140
Table 4.4 Step 3-2(c) - Polarity scenarios of pre-fault and fault phase currents and voltages are compared to detect the directional change of currents 162


Table 5.1 Faulted locations occur at different protection zones in a typical DC-microgrid
170
Table 5.2 Common fault characteristics of three grounding configurations of DC microgrids
173
Table 5.3 Primary and back-up protection devices of the faults F7F12 on load branches
176
Table 5.4 Protection devices are selected to protect DC power sources and power converters
177
Table 5.5 Faulted sections (or fault locations) are identified by comparing the parameter di/dt (or dv/dt) of adjacent digital relays 189


Table 6.1 Fault simulation situations of Zone 1 at the grid-connected operation mode 199
Table 6.2 Fault simulation situations of Zone 1 at the islanded operation mode 200
Table 6.3 Tripping settings of fault protection modules in the AC-microgrid digital relay corresponding with different fault locations (F1~F6) 208
Table 6.4 A comparison on the total downstream fault current across the MDR3 calculated by the simplified fault analysis approach and simulation method 210
Table 6.5 A comparison on total downstream/upstream fault currents going across MDR7, MDR4 &; MDR5 calculated by a simplified estimation approach and a simulation method 215
Table 6.6 A comparison on total downstream/upstream fault currents going across MDR6 and MDR7 calculated by the simplified estimation approach and the simulation method
221
Table 6.7 A comparison on total downstream/upstream fault currents going across MDR15, MDR1 &; MDR7 calculated by a simplified estimation approach &; the simulation method 227
Table 6.8 A comparison on total downstream fault currents across MDR1 and MDR4 calculated by the simplified fault current estimation approach and the simulation method 231
Table 6.9 Fault simulation cases of Zone 1 for the sensitivity test of the novel AC-microgrid fault protection system 235
Table 6.10 Different staged fault tests performed at the multi-grounded 380V AC INER MG
248
Table 6.11 Results of the single-phase to ground fault test at the trunk line-F1 of Zone 1 250
Table 6.12 Results of the three-phase to ground fault test at the trunk line-F1 of Zone 1 255
Table 6.13 Results of the single-phase to ground fault test at the trunk line-F7 261
Table 6.14 Results of the three-phase to ground fault test at the trunk line-F7 262
Table 6.15 Results of the single-phase to ground fault test at the source feeder-F8 264
Table 6.16 Results of the three-phase to ground fault test at the source feeder-F8 265
Table 6.17 Fault characteristics of two IBDG types in the INER microgrid 268
Table 6.18 Results of the operation transition tests of Zone 1 274


Table 7.1 Fault simulation cases corresponding with five different protection zones in a 48V DC-microgrid model simulated and their significance 286
Table 7.2 Setting parameters of two digital relays, DCMDR16 and DCMDR5(5) 295
Table 7.3 Different fault cases are performed at the 48V DC-microgrid test-bed and its PSCAD simulation model 299
Table 7.4 Setting parameters of the digital relays at P1, P2 and P5 300
Table 7.5 Setting parameters of a DC-microgrid digital relay-P3 307
Table 7.6 Setting parameters of digital relays-P7, -P9, -P10, and –P13 313
Table 7.7 The rates of DC current and voltage change are observed by the relay-P13 with regard to P-G and P-P faults at F6 and F7 right after the fault inception time 320
Table 7.8 Setting parameters of the digital relays–P14 and –P15 321
Table 7.9 The rates of DC current and voltage change are observed by the relay-P15 with regard to P-G and P-P faults at F8 and F9 right after the fault inception time 322
Table 7.10 The rates of DC current and voltage change are observed by the relay-P15 with regard to P-G and P-P faults at F8, F10, and F11 right after the fault inception time
323

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