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2
OVERCURRENT PROTECTION PRINCIPLES
Most electrical power system faults result in an overcurrent of one kind or another. It is the job of protection
devices, formerly known as 'relays' but now known as Intelligent Electronic Devices (IEDs) to protect the power
system from faults. The general principle is to isolate the faults as quickly as possible to limit the danger and
prevent fault currents flowing through systems, which can cause severe damage to equipment and systems. At
the same time, we wish to switch off only the parts of the power grid that are absolutely necessary, to prevent
unnecessary blackouts. The protection devices that control the tripping of the power grid's circuit breakers are
highly sophisticated electronic units, providing an array of functionality to cover the different fault scenarios for a
multitude of applications.
The described products offer a range of overcurrent protection functions including:
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Phase Overcurrent protection
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Earth Fault Overcurrent protection
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Negative Sequence Overcurrent protection
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Sensitive Earth Fault protection
To ensure that only the necessary circuit breakers are tripped and that these are tripped with the smallest possible
delay, the IEDs in the protection scheme need to co-ordinate with each other. Various methods are available to
achieve correct co-ordination between IEDs in a system. These are:
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By means of time alone
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By means of current alone
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By means of a combination of both time and current.
Grading by means of current alone is only possible where there is an appreciable difference in fault level between
the two locations where the devices are situated. Grading by time is used by some utilities but can often lead to
excessive fault clearance times at or near source substations where the fault level is highest.
For these reasons the most commonly applied characteristic in co-ordinating overcurrent devices is the IDMT
(Inverse Definite Minimum Time) type.
2.1
IDMT CHARACTERISTICS
There are two basic requirements to consider when designing protection schemes:
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All faults should be cleared as quickly as possible to minimise damage to equipment
●
Fault clearance should result in minimum disruption to the electrical power grid.
The second requirement means that the protection scheme should be designed such that only the circuit
breaker(s) in the protection zone where the fault occurs, should trip.
These two criteria are actually in conflict with one another, because to satisfy (1), we increase the risk of shutting
off healthy parts of the grid, and to satisfy (2) we purposely introduce time delays, which increase the amount of
time a fault current will flow. With IDMT protection applied to radial feeders, this probem is exacerbated by the
nature of faults in that the protection devices nearest the source, where the fault currents are largest, actually
need the longest time delay.
IDMT characteristics are described by operating curves. Traditionally, these were defined by the performance of
electromechanical relays. In numerical protection, equations are used to replicate these characteristics so that
they can be used to grade with older equipment.
The old electromechanical relays countered this problem somewhat due to their natural operate time v. fault
current characteristic, whereby the higher the fault current, the quicker the operate time. The characteristic typical
of these electromechanical relays is called Inverse Definite Minimum Time or IDMT for short.
Chapter 9 - Current Protection Functions
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Содержание P642
Страница 2: ......
Страница 18: ...Contents P64x xvi P64x TM EN 1 3 ...
Страница 24: ...Table of Figures P64x xxii P64x TM EN 1 3 ...
Страница 25: ...CHAPTER 1 INTRODUCTION ...
Страница 26: ...Chapter 1 Introduction P64x 2 P64x TM EN 1 3 ...
Страница 36: ...Chapter 1 Introduction P64x 12 P64x TM EN 1 3 ...
Страница 37: ...CHAPTER 2 SAFETY INFORMATION ...
Страница 38: ...Chapter 2 Safety Information P64x 14 P64x TM EN 1 3 ...
Страница 50: ...Chapter 2 Safety Information P64x 26 P64x TM EN 1 3 ...
Страница 51: ...CHAPTER 3 HARDWARE DESIGN ...
Страница 52: ...Chapter 3 Hardware Design P64x 28 P64x TM EN 1 3 ...
Страница 87: ...CHAPTER 4 SOFTWARE DESIGN ...
Страница 88: ...Chapter 4 Software Design P64x 64 P64x TM EN 1 3 ...
Страница 98: ...Chapter 4 Software Design P64x 74 P64x TM EN 1 3 ...
Страница 99: ...CHAPTER 5 CONFIGURATION ...
Страница 100: ...Chapter 5 Configuration P64x 76 P64x TM EN 1 3 ...
Страница 121: ...CHAPTER 6 TRANSFORMER DIFFERENTIAL PROTECTION ...
Страница 122: ...Chapter 6 Transformer Differential Protection P64x 98 P64x TM EN 1 3 ...
Страница 165: ...CHAPTER 7 TRANSFORMER CONDITION MONITORING ...
Страница 166: ...Chapter 7 Transformer Condition Monitoring P64x 142 P64x TM EN 1 3 ...
Страница 189: ...CHAPTER 8 RESTRICTED EARTH FAULT PROTECTION ...
Страница 190: ...Chapter 8 Restricted Earth Fault Protection P64x 166 P64x TM EN 1 3 ...
Страница 215: ...CHAPTER 9 CURRENT PROTECTION FUNCTIONS ...
Страница 216: ...Chapter 9 Current Protection Functions P64x 192 P64x TM EN 1 3 ...
Страница 249: ...CHAPTER 10 CB FAIL PROTECTION ...
Страница 250: ...Chapter 10 CB Fail Protection P64x 226 P64x TM EN 1 3 ...
Страница 259: ...CHAPTER 11 VOLTAGE PROTECTION FUNCTIONS ...
Страница 260: ...Chapter 11 Voltage Protection Functions P64x 236 P64x TM EN 1 3 ...
Страница 274: ...Chapter 11 Voltage Protection Functions P64x 250 P64x TM EN 1 3 ...
Страница 275: ...CHAPTER 12 FREQUENCY PROTECTION FUNCTIONS ...
Страница 276: ...Chapter 12 Frequency Protection Functions P64x 252 P64x TM EN 1 3 ...
Страница 286: ...Chapter 12 Frequency Protection Functions P64x 262 P64x TM EN 1 3 ...
Страница 287: ...CHAPTER 13 MONITORING AND CONTROL ...
Страница 288: ...Chapter 13 Monitoring and Control P64x 264 P64x TM EN 1 3 ...
Страница 306: ...Chapter 13 Monitoring and Control P64x 282 P64x TM EN 1 3 ...
Страница 307: ...CHAPTER 14 SUPERVISION ...
Страница 308: ...Chapter 14 Supervision P64x 284 P64x TM EN 1 3 ...
Страница 322: ...Chapter 14 Supervision P64x 298 P64x TM EN 1 3 ...
Страница 323: ...CHAPTER 15 DIGITAL I O AND PSL CONFIGURATION ...
Страница 324: ...Chapter 15 Digital I O and PSL Configuration P64x 300 P64x TM EN 1 3 ...
Страница 336: ...Chapter 15 Digital I O and PSL Configuration P64x 312 P64x TM EN 1 3 ...
Страница 337: ...CHAPTER 16 COMMUNICATIONS ...
Страница 338: ...Chapter 16 Communications P64x 314 P64x TM EN 1 3 ...
Страница 397: ...CHAPTER 17 CYBER SECURITY ...
Страница 398: ...Chapter 17 Cyber Security P64x 374 P64x TM EN 1 3 ...
Страница 415: ...CHAPTER 18 INSTALLATION ...
Страница 416: ...Chapter 18 Installation P64x 392 P64x TM EN 1 3 ...
Страница 429: ...5 2 CASE DIMENSIONS 60TE E01409 Figure 167 60TE case dimensions P64x Chapter 18 Installation P64x TM EN 1 3 405 ...
Страница 431: ...CHAPTER 19 COMMISSIONING INSTRUCTIONS ...
Страница 432: ...Chapter 19 Commissioning Instructions P64x 408 P64x TM EN 1 3 ...
Страница 454: ...V01505 Figure 173 Harmonic Restraint Test Plane Chapter 19 Commissioning Instructions P64x 430 P64x TM EN 1 3 ...
Страница 460: ...Chapter 19 Commissioning Instructions P64x 436 P64x TM EN 1 3 ...
Страница 461: ...CHAPTER 20 MAINTENANCE AND TROUBLESHOOTING ...
Страница 462: ...Chapter 20 Maintenance and Troubleshooting P64x 438 P64x TM EN 1 3 ...
Страница 477: ...CHAPTER 21 TECHNICAL SPECIFICATIONS ...
Страница 478: ...Chapter 21 Technical Specifications P64x 454 P64x TM EN 1 3 ...
Страница 507: ...APPENDIX A ORDERING OPTIONS ...
Страница 508: ...Appendix A Ordering Options P64x P64x TM EN 1 3 ...
Страница 512: ...Appendix A Ordering Options P64x A4 P64x TM EN 1 3 ...
Страница 513: ...APPENDIX B SETTINGS AND SIGNALS ...
Страница 515: ...APPENDIX C WIRING DIAGRAMS ...
Страница 516: ...Appendix C Wiring Diagrams P64x P64x TM EN 1 3 ...
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