For solidly earthed systems, the operating current for the transformer differential protection is still significant for
faults over most of the winding. For this reason, independent REF protection may not have been previously
considered, especially where an additional device would have been needed. But with this product, it can be
applied without extra cost.
2.3
THROUGH FAULT STABILITY
In an ideal world, the CTs either side of a differentially protected system would be identical with identical
characteristics to avoid creating a differential current. However, in reality CTs can never be identical, therefore a
certain amount of differential current is inevitable. As the through-fault current in the primary increases, the
discrepancies introduced by imperfectly matched CTs is magnified, causing the differential current to build up.
Eventually, the value of the differential current reaches the pickup current threshold, causing the protection
element to trip. In such cases, the differential scheme is said to have lost stability. To specify a differential scheme’s
ability to restrain from tripping on external faults, we define a parameter called ‘through-fault stability limit’, which
is the maximum through-fault current a system can handle without losing stability.
2.4
RESTRICTED EARTH FAULT TYPES
There are two different types of Restricted Earth Fault; Low Impedance REF (also known as Biased REF) and High
Impedance REF. Each method compensates for the effect of through-fault errors in a different manner.
With Low Impedance REF, the through-fault current is measured and this is used to alter the sensitivity of the REF
element accordingly by applying a bias characteristic. So the higher the through fault current, the higher the
differential current must be for the device to issue a trip signal, Often a transient bias component is added to
improve stability during external faults.
Low impedance protection used to be considered less secure than high impedance protection. This is no longer
true as numerical IEDs apply sophisticated algorithms to match the performance of high-impedance schemes.
Some advantages of using Low Impedance REF are listed below:
●
There is no need for dedicated CTs. As a result CT cost is substantially reduced.
●
The wiring is simpler as it does not require an external resistor or Metrosil.
●
Common phase current inputs can be used.
●
It provides internal CT ratio mismatch compensation. It can match CT ratios up to 1:40 resulting flexibility in
substation design and reduced cost.
●
Advanced algorithms make the protection secure.
With High Impedance REF, there is no bias characteristic, and the trip threshold is set to a constant level. However,
the High Impedance differential technique ensures that the impedance of the circuit is sufficiently high such that
the differential voltage under external fault conditions is lower than the voltage needed to drive differential current
through the device. This ensures stability against external fault conditions so the device will operate only for faults
occurring inside the protected zone.
High Impedance REF protection responds to a voltage across the differential junction points. During external faults,
even with severe saturation of some of the CTs, the voltage does not rise above certain level, because the other
CTs will provide a lower-impedance path compared with the device input impedance. The principle has been used
for more than half a century. Some advantages of using High Impedance REF are listed below:
●
It provides a simple proven algorithm, which is fast, robust and secure.
●
It is less sensitive to CT saturation.
2.4.1
LOW IMPEDANCE REF PRINCIPLE
Low Impedance REF can be used for either delta windings or star windings in both solidly grounded and resistance
grounded systems. The connection to a modern IED is as follows:
Chapter 8 - Restricted Earth Fault Protection
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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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