Overcurrent and Ground Fault Protection

Transcription

1 Over and Ground Fault Protection Announcement: You are not supposed to prepare a pre-report. But there will be an oral examination, so you are strongly advised to study this note regarding to the pre-study questions below. After the lab, you will need to deliver a post-report which contains what you have done in the lab, data, related graphs and answers of the questions. Pre-Study Questions 1. Why do we use relays in the power systems? 2. What are the ANSI/IEEE codes of over relays? 3. What type of relay is over relay? 4. What is pick-up? How can it be selected regarding to fault and load? 5. How many types of over relays are there? 6. What are the differences between the types of over relays? 7. What is the purpose of using time delay in the over relays? 8. What are the types of inverse time over relay? 9. What is meant by time dial setting? 10. How the directionality of flow can be found in a power system? 11. Which types of over relays can be used for ground fault protection? 12. What are the meanings of DMT and IDMT? 1. Objective To study the principles of over and ground fault protection. 2. Theory [1, 2] Relays are used to detect abnormal conditions in the power systems. After detection of a fault, relays close circuit breakers and disconnect faulty circuits from the general supply system in order to minimize the damage. There is a list of ANSI/IEEE codes of different types of protection relays as follows [3, 4]: ANSI /IEEE Standard Device Numbers 2 - Time Delay Starting or Closing Relay 67 - AC Directional Over Relay 21 - Distance Relay 68 - Blocking or out of step Relay 25 - Synchronizing or Synchronism-Check Device 69 - Permissive Control Device 27 - Undervoltage Relay 74 - Alarm Relay 30 - Annunciator Relay 76 - DC Over Relay 32 - Directional Power Relay 78 - Phase-Angle Measuring Relay 37 - Under or Underpower Relay 79 - AC-Reclosing Relay 38 - Bearing Protective Device 81 - Frequency Relay 40 Field (over/under excitation) Relay 85 Pliot Comm., Carrier or Pilot-Wire Relay 46 Rev. phase or Phase-Bal. Current Relay 86 - Lockout Relay 47 - Phase-Seq. or Phase-Bal. Voltage Relay 87 - Differential Protective Relay 49 - Machine or Transformer Thermal Relay 94 - Tripping or Trip-Free Relay 50 - Instantaneous Over 51 - AC Time Over Relay B Bus 59 - Overvoltage Relay F - Field 60 - Voltage or Current Balance Relay G Ground or generator 63 - Pressure Switch N Neutral 64 - Ground Detector Relay T Transformer 1

2 2.1. Over Protection Over protection is practical application of magnitude relays since it picks up when the magnitude of exceeds some value (setting value). Over relays can be used to protect practically any power system elements, i.e. transmission lines, transformers, generators, or motors. As an example, a radial transmission line can be used. For a fault within the zone of protection, the fault is smallest at the end of the line and greatest at the relay end. If the minimum fault possible within the zone of protection is greater than the maximum possible load, it would be possible to define the operating principle as follows: fault zone, trip no fault in zone, do not trip. where is the in the relay and is the pickup setting of the relay. should be selected as: A B Ifault Iload max. R A location B (a) (b) Figure 1. Over protection of transmission lines. (a) Radial system protection. (b) Fault magnitude as a function of fault location. There are four types of over relays; instantaneous, definite time, inverse time and directional over relays Instantaneous Over Relays Its operation criterion is only magnitude (without time delay). This type is applied to the outgoing feeders. time no trip trip Ip Figure 2. Characteristic of instantaneous over relays Figure 3. Connection diagram of instantaneous over relays 2

3 Definite Time Over Relays In this type, two conditions must be satisfied for operation (tripping), must exceed the setting value and the fault must be continuous at least a time equal to time setting of the relay. Modern relays may contain more than one stage of protection each stage includes each own and time setting. time T3 T2 T1 I1 I2 I3 Figure 4. Characteristic of definite time over relays Definite time over relay is the most applied type of over. It is used as: 1. Back up protection of distance relay of transmission line with time delay. 2. Back up protection to differential relay of power transformer with time delay. 3. Main protection to outgoing feeders and bus couplers with adjustable time delay setting. Figure 5. Connection diagram of definite time over relay with internal timer Inverse Time Over Relays In this type of relays, operating time is inversely changed with. So, high will operate over relay faster than lower ones. There are standard inverse, very inverse and extremely inverse types. time inverse very inverse extremely inverse Figure 6. Characteristic of inverse time over relays 3

4 The operating time of an over relay can be moved up (made slower) by adjusting the time dial setting. The lowest time dial setting (fastest operating time) is generally 1/2, and the slowest is Operating time in seconds /2 Time dial setting Multiples of pickup setting (I f/i p) Figure 7. A typical commercial time over relay characteristic Directional Over Relays When the power system is not radial (source on one side of the line), an over relay may not be able to provide adequate protection. This type of relay operates in on direction of flow and blocks in the opposite direction. Three conditions must be satisfied for its operation: magnitude, time delay and directionality. The directionality of flow can be identified using voltage as a reference of direction Ground Fault Protection Figure 8. Connection diagram of directional over relay Ground fault s are dependent upon system grounding and they produce zero sequence s whereas there is very little zero-sequence during normal operation. Thus the pick-up settings of the ground fault relays can be made more sensitive than those of phase fault relays. A separate relay responding to the zero sequence is provided for ground fault protection. Types of ground fault relays are similar to over relays but with only one coil for in the case of instantaneous, definite time or inverse time ground fault types. One voltage coil is added in the case of directional ground fault relays. 4

5 There are different connection types of circuits: Current coil may be connected to return path (neutral) of a transformer Current coil may be connected to the secondary side of transformer which is installed at the star point of power transformer Current coil may be connected to secondary side of ring type transformer installed at power cables Figure 9. Ground fault relay connected at the neutral point of transformer Figure 11. Ground fault relay connected to ring type transformer Figure 10. Ground fault relay connected star point of power transformer Figure 12. Connection diagram of directional ground fault relay 3. Practical Information In this lab, the over functions (50, 51) of SIPROTEC Compact 7SJ80 Multifunction Protection Relay will be used. Order number is as follows: 7SJ8031-1EB90-1FC1 Order number includes many information about the relay, such as; housing, binary inputs and outputs, measuring inputs, language settings and its functions. It can be seen in the catalog of the relay[5]. In the relay, three definite time over (DMT) protection elements are available both for the phase and ground elements. The threshold and delay set time can be set. Inverse time over protection characteristics (IDMT) can also be selected and activated. 5

6 4. Procedure You don t need to set up any connection. Only study the connection scheme. In the lab, you will measure several values and related time delays in the context of over protection. 5. Connection Scheme SIPROTEC Compact 7SJ80 -S2 190V AC (L-L) LA N P1 + P1 - A IL1 IL2 IL3 IE F1 F2 F3 F4 F5 F6 F7 F8 Circuit Breaker K1 A1 A2 B01 C9-NO C10-NC C11-COM NC Circuit Breaker CB off KE KE BI1 BI2 NC C3 C4 C5 C S4 24V DC 2L+ 2L- C1(+) PS C2(-) Figure 13. Connection scheme of the test set-up 6

7 6. References 1. Ahmed Safie Eldin, Practical Introduction to Power System Protection and Control, Arun G. Phadke, James S. Thorp, Computer Relaying for Power Systems, John Wiley & Sons, IEEE Standard C : IEEE Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations Catalog of SIPROTEC Compact 7SJ80 Multifunction Protection Relay 6. IEC :2009 : Functional requirements for over/under protection 7. Questions for the Post-Report 1. What do you understand by Primary relay? Secondary relay? Auxiliary relay? 2. What is meant by Pick-up? Drop out? Dropout ratio? 3. What are the equations of IEC normal inverse, very inverse and extremely inverse over relays? 7