Distance 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 is the ANSI/IEEE code of distance relays? 3. What is the basic principle of distance protection? How it works? 4. What is the purpose of reach point? How will relays operate regarding to this setting? 5. What are the basic distance protection zones? Why different zones should be defined? 6. What are the main tripping characteristics for distance protection? 7. What is the difference between impedance and mho characteristic? 8. Why modern distance relays offer quadrilateral characteristics? 9. Why R-X diagram is commonly used? 10. What do the four quadrants of the R-X diagram show? 1. Objective To study the principles of distance protection. 2. Theory [1-4] 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 [5, 6]: ANSI /IEEE Standard Device Numbers 2 - Time Delay Starting or Closing Relay 67 - AC Directional Overcurrent 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 Overcurrent Relay 32 - Directional Power Relay 78 - Phase-Angle Measuring Relay 37 - Undercurrent 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 Overcurrent 51 - AC Time Overcurrent 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
2.1. Basic Principle A distance relay has the ability to detect a fault within a pre-set distance along a transmission line or power cable from its location. Every power line has a resistance and reactance per kilometer related to its design and construction so its total impedance will be a function of its length. A distance relay therefore looks at current and voltage and compares these two quantities on the basis of Ohm s law. Since the impedance of a transmission line is proportional to its length, for distance measurement it is appropriate to use a relay capable of measuring the impedance of a line up to a predetermined point (the reach point). Distance relay is designed to operate only for faults occurring between the relay location and the predetermined (reach) point, thus giving discrimination for faults that may occur in different line sections. The basic principle of distance protection involves the division of voltage at the relaying point by the measured current. The calculated apparent impedance is compared with the reach point impedance. If the measured impedance is less than the reach point impedance, it is assumed that a fault exists on the line between the relay and the reach point. 2.2. Zones of Protection Careful selection of the reach point settings and tripping times for various zones of measurement enables correct coordination between distance relays on a power system. Basic distance protection will comprise one instantaneous (Zone 1) and one or more time delayed zones (Zone 2, Zone 3, Zone 4 ). Typical reach and time settings for a 3-Zone distance protection are shown below: Zone 1: this is set to protect between 80% of the line length AB and operates without any time delay. This under-reach setting has been purposely chosen to avoid over-reaching into the next line section to ensure selectivity since errors and transients can be present in the voltage and current transformers. Also manufacturing tolerances limit the measurement accuracy of the relays. Zone 2: this is set to protect 100% of the line length AB, plus at least 20% of the shortest adjacent line BC and operates with time delay t2. ( 0.5s) It not only covers the remaining %20 of the line, but also provides backup for the next line section. Zone 3: this is set to protect 100% of the two lines AB, BC, plus about 25% of the third line CD and operates with time delay t3. ( 1.5s) It should be noted that, digital distance relays may have up to six zones, some set to measure in the reverse direction. 2
2.3. Tripping Characteristics The shape of the operation zones has developed throughout the years. An overview of relay characteristics can be seen below: i. Impedance Characteristic: If the relay s operating boundary is plotted on an R-X diagram, its impedance characteristic is a circle with its center at the origin of the coordinates and its radius will be the setting (the reach point) in ohms. The relay will operate for all values less than its setting i.e. for all points within the circle. This type of relay, however, is non-directional. It can operate for faults behind the relaying point. It takes no account of the phase angle between voltage and current. It is also sensitive to power swings and load encroachment due to the large impedance circle. ii. Mho Characteristic: The limitation of the impedance characteristic can be overcome by a technique known as selfpolarization. Additional voltages are fed into the comparator in order to compare the relative phase angles of voltage and current, so providing a directional feature. This has the effect of moving the circle such that the circumference of the circle passes through the origin. Angle θ is known as the relay s characteristic angle. It appears as a straight line on an admittance diagram. 3
By use of a further technique of feeding in voltages from the healthy phases into the comparator (known as cross polarization) a reverse movement or offset of the characteristic can be obtained. This is called the offset mho characteristic. iii. Combined Characteristic: Relays with combined characteristics are obtained by added a mho circle with lines parallel to the resistive and reactive axes which cross each other at the setting point. iv. Quadrilateral Characteristic: Modern distance relays offer quadrilateral characteristic, whose resistive and reactive reach can be set independently. It therefore provides better resistive coverage than any mho-type characteristic for short lines. This is especially true for earth fault impedance measurement, where the arc resistances and fault resistance to earth contribute to the highest values of fault resistance. Polygonal impedance characteristics are highly flexible in terms of fault impedance coverage for both phase and earth faults. For this reason, most digital relays offer this form of characteristic. 2.4. Significance of R-X Diagram In general, all electromechanical relays respond to one or more of the conventional torqueproducing input quantities: (a) voltage, (b) current, (c) product of voltage, current and the angle θ between them and (d) a physical or design force such as a control spring. Similar considerations hold for solid-state relays as well. For distance relay, analyzing the response of the relay for all conditions is difficult because the voltage varies for each fault, or varies for the same fault but with different system conditions. To resolve this difficulty, it is common to use an R X diagram. By utilizing only two quantities, R and X (or Z and θ), we avoid the confusion introduced by using the three quantities E, I and θ. There is an 4
additional significant advantage in that the R X diagram allows us to represent both the relay and the system on the same diagram. Consider an ideal (zero resistance) short circuit at location F in the single-phase system shown below. The distance relay under consideration is located at line terminal A. The subscript p represents primary and s represents secondary quantities. In terms of the secondary quantities of voltage and current transformers, the relay sees as. where and are the current transformer (CT) and voltage transformer (VT) turns ratios. When a fault occurs, this impedance may be plotted as a point on the complex R X plane. Now consider the fault at location F as shown in the figure below. The line AB makes an angle θ with the R axis, where θ is the impedance angle of the transmission line. (For an overhead transmission line, θ lies between 70 and 88 ). When the fault is on the transmission line, the apparent impedance plots on the line AB; for all other faults or loading conditions, the impedance plots away from the line AB. Often it is convenient to plot the source impedance also on the R X diagram. 3. Practical Information In this lab, the distance protection function (21) of SIPROTEC 4-7SA610 [7] and Kingsine K68i Protection Relay Test Set [8] will be used. Order number of the relay is as follows: 7SA610 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
The relay supports distance protection (21/21N), high resistance earth-fault protection for single and three phase tripping (50N, 51N, 67N), pilot protection (85), fault locator (FL), phase overcurrent protection (50/51/67), over/under voltage protection (59/27), over/under frequency protection (81O/U) and auto-reclosure function (25). For communication, the relay supports IEC 61850 Ethernet, IEC 60870-5-103, Profibus, RS232 and RS 485. For distance protection, the relay provides 6 zones that can be set to forward, reverse, nondirectional and inactive. Load zones for phase-phase and phase-earth can be set separately. 4. Procedure You don t need to set up any connection. Only study the connection scheme. In the lab, you will measure several distance and trip time values from the relay regarding to specified fault impedances in the relay test set. 5. Connection Scheme Connection scheme will be provided soon. 6. References 1. L. G. Hewitson, M. Brown, R. Balakrishnan, Practical Power System Protection, Elsevier, 2005. 2. S. H. Horowitz, A. G. Phadke, Power System Relaying, John Wiley & Sons, 2008. 3. H. M. Tran, H. Akyea, Numerical Distance Protection Relay Commissioning and Testing, MSc Thesis, Chalmers University of Technology, Göteborg, Sweden, 2005. 4. http://www.fecime.org/referencias/npag/chap11-20-170-191.pdf 5. IEEE Standard C37.2-2008 : IEEE Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations. 6. http://www.ee.uidaho.edu/ee/power/ee525/lectures/l9/relaydevicenumbers.pdf 7. Catalog of Siemens SIPROTEC 4-7SA6 : Distance Protection Relay for all Voltage Levels. 8. http://www.kingsine.com.cn/userfiles/download/1713062151.pdf 7. Questions for the Post-Report 1. What is the meaning of mho? Is there any relation between the units mho and ohm? 2. What is the key advantage of distance protection over overcurrent protection? (look up in [4]) 3. What are the under-reach and over-reach concepts? (look up in [4]) 6