Electricity and New Energy Overcurrent and Overload Protection Using Protective Relays
Course Sample 589888 Order no.: 589888 First Edition Revision level: 11/2016
By the staff of Festo Didactic
© Festo Didactic Ltée/Ltd, Quebec, Canada 2015 Internet: www.festo-didactic.com e-mail: [email protected]
Printed in Canada All rights reserved ISBN 978-2-89747-450-8 (Printed version) ISBN 978-2-89747-452-2 (CD-ROM) Legal Deposit – Bibliothèque et Archives nationales du Québec, 2015 Legal Deposit – Library and Archives Canada, 2015
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All other trademarks are the property of their respective owners. Other trademarks and trade names may be used in this document to refer to either the entity claiming the marks and names or their products. Festo Didactic disclaims any proprietary interest in trademarks and trade names other than its own. Safety and Common Symbols
The following safety and common symbols may be used in this manual and on the equipment:
Symbol Description
DANGER indicates a hazard with a high level of risk which, if not avoided, will result in death or serious injury.
WARNING indicates a hazard with a medium level of risk which, if not avoided, could result in death or serious injury.
CAUTION indicates a hazard with a low level of risk which, if not avoided, could result in minor or moderate injury.
CAUTION used without the Caution, risk of danger sign , indicates a hazard with a potentially hazardous situation which, if not avoided, may result in property damage.
Caution, risk of electric shock
Caution, hot surface
Caution, risk of danger. Consult the relevant user documentation.
Caution, lifting hazard
Caution, hand entanglement hazard
Notice, non-ionizing radiation
Direct current
Alternating current
Both direct and alternating current
Three-phase alternating current
Earth (ground) terminal
© Festo Didactic 589888 III Safety and Common Symbols
Symbol Description
Protective conductor terminal
Frame or chassis terminal
Equipotentiality
On (supply)
Off (supply)
Equipment protected throughout by double insulation or reinforced insulation
In position of a bi-stable push control
Out position of a bi-stable push control
IV © Festo Didactic 589888 Table of Contents
Preface ...... IX About This Manual ...... XI To the Instructor ...... XIII
Introduction Overcurrent and Overload Protection Using Protective Relays ...... 1
DISCUSSION OF FUNDAMENTALS ...... 1 Distinguishing overcurrent protection from overload protection ...... 1 Evolution of the protective relay technology ...... 2 Protective relay testing ...... 5
Exercise 1 Overcurrent Protection ...... 9
DISCUSSION ...... 9 Introduction ...... 9 Overcurrent protection and overload protection using fuses ...... 10 Overcurrent protection and overload protection using LV circuit breakers (MCBs and MCCBs)...... 15 Overcurrent protection and overload protection using protective relays and HV circuit breakers ...... 19 Instantaneous overcurrent relay ...... 21 Definite-time overcurrent relay ...... 23 Inverse definite minimum time (IDMT) overcurrent relay ...... 24 Standard time-current characteristics of IDMT overcurrent relays ...... 25 Overcurrent protection of a power transformer using a numerical overcurrent relay ...... 28 Main features of protection implemented with numerical overcurrent relays and HV circuit breakers ...... 31
PROCEDURE ...... 32 Set up and connections ...... 32 Operation of an instantaneous overcurrent relay ...... 33 Operation of a definite time overcurrent relay ...... 39 Operation of an inverse definite minimum time (IDMT) overcurrent relay ...... 43 Overcurrent protection of a power transformer using a numerical overcurrent relay ...... 49
© Festo Didactic 589888 V Table of Contents
Exercise 2 Overcurrent and Overload Protection of AC Machines and Power Transformers ...... 57
DISCUSSION ...... 57 Introduction ...... 57 Thermal capability and power rating of ac machines and power transformers ...... 58 Machine or transformer thermal relays of the temperature-sensor type ...... 60 Machine or transformer thermal relays of the thermal- replica type ...... 61 OC and OL protection of an ac machine or power transformer using a numerical protective relay combining the functions of ANSI devices no. 50 and no. 49 ...... 66 Using the IDMT overcurrent relay (ANSI device no. 51I) function to achieve overload protection ...... 69
PROCEDURE ...... 71 Set up and connections ...... 71 Settings of a machine or transformer thermal relay of the thermal-replica type ...... 72 Operation of a machine or transformer thermal relay of the thermal-replica type ...... 76 Time-current characteristic of a machine or transformer thermal relay of the thermal-replica type ...... 83 Overcurrent protection ...... 86
Exercise 3 Overcurrent Protection of Radial Feeders ...... 91
DISCUSSION ...... 91 Radial feeders ...... 91 Protection of a radial feeder with overcurrent relays ...... 93 Discrimination based on coordination in current ...... 94 Discrimination based on coordination in time ...... 98 Current settings ...... 99 Time settings ...... 99 Discrimination based on coordination in both time and current ...... 101 Generic procedure to achieve time-current coordination of IDMT overcurrent relays ...... 101 Information about the system ...... 101 Time-current characteristics of the IDMT overcurrent relays .. 101 Current settings ...... 102 Time settings ...... 102 Coordination of IDMT overcurrent relays with different types of time-current characteristics ...... 109 Relays with different types of time-current characteristics ...... 110 Coordination between an IDMT overcurrent relay and a fuse ...... 114
VI © Festo Didactic 589888 Table of Contents
PROCEDURE ...... 116 Set up and connections ...... 116 Overcurrent protection of a radial feeder ...... 116 Operation of relay C ...... 119 Operation of relay B ...... 122 Operation of relay A ...... 125 Coordination verification ...... 128 Comparison with definite time overcurrent relays ...... 130 Ending the exercise ...... 130
Appendix A Equipment Utilization Chart ...... 133
Appendix B Glossary of New Terms ...... 135
Appendix C Introduction to the DIGSI 5 Software from Siemens ...... 139 Setting the language used in DIGSI 5 ...... 140 Opening a project file ...... 140 Displaying the single-line diagram ...... 142 Setting the frequency of operation of the protective relay ... 142 Setting the language used in the front panel display of the protective relay ...... 144 Accessing the settings of a specific protection function of the relay ...... 144 Accessing the parameters of a test sequence ...... 146 Changing the ratio of current (or voltage) transformers ...... 147 Enabling/disabling fault display ...... 149 Loading a new configuration to the protective relay ...... 150 Restarting the protective relay in the simulation (test) mode ...... 152 Updating the test environment ...... 155 Starting a test sequence ...... 155 Downloading a fault record from the protective relay ...... 157 Displaying the signals stored in a fault record ...... 158 Restarting the protective relay in the process (normal operation) mode...... 161
Appendix D Protective Relay LED Identification Labels ...... 163
Appendix E Electrical Graphic Symbols (IEC and ANSI) ...... 165
Index ...... 167 Acronyms ...... 169 Bibliography ...... 171
© Festo Didactic 589888 VII
Preface
The production of energy using renewable natural resources such as wind, sunlight, rain, tides, geothermal heat, etc., has gained much importance in recent years as it is an effective means of reducing greenhouse gas (GHG) emissions. The need for innovative technologies to make the grid smarter has recently emerged as a major trend, as the increase in electrical power demand observed worldwide makes it harder for the actual grid in many countries to keep up with demand. Furthermore, electric vehicles (from bicycles to cars) are developed and marketed with more and more success in many countries all over the world.
To answer the increasingly diversified needs for training in the wide field of electrical energy, the Electric Power Technology Training Program was developed as a modular study program for technical institutes, colleges, and universities. The program is shown below as a flow chart, with each box in the flow chart representing a course.
The Electric Power Technology Training Program.
© Festo Didactic 589888 IX Preface
The program starts with a variety of courses providing in-depth coverage of basic topics related to the field of electrical energy such as ac and dc power circuits, power transformers, rotating machines, ac power transmission lines, and power electronics. The program then builds on the knowledge gained by the student through these basic courses to provide training in more advanced subjects such as home energy production from renewable resources (wind and sunlight), large- scale electricity production from hydropower, large-scale electricity production from wind power (doubly-fed induction generator [DFIG], synchronous generator, and asynchronous generator technologies), smart-grid technologies (SVC, STATCOM, HVDC transmission, etc.), storage of electrical energy in batteries, and drive systems for small electric vehicles and cars.
We invite readers of this manual to send us their tips, feedback, and suggestions for improving the book.
Please send these to [email protected]. The authors and Festo Didactic look forward to your comments.
X © Festo Didactic 589888 About This Manual
Manual objectives
When you have completed this manual, you will be familiar with the operation and settings of the instantaneous (ANSI device no. 50), definite-time (ANSI device no. 51DT), and inverse definite minimum time (ANSI device no. 51I) overcurrent relays. You will be able to adjust the settings of an overcurrent relay to obtain a specific time-current characteristic. You will know applications where it is common to use overcurrent relays and high-voltage circuit breakers in conjunction to achieve overcurrent protection of electrical equipment. You will be familiar with the operation and settings of the machine or transformer thermal relay (ANSI device no. 49) of the temperature-sensor type or the thermal-replica type. You will know how to combine protection functions in a numerical protective relay to achieve overcurrent and overload protection of an ac machine or a power transformer. You will also know how to implement overcurrent protection of a radial feeder using either definite-time overcurrent relays or inverse definite minimum time (IDMT) overcurrent relays. You will be able to use the internal relay test system of a numerical protective relay to assess that the relay operates as expected.
Safety considerations
Safety symbols that may be used in this manual and on the equipment are listed in the Safety Symbols table at the beginning of the manual.
Safety procedures related to the tasks that you will be asked to perform are indicated in each exercise.
Make sure that you are wearing appropriate protective equipment when performing the tasks. You should never perform a task if you have any reason to think that a manipulation could be dangerous for you or your teammates.
Prerequisite
As a prerequisite to this course, you should have read the manuals titled DC Power Circuits, part number 86350, Single-Phase AC Power Circuits, part number 86358, Single-Phase Power Transformers, part number 86377, Three-Phase AC Power Circuits, part number 86360, and Three-Phase Transformer Banks, part number 86379.
Systems of units
Units are expressed using the International System of Units (SI) followed by units expressed in the U.S. customary system of units (between parentheses).
© Festo Didactic 589888 XI
To the Instructor
You will find in this Instructor Guide all the elements included in the Student Manual together with the answers to all questions, results of measurements, graphs, explanations, suggestions, and, in some cases, instructions to help you guide the students through their learning process. All the information that applies to you is placed between markers and appears in red.
Accuracy of measurements
The numerical results of the hands-on exercises may differ from one student to another. For this reason, the results and answers given in this manual should be considered as a guide. Students who correctly performed the exercises should expect to demonstrate the principles involved and make observations and measurements similar to those given as answers.
Detailed procedure in Exercise 1
It is recommended to perform the exercises in the order proposed in this manual, as Exercise 1 features more detailed explanations than the rest of the exercises.
© Festo Didactic 589888 XIII
Sample Extracted from Instructor Guide
Exercise 2
Overcurrent and Overload Protection of AC Machines and Power Transformers
EXERCISE OBJECTIVE When you have completed this exercise, you will understand the relationship between the power rating of an ac machine or a power transformer and its thermal capability. You will be familiar with the operation and settings of machine or transformer thermal relays (ANSI device no. 49) of the temperature-sensor type. You will also be familiar with the operation and settings of machine or transformer thermal relays of the thermal-replica type. You will be able to adjust the settings of a thermal relay of the thermal-replica type to obtain a specific inverse time-current characteristic. You will know how to use a numerical protective relay combining the protective functions of an instantaneous overcurrent relay (ANSI device no. 50) and a machine or transformer thermal relay (ANSI device no. 49) to achieve overcurrent and overload protection of an ac machine or a power transformer. You will also know that an IDMT overcurrent relay (ANSI device no. 51I) function can also be used to achieve overload protection of an ac machine or a power transformer. You will be able to use the internal relay test system of a numerical protective relay to assess that the machine or transformer thermal relay function operates as expected.
DISCUSSION OUTLINE The Discussion of this exercise covers the following points:
. Introduction . Thermal capability and power rating of ac machines and power transformers . Machine or transformer thermal relays of the temperature-sensor type . Machine or transformer thermal relays of the thermal-replica type . OC and OL protection of an ac machine or power transformer using a numerical protective relay combining the functions of ANSI devices no. 50 and no. 49 . Using the IDMT overcurrent relay (ANSI device no. 51I) function to achieve overload protection
DISCUSSION Introduction
In the previous exercise, you saw that an instantaneous overcurrent relay (ANSI device no. 50) used in conjunction with a high-voltage circuit breaker is well suited to achieve overcurrent protection of an ac motor or a power transformer (up to about 5 to 10 MVA). In this situation, the current setting of the instantaneous overcurrent relay is set to several times the nominal current of the protected ac motor or power transformer. This, however, has the disadvantage that little or no protection against overload is provided because overload situations often result in currents that are generally less than about 3 to 4 times the nominal current of the protected ac motor or power transformer. Consequently, using an instantaneous overcurrent relay alone is not sufficient to protect an ac motor or a power transformer against both overcurrent and
© Festo Didactic 589888 57 Exercise 2 – Overcurrent and Overload Protection of AC Machines and Power Transformers Discussion
overload. It is thus common to use a numerical protective relay combining the protective functions of an instantaneous overcurrent relay (ANSI device no. 50) and a machine or transformer thermal relay (ANSI device no. 49) to achieve overcurrent and overload protection of an ac motor or a power transformer. It is also possible to combine the protective functions of an instantaneous overcurrent relay (ANSI device no. 50) and an IDMT overcurrent relay (ANSI device no. 51I) to achieve overcurrent and overload protection. These two alternatives to achieve overcurrent and overload protection are covered in this discussion.
Thermal capability and power rating of ac machines and power transformers
The current flowing in an ac machine (e.g., an induction motor or a synchronous generator) or a power transformer when it is loaded causes electric power to be dissipated as heat. This loss of electric power is due to the resistance of the wire used in the windings of the device (ac machine or power transformer) and is known as the copper loss (RI2 loss). The heat caused by the flow of load current, in turn, causes the temperature of the device to increase until it stabilizes to a certain final value. The increase in the device’s temperature, i.e., the difference between the final temperature (Final) that is reached when it stabilizes and the ambient temperature (Amb.) is referred to as the temperature rise (). The higher the value of the load current, the higher the temperature rise . This is illustrated in Figure 32. Notice that the rate at which the temperature increases depends on the heating time constant H of the device. The longer the heating time constant H, the slower the device’s temperature increases. In Figure 32, the heating time constant H of the device is equal to 10 minutes, and thus, the device’s temperature stabilizes after about 50 minutes (i.e., after about 5·H).
ILoad 2 > ILoad 1 Load current starts
to flow in the device Final 1 at this instant ILoad 2
Final 2 ILoad 1 Temperature 1 2
Amb.
H of device = 10 min.
Time (min)
Figure 32. Temperature increase of an electric device such as an ac machine or power transformer as a function of time, caused by the flow of a load current of constant value.
The temperature rating of the insulation (often referred to as the insulation class) of the windings in an electric device such as an ac machine or a power transformer is generally the main factor limiting the maximum operating
58 © Festo Didactic 589888 Exercise 2 – Overcurrent and Overload Protection of AC Machines and Power Transformers Discussion
temperature of the device. This, in turn, limits the maximum temperature rise (Max.) that is possible, and thus, the maximum current that can flow in the device continuously without causing damage to the device. This ultimately determines the power rating of the device. The maximum current that can flow in the device continuously without causing damage is referred to as the maximum thermally-permissible current (ITherm. Max.). For instance, when the temperature rating of the insulation of a particular device is 105°C (221°F) and a maximum ambient temperature of 40°C (104°F) is assumed, the maximum temperature rise Max. that is allowed is 65°C (117°F), as shown in Figure 33. Any load current below current ITherm. Max. causes a temperature rise lower than the maximum temperature rise Max., and thus, can flow in the device continuously without causing damage. However, any load current above current ITherm. Max. leads to a temperature rise higher than the maximum temperature rise Max. and causes the maximum operating temperature of the device to be exceeded. Obviously, such a load current cannot flow in the device continuously as it will eventually cause damage to the device. Note that the higher the value of the load current, the shorter the time required to reach the maximum operating temperature of the device.
Maximum operating temperature of Final 3 = 140°C (284°F) device = 105°C = 100°C (180°F) (221°F) 3 ILoad 3 > ITherm. Max.
Final 2 = 105°C (221°F) 2 = 65°C (117°F) ILoad 2 = ITherm. Max.
Max. = 65°C Final 1 = 75°C (167°F) (117°F) 1 = 35°C (63°F) Temperature ILoad 1 < ITherm. Max.
Amb. = 40°C (104°F)
Load current starts to flow in the device at this instant
Time
Figure 33. The temperature rise occurring in an electric power device such as an ac
machine or power transformer exceeds the maximum temperature rise Max. when the load current flowing through the device exceeds the maximum thermally-permissible current ITherm. Max..
The important fact to remember from Figure 33 is that the current flowing through an electric device such as an ac machine or a power transformer can be used to determine whether or not the device is overheating (i.e., to determine whether or not the temperature rise produced by a load current of a given value exceeds the maximum temperature rise Max.). This is the basic principle used in machine or transformer thermal relays of the thermal-replica type. The operation of these thermal relays is studied later in this discussion.
© Festo Didactic 589888 59 Exercise 2 – Overcurrent and Overload Protection of AC Machines and Power Transformers Discussion
Machine or transformer thermal relays of the temperature-sensor type
Machine or transformer thermal relays are generally of the temperature-sensor type or of the thermal-replica type. Machine or transformer thermal relays of the temperature-sensor type are covered in this section of the discussion. Machine or transformer thermal relays of the thermal-replica type are covered in the next section of this discussion.
Figure 34 is a single-line diagram showing overload protection of an ac machine or a power transformer implemented with a thermal relay (ANSI device no. 49) of the temperature-sensor type used in conjunction with an HV circuit breaker.
AC machine or power transformer
AC power source Alarm Temperature 49 signal sensor
Figure 34. Overload protection of an ac machine or a power transformer implemented with a thermal relay (ANSI device no. 49) of the temperature-sensor type used in conjunction with an HV circuit breaker.
In a machine or transformer thermal relay of the temperature-sensor type, the temperature of the protected device is measured directly using a temperature sensor such as a resistive temperature detector (RTD) or a thermocouple. The thermal relay compares at regular time intervals the measured temperature of the device to a maximum temperature threshold set in the relay. When the measured temperature of the device reaches about 80% to 90% of the maximum temperature threshold (this percentage value is adjustable in the thermal relay), the thermal relay produces an alarm signal. This signal can be used to automatically initiate an action that will prevent the temperature of the protected device from further increasing or to warn an operator so that he or she can initiate the necessary action. Turning on cooling equipment (e.g., fans) on the protected device or reducing the load applied to the protected device (e.g., by shedding loads connected to a power transformer) are two examples of action that can prevent the device’s temperature from increasing further. When the measured temperature of the device exceeds the maximum temperature threshold for a certain predetermined time set in the relay (this is likely to occur when no action is taken after the initial alarm signal), the thermal relay produces a trip command that can be used to open an HV circuit breaker connected in series with the device to be protected. This prevents prolonged overheating of the protected device, thereby avoiding damage to the device.
The value at which the maximum temperature threshold of a thermal relay is set depends on the nature of the device to be protected. For instance, the maximum temperature at which most oil-immersed power transformers can safely operate (i.e., without damage or reduction in life expectancy) is generally between 95°C (203°F) and 105°C (221°F). On the other hand, the maximum temperature at which ac machines such as induction motors and synchronous generators can safely operate generally ranges from 80°C (176°F) to about 200°C (392°F). The maximum operating temperature of any particular ac
60 © Festo Didactic 589888 Exercise 2 – Overcurrent and Overload Protection of AC Machines and Power Transformers Discussion
machine depends on its insulation class as defined in the IEC and NEMA standards. In all cases, the maximum temperature threshold set in a machine or transformer thermal relay must be carefully determined in accordance with the specifications of the particular device to be protected.
Note that in the case of overload protection of a power transformer, it is common not to use the trip command of the thermal relay to disconnect the protected transformer even when the maximum temperature threshold is exceeded. This is because damage, in general, does not occur immediately when a power transformer is overloaded. This is also motivated by the fact that the cause of a transformer overload is generally elsewhere in the power system, and consequently, disconnecting the overloaded power transformer is not the solution to the overload. In fact, disconnecting the overloaded power transformer simply aggravates the overload problematic since less power is available in the system to meet the demand. The consequence of disconnecting an overloaded power transformer is thus to move the overload problem to other power transformers in the power system. This can eventually lead to instability in the power system, which is highly undesirable. Using the trip command of a thermal relay to disconnect an overloaded power transformer is a design criterion that depends on the particular application in which the power transformer is used.
Machine or transformer thermal relays of the thermal-replica type
A machine or transformer thermal relay of the thermal-replica type operates the same way as a machine or transformer thermal relay of the temperature–sensor type, i.e., it first produces an alarm signal when the device’s temperature nears the maximum temperature threshold. Then, it produces a trip command when the device’s temperature exceeds the maximum temperature threshold for a certain predetermined time. However, in a machine or transformer thermal relay of the thermal-replica type, the device’s temperature is estimated from the current flowing through the protected device instead of being measured directly using a temperature sensor. Figure 35 is a single-line diagram showing overload protection of an ac motor or a power transformer implemented with a thermal relay of the thermal-replica type used in conjunction with an HV circuit breaker.
AC machine or power transformer AC power source Alarm 49 signal
Figure 35. Overload protection of an ac machine or a power transformer implemented with a thermal relay of the thermal-replica type used in conjunction with an HV circuit breaker.
A thermal relay of the thermal-replica type uses a mathematical model to estimate the temperature of the protected device as accurately as possible from the measured value of the current flowing through the device. This mathematical model is based on the assumption that the protected device is a homogeneous body that produces and dissipates heat at a rate proportional to the temperature rise .
© Festo Didactic 589888 61 Exercise 2 – Overcurrent and Overload Protection of AC Machines and Power Transformers Discussion
When the protected device is loaded, current starts to flow in the device and its temperature begins to increase. When the value of the load current flowing through the device is constant, the device’s temperature at any instant can be calculated using the following equation: