Commissioning Systems, Protective Relay

Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramco’s employees.Any material contained in this document which is not already in the publicdomain may not be copied, reproduced, sold, given, or disclosed to thirdparties, or otherwise used in whole, or in part, without the written permissionof the Vice President, Engineering Services, Saudi Aramco.

Chapter : Electrical For additional information on this subject, contactFile Reference: EEX30203 W.A. Roussel on 874-6160

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Commissioning Protective Relay Systems

Engineering Encyclopedia Electrical


Saudi Aramco DeskTop Standards

Content Page

INTRODUCTION................................................................................................................ 1

SAUDI ARAMCO REQUIREMENTS................................................................................. 2

Types ........................................................................................................................ 2

Electro-Mechanical ........................................................................................ 3

Solid State ..................................................................................................... 6

Characteristics........................................................................................................... 8

EVALUATING PROTECTIVE RELAY SYSTEMS UPON RECEIPT ..............................11

Visual Inspection......................................................................................................11

Verification Against Specifications ...........................................................................12

EVALUATING PROTECTIVE RELAY SYSTEM INSTALLATION AND TESTING .....16

Mechanical Checks...................................................................................................16

Electrical Tests.........................................................................................................18

Insulation Test ..............................................................................................18

Pickup Test...................................................................................................19

Timing Test ..................................................................................................20

Miscellaneous Electrical Tests.......................................................................20

SYSTEM PRE-OPERATIONAL CHECK-OUT .................................................................21

PDD Point-to-Point Wiring Check............................................................................21

Subsystem Checks....................................................................................................21

WORK AID 1: REFERENCES FOR EVALUATING PROTECTIVE RELAYSYSTEMS UPON RECEIPT ....................................................................22

Work Aid 1A: Protective Relay System Ratings and Requirements ...........................22

Work Aid 1B: Standard Electrical Symbols and Device Numbers..............................26

Work Aid 1C: Device Suffix Letters .........................................................................32

Work Aid 1D: Saudi Aramco Pre-Commissioning Form P-021 Excerpts...................35

WORK AID 2: REFERENCES FOR EVALUATING PROTECTIVE RELAYSYSTEM INSTALLATION AND TESTING ...........................................38




Work Aid 2A: Procedures and Methods for Evaluating Instrument Transformerand Meter Installation and Testing.....................................................38

Mechanical Checks...................................................................................................38

Electrical Tests.........................................................................................................39

Work Aid 2B: Saudi Aramco Pre-Commissioning Form P-021 Excerpts...................39

GLOSSARY........................................................................................................................44




Table of Figures Page

Figure 1: Example Induction-Type Relay........................................................................ 4

Figure 2: Basic Electro-Mechanical Relay Circuit ........................................................... 5

Figure 3: Block Diagram of a Solid State Current Relay ................................................. 7

Figure 4: Protective Relay Time-Current Characteristics................................................. 9

Figure 5: Overcurrent Device Coordination .................................................................. 10

Figure 6: Typical Electro-Mechanical Overcurrent Relay .............................................. 12

Figure 7: Typical Saudi Aramco Bus Overcurrent Protection Scheme ........................... 15

Figure 8: Electro-Mechanical Overcurrent Relay That Has Been RemovedFrom Its Case............................................................................................... 17

Figure 12: Standard Current and Voltage Ratings for Relays(From ANSI/IEEE C37.90).......................................................................... 22

Figure 13: Saudi Aramco Protective Relay System General Requirements(From SAES-P-114) .................................................................................... 23

Figure 13: Saudi Aramco Protective Relay System General Requirements(From SAES-P-114)(Cont'd)........................................................................ 24

Figure 14: Relaying Electrical Symbols......................................................................... 26

Figure 15: NEMA Device Function Numbers ............................................................... 27

Figure 15: NEMA Device Function Numbers (Cont'd)................................................. 28




Figure 16: Sheet 1 (of 12) of the Saudi Aramco Pre-Commissioning Form,P-021, Protective Relays .............................................................................. 36








Saudi Aramco DeskTop Standards 1

INTRODUCTION

Protective relay systems are installed to cause the prompt removal from service of any element ofa power system when it suffers a short circuit or when it starts to operate in any abnormal mannerthat might cause damage or otherwise interfere with the effective operation of the rest of thesystem. Protective relay systems consist of three types of elements that work together to performthe desired protective function: the sensing devices, the protective relays, and the controlling (orisolating) devices.

The sensing devices (e.g., CT or VT) continuously monitor power systems from generation,through transmission and distribution, to utilization. Faults or abnormal conditions are detectedthrough use of the sensing devices. The protective relays receive signals from the sensing devicesand provide outputs to the controlling devices (e.g., circuit breakers). When faults or abnormalconditions are detected by the sensing devices, rapid corrective action is initiated to isolate thefaulted portion of the system. Rapid fault isolation provides a high degree of power continuity,reduces potential personnel hazards, and limits equipment damage.

The correct selection, installation, and inspection of protective relay systems will directly affectthe overall safety, accuracy, and performance of electric power generation, transmission, anddistribution systems. This Module provides information on the following topics that are pertinentto commissioning protective relay systems:

• Saudi Aramco Requirements

• Evaluating Protective Relay Systems Upon Receipt

• Evaluating Protective Relay Systems Installation and Testing

• System Pre-Operational Check-Out




SAUDI ARAMCO REQUIREMENTS

Relaying provides a critical function in electric power system protection. Relays are versatile andreliable, and they can perform a wide variety of functions. Relays of various kinds can be set todetect abnormal conditions, such as faults, overloads, power swings, frequency changes, andover/undervoltages. Through initiation of a trip of the appropriate circuit breakers, relays canisolate and deenergize a zone of protection to minimize the effect of the disturbance or fault onthe remainder of the electric power system. A zone of protection is an area in a power systemthat a protection relay is configured to protect.

The safe and proper operation of Saudi Aramco electrical generation, transmission, anddistribution system installation depends on the proper selection and installation of protective relaysystems. Because the protective relay system functions to isolate faulted portions of the electricalsystems to prevent hazards to personnel and damage to equipment, the improper selection orinstallation of a protective relay system can lead to disastrous or catastrophic results. The typesand characteristics of protective relay systems are described in Saudi Aramco standards and in thefollowing text. A detailed description of protective relay systems is provided in EEX 106.SAES-P-114, 16-SAMSS-513, and ANSI C37 provide specific guidance that pertains to thematerial requirements for protective relay systems. These requirements must be followed whenthe equipment is specified in initial project design and also when the commissioning processoccurs.

A protective relay gets voltage and/or current information from an external sensing element in theform of current flow. This current flow is some small percentage of the parameter that is beingsensed. The small representative current flow causes the relay operating element (usually a coil)to open or close. The relay operating element opens or closes contacts, which actuate a warningsignal or which complete the trip circuit of a circuit breaker. The relay usually includes someform of visual indicator, which is known as a target, to show that the relay has operated. SpecificSaudi Aramco protective relay design requirements are provided in Work Aid 1.

Types

The components that are used to construct the various types of relays are often similar in natureacross a wide spectrum of relays; such components can generally be grouped into the electro-mechanical type and the solid state type.




Electro-Mechanical

Electro-mechanical relays use a combination of electrical fields and mechanical motion. Anelectrical field that is proportional to the measured quantity (e.g., system voltage or current)causes mechanical motion within the electro-mechanical relay. Electro-mechanical units can befurther subdivided into magnetic attraction units and magnetic induction units.

A magnetic attraction unit uses instrument transformer secondary outputs (either voltage orcurrent) to create magnetic fields within the relay unit. An example of a magnetic attraction unitis a simple plunger relay. When the current or voltage magnitude that is applied to the coilexceeds a predetermined value, the magnetic field that is formed will cause a plunger to moveupward. The action of the moving plunger closes or opens a set of contacts. Plunger relays areoften used as components in overcurrent relays.

A magnetic induction unit uses the principle of an induction motor in which torque is developedthrough use of induction onto a rotor. Induction-type relays are only used in ac applications. Therotating element of an induction-type relay is usually a disk or a cylinder. The two basic designs ofa magnetic induction relay are the induction disk and the induction cup. A typical induction-diskrelay is shown in Figure 1.

In Figure 1, the operating torque on the disk is produced through use of an electromagnet that hasa main coil and a lag coil. The coil terminals receive current from the external sensing element.The current from the external sensing element becomes the main coil operating current. The lagcoil produces a magnetic flux that is out of phase with the main coil. The electromagnetic fieldflows in the laminated core of the relay, across the disk air gap, and through the keeper. Thecombination of the in-phase and out-of-phase magnetic flux across the disk air gap causes torqueto be produced on the rotating disk. The magnitude and direction of the input quantitiesdetermine the speed and direction of the rotating disk, which can be adjusted for a specific time-delay. As the disk rotates, trip contacts are opened or closed. The three-dimensional view of thedisk and the contacts, which is also shown in Figure 1, shows a simplified example of thestationary and movable trip contact operation.




Figure 1: Example Induction-Type Relay

The operational objective of an electro-mechanical relay is to actuate an alarm or to initiate acircuit breaker trip when a power system parameter exceeds a preset value. Each relay designachieves its operational objective through a variation of the basic electro-mechanical relay circuitthat is shown in Figure 2.

The basic electro-mechanical relay circuit receives an input signal from the secondary winding ofan instrument transformer. The instrument transformer primary winding receives an input signalfrom the power system. The electro-mechanical relay operating element uses the input signalfrom the secondary winding to produce a magnetic field. The electro-mechanical relay operatingelement compares the force of the produced magnetic field to a known value (e.g., gravity orspring). When the force of the produced magnetic field exceeds the force that is exerted bygravity or by a spring, the relay contacts close and complete the circuit for the output signal. Theoutput signal then flows from the power supply for circuit breaker tripping through the circuitbreaker trip coil and the alarm. This output signal trips the power system circuit breaker andactivates an alarm.




Figure 2: Basic Electro-Mechanical Relay Circuit

Electro-mechanical relays can be used as follows:

• Overcurrent Relays

• Overvoltage Relays

• Undervoltage Relays

• Undervoltage Frequency

• Directional Overcurrent Relays

• Directional Power Relays

• Differential Relays

• Distance Relays

• Phase Balance Current Relay

• Loss of Excitation Relays

• Lockout Relays

• Synchronism Relays




Solid State

Solid state relays are electronics-based units that perform the same function as the electro-mechanical units; however, solid state circuits replace the moveable elements of a relay. In solidstate relays, electronic logic circuits are used to determine whether the direction and magnitude ofthe input values are sufficient to require protective action. Although solid state and electro-mechanical relays use different methods to initiate protective action at pre-determined levels, theyaccomplish the same results.

Solid state devices are available for use as individual protective relays or as a protection package.A protection package is a group of individual protective relays that have been combined into asingle unit. All electro-mechanical devices are available in solid state versions. Although the solidstate devices are different in construction and operation, the solid state devices can perform thesame functions and can be applied in the same manner as the electro-mechanical devices.

Figure 3 shows a block diagram of a typical solid state current relay. The function of this relay isto provide instantaneous and time overcurrent protection for a load. The current flow to the loadis sensed through use of the main current transformer. The main current transformer transmitsthis current signal to the auxiliary transformer. The current flow through the auxiliary currenttransformer develops a voltage signal across the secondary resistor. The voltage across thesecondary resistor will be proportional to the original current flow through the main currenttransformer. The voltage signal provides the input signal to the inverse time circuit and theinstantaneous circuit of the solid state relay.

The inverse time circuit consists of a rectifier, a solid state switch, a solid state timer, a trigger,and a memory coil. The ac input voltage that is proportional to the line current is converted to dcthrough use of diodes in the rectifier of the inverse time circuit. The dc output from the rectifierprovides an input signal to the solid state switch. When the input signal exceeds a preset value,the solid state switch starts to conduct and provides an output signal to the solid state timer. Theinput signal to the solid state timer starts to charge a capacitor in the solid state timer. Thecapacitor charging time varies with the magnitude of the original input signal. When the capacitoris fully charged, the trigger circuit emits a pulse through the memory coil, which actuates the tripunit. The trip unit trips the circuit breaker to isolate power to the load.

The instantaneous circuit consists of a breakdown diode, a solid state switch, and a memory coil.The breakdown diode starts to conduct when the input voltage exceeds a preset value. The presetvalue is determined through the construction of the diode. When the diode conducts, thebreakdown diode converts the input ac voltage to a dc output voltage. The dc output voltageprovides an input to the solid state switch. When the breakdown diode conducts, the solid stateswitch emits a pulse through the memory coil, which actuates the trip unit. The trip unit trips thecircuit breaker to isolate power to the load.




Figure 3: Block Diagram of a Solid State Current Relay

Solid state relays can be used as follows:

• Overcurrent Relays • Differential Relays• Overvoltage Relays • Distance Relays• Undervoltage Relays • Phase Balance, Voltage, and Current Relays• Underfrequency Relays • Loss of Excitation Relays• Directional Overcurrent Relays • Lockout Relays• Directional Power Relays • Synchronism Relays

Some solid state relays contain microprocessors that enhance the versatility, the performance, andthe reliability of the solid state relay. Microprocessor relays have coded chips that have addedlogic capabilities. The addition of a microprocessor creates a solid state circuit that can makedecisions about output signals based on input signals. Microprocessors also provide the ability tocontinuously perform operational self-checks that ensure proper operation. If an electro-mechanical or simple solid state relay were to be faulty, the most likely time that this malfunctionwould be discovered is when the relay fails to operate properly when an actual system faultoccurs. Continuous self-checking uncovers relay malfunctions as they occur so that correctiveaction can be taken.




Characteristics

Protective relay systems are designed to trip the circuit breaker when a protective device sensesan overload or fault condition. Through variation of design and sensitivity, the protective devicecan be used to control the operation of a circuit breaker during overload or fault conditions.Because overloads or faults result in overcurrent conditions, overcurrent relays are the mostcommon electrical protective devices for use in electrical distribution systems. Overcurrent relaysare used as both primary and backup protective devices. The two main types of overcurrent relayprotection are the time-delay overcurrent and the instantaneous overcurrent. In general, overloadprotection is provided through use of time-delay overcurrent relays, and fault protection isprovided through use of magnetic instantaneous overcurrent relays.

Time-delay overcurrent devices have an inverse-time characteristic. An inverse-timecharacteristic means that as the fault current gets larger, the time it takes for the device to operategets smaller. The time delay device can be designed so that the length of this time delay can bevaried. Generally, time-delay overcurrent devices are selected and set to provide short timedelays or long time delays. Short time delays are used to provide selective tripping when a circuitbreaker is used in series with other circuit breakers, and long time delays are used for overloadprotection. The following are the three different classes of time-delay overcurrent devices:

• Inverse

• Very Inverse

• Extremely Inverse

Figure 4 shows the three different classes of time-delay overcurrent devices and the instantaneousovercurrent device. Figure 4 shows that as the fault current increases, the time it takes for thetime-delay overcurrent device to trip the circuit beaker decreases.

The inverse and very inverse time characteristics are usually applied to feeder circuits in whichproblems with coordination of in-line devices are not present. Feeder circuits can readily handlewide variations in levels of fault current. The extremely inverse characteristic is used when thereis a small variation in fault current from minimum to maximum because the extremely inversecharacteristic can provide faster clearing times for faults.




Figure 4: Protective Relay Time-Current Characteristics

The time-delay overcurrent device is used to coordinate with other protective devices or relays.The time before the trip signal is sent from the protective relay to the breaker varies inversely tothe magnitude of the fault current. Figure 5 shows the various time-delay coordinationcharacteristics for various multiples of the pickup current and for various time-delay responsetime settings.

In Figure 5, the fault is located downstream of power circuit breaker #1. If the fault were fartherdownstream of breaker #1, it would take longer for the breaker to trip because the impedance ofthe distribution lines would result in less current being sensed at the protective relay. A smalleramount of current that is sensed (with the inverse-time characteristic of the relay) means a longertime for the breaker to trip.




The time it takes for breaker #1 to trip is finite (for a finite fault current). If breaker #1 failed totrip, the backup protection is breaker #2, which would trip "S" time units after breaker #1 shouldhave tripped. To provide protection for the generator, each of these time delays creates acoordinated backup sequence of trips that would occur when any breaker fails to trip. In theexample, each breaker backs up the one directly to its right, and the "S" time units between tripsensures a definite breaker backup sequence.

Figure 5: Overcurrent Device Coordination

Instantaneous protection is provided through the use of an element that exhibits a very small timelag between the sensed overcurrent condition and the circuit breaker trip signal. For operationalcases, the time lag is so small that it is virtually instantaneous.




EVALUATING PROTECTIVE RELAY SYSTEMS UPON RECEIPT

When protective relays are received as a part of the total inventory of equipment for a protectiverelay system installation, a receipt inspection must be performed to ensure that the protectiverelays are of the proper type as specified in the installation drawings and that no damage hasresulted during the shipping process. The inspection process involves a physical inspection of theprotective relays and a verification of nameplate data against the installation specifications. Anydeviation from specifications and any observed damage to these items must be evaluated andaction must be taken to obtain the correct equipment or to replace any damaged equipment. Thereceipt inspection should be performed as soon as the equipment is received on the job site. Ifdiscrepancies are noted, immediate corrective action can be taken, which will prevent excessiveinstallation delays. This section provides information on the following topics that are pertinent toevaluating protective relay systems:

• Visual Inspection

• Verification Against Specification

Visual Inspection

When protective relay systems are received from the manufacturer, they generally arrive alreadyinstalled in the switchgear or in the control panels. If the protective relays are not included in theswitchgear or in the control panel, they will be shipped in cartons that are designed for protectionagainst damage. Upon initial receipt from the manufacturer, the protective relays must be visuallyinspected. If the protective relays are installed in the cabinet or switchgear when they are shipped,the protective relay visual inspection is performed during the cabinet or switchgear visualinspection. The visual inspection check lists are provided in Saudi Aramco Pre-CommissioningForm P-021, Protective Relays. The visual inspection portions of P-021 are provided in WorkAid 1.

Figure 6 shows a typical electro-mechanical time overcurrent relay. During the visual inspectionportion of protective relay system commissioning, each protective relay should be checked forphysical damage that could have occurred during manufacturing or shipping. The relay housingcover, the glass, and the gasket should be removed and inspected for damage. The relay housingitself should be inspected for the presence of cracks, foreign material, and moisture. The relayshould be inspected for tarnished contacts, loose screws, or other imperfections that could affectthe freedom of mechanism movement or proper mechanism alignment. In addition, all appropriatevendor operating and maintenance instruction manuals should also be verified to be present. Ifthe relay is installed in the switchgear or in the control panel, the connected wiring should bechecked for the proper creepage, clearance, bend radius, insulation, and tightness during thevisual inspection.




Figure 6: Typical Electro-Mechanical Overcurrent Relay

Verification Against Specifications

When a new facility or facility modification is at the equipment installation stage, the design of theinstallation has already been completed. The type of protective relays that are selected for aspecific protective relay system should be shown in the design package drawings, prints, orspecifications for the installation. The purpose of verifying protective relay systems against thespecifications during the commissioning process is to ensure that the equipment that is beinginstalled meets Saudi Aramco and industry standards. Minimum design package and generalrequirements for protective relay systems are provided in Work Aid 1.




Generally, the verification against specifications consists of a determination of whether the typeand rating of the equipment that is to be installed matches the size and type of the equipment thatis required for the installation. This determination is accomplished by reading up-to-date meteringand relaying one-line diagrams and relay and control ac and dc schematics. The Engineer inspectsthe manufacturer's nameplate data on each protective relay and compares them to therequirements on the electrical prints and schematics to determine whether the correct relay isbeing used. Saudi Aramco Pre-Commissioning Form P-021, Protective Relays, contains a checklist that itemizes what should be included in the verification of protective relays against SaudiAramco and industry specifications.

During the verification against specification, the Engineer should ensure that the protective relaysin the protective relay system meet the minimum requirements for Saudi Aramco installations inaccordance with SAES-P-114, 16-SAMSS-513, and industry standards. Saudi AramcoEngineering Standard SAES-P-114 provides drawings that show the specific applications forprotective relays in Saudi Aramco electrical systems. Each drawing shows the minimummandatory protection that must be provided for a particular portion of the power system or for aparticular piece of electrical equipment. The relays that are shown in these drawings are identifiedthrough use of standard device function designations. These device function designations havebeen developed over years of usage as an abbreviated method of designating the function of arelay. This abbreviated method allows the use of a few numbers to completely describe thefunction of a relay rather than writing a note to describe the function of each device on a drawing.

The device function numbers are used on drawings and elementary and connection diagrams, inrelay instruction books, in publications, in standards, and in specifications. In addition, thesenumbers are usually placed on nameplates that are adjacent to the device on switchgear orrelaying panels for identification of the designated function of that relay in the system. Thesedesignated functions may refer to the actual function the device performs when it is installed, orthey may refer to the electrical or other quantity to which the device is responsive. Because achoice of function numbers for a given device is sometimes available, the preferable choice is thefunction number that is recognized to have the narrowest interpretation.

When alternate names and descriptions are included under the function, only the name anddescription that applies to each specific case should be used. In general, only one name for eachdevice, such as relay, contactor, circuit breaker, switch monitor, or other device, is included ineach function designation. If the function of the relay is not inherently restricted to any specifictype of device, and if the type of device itself is incidental, any one of the alternative names maybe substituted as applicable.

Device function numbers and suffixes are found in the most recent revision of ANSI StandardC37.2, "IEEE Standard Electrical Power System Device Function Numbers." A portion of thestandard electrical device function designations is provided in Work Aid 1 to assist in theverification against specification during the commissioning process.




A detailed discussion of each system and equipment protection scheme is beyond the scope of thismodule; however, a typical Saudi Aramco bus overcurrent protection scheme is shown in Figure7. The protection devices that are shown in Figure 7 are identified through use of the followingstandard device function numbers:

• 51 is a time-delay overcurrent relay.

• 52 is a circuit breaker.

• 50/51 is an overcurrent relay with an instantaneous and a time-delay element.

• 50/51G is a residually connected ground-fault overcurrent relay with aninstantaneous and a time-delay element.

• 86B is a bus lockout relay.

The 51 device (time-delay overcurrent relay) provides overcurrent protection for bus 1 asmeasured at CT-1. When an overcurrent condition exists at CT-1 for a sufficient length of time,time-delay relay 51 operates and sends a signal to the bus lockout relay (device 86B). Device 86Bcauses the source breaker, 52-1, to trip (open).

The 50/51 device (instantaneous and time-delay overcurrent relay) provides overcurrentprotection against faults that occur downstream of load breaker 52-2 as measured at CT-2. Whenan overcurrent condition exists at CT-2 (either large enough to activate the instantaneous elementor of a sufficient length of time to activate the time-delay element) the 50/51 device will trip(open) load breaker 52-2.

The 50/51G device (instantaneous and time-delay ground-fault overcurrent relay) providesovercurrent protection against ground faults as measured at the wye of the three-phase, wye-connected current transformer (CT-2). Under ground fault conditions, unbalanced residualcurrent conditions cause either the instantaneous element or the time-delay element of the 50/51Gdevice to trip (open) load breaker 52-2.




Figure 7: Typical Saudi Aramco Bus Overcurrent Protection Scheme




EVALUATING PROTECTIVE RELAY SYSTEM INSTALLATION AND TESTING

After protective relay systems have been installed and prior to the equipment being placed intoservice, specific tests are required by Saudi Aramco. The tests that are performed are designed toprove that the protective relay system will perform its job under the conditions of use and withinthe accuracy that is required by Saudi Aramco and industry standards. Additionally, theinstallation of protective relays must be inspected to determine whether the wiring and associatedcomponents are correctly installed or assembled. If the results of testing during thecommissioning process indicate that the protective relay system does not meet Saudi Aramcorequirements, steps must be taken to rectify any deficiency.

This section provides information on the following topics that are pertinent to evaluatingprotective relay system installation and testing:

• Mechanical Checks

• Electrical Tests

Mechanical Checks

Before the mechanical checks of electro-mechanical relays are described, a brief overview of theconstruction of a typical electro-mechanical relay is necessary. Figure 8 shows a typical electro-mechanical overcurrent relay that has been removed from its case. The overcurrent relay has bothtime and instantaneous overcurrent units. In the overcurrent relay that is shown in Figure 8, themain moving shaft rotates an induction disk against the force that is applied to the disk from adrag magnet. The main disk shaft rotates between a top pivot and a bottom pivot (not shown).The degree and speed of disk rotational movement is a result of the sensed system current. Thecombination of the tap block and time dial settings will determine the operation of the timeovercurrent unit. The combination of tap block and adjustable pole piece settings will determinethe operation of the instantaneous unit. The calibration plate is used to set the adjustable polepiece. The main stationary brush and contact assembly will make contact if the system currentexceeds preset current levels, and the seal-in stationary contact assembly will operate the targetand seal-in unit.




Figure 8: Electro-Mechanical Overcurrent RelayThat Has Been Removed From Its Case

As shown in Figure 8, electro-mechanical protective relays have moving parts (e.g., disks,contacts, etc.) that are critically aligned and must be checked for proper mechanism operationduring the commissioning process. Proper mechanism operation includes freedom of movement,alignment, contact travel, contact wipe, and spring tension. During the mechanical check, therelay should be inspected for friction. Friction can be caused by a warped or bent disk, or byimproper clearance between the disk and the magnet poles. The disk rotation should be checkedby hand. The disk should move freely and should reset smoothly. If auxiliary target devices,plunger relays, or annunciator coils are present, they should also be checked for friction andfreedom of moving parts.




A detailed mechanical inspection procedure is provided in Work Aid 2. Mechanical inspectionchecks are provided on sheets 1 and 3 of the Saudi Aramco Pre-Commissioning Form P-021,Protective Relays. A representative portion of Saudi Aramco Pre-Commissioning Form P-021,Protective Relays, is provided in Work Aid 2. Solid state protective relays have no moving parts,so the commissioning process mechanical checks for solid state relays are limited to the properinstallation of the protective relay unit and the proper connection of the power supply and inputand output wiring.

Electrical Tests

After the protective relay system has been installed and inspected, it must be tested prior to beingcommissioned. During the electrical tests of protective relay systems, all electrical safetyprecautions must be taken to prevent personnel injury or equipment damage. Because the settingsof protective relays must be coordinated with other relays in the electric power system, the relaysmust be set by Saudi Aramco. It is assumed that all protective relay settings have been madeprior to the commencement of the commissioning process.

Directions for testing electro-mechanical and solid state relays are given in manufacturer'smanuals or instruction books for each device. In general, there are three specific types ofelectrical tests and some miscellaneous electrical tests that are performed on protective relays.The electrical tests that are performed on protective relays are as follows:

• Insulation Test

• Pickup Test

• Timing Test

• Miscellaneous Tests

Insulation Test

Insulation resistance tests are performed on each protective relay circuit. Insulation resistancetests are not performed on solid state protective relays because of the potential of damage to theelectronic components. The insulation resistance test that is recommended during thecommissioning is a megohmmeter test, which consists of application of a megohmmeter from eachprotective relay circuit branch to ground. The resistance value is measured in megohms.




Pickup Test

For both electro-mechanical and solid state protective relays, a pickup test should be performedon each operating element. During the pickup test, specific test equipment is used to determinethe pickup current that will cause the relay to operate. The manufacturer's instruction literatureshould be consulted for the test equipment that must be used, as well as time/current curves andtolerances for the specific protective relay that is being tested. In older electro-mechanicalsystems, test plugs are available to assist in protective relay system testing. Test plugs areprovided by the switchgear or protective equipment manufacturer, and they install directly into theswitchgear or panelboards.

When the pickup value of current on electro-mechanical (e.g., plunger and solenoid type) relays isreached, the ammeter reading at the instant the relay operates should be noted. Solid state relays(and solid state test equipment) generally contain electronic circuitry that will retain the systemparameters in chip memory at the time the relay operates. The system parameters can then berecalled by the operator for entry on the proper data forms. The pickup current value should bewithin 5% of the calibrated or tap setting value. An example would be an electro-mechanicaltime-overcurrent induction relay with an instantaneous unit. The induction unit of the exampleprotective relay has an operating coil with 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, and 6.0 ampere taps. Withthe operating coil set at the minimum tap, the coil should pick up at a test current that is between1.425 and 1.575 amperes. The manufacturer's technical manuals should be consulted for the exactvalue of the pickup current because deviations to the 5% tolerance do exist.

The pickup test should be conducted with the relay in its normal operating position. For electro-mechanical relays, once the relay has picked up, the relay should be checked for quiet operation.After the pickup current value is established, the current that is applied to the relay should beslowly reduced until the relay drops out (or resets). The current value at the point that the relaydrops out should be between 90% and 95% of the pickup current value. Pickup tests should alsobe performed on any target or seal-in units that are present in the protective relay. When thepickup tests are performed, a check should also be made to ensure that the proper relay positionindicators function.




Timing Test

With protective relays, not only must the relay pick up at a predetermined value of sensed current,but the relay operation must be coordinated with other relays or devices in the protection system.The coordination of a relay is accomplished by setting the pickup value at a specific point. If atime delay is incorporated into the operation of the relay, the coordination of the relay isaccomplished through a combination of the pickup value and the time delay of the relay. Inelectro-mechanical relays, the time dial determines the length of time that the unit requires toclose its contacts when the unit reaches a predetermined value. Most electro-mechanical relaytime dials have settings from zero (no time delay) to ten (the maximum time delay). The timedelay of a protective relay on some electro-mechanical relays can also be set by moving theposition of the drag (or permanent) magnet.

The timing test is conducted to determine whether the time delay of the relay is correct for theinstallation. During the pickup test of a protective relay, the relay operating coil is set to theminimum tap, and the induction unit time dial is set at some mid-point value. Because theinduction unit time dial generally has ten settings, a mid-point value of five is generally selected.Three different multiples of the tap pickup current are applied, and the relay operation time ismeasured. The relay operation time for various multiples of pickup current is provided in graphform in the manufacturer's technical manual. There is also a tolerance for pickup times (usually5%), but the manufacturer's technical manuals should be consulted for the exact value of thepickup time because deviations to the 5% tolerance do exist.

Miscellaneous Electrical Tests

Several other miscellaneous electrical tests are performed on protective relays, and such testsdepend upon the type relay that is installed in the system. A directional protective relay isdesigned to operate in only one direction of current (or power flow). In addition to the pickupand timing tests, a directional relay should be tested in the reverse direction to ensure that it doesnot operate. A differential relay is designed to operate with current (or power flow) in bothdirections. In addition to the pickup and timing tests, a differential relay should be tested in thereverse direction to ensure that it operates correctly. Voltage relays (e.g., undervoltage orovervoltage) should be tested at the dropout and pickup voltage levels in a manner that is similarto the pickup and timing tests that have been previously described.




SYSTEM PRE-OPERATIONAL CHECK-OUT

The system pre-operational checkout phase of the commissioning cycle for protective relaysystems provides an opportunity for Saudi Aramco personnel to perform wiring and subsystemcomponent checks. During the pre-operational check-out, each protective relay systemcomponent is checked to ensure that it works individually and as a complete system.

PDD Point-to-Point Wiring Check

Point-to-point wiring checks are performed to verify that the protective relay system complieswith the wiring diagrams and the manufacturers' specifications. Terminations and terminal blocksare also checked for routing and labeling. During the point-to-point wiring checks, the protectiverelays and their associated control and instrumentation transformers and fuses are checked forproper application and type.

Subsystem Checks

Subsystems are checked to ensure that electrical continuity exists for control and protectivedevices. The proper operation of all subsystems is tested through use of controlled operation andcheckout of the controls and protective devices. Each subsystem is performance tested throughthe application of full operational voltage. The full operational voltage is applied to eachsubsystem through the proper protective devices. With each subcircuit connected to its mainsystem, a complete operational test is performed on equipment controls, interlocks, protectivedevices, and components. During the performance test, the main systems are still isolated andindependent from plant systems. Before the equipment is connected to the plant system,subsystem performance testing is critical to ensure the proper and safe operation of the equipmentprotection and control subsystems.

Operational tests are performed after the protective relay system has been received and installed.These tests determine the operational acceptance of each protective relay in the system and mustbe conducted prior to commissioning and release of the protective relay system for operation.




WORK AID 1: REFERENCES FOR EVALUATING PROTECTIVE RELAYSYSTEMS UPON RECEIPT

Work Aid 1A: Protective Relay System Ratings and Requirements

Figure 12 shows the standard current and voltage ratings for protective relays in accordance withANSI/IEEE C37.90-1989.

Figure 12: Standard Current and Voltage Ratingsfor Relays (From ANSI/IEEE C37.90)




Figure 13 shows Saudi Aramco protective relay system general requirements from SAES-P-114.

GeneralRequirements

Non-directional overcurrent relays (Devices 51 or 50/51) must befurnished with both time-overcurrent and instantaneous units. A separatetarget indicator must be supplied for each.The standard voltage and current ratings for protective relays must be 120VAC, 125 VDC, and 5 A RMS.Overcurrent devices on three-phase circuits must be installed in all threephases, unless specified otherwise. The ground fault protective devicemust be a separate device from the phase fault device for all feeder andbranch circuit voltages rated 600 V or above.The approved electro-mechanical relays are listed in the Tables and RelaySelection paragraphs in SAES-P-114, and must be mounted in semi-flushdrawout type cases. Electro-mechanical relays that are not listed must beapproved by the Manager, Power Distribution Department.The approved solid-state relays are listed in the Tables and RelaySelection paragraphs of this standard. Solid-state relays that are not listedmust be approved by the Manager, Power Distribution Department.Solid-state or static-type relays must have an internal electro-mechanicaltrip output relay.

ApplicationRequirements

Time-overcurrent phase-fault relays on industrial or commercial powersystems must have an inverse or very-inverse characteristic.Time-overcurrent phase-fault relays on residential distribution powersystems must have a very-inverse or extremely-inverse characteristic.

Figure 13: Saudi Aramco Protective Relay SystemGeneral Requirements (From SAES-P-114)




ApplicationRequirements(Cont'd)

Time-overcurrent ground-fault relays on resistance grounded systemsmust have an inverse characteristic.

The time-overcurrent relays along a branch or feeder distribution systemmust have similar or compatible time-overcurrent characteristics that canbe coordinated over the range of normal load and fault currents.

Coordination& Settings ofMainProtection

The coordination time interval (CTI) between coordination pairs oftime-overcurrent relays must be within the range of 0.35 to 0.50 secondsat normal maximum transient fault current.

The maximum fault duration time allowed by the protection must notexceed the short-circuit withstand capability of the protected equipment.

LockoutRelays andTrip-IsolationTest Switches

Auxiliary lockout relays (Device 86) must be hand-reset type and haveseparate contacts connected in the closing and tripping circuits of thecircuit breakers.

The electrical continuity of the Device 86 operating coil circuit must bemonitored by a white indicating light located adjacent to the reset handle.A minimum of two normally-open and two normally-closed spare contactsmust be provided on each lockout relay.The lockout relays must be equipped with a mechanical target indicatingthe tripped and reset positions.Trip-isolation test switches (not ganged) must be installed in the lockouttrip circuits to the circuit breakers and breaker failure timers. The testswitches must be mounted on the front of relay panels in a clearly visiblelocation, and must be Westinghouse Type FT-1, or States Type MTS, oran approved equivalent.

Figure 13: Saudi Aramco Protective Relay SystemGeneral Requirements (From SAES-P-114)(Cont'd)




The design package for control and protection systems must contain, at a minimum, the followinginformation:

• Relay and Metering One-Line Diagrams (Saudi Aramco).

• AC and DC Elementary (Schematic) Diagrams (Saudi Aramco and Vendor).

• AC Three-Line (Connection) Diagrams (Saudi Aramco and Vendor).

• Interconnection Diagrams.

• Panel Wiring Diagrams.

• Synchronizing Diagrams.

• Relay and control panel layout drawings (Saudi Aramco and Vendor).

• Protective device data, such as manufacturer, style, model, type, range, and timecharacteristic curves.

• Nameplate data and ratings of motors, buses, generators, power conductors,instrument transformers, power transformers, and cables (including cableshort-circuit withstand limits).




Work Aid 1B: Standard Electrical Symbols and Device Numbers

Figure 14 shows standard electrical symbols for protective relaying systems.

Figure 14: Relaying Electrical Symbols




Figure 15 shows NEMA device function numbers and a description of the device.

Number Name Function

21 Distance Relay The distance relay functions when the circuitadmittance, impedance, or reactance increases ordecreases beyond a predetermined value.

24 Over Excitation Relay The over excitation relay operates when the ratio ofvoltage to frequency exceeds a preset value.

27 Undervoltage Relay The undervoltage relay operates when its input voltageis less than a predetermined value.

30 Annunciator Relay The annunciator relay is a non-automatically resetdevice that gives a number of separate visual indicationswhen a protective device functions. The annunciatorrelay can also perform a lockout function.

32 Directional PowerRelay

The directional power relay is a relay that operates at apredetermined value of power flow in a given direction,or on reverse power.

37 Undercurrent orUnderpower Relay

The undercurrent or underpower relay is a relay thatfunctions when the current or power flow decreasesbelow a predetermined value.

40 Field Failure Relay The field failure relay (also called "Loss of ExcitationRelay or Reverse VAR Relay ") is a relay that functionson a failure of machine field current, or on an excessivevalue of the reactive component of armature current inan AC machine (low field excitation).

44 Unit Sequence StartingRelay

The unit sequence starting relay is a relay that functionsto start the next available unit in multiple-unitequipment upon the failure or on the nonavailability ofthe preceding unit.

46 Reverse Phase Relay The reverse phase relay (also called "NegativePhase-Sequence Current Relay") is a relay thatfunctions when the polyphase currents are inreverse-phase sequence, or when the polyphase currentsare unbalanced or contain negative phase-sequencecomponents above a given amount.

Figure 15: NEMA Device Function Numbers





47 Phase-SequenceVoltage Relay

The phase-sequence voltage relay (also called "NegativePhase-Sequence Voltage Relay") is a relay thatfunctions at a predetermined value of polyphase voltagein the desired phase sequence.

48 Incomplete SequenceRelay

The incomplete sequence relay is a relay that returns theequipment to the normal (or off) position and locks itout if the normal starting, operating, or stoppingsequence is not completed within a predetermined time.

49 Machine orTransformer ThermalRelay

The machine or transformer thermal relay is a relay thatfunctions when the temperature of an AC machinearmature, or other load-carrying winding or element ofa machine, or the temperature of a power rectifier orpower transformer exceeds a predetermined value.

50 InstantaneousOvercurrent Relay

The instantaneous overcurrent relay is a relay thatfunctions instantaneously (without intentional timedelay) on an excessive value of current, or on anexcessive rate of current rise.

51 Ac Time OvercurrentRelay

The ac time overcurrent relay is a relay that operateswhen its ac input current exceeds a predeterminedvalue, and that either has a definite time delay, or has atime delay that is inversely proportional to the inputcurrent through a substantial portion of the performancerange.

52 Ac Circuit Breaker The Ac Circuit Breaker is a device that is used tocomplete and interrupt an ac power circuit undernormal conditions or to interrupt this circuit under faultor emergency conditions.

53 Exciter or DcGenerator Relay

The exciter or dc generator relay is a relay that forcesthe DC machine field excitation to build up duringstarting (field flashing) or that functions when themachine voltage has built up to a given value.

55 Power Factor Relay The power factor relay is a relay that operates when thepower factor in an ac circuit rises above or falls below apredetermined value.

Figure 15: NEMA Device Function Numbers (Cont'd)





56 Field ApplicationRelay

The field application relay is a relay that automaticallycontrols the application of the field excitation to asynchronous ac motor at some predetermined point inthe slip cycle.

59 Overvoltage Relay The overvoltage relay is a relay that operates when itsinput voltage is more than a predetermined value.

60 Voltage or CurrentBalance Relay

The voltage or current balance relay is a relay thatoperates on a given difference in voltage or current(input or output) between two circuits.

62 Time Delay Stoppingor Opening Relay

The time delay stopping or opening relay is a time-delayrelay that serves in conjunction with the device thatinitiates the shutdown, stopping, or opening operationin an automatic sequence or protective relay system.

64 Ground DetectorRelay

The ground detector relay is a relay that operates onfailure of a machine, or other apparatus insulation toground.

NOTE: This function is not applied to a device that isconnected in the secondary of current transformers in anormally grounded power system, where other devicenumbers with a suffix G or N should be used.

67 Ac DirectionalOvercurrent Relay

The ac directional overcurrent relay is a relay thatfunctions on a desired value of ac overcurrent flowingin a predetermined direction.

68 Blocking Relay The blocking relay is a relay that initiates a pilot signalto block tripping on external faults in a transmission lineor in other apparatus under predetermined conditions,or cooperates with other devices to block tripping or toblock reclosing on an out-of-step condition or on powerswings.

72 Dc Circuit Breaker The dc circuit breaker is a circuit breaker that is used toclose and interrupt a dc power circuit under normalconditions or to interrupt this circuit under fault oremergency conditions.






74 Alarm Relay The alarm relay is a relay (other than an annunciator ascovered under device function 30) that is used tooperate, or to operate in connection with, a visual oraudible alarm.

76 Dc Overcurrent Relay The dc overcurrent relay is a relay that functions whenthe current in a dc circuit exceeds a given value.

78 Phase-AngleMeasuring Relay

The phase-angle measuring relay is a relay thatfunctions at a predetermined phase angle between twovoltages or between two currents or between voltageand current.

79 Ac Reclosing Relay The ac reclosing relay is a relay that controls theautomatic reclosing and locking out of an ac circuitinterrupter.

81 Frequency Relay The frequency relay (also called "Overfrequency orUnderfrequency Relay") is a relay that responds to thefrequency of an electrical quantity, operating when thefrequency or rate of change of frequency exceeds (or isless than) a predetermined value.

82 Dc Reclosing Relay The dc reclosing relay is a relay that controls theautomatic closing and reclosing of a dc circuitinterrupter, generally in response to load circuitconditions.

83 Automatic SelectiveControl Relay

The automatic selective control or transfer relay is arelay that operates to select automatically betweencertain sources or conditions in an equipment, or toperform a transfer operation automatically.

85 Carrier or Pilot-WireReceiver Relay

The carrier or pilot-wire receiver relay is a relay that isoperated or restrained by a signal that is used inconnection with carrier-current or dc pilot-wire faultrelaying.

86 Lockout Relay The lockout relay is a hand or electrically resetauxiliary relay that is operated by tripping of anotherprotective relay upon the occurrence of abnormalconditions to maintain associated equipment or devicesinoperative until that are reset.






87 Differential ProtectiveRelay

The differential protective relay is a protective relay thatfunctions on a percentage, phase angle, or otherquantitative difference between two electrical quantities.

91 Voltage DirectionalRelay

The voltage directional relay is a relay that operateswhen the voltage across an open circuit breaker orcontactor exceeds a given value in a given direction.

92 Voltage and PowerDirectional Relay

The voltage and power directional relay is a relay thatpermits or causes the connection of two circuits whenthe voltage difference between them exceeds a givenvalue in a predetermined direction. The voltage andpower directional relay causes these two circuits to bedisconnected from each other when the power flowingbetween them exceeds a given value in the oppositedirection.

94 Tripping or Trip-FreeRelay

The tripping or trip-free relay is a relay that functions totrip a circuit breaker, contactor, or equipment, or topermit immediate tripping by other devices; or toprevent immediate reclosure of a circuit interrupter if itshould open automatically even though its closingcircuit is maintained closed.





Work Aid 1C: Device Suffix Letters

Suffix letters are used with device function numbers to further indicate the exact function of aparticular piece of equipment. A complete listing and a detailed explanation of suffixes arecontained in ANSI Standard Y1.1-1972, "Abbreviations for Use on Drawings and in Text." Thissection of the Work Aid contains lists of the suffixes that are most often used in Saudi Aramcodrawings and prints.

The following letters indicate the condition or electrical quantity to which the device responds orthe medium in which it is located.

• A Air or Amperes

• C Current

• E Electrolyte

• F Frequency or Flow

• L Level or Liquid

• P Power or Pressure

• PF Power Factor

• Q Oil

• S Speed

• T Temperature

• V Voltage, Volts, or Vacuum

• VAR Reactive Power

• W Water or Watts




The following letters denote the main device in the circuit or the type of circuit (BL for blockingcircuit) to which the numbered device is applied or related.

• A Alarm or Auxiliary Power

• AN Anode

• B Battery, Blower, or Bus

• BK Brake

• BP Bypass

• BT Bus Tie

• C Capacitor, Condenser, Compensator or Carrier Current

• CA Cathode

• DC Direct Current

• E Exciter

• F Feeder, Field or Filament

• G Generator or Ground **

• H Heater or Housing

• L Line

• M Motor or Metering

• MOC Mechanism Operated Contact (circuit breaker mechanism operated cellswitch)

• N Network or Neutral **

• P Pump

• R Reactor or Rectifier

• S Synchronizing

• T Transformer, Test or Thyratron

• THE Transformer (High-Voltage Side)

• TL Transformer (Low-Voltage Side)

• TM Telemeter

• U Unit




** Suffix "N" is generally used in preference to "G" for devices connected in theresidual connection of current transformers.

** Suffix "G" is more commonly used for those relays which are connected to acurrent transformer whose primary winding is located in the neutral of a machineor power transformer, and in the case of transmission line relaying, which operateson ground-faults.

The following is a list of miscellaneous suffix letters that are used in Saudi Aramco drawings andprints.

• A Accelerating or Automatic

• B Blocking or Backup

• C Close or Cold

• D Decelerating, Detonate, or Down

• E Emergency

• F Failure or Forward

• H Hot or High

• HR Hand Reset

• HS High Speed

• L Left or Local or Low or Lower or Leading

• M Manual

• OFF Off

• ON On

• O Open

• P Polarizing

• R Right or Raise or Reclosing or Receiving or Remote or Reverse

• S Sending or Swing

• T Test or Trip or Trailing

• TDC Time-Delay Closing

• TDO Time-Delay Opening

• U Up




Work Aid 1D: Saudi Aramco Pre-Commissioning Form P-021 Excerpts

Figures 16 and 17 show sheets 1 and 3 (of 12) from the Saudi Aramco Pre-Commissioning form,P-021, Protective Relays. Sheet 1 is used to evaluate protective relay systems upon receipt, andsheet 3 contains a checklist for specific relay or instrument visual and mechanical inspections. Forprotective relay systems that contain more than five instruments or relays, multiple copies of sheet3 may be used.




Figure 16: Sheet 1 (of 12) of the Saudi AramcoPre-Commissioning Form, P-021, Protective Relays




WORK AID 2: REFERENCES FOR EVALUATING PROTECTIVE RELAY SYSTEMINSTALLATION AND TESTING

Work Aid 2A: Procedures and Methods for Evaluating Instrument Transformer andMeter Installation and Testing

The procedures and methods for evaluating protective relaying system installation and testing aredivided into the following categories:

• Mechanical Checks

• Electrical Tests

Mechanical Checks

The mechanical checks of protective relays should be performed in accordance with themanufacturer's technical manuals that are provided with the protective relays. The specificmechanical checks are provided in the Saudi Aramco Pre-Commissioning Form, P-021, ProtectiveRelays. All mechanical check data must be entered in the appropriate section of P-021. Thegeneral procedure for performing a mechanical check of an electro-mechanical protective relayduring the commissioning process is as follows:

1. Any rust or filings must be removed from the disk and magnetic poles with a magnetcleaner or brush.

2. Check the relay for friction. The relay disk must not be warped or bent. The relay disk

must have proper clearance between the disk and the magnet poles. 3. The disk rotation must be checked by hand. The disk should move freely and reset

smoothly. 4. Relay contacts must be clean and tight. 5. If auxiliary target devices, plunger relays, or annunciator coils are present, they must be

checked for friction and freedom of moving parts.

Solid state protective relays have no moving parts, so the mechanical checks that are performedon solid state relays during the commissioning process are limited to checking for the correctinstallation and any indication of damage.




Electrical Tests

The electrical tests of protective relays should be performed in accordance with themanufacturer's technical manuals that are provided with the protective relays. The specificelectrical tests are provided in the associated section of the Saudi Aramco Pre-CommissioningForm, P-021, Protective Relays.

Work Aid 2B: Saudi Aramco Pre-Commissioning Form P-021 Excerpts

Figures 18 through 21 contain a representative portion of Saudi Aramco Pre-CommissioningForm P-021, Protective Relays from GI-2.710.




GLOSSARY

backup protection a form of protection that consists of protective devices that operateindependently of specified components in the primary protectivesystem and that is intended to operate if the primary protectivedevice or apparatus fails or is out of service.

circuit element a basic part of the circuit in question, e.g., conductors, interruptingdevices, transformers, etc.

current transformer An instrument transformer that is intended to have its primarywinding connected in series with the conductor that carries thecurrent to be measured or controlled.

fault a partial or total local failure in the insulation or continuity of aconductor.

instrument transformer A transformer that is intended to reproduce in this secondarycircuit, in a definite and known proportion, the current or voltageof its primary circuit with the phase relations substantiallypreserved.

line side the supply side of a circuit element from which the circuit elementreceives its source of voltage. This is also known as the "upstreamside" of a circuit element.

load side the side of a circuit element from which the voltage is supplied tothe load. This is also known as the "downstream side" of a circuitelement.

overlapping zones of the distance that one relay's reach extends into the zone ofprotection protection of another relay, generally to provide comprehensive

protection of components, such as circuit breakers.

protective zone the zone that lies between two or several sets of currenttransformers that, together with the relays, constitute the protectivesystem for that portion of the power system.

reach the maximum distance from the relay sensing device (CT or VT)location to a point of overcurrent for which a particular relay willoperate.

Commissioning Systems, Protective Relay

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