CALIFORNIA STATE UNIVERSITY, NORTHRIDGE

MOTOR PROTECTION USING

SEL-710 MICROPROCESSOR BASED RELAY

A graduate project submitted in partial fulfillment of the requirements

For the degree of Master of Science

in Electrical Engineering

By

Tanmay Kalra

December 2010

The graduate project of Tanmay Kalra is approved by:

Ali Amini, Ph.D. Date

Xiaojun Geng, Ph.D. Date

Bruno Osorno, Prof, Chair Date

California State University, Northridge

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Dedication

I would like to take the opportunity to thank the people who have been incredible sources of

support and encouragement throughout this journey.

Thanks are due to the almighty, without whom nothing is possible.

My parents and brother have been there for me through the trials of living away from home, and

inspire me every day to be the best I can be. Thanks to Shikhir and Shruti, who were here to

guide me and support me through everything.

I owe a debt of gratitude to my advisor and mentor, Professor Bruno Osorno. He goes above and

beyond what is required of him and is a “guru” in the true sense of the term in Hindi. I am lucky

to have him to guide me through this process. I also thank Dr. Ali Amini and Dr. Xiaojun Geng

for being on my thesis committee and offering their expertise and invaluable suggestions for

improvement.

Finally, thanks to my supportive cohort at CSUN, my lab mates, my team-mate Ronak and others

I met on this wonderful journey.

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Table of Contents

Signature Page…………………………………………………………………………………….i

i

Dedication......................................................................................................................................iii

Table of Contents............................................................................................................................iv

List of Figures.................................................................................................................................vi

ABSTRACT.................................................................................................................................viii

Chapter 1: Introduction to different types of Motors.......................................................................1

1.1 Motor types:...........................................................................................................................1

1.2 AC Motors:............................................................................................................................2

1.2.1 Advantages:.....................................................................................................................2

1.2.2 Disadvantages:.................................................................................................................4

1.3 DC Motors..............................................................................................................................4

1.3.1 Advantages......................................................................................................................7

1.3.2 Disadvantages..................................................................................................................8

Chapter 2: Motor Protection............................................................................................................9

2.1 Introduction:...........................................................................................................................9

2.1.1 Short Circuit Protection of Stator Windings:..................................................................9

2.1.2 Stator-Overheating Protection:......................................................................................10

2.1.3 Rotor Overheating Protection:.......................................................................................13

2.1.4 Loss of Synchronism Protection:..................................................................................13

2.1.5 under voltage Protection:...............................................................................................14

2.1.6 Loss of excitation protection:........................................................................................14

2.1.7 Field ground fault protection:........................................................................................14

Chapter 3: Introduction to Relays..................................................................................................15

3.1 Relay and its classifications:................................................................................................15

3.2 Electromechanical Relay:....................................................................................................15

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3.3 Static Relays.........................................................................................................................16

3.4 Microprocessor based relay:................................................................................................17

Chapter 4: Motor Protection Relay (SEL-710)..............................................................................20

4.1 Introduction:.........................................................................................................................20

4.2 Application:..........................................................................................................................20

4.3 Principle of Operation:.........................................................................................................21

4.4 Testing of relay SEL-710:....................................................................................................22

4.4.1 Procedure to test Motor Protection relay (SEL-710) using direct method:...................24

4.5 Device Metering and test results..........................................................................................36

Chapter 5: Conclusion...................................................................................................................45

References......................................................................................................................................46

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List of Figures

Fig 1: Family tree of electrical motors

Fig 2: Anatomy of AC motor

Fig 3: Anatomy of DC motor

Fig 4: Diagram of stator windings of a 3 phase AC motor

Fig 5: Illustrating the need for overcurrent protection in each phase

Fig 6: Thermal Imaging thermography showing stator overheating of a motor

Fig 7: Electromechanical relay

Fig 8: Static relay using solid state devices

Fig 9: Working inside a microprocessor based relay

Fig 10: Standard ANSI definitions for microprocessor-based relay

Fig 11: Applications possible using SEL-710 relay

Fig 11a: Motor starting adaptation in an SEL-710 relay

Fig 12: Flowchart for the testing of relay SEL-710

Fig 12a: Back panel for SEL-710 relay

Fig 13: AC connections-across the line starting

Fig 14: Front panel for SEL-710 relay

Fig 15: AcSELerator Quickset window

Fig 16: Terminal Window

Fig 17: Setting Window

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Fig 18: Over Current setting window

Fig 19: Under current settings

Fig 20: Current Imbalance element

Fig 21: Under voltage element

Fig 22: Over voltage element

Fig 23: Metering of the motor using AcSELerator

Fig 24: Phase component observed in the AcSELerator

Fig 25: Instantaneous values observed in the AcSELerator

Fig 26: Min/Max metering values in AcSELerator

Fig 27: Under voltage trip observed on AcSELerator

Fig 28: Over voltage trip as seen on AcSELerator

Fig 29: Shows a figure of over frequency fault occurrence

Fig 30: Observing the under frequency fault occurrence

Fig 31: Phase imbalance fault

Fig 32: Real time testing

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ABSTRACT

MOTOR PROTECTION USING

SEL-710 MICROPROCESOR BASED RELAY

By

Tanmay Kalra

Master of Science in Electrical Engineering

This paper provides details about testing a microprocessor based relays. The Relay used for testing is a microprocessor based multifunction motor protection Relay (SEL-710).This paper explains tests that are carried on the relay when different fault occurs on a motor.

 

Chapter 1 explains various protection devices used in power system

Chapter 2 deals with basic functionality of microprocessor based relay and

Chapter 3 deals with the circuit required for testing the relay.

 

Response of the relay is indicated by respective LED’s. The ON / OFF status of the LED indicate the fault in the line. If a fault exits in the line, the relay sends a signal to the circuit breaker which opens the line and clears the fault. The line is then restored upon clearance of fault.

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Chapter 1: Introduction to different types of Motors

1.1 Motor types:Industrial motors come in a variety of basic types. These variations are suitable for many different applications. The usage of a specific type of motor depends on the application and the performance that can be obtained from it. This chapter will give guidance about different types of motors and there advantages and disadvantages. The two basic types of motors are as follows.

AC MotorsDC Motors

Figure 1: Family tree of Electrical Motor [1]

1.2 AC Motors:The most common and simple industrial motor is the three phase AC induction motor, sometimes known as the “squirrel cage” motor.

Figure2: Anatomy of an AC Motor [2]

1.2.1 Advantages:Simple DesignLow CostReliable OperationEasily Found replacementsVariety of Mounting StylesMany Different Environmental Enclosures

Simple Design:

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The simple design of the AC motor—simply a series of three windings in the stator section with a simple rotating section (rotor).The changing field caused by the 50 or 60 Hertz AC line voltage causes the rotor to rotate around the axis of the motor.

The speed of the AC motor depends on three variables:

There are a fixed number of windings sets (known as poles) built into the motor, which determine the motor’s base speed.

The frequency of the AC line Voltage. Variable speed drives change this frequency which in turn changes the speed of the motor.

The amount of torque loading on the motor, which causes slip.

Low Cost:

The AC motor has the advantage of being the lowest cost motor for applications requiring more than about ½ Hp (325 watts) of power. This is due to the simple design of the motor. For this reason, AC motors are overwhelmingly preferred for fixed speed applications in industrial applications and for commercial and domestic applications where AC line power can be easily attached. Over 90% of all motors are AC induction motors. They are found in air conditioners, washers, dryers, industrial machinery, fans, blowers, vacuum cleaners and many, many other applications.

Reliable Operation:

The simple design of the AC motor results in extremely reliable, low maintenance operation. Unlike the DC motor, there are no brushes to replace. If run in the appropriate environment for its enclosures, the AC motor may need new bearings after several years of operations. If the application is well designed an AC motor can be expected to need new bearings after several years of operation. If the application is well designed, an AC motor may not need new bearings for more than a decade.

Easily Found Replacements:The wide use of the AC motor has resulted in easily found replacements.

Variety of Mounting Styles [3]

AC Motors are available in many different mounting styles such as:Foot MountC-FaceLarge FlangeVerticalSpecialty

Many Different Environmental Enclosures:

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Because of the wide range of environments in which people want to use motors, the AC motor has been adapted by providing a wide range of enclosures:[4]

ODP-Open Drip ProofTEFC-Totally Enclosed Fan ClosedTEAO-Totally Enclosed Air OverTEBC- Totally Enclosed Blower CooledTENV- Totally Enclosed Non-VentilatedTEWC-Totally Enclosed Water Cooled

1.2.2 Disadvantages:

Expensive speed controlInability to operate at low speedsPoor positioning control

Expensive speed control:

Speed control is expensive. The electronics required to handle an AC inverter drive are considerably more expensive than those required to handle a DC Motor. However, if performance requirements can be met – meaning that the required speed range is over 1/3 of base speed –AC inverters and AC motors are usually more cost-effective than DC motors and DC drives for applications larger than about 10 horsepower, because of cost savings on the AC motor.

Inability to operate at low speeds:

Standard AC motors should not be operated at speeds less than about 1/3rd of base speed. This is due to thermal considerations. A DC Motor should be considered for these applications.

Poor positioning control:

Positioning control is expensive and crude. Even a vector drive is crude when controlling a standard AC motor. Servo Motors are more appropriate for these applications.

1.3 DC Motors

The direct current motor is one of the first machines devised to convert electrical power into mechanical power. Permanent magnet direct current converts electrical energy into mechanical energy through the interaction of two magnetic fields. One field is produced by a permanent magnet assembly; the other field is produced by an electrical current flowing in the motor windings. These two fields result in a torque which tends to rotate the rotor. As the rotor turns, the current in the windings is commutated to produce a continuous torque output. The stationary

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electromagnetic field of the motor can also be wire-wound like the armature (called a wound-field motor) or can be made up of permanent magnets (called a permanent magnet motor).

Figure 3: Anatomy of a DC Motor [5]

In either style (wound –field or permanent magnet) the commutator, acts as half of a mechanical switch and rotates with the armature as it turns. When electrical energy is passed through, the brushes switch to adjacent bars on the commutator and this switching action transfers the electrical energy to an adjacent winding on the armature which in turn perpetuates the torsional motion of the armature.Permanent Magnet motors are probably the most commonly used DC motors. DC motor operate from a direct current power source. Movement of the magnetic field is achieved by switching current between coils within the motor. This action is called “commutation”. Many DC motors have built in commutation, meaning that as the motor rotates, mechanical brushes automatically commutate coils on the rotor. A simple, permanent-magnet dc motor is an essential element in a variety of products, such as toys, servo mechanisms, valve actuators, robots, and automotive electronics. There are several typical advantages of a PM motor. When compared to AC or wound field DC motors, PM motors are usually physically smaller in overall size and lighter for a given power rating. Furthermore, since the motor’s field, created by the permanent magnet, is constant, the relationship between torque and speed is very linear. A PM motor can provide relatively high torque and it’s speed is extremely linear. A PM motor can provide relatively high torque at low speeds and PM Field provides some inherent self-braking when power to the motor is shutoff. There are several disadvantages though, most of them being high current during a stall condition and during instantaneous reversal. These can damage some motors or be problematic to control circuitry. Some magnet materials can be damaged when subjected to excessive heat and some lose field strength if the motor is disassembled.

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High-volume everyday items, such as hand drills and kitchen appliances, use a dc servomotor known as a universal motor. These universal motors are series-would DC motors, where the stationary and rotating coils are wires in series. Those motors can work well on both AC and DC power. One of the drawbacks about series-wound DC motors is that if they are unloaded, the only thing limiting their speed is the wind age and friction losses.A brushless motor operates much in the same way as a traditional brush motor. However, as the name implies there are no brushes. The mechanical switching function, implemented by the brush and commutator combination in a brush-type motor, is replaced by electronic switching in a brushless motor. In a typical brushless motor the electromagnetic field, created by permanent magnets, is the rotating member of the motor and is called a rotor. The rotating magnetic field is generated with a number of the electromagnets commutated with electronics switches in a right order at right speed. In a brushless motor, the trick becomes to know when to switch the electrical energy in the windings to perpetuate the rotating motion. This is typically accomplished in a brushless-type motor by some feedback means designed to provide an indication of the position of the magnet poles on the rotor relative to the windings. A Hall Effect device is a commonly used means for providing this positional feedback. In some applications, a brushless motor is often used when high reliability, long life and wear out are factors of consideration. In applications where high speeds are required a brushless motor is considered a better choice. A brushless motor’s commutation control can easily be separated and integrated into other required electronics, thereby improving the effective power to weight and power to volume ratio. A brushless motor package will usually cost more than a brush-type yet the cost can often be made in other advantages. Brushless motor are seen nowadays in very many computer application, they for example rotate normal PC fans, hard disks and disk drives.Sometimes the rotation direction needs to be changed. In normal permanent magnet motors, this rotation is changed by changing the polarity of operating power .This direction changing is typically implemented using relay or a circuit called an H bridge. There are some typical characteristics on “brush-type” DC motors. When a DC motor is connected straight to a battery, it draws a large surge current when connected up. The surge is caused because the motor, when it is turning, acts as a generator. The generated voltage is directly proportional to the speed of the motor. The current through the motor is controlled by the difference between the battery voltage and the motor’s generated voltage. When the motor is first connected up to the battery there is no back EMF. So the current is controlled only by the battery voltage, motor resistance and the battery leads. When a motor speed controller is used, it varies the voltage fed to the motor. Initially, at zero speed, the controller will feed no voltage to the motor, so no current flows. At the motor speed controller’s output voltage increases, the motor will start to turn. At first the voltage fed the motor is small, so the current is also small, and as the motor speed controller’s voltage rises, so too does the motor’s back EMF. The result is that the initial current surge is removed, acceleration is smooth and fully under control.Motor speed control of DC motor is nothing new. The simplest method to control the rotation speed of a DC motor is to control its driving voltage. The higher the voltage, the higher speed the motor tries to reach. In many applications a simple voltage regulation would cause lots of power loss on control circuit, so a pulse width modulation method is used in many DC motor controlling applications. In the basic pulse width modulation method, the operating power to the motors is turned on and off to modulate the current to the motor. The ratio of “on” time to “off” time is what determines the speed of the motor. The reason is that a motor is mainly a large

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inductor. It is not capable of passing high frequency energy, and hence will not perform well using high frequencies. Reasonably low frequencies are required, and the PWM techniques will work. Lower frequencies are generally better than higher frequencies, but PWM stops being effective at too low a frequency. The idea that a lower frequency PWM works well simply reflects that the “on” cycle needs to be pretty wide before the motor will draw any current. A higher PWM frequency will work fine if you hang a large capacitor across the motor or short the motor out on the “off” cycle. The reason for this is that short pulses will not allow much current to flow before being cut off. Then the current that did flow is dissipated as an inductive kick- probably as heat through the fly-back diodes. The capacitor integrates the pulse and provides a longer, but lower, current flow through the motor after the driver is cut off. There is not inductive kick either, since the current flow isn’t being cut off. Knowing the low pass roll-off frequency of the motor helps to determine an optimum frequency for operating PWM.

1.3.1 AdvantagesEasy to understand designEasy to control speedEasy to control torqueSimple, cheap drive design

Easy to understand design

The design of the brushed DC motor is quite simple. A permanent magnetic field is created in the stator by either of two means:Permanent MagnetsElectro-magnetic windings

If the field is created by permanent magnets, the motor is said to be a “permanent magnet DC motor” (PMDC). If created by electromagnetic windings, the motor is often said to be a “Shunt wound DC motor” (SWDC). Today, because of cost-effectiveness and reliability, the PMDC is the motor of choice for applications involving fractional horsepower DC motors, as well as most applications up to about three horsepower. At five horsepower and greater, various forms of the shunt wound DC motor are most commonly used. This is because the electromagnetic windings are most cost effective than permanent magnets in this power range.Opposing the stator field is the armature field, which is generated by a changing electromagnetic flux coming from winding located on the rotor. The magnetic poles of the armature field will attempt to line up with the opposite magnetic poles generated by the stator field. The section of the rotor where the electricity enters the rotor windings is called the commutator. The electricity is carried between the rotor and the stator by conductive graphite-copper brushes which contact rings on stator. The motor rotates toward the pole alignment point. Just as the motor would get to this point, the brushes jump across a gap in the stator rings. Momentum carries the motor forward over this gap. When the brushes get to the other side of the gap, they contact the stator ring again and the polarity of the voltage is reversed in this side of ring. The motor begins accelerating again, this time trying to get to the opposite set of poles. This continues as the motor rotates.

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Easy to control speed [6]

Controlling the speed of a brushed DC motor is simple. The increase in armature voltage leads to faster rotation of the motor. This relationship is linear to the motor’s maximum speed. The maximum armature voltage which corresponds to a motor’s rated speed is available in certain standard voltages, which roughly increase in conjunction with horsepower. Thus, the smallest industrial motors are rated 90VDC and 180VDC. Larger units are rated at 250VDC and sometimes higher. Specialty motors for use in mobile applications are rated 12, 24, or 48 VDC. Other tiny motor may be rated 5VDC. Most industrial DC motors will operate reliably over a speed range of about 20:1—down to about 5-7% of base speed. This is much better performance than the comparable AC motor. This is partly due to the simplicity of control, but is also partly due to the fact that most industrial DC motors are designed with variable speed operation in mind, and have added heat dissipation features which allow lower operating speeds.

Easy to control torque

In a brushed DC motor, torque control is also simple, since output torque is proportional to current. If you limit the current, you have just limited the torque which the motor can achieve. This makes the motor ideal for delicate applications such as textile manufacturing.

Simple, Cheap drive design

The result of this design is that variable speed or variable torque electronics are easy to design and manufacture. Varying the speed of a brushed DC motor requires little more than a large enough potentiometer. In practice, these have been replaced for all but sub fractional horsepower applications by the SCR and PWM drives, which offer relatively precisely control voltage and current.

1.3.2 Disadvantages

Expensive to produceCan’t reliably control at lowest speedsPhysically largerHigh maintenanceDust

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Chapter 2: Motor Protection

2.1 Introduction:

This chapter explains the different types of protections which are used for motors. The protection used is similar for synchronous motors, induction motors, synchronous condensers, and the motors of frequency converters. Normally the motor in unattended stations must be protected against all harmful abnormal conditions. The protection of very small motors is similar as the basic principles of protection are the same. The practice described here for large motors are at least equal to those covered in the National Electricity Code and are generally more comprehensive.

2.1.1 Short Circuit Protection of Stator Windings:

Overcurrent protection is the basic type that is used for short-circuit protection of stator windings. The equipment for this type of protection ranges from fuses for motor voltages of 600 volts and lower, through direct-acting overcurrent tripping elements on circuit breakers, to separate overcurrent relays and circuit breakers for voltages of 2200 volts and higher.

Figure 4: Diagram of Stator Windings of a 3 Phase AC Motor [7]

Protection should be provided against a fault in any ungrounded conductor between the interrupting device and the motor, including its stator windings. Where fuses or direct acting tripping devices are used, there must be one protective element in each ungrounded conductor. Where relays and current transformers are used with so-called “a-c tripping” from the output of the current transformers, a CT and relay are required for each ungrounded conductor. However,

9

if battery or capacitor tripping is provided, three current transformers with two phase relays and one ground relay will suffice for a three phase circuit whether or not the source neutral is grounded. The types of protection provided are both inverse-time and instantaneous phase and ground overcurrent relays for automatic tripping. The inverse-time phase relays are generally adjusted to pick up at somewhat less than about 4 times rated motor current [8], but to have enough time delay so as not to operate during the motor-starting period. The instantaneous phase relays are adjusted to pick up a little above the locked-rotor current. The inverse-time ground relays are adjusted to pick up at no more than about 20% of rated current or about 10% of the maximum available ground-fault current, whichever is smaller [9]. The instantaneous ground relay pickup should be from about 2.5 to 10 times rated current and this relay may not be used if the maximum available ground-fault current is less than about 4 times rated current, or if the pickup has to be more than about 10 times rated current to avoid undesired tripping during motor starting or external faults. If a CT, like a bushing CT, is used with all three phase conductors of the motor circuit going through the opening in the core, a very sensitive instantaneous overcurrent relay can be used that will operate for ground faults within about 10% of the windings from the neutral end.

Percentage-differential relaying is provided for large motors. It is the practice of manufacturers to recommend such protection for motors of the following ratings: (a) 2200 volts to 4999 volts, inclusive, 1500 hp and higher; (b) 5000 volts and higher, 201 hp and higher [10]. The advantage of percentage-differential relaying is that it will provide faster and more sensitive protection than overcurrent relaying, but at the same time it will not operate on starting or other transient overcurrent’s.

2.1.2 Stator-Overheating Protection:

All motors need protection against overheating resulting from overload, stalled rotor, or unbalanced stator currents. For complete protection, three-phase motors should have an overload element in each phase this is because an open circuit in the supply to the power transformer feeding a motor will cause twice as much current to flow in one phase of the motor as in either of the other two phases, as shown in figure 5.

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Figure 5: Illustrating the need for overcurrent protection in each phase [11]

Consequently, to be sure that there will be an overload element in the most heavily loaded phase no matter which power-transformer phase in open-circuited, one should provide overload elements in all three phases. In spite of the desirability of overload elements in all three phases, motors rated about 1500 hp and below are generally provided with elements in only two phases, on the assumption that the open-phase condition will be detected and corrected before any motor can overheat. Single-phase motors require an overload element in only one of the two conductors. According to the NEC motors rated less than 1500 hp are required to provide either replica-type thermal-overload relays or long-time –overcurrent relays or direct-acting tripping devices to disconnect a motor from its source of supply in the event of overload. Other things being equal, the replica type will generally provide the best protection because the time-current characteristics more nearly matches the heating characteristic of a motor over the full range of overcurrent; also , it may take into account the heating effect of the load on the motor before the overload condition occurred. The inverse-time-overcurrent relay will tend to ‘overprotect’ at low currents and to “under protect” at high currents. However, the overcurrent relay is very easy to adjust and test, and it has built in self-reset. For continuous-rated motors without service factor or short-time overload ratings, the protective relays or devices should be adjusted to trip at not more than about 115% of rated motor current. For motors with 115% service factor, tripping should occur at not more than about 125% of rated motor current. For motors with special short-time overload ratings, or with other service factors, the motor characteristic will determine the required tripping characteristic, but the tripping current should not exceed about 140% of rated motor current .[12]

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Figure 6: Thermal Imaging Thermography showing stator overheating of a motor [13]

The overload relays will also provide protection in the event of a phase to phase short circuit, and in practice one set of such relays serves for both purposes wherever possible. A survey shows that a large number of power companies use a single set of long time inverse-time-overcurrent relays, adjusted to pick up at 125% to 150% of rated motor current[14], in used for combined short-circuit and overload protection of non-essential auxiliary motors; they are supplemented by instantaneous overcurrent relays adjusted as already described. Such inverse-time-overload relays must withstand short-circuit currents without damage for as long as it takes to trip the breaker. Also the minimum requirements as to the number of relays or devices for either function must be fulfilled.

Motors rated higher than about 1500 hp are generally provided with resistance temperature detectors embedded in the stator slots between the windings. If such temperature detectors are provided, a single relay operating from these detectors is used instead of the replica-type or inverse-time-overcurrent relays. Also, current-balance relays capable of operating on about 25% or less unbalance between the phase currents should be supplied. If the motor does not have resistance temperature detectors, but is provided with current-balance relays, a single replica-type thermal overload relay may be substituted for the resistance-temperature-detector relay.

Specially cooled or ventilated motors may require other types of protective equipment than those recommended here. For such motors the manufacturer’s recommendations should be obtained.

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2.1.3 Rotor Overheating Protection:

We will discuss the rotor overheating protection for squirrel cage induction motor, wound rotor induction motor, synchronous motors.

Squirrel Cage Induction Motors: The replica-type or the inverse-time-overcurrent relays recommended for protection against stator overheating, will generally protect the rotor except where high-inertia load is involved; such applications should be referred to the manufacturer for recommendations. Where resistance-temperature-detector relaying is used, a single replica-type or inverse-type-overcurrent relay should be added for rotor protection during starting.

Wound-Rotor Induction Motors: General recommendations for this type of motor cannot be given except that the rotor may not be protected by the stator –overheating protective equipment that has been described. Each application should be referred to the manufacturer for recommendations.

Synchronous Motors: Amortisseur-overheating protection during starting or loss of synchronism should be provided for all “loaded-start” motors. Such protection is best provided by a time-delay thermal overload relay connected in the field-discharge circuit.

Amortisseur [15]-overheating protection is not required for “unloaded-start” motors. An unloaded-start motor is not likely to fail to start on the application of normal starting voltage. Also, loss-of-synchronism protection that is provided either directly or indirectly will provide the necessary protection. An exception to the foregoing is a condenser or a motor that has an oil-lift pump for starting.

When stator-overheating protection is provided by current-balance-relaying equipment, the Amortisseur is indirectly protected also against unbalanced phase currents.

Protection against field-winding overheating because of prolonged over-excitation should be provided for synchronous motors or condensers with automatic voltage regulators without automatic field-current-limiting features. A thermal overload relay with time delay or a relay that responds to an increase in the field-winding resistance with increasing temperature may be used. In an attending station, the relay would merely control an alarm.

2.1.4 Loss of Synchronism Protection:

All loaded-start synchronous motors should have protection against loss of synchronism, generally arranged to remove the load and the excitation temporarily and to reapply them when permissible. Otherwise, the motor is disconnected from its source.

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For unloaded-start motors except the synchronous motor of a frequency converter, the combination of under voltage protection, loss-of-excitation protection, and the d-c generator overcurrent protection that is generally furnished will provide satisfactory loss-of-synchronism protection. Should additional protection be required, it can be provided by an inverse time overcurrent relay energized by the current in the running connection and arranged to trip the main breaker.

2.1.5 under voltage Protection:

All ac motors should have protection against under voltage on at least one phase during both starting and running. For polyphase motors larger than about 1500 hp, polyphase under voltage protection is generally provided in all the phases.

2.1.6 Loss of excitation protection:

All unloaded-start synchronous motors that do not have loss of synchronism protection and do not have automatic voltage regulators, should have loss of excitation protection in the form of a low set , time delay undercurrent relay whose coil is in series with the field winding.

If a motor has loss-of-synchronism protection, amortisseur-over heating protection, and stator overheating protection, this equipment’s indirectly provide loss of excitation protection.

2.1.7 Field ground fault protection:

This type of protection is used for motor rated above 1500 hp. normally field circuits are operated ungrounded, a single ground fault increases the stress on the ground and the chances of a second ground fault occurrence increases. If a second ground occurs, part of the field winding is bypassed and the current through the remaining portion is increased causing a lot of stress on the shaft of the motor.

The safest practice is to use protective relaying to trip the motors main circuit instantaneously after the first fault occurs.

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Chapter 3: Introduction to Relays

3.1 Relay and its classifications:IEEE defines relay as “an electrical device that is designed to respond to input conditions in a prescribed manner and, after a pickup value is reached, it causes contact operation.” Relay can be classified on various factors.

1. Number of operating quantities: single input relays, two input relays and multiple input relays. Example: an overcurrent relay is a single quantity relay; a differential relay is a two quantity relay.

2. Relays can be classified on the function they perform.Example: Overcurrents relay; over/under voltage relay; distance relay; directional relay; thermal relay; under power relay.

3. Relays can be classified on their time of operation.Example: instantaneous relay; time-delay relay; inverse time current relay.

4. Relays can be classified by their construction features.Example: attracted armature relays, induction disc type relays, induction cup type relays, balanced beam type relays.

5. The most important classification used is based on their principle of operation. Relays working on electromechanical principle are called electromechanical relays. Relays which have static (electronic) components in them are called static relays and those having microprocessor embedded in them are called microprocessor based relays.

3.2 Electromechanical Relay:Electromechanical relays are switching devices used to control high power devices. In such relays the switching mechanism that is part of an operating circuit, is activated by means of a relay solenoid coil through which the switching current flows.

Figure 7: Electromechanical relay [16]

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Electromechanical type relays have one or more mechanical displacement electrical contacts coupled to a mobile element of the magnetic circuit of an electromagnet. The electromagnet is controlled by supplying power to its coil which drives the movement of the mobile element of the magnetic circuit of an electromagnet. The electromagnet is controlled by supplying power to its coil which drives the movement of the mobile element and the closing or opening of the electrical contacts of the relay.

Advantages Simple design Economical

Disadvantages Bulky Less Reliable

3.3 Static Relays

Static relay generally refers to a relay wherein active semiconductor devices such as diodes, transistors, I.C.s are employed for processing the electrical input signals so as to obtain the desired relay characteristics. The operation of the final contact may be either a solid state device or an electromechanical device.

Figure 8: Static relay using Solid State devices [17]

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Advantages:

Low Burden No Moving Parts Fast response Precise characteristic Sensitivity Miniaturization Less maintenance Low resetting time Low overshoot Low transient over-reach High drop-off to pick-up ratio

Disadvantages:

Vulnerability to voltage transients Variation of characteristic with temperature and age Price Overload capacity Reliability

3.4 Microprocessor based relay:Low voltage and low current signals are brought into a low pass filter that removes frequency content above about 1/3 of the sampling frequency. This signal is then sent to the A/D converter which needs to sample faster than 2x per cycle of the highest frequency that it is to be monitored. The AC signal is then sampled by the relay’s analog to digital converter at anywhere from 4 to 64 samples per power system cycle. In some relays, the entire sampled data is kept for oscillographic records, but in the relay, only the fundamental component is needed for most protection algorithms, unless a high speed algorithm is used that uses sub cycle data to monitor for fast changing issues.

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Figure 9: Working inside a Microprocessor based relay [18]

The sampled data is then passed through a low pass filter that numerically removes the frequency content that is above the fundamental frequency magnitude and angle. Next the microprocessor passes the data into a set of protection algorithms, which are a set of logic equations which help in monitoring for abnormal conditions that indicate a fault. If a fault condition is detected, output contacts operate to trip the associated circuit breaker.

Impedance is calculated using following equation:

v=Ri+Ldidt

(1)

L=∫v−R∫ i

i (2)

X l=ωL (3)

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Internally microprocessor performs integration operation for calculation of impedances. Microprocessor based relay then compares the computed value with a preset value and thereby sends a signal to circuit breaker to act accordingly.The standard operations that a microprocessor based motor protection relay has to do are as follows:

Relay Function Function Description11 Multifunction device21 Impedance(21G implies ground impedance)24 Volts/Hz27 Under Voltage(27LL=line to line,27LN=line to ground)32 Directional power element46 Negative sequence current40 Loss of Excitation(Generators)50 Negative sequence voltage51 Instantaneous overcurrent(N or G implies ground)59 Inverse time overcurrent(N or G implies ground)62 Timer64 Ground Fault(64F=Field ground, 64G=Generator ground)67 Directional Over Current(controls 50/51 element)79 Auto-reclosure81 Under/Over frequency86 Lockout Relay/Trip circuit supervision87 Current differential(87L= transmission line differential,

87T=Transformer differential, 87G=Generator differential)

Figure 10: Standard ANSI definitions for microprocessor-based relay [19]

Advantages:

• More sensitive and scalable• Communication Options• Fault Oscillography and SER data• Better targeting and annunciation• More reliable; failure alarm also included• Advanced protection features all in one box• Economical both Financially and Physically Disadvantages:

• Shift in Thought: Digital Logic v. Circuitry• More Complex Logical Systems• Longer Commissioning Procedures• Additional Training Requirements

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Chapter 4: Motor Protection Relay (SEL-710)

4.1 Introduction:

Motor Protection relay are multifunction motor protection relay for protection of AC motors. The multifunction relay used here as a reference is SEL-710 relay. SEL 710 relay is an all in one relay featuring thermal overload protection, thermal start-up supervision, stall protection or time overcurrent protection. It also features high-set overcurrent protection, low-set earth fault protection, incorrect phase sequence and phase unbalance protection, loss-of-load protection and supervision of multiple start-up. It has a whole set of measured fault parameters recorded in memory at relay operation. This relays is categorized according to the performance as a multifunction relay and operation as digital relay.

4.2 Application:

The motor protection relay type SEL-710 employs the latest microprocessor techniques to provide protection of medium sized and large three phase motors in all types of ordinary contactor controlled or circuit breaker controlled motor drives. The motor protection features extensive data communication capabilities, a variety of I/O choices and programmable SELogic

control equations. The SEL-710 also provides many functions of a programmable logic controller.

Figure 11: Applications possible using SEL-710 relay[20]

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The SEL-710 provides the protection and starting opportunities for high-inertia starting applications because the real-time calculation of changing motor slip and rotor resistance is used to calculate motor thermal rise and maximize safe starting times. This results in additional start times for slow-starting motors. If we are using a constant value for motor resistance throughout the start sequence results in premature trip and reduced start opportunity.

Figure 11a: Motor starting adaptation in an SEL-710 relay [21]

4.3 Principle of Operation:

SEL-710 being a multifunction relay will have different principles for the different types of protections it offers. We would be discussing each principle of operation individually.

• Short Circuit (50/51): The stage with definite time delay protects against phase short circuit faults, which are responsible for overheating damages.

• Earth fault Overcurrent Protection (50G/51G): The earth fault current, which can appear following a stator earth fault, can be detected by non-directional overcurrent protection element using earth current measured from a sensitive current input. Each group and related time delay can be programmed to provide maximum selectivity. Protection element can be configured to make a trip or alarm.

• Loss of Load/Undercurrent (66): The current stage, along with a pick-up time delay, is used to detect a loss of load due to a shaft failure or a pump running unprimed.

• Current Unbalance and Phase loss (46): Running motors at unbalance conditions results in overheating, even at current values below the nominal. Principle of operation is based on monitoring of the percentage of the current unbalance in the three subsequent phases.

• Thermal Overload (49): The protection feature is based on mathematical model of motor thermal image. The model utilizes heating constant time values. It is input to the relay by

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means of T6xIB parameter, which describes the maximum period of time when actual motor current value is allowed to reach a six fold increase of its nominal value. Cooling constant time value is calculated on the basis of the heating constant.

• Start/Stall Protection (48/51LR): The relay monitors the starting sequence of the motor and provides excessive start time protection/locked rotor protection. The detection of the start-up is based on current criteria only. If Start-up phase is too long there is trip command or alarm signal. If the starting of motor is finished, start protection is blocked. During normal motor operation, an overcurrent threshold detects rotor stalling, and the shorter time delay is used. The normal motor operation, after starting, is detected by watching current stages.

• Locked Rotor during Starting (51S): During motor start-up, a locked rotor is detected with the help of a speed switch signal, connected via the logic input. If during start-up phase input is not shorted the shorter time delay setting is used as it recognizes the motor is stopped.

4.4 Testing of relay SEL-710:

Relay is required for the detection of fault and sending it to the circuit breaker. SEL-710 is used here for the protection of motor and also to detect faults.

There are various types of faults that can occur in motors. Some of them being overcurrent, undercurrent, over/under voltage, over/under frequency, phase imbalance, VAR element, under power and motor starting protection.

To test the relay SEL-710 all the types of faults were manually created and the relay’s response was tested.

Tests are conducted mainly in following way:1) Direct Method

The test is carried in direct method in order to test the relay directly, and since in real time these relays are placed on the lines carrying current to the motor the testing is done mainly by simulation.

Flow chart & procedure to test SEL-710 using direct method will be explained below.

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Apply Power to PC & Directional Over current Relay Protection Relay

Connect Device to Computer using cable”SEL-C234A”

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Set Parameters using software “SEL-5030” to default Values

Press <Enter> to Check Communication Link

Type ACC & enter default password (587) to go to access level 1

Type 2AC & enter default password (587) to go to access level 2

Type SET in order to change parameters (V, I, time etc.)

Change the value of pickup current or voltage or both to desired value (0.2-5000 amps) & voltage from (100-30000 volts) &

save the settings

Apply 48V dc from power supply to Control circuit terminals (A01, A02) on rear panel of relay

Figure 12: Flowchart for the testing of relay SEL-710

4.4.1 Procedure to test Motor Protection relay (SEL-710) using direct method:

Step 1: Software installation

SEL -5030 CD in inserted into drive of PC and it is automatically loaded.

SEL-5030 is called PC terminal simulation software.

Open “SEL-acSELerator” software.

Step 2: Relay Connections

Motor Protection Relay (SEL-710) is connected to pc using serial communication cable

i.e. SEL-C234A.

Apply 120 volts from power supply to Control circuit on terminals A01 & A02 (+/H -/H).

Connect Phase A, Phase B and Phase C on Z01, Z03 and Z05 and in parallel across to

E01, E02, and E03. The output for the phase currents is taken from Z02, Z04 and Z06.

To explain the back panel connections a back panel diagram is shown below.

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Apply voltage from power supply going to (E01, E02, E03 and neutral connected at E04) and current going from power supply on to ports Z01, Z03 and Z05 and

outputs from Z02, Z04 and Z06

Observe trip signal indicated by relay on its front panel and also the metering section which tells the

value of fault recorded and type of fault.

Figure 12a: Back panel for SEL-710 relay. [22]

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Voltage Input from the PT

Figure 13: AC connections –Across the line starting [23]

Step 3: Test Procedure

Relay”SEL-710” is powered from a 120V outlet & PC is powered ON. This is indicated by illumination of LED.

Click the communication pull down menu, and click on “parameters”, enter the default settings as shown below :

Description Default ValueBITS PER SECOND 9600DATA BITS 8PARITY NONESTOP BITS 1FLOW CONTROL NONE

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Figure 14: Front Panel of SEL-710 relay [24]

Figure 15: AcSelrator Quickset window

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Green light indicating relay is enabled

Communications

Terminal

Go to TERMINAL button press<Enter> and verify that SEL-710 returns an equals

to (=) prompt.

Type ACC<Enter> and enter the default password as 587.

Type 2AC<Enter> and enter the default password as 587.

Figure 16: Terminal Window

There is another method of accessing the parameters of the relay and is less complicated and more user friendly.

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Level 1 access

Enter Password: 587

Level 2 access

Enter Password: 587

Figure 17: Setting Window

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We can read the settings directly from the relay using this button

We can sent the settings to the relay by using this button

Main parameter box

CT ratio setting and Full load current setting

PT ratio setting and Nominal line voltage setting

This is the main window where we can make changes in the settings of the relay according to the requirements. There are 3 groups and each group can take different settings. For complex PLC operated networks all 3 groups are kept active and according to faults and the change in the flow of power the groups are selected.

The main window as shown in the figure displays settings for the CT and PT ratio selection. Also depending on the CT ratio we can select the full load current settings and the nominal line voltage settings. If a mistake is made in writing the full load current or nominal voltage depending on the CT ratio a red box indicating an error will appear and the setting cannot be imported to the relay until that error is corrected.

Settings are also provided for the type of PT ratio depending on its connections we can select a Wye or Delta type of transformer.

If two motors are being connected then the CT ratio for each input and full load amperes of both motors will have to be specified individually.

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Figure 18: Overcurrent setting window

In the phase overcurrent settings we set the value of the trip which is calculated upon the motor starting current and the full load amperes. We can also keep a time delay as the motor has high starting current during the start. The SEL-710 is adaptive to startup and so it doesn’t need the time delay. We have not considered Neutral, residual and negative sequence overcurrent.

The alarm trip can be switched on which gives us an alarm saying the current is exceeding the set values.

There is a delay for the alarm which can also be set depending on the factors and the type of motor.

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Overcurrent

Element

Overcurrent trip delay

Current settings=instantaneous trip

Phase overcurrent trip setting (i.e=1.37*1.6=2.192)

Phase Overcurrent Settings

Phase overcurrent Alarm Pickup

Current settings=OFF

Figure 19: Under Current Settings

The values of undercurrent are depending on two major values the full load amperes and the no load amperes. The value of undercurrent has to be set at more than the no load current value and lesser than the full load current value.

A minimum of 0.4 seconds of delay is required to be set if the undercurrent element is being used.

The alarm values can be set from 10% of the full load value to 100% of the full load value.

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Under Current element settings window

Under Current alarm=OFF

Load Loss Protection delay=0 sec

Under Current delay settings

Current value is=5 sec

Under Current Trip Pickup setting (i.e. = (0.8*1.6=1.28)

Figure 20: Current Imbalance Element

Current Imbalance element comes into play when the one of the phase of motor is down. Due to this there is an imbalance and it causes a lot of stress on the motor. This also leads to heating of the motor and can cause insulation problems.

In the current settings I have kept the alarm settings on 25% and trip settings on 33%.

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Current Imbalance element setting selection icon Under Current alarm=OFF

Load Loss Protection delay=0 sec

Current Imbalance trip settings

Current settings=33%

Imbalance trip delay

Current settings=2 sec

Current Imbalance alarm trip settings

Current Settings=25%

Figure 21: Undervoltage Element

Under voltage will cause the motor to slow down and the current would increase causing the heating effect in motor to take place. The current increase will also sometimes cause the over current relay to trip.

The over voltage settings are kept in such a way that the voltage doesn’t fall too much. Too much fall in the voltage would lead to a sudden rise in temperature which could lead to motor burning itself.

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Under Voltage element setting selection tab

Under voltage Trip pickup setting (i.e. =0.9*220=198v)

Under voltage trip delay setting

Current setting=5 sec

Under voltage alarm pickup and delay setting

Figure 22: Over voltage element

The overvoltage fault is due to bus voltage and does occur when the load is decreasing suddenly.

To protect the motor against such types of faults the trip settings have been set in such a way that no more than the maximum level the motor can handle is supplied to the motor.

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Over voltage trip pickup Current Pickup=1.1*220=242volts

Over voltage trip delay Current Pickup=2 sec

Over voltage alarm pickup Current Pickup=262 volts No time delay is selected

Over Voltage element selection tab

4.5 Device Metering and test results

The following is the window which is observed for reading the motor data which is changing at 9600 bits /second.

In the metering section we can observe the Input currents which are coming into the motor, the input voltages the active, reactive and total power, power factor and frequency.

It also indicates the relay is enabled and the motor is running with no faults.

Figure 23: Metering of the motor using acSELerator

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Here after the metering section we can go to the Phase component section and we can observe the currents and voltages and their phase shifts. This is a feature which has been given to observe the unbalanced faults that occur through a graphical interface.

Figure 24: Phase Components observed in the acSELerator

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Figure 25: Instantaneous values observed in acSELerator

The instantaneous metering value section would give us the instantaneous value of the current, average current magnitude, Motor load, Negative sequence current, current imbalance, differential phase current value, voltage magnitude, average phase voltage value, real power, reactive power, apparent power, Power factor and frequency.

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Figure 26: Min/Max metering values in acSELerator

The Min/Max metering values section would record all the current and voltage values that have occurred as the min or maximum for that section.

It also records the maximum and minimum of Power, reactive power, apparent power and frequency.

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This is the observation during an undervoltage fault that occurs and as we can see the value of voltage is quite lower than the set Vnominal value. The OUT103 light indicates the circuit needs to be tripped. The front panel shows TRIP indication and also the type of fault that has occurred.

Figure 27: Under voltage trip observed on acSELerator

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Figure 28: Over voltage Trip as seen on acSELerator

This is the observation during an over voltage fault that occurs and as we can see the value of voltage is quite higher than the set Vnominal value. The OUT103 light indicates the circuit needs to be tripped. The front panel shows TRIP indication and also the type of fault that has occurred.

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For faults analysis we can observe the fault occurrence in the relay and the trip signal and the trip in the relay which occurs. To observe these files we need to go to TOOLS in the acSELerator software and select EVENTS and select GET EVENT FILES. We can get the files in two formats the RAW files which has more data storage capacity for detailed analysis and the CEV files which give us the over view of the fault. These files would show us the detailed observation just at the occurrence of the fault and the operation of the relay in response to it.

Figure 29: Shows a figure of over frequency fault occurrence.

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Figure 30: Observing the under frequency fault occurrence

We can observe in the figure that during an under frequency fault occurrence the relay would trip within 5 cycles of fault detection and as a time delay was set the delay in fault detection is because of that.

We can observe all three currents and their values and see the distortion in it. The positive values are much higher than the negative values.

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Figure 31: Phase Imbalance fault

Phase imbalance fault occurrence happens when one or more phases is not supplying proper amount of current which leads to strain on the motor and causes heating.

As we can see in the above figure phase C is faulty and the value of it is fluctuating which leads to a fault occurrence and makes the relay trip.

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Chapter 5: Conclusion

The testing of fault on SEL-710 microprocessor based relay concludes that when the relay is tested on all major faults, a satisfactory tripping to protect the motor is potentially obtained. Several functions such as faster operation, multiple protections and setting selection makes SEL-710 relay significantly different from electromagnetic and the static relays.

Test was conducted in following method as explained in the paper:

1. Direct Method.

This method helps to understand the functionality and response of relay under fault conditions. Response of relay is determined by the indication of respective LED’s.

Further testing can be carried on SEL-710 for various faults such as VAR fault, Double motor differential current fault, under power fault etc. Unlike in the paper where fault is generated to test the relay, industrial testing of relays is done using a multi amp test kit which has all the fault generation parameters and only the connections need to be done to test the relay and observe the results. In real time, testing is done by by-passing relays and real time data is computed with the help of Communication Processor(Remote Terminal Unit) which collects the data from lines, passes it to a computer from which signals are given to PLC or building Automation system which then passes on the signals to the relays (like SEL-710,SEL-701, etc.) which are present as backup relays.

Figure 32: Real time testing

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References

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hp-laserjet-fuser.html18. http://www.pacw.org/no-cache/issue/winter_2009_issue/testing_philosophy/

testing_philosophy.html19. http://en.wikipedia.org/wiki/Digital_protective_relay 20. http://www.pdftop.com/view/

aHR0cDovL3d3dy5zZWxpbmMuY29tL1dvcmtBcmVhL0Rvd25sb2FkQXNzZXQuYXNweD9pZD0yODkw

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22. SEL motor protection relay flyer, page 4 23. SEL motor protection relay manual, page 26 24. SEL motor protection relay flyer, page 4

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