S.D. Loudon NewElec Pretoria (Pty) Ltd

www.newelec.co.za

Keywords – Motor protection, three phase protection relays, 

Abstract

This paper will provide guidelines to evaluate the ability of existing motor protection and control relays to meet the criteria to protect machines, maintain plant integrity and increase equipment longevity.  Key components will be identified in existing systems and a strategy suggested to supplement existing protection and control relays with add on equip­ment or by utilizing existing protection equipment features within the protection relay.

This opinion can be expressed by asking the questions what, when, how, and where.  It endeavors to provide a simple answer.

1.         Brief history and background

Application of motor protection in the 21^st century should be the simplest task when one considers the abundance of exceptionally good quality equipment that is available on the market.  The experience of any plant engineer may yield a different opinion due to inheriting an existing plant and equipment that may not appear to be the choice of the day.  The lack of budget and skilled manpower to implement the changes to modernize the plant often leads to piecemeal or firefighting upgrading strategies which never seems to get to a point of completion.  The next hurdle is the multiplicity of protection, control and communications features that every relay vendor offers.  It inclines to cloud the issue of what the prime requirement is and what a nice to have protection or control function is.

In the 20^th century a protection, relay configuration was primarily used to provide protection, to trip the motor.  Communication, via a communication bus and protocol, transferred relevant data relating to the trip or alarm status of the motor to the process control room.  Information extracted from these messages informed process control staff of the operational status of the motor. They would in turn take corrective action regarded as a control action, which in the event of an alarm would be to prevent the disruption of production or in the event of a trip to shut down all processes affected by the motor that has been trip and removed from services.

Control functions were not integrated locally within the protection relay. Control functions resided in software coded control routines in the DCS or SCADA systems.

This provided a structured and integrated approach to the control of the plant or system from a central point. 

A totally separate option of selecting a hardwired manual control at local MCC level if the DCS or SCADA system went off line for whatever reason was the standard typical of most mission critical and Petrochemical plants due to process certification relating to OHS act and process integrity.

Triple redundant control circuits and very strict risk analysis are the order of the day no deviations accepted and are still the standard today.

2.      Protection and Control in the 21^st century

The present configuration of substations, and low voltage motor control centers, is to use I.E.D.’s  (Intelligent Electronic Device), or smart motor protection relays.  These devices are able to provide the most advanced protection features as well as integrated control function blocks normally associated with a PLC.

The advantage of using an IED is that of distributed control.  Residing within a cubicle or MCC independent of the main DCS as well as faster scan times on the DCS since control functions have been removed from the DCS and placed at a local node on the network.

The other advantage that can be realized is that the traditional manual or local control mode can also use the control functions or I/O of the IED to simplify the control wiring.  It is even propagated in respected software design companies that the requirement for the old hardwired safety circuits can be regarded as obsolete and routed through the IED or PLC inputs with certain strict criteria on the software coding of the function (a little to advanced in my opinion).

The IED if utilized to its full capacity has advantages but the drawbacks are evident in that the requirement on the skill level of the maintenance personnel may require the relaxation and retraining of our traditional electrician since part of the control circuit is now buried within the IED that needs replacing if it has failed.  If the IED protection and control settings are not transferred correctly into the new or replacement device, it will keep the entire process locked out.  It is a little more serious than keeping a drive out because the default relay setting results in a trip on over-current or locked rotor during a normal start up event.

The current practice is that the EI control technician is responsible for the parametrization and downloading of new settings into the replacement IED.  This results in the increased pressure on a few individuals with no shortage of spectators.

The training of electricians to use a laptop, or the protection relay keypad, as well as a disciplined approach to storing of relay settings in read only type files. Files need to be stored in file names reflecting the drive name, or some foolproof method of matching the relay to the motor drive, which it is protecting.

A structured procedure outlines:

• The method of retrieving the relevant drive Relay settings data file.
• The entry of the relay settings and I/O parametrization data into the relay, followed by verification of actual values once the motor is returned to service.

The emphasis would be on providing a limited platform with user restrictions tailored to allow only the downloading of existing files with no editing features.

3.         Motor Protection

Having introduced the aspect of protection and control, it maybe worth discussing the protection features required by the motor and to separate these protection features into two categories namely:

• basic non negotiable and
• extended protection features offered by the present motor protection relays.

To further focus the discussion we will attempt to answer, the following questions where applicable.  What? Why? When? How? Where?

3.1       Basic essential non-negotiable protection features

These features are the minimum essential to protect the motor itself

• Thermal Overload full thermal memory
• Locked Rotor
• Unbalanced current or negative phase sequence current protection
• Single phasing

3.1.1    Thermal overload full thermal memory

What must be achieved ?

The purpose of the thermal overload protection element is to prevent the motor stator or rotor windings from ever exceeding the safe rated operating temperature of the insulation material used.

Why is it necessary?

The motor can provide 250% rated torque under normal running conditions before the pull out torque point is reach at which point the motor will stall.  This means that the motor is capable of driving a load of up to 250% nameplate rating.  This will result in the temperature of the motor windings exceeding their rated operating temperature resulting in insulation failure which will progress into a Phase to Phase or a Phase to earth fault at the moment of failure.  A useful rule of thumb to see at what tempe­ra­ture the windings will stabilize at above the designed temperature rise is:

Calculate: a 1% overload  2⁰C temperature rise

This formula can also be used to calculate the C.D.F. (cyclic duration factor) at which a motor operated above the S1 rated load current value by using the calculated temperature rise multiplied by C.D.F. equates to the rated temperature rise specified by the motor manufacturer. A word of caution in using this method is that if the entire repetitive load pattern period exceeds 10 minutes this rule of thumb assumption can result in temperatures during the cyclic loading exceeding the maximum rated insulation rating.

·         How can one maintain the operating temperature within design limits?

The relationship between the I²t load current levels flowing in the motor windings has a direct relationship to temperature rise of the motor windings.  A thermal memory model based on the stable as well as cyclic load current patterns of the load current, as well as the operational state of the motor standstill or running controls the move­ment of the selected thermal damage curve between a Hot and Cold curve charac­teristic.  The thermal capacity value must be maintained and restored to the actual value when cycling the auxiliary control power supply to the relay.

3.1.2    Full Thermal memory trip delay curve

IEC 60255-8

Where ;

t              trip time in seconds

t             heating time constant in seconds

I              actual load current flowing in motor windings

Ie            full load current setting of motor

Ip            time averaged load current level prior to present overload condition

Is            full load current service factor typical if used is 1.1 or 110%

3.1.3    Locked Rotor

Whatmust be achieved?

The purpose of the locked rotor overload protection element is to prevent the motor stator or rotor windings from ever exceeding the safe rated operating temperature of the insulation material used during the motor starting and accelerating up to opera­tional speed.

Whyis it necessary?

The motor load current at start-up on a DOL applications are in the order of 600% to 720% motor full load current rating and are maintained until the motor has accelerated to a rotor speed greater than the pull out torque or breakdown torque value.  The load current reduces afterwards to a value below the motor full load current value.  The period that the stator and rotor windings can withstand these current levels are referred to as the safe cold or hot stall times in seconds.  This is the maximum period that the motor stator and rotor windings may be subjected to these currents assuming that the rotor remains stationary or as is termed “locked rotor”.

The AC rotor resistance of a stationary rotor can be up to 300% the value of the rotor resistance of a motor running at operating speed. This is due to the higher frequency of the rotor current (50 hertz) at standstill versus the frequency of the rotor current at operating speed (2-3 Hertz).  At the higher frequency at standstill the rotor current flows on the surface of the conductor a condition referred to as “skin effect”.   This phenomena is used by the motor designer with deep bar rotor designs to achieved high starting torque characteristics, normally achieved with high resistance rotor windings with the low slip characteristic normally associated with low stating torque low rotor resistance winding resulting in lower operating temperatures and cooler running machines.

A very simplistic analogy to describe the operation of the rotor windings even in a standard rotor will be to visualize the rotor winding  having a 10mm² conductor during starting and during acceleration the rotor current transferring over to a 30mm² conductor and the “Locked Rotor” current / Time spec is given on the 10mm² conductor.

How is locked rotor detection being done?

• Vectorial stall rate of change power factor
• Detection via speed switch or tachometer

If one could detect that the motor is accelerating up to operational speed then we know that the rotor conductor resistance will be decreasing and the I² T rating of the rotor conductor will be increasing.  Methods of detecting that the motor is  accele­rating to speed are by a tachometer or a speed switch attached to the motor shaft or by the movement of motor power factor during the acceleration period.

The locked rotor trip delay can be extended to cater for conditions in which the motor run up time exceeds the allowable locked rotor trip time.  Extreme caution is the password in this application, ensure that the following conditions are met.  The tachometer output must be matched to the acceleration profile of the drive.  The simplest characteristic would be a linear characteristic monitoring shaft speed against total allowed acceleration time.  The basic principle is that the motor speed must always be increasing until the motor has reached operational speed.  If a speed switch is used, the shaft speed at which it will close is critical and the higher >20% would be the minimum level.  Especially on ID fan application where intake dampers can be opened prematurely and result in the motor failing to accelerate but entering a sub synchronous crawl whilst still drawing the acceleration current.

The safest method to my way of thinking is the” Vectorial Stall” Method, which employs the rate of change of power factor, integrated into the protection relay.

The Thermal trip curve set to the locked rotor time but being extended under control of the vectorial stall element.      

3.1.4      Unbalanced current or negative phase sequence current protection

What must be achieved?

The purpose of unbalanced current or negative phase sequence protection is to identify the magnitude of the negative phase sequence component.  The thermal capacity level must be modified to cater for the heating effect of the negative phase sequence currents.

Why must protection be provided?

The negative phase sequence current heating component on a running motor will be in the order of 6 to 7,2 times the actual value of the current. Since the negative phase sequence current induced in the rotor will be at twice the line frequency, a condition referred to as skin effect occurs.  It results in negative phase sequence current flowing on the surface of the rotor conductors. The result is higher copper losses and localized hot spot heating.

How are the values calculated?

The traditional method of measuring negative phase sequence current is to pass the line current flowing into the load current through a negative phase sequence filter.  It only requires the measurement of the I red (0⁰) and I blue (240⁰) currents. The filter delays the waveform of I red (0⁰) value by 60⁰ and subtracts the I red and I blue values.

An Alternative method which traces negative phase sequence value to 30% before deviating is to calculate the > deviation from the average value of the three phase current divided by the average x 100 %. for (load currents > Is) and divided by Is for (load current < Is).   

A variation that has been noted as well is to calculate the difference between the > and < line current value divided by the average x 100% for (load currents > Is) and divided by Is for (load current < Is).

When negative phase sequence current of significant levels are detected.

The option to integrate the heating effect into an IDMT curve, which will sum with the thermal overload element to provide thermal protection of the motor windings.  An alternative method is to provide a definite time trip once the predetermined Negative phase sequence level is exceeded.  Caution is to be exercised if the trip delay time is extended beyond a reasonable delay typically the safe cold stall time in seconds if not then an IDMT type characteristic must be used and integrated with the thermal overload model.

3.1.5    Single phasing

Whatmust be achieved?

Single phasing protection or phase loss results in a severe condition of unbalanced current or 100% negative phase sequence condition and the motor must be disconnected from the supply within a period of 5 sec maximum.

Why must protection be provided?

The negative phase sequence current heating component on a running motor will be in the order of 6 to 7,2 times the actual value of the current. Since the negative phase sequence current induced in the rotor will be at twice the line frequency a condition referred to as skin effect occurs.  It results in negative phase sequence current flowing on the surface of the rotor conductors causing higher copper losses and localized hot spot heating.

Howare the values calculated?

The traditional method of measuring negative phase sequence current is to pass the line current flowing into the load current through a negative phase sequence filter which only requires the measurement of the I red (0⁰) and I blue (240⁰) currents. The filter delays the waveform of I red (0⁰) value by 60⁰ and subtracts the I red and I blue values.

An Alternative method which traces negative phase sequence value to 30% before deviating is to calculate the > deviation from the average value of the three phase current divided by the average x 100 %.

A variation that has been noted as well is to calculate the difference between the > and< line current value divided by the average x 10

When a single phase condition occurs negative phase sequence current of significant levels are detected.

The option to integrate the heating effect into an IDMT curve, which will sum with the thermal overload element to provide thermal protection of the motor windings

With a trip characteristic of 5 sec for 100% negative phase sequence.

An alternative method is to provide a definite time trip once the predetermined negative phase sequence level for phase loss is exceeded.

Caution is to be exercised if the trip delay time must be less than 5 seconds.

3.2       Extended motor protection added value

• Locked rotor detection
• Vectorial stall rate of change power factor
• Detection via speed switch or tachomete

• Running stall / jam detection

• Minimum load or under load protection

• Under power protection

• Earth Leakage

• Over voltage

• Under voltage

• Phase rotation

• Starts per hour limitation

• Over temperature detection

• PTC over temperature detection

• RTD over temperature detection

 3.2.1    Running stall / jam detection

What is a running stall condition?

Once a motor has accelerated up to operation speed and the load current has reduced below full load current, the operation status of the motor is said to be in service and running at operational speed.  The load current is generally below Motor full load level when measured as an RMS value over a 10 min period.

Whilst operation in this state the motor will draw current related to the load applied to the motor shaft, The torque available is roughly 250% of rated torque and this torque will be used to maintain the motor speed between operational speed and the stall speed which corresponds to the pull out torque point on the speed torque curve,

When will a running stall occur?

Should the motor load increase above the pull out torque value, the motor will stall, the stator load current will increase to locked rotor current levels and the motor will trip after the thermal curve trip delay has elapsed.  This is not necessary since the trip delay can be shortened to 1 second.

Can we detect a running stall condition?

If we examine the speed / torque curve of the motor, one can see that once the motor is running at operational speed, the load current level (is >30%) and lower than 350% of motor full load current. Should the current increase above this level, the pull out torque point is exceeded and the motor enters a stall condition during normal motor. The simplest method of implementing a running stall protection is, once the motor is running at operational speed to arm a definite time trip on a set threshold between 110% to 300% of motor full load current.

Where would one use running stall protection?

On almost every application, particularly on compressor and jaw crusher applica­tions, that could be prone to jamming. The equipment used for interrupting the load current limits the trip delay selected.  If a contactor is used to interrupt the load current, the minimum trip delay should not be less than 1 second. The reason being that in the event of the motor windings or the cable end box developing a short circuit, the fault current will not be limited to the locked rotor current level but will be at the full system fault level which could vaporize the contactor (Faster is not better if the equipment doing the interrupting is not rated for the task).

Note:

The 1-second trip-grading margin is the trip delay one will get when using at HRC motor fuse at 10 x Fuse rating.

3.2.2   Minimum load or under load protection       

What is a minimum load condition?

A load current based minimum load condition is identified as the condition under which the motor load current drawn by a motor under operational conditions is below the minimum safe or normal load current indicating a faulty or abnormal operating condition.

When will a minimum load condition occur?

On loss of driven load, Typical examples are loss of suction on a centrifugal pump, snapping of “V belts” on a pulley driven load, shearing of motor shaft or coupling shearing pin to name but a few conditions

How is a minimum load condition detected?

Minimum load is detected by measuring and establishing that the motor load current being drawn by the motor windings (is > 15%) motor full load current but < the preset Minimum load value established by the process healthy condition. Should a Minimum load condition be detected, an alarm condition is generated and after a preset time delay ( 1 to 100 sec typical) the motor is tripped on minimum load.

Where should minimum load protection be used?

Minimum load protection should be used where the process load, the production line or process would be critical.

Typical examples are;

• A process being controlled by DCS or SCADA which use an auxiliary contact on the contactor via the I/O block is used to indicate a run signal (the contactor is closed the motor must be running?) and this will provide the interlocking to other drives downstream of the effected drive and result in chemical or material spillages since the faulted drive is not clearing the material through its part in the process. 
• A centrifugal pump, loss of suction or medium results, in the same medium circulating in the impeller housing raising the temperature of the medium to boiling point damaging the pump seals or if the medium is acidic the corrosivity will double for every 10° C (why does the medium boil. When a pump is operating it pumps from a low pressure to a high pressure. What happens to a liquid at a low pressure the boiling point decreases now if the same liquid is being spooled around at the suction intake of the centrifugal inlet of the pump it is eventually going to boil.

3.2.3      Under power protection

What is an under power condition?

An under power based minimum load condition is identified as the condition under which the motor load drawn by a motor under operational conditions is below the minimum safe or normal load power factor indicating a faulty or abnormal operating condition

Whenwill an under power load condition occur?

On loss of a driven load, typical examples are: loss of suction on a centrifugal pump, snapping of “V belts” on a pulley driven load, shearing of motor shaft or coupling shearing pin to name but a few conditions.

How is an under power condition detected?

Under power is detected by measuring and establishing that the power factor of the motor load being drawn by the motor windings is < the preset Minimum power factor value established by the process healthy condition. Should a under power load condition be detected an alarm condition is generated and after a preset time delay (1 to 100 sec typical) the motor is tripped on minimum load.

Where should under power protection be used? 

Under power protection should be used where the process load, production line or process would be critical.

Typical examples are:

• When oversized motors are used to resulting in insufficient current differential between a loaded and a unloaded motor in the production process or;
• A process being controlled by DCS or SCADA which use an auxiliary contact on the contactor via the I/O block is used to indicate a run signal (the contactor is closed the motor must be running?) and this will provide the interlocking to other drives downstream of the effected drive and result in chemical or material spillages since the faulted drive is not clearing the material through its part in the process.

3.2.4      Earth Leakage

What is an earth leakage condition?

An earth leakage condition can be identified as a condition under which a current generally not exceeding 1 amp flows to earth in a Health circuit as opposed to earth fault which a current flows to earth in a faulty circuit and is generally > 1 amp

When does an earth leakage occur?

Typically an earth leakage condition occurs when the insulation around the conductors are compromised by contact with human, animal, carbon tracking between the exposed conductor and earth, moisture, or lubricant ingress of windings

The moment the human, animal, carbon tracking or moisture is removed or dried out the insulation of the motor windings are once more ready for service. Thus the definition as earth leakage being in a Healthy insulated circuit.

How is an earth leakage condition detected?

Earth leakage is detected by using a summation or core balance current transformer through which all conductors supplying the protected circuit are passed.

Sensitivities of 0,030 to 1 amp are detected; the balance is maintained by the fact that in a healthy circuit all currents flowing into a circuit are balance against those currents returning from the circuit, thus resulting in the cancellation of all fluxes generated by the currents flowing in the circuit.

Where should earth leakage protection be used?

Where personal safety is required. In explosive atmosphere conditions or where moisture and carbon can result in leakage current paths to earth.

3.2.5    Over voltage

What is over voltage?

Over voltage in a motor circuit is defined as the maximum voltage level at which the supply phase to phase voltage can operate without the saturation of the stator core laminations resulting in the overheating of the stator windings,

How is over voltage detected?

Over voltage is detected by direct measurement of the main circuit line voltages and should the voltage exceed 115% of rated line voltage, an alarm flag is set and should the condition persist for longer than 10 sec with the motor running, the motor contactor is tripped.

When or where are an over voltage conditions likely to occur?

Over voltage, conditions are not common within the industrial metro areas. It occurs more frequently in outlying areas, feed by rural feeders, farm lines, or underground section feeders.  The over voltage condition occurs when the load demand on the feeder is lower than what it normally would be, or if the switching reticulation is altered to alternative feeders during maintenance periods or outages. Bottom line, it can occur at anytime in the rural or single line feeder networks.

3.2.6    Under voltage

What is under voltage condition?

Under voltage in a motor circuit is defined as the minimum voltage level at which the supply phase to phase voltage can operate without significant torque reduction of the motor, resulting in load currents > motor full load, being drawn from the supply resulting in the overheating of the stator windings, (the torque of the motor is proportional to the square of the voltage the maximum allowed by manufactures is –10% rated supply).

How is under voltage detected?

Under voltage is detected by direct measurement of the main circuit line voltages and should the voltage drop below 90% of rated line voltage an alarm flag is set.  Should the condition persist for longer than 10 sec with the motor running, the motor contactor is tripped.

When or where are an under voltage conditions likely to occur?

Under voltage, conditions are not common within the industrial metro areas. They occur more frequently in outlying areas feed by rural feeders, farm lines, or underground section feeders.  The under voltage condition occurs when the load demand on the feeder is greater than what it normally would be, or if the switching reticulation is altered to alternative feeders during maintenance periods or outages. Bottom line it can occur at anytime in the rural or single line feeder networks. 

3.2.6    Phase rotation

What is phase rotation protection?

Phase rotation protection will ensure that the incoming supply to the MCC is correct. In the event of the incoming supply phase sequence being altered, the motors on the protected MCC must not be allowed to start, since they will run in the opposite direction to the specified direction (the direction of a three-phase motor is altered by interchanging two phases of the voltage supply).

When will you require phase rotation protection?

In every three phase motor application, since it is critical to ensure that once a motor or plant has been commissioned, that the reversal of the supply to that plant will be correct under any reticulation configuration. In the event of portable equipment the phase rotation protection must be built into the starter panel of the drive since the phase rotation from location to location could be swapped

How is phase rotation detected?

Phase rotation and phase symmetry are generally combine to ensure that the phase voltage sequence will be Vr, Vw, Vb and that the voltage waveforms are displace by 120 degrees.  The frequency has to be between 30 hertz and 100 hertz.

Where is voltage phase rotation detected? 

It is detected above the main contactor. This enables the detection of phase rotation before the main contactor is closed.  IOW ensures that phase sequence is correct before allowing the closing of the main contactor.

3.2.7    Starts per hour limitation

What is a start per hour limitation?

Take the number of starts per hour allowed, divide into 60 min and we have a period specifying the time delay between starts. Example 6 starts per hour Starts per hour means that every (60/6) min = 10 min the drive can be started

When or where would one employ a start per hour limitation? 

One may be inclined to think, that it is for thermal protection from the motor manufacturer this will be the spec since they do not know what type of thermal overload or stall protection will been fitted to the drive.  It would more sense to fit starts per hour protection in an application where  “jogging” or “Inching” of the drive has to be prevented to save switchgear and mechanical drive chain of the machine from operator abuse.  On the other hand, if the motor employs a reactor or shared soft start starter that needs to be thermally protected from to frequent starts.

3.2.8    Over temperature detection

What types of over temperature protection are available?

Various types of over temperature sensors are available.  It can be divided into two main categories namely:

• Non-linear  -  PTC sensors (Positive Temperature Coefficient) normally referred to as Thermistors have a S type or switch characteristic normally use to indicate that a specific temperature has be exceeded the device is selected according to the temperature at which the element is to switch or alter the internal resistance on a exponential type switching characteristic. This is the default sensor fitted in LV motor and if the LV overload relay does not have NTC sensors (Negative temperature coefficient) these devices have a more linear type characteristic, which can be linearised to allow accurate temperature measurement.
• Linear Sensors  -  PT 100 normally referred to as RTD’s is the most common and when used in a standard 3 wire configuration or a 4-wire bridge configuration and can be used to measure actual temperature with accuracy with lead resistance compensation. This is the default sensor fitted in MV motors and most MV protection relays have PT 100 inputs

When would one use over temperature detection?

It would be ideal to fit over temperature detectors to every motor but the cost of cabling to connect the sensors in the field is high. A definite application will be when the motor is force cooled by external motorized fan or by circulation of water through water jackets, since the thermal model applied in the overload protection relay will be modeling the predicted motor temperature on the I²t model assuming that the shaft mounted fan will be operating efficiently and only altering the thermal decay model for “Motor running” standard 2 x Motor heating time constant or “motor cooling” standard 4 x Motor heating time constant. 

How and where should temperature sensors be fitted?

Generally, the PT 100 sensors are inserted into the winding slots with the stator winding. This is a very secure and stable location and being in a flat thin wafer type construction will expose a large surface area to the winding.

A point to remember is that the temperature measured in the slot will be up to 20°C lower than the hot spot temperature measured on the bottom of the stator winding overhang (reflected heat from rotor without having to pass through stator core laminations heat sink.

Locked rotor and stall conditions can not be protected with temperature sensors the thermal overload curve based on I²t characteristic and full thermal memory will be the first line of defense.

The temperature sensors can be very useful in modifying the thermal memory of the thermal curve and allow actual temperature to establish the available thermal capacity should the thermal model want to restore more capacity than is actually available.

PTC or NTC temperature sensors are bead type construction with leads attached and the entire sensor connection construct insulated with a heat shrink sleeve.  Theses sensors are ideal to fit into the stator-winding overhang and are secured in position with insulation putty with a good temperature coefficient to ensure rapid heat transfer.

3.3       Additional System protection features

• Short circuit

• Earth Fault

3.3.1    Short circuit

What is short circuit protection?

A short circuit condition occurs when the insulation between phase conductors fails and the load current level exceeds the normal locked rotor current levels that the motor would draw during start up.

The Fault level of the motor feeder supply determines the current level during a short circuit and will be as high as 15 x to 50 x the motor full load current value.

Whenand how can one implement a short circuit protection?

A short circuit fault is a system level fault, which means that it will compromise the stability of the entire network it will result in total power loss to healthy drives and shut down the entire process.

Fundamental in clearing a short circuit fault is grading and use of equipment that can interrupt the high-energy fault.

The main contactor on the motor drive is not suitable since at best a contactor should not try clear a fault current > 10 x contactor rating and upstream a MCCB or fuse should be the first line of defense.

Fuses are not as popular as in past but do provide a ideal solution since they do limit the fault level allowed through to the motor feeder circuit and the motor fuse (rated 200% Motor Ifl) will clear in 1 sec at 15 x motor Ifl and 0,020 sec at 30 x motor Ifl

Using a Motor feeder MCCB is neat clean solution with the ability to be shunt tripped from the protection relay as well as having its own thermal and fast magnetic trips.

The discipline required with maintenance staff is to be disciplined in the inspection of the contact sets and arc chute after clearing a high-energy fault to avoid the possibility of catastrophic failure on clearing the next fault.

The MCCB is a maintenance specific item that can still appear to be functional but is not and will result in a catastrophic failure on next fault clearance, hence my default selection of a fuse in applications that are critical.

3.3.2    Earth Fault

What is an earth fault?

An earth fault condition occurs when the insulation between a phase conductor and earth fails. The fault level of the motor feeder supply determines the current level during an earth fault condition and will be as high as 7 x to 30 x the motor full load current value.

When and how can one implement earth fault protection?

An Earth fault is a system level fault, which means that it will compromise the stability of the entire network it will result in total power loss to healthy drives and shut down the entire process.

Fundamental in clearing an Earth fault condition  is grading and use of equipment that can interrupt the high-energy fault.

The main contactor on the motor drive is not suitable since at best a contactor should not try clear a fault current > 10 x contactor rating and upstream a MCCB or fuse should be the first line of defense.

Fuses are not as popular as in past but do provide a ideal solution since they do limit the fault level allowed trough to the motor feeder circuit and the motor fuse (rated 200% Motor Ifl) will clear in 1 sec at 15 x motor Ifl and 0,020 sec at 30 x motor Ifl

Using a Motor feeder MCCB is neat clean solution with the ability to be shunt tripped from the protection relay with sensitive CBCT sensing can provide Earth leakage protection to trip main contactor and once Earth leakage level increases above 1 amp to transfer the trip to MCCB shunt

The discipline required with maintenance staff is to be disciplined in the inspection of the contact sets and arc shuts after clearing a high-energy fault to avoid the possibility of catastrophic failure on clearing the next fault.

The MCCB is a maintenance specific item that can still appear to be functional but is not and will result in a catastrophic failure on next fault clearance, hence my default selection of a fuse in applications that are critical

3.4       IED Control features

• Function blocks
• Starter functional control blocks
• Time and event recording
• The introduction of the IED (Intelligent Electronic Device) has introduced a new dimension of control, sequencing and interlocking to the traditional protection relay and made available functions which traditionally belonged to the PLC (Programmable logic controller) as well as metering function which traditionally belonged to the measurement and billing authorities.
• Inputs/Outputs
• Flexible control of all input and output elements whether they be contact, binary or analog signal
• Serial communications over all the popular protocols be they RS485, Ethernet or fibre. The main feature is not the protocol but the total transparency of the data stored in the IED to be used for fast seamless measurement and control
• The relay shall have selectable setting groups for dynamic reconfiguration of the protection elements due to changed conditions such as system configuration changes, or seasonal requirements.
• Voltage (phasors, symmetrical components), current (phasors, symmetrical components, true RMS values), and frequency.
• Differential and restraint currents shall be available in terms of magnitude and angle for easy testing, commissioning and troubleshooting.
• Selectable Setting Groups
• Metering and Monitoring Functions
• Currents and Voltages

3.4.1    Function blocks

What is a function block in a protection relay?

Traditionally relays have utilized a relay tripping and blocking matrix to route the trip signal to a specific output, which could serve as an input to block an upstream relay during a fault condition. These signals required the physical wiring from the output relay to the input 

Standard function blocks

• Truth Table 3 I/ 1O

• Truth Table 5 I/ 2O

• Truth Table 2 I/ 1O

• Counters
• Timers
• Signal Conditioner
• Non-Volatile elements
• Flashing
• Flickering
• Limit Monitor

 3.4.2    Starter functional control blocks

•  Overload Relay
• Direct Starters
• Reversing Starters
• Circuit Breaker
• Star Delta starters
• Reversing Star Delta
• Dahlander
• Reversing Dahlander
• Two Speed motor Starter
• Reversing Two speed starter
• Motorised Valve control
• Slider valve controller
• Soft Start starter
• Reversing Soft Start starter

 3.4.3    Time and event recording

• Event Logging
• Motor Start Local
• Motor Start PLC
• Motor Start Field Input
• Motor Stop Local
• Motor Stop PLC
• Motor Stop Field Input
• Motor Starting acceleration
• Motor Started running at speed
• Alarm Flag
• Trip Flag
• Setting change
• Reset
• Digital Input change
• Analog input change above / below Tset
• User defined event x4 link to Logic control blocks 
• Every Event, stored the following variables  
• RTC yyyy/mm/dd/hh/mm/ss
• Event
• Actual load current
• Thermal capacity
• Voltage
• Earth Leakage current
• Fault record 
• RTC yy / mm/ dd/ hh/ mm/ ss
• Fault
• Actual load current
• Thermal capacity
• Voltage
• Earth leakage current
• User defined Fault record