Types of 3 phase induction motor protection

 The three-phase asynchronous motor – also known as the 3 phase induction motor – is the most frequently used motor type for industrial application.

Especially in the form of a squirrel-cage induction motor, it dominates the field of industrial electrical drive technology.

Hence, motor protection is used to prevent damage to the electrical motor, such as internal faults in the motor.

Also external conditions when connecting to the power grid or during use have to be detected and abnormal conditions must  be prevented.

Additionally, the protection relay prevents the disturbance to spread back into the grid.

The motor thermal limits curves consist of three distinct segments, which are based on the three running conditions of the motor: the locked rotor or stall condition, motor acceleration and motor running overload.

Ideally, curves  should  be  provided  for  both  hot  and  cold  motor conditions. For  most  motors,  the  motor  thermal  limits  are  formed into one smooth homogeneous curve. 

The acceleration curves are an indication of the amount of current and  associated  time  for  the  motor to accelerate  from  a  stop condition to a normal running condition. 

Usually, for large motors, there are two acceleration curves: 

the first is the acceleration curve at rated stator voltage while the second is the acceleration at 80% of rated stator voltage (soft starters are commonly used to reduce the amount of inrush current during starting). Starting the motor on a weak system can result in voltage depression, providing the same effect as a soft-start)

The primary protective element of the motor protection relay is the thermal overload element and this is accomplished through motor thermal image modeling. 

This model must account for all thermal processes in the motor while motor is starting, running at normal load, running overloaded and if motor is stopped.

The algorithm he thermal model integrates both stator and rotor heating into a single model.

If the motor starting current begins to infringe on the thermal damage curves or if the motor is called upon to drive a high inertia load such that the acceleration time exceeds the safe stall time, custom or voltage dependent overload curves may be required.

Negative  sequence  currents  (or  unbalanced  phase  currents)  will cause  additional  rotor  heating  that  will  not  be  accounted  for  by
electromechanical  relays  and  may  not  be  accounted  for  in  some electronic protective relays.

The main causes of current unbalance 3 phase motor are:

• blown  fuses,  

• loose  connections,  

• stator  turn-to-turn  faults,

• system voltage distortion and unbalance, as well as external faults.

Thermal models can have following enhancements and additions: motor  start  inhibit;  standard,  custom  and  voltage  dependant overload  curves;  thermal  model  biasing  by  measured  current unbalance and RTD’s; separate thermal time constants for running and  stopped  motor  conditions;  independent  current  unbalance detector; acceleration limit timer; mechanical jam detector; start
and restart supervision

Motor differential Protection

This  protection  function  is  mostly  used  to  protect  induction  and synchronous motors against phase-to-phase faults.

This function requires  two  sets  of  CT’s,  one  at  beginning  of  the  motor  feeder, and  the  other  at  the  star  point .

Differential  protection  may  be considered the first line of protection for internal phase to phase or  phase  to  ground  faults.

In  the  event  of  such  faults,  the  quick response  of  the  differential  element  may  limit  the damage  that may have otherwise occurred to the motor.
     
The differential protection function can only be used if both sides of  each  stator  phase  are  brought  out  of  the  motor  for  external connection such that the phase current going into and out of each phase  can  be  measured. 

The  differential  element  subtracts  the current coming out of each phase from the current going into each phase and compares the result or difference with the differential pickup level.

If this difference is equal to or greater then the pickup level  a  trip  will  occur.  GE  Multilin  motor  protective  relays  support  both  three  and  six  CT  configurations.

For  three  CT  configuration both sides of each of the motors stator phases are being passed through a single CT. 

This is known as the core balance method and is the most desirable owing to it’s sensitivity and noise immunity. 

If six CTs are used in a summing configuration, during motor starting,the values from the two CTs on each phase may not be equal as the CTs are not perfectly identical and asymmetrical currents may
cause the CTs on each phase to have different outputs.

To prevent nuisance  tripping  in  this configuration,  the  differential  level  may have to be set less sensitive, or the differential time delay may have to  be  extended  to  ride  through  the  problem  period  during  motor starting.

The running differential delay can then be fine tuned to an application such that it responds very fast and is sensitive to low differential current levels. Biased  Differential  protection  method  allows  for  different  ratios for  system/line  and  neutral  CT’s. This  method  has  a  dual  slope characteristic.

To prevent a maloperation caused by unbalances between CTs during external faults.CT unbalances arise as a result CT accuracy errors or CT saturation. 

Ground Fault Protection for induction motor

Damage to a phase conductor’s insulation and internal shorts due to moisture within the motor are common causes of ground faults.

A strategy that is typically used to limit the level of the ground fault current is to connect an impedance between the neutral point of the 3 phase motor  and  ground.

This  impedance  can  be  in  the  form  of  a resistor or grounding transformer sized to ensure that the maximum ground fault current is limited to a level that will reduce the chances of damage to the motor.

There are several ways by which a ground fault can be detected.The most desirable method is to use the zero sequence CT approach, which  is considered  the  best  method  of  ground  fault  detection methods  due  to  its  sensitivity  and  inherent  noise  immunity. 

All phase conductors are passed through the window of a single CT referred to as a zero sequence CT.

Under normal circumstances, the three phase currents will sum to zero resulting in an output of zero from the zero sequence CT’s secondary.

If one of the motor’s phases were shorted to ground, the sum of the phase currents would
no longer equal zero causing a current to flow in the secondary of the zero sequence CT.This current would be detected by the motor relay as a ground fault .

If  the  cables  are  too  large  to  fit  through  the  zero  sequence  CT’s window or the trench is too narrow to fit the zero sequence CT, the residual ground fault configuration can be used.

This configuration is inherently less sensitive then that of the zero sequence configuration, owing to the fact that the CTs are not perfectly matched.

During the  motor  start ,  the  motor ’s  phase  currents  typically  rise  to magnitudes greater than 6 times the motors full load current .

 The slight mismatch of the CTs combined with the relatively large phase current magnitudes produce a false residual current , which will be seen by the relay.

This current can be misinterpreted by the motor relay as a ground fault unless the ground fault element’s pickup is set high enough to disregard this error.

Unbalance Protection of 3 phase induction motor

Unbalance Relay – Reliable Phase Loss Detection

                                                        
Unbalanced load in the case of AC 3 phase motors is mainly the result of an unbalance of  the power  supply voltages.

The  negative-sequence reactance  of  the  three-phase  motor  is  5 to7 times  smaller than positive-sequence  reactance,  and  even  a  small  unbalance  in  the power  supply  will  cause  high  negative  sequence  currents.

For example for an induction motor with a staring current six times the full  load  current. a  negative  sequence  voltage  component  of  1% corresponds to a negative sequence current component of 6%.

The negative-sequence current induces a field in the rotor, which rotates in  the  opposite  direction  to  the  mechanical  direction  and  causes additional temperature rise.

Main causes of current unbalance in motor are:

• system voltage distortion and unbalance, 

• stator turn-to-turn faults,

• blown fuses, 

• loose connections, as well as faults.

Miniature Circuit Breakers MCB for motor protection

Miniature circuit breakers are primarily designed to protect induction motor and lines against overload (thermal) and short-circuit (electromagnetic).

They thus care for protecting this electrical equipment against excessive temperature rises and destruction in the event of a short-circuit.

Miniature circuit breakers are used in distribution networks in homes and in industrial applications.

They meet the requirements for different applications by various designs and with the aid of a comprehensive range of accessories (for example auxiliary and signal contacts etc.).

Short circuit

The  short  circuit  element  provides  protection  for  excessively high overcurrent faults. When a motor starts, the starting current (which is typically 6 times the Full Load Current) has asymmetrical
components.

These asymmetrical currents may cause one phase to see as much as 1.7 times the RMS starting current .

As a result the pickup of the short circuit element must be set higher than the maximum asymmetrical starting currents seen by the phase CTs to avoid nuisance tripping.

The breaker or contactor that the relay is to control under such conditions must have an interrupting capacity equal to or greater then the maximum available fault current .

Under voltage

If an induction motor operating at full load is subjected to an under voltage condition, full load speed and efficiency will decrease and the  power  factor,  full  load  current  and  temperature  will  increase.

The undervoltage element can be considered as backup protection for  the  thermal  overload  element If  the  voltage  decreases,  the current will increase, causing an overload trip.

In some cases, if an undervoltage condition exists it may be desirable to trip the motor faster than the overload element .

The  overall  result  of  an  undervoltage  condition  is  an  increase in  current  and  motor  heating  and  a  reduction  in  overall  motor performance.

Over voltage

When  the  three phase motor  is  running  in  an  overvoltage  condition,  slip  will decrease as it is inversely proportional to the square of the voltage and efficiency will increase slightly.

The power factor will decrease because  the  current  being  drawn  by  the  motor  will  decrease  and temperature rise will decrease because the current has decreased (based  on I^2 t).

As  most  new  motors  are  designed  close  to  the saturation point , increasing the V/HZ ratio could cause saturation of air gap flux causing heating

The overall result of an overvoltage condition is an increase in current and motor heating and a reduction in overall motor performance.

Mechanical rotor Jam

The mechanical jam element is designed to operate for running load jams due to worn motor bearings, load mechanical breakage and driven load process failure.

This element is used to disconnect the motor on abnormal overload conditions before motor stalls.

In terms of relay operation, the Mechanical Jam element prevents the motor from reaching 100% of its thermal capacity while a Mechanical Jam is detected.

 It helps to avoid mechanical breakage of the driven load and reduce start inhibit waiting time.

Load loss detection

Undercurrent protection is useful for indicating the loss of suction in a pump application, or a broken belt in a conveyor application. The second method of load loss detection is to use of the underpower
protection element .

reference: www.GEMultilin.com