22 CHAPTER 3 CAUSES AND EFFECTS OF ELECTRICAL FAULTS 3.1 INTRODUCTION A large number of asynchronous motors are used in industrial processes even in sensitive applications. Consequently, a defect can induce high losses in terms of cost and can be dangerous in terms of security and safety. In the last few years, a great number of works relating to various aspects of failures of the three phase asynchronous motors were carried out. An IEEE transaction entitled, Report of Large Motor Reliability Survey of Industrial and Commercial Installation, Part I included both the results of an IEEE survey and the results of an EPRI survey. A summary of the survey is given in Table 3.1. In spite of different approaches and criteria both studies indicate very similar failure percentage associated with mechanical and electrical related machine failures. Analyzing the data from the Table 3.1, it is concluded that many positions are directly or indirectly related to failures caused by extensive heating of the different motor parts involved in motor operation.
23 Table 3.1 Summary of IEEE and EPRI motor reliability surveys IEEE Study EPRI Study Average Failure contribution % Failed Component % % Persistent Overload 4.20 Stator ground Insulation 23.00 Normal Deterioration 26.40 Turn Insulation 4.00 Bracing 3.00 Core 1.00 Cage 5.00 Electrical Related Total 30.60 Electrical Related Total 36.00 High Vibration 15.50 Sleeve Bearings 16.00 Poor Lubrication 15.20 Antifriction Bearings 8.00 Mechanical Related Total Trust Bearings 5.00 Rotor shaft 2.00 Rotor Core 1.00 30.70 Mechanical Related Total 32.00 High Ambient Temp 3.0 Bearing Seals 6.00 Abnormal Moisture 5.8 Oil Leakage 3.00 Abnormal Voltage 1.5 Frame 1.00 Abnormal Frequency 0.6 Wedges 1.00 Abrasive Chemicals 4.2 Poor Ventilation cooling 3.9 Other Reasons 19.7 Other Components 21.00 Environmental Related Maintenance Related & Other Reasons: Total 38.70 & Other Parts : Total 32.00 Electrical Related Failures 33% Mechanical Related Failures 31% Environmental, Maintenance & Other Reasons Related failures 36% Normally electric motors do not fail suddenly. It happens over time and regular inspection will detect a problem before a serious situation develops. Faults in ACIMs can occur in any of the three main components of the motor such as stator, rotor and bearings. Faults in the induction motor drives can be classified into three different groups: 1. Growing faults with only small effects on the operation.
24 2. Partial non-catastrophic faults with emergency operation possible. 3. Catastrophic faults with total drive breakdown. The causes of faults can be categorized in to two groups: i) Mechanical Causes of Failure: Misalignment Mechanical unbalance Soft foot Bearing fatigue Fractured rotor bars or end rings Overheating Loss of cooling Improper lubrication Electrical Causes of Failure: Poor power quality Resistance and Impedance unbalance Insulation failure Excessive loading and current as follows: The abnormal operating conditions of induction motor are classified i) Mechanical overloads : Sustained overloads, prolonged starting, locked rotor and stalling.
25 i iv) Abnormal supply conditions: Loss of supply voltage, unbalanced supply voltage, phase sequence reversal of supply voltage, over voltage, under voltage and under frequency. Faults in supply: Interruptions in phases, blowing of fuse and short circuit in supply cable. Internal Faults in Motor itself (caused by 1, 2, 3 above): Phase to phase faults, phase to earth faults, failure of phase (open circuit) and mechanical failure. It is preferable to find faults before complete motor failure. This is called incipient fault detection. Often the motor can run with incipient faults, but eventually it will lead to motor failure causing downtime and large losses. Power quality problems that affect the induction motor behavior are voltage sag (torque, power, speed and possible stall), harmonics (losses and torque), unbalance (losses), short interruptions (mechanical shock and possible stall), impulse surges (isolation damage), over voltage (expected life shortening) and under voltage (over heating and low speed). Principally, voltage distortions and amplitude variations cause the present power quality problems. Power supply variations may be classified into three categories: i) Frequency variation from rated i Unbalanced voltage between phases Balanced phase voltage with voltage variation from rated value.
26 Causes of failures in induction motor are as follows: a) Overheating of motor: i) Over load i iv) Under voltage Over voltage Ventilation duct clogged v) Fuse blowing (single phase rotating) vi) Unbalanced three phase voltages It can be seen that 44% of motor failure problems are related to heat. Allowing a motor to reach and operate at a temperature 10 o C above its maximum temperature rating will reduce the motor s expected life by 50%. Operating at 10 o C above this, the motor s life will be reduced again by 50%. The reduction in the expected life of the motor repeats itself for every 10 o C. This is referred to as the half life rule. Although there is no industry standard that defines the life of an electric motor, it is generally considered to be 20 years. The term, temperature rise means that the heat produced in the motor windings (copper losses), friction of the bearings, rotor and stator losses (core losses) will continue to increase until the heat dissipation equals the heat being generated. b) Abnormal electromagnetic sound: i) Single phasing i iv) Short circuit in windings Unbalanced air gap resulted from serious bearing wear Overheating of motor
27 c) Speed fall: i) Voltage drop i Sudden overload Single phase rotating Motor life depends on electrical, thermal, mechanical and environmental stresses. This thesis defines the loss of motor life as the reduction in stator winding insulation life due to thermal stresses considering single aging effect. Since the rotor is of squirrel cage type, there is no rotor insulation and hence the rotor structure is more robust. Hence, motor life is focused on stator winding insulation. This thesis examines the loss of motor life when it is supplied by unbalanced voltages in combination with over and under voltages, balanced under and over voltages, frequency variations, single phasing, overload and ground fault. Electrical faults are usually related to insulation failure. They are generally known as phase to ground or phase to phase faults. It is believed that these faults start as undetected turn to turn faults which finally grow and culminate into major ones. Almost 30-40% of all reported induction motor failures fall in this category. 3.2 OCCURRENCES OF ELECTRICAL FAULTS DURING OPERATION The squirrel cage induction motors are the most popular AC motor used in agriculture firms and industrial applications due to their reliability, low cost and high performance. However these motors experience several types of electrical/incipient faults. The main faults considered in this analysis are: Unbalanced supply voltage, Single phasing, Phase reversing, Over and
28 under voltages, Over load, Earth fault and Power frequency variations. All the above faults affect the motor performance and reduce the motor lifetime. 3.2.1 Unbalanced Supply Voltages Since the three phase induction motors have been widely employed in industrial, commercial and residential systems, the supplied three phase voltage significantly affects their operating performance. When the supplied three phase voltage is unbalanced, the startup transients, dynamic performance and steady state characteristics of the three phase induction motor will vary accordingly. The positive sequence voltage is developed during normal balanced operating condition of induction motor. When supply voltage is unbalanced, the negative sequence voltage is developed. The positive sequence voltage produces a positive torque, whereas the negative sequence voltage gives rise to an air gap flux rotating against the forward rotating field, thus generating a detrimental reversing torque. This negative sequence voltage produces a flux in the air gap rotating opposite to the rotation of the rotor, tending to produce high currents. When neglecting non-linearity, due to saturation, the motor behaves like a superposition of two separate motors, one running at slip s with terminal voltage V p per phase and the other running with a slip of (2-s) with terminal voltage of V n. The result is that the net torque and speed are reduced and torque pulsations are produced. Also, due to the low negative sequence impedance (R 2 / (2-S)), the negative sequence voltage gives rise to large negative sequence currents. The currents at normal operating speed with unbalanced voltages will be greatly unbalanced in the order of approximately 6 to 10 times the voltage unbalance. This introduces a complex problem in selecting the proper
29 overload protective devices particularly. Because devices selected for one set of unbalanced conditions may not be inadequate for a different set of unbalanced voltages. Hence, increasing the size of the overload protective device is not a solution. Figure 3.1 identifies the potential troubles which can result in voltage unbalance. The effects of voltage unbalance are stator and rotor heating with the ultimate failure mode being stator winding failure or bearing failure due to bearing lubricant overheating or rotor vibration. Further failure can only be prevented by derating the machine according to standards, allowing it to operate within its thermal limitations. Causes of unbalanced supply voltages are given below: i) Open delta transformers i iv) Unbalanced loading Unequal tap settings High resistance connections v) Shunted single phase load vi) Unbalanced primary voltage v Defective transformers In general, following effects will occur due to voltage unbalance: i) Reduction in motor efficiency i iv) Increase in stator and rotor copper losses Rise in temperature Serious reduction in starting torque v) Nuisance and overload tripping vi) Premature failure of motor winding
30 v vi ix) Excessive and unbalanced full load current Shaft vibration and noise Speed and torque pulsation x) Reduction of motor output torque POTENTIAL TROUBLES EFFECTS RESULTS Utility Distribution System Unbalanced Income Line Reduced usage of Transpositions in High Voltage Transmission Lines Primary Open Delta Transformer Bank Plant Distribution System Large Single Phase loads Unbalanced Three phase Lighting Loads Unequal conductor Impedance Blown Fuse on a Three phase Bank of P.F. Correction Capacitors Plant Expansions Without additional Distribution capacity Voltage Unbalance (Negative sequence effects) Stator heating Rotor heating Electrical Failures (33%) Rotor bar heating (Loose bars) Shaft/ Bearing Lubricant Over heating Stator Winding Failure Stator Induced current Bearing failure Rotor Vibration Mechanical Failures (31%) Figure 3.1 Causes and effects of unbalanced voltages
31 Unbalanced Supply voltage can further divided into two groups: Under Voltage Unbalance: It is defined as unbalance due to the positive sequence voltage lower than rated value. Over Voltage Unbalance: It is defined as unbalance due to the positive sequence voltage higher than the rated value. They are: Many unbalance voltage conditions are possible in electrical system. i) Single phase under voltage unbalance i iv) Two phase under voltage unbalance Three phase under voltage unbalance Single phase over voltage unbalance v) Two phase over voltage unbalance vi) Three phase over voltage unbalance In this thesis, all the above six types of unbalanced conditions are considered for analysis. 3.2.2 Single phasing The term single phasing means one of the phases is open. A single phasing condition subjects an electric motor to the worst possible case of voltage unbalance. If a three phase motor is running when the single phase condition occurs, it will attempt to deliver its full horsepower, enough to drive the load. The motor will continue to drive the load, until the motor burns out or the properly sized overload elements take the motor off the line.
32 When an induction motor loses one phase its slip increases but it is does not stall unless the resulting single phase supply voltage is below normal or the shaft load is more than 80% of full load. The losses increase significantly when loaded near or above its rating. Single phasing is a hazardous condition and steps should be taken to de-energize the motor. For lightly loaded three phase motors, say 70% of normal full load amperes, the phase current will increase by the square root of three under secondary single phase conditions. This will result the motor to draw a current of approximately 20% more than the name plate full load current. If the overloads are sized at 125% of the motor name plate, circulating currents can still damage the motor. That is why it is recommended that motor overload protection should be based upon the actual running current of the motor under its given loading, rather than the name plate current rating. When the motor is single phased, the current in the remaining two phases increase to 173% of normal current. Causes of single phasing are given below: i) Open winding in motor i Any open circuit in any phase anywhere between the secondary of the transformer and the motor Primary fuse open. The potential troubles which can lead to single phasing are shown in Figure 3.2. The effects of single phasing on three phase motors vary with service conditions and motor thermal capacities. The effects are similar to those identified for unbalance voltages, since single phasing represents the worst case of an unbalance voltage condition. An additional effect is the
33 remaining phase windings experience excessive over heating, there by creating a greater potential for stator winding failure. POTENTIAL TROUBLES EFFECTS RESULTS Utility Distribution System Transmission line Out on one phase Electrical Failures (33%) Utility fuse blown In one phase Transmission line Fault on one phase Plant Distribution System Unbalanced voltage Source Single phased Motor control Blown motor Winding lead Blown Fuse in one phase Disconnect switch Blade Malfunction in one phase Cable fault in One phase Cable disconnected In one phase Over heated/loose Electrical connection In one phase Single Phasing (Maximum Voltage Unbalance, Negative sequence effects) Remaining Phase windings experience Excessive over heating Rotor heating Rotor bar heating (Loose bars) Shaft/ Bearing Lubricant Over heating Stator Induced current Rotor vibration Stator Winding Failure Bearing failure Mechanical Failures (31%) Figure 3.2 Causes and effects of single phasing
34 3.2.3 Phase reversal The phase reversal occurs when two of the three phases (RYB) of supply line reverses. Most of motor will react very badly to such a situation. Motor suddenly in the wrong direction, causing major collateral damage. 3.2.4 Over and under voltages Over and under voltages are caused by load variations on the system. Over loaded circuits result in under voltages. The under voltage fault occurs when the motor is supplied by reduced voltage with rated mechanical load. An under voltage is a decrease in the RMS ac voltage to less than 90% at power frequency for duration longer than 1 min. Examples include load switching, capacitor bank switching off, Overloaded circuits, etc., Under voltage faults result in increased currents, excess heating of machine and increased stator and rotor losses. If any one of the line voltage in the motor is greater than 110% of rated value, over voltage fault occurs. An over voltage is an increase in the RMS voltage greater than 110% at power frequency for duration longer than 1 min. Examples include load switching, incorrect tap settings on transformers, etc., Over voltage faults result in harmful effects on machine insulation. 3.2.5 Overload When there is increase in mechanical load on the motor beyond the rated value, overload situation occurs. Due to high load torque, motor begins to draw more current. Over load condition results in increased phase currents, overheating of machine and high stator and rotor copper losses. Figure 3.3 illustrates the potential troubles which can result in over loading effects.
35 POTENTIAL TROUBLES EFFECTS RESULTS Utility Distribution System Voltage low on Incoming line Plant Distribution System Excessive cycling Or pulsating load Continuous Overloading Prolonged Acceleration Time Motor load Mechanical Troubles Low voltage Supply Repeated starting Repeated stalling Stator Winding Overheating Electrical Failures (33%) Insulation Deterioration Cracking Over heating Tracking Pulverizing Turn to turn short Coil to coil short Phase to phase short Coil to ground short Mechanical stresses on Iron winding end damage turns and individual coils Mechanical Failures (31%) Stator Winding Failure & Potential Iron core Damage Figure 3.3 Causes and effects of over loading The majority of troubles only generate stator winding over heating. Whereas two troubles: repeated starting and repeated stalling generate mechanical stresses on winding end turns and individual coils. These mechanical forces, generated via the production process (starting, stopping, etc.,) result in physical stress on the end turns of the copper winding, thereby increasing the potential for cracking and conductive paths. Another possibility is a substantial increase in insulation temperature as a result of
36 motor jogging operations or repeated starts within a short time period. This can be the result of normal production, start up of the process line or the implementation of new production equipment. 3.2.6 Ground fault Ground fault is more prevalent in motors than other power system devices, because of the violent manner and frequency with which they are started. These faults are detected by observing zero sequence current. The ground fault is monitored and detected by measuring leakage current. Almost 80% of the electrical faults in low voltage distribution systems are line to ground faults. Monitoring leakage to ground is a predictor of an impending ground fault. Most faults start with leakage to ground. When these leakage currents begin to increase, this eventually will lead to a ground fault. Early detection keeps the damage at the fault point very low. Therefore the cost and time required to repair the equipment is also significantly reduced and the fault damage will be limited. The fault can be easily located and repairs can be scheduled. Periodic monitoring of ground leakage current is a predictor of incipient failure and can be used effectively in preventive maintenance program. Ground fault sensing is normally applied on the line side of the drive. Aging and thermal cycling cause a decrease in dielectric strength of the insulation in the stator winding. This can produce a low impedance path from the supply to ground resulting in ground fault currents which can be quite high in solidly grounded systems. In resistance grounded systems, there is a resistance is series with the supply source to limit ground fault current and allow the system to continue operating for a short time under fault conditions. The fault should be located and corrected as soon as possible, however, since a second fault on
37 another phase would result in a very high current flow. In addition to damaging the motor, a ground fault can place the motor casing above ground potential thus presenting a safety hazard to personnel. On the occurrence of a ground fault caused by insulation break down, an unprotected motor will commonly suffer severe structural damage and have to be replaced. The fault could also shut down the power supply bus to which the faulty motor is connected. Ground faults occur in good motors because of environmental conditions. Moisture or conductive dust, which are often present in mines, can provide an electrical path to ground thus allowing ground fault current to flow. In this case, ground fault protection should shut down the motor immediately so that it can be dried or cleaned before being restarted. The amount of current flow due to fault depends on the location of fault in the motor winding. A high current flow will result if a short to ground occurs near the end of the stator winding nearest the terminal voltage. A low ground fault current will flow if a fault occurs at the neutral end of the winding, since this end should be a virtual ground. This low level of ground fault pickup is desirable to protect as much of the stator winding as possible and to prevent the motor casing from shock hazard. The two types of insulation failures occurred in a motor are: turn to turn and turn to ground. The amount of phase current unbalance is a very good indication of the turn-turn insulation conditions. Turn-turn insulation failure is a prelude to most insulation failure in motors and normally occurs before a fault propagating to a turn to ground failure. Therefore, in the proposed system current unbalance protection is included to avoid turn-turn insulation failure. Ground fault current leads to insulation failure in motor. Therefore, a considerable amount of attention is given to the ground current levels
38 available in the system. Very low level ground fault currents between 100mA to 2A can be used only as a possible indication of fault since capacitive charge currents can also indicate a false ground fault. A ground fault that propagates to a threshold level of 5A or more is imminent indication of a motor failure. Ground fault results in hazards for human safety, thermal stress due to fault current, voltage stress, interference with telecommunication and interruption of power supply. 3.2.7 Power frequency variations Power frequency variations are defined as deviation of the power system fundamental frequency from its specified nominal value. No Load, locked rotor and full load currents vary inversely with a change in applied frequency. Locked rotor, minimum pull up and break down torques vary inversely as the square of the frequency change. An increase in motor torque and /or speed with decrease in power frequency may damage the driven machine. A decrease in motor torque and/or speed with increase in power frequency may cause a reduction in output of the driven machine and slight increase in power factor. 3.3 CONCLUSION Poor power quality (unbalanced supply voltage, single phasing, over and under voltages), over load and earth fault will cause the motor to imbibe more current. Due to the above operating conditions, there is increase in the stator and rotor losses, reduction in efficiency, rise in temperature, shaft vibration and noises, etc., in induction motor. Phase reversal will cause a major collateral damage to the motor driven system. Over load condition leads to heavy temperature rise in the stator winding. Motor currents and torques vary inversely with frequency. All the above electrical faults affect the life of the stator winding insulation.