Motor Protection Application Considerations

Transcription

1 Motor Protection Application Considerations

2 About the Authors Paul Lerley has 28 years of utility and electronics experience, including 15 years at Central Maine Power Co. He is a graduate of the University of New Hampshire and was Director of Substations Electrical Systems at Central Maine Power prior to joining Basler Electric Company. Mr. Lerley is a Senior Member of the IEEE and a member of four working groups of the Power System Relaying Committee. He has authored articles on testing for the Doble Engineering Conference and Transmission and Distribution magazine. He was previously very active in the Electric Council of New England. Mr. Lerley was a Regional Application Engineer for Basler Electric from 1994 to Mike Young of Sanford, Florida, received his MBA from Rollins College in 1983 and BSET from Purdue University in He worked for Wisconsin Electric Power Company as a Relay Engineer for two years, and for Florida Power Corporation as a Field Relay Supervisor for 21 years. He authored the text "Protective Relaying for Technicians" and co-authored papers for the Georgia Tech Protective Relaying Conference. Mr. Young has been a Regional Application Engineer for Basler Electric since 1994 and is a member of the IEEE. This document contains a summary of information for the protection of various types of electrical equipment. Neither Basler Electric Company nor anyone acting on its behalf makes any warranty or representation, express or implied, as to the accuracy or completeness of the information contained herein, nor assumes any responsibility or liability for the use or consequences of use of any of this information. First printing 4/98

3 Motor Protection Application Considerations 1. INTRODUCTION When applying protective relays to motors or any other equipment, we always ask how much protection is enough. The answer depends on rewind cost, loss of production, effect on downtime, new versus old installation, need for communication, metering, control and the consequences of a motor failure on the electrical system and process. This publication presents an overview of motor hazards and a discussion of detection and protection options. Basler relay models are offered with typical setting value ranges and considerations to help designers and users select Basler relays for motor protection. Most of the protection functions apply to squirrel cage, wound induction motors and synchronous motors. Additional protection is usually provided for synchronous motors and will be mentioned in this document. 2. OVERVIEW OF MOTOR HAZARDS Motor protection is a challenge because there are so many different things that can go wrong with a motor and its associated load: Motor induced Insulation failure (within the motor) Bearing failure Mechanical failure Synchronous motors-loss of field Load induced Overload and underload Jamming High inertia Environment induced High ambient temperature High contaminant level or blocked ventilation Cold or wet ambient conditions Source induced Loss of phase or phases Voltage unbalance Overvoltage Undervoltage Phase reversal Out of step condition resulting from system disturbance Operation induced Synchronizing or closing out of phase High duty cycle Jogging Rapid reversing 3. PROTECTION 3.1 Stator Faults Phase Fault Overcurrent Protection Phase to phase and three phase faults are usually detected with nondirectional 1

4 instantaneous or definite time overcurrent relays. If the available 3-phase fault current is a low multiple of the relay setting (weak system), quick pickup is not assured. Differential relaying should then be considered. Instantaneous relays are typically applicable when the motor rating is less than one-half of the supply transformer KVA rating. The instantaneous phase relay should be set at no less than 1.6 times the locked rotor current using the value of locked rotor current at maximum starting voltage. This setting also assumes the relay is sensitive to the transient overreach (DC offset) of an asymmetrical fault. Lower settings are possible if the relay disregards the transient component or if a time delay longer than the transient time (6-15cy) is added. Verify that the minimum 3-phase fault current at the motor terminals is at least 3 times the relay setting. Fig. 1 illustrates the relay settings in relation to the starting current and the minimum short circuit current Differential Protection Differential protection is used on motors where the available short circuit current is close to the value of locked rotor current. It is also frequently used on very large motors because of its greater sensitivity. Differential protection is always preferred; however, it is generally more costly than instantaneous relaying because all six leads must be brought out of the motor and additional relays may be required. SELF BALANCING The most economical approach is self-balancing differential as shown in Fig. 2. Both ends of the winding are passed through a toroidal current transformer and connected to a 50 device. This CT has a maximum opening around 8 inches that may preclude its use on larger motors. FIGURE 1. Stator short circuit protection with 50 or 50P element. FIGURE 2. Self balancing differential. 2

5 With a fixed ratio of 50:5 and a sensitive instantaneous overcurrent, the self-balancing differential provides a pickup around 5 amps of primary current. This scheme is self-balancing and produces no current for starting or load variation and, because there is only one CT per phase, there is no concern about matching CT performance to eliminate unequal CT saturation. CT saturation is likely for large fault currents but is slow enough to allow the instantaneous relays to operate. PERCENTAGE RESTRAINT DIFFERENTIAL When the toroidal CT cannot be used, the percentage restraint differential circuit (Fig. 3) must be applied. Typically, all 6 CTs are the same ratio and accuracy class. A 2-winding differential relay can be applied with equal currents flowing in the restraint windings for normal load, starting, and external faults. For internal phase or ground faults, all of the current will flow through the operate windings. The scheme will also protect for cable faults between the motor and the motor breaker (52) by using the line side CTs of the breaker. If the motor and motor breaker are supplied separately, be certain to match the CT ratios and accuracy classes when specifying the equipment Ground Fault Protection Ground Sensor 50G The preferred and most sensitive method to detect stator ground faults is with a ground sensor CT. All three phase leads from the motor are passed through the opening of a toroidal current transformer supplying the instantaneous overcurrent 50G relay shown in Fig. 4. This arrangement leaves only the ground fault zero sequence currents in the CT. The typical application calls for a 50:5 CT ratio regardless of the size of the motor. Primary pickup values in the range of 4-12 amps are typical. If more sensitive settings are required, time delay may be necessary to avoid nuisance trips due to zero-sequence cable capacitance current flow during external faults. The ground fault sensor connection may be the only scheme providing sufficient sensitivity when the supply system is high-impedance grounded. If a large ground fault current is available in a solidly grounded system, the 50G relay must operate before the low ratio CT saturates. Fortunately, the low impedance of solid state relays reduces the CT burden. FIGURE 3. Conventional percent differential relay. FIGURE 4. Ground sensor relay and residual ground connection. 3

6 RESIDUAL CONNECTION 51N For larger motors, where the conductors will not fit through a ground sensor CT, the residual ground connection, shown in Fig. 4, must be used. The ground fault relay sensitivity is limited by the phase CT ratio. Since unequal CT performance must be expected, a 51 relay is used to avoid tripping on false residual current. This 51N relay must be coordinated against the 51G system ground protection relay (typically in the supply transformer neutral). In solidly grounded feeder applications, where the ground fault is usually high and the CT quality good, an instantaneous relay (50N) can be added to accelerate the tripping. This relay should be set at 4 x Full Load Current or higher to avoid tripping on starting. 3.2 Thermal Damage SHORT START TIMES When the margin between the maximum start time and the hot stall time is at least 2 to 5 seconds, locked rotor protection can easily be achieved with a definite time overcurrent (50TP) as shown in Fig. 5. By setting this relay close to the Full Load Current, good protection against failure to accelerate is obtained. To prevent the 50TP relay from operating under temporary overloads once the motor is running, it is supervised by the 62 timer. The time delay on the 50TP should be set at the maximum start time plus 25% of the thermal limit margin time. The delay on the 62 timer should be set slightly higher than the 50TP time delay to allow a one or two second window for the locked rotor protection to operate. This protection is easy to implement in the Basler 851 and MPS multifunction relays Locked Rotor Protection When a motor stator winding is energized with the rotor stationary, stator winding currents may range from three to seven times rated full-load value depending on motor design and supply system impedance. Actual values of locked rotor currents are part of the motor data supplied by the motor manufacturer. Heating in the stator winding, proportional to I 2 t, is 10 to 50 times rated conditions and the winding is without benefit of the ventilation normally produced by rotation of the rotor. Depending on the design, a motor may be stator limited (thermally) or rotor limited (thermally) during locked-rotor conditions. The motor manufacturer can furnish the allowable locked-rotor time only after the motor design is completed. This is given as time at rated lockedrotor current starting from either rated ambient temperature or rated operating temperature also referred to as cold stall time or hot stall time. It also is given as part of the motor time-current curve defined by IEEE Standard Starting times depend on motor design and load torque characteristics and must be determined for each application. Although starting times of 2 to 20 seconds are common, high inertia loads may take several minutes to bring to full speed. Starting time is increased if bus voltage is less than nominal. FIGURE 5. Locked rotor protection Short starting times. Another approach often used with singlefunction relays is shown in Fig. 6. The 50S or 12 (speed switch) device is used to supervise the 51S relay which is set for locked rotor protection. The speed switch is set at 10%-50% of full speed and the 50S is set about 85% of Locked Rotor Current (at minimum allowable voltage). 4

7 The 51S should be set between the hot stall time and the start time. The 51P relay is a second 51 relay set for running thermal overload. If no transient overloads are expected, the 51P and 50S relays may not be required. The 51S will then provide starting and running protection. When the start times approach or exceed the maximum safe stall time, protection against locked rotor requires a 51S relay that must be prevented from tripping soon after the motor has successfully started as shown in Fig. 7. (The 51S contact is likely to close due to the intensity of the starting current following the locked rotor current.) A speed switch set at 10%-50% of nominal speed or a 50S relay set at about 85% of Locked Rotor Current (at minimum voltage) are commonly used to supervise the 51S relay. The 51S curve must be set to operate below the hot stall time. When the motor starts successfully, the 12 or 50S device drops out and prevents the 51S from tripping the breaker. If the motor starts but does not accelerate to nominal speed, this protection may not trip since the 51S relay is cut out early in the start sequence. The failure to accelerate would have to be detected by the thermal overload 51P, set for running conditions (shown in Fig. 6). However, the 51S may be used to alarm for subsequent overloads, including failure to accelerate once the motor has started. FIGURE 6. Locked rotor protection short start times Single function relays. LONG START TIMES The starting current of a motor falls between the locked rotor value when the rotor begins to turn. Therefore, the stator heating is reduced when the motor accelerates. For some large induction motors with low starting voltages or with high inertia loads and long starting times, the starting time may exceed the allowable locked-rotor time without excessively heating the rotor. FIGURE 7. Locked rotor protection Long start times Thermal Overload Protection STATIC REPRESENTATION The life of the motor is reduced if the winding temperatures are allowed to exceed their insulation class levels for a significant time. It is usually assumed that for every 10 degrees C above the design temperature limit the life of the motor is reduced by a factor of 2. 5

8 When normal cooling and ambient temperatures are present the temperature of the stator winding is directly related to the stator current, and the running thermal overload limit can be stated on a time-current plot as recommended in IEEE STD 620. Running thermal overload can thus be provided by an overcurrent relay which has a time-current characteristic similar to the thermal overload limit. The Minimum Pick Up of this relay is the continuous overload specification of the motor, i.e. the (Full Load Current)x (Service Factor). The characteristic is usually an I 2 t curve. The time dial is chosen to coordinate against the thermal limit and allow short duration overloads predictable from the process analysis. Fig. 8 shows two IEEE device numbers (51 or 49). These devices may have nearly identical static characteristics, but will differ in their dynamic response and, therefore, in their ability DYNAMIC CONSIDERATIONS In order to force the static characteristic to pass through point P in Fig.8, the user adjusts the time dial in a 51 relay or the time constant in a 49 relay. These terms imply that the dynamic response of a 51 element is linear, whereas the 49 element has an exponential response. When the 49 element, found in dedicated motor relays (as opposed to general purpose overcurrent relays) takes the load level into account, it becomes a realistic thermal model of the motor. In this case, the 49 element does not reset to zero when the current is below the overload limit (as does the 51 relay) but settles at percent of pickup value corresponding to the used thermal capacity at the given load level. Fig. 9 compares the 51 and 49 response for nearly identical static settings. The 51 relay is a more conservative choice since it tends to trip faster than the 49 relay. The 51 is an acceptable choice for any process where temporary overloads are abnormal. The 49 is preferred when the process requires the tolerance of temporary overloads. FIGURE 9. Compare 51 and 49 dynamic response. to track the motor temperature over time. FIGURE 8. Thermal overload Running. TEMPERATURE SENSING Motors are typically cooled by means of a rotormounted fan blade that forces air through the motor frame while the motor is running. Thermal limits and temperature rise are based on this cooling functioning as designed with a known level of ambient air temperature. If normal 6

9 cooling is blocked, overheating at normal load current is possible. The only protection will be temperature-measuring devices located in the motor such as RTD s or thermocouples. Basler MPS100 and 200 series relays provide this protection with inputs from RTD s or thermocouples imbedded in one or more of the winding slots. The MPS relays monitor the RTD resistance and accept two setting levels for each monitored point: a low setting for alarm and a high setting for shutdown. The specific settings are derived from the winding insulation class, defined in NEMA MG-1, and judgment based on the plant operating conditions. The recommended setting for alarm temperature level is the sum of the maximum ambient, plus 10 degrees hotspot allowance, plus the full load temperature rise. This value should be below the insulation class rating. The trip level can be up to 50 degrees C above the class rating if the process is critical, since the loss of life from occasional short overload periods is insignificant. Setting the trip temperature at the insulation class limit is a conservative setting Repetitive Starts and Jogging Protection In repeated starting and intermittent operation very little heat is carried away by the cooling air produced by a turning rotor. Repeated starts can build up temperatures to dangerously high values in either stator or rotor windings unless enough time is provided to allow the heat to be dissipated. The NEMA MG (Motor Guide) sections 12.50, and provide guidelines for typical installations. These standards allow two starts in succession, coasting to reset between starts with the motor initially at ambient temperature, and for one start when the motor is at a temperature not exceeding its rated load operating temperature. This assumes that the applied voltage, load torque during acceleration, method of starting, and load inertia are all within values for which the motor was designed. The application and protection of motors having abnormal starting conditions must be coordinated with the manufacturer. The Basler MPS relays have protection for too many starts. The user selects a setting for number of starts and time period to match manufacturer recommendations. Exact determination of starting frequency is a very complex calculation that is affected by many factors including motor size, enclosure, voltage, ambient temperature, inertia, load-speed-torque characteristic, and running time. Motor restarts will typically depend more on the stator thermal capacity than on rotor thermal capacity and stall time. The best rule, by far, is to minimize the number of starts since each start reduces the life of the motor. Motors protected by Basler MPS relays include a protective element for thermal overload protection. Unlike their inverse time electromechanical counterparts, these relays can remember the stored value of the accumulated thermal capacity. Motor starting alone may use up 50%-65% of the available thermal capacity. These multifunction devices also recognize a stopped motor will cool slower than a running motor because there is no cooling air produced by the rotor. Therefore, it is possible that attempting to start a motor twice in rapid succession may cause a protective trip on thermal overload. However, we should still adhere to the manufacturer s recommendation for frequent starts Unbalance Protection CAUSE AND EFFECTS Unbalance in the feeder phase voltages or motor winding impedance will cause unbalanced currents to flow to the motor. The negative sequence current from the unbalance will cause rotor heating and additional copper losses in the stator windings due to an increase line current. Due to the low negative sequence motor impedance the % negative sequence current is typically about five times larger than the % negative sequence voltage. Unbalanced conditions must be detected to avoid thermal damage to the running motor. DETECTION Although the current unbalance is the parameter directly responsible for the temperature increase in the motor, two detection methods are available: voltage and current unbalance. 7

10 Voltage Sensing (47) This method has the advantage of detecting the unbalance voltage for a complete bus to which several motor loads may be connected, but has the disadvantage of requiring that all motors be tripped when an unbalance exceeds the setting. The bus voltage unbalance may be tolerated by a motor if its load is lighter at the time of the unbalance. Two common measuring techniques have been implemented: the NEMA defined unbalance and negative sequence voltage measurement. The NEMA definition, found in MG1 is: %Unbalance=(Max Deviation from Avg.)/Avg. The negative sequence voltage is usually defined in % of nominal voltage. Current Sensing (46) Current unbalance is measured in the motor feeder itself and has the advantage of being adapted to each motor. It is easy to implement in multifunction and dedicated motor protection relays. Measuring algorithms include the true negative sequence measurement and the difference between the maximum and minimum phase currents. SETTINGS Voltage Relay NEMA recommends in MG1 that continuous voltage unbalance should never exceed 5%. For small to moderate unbalance, the NEMA and negative sequence formulae yield approximately the same result. A voltage unbalance relay can, therefore, be set at an MPU of 5%. To set the time delay to trip, consider the thermal damage by the corresponding negative sequence current. To this voltage unbalance of 5% corresponds an I 2 of about 25%, provided the voltage is measured at the motor terminals. Assuming the motor can tolerate I 2 t=k, the maximum time delay for a 5% voltage unbalance and K=40 would be 640 seconds. Although no standard exists for motors, a value of K=40 is often used. Unfortunately the 47N relay does not offer an extremely inverse characteristic that could emulate the I 2 t characteristic. It is suggested to base the time delay on the worst case expected unbalance, i.e. open phase in the motor feeder cable. The positive and negative sequence currents are then equal (1pu at full load). The trip time for this unbalance condition would thus be equal to K (I 2 =1pu). For K=40, the maximum delay for an open phase should be 40 seconds. If the relay uses a definite time, this will have to be the setting, and result in overprotection if the unbalance is less severe. If the timing curve is inverse, the time dial should be selected to cause tripping when the voltage unbalance, at the motor terminals, corresponding to the 1 pu I 2 is equal to 20%. In most applications the voltage seen by the 47N will not come from the motor terminal, but from the bus. Depending on the size and nature of other loads (static Vs motor) connected to the bus, the 47N may not sense the open phase in the motor feeder. Therefore, 47N application requires careful analysis. Current relay In order to relate the current unbalance MPU setting to the 5% NEMA voltage unbalance limit, it is necessary to establish the correlation between the current unbalance algorithm and the unbalanced voltage. For a negative sequence type element, the I 2 % MPU setting is approximately 5 times the % voltage unbalance for the worst case nominal load condition. For other algorithms, the Instruction Manual must be consulted. The current unbalance measuring elements have an I 2 t=k like characteristic which makes the time delay settings easier to apply than with the voltage relay. If no other information is available choose K=40. The worst case unbalance occurs for an open phase at full load. The negative sequence current is then equal to the positive sequence current, i.e. 1 pu. The time dial should be set to cause tripping in 40 seconds in this case where K= Abnormal Supply According to the NEMA MG section 20.45, motors are generally expected to operate successfully under running conditions at rated load with a variation of plus or minus 10% of rated voltage, plus or minus 5% of rated frequency, or a combination of the two, provided the sum of the absolute values of the deviations does not exceed 10% and the frequency variation does not exceed plus or minus 5%. For synchronous motors, rated excitation current must be maintained. 8

11 Fig. 10 shows the effects of voltage and frequency variations on induction motor characteristics. Given these limits, there is no one protective device that can make a direct determination of these quantities simultaneously. However, variation in voltage or frequency will usually result in an increase in stator winding temperature over a long period of time. Direct temperature measuring devices, such as RTDs, will detect the change and provide adequate warning or tripping, provided the abnormal condition is not extreme. A large induction motor rotating at rated speed or a large synchronous motor with fixed excitation may be approximated at steady-state conditions as a constant kilovoltampere device for a given shaft load, and, therefore, current variations follow voltage variations inversely. An undervoltage condition will result in an overcurrent condition. Single phase over- or undervoltage is likely to be detected by unbalanced voltage or current protection if so equipped. Three-phase undervoltage will be protected by thermal overload protection since the current will be higher than normal for a given load. Voltage relays, per se, are generally not always sensitive enough to provide reliable protection, especially on busses where several motors are connected, since the spinning motors will support the voltage on the low or missing phase. However, an inverse time or definite time undervoltage relay is recommended to trip when a prolonged undervoltage condition exists and as a backup. Pickup settings of per unit will provide adequate protection. The time delay should be set slightly longer than the maximum starting time with minimum allowable voltage to ensure undervoltage will not trip for a start. A separate concern of undervoltage is its impact on starting a motor. Unlike a running motor, low voltage on starting of a motor produces lower starting current and, hence, lowers torque. If the torque is too low to overcome the torque requirements of the load, the motor will not successfully start. The MPS210, equipped with control functions, checks the supply voltage before starting; if the voltage is too low, the relay prevents starting Voltage Drop During Starting Another concern during motor starting is the voltage drop caused by the locked rotor current flowing through the supply transformer. A weak system or undersized supply transformer will only aggravate the situation. When the supply voltage decreases during start, then so does the current and starting torque. If there are other running motors on the bus, the reduced voltage will cause higher currents and further increase the voltage drop. Should the voltage drop low enough, it is possible for the motor torque to be low enough to prevent a successful start of the motor. Whether motor starting or system weakness is the problem, reduced voltage may cause trouble at times other than during acceleration. Reduced voltage running will cause overheating with time. Short term voltage dips may also cause an already running motor to stall. The user should also consider the effect of trying to start more than one motor at the same time, which will only aggravate the undervoltage condition. Many motors use motor contactors powered by the ac line voltage. Reduced voltage could drop out the motor contactor and cause an already running motor to be dropped off line when the motor contactor drops out. Voltage drop calculations should be performed to determine what the motor voltage conditions will be during starting. The calculation should be checked at maximum and minimum expected bus voltage before start. In a properly designed power system, with a good match between bus and motor design voltages, starting voltage dips of 15%-20% are not uncommon. Designers frequently assume that an accelerating motor draws its full voltage inrush current and calculate the upstream voltage drops on that basis. Clearly, any voltage drop in the supply system means that full voltage and corresponding inrush current cannot be present Reduced Voltage Starting When voltage drops are excessive during starting, reduced voltage starting techniques may be employed. These add to the motor controls but may be less expensive than changing transformers and cables. All of these tech- 9

12 Characteristic Voltage Frequency 110% 90% 105% 95% Torques* Increase 21% Decrease 19% Decrease 10% Increase 11 % Starting and Max Running Speed Synchronous No Change No Change Increase 5% Decrease 5% Full-load Increase 1% Decrease 1.5% Increase 5% Decrease 5% Percent Slip Decrease 17% Increase 23% Little Change Little Change Efficiency Full-load 3/4-load 1/2-load Power Factor Full-load 3/4-load 1/2-load Current Starting Full-load Temperature Rise Max Over-Load Capacity Magnetic Noise Increase 0.5 to 1 point Little Change 1 to 2 points Decrease 3 points Decrease 4 points Decrease 5 to 6 points Increase 10 to 12% Decrease 7% Decrease 3 to 4 C Decrease 2 points Little Change Increase 1 to 2 points Increase 1 point Increase 2 to 3 points Increase 4 to 5 points Decrease 10 to 12% Increase 11% Increase 6 to 7 C Slight Increase Slight Increase Slight Increase Slight increase Slight Increase Slight Increase Decrease 5 to 6% Slight Decrease Slight Decrease Slight Decrease Slight Decrease Slight Decrease Slight Decrease Slight Decrease Slight Decrease Increase 5 to 6% Slight Increase Slight Increase Increase 12% Decrease 19% Slight Decrease Slight Increase Slight Increase Slight Decrease Slight Decrease Slight Increase * Torques of an induction motor will vary as the square of the voltage. The speed of an induction motor will vary directly with the frequency. FIGURE 10. The effects of voltage and frequency variation on induction-motor characteristics. niques use some method to apply partial voltage to the motor during the initial starting sequence, then when the motor is at partial speed, full voltage is applied to finish the start sequence. The Basler MPS210 supports reduced voltage starting. Wye-Delta starting applies a reduced voltage at the beginning of the start sequence with a wye connection of the motor and then changes to the delta connection of the motor to complete the start sequence. This arrangement reduces starting torque and voltage drop on the motor bus. Another method of reduced voltage starting is autotransformer start. The autotransformer is connected in wye with the supply voltage and, during starting, the tapped partial voltage is applied to the motor. When the starting contactor makes its transition, the partial voltage source is opened, and full supply voltage is applied. Detailed descriptions of these schemes may be found in the Instruction Manual for the Basler MPS210 relay Frequency Protection Frequency in excess of rated frequency but not in excess of 5% over the rated frequency without a corresponding voltage increase is not considered to be a hazardous condition for synchronous or induction motors provided the driven equipment does not overload the motors at the higher frequency. At decreased frequency without a corresponding voltage drop, the flux requirements of a motor are increased, thus increasing the hysterisis and eddy current losses and heating. Sustained operation at 5% below nominal frequency and rated or overvoltage is not permissible per NEMA MG section Protection against this type of operation is 10

13 typically thermal overload or RTD temperature measurement. However, more refined protection can be obtained with the Basler 81O/U over/under frequency relay. Time delay settings of seconds will allow it to ride though transient conditions without nuisance tripping. Many utility substations are equipped with underfrequency load shedding relays to reduce the system load during a loss of generation and subsequent decay in system frequency. Large motor loads connected to the distribution substation may interfere with the normal operation of the underfrequency relay by allowing it to see a decline in frequency without a complete loss of voltage. This can happen when the distribution bus is disconnected from the supply transformer and the underfrequency relay is connected to the distribution bus. The relay will then see the residual voltage from the motor load and may operate incorrectly. Relocating the underfrequency voltage transformer to the high side of the supply transformer or adding additional time delay to the underfrequency time delay may solve the problem. 3.4 Mechanical or Process Protection Undercurrent We generally think of protective relays as devices that protect electrical equipment. In the case of motor protection, there may be times when they are used to protect the process. For example, the water pumping station that is intended to operate continuously at 90% of full load current. If the pump were to be damaged, lose its prime, or the shaft break, the load on the motor would be drastically reduced. The Basler MPS relay monitors for undercurrent or under power conditions. These elements are not in service until the motor is running and can be set to detect these loss-of-load conditions to alarm or trip Bearing Protection To minimize damage caused by bearing failure, protective devices should be used to sound an alarm or de-energize the motor. Bearing protective devices responsive to one or more of the following conditions may be included: (1) Low oil level in reservoir: (device 71) level switch (2) Low oil pressure: (device 63) pressure switch (3) Reduced oil flow: (device 80) flow switch (4) High temperature: (device 38) thermocouples or resistance temperature detector (5) Rate of temperature rise (6) Vibration (used on motors with anti-friction bearings in place of thermal devices) Large motor bearings are usually monitored by a resistance temperature detector (RTD) which can be used as one of the inputs to the Basler MPS200 or 210 relay. The dual-setpoint of the RTD function of the MPS allows for alarm and trip settings at two different temperatures. 4. BUS TRANSFER AND RECLOSING Many motor busses are critical to process or plant operation and, therefore, must be maintained if at all possible. For static loads, high speed reclosing or transfer to an alternate source is appropriate. Motor loads require special considerations. When the motor is disconnected from the voltage supply, the voltage at the motor terminals does not go to zero. The machine generates a voltage at its open-circuited terminals that decays with time. A fast reclose applies the full bus supply voltage in series with the residual motor voltage, producing a total winding voltage that can be dangerously high. Capacitors in the circuit only make the situation worse. A second complication is the decay in motor speed with respect to the supply system. The frequency of the residual voltage in the motor will be a decaying value of frequency as the motor begins to slow down. The worst case could be nearly 2.0 per unit voltage and 180 degrees out of phase with the supply voltage. The possibility of damage exists for local reclosing of the motor, high side reclosing from the utility, transferring to an alternate source, or reduced voltage motor starting; they all mean the motor will be re-energized after some dead time and the same principles apply. 11

14 4.1 Parallel Transfer Parallel transfer is a method of transferring process loads from one source to an alternate source. In this method, the bus tie breaker is closed before the normal source breaker is opened. This method has gained wide acceptance because the transient on the motor bus is eliminated, assuming the two sources are in phase. However, the bus system designed for this transfer method may violate the interrupt rating for the circuit breakers and the short-term withstand ratings of the normal and alternate source power transformers. A fault in a motor or its leads occurring during the time the sources are paralleled may produce fault current levels in excess of the circuit breaker ratings. The probability of this happening may be viewed as small; however, the consequences of such a fault should be thoroughly understood before the parallel transfer system is used. Parallel transfer requires a high-speed synccheck relay such as the Basler BE1-25 as shown in Fig. 11 to ensure that the phase difference across the bus tie breaker is within acceptable limits prior to transfer. Without this permissive relay, a large phase angle would cause a power surge through the bus system that could cause damage to the bus system components. An angle setting of degrees with no time delay may be used. FIGURE 11. High-speed sync-check relay. 4.2 Fast Transfer Fast transfer involves opening the normal source breaker prior to closing the tie breaker, thus avoiding the problems associated with parallel transfer. This method is intended to minimize the transfer time between sources. However, the bus must always be completely disconnected from both sources for a short period of time. One technique involves issuing simultaneous trip and close commands to the normal source and bus tie breaker. If the tripping breaker is abnormally slow, the sources can be briefly paralleled, introducing the problems of parallel transfer. Another method involves using a b contact from the normal source breaker to close the bus tie breaker. Especially during abnormal transient conditions, supervision of the fast transfer requires a highspeed sync-check relay such as the Basler BE1-25 to ensure that the phase angle between the motor bus voltage and the alternate source 12

15 voltage is within acceptable limits prior to closing the bus tie breaker. An angle setting of degrees with no time delay may be used. 4.3 Delayed Transfer on Residual Voltage Residual voltage transfer involves waiting until the bus voltage drops below a predetermined point before closing the alternate source breaker. This technique is the slowest of the methods in that the open-circuit time of the bus is the greatest. By waiting until the voltage is 33% of rated voltage, the resultant voltage across the alternate source breaker is reduced to a maximum of 1.33 p.u. This supervision can be achieved with a 27 relay set at.33 per unit with no time delay or by adding a fixed time delay to the closure of the alternate source. Typical residual voltage decay is shown in Fig. 12. The length of time required for the voltage to decay depends on how quickly the stored electromechanical energy dissipates. The motor s open circuit time constant may be defined as follows: where: f = frequency Xm = per unit magnetizing reactance of the motor X2r = per unit rotor reactance at running speed R2r = per unit rotor resistance at running speed At a value of one time constant, the voltage will have decayed to 36.8% of its initial value. Each successive time constant will drop the voltage and additional 36.8% until no voltage remains. A safe value of residual voltage is considered.33 per unit per ANSI and IEEE. Meeting that requirement requires a delay in circuit reclosure of at least one to one and one-half time constants. When auto-reclose of the motor feeder or autoreclose of the utility source takes place, the residual voltage considerations should be used. Either the motor should be disconnected prior to reclose by using an 81O/U relay, or the reclose should be delayed until the voltage has decayed to.33 per unit. FIGURE 12. Decay of open circuit voltage and phase angle. When the user does not wish to reclose or transfer the motor load but wants to protect it from being re-energized out of phase or with high residual voltage, a Basler BE1-81O/U set at 97 to 98% of rated frequency with a time delay of cycles will protect the motor by detecting and underfrequency condition as the motor is decelerating and tripping the supply breaker. The time delay will have to be shortened if high speed reclosing is being used. The same relay can be used for automatic load shedding of the motor at abnormally low frequencies. In both cases potential transformers must be located between the motor supply breaker and the motor leads. For synchronous motors, reclosing must not be permitted until proper resynchronization can be performed. This means tripping the supply breaker with an undervoltage or underfrequency relay. 13

16 5. SYNCHRONOUS MOTORS Protection of the synchronous motor is similar to that of the induction machine with additional requirement for field, loss of excitation and out of step conditions. The field may have its own protection for loss of field or field undervoltage. starting times are short and a significant time margin exists between the maximum start time and the hot stall time. The load is assumed to remain within the motor rating during normal process conditions, allowing the use of one 51 element for locked rotor and running thermal overload protection. Out of step protection is applied to synchronous motors and synchronous condensers to detect pullout resulting from excessive shaft load or too-low supply voltage. Small synchronous machines with brush-type exciters are often protected against out of step operation (or loss of excitation) by ac voltage devices connected in the field. No ac voltage is present when the motor is operating synchronously. Synchronous motors can be protected against loss of excitation by a low-set undercurrent relay connected to the field. This relay should have a time delay drop out. On large synchronous motors an impedance relay is frequently applied that operates on excessive var flow into the machine, indicating abnormally low field excitation. If an undervoltage unit is part of the relay, its function should be shorted out because loss of motor field may produce little or no voltage drop. Operation of synchronous motors drawing reactive power from the system can result in overheating in parts of the rotor that do not normally carry current. Some loss-of-field relays (device 40) can detect this phenomenon. The Power Factor Relay (device 55) can also be used to detect an out of step or loss of excitation condition in a synchronous motor. When the motor loses synchronism or loss of field it will produce watt flow out of the motor and var flow into it. A short time delay is typical, and the relay is generally not in service until the motor is running at synchronous speed. 6. TYPICAL PROTECTION FOR MOTORS CASE 1 - Small Motor ( HP) This example suggests the relay selection and typical settings for motors in the HP range. This range is somewhat arbitrary. Cost and process considerations will ultimately determine the choice of protection level. The proposed scheme shown in Fig. 13 applies to situations where the load has low inertia, the FIGURE 13. Typical small motor protection ( HP) CASE 2 Medium Size Motor ( HP) Single Function Relays This example suggests the relay selection and typical settings for motors in the HP range. This range is somewhat arbitrary. Cost and process considerations will ultimately determine the choice of protection level. The proposed scheme shown in Fig. 14 applies to situations where the load has high inertia, the starting times are long and a small time margin exists between the maximum start time and the hot stall time. The load is assumed to periodically exceed the motor rating during normal process conditions, requiring the use of two separate 51 elements for locked rotor and 14

17 running thermal overload protection. For the 51S locked rotor protection, an Extremely Inverse characteristic will best match the hot stall time curve. If the time dial range is insufficient, the trip time can be adjusted by raising the tap setting to decrease the effective multiple of tap. The 51P, running thermal overload relay must have a MPU equal to the continuous overload limit. A time-current coordination should be performed if the protection is to be optimized. CASE 2A Medium Size Motor ( HP) Multifunction Relay This example suggests the relay selection and typical settings for motors in the HP range. This range is somewhat arbitrary. Cost and process considerations will ultimately determine the choice of the protection level. The functions are similar to Case 2, except that they are integrated into the BE1-851 multifunction overcurrent relay shown in Fig. 15. The 851 offers one time overcurrent and two instantaneous overcurrent for phase, ground and negative sequence. We will also take advantage of multiple setting groups, independent timers, and programmable time overcurrent curves. Programmable alarms, metering, and oscillography will help monitor the motor performance. The 851 uses programmable BESTLogic to customize the relay operation for each application. Two basic schemes are presented here, one for normal loads and one for high inertia loads. Full details of the 851 programming and setting for each scheme can be found in the 851 instruction manual. For low inertia loads, Locked Rotor protection is covered with a maximum start time logic. As shown in the Fig. 15 logic diagram when the motor starts, the 62 starts timing and the 50P is above pickup and timing. The definite time delay of the 50P is set at the motor maximum start time with the 62 set a second or two longer. If the motor starts successfully, the 50P will drop out before its definite timer elapses. Once the motor is running, the 62 timer times out and blocks the logic AND gate from nuisance tripping the motor on temporary overloads if the 50P should pick up again. If the motor does not start successfully, the 50P will stay picked up until it times out and will trip for locked rotor conditions. FIGURE 14. Typical medium size motor protection Single function relays ( HP), High inertia-discrete relays. Thermal overload protection is provided by the 51P element of the 851. The user programmable time overcurrent curve is used to simulate the I 2 t heating. The constants shown in Case 2A settings table will give an approximate range of 2.5 to 25 seconds at 6 times tap for time dials 1 and 10, respectively. If a different range is required, change the value of constant A. The stator short circuit element (150TP) is often applied with a short time delay to overcome asymmetrical current during fault conditions. This is not necessary in the 851 since it only measures the symmetrical current. 15

18 Current unbalance (46N) detection provides rotor thermal protection. The negative sequence (51Q) MPU setting in Amperes is approximately 5 x (max Continuous Voltage unbalance, pu) x (Full Load Current, secondary). The time dial is set to cause tripping in K (the assumed I 22 t value) seconds for I 2 =Full Load Current. The programmable alarm feature of the 851 can be used to provide pre-trip alarms for thermal overload and current unbalance. For high inertia loads the 851 switches setting groups for locked rotor protection since maximum start time is not feasible. When the motor breaker is open, the 851 is using setting group 0 which has the 51 set with a lower time dial to match the locked rotor thermal limit. This is shown as the 51S curve. As shown in the Fig. 15 logic diagram when the motor starts the breaker is closed and the 50P is picked up which keeps the relay in group 0 settings. When the motor starts successfully, the 50P will drop out and the 851 will change to setting group 1. Setting group 1 raises the time dial on the time overcurrent to match the running overload characteristics of the motor. This is shown as the 51P curve. When the motor breaker is opened, the 851 returns to group 0 settings. CASE 3 Comprehensive protection for medium and large motors (>600HP) This protection uses dedicated microprocessor MPS200 or MPS210 relays which, in addition to the essential 50P, 50G, 49, 46,47, 27 functions, offers undercurrent (27), underpower (32U), low PF (55), overvoltage (59), jam protection, and 10 RTDs. There are dual setting levels for trip and alarm for most functions. These relays track the motor temperature accurately (thermal modelling) and offer calculated, statistical and fault data to help the operators and maintenance personnel. See Fig. 16. FIGURE 15. Typical medium size motor protection Multifunction relays ( HP), High or low inertia-multifunction overcurrent. Model variations allow the users to choose among integrated protection, control and metering. 16

19 FIGURE 17. Multifunction protection for medium motors. FIGURE 16. Comprehensive protection for medium and large motors (greater than 600HP) CASE 3A Multifunction Protection for Medium Motors ( HP) This protection option is similar to Case 3, using the MPS100 relay. Protective functions are the same as the MPS200 except without voltage or power functions and with only one RTD. Phase sequence is checked upon energization and is detected in less than 500ms. The motor temperature is tracked through the thermal model, assuring correct dynamic performance. See Fig. 17. COMMUNICATIONS Communications ports are standard in the MPS100, MPS200 and MPS210. Each relay has one RS-485 with MODBUS protocol standard. The BE1-851 relay comes standard with one RS-485 and a front and rear RS-232. ASCII protocol is standard in the 851, MODBUS protocol is optional. 7. BIBLIOGRAPHY 1) Guide for AC Motor Protection, ANSI/IEEE Standard C ) Blackburn, J.L., Protective Relaying Principles and Applications, Marcel Dekker, ) Hornak, D. L. And Zipse, D. W., Automated Bus Transfer Control for Critical Industrial Processes, IEEE Transactions on Industry Applications, Sept/Oct ) Motor Guide, NEMA Standard MG ) Nailen, R. L., Motors, Electric Power Research Institute, ) Dymond, J. H., Stall Time, Acceleration Time, Frequency of Starting: The Myths and the Facts, IEEE Transactions on Industry Applications, Jan/Feb ) IEEE Guide for the Presentation of Thermal Limit Curves for Squirrel Cage Induction Machines, IEEE Standard ) Boothman, D. R., Thermal Tracking A Rational Approach to Motor Protection, IEEE Power System Relay Committee, Jan

20 CASE 1 ANSI QTY Basler Model/ Description Basler Typical No Function Style Number Settings 50P 3 BE1-50/51 Detects stator BE1-50/51B xI LR Instantaneous short circuits 51: H2E-Z3P-A1C0F Or BE P 3 BE1-50/51 Locked Rotor and BE1-50/51B-207 MPU=1.2xFLC Inverse time thermal overload 51: H2E-Z3P-A1C0F Curve: E Or BE1-51 TD: fit below max. safe stall time with 2-5s margin above start current 51N 1 BE1-50/51 Stator ground faults BE1-50/51B-207 MPU=0.5A Inverse time 51: H2E-Z3P-A1C0F Curve: E Or BE1-51 TD: 4xFLC Must coordinate against upstream 51N relay 50N 1 BE1-50/51 Stator ground faults BE1-50/51B-207 MPU=4xFLC Instantaneous (Residual connection) Or BE G 1 BE1-50/51 Stator ground faults BE1-50/51B-207 MPU=0.5A Instantaneous (Alternate to 50/51N) Consider 3I 0 from Or BE1-50 (Toroidal CT) cable capacitance before setting to max. sensitivity 27 1 BE1-27 System undervoltage BE1-27: MPU=0.8xVnom. H3E-E1J-B0H0F Delay: 1-10s Consider slow clearing system faults. NOTE: Quantities correspond to single-function relays. Functions may be combined, as in 50/51 relay. 18