Showing posts with label Motors. Show all posts
Showing posts with label Motors. Show all posts
MOTOR OVER TEMPERATURE THERMAL PROTECTIVE DEVICE (49) INFORMATION SHEET
There are two main classes of overtemperature thermal protective devices. One is a line break type, which interrupts load current directly. The second is a control circuit system using detector devices, which interrupts the motor current through its controller.
Thermal protectors are intended to limit motor-winding temperature and motor current to predetermined values during abnormal motor operating conditions. This prevents premature motor insulation failure.
Abnormal conditions that can result in overheating include overload, stalling, failure to start, high ambient temperature, restricted motor ventilation, reduced speed operation, frequent starting or jogging, high or low line voltage or frequency, mechanical failure of the driven load, improper installation, and unbalanced line voltage or single phasing.
Current sensing alone cannot detect some of these conditions, such as restricted ventilation. Temperature sensing alone may be inadequate, for example, with frequent starting or jogging.
For some conditions, a coordinated arrangement of current and temperature sensing may be required. The temperature sensing capability of thermal protectors depends on their location with respect to the motor windings.
The protectors should be installed within or on the motor frame in such a manner that the temperature at the device changes in proportion to the winding temperature, and they should be matched to the motor's insulation class.
The advent of microprocessor-based protection systems has made it possible to provide a more refined thermal protection than was possible with electromechanical relays. Since no attempt is made with electromechanical relays to match the thermal time constant of the motor, thermal coordination is achieved under limited operating conditions.
Conversely, microprocessor-based protection systems make it possible to match thermal properties identified by motor data and, as a result, tracks the motor temperature more accurately over a wider range of operating conditions (Zocholl [B115]).
Microprocessor systems can also monitor resistance temperature detectors (RTDs) embedded in the motor stator windings to detect temperature rise caused by impaired or lost ventilation not indicated by current. In addition, these systems can monitor temperature detection devices for motor bearings.
MOTOR STATOR WINDING PROTECTION BASICS
Deterioration of the electrical insulating system of stator windings is a common cause of reduced motor life and failure. This may result from numerous causes, such as subjecting the insulation to moisture, excessive dielectric stress, and mechanical or thermal damage.
The physical and dielectric properties of an insulation system deteriorate with age, and like other chemical activity, this process is accelerated by an elevation in temperature.
A rule of thumb has been developed from tests and experience to indicate that the life of an insulation system is approximately halved for each 10 °C incremental increase of winding temperature, and approximately doubled for each 10 °C decrease (the range of 7 °C–12 °C is indicated for modern insulation systems).
Thus, insulation life is related to the length of time the insulation is maintained at a given temperature.
In practice, winding failures resulting from dielectric breakdown are usually attributed to conditions such as impulse or switching surge voltage, moisture, penetration, or conducting contaminants. Mechanical stresses, such as vibration or distortion forces, can also cause winding failures.
These forces occur most often when starting the motor, or possibly during a transfer to another electrical source. Regardless of the reason associated with the failure, the effect of elevated temperature is to reduce the ability of the insulation to withstand electrical or mechanical abuse.
The temperature level at which an insulation system should be protected is subject to engineering judgment and applicable standards. (For limits established by the motor industry, see NEMA MG1-1998, Articles 12.41, 12.42, and 12.52. For induction motors, see NEMA MG1-1998, Article 20.40. For synchronous motors, see NEMA MG1-1998, Article 21.40.)
It should be noted that deriving increased output at the price of higher temperatures for any given motor means accepting a shorter life. However, when motors are used in essential or critical service, such as for fire pumps or boiler-feed pumps, it is often desirable that the operator be given time to correct an overload condition before a motor is stopped.
Such service may require the motor to run overloaded for prolonged periods, in situations where the overload does not exceed the breakdown torque rating of the motor. In these cases, the cost of reduced motor life due to the overload conditions must be weighed against the expense and damage that would result from a service interruption.
TURBINE UNDER FREQUENCY PROTECTION RELAY SCHEMES AS PER IEEE C37.106-2003
The turbine underfrequency protection scheme may be accomplished by one or more relays. Digital or solid state relays are preferred for their accuracy over a broad frequency range. The required number of frequency setpoints and their associated time delays are dependent upon the characteristics of the turbine.
This relay function may be included in a multifunctional protection package. The first step in designing an underfrequency protection scheme is determining the turbine’s abnormal frequency operating characteristic.
Consultation with the manufacturer should provide the initial design parameters. Modifications to the turbine as well as the known condition from turbine inspections may result in changes to the either the resonant frequency or the allowable abnormal frequency operate time or to both.
From this information, the number of frequency levels that require action can be identified. It should be noted that extreme frequency variations may not require underfrequency relay action as other plant equipment will force the plant to trip.
Once the number of frequency steps is known, the time delay for each step must be determined. Because the allowable underfrequency operation time cannot be identified exactly, some margin should be included in the time delay.
This would allow tripping of the unit prior to damage, with the opportunity to inspect the turbine at the owner’s convenience during a future outage. This allows for application of underfrequency protection, even if the unit has been in operation for many years without having accumulated previous underfrequency operational data.
The time delay margins should consider the importance of the unit, the susceptibility of the system to an underfrequency event and operating agreements with local or regional power authorities. A range of 50–90% of the allowable time per expected event over the blading life is reasonable.
Settings of 50% should be considered if the turbine is in poor condition, there is a high possibility of an underfrequency event or if the unit is not system critical. If the unit is in good condition, an underfrequency event is unlikely, and the unit is critical to the system, a setting near 90% of the allowable underfrequency time should be considered.
It should be recognized that some underfrequency relay timers have an instantaneous reset once the frequency rises above the trip setting, while others accumulate the underfrequency operate time in a memory function (zero reset). The time delay setting should be a smaller percentage of the allowable time if the relay is of the instantaneous reset type, whereas the zero reset relay can be set at a greater percentage of the allowable time.
Figure below will be used to demonstrate an underfrequency relay setting. The example indicates the turbine is capable of continuous operation at frequencies above 58.5 Hz and is limited to a maximum of 10 minutes accumulated over the blading life at 56.0 Hz.
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| Example of turbine abnormal frequency limitations and settings |
If the unit is in good condition and is critical to the system, a longer time delay of nine minutes could be used to allow the maximum opportunity for system recovery, prior to tripping the unit assuming these events are very rare so that 90% of the fatigue life can be expended on it. An alarm should be provided when the underfrequency relay begins to time out, providing operating personnel a warning of the impending underfrequency trip.
Many operators apply a single multilevel relay having four frequency set points. In many cases, one of the frequency set points will be used to alarm at a relatively high frequency (59.4 Hz for example) to indicate the inception of an abnormal frequency event, with the remaining three set points are at lower frequencies corresponding to the turbine characteristics.
If IEC 60034-3:1996 is applicable, time-frequency protection could be required by a manufacturer for the turbine generator. If such a situation was to develop, the more restrictive of the generator and turbine frequency requirements, over a frequency band, should be used in the determination of appropriate frequency settings for underfrequency relaying. The frequency capability of the generator, when applicable, must be considered in the development of the frequency relay protection scheme.
STATOR WINDING PROTECTION RELAYING
Deterioration of the electrical insulating system of stator windings is a common cause of reduced motor life and failure. This may result from numerous causes, such as subjecting the insulation to moisture, excessive dielectric stress, and mechanical or thermal damage.
The physical and dielectric properties of an insulation system deteriorate with age, and like other chemical activity, this process is accelerated by an elevation in temperature. A rule of thumb has been developed from tests and experience to indicate that the life of an insulation system is approximately halved for each 10 °C incremental increase of winding temperature, and approximately doubled for each 10 °C decrease (the range of 7 °C–12 °C is indicated for modern insulation systems). Thus, insulation life is related to the length of time the insulation is maintained at a given temperature.
In practice, winding failures resulting from dielectric breakdown are usually attributed to conditions such as impulse or switching surge voltage, moisture, penetration, or conducting contaminants. Mechanical stresses, such as vibration or distortion forces, can also cause winding failures.
These forces occur most often when starting the motor, or possibly during a transfer to another electrical source. Regardless of the reason associated with the failure, the effect of elevated temperature is to reduce the ability of the insulation to withstand electrical or mechanical abuse.
The temperature level at which an insulation system should be protected is subject to engineering judgment and applicable standards. (For limits established by the motor industry, see NEMA MG1-1998, Articles 12.41, 12.42, and 12.52. For induction motors, see NEMA MG1-1998, Article 20.40. For synchronous motors, see NEMA MG1-1998, Article 21.40.)
It should be noted that deriving increased output at the price of higher temperatures for any given motor means accepting a shorter life. However, when motors are used in essential or critical service, such as for fire pumps or boiler-feed pumps, it is often desirable that the operator be given time to correct an overload condition before a motor is stopped.
Such service may require the motor to run overloaded for prolonged periods, in situations where the overload does not exceed the breakdown torque rating of the motor. In these cases, the cost of reduced motor life due to the overload conditions must be weighed against the expense and damage that would result from a service interruption.
Motor overloading
Overloads can produce stator total hot spot temperatures in excess of the designed thermal limits of the winding insulation system. However, in all cases of operation that result in overtemperature, time is an important factor.
The heat storage capacity of an induction motor is relatively large. Slight overloading for short periods of time does not result in damaging temperature excursions, because the extra heat is stored in the mass of the conductor, core, and structural members. In contrast, for locked-rotor conditions, the rate of temperature increase is very rapid due to the large currents.
Since very little heat is transmitted (in this short time interval) from the conductors to the more massive parts of the motor, the thermal limits of the winding insulation may be reached within seconds.
National Electrical Code ® (NEC ® ) (ANSI/NFPA 70-1999) & OSHA
The National Electrical Code® (NEC®) (ANSI/NFPA 70-1999) has as its purpose the practical safeguarding of persons, buildings, and their contents from hazards arising from the use of electricity. It contains provisions considered necessary for safety.
Its scope includes the electric conductors and equipment installed, for example, within or on public or private buildings, industrial substations and mobile homes.
It does not cover installations under the exclusive use of electric utilities, mines, and certain other exceptions. The NEC as a recommendation for safe practice is adopted by most of the states, cities, and towns in the United States as the governing electrical code, and is enforced by the local or state-approved authority.
OSHA, 29 CFR, Chapter XVII, Part 1910 is concerned with all establishments engaged in the manufacture of products for interstate commerce. Part 1910 (Subpart S—Electrical) of the regulation has essentially adopted the NEC and incorporates its requirements for electrical installations.
The edition of the NEC adopted by OSHA is not generally the same as that enforced by local authority. NFPA Std 70E is generally the standard that influences the OSHA changes.
The adoption of the NEC in the public and private sector, even though excluding installations under the exclusive control of electric utilities when used in connection with the generation, transmission, and distribution of electric energy, essentially represents requirements for practically all residential, commercial, and industrial installations. For this reason, NEC requirements, as they apply to motors and motor circuits, are included.
The NEC specifies overload devices used to protect motors, motor-control apparatus and motor branch-circuit conductors against excessive heating due to overload and failure to start (see ANSI/NFPA 70-1996, Article 430, Part C).
The NEC further specifies devices intended to protect the motor, motor-control
apparatus, and branch-circuit conductors against overcurrents due to short circuits and grounds (see ANSI/NFPA 70-1996, Article 430, Part D).
In motor branch circuits, it is customary to provide for these functions separately, where the running overload protective device protects against motor overloads and locked rotor, and the overcurrent protective device (as a separate device) provides protection against short circuits and ground faults.
The NEC specifies maximum current rating or setting in relation to motor nameplate full-load current for the motor running overload protective device, since the requirement includes the need to monitor the maximum continuous motor branch-circuit current.
Of the several means recognized for providing motor running overload protection, the two basic approaches in common use are a separate overcurrent device that is responsive to motor current, and a thermal protector integral with the motor (see ANSI/NFPA 70-1996, Articles 430–432).
For continuous duty motors, a separate overload device responsive to motor current is specified in ANSI/ NFPA 70-1996, Articles 430–432, to be rated or selected to trip at no more than 125% of rated full-load current for motors with a marked temperature rise not over 40 °C, or with a service factor not less than 1.15 and not higher than 115% of full-load current rating for all other motors.
For continuous duty motors having a thermal protector integral with the motor, ANSI/NFPA 70-1996, Articles 430–432 specify that the thermal protector shall be approved for use with the motor that it protects on the basis that it will prevent dangerous overheating of the motor due to overload and failure to start.
For motors rated more than 1 hp, in addition to protecting against excessive temperature, the thermal protector is specified to limit the combination of motor and protector to an ultimate trip current in the following manner.
Where full-load current does not exceed 9 A, the trip current should not exceed 170% of motor full-load current. For motors with full-load current values between 9.1 A and 20 A, the trip current should not exceed 156% of motor full-load current. For motors with a full-load current value of 20 A, the trip current should
not exceed 140% of motor full-load current.
Other overload protection recognized by ANSI/NFPA 70-1996, Articles 430–432(a) (4), involves the use of embedded temperature detectors used in conjunction with intermediate devices that cause motor current to be interrupted.
AC MOTORS PULL OUT AND STALL PROTECTION RELAYING
Induction-motor stalling
An induction motor stalls when the load torque exceeds the breakdown torque and causes its speed to decrease to zero, or to some stable operating point well below rated speed. This occurs when the applied shaft load is greater than the producing motor torque due to the suppression of the motor terminal voltage.
Also, a stall condition can occur when an excessive mechanical load is applied beyond motor torque capability. This condition will develop motor current equal to or approaching locked-rotor current.
Synchronous motor loss of synchronism (pullout)
When a synchronous motor loses synchronism with respect to the system frequency with which it is connected, it is referred to as “out of step.” This condition occurs when the following actions take place individually or in combination:
a) Excessive load is applied to the shaft
b) The supply voltage is reduced excessively
c) The motor excitation is too low
Torque pulsations applied to the shaft of a synchronous motor are also a possible cause of loss of synchronism, if the pulsations occur at an unfavorable period relative to the natural frequency of the rotor with respect to the power system.
A prevalent cause of loss of synchronism is a fault occurring on the supply system. Fault-clearing time, fault location, fault type, and system configuration are significant factors relating to the stability of the motor. Fast fault clearing, multiple ties, and remoteness of faults favor stability.
Underexcitation of the machine is a common cause of out-of-step operation. This may be caused by incorrect tripping of the rotor field circuit breaker (or contactor), or by opening or short circuiting the field circuit.
When loss of synchronism (pullout) occurs, and the motor is not separated from the system on the first pole slippage, field excitation must be disconnected and the field connected to the discharge resistor immediately. This minimizes the current that flows until the motor can be isolated. The motor should then be isolated as quickly as possible, because this is not an acceptable long-term operating condition.
Electrical quantities change during a stall
For an induction motor to stall during normal operation, the load torque must exceed the breakdown torque as described above. During this process, the motor current will increase rapidly (which is called “inrush current” or “locked rotor”) until the breakdown torque is reached.
Beyond breakdown torque, the motor current continues to increase approaching locked-rotor current. Along with the increase in current, the speed of the motor decreases and the impedance of the motor approaches the locked-rotor impedance. There are two types of stall causes, as follows:
a) Excess shaft load torque prior to a motor startup (e.g., failure to open the pump’s discharge gate)
b) Sudden change of increased shaft load torque during normal operation (e.g., bearing failures)
For a synchronous motor, loss of synchronism is a gradually evolving phenomenon rather than an instantaneous occurrence. During the initial phase of pullout, stator current increases, terminal voltage decreases, and a voltage is induced in the rotor circuit at the slip frequency. Power flow into the motor increases until approximately a 90° angle is reached between the equivalent machine voltage and the system voltage.
At approximately the 180° point, current is maximum and lags the system voltage by the angle of the total impedance between the motor and the system (including the stator resistance and transient reactance of the motor). Also at this point, the direction of power flow reverses, with the motor mass supplying energy to the system.
When resistance is significant, this reversal occurs prior to the 180° point. The reactive power flow for virtually the full slip cycle is into the motor, but it may be provided by the motor for a small part of the slip cycle, depending on the machine excitation.
Protective devices for detecting abnormal motor conditions
Stall detection for an induction motor is usually provided by an overcurrent relay, with an inverse characteristic set to detect current above the breakdown torque level. Since motor starting can result in a stall or locked-rotor condition, this protection is usually covered by setting the motor-starting relays above the motor-starting time-current curves and below the running and accelerating thermal limit for the motor.
In cases where motors are applied to high-inertia loads, overcurrent protection may need to be combined with speed switches, distance relays, or additional rotor thermal protection to fully protect the motor. Out-of-step detection devices for synchronous motors usually operate on the stator power-factor angle.
Impedance-type devices are available for detecting loss of field, and they may also be set to operate on out of-step conditions without field failure, where the motor transient reactance exceeds the system impedance viewed from the motor terminals (the usual case).
For very large synchronous motors or synchronous condensers, a loss-of-field relay is often used to detect VAR flow into the machine. Accidental tripping of the rotor field circuit breaker (or contactor) or loss-offield current can be accurately detected by this device.
There have also been successful applications of rotor field current devices operating from a rotor field current shunt and of notching relays that count pole slips based on power reversals.
A device sensing alternating current in the rotor field circuit may also detect the motor out-of-step condition. These devices used in the rotor field circuit usually consist of a current transformer (CT) with an ac relay on its secondary.
When the machine is operating synchronously, there is no ac component of rotor field current and, therefore, no relay current. If the machine is out-of-step with the system, a current of slip frequency exists; if it is of sufficient magnitude, the relay picks up. During the starting period, this relay must be blocked. This scheme is not adaptable to motors with a brushless excitation scheme.
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