LARSEN & TOUBRO LIMITED TYPICAL SCHEME FOR PROTECTION OF SYNCHRONOUS MOTORS R.Seshadri Larsen & Toubro Limited Mysore works
1.0 GENERAL DETAILS 1.1 Synchronous Motors: Synchronous motors come in large sizes like 1 MW and above. Protection of a synchronous motor is different from a normal induction motor protection since a synchronous motor has DC rotor in addition to stator. This document gives details of a protection scheme for large size synchronous motors. 2.0 Relays to be used for protection of Synchronous motor, typical size ( 1750 kw) L&T recommends following relays : a) Comprehensive Motor Protection Relay type MM30W b) Stator differential relay type MD32-G c) Rotor Earth fault relay type UBO/CR d) DC under voltage relay for rotor side, Type UBC-80 e) DC under current relay for rotor side, Type UBC/A/37 One each of above relays are required to offer a complete protection to the synchronous motor. The SLD for the protection is enclosed. Section 3 gives details of individual protections offered by the above relays. 3.0 Protections offered by Motor Protection Relay MM30W MM30W is completely microprocessor based and uses state of the art technology. It provides totally 12 protections. It receives inputs from three phase CTs, one bus PT and gives trip contact as outputs. Following are the protections offered by MM30W: 3.1 Thermal overload This protection will save the motor from any over load conditions from 105% to 600%. If at any time, the motor draws more current than its rated current, thermal over load protection will first give alarm and then eventually trip after a time delay which is defined by the over load trip time characteristics of the motor. Relay has facility to match closely its trip time characteristics with the over load characteristics of the motor. 2
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3.2 Locked rotor protection : -3- MM30W offers protection against a locked rotor condition both while starting as well as while running. The LR trip time delay while running can be set by the user. When starting the trip time delay is automatically made as twice that of the starting time. This will save the motor from frequent momentaer over loads ( like crusher moros etc.) 3.3 Repeat Starts Protection: Large machines can be damaged when the operator tries to start them again and again without knowing the cause of trip. It should be noted that every time a start impulse is given, the motor experiences a high current impulse, which progressively increases heating in the stator. This protection saves the motor from damage due to frequent starts. If the number of starts have exceeded a set value within a set time, the start command is inhibited by the relay. 3.4 Negative sequence protection : The motor can get over heated whenever there is unbalance current flowing in the stator. Even a small unbalance in voltage, of the order of 2%, can cause a temperature rise by 10% for the same load. This will not be detected by the thermal overload element since the motor current is still at rated current. This protection monitors the unbalance level and trips the motor if unbalance level exceeds say 30%. 3.5 Over current Protection : This is a protection against external short circuits in the feeder. 3.6 Earth Fault Portection : This is a protection against E/F which can happen frequently in an industry. E/F pick up levels can be set to as low as 10% levels there will not be any spurious tripping since the E/F element in MM30W has a third harmonic filter. 3.7 Under current protection : This is a protection against motor running without load. This saves the motor from conditions of sudden load throw off - like gear box detachment etc...4
3.8 Prolonged start protection : -4- When the motor is starting, some times it may take a longer time than specified, to start. This happens in high inertia motors. This may also be due to some maintanence problems. If this happens frequently, then motor can get damaged. MM30W monitors the starting current and starting time if the current does not reduce to a preset level at the end of the starting time, it will initiate the trip, thus savung the motor from further damage. 3.9 Single phasing protection : Single phasing is condition where a large amount of negative sequence will be present, of the order of 53% and above MM30W has inverse time characteristics for trip for negative sequence. It will trip on negative sequence, with shorter time delay, in shorter timehence, whenver there is single phasing. 3.10 Phase reversal Protection: Motors can run in reverse direction when there is phase reversal. Phase reversal is 100% negative sequence condition. MM30W will protect the motor feeder by tripping on negative sequence whenever there is a phase reversal.while starting 3.11 Under Voltage / Over voltage Protection : This protects the motor from stator side under / Over voltages. 3.12 Under / Over frequency protection : This protects the motor from stator side under / Over frequencies. 3.13 Low PF protection : Synchronous motors can easily go into a low PF condition since their rotor excitation is externally controlled. MM30W monitors the PF level of the motor and gives an alarm contact under low PF conditions. 3.14 Auxiliary supply failure protection: If the auxiliary supply to relay is absent, then there is no protection MM30W has a facility where by it will generate a contact whenever its auxiliary supply fails. This contact can be used for alarm / trip. 3.15 Protection against unathorised change in settings : MM30 generates an alarm contact whenever it is taken into programme mode. Consequently it is difficult to change the settings without raising an alarm...5
4.0 Differential protection : -5- Relay MD32-G offers a comprehensive differential protection for the synchronous motor. In large machines, it is advaisable to monitor the internal short circuits with a separate relay which will trip instantaneously. MD32 G provides this protection. With a dual slope trip characteristics, MD32G offers a very high through fault stability and avoids spurious trippings under fluctuating loads and external faults. 5.0 Rotor side protections : Rotor of a synchronous machine is a DC winding- hence it needs DC relays. 5.1 Relay UBO-CR protects the rotor from any earth faults. 5.2 Relay UBC-80 provides protection against under voltage for rotor. 5.3 Relay UBC/A/37 provides protection against under excitaion to rotor. All these protections take care of the motor from any loss of field condition also. 6.0 Out of step protection : The normal method is to use a separate relay for out of step protection. L&T has a separate relay IM30-GOS for this application. This is basically caculating the out of step condition by evaluating the position and magnitude of the impedance vector ( from the stator side measurement of current and voltage). While this may be good for a generator, it may not be effective for a motor. MM30W provides a more direct monitoring for an impending out of step condition. Out of step occurs due to sudden over load ( exceeding peak torque rating )on the machine or fall in the applied voltage to stator. Under these conditions motor draws excessive current - it produces a non uniform or pulsating torque - and goes into stalling mode. This current drawn during out of step is of a low PF. 6
-6- b) MM30W monitors all above conditions independently and on line. - it has current measurement and a LR setting to save motor from momentary over loads - it has U/V element - to save motor from Under voltage conditions - it has PF element to monitor quality of current and recognise out of step condition. Only thing is motor PF is low during starting - hence PF element should be inhibited during starting. This can be automatically done by ITR/TTR contact. 7. PC Communication software : Many of the relays ( except the DC relays) proposed above have RS485 port through which they can communicate all data with a central PC system. MSCOM communication software is available this will help in continuous polling of the relays from a remote point and log all the relevant feeder data. 8. Control & Relay Panel for Synchronous Motor Protection ; Since there are a total of 5 relays ( MM30W, MD32G, UBO/CR, UBC/80, UBC/A/37), it is advisable to mount all these rel;ays on a small panel with all associated control accessories duly wired together. L&T can offer a total solution for synchronous motor protection in the form of a pre wired C&R panel. ---oooooo@&@oooooo---