Protection of asynchronous and synchronous electric motors with voltages above 1 kV

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Section 5. Electric power plants

Electric motors and their switching devices. Protection of asynchronous and synchronous electric motors with voltages above 1 kV

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Encyclopedia of radio electronics and electrical engineering / Rules for the installation of electrical installations (PUE)

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5.3.43. Electric motors shall be provided with protection against multi-phase faults (see 5.3.46) and, in the cases specified below, protection against single-phase earth faults (see 5.3.48), overcurrent protection (see 5.3.49) and undervoltage protection. (see 5.3.52 and 5.3.53). On synchronous motors, protection against asynchronous operation (see 5.3.50 and 5.3.51) must also be provided, which may be combined with protection against overload currents.

The protection of variable speed motors must be carried out for each speed as a separate kit acting on its own circuit breaker.

5.3.44. On motors with forced lubrication of bearings, protection should be installed that acts on the signal and shutdown of the motor when the temperature rises or the lubrication stops.

On motors with forced ventilation, protection should be installed that acts on the signal and shutdown of the motor when the temperature rises or the ventilation stops.

5.3.45. Electric motors with water-cooled windings and active stator steel, as well as with built-in water-cooled air coolers, must have protection that acts on the signal when the water flow drops below the set value and to turn off the electric motor when it stops. In addition, an alarm should be provided that operates when water appears in the motor housing.

5.3.46. To protect electric motors from multi-phase short circuits in cases where fuses are not used, the following should be provided:

1. Current single-relay cutoff without time delay, detuned from starting currents when the starting devices are removed, with a direct or indirect relay connected to the difference in two-phase currents - for electric motors with a power of less than 2 MW.

2. Two-relay current cutoff without time delay, detuned from starting currents when starting devices are removed, with a relay of direct or indirect action - for electric motors with a power of 2 MW and more, having protection against single-phase earth faults acting on tripping (see 5.3.48) , as well as for electric motors with a power of less than 2 MW, when the protection according to paragraph 1 does not meet the sensitivity requirements or when a two-relay cutoff is appropriate for the design of the complete protection or the drive used with a direct-acting relay.

In the absence of protection against single-phase earth faults, the current cutoff of electric motors with a power of 2 MW or more should be three-relay with three current transformers. Two-phase protection is allowed with the addition of protection against double earth faults, made using a zero-sequence current transformer and a current relay.

3. Longitudinal differential current protection - for electric motors with a power of 5 MW and more, as well as less than 5 MW, if the installation of current cutoffs according to clauses 1 and 2 does not ensure that the sensitivity requirements are met; longitudinal differential protection of electric motors in the presence of earth fault protection must have a two-phase design, and in the absence of this protection - three-phase, with three current transformers. Two-phase protection is allowed with the addition of protection against double earth faults, made using a zero-sequence current transformer and a current relay.

For electric motors with a power of 5 MW or more, made without six stator winding leads, a current cutoff must be provided.

5.3.47. For blocks transformer (autotransformer) - electric motor, general protection against multi-phase short circuits should be provided:

1. Current cutoff without time delay, detuned from starting currents with the starting devices removed (see also 5.3.46), - for electric motors up to 2 MW. When the transformer windings are connected, the star-delta cutoff is performed from three current relays: two switched on for phase currents and one switched on for the sum of these currents.

If it is impossible to install three relays (for example, with a limited number of direct-acting relays), a circuit with two relays connected to the secondary windings of three current transformers connected by a triangle is allowed.

2. Differential cutoff in a two-relay version, detuned from transformer magnetizing current surges, - for electric motors with a power of more than 2 MW, as well as 2 MW or less, if the protection according to clause 1 does not meet the sensitivity requirements for phase-to-phase short circuit at the motor outputs.

3. Longitudinal differential current protection in a two-relay version with intermediate saturable current transformers - for electric motors with a power of more than 5 MW, as well as 5 MW or less, if the cut-off setting according to paragraphs 1 and 2 does not meet the sensitivity requirements.

Sensitivity assessment shall be carried out in accordance with 3.2.19 and 3.2.20 in case of short circuit at the motor terminals.

The protection must act to turn off the block circuit breaker, and for synchronous electric motors - also to the AGP device, if it is provided.

For units with motors larger than 20 MW, as a rule, earth fault protection must be provided, covering at least 85% of the turns of the motor stator winding and acting on the signal with a time delay.

Instructions for the implementation of other types of protection of transformers (autotransformers) (see 3.2.51 and 3.2.53) and electric motors when they are operated separately are also valid when they are combined into a transformer (autotransformer) - electric motor unit.

5.3.48. Protection of electric motors with a power of up to 2 MW from single-phase earth faults in the absence of compensation should be provided for earth fault currents of 10 A or more, and in the presence of compensation - if the residual current under normal conditions exceeds this value. Such protection for electric motors with a power of more than 2 MW should be provided for currents of 5 A or more.

The tripping current of electric motor protection against earth faults should be no more than: for electric motors with a power of up to 2 MW 10 A and for electric motors with a power of more than 2 MW 5 A. Lower trip currents are recommended if this does not complicate the protection.

Protection should be performed without time delay (with the exception of electric motors that require protection deceleration due to the condition of detuning from transients) using zero-sequence current transformers installed, as a rule, in the switchgear. In cases where the installation of zero-sequence current transformers in the switchgear is not possible or may cause an increase in the protection time delay, it is allowed to install them at the motor terminals in the foundation pit.

If the protection against transients must have a time delay, then an additional current relay with a primary operating current of about 50-100 A should be installed at various points to ensure the fast tripping of double earth faults.

The protection must act to turn off the electric motor, and for synchronous electric motors - also to the AGP device, if it is provided.

5.3.49. Overload protection should be provided for electric motors subject to overload for technological reasons, and for electric motors with especially difficult starting and self-starting conditions (the duration of direct start directly from the network is 20 s or more), the overload of which is possible with an excessive increase in the duration of the starting period due to a decrease in voltage in networks.

Overload protection should be provided in one phase with a current-dependent or current-independent time delay, detuned from the duration of starting the electric motor under normal conditions and self-starting after the action of ATS and AR. The time delay of protection against overload of synchronous electric motors in order to avoid unnecessary trips during long-term forcing of excitation should be as close as possible to the maximum allowable thermal characteristic of the electric motor.

On electric motors subject to overload for technological reasons, protection, as a rule, should be carried out with an effect on the signal and automatic unloading of the mechanism.

The action of protection to turn off the electric motor is allowed:

on electric motors of mechanisms for which there is no possibility of timely unloading without stopping, or on electric motors operating without constant personnel on duty;
on electric motors of mechanisms with difficult starting or self-starting conditions.

For electric motors that are protected against short-circuit currents by fuses that do not have auxiliary contacts for signaling their burnout, overload protection in two phases must be provided.

5.3.50. Protection of synchronous motors from asynchronous mode can be carried out using a relay that reacts to an increase in current in the stator windings; it must be tuned in time from the starting mode and current during the excitation forcing action.

Protection should normally be carried out with a current-independent time delay characteristic. It is allowed to use protection with a current-dependent characteristic on electric motors with a short circuit ratio of more than 1.

When implementing the protection scheme, measures must be taken to prevent failure of protection during asynchronous current beats. It is allowed to use other protection methods that provide reliable protection in the event of an asynchronous mode.

5.3.51. Protection of synchronous electric motors from asynchronous mode must act with a time delay on one of the circuits, providing for:

1) resynchronization;

2) resynchronization with automatic short-term unloading of the mechanism to such a load, which ensures the retraction of the electric motor into synchronism (with the admissibility of short-term unloading according to the conditions of the technological process);

3) shutdown of the electric motor and repeated automatic start;

4) shutdown of the electric motor (if it is impossible to unload or resynchronize it, if there is no need for automatic restart and resynchronization according to the conditions of the technological process).

5.3.52. To facilitate the conditions for restoring voltage after a short circuit is turned off and to ensure self-starting of electric motors of critical mechanisms, it is necessary to provide for switching off the minimum voltage protection of electric motors of non-critical mechanisms with a total power determined by the capabilities of the power source and the network to ensure self-starting.

The undervoltage protection time delays should be selected in the range from 0,5 to 1,5 s - one step longer than the action time of high-speed protection against multi-phase short circuits, and the voltage settings should, as a rule, not exceed 70% of the rated voltage.

In the presence of synchronous electric motors, if the voltage on the disconnected section decays slowly, in order to speed up the action of the ATS and AR, the field of synchronous electric motors of critical mechanisms can be extinguished using the minimum frequency protection or other methods that provide the fastest fixation of power loss.

The same means can be used to disconnect non-responsible synchronous motors, as well as to prevent non-synchronous switching on of disconnected motors if the switching currents exceed the allowable values.

In electrical installations of industrial enterprises, in cases where simultaneous self-starting of all electric motors of critical mechanisms cannot be carried out (see 5.3.10), shutdown of a part of such critical mechanisms and their automatic restart after the completion of self-starting of the first group of electric motors should be used. The inclusion of subsequent groups can be carried out by current, voltage or time.

5.3.53. Undervoltage protection with a time delay of not more than 10 s and a voltage setting, as a rule, not higher than 50% of the rated voltage (except for the cases specified in 5.3.52) should be installed on the electric motors of critical mechanisms also in cases where self-starting of mechanisms after a shutdown is unacceptable according to the conditions of the technological process or according to safety conditions and, in addition, when self-starting of all electric motors of critical mechanisms cannot be ensured (see 5.3.52). In addition to the indicated cases, this protection should also be used to ensure the reliability of starting the ATS of electric motors of mutually redundant mechanisms.

On electric motors with a variable speed of critical mechanisms, the self-starting of which is permissible and expedient, the undervoltage protection should automatically switch to a lower speed.

5.3.54. On synchronous electric motors, automatic field damping must be provided. For electric motors with a power of 2 MW or more, AGP is carried out by introducing resistance into the excitation winding circuit. For electric motors with a power of less than 2 MW, it is allowed to carry out AGP by introducing resistance into the circuit of the excitation winding of the exciter. For synchronous motors less than 0,5 MW, AHS is generally not required. On synchronous electric motors, which are equipped with an excitation system made on controlled semiconductor elements, AGP, regardless of the motor power, can be carried out by inverting, if it is provided by the power circuit. Otherwise, AGP must be carried out by introducing resistance into the excitation winding circuit.

See other articles Section Rules for the installation of electrical installations (PUE).

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