Protection of turbogenerators operating directly on generator voltage busbars

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Section 3. Protection and automation

Relay protection. Protection of turbogenerators operating directly on generator voltage busbars¹⁾

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

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3.2.34. For turbine generators above 1 kV with a power of more than 1 MW, operating directly on the generator voltage busbars, relay protection devices must be provided against the following types of damage and disruption of normal operation:

1) multi-phase short circuits in the generator stator winding and on its terminals;

2) single-phase ground faults in the stator winding;

3) double ground faults, one of which occurred in the stator winding, and the second - in the external network;

4) short circuits between turns of one phase in the stator winding (in the presence of output parallel winding branches);

5) external short circuit;

6) reverse sequence current overloads (for generators with a capacity of more than 30 MW);

7) symmetrical overload of the stator winding;

8) overload of the rotor winding by excitation current (for generators with direct cooling of the winding conductors);

9) ground fault at the second point of the excitation circuit;

10) asynchronous mode with loss of excitation (according to 3.2.49).

1. The requirements given in 3.2.34 - 3.2.50 can be followed for other generators.

3.2.35. For turbine generators above 1 kV with a power of 1 MW or less, operating directly on the generator voltage busbars, a relay protection device should be provided in accordance with 3.2.34, clauses 1-3, 5, 7.

For turbine generators up to 1 kV with a power up to 1 MW, operating directly on the generator voltage busbars, protection is recommended in accordance with 3.2.50.

3.2.36. To protect against multi-phase short circuits in the stator winding of turbogenerators above 1 kV with a power of more than 1 MW, having outputs of individual phases on the neutral side, longitudinal differential current protection must be provided (for an exception, see 3.2.37). The protection must act to turn off all generator switches, to extinguish the field, and also to stop the turbine.

In addition to the generator, the protection zone should include the connections of the generator with the busbars of the power plant (up to the circuit breaker).

Longitudinal differential current protection must be performed with a trip current of not more than 0,6 Inom.

For generators with a power of up to 30 MW with indirect cooling, it is allowed to perform protection with a trip current of 1,3-1,4 Inom.

Monitoring of the failure of the current protection circuits should be provided for when the protection operation current is more than Inom.

Longitudinal differential current protection must be implemented with detuning from transient values of unbalance currents (for example, relays with saturable current transformers).

Protection should be performed by three-phase three-relay. For generators with power up to 30 MW, it is allowed to perform two-phase two-relay protection in the presence of protection against double earth faults.

3.2.37. To protect against multi-phase short circuits in the stator winding of generators above 1 kV with a power of up to 1 MW, operating in parallel with other generators or the electric power system, a current cut-off without time delay must be provided, installed from the side of the generator outputs to the busbars. If the current cutoff does not meet the sensitivity requirements, it is allowed to install longitudinal differential current protection instead.

The use of current cutoff instead of differential protection is also allowed for larger power generators that do not have phase leads from the neutral side.

For single-operated generators above 1 kV with a power of up to 1 MW, protection against external short circuits should be used as protection against multi-phase short circuits in the stator winding (see 3.2.44). The protection must act to turn off all the generator switches and extinguish its field.

3.2.38. To protect generators above 1 kV from single-phase earth faults in the stator winding at a natural capacitive earth fault current of 5 A or more (regardless of the presence or absence of compensation), current protection must be provided that responds to the full earth fault current or to its components higher harmonics. If necessary, to enable it, zero-sequence current transformers can be installed directly at the generator outputs. The use of protection is also recommended for capacitive earth fault currents less than 5 A. The protection must be transient detuned and act as in 3.2.36 or 3.2.37.

When the earth fault protection is not installed (because it is insensitive when the capacitive earth fault current is less than 5 A) or is not active (for example, when compensating the capacitive current in the generator voltage network), the set ground fault protection of the generator can be used. on the busbars and signal-acting insulation monitor.

3.2.39. When installing a zero-sequence current transformer on generators for protection against single-phase earth faults, current protection against double earth faults must be provided, connected to this current transformer.

To increase the reliability of operation at high current values, a relay with a saturable current transformer should be used. This protection shall be carried out without time delay and act as the protection specified in 3.2.36 or 3.2.37.

3.2.40. For protection against short circuits between turns of one phase in the stator winding of a generator with removed parallel branches, a single-system transverse differential current protection without time delay shall be provided, acting as the protection specified in 3.2.36.

3.2.41. To protect generators with a power of more than 30 MW from currents caused by external asymmetrical short circuits, as well as from negative sequence current overload, negative sequence current protection, acting on tripping with two time delays, should be provided (see 3.2.45).

For generators with direct cooling of the winding conductors, protection should be carried out with a stepped or dependent time delay characteristic. In this case, the step and dependent characteristics at the second (higher) time delays should not be higher than the characteristics of the permissible overloads of the generator by the negative sequence current.

For generators with indirect cooling of winding conductors, protection should be carried out with an independent time delay with a trip current not exceeding that allowed for the generator when the negative sequence current passes through it for 2 minutes; a shorter protection time delay should not exceed the allowable duration of a two-phase short circuit at the generator outputs.

Negative sequence overcurrent tripping must be supplemented with a more sensitive element acting on the definite time signal. The operating current of this element must be no more than the continuous negative sequence current for this type of generator.

3.2.42. To protect generators with a power of more than 30 MW from external symmetrical short circuits, overcurrent protection with a minimum voltage start must be provided, performed by one current relay connected to the phase current and one minimum voltage relay connected to the phase-to-phase voltage. The protection tripping current should be about 1,3-1,5 Inom, and the tripping voltage - about 0,5-0,6 Unom.

On generators with direct cooling of the winding conductors, instead of the specified protection, one-relay distance protection can be installed.

3.2.43. To protect generators with a power of more than 1 MW up to 30 MW from external short circuits, overcurrent protection should be used with a combined voltage start, made with one undervoltage relay connected to phase-to-phase voltage and one negative sequence voltage filter-relay device that breaks the circuit of the undervoltage relay .

The protection trip current and the trip voltage of the minimum voltage element should be taken equal to those specified in 3.2.42, the trip voltage of the negative sequence voltage filter-relay device is 0,1-0,12 Unom.

3.2.44. For generators above 1 kV with a power of up to 1 MW, maximum current protection should be used as protection against external short circuits, connected to the current transformers from the neutral side. The protection setting should be selected according to the load current with the necessary margin. It is also possible to use simplified undervoltage protection (without current relay).

3.2.45. Protection of generators with a power of more than 1 MW from currents caused by external short circuits must be carried out in compliance with the following requirements:

1. Protection should be connected to current transformers installed at the generator terminals on the neutral side.

2. If there is a sectioning of the generator voltage buses, protection should be performed with two time delays: with a shorter delay - to turn off the corresponding sectional and bus-connecting switches, with a longer one - to turn off the generator switch and extinguish the field.

3.2.46. On generators with direct cooling of the winding conductors, protection of the rotor against overload should be provided when the generator is operating with both main and backup excitation. Protection should be carried out with a definite or current-dependent time delay and responding to an increase in voltage or current in the rotor winding. The protection must act to open the generator circuit breaker and extinguish the field. With a shorter protection time delay, the rotor should be unloaded.

3.2.47. Protection of the generator against currents due to symmetrical overload must be implemented in the form of overcurrent protection acting on the signal with a time delay and using the current of one phase of the stator.

For unloading and, if necessary, for automatic shutdown of the generator with direct cooling of the winding conductors in case of symmetrical overloads, it is allowed to use rotor protection, performed in accordance with 3.2.46 and reacting to rotor overloads accompanying symmetrical overloads of turbogenerators.

3.2.48. Protection against earth faults at the second point of the excitation circuit of turbogenerators must be provided in one set for several (but not more than three) generators with close parameters of the excitation circuits. Protection should be activated only when a ground fault occurs at one point of the excitation circuit, detected during periodic insulation monitoring (see Ch. 1.6). The protection must act to open the generator circuit breaker and extinguish the field on generators with direct cooling of the winding conductors and to signal or disconnect on generators with indirect cooling.

3.2.49. On turbogenerators with direct cooling of the winding conductors, it is recommended to install protection devices against asynchronous operation with loss of excitation. Instead, it is allowed to provide for automatic detection of the asynchronous mode only by the position of the automatic field suppression devices. When the specified protection devices are in operation or when the AGP is turned off on generators that allow asynchronous operation, a signal about the loss of excitation should be given.

Generators that do not allow asynchronous mode, and in conditions of a shortage of reactive power in the system, other generators that have lost excitation must be disconnected from the network when the specified devices (protection or automatic field extinguishing) are in operation.

3.2.50. Protection of generators up to 1 kV with a power of up to 1 MW with an ungrounded neutral against all types of damage and abnormal operating modes should be carried out by installing an automatic circuit breaker with maximum releases or a circuit breaker with overcurrent protection in a two-phase version at the terminals. If there are neutral side terminals, this protection, if possible, should be connected to the current transformers installed on these terminals.

For these generators with solidly grounded neutral, this protection must be provided in a three-phase version.

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

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