Generator Protection

 

 

1- Rotor malfunctions

In the event of more than one ground fault in the circuit of the rotor (field), magnetic imbalance occurs and then vibrations occur in the machine, which makes the detection of ground faults in the circuit of the rotor of fundamental importance, and Figure (1) shows the model used, which uses constant current and the principle of dividing the voltage between R2 and R1  as two constant resistors (often taken for the excitation circuit at a rate of 250 volts), noting that RN It is a resistance that changes with the change of voltage applied to it (traditionally 45 kΩ at 60 V and 4.7 at 150 V).

Figure (1): Diagram of a system for the protection against ground faults of the field circuit

 

 

When the field circuit (excitation ) is connected, a voltage will appear between point M and the ground (the voltage value depends on the voltage of the excitation circuit and the failure point on the field circuit, and the value of the voltage VMO is  the highest possible value if the failure is on any side of the field circuit and the point of the field circuit, at which point (VMO=0) is called the NULL POINT, and in the case of:

 R2 = R1 + RN

 The idle point in the center of the field coil and the DC relays will detect abnormal voltage values of V0 and then the rotor failures.

 

2-  Different ways to prevent the birth

Figure (3) shows all the prevention methods discussed that are designed to detect the faults that occur to the generators, in addition to some preventive measures that are taken, such as lightning arresters, which are connected between the phases of the entrances and the star point of the coil and the ground and its function is to reduce the stresses that the coils are exposed to due to sudden weather changes.

In the event of an increase in the turbine speed, the voltage increase unit works to suppress and reduce the field, and this importance also appears for the faults that occur in the voltage regulator and the excitation system, while in the water units, voltage increase relays are used with a time lag with high speed relays, while in turbo generators, the use of instantaneous voltage increase relays is the best choice.

 

Thermal relays use protection against load surges that cause an unacceptable increase in temperatures, and these relays intervene to operate when the coil to be protected reaches the limits of the temperatures designed for and the transformer has the same thermal curve as the generator with the compatibility in the time constant of the generator. The relay is able to withstand the increase in load for a short time, so the load of the generator can increase to its highest thermal capacity.

 

As for the thermal current surge relays, which have intermediate time constants, they are used in the output circuits of the voltage regulator, but in the event of a permanent positive or negative excitation as a result of a failure in the regulator, the generator is converted to the use of a manual regulator.

 

As for  the negative sequence relays, they are used in the case of a single-phase load of a three-phase generator higher than the allowable increase, and the same relay is able to perform thermal protection for the moving part of the generator, because in the event of an asymmetric load of the machine, different currents are generated in the phase and with a double frequency in the moving part, which leads to a harmful increase in the heat to which the heart is exposed.

 

Figure (2): Locations of the protective devices for the fixed and moving part and the circuit of the main motor

 

As for the relays that operate with the least capacitive reactor , they are used in generators when there is a danger to that, and this relay is linked to the voltage surge relay and is not delayed in time in a way that allows that when the process of self-excitation begins, both relays start working and send the signal to the circuit breaker of the generator to work immediately, and during the second output, a relay of a late type is connected, and when the time delay ends, it causes the closure of the hydrogenerators and suppression of the field for the turbogenerators, with the possibility of giving permanent excitation For asynchronous operation.  This relay also has the ability to detect harmonies or minor malfunctions that occur in the excitatory circuit. For such failures, either the generator is immediately out of service or stays in the network while trying to restore the network to synchronization again.

 Figure (3) shows the diagram (X-R) of the less capacitive reactant when it is:

XD               is the synchronous reactor of the generator

X'_d               is the trans-generator reactor

a, b               constants for adjustment

 

Figure (3): Diagram showing the geometric location of the operation of a less capacitive reactor in the negative reactor region on the diagram (R-X)

 

 

Reverse power relays stop the generator when the generator is operating as a motor at full speed due to a turbine malfunction and the generator is still connected to the grid, but in some systems a high sensitivity directional power relay is used that is set at a small value instead of using a reverse power relay.

 

In order to remove the currents that occur in the loading piles,  at least one loading substrate is isolated from the ground with its auxiliary piping system, and then faults in the insulation or shortages in the circuit are detected by the presence of an object connected to the current protection system in the loading piles, which determines the current that flows directly in the loading piles, and is often applied in two stages.