Generator Protection - Class-B Protection

Class-B Protection:

It initiates the closing of turbine stop and interceptor valve. The load reduces to 1% or less within a few seconds; this situation is detected by a: Low Forward Power” relay which, after a short delay, initiates a main CB trip. The process takes 4 to 5 sec from the original initiation and is intended to prevent a possible speed run away if over speed governor does not function correctly (Tripping of unit after a time delay).

Class B trip through unit lock out relay 86G-3 under following conditions

Distillate flow to generator stator winding decreases to 13 tons/hour.

Distillate specific resistance is very low (50K-ohms-cm).

Generator transformer winding temperature increases to 1250 C.

UAT A&B winding temperature increases to 1250 C.

Generator transformer back up over current and earth fault protection.

Generators overload protection.

Generator loss of excitation protection.

Generator negative phase sequence protection.

Generator back up distance protection.

10.

GT/UAT A & B phase over current protection.

11.

Excitation trouble- excitation transformer temperature high, failure of power supply to grid control or manual regulator.

12.

Generator seal oil level in damper tank decreases to “Emergency Level”.

13.

Steam temperature before ESV drops down to 4500 C.

14.

Boiler master fuel trip.

Tripping of 86G-3 will result in the following operations :-

86G-3 will trip out the turbine lock out relay which in turn trip 86G-1 through low forward power relay when ESV and IV are closed.

Comments

One & Half Breaker Arrangement - Substation design (Bus Bar Schemes)

In this scheme, three circuit breakers are used for controlling two circuits which are connected between two bus bars. Normally, both the bus bars are in service. A fault on any one of the bus bars is cleared by opening of the associated circuit breakers connected to the faulty bus bar without affecting continuity of supply. Similarly, any circuit breaker can be taken out for maintenance without causing interruption. Load transfer is achieved through the breakers and, therefore, the operation is simple. However, protective relaying is somewhat more involved as the central (tie) breaker has to be responsive to troubles on either feeder in the correct sequence. Besides, each element of the bay has to be rated for carrying the currents of two feeders to meet the requirement of various switching operations which increases the cost. The breaker and a half scheme is best for those substations which handle large quantities of power and where the orientation of out g

Generator Protection - Rotor Earth Fault Protection (64R1/ 64R2)

Rotor Earth Fault Protection (64R1/64R2) The field circuit of generator (i.e. rotor winding) is an isolated D.C. circuit and not earthed anywhere. The field can be exposed to abnormal mechanical or thermal stresses due to vibration, excessive currents or choked cooling medium flow. This may result in a breakdown of the insulation between the field winding and the rotor iron at one point where the stress has been too high. A single earth fault in the field winding or its associated circuits, therefore, gives rise to a negligible fault current and does not represent any immediate danger. If however a second ground fault should occur, heavy fault current and severe mechanical unbalance may quickly arise and lead to serious damage. It is essential therefore that any occurrence of insulation failure is discover and that the machine is taken out of service as soon as possible. Normally the machine is tripped instantly on occurrence of second rotor earth fault. Three method are avai

Generator Protection - Stator Earth Fault Protection

Stator Earth Fault Protection In most countries, it is a common practice to ground the generator neutral through a Grounding Transformer having a loading resistor across its secondary. This method of earthing is called high impedance earthing where the earth fault current is limited to 5-10 Amps. Tuned reactors which limit the ground fault current to less than 1.0 A are also used. The generator grounding resistor normally limits the neutral voltage transmitted from the high voltage side of the unit transformer in case of a ground fault on the H.V. side to maximum 2-3% of rated generator phase voltage. Short circuits between the stator winding in the slots and the stator core are the most common electrical fault in generators. Interturn faults, which normally are difficult to detect, will quickly develop into a ground fault and will be tripped by the stator ground fault protection. 95% stator ground fault relay for generator For generators with unit transformer

ELECTRICAL ENGINEERS

Search This Blog