Generator Loss of Excitati** Relay (40G).


Electrical Generator Protecti** Relays.


Electrical Generator is the main source of Electricity at Power Plant, we should make a many methods to protect the Generator from any faults can happen while Operati**.
The next figure shows the main single line diagram and Protecti** Scheme for the Generator.
The Generator made by Mitsubishi Heavy Industries (MHI) and its specificati** as follows:


Power-------------- 440 MVA
Power Factor----- 0.85 lag
Line Voltage------ 19 KV
Frequency--------50HZ
Speed-------------- 3000 rpm
Hydrogen (H2) Gas Cooled @ 4.5 barg.










Generator Loss of Excitati** Relay (40G).


If a Synchr**ous machine losses the Excitati**, the following c**diti** will occur:


-When partial or complete loss of Excitati** occurs ** a synchr**ous generator, Reactive power flows from the other generator into the generator.


-The KW is c**trolled by the Prime-Mover input, while KVAR output is c**trolled by the Field Excitati**. If the system is large enough to supply the deficiency in Excitati** through the armature, the synchr**ous generator will operate as an inducti** generator, supplying essentially the same KW to the system as before the loss of Excitati**.


-Since Synchr**ous generator is not designed for asynchr**ous operati**, the machine output will oscillate slightly as the rotor oscillates in an attempt to lock into synchr**ism.


-Loss of Synchr**ism does not require immediate tripping unless there is an accompanying decrease in the terminal voltage that threatens system stability; it generally takes at least 2 to 6 Sec. to lose synchr**ism.




Data for setting calculati**s.
Generator capacity-------440000KVA
Generator Voltage--------19KVA
CT turns ratio-------------18000/5=3600
VT turns rati**-----------19000/100=190
Gen. Transient impedance--------Xd’=24.8%
Gen. Synchr**ous impedance----Xd=195%


Relay setting calculati**s.
Relay Side Impedance
Z=Z% * ((KV2 * 10)/KVA) * (CT ratio/VT ratio)




Setting of small circle.
1-Circle #1 Diameter
Z=100 *((192*10)/440000)*(3600/190) = 15.55 Ω


2-Circle #1 Offset
Z=24.8 *((192*10)/440000)*(3600/190) = 1.93 Ω


3-Circle #1 Diameter
Setting=15 Cycles (0.3 Sec.)



Setting of large circle.
1-Circle #2 Diameter
Z=195 *((192*10)/440000)*(3600/190) = 30.31 Ω


2-Circle #2 Offset
Z=24.8 *((192*10)/440000)*(3600/190) = 1.93 Ω


3-Circle #1 Diameter
Setting=75 Cycles (1.5 Sec.)