Generator Loss of Excitation Relay (40G).

Electricity , Power generation ,Thermal power plant , Solar Energy , Generators , Green Energy: Generator Loss of Excitation Relay (40G).

Electrical Generator Protection 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 Operation.
The next figure shows the main single line diagram and Protection Scheme for the Generator.
The Generator made by Mitsubishi Heavy Industries (MHI) and its specification as follows:

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


Generator Loss of Excitation Relay (40G).

If a Synchronous machine losses the Excitation, the following condition will occur:

-When partial or complete loss of Excitation occurs on a synchronous generator, Reactive power flows from the other generator into the generator.

-The KW is controlled by the Prime-Mover input, while KVAR output is controlled by the Field Excitation. If the system is large enough to supply the deficiency in Excitation through the armature, the synchronous generator will operate as an induction generator, supplying essentially the same KW to the system as before the loss of Excitation.

-Since Synchronous generator is not designed for asynchronous operation, the machine output will oscillate slightly as the rotor oscillates in an attempt to lock into synchronism.

-Loss of Synchronism 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 synchronism.

Data for setting calculations.
Generator capacity-------440000KVA
Generator Voltage--------19KVA
CT turns ratio-------------18000/5=3600
VT turns ration-----------19000/100=190
Gen. Transient impedance--------Xd’=24.8%
Gen. Synchronous impedance----Xd=195%

Relay setting calculations.
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.)



  • narayana murthy
    narayana murthy
    nice post buddy
  • freak16
    Hello kahled
    can you please explain me term like
    NGR and its significance in power plant
    which type of circuit breaker is SWYD ?
    What the transformer
    UATR and extr doing?
    Also can you tell me various classes of insulation , their significance and range?
    Please help me i am in dire need of above doubts..
  • freak16
    also what is the significance of power factor of generator ?
    It is the input power factor?
    also power factor is defined at load end so wat is the significance of power factor 0.85 lagging?
    can you tell me displacement factor??
    Please i will be really gratefull.
  • khaled hamza
    khaled hamza
    how are you , now ? i see your inquiries m and i will try to understand you and give you the answer.

    ** NGR , is neutral ground , this is the urgent earthing method to take the leakage charges to a point called neutral ground to protect the humans and who operate equipments at power plant , this NGR is low resistance point which is connected to an equipotential grid among the entire power plant.

    **Type of breaker at SWYD , we use at SWYD 500 KV C.B. , SF6 insulated at champers to insulate and interrupt the arcs at making and closing of C.B. , we use at our power plant , ( one and half circuit breaker ) which provide more reliability to continue power delivering to loads.

    **UATR ( Unit Auxiliary Transformer ) , this is very important transformer , this Transformer provides Electric power for the power plant to operate :-
    - all Auxiliary Equipments at power plant ( Pumps , Fans , Blowers , Compressors,..
    - all essential Lighting for the entire power plant
    UATR is a step down tertiary transformer ( 19 K.V. / 6.6 K.V. ) take the power from the outgoing busbar of the generator of the plant.

    **Excitation Cubicle ( Excitr) , this is a cubicle is responsible for provide D.C. current for the Generator's Rotor to provide flux which cuts by stator coils , therfore produce Electric Power. This Excitation Cubicle Takes A.C. voltage from the Generator , then it converted to D.C. Voltage by RECTIFIER Panel , after that D.C. Current is supplied to Generator's Rotor by help of Slip ring and carbon brushes.
    you should know that Excitation Current is responsible for providing output Reactive Power to the outgoing system.

    **Power Factor of generator , is the measurement of COS(Phie)= P/S
    it is a measurement of how much reactive power and active power is supplied by the generator , we should provide Active power to the system , Also we have to provide Reactive Power , the relation between those parameters are linked together with the power factor term.
    you should have at your eye's , the Capability Curve of the Generator , which indicates how much Active and Reactive Power is produced by the generator for the system , if you increase power factor ( to unity ) , you realize that more Reactive Power is pumped to the system , and vice verse .

    I hope i Could help you , and i wait your comment at any time , I will also attach files about these topics at my blog , you can visit. thank you
  • freak16
    I am your fan..and following your blog also.
    how you do neutral grounding of generator in power plant.
    AS in generator is three phase star connected so you ground its i right?
    So what is the difference btw earthing and neutral??is it the same thing in this case.
  • khaled hamza
    khaled hamza
    You are right , the generator has three phase winding in Star connection , at the center of the three phases , there is a point called Star point , Usually see this point grounded for measuring relays and protection of the generator from Earth faults.

    but There are big difference between ( earthing and neutral )

    Earthing--- if we talk about a small machine , Earthing conductor is a small sire connected to the outer metallic body of the machine to protect people who working with this machine from leaked charges from the machine , and this Earth wire is connected to the Earth Grid ( Equipotential Points)

    Neutral---- if we talk about a small machine ( two phases ) , there are two conductor to supply this machine with ( Live Wire and Neutral Wire ) , Live neutral carry the supply current , but this Neutral Wire is completion of the circuit , So it isn't a protective wire , but it is a necessary wire to complete the circuit.

    I hope you benefit from this post , my friend. , thank you for your contribution.
  • freak16
    Thank you my friend.
    You are great..
    I am clear about things now..
    I am also interesred to work in a power plant so can you please refer me books for the knowledge i should have for the same though i am following your blog daily.
    But i need to be more clear about the things..
  • freak16
    also what is the significance of dpr 1 and dpr 2 i think they are the control rooms..
    and what the number 51 VG , 40 G etc signifies?
  • Arp
    in excitation cubicle(you have mentioned) we should also have some battery bank to provide dc while starting the generator..
    i'm right or we are employing some dc power by some other means while starting??
    also can u brief me about oil surge relay of transformer??
    is it different from buccholz relay or same??
    please reply
  • Arp
    @ khaled NGR u mentioned is the earthing provided to all equipments(like LA body , transformer neutral and body, panels body) through some galvanised pipe whose one end put in ground and other connected with a mesh like thing which is covering the entire substation or a plant and then that is also grounded(plate earthing) in that case??
    SIR, please correct me wherever i'm wrong.
    i've just visited a substation and observe this thing so i'm not sure....
    please reply..
  • khaled hamza
    khaled hamza
    look Mr.Freak
    DPR1 , DPR2 --- are abbreviations for "Digital Protection Relay #1 and #2 "

    It is the relay cubicle which we put all the function relay with its different functions , between the two Current transformer , As you see at the picture above, we use two big relay cubicle ( DPR1 & DPR2 ) used for Generator's Protection.

    but for the names like ( 40 G , 32 G , .....,etc) , it is a united code name for the protection relay name , I mean all the universe are united at naming the Generator Loss of Excitation as ( 40 G), to be easily understood from all who deal with Generators.
  • khaled hamza
    khaled hamza
    Look at the picture above , you can see that the Excitation Cubicle is supplied by the A.C. Voltage Power from the Generator. So at the Start up of the generator , there is no output Voltage , So How can we provide Excitation Current at start up ?
    It is so simple method is Called ( Field Flashing ) , this method is done by Battery bank provide a field current for Excitation until the MW output is 5% until that , the Generator now can provide DC Current for the Exciter.

    Really I Think oil surge relay , is the same operation like Bucchloz relay is protection the transformer from short ciruting by observing the oil dissipated at the transformer main tank.
  • khaled hamza
    khaled hamza
    about the substation Grounding , there is many ways to make earthing. but we should know the principle of grounding , That we put a big electrode and connected to all the entire substation , ok now we have a low resistance point , and we will make a grid covers all the substation , then for the machine , we connect the metal outer bod of the machine to the earthing grid .
    So now we connect the body of the machine with a low resistance point , this is ( earthing of the machines , and Grounding of Substation )
  • Arp
    thanx alot khaled hamza
    u r great..
  • Arp
    are we using Brushless excitation for synchronous generator anywhere yet now?
    i know its costly but it eliminates the commutation problem totally..
    shouldn't we should use it in all major plants because it just involve one time investment and then the results it give are good.
  • Arp
    sir, please tell me about the oil surge relay of transformer?
    its working i'm not getting?
  • dhani0112
    nice post i'm new of this thing, i dont understand and i want to ask you some question
    from where do you get the value of
    Gen. Transient impedance Xd’=24.8%
    Gen. Synchronous impedance Xd=195%
    please answer thanks

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