Generators and Excitation Protection

Have you tried to reset the under-excitiation relay and restart the unit ?

What results ?

Is the exciter a rotating or static ? Manufacturer and model number ?

What vintage is the relay ? Manufacturer and model ?

What is the prime mover ?

Is the unit on a grid or isolated ?

How big is this generator relative to the grid size ?

If a small grid, biggest load ?

More detail will help provide a better answer to your question, and at some point you may answer yourself.

Main reasons for excitation protection to trigger is shortcircuit in excitation winding and maybe overheating of the excitation. The electric fault may occur as ground fault for one winding in which case you need differential protection for rotor; the other case is fault between two rotor windings, triggered by overcurrent protection.

the schemes are just like ordinary overcurrent and differential protections.

Dear ZM

Thanks to your good comment.

BRN

Answer of ZM eng is absolutly true. This is more theoritical answer & explaining exactly what is going on in the gerertor.

Practically,

If you are running generator in parallel with Grid. In that case, excitation governs pf of the Generator.

If you are running Generator in isolation then Excitation governs Terminal Voltage.

Now I am assuming your generator is running with Grid. Normally, when it is running with grid one contact of Grid breaker is wired in AVR. It gives comand to AVR for controlling pf not terminal voltage. This pf is being set by opertor on AVR by some variable port or any other mathod as per AVR. Now By chance that Grid breaker contact to AVR goes off, but actually Generator is running with Grid. in that case AVR will shift in isolation operation mode. & it will vary excitation current in such that it maintain terminal voltage as 11 kV. Now at that time if Grid voltage is more then 11 kV, say 11.3 kV. Hence AVR will reduce excitation current but actually teminal voltage can not reduce COZ it is running with Grid. Hance pf will go toward 1 & then to leading side & ultimate excitation current will reduce very low & it may cross beyond Generator excitation stability curve. In this case your excitation monitoring relay will operate & trip the generator.

This is only one type of case. Tell more about youe system then one can give more reasonse for tripping.

Thanks to your reply.

The generator is connected to Grid. We have 3 Generators running in the Power House and all the Generators are connected to the Grid. The frequency is 50 Hz. Capacity of each generators are 55 MW each. The excitor is static excitor system. This is a hydroelectric Plant.

The machines are normally running with pf = 1 and MVAR = 0. Sometime the load despatch centre is requesting to incrase the MVAR. In what cercumstance MVAR is required for the Grid?. How we can control MVAR ?.

Can you please discribe the relation between MVAR, pf, terminal voltage and excitation current and voltage ??.

BRN

Dear Waco,

I need your comment about Mr.Ajay's views.

Rgds

BRN

Dear Byjuram,

As you said that you are running Generators at unity pf, & your load is also being control by LDC, I can conclud that you are Power Generating Plant & Feeding your all Generation to Grid.

It is really surprizing that you are running your generators are 1 pf. Be careful, you are near to leading pf. As soon as your excitation current will reduce little your generator will be in leading pf. And please see Generator stability curve given by your generator manufacturer. In leading operation, it may possible that your roator will start heating. It is always better to run Generator in lag.

That's why we provide loss of excitation relay. We can also say that it is reverse reactive Power Relay. Both are doing same function & Relays are connected to Statos Voltage & Stator Current, not on DC Field side. Please check your relay which was operated, whether it is connected to Excitation side (Rotor side) or Stator Side & let me know.

Please let me know, whether your generator trips frequently on excitation fail?

I am in hurry, I will wite some thing more on this.

Ajay

Dear Mr.Ajay

Thankyou for your reply

You are right, the Generators are connected to very huge grid. We have total 6 generators with 55 MW for 5 generator and 60 for one generator. Out of 6 only three in service and total load is 55+55+60 = 170 MW.

Generation voltage is 11KV. Stepup transformers are using to stepup the volttage to 220KV. The generators are running at pf=1. I didn't noticed any variation in pf and MVAR. When we reduce the load of the machine, the MVAR is slightly varying but within seconds it came to zero. We dont have capacitor back in the generating station.

Mr.Ajay - What are the additional parameters or measurements required for you to analyse my situation. Write me. I will provide complete information.

Rgds

BRN

Byjuram

As ajay said i know your generators are on grid but running your generator close to unity p.f is not o.k,maybe thats why it trips. Have you been runnig this generator under these conditions for a long period? And are all generators behaving the same, if not all then am of a view that since you have a very small variation between the Excitation current and the Mvar you consider checking the tap positions of your generator transformers. I think thats why you manage to run your generators at 0 Mvars, of late we had a similar situation when one of our generators stator voltage is around 11.4 Kv with our referance Mvar at 3Mvar, so we managed to get rid of that by varring the tap position of the generator transformer to much the other generator transformers, but i do not remember which tap position was the calprit generator transformer but what i remember is that the other generators are at tap position 2.

Dose the generator trip out during a system disturbance? Also try to varry the excitation or Mvar referance point but don't use the load referance, then note the field current and the stator voltage. Since you run it at referance 0 Mvars try to adjust it to 3 or 5 and note the field current and the stator voltage, Do the same to another generator.If there is a big varriation then consider changing the transformer tap position.

Note:

If you increase Excitation or Mvar

-field current will increase

-Stator voltage will increase

Power factor moves away from Unity.

And the reverse is true. So at 0Mvars that's why you have Unity p.f. The generator will run fine because other generators on the system are providing the reactive power needed by the system and your generator will tend to draw part of this to maintain it's self but if the generators on the system are under excited or more inductive loads are put to the system, they will reach their maximam point that they will not supply any more reactive power thus your generator to be under excited because it's excitation was being supported by the grid, thus it will trip out.

And if you reduce LoaD (MW referance)

- field current will reduce

- Power factor moves further away from Unity

Dear Ajay & Waco

The following are details of my system

1. Unit #5 machine running with 55 MW, Iexc=539 A, Uexc=139.3 V, PF=1, MVAR = 0, MW=55.1, F=50.3 Hz, Speed=502 RPM

2. Unit #6 machine running with a reduced load 30 MW (Actual capacity is 60MW), Iexc=602.8A, Uexc=110.6V, pf=1.0, f=50.2, speed =501 RPM

3. All machines are connected to Grid

4. I understood from our relay testing team, the excitation setting is done as below

a) Rotor angle 75 - Stage 1

b) 14 Amps DC current measurement. Transducer 4-20 mA - Stage 2

Instantaneous trip.

5. The excitation system is :

Type : static Elin-Thyre 5 IEEE 421

Rated voltage = 250 V

Rated current = 800 A

6. Exciter Transformer

438 KVA, Volt HV 11000, LV 350, Current HV 22.98, Current LV 722.53

Phase 3, f=50Hz, Dy5.

So the excitation setting is based on the rotor angle for the stage 1 and DC excitation current for stage 2.

The excitation system here is keeping the power factor unity. If any reduction in load the MVAR will increase but within seconds it will become zero. We are using SCADA system to change the MW & MVAR if the LD insists.

I hope you got information about our system. Please comment based on this information.

The generator is not tripping frequently with under excitation protection.

Rgds

BRN

Dear Ajay,

Please reply based on my previous mail.

Rgds

BRN

Unity PF is when the generator excitation is correct to produce minimal amperage and HEATING in the generator windings, as the generator is producing KW only. Observe this when the generator is online, vary the excitiation up and down slightly and you will see that the generator amp meter "dips" at unity PF, the KVA meter will also read zero. This is how to do a metering check to verify proper wiring.

Producing KVA to have a power factor other then unity, does take power and does create more current flow in your generators windings which produces more HEAT which is cumulatively detrimental to the winding insulation, requires more HVAC to remove the heat, thus reducing efficiency, and may limit the generators ouput during hot weather.

So unless you are the utility it is always in your best interest to run at UNITY PF to reduce heating in the generator windings. I have started 15 hydro plants and always set the Auto Voltage Regulator/Power Factor Controller to run at unity PF.

One thing you may need to look into is the PF after your station transformer to make sure that the plant out at the utility metering is metered at 0 KVAR.

Thru the course of questions and statements some of the questions I initially asked have been answered, regarding the Loss of Excitation relay operation.

But you still don't know why it operated if I am following the string ?