Electrical

Maybe a stupid question... A 1.5MW motor driving a 6MW turbine ? Which is also a prime mover ? What is driving what ? Ah, maybe you mean the 11kV supply is captive and available from a turbogenerator ?

In any case, why is starting pf important to you ? It will be low due to magnetising current.

i am not sure if this is any help, but check out the cowern papers

Dear Mr. Sridhar,

Possibly my expression is not clear. We have a 6 MW Turbo-alternator, of 11 kV used as captive power set and while the system is working we donot need power from the state grid. While starting the factory the boiler is started with power from Grid/D.G set and turbine is started.

The single Largest Load is 1500 kW, 750 RPM Motor as explained above. This motor does not drive Turbine but drives a fibrisor, which has high Gd^2 Value. We start this motor first and start other loads. This motor has auto slip resistor starter, have 800 kVAR capacitor, switched on manually after loading the motor.

The problem is, some times, while starting the motor, the Turbine trips. My point is only the electrical circuit by way over load/high starting current, thermal protection relay etc. can trip but not the turbine.

I request cr4 members to throw some light on this issue and advise, what could be the problem and how to over come the problem.

Thanks,

Rajeswari.

Thank you for a very succinct explanation young lady. i am sure you have come to the right place as there are many experts on various aspects of electrical engineering here on CR4.

Your apprehension is quite right. The starter of the motor should trip first, if there is any overload condition...not the relay/CB on the alternator. In any case, even the motor protector should not trip unless the current drawn is dangerous for the motor. Is it properly set ?

The most likely explanation is wrong (too low) setting on the turboalternator protection. But then, it is so obvious that you would have already checked it out...both the motor protector and the turboalternator protection relay.

Good luck.

Thank You Mr. Sridhar, for instant reply.

The relay setting etc is set asper the Motor Manufacturer's Advise, whose Engineer was present at the time of commisioning. One point I forgot to mention that the motor is designed for 15% slip, which will develop more Torque as the Torque is proportional to slip.

As you said the starter should trip, but that is not happening. Once turbine trips it takes 50 to 55 min. to restart and excite the alternator and change over.

Thanks.

RAJESWARI.

A 6MW turbine needs 50-55 minutes for hot rolling? Doubtful.......

Generally, there are different start-up times (rolling time) specified for hot, cold and very cold conditions.........

YES Mr. SAJIPV,

The tripping occurs as if we manually trip. From the full speed, the rotor of the turbine has to reach zero speed and then start the Turbine - the governor will not allow to increase the speed manually as we wish and from zero speed to full speed of 8450 RPM you can calcuate time required at the rate of 3 rpm/Sec. i.e. 0, 3, 6, 9.... RPM.

At critical speed zone the speed increase at the rate of 45 RPM/Sec. in the range from 4200 to 4650 rpm. At 3000 RPM level it holds constant for 60 seconds.

In all it takes 50-55 min to come to loading level.

Thanks,

RAJESWARI.

Madam,

i am not an expert but being a maintenance guy was just thinking about your problem.

Any generator set experiences a dip in its speed when a huge load is suddenly thrown on to it and its governor recovers / brings back its speed to its set value. I think your high GD^2 load is not allowing your turbine to recover its speed. The time settings in your auto slip resistor starter steps may have to be adjusted to match your turbine's stability/recovery time.

Regards,

sitaram.

Thank you Mr. Sitaram, for giving your views.

The Gd^2 value is more important for the starting motor and the electronic governor should and will handle speed or load demand by allowing more steam to flow.

As Mr. Sridhar said, the starter should trip and this is not happening.

Thanks,

RAJESWARI.

Madam,

you said the turbine is tripping when the motor is started. Any prime mover pullout happens only when it is unable to recover its speed to meet the demand.

All protection features of electrical starters are generally provided with inherent delays to take care of starting overloads.

Is your alternator breaker is tripping or your turbine tripping? If your breaker is tripping, what relay is causing the tripping? Long back i faced similar trouble of generator tripping on Instaneous earth-fault and only when i deliberately introduced a 2sec. delay to this relay (specialists will definitely object for this) my problem was solved.

Regards,

sitaram.

Thank You Mr. Sitaram, for continued effort.

The turbine is tripping. If ACB is tripping and the turbine is RUNNING, immediately we can restore the supply and start. Even if excitation is lost it can be restored within few minutes.

If turbine trips, we have to wait till it comes to zero speed and start the turbine. The turbine full speed is 8425 RPM and with speed rising rate is 3RPS (Revolution Per Second) we loose 50 to 55 minutes.

The Full Load Current is 105 Amps at 11 kV, capacitor added is 800 kVAR initially and I insisted to add 300 kVAR subsequently, where the line current has come down by 5 amps.

Capacitor Addition - how much to be added depends upon the inductive reactance, Whether the capacitor is to be added further, for that only I asked how to estimate the inductance of 750 RPM, 1500 KW MOTOR - refer my question above.

ALTERNATIVELY CAN WE INCREASE THE RESISTANCE IN THE ROTOR CIRCUIT, at the time of starting.?

Thank you,

RAJESWARI.

What kind of motor are you using?

• If it is an induction motor you have to make a no load test, a blocked rotor test and a DC test in order to meassure the resistance of the stator winding. With those test you can determine all the parameters of the equivalent circuit.
• If your motor is a synchronous one, (less usually, but still a choice for some apps), you will need to assess the Syncronous Reactance by an Open Circuit test and a Short Circuit test.

Hope this will help you

Thank you Mr. Fortiz,

I shall furnish the details of TEST CERTIFICATE provided by the Motor Manufacturer.

How to up-load the scanned copy of the Report, I need guidance from the CR4 FORUM MEMBERS. OR ELSE, I will type the details.

Thanks,

RAJESWARI.

I am sure that adding more details will help, please do excuse me for not reading the above coments on the issue, as I just did. You might have two things to consider: a) The impedance of the line-Transformers-and all the way connecting the motor and generator can make a big dropp in voltaje as the motor starts, you can solve this by adding more reactive power (capacitors) as you are considering, but then b) you have to be careful of the impedance of the motor since you can end up having a resonant circuit between the capacitor and the inductance of the motor, therefore I suggest you meassure the actual inductance of the motor.

As mentioned above, if you can change the torque-speed at starting (ie, if you have the terminals of the rotor available) or have the starter to "soft" start the motor, would also help avoiding the more than 10X nominal current that would make a voltage drop in the lines.

Hope I have read all the important issues of the subject and have a more clear sight of the issue.

Regards

Thank you Mr. Fortiz for expressing your views.

There is NO TRANSFORMER in this system and the Motor it is connected through a proper circuit braker with SOFT STARTER (AUTO SLIP RESISTOR TYPE) since the Voltage generation is at 11 kV.

The Motor Manufacturer recommeded 705 kVAR and we connected one No. 800 kVAR initially, and added one no 300 kVAR later so s to get 1100 kVAR, totally. The reduction in current is 5 Amps after adding 300 kVAR. While the full load current is 105 Amps, the kick load goes upto 295 Amps voltage drop 11kV to 10.2 kV and comes back to 11 kV IN 22 Seconds, but the turbine trips.

That is why I put in this forum " HOW TO ASSESS THE INDUCTANCE EFFECT"

We are considering to add more resistance to the ROTOR CIRCUIT SO THAT TORQUE DEVELOPED IS MORE as Torque is proportional to s/R for which we are discussing with the Soft Starter manufacturer who supplied it. I hope I have cleared your doubt

RAJESWARI.

You can upload a jpg or similar image using the camera icon above. However, an A4 sheet will become minimised to such an extent that it will not be readable. Best bet for you to type the salient data.

This thread by you makes me think i can expand my knowledge, though i may not be able to help you . Sorry, some questions below ...

Being an electrical engineer, i associate 'tripping' with a relay or a Circuit breaker. What is the 'tripping' of a turbine ? What happens ? Is it a mechanical phenomenon ?

i assume your generation is at 11kV and so is the motor, so, no transformers are involved. Yes?

You mention 750rpm. Which implies synchronous speed of the motor. You also mention working slip as 15% which would make the actual motor a slip-ring-induction motor with a resistor starter, automotic. So far so good? Just asking to make sure as many posts are implying something else. i am embarrassed to ask .. what is an Auto-slip-resistor starter ? i haven't come across this name before, some details would be welcome.

.....What is the 'tripping' of a turbine ?.......

Tripping of a turbine means closing the ESV (Emergency Stop Valve) which cuts off all steam input to the turbine. Turbine tripping happens due to action of Turbine protections (some of which can be from generator side too).

Thank you Mr. Sridhar for your views and my reply is as follows.

1. Tripping I meant, the Turbine instantly trips mechanically, just like we trip manually by pressing emergency stop lever. If electrical circuit trips due to variety of reasons, the ACB alone isolate the supply and the Turbine should run at rated speed and excitation and voltage should not undergo disturbance.

2. Voltage generation is at 11 kV and motor is of 11kV and hence NO TRANSFORMER.

3. 750 RPM is the synchronous speed and hence 8 Pole Motor, hence Inductance will be more than 1500 RPM motor.

4. AUTO-SLIP RESISTOR the full details I will post here tomorrow.

In Earlier days there will be EXTERNAL RESISTANCE added to the ROTOR CIRCUIT CONTINUOUSLY TO PROVIDE MORE SLIP and in turn it will DEVELOPE MORE TORQUE as TORQUE IS PROPORTIONAL TO THE SLIP. This external rotor resistance depending on Rotor current generates heat and wasted by radiation and this energy is totally wasted. You cannot stand by the side of the Resistance assembly - radiation of heat will be severe.

Some times this energy wasted is as high as 15% and in the AUTO SLIP RESISTOR STARTER the WASTAGE OF ENERGY is TOTALLY AVOIDED, as the resistance coming in to circuit and cutting out of circuit is AUTOMATICALLY DONE as against remaining in circuit at all times.

RAJESWARI.

Let me throw in my bit as well!

Dear OP! Does the motor manufacturer know that the motor is connected to a finite source (a captive generator) - while recommending the motor feeder protection relay settings?

I would strongly suggest you to undertake a relay co-ordination study.

Dear Elec.Expert65,

Thank you for your comment.

The motor and VFD supplier is fully aware of the source of power supply as 6MW, 11 kV TURBO ALTERNATOR SUPPLY SOURCE.

In my earlier thread I informed the kick load as 290 amps, and it is actually 190 Amps only which less than twice full Load current and the full load current is 105 Amps at 11 kV. The mistake of 290 amps is a typing error and I regret for the error in typing

Thanks,

RAJESWARI.

The thing about this problem that puzzles me is the turbine trip. Most turbine control systems are designed to be capable of resynchronizing after a full load rejection. There are cases where the turbine is tripped on a breaker opening but this is the exception not the rule. The expectation is that the turbine will increase speed to the level dictated by the system speed regulation. In the states this is near 5% speed increase for a full load rejection at this point the valves are completely shut a further increase in speed will try to drive the valves further shut.

By the same token if you increase load on the turbine by 100% the speed will decrease by 5% and the valves will be wide open. Further load application will decrease speed below this value because the valves have done all they can do.

I don't understand how your turbine is tripping.