Starting of large Induction Motor on Gas Turbine Generator


Thread Starter

Arjun Sharma

Dear all,

We have a small in house CPP consisting of two gas turbines of Hitachi make, Mark V equivalent machines, with 22MW capacity each and a steam turbine. We have commissioned two gas turbines successfully, but recently we encountered a problem with one of the gas turbine, which tripped due to contamination of lube oil with fuel oil, which is a different issue.

I have one gas turbine running with the in house load of 8MVA present on the generator. The 22MW gas turbine is rated 27.5 MVA with 22MW generation at 11kV, 0.8pf lagging. I need to start one Induction motor of 7.6MW in decoupled condition and I have a soft started to start it. But, as per the vendor's data, the initial MVA requirement by the motor shall be 20.12 MVA on 6.6kV level with 0.1pf lagging for 10~20 seconds even with the soft starter in decoupled condition. During design stage it was considered that coupled condition of motor shall be started with any two generators running in synchronism. Since, I need to take a no load trial of the motor before coupling it, I need to start the motor on my gas turbine generator.

My machine is running with HSD currently. I am running my machine presently on Isochronous mode. My questions are:

1. Is it possible to start the motor on my GT without tripping of my complex? My U/V settings in the downstream are set at 80% for 1 seconds.

2. If at all if it is possible, what are the changes in the running parameters I should change.

3. I have a grid power also, from which I can extract upto 7MVA (above that my upstream supplier substation trips at overcurrent). So, is it possible to start my motor with grid synced with GT?

Please help me in this regard. Thanks in advance.

P.S.: My concerns are towards the response time of AVR, Voltage drop of the generator and the MVAr capability of my alternator.
Arjun Sharma... to start the investigation into your problem:

1) Has the motor ever been started while uncoupled from the driven machine?

2) Do you know what the bus-voltage was at the instant the Soft-Start controller was energized?

Phil Corso

Arjun Sharma

Dear Phil Corso,

> 1) Has the motor ever been started while uncoupled from the driven machine?

No, this will be the first site No Load Test run to be done.

> 2) Do you know what the bus-voltage was at the instant the Soft-Start controller was energized?

The soft starter is an FCMA Soft Starter. These are the vendor provided details available:

Motor Rating: 7.6MW
System Voltage: 6.6kV
Motor Efficiency: 97.26%
Motor running pf: 0.92
Motor current: 762A
Motor DOL current on Full Voltage: 4.34pu
Motor starting pf: 0.13
Starting current with FCMA 1st stage: 1.56pu
Starting current with FCMA 2nd stage: 2.32pu

The motor is connected to generator through the following components.

Generator outcomer is 11kV connected directly to a step up transformer of 33kV and the supply is connected to 33kV bus which supplies the complex. The 33kV bus has a tapping to a step down transformer present in another substation bringing the bus voltage to 6.6kV. The 6.6kV bus is dedicated to this only motor.

I tried to calculate the voltage drop considering the line drops, transformer impedance and the motor impedances. When it comes to generator I am confused as to whether to consider Xd, X'd or X"d . And whether to consider the armature reaction at the moment of starting as MVAr demand is like a step function.

For 10-20 seconds the IM shall consume 20MVA at 0.13pf lagging. And later, the total MVA required drops down to 4-5MVA. Hence, how my GT behaves, whether there will be a drastic drop in voltage or not are my concerns.

Thanks in advance for your guidance.

Arjun Sharma
Arjun... can you provide additional data:

1) Was the UV-relay measuring the 11kV, 33kV, or 6.6kV voltage?

2) What is size and %impedance of the 11/33kV Xfmr?

3) What is size and % impedance of 6.6kV Captive-xfmr between 33kV Xfmr and Mtr?

4) What is size and distance of 11kV line between Gen’r and step-up Xfmr?

5) What is size and distance of 6.6kV line between 33kV and 6.6kV Captive-xfmr

6) What is size and distance of 6.6kV line between Captive Xfmr and Mtr?

7) Other than fact that UV relay tripped can you provide V, A, kW, Var, and duration (real or pu measurements)elsewhere?

Phil Corso

Arjun Sharma

Dear Sir,

My apologies for late reply.

We have started the motor successfully yesterday. The No Load Trial took place with the following increase in parameters.

During Starting (following were increase/Decrease in values)
MW -- 2.225 MW (inc)
Freq -- 0.3 Hz (dec)
MVAr -- 18.25 MVAr (inc)
Pf -- 0.4 lag from 0.85 lag (dec)
Gen I -- 826.25 A (inc)
Gen Vg -- 0.5kV (dec)

So, The voltage dipped by around 0.5 kV max during starting and recovered within 2 seconds. The AVR controller did the rest. The power factor was low because of high MVAr as compared to MW requirements. For 10 seconds the MVAr and MW parameters remained the same. The generator current was well within its rated maximum current. Hence, the fear of voltage drop was ruled out and the dip in voltage was as usual and the recovery time was found to be consistently less.

Thanks for all your inputs. It was a great success.
Arjun Sharma...

as a rule of thumb, if the Generator is a least twice the capacity of the motor, a soft-starter should work! The principle constraints are:

a) The actual acceleration torque (related to kW) during run-up is usually much less than the prime-mover is capable of handling!

b) The kVAr capability of the generator is sufficient to handle the motor's inrush-current (mostly inductive) as permitted by the Soft-Starter.

c) The AVR's response is fast enough to handle the initial voltage-drop of the generator's terminal! (Remember the impact of Armature Reaction!)

d) The driven load, compressor, pump, etc, is unloaded!