Using series capacitors to start a big induction motor 4

[Bronzeado]

Hi folks,

I wonder if someone has experience in using series capacitors (BCS in the diagram below) to start big induction motors (around 13 MW) in a pumping station.
What kind of caution I should to take into account?

Cheers,

Herivelto S. Bronzeado
Brasília, Brazil

https://www.linkedin.com/in/heriveltobronzeado/

https://www.researchgate.net/profile/Herivelto_Bronzeado

 

[magoo2]

I see 230 kV on the left side of your drawing, but what is the voltage level supplied to the motor? 25 kV, 35 kV? Also, what is the rating of the substation transformer and its impedance?

A series capacitor will reduce the reactance to the motor and thus reduce the starting voltage flicker.

 

[RRaghunath]

I suppose you have 13MW motor connected at the end of a long 11kV OHL. If your concern is voltage drop due to the VArs drawn by the motor while in service, I feel it is best addressed by PF correction Capacitors (Shunt connected) near the motor.
Voltage drop during start may not be a concern as the required torque for pump duty during start (with discharge valve closed) is low.
Series capacitors at 11kV level - I have not heard of.

 

[Bronzeado]

Sorry guys!

Thanks magoo2 and RRaghunath for the quick repply!

The motor is a synchonous motor. It starts as a induction motor. The secondary voltage is 6.9 kV.
The transformer power is 23 MVA, 14% impedance. It is operating at +5% tap to reduce the voltage drop.
The length of the 6,9 kV OHL is around 700 m.

We have 2 motors on the same 6.9 kV busbar. So when the second motor starts the first one trips.
We have a soft start but it doesn't work as spected.

I am thinking in putting a series capacitor in oder to to avoid the voltage drop, but I am afraid about this "inovation".

I would very much appreciate the expert imputs in this matter. Thank you!

Cheers,

Herivelto S. Bronzeado
Brasília, Brazil

https://www.linkedin.com/in/heriveltobronzeado/

https://www.researchgate.net/profile/Herivelto_Bronzeado

 

[LionelHutz]

You'd be better off just making sure your first motor is at full excitation and doesn't trip when you start the second motor.

 

[RRaghunath]

"We have a soft start but it doesn't work as spected." - If so, is this not an issue to be taken up with soft starter vendor??
Soft starter is supposed to limit the starting current and in turn limit the voltage drop.
Changing the Power factor set point of the first motor to leading side should also help reduce the voltage dip and also help the motor to remain stable during second motor start (as mentioned also by LionelHutz).
Key points are -
1) minimise the motor starting current magnitude with softstarter and
2) reduce the lagging VArs drawn (during motor start) through the transformer / OHL by locally producing them using the excitation of first motor.

 

[Bronzeado]

LionelHutz and RRaghunath,

I will change the power factor set point of the first motor. Thanks!

Unfortunately, I heard that the soft starter vendor went bankrupt.

Cheers,

Herivelto S. Bronzeado
Brasília, Brazil

https://www.linkedin.com/in/heriveltobronzeado/

https://www.researchgate.net/profile/Herivelto_Bronzeado

 

[LionelHutz]

I'd be rather curious to know what company that is. I happen to know that the 2 x 17000hp, 6.9kV starters sold to Brazil for the PISF project had all support cut off because they weren't paid for.

 

[bacon4life]

Check out these threads on capacitor assisted starting:
thread237-124926: Starting large motors/loads with capacitor assist
thread237-27171: Capacitor assisted starting

https://www.eng-tips.com/viewthread.cfm?qid=224143

:Series Capacitor Help Diminish Vdrop Due to Motor Starting???