Motor Start-up Testing

[Designer69]

Gents,

we are testing 8000HP 13.8KV motors with load connected.

The mfg says they must bypass the CT readout initially on start-up since the inrush current is so high and out of range of the test CT's. They will then monitor steady-state.

Has anyone heard or run into this problem before? Do CT's actually have a current measuring "range"?

I am not allowing the mfg this option btw.

thanks!

 

[mull982]

Yes CT's esentially have a maximum range for which they become saturated for primary currents exceeding their maximum design values. Once the CT's go into saturation they will no longer provide any usefull current feedback.

However the CT's for a motor should be sized and specified as such that they are capable of detecting the starting current of the motor without going into saturation.

With that being said there is no reason that these CT's should be taken out of the circuit during starting. They are not only providing protectin for transient events during startup, but they are also providing the starting current magnitudes for with the motor protection relay uses to build its thermal model and protect the motor from crossing the thermal damage curves of the motor during prolonged starting. So long story short, leave the CT's in during starting and if they are not sized properly for starting current then this needs to be addressed by the motor manufacturer.

 

[BJ Conner]

Gents,

we are testing 8000HP 13.8KV motors with load connected.

The mfg says they must bypass the CT readout initially on start-up since the inrush current is so high and out of range of the test CT's. They will then monitor steady-state.

Has anyone heard or run into this problem before? Do CT's actually have a current measuring "range"?

I am not allowing the mfg this option btw.

thanks!

Test CTs?
I would not start this motor with Test CTs.
Real CTs are rated as primary current to 5amps. ie 400/5. 200/ etc. The ratio is selected so the normal primary curret produces a secondary current of 3.5-4 amps (YMMV).

Where are the real protection system CTs? That is the system you need to use. Full load for this motor is around 300 amps, starting current about 1,800
The real CTs may saturate and not accurately give you the right current.
The "real" CTs should be 450/5 -600/5 that will give your a ct current of 3-3.5 amps in normal operation. In starting the overcurrent (51) function will be out. Ground fault protection should be in.
You don't bypass the CTs you just ignore them (for overcurrent tripping ) in starting. Solid state motor management relays will have an acceleration time build in so they will not trip when starting. IF your starting with a heavy load connected the time to speed will be longer.

If it was my motor I would make sure someone has throughly tested the control and protection system, calibrated and set all the relays and injection tested all the CTs. A motor this big should a differential protections scheme as well as ground.

 

[Jraef]

Sometimes it is necessary to bypass the Over Load protection on start up, especially if it is a high inertia load. This is allowable under the NEC with the proper precautions (430.35). So if your OL is using CTs, as it invariably is in MV motor controllers, then the simplest way to bypass the OL is to short the CTs during start up. Otherwise the OLR trips and has to reset before it starts protecting the motor, whereas if you short the CTs the OLR functionality is resorted immediately when the shorting contactor is opened.

If by "test CTs" you mean some that are not associated with the OLR but are connected to test instruments, it could be that the test instruments will suffer if the CTs are driven into saturation by start-up current.

 

[Designer69]

The "real" CTs should be 450/5 -600/5 that will give your a ct current of 3-3.5 amps in normal operation. In starting the overcurrent (51) function will be out.

thanks BJ, so if the expected inrush is around 1800A why not size the CT's say 2000/5 so that the inrush can be captured?

if that is not possible then how do you measure the magnitude of the inrush current on startup to verify motor characteristics??

Thanks again

 

[jim dungar]

thanks BJ, so if the expected inrush is around 1800A why not size the CT's say 2000/5 so that the inrush can be captured?

if that is not possible then how do you measure the magnitude of the inrush current on startup to verify motor characteristics??

Thanks again

There is nothing wrong with over driving most CTs, however there can be real problems with the accuracy of lightly loaded (i.e. <10%) ones.

When a CT saturates its output is no longer linear with its input, but as soon as the current levels drops below the saturation point it linearity is restored.

Unless the CTs are really small, in regards to their burden, it is unlikely that a 3-4 times overload will cause them to be inaccurate, however the test equipment must be able to handle the >5A CT output that will exist.

 

[BJ Conner]

thanks BJ, so if the expected inrush is around 1800A why not size the CT's say 2000/5 so that the inrush can be captured?

if that is not possible then how do you measure the magnitude of the inrush current on startup to verify motor characteristics??

Thanks again

You want the normal running current to be in the linear range of the CT. The CT current ratio is a "S" shaped curve with the low and high ranges in the curved portion of the curve.
If your going to have trouble in the start there are other ways to catch it.

This sounds like an FD fan, yes, no?

 

[markstg]

thanks BJ, so if the expected inrush is around 1800A why not size the CT's say 2000/5 so that the inrush can be captured?

if that is not possible then how do you measure the magnitude of the inrush current on startup to verify motor characteristics??

Thanks again

There are 2 Classes of CT's; Metering and Protection Relay.

Metering CT's are built to be accurate within its rated output range (say 0-5A).
Protection Relay CT's are built to perform with a reasonable degree of accuracy over a wider range of current. The wider range of current allows the protective relay to operate at different fault levels.

 

[Designer69]

This sounds like an FD fan, yes, no?

it's actually a feedwater pump motor. 8000HP 13.8KV

If your going to have trouble in the start there are other ways to catch it.

would you be kind enough to share some of those ways? Thanks!

 

[BJ Conner]

it's actually a feedwater pump motor. 8000HP 13.8KV



would you be kind enough to share some of those ways? Thanks!

The motor differential (87) scheme and the ground fault scheme (50) should catch a fault in the start up.
The solid state motor management relays like the GE M60
http://www.gedigitalenergy.com/multilin/catalog/m60.htm
can do it. they even learn how your motor starts, they can learn when the starting current is in the normal operating range. They learn to read the starting current even though the waveform is distorted by the core being saturated. Trouble is the relays are so smart and have so many features it could become a career just to learn to use it.

Once upon a time allmost all power plant motors over 4,000HP had differential protection, now I see them without. I don't know if owners are getting cheaper or motors are getting better. You may or may not have differential protection.