Synchronous motor out of step relay

[obut4]

Hi,

I want to replace an Out of Step Relay for Motors "MFOS 24" from Alstom.
I’am looking for a relay with the latest technology.

I can only find numerical protection relays who monitor the reactive power or the power factor of the motor to perform a motor” pull out” or “out step” protection.

Can anyone advice a recent motor out of step relay?

Thanks for your help

 

[glendale]

I think CEE Relays GTM7111 Out of Step Relay is still available, give them a try.

 

[obut4]

Thanks for your reply Glendale.

I'am wondering if i can use the loss of excitation protection of a generator protection relay to perform an motor out of step protection.

I'am speaking about the loss of exictation protection with 2 mhos circles.

If not why ?

 

[rhatcher]

One old-school option is the rotor frequency relay. I do not know the device number but I recall this is used on older style Westinghouse sync. controllers. There needs to be a tie with the sync. controller so the relay is inactive during startup (ie. when the excitation is off).

 

[glendale]

Under excitation or even a loss of excitation does not allways lead to a loss of synchronism, for instance if the motor is lightly loaded. An under excitation relay can be used but there is allways a risk of spurious tripping during motor supply voltage dips. Most modern protection systems for large synchronous motors do not now employ "Out of Step" protection (partly because of the scarcity of a suitable relay)and as spurious tripping from under excitation relays is actually quite rare in practice.
What size is the motor and what is it driving?

 

[RRaghunath]

Out-of-step protection is also known as pole slip protection. There is no discrete relay available for this function these days. However, motor protection relays like P243 and 7UM have this function included. The function in these relays is based on measurement of impedance change and thus is more comprehensive unlike the ones based on loss of excitation or change of VAR / PF measurement.

 

[ScottyUK]

You might be able to get an older relay such as an ABB RXZK 23H which is an under-impedance relay designed for pole-slip protection.


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If we learn from our mistakes I'm getting a great education!

 

[obut4]

The synchronous motor is 6MVA and is driving a compressor.

As i see that most motor numerical protection relay perform the out of step protection with the Power factor protection 55, i was wondering if i can use a transducer (with fast cycle time) measuring the motor power factor to perform this protection.

Note that i have only a single phase CT and single phase VT to perform the protection which eliminates most numerical relays.

The trip coming from the transducer would be inhibited until the motor is running at full speed.

This would be equivalent to the PF protection in the MICOM P243.

Hereunder an extract from the IM:

"Upon loss of synchronism a heavy current at a very low power factor is drawn from the supply. The P24x is able to monitor this change in power factor when the motor pole slips, thus allowing appropriate action to be taken. If the power factor passes under a defined threshold for a defined time, a trip is generated."

Setting Guidelines :
The ability of a motor to run at a low power factor will be design dependent.
For a unity power factor machine, the following settings will be typical :
power factor = 0.9,
time delay = 50 ms,
drop-off time delay = 1 or 2 seconds above the start time of the machine.

Do you have any comment on 50ms time delay? should it be longer?

Thanks for your help

 

[JG2828]

Can you use a small generator protection relay like the SEL-700G? It has Out-of-step protection as an option.

http://www.selinc.com/SEL-700G/