Transformer differential relay trips when energizing a transformer ? what is a restraint winding?

[bdn2004]

In the Red Book there is this statement about the below circuit.

"It is good practice for transformers of the size shown on the incoming service, where a circuit breaker is used on both the primary and secondary sides, to install percentage differential relays and inverse characteristic overcurrent relays for backup protection. To prevent operation of the differential relays on magnetizing inrush current when energizing the transformer, the large proportion of currents at harmonic multiples of line frequency contained in the magnetizing inrush current are filtered out and passed through the RESTRAINT WINDING so that the current unbalance required to trip is made much greater during the excitation transient than during normal operation."

What is a restraint winding? Has anyone ever seen this happen?

 

[davidbeach]

The restraint winding is internal to the relay. Today it is more a virtual winding rather than a real winding.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[bacon4life]

Electromechanical differential relays had multiple windings to allow the connection of several separate voltage and/or current inputs to act on the same physical lever/dial. The OPERATE winding exerts force to move the lever/dial towards trip, and the RESTAINT winding exerts force in the opposite direction.

In modern relays there are programable coefficients to determine how much restraint is provided by 2nd, 4th, and/or 5th harmonics. This harmonic restraint can also be used on feeder relays to desensitize the relay when re-energizing a distribution circuit.

 

[Palletjack]

Best advice I ever received for protection relays..
Learn the IAC and BDD relays. They will help with the modern relays.

When you can “see” a operate and restraint winding working in an old relay it helps to understand what’s going on in the new ones.

 

[RRaghunath]

Magnetising inrush is known to be rich in 2nd harmonic component. Restraining the Transformer differential protection on sensing the threshold of 2nd harmonic is common in most of the transformer differential relays, the old static relays as well as in the latest numerical ones.

 

[mvcjr]

If your transformer differential protection trips during transformer energization, most probably the inrush restraint feature is disabled.

 

[bdn2004]

Thanks for clearing this up.

And Palletjack.....

"Best advice I ever received for protection relays..
Learn the IAC and BDD relays. They will help with the modern relays.

When you can “see” a operate and restraint winding working in an old relay it helps to understand what’s going on in the new ones."

I've often thought about buying an old electromechanical relay off of Ebay just cause they are kind of a work of master craftsmanship imho. But would there be a way to simulate on such a functioning relay these types of phenomena?

 

[thermionic1]

"But would there be a way to simulate on such a functioning relay these types of phenomena?"

In the HU and BDD books, they show a 'homebrew' test set up consisting of a variable resistor, ammeters and DPDT switches. For the harmonic restraint tests the current is fed through a diode to create the second harmonic. There are conversion tables for how one measures the current and how to calculate the % harmonic.

Modern relay test equipment can generate sinusoidal currents of system frequency and beyond. Some are able to create a complex waveform with defined harmonic content. Others accomplish this by paralleling the system frequency from one current channel with the nth harmonic from a second channel.

Most modern test sets are also capable of playing back event reports for fault recorders or relays in COMTRADE format.