Motor Differential Relay across captive motor transformer
Motor Differential Relay across captive motor transformer
(OP)
I have an application where 12.47kV switchgear has a feeder breaker which feeds a motor that has a captive transformer with the transformer secondary and motor operating at 4.16kV. This motor is a synchronous motor.
The existing Switchgear is being replaced and an SEL-710 relay will be used to provide motor protection. It is being requested that motor differential protection be included and that the differential zone extend from the motor neutral point all the way up to the line side of the feeder breaker (not core balance CT's at motor).
Does anyone have any experience with this type of application or know of any pitfalls to look out for? My initial concern is how to account for the phase shift across the Delta-Wye transformer that will be in the zone of protection. With a dedicated transformer differential relay you can program the relay to account for this shift but with this motor protection relay there doesn't appear to be that option. The only thing I can think of would be to wire the two sets of CT's in wye and Delta respectively in order to account for this shift external to the relay?
The existing transformer is LRG on the secondary to 400A. The CT on the transformers secondary neutral will be connected to the ground input on the relay. Are there any issues of using a CT on the transformer secondary for ground fault detection in a motor relay?






RE: Motor Differential Relay across captive motor transformer
The existing differential that is being used in an ABB CA differential relay. The CT's in the switchgear are 400/5 while the CT's at the motor are 2000/5. I believe this relay is a generator/transformer % type relay which will be different than the SEL-710 relay.
RE: Motor Differential Relay across captive motor transformer
It took a lot of head-scratching to identify the problem.
old field guy
RE: Motor Differential Relay across captive motor transformer
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Motor Differential Relay across captive motor transformer
RE: Motor Differential Relay across captive motor transformer
I don't see why a standard transformer 87 relay would not work here. I will assume that you have all 6 leads available from the motor, correct? This implies that a set of CTs can be installed on the load side of the motor winding (just prior to them being connected in to a wye.
I'm not aware of a single MPR relay that has this type of 87 protection; I am sure that you will require two relays (ie an MPR and a 87 relay).
GG
ps What size is the motor (10,000hp)?
pps 400A is rather large for a 2400V NGR (ie why not 50A?)
RE: Motor Differential Relay across captive motor transformer
As oldfieldguy said, the TX Primary side CTs will see a high inrush current which won't proportionately be reflected in the motor / secondary side of the TX, so unless you have a diff relay that can have Bias settings and 2nd Harmonic inrush restraint, then you will probably have tripping on energisation. The old ABB relay that you mentioned probably has those features. You referred to the ABB relay as a generator / transformer % type relay which suggests that it is a Biased diff relay with adjustable (or sometimes fixed) Bias and probably Harmonic restraint also.
With regard to the Neutral CT on the TX secondary, you would have to wire the CT to the "Measured" E/F input on the relay (assuming it has one) and be able to set that input CT ratio (in the relay) to be different to the Phase CT ratio, though most relays have this feature. The ground fault current in this case can only circulate in the Tx secondary / motor zone.
RE: Motor Differential Relay across captive motor transformer
The relay must be able to accommodate several parameters:
1> Voltage ratio.
2> Transformer no-load losses.
3> Transformer load losses.
You may have better luck looking for a transformer protection relay that provides motor protection, rather than trying to add transforrmer protection to a motor protection relay.
You may need two relays.
This may help:
Bill
--------------------
"Why not the best?"
Jimmy Carter