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Motor Differential Relay across captive motor transformer

Motor Differential Relay across captive motor transformer

Motor Differential Relay across captive motor transformer


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

I should add that the existing switchgear does have the same differential scheme using CT's in the Switchgear and the neural side of the motor.

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

One problem I faced with switching from an electromechanical to a microprocessor relay was that the electromechancial was an induction disk relay. During the course of startup, we found that the CT's at the motor end of the motor diff zone and the CT's at the switchgear end had two entirely different inrush characteristics. The electromechanical, with its small time delay, had no trouble handling this, but the new relay saw the difference instantly and tripped us. The inrush was characterized by a big DC offset.

It took a lot of head-scratching to identify the problem.

old field guy

RE: Motor Differential Relay across captive motor transformer

Your original relay may have had a provision to compensate for the transformer losses. If your transformer losses exceed the differential trip point this may be an issue.

"Why not the best?"
Jimmy Carter

RE: Motor Differential Relay across captive motor transformer

Your answer seems appropriate, but I would ask SEL to comment. You may want to add a ground differential that compares sum of xfmr neutral and motor neutral.

RE: Motor Differential Relay across captive motor transformer

Howdy Rockman,
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).
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

I'm not familiar with either of those relays that you mention, but if the SEL-710 is a dedicated motor protection relay then you are probably going to have problems even if you do solve the phase shift issue. The conventional way to phase shift without having to connect the Wye side CTs in Delta was to have Delta connected interposing CTs on the Wye side (which also allowed ratio matching to be corrected).

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

As I remember my studies of transformer differential protection relays:
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:

Quote (https://selinc.com/products/387/)

The SEL-387 Current Differential and Overcurrent Relay provides protection, control, and metering for transformers, buses, breakers, and feeders. Features include four three-phase current inputs with independent restrained and unrestrained differential protection, programmable single- or dual-slope differential characteristic, circuit breaker monitor, battery voltage monitor, and enhanced SELogic control equations.

Multiwinding Current Differential Protection—Protect power transformers with as many as four windings by using a combination of single- or dual-slope percentage differential characteristics, overcurrent protection, and restricted earth fault (REF) protection. The SEL-387A Current Differential and Overcurrent Relay is a low-cost solution for two-winding transformer protection.

Beyond Typical Transformer Protection—Protect reactors, generators, large motors, and other multiterminal power apparatus. Provide backup distribution bus or feeder protection with low-side overcurrent elements.

"Why not the best?"
Jimmy Carter

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