transformer REF HI protection

[cesgibis]

Hi,
Client wants transformer 30MVA 33/6.6kV DY to be protected by REF High Impedance on sec. side.
earth fault is limited by resistance to 300A.
CTs ratio 3000/1A (no additional details for now)
I think that REF is not sensitive enough and operating primary current will be close to the 300A and not providing proper protection for transformer windings (and maybe not at all).
Client is not convinced.
any feedback on similar experience?

 

[RalphChristie]

With more types of protective schemes on a transformer, the more different faults-conditions you will be able to detect. See REF-protection as a scheme to protect for faults from near the neutral towards the phases and diff-protection for protection from the phases towards the neutral. With a combination of both you can protect the whole winding, while with only one type you'll just protect a certain portion of the winding.

From IEEE C37.91-1985: (sorry, a little outdated)
On a grounded neutral system, it is possible to isolate the transformer case from ground except for a single point. A CT and O/C relay at the grounding point would detect any internal ground fault or bushing flashover. Although effective, several problems are encountered. The system shall be tested periodically to determine that no accidental grounds have been added. Incorrect operation can result from accidental grounds from power tools and trsf auxiliary equipment, or from a failed lightning arrester discharge if the arrester is mounted on the trsf.

KJvR:
The trend today is towards low impedance REF-schemes. A few years ago a lot of problems were encountered with low impedance REF-schemes, but these faults were largely minimized with the era of microprocessor relays. One must not stagnate with old stuff (although nothing wrong with it) but you can do so much more with the new technology.
I am not sure if you can use metering CTs on the low impedance schemes - normally are protection CTs recommended. (and not necessarily class X CTs like on high impedance systems) I think you might be able to get away with metering CTs, but the burden imposed on the CTs must be very low. Thus, during through-fault conditions, the CTs must still be able to drive the relay.

Regards
Ralph


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[davidbeach]

I don't know how IEC metering class CTs might differ from IEEE metering class CTs, but the IEEE metering class CTs operate much closer to the saturation knee than protection CTs, using saturation to protect the metering equipment from fault currents. I wouldn't trust any differential scheme that used metering class relays unless it could be proved that the maximum expected fault would stay out of the saturation region.

 

[RalphChristie]

In my opinion, if the knee-point voltage of any CT is high enough to overcome the winding resistance and the relay burden at the maximum fault level, it should be able to work - whatever it is called (metering or protection)

But yes, I would also stay with protection CTs in a differential scheme.

Regards

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[davidbeach]

Ralph, no disagreement with your 17:28 post; I just doubt that the knee-point voltage on a metering CT will be high enough for practical applications.

 

[KJvR]

Ralph,

Regarding the metering CT for REF. The reason for REF is to detect small fault currents on the transformer windings. The current you will require trips at will all be smaller than the nominal current of the transformer - maybe slightly higher. Larger fault currents (fault on windings closer to phase terminals) will be detected by the normal transformer diff protection. The low imp relay should be able to block the trip if the measuring CTs saturate for large external fault currents (as I understood from the paper referenced by you). Will this be a workable scheme?

 

[RalphChristie]

The current you will require trips at will all be smaller than the nominal current of the transformer - maybe slightly higher.

Depending where you are seeing the current. For a fault near the neutral, the current might be very low as seen from the phases, but it will be very high from a neutral perspective. See it as a kind of an autotransformer (just the faulted winding) during fault-conditions.
Refer again to the referenced paper in my 5 Apr 06 12:46 post, Fig 1 pg 2.

To answer your question regarding the CTs:
Theoretical I would say yes, if you meet all the criteria, like high enough kneepoint voltage, low enough burden, etc. In practice? I do not know. My experience on low impedance systems is very limited - we are using it only for about two years on some of our transformers. I would use the manufacturer's recommendations, and that is to use at least protective CTs.

Hope it helps, maybe somebody else can give their thoughts or experience.

Regards
Ralph




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[stevenal]

Don't confuse sensitivity with accuracy. If low impedance REF is used, the protection CT on an unloaded resistive grounded neutral can be sized with a low ratio to be very sensitive.

If metering accuracy is also required (I question this), CTs can be specified that meet both ANSI metering and protection standards.