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transformer REF HI protection

transformer REF HI protection

transformer REF HI protection

(OP)
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?


RE: transformer REF HI protection

In my opinion your client is correct. REF-protection (if correctly applied) is extremely sensitive, especially on resistance or impedance earthed systems, where the current available on an internal fault is disproportionately low. (Protect over 90% of the transformer windings if correctly applied) Simple O/C and E/F relays will not provide adequate protection for winding earth faults, and even with biased diff relays installed, the biasing de-sensitises the relay such that it is not effective for certain earth faults in the winding. It might be an old type of scheme, but it is very effective and proven over the years.

What kind of scheme did you recommend?

Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon

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RE: transformer REF HI protection

(OP)
1- I thought an earth-tank protection good do the job. being very easy to implement (except that transformer needs to be isolated from earth). But on a protection point of view: a standard CT with instantaneous o/c that can be set very low, around 5% of 300A = 15A (means 95% of winding protected)

2- my assumption for REF is roughly:
 the protection draws 20mA (typical)
 each CT magnetising current is say 15 mA
Primary operating current would be 20 + n x 15 (with n=4 : number of CTs involved) ==> Iprim = 80 mA x 3000= 240A
Am I missing something ?( maybe the CTs magnetising currents are too high ?)
Thanks for your answer.  

RE: transformer REF HI protection

As Ralph said, "if correctly applied". Instantaneous OC set sensitively will not coordinate with downstream devices. REF does not need to. If 240A primary is not sensitive enough, you can change CTs, loading resistors, or relay taps to achieve better sensitivity. Any reason why high impedance is a requirement?

RE: transformer REF HI protection

Sorry - was last week not in the office.

Agree with Steve.

The problem with the earth-tank protection is that for any downstream earth fault, the current will be very near 300A, depending on the location of the fault. This will result in a lot of nuisance trips on a relay without a time-delay, because the earth fault-current will always return through the resistor/s to the transformer. You'll have to use a relay with a relative long time-delay to provide time for the downstream relays to clear the fault. Due to the fact that the REF-protection is a zone-type of protection, it needs not to coordinate with any other devices, and can trip instantaneously if it detects a fault.

Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon

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RE: transformer REF HI protection

Hi Cesgibis,

I had almost the same problem but on a high impedance / circulating current bus diff protection scheme.  I was forced to use a Class X CT with an excitation current of 40mA (1250/5).  An experienced protection engineering did the verification and did not like the low excitation current I have specified.  Is a 15mA excitation current not a bit to low for a practical design?  I would have aggreed with you if I had more experience but revering to stevenal's comments:
(a)  I am not sure if you can have a lower tap setting on electro mechanical High imp REF relays - it is normally fixed and you can just change the resistors in serie with the trip coil;
(b)  The resistor taps only influence the knee-point voltage and stability of your scheme and not the sensitivity;
(c)  I am sure you will not be able to reduce the CT ratio because of full load current.  You have also used a very high CT spec with the very low excitation current and I assume you can not improve on this;

Maybe Ralph or stevenal can elaborate more on how to improve the REF scheme to be more sensitive.  I would also appreciate.

KJvR

RE: transformer REF HI protection

High impedance REF-protection is voltage operated - by changing the resistance (stabilizing resistors) in series with the relay-coil you can change the operating voltage. Some type of electromechanical relays required an external resistor, but there are types with links or some with potentiometers.
A rule of thumb is to have a kneepoint voltage of at least twice the voltage (trip) setting. The setting is normally fixed - a too low setting can result in relay operation during heavy through-faults. By doing some measurements (wire resistance, CT internal resistance etc) and calculations a lower operating setting can be obtained and will still be stable during through-faults, than by using the rule of thumb of a setting half the knee-point voltage of the CT.
Class-X CTs are preferably required, however most protection CTs are suitable for use with high impedance relays, providing that the following basic requirements are met:
CTs should have identical turns ratio. When turns error is unavoidable, it may be necessary to increase the fault setting to cater for this.
To ensure positive operation, the relay should receive a voltage of at least twice its setting.
CT should be of low reactance type.

We use on our 20MVA transformers on the 11kV side Class-X CTs with a magnetizing current of 0.01A (10mA) - a low magnetizing current is thus obtainable. Unfortunately, class-X CTs are more expensive than normal protection CTs.

I hope this answer some of your questions.

Regards
Ralph



Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon

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RE: transformer REF HI protection

a) Missed the electromechanical requirement.
b) See Ralph's answer above.
c) True.
Suggestion: Speak to client of the benefits of using a multifunction transformer protection relay, combining differential, REF, overcurrent, etc. Current inputs will be low impedance, so magnetizing current is not an issue. I've achieved very good sensitivity with these applied on impedance grounded transformers while maintaining high speed protection and coordination.

RE: transformer REF HI protection

Any reason a low ratio core balance CT couldn't be used in this application?

RE: transformer REF HI protection

30MVA at 6.6kV won't be brought out on a multicore cable - more likely multiple single cores per phase, or perhaps a bus duct. Neither is especially amenable to a CBCT installation. Individual CTs located at the line and neutral bushings is  easier from a practical point of view.

----------------------------------
  I don't suffer from insanity. I enjoy it...

RE: transformer REF HI protection

Thanks RalphCristie for that 10mA practical example.  That is the type of info you can't obtain from books.  

What percentage of the bolted earth fault would your target operating current be to ensure 80%-90% of your windings are protected?  Above example given by cesgibis would probably only provide a FAST trip at 2 x 240A fault current and a secure trip at 1.5x240A (sorry, I again refered to an electro mechanical relay)?  

If you do use a low impedance scheme - how accurate is a CT under such low currents?  I am refering to the inaccuracies due to magnetising energy every time the current crosses zero - a previous thread was recently posted on this subject.  I have seen some literature suggest that your error increase exponentially for currents smaller than 5% of the nominal CT current.  For this example you need a trip for a secondary current of 0.5% (and even smaller to protect let say 80% of your Y - windings?)

I would value your feedback to see the mistake in my argument as I know REF is supose be OK.

RE: transformer REF HI protection

ScottyUK,

I like BCTs, but a core balance CT could be very senstive while still allowing lots of load current. I don't see why multicore cable would be required, just make sure all cables pass through the window, including the neutral which is insulated from ground until it passes the CT and hits the grounding resistor on the load side of the CT. Here's a CT with a 42" window: https://www.kuhlman.com/clientdata/PSG981SpecSheets.pdf

RE: transformer REF HI protection

KJvR,

In the low impedance scheme I'm familiar with, The neutral CT uses a small ratio and determines whether pickup occurs. Current from the matched phase CTs is summed and directionally compared with the neutral current to determine whether or not tripping is allowed. The neutral current should be accurate due to the low ratio, and the phase summation current would need to be 90 degrees in error for the relay to mis-operate. Further logic blocks against false I0 due to phase CTs saturating on three phase faults. Except for this blocking, the magnitude of the phase currents does not affect the protection.

RE: transformer REF HI protection

I guess if you managed to avoid any local core saturation effects caused by the bundle of cables passing through the window it might work. Not sure how the economics would work out compared with a more conventional scheme. It's certainly not typical of how UK transformers are protected - where multiple cables are used for connection, the CT is usually in or just behind the LV cable box. What is done in the US?

Anyway thanks for the link - it's useful to have the name of a supplier of such things. We have a number of huge Class X CT's fitted one per phase on our generator LV bushing turrets. One core forms part of the generator & IPB overall differential scheme, the other is used for the transformer diff. relay. They aren't as large as 42" diameter though, more like 30" or so. Big enough!

----------------------------------
  I don't suffer from insanity. I enjoy it...

RE: transformer REF HI protection

ScottyUK,

Don't know if I can speak for the entire USA, but installations I'm familiar with use BCTs with low impedance REF. I'm just throwing some wild donkey ideas into the mix for others to shoot down. What good is the forum if I can't learn something new?

RE: transformer REF HI protection

KJvR and cesgibis:

This following link might be helpful:
http://www.selinc.com/techpprs/6207_ComparisonHigh_CL_20051021.pdf

High Impedance REF ain't the only option - you might look into Low Impedance REF also. With the modern technology Low impedance REF schemes have improved a lot.

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RE: transformer REF HI protection

Ralph,

Good link. I'm confused about your first post of April 4. A downstream ground fault will return through resistor and to transformer neutral bypassing the earth tank protection. The in zone phase to tank wall fault, however, returns through the earth tank protection CT on its way to ground, resistor, and transformer neutral. The zone is strictly defined and high speed tripping can occur with no coordination issues.

RE: transformer REF HI protection

Steve:

Yes - you are correct - I mixed earth tank protection and standby earth protection. (CT in neutral wire)
However, and please correct me if I am wrong, wouldn't earth tank protection be more applicable to three limb transformers? (where there is not a path for the flux distribution or the circulating zero-sequence currents, except through the tank) With a 5-limb transformer you create a path for the currents and earth tank protection would be fairly useless. So in my opinion, if I am correct with the statement above, earth tank protection will be fine with a three limb (or a cheaper) transformer but not necessarily with a five limb transformer. Unfortunately I have never actually worked on such a scheme and am not familiar with it.

KJvR:

Regarding REF-protection:
With high impedance REF the big issue is not necessarily the knee-point of the CT, but actually the ankle region. That is why such a scheme prefers class X CTs, and especially CTs from the same batch - to ensure that the CT magnetizing current is very low and that all the CTs have the same (or as close as possible) magnetizing currents.
I also want to point out to look again at fig 2 in the pdf-file I have mentioned in my previous post. From the graph you'll see that the neutral current (and just note it is the neutral current and not the phase current) is very high for faults near the neutral. This is the current flowing through the neutral CT, and this is why the scheme is so effective near the neutral point. With modern technology we can use low impedance schemes much more effectively - actually the trend today is to move away from high-impedance schemes. Low impedance schemes are as good as high impedance schemes but much cheaper. (costs of metrosils, class X CTs, etc) Eskom (energy supplier in South Africa) did some tests and found that High impedance schemes for just 5% of in-zone faults better perform than low-impedance REF schemes, thus, for 95% of the faults Low impedance schemes perform better than High Impedance schemes. (just note that the low impedance schemes I am referring to are these used on the new microprocessor based relays)

Hope it helps

Regards
Ralph

Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon

Make the best use of Eng-Tips.com
Read the Site Policies at FAQ731-376

RE: transformer REF HI protection

Ralph,

I think you're taking about the magnetic circuit and it's reluctance to zero sequence flux? I'm a little rusty on all that, but once you've moved to the wires it's all current and impedance. Whatever the transformer's zero sequence impedance is in this case, is insignificant compared with that of the neutral grounding resistor. A bolted phase to tank wall fault will produce 300 A of fault current in the neutral unless there is a simultaneous neutral to tank wall (or core)fault. The trick is making sure that all the return current is seen by the protection, and no control wires are are inadvertently completing the circuit.

It also bothers me that a possible source of impedance is purposely placed where it could affect touch potential. I know it's only 300A and I believe an open CT secondary will cause the CT to saturate resulting in very little impedance showing up on the primary side. Still bothers me. I'd opt for the low impedance REF system.

RE: transformer REF HI protection

Ralph,

My orr. thought too about low magnetising currents because you operate in the ankle region.  However, you need a minimum current to drive your trip coil and this also pass through the stabilising resistors.  This voltage will be at least 75% of knee point voltage (if you follow standard practice in sizing the knee-point voltage as 2xtrip voltage).  From your link you need a trip at 15A to protect 95% of the coils for this thread.  I am now more convince that the only way will be low impedance diff protection.  I have never use this type of scheme before but 15A/3000A = 0.5% (phase CTs).  This type of accuracy is required because of current sumations.  Is this type of sensitivity obtainable or must you use metering CTs for your REF protection?

RE: transformer REF HI protection

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

Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon

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Read the Site Policies at FAQ731-376

RE: transformer REF HI protection

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.

RE: transformer REF HI protection

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

Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon

Make the best use of Eng-Tips.com
Read the Site Policies at FAQ731-376

RE: transformer REF HI protection

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.

RE: transformer REF HI protection

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?

RE: transformer REF HI protection

Quote:

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


Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon

Make the best use of Eng-Tips.com
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RE: transformer REF HI protection

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.

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