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]

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?



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[cesgibis]

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.

 

[stevenal]

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?

 

[RalphChristie]

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.

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[KJvR]

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

 

[RalphChristie]

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





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[stevenal]

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.

 

[stevenal]

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

 

[ScottyUK]

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.


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[KJvR]

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.

 

[stevenal]

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

 

[stevenal]

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.

 

[ScottyUK]

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!



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[stevenal]

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?

 

[RalphChristie]

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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[stevenal]

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.

 

[RalphChristie]

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

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[stevenal]

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.

 

[KJvR]

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?