Differential Protection

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redbluff

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Came across a situation in which the project engineer has a differential protection scheme that measures the incoming 13.8Kv to a transformer and then is measuring the (secondary) 480V bus (in some ratio I assume) for the differential protection which appears fine for the equipment in the substation. Then he used the same CT's C200, 0.3B0.1 (.3% @ .1 ohm allowable burden) for remote located transformers and 480V switchgear containing the low voltage CT 350 feet away from the 13.8Kv CT. Seems the remote transformers would need C200,0.3B0.5 or better to be able to maintain a Z that will accurately trip or not have a false trip. This unit is stretched thin on incoming power as it is and seems a critical portion of the new substation protection. I'm very uncomfortable with this situation. Guru's thoughts?
 

jdsmith

Senior Member
Location
Ohio
Came across a situation in which the project engineer has a differential protection scheme that measures the incoming 13.8Kv to a transformer and then is measuring the (secondary) 480V bus (in some ratio I assume) for the differential protection which appears fine for the equipment in the substation. Then he used the same CT's C200, 0.3B0.1 (.3% @ .1 ohm allowable burden) for remote located transformers and 480V switchgear containing the low voltage CT 350 feet away from the 13.8Kv CT. Seems the remote transformers would need C200,0.3B0.5 or better to be able to maintain a Z that will accurately trip or not have a false trip. This unit is stretched thin on incoming power as it is and seems a critical portion of the new substation protection. I'm very uncomfortable with this situation. Guru's thoughts?

Let me see if I'm following this: you have a transformer differential relay located near a 13.8/480 transformer. The CT leads from the primary of the transformer are fairly short, but the CT leads on the 480V side of the transformer do not go from the secondary bushings to the relay, the 480V CTs are actually located 350 ft away from the relay.

Am I understanding this correctly?

The burden on the CT is important, but it is a lot less important with modern microprocessor relays than it was with electromechanical relays. The burden of a microprocessor relay is nearly zero, so the only meaningful burden is the CT wiring and any analog meters that may be installed on the same CT.
 

redbluff

Member
Yes, the 13.8Kv CT is located in the 13.8v switchgear and the 480V CT is located 350 feet away. I see what you are saying about the relay burden which is a SEL for the 87 as opposed to a meter which would be a lot more or about 75va or so. The burden then would be essentially in the wire between the CT and the SEL and the CT Z?
 

jdsmith

Senior Member
Location
Ohio
Yes, the 13.8Kv CT is located in the 13.8v switchgear and the 480V CT is located 350 feet away. I see what you are saying about the relay burden which is a SEL for the 87 as opposed to a meter which would be a lot more or about 75va or so. The burden then would be essentially in the wire between the CT and the SEL and the CT Z?

Right, the only burden that matters with an SEL relay is the resistance of the wire. The impedance of the CT itself is a significant value, but typically CTs are rated on how much burden they can drive in addition to their internal burden - the internal burden is already factored in by the manufacturer. The rule of thumb is something like 1000 ft of wire has 1 ohm of resistance, but I don't remember if that was #10, #12, or #14 wire. In any case 700 ft of wire (you have to count the whole loop or wire, not just the distance) as the only burden on a C200 going into an SEL relay shouldn't cause a problem. This of course assumes that the C200 is appropriate for the level of fault current available and that the relay has an appropriate percentage restraint slope setting.
 

redbluff

Member
Thanks so much for the clarity, the fog is clearing a bit. So the CT should be selected for the burden it can handle which in our case is the 0.1 portion of the 0.3B0.1 metering accuracy rating? The fault calc he has looks correct for the CT. What I'm not quite clear on is the burden number on the CT and does it matter? Looking at the data it looks like it (the farthest) CT should be a CT capable of handling almost .5 ohms which would be a 0.3B0.5. The BOM says it is a C200,0.3B0.1 for relaying or metering.
 

SG-1

Senior Member
For differential protection all the CTs in the system usually have the same characteristics except for ratio when required.

I would expect the trip level would be fairly high because the relay must ignore the inrush current, which would only be seen by the transformer primary CT. So, any imbalence or CT error, during normal operation would be much less than the inrush.

Welcome to the forum !
 
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redbluff

Member
The 480V switchgear we are looking at happens to be SG-1 - ha! So for a fault event the C-200 is way more than adequate. If we also use the CT for a metering or data input for the fiber Power Net it seems the CT's need to be suitable for that purpose as well? All the SEL's are connected to the power fiber network and will be monitored in several offices. Almost sounds like there should be two systems.

I've been reading these forums for quite some time and have benefited greatly as well a few laughs.

Thanks, Bill
 

SG-1

Senior Member
Do you talk to your switchgear often ?

Do you talk to your switchgear often ?

Has your switchgear ever posted to you before ?? :grin:

Differential CTs are usually dedicated to that one purpose alone. If metering or other protective relays are used they get their own CTs.

If the one SEL relay is doing everything, then never mind, but the accuracy class of the CT must be considered & suitable for the purpose. I do not know the miminum standard for a revenue metering CT, others here might & it may vary from poco to poco. I am specialized in medium voltage switchgear, not low voltage.
 
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redbluff

Member
My switchgear has talked to me in many ways but this a first! HAR!:) The decision making in the system is by PLC's and not the SEL's. In this case there are two involved and this is where I left out some important info while over thinking the issue. You can overcome some offset with scaling in the plc but probably makes zero difference to the differential trip as you pointed out. The monitoring of this system will be a little off which will prompt the question from the boss - why? after all the money he spent. The new MV sub we are starting now will use the SEL's for the decision making and monitor with transducers so none these issues are present.

Thanks, Bill
 

jdsmith

Senior Member
Location
Ohio
Metering off of CTs designed for protection is an interesting topic that many do not fully understand.

Protection CTs are not specified or designed to accurately reproduce currents at the low end of their range, say less than 20% of their full load rating. Accuracy at the low end doesn't matter to relays, which are generally concerned with currents from around 75-100% of full ratio for a thermal overload element and many times full ratio for overcurrent and other relay elements. There is some linearity error to be expected between 10-20% or full load, with a much higher error below 10%.

A number of protective relays now offer utility class metering capability, which means that the analog to digital converters and the rest of the electronics inside the relay have less than 0.2% error (off the top of my head, this number may not be correct) taking the analog current signal, digitizing it, and scaling it. Some people assume that utility class metering capability on a relay automatically means that the kWh and kVARh figures coming out of the meter meet that accuracy requirement, but it is not necessarily true unless the CTs are also reproducing currents accurately. Metering CTs are designed to reproduce current accurately from a very low value up to their rating, while relaying CTs are not concerned with accuracy at the low end of their range. If you need accurate metering at the lower end of the CT range you may need to explore this a little more and consult your relay manufacturer for guidance.
 

SG-1

Senior Member
One interesting point that did not come up is the use of harmonics to distinguish between inrush & fault current. Transformer inrush current is rich in ( if I am not dain bamaged ) the third harmonic. This is used to restrain the protective relay from false tripping.

With the old electro-mechanical relays like the Westinghouse HU ( Harmonic Unit ) you knew what the relay was "thinking". Now many relays like the SEL use "secret" algorithms.
 
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