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Current Transformer Question

Current Transformer Question

Current Transformer Question

(OP)
I have a generator control unit on a 500kva 415v generator. The control unit has short-circuit protection settings, but short-circuit protection is covered by a circuit breaker with an electronic trip unit. The current transformers feeding the control unit are 800/5 metering CTs, so they are used for long-time overload protection and metering.
The short-circuit settings in the control unit were left at their default values of 300% and 0.01 seconds.
After a downstream fault, the control unit picked up a short-circuit fault.
My question is why did the control unit register the short-circuit current? I thought that the measurement CTs would only respond up to around 120% of their rated value. The control unit has the correct CT ratio entered.
Thanks.

RE: Current Transformer Question

It likely within the VA rating of the CT and the burden of the trip unit is very small and all else being local at the Gen set.

RE: Current Transformer Question

Perhaps both protection pickup but only control unit trip and long-time overload protection did not conclude trip.

RE: Current Transformer Question

It is certainly not a given that the metering CT will fall flat for currents beyond 120%. For currents > 120% of rated, the CT starts going into saturation by design. All this means is that the CT error starts to increase beyond the declared limits and the output starts to get distorted but there will still be some output.

How much output and how much distortion depends on many factors - fault current magnitude, X/R of fault current, point on wave at which fault inception occurrs, connected CT burden and their pf (leads, measurement devices, etc.), duration of fault current, etc.

It appears that in your case there was sufficient output to operate the control unit. If you do not wish to have the control unit operate then I would probably recommend maximum settings or disable the protection.

If fed from a metering CT then the intention is that the control unit should not respond to fault currents. The metering CT is meant to saturate for I > 120% of CT rated so as to prevent the measurement equipment being gored to death by the high fault currents. But it sounds like your control unit could be fault rated?

In your case you probably did not have maximum fault current or the fault was some distance away or maybe a phase fault with some resistance such as arcing.

RE: Current Transformer Question

(OP)
Thanks for the replies.
The controller CT input can tolerate 150 amps for 1 second, so no concerns there. I can increase the controller short circuit settings to maximum to get it "out of the way".
But it leads me to another question - The metering CT is obviously capable of delivering substantially more than 120% of its rated value, so is the saturation of the CT mainly dependant on its VA rating? ie if the normal load burden on the CT is small relative to the CT VA rating, then a fault on the primary will result in a higher secondary current than if the normal load burden is closer to the CT VA rating? I hope my question makes sense.
I have never delved too deeply into the finer details of CTs, but I remember a colleague attending "A Short Introduction to Current Transformers" and the course notes were about 200 pages!

RE: Current Transformer Question

Metering class CT VA rating is its continuous power rating based on nameplate burden and current rating factor and be within its guaranteed accuracy. If you can perform a saturation test, the knee point voltage is more telling than the VA rating since you can calculate better what the max secondary fault current RMS limit would be knowing the burden or reduced burden, without distortion.

Because VA is a continuous rating it is not as simple to calculate what a maximum RMS short time current is. I think it is more of an educated guess.

GE has a write up about this, I'll see if I can find it and post.

RE: Current Transformer Question

I thought that metering CTs will maintain accuracy up to 200% rated current. Is/Was this an old standard?

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Current Transformer Question

Up to what level a metering CT will maintain accuracy is based on the rating factor of the CT. There is no "standard" and it can range from 1.0 up to 4.0 typically. In the ANSI/IEEE market it is common to have rating factors up to 4.0 (400%Inom) as the normal revenue meter used is a class 20 (20A continuous input). Assumptions that metering CTs will saturate above 120% or 200% are not correct.

Now, at what point the CT will actually saturate is not a direct function of the rating factor, but rather the knee-point voltage of the CT. For a metering CT, you cannot assume that the CT will saturate when operated above it's rated burden. You really need to find out the saturation voltage point and then work backwards from that.

RE: Current Transformer Question

This write up is GE so it promotes GE relay functioning using small size CT when the instantaneous element can work with saturated CT. Schweitzer uses current peak detector circuits to enable their 50 elements to work during such conditions. For true timed OC functions, CT must still operate in the linear region.

RE: Current Transformer Question

CT saturation is really a relative term.

Kneepoint of a CT is only defined for Class X CT's (as per IEC 60044). I do not know if there is an equivalent definition in the ANSI world. Typically we are looking at around 1.4Tesla of flux at Ek for a Class X. For a Class 5P it is around 1.6T and 1.9T for a Class 10P protection CT.

It all boils down to CT error at the end of the day. For a Class X CT it is typically < 0.25% whilst it is 5% and 10% for a 5P or 10P CT (not going into compound vs ratio and angle error here but for ease of calcs it is usually assumed to be ratio error). Thus the same CT can be badged as a Class X, 5P or 10P all depending what you wish to use it for. Once the error is exceeded for a declared limit (e.g. 5P) then the CT is considered to go into saturation. For example if a 5P CT is operated at around 1.8T. But if this same CT was rebadged as a 10P then a 1.8T flux density is not considered as the CT having gone into saturation. Thus the saturation voltage point is not one fixed point for a given CT but rather depends on the declared error of the CT as explained above.

For any CT, whether metering or protection, if the current exceeds rated or ALF current, it does not automatically imply that the CT is going into saturation. It depends on whether the connected burden is greater or less than rated burden, offset in fault current, etc.

collies99 - thanks for the GE article. Will give it a read. Looks good.

genhead - But it leads me to another question - The metering CT is obviously capable of delivering substantially more than 120% of its rated value, so is the saturation of the CT mainly dependant on its VA rating? ie if the normal load burden on the CT is small relative to the CT VA rating, then a fault on the primary will result in a higher secondary current than if the normal load burden is closer to the CT VA rating? I hope my question makes sense.

Maybe rephrase as I do not quite understand what you are asking.

RE: Current Transformer Question

(OP)
Thanks all.
Veritas,
Firstly, I was incorrect in my belief that the maximum secondary current of a measurement CT would be 120% of rated. I had read that this was so, in order to protect meters from high current, in the event of fault current on the primary.

For my 800/5 measurement CT.

a) As primary current increases, at what point will the secondary current stop increasing?

b) What impact does the burden have on this?






RE: Current Transformer Question

a) when the CT core begins to saturate it can no longer support a changing magnetic flux which is required for the transformer action to take place. So as the primary current increases on its sine wave, the output current begins to collapse towards zero. we are talking about real time instantatneous values. The point at which the CT begins to saturate is called the knee point voltage or the saturation voltage level and related to b)

b)The burden have no impact of the saturation voltage level as it is a design issue. However, the burden have an impact at the level of secondary current which can be reproduced without distortion as a function of the primary current flowing. Example: A ct has a design knee point voltage of 10V, nampelate ratio is 50:5A or 10:1A. With 50 amps flowing in the primary, the secondary can be reproduced without distortion at 5A RMS into a maximum burden of 2 ohms, 2 x 5 = 10.

If the burden is increased to 4 ohm, then it can only reproduce without distortion 2.5A RMS secondary or with 25A primary flowing. If 50A was to flow in the primary you would not get 2.5A RMS since the waveform would be distorted and may be equivalent to .5A RMS (that is just a figure out of the air).


Hope that helps

RE: Current Transformer Question

genhead - look, you're not far off. For a metering CT you would typically have it being good up to a factor k x rated current. Why I say k is because k will vary from one standard and country and practice to the next. For argument's let's say k  =  1.5. So the CT is good for 1.5 x 800  =  1200A in your case. What I mean by "good" is that the CT error will be less than the specified error up to 1200A.

It is important to note that the above is true only if rated burden is connected. This is the total external secondary load. Basically, the secondary current is not a function of the connected burden but of the primary current. After all it is a CT, the secondary must be a faithful replica of the primary. To achieve this, the CT must be able to generate enough voltage to drive the secondary current through the secondary circuit. This is where the burden comes in.

Let's say rated burden is 5ohms. At 150% of rated current, you will have 1.5A secondary. Thus the CT must be able to generate a voltage of 5 x 1.5  =  7.5V (this is the induced secondary emf). If the current is further increased to say 5 x rated then the voltage required is 25V and so on. There comes a point where the CT is not able to generate the required voltage as the core is starting to saturate.

When this happens, there is still an output from the CT but the secondary current is now starting to distort. But there may still be sufficient energy to operate a relay, if the curren is above it's setting.

If the current is increased even further, the output becomes more distorted and less in terms of energy (area under the current waveform is less). With very high currents there is hardly any output at all and one gets spikes at the zero crossover points of the current waveform. The CT is now in deep saturation - and it's not a good place for it to be in as it can seriously overheat, etc.

Now if the burden is halved to 2.5ohms then at 150% of rated current the CT voltage required is 1.5 x 2.5  =  3.75V. At 5 x rated, V required  =  12.5V. Thus with a reduced burden there is a less onerous requirement on the CT which means it can accommodate more current before going into saturation.

The converse is of course true if the burden is increased.

I hope this explains somewhat the effect of the burden on CT performance.

Kind regards.  

RE: Current Transformer Question

The idea that a metering CT should saturate to protect the attached meter from high current sounds like this may be IEC stuff? I believe this is Dependant on the CT's instrument security factor.

RE: Current Transformer Question

(OP)
Veritas,
That was a very helpful explanation.
Thank you.

RE: Current Transformer Question

Genhead

You're very welcome.

Kind regards.

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