CT's in parallel question

[dgarayco]

Hey there!

I have a low voltage transformer with CT's on the low side of this.
The primary conduit on the low side transformer (5000A per phase) splits into a parallel connection, designed to have 2 lines per phase (2500A per line), there are two CT's on each line rated at 2500:2.5 C200 with RF 1.0. The secondary's connect back into a series connection, resulting in a 5A output.

Since the lines split for a parallel connection, we should see 2500A primary current per CT. But can anyone explain the rating of the C Class of the CT? Would it stay C200 or doubles to C400? does the RF of the CT's double?

As far as I can surmise, C class would stay C200, and RF would stay 1.0. I need a solid explanation on this of why these specifications stay the same mathematically as our specifications of 1 CT.

Thank you!

 

[System Protection]

Are you sure your CT secondaries are in series and not in parallel?

 

[waross]

Assuming that your meter or relay is rated for 5 Amps;
With 5000:5 CTs in parallel.
The CTs have a very high impedance to injected current and the meter has a very low impedance.
The current from each CT will flow through the meter element rather than through the other CT.
With 5000 Amps line current, each 2500:5 CT will output 5 Amps and the meter will see 10 Amps.
With 2500:2.5 CTs you will develop 5 Amps at full load. No need for 5000:5 CTs

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

"...The primary conduit on the low side transformer (5000A per phase) splits into a parallel connection, designed to have 2 lines per phase (2500A per line), there are two CT's on each line rated at 2500:2.5 C200 with RF 1.0. The secondary's connect back into a series connection, resulting in a 5A output.."

I have the following opinion for your consideration.
1. Two CTs on each line rated 2500:2.5 ...the secondaries connect back into a series connection, resulting in a 5A output; the C200 RF 1.0 remined the same. BTW: A 2500/2.5 A CT is not the standard rating on the market. The standard rating 2500/5 A may be is a better choice/ at a lower cost?.
2. With two CTs each 2500/5 A , the secondaries should be in parallel. The output is 2500/5 A, with the burden increased by 2.
Che Kuan Yau (Singapore)

 

[System Protection]

I have never heard of a 2.5A nominal CT.

Are you sure you don't have a 5000:5 CT? It's the same ratio as 2500:2.5

Two 5000:5 A CTs with 2500A primary in each phase with a SERIES connection will result in 2.5A flowing in the secondary. (With half the voltage across each CT compared to a single CT)

With a PARALLEL connection you will get 5A secondary.

You are not changing the C rating by paralleling or putting the secondary in series. Putting two CTs in series will drop the voltage across them by half and thus improve the performance during fault conditions.

 

[stevenal]

Interesting concept, parallel primary with series secondary. My first thought is with System Protection and che12345... don't. If one tries to output 2.5 and the other 2.4, who wins? But perhaps this is a way to force the primary lines to carry more equal current to a degree. If this is truly the case, the current does not double to 5 amps, it remains at 2.5 for a 5000 A load (2500 per CT primary). Please confirm your secondary connection.

 

[che12345]

I have the following alternative opinion to my earlier submission on 19th inst.
1. Previous opinion was to use two standard 2500/5 A CTs with secondaries in parallel. When both branches are each load to 2500A, the paralleled CT output would be 10A. Attention: 10A could be too high for standard ampere meters and protection relays, which are usually rated 5A. Possible solution: install a 5A+5A/5A summation CT. The output is 5A. This CT is low cost and small in size.
2. Alternatively, use two standard 5000/5 A CTs which may be bigger in size and more costly on each branch; connect the secondaries in series. The output would be 5A; which would suit the meters and relays directly.
Che Kuan Yau (Singapore)

 

[waross]

A question Mr Che;
We know that the current must be equal in all parts of a series circuit.
There are valid reasons why the primary current may not split equally on all phases.
In such a case, the secondary currents would not be equal, but the secondaries are in series and must be equal.
Mr Kirchhoff will cry; "Foul".
How do you resolve this issue?
The load in this case is the input impedance of the relay or meter.
Normally to share a load between two transformers, the transformers are connected in parallel.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

@ Mr. Waross
1. Agreed on your learned advice that "Normally to share a load between two transformers, the current transformers are connected in parallel".
2. CT secondaries connected in Series is also a common practice. The advantage is each CT will only half the excitation voltage, leading to large reduction in excitation current, thus improving the accuracy. Acknowledged that there are drawbacks.
3. Conclusion: Both series and parallel connection are possible/commonly used, depending on the actual/practical need.
Che Kuan Yau (Singapore)

 

[waross]

The closest that I have seen to CT secondaries in series was CT secondaries in delta.
When I was doing revenue metering for a small utility, I had few resources.
The only KWHr meters I had were two element meters.
Did you know that it is possible to meter unbalanced loads on a four wire wye circuit with acceptable accuracy with a two element meter? (eg: 120/208V, 277/480V circuits.)(Form 5S meters)
This is not a Blondel solution and makes an assumption that is generally but not always valid.
The metering scheme assumes that the phase voltages are equal. Unequal phase voltges may lead to metering errors.
The same may be said of the standard metering scheme for millions of residential meters on single phase, center tapped circuits. (eg; 120/240V circuits.)(Form 2S meters)
This is not a Blondel solution and makes an assumption that is generally but not always valid.
The metering scheme assumes that the loads are balanced. In the event of neutral currents causing neutral voltage drops due to unbalanced loads, the metering will be inaccurate.
The error is small and millions of single phase customers in North America have never suspected this.

Tescometering.com[/url]Tesco Metering]BLONDEL’S THEOREM
tescometering.com 21
Why are non-Blondel meters used?
• Fewer elements (meters) = lower cost
• Especially true for electro-mechanical meters
• Fewer CT’s and PT’s = lower cost
• Less wiring and cheaper sockets

These comments are true for both electro-mechanical meters and electronic meters.
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[davidbeach]

If the CT primaries are in series, the secondaries of two matched CTs may also be in series. That’s it. Ratio doesn’t change. Secondary current doesn’t change. But the voltage across the CT secondary will be halved.

Something to have be considered once upon a time when faced with a long run of secondary to a whole bunch of EM relays. But now that the relays contribute practically no burden there’s little reason for weird connections.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[che12345]

@ Mr waross
let us get it right.
1. SI units of measurement has been accepted in Europe IEC, in the US IEEE and in Canada CSA, without exception.
2. KWHr is incorrect. Should be kWh.
Che Kuan Yau (Singapore)

 

[waross]

Different strokes;
I use the English language Capitalization rules that I was taught in school.
The iec and ieee are a small subset of the users of the English language.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[davidbeach]

Oh, wow. News to me that KWHr doesn't include a "r". Guess I've been too busy with trying to get the late '70s transition from PT to VT incorporated into everything. Good to know that somebody's up on all the latest. And has that much spare time.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[wcaseyharman]

My team Just dealt with a similar issue on a 1993 vintage power plant, but used the CTs in series. First time we have come across this.

Our research discovered what others have already said, CT in series effectively increased the C rating as the voltage on each CT was half and thus would saturate at a much higher output.

It then follows that CTs in parallel effectively decreases the C rating because the apparent impedance of the relay burden is much higher due to the contribution of the parallel CT. T

Does it matter? My take is it Depends on the available fault current and the connected “burden.”

 

[che12345]

@ Mr wcaseyharman (Electrical)29 Jun 24 03:40
"...Does it matter? My take is it Depends on the available fault current and the connected “burden.” "
I am in full agreement with your learned advice. I would try to illustrate with the following example.
1. In the IEC world, e.g. CTs rated Voltage (11kV), Current ratio (1000/5 A), Class (5P10), Burden (10VA). Where Class 5P10 Burden 10VA, indicate that the accuracy is within 5% at 10 times the primary current, with Burden of 10VA.
2. Case A, two abovementioned Cts with their secondaries connected in Series. This is equivalent to a single CT rated Class 5P20 burden 10VA . Note: Class from (5P10) is improved to (5P20) , but the Burden remained at 10VA.
3. Case B, two abovementioned Cts with their secondaries connected in Parallel. This is equivalent to a single CT rated Class 5P10 Burden 20VA . Note: Class remained at (5P10), but the Burden is increased to (20VA).
4. Attention, Case C, neither of these CTs connected in Series nor in Parallel would be rated for Class 5P20 Burden 20VA.
Che Kuan Yau (Singapore)