ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
ShoutChat
Recent Posts
Increasing demand factors in residential
by gfretwell - 03/28/24 12:43 AM
Portable generator question
by Steve Miller - 03/19/24 08:50 PM
Do we need grounding?
by NORCAL - 03/19/24 05:11 PM
240V only in a home and NEC?
by dsk - 03/19/24 06:33 AM
Cordless Tools: The Obvious Question
by renosteinke - 03/14/24 08:05 PM
New in the Gallery:
This is a new one
This is a new one
by timmp, September 24
Few pics I found
Few pics I found
by timmp, August 15
Who's Online Now
2 members (Scott35, gfretwell), 269 guests, and 15 robots.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
Page 2 of 3 1 2 3
Joined: Dec 2005
Posts: 869
Likes: 4
R
Member
Quote
Ratio's of 100:5, 200:5, 300:5, 500:10, 700:10, 1000:20 are common here at 400V

Trumpy:
Very interesting are the 500:10,700:10 and 1000:20 values. Are these metering CT's or protection CT's? [Linked Image]

I have done a lot of CT metering work in the North Island and the secondaries were always referred to as 5 Amps for 400 Volts metering. The Watthourmeters are very accurate between 0.5 and 6.5 Amps loading. With current in the 10 to 20 Amps range the current coils in the 5 Amp meter would be saturated and big under readings recorded on the watthourmeter.
Most CT meters class 1 and 2 were rated at 5 Amps ( Sangamo, Ferranti, English Electric, Landis&Gyr ).
Some Email meters had nameplate ratings of 2.5 / 10 Amps. ( the only ones I'm aware off ) which were designed to take 10 Amps from the CT's.
CT accuracies were CM, BM or class 1 and common maximum burdens were 5VA or 10VA for CT's.
A Trivector kWh / kVArh meter would have a burden of about 4 VA.
Auckland Central used thermal MDI meters in series with a standard kWh meter which required 10 or 15 VA rated CT's
Ratio's used were 100,150,200,250,300,400,500,600,800,1000,1200,1600,2000,3000 / 5 Amps.
In installations over 1600 Amps often summation CT's were used because double feeders were provided for in large pumpstations or shopping malls.
The multitap CT's were most accurate on the highest tapping while being tested.( these CT's were e.g. 150/250/400/5A ).

From 2000 onwards through the MARIA standards a lot of multitap CT's are being replaced to single tap class 0.5 or 0.2 CT's for the T.O.U. metering.


The product of rotation, excitation and flux produces electricty.
Arc Flash PPE Clothing, LOTO & Insulated Tools
Joined: Dec 2005
Posts: 869
Likes: 4
R
Member
A thing I forget to mention is the testblock.

The CT secondaries were earthed at the common point at the testblock. Also here were the shorting links provided to short out the CT's and the PT fuses to isolate the voltage to the meter if a meter change was required for a different tariff.

The non standardising of CT wiring in different networks was a bit of a worry sometimes. Waitemata Area had CT's earthed, and Auckland not, Hamilton had different colour CT wires while Thames Valley had the testblocks the other way around.
Through the new MARIA standards at least earthing of CT secondaries is now mandatory which will improve metermen safety and on new installations the wire colours are standardised too.
Current S1-S2 (M-L) Blue-Yellow(White) and Potential Red. The 3 core usually TPS sleeved with the appropiate phase colour.
Neutral Black and Earth Green / Yellow.

So a typical CT installation had 11 wires coming from the CT chamber.


The product of rotation, excitation and flux produces electricty.
Joined: Sep 2001
Posts: 806
N
Member
And which direction does an AC current flow in, exactly?

The polarity markings on a transformer are to determine proper phasing. On a schematic diagram, this is usually indicated by a dot at one end of each winding. When the marked end of the proimary goes positive, so do the marked ends of all the secondaries...

[This message has been edited by NJwirenut (edited 01-07-2006).]

Joined: Sep 2001
Posts: 806
N
Member
Quote
The dot represents the line side, and the current must pass through in that direction.

In an AC circuit, the current flows in BOTH directions, reversing at 2X the line frequency.

The markings are there to make sure that the METER connected to the CT secondaries turns in the proper direction. It makes no difference at all to the CT which side of the donut is line or load. If you were to flip the CT connections (primary AND secondary) the system would operate just fine.

In a simple system using a CT to monitor a single conductor with an ammeter across the CT secondary, these "polarity" markings become completely meaningless. The "polarity" (or "phasing") of the connections only becomes important when multiple legs are being metered by a single device. Then you need to maintain the proper phase relationships between the current and voltage signals from all 3 phases in order for the meter to register correctly.

Joined: Dec 2005
Posts: 869
Likes: 4
R
Member
In New Zealand the newer CT's (1990's onwards) are marked:

Primary: P1 lineside - P2 loadside.

Secondary: S1 polarity to meter - S2 return from current coil from meter. (S3 - S4 may be there on multitap CT's).

Older CT's were marked as:

Primary: P1 lineside - P2 loadside.
or manufactures CT label on primary side.
Sometimes with a dot.

Secondairy: com. 150 / 250 / 400 on a multitap CT.
also markings as M - L were used, M to meter, L from meter. ( these were single tap CT's ).

It is very important that the correct phasing relationship exists between the voltage circuit and current circuit.

e.g. Red potential, Red current S1 - S2 and so on.
With Yellow potential for example and Red current S1 - S2 the meter will go backwards.
Red current and S2 - S1 meter will go backwards too. + various other combinations possible.

On CT checks with testblock links open and meter connected and 3 phase load a meter potential was livened one at the time and the meter has to make a forward movement of the disc or pulsing LED. Then one fuse was inserted, second fuse, third fuse and an increase of meter has to be noticed. If not there may be wrong wiring.

Also the primary current / secondary current has to be measured with accurate clamp meters and the ratio confirmed.

Say for 250 / 5 CT tap.
Primary current 200 Amps / secondary 4 Amps.
usually there are some minor reading errors in the clamp meter but close enough to see if the CT is at the correct tap. ( 201A / 3.92A ). multiplier x 50.

For TOU metering a half hour load check was done and primary readings compared with readings obtained from standard.


The product of rotation, excitation and flux produces electricty.
Joined: Dec 2005
Posts: 869
Likes: 4
R
Member
It's important to do a job properly !!
CT's in the correct way.

I case of multitap CT's it is very important that P1 - P2 sides S1 - S2 sides are connected up correctly if high accuracy is required.
On the lower tappings there is a compensating wire in parallel with the main CT current wire. This one is of smaller diameter. It's there to allowe for compensation to have good acuracy.
Some classes are stated as followes
500/750/1000/5 Amps class 1.0/1.0/0.5
The compensation is for the 500 and 750 tap.


The product of rotation, excitation and flux produces electricty.
Joined: Sep 2001
Posts: 806
N
Member
XtheEdgeX:

Your original quote:

Quote
And don't forget that ct's are directional. There is a polarity mark on them, to distinguish the direction of the flow of current.

CTs are NOT "directional". Current flows equally well through them on either half of the sine wave cycle. If you want to see a "directional" device, how about a diode? The polarity markings on a CT have nothing to do the "direction of the flow of current". The current flows in BOTH DIRECTIONS in a transformer, because it is an AC device.

The markings are there to establish the PHASE RELATIONSHIP between the primary and secondary sides.

No, I haven't wired any substations, but I have installed a lot of CTs on control systems, motor drives, energy management systems, and the like. And I always follow the polarity markings when doing so. if nothing else, it makes for easier troubleshooting down the line. Nobody is arguing the need for following industry standard practices, only your overly simplified explanation of the reasons BEHIND these practices.

Knowing the WHYS as well as the HOWS behind the work we do is what separates the electrician from a monkey pulling wire through a pipe.



[This message has been edited by NJwirenut (edited 01-08-2006).]

Joined: Sep 2001
Posts: 806
N
Member
Quote
The ratio of the current transformer can be modified by adding more primary turns to the transformer. By adding primary turns, the current required to maintain five amps on the secondary is reduced.
The ratio of the current transformer can be modified by altering the number of secondary turns by forward or backwinding the secondary lead through the window of the current transformer. By adding secondary turns, the same primary current will result in a decrease in secondary output. By subtracting turns, the same primary current will result in greater secondary output.

So exactly what there makes a CT "directional"? It behaves like ANY OTHER TRANSFORMER.

Adding secondary turns to a normal transformer will increase secondary VOLTAGE, while dropping secondary CURRENT. Adding primary turns will DROP secondary voltage, while increasing current (subject to limitations of wire size and core saturation).

The fact that added windings (such as those through a CT window) all need to be wound in the same direction is nothing specific to a CT, either. Again, it has to do with the relative phase relationships between the windings. The same way that the LV windings on a buck/boost transformer can be connected to add or subtract voltage, by connecting them series-aiding or series-opposing.

Not an issue of "direction of current flow", but one of relative phase relationship.

Joined: Sep 2001
Posts: 806
N
Member
Quote
If you wind the conductor through the window one direction, you get a result, if you wind the same conductor through the window in the opposite direction, you get a different result, making a CT directional to the flow of the current.

Take a 100:5 CT. Place a 5A ammeter across the secondary. Run 2 loops of wire through the window, and pass a 50 A current through the wire. What does the ammeter read? 5A, of course.

Now reverse the direction of the 2 loop winding. What does the ammeter read now? Still 5A? Hmmm...

The CT itself isn't "directional". It is just that the majority of applications involve more than one CT, and the MEASURING EQUIPMENT connected to the CT (and PT) secondaries depends on a precise phase relationship in order to read correctly.

Joined: Sep 2001
Posts: 806
N
Member
Quote
You are wrong in that assumption.

WHAT assumption?

Quote


Hmmm. That page shows the EXACT example I gave, of a 100:5 CT converted to 50:5 (or 25:5) with additional primary turns. I don't remember saying ANYTHING about added secondary turns, but the Simpson page gives a siimilarly good, clear explanation of those installations.

[quote]And what about the bar type CT's here. You better mount them with polarity in mind. ]http://www.kilowattclassroom.com/Archive/CurrentTransf.pdf[/quote]

Yeah, kinda hard to put an extra turn through one of those, ain't it? But again, when used with a single ammeter as an indicator, it really doesn't make a bit of difference which way the thing is oriented. The polarity markings are only important when the downstream equipment cares about the phase relationship between multiple phases, or between volatge and current signals from the same phase. This would include MOST CT applications, as they are USUALLY used in sets of 3, to feed a polyphase metering setup.

I don't think we disagree AT ALL about the need for observing proper polarity in a CT metering installation. But where I did disagree is in calling a CT a "directional" device. It works just the same no matter what "direction" the current is flowing, same as any other transformer. But the equipment DOWNSTREAM of the CT usually cares very much about such issues.

[This message has been edited by NJwirenut (edited 01-08-2006).]

Page 2 of 3 1 2 3

Link Copied to Clipboard
Powered by UBB.threads™ PHP Forum Software 7.7.5