I need help with a current transformer diagram

[weressl]

I think we're talking about two different things . What I'm thinking about is multiple breakers on a bus , with a " sum point " . Each phase current then goes to a bus relay , Example , an ABB type KAB . Maybe I got away from the original question . :smile:

I posted a transformer differential relay wiring, but a bus diferential relaying will work the same way.

GE Power Management publication GET8386.

CONCLUSION

​

​

External summation of the CTs may be considered under the following conditions:​

1.​

​

Neither of the two circuits that are paralleled is capable of producing any significant
fault current. Ideally they should feed pure loads.​

2.​

​

Even if the two circuits feed loads, some infeed is possible on ground faults through
the zero-sequence current unless the loads are connected in delta. The effect of the
zero-sequence infeed current on CT saturation must be considered when calculating
settings.​

3.​

​

The paralleled CTs are guaranteed to operate correctly (with minimal errors) in the
situation when there is not extra restraint and the fault current flows in and out of the
zone ?unnoticed?. Or, in other words, the pickup setting is high enough to prevent misoperation
due to saturation of the paralleled CTs.​

4.​

​

If needed, the situation shown in the above figure shall be identified through input
contacts, and appropriately settings shall be applied through multiple setting groups
in order to ensure security.
Overall, paralleling CTs for low impedance differential applications is always risky and shall be approached with maximum caution.
​

[FONT=Arial,Bold]

New York State Electric Meter Engineers? Committee Guide for Uniform Practices in Revenue Quality Metering
​

Rev. 4: August 20, 2003
......
3.8 CT secondary windings should not be connected in parallel.
......

There are schemes and special devices that do use multiple CT's for a single input, but it is not commonplace and their use is very selective and requires special care in selecting the CT's, matching their burden and having identical saturation curves.

​

[/FONT]

 

[jim dungar]

There are schemes and special devices that do use multiple CT's for a single input, but it is not commonplace and their use is very selective and requires special care in selecting the CT's, matching their burden and having identical saturation curves.

​

​

​

​

This is very true in the world of protective relaying. However, in the world of electronic metering/monitoring (which the OP has), things are often a lot looser. Many people say "its just metering" and they often ignore alot of the basics of applying CT's.
​

 

[jim dungar]

This is using Square D products but it is applicable to any device. Based on what "normal" value is not correct it becomes an easy task to narrow down the potential proble areas (i.e. CT's pointing in different directions).​

What is "Normal"?
​

Most power systems have a lagging (inductive) power factor. The Circuit Monitor and PowerLogic software displays lagging power factor as negative, leading (capacitive) power factor as positive. The only time a leading power factor would be expected would be if the customer had power factor correction capacitors switched in or over-excited synchronous motors with enough capacitive KVARS on-line to overcorrect the power factor to leading.​

"Normal" lagging power system readings are as follows:
Positive KW = ( 3 * VAB * I3[FONT=Symbol,Italic][FONT=Symbol,Italic]f[/FONT][/FONT]Avg * PF 3[FONT=Symbol,Italic][FONT=Symbol,Italic]f[/FONT][/FONT]Avg )/1000
Negative KVAR = ( (KVA )2-(KW)2 )/1000
KVA (always positive) = ( 3 * VAB * I 3[FONT=Symbol,Italic][FONT=Symbol,Italic]f[/FONT][/FONT]Avg )/1000​

PF

​

​

3[FONT=Symbol,Italic][FONT=Symbol,Italic]f[/FONT][/FONT]Avg = lagging (negative) in the range -0.70 to 1.00 (for 4W systems, all phase PF's about thesame)

Phase currents approximately equal

Phase voltages approximately equal​

Quick Check

A quick check for proper Circuit Monitor readings consists of KW comparison (calculated per equation above and compared to the Circuit Monitor reading) and a reasonable lagging 3-phase average power factor reading. If these checks are OK, there is no reason to do anything more.
If your calculations indicate that Circuit Monitor readings are correct but the customer disagrees, check CT ratio, PT ratio, and system type (3-wire or 4-wire) programmed into Circuit Monitor versus what is actually installed. If this is not the problem, ask customer what he based his estimate of the "correct" reading on. ​

Be aware that there is a surprisingly high percentage of mis-wired watt-hour and power factor meters installed which make the Circuit Monitor appear wrong. Many times old meters that "never have worked right" have been in reality mis-wired from the start and the new Circuit Monitor is using the same instrument transformers.​

 

[weressl]

This is very true in the world of protective relaying. However, in the world of electronic metering/monitoring (which the OP has), things are often a lot looser. Many people say "its just metering" and they often ignore alot of the basics of applying CT's.
[/left]

Yep, its only money, right?! That is what is metering used for be it revenue metering or used only for intra-company cost accounting which is used to determine the cost of goods. Why meter if you can afford being wrong or inaccurate. Then they will try to ballance the reading and the beancounters realize that the meters do not sum up and just go and ignore the readings and use the previous percentage split between the cost centers as before. So now you have this fancy metering system installed and its basically ignored. Seen it happen.

 

[beanland]

CT by the Dots

CT by the Dots

The dot indicates that as current is flowing into the CT from line toward load, the CT will produce current flowing out of the dot. If the CTs are installed upside down, just reverse the connections at the CT. The CT ratio is not affected.

 

[wirenut1980]

Thanks to everyone that has responded so far. :smile:



Well I am going to need a bit more than 'Laszlo says it is an ABSOLUTE no-no.'

The equipment and design was by a Siemens Engineer using Siemens meters. An EC walking in and saying it's all wrong without something to back that up will be laughed all the way out the door.

This metering is not for protection, it is to monitor energy consumption of various feeders to assign electrical costs to the right departments in a large company.

I would suggest monitoring voltage, current, kW, KVA, etc at a metering point and compare your results with the metering. This would be easier than getting into an electrical theory war with a Siemens engineer.

You should not have to do this if everyone knows the metering is wrong, but sometimes people have to see proof.

I think Jim is right on the money that some CT's might be wired backwards, r maybe some are not wired up at all. I have been to a school where they had a nice voltage and current monitor installed at the service and was worried about their power factor. I noticed that the monitor was not measuring any current. The CT's were either never installed or connected properly.

 

[gar]

081204-1130 EST

Iwire:

I assume you know that you do not want to open circuit the ouput of a current transformer with current flowing thru the current transformer. This comment is for primarily for those unfamiliar with current transformers. But there is no problem if you short the output of a current transformer.

If you short the outputs of two current transformers at the same time, then only one is feeding the instrument. If you know that there is a power load on that phase, then your metering system should display positive power. If not, meaning negative power, then either the current transformer is reversed or the potential transformer is reversed. Assume for the moment that the correct phases are associated with each other.

If positive power is read from the first one you test this way, then proceed to a second one, and then to a third.

If each of these show positive power, then there may be a question of whether the potential and current circuits are correctly associated.

There is always a possibility of a problem in the power measuring equipment.

.

 

[iwire]

Iwire:

I assume you know that you do not want to open circuit the ouput of a current transformer with current flowing thru the current transformer.

Yes I do know that, but thank you, letting the smoke out is something I want to avoid. :smile:

If you short the outputs of two current transformers at the same time, then only one is feeding the instrument. If you know that there is a power load on that phase, then your metering system should display positive power. If not, meaning negative power, then either the current transformer is reversed or the potential transformer is reversed.

That is a great idea, and one that can be accomplished easily and without having a shutdown.

Thanks.

There is always a possibility of a problem in the power measuring equipment.

I think there is a strong possibility that the metering equipment software settings are wrong.

 

[Cold Fusion]

(cut)That is a great idea, (shorting two of three CTs)and one that can be accomplished easily and without having a shutdown. (cut)

Iwire -
As you likely already knew, this won't work with paralleled CTs. Shorting one CT shorts them all. I'm sure gar was refering to a standard 3ph metering input to a power monitor.

cf

 

[gar]

081204-1307 EST

Yes I am assuming these current transformers are independent of one another based on the original circuit drawing and the later information that this is a power monitoring application.

.

 

[iwire]

081204-1307 EST

Yes I am assuming these current transformers are independent of one another based on the original circuit drawing and the later information that this is a power monitoring application.

.

Some are independent of each other and some are not.

I can short and disconnect one set of CTs while I experiment with another set.

 

[LarryFine]

I can short and disconnect one set of CTs while I experiment with another set.

I'm picturing doing that while loaded. It could be tricky to accomplish.

 

[iwire]

I'm picturing doing that while loaded. It could be tricky to accomplish.

Short CT first (Milli-volts) then remove connection to meter. :smile:

 

[steve66]

How is power factor determined in that case? Average?

The CT's measure the total current, and the voltmeters measure the total voltage. So the total PF is also figured into the calculation - no approximations or averages are necessary.

Are any readings on the meters with single sets of CT's messed up??

Since you have looked at the CT's, and they all seem to be installed right, any chance the voltage leads don't have the correct phasing?

Steve

 

[gar]

081204-1623 EST

steve66:

The circuit diagram shown in post #1 is a Y system, at least on the source side. There is a voltage reference terminal from the neutral. For power measurement and analysis this can be considered as three independent single phase circuits wherein we measure the power independently in each by multiplying the instantaneous voltage of said circuit by the instantaneous current of that circuit. Average the power for each and sum the three power values to get the total power.

It is my assumption that this description is how the black box power meter works.

If this black box also measures energy used, KWH, then the power measurement needs to be integrated. Integrated in this case means to accumulate the sum of small increments of power times small time intervals. For example in a 1 secnd time the average power was 4 W, and in the next 1 sec it was 20 W, then for those 2 seconds 24 W-sec are added to the accumulator. The next second has 1 W and at the end of the 3 seconds the total accumulation is 25 W-sec.

Current and voltage are vector quantities, and power is not. You obtain the total power from the numeric sum of the individual power values.

By shorting all but one of the current inputs I am measuring the power in one circuit. The total power is the sum of the power in each of the three circuits.

I have no need to know the power factor to measure power when I measure the average of the product of the instantaneous voltage and current.

If you calculated power by measuring average voltage and independently measure average current, then you need a value for power factor. Where does this power factor value come from? Out of the thin air?. Suppose I have a complex waveform rather than a pure inductance with a series resistance. How do I determine the power factor?

The CT's measure the total current, and the voltmeters measure the total voltage. So the total PF is also figured into the calculation - no approximations or averages are necessary.

I do not know what this paragraph says.

The circuit diagram of post #1 does not allow for an inversion of a voltage input. It is possible that if potential transformers are used internally that an inversion could occur. But extremely unlikely.

What is possible on the outside is to have a mixup of the relationship of voltage leads and current transformers, and current transformer polarity.

.

 

[gar]

081204-1704 EST

Iwire:

I do not think that you need to remove any current transformers. Just short the two that you want to inhibit.

.

 

[iwire]

Here is some info about the meter itself if anyone is interested. :smile:

Siemens 9200 Installation & Basic Setup Instructions

9200 Power Meter Installation & Operation Guide

 

[steve66]

081204-1623 EST

steve66:

The circuit diagram shown in post #1 is a Y system, at least on the source side. There is a voltage reference terminal from the neutral. For power measurement and analysis this can be considered as three independent single phase circuits wherein we measure the power independently in each by multiplying the instantaneous voltage of said circuit by the instantaneous current of that circuit. Average the power for each and sum the three power values to get the total power.

It is my assumption that this description is how the black box power meter works.

If this black box also measures energy used, KWH, then the power measurement needs to be integrated. Integrated in this case means to accumulate the sum of small increments of power times small time intervals. For example in a 1 secnd time the average power was 4 W, and in the next 1 sec it was 20 W, then for those 2 seconds 24 W-sec are added to the accumulator. The next second has 1 W and at the end of the 3 seconds the total accumulation is 25 W-sec.

Current and voltage are vector quantities, and power is not. You obtain the total power from the numeric sum of the individual power values.

By shorting all but one of the current inputs I am measuring the power in one circuit. The total power is the sum of the power in each of the three circuits.

I have no need to know the power factor to measure power when I measure the average of the product of the instantaneous voltage and current.

If you calculated power by measuring average voltage and independently measure average current, then you need a value for power factor. Where does this power factor value come from? Out of the thin air?. Suppose I have a complex waveform rather than a pure inductance with a series resistance. How do I determine the power factor?

"The CT's measure the total current, and the voltmeters measure the total voltage. So the total PF is also figured into the calculation - no approximations or averages are necessary."

I do not know what this paragraph says.

.

It says the same thing it took you 7 paragraphs to say.

What is possible on the outside is to have a mixup of the relationship of voltage leads and current transformers, and current transformer polarity.

.

Again, thats what I said.

 

[gar]

081204-1757 EST

steve66:

What does "The CT's measure the total current" mean? Are you saying that if I have 3 Amprobes in place of the current transformers that the sum of these three readings is "total current"? If not what does "total current" mean?

To measure average power of a load I need the average of the product of the instantaneous voltage across the load and its associated instantaneous current.

.

 

[gar]

081204-2001 EST

iwire:

I glanced thru your references. I believe this instrument works as I have suggested. Basically as three independent single phase meters and with the ability to sum the independent power measurements to obtain a total power reading.

Having seen the various connection drawings and by your selection of the one in post #1 I am assuming that you do not have any external potential transformers. Thus, the only possibility of miswiring of the voltage leads is relative to their relationship to the current transformers.

.