Tag: Current transformer

Extended Range CTs VS High Accuracy CTs

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Extended range CT’s are all the rage with CT sales people now. But what exactly is and extended range current transformer? Don’t we already have high accuracy CT options out there? Is there a difference in extended range current tranformers and high accuracy current tranformers? I will try and break these terms down and explain what the difference is between extended range CTs and high accuracy CTs.

What are Extended Range CTs?

To understand what extended range CTs are we need to understand what standard range CTs are. Standard range CTs have a measurement range from 0 amps through their rating factor. However, notice that I said measurement and not accuracy. That is because if we remember correctly that standard CTs are only certified as being accurate to 0.6% from 10% of their nameplate rating through the nameplate rating. Furthermore, at the nameplate rating up through the rating factor they are accurate to within 0.3%.

So, obviously an extended range CT would be better than that right? Well, yes. However, there is a caveat. That being that there is no standard when it comes to the term “extended range CT.” These extended ranges are noted in the literature that comes from the manufacturer of the CT. This means that the CT could be accurate down to 3% of the nameplate value or even down to 1%. This however all depends on the manufacturer. Which leads us to our next point.

Don’t we Already Have High Accuracy CTs?

The answer to that question is yes. But let’s remember that to be classified as a high accuracy CT that there are certain guidelines that must be adhered to. The IEEE is the body that defines that standard. If you want to learn more about high accuracy CTs follow the link. So the next question then is why do we have extended range CTs if high accuracy CTs already exist and is there a difference?



What is the difference between extended range CTs and high accuracy CTs?

The first difference is that high accuracy CTs actually have a standard that must be met to be called high accuracy. Extended range CTs however do not have this standard. However, most extended range CTs are high accuracy CTs. The reason for this is because manufacturers need a way to separate themselves from the competition. It is not good enough for them to just meet the standard. They need to go above and beyond the standard to attract more customers.

This is good for utilities because it means that in the end they have access to more choices and better products. Utilities who want high accuracy CTs will find that not only are they available but they are available with more extended ranges than the standard requires. Just remember also that an extended range CT is not always high accuracy. Check and make sure that the CT has a rating for high accuracy before believing the salesman. Extended range is something that the manufacturer decides.

Conclusion

Extended range CTs can be used anywhere standard CTs are used. They allow you the ability to meter lower levels of current than you would have before now. This also permits greater revenue over the life of the CT. However, make sure that the CT is high accuracy before you jump on the extended range bandwagon.









Testing a Form 9s transformer rated installation

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The form 9s meter is perhaps one of the most popular meter forms used in metering. It is a versatile meter that can be used to meter either a 120/208 three phase four wire wye service or a 277/480 three phase four wire wye service. Can the form 9s be used to meter other services? Yes but I am only going to cover these two here. When testing a form 9s transformer rated installation you will be looking at a few different things. You will check the meter, the wiring, the CT’s and/or PT’s, and the voltage as well as the transformer.

First a disclaimer. If you are not a meter tech or a qualified person then you do not need to attempt anything that is written below in regard to the form 9s. It can kill you if you do not know what you are doing.

When we perform a load check the form 9s meter in a transformer rated installation there are a couple of things that we will look at and take note of. First we want to check the meter number. I know this sounds simple but you want to make sure that you are in the right place. Now write down the readings of the meter. There should be a simple kwh reading as well as a kw reading. It is also possible that your utility uses more than these such as kva. You may also be required to download the information from the meter as well. If the meter is a solid state meter you want to make sure that there are no diagnostic codes in the meter. If there are you will need to check them and find out what the problem is.

Solid state meters have come a long way. If you do not have a piece of test equipment that is capable of showing you the vector diagram of your form 9s meter you can logon to the meter and view the vector diagram there. The vector diagram can alert you to things that you may not be able to see right away. Vector diagrams can also let you know if you have any wires crossed as well as the amplitude of the current and voltage on all three phases. Using vector diagrams you can also view all of your phase angles.

We are still talking about the meter here. If you have an electro-mechanical form 9s meter then the procedure is a little different. There is nothing to logon to. The meter may have lights for each voltage phase. If so, you want to make sure that they are all lit up. Next, you want to do what is know as an element check. You will also do this with the solid state meter but you will have to leave all of the voltage switches in as the display likely comes off of A or C phase. To do the element check you will open all of the switches with the exception of the neutral switch. Then you close the voltage switch and the current switches for the phase your will be working on. You are doing this to ensure that each element in the meter causes the disk to rotate in a forward direction. Remember that the disk rotates in a counter-clockwise rotation.








After checking the meter you will want to check the wiring of the form 9s. Give the wiring a good visual check. Look at all of the terminations that you can see and make sure that the color code is correct. If you suspect overheating and think the wire needs to be replaced do so as soon as possible. Wires that are out in the sun tend to crack after several years so you want to make note of this. If the wiring needs to be replaced make a work order to do so. Look at the wiring on the CT’s and PT’s. Does everything look good? Improper wiring is one of the biggest causes of lost revenue with a form 9s metering installation. If the wiring looks good, let’s move on to the CT’s and PT’s.

I am not going to go in depth in this post about testing the CT’s and PT’s, that is for a later date. What I want to tell you here is to visually inspect the CT’s. If you can see the nameplate then that is great. Make sure that the multiplier on the meter matches whatever the CT ratio says that it should be. After that, make sure that the multiplier on the meter is what the multiplier is in the computer system.

This is one of the other big mistakes that you will find with the form 9s. It is so easy to input a multiplier incorrectly into the system. That is why you need to check it for every transformer rated meter that you test. After you verify the CT ratio on the nameplate use test equipment to test the CT. This may be a CT burden test or an admittance test. You can also do a ratio test.

After the CT’s you will want to check the PT’s. Verify the wiring and make sure that you do have voltage on all three phases. The PT’s are so much easier than the CT’s. On the subject of PT’s we always want to check the voltage and make sure that it is the correct voltage for the service.

Next we want to check the transformer. Look at the transformer and make sure that it is not leaking oil anywhere. Also, visually inspect all of the secondary connections that you can see and make sure there is no overheating. If you have an infrared camera now is a good time to use it and check the connections for hot spots. Be very careful when working around transformers as it is possible to have the primary connections very close to the secondary. Make sure that you are wearing all of your PPE before doing any of the aforementioned work.

You will want to check the KVA of the transformer and compare it against the KW reading of the meter which is called demand. If you cannot get the power factor from the meter either because it is a mechanical meter or you do not have a way with another piece of equipment you can use 80% for the power factor.

To convert KW to KVA you will divide the KW by the power factor. This will give you KVA. Let’s do the calculation. Let’s say that you have a form 9s meter that has a KW reading of 0.8 with a multiplier of 80. First, multiply 0.8 x 80 = 64 KW. Then if you cannot get a power factor reading assume 80%. 64/.80 = 80 KVA. Now that you have the KVA check to make sure that it is in the limits of the transformer. If it is you are good. If not get with engineering to make a change.

There it is. That is how to can check a form 9s metering installation. As always, be careful and always wear your PPE.