Tag: CT

How to Perform a Load Check on a Polyphase Transformer-rated Watt-hour Meter

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How to Perform a Load Check on a Polyphase Transformer-rated Watt-hour Meter

Introduction

Polyphase transformer-rated watt-hour meters make up the bulk of the industrial metering systems installed in the field. They are also installed in a variety of commercial applications as well. In this post, I want to show you how you can perform a load check on polyphase transformer-rated metering applications. Learning how to perform a load check on polyphase tranformer-rated metering installations will help you verify that everything is installed and working correctly. Let’s get started.

Tools

To perform a load check on a polyphase transformer-rated metering installations, you only need a few essential tools. There are also a couple of tools that will help you, but are not absolutely necessary.

  • Pliers: The first tool you are going to need is a good set of pliers. These will help you cut the seal and get into the meter base.
  • Screwdriver: The next tool you may need is a good screwdriver. You may need a screwdriver to help you open the meter base and you may need a good screwdriver to help you open up the CT cabinet. Normally a slotted screwdriver does the trick, but sometimes CT cabinets are secured with Philips head screws.
  • Nut Driver: Sometimes you need a nut driver to get into the meter base or CT cabinet.
  • Voltmeter: You will need a voltmeter to check the voltage and current on the site. I recommend a good clamp meter so you can easily check the amps on the site.
  • Optional: A laptop is an optional tool to help you check transformer-rated metering installations. If you have a laptop and vendor specific software along with the appropriate optical probe, you can verify that your metering installation is wired correctly using a computer and vector diagrams.

This is just a small list of tools that will help when you are performing a load check on a transformer-rated metering installation. If you want to see more tools that I recommend, visit my Tools for Meter Techs page.

PPE

Sorry, but I have to mention PPE. If you are going to perform load check on a transformer-rated metering installation, you are going to be working on live circuits. Anytime you work on a live circuit you must wear all the appropriate PPE. This includes hard hats, safety glasses, rubber gloves rated for the voltage at the installation, boots, and FR clothing with the appropriate calorie rating for the available arc flash potential. Working on live circuits is dangerous, and you should not perform any work on a live circuit without proper training. This post in no way qualifies anyone to perform this work. You must be qualified by your employer. I take no responsibility for the actions you may take while working on live circuits based on the information in this post.

Step 1

The first step is to verify that you are at the correct location. This involves looking at your paper work and comparing the address with the meter number and making sure they match. You are also going to look at the form number of the meter and verify it is the correct form number for the installation.

Step 2

Now that you have verified the address and you know you are in the right place you are going to go ahead and remove the cover from the meter based. In a transformer-rated metering installation, there should be a test switch. I am going to be covering a form 9s metering installation here, but the basics apply for other transformer-rated metering forms as well.

Step 3

Inside the meter base there is a test switch. We are going to start by identifying the voltage switches and the current, and current return switches. Get out your voltmeter and go ahead and check the voltage switches for the proper voltage. Let’s go ahead and say that we are checking a 120/208v 4 wire wye service with a form 9s meter and 200:5 CTs. There should be 3 voltage switches on the test switch. A common test switch setup for form 9s meters is to use a 10 pole test switch. There will be 3 voltage switches, 3 current switches, 3 current return switches, and one neutral switch.

When you check voltage between all three voltage switches, you should get about 208v. When you check each switch to ground, you should get about 120v. This normally runs a little high so it is not uncommon to see something around 215v and 125v.

You can now go ahead and clamp your clamp meter around each of the current wires. This will tell you how much current is on the service. Since you know what the multiplier is, you can multiply the amps on your clamp meter by the multiplier to tell you how many amps you should get on the primary side of the CT. Now take your clamp meter and put it around the primary wire in the CT cabinet or transformer. You should be able to do the math to verify that the CT ratio is correct and that the multiplier is correct.

Step 4

Now that you have used your voltmeter to determine which switches are the voltage and current switches and used you clamp meter to verify the multiplier and CT ratio, you now want to verify the phasing. This works better with an old electromechanical meter but you can do it with solid state meters as well. A phase voltage has to match up with A phase current for a form 9s metering installation. You want to open all the switches and then close the neutral, A phase voltage, and A phase current and current return switches. If A phase voltage and current are properly wired, the meter will run forwards. If not, you have a problem and you will need to trace the wires to see what is going on. Repeat this for the other two phases.

Step 5

Once you have done all the verification steps above, you are done with performing a load check on a transformer-rated metering installation. Now all you need to do is close up the meter base and CT cabinet.

Conclusion

I hope this helps understand what all goes into performing a load check on a transformer-rated metering installation.

Further Resources

If you want more information on some of the topics above, be sure to visit:

Increasing Revenue with High Accuracy CT’s

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Increasing revenue is something that we all want to do. High accuracy CT’s can help us achieve that goal. When it comes to instrument rated metering installations we have to remember that our meter is only as accurate as our CT’s. So, the first step in making sure we are accurately measuring our metering installations is making sure we have the most accurate CT’s installed.

Standard Vs. High Accuracy

This is where high accuracy CT’s come into play. Remember that standard current transformers are only guaranteed accurate to within 0.3% from the nameplate value to the rating factor. Also, standard CT’s are 0.6% accurate from 10% of the nameplate value up to the nameplate value. This means that for a common 600:5 CT that anything below 60 amps is not guaranteed to be measured accurately. So, what do you do?

You install a high accuracy CT that is 0.15% accurate from 5% of its nameplate rating through the rating factor. Now, using the same 600:5 ratio as an example, we are now able to measure down to 30 amps with 0.15% accuracy. This is high accuracy CT’s are used to help with increasing revenue.

Where can high accuracy CT’s help with increasing revenue?

One of the places where high accuracy CT’s can help with increasing revenues are department stores or industrial factories that have been converted to warehouse space. These types of customer changes can cause revenue losses because the infrastructure that was put in for these larger customers who have now left or have converted their spaces is no longer being utilized.








Let’s say that the customer was doing some small scale manufacturing and decided that the space was no longer big enough for its manufacturing needs. So, they decide to move into a larger space and convert this previous space into warehousing. Let’s keep with the standard 600:5 for this example. For this example let’s also assume that the rating factor of this standard CT was three. We are also going to assume that we were using this rating factor. Meaning that this was a 1200 amp service and we installed 600:5 CT’s with a rating factor of 3, to make sure that the CT’s were as saturated as possible.

The customer in this example typically pulls around 750-800 amps. This is well within the range of our standard accuracy 600:5 CT’s. Now when they leave, they take all of the manufacturing equipment with them and only leave a few lights. There new load is now only around 40 amps. They do not need to heat or cool the space because what they are storing does not require it. So now, the standard 600:5 CT is not guaranteed to measure the 40 amps accurately.

What if we had installed a high accuracy CT?

If we had installed a high accuracy CT to begin with then we would not be worried. The high accuracy 600:5 CT is able to measure down to 30 amps with 0.15% accuracy. This means we capture those 40 amps and we do so accurately. This is how high accuracy CT’s play a vital role in increasing revenue.

Conclusion

In conclusion, if you are concerned with increasing revenue make sure that you are giving a look to high accuracy CT’s. They can be placed anywhere your standard accuracy CT’s are and can instantly start increasing revenue. High accuracy CT’s help mitigate low current situations such as when large customers move out or convert. If you are in the market for high accuracy CT’s check out Peak Demand.








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Reducing Inventory with High Accuracy CTs

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Reducing inventory is a big concern with utilities these days. This is primarily driven by the fact that reducing inventory means reduced costs. Reducing costs means that the profit margin can be higher. So, what does that have to do with high accuracy CT’s? How can high accuracy CT’s reduce inventory?

Use the Extended Range of High Accuracy CT’s

When placing new CT’s in service we traditionally had a variety of different CT ratios to choose from. Among the most popular are 200:5, 400:5 and 600:5 variants. This means that when a new service is built the meter tech or engineer will look at the service and size the CT’s according to the projected load. This is done because we want the amps on the service as close to the nameplate rating as possible. We do this because standard CT’s are most accurate at their nameplate rating up to their rating factor.

So, if we have a 400 amp service that is projected to normally run around half of that or less we may use 200:5 CT’s with a rating factor of at least two. This ensures that when the amperage is low we can measure it as accurately as possible.

Enter the new kid on the block. The high accuracy CT. With the high accuracy CT, with its extended range features we can now use one size CT for most of our installations. For instance a 600:5 high accuracy CT will allow us to measure down to 30 amps. But there is more. Many manufacturers are offering these high accuracy CT’s with extended ranges that can measure down even further.



How can High Accuracy CT’s be useful in Reducing Inventory?

High accuracy CT’s can reduce inventory by reducing the number of different ratios that you need. You can now purchase a 600:5 CT with a rating factor of 2 or more that can cover all your needs. This means that you no longer need to use 200:5 or 400:5 CT’s. You can stock one size. This allows you to purchase less. Even if you do not have a new job coming up, in the past you would have needed to keep a certain number of 200:5 and 400:5 CT’s on the shelf for emergencies.

So, instead of having a couple of sets of each size, you can have just a couple of sets of 600:5 CT’s that can be used for each service type. Instead of having purchased four sets of CT’s now you have only purchased two. This is beneficial to reducing inventory and cutting costs. Also, it leads to a reduction in storage space as well.

Conclusion

High accuracy CT’s can be a great way of reducing inventory in your meter shop or warehouse. Look at what you have stored in your warehouse and see if you could benefit from reducing the number of CT’s that are just sitting on the shelf as spares. If you are interested in high accuracy CT’s check out the offerings from Peak Demand.









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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.









What are High Accuracy CT’s?

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High accuracy CT’s are becoming more and more popular with utilities these days. Because of new manufacturing processes and materials, manufacturers such as Peak Demand are now able to make their CT’s more accurate. But, more accurate than what? Just saying that a CT is high accuracy still does not tell us what it is. So, what is a high accuracy CT?

What is a Standard Accuracy CT?

To understand what makes a CT “high accuracy,” we first need to know what the standard accuracy of a revenue grade CT is. To be classified as a revenue grade CT the IEEE states that it must be a Class 0.3. Well, that really does not tell us much. Class 0.3 means that the CT must be accurate to within 0.3% of the nameplate rating up to the rating factor.

We can use a common Class 0.3,  600:5 CT with a rating factor of 2 to create an example. From the nameplate value of 600 amps all the way up to 1200 amps when using the rating factor, the CT has to be accurate to within 0.3%. However, below 600 amps, more specifically from 10% of 600, which is 60 amps, all the way to 600 amps the CT only has to be 0.6% accurate. Below 60 amps the accuracy is not guaranteed. Thus the need for a higher accuracy solution.

What is a High Accuracy CT?

A high accuracy CT is a Class 0.15S. This means that the CT must be accurate to within 0.15%, but there is more. Not only does the Class 0.15S have a closer accuracy tolerance range, the ampere range is greater as well. Now, instead of only being accurate from the nameplate rating up to the rating factor, the CT must be accurate all the way down to 5% of the nameplate value.




We can use a Class 0.15S, 600:5 CT with rating factor of 4 to create another example. In the previous example the CT was accurate to within 0.3% from 600 amps all the way to 1200 amps. From 60 amps to 600 amps the CT was accurate to 0.6%. And below 60 amps there was no guaranteed accuracy. Now with the Class 0.15S 600:5 CT the accuracy is guaranteed from 5% of the nameplate value all the way to the rating factor. So, from 30 amps all the way to 1200 amps the CT is 0.15% accurate. What a difference. This offers a very big advantage to utilities.

Conclusion

A high accuracy CT can be a great solution for any utility that is looking to increase their revenue. Manufacturers like Peak Demand have high accuracy solutions available such as their Alta Series™ of High Accuracy 600V Metering Current Transformers line. The next time you are in the market for CT’s think about using high accuracy CT’s instead of the standard Class 0.3 CT’s you have been using all these years.









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CT Meters: Understanding Current Transformer Meters and Their Applications

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CT meters, or current transformer meters, are essential for measuring electrical consumption in large-scale systems where direct metering is impractical. Typically used in commercial and industrial settings, CT meters work in conjunction with current transformers (CTs) to provide accurate measurements. Understanding how to calculate CT ratios and multipliers is crucial for ensuring accurate billing and system monitoring.

What is a CT meter?

A CT meter is simply a meter that is used in conjunction with instrument transformers known as current transformers. These are also known as CTs. In electrical metering, meters are divided into two types. There are self-contained meters. In addition there are transformer-rated meters. Transformer-rated meters are also known as CT meters.

What are the characteristics of a CT meter?

The characteristics of a CT meter include its ratings. CT meters now are generally rated at 20 amps. This means that the current coils of the meter are capable of handling 20 amps. You may think that this is low. But, remember that CT meters are used with CTs. Also remember that CTs have outputs determined by their ratios. They are rated with an output on the secondary side of 5 amps. If you remember, when using the rating factor of a CT it is possible for the CT to put out 20 amps.

CT meters also have voltage ratings. Many of the meters now are multi-range. This means that the meter can sense the incoming voltage and adjust its calculations based on the incoming voltage. Most meters now show the voltage on the display. Before digital meters, one had to be careful to make sure that the meter with the correct voltage rating was chosen.

What types of CT meters are there?

When talking about meter types what we are really referring to are the meter forms. Meter form numbers are used to designate what type of meter we have. These meter form numbers help us to decide which meter to use in which installation based on Blondel’s Theorem.

The normal transformer-rated meter forms are as follows:

Form 3s

Form 4s

Form 5s

Form 9s

Now remember that these are the most common. There are more.








Where will you find CT meters?

CT meters are installed on services that are too large for self-contained services. This normally means services that are larger than 200 amps. Although there are now self-contained 320 amp meters as well as 400 amp bolt in meters. CT meters are also used whenever PTs, potential transformers, are used to step down the voltage.

Large residences, commercial and industrial buildings, hospitals and schools are all examples of where you will find a CT meter installed.








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.








What are test switches for?

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Every now and then in the electric metering field you will run across a transformer rated metering installation that does not have a test switch. Is this a good thing or a bad thing? Most CT rated installations require that test switches be installed. These switches can be used for a few different things, like performing a load check. To test the meter, to shunt the CT’s, to safely remove the CT rated meter from service and they can also be used to check the voltage and the amperage on the service without actually having to open an enclosure or go into a fence.

Why do certain installations not utilize test switches? The answer here would most likely be cost. The meter bases or CT rated installations that you will typically find without test switches are form 3s and form 4s meter bases. Form 3s and form 4s meters are many times found on large residences and sometimes large temporary services such as construction services or school trailers that are not thought to be in service very long. The cost of installing the test switch along with the cost of the larger meter base to hold the test switch is often times a deterrent. Also, in the case of residences one could argue that they just do not use enough power to justify putting a large meter base and test switch on the wall because the meter will be changed out when all of the form 2s meters are changed on their neighbors homes.

Installing a transformer rated service without a test switch can be a bad thing. One thing that you have to remember is that when you pull the meter in a CT rated service and you do not use a test switch is that you are opening the circuit of the secondary side of the CT. This leads to a build up of voltage on this circuit which is dangerous to metering personnel. The proper procedure without a test switch would then be to shunt the secondary side of the CT before pulling the meter.

Test switches can be used to test the meter. With different types of test equipment they can be used to test the meter in service using the load that is available at the customer’s site. This can be a good test to show exactly how the meter is metering the service under the load that is currently on the service. You can also test the meter using a phantom load while it is still in the meter base using the test switches.








Test switches are also used to test the CT’s in the service. You can use various different types of test equipment to test the CT’s. You can test the burden on the CT circuit as well as determine how many amps are on the CT circuit as well.

To remove the meter from service you need to shunt the CT. There is a switch that does this for you. Shunt the CT out and you can safely remove the meter from service. You can also use the test switches to remove all voltage from the meter as well before removing the meter from service and before installing the meter in service.

With the new regulations regarding arc-flash hazards and safety, many utilities have adopted safety policies that no longer allow their personnel to work inside energized cabinets, pad mount transformers or other enclosures if the service is too large or if the voltage is too high. This is yet another thing that the test switch can be used for. It can be used to check and make sure that the customer is getting the proper voltage. You can also check the rotation in the meter base at the test switch as well.

All in all, it is a good practice to install test switches in all of your CT rated metering installations. They will allow you to test the meter in service, test the CT’s in service as well as allow you to check voltage and rotation. Test switches also allow you to safely install and remove meters from service by isolating the blocks of the meter base from current and voltage.