Current and Voltage Transformers

Current and Voltage Transformers

By Stephen Schaefer
Stephen is a guest author for Peak Demand’s Knowledge Center and editor of Learn Metering found at www.learnmetering.com

 

CT’s, or current transformers, and PT’s, or potential transformers are used in metering to step down current and voltage to safer and more manageable levels. Many people want to know what is a current transformer and potential transformer. Here I will try to demystify the CT PT confusion. One thing that I want to note also is that CT rated meters are not only used as a secondary electric meter, they are also used as a primary electric meter as well. CT rated meters are also typically demand meters as well.

When CT’s and PT’s are used in a metering installation, the installation is known as being transformer-rated. Some people refer to the meters that use a CT PT combination or just CT’s as a current transformer meter. Transformer-rated services run in parallel with the service. This means that unlike self-contained services the customer’s power is not interrupted when the meter is removed. The reason that they are needed is that either the current and/or voltage of the service to be metered is too high. This also depends on the policies and procedures of the utility. For example, some utilities require anything over 480v to be transformer-rated. While other utilities do not.

Also, some utilities do not use PT’s in 480v services at all. I recommend against this practice for the safety of the meter tech or lineman who may need to install or remove these meters from service.

So, what do CT’s do? As stated before they serve to step down high current to a safe a manageable level. Revenue grade CT’s are engineered to produce 5 amps when the amps on the service are at the rated value. For example, a typical installation in a 120/208 service 400 amp service contains 200:5 CT’s. When 200 amps are flowing through the primary side of the CT, 5 amps are coming out of the secondary terminals.

CT’s have nameplates and ratings just like any other piece of electrical equipment. The most important things to note on the nameplate are the ratio and the rating factor. The ratio will be printed in large letters on the side of the CT. Typical ratios are 200:5, 400:5, 600:5, 800:5 and so on. Again, what this means is that when the stated value of amps is flowing through the primary side of the CT, 5 amps is flowing through the secondary side.

The rating factor is used when determining what size CT to use in a particular installation. Some CT’s have a rating factor of 4, 3, 2, or 1.5. What this means is that the manufacturer says the CT is accurate beyond its nameplate value. For example, a 200:5 CT that has a rating factor of 4 will accurately measure a service up to 800 amps. So, if that particular service were to have 800 amps on it, there would 20 amps coming out of the secondary side of the CT and in the meter base. This is important because we want to size our CT’s so that they are fully saturated. Meaning that we want a 200:5 CT to be sized so that the amps flowing through the primary side has as close to 200 amps as possible. When the core of the CT is fully saturated it is the most accurate. CT’s tend to lose some of their accuracy at lower amp levels.

Most transformer-rated meters today are class 20 meters. This means that the current coils inside the meter are rated to carry a continuous 20 amps. You do not want to overdrive the meter by placing more than 20 amps in the meter base because you sized the CT’s incorrectly. For example, you would not want to place 200:5 CT’s in service that you know will be pulling 1000 amps on the primary side. This would place 25 amps in the meter base going over the meter’s rated capacity. This leads to lost revenue.

To properly size CT’s it is important to know what the actual connected load will be. The best way to do this is to consult with the engineer. If the CT’s are to be placed in a pad mount transformer or on the pole and there is only one service coming off of those transformers, it is best to size the CT’s to handle the maximum amps that the transformer is good for. This does two things, one, it makes sure that your CT’s are never overloaded and two, it is a way to find overloaded transformers.

Another thing that many people want to know is what is the current transformer sizing calculation. I know that I said before that you should consult with the engineer and you should but the formula that we use for current transformer sizing for a single phase transformer is:

 

KVA x 1000

line to line voltage

 

Now, to find the correct current transformer size for a three phase service we use this current transformer sizing calculation.

 

KVA x 1000

line to line voltage x √3

 

This is actually the formula to find the maximum ampacity of transformers. With this information we can then size the current transformers based on the information that is given.

Enough about CT’s, let’s talk about PT’s. PT’s are potential transformers. They are also called VT’s or voltage transformers. They are used to step down the voltage to a safe level so that it can be metered. PT’s are typically used in any installation where the voltage on the service is 480v or higher. Some typical PT’s are 2.4:1 and 4:1.

Now that we know what CT’s and PT’s are, we can talk about meter multipliers. Meter multipliers are used when meters are installed in transformer-rated installations. If the CT ratio is 200:5, then the meter multiplier is 40, which is simply 200/5. If a service has both CT’s and PT’s then the two values are multiplied together to give the billing multiplier. For example if a service has 200:5 CT’s and 2.4:1 PT’s, the multiplier will be 96. This is because 40 x 2.4 = 96.

We also know much about CT’s and meters because of Blondel’s Theorem. Follow the link to find out more about this theorem.

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