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New CT Online Ordering Website

J&D Electronics is a South Korean manufacturer that has a new online presence at jdmetering.com.  I’ve perused their website and found it pretty easy to use.

jd-screenshot

Here are a few things that stood out to me:

  1. They have a 100/5A CT that is very small (24 mm opening), which is unique.
  2. Their spec sheets are typically very detailed; it’s always nice to have more information rather than not enough.
  3. They have a very economical Aac to Vdc transducer.  See the JSXXX-V and JSXXX-VH series.
  4. They offer a lot of products and product categories.  They seem to be quite the one-stop place to go for anything current transducer or transformer related.

Interestingly enough the website is available in Spanish as well.

Currently they don’t seem to offer very many power meters, but I suppose that could change.

More on CT Accuracy Classes

A couple of days ago I posted an article called Understanding Current Transformer Accuracy Classes.  I recommend reading that blog first. Today I want to elaborate a little more.

First, I would like to share the exact tables found in the IEC 61869-2 standard.

IEC 61869-2 Table 201

Accuracy Class ± Current ratio error permissible at rated current shown below ± Phase displacement (in mins) at rated current shown below
5% 20% 100% 120% 5% 20% 100% 120%
0.1 0.4% 0.2% 0.1% 0.1% 15 (0.25°) 8 (0.133°) 5 (0.083°) 5 (0.083°)
0.2 0.75% 0.35% 0.2% 0.2% 30 (0.5°) 15 (0.25°) 10 (0.167°) 10 (0.167°)
0.5 1.5% 0.75% 0.5% 0.5% 90  (1.5°) 45 (0.75°) 30 (0.5°) 30 (0.5°)
1.0 3.0% 1.5% 1.0% 1.0% 180 (2.0°) 90 (1.5°) 60 (1.0°) 60 (1.0°)

IEC 61869-2 Table 202

Accuracy Class ± Current ratio error permissible at rated current shown below ± Phase displacement (in mins) at rated current shown below
1% 5% 20% 100% 120% 1% 5% 20% 100% 120%
0.2S 0.75% 0.35% 0.2% 0.2% 0.2% 30 (0.5°) 15 (0.25°) 10 (0.167°) 10 (0.167°) 10 (0.167°)
0.5S 1.5% 0.75% 0.5% 0.5% 0.5% 90 (1.5°) 45 (0.75°) 30 (0.5°) 30 (0.5°) 30 (0.5°)

Note that Table 202 has two differences compared to 201:
1. It specifies behavior at 1% of the rated current.
2. It is more rigorous (difficult to achieve) at lower current percentages.

Of course we can visualize the data in the tables as well. In this graph I’m depicting the 0.5 and 1.0 classes.

0.5 Class versus 1.0 Class

0.5 Class versus 1.0 Class

Perhaps even more interesting, here’s the difference between 0.5 and 0.5S Classes.  Note that the standard doesn’t specify how a 0.5 Class CT should behave below 5%, and that the 0.5S class is more rigorous below 100% of the rated current.

0.5 Class versus 0.5S Class

0-5s-versus-0-5-class

I’m too lazy to draw similar graphs for the allowed phase shift, but hopefully you get the idea.  It is important however to realize that it is the combination of compliance with the accuracy AND the phase displacement tables that allows a unit to be IEC 61869-2 compliant.

Understanding Current Transformer Accuracy Classes

February 21, 2017 Leave a comment

IEC 61869-2 defines the new current transformer accuracy classes intended to replace the old standard, IEC60044-1 (note that IEC 61869-1 is designed for instrument transformers) .  The new and old standards are essentially identical, but IEC 61869-2 consolidated two parts of the older standard:

  • IEC 60044-1 : Instrument transformers – Part 1: Current transformers
  • IEC 60044-6 : Instrument transformers – Part 6: Requirements for protective current transformers for transient performance

While IEEE/ANSI C57.13 remains active, most current manufacturers prefer the IEC standard because it is not specific to instrument transformers with a 5A output.  Therefore, instead of dealing strictly with 5A output devices, the IEC standard is more general, covering devices with a mA output or voltage output.

The next thing to be aware of is that the 0.5 Class (of IEC 61869-2) and the 0.5s Class (of IEC 61869-2) are different.  The 0.5s version is a more rigorous standard when it comes to performance on lower current ratios.  For example, at 5% of the rated current, the percentage current ratio error for the 0.5 Class is 1.5% whereas 0.5s requires 0.75% or better.

The biggest single mistake I see people make when comparing accuracy of CTs is to assume that 0.5% accuracy = IEC 61869-5 0.5 Class (or some other standard).  THIS IS NOT NECESSARILY THE CASE!

I recently tested several CTs from a manufacturer (that I’ll leave unnamed).  The website specified the CTs were 0.5% accurate from 10% to 120% of the rated current.  I tested the CTs and I found they were correct.  However, they were not 0.5 Class!  Here’s why:

IEC 61869-2 0.5 Accuracy

This particular unit, which was rated at 20A, has an error at 20A that is below 0.5%.   In addition, I tested the part at 10%, 25%, 50%, 75% and 120% of the rated current, and in each case the linearity was below 0.5%.  Therefore, the manufacturer’s claim is true; however, the unit’s phase shift is too large to meet the 0.5 Class.  This is because it would have to be less than 30 minutes (0.5 degrees) at 100% of the rated current.

In conclusion, 0.5% accuracy ≠ 0.5 Class ≠ 0.5s Class.  Be sure to look at the standard the CT complies with when comparing products.  And, if it doesn’t state any standard, be wary!

Purchasing a Current Transformer

When purchasing a current transformer or transducer, the most important considerations are:

  1. What type of input are you expecting?  This may include an AC amperage input, DC amperage input, DC voltage input, etc.
  2. What type of output is the meter/monitor you are working with going to expect?  In the power monitoring industry the most common output required is 333 mVac, but others I’ve seen include 5A, 0-5Vac or 0-2Vac.

True current transformers have the same type of input as they do output.  In this sense, current “transformers” that output an AC voltage from an AC amperage input, are not true transformers but rather what is called a current transducer.  Anyway, if you get either of these two elements wrong, you’re going to be disappointed when you install and use the current sensor. Other considerations, such as size, style, etc. are secondary.

You can get a wide assortment of current transducers from Aim Dynamics including these options:

  1. Aac input -> Vac output
  2. Aac input -> Vdc output
  3. Aac input -> 4-20 mA output
  4. Adc input -> Vdc output
  5. Adc input -> Vac output
  6. Vac input -> 4-20 mA output
  7. Vac input -> Vdc output

Back to current transformers, as explained on Aim Dynamics’s website, a current transformer is a device that “transforms” or “steps down” the current input on the “primary winding” to an alternating current of equal proportion on its “secondary winding,” or output. In this way current transformers can convert a potentially dangerous current to one that is more manageable and easier to work with. Because the output current is proportional to the input, it’s ideal for power monitoring, controlling devices, etc. because we can know what the actual current is on the primary conductor by measuring the corresponding current on the secondary output.

True current transformers are passive devices, meaning that they do not require external power. Rather, they use electromagnetic principles to function. More specifically, they typically contain a laminated core of low-loss magnetic material. Next, a wire is wound around the laminated core. The number of windings, or “turns,” is inversely proportional to the current desired on the secondary winding, as expressed by this equation:

(Secondary Current) = (Primary Current) * (Number of turns on the primary conductor / Number of turns on the secondary conductor). We abbreviate this as Is = Ip * (Np/Ns)

In most situations with power monitoring current transformers the number of turns on the primary conductor = 1, that is to say, the conductor is simply passed through the center hole of the transformer, so in this situation we get:

Is = Ip * (1 / Ns), or Is = Ip / Ns.

The most common “true” current transformer used for power monitoring and power controls has a 5 Amp AC current output, but 1 Amp AC currents also exist. Having said this, many current sensors in use today use a large number of windings, resulting in a very low current output. Many industries are preferring this type of output because it’s easier to work with. Instead, what they often do is add a “burden” resistor to the secondary winding to create voltage. Voltage is defined by this equation:

Voltage = Current * Resistance, abbreviated V = I * R

Using this formula, let’s come up with a hypothetical current sensor. Let’s say that we want to produce 333mV when 1000 Amps are “sensed” on the primary conductor, which in our scenario will be a bus bar passing through the center. If the current sensor has 7500 turns, we would expect 1000/7500 Amps, or 133 mA of current if no burden resistor existed. But in our case we want 333 mV of output, so we can divide 333 mV / 133 mA (or .333 V/ .133 A) and we find that the needed burden resistor should be 2.5 Ohms. Once burdened in this way, we can ignore the amperage output (it’s pretty small after all) and consider this a “voltage output” device. Because the current output is alternating current (AC) the output voltage is also alternating, abbreviated Vac.

Current sensors with a 333 mV output don’t have this risk because the current output is extremely low.

Current sensors that change the output type are called current transducers. The hypothetical current sensor described earlier would be most accurately called a current transducer, yet they often simply get called current transformers because they operate using the same basic principles as a current transformer.