Transcript

Introduction to the Harmonic Test

Today, we’re testing the harmonic measurement ability of the Bolt, and it’s also a test of the analog front end, testing any non-linearities in the front end, and the frequency response, and flatness of the front end for both voltage and current in the Bolt.

So we have a three-phase power supply that can supply a programmable harmonic pattern. For voltage, we have resistive load for current, and we have a Bolt connected with all channels tied together for voltage, using TELAR CTs for the current, and a 10 amp resistive shunt in series with this resistive load space heater to give us a current measurement that we can run into the spectrum analyzer.

We have voltage on the top plot, current on the bottom plot, with a 20:1 probe for voltage. And we’re measuring on a shunt that is designed for 10 amps full scale, with 100 millivolt output to that current. And also looking at the volts and current waveforms here on the scope, so we can make sure that we’re generating what we think we generate, and make sure the Bolt agrees with the spectrum analyzer. And we’re using the PMIVUE app on an iPad, connected through wifi.

Clean Sine Wave Test

The first test is a secure sine wave. It is a clean sine wave, should have no harmonic content at all. So in theory, the THD should be zero for voltage and for current. We can see it looks like a clean sine wave on the scope.

Let’s take a look at the harmonic meter readings. We can see we’re at 0.1% distortion for voltage and for current. That’s really the limit of the power supply. That’s a very low distortion, very difficult to get below that with a large power supply.

We can see on the spectrum analyzer, we have 60 hertz fundamental. Let’s put the marker on 60 hertz for voltage. We have 5.02 volts RMS as read by this 20X probe, which corresponds to 100 volts on the volt input, and that’s what we see. We have 99.9, 100, 100 for three voltage channels. And with this size of resistive load, we have 6.07 to 6.0 amps, 6.08 amps on current. And the harmonics are all zero. And with the THD of 0.1%, very low distortion, here at the limits of the power supply itself.

Pass Band Flatness Test

With our next test, we’re gonna test the flatness of the pass band for the Bolt, to make sure that each harmonic, all the way up to the fifth first harmonic is the correct level. We have a special pattern where every fifth harmonic, we have aligned. These show exactly five volts in amplitude for voltage, and the proportional amplitude for current. And you can see on the spectrum analyzer, all the way up to three kilohertz, these are flat, and this is the special waveform that does this.

Let’s check the harmonic levels on the Bolt harmonic meter display. We can see 100 volts on the first harmonic, five volts for the fifth harmonic, and then the next one, we have the 11th harmonic, and so forth up the spectrum, all five volts and proportional here for current.

Let’s corroborate that with the spectrum analyzer. We have the marker at 300 hertz, the fifth harmonic, and we’re reading 250 millivolts, which times 20 is five volts. And you can see, these are all the same level.

We can look at the harmonics on the bar chart. So here, we have voltage. Again, we have a line on periodic harmonics all the way up, and we can just pick any one up. Pick any one here, the 35th harmonic, for example, here, we’re at 5.0 volts, 4.9 volts here. We see that’s flat across the pass band. Analog front end of the Bolt is flat all the way up to three kilohertz, as we see with the special waveform.

Two-Tone Linearity Test

Now, another test is the two-tone test. This is a test of linearity of the Bolt front end. With a two-tone test, you put two sine waves at different frequencies, and you check for the sum and difference frequencies. Any non-linearity will manifest itself as mixing in the frequency domain.

Here, we have 100 volts at the first harmonic. We have 20 volts at the seventh harmonic, and we see the same thing on the spectrum analyzer. Let’s turn off the fundamental to make sure there are no other frequencies present.

The two-tone test will produce other frequencies, sums and differences of 60 hertz to 420 hertz that would manifest themselves here as other harmonics. But we see these levels are all close to zero. We can look at the meter display to verify that. We have 100 volts at the first harmonic. We have 20 volts at the seventh harmonic, and all the other harmonics are zero for voltage and for current, showing that it’s a very linear front end. There’s no mixing going on, no non-linearities in the Bolt.

Spectral Leakage Test

With this test, we have two closely spaced harmonics, and the test is to make sure that the harmonic in between is close to zero or absolutely zero, to make sure there’s no leakage from one frequency bin to an adjacent bin.

We can see the waveform here on the scope. We have, of course, 60 hertz at 100 volts again, and we have two closely spaced harmonics here. We have the fifth and seventh harmonic. The test is to make sure that it’s zero in between, that there’s no leakage from two high harmonics to one that’s zero in between.

We can see these are 30 volts, proportionally sized currents, and in between we have basically zero. Have a look at the meter reading to corroborate. 30, 30, 49.9, and sixth harmonic is exactly zero for voltage and for current. So we passed the two-tone test, or the spectral leakage test. There is no leakage from high harmonics to an adjacent harmonic. The adjacent harmonics still read zero, and we can see that on the spectrum analyzer as well.

Small Harmonic Resolution Test

Now for our next test, we are going to test the ability to resolve a very small harmonic, especially one next to a large harmonic. And with this pattern, we have a very large seventh harmonic and a very small ninth harmonic. We want to make sure the ninth harmonic is being measured correctly, even in the presence of a nearby large harmonic.

On the waveform and the scope, we can see it’s mostly 60 hertz and seventh harmonic. The ninth harmonic is going to be impossible to see in the time domain, though we can see 60 hertz, we can see the seventh harmonic, which is large, the ninth harmonic is very small here.

Let’s take a measurement, 540 hertz. We have roughly 10 millivolts, times 20, that corresponds to 0.2 volts. And here on the meter screen, we see we’re at 100 volts at the first harmonic. We see 19.9 to 20 volts at the seventh harmonic, and the ninth harmonic is 0.2 volts. So even in the presence of other large harmonics, we still have good resolution down to very small levels for voltage.

And for current, we see the same thing. Proportionately here on the seventh harmonic, we have 1.2 amps, and then on the ninth harmonic we have 101 amps, which is the current value, given that it’s six amps on the fundamental. And we can also check the THD as a sanity check. Should be 20% THD roughly.

Summary

So we’ve stress tested the Bolt front end to test the harmonic measurements. We’ve tested the harmonics up to the 50th harmonic to make sure that the Bolt front end is flat all the way up in frequency. We’ve also done some linearity tests, the two-tone test, the spectral leakage test, and the small harmonic adjacent to a very large harmonic to make sure there are no non-linearities in the analog section of the Bolt, and also in the digital section, the computations for the Bolt.

The Bolt harmonic measurements pass all the tests, and very good performance.