Total Harmonic Distortion

dfm379

Joined Sep 21, 2020
3
I live in an off-grid home. I have an PV system and a 16KW commercial generator (1800rpm / 4 cylinder/ 4 pole Rotating Field, Brushless, Electronically Regulated ) . I noticed that when I run the generator a few of my LED light bulbs flicker at a high rate. After much research, I found the issue to be Harmonic Distortion. The Total Harmonic Distortion in the system goes up significantly when the generator runs. Voltage and frequency coming from the generator are near perfect at 120 +/- 2 volts and 60Hz +/- .4Hz. However, THD goes from under 4% to about 12 -13 %. I tried connecting the generator in bypass mode and shutting off the PV inverters. I found that THD remains under 4% when just running all the loads in the house on the generator. I believe my issue is the non-linear loads created by the Inverters’ battery chargers which are converting the generator power from AC to DC voltage. When the generator runs, the inverters' battery chargers draw 56 amps AC to convert to DC voltage used to charge the batteries. Does anybody know of a solution to this problem? Is there something else to look at other than THD to assess the distortion, i.e. is it really a problem that may result in damaging devices in the power network or just an annoyance....

Also - I am confused by THD ratings on generators and my inverters. It seems to me that THD is a result of the non-linear loads in the power system which can vary a great deal. So how can a generator or inverter carry a rating for THD% without knowing the particulars of the power system. My THD generally remains below 3% running off the inverters, but it goes up to 7% or 8% when my RO system (which has a 5HP motor driven by a VFD (another source of non-linear loads)) running. It's rated THD is typical - 2% Maximum -5%. But in my real world system its going higher than that so how can you put a limit on THD without adding something to the power system that will compensate for the non-linear loads.

Marc Sugrue

Joined Jan 19, 2018
222
Voltage Distortion of a Generator would tell you the purity of the voltage of the sine wave from the Generator Itself. As you point out in your note, if the issue is your non linear loads its because they are not pulling a pure resistive current and are therefore distorting the supply current. To avoid this a good quality non-linear load would typically use a power factor correction front end to ensure current from the source is sinousoid with low harmonics distortion.

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Papabravo

Joined Feb 24, 2006
20,992
I'm going to guess that THD ratings on generators and inverters are measures of deviation from ideal behavior with SPECIFIED loads. It could hardly be otherwise since why would a manufacturer want to advertise worse behavior under less than ideal conditions. AFAIK the THD is costing you some efficiency but does not "appear" to be harmful to any of your loads. With the LED lamps, their nonlinearity is due to the rectifier. So because rectifiers work with both AC and DC you could run 169.7 VDC to the lights without undue difficulty. Then you would have fewer rectifiers and possibly less flickering. If you object to running 167 VDC through your house you could switch to 12 VDC lighting for some or all of your needs.

From a systems perspective I'm not sure there is any convenient way to linearize a non-linear load. The best you can do is linearize around an operating point and a DC source makes a pretty good operating point.

dfm379

Joined Sep 21, 2020
3
Voltage Distortion of a Generator would tell you the purity of the voltage of the sine wave from the Generator Itself. As you point out in your note, if the issue is your non linear loads its because they are not pulling a pure resistive current and are therefore distorting the supply current. To avoid this a good quality non-linear load would typically use a power factor correction front end to ensure current from the source is sinousoid with low harmonics distortion.
The sine wave from the generator without a load is good - 2% THD. It would seem my battery charges and the VFD that converts single phase 240 / 60 to 3 phase 55hz power to the motor are the biggest causes.

dfm379

Joined Sep 21, 2020
3
I'm going to guess that THD ratings on generators and inverters are measures of deviation from ideal behavior with SPECIFIED loads. It could hardly be otherwise since why would a manufacturer want to advertise worse behavior under less than ideal conditions. AFAIK the THD is costing you some efficiency but does not "appear" to be harmful to any of your loads. With the LED lamps, their nonlinearity is due to the rectifier. So because rectifiers work with both AC and DC you could run 169.7 VDC to the lights without undue difficulty. Then you would have fewer rectifiers and possibly less flickering. If you object to running 167 VDC through your house you could switch to 12 VDC lighting for some or all of your needs.

From a systems perspective I'm not sure there is any convenient way to linearize a non-linear load. The best you can do is linearize around an operating point and a DC source makes a pretty good operating point.
I have over 300 LED lamps in the house and only 7 flicker when the generator runs so I can fix that easily enough. However, I would like to better understand what bad things other than some efficiency loss and light flicker can happen?

Papabravo

Joined Feb 24, 2006
20,992
I have over 300 LED lamps in the house and only 7 flicker when the generator runs so I can fix that easily enough. However, I would like to better understand what bad things other than some efficiency loss and light flicker can happen?
Take a look at the Wikipedia article:

https://en.wikipedia.org/wiki/Harmonics_(electrical_power)

Scroll down to the waveform picture for a CFL bulb (2nd frame down on the right hand side) that uses a rectifier and notice the highly distorted waveform that results. I'm going to say that I don't think you have much to worry about unless you are getting unusually short lifetimes for things that should last a while. I think you need a good solution to the flickering lights problem. If it were me it would drive me batshit crazy.

sparky 1

Joined Nov 3, 2018
755
Harmonics are very undesirable. A large heavy flywheel at 1800 RPM will overcome the drag on magnetic induction being driven adequately by the 4 cylinder motor. Our perception can be a smooth running system but the integrity of the power output can be lacking a proper cohesive fundamental wave. In order to deliver strong current we use attenuation methods to reduce both current harmonics and voltage harmonics. The (fundamentals) the basic considerations on wave reconstruction and phase sync will be achievable on a serious mid size power plant rated for your off grid house. A good generator head can make a difference. An important discussion for off grid.

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Delta Prime

Joined Nov 15, 2019
1,311
Hello there Power Inverters, drives, VSDs, variable speed drives, variable frequency drives – the category of products used to control an electric motor’s speed and thus reduce energy usage in a variable speed application has more names than
ower inverters can be completely electronic or combine mechanical effects with electronic circuitry. Since inverters convert DC electricity from batteries, solar panels or fuel cells into AC electricity.
Inverters rely on capacitors to provide a smooth power output at varying levels of current; however electrolytic capacitors have a limited lifespan and age faster than dry components
Capacitors are also extremely temperature sensitive. Temperatures over the stated operating temperature, often caused by high current, can reduce the life of the component. However, as the electrolytes evaporate faster at higher temperatures, capacitor life increases when they are run at lower than operating temperature. Happily, keeping a consistent maintenance regime and regularly replacing capacitors avoids most problems caused by wear and tear
Using inverters beyond their operating limit, either by choice or due to oversight or lack of knowledge, can contribute to inverter bridge failure.
inverter failure are over-current and over-voltage. If either current or voltage increases to a level that the inverter is not rated for, it can cause damage to components in the device, most frequently the inverter bridge. Often this damage will be caused by the excess heat generated by the spike in voltage or current. Over-current can be avoided with fuses or circuit breakers, but avoiding over-voltage can be tricky. Sometimes voltage spikes are man-made, but they can also be caused by lightning or solar flares.
Ultrasonic vibrations originating in the cores of inductive components cause friction,that contributes to the mechanical stress placed on an inverter, adding to the unwanted heat generated by the device and further damaging components in the inverter
with time, electrical connections tend to loosen or corrode cleaning the terminals in the battery box, fuses and the inverter connection at least once every six months is crucial
Cleaning has to be performed correctly, or it might end up doing more damage than good. Ideally, a wire brush and grease dissolvent agent should be used,a protective sealant must also be used on all battery terminals. When deciding which protective coating to use, avoid grease-based ones, because they tend to attract contaminants like dust, which leads to an increased decay of the connections, while also hiding the degradation from further visual inspections.
phase rectifier, rectifies (converts) the pulsing A/C (alternating current) into D/C (direct current). The D/C output voltage (of the rectifier) will be 1.3 times higher than the RMS A/C that is entering the rectifier. For example, if you measure the incoming A/C voltage on any two A/C wires, and your meter reads 18 volts A/C, then you can expect to see about 23.4 volts D/C leaving the rectifier (on the D/C terminals). A rectifier does not downshift or up-shift the voltage. The 1.3 times is simply because you have three current carrying wires coming in, verses two leaving. There are other factors including the conversion from RMS (root mean square), but to keep it simple, 1.3 times any incoming pair will equal the D/C out.
Leaving the rectifier, the D/C current is routed though a shunt resistor connected to an ammeter and voltmeter if possible so you can monitor your battery Bank.Leaving the shunt, we route our positive wire to a breaker or fuse.to protect everything (including the battery) from the battery. Batteries are capable of supplying massive amounts of current (in excess of 900 amps!) that can weld metal, start fires and given the right conditions, electrocute people. Basically, anytime you run a wire from the positive post of the battery it should go though a breaker or fuse. The fuse should be rated higher than the anticipated current that the leg will handle. I forgot to mention that a breaker or fuse is too slow to react to a short circuit meaning if your body is the cause of shorting the power rails it will not only kill you... It Will Hurt the Whole Time YOU ARE DIEING!!
Sorry for yelling guys.