1/20/2022 – Initial post
1/25/2022 – major addition: “IS ANY OF THIS STUFF REAL?”
This article is about “Power Quality.” “Power Quality” is a term that refers to the shape of the AC sinusoidal voltage waveform in an AC utility power system being both 1) free of distortion and 2) within the rated voltage and frequency (60 Hz throughout North America) specified for the host system. When the waveform isn’t “perfect,” end user electrical equipment can be negatively affected; particularly, equipment that contains digital control circuitry.
Those who have followed my articles over the years know that I constantly remind boaters that boat electrical systems are different from residential systems. Because of several differences related to commercial utility power distribution systems, the power delivered to residential premises is “cleaner” (fewer noise components) than the power that boats might receive at marinas. And, this is not about the difference between 120V/240V and 120V/208V power; this topic has to do with distortion of, and noise on, the AC waveform. Waveform distortion on boats occurs more commonly when running on generators (and inverters) than shore power.
Figure 1 shows a conceptual portrayal of some common forms of voltage events that can lead to customer premises equipment failures:
Fortunately, many of these situations are fairly easy to identify and observe through simple measurement means. But, those labeled “Miscellaneous Waveform Distortions” can be quite technical and very difficult to diagnose, pinpoint and confirm without expensive, professional measuring tools. Two of those are:
- Common Mode Currents and
- Harmonic Distortion
I realize that these two terms are probably new to most readers. Much of the content of this article is likely new to most people, including many electricians and many marine electrical technicians. The reason boat owner/operators should be aware of these concepts is that when they occur, they can and do affect the performance of electrical equipment onboard boats, and often with intermittent and obscure symptoms. “Honey, it never did THAT before!” Symptoms of these issues DOES NOT mean that affected equipment, itself, is necessarily faulty or failing.
Figure 2 expands upon the conceptual depictions of Figure 1. Here we see the waveform detail that characterizes and defines “Power Quality” problems. These voltage waveform distortion “anomalies” can be present one-at-a-time or in random combination of several-at-a-time. All voltage waveform anomalies can cause customer premises equipment to fail to operate correctly or to fail to operate at all.
When and where can this stuff happen to you?
- Anywhere, any time of day, any season of the year, randomly…
- random equipment shutdowns on boats in a marina…
- random equipment malfunctions on genset power but not on shore power…
- random HVAC equipment power errors…
- random refrigeration system shut down…
- random TV picture distortion anomalies…
What can the cause(s) be…
- equipment not designed or intended for use in mobile (marine) applications…
- ungrounded or improperly grounded equipment on your own boat…
- faulty equipment on other boats on the dock,…
- anomalies in the campus power delivered to a dock by the facility power distribution system, including inadequate dock wiring…
- anomalies in the power provided to the facility by the local electric utility at it’s “Point of Common Connection” (PCC) to the grid…
In order to understand “Power Quality” issues, some introduction to technical concepts is helpful and necessary. Following, I’m trying to write to my dad (banking and finance), my dock neighbor (911 dispatcher) and my best friend (printer) as I discuss these topics. In other words, I’ll be trying to write to people with little or no electrical background. The next three topics are building blocks for understanding and becoming familiar with the larger issues that cause us problems. The goal is to understand what “poor quality” AC power is, and how it can cause problems for us as consumers.
LINEAR vs NON-LINEAR LOADS:
While much of what follows is new, most readers will (I assume) have heard of “Ohm’s Law.” In my era, Ohm’s Law was a high school science topic. Ohm’s Law is a fundamental law of electro-physics, that describes the inter-relationships of resistance (Ohms), voltage (Volts) and current (Amps). Ohm’s Law recognizes that electrical circuit components all have the property of “resistance” (“impedance” in AC circuits). In a circuit, as one quantity changes, the others follow in direct linear or inverse proportion. In our homes and on our boats, we expect the incoming electrical voltage (120V/240V) to stay mostly constant, so as the resistance of the circuit changes, current follows. For example, in a 120V/240V residential system:
- turn lights “on” throughout the house, total current used goes up;
- temperature satisfied in water heater, water heater shuts “off,” current use goes down;
- thermostat tells Air Conditioner to turn “on,” current use goes up;
- toaster completes your breakfast muffins, current use goes down;
- induction fry pan turned “on” to make hash-browns, current use goes up.
Ohm’s Law describes the relationship between current and voltage as “Linear.” As shown in Figure 3, the current waveform follows voltage waveform in a perfectly proportional and aligned manner. In technical literature, electrical loads that elicit this behavior are characterized as “linear loads.”
There are many kinds of electrical equipment and appliances that are not linear in behavior. Electrical circuits where current does not follow voltage are characterized as a “non-linear;” simply stated, they do not adhere to the simple proportionality of Ohm’s Law. Figure 4 shows an example of a non-linear load, where current behaves quite independently of voltage.
in homes and on boats, we find many examples of both linear and non-linear loads. Water heaters, clothes dryers, cook tops, crock pots and “old fashioned” incandescent lamps are “purely resistive” linear devices. HVAC and refrigeration compressors, florescent lighting ballasts, “new fangled” LED lighting fed by AC power bricks, microwaves, inverter/chargers, DC-to-DC converters, engine alternator Voltage Regulators and Switched-Mode Isolation Transformers are non-linear devices.
SWITCHED MODE POWER SUPPLY (SMPS):
A “Switched Mode Power Supply” (“SMPS”) is now by far the most common type of power supply found in modern electronics, especially digital electronics, both AC and DC. As shown below in Figure 5, an SMPS is a “non-linear device” that utilizes solid state switching devices (IGBT – Insulated Gate Bipolar Transistor) to continuously switch power “on” and “off” at very high frequencies (more on that in the section on “Pulse Width Modulation”).
A sidebar of “geek speak” follows, for those interested, to illustrate the cause of the non-linear current. Others can “skip it.”
In an SMPS, incoming power is fed to “energy storing devices” (capacitors and inductors). The energy storing devices smooth the DC and supply power during the non-conduction state of the switching transistors.
The basic SMPS design variations are categorized based on input and output voltage type. The four principle groups are:
- AC to DC – DC power supplies as found in many end-user devices
- DC to DC – Converter to change or regulate DC voltage
- DC to AC – Inverter
- AC to AC – Cyclo-converter (“frequency changer;” i.e., 60Hz AC to 50 Hz AC or vice versa)
- More compact and use smaller transformers; smaller size and lighter weight is an advantage for electronic devices with limited space and in mobile applications
- Regulated and reliable voltage outputs regardless of variations in input supply voltage
- >High efficiency: 70% to 90% vs 45% for traditional power supplies
- Generate Electro-Magnetic Interference (EMI/EMC) and electrical waveform noise/distortion.
- Complex electrical designs
- More components resulting in greater expense vs traditional linear supplies
The main internal components of an SMPS are:
- Input rectifier and filter
- Inverter (consisting of a high frequency signal and switching devices)
- Power transformer
- Output rectifier and filter
- Feedback system and circuit controller
Figure 6 is a very highly conceptualized block diagram showing power flow through a SMPS. This example is typical of a DC power supply in a TV, VCR, computer/printer/copier power brick, all kinds of battery chargers and other electronic equipment. This example uses 120V AC wall input and produces clean, highly regulated DC Output.
Versions of this same technology can use DC input power to generate Pure Sine Wave AC, can be used to change AC line frequencies (50hz to 60Hz, or vice versa). And, versions of this same technology are used extensively in DC-to-DC applications, like the Balmar external alternator voltage regulator, Victron solar DC-to-DC controllers or Sterling Power DC-to-DC Converters used in battery charging, voltage doubling or voltage halving applications. These power supply designs are also used in DC navigation equipment, VHF radio equipment, and other DC equipment since the internal “high speed switch” is, electrically, an inverter.
Figure 6 shows an “InAC” Input and an “InDC” input; likewise, an “OutAC” and “OutDC” output. All of these input and output types do not usually appear in the same device, but different mix-’n-match combinations of AC input and output, and DC input and output designs are manufacturer’s-choice product alternatives. Because of the PWM signal generator, the “AC” output is an AC Pure Sine Wave, such as what is found in 12V/24V PSW inverters on boats. The “DC” output is a highly-regulated DC voltage, such as found in a DC-to-DC Converter, or in the power supply inside sensitive made-for-purpose navigation electronics.
Pulse Width Modulation in a SMPS:
Figure 6 shows a PWM Signal Generator (the signal in the red oval). This is both “the heart of the magic” and the source of some of its problems. A “PWM Signal Generator” produces a DC Square Wave, where the individual pulses have varying widths. A DC Square Wave is technically also an AC waveform, so it can be fed into a transformer just like any other AC waveform.
PWM Signal Generators use very high internal DC square-wave signal frequencies (50kHz). This enables the use of smaller, lighter transformers in power supply applications, and greatly simplifies filtering of the DC output voltage. A significant potential penalty of this technology is electrical noise and waveform distortion reflected backwards into the local dockside electrical system, as well as local RF interference, which is very common with LED lighting that isn’t filtered well enough.
Another sidebar of “geek speak” here, for those interested, to frame the operation of “Pulse Width Modulation.” Others can “skip it.”
In a SMPS circuit, a PWM signal is generated by feeding a reference signal and a carrier signal through a comparator. The output signal is based on the difference between the two inputs. In an inverter application, the reference is a sinusoidal wave at the frequency of the desired output signal. The carrier wave is a triangular, or “sawtooth,” waveform which operates at a frequency significantly greater than the reference. During times when the carrier signal voltage exceeds the reference signal voltage, the output square wave is in one state, and at times when the reference voltage exceeds the carrier signal voltage, the output square wave is in the opposite state. Figure 7 shows the signals, with the carrier signal in blue, the reference wave in red, and the PWM DC output square wave in green.
COMMON MODE CURRENTS:
Two electrical concepts with which I would expect most laymen to be unfamiliar are “Differential Mode” and “Common Mode” voltages and currents in a circuit. Common Mode currents are usually noise; that is, an AC disturbance between one or more signal or power conductors and an external conduction path, such as an earth or chassis ground or miscellaneous conductive material not intended to conduct the power or signals (including ground fault current). Even to power engineers, this is arcane stuff. Arcane, that is, outside the marine environment. Then, it can rear its ugly head as power quality issues in onboard electrical equipment.
Pictures will make this discussion much easier to follow, so the next several drawings are a progressive sequence of views of the same thing, each building on the previous one, to help understand Differential Mode currents and Common Mode currents.
Figure 8 is the starting point; the same basic electrical circuit shown in Figure 6, but this time, with some of the internal circuit details shown. The voltage waveforms in the various parts of the circuit are as shown earlier, and have the same meanings here.
In Figure 9, the very same circuit diagram is repeated, but here, the metallic equipment case of the device is portrayed as a grey box in the background of the diagram.
Notice (lower left) that the equipment case is grounded to the incoming AC power source, but the logic circuit itself is electrically isolated from the case. The electrical isolation from the case of the unit is to help minimize the presence of Common Mode signals and other types of electrical noise.
in Figure 10, we begin to see the emergence of the “noise problem” (or “magic,” as some might see this).
Capacitors are electrical components that block DC but pass AC. It’s actually much more complex than that, but that’s enough for now.
A Switch Mode Power Supply (SMPS) develops high frequency signals that also vary in frequency, cyclically over fixed time intervals, in normal operation. These high frequency AC and AC-like DC signals couple through internal parasitic capacitances (stray capacitance between electrical circuit components) directly to the equipment ground, and also couple through the inverter circuit via magnetic field coupling. This generates undesirable noise currents which find their way back to the external power supplying source. Here, the little red capacitors show the parasitic capacitive connections between the darker grey component heat sinks and component metal part content and the metallic case of the equipment.
And following the electrical path of noise currents from the parasitic capacitances shown in Figure 11, we see the noise currents reaching the power source’s “safety ground” conductor.
For simplicity, the Earth connection appears on this drawing, but remember from many previous discussions that the actual earth connection is back at the “derived source” in the facility’s infrastructure.
Finally now, we can see, and point to, the distinctions between Differential Mode signals (voltages and currents) and Common Mode signals in a Power Distribution System on a dock.
Figure 12 shows both kinds of signals in the distribution system and attached equipment. In this case, Differential Mode signals are the desirable, wanted voltages and currents that make attached equipment work. They are portrayed in blue. They originate at the facility power source, travel to loads on the line conductor, and return from loads on the neutral conductors.
I want to emphasize that Differential Mode currents and voltages are the same old AC currents that we know and love and have always talked about. They are the currents flowing from the source to the load in the Line conductors (L1 and L2) and returning from the load to the source in the Neutral (N) conductor. We have just never needed to talk about them as “Differential Mode Currents” (or “Differential Mode Voltages”) before. It’s not language that’s commonly found in the ordinary course of “electricity” discussions, because we have never needed to differentiate these normal currents from anything abnormal; until now.
Common Mode currents are undesirable and engineers work hard to minimize and eliminate them. They are portrayed in red in Figure 12. The electrical “source” of Common Mode signals is in the SMPS of the premises equipment. In most marine environments, there are many, many, many of these devices on any given dock. AC non-linear loads produce undesirable Common Mode signals that
require suppression by complex and costly circuits designed specifically for noise filtering and suppression.
RULE 1: ALL ELECTRIC CURRENTS RETURN TO THEIR SOURCE.
RULE 2: ELECTRIC CURRENTS RETURN TO THEIR SOURCE ON ALL AVAILABLE PATHS.
Again in Figure 12, the high frequency Common Mode Noise Currents ORIGINATE in the Rectifier and Inverter sections of the SMPS, capacitively couple to the equipment ground, and flow along the ground conductor into the external system’s power source. From there, they can flow as electrical noise in many directions. Above, they are shown flowing in phase with each other (which is the technical definition of Common Mode Currents) back to the SMPS in which they originated (Rule 1). They are flowing in the same direction on BOTH the Line and Neutral conductors of the device (Rule 2). But, since the line, neutral and ground conductors are shared in parallel across many, many end-user circuits, that noise will ALSO flow on those parallel paths (Rule 2). And since the device ground is connected to earth ground, Common Mode currents can also flow through the earth/water to impact other parts of the common, shared system. And so, a noise-producing fault on one boat can and will propagate to other nearby boats.
And folks, that’s the reason to care about any of this stuff in the first place!
What does “electrical noise” look like? Figure 13 shows screenshots of electrical noise from articles I’ve found online.
Instead of a nice, clean waveform, it’s a distorted jumble of spikes, ripples and gaps.
In systems with Common Mode Currents causing electrical noise, both the line conductors and the neutral conductor will be oscillating up and down at the noise frequency, so Differential Mode Currents can appear almost normal. But excessive Common Mode noise can cause equipment circuits to malfunction. Lots of money is spent to design power supplies to recognize and suppress these undesirable noise components. That works; to a point. But, sometimes under some conditions in some places, the noise components become too large to be fully suppressed by “standard” means, and then end-user equipment may fail. Noise filtering adds cost to components, and so may not be found in all equipment. Buyer beware.
Equipment manufacturers do know about this kind of noise and its causes, and try to design filtering circuits to minimize it. Let’s look quickly at another source of Common Mode noise that is found on many, many cruising boats.
Refrigerators with the ever-so-common BD35, BD50 Danfoss/Secop compressors are advertised as 12V and/or 24V DC appliances. And as far as the power supplied to the refrigerator is concerned, that’s true. However, the compressor motor itself IS NOT a 12VDC or 24VDC motor. That little compressor motor is a 3-phase, variable frequency motor that runs at nominal 277VAC.
WHAT DID HE SAY? DID HE SAY, “277V AC 3-PHASE VARIABLE FREQUENCY DRIVE?”
Indeed, I did say that. Cowabunga, dude!
The Danfoss/Secop power module (101N0510) in my Vitrifrigo fridge accepts either 12VDC/24VDC or 120VAC and converts that input voltage into 3Ø, 277VAC to run the little compressor. The conversion is via a 3Ø AC SMPS. Yes, that makes the fridge a non-linear device to incoming 120V AC. Figure 14 is a highly conceptualized diagram of the 101N0510 SMPS:
The 3Ø SMPS is a Variable Frequency Drive application which spins the compressor at slower speeds when cooling demands are low and higher speeds when cooling demands are high, all while regulating the AC Voltage required by the motor.
For those familiar with three-phase systems, the Danfoss/Secop compressor motor is a Wye-connected 3Ø circuit with a totally isolated, floating neutral. The 3Ø electricity is created by 6 IGBT switches, so as the phases rotate, the motor’s neutral star-point DOES NOT stay at 0 v with respect to frame ground. Great pains are taken to avoid capacitive coupling from the motor’s star point neutral to the frame of the motor, because that becomes the source of a Common Mode Current. But yes, there is capacitive coupling from that neutral to the frame of the fridge… And. Insulation does break down as it ages and goes through thousands of heating/cooling cycles…
Here’s a link to an 11-minute Danfoss video, explaining the above, that readers may find interesting:
“Harmonic Distortion” is another true AC sine wave malformation. The math describing Harmonic Distortion is called “Fourier Analysis,” or a “Fourier Transform.” No, we’re not going to look at Fourier math in this article! The concept is, if a waveform is a true sine wave, it is made up of one, and only one, fundamental frequency. So, ANY sine wave that is not “prefect,” is actually not a “sine wave.” That waveform contains, by definition, “harmonic frequency components.” Harmonic frequencies are even and odd multiples of the fundamental sine wave frequency. For a 60-Hz sine wave, the 1st harmonic is 120 Hz, the 2nd harmonic is 180 Hz, the 3rd harmonic is 240 Hz, the 4th harmonic is 300 Hz, the 5th harmonic 360 Hz, etc, etc, etc. Fourier analysis will tell engineers exactly what harmonics are present, as well as their real and relative amplitudes. There is expensive handheld test equipment, such as the Fluke 40, 41 and 345, AEMC 8336, and others, that can identify harmonics for electrical technicians working on Power Quality problems at customer premises.
Harmonic waveform voltages combine with (add to and subtract from) the fundamental waveform voltage to produce the observed “apparent waveform.” OK. Time for a picture:
Figure 15 shows a 60Hz fundamental frequency waveform (solid black line) and for simplicity, only 3rd and 5th harmonics (dotted lines) of the fundamental frequency. Instantaneous voltages of the fundamental and harmonic waveform voltages “add up” to produce the resulting voltage waveform that is actually experienced by equipment in the system.
Depending on the specific mix of harmonics and their amplitudes, many variations of malformed voltage wave shapes are possible. Two additional, “typical” malformed AC mains power waves are shown in green and blue in Figure 16. The red wave shape above is called “Flat Topping,” for obvious reasons. Flat topping is characteristic of harmonics injected into the mains power by SMPS power supplies. Other malformed waveforms are characteristic of inadequately filtered Variable Frequency Drives (VFD). There are many more harmonics than are shown in this highly simplified chart.
All electrical equipment is rated to several industry quality standards by its manufacturer for its tolerance of Harmonic Distortion and its contribution to back-feeding distortion into the AC service mains and onto the grid in the neighborhood. The applicable utility company Power Quality standards (IEC 1000-3-2 or EN61000-3-2 and IEEE-519) are to enhance the quality, reliability and stability of the electrical power grid and it’s infrastructure.
The issue of Harmonic Distortion can become symptomatic on boats when digitally controlled equipment is running on generators vs when running on shore power. The reason traces back to Ohm’s Law. ALL ELECTRICAL CIRCUITS have the property of electrical resistance. In AC circuits, its called “Impedance,” but it works the same way as resistance in Ohm’s Law math.
Another sidebar of “geek speak” follows, but please don’t skip this one, because the punch line may be worth the price of reading through it.
Figure 17, left side, represents “source impedance” as a combination of the source’s electrical resistance, Rg, and inductance, Lg. Ohm’s Law predicts there will be a voltage drop, Vdrop, across that source’s impedance as a result of current, Ig, flowing from the source into attached loads. The effects of that current flow is reflected in the waveform corruption seen on the drawing. In the case of commercial utility distribution systems, the subscript “g” means “grid.” In the case of a boat’s onboard genset, the subscript “g” means “genset.” For utility systems, the heavy vertical line in the center of the drawing is the “Point of Common Connection” between the utility grid and a collection of shared loads, such as residential neighborhoods or commercial facilities. On boats, the PCC is where the genset connects to the onboard electrical panel; it’s easy to think of that point as being the Generator Transfer Switch. The effective impedance of the AC shore power utility grid system is proportionally much smaller than the effective impedance of an onboard genset. Per Ohm’s Law, the effect of harmonic currents is therefore much greater and more significant within the proportionally larger impedance of the proportionally smaller capacity generator power source. Especially when that small source is running above 60%-70% capacity, where magnetic saturation issues begin to come into play. The result of all of this is that onboard AC equipment may run just fine when on shore power, but experiences intermittent failures when running on the genset.
IS ANY OF THIS STUFF “REAL?”
I’m sure some readers ask themselves that question from time to time. Well, how’s about I show you some oscilloscope waveforms and you decide for yourself?
Aboard Sanctuary, we have a Dometic model DTU16-410A, 16kBTU, 120V, 1∅, mfg. no. 205160160, heat pump installed. User input is provided by a Dometic model SMX II remote control unit. The SMX II stages loads “on” in order to more evenly distribute inrush currents and avoid tripping the HVAC branch circuit breaker. When calling for heating or cooling, first the fan come “on,” then after short delay, the raw water circulator stages “on,” and again after a short delay, the compressor stages “on.” These units are staged “off” in reverse order once the thermostat becomes satisfied. Aboard Sanctuary, this heat pump provides for our main heating and cooling needs. Many readers will have this unit, or very similar and equivalent units, aboard their boats.
The blower in this heat pump is a variable speed drive motor that speeds up when heating or cooling demand is high and then slows when the thermostat is satisfied. Figure 18 show the 120V voltage waveform (yellow) and the current waveform (blue) with the blower-only running, on its “slow speed:”
Clearly here, the current drawn by this blower IS NOT a sine wave, with somewhat more than 50% of the leading portion of the waveform missing. This is reminiscent of what we saw above in Figure 4. This motor is a typical example of a non-linear load. Because it’s a non-linear load, it contains many harmonic frequencies.
Figure 19 shows the current waveform (blue) when the raw water circulator stages “on,” in this case, in a “heating” cycle example:
In this waveform, we can see the current drawn by the non-linear blower superimposed on top of the current drawn by the linear raw water circulator pump. The pump motor’s waveform is a sine wave, and this composite waveform shows the “bump” of the fan’s non-linear load atop the motor’s linear sine wave. This resultant waveform is non-linear, itself with harmonics, but different harmonics than were found in the fan waveform by itself.
Finally, Figure 20 shows the current waveform when the fan, raw water circulator pump and compressor are all “on,” running, at the same time.
This waveform is the composite result of the current drawn by the Blower, the current drawn by the raw water pump and the current drawn by the compressor motor. This waveform is still not a sine wave, distorted as it is by the blower’s non-linear components. And, as can easily be seen here, the top of the wave is “flattened” by the fan component. And again, there are lots of harmonics here.
My oscilloscope has a built-in Fast Fourier Transform (FFT) math function to analyze the harmonics present in waveforms under analysis. Figure 21 shows the FFT Analysis for the composite waveform of the Fan, the circulator pump and the compressor:
This FFT waveform shows the 60 Hz “fundamental frequency” as the highest peak on the left side of the screen. Then, the peaks show several succeeding harmonics, each with their characteristic descending magnitude.
ALL OF THESE HARMONICS contribute to the shape of final waveform shown in Figure 20 and experienced by the equipment on this particular power inlet to our boat. This (and one other, smaller, heat pump) is the only equipment on this 120V line into our boat, so what happens here is exactly what is drawn from the dock feeder by our boat.
The day I took these screen shots was “chilly” and rainy (downright “cold” for La Florida), and many live-aboard boats on the dock feeder probably had heating systems in various stages of “working.” Notice that there is minimal distortion of the voltage (yellow) sinusoid. That’s because the dock infrastructure is capable of providing power to the loads attached to the dock feeder; in electrical speak, the feeder has a “low internal impedance.”
One final waveform is shown in Figure 22. I have not described this at all in the preceding text because it is generally not an issue for end users of electric power, but it does contribute to waveform distortion if/when aggregated into the electric utility.
In this screenshot, I adjusted the amplitude of the current waveform (blue) to show it visually to the same scale as the voltage waveform (yellow). Notice the obvious non-sinusoidal shape (distortion) of the current wave form; proof positive of the presence of Harmonic Distortion. But also notice that the current waveform lags the voltage waveform in time; that is, these two waveforms are slightly out-of-phase with one another. The voltage waveform makes its “zero crossing” before the current waveform makes its “zero crossing.” This phenomena is an electrical “characteristic result” of “inductive loads” (motors, transformers) in AC systems, and it’s called “power factor.” It doesn’t mean much to boaters, and there is nothing end users of electric power can do about it, which is why I haven’t discussed it in more detail, but it can and does mean a lot to the utility supplying the docking facility with electric power. It means the electric feeder cables need to be bigger – sometimes significantly bigger – than they otherwise would in order to carry the total load of the facility.
What was the impetus for this article? Consider the question, “what is an acceptable limit for neutral-to-ground voltage?” The National Electric Code says 2.0 VAC is the max, and that’s based on resistive voltage drops that can occur in the neutral conductor in long cable runs as found on boat docks. But, since some of these Power Quality signals can look like Differential Mode currents between neutral and ground, it is possible in unusual circumstances to experience very high neutral to ground voltages. Anything higher than 2.0VAC should be treated as suspicious, and should be investigated. These waveform distortion faults are not simply aggravating and frustrating, they can be economically significant.” They degrade wire insulation and the mechanical parts of equipment (especially, motor bearings), so they can have real economic impacts to equipment service life.
So, to wrap it up, there are several power quality issues that can occur on boats, and particularly when running on relatively “small” power sources (compared to facility shore power), like inverters and generators. Power Quality issues can cause attached equipment to experience intermittent or continuous failures. These problems can appear alone or in combination with other types of electrical
disturbances. Some of these disturbances are intermittent and very complex and arcane. When they happen, they can be enormously frustrating, time consuming and expensive.
In our “modern” “throw it away” vs “fix it” culture, it’s easy to fall into a “replace it” trap without identifying or realizing the actual root cause of the problem. Since the equipment itself may not be the cause of the fault symptom(s), replacing equipment with new, like-for-like equipment is just as likely to result in continued exposure to the same old performance issues with the new, replacement equipment. So I bring this to your attention for awareness. I haven’t talked much about how to identify these issues, because that is definitely an advanced skill requiring expensive, time-interval and data recording test equipment. But simply being aware of these issues can enable boaters to ask the right questions and get to qualified service technicians to reach a good, and probably permanent, problem correction.