The information that follows may also be useful for interference reduction of other types of power systems that have been found to generate radio frequency interference, such as grid-tie inverters and other "battery back-up" systems that function like the Tesla Powerwall.
Note that RFI that is radiating directly from solar devices such as microinverters and optimizers require different mitigation techniques - See the blog post "The solar saga - part 1: Avoiding interference (Why I did not choose microinverters!)" for links to information about reducing this type of interference.
As a follow-on to the article "Does the Tesla Powerwall 2 produce RFI (Radio Frequency Interference)?" , this post describes some of the mitigation techniques to knock down what little interference the Tesla Powerwall might produce.
A recap: Does it produce RFI?
But first, a possible spoiler.
The answer is: It depends - but the fact that this article exists should be a big clue. In short:
- When it is neither charging or discharging: No interference at all on any band.
- On the 80 meter amateur band and higher frequencies: Not that I can tell.
- On the 160 meter band, AM broadcast bands and lower amateur bands: Maybe.
Mitigating interference from the Powerwall 2:
If we were dealing with a normal switching power supply the mitigation of interference would be quite straightforward: Apply "brute force" L/C filters to all of the AC connections in and out of the device - a topic that has previously been discussed in great detail at this web site (see the links to related articles at the end of this blog posting.)
Applying filtering to a plug-in device that is capable of up to a kilowatt or two is one thing, but mitigating interference issues on a device that is permanently wired in to the house's electrical system and capable of tens of kilowatts is an entirely different matter! For example, my Powerwall 2 system consists of a two battery/inverter modules that, together, are rated for 14 kW for brief periods, or over 10 kW continuously, representing over 58 and 41 amps at 240 volts, respectively.
To afford a wide safety margin any added inductive filtering would need to be capable of handling at least 100 amps with any capacitors being conservatively rated for the voltage. Finding and installing a commercially-available AC mains filter with such ratings could be difficult, expensive and awkward, probably requiring a separate enclosure - not to mention appropriate sign-off by inspectors. What's more is the fact that on a battery-inverter system like this, two such filters would be required: One on the AC mains feed-in from the utility to the Powerwall and another on the AC mains from it.
A more practical solution - and one that works effectively for 160 meters - is to install snap-on ferrite sleeves on these six conductors (e.g. the two "hot" phases and the neutral for each of the lines.) It so-happens that readily-available devices that will fit over RG-8 coaxial cable will also fit nicely over power cable that is appropriately sized for 125 amp circuits. (The dimensions of these devices is approximately 1.55" [39.4mm] long, 1.22" [31mm] diameter and are made to accommodate cables up to about 0.514" [13.05mm] - but could be modified to go over cables that are nearly 0.6" [15.24mm] diameter).
For exclusively HF, the so-called "Mix 31" ferrite material a reasonable choice, each device providing equivalent resistance as follows:
- 1 MHz: 25Ω
- 5 MHz: 71Ω
- 10 MHz: 100Ω
- 25 MHz: 156Ω
- 100 MHz: 260Ω
- 250 MHz: 260Ω
Intended for lower-frequencies, the equivalent resistance of each of these devices is:
- 200 kHz: 20Ω
- 500 kHz: 58Ω
- 1 MHz: 102Ω
- 2 MHz: 70Ω
- 5 MHz: 50Ω
At this point, there are a few "weasel words" that I must include:
- While it is possible to put these ferrite devices (or anything at all!) inside the Tesla Powerwall's gateway box, doing so would probably require the "official" permission of Tesla's engineering department to avoid the possibility of voiding a warranty/service agreement. Because of this, it is better to mount them on the conductors outside the gateway. Filtering could also be installed at the disconnect and/or circuit breaker between the Gateway and the Powerwalls, but this, too, may require appropriate approval and sign-off by Tesla engineering to avoid warranty issues.
- Placing any ferrite devices outside the Gateway box will not affect its operation and would be less intrusive than, say, installing a whole-house surge protector as no physical connections are being made. Because of the wide difference between the mains frequencies (50/60 Hz) and the lowest RF frequencies of interest (136 kHz-1.8 MHz) for which these devices are designed, these ferrites will have no measurable effect at mains frequencies.
- The installations described below involve the exposure of high voltage, high-current circuits inside a breaker panel. DO NOT even think of opening such a panel when it is "live", let alone installing any such devices inside it.
- DO NOT even think of installing such devices in a panel - even if it is powered down - unless you have experience working with electrical circuits. If you do not have such experience, refer to a licensed electrician to install such devices.
- Where I live it is permitted for me (the homeowner) to make modifications to the home's electrical system, but it is up to YOU to determine the safety and legality of any sort of modification of your electrical system and determine if you are competent to work with it. Do not presume some/any of the described modifications to be legal or in compliance of safety regulations in your (or any) jurisdiction!
- I cannot be responsible for injury or damage and no warranties as to suitability or safety should be implied related to the content of this and related pages. You have been warned!
First off, note that all of the units (the two Powerwalls, breaker panels, etc.) in my installation are connected together with metallic conduit and if properly installed, this conduit will quite effectively bond all of the various boxes together electrically. This means that it is likely to be quite effective in both preventing direct radiation of RF energy from the contained conductors as well as minimizing differential RF currents between the various boxes.
What this de-facto shielding will not do is stop RF from being conducted on the wires that leave this system - notably those that go into the house or to the power utility. In my case, mains power is fed from underground which means that the most likely source of interference from the Powerwall is likely to be conducted into it from the main breaker panel and onto the house wiring.
Visible in Figure 2 (above) is a channel that runs underneath several of the boxes and in this channel are the conductors that, in my installation, go from the utility mains panel to the Powerwall's Gateway - and I installed one set of the ferrites (a total of 12 devices) in it as depicted in Figure 2. Because there are no exposed electrical connections in this channel, these devices can be safely installed without turning off power.
These ferrite devices are, by their nature, quite magnetic and as such the magnetic field associated with the AC current flowing through the wires over which they are slipped will cause mechanical movement. When I installed the first of these devices I could hear them buzzing slightly, the apparent result of the two halves of the ferrite moving with respect to each other.
Installation in the main breaker panel:
In my installation there was another location at which these ferrites were to be installed: On the power feed from the Powerwall to the household circuits where the majority of RF noise is likely to be conducted - but instead of being in a raceway where there are no "live", exposed connections, the only place that this wiring appears is in the main circuit-breaker panel.
It should be noted that some ferrite mixes can be slightly conductive which means that the material itself should not be allowed to touch any metal that may carry a voltage. The "snap-on" devices have plastic covers that effectively insulate the ferrite within, but this should be noted if "bare" toroidal cores are used: Good-quality polyester tape is likely suitable to provide good insulation and protection.
Figure 3, above, shows the installation of the ferrites on the conductors within the breaker panel. As can be seen, there are "live" exposed connections that pose a shock hazard which means that these devices can be safely installed only if the power is turned completely off. As was done with the other devices, an extremely thin layer of RTV was put on the mating surfaces of the ferrites' halves to prevent their buzzing.
It would be preferable to be able to wind several turns of the large power cables together through large ferrite cores (such as toroids) to achieve much higher effective resistance at the frequencies of interest, but this is simply not possible in the available space with the existing wiring. Because the conductors were already in place and routed, it was deemed to be too awkward to disconnect one end of the (heavy!) cable to allow ferrite devices to be slid over it, so "split" devices were used instead.
If one is starting from "scratch" - or has the ability to add it later with some rewiring - enough extra cable length added to allow the winding of multi-turn chokes through large ferrite (toroidal) "non-split" cores inside a dedicated, metal junction box would be desirable. Doing this can greatly increase the series inductance and provide a commensurate reduction of conducted RFI.
It would also be preferable to pass all of the power cables through the center of a single ferrite (of ferrites) as a single bundle to provide a "common mode" impedance path, but this is difficult to do as I have not found a source for split ferrites of 31, 75 or 77 mix that would accommodate three cables that are about 0.5 inch (approx. 12 mm) diameter. The obvious alternative would be to pass the conductors through a stack of adequately large ferrite beads/cylinders or toroidal cores, but doing this would require that the conductors be disconnected from one end and temporarily pulled back. The preference would be to have this done at the time of the original installation, particularly if several turns could be passed through some large cores, but again, this is much harder to do after the fact, particularly with the limited length of wire in an already-installed system.
If you are able to put all of the wires with the ferrite cores in a single box, it is a good idea to keep the "input" and "output" wires (e.g. "before" and "after" the ferrite) away from each other - and from other conductors and ferrite devices as well. If the "clean" and "dirty" (from an RF standpoint) wires are run together in the same conduit, it is possible that RF energy could couple from one to another and partially negate the effects of the RFI mitigation. Note also that the wires themselves - and the ferrites - of different sets of wires should be kept apart to prevent capacitive/magnetic coupling as well - but only a few inches/cm of distance should suffice.
Finally, while there is plenty of room in the raceway to accommodate the bulk of a number of these cores, there is much less available space within the cramped confines of the breaker panel to accommodate a large stack of ferrite rings/sleeves, particularly if one were to wind several turns of wires through them. If you are contemplating a brand new installation, or if you are willing to pull wire out and do mechanical re-work, by all means put several turns of the three wires (both "hot" and the neutral leads) through common cores to maximize common-mode impedance.Other RF interference paths:
In addition to the power connections to/from the Powerwalls, there are two other possible egress paths for radio frequency interference:
- The Ethernet connection from the Gateway. It is common to "hard wire" a CAT5/6 cable from the Powerwall's Gateway to an Ethernet switch (behind a firewall) to provide internet connectivity. While an Ethernet interface is, by its nature, galvanically isolated from its support circuitry, it does have some capacitive coupling. It is possible to wirelessly (via either WiFi or via a cellular network) connect the Powerwall to the Internet - which would avoid such cabling - so one would have to determine the nature of the specific installation.
- Serial power cable to voltage/current monitoring. A typical Powerwall 2 installation uses devices made by Neurio to monitor the voltage and current at both the connection to the power mains and at the PV (solar) electrical connection. While a wireless connection between some of these devices is possible, there may be a (more reliable!) 2-wire (half-duplex, RS-485 serial) connection between some of these devices and RF egress could occur on this cabling as well.
While using ferrite devices on CAT5/6 cable will not normally affect the high-speed Ethernet signals within, CAT5/6 cable should not be coiled extremely tightly as doing so will distort the geometry of the twisted pairs and the integrity of the signals. While this is unlikely to have much of an effect on 10 or 100 Megabit connections unless the cable is very tightly wound, it can degrade a "Gig-E" (1 gigabit) Ethernet connection (the Powerwall only uses a 100 Mbps connection) if the coil is smaller than 3-5 inches (about 8-12cm) in diameter or if the outer jacket of the Ethernet cable is "kinked".
Adding RF bypass capacitors:
As mentioned above, I did (later) add some RF bypass capacitors to the system: Two capacitors (one for each phase) on the "house" panel connected to the output of the Powerwall and another pair of these same capacitors on "utility" side of the world in the original distribution panel. These capacitors are 4uF plastic film units designed specifically for bypass and pulse service and are rated for at least 630 volts AC. Their connection to the power bus is made by dedicated 15 amp circuit breakers and the other end of these capacitors is to the ground bus inside the panel - See figures 5 and 6. In theory, the 4uF capacitors will present around 660 ohms at the 60 Hz mains frequency, implying a current of 180mA through each at 120VAC for a total of about 22VA, but since this is reactive-only, no heat will be generated. This high capacitance was chosen for its ability to effectively shunt energy at 137 kHz where its reactance will be on the order of 0.3 ohms, although the overall reactance+resistance of the leads and circuit breakers will be higher than this.
Added breakers to connect EMI capacitors to the panel's
power bus. Using a separate breaker for each capacitor
is the easiest and safest way to connect the it to the
main bus bar in the panel.
Click on the image for a larger version.
Installation of these capacitors significantly reduced the amount of inverter noise present on 630 meters and "noticeably" reduced it on 2200 meters. Eyebrows might be raised about the rather long lead lengths to connect these capacitors - both on the "ground" and the "hot" side. At these low frequencies (e.g. <500 kHz) this isn't as critical as it might be at HF, but lead length should be minimized.
Another pair of capacitors (not shown in the attached pictures) was similarly installed in the breaker panel that is on the "unprotected" side of the isolation switch: Whereas those in the picture are on the "house" side of the Powerwall, the others are on the "utility" side.
In the U.S., electrical code typically requires wiring of electrical circuits inside a grounded metal enclosure with a large bus bar for carrying the current from the (usually) two phases found in residential wiring - which is likely going to be a 240 volt, center-tapped source from the utility's mains transformer. In North America and some other places, "small" items (up to about 1800 watts) are powered from a 120 volt circuit against the common "neutral" wire (the center-tap) while larger items will operate from a 240 volt circuit, often in a balanced fashion.
In many other parts of the world there is a single neutral connection to the house and a 240 volt "hot" wire which means that one would need only apply interference mitigation to these two wires instead of three as depicted elsewhere on this page. It is also possible that in some areas electrical codes may not require an enclosure that provides a convenient common power "bus" and large ground plane (e.g., metal enclosure that also acts as a shield) per se, requiring different techniques to apply the described mitigation.
While it may be a bit of overkill, the addition of the two types of snap-on ferrites (e.g. two of each type on each conductor for a total of 24 snap-on devices) has reduced the interference on 160 meters to the point of inaudibility - and pretty much the same thing on 630 meters.
On 2200 meters the interference is significantly reduced - only being "just visible" that band as the spectral display below shows.
Annotated spectral display showing the harmonic (and other) energy from various devices, including the Powerwall 2 after the described RFI mitigation techniques.
Click on the above image for a larger version
This spectral display also depicts the harmonics from a pair of SunnyBoy grid-tie inverters (only the first several harmonics of the 16 kHz switching frequency are even visible) plus some rather strong harmonics from a plug-in switching wall-wart that operates at a fundamental frequency of about 73 kHz: It is this wall wart - and (potentially others like it) that is likely to cause more QRM to reception on amateur bands than the Powerwall 2!
To completely quash interference at the 2200 meter frequency (around 137 kHz) it would probably be necessary to increase the inductance in the offending leads between each of the three conductors (ground, L1 and L2) even more than has been done with the ferrite devices.
What about RF interference to the Powerwall?
The Powerwall itself is a computer-based system with a number of analog monitoring points and as such, it is theoretically possible for external RF to cause it to malfunction if that energy somehow "glitches" one of its computers and/or causes one or more of its many sensors to read incorrectly. To provide protection, the Powerwall is designed very conservatively and in the event of a serious discrepancy or fault, it will shut itself down.
The question should be asked: Is it possible for external RF to cause such a shut-down?
The answer is: Maybe.
About a week after my Powerwall was installed and running I happened to tune up on 40 meters using my 1.5kW amplifier. While I was doing this, the power to my entire house "blinked" several times and went off with the Powerwalls indicating some sort of error condition. Unfortunately, the isolation relay had tripped and my house was disconnected from the mains and the Powerwalls did not reset themselves even after turning them "off" for over 15 minutes. After a bit of hassle, I was able to get the Powerwalls reset - but the question remained: What happened? I opened a ticket with Tesla support and they came out to investigate a few days later.
It was determined that a possible cause of this "loss of power" event wasn't due to RF, but instead due to arcing at one or more connecting clamps on the mains side of the isolation relay in the gateway that had not been properly tightened when it was installed. The extra 2+ kW of load on the AC mains from the RF amplifier may have been enough to cause arcing in that loose connection and the Powerwall, detecting this as a potentially dangerous fault (as arcs can be!) killed all of the power for reasons of safety.
Since the clamps were tightened I have never been able to recreate this event, but being "gun shy" I immediately started installing the various ferrite devices on the power and data communications cables - not only to keep RF interference from the Powerwall from radiating, but also to prevent RF from getting in. In other words, if it had been sensitive to external RF before, it certainly is not sensitive anymore now that I made the above additions!
There are several sources of snap-on ferrite devices described on this page, including:
- KF7P Metalwerx - link - Supplier of a variety of Ferrite devices and many other things. At the present time he stocks the "Mix 31" devices, but does not stock "Mix 75" snap-on cores at the time of posting.
- Mouser Electronics - link - The "Mix 31" snap-on cores - P/N: 623-0444164181 (Fair-Rite P/N: 0444164181); "Mix 75" snap-on cores - Mouser P/N: 623-0475164181 (Fair-Rite P/N: 0475164181). Mouser Electronics has other sizes and mixes of these various devices. Also obtained from Mouser Electronics were the Kemet 4 uF, 600 VAC "pulse" capacitors Mouser P/N: 80-C4GAMUD4220AA1J (Kemet P/N: C4GAMUD4220AA1J) depicted in Figure 5.
Links to other articles about power supply noise reduction found at ka7oei.blogspot.com:
- Containing RF noise from a "pure" sine wave UPS. Even when it is not operating your sine wave UPS may be producing a lot of HF radio interference!
- Completely containing switching power supply RFI - link. Sometimes it can be difficult to quiet a switching power supply, so it may be necessary to put it in a box with strong filtering on all of the conductors that enter/leave.
- Minimizing VHF (and HF) RFI from electronic ballasts and fluorescent tubes - link. Electronic light ballasts, like many switching power supplies, operate in the LF frequency range so "cleaning them up" at VLF/LF/MF frequencies can be a challenge.
- Quieting high current switching power supplies used in the shack - link. This page describes techniques that can be used to reduce the amount of RF energy produced by switching power supplies that you may be using to power your radios. Again, higher-inductance chokes may be required at VLF/LF/MF frequencies.
- Reducing switching supply racket - link. This describes techniques that can be used to beef up the filtering for switching supplies in general.
- Teasing out the differences between the "AC" and "DC" Powerwalls - link. In this post I discuss generally how "AC Battery" systems like the Powerwall and how they work as well as the general differences between the so-called "DC" Powerwall and the AC Powerwall.
- The solar saga part 1: Avoiding Interference (Why I did not choose Microinverters) - link. Having had first-hand experience observing a microinverter-based PV system, I discuss why I went the route of the series-string inverter.
- The solar saga part 2: Getting the system online - link. Like most large projects something intervenes that it makes it take longer to complete - and that was the case here, but it was successfully completed... eventually!