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| Figure 1: A typical Powerwall 2 installation. Left to right: Utility meter/original load center fed from an underground power feed, the"new" load center to which the household circuits now connect, the Powerwall "Gateway" (with two 4G antennas on top), AC disconnect for the Powerwalls, sub-panel for the Powerwalls (containing two circuit breakers), and finally the two Powerwalls. This type of system is typically installed outside, near the utility's connection to the house. Click on the image for a larger version. |
What is a Powerwall?
A Powerwall is the Tesla-specific name for what amounts to a "whole house UPS"(Uninterruptible Power System): There are other manufacturers of similar systems and they have their own nomenclature.
As the name implies, if the mains power disappears, this system can provide electricity to the entire house (or a portion of it) during the power outage. As you might expect, very large, high duty-cycle loads such as whole-house air conditioning, electric water heaters and electric furnaces are typically not backed up by a system like this as they would draw down the battery very quickly.
When integrated with a PV (solar electric) system, it can be charged from the solar energy. If the grid remains unavailable, the house can run indefinitely, provided that the power budget is positive - that is, more solar power is produced than is being used and the battery is not discharged so much between charges (e.g. overnight, on cloudy days) that one does not reach the point of cut-off.
In areas where there are significant electric rate (tariff) differences between "peak" and "off-peak" hours, this system can be used to "zero out"(or reduce) utility usage during peak hours and charge during off-peak hours from the grid and/or with solar. In my area, this is not relevant as the power rates remain constant throughout the day and it is configured to charge only from solar.
This type of system often called an "AC Battery" because it contains both a high-power inverter and battery charger, each unit capable of being charged/discharged continuously at about 5kW. When multiple units are connected in tandem, the potential discharge/charging rate increases proportionally.
Having one of these systems is a bit like having a back-up generator, except that if the sun is shining, the gas tank is refilled. Practically speaking, this system is unlikely to save me any money in the same way that a back-up generator probably wouldn't, so I would consider it to be a sort of extravagance - like owning an RV or some 4 wheelers - a bit like a somewhat expensive "hobby" - but a bit more utilitarian. Being an amateur radio operator, I'm also concerned a bit about having back-up power in case there is some sort of event that causes the loss of the grid for a period of time, hence the concern about possible interference.
Figure 2, below, shows how a typical system might be wired into a household power system and integrated with a PV inverter.
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| Figure 2: A generic block diagram of an "AC Battery" type of back-up power system. In "Tesla speak", the "Gateway" comprises the functions depicted in the box labeled "Supervisory Control" while the Powerwall(s) themselves are depicted by the boxes labeled as "Battery-backed inverter/charger system(s)". Click on the image for a larger version. |
As can be seen, in normal operation it is simply in parallel with the house's power and the power grid and when "charging" from the solar, it simply monitors the output of the PV system and the power to/from the grid and adjusts its charge rate to match. Likewise, in the "Self-Powered" mode (described below) when there is a grid connection it will charge/discharge at a rate that precisely matches the house's usage, effectively zeroing-out the power going to/from the grid.
Why I care:
Being an amateur radio operator that uses a wide range of frequencies across the electromagnetic spectrum (from below 137 kHz to at least 24 GHz) and often "listens" over wider ranges than that I'm always on the look-out for devices that unintentionally produce radio frequency energy which will be manifest as radio interference, reducing my ability to receive signals.
This sort of interference is increasingly commonplace, the incidence having accelerated with the increased prevalence of "switching" type "wall-warts"(a.k.a. "power cubes") that ubiquitously power nearly anything that is plugged into the wall. As part of their power conversion, these small devices contain powerful oscillators - typically operating in the 20-100 kHz range - that have the potential to cause radio interference at frequencies far removed from that.
What this means is that the inclusion of even more of these devices in my household - including a Tesla Powerwall 2, which is a really big switching power converter - have the potential of adding to this sea of noise.
Does the Tesla Powerwall 2 cause radio frequency interference:
Yes and no.
The "no" part:
On the HF bands, I have determined that in my particular case(and prior to mitigation techniques described later) the interference potential on the HF bands to be minimal or negligible. When the unit is operating (either charging or discharging) and I am using my normal HF antenna systemI cannot detect any interference from it on the HF amateur radio bands of 80 through 10 meters (e.g. 3.5-30 MHz). Additionally, I cannot detectany interference from the Powerwall 2 system on any VHF or UHF band, either.
If I walk up to the Powerwall 2 system with a portable shortwave radio while it is operating I can hear a bit of noise when I am within a foot or so (less than a meter) but this noise energy doesn't seem to be being coupled on the connecting wires outside the unit.
The "Yes" parts:
160 meters:
Prior to noise mitigation techniqueson the highest MF band, 160 meters (1.8-2.0 MHz), the story is a bit different: When the unit is operating, I can just detect a bit of noise from the unit in the far background, just below the local noise floor - but whether or not I can hear this at all depends on which antenna I'm using for receive. For example, on an active E-field whip I can just hear this noise - but it is not at all audible when using a wire antenna.
On lower frequencies:
Going down lower in frequency - into and below the AM Broadcast band (e.g. below 1.7 MHz) - the RF noise being produced by the Powerwall 2 (again, when it is charging or discharging) gradually increases, being fairly obvious by the time one gets to the bottom of the AM broadcast band (e.g. 530 kHz). Below the AM broadcast band are two more amateur bands - relatively recent additions to amateur radio in the U.S. - and both of these are bands on which I operate: The 630 meter band (472-479 kHz) and the 2200 meter band (135.7-137.8 kHz).
At these frequencies the interference from the Powerwall 2 (when it is operating) ranges from "significant" at 630 meters to "considerable" at 2200 meters - but this is not surprising. It would appear that the main power converter(s) inside the Powerwall(s) operate in the area of 20-30 kHz - and the 2200 meter band is at only about 4-5 times this frequency. Because the 2200 meter band's frequencies are comparatively close to the operating frequency of the inverter - and because the RF interference filtering built into the unit works better as frequency is increased while the harmonics of these converters also decrease in amplitude - the amount of energy at 2200 and 630 meters will, of course, be higher than it would be on the HF bands.
In short: If you do not plan to operate on the 160, 630 or 2200 meter bands, you will likely not experience any interference at all, even if no mitigation techniques are used.
An interference source that can be controlled:
If the RF interference from the Powerwall 2 were to be of great concern, its worth noting that the user has pretty good control of when this might happen as interference from the Powerwall 2 seems to occur only in two possible states: When it is charging, or when it is discharging. What this means is that even if you use any of the MF (160 or 630 meters) or LF (2200 meters) bands it will not cause interference when it is "idle." The Powerwall will, of course, be "active" when it is backing up the house during a grid failure, but this is (hopefully) an uncommon occurrence.
A typical Powerwall 2 owner would operate it in one of two modes, selectable from a phone app:
- Backup-only. In this mode the Powerwall 2 operates only as a "whole house UPS" - that is, it is not producing power except when the utility mains is offline (e.g. a power failure). In a typical installation, charging of the Powerwall 2's battery is done only with energy from the PV system (solar + inverter) when it needs to do so - and this usually occurs only if the battery has been discharged below 95% or so.
- Self-powered. In this mode the Powerwall 2 monitors the net inflow and outflow of power from the house. In this configuration the Powerwall will either output enough power to "zero out" the usage of the house so that there is, on average, no power going to/from the utility and/or it will take excess power from the PV system to charge its battery which will also "zero out" the power to/from the utility. If the battery is fully-charged, excess power from the PV system will be fed back into the Grid - just as is done in a normal "Net Metering" situation.
In the "Backup-only" mode the Powerwall 2 system is not usually operating (charging/discharging) and will thus not typically produce any noise on any amateur band - but in the "Self Powered" mode, the only time that interference would not be being produced would be when the Powerwall 2's battery is fully-charged and the excess PV power is being exported to the utility grid.
What this means is that if there is the possibility of interference, one would typically operate in the "Backup-only" mode where it is fairly rare for the unit to operate at all. In my case, the charging portion of the inverter will operate only for a few hours in the morning as soon as the PV system starts to produce power, one or two days a week when it "tops off" the battery.
If, for some reason one wanted to completely eliminate the possibility of the unit going active - say, during some sort of contest - the Powerwalls could simply be turned off, but this would be done at the risk of losing the power back-up capability in the event of a grid failure
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 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 killowatt 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 many 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 short periods, or over 10 kW continuously representing over 58 amps 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 a commercially-available AC mains filter with such ratings is difficult and purchasing and installing such a filter would be both 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 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" 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
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| Figure 3: Beneath many of the boxes is a raceway/channel that contains some of the conductors, including data lines and, as depicted above, the wires coming from the utility mains and connecting to the Powerwall's gateway. In my installation there are no exposed conductors in this raceway and there is plenty of room for the installation of the ferrites. The marked ferrite devices are the "Mix 75" while the unmarked are the "Mix 31." While it probably doesn't make a difference, I placed the Mix 75 ferrites on the end of the leads closest to the Powerwall. Click on the image for a larger version. |
Warnings:
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.
- Placing any ferrite devices as described here outside the Gateway box will not affect its operation and would be less intrusive than, say, installing a whole-house surge protector. 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 interaction at mains frequencies.
- The installation described below involves 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 the homeowner to make modifications to the electrical system, but it is up to YOU to determine the safety and legality of any sort of modification of your electrical system. 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 related to anything on this page. 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 which 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, utility 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 1(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 3. Because there are no exposed electrical connections in this channel, these devices can be safely installed without turning off power.
Vibration prevention:
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.
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| Figure 4: This is a view inside my main house's breaker panel with the "dead front" cover removed. In the upper-right corner is a 125 amp circuit breaker that is the main feed-in from the Powerwall Gateway (the partially-visible box to the right) which can carry the power from the utility and/or from the Powerwall. The space for these ferrite devices is a bit crowded, but they do fit. As noted in the warning, this panel has exposed, live connections and you should not even think about working in it unless you have experience in working on electrical systemsand the power is turned completely off! Click on the image for a larger version. |
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 - but instead of being in a raceway where there were no "live", exposed connections, the only place that this wiring appears is in the main circuit-breaker panel.
Figure 4, 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.
Comment:
It would be preferable to be able to wind several turns of the large power cables through ferrite cores 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 - particularly in the preferred common-mode fashion (e.g. all conductors going through the same core(s)). 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 the (more expensive!)"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.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:
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| Figure 5: Also contained in the raceway is the CAT 5/6 cable for the Ethernet cable that provides the Powerwall with internet connectivity. In my installation there is also another data cable that goes to current/voltage monitoring equipment where the PV (solar) equipment feeds into its sub-panel. Multiple turns and conductors of wire were fed through several ferrite devices to choke any RF that might egress. The upper device consists of three square snap-on ferrite cores while the bottom device is the ferrite core from the yoke of a scrapped CRT computer monitor. Click on the image for a larger version. |
- 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. 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 4G 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 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, it should be noted that CAT5/6 cable should not be coiled extremely tightly as doing so will distort the geometry of the twisted pairs. While this is unlikely to have much of an effect on 10 or 100 Megabit connections unless the cable is tightly wound, it can degrade a "Gig-E"(1 gigabit) Ethernet connection if the coil is smaller than 4-5 inches (about 10cm) in diameter and/or if the outer jacket of the Ethernet cable is "kinked".
The results:
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 both 160 and 630 meters to the point of inaudibility.
On 2200 meters the interference is reduced, but still significant: To completely quash interference at this frequency it would probably be necessary to, at the very least, install bypass capacitors (perhaps 1uF or greater) between each of the three conductors (ground, L1 and L2). If I do this I'll do so using a low-current (15 amp) circuit breaker to provide the connection between L1 and L2 and the ground, the breaker being a safe and simple way to make the connection. If adding such capacitors were found to be insufficient to reduce the interference to inaudibility and working "around" the operation of the Powerwall were not practical (e.g. when it was not active) the next step would be a rather awkward and potentially expensive one: The addition of the aforementioned extra junction box and re-running of the cables to allow the installation of multi-turn chokes.
What about RF interference to the Powerall?
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 of its many sensors to read incorrectly. To provide protection, the Powerwall is designed 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 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 mains 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 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" even was 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, killed all of the power for reasons of safety.
Since the clamp was 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 escaping, but also to prevent RF from getting in!
Parts sources:
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 has the "Mix 31" devices, but does not stock the "Mix 75" snap-on cores.
- 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.
- 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.
- The article "Common Mode Chokes" - link by W1HIS. This paper describes techniques and materials used for interference mitigation.
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This page stolen from ka7oei.blogspot.com