Home Energy Management System Part 4: Characterising the Hypervolt Current Transformer

The Hypervolt EV Charge Point is a UK-manufactured 7kW single phase unit. It is one of the smartest looking EV chargers on the market. It has no controls on the unit and can only be controlled via a smartphone or similar via the cloud server.

Unfortunately the app is rather poor. It often loses connection with the server and has to be stopped and restarted. The Charging/Stopped status display is frequently wrong. However, it does provide solar integration, with three modes:

  • Super Eco (only charge if there is enough solar power to provide the minimum 6A charging current, and then use all available solar power up to the maximum 32A)
  • Eco (use grid power if necessary to make up the required 6A, but use up to 32A solar power if available)
  • Boost (ignore solar, always charge at 32A).

Unfortunately, in the solar modes there is no display showing the actual charging power. There is a graph of charging power shown on the Analytics page, but it is misleading because it is very heavily smoothed and it does not show the gaps when charging was suspended. Also there is no facility to specify the maximum amount of grid power to be imported to make up the 6A; you can only choose zero (Super Eco mode) or 6A (Eco mode). So when my excess solar PV drops to 4A, if I want to avoid exporting that solar power and I am happy to import 2A to continue charging my car at one third of the price of fully imported electricity, I have to switch it out of Super Eco mode into Eco mode. Then I have to monitor the solar output as it drops further at the end of the day and turn it off when the amount of imported current it needs is more than I want. Of course I often forget to do this.

Sensible EV charger software would allow me to specify that I am happy to import up to e.g. 750W (3A) to make up the solar output to 6A. I am told that the Zappi EV charger provides this facility.

All said, I honestly cannot recommend the Hypervolt until they improve its functionality. But I’ve already had it installed, and I am not going to spend another £1000 or so for an alternative unit that might or might not be better overall.

In the absence of any published API to control the Hypervolt, and not wishing to modify the Hypervolt itself, it seems my only option to exert greater control over it is to modify the input that it receives from the current transformer. This input tells the unit how much current is being imported from or exported to the grid, and hence how much charge current to provide to the EV when the unit is in solar mode. In order to be able to show the real-time EV charging power, I will use an additional current transformer to monitor the power draw of the Hypervolt.

Caution! In some installations, the current transformer is also used to sense the amount of grid import, and to reduce the charging current if necessary in order to limit the grid import to some maximum value. This may be needed when the house electricity supply does not have a 100A fuse and cannot easily be upgraded, for example because it is a looped supply (cable shared with an adjacent property). In such an installation it could be dangerous to modify the signal from the current transformer to the Hypervolt. Fortunately for me, I have a modern supply with a 100A fuse and my local distribution company has not applied an import limit to my property.

How current transformers work

A split core current transformer comprises a pair of ferrite cores which are clamped around either the line or the neutral wire that carries the current to be monitored. Around one of the cores is wound many turns of fine wire. This acts as the secondary of the transformer. The wire carrying the current to be measured passing through the core acts as a single-turn primary.

The current is stepped down by (ideally) the turns ratio of the current transformer. For example, if the current transformer has a ratio of 1:1000 then a current of 10A in the wire would give rise to a current of 10mA in the secondary.

In order to measure the secondary current, it is normally loaded with a “burden resistor”. For example, a burden resistor of 100 ohms would convert the 10mA current into 1V.

Sometimes the burden resistor is integrated into the current transformer itself (this is the case for the current transformer used by the iBoost sender). If it isn’t, then when the current transformer is disconnected from the equipment that provides the burden resistor, a dangerously high voltage can appear across the secondary of the current transformer. To avoid this, current transformers without internal burden resistors normally have metal oxide resistors or other transient suppression devices incorporated, to carry the secondary current safely and limit the output voltage to a few volts or a small number of tens of volts.

Modifying the sensed current

We could modify the current sensed by the CT in a number of ways:

  • Modify the CT itself, for example by adding an additional winding. This is not likely to be practical.
  • To make the CT less sensitive, we could switch an additional burden resistor in parallel with the existing one to shunt some of the current
  • To make it more sensitive, we could substitute a current transformer with a higher turns ratio
  • If the current transformer has an internal burden resistor, then to add an offset we could insert a voltage in series with the current transformer
  • We could apply an offset by injecting a current in parallel with the current transformer (this method can be used regardless of whether the burden resistor is internal or external)

Whatever method is used, we have no control over the static voltage at the Hypervolt current transformer connections; so we had better ensure that whatever means we use includes an isolating transformer, or some other form of electrical isolation from our control hardware.

In order to achieve full control of the amount of charging current provided by the Hypervolt from zero to its maximum, I will need to put it in Super Eco mode; then I can control how much power it takes by adjusting the apparent grid import, by adding an offset to the current transformer output. To prevent the Hypervolt from taking charge from the house battery, I will need to reduce the apparent grid export power by the amount of power drawn from the battery. To put the Hypervolt in Boost mode, I will need to pretend that there is 7kW of solar export available (and then prevent the house battery trying to supply any of it).

The first step was to establish the ratio of the current transformer and whether it has an internal or external burden resistor. To do this, I carefully disconnected the current transformer secondary for the Hypervolt. Then I looked at the voltage across the secondary using an oscilloscope. This showed a clipped waveform – almost a square wave – with an amplitude of 15V peak to peak, indicating that the CT uses an external burden resistor and has a metal oxide resistor or other internal transient suppression device.

I then connected a multimeter set to measure AC current across the secondary. I monitored the reading while watching the solar export reported by the inverter app. The inverter app has some lag and at times the CT current reading was changing rapidly as clouds passed in front of the sun; but eventually I got a set of readings. Here are some of them:

Export powerCurrent mACalculated CT ratio
Some readings of Hypervolt CT current and export power

From this I conclude that the Hypervolt CT has a ratio of about 2500:1. Current transformers are not exact devices because the actual ratio depends on how well the wire fills the aperture and how thick the insulation is. Small wires and thick insulation cause some of the magnetic field from the wire not to be captured by the ferrite core, leading to a reduction in the ratio achieved.

I also estimated the value of the burden resistor in the Hypervolt, both by attempting to measure it directly (testing it with both polarities of the multimeter leads) and by measuring the voltage when the CT was again connected to the Hypervolt and the export power was stable. Both led to a value of around 10 ohms.

So to control the Hypervolt up to 32A charging current, we will need to inject current of up to (32/2500)A = 12.8mA. The maximum voltage that the injection device may need to overcome, assuming 70A maximum load, is about (70/2500)*10 = 0.3V.

Providing an isolated source of injection current

I have already mentioned the need for the injection device to be electrically isolated from the CT in case of high common mode voltages in the Hypervolt (for example, before a PEN fault is detected). Additionally we require that when the injection device is not powered, it must not affect the CT output read by the Hypervolt significantly. To meet these requirements I propose to use an Oxford Instruments A262A6E transformer. This is a 1+1:1+1 audio transformer capable of carrying frequencies down to 30Hz, so it covers the 50Hz frequency that we need. It provides isolation up to 1kV. With the two secondaries connected in series, it has 500mH secondary inductance and 40 ohms resistance. The reactance at 50Hz will be 2*pi*50*0.5 = 157 ohms, which if not corrected could cause a phase shift and up to 6% error in the Hypervolt reading. We can tune that out by connecting 20uF of capacitance in parallel with it so as to form a parallel resonant circuit tuned to 50Hz. The split primaries allow us to drive it easily in push-pull fashion from constant-current outputs.

The Vigortronix VTX-101-006 appears to be a very similar transformer at lower cost but its datasheet does not specify the inductance; so I chose the Oxford Instruments one.

The remaining consideration is whether the transformer can tolerate the maximum required secondary voltage at 50Hz without the magnetic core saturating. The datasheet gives the power carrying capacity as 2.5mW @ 50Hz. Although not clearly stated on the datasheet, I believe this is with the windings connected in series and a load of 600 ohms. This corresponds to a voltage of sqrt(0.0025 * 600) = 1.22V RMS, well above the maximum 0.3V that we expect (thanks to the low burden resistance in the Hypervolt).

I will publish the full schematic of the current injection circuit in a later instalment of this series.

[Series to be continued]

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