High DC voltage relays

Re: High DC voltage relays

lazza wrote: »

Hi Stephen

Thanks for the link, looks good and within the right price range... However:

1. Surely the accuracy will be questionable, using this type of clamp.

Yep, accuracy will depend on the clamp, not sure what the error margin on that particular clamp is. These types of clamps could have a 1% error on the current reading, so if you have 1kW of PV the error could be 0.04A, so 10W. During a 16 hour summer day, assuming a constant 1kW all day for arguments sake, that's 160Wh at 15cents/kWh that's 2 cents/day. Probably worth it.

lazza wrote: »

2. It requires a resistance heater:

What resistance heater are you using? What happens if it breaks, where does the energy go then? The energy of the resistence heater wont match the excess and you could end up consuming alot of unecessary electricity.

Says it can dissipate up to 4.5kW with a single unit, use multiple units if you need to dissipate more. It's a PWM diversion unit so it will dissipate exactly the right amount energy, no more no less (within the error margin of the sensor of course).

lazza wrote: »

Have you spoken to them? Do you know how it works?

Haven't spoken to them, as I don't need such a device It's similar to how the Immersun unit available in the UK works, except it doesn't have an LCD display and probably lacks other features in the immersun. You'll find the inventor at the bottom of this thread (in Spanish): http://www.solarweb.net/forosolar/fotovoltaica-sistemas-autoconsumo-energetico/31128-sistemas-evitar-inyectar-la-red-7.html

Re: High DC voltage relays

lazza wrote: »

It has been mentioned that they produce noise and could have a negative effect on the power factor?? Can anyone enlighten me on this?

Anything that turns on suddenly and handles a lot of current has the potential to produce AC hum and RF noise, as will as voltage spikes if an inductive load it present somewhere. Usually not a concern if suitable RF filtering is used on the input and output wires and the device has a metal enclosure.

The power factor issue is that if you use a triac just as an on-off switch to turn a heater on full for some period of time then turn it off, or several of them to control multiple heaters on/off you are just fine. The current and voltage waveforms track just as for a pure resistive load.

But if you use the triacs for proportional control, you will have to do it by delaying firing the triac until some point in the middle of the voltage curve instead of right at the zero crossing. The result will be that current only flows during part of the AC waveform, even though it is proportional to the voltage during that time. This distortion is harder to characterize mathematically, and is referred to as "distortion power factor" rather than the "reactive power factor" in which the current is still a sine wave like the voltage, only shifted in time.

Meters which are designed to measure and record PF will react differently to distortion power factor depending on their design, but will still notice something.
A good meter designed for real power will still read only real power when faced with a distortion power factor.

For a three phase network, this time of time-sliced distortion power factor can cause currents in the neutral wire which are up to three times the current in the phase wires, instead of canceling out to zero. This can overload the neutral.
For 120/240 split phase the same thing can happen, but with less severe effects, and with no affect at all if the two 120 volt triacs are properly synchronized.