13 hours ago, BritishRacingGreen said:

our resident guru @Coulomb

Warning: I'm no expert in this area.

13 hours ago, BritishRacingGreen said:

Let us assume we have a grid-tied inverter eg SunSynk  (SS) that is allowed to export , but limited  to export only within the non-essential domain . In this domain their exist a geyser that has a phase angle chopper to derate from 4kW to 2kW  using a 4kW resistive element. So in practice the first 90 degrees of each half cycle is wasted , whereupon the last 90 degrees carry the full burden of 4kW power. 

I am of the opinion that the SS cannot handle the vast disruptive changes in power on a per 50hz cycle basis.

I would think that it will be difficult for the inverter to manage its DC bus voltage, and therefore its AC output amplitude. That will cause the instantaneous Q (reactive power) to vary wildly. All in all, I agree with BritishRacingGreen's unease, though I can't be specific about it.

I do know that Axperts hate certain loads, like hair dryers on half power (that causes DC offset in the current draw; TRIACs won't have that problem at least), and my Atten reflow soldering machine (basically a slightly more sophisticated hair dryer with a nozzle, and what I assume is TRIAC control). Lights flicker when these types of loads are on, indicating to me that the AC output voltage is slightly unstable.

I'm a long term critic of Axpert control algorithms; perhaps SunSynk have better. 

@BritishRacingGreen, I don't immediately see why the bus capacitors would suffer extra ripple with a TRIAC controlled load. Yes, at the instant of the TRIAC , there is a sudden change of load, but a sine wave has a periodic change of load too, just spread out more smoothly over a 50 Hz cycle. It's not clear to me whether that sudden current after a half cycle rest is worse than the same current times time spread out over a whole cycle.

It's a tricky one, and not something that can easily be tested on the bench.

On 2023/10/24 at 12:04 PM, JustinSchoeman said:

I think you may not understand how RMS works.  RMS is a technique for time-averaging a changing reading in such a way that it preserves the effective power transmitted.

If you look at your typical AC sine wave, and you measure power at the zero crossing, you will see V=0V, I=0A and P=0W.  If you measure at the peak, you will see V=325V, I=25A and P=8125W.

100 times every second, power is 0W and 100 times every second, power is 8125W.  If you take the RMS average over one second though, then you will see an average power of 4000Wrms.

RMS is an average power over time, whether you take that average over 1 second or 10 minutes does not change it from power to energy.

Well, I coulkd say the same about you.

As you already said, RMS, effective power or whatever you want to call it is a kind of average power transmitted by a wave over time cause, as you also said, there are parts of the wave period wher power is 0. You are thinking about second-long averages but, in fact RMS is considering wave period average (20ms @ 50Hz).

The longer part of the period the voltage remains at 0 volt, the lower energy the wave can transmit to the load. This is what a triac phase control system is doing, trimming part of the wave and keeping voltage at 0 volt for longer part of the period thus lowering the average energy that the load receives.

It is the same thing that your old fashioned on-off thermostat is doing every few seconds but doing it on every wave period.

Please, remember that what is heating a geyser y energy, not power. Power is an instant magnitude and energy is a time related magnitude.

 

 

On 2023/10/24 at 9:58 AM, P1000 said:

@JustinSchoeman Is correct on this. Many inverters warn against the use of phase angle devices like this in their documentation - in fact you can cause far more complicated issues than is covered here. Luckily most inverters handles these issues remarkably well, but in general phase angle triggering is a bad idea on this power level even if you are just on the grid.

Can you share any paper stating this?

Please, quote also the source.

I fully agree with @BritishRacingGreenand @JustinSchoeman regarding the use of triac for phase angle control.Triac is a rugged device but best used as a switch like a in a relay or in place of a relay. Phase angle controlled voltage voltage regulating devices like in a fan regulator or a light dimmer throws a large amount of harmonics into the mains. It is a headache to the wiring and power distribution system, and the neighboring loads. Using such device increases the neutral current and increases phase - neutral voltage. Using the mini fan regulators (Triac based) heats up the fan motor excessively and reduces its life. My advice to all use either a capacitor or inductor based fan speed regulator. Even a resistor based regulator is preferable than a triac based one. Use triac only as a switch and not as a voltage regulator except for experimentation or in a localised circuit where the noise is not thrown back into the mains. Be very aware of its shortcoming while using it.

4 hours ago, Derek3 said:

I fully agree with @BritishRacingGreenand @JustinSchoeman regarding the use of triac for phase angle control.Triac is a rugged device but best used as a switch like a in a relay or in place of a relay. Phase angle controlled voltage voltage regulating devices like in a fan regulator or a light dimmer throws a large amount of harmonics into the mains. It is a headache to the wiring and power distribution system, and the neighboring loads. Using such device increases the neutral current and increases phase - neutral voltage. Using the mini fan regulators (Triac based) heats up the fan motor excessively and reduces its life. My advice to all use either a capacitor or inductor based fan speed regulator. Even a resistor based regulator is preferable than a triac based one. Use triac only as a switch and not as a voltage regulator except for experimentation or in a localised circuit where the noise is not thrown back into the mains. Be very aware of its shortcoming while using it.

I fully agree about harmonics on inductive loads, which is not the case for a purely resistive heating element mounted on a geyser.

By definition, any inductive element will store energy in form of magnetic field when connected to a voltage source and will send back that energy in form of an opposite sign voltage to the one that charged it when disconnected. That is why flyback diodes are mounted on relay coils or wherever that opposite sign voltage can be generated by an inductor.

Unfortunately, we cannot use flyback diodes in AC and commuting of big inductive loads is one of the main harmonics and noise generators on the distribution grid. It is still possible to use some snubber circuits in AC to reduce some ringing and spikes.

I highly recommend reading some literature on how & why harmonics are generated on AC systems.

Harmonics (eaton.com)

Harmonic sources | What causes harmonics (eaton.com)

In addition I would like to point out that PF lower than 1 is the main reason for having increased currents on ditribution lines. A triac based system is not able to increase any voltage by definition when connected to a resistive element cause the resistive element is not able to store energy in any form.

I still repect that you personally prefer other control systems.

 

 

 

Edited October 26, 20232 yr by Mauritius B
Diode correction

16 hours ago, Mauritius B said:

I fully agree about harmonics on inductive loads, which is not the case for a purely resistive heating element mounted on a geyser.

The harmonics come from the high frequency slope you introduce when you switch on the element mid-wave. Think of it in the frequency domain.

25 minutes ago, P1000 said:

The harmonics come from the high frequency slope you introduce when you switch on the element mid-wave. Think of it in the frequency domain.

Please, can you elaborate a bit more what is the frequency of a slope? What is the definiton of frequency?

 

1 hour ago, Mauritius B said:

Please, can you elaborate a bit more what is the frequency of a slope? What is the definiton of frequency?

Somewhat sloppy use of language that most engineers suffer from.  Poster probably meant to say 'frequency component':

https://math.stackexchange.com/questions/1920602/how-does-one-derive-the-fourier-transform-of-the-ramp-function

Whenever you are dealing with electronics, you pretty much always look at the Fourier decomposition of the wave form, as you need to be able to handle all the significant frequency components of the wave form.

23 hours ago, Mauritius B said:

You are thinking about second-long averages but, in fact RMS is considering wave period average (20ms @ 50Hz).

No where, anywhere, ever does 'RMS' limit something to a single wave period average.  Pretty much the only time you will ever see that, is in a mathematical analysis of a perfect waveform, in order to determine the theoretical RMS value of that wave form.

You can happily plug a true RMS meter into a noise source, and it will tell you the RMS value of that signal, even although there are no repeating periods.

23 hours ago, Mauritius B said:

It is the same thing that your old fashioned on-off thermostat is doing every few seconds but doing it on every wave period.

Which is pretty much what I was saying. Thermostat or TRIAC - they both work by zeroing the waveform for a period.  The periods are just shorter in one case. And in both cases, the source (inverter) can be called upon to provide the full instantaneous power of the load.

On 2023/10/26 at 10:20 AM, Mauritius B said:

Please, remember that what is heating a geyser y energy, not power. Power is an instant magnitude and energy is a time related magnitude.

I am not sure why you keep dragging energy into this. We are, and always have been, discussing power.

1 hour ago, JustinSchoeman said:

No where, anywhere, ever does 'RMS' limit something to a single wave period average.  Pretty much the only time you will ever see that, is in a mathematical analysis of a perfect waveform, in order to determine the theoretical RMS value of that wave form.

You can happily plug a true RMS meter into a noise source, and it will tell you the RMS value of that signal, even although there are no repeating periods.

Which is pretty much what I was saying. Thermostat or TRIAC - they both work by zeroing the waveform for a period.  The periods are just shorter in one case. And in both cases, the source (inverter) can be called upon to provide the full instantaneous power of the load.

I am not sure why you keep dragging energy into this. We are, and always have been, discussing power.

I do not understand why you refer to non periodical signals now. We are talking about sinewave produced by the inverter. Of course you can measure RMS parameters over a non repeating signal and, of course you will be obtaining non repeating RMS values on every analisys interval.

The inverter is called to provide the full instant peak power on every wave period unless you trim the peaks of the wave with the triac. We agree on that. I am still wondering why we keep discussing this. Additionally, the rating of the inverter is expressed in RMS Watts and/or VA. No one cares about the peak power of the wave.

I keep talking about energy because there is no way to derate the geyser unless you replace its heating element. If the heater is rated 3Kw at 220Vac 50Hz it will always use 3kw at 220Vac. Nominal instant power will always be 3Kw, no matter what you do. The thing is that you keep mixing power and energy usage. I do not care what the nominal power of the heating element is, but care how much energy it draws every second, minute, hour or day, which in the end, is my limitation cause I do not want to use too much of my PV production on it.

To reduce the energy the heater takes I can use several options like phase control, on-off thermostat, capacitors to increase reactance or even pure resistors, as somebody also mentioned. Each method will have pros and cons. None of them are perfect. Some will produce wave shape variation, some will change PF, some will dissipate a lot of energy as heat.

Some people prefer capacitors and I prefer a triac for a pure resistive load. If we do not agree, that's life.

All good and well but why do you want to derate a geyser element? For the effort and cost of modifying an existing geyser element why not just replace the element with a lower wattage element or a ac ptc ceramic element. I respect the efford in the name of creativity but to me i would just replace with suitable wattage element and be done with it 

1 hour ago, Derek3 said:

All good and well but why do you want to derate a geyser element? For the effort and cost of modifying an existing geyser element why not just replace the element with a lower wattage element or a ac ptc ceramic element. I respect the efford in the name of creativity but to me i would just replace with suitable wattage element and be done with it 

The major plus is as per the OP requirement to use excess PV is you can adjust the power to suit. It might even differ from season to season. 

I use a digital version with 1% adjustment from 0-100%. That is what I used for my 25-50-75% settings placed a while ago. Also works very well as a softer type of starter on universal motors and start (inrush) current much reduced. 

Edited October 27, 20232 yr by Scorp007