Blinking Downlinghts

[Muchachos]

We had load shedding last night at 22:00 and my daughter decided to switch on the hairdryer to dry out something. To my surprise the dimmable downlights in the lounge started flashing.

 

What could be the cause, are the 3xUS3000B pylontech batteries not enough or is this a problem with the dimming switch?

 

[Richard Mackay]

Hairdryers are as hectic a load as electric kettles. Murder for any battery backed up system!

[Stanley]

Dimmers can be affected in a couple of ways. Most dimmers look for the zero-crossings of the AC signal and use that for their timing. Then depending on if they are leading edge or trailing edge dimmers, they either wait some time after the zero-crossing before turning on or they turn on at the zero-crossing and then wait some time before turning off. Either way, distortion of the AC waveform can cause the zero-crossing detection to trigger a little earlier or later on different cycles, making the brightness fluctuate. So if the brightness was constantly changing then it is most likely caused by some distortion when the hair dryer was on.

Another possibility with bell-press dimmers (the ones that you turn on and off by pressing the same button that you hold to change the brightness) is that they have a small capacitor between live and the button input and high frequency noise can go through that capacitor making the dimmer think that the button has been pressed briefly. This will cause the dimmer to turn on and off. So if it was turning on and off then it is most likely caused by high frequency noise.

Hair dryers are particularly nasty if you use them on their half power or half speed setting, because most hair dryers use a diode rectifier for the half speed or half power setting, which means they only draw current for one half of the AC wave. This causes transformers to saturate and then the waveform can get quite badly distorted.

[Muchachos]

Thanks @Stanley, this is very informative. I believe I need more schooling about downlights as I have a lot of them in the house.

The dimmers also switch on when load shedding starts/stops. I am assuming this because of the distortion of the waveform.

6 hours ago, Stanley said:

Another possibility with bell-press dimmers (the ones that you turn on and off by pressing the same button that you hold to change the brightness) is that they have a small capacitor between live and the button input and high frequency noise can go through that capacitor making the dimmer think that the button has been pressed briefly. This will cause the dimmer to turn on and off. So if it was turning on and off then it is most likely caused by high frequency noise.

I have the button press dimmers and it seems like this is what is happening. .

I do not have enough space on the light switch to change to a dimmer on/off switch  and a dial for dimming. 

What is the fix for this problem?

 

[Stanley]

2 minutes ago, Muchachos said:

The dimmers also switch on when load shedding starts/stops. I am assuming this because of the distortion of the waveform.

Switching on or off is caused by high frequency noise or transients. When load shedding starts or stops, it will be transients causing the switching on or off.

I am not sure what solution there is for this problem. I would suggest asking the dimmer manufacturer if they know of anything.

[NigelL]

Hi @Muchachos

Some dimmer modules (e.g. the type covered in blue heat-shrink) can flicker, or even turn off, if there is noise or dips on the mains supply. I recommend changing the dimmer module to the "Shuttle" brand (see http://shuttlelighting.com/) - these dimmers work very well, especially with LED downlighters.

 

[Bobster.]

1 hour ago, Stanley said:

Switching on or off is caused by high frequency noise or transients. When load shedding starts or stops, it will be transients causing the switching on or off.

I am not sure what solution there is for this problem. I would suggest asking the dimmer manufacturer if they know of anything.

Is it not just that when there's no shedding the inverter synchronises with the grid, when there is shedding the inverter disconnects from grid and now generates AC according to it's own rules and so the frequency changes?

[Stanley]

29 minutes ago, Bobster said:

Is it not just that when there's no shedding the inverter synchronises with the grid, when there is shedding the inverter disconnects from grid and now generates AC according to it's own rules and so the frequency changes?

No, a small change in frequency wouldn't make the dimmers switch on or off. It may cause the brightness to change briefly though.

When the grid fails, it takes time for the inverter to realize that the grid is gone and time for it's contactor / relay etc. to open and disconnect it from the grid.

In that time. the grid voltage has dropped to 0 very quickly, this is the first transient. Then the inverter needs to take over, ramping up it's voltage very quickly which would be the 2nd transient. Note that this is different from just turning off the circuit breaker supplying the inverter because during an actual power failure (i.e. Load shedding etc.) the supply to your entire neighborhood has been disconnected, so your inverter which is connected to the grid is essentially trying to supply the whole neighborhood for a few mS. So this appears as a short circuit very briefly until the contactor or relay can open to disconnect it from the grid. During that time the load will see the voltage drop to 0 and then climb very quickly again. This dip is usually about 20mS, but may be a bit more or less depending on the inverter and how it detects a grid loss and also what it uses as it's disconnecting device.

For off-grid inverters where the inverter isn't actually inverting while the grid is present you will see something similar, but this will happen even if you just turn off the breaker supplying the inverter and not only during a real power outage. This is because the inverter also needs to detect the grid loss then disconnect from the grid and then start inverting. (A true hybrid inverter is always inverting even while the grid is present, so the problem is only that the grid becomes a short circuit when the power fails)

[Bobster.]

1 hour ago, Stanley said:

No, a small change in frequency wouldn't make the dimmers switch on or off. It may cause the brightness to change briefly though.

When the grid fails, it takes time for the inverter to realize that the grid is gone and time for it's contactor / relay etc. to open and disconnect it from the grid.

In that time. the grid voltage has dropped to 0 very quickly, this is the first transient. Then the inverter needs to take over, ramping up it's voltage very quickly which would be the 2nd transient. Note that this is different from just turning off the circuit breaker supplying the inverter because during an actual power failure (i.e. Load shedding etc.) the supply to your entire neighborhood has been disconnected, so your inverter which is connected to the grid is essentially trying to supply the whole neighborhood for a few mS. So this appears as a short circuit very briefly until the contactor or relay can open to disconnect it from the grid. During that time the load will see the voltage drop to 0 and then climb very quickly again. This dip is usually about 20mS, but may be a bit more or less depending on the inverter and how it detects a grid loss and also what it uses as it's disconnecting device.

For off-grid inverters where the inverter isn't actually inverting while the grid is present you will see something similar, but this will happen even if you just turn off the breaker supplying the inverter and not only during a real power outage. This is because the inverter also needs to detect the grid loss then disconnect from the grid and then start inverting. (A true hybrid inverter is always inverting even while the grid is present, so the problem is only that the grid becomes a short circuit when the power fails)

Thanks for the detailed explanation. Particularly the description of what happens as the grid goes down.
 

[Muchachos]

2 hours ago, NigelL said:

Some dimmer modules (e.g. the type covered in blue heat-shrink) can flicker, or even turn off, if there is noise or dips on the mains supply. I recommend changing the dimmer module to the "Shuttle" brand (see http://shuttlelighting.com/) - these dimmers work very well, especially with LED downlighters.

Thank you. Will check them out. 

[Richard Mackay]

1 hour ago, Stanley said:

For off-grid inverters where the inverter isn't actually inverting while the grid is present you will see something similar, but this will happen even if you just turn off the breaker supplying the inverter and not only during a real power outage. This is because the inverter also needs to detect the grid loss then disconnect from the grid and then start inverting. (A true hybrid inverter is always inverting even while the grid is present, so the problem is only that the grid becomes a short circuit when the power fails)

An off grid system isn't connected to the grid..

And if the grid fails the supply doesn't become a short circuit. There will be the substation transformer winding and all the other users who are connected to that supply. Yes, the voltage drops to zero but it's not a short circuit! 

[Muchachos]

3 hours ago, NigelL said:

I recommend changing the dimmer module to the "Shuttle" brand (see http://shuttlelighting.com/) - these dimmers work very well, especially with LED downlighters.

I have been thinking of getting a few smart lights. Would the SONOFF SMART DIMMER SWITCH work?

[Stanley]

12 hours ago, Richard Mackay said:

An off grid system isn't connected to the grid..

And if the grid fails the supply doesn't become a short circuit. There will be the substation transformer winding and all the other users who are connected to that supply. Yes, the voltage drops to zero but it's not a short circuit! 

Not necessarily. According to the CoCT rules, a passive standby UPS may be registered as off-grid embedded generation. A passive standby UPS is one that is not inverting while grid connected. i.e. It just passes the grid through to the load (it may however include a battery charger).

As far as the inverter is concerned, the grid may as well be a short circuit when it is off. If the inverter is trying to produce 230Vac onto the grid, it will be trying to power the whole local grid. i.e. Transfromers and all the other users connected. This could be hundreds of kW or even MW of load.

[Speedster]

1 minute ago, Stanley said:

Not necessarily. According to the CoCT rules, a passive standby UPS may be registered as off-grid embedded generation. A passive standby UPS is one that is not inverting while grid connected. i.e. It just passes the grid through to the load (it may however include a battery charger).

As far as the inverter is concerned, the grid may as well be a short circuit when it is off. If the inverter is trying to produce 230Vac onto the grid, it will be trying to power the whole local grid. i.e. Transfromers and all the other users connected. This could be hundreds of kW or even MW of load.

Which is a different beast to a short-circuit

[Stanley]

Just now, Speedster said:

Which is a different beast to a short-circuit

Not to a small inverter of only a few kW

[Speedster]

16 minutes ago, Stanley said:

Not to a small inverter of only a few kW

How is overload and short circuit similar?

[Stanley]

1 minute ago, Speedster said:

How is overload and short circuit similar?

An overload is a load that is greater than the inverter's rating. Depending on how much the inverter is overloaded it may be able to handle it for a while. A short-circuit on the other hand is a bit more difficult to define, and is just a very low impedance. A short circuit (much like an overload) depends on what is supplying the power. You would agree that a 10kW load would be an overload for a 5kW inverter, but not for a 10kW inverter. Similarly a 20kW load might be so much of an overload for a 5kW inverter that it's over-current protection kicks in to protect it from blowing up. This would essentially be seen as a short circuit to the 5kW inverter, while a 10kW inverter would just see it as an overload. Now in the case of a power failure, we are talking about a load of hundreds of kW or even MW. Let's say for the sake of this discussion that your inverter is trying to power a load of 100kW on one phase of the supply. The impedance of that would be (230^2 / 100000) = 0.529 Ohms (Since P = V^2 / R so R = V^2 / P)

Now I don't know about you, but I would regard an impedance of 0.529 Ohms to be pretty much a short circuit, but again that is subjective because a 100kW inverter would be happy supplying that load. All I was trying to do earlier was describe what happens during a power failure, and why certain things happen. i.e. Why the inverter output collapses to 0V before it can disconnect from the grid. I don't think any of us have inverters capable of supplying hundreds of kW to keep the grid up during a power failure. Although it could become a problem if enough people in a small area have inverters, as they could collectively keep the grid going which is one of the reasons why the anti-islanding protection in inverters has to be very good.

[NigelL]

17 hours ago, Muchachos said:

I have been thinking of getting a few smart lights. Would the SONOFF SMART DIMMER SWITCH work?

I suspect they will work okay, but I have never used them.

A quick check of the specs show that they have a Neutral connection, so should be able to operate more reliably than the old-style dimmer modules (without Neutral connection). The Neutral allows them to remain powered even when the output to the light is turned off or dimmed right down to zero.  

[Muchachos]

9 hours ago, NigelL said:

A quick check of the specs show that they have a Neutral connection, so should be able to operate more reliably than the old-style dimmer modules (without Neutral connection). The Neutral allows them to remain powered even when the output to the light is turned off or dimmed right down to zero.

Thanks will give the SONOFF a shot.

 

[Bobster.]

On 2021/02/09 at 6:06 AM, Speedster said:

Which is a different beast to a short-circuit

I'd think so, but it would have the same affect on my inverter. Trying to power the whole suburb or an actual short will both result in a very high current demand.