Hi folks,
I am trying to design a crowbar circuit that will open an MCB to protect equipment from high voltage.
Yes this is work but I am in a bind. All test will be performed using a variac fed via an isolation transformer.
I have a site with a turbine, hydro, driving a SEIG. The capacitors are switched to manage the speed and voltage as this alters the frequency to voltage relationship. There ere 2, well technically 3, loads, heaters and an isolation transformer, with 2 separate secondary's, the output of which is rectified and fed to grid tie inverters.
There is an over voltage relay to disconnect the generator, SEIG, from its caps and the loads.
With just the heaters on all is well but if the inverters are running, and abruptly disconnect, we are seeing a voltage spike big enough to damage the energy meter. I do not know the origin of the spike but I suspect it has something to do with collapsing flux in the transformer.
Whatever the cause of the spike I cant muck about with caps as that would mess with the voltage regulation, the generator being an islanded SEIG, and the secondary voltage is none standard to avoid excessively high DC voltages after the rectifiers.
That is a summery of the issue, assuming I have to mitigate the spike as opposed to preventing it n the first place then ...
I need a way to protect both the primary circuit, to protect the energy meter and the two secondary circuits as these are blowing rectifier fuses when the inverters shut down, presumably because the voltage spikes and the current associated with lifting the voltage of the caps on the DC side.
The energy meter can be protected with a 1A MCB, the rectifiers will require at leas 10A and if I decided to protect the main circuit anywhere it would need to be at least 25A, preferably 32A
I would rather not use MOV's because of the ageing. TVS diodes are a possibility but voltage matching will likely be an issue as will current limiting. I looked as SIDAC's found they were not easily available in the voltages I need and based on that decided a TRIAC would probably do the job well. Or at least I originally thought it would, perhaps not
The problem I have is that running in QI and Q3 I need an IG of 50mA or more and if I do that with a simple divider and a DIAC the power requirement of the resistors it quite high.
I may be missing something obvious but if the diac VBO is 45V any voltage decider could be dropping 210V, at the top and 40V, or so, on the ref side whilst supporting a current in excess of 50 mA when the DIAC is off.
Obviously I can use higher value resistors to limit current in the voltage divider, and thus power, but as this would also limit IG I do not think that is an option, hence the question.
In short how do I drop 250VAC to 45VAC in a way that can provide sufficient current to turn on a TRIAC, Qxx30LH5, without the resistors having to handle 15W between them?
this feels like an asked and answered thing and must be a common design issue so what am I missing?
I am planning to have a current limit resistor that will open the MCB in its MAG region but also limit the pulse current through the TRIAC.
I realise that the IG will only be a short pulse and because the supply voltage will fall as soon as the TRIAC turns on, even before the MCB opens.
I also appreciate that I have probably answered my own question RE power dissipation over the restive divider and the 'normal' off state current needs to be significantly lower than the required IG with a cap providing IG when the DIAC is pushed above VBO...
What I don't know is how to calculate all that to actually decide if the plan is viable specifically with respect to the delay that will be added to the trigger pulse. It is also fair to say that there is likely quite a bit more that I don't know I don't understand yet...
Ignorance is not bliss in this case.
To be honest although I have been talking about RMS voltages here I expect that I need to be thinking in terms of peak voltages and transients for everything but the power dissipation...
My idea may be unworkable but if it is I would like to understand why so I can come up with a workable one...
Any and all comments are welcome and don't feel the need to pull any punches... I can live with feeling silly if I am learning something.
I am trying to design a crowbar circuit that will open an MCB to protect equipment from high voltage.
Yes this is work but I am in a bind. All test will be performed using a variac fed via an isolation transformer.
I have a site with a turbine, hydro, driving a SEIG. The capacitors are switched to manage the speed and voltage as this alters the frequency to voltage relationship. There ere 2, well technically 3, loads, heaters and an isolation transformer, with 2 separate secondary's, the output of which is rectified and fed to grid tie inverters.
There is an over voltage relay to disconnect the generator, SEIG, from its caps and the loads.
With just the heaters on all is well but if the inverters are running, and abruptly disconnect, we are seeing a voltage spike big enough to damage the energy meter. I do not know the origin of the spike but I suspect it has something to do with collapsing flux in the transformer.
Whatever the cause of the spike I cant muck about with caps as that would mess with the voltage regulation, the generator being an islanded SEIG, and the secondary voltage is none standard to avoid excessively high DC voltages after the rectifiers.
That is a summery of the issue, assuming I have to mitigate the spike as opposed to preventing it n the first place then ...
I need a way to protect both the primary circuit, to protect the energy meter and the two secondary circuits as these are blowing rectifier fuses when the inverters shut down, presumably because the voltage spikes and the current associated with lifting the voltage of the caps on the DC side.
The energy meter can be protected with a 1A MCB, the rectifiers will require at leas 10A and if I decided to protect the main circuit anywhere it would need to be at least 25A, preferably 32A
I would rather not use MOV's because of the ageing. TVS diodes are a possibility but voltage matching will likely be an issue as will current limiting. I looked as SIDAC's found they were not easily available in the voltages I need and based on that decided a TRIAC would probably do the job well. Or at least I originally thought it would, perhaps not
The problem I have is that running in QI and Q3 I need an IG of 50mA or more and if I do that with a simple divider and a DIAC the power requirement of the resistors it quite high.
I may be missing something obvious but if the diac VBO is 45V any voltage decider could be dropping 210V, at the top and 40V, or so, on the ref side whilst supporting a current in excess of 50 mA when the DIAC is off.
Obviously I can use higher value resistors to limit current in the voltage divider, and thus power, but as this would also limit IG I do not think that is an option, hence the question.
In short how do I drop 250VAC to 45VAC in a way that can provide sufficient current to turn on a TRIAC, Qxx30LH5, without the resistors having to handle 15W between them?
this feels like an asked and answered thing and must be a common design issue so what am I missing?
I am planning to have a current limit resistor that will open the MCB in its MAG region but also limit the pulse current through the TRIAC.
I realise that the IG will only be a short pulse and because the supply voltage will fall as soon as the TRIAC turns on, even before the MCB opens.
I also appreciate that I have probably answered my own question RE power dissipation over the restive divider and the 'normal' off state current needs to be significantly lower than the required IG with a cap providing IG when the DIAC is pushed above VBO...
What I don't know is how to calculate all that to actually decide if the plan is viable specifically with respect to the delay that will be added to the trigger pulse. It is also fair to say that there is likely quite a bit more that I don't know I don't understand yet...
Ignorance is not bliss in this case.
To be honest although I have been talking about RMS voltages here I expect that I need to be thinking in terms of peak voltages and transients for everything but the power dissipation...
My idea may be unworkable but if it is I would like to understand why so I can come up with a workable one...
Any and all comments are welcome and don't feel the need to pull any punches... I can live with feeling silly if I am learning something.