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Generator voltage vs load current


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Good day.

I need some advice on generator avr settings.  I inherited a 250kva genset with indicated 300a per phase and 200kw full load capacity.  I noticed a very high voltage output +/-250v LN and 450v LL.  When running under or without load all is well with rpm at 1500 and 50hz.  This set has been in service for many years with the same connected loads though i wonder why the avr was adjusted to such a high voltage.  Currently loads of between 200 and 220a per phase have been recorded so probably a stupid question:  If i adjusted the avr down to more acceptable voltage levels will this increase or decrease load current?  As i understand, when generating electricity - high voltage = low current and visa versa.  Just need some confirmation, trying to avoid overloading.

Thank you.  

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24 minutes ago, Inquest said:

Good day.

I need some advice on generator avr settings.  I inherited a 250kva genset with indicated 300a per phase and 200kw full load capacity.  I noticed a very high voltage output +/-250v LN and 450v LL.  When running under or without load all is well with rpm at 1500 and 50hz.  This set has been in service for many years with the same connected loads though i wonder why the avr was adjusted to such a high voltage.  Currently loads of between 200 and 220a per phase have been recorded so probably a stupid question:  If i adjusted the avr down to more acceptable voltage levels will this increase or decrease load current?  As i understand, when generating electricity - high voltage = low current and visa versa.  Just need some confirmation, trying to avoid overloading.

Thank you.  

Yes the loads that are switched on needs X kW. If you lower the voltage the current will increase by the same %. Normally a sweet spot is around 80% power rating. 

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Ok.  So evidently someone raised voltage to avoid overloading in the past.  I'm basically stuck then.  I'm worried about damage to electronics.  Also with the higher voltage i'm probably running closer to max load than i thought.  Oh well.  Thank you.

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No, no.  Think back to school science and ohms law.   Power = [V^2]/R.

Let's assume you want to use a kettle or heat water in your geyser.  These devices use a element which has a fixed resistance.  If the applied voltage increases, the power dissipated in the element will increase and be in danger of overheating and burning out.

Most loads are not purely resistive and it get's more complicated, but many motors and other devices would be capable of higher output power at elevated supply voltages.  For instance, a pool pump rated at 0.8KW at 230VAC, would likely be able to produce much more power and torque at higher supply voltages.  The higher voltage in itself would produce higher currents and more I2R dissipation and would likely lead to motor damage.  The higher mechanical power can be problematic in many other ways.

However, there is always a however.  Nothing is ever that easy.  Most electronics use SMPS (switched-mode power supplies).  Let's take a TV as an example.  The TV electronics might require 15VDC and draw 3A.  This voltage must be quite accurate (within, say 300mV) or the TV might malfunction or get damaged.  It is the duty of the SMPS to supply a constant 15VDC to the TV internals.  The SMPS has a control chip that uses a negative feedback control loop.  It constantly measures the 15V output.  If the output rises, the control loop will reduce the PWM duty-cycle of the switching transistor and vice-versa.  The result here is that the ***power*** take-up is constant.  If the supply voltage rises, the SMPS will consume less current and the V*I (power) product will be constant.  If the supply voltage drops, the SMPS will draw more current.  All this happens by changing the duty-cycle of the PWM controller.

Note that the SMPS has certain design limits.  You cannot expect a 230VAC supply to still work at 70VAC, or at 500VAC.  At some point, the duty cycle will reach 0 or 100% and the whole thing will become unstable and emit smoke signals.

It is true that most devices use SMPS supplies....  however, these are normally small loads, normally under 100W.  Any large load such as heaters, motors, etc. are as described by ohms law.  And don't think you don't have heaters.  Heaters hide in many fridges, washing machines and other devices where one might not expect them to be.

Brownouts (under-voltage) and over-voltage can be very bad for some equipment while the same conditions might be perfectly fine for others.  So it is important to keep you AC within +/-10% or better, or you will have random failures and/or malfunctioning equipment.

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50 minutes ago, Inquest said:

Ok.  So evidently someone raised voltage to avoid overloading in the past.  I'm basically stuck then.  I'm worried about damage to electronics.  Also with the higher voltage i'm probably running closer to max load than i thought.  Oh well.  Thank you.

250V is very close to the top range for equipment. Based on many country grid codes inverters should switch off if the voltage reaches 260V. This is stipulated by various countries in the grid code they apply. 

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