Data Log figures not making sense

[BillP]

We're getting a lot of over-voltage alerts from one of our UPS's. I've been looking at the figures in the Data Logs trying to see if I can spot what is going on but the figures don't seem to make sense to me.

The device is a Smart-UPS SC450 RM in the UK.

Output Voltage is set to 230V

High Transfer Voltage is set to 253V

Low Transfer Voltage is set to 208V

Power Quality Sensitivity is set to Low

Now I know that despite the UK and mainland Europe being 'harmonised' at 230V, mains power in the UK remains around the old 240V level in most cases and this is reflected in the figures I'm seeing from the device. For the weekend period (6th April to 8th April) I can see a high of 256.30 and a low of 237.60. Both these figures fall within the +/-10% from 240V to be within the permitted voltage regulations for the UK.

What doesn't make sense to me though is this: If the Output Voltage is set to 230V, why can I see a fluctuation of 33.20V (low 218.80, high 252.00)? I was under the impression that a Smart-UPS was an 'online' device which should be 'cleaning' the mains power and delivering a smooth supply to the load.

What also doesn't make sense is how the Output Voltage can be higher than the Input Voltage yet there's no corresponding AVR Boost action logged in the Event Log?

I've also seen both the Output and Input Voltage levels fall outside of the Max and Min line values. How does this happen?

Interpreting these logs is turning out to be a bit of a minefield and I need to understand them. Can anyone shed any light in this dark cupboard please?

[voidstar_apc]

On 4/9/2018 4:26 AM, Richard said:

What doesn't make sense to me though is this: If the Output Voltage is set to 230V, why can I see a fluctuation of 33.20V (low 218.80, high 252.00)? I was under the impression that a Smart-UPS was an 'online' device which should be 'cleaning' the mains power and delivering a smooth supply to the load.

Your SmartUPS is a line-interactive model. It has an internal transformer that can boost or trim the voltage by a fixed percentage. The UPS uses this transformer to ensure the output stays within the low<->high transfer voltage range but not exactly 230V.

Online UPSes are nice but more expensive and less efficient.

On 4/9/2018 4:26 AM, Richard said:

What also doesn't make sense is how the Output Voltage can be higher than the Input Voltage yet there's no corresponding AVR Boost action logged in the Event Log?

That would be odd. I suppose its possible the voltage changed during measurement.

On 4/9/2018 4:26 AM, Richard said:

I've also seen both the Output and Input Voltage levels fall outside of the Max and Min line values. How does this happen?

Probably sampled at different times. The "max and min" capability was removed on newer UPSes because of that kind of issue.

[BillP]

On 4/9/2018 6:01 PM, voidstar said:

Your SmartUPS is a line-interactive model. It has an internal transformer that can boost or trim the voltage by a fixed percentage. The UPS uses this transformer to ensure the output stays within the low<->high transfer voltage range but not exactly 230V.

While what you say here makes sense in operational terms, what would be the point of this? I would have thought that the equipment connected to the UPS would require a fixed voltage/amperage draw and the UPS would regulate the fluctuating incoming power to deliver this smooth 'cleaned' supply. If the incoming current and outgoing current are both fluctuating, what's the point of the UPS 'cleaning' the power at all? I'm obviously missing something here but do not currently understand enough to see the point of this.

On 4/9/2018 6:01 PM, voidstar said:

Probably sampled at different times. The "max and min" capability was removed on newer UPSes because of that kind of issue.

These are on the same line in the logs and logged against the same time entry indicating that they happened together.

[voidstar_apc · Accepted solution]

On 4/10/2018 5:11 AM, Richard said:

On 4/9/2018 6:01 PM, voidstar said:

Your SmartUPS is a line-interactive model. It has an internal transformer that can boost or trim the voltage by a fixed percentage. The UPS uses this transformer to ensure the output stays within the low<->high transfer voltage range but not exactly 230V.

While what you say here makes sense in operational terms, what would be the point of this? I would have thought that the equipment connected to the UPS would require a fixed voltage/amperage draw and the UPS would regulate the fluctuating incoming power to deliver this smooth 'cleaned' supply. If the incoming current and outgoing current are both fluctuating, what's the point of the UPS 'cleaning' the power at all? I'm obviously missing something here but do not currently understand enough to see the point of this.

One tricky thing about AC power is that it's not a fixed voltage but rather time varying. There's two different time-scales to consider:

At the tens of millisecond time-scale, we have an AC voltage (say, 230VAC). This is an RMS average of the changing voltage. Your equipment is required to accept voltages in the range 230VAC +/- 10% and regulate it to whatever voltages it needs internally. For computers, the power supplies within are often universal and can go down to 100V if needed. But outside their designed voltage range, either the supply will shut down and/or the internal components become over-stressed which shortens their lifespan.

The transformer AVR allows equipment to continue working even when the incoming voltage exceeds the equipment's design tolerance. If you live in a region where the voltage is stable and within spec, this provides little value. But even in the UK people sometimes experience their electricity running above 253V.

Below tens of milliseconds, we have electrical noise and power surges. Noise is something PC power supplies are relatively immune to. Power surges can be damaging -- the UPS has three mitigations:

 - The AVR transformer, due to the physics of magnetic saturation, limits the amount of energy transferred in a power anomaly.

 - Surge suppression circuit -- similar to what's in a surge strip -- shunts excess energy to ground or sacrifices itself.

 - The UPS itself transfers to battery during a voltage anomaly disconnecting the equipment from the incoming AC entirely.

Also below tens of milliseconds are lightning strikes. These have so much energy its hard to mitigate because they can basically choose their own path around the surge protection.