UPS Overload and Bypass: CT Scanner Load

A quick consulting project came over the transom this week. A 150 KVA UPS, protecting a CT scanner, was occasionally overloading and transferring to bypass.

UPS Bypass 02

Here, the transition to Bypass is evident by the step change in voltage from a rock solid 480 VAC (UPS Inverter) to a very high 515 VAC (Bypass)

UPS Bypass 01

Drilling in a bit more, we see the CT Scanner switch on (point “A”) with a maximum current of 245 Amps and a resultant collapse of the UPS output, a short period where the CT current drops and the UPS output stabilizes, then a transition to Bypass (point “B”). Note the increase in voltage while operating on Bypass.

At the end of the CT scan (point “C”) the voltage rises due to impedance. And the UPS stays in Bypass for an extended period (point “D”) needing to be manually reset.

UPS Bypass 03

A close-up of the “start of scan” waveform shows the nature of the inrush current (higher for just once cycle) – although the UPS voltage drops more than usual, it does not really fold or collapse.

Nothing really unusual here – some finger pointing at impedance (not really an issue, the voltage drop on the unregulated bypass was just 2.7% at full load) and voltage distortion (under 3% voltage distortion on the UPS input) – neither of which is a problem. The UPS got sized in based on power monitoring, which apparently did not capture peak load condition.

I suggested that the higher voltage on Bypass (515 VAC = 7% higher than nominal) would mean lower observed current, although that did not factor into the calculations (they were monitoring further upstream on a 208 VAC source). The UPS vendor is going to see if they can tweak the protection circuitry a bit to be able to survive and supply this short overload without a bypass transition.