UPS cable lengths

[matthewm1965]

I am aware of the potential problems of using long cables on the output of VFDs with the IGBT output switching causing a high voltage standing wave, stressing cable and motor insulation. Also problems caused by common mode noise from such systems.

Are the same problems likely with transformerless design UPS systems? The output stages are similar with high frequency switching IGBT devices, albeit with a fixed frequency and voltage output.

I have not seen any reference to maximum cable distance recommendations from UPS manufacturers. Should this be taken into consideration when designing a centralized UPS system where some of the loads could be a few hundred metres away?

Thanks

 

[olavurthomsen]

Hi Matthewm,
I have a related problem that is high initial voltage on a series of lights in a new tunnel. We installed some 50 pcs 8W flourcent lighting fixtures with electronic ballast and integrated charger for a back up battery for emergency. These fixtures are actually meant for exit emergency lighting, often used in hotels etc.
The relevant data I can think of are:
Grid: 3x400/230V 50Hz, 4 wire system.
Lighting fixpures are connected between phase and neutral that measure 230V. They are evenly connected on the three phases, with 50m between, total 2000m.
Feed cable is 4x10mm2 equivalent to AWG 7.
The problem is that each time we turn on the voltage, there are up to 7 lights that blow a fuse on the circuit board inside, and sometime a transistor or diode blow up. We tested with two different makes with roughly same devestating experience. I grew up in electricity, and this is new to me, and I can only imagine that there is a transient causing the faults. The cable selected is only this thickness because of avoiding voltage drop, as the installation is 2km long. I wonder if he problem is of same character?
Well I didn't want to bother you with my problems, but I hope this widened your perspective on this problem. Take care.

 

[Warpspeed]

My experience has been that VFDs are usually designed only to dive one induction motor, and there is zero attempt within the VFD itself to attenuate or filter out the high frequency PWM switching waveform.

While the mains power input may have some fancy power factor correction inductors, and transient voltage protection, the PWM switched output from the IGBTs goes straight to the motor.

Pulse width modulated UPS on the other hand, are designed to drive almost "anything", including sensitive loads.
They usually (if not always) have some type of low pass filter on the output, such that the output waveform is essentially sinusoidal at mains frequency with only a small percentage of high frequency PWM ripple.

 

[matthewm1965]

Thanks Warpspeed, that makes good sense.

Olavurthomsen, It sounds an interesting problem you have there! I can well imagine it hapenning, but not sure of a cure. Maybe a large resistive load connected at switch-on and then turned off via a timer would absorb the transients, or MOV's connected to each luminaire?

cheers
Matt