In this issue:
Waveform of the Month
~ Notice how the voltage waveform is flat topped due to the high line impedance of the feeder. Also notice how the current pulses is on for a longer duration, it should be tall and skinny but due to the impedance, it is short and fat! This indicates that the load is struggling to get enough power to operate properly and increasing current draw. This increased current can cause parts in the power supply to over heat and could degrade the system over time.
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Causes of Line impedance
~ Transformers, copper conductors, contactors, fuses, terminals, anything that exists between the source and the final electrical device can increase the total line impedance. Common sources that can greatly increase line impedance include poor quality transformers or power conditioners, faulty circuit breakers, excessive loads on the line, undersized wire installed for the load and the increase in wire resistance due to heat and I2R loses.
How to test for Line impedance
~ RxMS has the Z-Brick that works in conjunction with the EPA monitors. The Z-Brick is a 3 phase load that precisely introduces a resistive load on the line and measures the voltage drop before, during and after the load is switched in.
~ There are other single phase testers (Siemens and Panensa come to mind) but both of these manufacturers are no longer supporting the product in the
Solutions for Line impedance
~ Remove undersized wires and high impedance transformers and replace with larger gauge wire and or low impedance transformer, this is the most ideal method. A solution could also be to provide by a new source such as an on-line UPS or voltage regulators that rebuild the waveform through an adjustable frequency drive or inverter. While this solution is not ideal, it will work as long as the device is properly sized and the impedance feeding it is not to large. This won’t eliminate the impedance problem; it will just mask the adverse affect to the sensitive electrical device.
Why does Line impedance occur?
1. When obsolete systems are replaced with a newer version, it often has a much different current draw then the system it is replacing. Though these systems usually fit into the same space, there power requirements change and there has been no upgrade to the feeder supplying it.
2. Intermittent duty equipment, there are several pieces of equipment that has an operating current of only 12 KVA but an intermittent duty (inrush) can be as high as 150 KVA(i) or 150 Amps at 480V. NEC requires that the circuit breaker be sized at least 50% of the intermittent duty (or 75Amps) so the feeder has to be sized for this intermittent duty and sometimes that is misunderstood. If this happens, serious flat topping of the voltage waveform can occur.
3. Excessive loads get added to an existing panel. This can also have and affect on the feeder’s impedance.
4. Complex impedance of the feeder (and loads) are greater than the reactance of the components, causing a much greater voltage distortion than was expected.
RxMS's Experience with Line impedance
~ There are several manufacturers that RxMS have run across that buy power supplies and components from other vendors, integrating them into their system. Over the years due to IEC, CE, UL and other manufacturing changes, the power draw of many of these components has changed. Unknown to RxMS, it doesn’t seem that the vendors properly notified the manufacturer and the site prep manual was never updated, leaving improperly sized conductors fuses etc.
~ Another situation is a manufacturer that states it has a 53KVA electrical load, this stated load according to NEC, allows for a 70 Amp circuit breaker, but their intermittent current draw is actually over 400Amps! This means that they are at least 130Amps below the minimum that NEC allows and this causes major problems for any power protection company that is trying to protect their system. As they are so far outside the allowable guideless, it is very possible to incorrectly size the power treatment device and cause more problems then good. Properly dealing with inrush current is a great way to save customers money on the potential need for power treatment devices.
~ Replacement of systems can always be a problem for line impedance issues. Systems today have shrunk in total size so greatly that places that were never expected to have large electrical loads are now being used, overloading feeders and panels without any upgrading to building wiring. Newer systems have more stringent power requirements, including larger current draws in some cases, and these are placed where the old systems used to be without any though of changes in increase draw from the feeder. The increased current draw can create wave shape distortion, voltage sags and transients. All can contribute to system resets, lockups, shutdowns, and eventually component failure.
