Our Expert Offers Answers to Frequently Asked Questions About Power Quality

by David Windley, P.Eng., C.I.M.

Question:
We have a number of resistance welding machines in our plant. I have heard that they can create power quality problems. What kind of problems can occur and how can I determine if I have them?

Answer:
A resistance welder will typically perform a series of welds in its cycle, each of which can draw several hundred amperes for a very short time. If insufficient attention was paid in the design of the distribution system, some significant problems can occur.

First, one must consider the effect of voltage drop on the feeders supplying the welder. If the conductors have too much resistance for the peak current, the voltage will degrade momentarily during the weld such that other loads on the circuit may malfunction or 'drop out'.

Second, the conductors feeding the welder must be of sufficient size to handle the heavy ampere duty cycle of the welder. If the feeder is too small, the cable and connections may overheat and fail.

The best way to avoid problems is to supply each welder with its own properly sized feeder connected to a 'stiff' bus in the plant. This may be impractical in some cases and thus some compromises must be made.

If multiple welders are connected to one feeder, it may be necessary to schedule the occurrence of the welds with a PLC or some means of control. In this way two or more welds will not 'hit' at the same time creating a severe voltage drop or current draw beyond the feeder cable capacity. Other sensitive loads should be removed and placed on a more stable bus.

Power quality problems associated with resistance welders are electronic equipment malfunctions, contactors dropping out, bad welds, and overheated connections.

An audit of the distribution system by a professional engineer should uncover any potential problems. Monitoring equipment capable of capturing fast transients can also identify damaging situations. Proper design in the first place can save a lot of woe down the road.

Question:
We have a delta connected secondary distribution system in our facility. I've been told that this is an undesirable type of distribution from a power quality standpoint. Can you explain this?

Answer:
The delta-delta transformer was a very popular means of providing the main system voltage in process plants and other critical processes where continuity of service was important. Because the delta-connected secondary has no hard connection to ground, one phase could go to ground without affecting the phase to phase voltages on the system. The theory was that if a fault occurred, the system remained operational while one looked for and repaired the fault.

However, with the explosion of electronic equipment on our power systems, some concerns about power quality have arisen.

Because the secondary is effectively floating with respect to ground, disturbances have the potential for raising voltage levels. The creation of noise is more prevalent. In severe cases, arcing ground faults have raised system voltages to more than twice normal levels, destroying motors and lighting throughout the plant.

The modern day practice for critical systems is to utilise a grounded wye secondary with a high resistance in the ground leg. This allows a phase to ground fault without tripping off the system. The fault current is restricted to around 10 amperes, which can activate an alarm to indicate a ground fault.

The difference is, that because the secondary is grounded, the susceptibility to electrical noise and elevated voltage levels is greatly reduced.

The same effect can be achieved on older delta systems by installing grounding resistors or a zigzag transformer to tie the system to ground and still maintain system continuity.