Now that you've spent considerable time on the neutral and ground issues, you find that your power quality analyzer still shows flat-topping. What's going on?
To answer this, we'll assume a 4-wire system in this scenario. The typical transformer supplying single-phase loads is of dry-type construction, and it has a 4-wire wye secondary stepping down from 480V. We'll assume that, too. Not every facility has a 4-wire system, and sometimes loads are on a transformer with a delta secondary for power quality reasons.
A common source of flat-topping is transformer overloading. Typically, it occurs through the addition of loads over time. Fortunately, overloading is easy to confirm or disprove.
- Use a current probe to measure the load on each phase.
- Total the readings to determine actual kVA.
- Compare actual kVA to your transformer's rated kVA.