The Cyltec Facility
The Cyltec facility in Tecumseh, MI had an “ungrounded” DELTA power distribution system with a 13,800 volt DELTA primary. The problem, is Indramat and other controls and drives are designed to operate with a balanced phase to phase voltage of 480 volts plus or minus 10% and phase to ground imbalance not exceeding 10%.
The Indramat power supplies test for a phase loss by testing voltage phase to ground, if the voltage imbalance exceeds 10% the power supply will not provide the DC voltage and current required to operate the servo drives. The spindle drives have this same “feature” and cannot be bypassed or shut off.
The second problem with adjustable speed drives, they are solid state and simply not standup to high transient overvoltage phase to ground. All solid-state power supplies and adjustable speed drives create a high harmonic content causing distortion affecting their reliability, creating most of their own problems. High frequency noise causes them to lockup. Powering down and powering up resets the problem.
Periodic machine dropout may well be caused not by an over voltage but an under voltage. The phase voltages were a nominal 270 volts (260 to 274 volts) and the line voltages measured a nominal 467 volts (464 to 469 volts). The nominal voltage calculated to be about 2.7% low using 480 volts as the target or nominal voltage. This is the requirement indicated in the Indramat manuals. It has also been noticed that when a capacitor bank switches, the lights flicker in the plant. Lights typically do not flicker from an over voltage, rather they flicker from an under voltage and if they see a cycle with no voltage the arc stops and the lamp cools down before it will come back on. The requirements are 480 volts line to line within 10% and with the voltage already 2.7% low, it does not take much to cause the dropout problem.
The high transient overvoltage can be an intermittent ground in an ungrounded distribution system. The phase voltages become unbalanced to ground causing an imbalance exceeding the 10% requirement and stopping most, if not all the machines powered by the power transformer. As the system has the power transformers connected in parallel, the problem is twice as large and is seen everywhere both transformers feed.
Analyzing the data, we discovered, the voltage phase to ground had a large harmonic content. Further data showed the current having an even higher harmonic content through 528 MHz. The noise energy was most likely the nonlinear loads; power supplies, servo drives, spindle drives and controls are the cause.
Phaseback was installed to stabilize the voltage between phase and ground to maintain the voltage requirements of the drives, power supplies and prevent damage caused by arcing ground faults.
Running a harmonic spectrum revealed the level of noise being filtered out with the Phaseback.
Implementation of Phaseback produced the following results.
- The Phaseback unit is filtering noise while stabilizing and balancing voltage to ground while it was on, by simply converting the voltage imbalance and system noise, into heat.
- The data recorded clearly shows the noise but also shows the phase voltages about
2.7% low with little room for error until machines dropout or fault.
- Power transformers have primary tap changers for adjusting the voltage, however the 13,800 volt primary should be verified to be within Utility standards. Raising the voltage should reduce problems due to the momentary low voltage conditions during certain times of the day or during certain switching events.
- The Delta power system is rugged, robust and reliable, and it can generally survive overload conditions without damage. By adding the Phaseback EMTVSS you have the best of both worlds as it creates (derives) a neutral reference point desired by many drive systems, balances phase voltages, does not allow them to arc, float or raise over system voltage (480 volts in your system) and filters harmonic noise through 528 MHz. The distribution system is even continuously protected during a fault or short to ground and the unit can continue to do this for about 35 years.