Power Quality : Voltage Irregularities
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Voltage Irregularities

Voltage irregularities are one of the greatest power quality issues facing industry today.  In fact, about 95% of the problems revealed in electrical networks stem from voltage problems.

The following discusses some common voltage problems and the Hershey approach to solving them.

Over/Undervoltage: Technically speaking, an over/undervoltage condition is reached when the voltage exceeds/lags the nominal voltage by 10% for more than 1 minute.  Both of these conditions result in voltage that falls outside the acceptable power envelope as defined by the CBEMA Curve pictured here.

Short-Term Voltage Fluctuations:  Short duration voltage events can also occur such as transients (both impulsive and oscillatory), sags/dips and swells.  Although shorter in duration (by definition they are all shorter than 1 minute), these events can reach a magnitude that falls outside the acceptable power envelope as shown in the CBEMA curve. 

When this happens, events above the upper asymptote will lead to insulation breakdown, overvoltage tripping and over excitation.  Events below the lower asymptote may cause loads to drop out altogether.

Voltage Imbalance: Voltage imbalance is most often seen as a result of single-phase motors installed on a three-phase circuit. It is measured by averaging the voltages on the 3 phases, and then measuring the maximum deviation for any 1 phase from that average. Voltage imbalance that exceeds 2% is detrimental to reliable long-term 3-phase motor operation.

Furthermore, voltage imbalance generates unwanted heat in motors. This, in turn, results in wasted energy, insulation breakdown, and improper/inefficient motor operation. As we can see from the opposing motor overheating graph, a 5% voltage imbalance would result in a 50% increase in temperature. Per NEMA limits, a motor should not be operated with a voltage imbalance at or above 5%.

Intermittent Supply Failures: Short duration intermittent supply failures can last anywhere from .5 cycles up to 1 minute and can be caused by a number of occurrences such as supply system faults (caused by lightning or other natural phenomena), equipment failures, or malfunctions in control equipment. These intermittent supply failures can be very costly to industry and can have the following adverse effects on a distribution system:

  • Voltage control relay tripping
  • Phase imbalance relay tripping
  • Production line shutdowns
  • Loss of microprocessor memory
  • Jogging, pinching and stalling of motors
  • Loss of control equipment

The Hershey Solution: The standard Hershey system will boost the secondary voltage on a given electrical network and in so doing, protect it against undervoltage conditions and intermittent voltage sags that may occur on that network.

Furthermore, the Hershey System utilizes a lower impedance value to trap transients and swells and prevents them from harming sensitive equipment on the plant floor. 

Unlike MOV’s or other surge suppressors that have limited joules/second capacity and simply channel the absorbed energy to ground, the Hershey System is able to absorb an infinite number of transients and/or swells and will re-circulate the swell or transient energy 120° out of phase. This produces energy savings in addition to protecting sensitive equipment.

The standard Hershey System is also designed to balance the voltage across the 3-phases to within 2%. This provides a very stable electrical network for long-term, efficient operation of plant equipment.

Finally, the Hershey System can also be designed to provide protection against intermittent supply failures. This option, also called the “1-Second Carry Through”, provides 59/60 cycles of ride through power to a distribution system. Note that the system is designed with increased capacitance to handle this situation, and will weigh significantly more than the standard system.

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