Friday, March 18, 2011


An industrial customer of a local electric distribution utility that is situated at a distance of approximately 14.5 km from the substation with a nominal distribution voltage of 23 kV has complained on an intermittent undervoltage condition from the main incoming electrical supply line. Undervoltage problem can blow out the factory high voltage main fuse or trip low voltage main circuit breaker resulting in an entire plant shutdown. The system shutdowns interrupt a production line and will cost them an output opportunity loss of around $15,000 per hour every time the disturbance happens. In addition, $3,000 of work in process needs to be scrapped.

The initial investigation revealed that the undervoltage condition occurs both during factory full load and no load periods. In addition, the undervoltage condition appears almost every day in the morning and afternoon. So, the factory draws a conclusion that the utility supply line may have some problems. This implies that customer transformer tap has already been exhausted but still the undervoltage persists.

Figure 1. One-Line Diagram From Substation to Factory
Problem Statement

What are the actions needed by the utility to improve the voltage profile at the industrial customer’s site?


1.    To determine the root cause of the undervoltage
2.    To bring voltage level to desired value

Areas of Consideration

Utility Voltage Limits
Table 1. Utility Voltage Limits
Note: The voltage limits shown are only applicable to the utility mentioned in this case.

Substation Bus Voltage Profile

Figure 2. Substation Sending Voltage
Minimum Voltage: 22.7 kV
Maximum Voltage: 23.5 kV

Feeder Loading Profile
Figure 3. Feeder Loading Profile

Customer Voltage Profile

Figure 4. Factory Receiving-End Voltage (Before Improvement)

Minimum Voltage: 21.0 kV
Maximum Voltage: 23.0 kV

Summary of Findings and Analysis

  1. The supplying feeder is over extended.
  2. It can be observed from Figure 4 that the occurrence of the undervoltage problem is recurring.
  3. The substation bus voltage profile has no problem. The minimum and maximum values are within the normal range of the utility’s voltage limits.
  4. At times, the factory’s receiving voltage profile falls down below limits. Figure 4 indicates this drop.
  5. Undervoltage occurs during peak loading of supplying feeder. Looking at Figures 3 and 4, it is noticeable that when the feeder load (current) increases, the factory’s receiving voltage profile shows declining values.

The above findings can be analyzed that the undervoltage problem is due to the high voltage drop in the distribution line during peak hours – primarily due to the increase in the feeder load and also the considerable length of the feeder.


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About Me

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I am a Professional Electrical Engineer with a Masters Degree in Business Administration. My interest is in Power Quality, Diagnostic Testing and Protective Relaying. I have been working in an electric distribution utility for more than a decade. I handle PQ studies, power system analysis, diagnostic testing, protective relaying and capital budgeting for company projects.