UNDERVOLTAGE CASE STUDY: LONG DISTRIBUTION FEEDER PART 2

Friday, March 18, 2011



Alternatives

The following are the general solutions to mitigate undervoltage problems as discussed in this site (Please refer to Power Quality Basics):

1.  Reduce the system impedance - increase the size of the transformer, reduce the line length, add series capacitors or increase the size of line conductors.
2.   Improve the voltage profile - adjust transformers to the correct tap setting (for manual tap changers) or install voltage regulators or automatic on-load tap changers.
3.   Reduce the line current – deload the feeder or circuit by transferring some loads to other substations or load centers, add shunt capacitors or static VAR compensators, or upgrade the line to the next voltage level.

The specific alternatives for this case are:


  1. Adjust substation tap setting to compensate voltage drop within upper voltage limit of Table 1.
Advantages:
Quick, easy and cheap to implement.
Disadvantages:
Major concern for this option is when the tap setting is adjusted, how high would the bus voltage become. Exceeding the upper voltage limit would make this alternative impractical.

  1. De-load feeder by transferring some loads to adjacent feeders, if available.
Advantages:
Quick, easy and cheap to implement.
Disadvantages:
The absence of adjacent feeder would make this option unrealizable. Also, this would require about half an hour of interruption losing energy sales for the duration of the transfer. However, if paralleling is practiced, then, interruption can be avoided.

  1. Install capacitor banks to serve as voltage support while maintaining power factor close to unity as much as possible.
Advantages:
Aside from voltage support, this would reduce line technical losses and can be implemented without feeder interruption.
Disadvantages:
This would require expenses for the capacitor installation.

  1. Install automatic voltage regulators (medium voltage) along the feeder.
Advantages:
Properly regulated voltage.
Disadvantages:
Cost is relatively high compared to capacitor banks.

  1. Upgrade size of feeder conductor
Advantages:
Lessens voltage drop and losses
Disadvantages:
Costly

  1. Put up new substation near the factory
Advantages:
Lessens voltage drop and losses
Disadvantages:
Costly


Recommendations

The root cause of the problem was found out to be that the voltage drop along the utility’s distribution feeder is significant enough to cause an undervoltage condition at the location of the industrial customer. To quickly address the undervoltage problem, it is recommended to implement the following economical alternatives:

1.   Deload feeder by transferring some loads to adjacent feeders, if available.
2.   Install capacitor banks to serve as voltage support while maintaining power factor close to unity as much as possible.



The recommended actions are expected to improve the factory’s receiving voltage to desired levels at the least cost. In this case, a load flow study through software simulation (i.e. ETAP) would come in handy to verify whether implementing the propose solutions are able to correct the voltage. Then, a cost analysis would approximate the expenditure of the chosen projects as compared to the other options.

Figure 5. Factory Voltage Profile After Improvement

Minimum Voltage: 21.8 kV
Maximum Voltage: 23.4 kV

Figure 5 illustrates that after the recommendations were implemented, the voltage at the industrial customer's location improved and are now within the limits set by the utility.

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I am an 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.