Buck and Boost Transformers

1. Buck and Boost transformers are isolation type transformers which can be used as an autotransformer for small adjustments in voltage (either up or down). When used as an autotransformer to slightly step up or down voltage, the Buck and Boost transformer can carry loads in excess of its nameplate rating. This method for slight voltage adjustment is one of the most economical and compact means available; however, there are some critical factors which must be considered for proper operation. These are load kVA, available voltage and desired load voltage. In addition, there are some common mis-applications, which should be avoided by observing both common sense and the restrictions for autotransformers in the National Electrical Code. Some common mis-applications are as follows:

1. Creating a 240/120V single-phase service from a 208Y/120V source; This creates unbalanced line-to-line neutral voltages. This application is proper only for 240V 2-wire loads.
2. Bucking or boosting 3-phase, 2-wire systems for 3-phase, 4-wire loads; This uses three Buck and Boost transformers in a 3-phase wye connection. The neutral created by this connection is not stable and will not yield proper line-to-neutral voltages under load. This connection violates NEC Article 210.9, Exception No.1. The wye connection can be used for 3-wire to 3-wire, 4-wire to 3-wire, and 4-wire to 4-wire applications.
3. Correcting long-line voltage drop where load fluctuates; Line drop will vary with load. If Buck and Boost transformers are used to correct voltage drop during peak load cycle, dangerously high voltages may result under lightly loaded conditions.
4. On a recent project, the AE designed an emergency feeder (277/480V) to a building using Buck and Boost transformers (since the emergency source was approximately 2300 feet away in another building) to boost the voltage back to normal due to the extra long feeder length. The feeder terminated in Buck and Boost transformers and then ran to the building's automatic transfer switch. The emergency feeder was constantly energized (without load) while the building's normal service was energized. Upon failure of the normal service, the automatic transfer switch would transfer to the emergency source. Under normal conditions, there was no load and thus no voltage drop on the emergency feeder, so the Buck and Boost transformers increased the voltage in excess of the ratings of the transformers and all three failed. This mis-application (#3 above) had to be corrected by a change order, which included the installation of a generator at the building.
5. For additional information contact Larry Lau, Consulting Support Service, 202-565-6436

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