Electric Power Distribution Handbook, T. A. Short

5. Transformers

Questions

  1. Which of the following effects the amount of inrush? Select all that apply.

    1. Load prior to the fault
    2. Distance of the fault from the substation
    3. Connected kVA
    4. Phase angle of voltage at fault initiation
    5. Phase angle of voltage at reclose

  2. Mark any of the following that are “problem” connections for distribution transformers because of overvoltages. Notation is highside to lowside.

    1. Delta - grounded wye
    2. Delta - floating wye
    3. Grounded wye - delta
    4. Floating wye - delta
    5. Grounded wye - grounded wye
    6. Floating wye - grounded wye
    7. Grounded wye - floating wye
    8. Open wye - open delta

  3. Are grounded-wye – grounded-wye transformers immune from ferroresonance?

    1. Yes
    2. No

  4. True/false: Open-wye – open-delta transformers are susceptable to voltage unbalance even when loading is balanced.

    1. True
    2. False

  5. Which transformer connections block zero-sequence harmonic currents created on the low side? Mark all that apply.

    1. Delta - grounded wye
    2. Delta - floating wye
    3. Grounded wye - delta
    4. Floating wye - delta
    5. Grounded wye - grounded wye
    6. Floating wye - grounded wye
    7. Grounded wye - floating wye
    8. Open wye - open delta

  6. Mark all items that reduce the chance of ferroresonance.

    1. Higher-voltage systems
    2. Smaller transformers
    3. Transformers with higher no-load losses
    4. Lower customer load
    5. Longer customer secondaries
    6. Shorter cable lengths
    7. Grounded secondary connections

  7. Are grounded-wye – grounded-wye transformers immune from ferroresonance?

    1. Yes
    2. No

  8. Which is likely to have higher no-load losses?

    1. 25-kVA single-phase overhead transformer
    2. 50-kVA single-phase padmount transformer

  9. Which scenarios are susceptable to backfeed to open conductors? Mark all that apply.

    1. Grounded-wye grounded-wye that’s unloaded
    2. Grounded-wye grounded-wye that’s typically loaded
    3. Delta grounded-wye that’s unloaded
    4. Delta grounded-wye that’s typically loaded

  10. True/false: ferroresonant overvoltages always increase as capacitance on the open phase(s).

    1. True
    2. False

  11. In comparing delta – grounded-wye transformers to grounded-wye – grounded-wye transformers, mark each case where delta – grounded-wye transformers have an advantage:

    1. Ferroresonance
    2. Harmonics
    3. Coordination of high side and low side ground relays

  12. Mark any of the following that act as grounding transformers. Notation is highside to lowside.

    1. Delta - grounded wye
    2. Delta - floating wye
    3. Grounded wye - delta
    4. Floating wye - delta
    5. Grounded wye - grounded wye
    6. Floating wye - grounded wye
    7. Grounded wye - floating wye

  13. On a 12.5-kV circuit, a lineman opens a fuse cutout from a bucket. The only load downstream of the cutout is a pump facility fed by a grounded-wye – grounded-wye transformer. Is the voltage likely to be above 100 V downstream of the cutout?

    1. Yes
    2. No

Problems

  1. Calculate the rated load current on a 7.2-kV, 50-kVA, single-phase transformer.

  2. Draw sequence diagrams of a transformer with a line-to-ground fault on the secondary side of the transformer for each of the following cases:

    • Grounded wye – grounded wye
    • Grounded wye – delta
    • Delta – grounded wye

  3. Calculate the critical cable length for ferroresonance on a three-phase, five-legged-core transformer with the following conditions:

    • Transformer rating of 100 kVA and no-load losses of 0.3%
    • Fed by a 1/0 XLPE cable insulated at 133%
    • No customer load

  4. For a 500-kVA transformer with load losses of 0.55% and no-load losses of 0.1%, find the following:

    • Load losses in watts at 100% loading with a power factor of 1.0
    • Load losses in watts at 40% loading with a power factor of 0.9
    • No-load losses in watts

Projects

  1. Find two technical papers on transformer load management. Summarize approaches to sizing transformers. Describe inputs needed to a transformer load management program. If you are at a utility, find out what your utility does for transformer load management.


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To the extent possible under law, Tom Short has waived all copyright and related or neighboring rights to these study questions. This work is published from the United States. Please use this material however you want.