Essential Piping Engineering: Design, Materials, and Standards

A: Correct answer: High soil resistivity and untransposed conductors. Simplified models assume symmetry and often neglect earth return effects, which become significant when soil resistivity is high and the line is untransposed. Balanced loading and nominal frequency reduce error, and short lines limit accumulated impedance differences.

A: Correct answer: Zero-sequence current flows on both primary and secondary sides. A YgYg0 transformer provides a grounded neutral on both windings, allowing zero-sequence currents to pass through. A YD1 transformer would block zero-sequence current from transferring to the delta side, which is a common modeling mistake.

A: Correct answer: Because the delta winding traps zero-sequence currents. In a YD1 configuration, zero-sequence currents circulate in the delta and do not propagate, leading to lower calculated ground fault currents. Positive- and negative-sequence behavior is largely unaffected by the delta, and tap changers are unrelated to zero-sequence blocking.

A: Correct answer: Incorrect transformer vector group modeling. Misrepresenting phase shifts and grounding in transformer models can bias estimated node voltages. Convergence tolerance affects numerical precision, not systematic bias. High X/R ratio and per-unit normalization do not inherently cause consistent overestimation.

A: Correct answer: Single-phase voltage magnitudes at multiple nodes. Phase-specific voltage measurements directly reflect unbalance conditions. Aggregate three-phase power masks phase differences, positive-sequence quantities suppress unbalance information, and nameplate data is static and non-observational.

A: Correct answer: Homogeneous soil resistivity. Urban environments often have layered or non-uniform soil, making the homogeneous assumption a major source of error in Carson-based calculations. Balanced currents and shunt capacitance assumptions are secondary for distribution lines, and temperature mainly affects resistance magnitude.

A: Correct answer: It captures mutual coupling and phase-specific loading. A three-phase pi-model explicitly represents unbalance and mutual impedance effects. It increases, not reduces, computation time, still relies on Carson-based parameters, and does not assume transposition.

A: Correct answer: Verify instrument transformer polarity and scaling. Phase-specific errors are often caused by CT polarity or ratio issues. Removing all measurements is excessive, pseudo-measurements reduce observability, and increasing iterations does not correct bad data.

A: Correct answer: Underestimation of phase coupling and voltage unbalance. Mutual impedance terms represent electromagnetic coupling between phases. Neglecting them reduces modeled interaction, leading to underestimated unbalance effects. Losses may be misestimated in either direction, and numerical stability does not imply physical accuracy.