Transformer Engineering Quiz
Transformer Noise & Harmonics
1. Transformer noise is mainly caused by:
Explanation: Transformer noise is primarily caused by magnetostriction in the core.
Magnetostriction is the phenomenon where the core laminations change shape slightly when magnetized,
causing vibrations at twice the power frequency (100Hz or 120Hz). This produces the characteristic
humming sound of transformers.
2. Humming sound in a transformer is due to:
Explanation: The humming sound in transformers is caused by vibrations resulting from
the alternating magnetic flux in the core. These vibrations occur at twice the power frequency
(100Hz or 120Hz) due to magnetostriction and magnetic forces between laminations.
3. Harmonics in transformers are primarily caused by:
Explanation: Harmonics in transformers are mainly caused by non-linear loads such as
electronic devices (computers, LED lights, VFDs) that draw current in non-sinusoidal pulses. These
loads introduce harmonic currents (integer multiples of the fundamental frequency) which can cause
overheating and other issues in transformers.
4. Triplen harmonics (3rd, 9th, 15th) in a 3-phase transformer:
Explanation: Triplen harmonics (multiples of 3) are zero-sequence harmonics that
behave differently in three-phase systems. In delta windings, they circulate and cancel out, while
in wye connections they add up in the neutral conductor, potentially causing overheating. This is
why neutral conductors in systems with harmonics are often oversized.
5. To reduce harmonics, transformers use:
Explanation: Zigzag windings are specifically designed to mitigate harmonics.
They provide a path for zero-sequence currents (including triplen harmonics) to circulate within
the transformer rather than propagating through the system. The phase displacement in zigzag
windings helps cancel harmonic currents.
Equivalent Circuit & Auto-transformers
6. The equivalent circuit of a transformer includes:
Explanation: The complete equivalent circuit of a transformer includes both
leakage reactance (representing the imperfect coupling between windings) and a magnetizing branch
(representing core losses and magnetizing current). The circuit also includes resistances
representing copper losses in the windings.
7. The magnetizing branch in the equivalent circuit represents:
Explanation: The magnetizing branch in the equivalent circuit consists of two
components: a reactance representing the magnetizing current needed to establish flux in the core,
and a resistance representing the core (iron) losses due to hysteresis and eddy currents.
8. Auto-transformers are more efficient than conventional transformers because:
Explanation: Auto-transformers are more efficient because they have only one
winding serving both primary and secondary functions, resulting in lower copper losses and less
material. However, this comes at the cost of no electrical isolation between input and output,
which can be a safety concern in some applications.
9. Auto-transformers are unsuitable for:
Explanation: Auto-transformers are not ideal for high voltage ratios because
the common portion of the winding must be rated for the higher voltage, reducing the cost
advantage. They're also unsuitable where electrical isolation is required (e.g., for safety
or to prevent ground loops) since input and output share a common connection.
10. The main disadvantage of an auto-transformer is:
Explanation: The primary disadvantage of auto-transformers is the lack of
electrical isolation between primary and secondary circuits. This can be a safety hazard in
many applications and makes them unsuitable for situations where isolation is required to
prevent ground loops or for protection against faults.
Phase Shifters & Testing
11. A phase-shifting transformer is used to:
Explanation: Phase-shifting transformers are specialized devices used in power
systems to control active power flow between parallel transmission lines by introducing a
controllable phase shift between input and output voltages. This helps balance loads and prevent
overloading of specific lines in complex grid networks.
12. Sumpner's test measures:
Explanation: Sumpner's test (or back-to-back test) is conducted by connecting
two identical transformers back-to-back. It measures the total losses (both iron and copper losses)
under load conditions without actually supplying any external load. This test is useful for
determining efficiency and temperature rise of large transformers without requiring full load.
13. Polarity test in transformers ensures:
Explanation: Polarity testing verifies both the correct phase relationship
between windings (important for parallel operation) and proper winding connections (additive
vs subtractive polarity). Getting polarity wrong can lead to short circuits when transformers
are connected in parallel or incorrect voltage outputs.
14. Inrush current occurs during transformer energization due to:
Explanation: Inrush current occurs when a transformer is energized due to
temporary core saturation. If switching occurs when the AC voltage is at zero crossing, the
flux needs to reach twice its normal peak value to oppose the remanent flux, causing saturation.
This results in a current that can be 8-30 times the rated current, lasting several cycles.
15. Inrush current can be reduced by:
Explanation: Inrush current can be mitigated by using soft starters that
gradually apply voltage or by controlled switching that energizes the transformer at the
voltage peak (not zero crossing). Point-on-wave switching devices can precisely control the
switching instant to minimize flux transients and reduce inrush current.
Per-Unit Calculations & Maintenance
16. Per-unit system simplifies calculations by:
Explanation: The per-unit system normalizes all quantities by dividing them
by base values, resulting in dimensionless numbers. This eliminates units, makes values easier
to compare (e.g., impedances at different voltage levels), and simplifies calculations in
systems with multiple voltage levels by eliminating the need for voltage ratio conversions.
17. Dry-type transformers use:
Explanation: Dry-type transformers use air or sometimes inert gases like
SF6 for cooling and insulation, as opposed to liquid-filled transformers. They are preferred
for indoor applications where fire safety is a concern, as they eliminate the risk of oil leaks
or fires associated with liquid-cooled transformers.
18. Advantages of dry-type transformers include:
Explanation: Dry-type transformers offer several advantages including no
fire risk (as there's no flammable oil), lower maintenance (no oil testing or leaks to worry
about), and environmental benefits (no oil disposal issues). However, they typically have lower
efficiency and power ratings compared to oil-filled transformers of similar size.
19. Smart transformers feature:
Explanation: Smart transformers incorporate advanced features like IoT-based
monitoring (real-time data collection on temperature, load, etc.), self-healing capabilities
(automatic fault detection and response), and often power electronics for voltage regulation.
They play a key role in smart grids by enabling bidirectional power flow and better integration
of renewable energy sources.
20. Dissolved Gas Analysis (DGA) detects:
Explanation: Dissolved Gas Analysis is a powerful diagnostic tool that analyzes
gases dissolved in transformer oil to detect various problems: hydrogen and acetylene indicate
arcing; methane and ethylene suggest overheating; carbon oxides reveal paper insulation
degradation; and nitrogen or oxygen may indicate oil contamination or leaks. The ratios of
different gases help pinpoint the specific issue.
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