Understanding Power Transformers: Types, Applications, and Operation
Overview of Power Transformers
Electric power transformers are robust and expensive static electrical machines found throughout the electrical system. They raise and lower voltage, enabling efficient AC electricity transmission.
Power Transformer Classification
Power transformers are categorized based on various factors:
- Location
- Power rating
- Dielectric used for insulation
- Number of phases
- Connections
- Cooling type
Power Transformers by Location
Transformers experience different thermal conditions depending on their location:
- Power Plant Transformers: Used in power plants to raise voltage for grid connection.
- Substation Transformers: Used in substations to increase or decrease grid voltage.
- Distribution Transformers: Found in electrical distribution lines.
Power Transformers by Power Rating
- Low Power Transformers: Primarily distribution and lighting transformers with lower power ratings.
- Medium Power Transformers: Used in distribution substations and industrial plants, typically below 10 MVA.
- High Power Transformers: Used in large substations and power plants, with ratings reaching Gigavolt-amperes (GVA).
Power Transformers by Dielectric
- Oil-Immersed Transformers: The most common type, with windings submerged in insulating and cooling oil.
- Dry Transformers: Used mainly in underground substations, with windings insulated by solid dielectrics like paper.
Power Transformers by Number of Phases
- Single-Phase Transformers: Operate on single-phase circuits. A bank of three single-phase transformers can function as a three-phase unit.
- Three-Phase Transformers: Transform three-phase circuits as a single unit and are common in substations.
Power Transformers by Connections
- Star-Star: Widely used in distribution substations. Voltages in primary and secondary stages have minimal phase shift.
- Star-Delta: Common in generation and distribution substations. A 30-degree phase shift exists between primary and secondary voltages and currents.
- Star-Star Grounded: Used in distribution substations. Grounding the secondary winding provides protection.
- Delta-Star: Impedances can limit ground faults. This connection is common in distribution substations and has a 30-degree phase shift between primary and secondary voltages and currents.
- Three-Winding Transformers: Can mitigate harmonic pollution by using different winding connections.
Power Transformer Cooling Types
- Natural Cooling: Uses natural air circulation through radiators or ribbed tanks to cool the oil.
- Forced Cooling: Employs fans or pumps to enhance cooling during overloads. Some systems use water circulation for cooling.
Normal Operating Regimes
Operation Under No Load
When energized with no load, a small magnetizing current flows, generating a magnetic flux and inducing voltage in the windings.
Equivalent Circuit
A T-shaped equivalent circuit simulates transformer operation under load. Parameters are calculated from no-load and short-circuit tests.
Hysteresis Curve
The B-H (flux density vs. magnetic field strength) curve is nonlinear. Saturation occurs when the core cannot handle more flux.
Magnetizing Process and Inrush Current
Energizing a transformer can cause a large inrush current due to residual flux in the core. This current decays rapidly.
Inrush Current Causes
- Transformer energizing
- Connecting transformers in parallel (sympathetic inrush)
- Voltage recovery after a short circuit
- Connecting an out-of-step generator
Cold Load Pickup
High currents can occur when energizing transformers with loads like compressors or refrigeration units. This is a normal phenomenon but can trigger protection devices.
Abnormal Regimes
Abnormal regimes, such as overloads and surges, can damage a transformer if not addressed. These conditions can be harder to detect than large faults.
Examples of Abnormal Regimes
- Sinusoidal and non-sinusoidal overloads
- Symmetrical and asymmetrical overloads
- Overvoltage
- Electromagnetic surges
- Oil problems
Overloads
Transformers have a surge capacity, but the duration and magnitude of the overload affect the extent of damage. The damage curve describes this relationship.
Asymmetrical Overloads
Oil-immersed transformers handle asymmetrical overloads better than dry transformers due to oil circulation and cooling.
Overheating
Overheating damages insulation and can lead to internal short circuits. Oil-immersed transformers have thermal inertia, delaying the temperature rise.
Non-Sinusoidal Overloads
Non-sinusoidal currents from electronic devices increase losses and can saturate the transformer, leading to overheating.
Overexcitation
Overexcitation, often caused by voltage regulator issues or synchronization problems, can saturate the transformer and distort output voltage.