Transformer – Overview
What is a Transformer?
A transformer is a static electrical device that transfers electrical energy between two or
more circuits through electromagnetic induction. It is used primarily to increase (step up)
or decrease (step down) AC voltage levels in power systems.
Basic Components
1. Core
oMade of laminated iron or steel to reduce energy losses.
oProvides a path for the magnetic flux.
2. Primary Winding
o Connected to the input voltage source.
o Generates a magnetic field when AC current flows through it.
3. Secondary Winding
o Receives the induced voltage via magnetic coupling.
o Connected to the output (load).
Working Principle
Faraday’s Law of Electromagnetic Induction:
When an alternating current (AC) flows through the primary coil, it creates a changing
magnetic field in the core. This changing magnetic field induces a voltage in the secondary
coil.
Types of Transformers
1. Based on Voltage Change:
o Step-Up Transformer: Increases voltage (secondary > primary).
o Step-Down Transformer: Decreases voltage (secondary < primary).
2. Based on Application:
o Power Transformers (used in transmission).
o Distribution Transformers (used near end-users).
o Isolation Transformers (for safety, same voltage in/out).
3. Based on Core Design:
o Core-type Transformer
o Shell-type Transformer
Advantages
• Efficient energy transfer over long distances
• Voltage regulation for safety and equipment protection
• No moving parts → low maintenance
Limitations
• Works only with AC, not DC
• Can be large and heavy for high-power applications
• Loses energy through heat, eddy currents, and hysteresis
Parallel Operation of Transformers
What It Means
Paralleling transformers means connecting two or more transformers to the same primary
and secondary buses so they share the load. This is often done in power substations or
industrial plants to:
• Increase load capacity
• Improve reliability and flexibility
• Enable maintenance without service interruption
Conditions for Parallel Operation
To operate transformers in parallel safely and efficiently, the following conditions must be
satisfied:
1. Same Voltage Ratio (Turns Ratio)
• Ensures that the secondary voltages are equal.
• If not equal → circulating currents will flow between transformers even without load.
2. Same Phase Sequence
• Especially important in 3-phase transformers.
• Incorrect sequence leads to short circuits or reverse phase rotation.
3. Same Polarity
• Ensures the voltages are in phase.
• Wrong polarity → voltages oppose each other → possible short circuit.
4. Same Phase Angle (Vector Group)
• For 3-phase systems, the phase displacement between primary and secondary must
be the same.
• Use transformers with matching vector groups (like Dyn11, Yyn0, etc.).
5. Same (or Very Similar) Per Unit Impedance
• Impedance affects how much load current each transformer carries.
• If unequal, one transformer will be overloaded, even if the total load is within limit.
Why Is Matching Impedance Important?
• The one with lower impedance takes more load, possibly overloading it.
• The one with higher impedance remains underutilized.
Advantages of Paralleling Transformers
• Handles future load growth
• Enables maintenance without total shutdown
• Improves system reliability and flexibility
Risks if Not Done Correctly
• Circulating currents → heat, losses
• Voltage imbalances
• Overloading one unit
• Protection system failure
