Multiple Winding Transformers

Transformers Multi-Winding & Tertiary Designs

Multiple Winding Transformers: Complete Guide to Multi-Winding & Tertiary Designs

While basic transformers have one primary and one secondary winding, many practical applications require transformers with multiple windings on either the primary, secondary, or both sides. These multiple winding transformers provide flexibility in voltage selection, enable complex power distribution schemes, and serve specialized functions in electrical systems.

Multiple winding transformers are essential for:

Dual voltage operation: Serving both 120V and 240V loads
Multiple voltage outputs: Supplying different voltage levels from one transformer
Harmonic mitigation: Using tertiary or zigzag windings
Phase shifting: Creating phase displacement for special applications
Load balancing: Distributing loads across multiple windings
Specialized applications: Welding, furnace, and rectifier transformers

Understanding multiple winding transformer configurations is crucial for electrical engineers working with power distribution, industrial systems, and specialized equipment. This comprehensive guide will explore dual voltage transformers, multi-winding configurations, tertiary windings, connection methods, and practical applications.

What is a multiple winding transformer?
A multiple winding transformer has more than one winding on the primary, secondary, or both sides. Common configurations include dual-voltage primaries (120/240V), multiple secondaries (providing different voltages), and tertiary windings (for harmonic suppression or auxiliary power). These transformers offer flexibility in voltage selection and specialized functionality.

Dual Voltage Transformers

Dual voltage transformers are the most common type of multiple winding transformer, allowing operation at two different voltage levels.

Primary Winding Configurations

Series-Parallel Primary Windings:

The primary winding is divided into two equal sections that can be connected in series or parallel:

Series Connection (High Voltage):

Two winding sections connected end-to-end
Total turns = N₁ + N₂
Voltage rating = V₁ + V₂
Example: Two 120V windings in series = 240V operation
Current rating = rated current of one section

Parallel Connection (Low Voltage):

Two winding sections connected in parallel
Total turns = N (same as one section)
Voltage rating = V (same as one section)
Example: Two 120V windings in parallel = 120V operation
Current rating = 2 × rated current of one section

Terminal Markings:

Standard terminal identification (per NEMA/ANSI):

H1, H2: First winding section
H3, H4: Second winding section

Series Connection (240V):

Connect H2 to H3 (jumper)
Apply power to H1 and H4
Voltage: 240V

Parallel Connection (120V):

Connect H1 to H3 (jumper)
Connect H2 to H4 (jumper)
Apply power to H1/H3 and H2/H4
Voltage: 120V

Secondary Winding Configurations

Similar to primaries, secondary windings can also be configured for dual voltage operation:

Example: 120/240V Secondary

Two 120V secondary windings (X1-X2 and X3-X4):

Series Connection (240V):

Connect X2 to X3
Output across X1 and X4
Voltage: 240V
Current: Rated current

Parallel Connection (120V):

Connect X1 to X3
Connect X2 to X4
Output across paralleled terminals
Voltage: 120V
Current: 2 × rated current

Center-Tapped Configuration:

A special case where the connection point becomes a neutral:

Series connection with center tap brought out
Provides: 240V across ends, 120V from either end to center
Common in residential service transformers
Neutral carries unbalanced current

How do you connect a dual voltage transformer?
For high voltage operation, connect the two winding sections in series (add voltages). For low voltage operation, connect them in parallel (same voltage, double current). Always verify terminal markings and follow manufacturer connection diagrams to prevent damage.

Multi-Winding Transformers (Multiple Secondaries)

Transformers with multiple independent secondary windings provide different voltage levels from a single primary, serving diverse load requirements.

Basic Configurations

Two Secondary Windings:

Common configuration providing two different voltages:

Example:

Primary: 480V
Secondary 1: 240V (for motors)
Secondary 2: 120V (for controls/lighting)

Advantages:

Single transformer replaces multiple units
Space and cost savings
Simplified installation
Common magnetic core

Considerations:

Total kVA = Sum of all secondary kVA ratings
Each winding must be sized for its specific load
Load imbalance affects regulation
Fault in one winding can affect others

Three or More Secondaries:

Complex transformers serving multiple voltage requirements:

Example – Industrial Control Transformer:

Primary: 480V
Secondary 1: 120V (control circuits)
Secondary 2: 24V (relays, sensors)
Secondary 3: 12V (electronics, indicators)

Applications:

Machine tools
Process control systems
Testing equipment
Power supplies

Power Rating and Load Distribution

Total Transformer Rating:

The transformer kVA rating must accommodate all secondary loads:

$S_{total} = S_1 + S_2 + S_3 + … + S_n$

Where:

$S_{total}$ = Total transformer kVA rating
$S_1, S_2, …$ = Individual secondary kVA loads

Example:

Secondary 1: 10 kVA
Secondary 2: 5 kVA
Secondary 3: 2 kVA
Total: 17 kVA minimum (use 20-25 kVA transformer for margin)

Load Sharing:

Each secondary winding operates independently but shares the common magnetic core:

Primary Current:
$I_p = \frac{S_1 + S_2 + S_3}{V_p}$

Where the sum includes all secondary loads referred to the primary.

Voltage Regulation:

Loading on one secondary affects voltage on others due to:

Common leakage reactance
Mutual coupling
Core saturation effects

Design Consideration: Size windings with adequate regulation margins.

Isolation Between Windings

Electrical Isolation:

Multiple secondaries provide isolated voltage sources:

Advantages:

Prevents ground loops
Isolates sensitive equipment
Improves safety
Reduces noise coupling

Insulation Requirements:

Between windings: Full voltage insulation
Winding to core: Ground insulation
Between secondary windings: Based on voltage difference

Creepage and Clearance:

Adequate spacing between terminals
Insulation barriers between windings
Compliance with safety standards (UL, IEC)

Tertiary Windings

Tertiary windings are additional windings beyond the primary and secondary, serving specialized functions in three-phase transformers.

Purposes of Tertiary Windings

1. Harmonic Suppression (Most Common):

Problem:

Three-phase transformers with delta-wye connections
Third harmonic currents cannot flow in wye without neutral
Causes voltage distortion and overheating

Solution:

Delta-connected tertiary winding
Provides path for third harmonic (triplen) currents
Circulates within delta, doesn’t appear in lines
Sinusoidal voltage waveform maintained

How It Works:

Third harmonic voltages are in-phase in all three phases
Delta connection allows circulating current
Harmonic MMF cancels fundamental MMF
Reduces harmonic distortion

2. Stabilization:

Unbalanced Load Conditions:

Wye-wye transformers without delta tertiary
Zero-sequence currents have no path
Neutral shift under unbalanced loads
Voltage imbalance

Tertiary Provides:

Path for zero-sequence currents
Stabilizes neutral point
Improves voltage balance
Limits fault currents

3. Auxiliary Power Supply:

Substation Applications:

Main transformer: High voltage to distribution voltage
Tertiary winding: Low voltage for station service
Powers: Control systems, cooling fans, lighting, battery chargers
Eliminates separate service transformer

Typical Ratings:

Main secondary: 13.8 kV or 34.5 kV
Tertiary: 480V or 240V
Tertiary kVA: 50-500 kVA (depending on station needs)

4. Reactive Power Compensation:

Shunt Reactor Connection:

Tertiary winding connects shunt reactors
Controls system voltage
Absorbs reactive power
Prevents overvoltage during light load

Capacitor Bank Connection:

Connects power factor correction capacitors
Supplies reactive power
Improves system power factor
Reduces losses

Tertiary Winding Configurations

Delta Connection (Most Common):

Advantages:

Provides path for triplen harmonics
Circulating currents stay within delta
No external connection required (often)
Simple construction

Rating:

Typically 25-40% of main transformer rating
Sized for harmonic current circulation
Must handle fault currents

Wye Connection:

Applications:

When tertiary supplies auxiliary loads
Provides neutral for grounding
Can supply single-phase loads

Considerations:

Must provide harmonic path (may need zigzag)
Neutral grounding required
More complex protection

Zigzag Connection:

Special Configuration:

Provides grounding without delta
Low zero-sequence impedance
Harmonic suppression
Ground fault current path

Applications:

Grounding transformers
Arc suppression
Harmonic mitigation

Sizing Tertiary Windings

For Harmonic Suppression:

Minimum: 25% of main rating
Typical: 33-40% of main rating
Based on harmonic current magnitude
Must not overheat from circulating currents

For Auxiliary Power:

Based on connected load
Diversity factor applied
Future expansion considered
Typical: 50-500 kVA

For Stabilization:

Based on unbalanced load requirements
Fault current considerations
System grounding requirements
Engineering analysis required

Example Calculation:

Given:

Main transformer: 10 MVA
Purpose: Harmonic suppression
Harmonic content: 5% third harmonic

Tertiary Rating:
$S_{tertiary} = 0.33 \times S_{main}$
$S_{tertiary} = 0.33 \times 10,000 = 3,300 \text{ kVA}$

Use: 3.3 MVA or 4 MVA tertiary winding

What is the purpose of a tertiary winding?
Tertiary windings serve multiple purposes: (1) Suppress third harmonics by providing a delta path for circulating currents, (2) Stabilize neutral point under unbalanced loads, (3) Supply auxiliary power for substation equipment, and (4) Connect reactive power compensation equipment. Most common in large three-phase power transformers.

Autotransformers

Note: Autotransformers are covered in detail in Article 5. Brief overview here:

Definition:
Transformer with single winding serving as both primary and secondary, with taps providing different voltage ratios.

Advantages:

Smaller size and cost (less copper)
Higher efficiency (lower losses)
Better voltage regulation

Disadvantages:

No electrical isolation
Higher fault currents
Limited voltage ratio

Applications:

Voltage regulation
Motor starting
Interconnecting systems

Special Multiple Winding Configurations

Zigzag Transformers

Configuration:

Six windings on three-phase core
Interconnected in zigzag pattern
No secondary output (grounding only)

Applications:

Ground fault current path
Harmonic suppression
Neutral grounding
Arc suppression coils

Advantages:

Low zero-sequence impedance
High positive/negative sequence impedance
Compact size
Effective grounding

Split Winding Transformers

Configuration:

Secondary winding split into two or more sections
Electrically isolated from each other
Common primary

Applications:

Dual power supplies
Redundant systems
Load separation
Noise isolation

Example:

Primary: 480V
Secondary A: 120V (critical loads)
Secondary B: 120V (non-critical loads)
Isolated to prevent fault propagation

Scott-T Connection

Purpose:

Convert three-phase to two-phase
Or two-phase to three-phase

Configuration:

Two single-phase transformers
Main transformer (86.6% tap)
Teaser transformer (50% tap)

Applications:

Legacy two-phase systems
Special industrial applications
Railway electrification (historical)

Practical Applications

1. Residential Service Transformers

Configuration:

Primary: 7200V or 13.8kV (distribution voltage)
Secondary: 120/240V split-phase
Center-tapped secondary

Connections:

Series secondary with center tap
Provides: 240V across ends, 120V to center
Center tap grounded (neutral)

Loads:

240V: Water heater, A/C, dryer, range
120V: Lighting, outlets, appliances
Neutral carries unbalanced current

2. Industrial Control Transformers

Configuration:

Primary: 480V or 600V
Secondary 1: 120V (control circuits)
Secondary 2: 24V (relays, PLCs)
Secondary 3: 12V or 5V (electronics)

Features:

Multiple isolated secondaries
Fuse protection on each secondary
Compact design
High reliability

3. Rectifier Transformers

Configuration:

Primary: Three-phase
Multiple secondaries with phase shift
Feeds rectifier bridges

Purpose:

DC power supply (electroplating, aluminum smelting)
Reduce harmonic distortion
Increase pulse number

Example – 12-Pulse Rectifier:

Primary: Delta or Wye
Secondary 1: Wye (0° shift)
Secondary 2: Delta (30° shift)
Two 6-pulse bridges
Combined output: 12-pulse DC
Reduced 5th and 7th harmonics

4. Welding Transformers

Configuration:

Primary: Multiple taps for voltage adjustment
Secondary: Low voltage, high current
Movable core or winding for current control

Characteristics:

High leakage reactance (current limiting)
Low secondary voltage (20-80V)
High secondary current (100-1000A)
Adjustable output

5. Furnace Transformers

Configuration:

Primary: High voltage
Secondary: Very low voltage, very high current
Multiple taps for voltage control
Often water-cooled

Applications:

Arc furnaces (steel making)
Resistance furnaces
Induction heating

Characteristics:

Secondary voltage: 100-1000V
Secondary current: 10,000-100,000A
Robust construction
High short-circuit strength

Connection and Installation Considerations

Terminal Identification

Standard Markings (NEMA/ANSI):

Primary (High Voltage):

H1, H2, H3, H4 (single-phase)
H1, H2, H3 (three-phase)

Secondary (Low Voltage):

X1, X2, X3, X4 (single-phase)
X1, X2, X3 (three-phase)

Tertiary:

Y1, Y2, Y3 (if applicable)

Polarity:

Dot convention or additive/subtractive
Critical for parallel operation
Must verify before connection

Parallel Operation

Multiple winding transformers in parallel:

Requirements:

Same voltage ratios
Same polarity
Same phase sequence (three-phase)
Same percentage impedance
Same X/R ratio

Challenges:

Complex winding configurations
Circulating currents between windings
Load sharing imbalances
Harmonic interactions

Solution:

Careful design and matching
Impedance measurements
Load testing
Professional engineering

Protection Requirements

Overcurrent Protection:

Primary side: Based on total kVA
Each secondary: Individual protection
Coordination between protections
Time-current curves

Ground Fault Protection:

Especially important for multi-winding
Detect faults in any winding
Prevent damage propagation
Ground fault relays

Overload Protection:

Thermal protection
Temperature monitoring
Load monitoring
Alarm and trip functions

Differential Protection:

For large multi-winding transformers
Compare currents in all windings
Detect internal faults
Fast operation

Practical Examples and Calculations

Example 1: Dual Voltage Transformer Connection

Problem: A 5 kVA transformer has dual primary windings rated 120V each (H1-H2 and H3-H4) and dual secondary windings rated 12V each (X1-X2 and X3-X4). Show connections for:

240V primary, 24V secondary
120V primary, 12V secondary
120V primary, 24V secondary

Solution:

Configuration 1: 240V Primary, 24V Secondary

Primary: Series connection
Connect H2 to H3 (jumper)
Apply 240V to H1 and H4
Secondary: Series connection
Connect X2 to X3 (jumper)
Output 24V from X1 and X4
Rating: 5 kVA at 24V = 208A secondary current

Configuration 2: 120V Primary, 12V Secondary

Primary: Parallel connection
Connect H1 to H3 (jumper)
Connect H2 to H4 (jumper)
Apply 120V to H1/H3 and H2/H4
Secondary: Parallel connection
Connect X1 to X3 (jumper)
Connect X2 to X4 (jumper)
Output 12V from paralleled terminals
Rating: 5 kVA at 12V = 417A secondary current

Configuration 3: 120V Primary, 24V Secondary

Primary: Parallel connection (as above)
Secondary: Series connection
Connect X2 to X3 (jumper)
Output 24V from X1 and X4
Rating: 5 kVA at 24V = 208A secondary current

Verification:

Configuration 1: 240V × 20.8A = 5 kVA ✓
Configuration 2: 120V × 41.7A = 5 kVA ✓
Configuration 3: 120V × 41.7A = 5 kVA primary, 24V × 208A = 5 kVA secondary ✓

Example 2: Multi-Winding Transformer Loading

Problem: A transformer has three secondary windings:

Winding 1: 120V, 50A rating
Winding 2: 24V, 100A rating
Winding 3: 12V, 50A rating

The primary is rated 480V. Calculate:

Total transformer kVA rating
Primary current at full load
If winding 1 supplies 4 kVA, winding 2 supplies 1.5 kVA, and winding 3 supplies 0.3 kVA, calculate primary current

Solution:

Part 1: Total kVA Rating

Winding 1: $S_1 = 120V \times 50A = 6,000 \text{ VA} = 6 \text{ kVA}$

Winding 2: $S_2 = 24V \times 100A = 2,400 \text{ VA} = 2.4 \text{ kVA}$

Winding 3: $S_3 = 12V \times 50A = 600 \text{ VA} = 0.6 \text{ kVA}$

Total: $S_{total} = 6 + 2.4 + 0.6 = 9 \text{ kVA}$

Use 10 kVA transformer for margin.

Part 2: Primary Current at Full Load

$I_p = \frac{S_{total}}{V_p} = \frac{9,000}{480} = 18.75 \text{ A}$

Part 3: Primary Current at Actual Load

Actual loads:

Winding 1: 4 kVA
Winding 2: 1.5 kVA
Winding 3: 0.3 kVA
Total: 5.8 kVA

$I_p = \frac{5,800}{480} = 12.08 \text{ A}$

Loading percentage: $\frac{5.8}{9} \times 100\% = 64.4\%$

Result: Transformer is operating at 64.4% load with 12.08A primary current.

Example 3: Tertiary Winding Sizing

Problem: A 15 MVA, 138/13.8 kV transformer requires a tertiary winding for harmonic suppression. The third harmonic content is estimated at 4% of fundamental. Calculate the required tertiary rating and verify it can handle the harmonic current.

Solution:

Given:

Main rating: 15 MVA
Harmonic content: 4% third harmonic
Purpose: Harmonic suppression

Tertiary Rating (Rule of Thumb):
$S_{tertiary} = 0.33 \times S_{main}$
$S_{tertiary} = 0.33 \times 15 = 4.95 \text{ MVA}$

Use: 5 MVA tertiary winding

Harmonic Current Calculation:

Fundamental current (secondary side):
$I_{fund} = \frac{S}{\sqrt{3} \times V} = \frac{15,000}{\sqrt{3} \times 13.8} = \frac{15,000}{23.9} = 627.6 \text{ A}$

Third harmonic current:
$I_3 = 0.04 \times I_{fund} = 0.04 \times 627.6 = 25.1 \text{ A}$

Tertiary Current (Delta Connection):

For delta-connected tertiary at 13.8 kV:
$I_{tertiary} = \frac{S_{tertiary}}{\sqrt{3} \times V} = \frac{5,000}{\sqrt{3} \times 13.8} = \frac{5,000}{23.9} = 209.2 \text{ A}$

Verification:
Tertiary rating (209A) >> Harmonic current (25A) ✓

The 5 MVA tertiary can easily handle the 4% third harmonic current with significant margin.

Result: Specify 5 MVA tertiary winding, delta-connected.

Multiple winding transformers provide essential flexibility and functionality in electrical power systems. From simple dual-voltage configurations to complex multi-winding arrangements with tertiary windings, these transformers serve diverse applications across residential, commercial, and industrial sectors.

Key takeaways from this guide:

Dual Voltage Transformers:

Series connection for high voltage operation
Parallel connection for low voltage, high current
Common in distribution and control transformers
Proper terminal identification critical

Multi-Winding Transformers:

Multiple independent secondary windings
Provide different voltage levels from one primary
Total kVA = sum of all secondary kVA ratings
Electrical isolation between windings

Tertiary Windings:

Suppress third harmonics (delta connection)
Stabilize neutral under unbalanced loads
Supply auxiliary power in substations
Typically 25-40% of main transformer rating

Special Configurations:

Zigzag for grounding
Split windings for redundancy
Scott-T for phase conversion
Rectifier transformers for DC power

Applications:

Residential service (120/240V split-phase)
Industrial controls (multiple low voltages)
Power rectification (phase-shifting secondaries)
Welding and furnace (high current, low voltage)

Design Considerations:

Proper sizing for all windings
Voltage regulation under varying loads
Protection coordination
Thermal management
Insulation requirements

Installation:

Correct terminal connections
Polarity verification
Grounding requirements
Protection devices
Load balancing

Mastering multiple winding transformer concepts enables engineers to design efficient power distribution systems, select appropriate transformers for complex applications, and troubleshoot multi-winding installations. Whether working with simple dual-voltage transformers or complex multi-winding power transformers, understanding these configurations is essential for modern electrical engineering practice.