Transfer switches are critical components in power distribution systems, ensuring continuous power in facilities where interruptions can lead to significant disruptions. These switches manage transitions between different power sources, typically from utility power to onsite generators. This article examines the types of transfer switches, their applications, and key design considerations.

Types of Transfer Switches and Their Applications

1. Manual Transfer Switches (MTS)

Characteristics:

• Require human operation for source switching
• Lower cost compared to automatic systems
• Transfer time depends on operator response

Applications:

• Small businesses or residential settings where immediate power restoration is not critical
• Non-critical backup power systems

2. Automatic Transfer Switches (ATS)

Characteristics:

• Detect power loss and initiate transfer automatically
• Typical transfer time: 10-20 seconds
• Programmable for various scenarios and timing sequences

Applications:

• Commercial and industrial facilities
• Healthcare institutions
• Data centers and other critical infrastructure

Types of ATS: 

a) Open Transition (Break-Before-Make):

• Momentary power interruption during transfer
• Prevents source paralleling
• Suitable for most applications

b) Closed Transition (Make-Before-Break):

• No power interruption during transfer
• Momentarily parallels sources (typically <100ms)
• Used for sensitive loads

c) Delayed Transition:

• Includes a programmable neutral position
• Allows for motor residual voltage decay
• Prevents out-of-phase reclosing

3. Breaker Pair ATSs

Characteristics:

• Utilize two circuit breakers instead of a switch mechanism
• Higher interrupt ratings (up to 200kA)
• Compact design

Applications:

• High-capacity systems where standard ATSs may not meet interrupt ratings
• Space-constrained installations

4. Static Transfer Switches (STS)

Characteristics:

• Use SCRs or IGBTs for switching
• Transfer time: <4ms
• No moving parts

Applications:

• Mission-critical facilities requiring near-instantaneous power transfer
• Data centers and telecommunications infrastructure

SCRs and IGBTs in Static Transfer Switches

Static Transfer Switches employ solid-state devices, primarily SCRs (Silicon Controlled Rectifiers) or IGBTs (Insulated Gate Bipolar Transistors), to achieve rapid power source transitions.

SCRs (Silicon Controlled Rectifiers):

Characteristics:

• Three-layer semiconductor devices
• Can handle high voltages and currents
• Once triggered, remain conductive until current falls below holding current
•   High surge current capability (up to 20 times rated current)
•   Low forward voltage drop (1.5V to 2V)
• Advantages:
  • Robust and reliable
  • High surge current capability
• Limitations:
  • Cannot be turned off by gate control
  • Generate more heat compared to IGBTs

Types:

1. Phase Control SCRs:
   • Commonly used in STS applications
   • Voltage ratings: 200V to 6500V
   • Current ratings: 20A to 5000A
   • Key features:
     • High surge current capability (up to 20 times rated current)
     • Low forward voltage drop (1.5V to 2V)
2. Inverter-grade SCRs:
   • Used in high-frequency switching applications
   • Faster turn-off times compared to phase control SCRs
   • Voltage ratings: 200V to 2500V
   • Current ratings: 20A to 1000A
3. Asymmetrical SCRs (ASCRs):
   • Lower forward voltage drop
   • Faster turn-on times
   • Used in applications requiring higher efficiency

Applications:

• High-power STS systems (up to 1000A)
• Systems requiring high surge current capability

IGBTs (Insulated Gate Bipolar Transistors):

Characteristics:

• Combine features of MOSFETs and bipolar transistors
• Voltage-controlled devices with high current handling capability
• Can be turned on and off by gate control
• Advantages:
  • Faster switching speeds than SCRs
  • Lower conduction losses at high currents
  • Easier to control
• Limitations:
  • More susceptible to damage from voltage spikes
  • Generally more expensive than SCRs

Types:

1. Non-Punch Through (NPT) IGBTs:
   • Traditional IGBT structure
   • Higher short-circuit withstand capability
   • Suitable for high-voltage applications (>1200V)
2. Punch Through (PT) IGBTs:
   • Lower conduction losses
   • Faster switching speeds
   • Typically used in applications <1200V
3. Trench Gate IGBTs:
   • Higher current density
   • Lower on-state voltage
   • Improved switching performance
4. Field Stop IGBTs:
   • Combine advantages of NPT and PT structures
   • Lower losses and improved temperature characteristics
   • Commonly used in modern STS designs

Applications:

• STS systems requiring frequent switching
• Applications needing bi-directional power flow
• Systems prioritizing lower losses and higher efficiency
• Voltage ratings: 600V to 6500V 
• Current ratings: 15A to 3600A

Comparison in STS Applications:

• SCR-based STS:
  • More common in high-power applications
  • Typically used in systems up to 1000A
  • Require additional circuitry for turn-off
• IGBT-based STS:
  • Growing in popularity due to improved control
  • Better suited for frequent switching operations
  • Often used in systems requiring bi-directional power flow

Selection Considerations:

Both SCRs and IGBTs in Static Transfer Switches enable transfer times of less than 4 milliseconds, which is crucial for maintaining power quality in sensitive loads. The choice between SCR and IGBT-based systems depends on factors such as required switching frequency, power rating, and system complexity.

• SCRs are often preferred for very high power applications due to their robust nature and high surge current capability.
• IGBTs are chosen for applications requiring frequent switching or where lower losses are critical.
• The choice between SCR and IGBT often depends on the specific requirements of the STS, including transfer frequency, efficiency, cooling capabilities and cost considerations.

Special Considerations for IT Loads

IT facilities, particularly data centers, require switches that can transition power sources without interrupting the power supply. Closed transition switches, which perform make-before-break operations, are critical here. These switches connect to a new power source before disconnecting from the old one, ensuring a seamless power supply.

Characteristics:

• “Make-before-break” operation
• Parallel time typically <100ms
• Requires source synchronization

Applications:

• Data centers and server rooms
• Medical imaging equipment
• Continuous industrial processes

Design Considerations

Integrating transfer switches into a power management system requires careful consideration of several factors:

1. Capacity:
   • Must match or exceed maximum load current
   • Consider future load growth
2. Configurations:
   • Standard: No redundancy
   • Bypass-isolation: Allows maintenance without power interruption
   • Service-entrance rated: Combines transfer switch and main disconnect
3. Neutral Arrangement:
   • Solid neutral: No neutral switching
   • Switched neutral: Neutral switches with phase conductors
   • Overlapping neutral: Ensures neutral is always connected
4. In-rush Current Handling:
   • Must withstand motor starting currents (up to 20x full load current)
   • Consider transformer inrush (up to 25x full load current)
5. Withstand and Closing Rating (WCR):
   • Short-time rating: Ability to carry fault current for a specified time
   • Closing rating: Ability to close into a fault
   • Must coordinate with upstream and downstream devices
6. Controls and Monitoring:
   • Microprocessor-based controls for programming and monitoring
   • Communication protocols: Modbus, BACnet, SNMP
   • Remote monitoring and control capabilities

Importance of Maintenance

Routine maintenance is vital to ensure that transfer switches function as expected during power failures. Maintenance activities include:

1. Visual Inspection
2. Operational Testing
3. Electrical Testing
4. Lubrication
5. Battery Maintenance (if applicable)
6. Documentation

Regular maintenance prevents failures and ensures system reliability when switching between power sources is necessary.

Manufacturers of Mechanical Transfer Switches (Manual and Automatic)

The transfer switch market includes several established manufacturers producing both mechanical and static transfer switches. Here are some of the prominent manufacturers in the industry:

1. ASCO Power Technologies (Schneider Electric)
   • Known for: Wide range of ATSs, from residential to industrial applications
   • Notable products: ASCO 300 Series, ASCO 7000 Series
2. Eaton Corporation
   • Known for: Extensive line of transfer switches for various applications
   • Notable products: Eaton ATC-300, Eaton ATC-900
3. Cummins Power Generation
   • Known for: Transfer switches often paired with their generator sets
   • Notable products: PowerCommand ATS
4. Kohler Power Systems
   • Known for: Transfer switches for residential, commercial, and industrial use
   • Notable products: Kohler Automatic Transfer Switches
5. Generac Power Systems
   • Known for: Residential and light commercial transfer switches
   • Notable products: Generac Smart Switch
6. ABB
   • Known for: Industrial-grade transfer switches
   • Notable products: ABB Zenith ATS
7. Siemens
   • Known for: Comprehensive range of transfer switches
   • Notable products: Siemens Sentron ATSs

Manufacturers of Static Transfer Switches

1. Schneider Electric
   • Known for: High-end static transfer switches for data centers
   • Notable products: Galaxy STS
2. ABB
   • Known for: Industrial and data center static transfer switches
   • Notable products: ABB CMS STS
3. Eaton Corporation
   • Known for: Static transfer switches for critical power applications
   • Notable products: Eaton STS PDU
4. Vertiv (formerly Emerson Network Power)
   • Known for: Static transfer switches for IT and data center applications
   • Notable products: Liebert STS2
5. Socomec
   • Known for: Wide range of static transfer switch solutions
   • Notable products: STATYS XS, STATYS
6. Delta Electronics
   • Known for: Static transfer switches for data centers and industrial applications
   • Notable products: Delta STS
7. Piller Power Systems
   • Known for: High-power static transfer switches
   • Notable products: Piller UNIBLOCK UBT+

These manufacturers offer a variety of features, capacities, and price points to meet different application needs. When selecting a transfer switch, it’s important to consider factors such as reliability, after-sales support, compatibility with existing systems, and specific features required for the application.

Manufacturers of SCRs and IGBTs

SCRs: ABB Semiconductors, Infineon Technologies, STMicroelectronics IGBTs: Infineon Technologies, Mitsubishi Electric, Fuji Electric

When selecting a transfer switch, it’s important to consider factors such as reliability, after-sales support, compatibility with existing systems, and specific features required for the application.

Conclusion

Transfer switches are essential for managing power in facilities that require a reliable electricity supply. Understanding the types of transfer switches, their specific uses, and the detailed considerations for designing and maintaining these systems is crucial. Effective management and regular maintenance of these systems help maintain operational stability and prevent power outages in critical environments. As power requirements evolve, staying informed about transfer switch technology and best practices remains vital for facility managers and engineers.

Need Expert Assistance with Transfer Switch Design?

Mission Critical Engineers Ltd. (MCE) specializes in the design and implementation of transfer switch systems for a wide range of applications. Our team of experienced engineers can help you:

• Select the optimal transfer switch type for your specific needs
• Design custom transfer switch solutions for complex power systems
• Ensure compliance with relevant codes and standards
• Optimize your power distribution system for reliability and efficiency
• Provide expert consultation on maintenance and upgrade strategies

Whether you’re planning a new installation, upgrading an existing system, or need expert advice on power distribution challenges, MCE is here to help. Our expertise in both mechanical and static transfer switches, combined with our deep understanding of various industries’ power needs, enables us to deliver tailored solutions that ensure seamless power transitions and system reliability.

Contact MCE today to discuss your transfer switch and power distribution needs. Let us help you design a system that keeps your operations running smoothly, no matter what.Don’t leave your critical power needs to chance. Reach out to MCE and ensure your transfer switch system is designed for optimal performance and reliability.