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Copyright AIST Reprinted with Permission. Presented at the 2013 Iron and Steel Technology Conference and Exposition (AISTech 2013). Unit substations have long served as the method for reducing voltage from higher distribution levels to lower, locally usable voltages. One of the main considerations affecting the design of the substation and the electrical protection and distribution system as a whole is the primary switching device of a substation. By comparing the abilities and options inherited with both types of primary switching devices, an informed decision can be made for specific system designs.
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Joe RichardSenior Marketing Specialist
Schneider Electric
A Consideration of Medium Voltage Substation Primary Applications
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James StacyManager, Strategy and Innovation
Schneider Electric
Substation Design
Medium Voltage
4160 V
Low Voltage
480 V
Substation Design
TransformerPrimary Secondary
Power Flow
Choosing a Primary
Protection
Cost
Application
Protection
Transformer Inrush Current to the left of the Fuse Curve
Primary Fuse and Secondary Breaker Overlap at High Overcurrent
Transformer Damage Curve and Fuse Curve Overlap.
Protection
Transformer Differential (ANSI 87):• Phase Currents are summed on
the Source and Load sides to see if there is any ground current leakage.
• Effective at detecting low-level ground faults that damage equipment over time.
CostBudget Example
MV Fused Switch MV Circuit Breaker
5kV Switch Section $10,500 $28,000 5kV Breaker Section
5kV Fuses $1,800 $4,000 Protection Relay
Installation $2,500 $1,500 Relay Commissioning
$2,500 Installation
Total $14,800 $36,000 Total
Application
Virtual main:• Use a Protection Relay at the
Secondary Terminal to detect faults and overcurrent on the secondary side of the transformer.
• Relay signals Primary Circuit Breaker to trip in the event of a fault.
• Normally includes a switch to increase instantaneous trip current during maintenance to lower arc flash incident energy.
• Particularly useful in retrofit applications
Application
Primary Loop System:• Application allows for primary
source selection at the unit substation.
• Resistant to cable failures within the loop as the system can be self-healing.
• Prime example of Medium Voltage Circuit Breaker application.
Application
Secondary Selective System:• Source selection is in the
secondary switchgear.• No automation in the primary
switching device of the substation• Example of Fused Switch
Application
Metal-Enclosed Fused Switches
Benefits:
• Lower Capitol Cost
• Simple Protection Application
• Smaller Footprint
• Fast High-level Fault Protection
Disadvantages:
• Stock of Fuses
• Less Customizable Protection
• Higher Arc Flash Energy in Transformers above 750kVA
• Low Lifetime Operations
Metal-Clad Circuit Breakers
Benefits:
• Customizable Protection
• Higher Lifetime Operations
• Broader Applications (Automated Transfer, Virtual Main, Higher Primary Amperages, Differential Protection, etc.)
• Better Coordination
Disadvantages:
• Increased Capitol Cost
• Relay Commissioning
• Larger Footprint
Thank You
Joe RichardSenior Marketing Specialist
Schneider Electric
James StacyManager, Strategy and Innovation
Schneider Electric
