Substation Design: Complete Professional Guide (HV/MV/EHV)

Substation Design: Complete Professional Guide (HV/MV/EHV)

A full engineering reference for electrical and power system engineers designing AIS, GIS, and hybrid substations.

Table of Contents

1. Introduction to Substation Design

Electrical substations are critical facilities that perform voltage transformation, switching, protection, SCADA monitoring, and interconnection of power systems. A well-designed substation ensures reliability, safety, and operational stability.

Main functions include:

  • Voltage transformation (e.g., 69/13.8 kV, 230/115 kV)
  • Protection & isolation of faults
  • Power flow control and switching
  • Metering, monitoring, and SCADA integration

2. Types of Substations

Substations are classified based on insulation type and voltage level.

A. AIS (Air Insulated Substation)

Traditional outdoor switchyard with large clearances.

B. GIS (Gas Insulated Substation)

Compact indoor/outdoor design using SF₆ gas. Ideal for cities and industrial areas.

C. Hybrid Substation

Combination of AIS bus with GIS breakers or bays.

D. Transmission vs Distribution

  • Transmission: 115 kV, 230 kV, 500 kV
  • Distribution: 13.8 kV, 34.5 kV, 69 kV

3. Substation Design Workflow

Professional workflow includes:

  1. Load flow & short-circuit studies
  2. Bus scheme selection
  3. Equipment sizing
  4. Ground grid design
  5. Protection & control system
  6. Civil/structural design
  7. SCADA & communication
  8. Construction planning
ASCII Overview Diagram:
 HV Line → Breaker → CT → Bus → Transformer → LV Feeders

4. Substation Layouts

4.1 Single Bus

Simple and inexpensive, but no redundancy.

4.2 Double Bus

Allows transfer switching for maintenance.

4.3 Breaker-and-a-Half (B&A1/2)

High reliability; used in 230/500 kV systems.

ASCII: Breaker-and-a-Half
Line A --[B1]-- Bus A --[B2]-- Bus B --[B3]-- Line B

4.4 Ring Bus

Every circuit has two paths; excellent reliability.

5. Major Substation Components

5.1 Power Transformer

Steps voltage up/down. Key specs:

  • MVA rating
  • Impedance
  • Cooling (ONAN/ONAF/OFAF)
  • Taps (OLTC)

5.2 Circuit Breakers

Interrupt faults; types include SF₆ and vacuum breakers.

5.3 Disconnect Switches

Provides visible isolation; not used for interrupting load.

5.4 CTs and VTs

Provide feedback for protection and metering.

ASCII CT/VT Layout:
Line → CT → Breaker → VT → Bus

5.5 Surge Arresters

Protects equipment from lightning and switching surges.

5.6 Substation Bus

Can be tubular, rigid, or GIS busbars.

5.7 Grounding Grid

Ensures step and touch voltage safety.

6. Required Power System Studies

6.1 Load Flow Study

Determines transformer sizing and voltage behavior.

6.2 Fault Analysis

Used for breaker sizing, CT knee points, and bus bracing.

6.3 Insulation Coordination

Ensures withstand capability against lightning/switching surges.

7. Protection & Control System

7.1 Main Protection

  • Differential protection
  • Distance protection
  • Overcurrent/earth fault
  • Breaker failure protection

7.2 SCADA and Automation

Using IEC 61850, IEDs, and RTUs.

ASCII Protection SLD:
CT → Relay → Trip Coil → Breaker

7.3 Communication Links

  • OPGW
  • Microwave
  • PLCC

8. Grounding Grid Design

Compliant with IEEE 80.

Design Inputs:

  • Soil resistivity (Wenner test)
  • Fault current
  • Grid conductor size
  • Mesh spacing
ASCII Ground Grid:
# # # # # # 
# # # # # #
# # # # # #

9. Civil & Structural Design

Includes gantries, foundations, cable trenches, and control building layout.

9.1 Control Building

  • Relay room
  • Battery room
  • Switchgear
  • SCADA room

9.2 Gantries & Support Structures

Designed per ASCE/SEI standards.

10. Electrical Clearances

Typical 230–500 kV AIS clearances:

  • Phase-to-phase: 3–6 m
  • Phase-to-ground: 4–7 m
  • Switching surge: IEC 60071

11. Commissioning Tests

  • CT/VT polarity test
  • TTR (transformer turns ratio)
  • Primary and secondary injection
  • Relay logic testing
  • SCADA end-to-end test

12. Standards

IEC

  • IEC 60071 – Insulation coordination
  • IEC 62271 – High-voltage switchgear

IEEE

  • IEEE 80 – Grounding
  • IEEE C37 – Breakers & protection

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