Preface
At the turn of the nineteenth century, a revolution took place in electrical engineering. In a rather short time, the transformer was invented, electric generators and motors were designed,and the step from DC to AC transmission was made. At the beginning of the twentieth century,the transmission voltages were steadily increased to reduce transmission losses. To improve operating effciency, power systems began to be interconnected. Reserve power or spinningreserve could be then shared and capital expenditure could be reduced.
This is where “power” switching came in with its major task: isolating the faulted section of the system while keeping in service all healthy parts. Nowadays the power system can be regarded as one of the most complex systems ever designed, built and operated. Despite its complexity and robustness, the switching technology facilitates consumers to connect and disconnect electric loads in a rather simple and reliable way. Moreover, it protects the system from the effects of faults. However, this comes at a price since every change in the state of a system generates transients that may affect both the operating conditions of the system and it's components.
With the frst application of power switching in early electric systems, the development of standards for rating, testing and manufacturing high-voltage circuit-breakers began. In the United States, initiative was taken by a number of engineering and manufacturer trade organizations, such as the American Institute of Electrical Engineers (AIEE), dating back to 1884, later on merged into the Institute of Electrical and Electronics Engineers (IEEE) in 1963.
In Europe, the International Electrotechnical Commission (IEC) was founded in 1906,and the International Council on Large Electric Systems (CIGRE) started in 1921. CIGRE structured its organization by means of study committees in 1927. These study committees are responsible for the operation of working groups and task forces. Both collect feld data and perform system studies, and their reports are used as input for creation and revision of IEC standards.
Over the years, many books and publications have been written on switching in electric transmission and distribution systems. A great deal of this knowledge results from the work of CIGRE and IEEE working groups, published as standards, technical brochures, reports and scientifc papers.
For a utility engineer who wants to familiarize him- or herself with switching technology and the system aspects, the available literature is not easily accessible and sometimes difficult to comprehend. This book has been written to bridge the gap between the daily practice of utility engineers and the available literature.
Switching in Electrical Transmission and Distribution Systems
Switching is a fundamental process in electrical transmission and distribution systems, used to control, isolate, or redirect power flow to maintain system reliability, safety, and efficiency. It involves the operation of various switching devices such as circuit breakers, disconnectors, switches, and reclosers. Proper switching practices ensure uninterrupted power delivery, minimize equipment damage, and facilitate maintenance activities.
1. Importance of Switching
Power Flow Control
- Directs electricity through desired paths in the grid.
- Balances load distribution across the network.
System Protection
- Isolates faulty sections of the grid to prevent widespread outages.
- Protects equipment from overloads and short circuits.
Maintenance and Upgrades
- Facilitates safe working conditions by de-energizing specific sections.
Restoration of Service
- Quickly restores power after faults or outages through alternate pathways.
2. Types of Switching
a. Load Switching
- Purpose: To connect or disconnect loads without interrupting the flow of electricity.
- Devices Used: Circuit breakers, load break switches.
b. Fault Switching
- Purpose: To isolate faulted sections of the system and protect equipment.
- Devices Used: Circuit breakers, reclosers.
c. Maintenance Switching
- Purpose: To isolate equipment for inspection, repair, or replacement.
- Devices Used: Disconnectors, grounding switches.
d. Transfer Switching
- Purpose: To transfer loads between different power sources or feeders.
- Devices Used: Automatic transfer switches (ATS), manual transfer switches.
3. Switching Devices in Transmission and Distribution Systems
a. Circuit Breakers
- Function: Interrupt current flow during faults or abnormal conditions.
- Types: Air, vacuum, SF₆, oil.
b. Disconnectors (Isolators)
- Function: Provide visible isolation of circuits during maintenance.
- Features: Operated under no-load conditions to prevent arcing.
c. Load Break Switches
- Function: Operate under normal load conditions to connect or disconnect circuits.
d. Reclosers
- Function: Automatically close a circuit after a fault is cleared, improving service continuity.
e. Switchgear
- Function: Combines various switching and protection devices into a single enclosure.
f. Automatic Transfer Switches (ATS)
- Function: Automatically switch between primary and backup power sources.
4. Switching Operations in Transmission Systems
a. High-Voltage Switching
- Performed at voltage levels of 69 kV and above.
- Used in substations, interconnections, and transmission lines.
b. Switching for Grid Stability
- Controls reactive power devices like shunt reactors and capacitor banks.
- Manages power flow using phase-shifting transformers or HVDC systems.
c. Synchronization Switching
- Ensures generators or transmission lines are connected to the grid at the correct phase, frequency, and voltage.
5. Switching Operations in Distribution Systems
a. Sectionalizing
- Isolates faults to minimize outages.
- Uses sectionalizers and reclosers.
b. Load Balancing
- Redistributes loads to avoid overloading feeders.
- Achieved using load break switches and tie switches.
c. Distributed Generation Integration
- Manages connections of renewable energy sources like solar or wind farms.
6. Switching Methods
a. Manual Switching
- Performed by operators using physical controls.
- Suitable for planned operations but time-consuming during emergencies.
b. Remote Switching
- Operated remotely via SCADA systems, improving response times and safety.
c. Automatic Switching
- Performed by intelligent devices like reclosers and relays based on predefined conditions.
7. Challenges in Switching
Arcing
- High-energy arcs during switching can damage equipment and cause safety hazards.
- Mitigated by arc-quenching mediums like SF₆ or vacuum.
Transient Overvoltages
- Switching operations can generate voltage surges, damaging sensitive equipment.
- Surge arresters and snubber circuits help mitigate this issue.
Coordination and Timing
- Ensuring proper timing between devices to prevent cascading failures.
Grid Complexity
- Increased use of renewable energy and distributed generation adds complexity to switching operations.
8. Safety Measures in Switching
Clearance and Lockout Procedures
- Ensure circuits are de-energized and grounded before maintenance.
Personal Protective Equipment (PPE)
- Use insulated gloves, tools, and face shields during switching operations.
Switching Logs and Communication
- Maintain detailed records and communicate switching actions to all relevant personnel.
Training and Certification
- Train operators on equipment handling and emergency protocols.
9. Future Trends in Switching
a. Smart Grid Integration
- Advanced sensors and automation enable adaptive switching based on real-time grid conditions.
b. Use of Solid-State Switches
- Solid-state devices like IGBTs offer faster and more reliable switching.
c. AI and Machine Learning
- Predictive algorithms optimize switching decisions to enhance grid reliability and efficiency.
d. Renewable Energy Switching
- Special systems are being developed to handle variability and intermittency in renewable generation.
Conclusion
Switching in electrical transmission and distribution systems is a critical aspect of grid operation, ensuring efficient power flow, system protection, and reliable service delivery. By leveraging advanced technologies, adhering to safety protocols, and integrating intelligent systems, utilities can enhance the performance and resilience of modern power networks.
