Introduction
HVDC transmission lines have become commercially successful in India and many other nations after 1980. High Voltage Direct Current Transmission is an alternative to 3 phase 50 Hz AC transmission. Particular applications of HVDC transmission lines are as follows:
Long 2 terminal Bipolar High Power HVDC systems – They have following advantages
• Economy in capital cost.
• Better power control.
• Lower transmission losses.
• Energy conservation.
• Higher stability limit.
Back to back HVDC Coupling Stations between two independently controlled AC networks
• Technically superior.
• Better stability of AC networks at both ends.
• Excellent interconnection.
• Large scale blackouts in interconnected ac networks are prevented.
Long High power submarine cable
• No continuous charging currents.
Multi terminal HVDC interconnection system between 3 or more independently controlled ac networks
• Accurate and fast control of power exchange between 3 or more ac networks.
• No total blackouts.
• Higher stability limits.
• Lower losses.
• Energy conversation.
Protection and switch gear requirements
The protection and switch gear requirements of HVDC systems is quite different from that of AC systems. In HVDC systems, protection and control functions are integrated with the thyristor converter control. There are no HVDC circuit breakers. For normal operation and control and for protection from abnormal currents and voltages etc. thyristor control is employed. In the event of single pole to ground faults which are beyond the capability of thyristor control; the AC circuit breakers of the faulty pole are tripped after reducing the power flow and fault is isolated. All the present HVDC systems are without HVDC circuit breaker in the DC poles. Circuit breakers are provided on AC side of converter transformers.
However, the HVDC Switching Devices in form of Metallic Return Transfer Breaker are necessary in the earth return path in present 2 terminal HVDC systems for interrupting earth return currents during change over from earth return to pole return.
Artificial current interruption principle
The artificial current zero principle must be employed in HVDC switching devices for interrupting of DC arcs. The artificial current zeroes are produced in the LC oscillatory circuit in the loop of circuit breaker while opening the contacts. The arc is extinguished by the circuit breaker. The HVDC circuit Breaker Pole has ZnO arresters in parallel with the main CB for absorbing switching overvoltages. Even though the development of HVDC Circuit breakers have been technically successful, they are not commercially used due to their high cost.
Schematic of DC Switching System and Waveform of Idc with Artificial Current zeroes.
As the main breaker opens at t1, the DC arc is initiated between the contacts. Idc flows through the arc in the MB. As the Triggered Vacuum Gap sparks over, the parall
2. Introduction
• Commercial success – after 1980
• Alternative to 3Ø, 50 hz, AC
• Applications
• Long 2 terminal bipolar high power HVDC systems
• Back to back HVDC coupling stations
• Long high power submarine cable links
• Multi terminal HVDC interconnection between AC systems
8. Protection and Switch gear requirements
• Different from AC
• Integrated with thyristor convertor control
• Normal operation – thyristor control
• Single pole faults – AC circuit breakers are tripped
• At present HVDC systems are without HVDC CB’s in DC poles.
19. Control of dI/dt and dV/dt
• Lsat is connected in series to reduce dI/dt
• R-C1 in parallel to reduce dV/dt
• Resistance R across load side
• Switching Stress Factor
21. Types of HVDC circuit Breakers
• Identified with reference to –
• Switching time
• Current to be interrupted
• Switching energy
• Voltage at which current is interrupted
• TRV
• Eg:- type A CB, Type B CB (based on operating time)
22. Solid State Circuit Breakers
• To lower on state losses.
• No failure – Current flows through GCT
• Failure – Semiconductors switched off.
• TRV – Varistor starts conducting.
• Demagnetizes line inductance.
23. Hybrid HVDC Circuit Breaker
• Hybrid of mechanical actuator and IGBT.
• Have very low losses and is ultra fast.
• Normal operation – current flows through mechanical breaker & power
electronics breaker in series.( least resistance).
24. Fault operation of Hybrid
HVDC Circuit Breaker
• 3 stages
• Power electronics breaker is tripped.
• High resistance path for current.
• Mechanical breaker is fired.
• Main power electronics completes
breaking process.
28. References
• Switchgear Protection and Power Systems by Sunil S. Rao
• Wikipedia
• DC circuit breakers and their use in HVDC grids by N Kostoulas
• The Hybrid HVDC Circuit Breaker at new.abb.com
