Functions and Components of HV Circuit-Breakers

 Functions and Components of HV Circuit-Breakers

HV circuit-breakers are among the most important equipment in power systems. They are designed to use as interrupting devices both in normal operation and during faults. It is expected that HV circuit-breakers must be operated in any applications without problems. Moreover, it is expected that they must be ready to be operated at anytime, even after a long period of non-operating time. The main functions of HV circuit-breakers can be categorized into four functions:

• Switching-off operating currents
• Switching-on operating currents
• Short-circuit current interruption
• Secure open and closed position

Apart from the main functions, they are required to fulfil the physical requirements as follows:

• Behave as a good conductor during a closed position and as a good isolator during an open position.
• Change from the closed to open position in a short period of time.
• Do not generate overvoltages during switching.
• Keep high reliability during operation.

Components of HV circuit-breakers regarding basic functions can be divided into five groups : 

1. Insulation :

The electric insulation of HV circuit-breakers is provided by a combination of gaseous, liquid and solid dielectric materials. The failure of insulation can lead to severe damage such as flashover between phases, to ground or across the opening poles resulting in major repair or replacement. In order to prevent such failures, the insulation must be maintained and monitored. For example, the quantity of insulating medium must be continuously monitored; the quality of insulation has to be checked by diagnostic techniques periodically and the insulation distance should be monitored by using position transducers and visual inspection.

2. Current carrying : 

The current carrying parts are significant components that assure the flowing of current in the closed position. The failure of these parts can lead to catastrophic events such as contact welding and severe deterioration of the insulation system. It is however found that it takes several years until the contact degradation process reaches the final states. Practically, the most contact problems can be prevented by using periodic diagnostic testing. The techniques of current carrying testing can be accomplished by monitoring or diagnostic testing of contact resistance, temperature of contacts, load current and content of gas decomposition.

3. Switching : 

During operation of HV circuit-breakers, they are subject to electrical, thermal and mechanical stresses. It is required that they should be able to make and break large amount of power without causing failures. The parameters used to monitor and diagnose switching are composed of position of primary contacts, contact travel characteristics, operating time, pole discrepancy in operating times, arcing time and arcing contact wear. Contact travel characteristics are the most widely used parameters in periodic testing in order to investigate the contact movement.

4. Operating mechanism : 

The operating mechanism is a part used to move contacts from open to closed position or inversely. The operating mechanism failures account for a large proportion of total failures of HV circuit-breakers. For example, leakage of oil and gas in the hydraulic and pneumatic systems is very common but it can be handled without system interruption. On the other hand, breakdown of shafts, rods and springs could lead to serious failures resulting in the interruption of systems.

5. Control and auxiliary functions :

Control and auxiliary components are the parts controlled by 110-220 volts d.c. The signal is sent to the coil to move a latch or open a valve leading to energy release of a mechanical drive. The control and auxiliary parts, composed of electrical circuits and latches or values, are exposed to failures relatively frequently according to reliability surveys. Typical failures in these parts are failing to close or open on demand as well as delays in the operation. Coil current, voltages, status of auxiliary switches, circuit continuity and the environment of the control cabinet are the parameters relating to control and auxiliary systems which must be
monitored.