Electrical High Voltage Circuit Breakers
High Voltage Pylons
HV Electrical Circuit Breakers
A high voltage circuit breaker is a piece of electrical switchgear that is intended to switch-on or switch-off a circuit under normal or short circuit fault conditions. A circuit breaker and fuse a fuse provide the same function except that a circuit breaker, unlike a fuse, will allow for quick restoration of current flow after operation.
The design and construction process of a circuit breaker is concerned with the ability to switch-off short circuit currents and to withstand the stresses that come from breaking such large currents.
The rudimentary structure of a circuit breaker consists of interlocking contacts enclosed in a chamber with an assemblage of triggering circuits. When the contacts are closed the current can flow and when required the contacts can part to break the current.
Current Breaking Process
When a short circuit is detected and the circuit breaker is triggered to operate the contacts starts to part in order to open the circuit. As the contacts start to part an arc start to form. We are familiar with electrical arc and lighting flashes, it is the same phenomena in a circuit breaker. In the circuit breaker chamber the arc must be quenching fast enough to break the current.
The arc quenching mechanism involves elongating the arc and cooling it rapidly. The chamber contains either oil, air or is a vacuum. The arc quenching properties of these mediums is a subject of intense study in the field of high voltage engineering.
One important aspect of the arc quenching process is the ‘natural zero’ of the A.C. current. The arc is quenched at the point when the A.C. current reaches its zero point and modern breakers operate within 2 cycles i.e. around 40ms.
When the contacts are open and the arc is quenched the systems voltage appears across the tips of the contacts resulting is a very strong electric field. The intense electric field can ionize the quenching mechanism resulting in the re-establishment of the current. This is called restricting.
To dielectric strength across the circuit breaker contacts must reestablish faster than the rate of rise of the ‘transient recovery voltage’, TRV and must be greater than the peak TRV. Only then can the circuit breaker open the circuit after first current zero.
Understanding the Transient Recovery Voltage
The transmission line can be modeled as a resistance in series with inductance and shunt impedances (capacitance and insulation resistance of overhead lines). The shunt impedances can be ignored since they are of much higher than the series impedances. Moreover the resistance of transmission lines can be considered to be around 5% to 15%. Short circuit currents on transmission lines are purely inductive with a very low power factor.
For highly inductive circuits the voltage is very high at current zero and therefore there is a tendency for the arc to restrike. Power engineers have come up with detailed transients analysis to characterize the TRV after the first current. This analysis is used to check whether or not a restrike is possible. As indicated above the dielectric strength of the medium between the contacts must recover faster than the rate of rise of TRV. And the dielectric strength must withstand the peak TRV.
Circuit Breakers And Isolators
A discussion of circuit breakers is not complete before a distinction is made with isolators.
A circuit breaker is intended to break load and short circuit currents whereas an isolator is used for off-load connections or disconnections. This is the key distinction in performance between the two components. Another important factor must be discussed regarding the functions of the isolator in the safe operation of high voltage systems.
The contacts of an isolator must be visible and lockable in the off-position whereas those of a circuit breaker are necessarily enclosed and their operation is automated with the systems monitoring and control. A workman must be positive that the circuit is open before he can repair the systems.
Utilization Of Circuit Breakers
Primarily circuit breakers are used as switches that can either be used for normal closing of live loads and for interruption of circuits under abnormal conditions. For this reason the circuit breaker plays a very important part in high voltage engineering. There is another aspect of circuit breakers that requires further discussion, that of network flexibility and hence reliability.
Circuit breakers as switches can be used to route the flow of energy in the grid. They can also be used as sectionalizors. The use of circuit breakers in this way allows flexibility in the provision of energy to consumers and reliability in case of faults or operational requirements. Through the use of breakers (and other components) the reliability of a system can be enhanced.
Circuit breakers are very expensive and they require specialized skill in operation and maintenance.
Hitherto we were concerned about AC circuit breakers only. DC circuit breakers at high voltages are still in research because there is no current zero in DC to allow for managed quenching of the current. However with the rising popularity of High Voltage DC Transmission, solution for switching direct current will be required.