Testing of circuit breakers
For primary injection testing, high current is injected on the primary side of the current transformer. The entire chain – current transformer, conductors, connection points, relay protection and sometimes circuit breakers as well is covered by the test. The system being tested must be taken out of service during primary injection testing. Testing is usually conducted in connection with commissioning. The only way to verify that a direct-acting low voltage circuit breaker operates properly is to inject a high current.
A high-voltage breaker is designed to interrupt short-circuit current in a controlled manner. This puts great demands on the mechanical performance of all components in the interrupter chamber as well as the operating mechanism. It has to operate at a specific speed in order to build up adequate pressure to allow for cooling stream of air, oil or gas (depending on the type of breaker) to extinguish the arc that is generated after the contact separation until the next zerocrossing. It is important to interrupt the current to prevent a re-strike. This is accomplished by making sure that the contacts move apart far enough from each other before the moving contact has entered the so-called damping zone. The distance throughout which the breaker’s electric arc must be extinguished is usually called the arcing zone. From the motion curve, a velocity or acceleration curve can be calculated in order to reveal even marginal changes that may have taken place in the breaker mechanics. The contact travel motion is captured by connecting a travel transducer on the moving part of the operating mechanism. The transducer provides an analogue voltage relative to the movement of the contact. The motion is presented as a curve where distance vs. time allows for further analysis. From the motion curve, a velocity or acceleration curve can be calculated in order to reveal changes in the breaker mechanics that may affect the breakers operation.
The travel trace indicates the instantaneous position of the circuit beaker contacts during an operation. This gives important information such as total travel, overtravel, rebound, undertravel, contact wipe or penetration of movingcontact or operating-rod position at the time of close or open, and anomalies which are evident from the trace.
Speed is calculated between two points on this motion curve. The upper point is defined as a distance in length, degrees or percentage of movement from a) the breaker’s closed or open position, or b) the contact-closure or contact- separation point. The time that elapses between these two points ranges from 10 to 20 ms, which corresponds to 1-2 zero-crossovers. The lower point is determined based on the upper point. It can either be a distance below the upper point or a time before the upper point. The single most important benefit derived from the instantaneous velocity and acceleration curves is the insight that they provide into the forces involved during the operation of a circuit breaker.
Average acceleration can be calculated from the velocity trace.
Damping is an important parameter to monitor and test as the stored energy an operating mechanism use to open and close a circuit breaker is considerable. The powerful mechanical stress can easily damage the breaker and/or reduce the breaker’s useful life. The damping of opening operations is usually measured as a second speed, but it can also be based on the time that elapses between two points just above the breaker’s open position.