gas insulated CB resistance

[Mlmlzx]

While going through dynamic contact resistance measurement of gas insulated substation circuit breaker, and trying to go through IEC62271-100 and subsequently to others (IEC 62271-1: talks about resistance but not about DCRM), i could not find DRMC test methods and range of values acceptable. It appears that since the arcing resistance of CB value is dynamic, it can range between 20 to 30 uohms; and should be less than 100 uhoms. Also, graphical representation of resistance should be free from bounces for a newly installed CB. Since DCRM is done normally on CO cycle of CB, and arcing resistance has to decrease from certain value to zero when the contact is closed and again to the same value when it is open. Can somebody please help!!

 

[thermionic1]

I grabbed this text our of our CB Test manual. Sorry, I couldn't past the accompanying photos.

"Measuring the dynamic contact resistance combined with a motion analysis is an approach that is
commonly used for determining the length and the condition of the arcing contact without disassembling
the main contacts of the circuit breaker. That contact underlies a natural abrasion, because each time a
circuit breaker is tripped during load, an arc is created which vaporizes parts of the arcing contact. Due to
this process-related abrasion the contact must be inspected regularly in order to avoid any malfunctions.
When the contact length goes below a reference value given by the manufacturer, the correct functionality
of the circuit breaker cannot be guaranteed any longer and the breaker must undergo maintenance."

"When the interrupter unit is closed, the current flows via the main and arcing contact of a circuit breaker
(see Figure 6). As soon as the opening process starts, the main contacts are separated first and now the
current flows only via the arcing contacts (see Figure 7). After a few milliseconds the arcing contacts also
move apart but the current continues flowing, because the voltage across the contacts initiates an arc
(see Figure 8). As the sinus-shaped current crosses the zero point the dielectric strength between the
contacts is sufficient enough to resist a re-initiation and the ionised gases are removed. At this point the
arc is extinguished.
The contacts continue to separate, increasing the dielectric strength threshold as the voltage across the
contacts increases, preventing any restrike (see Figure 10).
During each opening process the resulting arc destroys parts of the arcing contact material. The amount
of wear depends on the current level. With higher currents more contact material is burned away. During
the arcing stage the current is only limited by the supply circuit impedance.
Determination of contact length
The dynamic contact resistance method measures the contact resistance continuously during the opening
operation of the circuit breaker’s interrupter unit. This is done with the breaker being isolated from the HV
supply and performing a four wire resistance measurement test during the opening operation.
The dynamic contact resistance measurement goes through the phases shown in Figure 6 through 10.
They show an increase in resistance as the contacts open. As soon as the main contact is open, a steep
decrease can be seen.
Now the arcing contact is the only connection. The resistance level shows strong deviations. They are
caused by the noise produced during the arcing process.
When the arcing contact movement comes to an end, the resistance increases again until it exceeds the
measurable limit and becomes infinite (i.e. contact is open).
The arcing contact length is determined by measuring the time between Figure 7 and 8 (i.e. time of
current flow via arcing contact until interruption). This time is used for calculating the actual contact travel
(see Figure 11).
A closing operation is usually not used for dynamic contact resistance measurements, because the
mechanical forces applied to the contact fingers would result in an extremely „noisy“ resistance
measurement. "