Rated breaking current vs small inductive breaking current rating 2

[protkid1]

Hi friends,
My EHV CB (rated 145 kV) is rated for breaking capacity 40 kA. But the reactor breaking current is only 315 A.
Also, the Cable charging current breaking capacity is 160A only. However, the Back to Back Capacitor Switching current breaking capacity is 400A. Why the current values differ over such a high value from kA to A range ? Also what is the difference in breaking cable charging and capacitor switching currents?
Thnks

 

[bacon4life]

At least 2 factors at work here.
1. Interrupting inductive & capacitive loads are much more difficult than resistive loads.
2. The 40 kA fault rating is designed to be used just a few times, whereas the switching rating might allow for thousands of operations. For one of our 2000A/ 40kA breakers, inspection is required at:

Amps - Inspection period
40000 10 operations
20000 30 operations
10000 100 operations
2000 2000 operations

 

[protkid1]

Can the AC circuit breaker 40 kA (47.6 kA assymetrical) 50 HZ 145 kV rated to break 315A reactor current, break the attached waveform ? I understand ,the r.ms value of the wave comes to around 614 A.. Current zeroes are there but at the lower end of the waveform !! How will it effect the CB? Any comments ?

 

[bacon4life]

Usually each thread focuses on just one question, and each question is only asked in one thread. It looks like you have the details on the breaker over at

http://www.eng-tips.com/viewthread.cfm?qid=341765,

so I'll just make a general comment in this thread.

The IEEE/IEC consensus standards are developed with typical applications in mind, and do not necessarily spell out the information needed to determine if your circuit breaker will work in this application. You are dealing with a highly specialized situation and will probably need to enlist the aid of your circuit breaker manufacturer and/or a simulation expert. In order to meet both IEEE and IEC requirements, the manufacturer probably went above and beyond the standard values on the nameplate stamp. For example, a breaker model we purchased passed design tests at 46 kA, but was nameplated down to 40 kA to match the IEEE "preferred" interrupting ratings.

It is quite possible that breakers with identical nameplate ratings would react very differently to your waveform.

 

[protkid1]

Thanks bacon4life,
Sorry for the details spread over different threads. Will take care.

 

[rocmor]

As far as your original question, the ratings you quoted for short-circuit breaking, capacitor switching, cable charging, etc. represent the levels to which the CB was tested and "certified" in an independent high-power test facility, in accordance with IEC/ANSI/IEEE procedures. As bacon4life mentioned, there are preferred values for each application. The manufacturer selected these values for their testing based on what they perceived as end-user needs. Each rating requires a separate test protocol. As I recall, the protocols for single-bank cap switching and cable charging are similar. The separate ratings reflect the fact that the applications differ: the one addressing typical situations in systems where components are connected via cables (not prevalent in the US), the other addressing the switching in and out of capacitor banks used for power-factor adjustment.

 

[protkid1]

In the specs, it is mentioned that the max overvoltage factor due to any switching will not exceed 2.3 p.u . What I understand is that the peak TRV that appears as soon as the arc is extinguished should not be more than 2.3 times the peak value of the phase voltage corresponding to 145 kV. Pls,correct me if I am wrong.

Thanks

 

[rocmor]

The overvoltage could be TRV. Overvoltages can also occur during capacitor switching and other duties, and can reach values of 2.5 pu in normal conditions. Depending on the type of breaker you're using (I'm assuming it's an SF6 CB though vacuum CBs have been produced for 145kV application), there is the possibility of further voltage escalation in certain circumstances. In all cases, proper system design should be able to assure overvoltages don't exceed the rated 2.3pu.