Hello everyone, in this Article we are going to have an overview of the parameters which are mentioned on nameplate of a 420kV SF6 circuit breaker, which you will also find on the other high voltage SF6 circuit breaker including 72.5kV, 145kV, 245kV, 300kV, 550kV, 800kV voltage levels, so read the below article to get the complete details.
In most of the countries high voltage switchgears including circuit breakers are manufactured based on the IEC standard, i.e. IEC 62271-100. All the parameters marked on the nameplate of high voltage circuit breaker are also as per this IEC and also as per IEC 62271-1. So, let’s first have a look at the IEC.
As per IEC, above are the parameters which should be mentioned on the nameplate of high voltage circuit breaker. Some of them are mandatory and some of them are optional based on some specific condition. Parameters marked with “X” are mandatory as per the standard. All the manufactures who are producing circuit breaker based on this standard, should mention all the parameters which are marked with X on the nameplate. But of course some of these parameters may be skipped, if it is mutually agreed between manufacturer and customer. Parameters marked with “y” are based on the condition stated in the column 6. If the breaker satisfies the condition mentioned in the column 6, then these parameters has to be on nameplate. For example, “Rated switching impulse withstand voltage”, this is only applicable for the circuit breaker which are rated above 300kV. So, for 420kV and above circuit breaker manufacture should mention this parameter on the nameplate, which here you can see. Now parameters marked with “(x)” are totally optional, which a manufacture can decide whether to mention them or not. So for example “Rated out of phase breaking current” is completely an optional parameter. One manufacture may include this on his nameplate and the other may not. We will first have a look on Mandatory parameters and then condition based parameters and then optional ones.
1. Manufacturer – Name of the manufacturer
2. Type designation and serial number – Type of Circuit Breaker and its serial number
3. Year of manufacture – year in which the breaker is manufactured
4. Relevant standard – This is the standard as per the breaker is manufactured
5. Rated voltage (kV) –
Rated voltage is the “Highest system voltage” for which breaker is designed. This voltage is in kV rms and refers to phase to phase voltage of 3 phase system. Most of the time people gets confused between rated voltage and normal voltage. Rated voltage is the highest voltage of a system for which the system is designed, whereas normal voltage is the normal voltage which will remain on the system normally. So, in this case 420kV is the rated voltage and 400kV is it’s normal voltage. Similary for 245kV voltage level, rated voltage is 245kV and the normal voltage is 220kV. For 145kV, rated voltage is 145kv and normal voltage is 132kV.
6. Rated frequency (Hz) – It is the power frequency on which electricity is generated transmitted and distributed. In some countries it is 50Hz and in some it is 60Hz.
7. Rated normal current (A) –
It is the rms value of rated current which circuit breaker can carry continuously. Or simply we can say this is the normal current of the system. It is given in Ampere.And the preferred values of rated current are – 400, 630, 800, 1250, 1600, 2000, 3150, 4000 A rms
8. Rated short circuit breaking current (kA) –
It is the highest rms value of short circuit current which circuit breaker is capable of breaking. It is given in kA rms. Rated short circuit current sometimes also called as symmetrical breaking current. On some nameplates you’ll find that short circuit current is given as symmetrical and asymmetrical current, like here you can see. The difference is that, Symmetrical current is the ac component of short circuit current which is equal to rated short circuit current. Whereas asymmetrical current is the combination of ac and dc components of short circuit current. So in the figure 1.1 you can see, asymmetrical current is greater than the symmetrical. So 61.2kA is the actual amount of current, this circuit breaker can break.
9. Rated duration of short circuit (sec)-
It is the time in sec for which the breaker can withstand the short circuit current. As per standard it can be 3 sec or 1 sec.
10. Rated peak withstand current or rated making current (kAp)-
If the circuit breaker closes during the existing fault, current may increase to a very high value during the first cycle, and the breaker has to withstand this high current and the mechanical forces caused by this current, this current is called as “short circuit making current”. Or it is also called as rated peak withstand current.
It is generally 2.5 times the rated short circuit current. It is referred in kA peak, as it remain for very short time. So if you multiply 2.5 with 50 which is rated short circuit current you’ll get 125 and that’s the making current.
12. Rated short duration power frequency withstand voltage (kV) & Rated lighting impulse withstand voltage (kVp)
These are the highest system voltages use to check the insulation properties of the equipment. It can also be called as insulation levels, if combined. Insulation level also includes switching impulse withstand voltage, which we’ll see shortly.
Power frequency withstand voltage can be caused by these reasons
- Phase to earth faults
- Load rejection
- Ferro resonance
- Ferranti effect
And hence breaker shall withstand power frequency voltage caused by these reasons. IEC has defined the level of power frequency voltage that can appear across breaker contact. So for example, for 420kV CB the power frequency voltage defined by IEC is 610kV rms. Circuit breaker has to undergo power frequency withstand test, in which power frequency voltage is applied to the circuit breaker for 1 min.
Lighting impulse voltage is generally generated due to lighting strokes, and of course breaker has to withstand these voltages too. For this also IEC has defined the values, based on the experience and system studies. For 420kV voltage level, lighting impulse voltage defined by IEC is 1425 kVpeak. Breaker has to undergo test for this also.
13. First pole to clear factor-
In 3 phase CB, arc extinguishes during current zero. As in 3 ph AC circuit currents are out of phase by 120°, current interruption in breaker is not simultaneous. Contact of one pole will open before the other two, and hence the power frequency recovery voltage across the first pole to open is more than the others. And this is called as first pole to clear factor. It is given as times the normal system voltage.
So, on the nameplate you’ll find it is mentioned as 1.3. This means first pole to open will have 1.3 times the normal system voltage across it, and the pole can sustain that.
14. Rated operating sequence-
This is one of the important parameter of the breaker, it is also known as “Auto reclosing duty”.
Operating sequence denotes the opening & closing operation breaker is capable of performing under specified conditions.
As per IEC 62271-1 there are two alternatives for operating sequence,
O – t – CO – t’ – CO
CO – t” – CO
O = Opening operation
C = closing operation
t,t’,t” = time intervals between successive operations
Let me tell you how auto reclosing works. 90% of the faults (like ) on the system are transient in nature, which remain in the system for a very short time and then the system goes back to normal. In such case it is beneficial to put the system live again, and here the auto reclosing system comes into picture.
We’ll consider the auto reclosing duty which is mentioned on our nameplate i.e. O-0.3 SEC-CO-3 MIN-CO. So, let’s say there is fault on the system the breaker will open then it will remain open for 0.3 sec. after 0.3 sec, it will close and if the fault is cleared it will remain close, but if the fault is still there then the breaker will open immediately. Now breaker will remain in open condition for 3 mins, after 3 mins the breaker will close again, and if the fault is cleared it will remain close, but if the fault is still there then the breaker will open immediately, and now breaker will remain open until it is closed manually.
So, this about operating sequence or auto reclosing duty.
15. Rated pressure of SF6 gas (Bar / Mpa) mega pascal-
This is the rated pressure of the SF6 gas in the breaker. It can me mentioned in bar or mega pascals or in kg/ sq. cm. This will vary manufacturer to manufacturer.
16. Total weight of SF6 gas (kg) –
Again this will vary manufacturer to manufacturer.
17. Total weight of CB (kg)
This shows the total weight of CB in kilo grams.
18. Rated control voltage (V DC) –
This is the DC voltage on which closing and tripping coil works.
These were the mandatory parameters as per IEC, which you’ll generally find on every nameplate of the high voltage SF6 circuit breaker.
Now let’s the condition based parameters.
Condition based –
1. Rated switching impulse withstand voltage (kVp) –
Switching surges are generally occurs above 245kV voltage level, and hence you’ll only find this parameter on the circuit breaker above 245kV level. It becomes important to test the breaker above 245kV voltage level for switching surges. Switching surges are generally caused by energization of lines or switching of transformers, etc. For 420kV voltage level switching voltage is specified as 1050kV peak. Here on the nameplate you can see. This also a part of basic insulation level.
2. DC component of short circuit current –
DC component is a component of short circuit current, and if it is more than 20% at the time of contact separation of CB, then it has to be on name plate. Which here you can see.
3. Rated line charging current (A) – above 72.5kV voltage level
This is the highest amount of line charging current a circuit breaker is capable of breaking. This type of current is generated because of the switching of loaded or unloaded overhead lines. So, for 420kV CB IEC has defined the rating equal to 600A, here you can see.
4. Classification – IF different form E1 – M1
IF the circuit breakers mechanical endurance class and electrical endurance class is different from M1-E1, then it has to be on nameplate.
Let me tell you what is mechanical endurance class and electrical endurance class. If the breaker is of M1 class which is also called as normal mechanical endurance class, then the breaker has to withstand 2000 no load operations during type test. If the breaker is of M2 class which is also called as extended mechanical endurance class, then the breaker has to withstand 10,000 no load operation during type test.
E1 stands for Electrical endurance. Generally most of the breaker nowadays are of E2 class. E2 class is an extended electrical endurance class which indicate that the interrupting parts of the breaker does not require maintenance during its expected operating life. On the nameplate if you see it is mentioned as M2 & C2. M2 class we just saw. But what is C2 class? Let’s see that. Breaker with class C1 indicates low probability of restrike during breaking of capacitive currents. Whereas C2 class breaker indicates very low probability of restrike during breaking of capacitive currents. Yes, low and very low that’s the only difference between C1 & C2 class CB.
So, this was about the condition based parameters. Now let’s look at the optional parameters.
Optional parameters –
1. Rated out of phase current (kA)-
If the breaker is used for synchronising two different system, it may happen that the breaker has to open when the systems are in synchronism procedure, and the current generated during this condition is called as out of phase current. This current gives the highest amount of transient recovery voltage across the breaker contacts and hence it is one of the critical duty to break. Generally out of phase breaking current is 25% of rated short circuit breaking current. Here on the name plate you can see the out of phase current is 12.5kA which is 25% of rated short circuit breaking current I.e. 50kA.
2. Rated cable changing (A) –
This is the highest amount of cable charging current a breaker is able to break. Cable charging current can occur while switching the unloaded cables. Don’t get confused between line charging and cable charging both are different. Line charging refers to overhead lines whereas cable charging refers to underground cables.
3. Rated single capacitor bank breaking current (A) –
This is the highest amount of a single capacitor bank current a breaker is capable of breaking. Switching of capacity and inductive current is a bit difficult task for the breaker, because voltage and current in capacity and inductive circuit is not in phase with each other. So, this is the single capacitor bank breaking current.
4. Rated back to back capacitor bank breaking current (A)—
Back to back capacitor bank switching is a special application, and not all the breakers are intended for this. While switching back to back captor banks inrush current is very high and hence there is a high possibility that the arc will restirke. So, if the breaker is made for back to back capacitor bank switching you’ll find Rated back to back capacitor bank breaking current in Ampere on the nameplate.
So, these are the optional parameters you can find on the name plate of HV or EHV circuit breaker.
I hope this article helped you and now you can identify the different parameters from the nameplate of Circuit breaker.
Watch my video on “Nameplate Details of HV & EHV Circuit Breaker”