Short Circuit Rating Selection Criteria For Circuit Breaker in PV Plants
Short Circuit Rating Selection Criteria For Circuit Breaker in PV Plants
Short Circuit Rating Selection Criteria For Circuit Breaker in PV Plants
By Pratim Dutta, Pradeep Solanki, Vishnu Shanker Vivek and Satnam Mahley
www.sterlingandwilsonsolar.com
Abstract:
A Circuit Breaker is the main component in a switchgear that breaks the circuit and isolates the protected equipment
from power system in case of a fault. However, while selecting the rating of a circuit breaker we normally specify only
the continuous current rating and short time current rating of the breaker, that might not guarantee the suitability of the
breaker for a given application.
In this paper the author intends to describe the other required parameters and methodology for calculation of these
parameters that proves the suitability of the breaker for being used in the power system.
Introduction:
A three-phase short circuit fault is considered to be the most severe fault in any power system. When a three-phase
short circuit fault occurs, the fault current in the power system comprises two components, i.e. symmetrical AC
component and the DC component of the fault current. Due to the presence of the DC component, the three-phase
short circuit fault current becomes asymmetrical in nature for the initial few cycles since the DC component of the fault
current practically lasts only for a few cycles from the inception of the fault.* Also, the combination of the DC
component and symmetrical AC component lead to a very high instantaneous value of the asymmetrical fault current
when it reaches the RST peak after the inception of the fault.
* Although the DC component of the fault current decays with time, it can never become zero theoretically. But in
practical applications it lasts in the system only for four-five cycles from the inception of the fault and then
disappears.
Note: - The purpose of this paper is to establish the parameters of the breaker, that are associated with its rated
interrupting capacity. Other fault ratings of a breaker like a short line fault current breaking capacity, small fault current
breaking capacity, out of phase fault current breaking capacity etc., are beyond the scope of this paper and hence not
mentioned here.
Figure - 1
The DC component of the fault current and asymmetrical fault current varies with time and depend on the time instant
at that they are measured and also on the symmetrical fault current of the system, system frequency and the value of
the X/R ratio of the system at the location of the fault. On the other hand, the peak short circuit current depends on the
symmetrical fault current of the system and factor ‘k’, that is further dependent upon the value of X/R ratio of the
system at the location of the fault. Hence, the very first step to calculate the DC component of the fault current and the
asymmetrical fault current and peak short circuit current of the system is to calculate the symmetrical fault current at
the fault location.
The symmetrical short circuit current at any particular location of the power system can be calculated by the MVA
method or by using equation No. 29 of IEC 60909-0 as mentioned in Eqn. A below.
The DC component of the fault current is calculated as per equation No. 64 of IEC 60909-0 as mentioned in
Eqn. B below.
Where,
idc = DC component of the fault current in kA
f = System frequency in Hz
t = Time at that idc is calculated in seconds
R/X = Reciprocal of the X/R ratio of the system at the location of fault
The asymmetrical fault current (iasym in kA) is calculated as per Eqn. C below.
The peak short circuit current is calculated as per equation No. 54 of IEC 60909-0 as mentioned in Eqn. D below.
ip = k * √2 * Ik” …..………..Eqn. D
Where,
Alternatively, the factor ‘k’ can also be found out from the Figure 2 below.
Figure - 2
The percentage DC component of the rated interrupting capacity of the breaker (denoted as ‘p’) is dependent on the
system X/R ratio or time constant and the rated breaking time of the circuit breaker as per Figure 9 of IEC 62271-100,
that is depicted in Figure 3 below.
Figure - 3
Once the value of ‘p’ is determined from the above figure, the DC component of the fault current withstand capacity
(Idc in kA) of the breaker can be determined from the following equation
Where,
Ist = Short time current rating of the circuit breaker in kA
The asymmetrical fault current withstand capacity (Iasym in kA) of the breaker can be determined from the following
equation
The peak withstand current capacity of the breaker (Ip ) is 2.5 times the short time current rating of the breaker for a
50 Hz system i.e
Figure - 4
Suppose a three-phase short circuit fault occurs on the bus bar of 33kV switchgear, in that case, the fault current
contribution from the power grid shall flow through the power transformer, 33kV cable between the transformer and
33kV switchgear and 33kV outgoing feeder circuit breaker up to the bus bar of the switchgear. Therefore, the 33kV
outgoing feeder breaker needs to interrupt the fault current to protect the 33kV switchgear bus bar. Hence in this
example we will evaluate the adequacy of the values of Idc, Iasym and Ip ratings of the breaker (as mentioned above in
Eqn. F, G and H respectively) with the corresponding values of the 33kV SWGR bus bar fault current i.e. idc, iasym and
ip respectively (as mentioned above in Eqn. B, C and D respectively).
The following assumptions are made for the purpose of this calculation.
Rated power output of the power transformer = 160MVA
Percentage impedance of the power transformer = 15%
X/R ratio of the system up to 33kV switchgear bus bar = 45
Rated breaking time of the breaker = 60ms
As mentioned earlier the very first step for the calculation of DC component of the fault current and the asymmetrical
fault current and peak short circuit current of the system is to calculate the symmetrical fault current at the fault
location, that can easily be calculated by MVA method. Therefore, for a fault on the 33kV switchgear bus bar, the value
symmetrical or steady state fault current (Ik”) can be calculated as follows: -
Ik” = Power transformer rated MVA / (percentage impedance of transformer * √3 * Nominal system voltage at the fault
location) = 160 / (0.15 * 1.732 * 33) kA = 18.66kA
Based on the above calculated value of Ik”, we can easily select the required short time rating of the breaker as 25kA,
i.e. Ist = 25kA. But selecting a 25kA breaker may not be sufficient for this application as the breaker rating needs to
pass the DC component of the fault current and asymmetrical fault current and peak short circuit current of the system
also to guarantee the successful fault interruption by the breaker as explained earlier. All the following criteria are
required to be fulfilled to prove the suitability of the 25kA rating breaker for this application.
Let’s now calculate the values of the above parameters individually by using their respective equation.
From the above calculations we can find that although the asymmetrical fault current withstand capacity and the peak
withstand current capacity of the breaker are higher than the asymmetrical fault current and peak short circuit current
of the system respectively, the selected breaker having rated short time withstand capacity of 25kA is not suitable for
this application since the DC component of the fault current withstand capacity of the breaker is less than the DC
component of the fault current of the system.
2 And/Or, We can select a circuit breaker that has a lesser rated breaking time, due to that the percentage DC
component of the fault current withstand capacity of the breaker shall be higher to meet the system
requirements.
3 And/Or, We can ask the breaker manufacturer to consider a higher X/R ratio value for the breaker design, although
the actual X/R ratio value of the system shall remain the same since it is dependent upon the system parameters.
In this case, the breaker manufacturer shall consider a higher X/R ratio to design the breaker interrupter, that will
increase the value of the parameter ‘p’ for the same rated breaking time of the breaker, thereby achieving a higher
value of Idc rating of the breaker with the same Ist rating of the breaker
For instance in Example-1, if we choose a circuit breaker having Ist rating of 40kA and rated breaking time of 40ms,
then we achieve Idc = 23.25 kA, Iasym = 46.27 kA and Ip = 100 kA against idc = 19.96 kA, iasym = 27.33 kA and ip =
51.12 kA that means all criteria are met and the breaker is suitable for the application.
Alternatively if the circuit breaker is designed considering X/R ratio value of 75, then having a breaker even with rated
breaking time of 75ms and Ist rating of 31.5kA will suffice all the requirements as we achieve Idc = 16.38 kA, Iasym =
35.51 kA and Ip = 78.75 kA against idc = 15.64 kA, iasym = 24.34 kA and ip = 51.12 kA that means the breaker is
suitable for the application.
Conclusions:
From the above discussion and worked out examples it can be concluded that while selecting the fault current ratings
of circuit breakers we need to crosscheck the rated short time withstand current, DC component of fault current
withstand capacity, asymmetrical fault current withstand capacity and peak withstand current capacity of the breaker
with the corresponding values of the system fault current to guarantee the suitability of the breaker for being used in
the system. Defining only the rated short circuit withstand capacity of the breakeris not sufficient.
Reference standard:
1. IEC 60909: Short circuit currents in 3 phase AC system: -
Part 0 – Calculation of currents
Part 1 – Factors for the calculation of short-circuit currents according to IEC 60909-0
SHORT-CIRCUIT REPORT
HT Panel Bus (PV Plant) TotalBus1 0.00 0.432 -18.313 42.4 18.318
Bus1 HT Panel Bus (PV Plant) 0.54 0.432 -18.313 42.4 18.318
SHORT-CIRCUIT REPORT
Making
ID kV ID Type Peak Ib sym Ib sym Idc I"k ip Ib sym Ib asym Idc Ik
HT Panel Bus (PV Plant) 33.000 HT Panel Bus (PV Plant) Bus 18.318 50.079 18.318
33.000 3150A, VCB CB 62.500 25.000 26.681 9.320 18.318 50.079 18.318 24.730 16.614*
Short Circuit Rating Selection Criteria for Circuit Breaker in PV Plants Published January 2021