Circuit Breaker Interrupting Rating Calculations
Circuit Breaker. While the determination of the closing and latching duty is fairly a straightforward process, the calculation of the interrupting duty for medium- and high-voltage circuit breakers is a bit trickier. There are a few more things to consider in the calculation such as circuit breaker interrupting time and contact parting time, remote and local short circuit current sources (for Synchronous Generators), and circuit breaker rating structure.
Medium- and High-Voltage Circuit Breaker Rated Cycle
The calculation of circuit breaker interrupting duty depends on how fast it can clear the fault, more specifically, from the time of short circuit initiation up to the time when breaker primary contacts part. This duration is commonly referred to as the circuit breaker contact parting time or CPT.
Figure 1 shows the sequence of events from the initiation of the short circuit up to the extinction of arc on primary arcing contacts. It is important to note that the contact parting time assumes a relay time (time from fault initiation up to the time the trip command is given) of 0.5 cycles. The default contact parting time is dependent on the breaker interrupting speed. Table 1 shows the breaker interrupting time and its corresponding contact parting time.
Practical experience: Circuit breaker contact parting time is the rounded up (nearest whole number) value of half its interrupting (except 2 cycle CB). For example, the contact parting time of a 5 cycle circuit breaker is 2.5 rounded up, 3 cycles!
AC Decay in Short Circuit Calculations
Synchronous generators and induction motors’ short-circuit contribution varies with time. To account for this, an equivalent circuit is used with time-varying impedances driven by a constant voltage source. This approach simplifies the calculation process while still providing an adequate estimation of the short circuit current. Figure 2 shows a typical short circuit current waveform and its representation based on time-varying impedances.
Remember that the closing and latching duty calculations are based on symmetrical short circuit current from the 0.5 cycle network (also referred to as the subtransient network). The multiplying factors were derived from the ‘half-cycle’ current assumption. The calculation of the medium- and high-voltage circuit breaker interrupting duty, on the other hand, is based on its contact parting which ranges from 1.5 to 4 cycles as shown in table 1.
For this reason, the impedances to be used in the calculation of the symmetrical short circuit current should be based on the 1.5-4 cycle network (also referred to as the transient network). Table 2 shows the rotating equipment reactances from 0.5, 1.5-4, and 30 cycle networks.
DC Decay in Short Circuit Calculations
Short circuit current asymmetry results from the transient dc component that decays exponentially with time. A detailed calculation will require different rates of decay for various X/R ratio between one source and the fault point. For multisource systems, this could be quite cumbersome.
The use of a single equivalent X/R ratio was recommended in order to simplify the calculation process. However, this single X/R ratio is not your typical X/R derived from the Thevenin’s equivalent impedance. Instead, the single X/R ratio is to be calculated from the ‘separate X and R’ networks.
Separate X and R Networks
The rationale behind this is that the X/R ratio from the ‘separate X and R’ network will generally be greater than the Thevenin’s equivalent. Hence yielding a certain degree of conservatism. To illustrate this, consider the network shown in figure 3. The utility and generator are parallel sources to the fault.
Figure 4 shows the impedance diagram of the network shown in figure 3.
A Thevenin’s equivalent circuit will yield and equivalent impedance, Zth, as shown in figure 5.
Using the ‘separate X and R’ reduction, the network shown in figure 3 is decomposed into separate X and R networks as shown in figure 6.
From this network, the fault point X/R ratio is calculated.
DC Decay Multiplying Factors
DC decay in ANSI short circuit studies is accounted for by introducing multiplying factors to the symmetrical current. In the previous article, multiplying factors in the calculation of circuit breaker closing and latching duty were introduced. The instantaneous short circuit current is composed of two components, the transient dc component, and the steady-state ac component.
While the steady-state ac component is symmetrical, the transient dc component decays exponentially with time-based on the system X/R ratio. This creates asymmetry consequently increasing the magnitude of the fault current in the first few cycles of its inception. The multiplying factors in determining the first cycle asymmetrical peak and RMS value of the short circuit current are based on the ‘half-cycle’ current and a purely reactive circuit assumption.
Remote and Local Short Circuit Current Sources (for Synchronous Generators)
Synchronous generators have a special treatment in the interrupting duty calculation for medium- and high-voltage circuit breakers in that their short circuit contribution varies depending on their proximity to the fault. Generator contribution can either be local or remote. Generators are considered remote if
the generator contribution, Ig, to the fault is lesser than 0.4 times the value of a hypothetical three-phase fault, It, at its terminal, or
the per-unit impedance external to the generator up to the fault point is at least 1.5 times its per unit subtransient impedance on a common system MVA base or
the generator is located at least two transformers away from the fault point.
Otherwise, they are considered local to the fault.
Importance of Determining Remote and Local Contributions
The identification of generators as local or remote is important in determining the correct multiplying factors used in the calculation of breaker interrupting duty. For a specific generator, the multiplying factor, if it was identified as remote is greater than if it was local.
Why is this so?
If we inspect the network reactances for synchronous generators in table 2, the values are the same for the 0.5 cycle and 1.5-4 cycle networks. This is intentional since generator AC decay is conditional on its proximity to the fault. AC decay is considered only for local generators while remote generators are assumed to feature no AC decay. In other words, the remote multiplying factor for generators is higher in magnitude than its local counterpart.
Remote Multiplying Factors
Since the remote multiplying factor only accounts for the DC decay, it can be calculated analytically using the instantaneous short-circuit current equation with time t set to the circuit breaker contact parting time (CPT).
Local Multiplying Factors
The local multiplying factors, however, are dependent on a list of curves provided under the IEEE Std C37.5 for totally rated circuit breakers and IEEE Std C37.010 for symmetrically rated circuit breakers. The following figure shows the local multiplying factors for totally rated circuit breakers.
Totally and Symmetrically Rated Circuit Breakers
Medium- and high-voltage circuit breakers are rated either on a total current or symmetrical current basis under the applicable standard, IEEE Std C37.5 and C37.010, respectively. Totally rated circuit breakers reflect an earlier breaker rating structure while symmetrically rated circuit breaker reflect a more recent rating structure.
Both rating structures quantify the DC decay by applying local and remote multiplying factors. The difference between these rating structures is that symmetrically rated circuit breakers already have an embedded asymmetry factor, S. This asymmetry factor is based on a required percent value of the DC component on a standard time constant of 45ms corresponding to an X/R ratio of 17 for 60Hz system.
where
Figure 9 shows the required %dc component for different circuit breaker contact parting times.
Remote and Local multiplying factors for symmetrically rated circuit breakers are obtained by dividing the multiplying factors for totally rated circuit breakers by the applicable asymmetry factor, S. Table 3 shows the S factor for the typical circuit breaker contact parting times.
Adjust both the calculated remote multiplying factor and local multiplying factors obtained from the local curve to account for this embedded asymmetry using,
Calculate the circuit breaker interrupting duty using the ‘No AC Decay’ approach as recommended in IEEE Std 551.
where
References
ETAP Enterprise Solution for Electrical Power Systems Online Help
cual es la diferencia entre..INTERRUPTING RATING and SHORT-CIRCUIT CURRENT RATING
Hi Andres,
Short-circuit rating is a general term and is used interchangeably with interrupting ratings when you talk about low-voltage circuit breakers and fuses. For medium- and high-voltage circuit breakers, the short-circuit current rating actually refers to two specifications, (1) the close and latch rating, and (2) the interrupting rating. Let us know if you have further questions.
Thank you!