Parallel switching phenomenon when MV/HV circuit breakers interrupt a shared fault current

What is the parallel switching?

Parallel switching occurs when two or more circuit-breakers are tripped to interrupt a shared fault current. This is typically the case for such bus arrangements as a double breaker, breaker-and-a-half, breaker-and-a-third, and ring buses. Ideally, all of the circuit-breakers should interrupt at the same current zero and this would probably be the case if all circuit-breakers were of the same type and technology, and contact parting was simultaneous.

In reality, the evolution of circuit-breakers has resulted in different circuit-breaker types and technologies being mixed quite randomly as stations are extended.

The purpose of this technical article is to examine this switching case and to relate it to oil, air-blast, SF6, and vacuum circuit-breakers operating in parallel in various combinations.

Parallel switching is influenced primarily by circumstances relating to the progressive development of circuit breakers and their actual application by and under the control of the users. It is therefore a user matter and this article provides guidance as to how the user can assess parallel switching with the necessary technical support from the circuit-breaker manufacturers.

Note that parallel switching should not be confused with evolving faults, the latter being a different phenomenon.

Table of Contents:

1. Circuit-breaker characteristics
2. Analysis and Rules
3. Parallel switching in practice
4. Conclusions

1. Circuit-breaker characteristics

Parallel switching is influenced by the types of circuit-breakers involved and their respective characteristics as follows:

• Relative mechanical opening times;
• Relative current levels;
• Relative arc voltages;
• Relative arcing windows;
• System and local impedances;

However, as will become apparent in the next Section “Analysis and Rules”, the factors that need to be considered are the relative mechanical opening times, the relative arc voltages and to some degree the system and local impedances.

Experience shows that the ranking of mechanical opening times from fastest to slowest most probably is as follows:

1. Air-blast circuit-breakers;
2. SF6 circuit-breakers;
3. Oil circuit-breakers;
4. Medium voltage circuit-breakers of all types.

As to the system and local impedances, the former will dominate and will give the same phase angle for the currents in the two circuit breakers in parallel.

The local impedances will determine the current division between the two circuit breakers. If the circuit breakers remain closed that current division will stand; however, once at least one of the circuit breakers contacts part, it is the arc voltage that will dominate in determining the current division and the ultimate outcome.

Recommended Reading – Fundamentals of High Voltage Circuit Breakers

Fundamentals of High Voltage Circuit Breakers, Switching Stresses and Failure Modes

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2. Analysis and Rules

The equivalent circuit for parallel switching analysis is shown in Figure 1.

Figure 1 – Equivalent circuit for parallel switching analysis

Where:

• V_s – Source voltage
• X_s – Remote source impedance
• X₁, X₂ – Local impedances
• I_F – Fault current
• I₁, I₂ – Current in circuit-breakers CB1 and CB2
• V_a1, V_a2 – Arc voltages of circuit breakers CB1 and CB2

The system can be treated essentially as a constant current source. Ignoring resistance for simplicity and thinking in terms of absolute values, and assuming that the contacts have parted on both circuit-breakers, the following applies (Equations 1 and 2):

I_F = I₁ + I₂ and I₁X₁ + V_a1 = I₂X₂ + V_a2

Combining these equations we get:

(I_F − I₂)X₁ − I₂X₁ = V_a2 − V_a1
I₂(X₁ + X₂) = I_F X₁ + V_a1 − V_a2

The condition for I₂ to go to zero, i.e. to commutate to CB1 is:

I_FX₁ +V_a1 −V_a2 = 0 or V_a2 = I_FX₁ +V_a1

Likewise for I₁ to commutate to CB2, the condition is: V_a1 = I_FX₂ +V_a2

The rule is illustrated in Figure 2 which shows an actual case of a disconnector being used to parallel switch between two transmission lines. After contract parting, it can be seen that the arc voltage builds as the arc is elongated and the current gradually transfers to the parallel line.

The current in the disconnector goes finally to zero when the arc voltage equals the open-circuit voltage across the switch. Note that this switching event relies on a natural commutation and thus takes a number of cycles depending on the series and parallel path impedances.

In the case of circuit-breakers, the commutation is forced and rapid and will take place within one half-cycle.

Figure 2 – Parallel switching between transmission lines with disconnector

Where:

1. Voltage across disconnector
2. Current in the disconnector
3. Current in parallel transmission line

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3. Parallel switching in practice

Clearly, many parallel switching scenarios are possible in practice. A base case will be considered and then variations will be discussed in relation to this case. The base case assumes simultaneous contact parting (in reality this may not always be the case particularly in fault events which cause several protection systems to operate) and about equal current in both circuit-breakers.

Actual sharing of current, in reality, is most likely to be reasonably close, say in the 60 % / 40 % range at worst. Table 1 shows the direction of current transfer for the base case (refer to Figure 2) with regard only to the hierarchy of arc voltages and the rule discussed above.

Table 1 – Current transfer direction for parallel circuit breakers with same contact parting instant and based on arc voltage

For high-voltage circuit-breakers, the mechanical opening times can vary from 16 ms for air blast and some SF6 circuit-breakers to about 25 ms for oil circuit-breakers. For medium-voltage circuit-breakers, mechanical opening times can be quite long and in the range of 40 to 60 ms.

Applying the rule, the following outcomes are probable:

Rule Outcome #1

If the contact parting on the same phase of both circuit-breakers is within the same half-cycle, current will start to transfer from the circuit-breaker that opens first (with reference to Equations 1 and 2, the arc voltage needs only to achieve a voltage equal to the voltage across the parallel local impedance).

If the transfer is not complete by the time the contacts part on the second circuit-breaker, then the transfer will be determined by the relative arc voltages.

Rule Outcome #2

If the contacts parting of the two circuit-breakers are in consecutive half-cycles, two outcomes are possible. If the contact parting of the first circuit-breaker to open is well prior to the current zero-crossing but less than the minimum arcing time, then the current will transfer to the parallel circuit-breaker during the period of significant change of arc voltage at the latest.

However, if the contact parting instant of the first circuit-breaker to open is close to the current zero-crossing, it may not develop sufficient arc voltage to effect a transfer and the transfer may then be determined by the relative arc voltages once the contacts part on the parallel circuit-breaker.

Rule Outcome #3

For medium-voltage circuit-breakers, where the difference in contact parting times can span about one cycle at 50 Hz or more than one cycle at 60 Hz, the above outcomes also apply with the added possibility of two current zeros between the respective contact parting times.

In the event of the latter possibility, the current will transfer from the first circuit-breaker to open at the second current zero at the latest.

Rule Outcome #4

In the event of a large current imbalance but dependent on the relative contact parting instants and the circuit-breaker types, the circuit-breaker with the lower current will probably commutate its current to the other circuit-breaker. The reason for this is the negative characteristic of the arc voltage.

An example of such a characteristic is shown in Figure 3.

Figure 3 – Arc voltage-current characteristic for an SF6 puffer type interrupter

Two references describing actual parallel switching tests between different circuit-breaker types are available and the above discussions can be tested against these tests [1, 2]. This is done in Table 2.

Table 2 – Analysis of actual parallel switching tests

Based on the limited information in the reference, the outcomes appear reasonably predictable, although it may be argued that some cases fall into grey areas. However, it can be stated with certainty that, if the characteristics of the arc voltages of both circuit-breakers is known, then the outcome can be definitively predicted.

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4. Conclusions

The perceived risk with parallel switching is that current transfer will occur after an arcing time greater than one loop and thus approach an evolving fault-type situation. To avoid this, it is often suggested that one or other of the two circuit-breakers be staggered by a time equal to one loop of current. However, introducing a time delay into a protective trip operation is unlikely to find favor with any utility and, on a balance of probabilities basis, appears to be unnecessary.

The reasons for this are discussed below bearing in mind that sometimes the unpredicted can indeed happen if only on a very low probability basis.

On the basis that the current will tend to transfer from the circuit-breaker which opens first, Table 1 can be revised as shown below in Table 3.

Table 3 – Current transfer directions for parallel circuit breakers with inherent opening times and arc voltages

* For like circuit-breakers with the same opening time, the current will transfer from the circuit-breaker with the highest arc voltage.

** Applies at medium voltage only and dependent on which circuit-breaker opens first.

At high voltages:

At high voltages, only in the case of an oil and an SF6 circuit-breaker is there an apparent conflict between the hierarchy of the arc voltages and the opening time. Dependent on the difference between the contact parting times it follows that:

Conclusion #1

If the contact parting time spread between the two circuit-breakers is at least a quarter-cycle, then the current in the SF6 circuit-breaker will probably transfer totally to the oil circuit-breaker before it opens.

Conclusion #2

If the contact parting time spread is less than a quarter-cycle, then either the above scenario will occur or the current will start to transfer to the oil circuit-breaker and then reverse direction due to the arc voltage in the oil circuit-breaker.

At medium voltages:

At medium voltages, the same reasoning applies for oil and SF6 circuit-breakers as discussed above. Where an oil or SF6 circuit breaker is paralleled with a vacuum circuit-breaker, the following is likely to occur:

Conclusion #1

If the vacuum circuit-breaker opens first, the current will transfer to the still closed oil or SF6 circuit-breaker if a current zero-crossing occurs before the latter circuit-breakers open. If no current zero-crossing occurs, then the current in the latter circuit breakers will transfer to the vacuum circuit-breaker on opening.

Conclusion #2

If the vacuum circuit-breaker opens last, the total current will be transferred to it.

Finally, what is the influence of frequency?

The answer to this is that 60 Hz is more favorable to successful parallel switching than 50 Hz. The reason for this is the more frequent occurrences of current zero crossings and their associated significant changes in arc voltage which are conducive to current transfer in one direction or the other.

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References:

1. GUIDE FOR APPLICATION OF IEC 62271-100 AND IEC 62271-1 – Cigre Working Group A3.11