Data Analysis is the final step of the timing test.

The professional in charge has to have good knowledge of both the circuit breaker being timed and the network requirements. Since timing machines, nowadays, incorporate powerful analysis tools, a good knowledge of these tools is very helpful and time saver.

He has to have also a developed analytical sense and to be able to distinguish between the importance of the requested results and the consequences of non-conformity.

In addition to the entire above, one of the important factors in good data analysis is always experience.
 

 4.1  Timing Chart

The circuit breaker has to comply with user requirements, in addition to the international standards specifications.

The designer takes into account these requirements when designing a particular circuit breaker. The operation time references and tolerances are established, based on tests and a reference table, called a timing chart, is drawn up.

The timing chart contains time references for all the operation cycles the breaker is meant to accomplish. In addition to these time references, the designer may consider it useful to note other times to insure proper function of the breaker or any of its subassemblies.
 

 4.2  Priority

Following the data analysis important decisions are to be taken:

1. Put the breaker into service
2. Suspend commissioning operations and take further actions in order to correct any faulty condition.

In the first case, putting an unsuitable breaker in service would have disastrous consequences, either to the equipment or to the maintenance personnel.

In the second case, the cost would be enormous if it turned out to be unnecessary as a result of an incorrect analysis.

Good analysis and distinction of priorities are crucial.
 

 4.2.1  Priority levels of the operation times

There remains the question of determining the priorities in order to perform a successful analysis. The priority levels are described from high to low as follows:
 

 4.2.1.1  "TRIP" Time

Reducing the duration of short circuit currents on power transmission lines is a permanent objective.

The main advantage is higher transmitted power, since the power transmission stability threshold is higher when the short circuit duration is shorter.

The user determines the current interruption duration, For example 2 cycles.

The duration of the current interruption is counted starting from the instant the main mechanism coil energizes until the final interruption of the current in the main contact, including the arc duration.

The interrupting time is then equal to the mechanical time (operation time) plus the arc duration.

For a 60 Hz network, 2 cycles are equivalent to 2/60 = 0.03334 s = 33.34 ms (ms = milliseconds)

Since the timing test is done with the breaker out of circuit (no load), the arc duration is not measurable.

The arc duration depends on several factors: voltage level, interrupting medium, interrupting techniques, etc. It is determined during the design testing. The "TRIP" time is then adjusted to obtain the interrupting time.

The "TRIP" time is then, first and foremost, a user requirement the designer has accepted to respect. Nevertheless, an inappropriate "TRIP" time may cause important risks of different natures for a longer time or shorter one.
 

 4.2.1.2  Longer "TRIP" time

Many risks can be caused by a too long "TRIP" time. It can be anything from a simple anomaly in the tripping control circuit to a major failure in the main interrupting circuit.

The analysis should take care to examine all details to reach a precise conclusion. Breakers' characteristics play a fundamental role. Listing all probable cases is impossible.

In general, and independently of breaker's type, a longer "TRIP" time can be caused by a slower transition speed, the arc duration may be longer and premature contact wear may take place.

For small capacitive currents, the voltage spikes are strong and may cause a consecutive fault.

Consecutive faults are line to ground short circuits, consecutive to an interrupting of small capacitive or inductive currents.

The breaker that is interrupting a small current sees its current growing instantly to full short circuit current. Some breaker types have difficulty in correcting this situation.

A known method to overcome the consecutive fault is to interrupt with high speed breakers. Lower speed can be crucial in this case.

Conclusion:

Reduced tripping speed can compromise the operation of the breaker and possibly that of the power network itself, not to mention the risk of consecutive faults.
 

 4.2.1.3  Shorter "TRIP" time

Following a short circuit, the nominal alternating current flowing in the circuit grows instantly to a huge value, of the same nature and frequency called the symmetrical short circuit current.

Figura 4.2.1.3a - Short circuit current

Due to the network's inductive nature, a temporary DC component adds itself to the symmetrical short circuit value. The result is called an asymmetrical short circuit.

The initial value equals the instantaneous value of the symmetrical short circuit at the point of the short circuit with a negative sign. It decreases afterward, following a damped exponential curve, with a speed determined by the time constant of the circuit.

The breaking capacity of a particular breaker, is the highest value of current the breaker is capable of interrupting.

The breaker is supposed to interrupt successfully every current equal or less to its breaking capacity, whether the current is symmetrical or asymmetrical.

Considering the curve in figure 4.2.1.3b, one notes that the asymmetrical value is function of the interrupting time.

Figura 4.2.1.3b - Nominal Value of aperiodic component

If it is higher, if the interrupting time is shorter. As a result, if the breaker is too fast the asymmetrical value can exceed its breaking capacity, and breaking is no longer ensured..

The curve of figure 4.2.1.3b shows the nominal value of the aperiodic component as a function of the opening time of the breaker. This curve uses a damping time of 20% per hundredth of a second.

Conclusion:

The "TRIP" time should never be less than the reference value, otherwise the asymmetrical short circuit value can exceed the breaker's breaking capacity.
 

 4.2.2  Contacts discrepancy

High voltage breakers are three-phase apparatus. It contains at least one contact per phase, and in some cases, multiple contacts in series per phase, up to 12 per phase for certain air blast breakers at 765 kV.

It is crucial for the proper operation of the breaker and of the network to limit the time discrepancy between the contacts.

The types of discrepancies can be divided into 2 groups:

1. Contact discrepancy between poles,

2. Contact discrepancy in the same pole.
 

 4.2.2.1  Contacts discrepancy between poles

On Trip:

According to IEC 56 (parag.3.3.1) the phase is considered open when the first contact of the pole is open.

The biggest discrepancy measured should not exceed a maximum value set by the designer, the user, or by an agreement between them.

If not conforming:

Poles' contacts separation has to be simultaneous to prevent high voltage transients, otherwise it would attain double the rated value on the first parting pole. The maximum discrepancy allowed is 1/6th of a cycle.

On Close:

According to IEC 56 (parag.3.3.2) the phase is considered closed when the last contact of the pole is closed.

The biggest discrepancy measured should not exceed a maximum value set by the designer, the user or by an agreement between them.

If not conforming:

The sudden energizing of circuits is always followed by a moderate voltage increase, with the exception of long, unloaded transmission lines, where the voltage rise can be critically dangerous.

When a line is connected to an energized network, a voltage wave is forced on the line. This wave is reflected back at the end of an open line, and returns with double the amplitude.

Even higher voltages may be encountered when the line has a load before being reenergized, and if the breaker closes at the moment that the polarity of the network is opposite to that which was present on the line.

The voltage may then be three times the network voltage, after reflection of the wave. This situation may be produced with a rapid reclose of a line.

Still higher voltages may be encountered on three-phase lines, when the three poles of the breaker do not close simultaneously.

A wave on one phase will produce induced waves in the other phases, and under unfavorable conditions, will increase the voltage on another phase.

Higher transition voltage rises can be encountered if the discrepancy on closing is too high.

Note that on networks where the nominal voltage is 500 kV and higher, the isolation of the lines is determined by the operation voltage spikes.
 

 4.2.2.2  Discrepancy between contacts of the same pole

For multiple-contact-per-pole breakers, grading capacitors are installed in parallel with each contact to equalize the voltage when contacts part.

In general, the fastest contact has the longest arc duration and higher contact wear.

In case of excessive discrepancy, the fastest contact on close and slowest on trip would cause higher voltage shocks to their grading capacitors, thus reducing their life expectancy and that of the contacts.
 

 4.2.3  "CLOSE" time

During closing, especially on short circuits, opposite forces are considerable. In case of slow moving contacts the pre-arc has a longer duration thus causing more contact deterioration.

If the closing time is not respected, this would compromise the relative guarantee to the closing capacity.

This time is usually supplied by the designer of the circuit breaker.
 

 4.2.4  Operation cycles

An operation cycle is a sequence of basic "CLOSE" and "TRIP" operations in specified time intervals.

The most common sequences are conform to the following formula:

O -> T -> CO -> T' -> CO

O: Trip operation

CO: Trip-Free cycle

T: Time delay of 0.3 seconds or 3 minutes

T': Time delay of 3 minutes
 

 4.2.4.1  Trip-Free (CO) cycle, short circuit time

CO cycles simulate closing on a short circuit. In the actual event, the breaker closes first, then the protection relay system detects the short circuit and trips the breaker.

In the test event the timing machine can be programmed to launch a trip command as soon as the contacts close. This gives the fastest short circuit time the breaker is capable of doing.

This value is compared with the designer's references.
 

 4.2.4.2  Reclose-Open (OCO) cycle, isolation time

Experience shows that a great number of short circuits are temporary. It means they are caused by an event that disappears shortly after the breaker opens. A few examples are: short circuits caused by lightning, a bird, fallen trees or branches, etc.

The purpose of fast Reclose is to reduce the duration of power interruption.

It is important to reduce this duration to a minimum. For the out of service circuit, it is important to give it enough time to clear the fault.

In effect, temporary faults create arcs; once the power feeding this arc is cut, enough time needs to pass for the arc plasma to deionize before reconnecting power, or another trip may occur.

Statistics show that a 0.3 sec duration between the contacts opening and the contacts closing is enough to achieve this goal.

If on closing another trip occurs, the breaker will have to interrupt the short-circuit a second time. There will have to be a sufficient delay between the interruptions for the interrupting medium to regenerate, so the second interruption will be performed correctly. If the breaker trips a second time, it should remain open.

High-voltage transport and interconnecting networks

Automatic fast reclosing avoids tripping between two interconnected sources.

In effect, when breakers on an interconnecting line between two networks trip, there is a rapid loss of phase synchronization if this line is the only one interconnecting them. If there is another line running in parallel, it may trip in turn by overloading, which causes a loss of synchronization.

This de-synchronization may be avoided, when the faults are temporary, by quickly reclosing the breakers before the phase shift becomes too great.

If the tripped circuit is three-phase, the reclosing time must be very short, on the order of 0.3 seconds. If the reclose time is longer, there is a risk of closing on a phase discordance.

During timing tests, the O-0.3s-CO test verifies the behavior of the circuit breaker in this particular type of operation.

On Reclose-Open timing test event, the timing machine is programmed to delay the closing command until the isolation time of 0.3 seconds is obtained. This delay must not be confused with that of certain type of delaying devices installed on some breakers, even if these times are similar.