PROTECTION AND CONTROL

Circuit Breaker Timing Tests - Part 2: Timing Tests Are a Powerful Tool for Troubleshooting Faulty Breakers

Mechanical operation means all operation or cycle of operation the breaker is intended to do without being connected to the power grid.

It is necessary to time each breaker in order to have its functional signature. Incorrect operation can have disastrous consequences on the equipment or the substation personnel., not to mention the out of service losses of revenue and the repair costs.

Timing tests are first done in the factory during routine testing and after installation, during commissioning.

These tests have to be done periodically in order to validate the good working order and reliability of the breaker. Timing tests are also a powerful tool for troubleshooting faulty breakers.

Description
To measure the operation times of a breaker, we need a device capable of detecting the instant the contact's state starts changing after the operation order was launched.

This device sends electric signals via cables connected to each contact, in the case of several contacts in series each signal has its own source to eliminate interference.

The signal has two possible states. The first is when the contact is closed, the second, when the contact is open. All is recorded for consultation and analysis.

These special devices are called timing machines or circuit breaker analyzers. They are designed to generate all the needed signals and incorporate a data acquisition system.

Four steps constitute the main activities to accomplish a timing test:
1. Cable installation and connection
2. Data Acquisition
3. Data interpretation
4. Data analysis

Installation and connection
The connection, mainly between the timing machine and the breaker, has to be done correctly, taking into account certain external factors such as magnetic induction by nearby high voltage transmission lines.

Another connection has to take place between the timing machine and the mechanism coils responsible for launching the operation order. Some other connections may also be required such as a travel transducer, a pressure transducer, auxiliary contacts, etc.

There are no special rules applied as long as the wanted information is obtained. But, in order to avoid any unpleasant surprises, some precautions should be observed.

Connection to main contacts: each breaker contact has to be verified separately. For multi-contacts per phase, each contact has to have its own verification circuit. Each verification circuit includes a voltage source that injects current on contact making; a detection circuit to detect the current and determine if the contact is closed; two-wire shielded cable to carry the signal. The timing machine supplies the verification circuits. Each verification circuit is called a channel.

Figure 3.321 shows en example of how the main contacts are connected to the channels.

For multiple contacts per phase, special care has to be observed to avoid the mirror effect (treated later in this article), where the data for some contacts will be corrupted by the data from other contacts.

Connection to mechanism (command) coils: the circuit breaker's control circuit has various safeguards against malfunctions, for instance against phase discrepancy, or against pumping. It is important not to bypass any of these protection circuits. A connection that bypasses the auxiliary contacts may cause the destruction of the coils.

Mirror effect: No intervention on high voltage equipment should take place without proper protection of the intervening personnel first and of the intervened equipment. One of the most important protection measures is the installation of ground cables. Ground cables connect the line to the ground. In case of accidental energy supply to the line the current will pass through the ground cable and avoid the working personnel. Ground cables should be installed on each side of the breaker.

In the case of timing a multi-contact phase, the presence of ground cables on both sides of the contacts may mask the signal of a faulty contact, making the fault undetectable.

Figure 3.3.13 shows this phenomenon.

If contact 2 stays open, while contact 1 closes, the channel 2 circuit will detect the current passing through the ground cables and will show contact 2 as being closed.

The solution would be to break the return circuit of the ground by disconnecting the breaker from the line, between the extremity of the breaker and the grounding cable next to it. By no means should the grounding cable be removed, under the risk of injury.

Data Acquisition
In addition to the signal generation, timing machines are in charge of contact transition detection and recording.

The earliest timing machines used light reflected on moving mirrors driven by the current signal coming from the contacts. The light is projected on paper film, thus creating a visible trace on the paper.

Today, in the computer age, timing machines have undergone a tremendous evolution.They use electronics and computer technology for data acquisition. Powerful software serves to analyze and conserve data for future studies. Data transmission has never been easier. Decisions can be made quickly and accurately.

Data Interpretation
Many external agents can influence greatly the collection of information. It is important to distinguish from the collected information, between useful data and external noise.

Good interpretation is based on three main principles:

• Know the breaker and the surrounding environment: induction due to the proximity of overhead power transmission lines, or a bad cable connection could alter the collected signal.
• Know the timing machine: incorrect programming or a failed circuit can cause unnoticed signal alteration as well.
• Know the significance of the values looked for: when timing a breaker, we are looking for certain values, such as: contact switching times, coil energizing times, etc. that can allow quick identification of a problem, and if a value seems excessive, we can look to it in time and repeat the test before taking all the cables down.

As mentioned before, these are not the only operations that the breaker is meant to do. In most cases it has to execute on demand a combination of the basic operations, called cycles. The most popular are the following:

• 'TRIP FREE', (C-O): simulates a trip on short circuit after a 'CLOSE'. The breaker must open instantly.
• 'RECLOSE', (O-C): Simulates a fast close after short circuit trip to re-establish the current.
• 'RECLOSE-OPEN', (O-0.3s-C-O): Simulates a 'RE-CLOSE' on short circuit. The breaker should clear the fault successfully.
• (C-O)-15 sec-(C-O)-15-sec-(C-O): Simulates a multiple close after short circuit trips for the purpose of r-eestablishing the current, hoping that the short circuit has disappeared. This cycle is used mostly in medium voltage applications.

General Definitions
Before proceeding with the operation times, here are some of the most used terms in the breaker timing field.

• Interrupting element (or unit): also called arcing chamber, is a closed volume containing the main contacts and in which current interrupting occurs.
• Pole: a pole is a part of the breaker that is installed on a line phase. The breaker is installed on three-phase line. It necessarily has three poles. Each pole includes at least one interrupting element. For high voltage levels, multi-element poles are most common.
• Main contact: these are the contacts in charge of establishing or interrupting the current flow in power circuits. They include a fixed and a moving contact. The contact's material has to be chosen to have minimum resistance when closed, to minimize the losses by joule's effect when transiting nominal currents. The best-suited material is silver plated copper.
• Auxiliary contact: as mentioned earlier, insertion resistors are usually used on high voltage circuit breakers, for closing, opening, or both. These resistors minimize the transition voltage before closing or after opening by engaging or disengaging auxiliary contacts in two or three steps.
• Arcing contact: for breakers, the arc can be a powerful generator of heat energy. This heat can cause fast deterioration of the main contact's material. To extend the life of the main contacts, breaker designers tend to separate the permanent current carriers, called 'Main Contacts', from those subjected to the arc effect, called 'Arcing Contacts'. The most common material for arcing contacts are, in general, tungsten alloys, reputed to have high arc resistant quality, but are less conductive than silver-plated copper.
• Indicating contact: in order to control the breaker on low voltage side, two types of contacts were created, (a) contact and (b) contact. They are both driven by the operation of the breaker and switch states with the main contacts. They are mainly used to indicate the breaker's position and to electrically interlock between basic operations.
  • 'a' contact: it has the same state as the main contact. It closes when the main contact closes, and opens when the main contact opens.
  • 'b' contact: it has the opposite state of the main contact. It closes when the main contact opens, and opens when the main contact closes.

Since we are talking time, we should determine the same reference points in reading the information.

Earlier, the designers determined reference values. The users also had their reference values. This created confusion between the designer's reference values and the user's interpretation to the collected values. In order to solve this problem, professionals tended to use the definitions as stated by international standards. The most widely used is the IEC 56 international standard.

IEC 56 INTERNATIONAL STANDARD
The IEC 56 international standard defines these times as follows:

Opening Time (IEC 56 3.105.32)
For a circuit breaker tripped by any form of auxiliary power, the opening time is the interval of time between the instant of energizing the opening release, the circuit breaker being in the closed position, and the instant when the arcing contacts have separated in all poles.

Notes:

• The opening time may vary significantly with the breaking current.
• For circuit breakers with more than one interrupting unit per pole the instant when the arcing contacts have separated in all poles is determined as the instant of contact separation in the first unit of the last pole.
• The opening time includes the operating time of any auxiliary equipment necessary to open the circuit breaker and forming an integral part of the circuit breaker.

Note: The closing time includes the operating time of any auxiliary equipment necessary to close the circuit breaker forming an integral part of the circuit breaker.

Open-Close Time, O-C or Isolation time (IEC 56 3.105.38)
The interval of time between the instant when the arcing contacts have separated in all poles and the instant when the contacts touch in the first pole during a reclosing operation.

Close-Open Time, or short-circuit time (IEC 56 3.105.42)
The interval of time between the instant when the contacts touch in the first pole during a closing operation and the instant when the arcing contacts have separated in all poles during the subsequent opening operation.

Note : Unless otherwise stated, it is assumed that the opening release incorporated in the circuit breaker is energized at the instant when the contacts touch in the first pole during closing. This represents the minimum close-open time.

Minimum Trip Duration (IEC 56 3.105.44)
The minimum time the auxiliary power has to be applied to the opening release to ensure complete opening of the circuit breaker.

Minimum close duration (IEC 56 3.105.45)
The minimum time the auxiliary power has to be applied to the closing device to ensure complete closing of the circuit breaker.

DATA ANALYSIS
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 saving.

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

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

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.

Priority
Following data analysis a decision must be made on whether to put the breaker into service or suspend commissioning operations and take further actions 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.

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

'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. Nevertheless, an inappropriate 'TRIP' time may cause important risks of different natures for a longer time or shorter one.

Longer 'TRIP' time: Many risks can be caused by a 'TRIP' time that is too long. It can be anything from a simple anomaly in the tripping control circuit to a major failure in the main interrupting circuit. The analyst should take care to examine all details to reach a precise conclusion. Breakers' characteristics play a fundamental role. Listing all probable causes 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.

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. 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 every current equal to or less than 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 a function of the interrupting time.

If it is higher, 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.

Contacts discrepancy
High voltage breakers are three-phase apparatus. They contain 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:
Contact 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: a pole's contact 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.

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.

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.

'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.

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

The most common sequences conform to the following formula:

O --> T --> CO --> T' -->CO
where:
O: Trip operation
CO: Trip-free cycle
T: Time delay of 0.3 seconds or 3 minutes
T': Time delay of 3 minutes

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.

Reclose-Open (OCO) cycle, isolation time
Experience shows that a great number of short circuits are temporary. This 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 desynchronization 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 types of delaying devices installed on some breakers, even if these times are similar.

Conclusion:
Accurate analysis makes it possible to make decisions that are profitable to the breaker, the network and to maintenance personnel. In order to achieve this, knowing the timing machine and the significance of the operating times is important but not enough. Knowing the breaker itself, the reference values (timing chart) and the network characteristics is also necessary and should be backedup with the experience and judgment of the testing personnel.