Authors: Nils Wäcklén, Technical Product Manager, and Niclas Wetterstrand, Business Development Director - Protection
Reliability of high-voltage circuit breakers is a critical issue for all operators of power networks, as failures of these almost ubiquitous devices can have consequences that are not only costly, but also hugely disruptive. Reliability-centered maintenance (RCM) is, as the name suggests, the key to maximizing circuit breaker reliability, but implementing RCM depends on having dependable and up-to-date information about the condition of the breaker, as without this information it is impossible to decide where maintenance efforts and resources should be targeted.
In an ideal world, the best way to gather information about the condition of a circuit breaker would be to take it out of service and subject it to a comprehensive battery of tests. In reality, taking a breaker out of service for testing almost always presents challenges, given today’s pressing uptime demands and limited maintenance resources.
For this reason, more and more network operators are adopting procedures that involve performing tests on a breaker while it is in service - an approach that is usually designated as ‘on-line testing’. This cannot provide as much information as off-line testing, but it is fast and inexpensive to perform, and it does provide sufficient information to decide if and when the breaker needs to be scheduled for more comprehensive testing and maintenance.
There are two principal ways in which on-line testing can be implemented. The first is to permanently equip each breaker with instruments that monitor its performance. This has the benefit that data is continuously available, but the downside is that the cost of the instruments needed for comprehensive monitoring can be as much as half the cost of the breaker itself. Given the huge numbers of breakers in service, the high cost makes this option unattractive for implementing across a whole breaker fleet, although it might be appropriate for a small number of breakers in exceptionally critical applications.
The second option is to use portable test equipment to carry out measurements on energized breakers. This article focusses on this more affordable second option, although much of the information provided is equally applicable to testing with permanently installed instrumentation. The article looks at two important aspects of on-line testing for circuit breakers: first-trip testing and testing through the voltage detection system (VDS) output, and then offers a brief discussion of how condition monitoring achieved through online testing contributes to the implementation of RCM.
On-line testing of an in-service breaker without performing a trip operation guarantees absolutely minimal disruption but provides little information. A practical alternative – first-trip testing – can, however, do rather better. First-trip testing involves connecting the portable test equipment to the breaker while it is in service, tripping the breaker to allow the test equipment to make performance measurements, and then immediately reclosing it.
This form of testing not only minimizes disruption, it also provides invaluable information about breaker performance that cannot be obtained in any other way, even with off-line testing. This is because it captures data relating to how the circuit breaker operates after months or even years or inactivity. It is particularly useful for revealing lubrication and corrosion problems that might be missed by other forms of testing, and for measuring the ‘real-life’ trip time. In addition, first-trip testing is quick to set up, and the immediate reclose operation means that the breaker is out of service for only a few hundred milliseconds.
During a first-trip test, coil current and operating voltage are recorded, and through that, armature, or plunger, movement times and coil resistance is evaluated. Additional tests can include main and auxiliary contact timing and travel analysis for the trip mechanism. Arguably the most important, and certainly the most revealing of the measurements, is coil current, and a typical coil current trace is shown in Figure 1.
The mode of operation is as follows. When the trip coil is energized (1), current starts to flow through its windings. Initially (1-2) the current rises to a value that is proportional to the energy needed to move the plunger from its rest position. The plunger starts to move (2), operates the trip latch which triggers the trip mechanism (3-4), completes its travel (4-5) and hits a stop (5). During the movement of the plunger (2-5), the coil current decreases due to the opposing current generated by the plunger moving in the coil’s magnetic field. The portion of the curve from 4 to 5 is of particular interest as the plunger moves from the point where the trip mechanism is unlatched (4) to the stop (5). This section of the curve gives an indication of plunger speed: the steeper the curve, the faster the plunger is moving. After the plunger has completed its travel, the current signature changes. The rate of current increase now depends on the inductance of the coil and finally, when the current reaches a steady value (7), its magnitude is proportional to the DC resistance of the coil. At (8) the auxiliary contact opens to de-energize the trip coil, the current falls to zero (9) and the plunger retracts to its rest position.
The most useful way to interpret the coil current trace obtained during a first-trip test is to compare it with a reference trace obtained when the breaker was put into service or newly overhauled. Figures 3 to 7 show common problems that such comparisons can reveal. In each case, the reference trace is shown in black and the first-trip trace in red.
As part of a first-trip test, it is important to monitor the DC coil voltage as well as the current because proper operation of a breaker will only be achieved if the voltage applied to the coil is stable and of the correct value. Monitoring the coil voltage while the breaker operates gives a quick test of the battery system, including the wiring and connections between the batteries and the circuit breaker.
The most important information associated with a firsttrip test can be summarized as:
- Peak current
- varies with coil resistance and control voltage
- Control voltage
- must be measured to ensure comparability of coil current and timing measurements
- increased voltage drop indicates increased resistance in the coil wiring
- the reaction time of the plunger depends on the control voltage
- Coil resistance
- can be calculated from the control voltage and the peak current (if saturated)
- changes indicate shorted turns or other coil damage
- Plunger movement start and stop time
- increased times indicate increased mechanical
- resistance to plunger movement
- Plunger movement start current
- increased current indicates increased mechanical resistance
- start current gives an indication of the lowest operating/coil pick-up voltage
First-trip testing can generally be performed on all breakers using on-line test connections, which do not require the breaker to be de-energized or isolated. The connections needed to carry out first-trip tests are straightforward and, in most cases, they can be put in place in a very short time, usually no more than ten minutes. Figure 8 shows typical connections. Note that main contact timing connections (not shown in Figure 8) can easily be added either by means of current clamps used in conjunction with the secondary CT wiring or via VDS terminals.
Conventional timing measurements are off-line tests and, therefore, only performed on isolated circuit breakers. Out of necessity then, the switching commands for these tests are issued by the testing device. However, when performing on-line measurements, the commands come from the on-site control system or the control room, so the circuit breaker analyzer is set to behave as a passive recording device that acts on these trig signals instead of issuing the switching commands. Trig signals should be connected to the analyzer’s trig input and when the analyzer is armed, it will start the recording when a trig signal appears.
Voltage Detection System (VDS) enabled tests
In the case of circuit breakers in enclosed medium voltage (MV) systems, on-line test connections happen to be the only way in which main contact timing can be performed. Off-line (conventional) main contact timing is impossible on these type breakers because there is no access to the primary circuits. While on-line main contact timing tests can, of course, be made on these breakers using current clamps on the secondary CT wiring, Syna and Westnetz, working in conjunction with Megger, developed a simpler and safer alternative that makes use of a breaker’s voltage detection system (VDS) that is typically found on these type breakers. VDS is a capacitive voltage detection system, compliant with IEC 61243-5, which forms an integral part of the switchgear in which it is used; it provides a reliable, safe and responsive indication of whether a circuit is live.
When carrying out tests with VDS-equipped switchgear, connections are made to the VDS measuring point, which can be done while the equipment remains in service. Tests using VDS connections can be carried out with standard instruments like Megger’s TM1700 or TM1800; the only additional requirement being an inexpensive VDS adapter box (Figure 9) for main contact timing measurement.
As with first-trip measurements made with conventional test hook-ups, in a VDS-enabled first-trip test, an external trig signal is needed to initialize the measurement and the coil current and voltage should be measured. These connections are carried out the same way as shown in Figure 8. While not the case for circuit breakers in enclosed MV systems, even when safe access to the highvoltage components of a breaker is possible, if a VDS connection is also available, it is safer, faster and more convenient to use VDS test connections to test and time the breaker.
An advantage of VDS-enabled tests is that main contact timing is included with minimal effort. Conversely, and as mentioned earlier, with conventional first-trip test hook-ups, main contact timing (if desired) requires an additional step of measuring current in the CTs’ secondary windings by means of current clamps.
The drawbacks of this include:
- Current must be flowing through the breaker as there must be a load in order to get a signal on the CT. This is not optimal both for safety reasons and for the fact that you will probably disconnect a consumer during the operation. Even given a reclose with minimum delay, there will be a short interruption. Of course, this can be solved if the configuration of the substation allows the breaker to be placed in parallel with another.
- Three additional current clamps are required, and these are costly.
- It requires more work to find the CT’s secondary wiring and attach the current clamps. The wires are probably located behind the front plate of the breaker and are not straightforward to find. However, it is noted that you will need to open the cabinet and review a wiring diagram in any case to attach the current clamp for the coil current, the trig and voltage signal.
With a VDS-enabled first-trip test, the timing is measured through the VDS interface instead of the CTs. This means:
- Main contact timing is a bonus, requiring only connection to the banana connector interface on the front panel of the breaker. Except to measure the coil current, no current clamps are required.
- Most importantly, the breaker does not have to carry any load, i.e. the measurements work with or without load, but are of course safer to make without load. Provided that you test without load, the arcing time is minimized which gives you a more accurate timing as opposed to when you measure the CT’s current, which includes the arcing time. This can be up to a half period in a worst-case scenario.
- The need for three additional analogue inputs is eliminated since the VDS voltage is measured by the analyzer’s timing channels.
The evaluation methods are the same for both, with the small difference that in one case you analyze an AC current and, in the other case, an AC voltage. Air-insulated breakers, which are often equipped with VDS, provide the opportunity to perform on-line measurements using either VDS connections or off-line conventional test connections. These breakers enable a comparison between test results obtained with both methods, which in turn has shown that the results acquired with VDS connections are in line with the normal tolerances that would be expected when carrying out comparable tests using conventional connections. The results given by VDS tests are a little different in appearance from those obtained with conventional testing, as can be seen in Figures 10 and 11, but as these figures also show, interpretation of the results presents no problems.
Reliability centered maintenance (RCM)
Few would argue against the statements made earlier in this article that circuit breaker failures are both disruptive and costly, and that effective maintenance is the key to minimizing the risk of failure. There is not necessarily the same level of agreement about how that maintenance is implemented. The least proactive option is to carry out no maintenance at all, simply fixing faults as they arise. In today’s operating environment this is rarely, if ever, acceptable.
Some form of preventative maintenance is almost invariably adopted, but there are many variations. A traditional example is time-based preventative maintenance where circuit breakers are maintained at fixed intervals. The shortcomings of this approach are all too clear: some breakers are likely to be maintained when no work is needed, while others will develop problems before they are due for their next maintenance intervention. By today’s standards, time-based maintenance is inefficient and ineffective.
A much better option is condition-based maintenance, where the condition of the breakers is regularly assessed, and maintenance performed when – and only when –this assessment shows it to be necessary. An extension to this concept is reliability centered maintenance (RCM), which adopts the same ideas and techniques as condition-based monitoring but adds in consideration of the importance of the breaker in the network. Essentially, breakers that play an important role will have their condition weighted more discriminately than those in lesser roles.
There is currently a strong trend among network operators to adopt RCM but, in the present-day competitive environment, there is also a need to minimize costs. On-line testing of circuit breakers, as discussed throughout this article, plays a major role in making this possible and it is a particularly inexpensive and effective way of highlighting faults in the operating mechanism which, according to CIGRE international surveys on circuit breaker reliability, account for 70 % of circuit breaker problems.
On-line testing provides a reliable way of assessing circuit breaker condition and performance, without the cost and inconvenience of taking the breaker out of service. This makes it economically feasible for network operators to get a quick but accurate overview of the condition of the whole of their circuit breaker fleet.
First-trip testing is particularly valuable as it provides the comprehensive information needed to support RCM strategies with a minimum of disruption and in a very short time. Better still, the information relates directly to real-world conditions: it shows how the breaker will operate after a long period of inactivity. Finally, VDS is making online testing safer, as well as allowing it to be implemented in many applications where it was previously not possible. In summary, RCM supported by online testing is probably the most dependable and cost-effective way to guard against circuit breaker failures with their attendant potential for calamitous costs and dire consequences.